Incineration Bottom Ash Leaching Properties

23th August. 2010, DTU no. 50443 Appendix A1 Literature review – Incineration bottom ash leaching properties

Energinet.dk project no. 2006 1 6368

Improved electrical efficiency and bottom ash quality on waste combustion plants

Igor Nesterov, Peter Arendt Jensen, Kim Dam-Johansen

Department of Chemical and Biochemical Engineering

Technical University of Denmark

Søltofts Plads, Building 229, DK-2800 Lyngby, Denmark

CHEC no. R1003

Table of content

1. Introduction and objectives 3 2. Short description of a typical Danish waste incineration plant 4 3. Composition and microstructure of MSWI bottom ash 5 4. Waste residual characterization methods 10 5. Utilization and leaching properties of incineration plant bottom ash 15 6. Influence of different parameters on slag leachebility 19 7. The influence of thermal treatment on slag parameters 23 8. Resume and conclusion 32

References 35

1. Introduction and objectives

Municipal solid waste incineration (MSWI) plays an important role in the Danish waste management system. Usage of incineration as next to recycling and in prior to land filling is decided as the Danish waste handling policy. By waste combustion (incineration) the waste is turned into a smaller amount of residue (up to 5-6 times decreasing in weight) and electricity and heat are also produced through the process. The most abundant residue from a waste combustion plants is the bottom ash, also often called slag. The slag can be utilized to some construction purposes, if mainly the leaching of heavy metals from the slag can be controlled to within some specified limits. One of the objectives of this research project is to investigate the possibilities to reduce the leaching of heavy metals from the slags by a thermal treatment of the slag in a rotary kiln. The present report is made to make a basis for the preformed slag treatment experiments.

Data presented at the table 1.1 shows that in 2005 incineration produced amount of residues five times less to the waste input. Taking into account average production of heat and electricity per ton of waste (2 MWh of heat and 0,67 MWh of electricity) (RenoSam, 2006) it could be calculated that about 2300 GWh of electricity have been produced in Denmark in 2005 from waste incineration plants. A total amount of 588.354 ton or 87% of the waste incineration residues were recycled that year. All the recyclable residues belong to the slag fraction which includes MSWI bottom ash and grate siftings (further referred as slag or MSWI bottom ash). High recycling potential of that fraction is caused by the mechanical properties of bottom ash close to those of natural gravel and sand (Rogheck, Hartlen, 1996) and a relatively low mobility of heavy metals and other pollutants from the bottom ash.

Table 1.1. Data on the waste and waste incineration residues treatment for the year 2005 in Denmark (Danish Ministry of the Environment, 2007)

Tons % Total production of waste 14.210.000 100 Recycled 9.545.000 67 Incinerated 3.473.000 24 Land filled 983.000 7 Production of incineration residues Total amount of incineration 678.236 100 residues Slag (bottom ash and grate 595.272 88 siftings) Fly ash and flue gas cleaning 82.964 12 products Waste incineration residues treatment Slag land filled 6.918 1.02 Fly ash and flue gas cleaning 260 0.04 products land filled in Denmark Fly ash and flue gas cleaning 82.704 12.19 products land filled abroad Slag sent to reprocessing plants 102.432 15.10 Slag sent directly to recycling 485.922 71.64

Meanwhile environmental legislation becomes stricter in terms of ground water pollution prevention and this fact makes recycling of MSWI residues more difficult. Statutory Order BEK nr 1480 of 12/12/2007 (Danish EPA, 2007) provided the restrictions for construction materials based upon both content and leaching ability of heavy metal; limitation values for organic substances content were also planned in the Waste Strategy 2005-2008 for Denmark. Thus, as higher leaching rates for some heavy metals from MSWI bottom ash in comparison to natural gravel have been proved by many laboratory and field studies (Åberg et al., 2006; Bruder-Hubscher et al., 2001) treatment of bottom ash is essential to allow its further utilization. Thermal treatment seems to be a promising technique as it can be done directly at the waste incineration plant. The changes in bottom ash structure occurring at high temperatures can provide both physical and chemical fixation of the heavy metals and making them less accessible to the leaching. The aspects of the MSWI bottom ash thermal treatment to produce a more ecologically safe construction material will be discussed in this literature report.

2. Short description of a typical Danish waste incineration plant

For the year 2006 a total number of 29 Danish waste incineration plants were reported (RenoSam, 2006). All of them are waste-to-energy facilities producing heat for district heating or heat and electricity. Typically waste coming to incineration has a composition which can be summarized as (Chandler et al., 1997): - organics – 40% - paper – 35% - metal – 5% - plastics – 5% - glass – 4% - textiles – 3% - Other – 8%. The capacity of Danish incineration plants varies from 2.5 t/h for the facility in Grenå to 83 t/h for the Glostrup Plant, but typically the Danish incineration plant has the capacity of about 20-30 t/h for two boiler lines (RenoSam, 2006) and consists of the following blocks (RenoSam, 2006): - waste reception and feeding system - incineration units equipped with an air-cooled grate system

- a steam boiler with de NOx system - A flue gas treatment system.

Fig. 2.1. Schematic view of the waste incineration plant in Aalborg as example of a typical Danish waste incineration plant (Skovhaug, 2007, p. 50 of the document)

Taking the Reno Nord waste incineration plant in Aalborg (see Figure 2.1), Denmark as an example of a typical incineration facility some plant parameters can be explained in more details. The MSWI plant in Aalborg has two oven lines of 10 and 20 t/h capacity with air-cooled BS grate, receiving waste with a typical calorific value of 12 MJ/kg. The feeding system is equipped with a hydraulic pusher. The boiler equipped with an economizer generates 80 t/h steam at 50 bar, 425 oC. The plant produces 18 MW of electricity and

43 MW of heat for district heating. The flue gas treatment system includes a de-NOx system (dosing of ammonium to the boiler) and an electrostatic precipitator. (Babcock&Wilcox Vølund, 2008). The waste incineration in Aalborg provides the residues in the following amounts (Skovhaug, 2007): - bottom ash – 4-5 t/h - fly ash – 400 kg/h

- gypsum from the SO2 removal system – 75 kg/h - Sludge and wastewater from wastewater treatment.

3. Composition and microstructure of MSWI bottom ash

A problem regarding MSWI bottom ash studies is the high variability in terms of chemistry and mineralogy. Both chemical composition and microstructure of bottom ash depend on such factors as composition of the received waste and the incineration process properties. Composition of waste sending to incineration certainly varies from one region and one country to another and incineration conditions vary among the incineration facilities, but even for one incineration facility variations in bottom ash structure affecting its’ mechanical properties have been found as seasonal changes in the received waste composition occur(Arm, 2004). Thus, understanding the variability of MSWI bottom ash chemistry and mineralogy is a key to study its’ properties and influence of various treatment methods. 6

Data from the following sources representing the MSWI bottom ash of Europe have been selected: - (Hjelmar, 1996) The elements’ concentration ranges for bottom ashes from Danish and Swedish waste incineration plants; - (Selinger et al. 1997) - Original data and cited from (Faulstich, 1993) for the German waste incineration plants; - (Barbieri et al., 2000) - Bottom ash compositions for the Reggio Emilia, Italy; - (Izquiedro et al., 2001) – Ranges of the bottom ash compositions for the five waste incineration plants in Catalonia, Spain; - (Bethanis et al., 2002), (Cheeseman et al., 2003), (Cheeseman et al., 2005) – Data on the bottom ash compositions for the waste incineration plant in SE England. The data were obtained during the period of some years. - (Rendek et al, 2007) – Data on the bottom ash compositions for five waste incineration facilities in France. - (Hyks et al., 2009) – composition of the bottom ash for the Danish MSWI plant (Vestforbrænding). All data is shown in the table 3.1 as concentration ranges for each element. Composition of the Danish MSWI bottom ash from the Vestforbrænding incineration facility is given separately to show its’ position within the European concentration ranges. Limiting values for the Category 1 of the Danish statutory order on utilization of soil and incineration residues are also presented at the table. Soil and residues of Category 1 are considered as environmentally neutral and could be directly used as topsoil. It can be seen from the table 3.1 that neither average European nor Danish MSWI bottom ash suits the conditions of Category 1. Variability of bottom ash composition for the selected sources is presented at fig. 3.1 as the average deviation of an elements’ concentration relative to their average values (%). As it can be seen the most abundant elements as silicon, calcium and aluminum have relatively low variability, but concentrations of heavy metals mentioned in the Danish regulations as copper, lead, chromium and cadmium vary in wide ranges. Another important fact regarding chemical composition of MSWI bottom ash is that the distribution of the elements among the MSWI bottom ash particles of different size is not uniform. Coarse grains contain more SiO2, Al2O3, Fe2O3 and less CaO relative to the fine ones. (Pfrang-Stotz et. al, 2002), (Liu et al., 2008) and (Speiser et al., 2000). Study of MSWI bottom ash mineralogy is a difficult task. Only basic generalization can be made from the studied data because sets of mineral phases given by different authors vary in wide ranges due to the following reasons: 1. MSWI bottom ash is not a compound with a uniform structure Pfrang-Stotz and Schneider (Pfrang- Stotz, Schneider, 1995) consider three types of bottom ash components: - vitreous bottom ash (fig. 3.2) – black glassy or obsidian-like particles with a bright surface; - porous bottom ash (fig. 3.3) – light/dark brown, grey or reddish with the degassing voids of different size, may be the predecessor of the vitreous type; - Microcrystalline bottom ash (fig. 3.4) – dense particles of grey or brown/black color. 2. Changes in the mineralogical composition because bottom ash is considered to be thermodynamically metastable. Different reactions and alteration of the phases are observed from the time right after the incineration. (Speiser et al., 2000), (Piantone et al., 2004) 7

Table 3.1 Chemical composition of MSWI bottom ash

Element Element content, wt % Dry Matter Minimum Maximum Average For Denmark (Hyks et al., Limiting values for 2009) – MSWI bottom ash Category 1 of Danish from the Vestforbrænding statutory order on plant utilization of soil and incineration residues Si 14,9 29,0 19,8 24,8 Ca 6,50 16,3 12,0 10,8 Na 0,789 4,70 2,60 2,32 Al 2,50 7,52 5,28 3,91 Fe 1,50 8,94 4,92 7,99 Mg 0,503 2,03 1,27 1,00 K 0,355 2,20 0,923 1,06 Cl 0,120 0,641 0,359 0,300 Ti 0,320 0,803 0,571 P 0,290 1,30 0,674 0,445 S 0,130 0,800 0,362 Cu 0,0650 0,593 0,264 0,206 0,05 Mn 0,0542 0,280 0,109 Ba 0,0450 0,270 0,124 0,160 Zn 0,0380 0,620 0,316 0,266 0,05 Pb 0,0233 0,540 0,197 0,110 0,004 Cr 0,0197 0,120 0,0404 0,0449 0,05 Sr 0,0170 0,0450 0,0322 0,0375 Sn 0,0080 0,130 0,0455 V 0,0030 0,0302 0,0083 Zr 0,0115 0,0200 0,0164 Ni 0,0038 0,0356 0,0145 0,0356 0,003 Co 0,00090 0,0040 0,00193 As 0,00070 0,0080 0,00306 0,002 Y 0,00060 0,0010 0,00080 Sc 0,00020 0,00040 0,00030 Mo 0,00025 0,0040 0,0013 Cd 0,00010 0,0079 0,0026 0,0001 Be 0,00008 0,00014 0,00011 Hg 0,000001 0,00030 0,00015 0,0001

From the above mentioned observations some notes on the study of data from the literature can be made - Preparation of the samples should be noted. General mineralogical composition is provided by fine milling and mixing of bottom ash, but no specific information on the organization of the compounds into the ash grain can be provided in this way. To obtain such information study 8

including the petrography analysis of individual ash grains should be made like it has been done by Eusden et al. (Eusden et al., 1999). - Data on mineralogical composition obtained using a set of methods (like X-ray diffraction with manual recognition of the phases supported by scanning electron microscopy) are preferable to the results obtained by a single analytical technique.

120

100

60 Concentration)*100% 40

deviationfrom level (deviationConcentration/Mean 20

K P

Al Sr

Cr Sc

Pb Be

Cu Co Cd

Mn Mo

Fig 3.1. Representation of variability in chemical composition of European MSWI bottom ash

Generalizing the data on bottom ash mineralogy from (Pfrang-Stotz, Schneider, 1995), (Kirby, Rimstidt, 1993), (Speiser et al., 2000), (Speiser et al., 2001), (Krzanowski et al., 1998), (Eusden et al., 1999) and (Piantone et al., 2004) it can be concluded that

- The main phase of MSWI bottom ash is a glass formed through the incineration in which the crystalline particles are embedded. Kirby and Rimstidt give the value of 74% as the sum for the amorphous phase and the phases beyond the detection limit of XRD analysis (Kirby, Rimstidt, 1993). In (Pfrang-Stotz, Schneider, 1995) a glassy phase was mentioned as the main presence (>10%) for the eight of eleven samples. As I was reported by (Speiser et al., 2001) “The vitreous phases contain low concentrations of Si and high concentrations of Ca”. This observation was confirmed by (Krzanowski et al., 1998) where the composition of a glassy phase - Ca – 38%, P – 9

22.1%, Sr – 19.3%, Si – 10.7%, Fe – 8%, Na – 6.8% and Mg – 2.3% was given. Eusden et al.

mentioned two types of glass: an opaque with high content of Si (62% of SiO2) and an isotropic

with relatively low Si content (39% of SiO2) (Eusden et al., 1999). - Composition of the crystalline phase provided in the sources varies; most of the authors

consider the main crystalline compounds as quartz (SiO2), ettringite (Ca6Al2(SO4)3(OH)12∙26H2O),

calcite (CaCO3), hematite (Fe2O3) and spinel (XY2O4, X and Y – are some metallic elements). - As shown by (Speiser et al., 2000), (Speiser et al., 2001) phase alteration processes occurring after incineration led to hydrate phases formation. Ca-hydrates such as calcium silicate hydrate (C-S-H – amorphous gel containing CaO, SiO2 and H2O) are considered as compounds with ability for heavy metals sorption (Ziegler et al., 2001a), (Ziegler et al., 2001b), (Poon et al., 1985). In (Speiser et al., 2001) micrographs showing the absorption of the metal particle corrosion products by the Ca-hydrate phase are given.

Fig. 3.2. Vitreous type of bottom ash (Pfrang-Stotz, Schneider, 1995, p. 280)

Fig. 3.3. Porous bottom ash (Pfrang-Stotz, Schneider, 1995, p. 280) 10

Fig. 3.4. Microcrystalline bottom ash (Pfrang-Stotz, Schneider, 1995, p. 281)

4. Waste residual characterization methods

A summary of the methods used to characterize waste incineration residues is given by Chandler et al. at the chapter 7 “Characterization methodologies” (Chandler et al., 1997). The most common methods for the study of MSWI residues given in that source are summarized in the table 4.1. References are provided if a description of a method is taken from another source.

Table 4.1. Summary of the MSWI residues characterization methods according to Chandler et al. (1997)

Name Procedure Results Physical characterization Visual Visual classification of the residues Division into observable fractions i.e. observation metal (magnetic/non-magnetic), glass, ceramics, stone etc. Also color and texture of the residues are noticed Particle size Material is sieved using a A chart showing the percent weight distribution consecutive set of calibrated sieves. passing a specified sieve size as Y- (ASTM 136) Material retained on values and particle or sieve size as X- each sieve is weighted. values (plotted logarithmically). To separate fine particles (d<75µm) the “wet sieves” technique is used (ASTM 117), which washes off the fine particles from the coarser fraction. Both fractions are then dried at 115 oC until constant mass is reached. 11

Agglomeration of fine particles is prevented by grounding the sieves or using the ultrasonic devices. Bulk density According to the ASTM C29/C29M Mass per unit volume. Value includes procedure container of known solid particles, water and air. volume is filled tightly with the sample then the weight of a sample within container is measured. Dry density The procedure includes drying of a sample at 105 oC for 24 h or until constant weight is reached. Specific gravity Mass of a pycnometer filled with Dimensionless value. Ratio of the water and oven dry specimen is weight of the sample to the weight of detected (ASTM C128). an equal volume of water. Water According to the ASTM C128 The difference between dry and the absorption standard the weights of oven dry wet specimen weights divided by the (dry) and water-saturated surface dry specimen weight. Shows the dry (wet) specimens are recorded. amount of water absorbed within the material’s pores. Water content The weight of the sample before and Geotechnical water content after drying at 105 oC to a constant weight _ of _ water w  weight. g weight _ of _ dry _ material Environmental water content weight _ of _ water w  e total _ weight _ of _ wet _ material Permeability Different techniques measuring the k – coefficient of water conductivity is (coefficient of water transport speed through the fined as the ratio between the speed water sample of transport and the hydraulic conductivity) concentration gradient. k>10-4 – the material is self-draining k<10-9 – the material is impervious k is affected by particle size distribution, particle shape and texture, mineralogical composition, void ratio, degree of saturation, type of flow, temperature, cementation and specimen size Analytical methods for solid phase chemical speciation Transmission Optical microscopy with transmitting Morphology of powders and particles light light in a thin section. microscopy (TLM) Scanning Detection of secondary and Precise information of the particles electron backscattered electrons to obtain a morphology. Can be coupled with X- microscopy picture of a sample surface. ray microprobe analysis to define the (SEM) Magnification is up to 5000x elemental composition at certain points of a particle.

Petrography Morphological analysis of the thin Provides morphology of a particle, (30 µm) sections embedded in epoxy exterior morphology and can also give resins. Samples of minerals with information on phase alteration. known morphology are used as standards. TLM and SEM can be used as observation techniques. X-ray powder Determines the diffraction patterns Semi-quantitative technique that diffraction of X-ray on the crystalline phases of gives information about the a sample. crystalline phases of a sample. Chemical composition Following methods are applied to the whole sample o Loss on ignition Weight of the sample dried at 105 C 푊퐷 − 푊푀 퐿푂퐼 = ∙ 100% (LOI) for 24 h and weight of the sample 푊퐷 o o after exposure to 550 C in a furnace WD – weight sample dried at 105 C until a constant weight is achieved. WM – weight of the sample exposed to 550 oC in a muffle furnace The weight loss is caused by organic matter decomposition and burning, hydrates such as gypsum decomposition and excess water evaporation. LOI can also be determined through Thermo gravimetric analysis can be thermo gravimetric (TG) analysis performed by heating samples from (van Zomeren, Comans, 2009) 25 to 900 oC at heating rate of 40 oC/min. It was found that organic carbon decomposes at 350 oC, elemental carbon – at 550 oC and carbonates decompose at 730 oC. The sum of weight losses obtained at those temperatures was interpreted as LOI.

Total carbon Detected by CNS (CHNS) combustion TC value includes organic carbon (OC),

(TC) analyzer. The quantity of CO2 after elemental carbon (EC) and carbonate the combustion at high temperature carbon (CC) (Ferrari et al., 2002) under oxidizing atmosphere is detected. (Rubli et al., 2000) Total organic According to the (Ferrari et al., 2002) TOC value includes OC and EC, to carbon (TOC) and (Rubli et al., 2000) total organic separate EC and OC the pyrolysis- carbon is detected as the difference oxidation technique is used (Rubli et between TC and inorganic carbon al., 2000) content which includes carbonates and carbides. Inorganic carbon detection is provided by colorimetric analysis.

Acid In (Johnson et al., 1995) two ANC is the capacity of material to neutralizing estimation procedures are resist changes in pH. The experiment capacity (ANC) explained: provides a resulting value of ANC 1. The automated titration which depends on the chemical and where water is added to a mineralogical composition of a sample with a liquid to solid sample. The titration curves which (L/S) ratio of 100:1 that show how the material resists means 1 g of a sample acidification on each titration step are placed into 100 ml of provided. deionized water. Then, small ANC is important to understand amounts of acid are added elements’ mobility through the

(0,1 ml of 1 M HNO3), each leaching. addition is made after the pH of the system is stabilized (variations are of less than 0,2 in 24 minutes). pH of the system is measured. 2. The batch titration where a sample is divided into portions and each portion is placed in a container with an acid solution of known strength. Initial pH of the solutions varies from one container to another. Resulting pH for all containers is recorded. Fourier According to (Smidt et al., 2009) FTIR Pellets for FTIR analysis were Transform methods can be used to identify prepared by mixing 2 mg of MSWI Infrared (FTIR) some mineral and organic bottom ash milled to 0,2 mm with 200 spectrometry compounds of bottom ash using mg of KBr. The wavenumber range for typical IR bands and comparison analysis was 4000-400 cm-1 using with standard mineral samples. transmission mode at a resolution of Methodology of Identification of 4 cm-1 (Smidt et al., 2009) carbonates to provide qualitative and quantitative analysis is given in (Tatzber et al., 2007) for soil samples.

The following methods are used to analyze the composition of solutions obtained by leaching or digestion procedures Atomic Spectrometry technique based upon AAS is a quantitative technique, Absorption the light of changing wavelength different variations of it are more Spectroscopy absorption by the vapors of a applicable to different groups o (AAS) sample. There are some variations of elements: the method: - FAAS – transition and alkaline - flame AAS (FAAS) – the elements sample is put into flame - GFAAS- Sb, As, Be, Cr, Cd, Pb, - graphite furnace AAS Se, Ti (GFAAS) – the sample - CVAAS – Hg only evaporates from the hot graphite walls of a furnace - cold vapor AAS (CVAAS) Inductively Spectrometry technique which uses Quantitative technique which can coupled plasma the plasma as the source of radiation analyze the content of most of the atomic emission. elements. emission spectrometry (ICP-AES)

Some digestion techniques used to provide samples for AAS or ICP-AES analysis can be described the following way (Chandler et al., 1997):

- Aqua-regia leach which utilizes digestion of sample in HCl:HNO3 mixture at a ratio of 10:3. This method cannot provide total digestion of the sample as it does not dissolve the silica matrix of bottom ash, so further analysis can provide reliable data only for the elements that are not strongly bound within the silica matrix. In (Chandler et al., 1997) aqua-regia digestion is recommended only for analysis of tin and vanadium concentration in MSWI bottom ash; for calcium, cadmium, cobalt and zinc this method is not considered to be suitable. - Real total digestion of samples can be done using a mixture containing hydrofluoric acid according to the APHA standard 3030I method utilizing a nitric acid/perchloric acid/hydrofluoric acid medium. The combination of acids used is very reactive, so some elements may be lost during the digestion procedure due to volatilization (silica and chromium – most probable, but loss of Ag, As, B, Ca, K, Mo, Sb, Se and V also may also happen). To prevent this hydrofluoric acid/aqua-regia/hydrogen peroxide mixture is used. The latter digestion technique is recommended for Ba, Cr, Pb and Ni determination and suitable for Al, B, Ca, Cd, Cu, Na, Zn.

- Another way to digest bottom ash samples is to fuse the sample with lithium metaborate (LiBO2) and dissolve the resulting flux in dilute nitric acid (Suhr, Ingamells, 1966). Sample is grinded to 75

µm (200-mesh) size and mixed with highly pure LiBO2 (0,1 g of sample and 0,5 g of lithium metaborate (Suhr, Ingamells, 1966)). The mixture is heated in a graphite crucible at 950 oC for 10- 15 min and molten material is dissolved in nitric acid solution (1:24 volumetric mixture of concentrated acid in distilled deionized water (Bankston et al., 1979)). This method provides a total dissolution of a sample avoiding problems which may be caused by acid digestion.

5. Utilization and leaching properties of incineration plant bottom ash

According to the statutory order no. 1480 (Danish EPA, 2007) all soil and incineration residues used for construction purposes are divided into three categories (table 5.1). Materials which suit the limitations of Category 1 could be considered as environmentally neutral. Materials of Categories 2 and 3 can be used for the construction purposes but some restrictions apply to their use. The classification of slag into the three categories is based on the leaching properties and for category 1 on maximum acceptable concentrations of some heavy metals as shown in Table 5.1.

Table5.1 Danish Environmental Protection Agency classification of soil and incineration residues used for construction purposes (Danish EPA, 2007) Category 1 Category 2 Category 3 Element Solid content, wt. % DS As 0 – 0,002

Pb 0 – 0,004

Cd 0 - 0,00005

Cr, total 0 – 0,05

Cr (VI) 0 – 0,002

Cu 0 – 0,05

Hg 0 – 0,0001

Ni 0 – 0,002

Zn 0 – 0,05

Concentration in leachate, μg/l  0 - 150000 Chloride 0 - 150000 150000 - 3000000 (15000000)* 0 – 250000 Sulphate 0 - 250000 250000 - 4000000 (2000000) Na 0 - 100000 0 - 100000 100000 - 1500000 As 0 - 8 0 - 8 8 - 50 Ba 0 - 300 0 - 300 300 - 4000 Pb 0 - 10 0 - 10 10 - 100 Cd 0 - 2 0 - 2 2 - 40 Cr 0 - 10 0 - 10 10 - 500 Cu 0 - 45 0 - 45 45 - 2000 Hg 0 - 0,1 0 - 0,1 0,1 - 1 Mn 0 - 150 0 - 150 150 - 1000 Ni 0 - 10 0 - 10 10 - 70 Zn 0 - 100 0 - 100 100 - 1500 * * - temporary values for salts concentrations in eluates from incineration slags are shown in brackets.

Slag complying with category 2 and 3 rules can be used for different kinds of construction works. Some general restrictions for the utilization of both Categories 2 and 3 slags are: - minimum distance of construction site from drinking water wells – 30 m (300 m for waste incineration slag); 16

- Materials must be placed above the groundwater table; - Land must be marked with linen net.

Limitations of use specific for each category are given at the table 5.2. Within the given limits incineration slag including MSWI bottom ash can be used without any permission, but as it can be seen Category 2 materials are more preferable for construction purposes as they can be used for more objects and with less environmental protection.

Table 5.2 Limitations for the Category 2 and 3 incineration residues use for construction purposes (Danish EPA, 2007) Limitations of use Category 2 Category 3 Roads Solid coatinga, material layer - max. 1 m Tight coatingb and drainage of surface water, material layer - max. 1 m Pathways Solid coating, material layer - max. 0,3 m Solid coating, material layer - max. 0,3 m Squares Up to 31 December 2009 slag from waste Not allowed incineration can be used with tight coating and drainage of surface water, material layer - max. 1 m. The distance to the nearest facility for water abstraction must be at least 300 m. Temporary values for chloride sulfate and sodium concentrations are used. Cable chutes and Solid coating Solid coating trenches Ramps Solid coating, material layer - max. 4 m. Not allowed Noise barriers Solid coating, material layer - max. 5 m, Not allowed maximum width at upper side – 2 m, Foundations and Max. 1 m layer of material under Max. 1 m layer of material under floors buildings. Soil shall not cause indoor buildings. Soil shall not cause the indoor pollution. pollution. a – Material should be coated with asphalt, concrete, at least 1 m of category 1 material etc. b – Coating material as asphalt, concrete etc. should reduce the amount of water percolating through the coating. The drainage should allow no more than 10% of rainfall to contact with material or soil.

According to the slag composition data given in chapter 3 it is almost impossible to have MSWI bottom ash as a material of Category 1 that can be use it without any limitation. The goal then is to see how Danish bottom ash suits the Category 2 and provide a treatment that can keep the slag within the limits of this category. Thus heavy metal leaching can be considered as the key-property to describe the environmental safety of the slag. Leaching from MSWI bottom ash can be described through some common leaching study techniques. An important controlling parameter for all those tests is the liquid to solid (L/S) ratio measured in l/kg which represents the amount of water coming into contact with the studied material. Leaching tests are often divided into two groups: - Batch tests – material is placed into some certain volume of water which remains constant through the whole experiment. L/S ratio for those is the ratio between volume of water and mass of the 17

sample. pH of the solvent may be controlled (pH-static experiment) during the experiment or several solvents with different pH may be used (multi-stage experiment). - Flow-through (Poon and Chen, 1999) tests – water percolating continuously through the sample during some certain time. L/S ratio is calculated from the whole amount of water passing through the material. Laboratory flow-through tests are column leaching techniques.

It will be ideal to know the leaching of a specific bottom ash from a specific real construction site. However in most cases laboratory tests are used to estimate the leaching. Leaching of elements from any material is a complex process which is controlled by phase precipitation/dissolution, molecular species sorption/desorption and transport processes occurring through interaction of the material with percolating water. pH of that interacting system is often considered as a main parameter which controls leaching. A typical theoretical dependence of the concentration of heavy metal containing ions on the pH level is shown at fig. 5.2. This curve represents only the balance between dissolution and precipitation of heavy metal species at some certain pH level. Even for this case the leaching process is rather complex as it summarizes different ionic species formation and precipitation/dissolution of heavy metal containing phases.

Fig. 5.2. Schematic diagram of theoretical solubility curve of cationic metal (Dijkstra et al., 2006a, p. 341)

According to present knowledge the most important controlling mechanisms for leaching of different elements can somewhat simplified be summarized by the following statements: - The leaching of Al, Fe, Zn, Cu and Pb follows typically a u-shaped curve as a function of pH as seen on Figure 5.2. However, for Zn, Cu and Pb there are only observed a moderate increase in leaching at high pH levels. - The leaching of Ca, Mg, Cr, Na, Cd and Ni generally decreases with increased pH level - The slag organic matter content influences the leaching of Cu, Zn and Pb. Oxidation of slag organic matter can lead to decreased leaching of Cu, Zn and Pb. - A more detailed discussion of leaching controlling parameters can be seen in chapter 6.

In many cases MSWI bottom ash is sieved and stored outdoors for some time (typically 1 to 12 month) before it is applied for construction work. A storage period of 3 month often reduces leaching levels, so 18

that the bottom ash can be utilized according to the category 3 rules. This process is often called weathering. During the weathering period the some oxides are converted to carbonates and pH is reduced. Often Danish raw slags have a pH value of 10.5 – 11.0 while they after weathering have pH values of 8.5 to 9.5. Often the leaching of Pb, Zn, Cd and Cu is reduced by the weathering process.

Batch leaching tests are presently used as a part of standard compliance tests, like (Danish EPA, 2007) which attributes materials to categories 2 and 3 (tables 3.1 and 3.2) utilizing CEN prEN 12457-3 step 1 leaching test. For the CEN prEN 12457-3 step 1 demineralized water is used as leachant, L/S is 2 l/kg and time of leaching is 6 h. In a study by Hjalmar the standard test was applied to bottom ash from 11 Danish incineration facilities (Hjelmar, 2004). The results of the study is shown in Fig. 5.3-5.4 as the ratios of the leaching tests results from relative to the limiting values of the categories 2 and 3 specifications. It is seen that most of the studied samples do not suit the limitations of Category 2. The main problem is high concentrations of copper in leachates, next by the significance is concentration of chromium and arsenic. Concentrations of lead exceeding the limitations were shown only for 3 incinerators from 11, but those concentrations were significantly higher than the limit. In some cases slags exceeding the category 2 specification were also seen for Cl, Na, Ni and Zn. Most of the studied Danish bottom ashes suit the limitations of Category 3, while high concentrations of copper and lead still remain to be the problem for some incinerators.

100,0

10,0

1,0

0,1 Concentration/Category limiting 2 value

0,0 Cl SO4 Na As Cd Cr Cu Ni Pb Zn

Fig. 5.3. Representation of MSWI bottom ash compounds’ leachability (Hjelmar, 2004) relative to the limitations of Category 2 (Danish EPA, 2007)

10,000

1,000

0,100

0,010 Concentration/Category limiting 3 value

0,001 Cl SO4 Na As Cd Cr Cu Ni Pb Zn

Fig. 5.4. Representation of MSWI bottom ash compounds’ leachability (Hjelmar, 2004) relative to the limitations of Category 3 (Danish EPA, 2007)

6. Influence of different parameters on slag leachebility

Leaching of elements from any material is a complex process which is controlled by phase precipitation/dissolution, molecular species sorption/desorption and transport processes occurring through interaction of the material with percolating water. pH level has often a large influence as seen on Figure 5.2. Explaining the Figure a starting point is natural pH level of a bottom ash sample. Addition of base to the system drives it to the region “I” where dissolution of amorphous phases and desorption processes overpower sorption and precipitation. At region “II” while acidity of environment increases sorption and precipitation processes become dominant and metal concentration in leachate decreases until some pH level is reached at which desorption and acidic dissolution of mineral phases become stronger (region “III”) and at region IV they increase leaching over the natural level. (Dijkstra et al., 2006a) Similar curve can be obtained for leaching of several elements from MSWI bottom ash, meanwhile it should be taken into account that real leaching process is more complicated as it includes sorption/desorption of ionic species to some other phases and precipitation/dissolution of those phases (Stumm and Morgan, 1996). In the review (Cornelis et al., 2008) a list of the minerals which can absorb heavy metals in bottom ash is given, those minerals includes: iron oxides, aluminium oxides, ettringite, monosulphate, hydrocalumite, hydrocalcite-like minerals, portlandite, calcium silicate hydrate (C-S-H), calcite, gypsum. Complexation with dissolved organic carbon (DOC) was also studied as the leaching control mechanism by many authors (Kersten et al., 1997), (Meima, Comans, 1999), (Olsson et al., 2007), (Arickx et al., 2007). It was found that Cu leaching ability mostly controlled by the organic acids like fulvic acid. 20

destruction of organic matter in MSWI bottom ash leads to lower Cu leaching. Also a decrease of Zn and Pb leaching ability with the decreasing DOC content was found (Arickx et al., 2007). Data obtained from pH-static tests is usually compared to the results of leaching modeling calculations or similar leaching tests for simpler mineral systems to provide better understanding of the leaching mechanisms (Meima, Comans, 1999; Dijkstra et al., 2006a, Dijkstra et al., 2006b). Another application of the concentration-pH dependencies data is search for material pH level at which leaching of heavy metals is minimal. For elements leaching abilities which follow the pattern shown at fig 5.1, there may be a favorable case if pH range containing leaching minima for all such elements is narrow enough. Then, keeping pH of the bottom ash within that range minimizes leaching. A comprehensive study of pH controlled leaching of elements from MSWI bottom ash was made in (Dijkstra et al., 2006a). Dependencies of leaching from pH levels were studied for the following elements and anions: -2 Na, Ca, Mg, Al, Si, Fe, Zn, Cu, Mo, Mo, Cd, Ni, Sb, Pb, SO4 and also for DOC. Batch pH-static procedure with pH values of 4, 6, 8, 10 and 12 was used. The results show that only leaching abilities of Al, Fe, Zn, Cu and Pb followed the theoretical solubility pattern described above. Minima points of these curves can be attributed to the following pH levels: Al – 7-8, Fe – 8-9, Zn – 10, Cu – 8-9, Pb – 8-9 (Dijkstra et al., 2006). Thus, taking into account that Danish environmental regulations pay attention to Zn, Cu and Pb, a “favorable” pH range for MSWI bottom ash can be defined as values from 8 to 10. From other elements studied in that work only Na, Cd, and Ni are mentioned in Danish regulations. Those elements have -2 leaching abilities which decrease with increasing pH. Leaching of SO4 also decreases with increasing pH while is independent of the DOC leaching is independent from pH level. Thus, according to the studied data, to minimize leaching of heavy metals pH of MSWI bottom ash should be kept at range of 8-10. Generalizing the data from (Dijkstra et al., 2006a), (Dijkstra et al., 2006b), (Meima, Comans, 1997) and (Meima, Comans, 1999) some information on the mechanisms of pH controlled leaching for different heavy metals can be summarized. - Zn leaching is most likely controlled by the solubility of the mineral phase only. Both studied works

point to willemite (Zn2SiO4) as the most probable leaching controlling mineral. It was found (Meima, Comans, 1999) that aging of bottom ash did not change Zn leaching pattern significantly. In (Meima, Comans, 1997) leaching of Zn from all the samples with different aging degree followed the same pattern of pH dependence and showed no dependence on the L/S ratio. Those data can be interpreted as an evidence of very small or even zero influence on Zn leaching coming from the presence of other phases and also high solubility of Zn-containing phase which does not alter during aging of bottom ash. - At acidic conditions copper leaching followed well a “surface complexation to Fe- and Al- (hydr)oxides” model (Dijkstra et al., 2006a), but at pH higher than 8 that model overestimated experimental concentrations significantly. No other model concerning surface complexation or phase dissolution was found to describe the Cu leaching at high pH values. Study of leaching from aged bottom ash shown that pH dependence pattern of Cu leaching changes significantly from pronounced pH dependence to nearly pH-independent behavior for 6 week aging (Meima, Comans, 1999). Batch leaching tests at natural pH and different L/S ratios (Meima, Comans, 1997) showed three different pH dependence trends for three weathered samples. That was clear evidence of changes in leaching abilities of Cu occurring through the phase alteration process, but it is not certain whether they are caused by changes in Cu-containing phases or in some phases which absorb Cu-containing ions. Thus, it may be assumed that leaching of Cu depends on a sum of some 21

different factors like surface complexation, dissolution of some copper-containing phases, complexation with organic acids (Arickx et al., 2007) and maybe more. There is not enough information from batch leaching tests to make an assumption on the main mechanism of copper leaching control. - For Pb it can be assumed (Dijkstra et al., 2006a), (Meima, Comans, 1997) and (Meima, Comans, 1999) that its leaching is controlled rather by surface complexation with some phases than by dissolution of Pb containing species. There were some suggestions according Pb-absorbing phases: Fe- or Al-(hydr)oxides (Dijkstra et al., 2006a) or amorphous iron (Chaspoul et al., 2008). For other studied elements the following suggestions on mechanisms controlling their leaching were made (Dijkstra et al., 2006a): - Al leaching was described well on wide range of pH by solubility of aluminous (hydr)oxide forms like

gibbsite (Al(OH)3) and ettringite (Ca6Al2(SO4)3(OH)12∙26H2O).

- Ca leaching was described by dissolution of gypsum (CaSO4∙2H2O).

- Mg leaching at pH above 8 may be described by dissolution of brucite (Mg(OH)2). No adequate description was proposed for lower pH values. in (Meima, Comans, 1997) it was suggested that there were no solubility controlled minerals at low pH values and all available Mg was leached from bottom ash. - Fe leaching was described well by solubility of Fe(III) (hydr)oxides. - Cd leaching was described adequately by surface complexation to Al- and Fe-(hydr)oxides on the whole pH range. - Mo leaching at pH values higher than 6 was described well by a surface complexation model. For

lower pH’s the suggestion on solubility of Fe2(MoO4)3 as leaching controlling factor was made.

It can be seen that the whole amount of information from pH-static tests is not enough to provide a model for MSWI bottom ash leaching behavior in real conditions. Thus, field tests is used to study the way the leaching of heavy metals is affected by natural conditions of building sites where MSWI bottom ash is used as construction material. Usually those field leaching tests are performed on road sections built with bottom ash. Such sections are equipped with sampling wells to collect the samples of water percolating through the material layer. Cross-section of such test road from (Åberg et al., 2006) is shown at fig. 5.3.

Fig. 5.3. Cross-section of the test road used for field leaching tests (Åberg et al., 2006, p. 2) 22

Results from five different field studies of MSWI bottom ash leaching were collected for this report, they were: 1. (Bruder-Hubscher et al., 2001) three-year study of leaching from the test road built of MSWI bottom ash from French waste incineration facilities. Results were compared to a reference road section built with natural gravel. 2. (Åberg et al., 2006) A one-year study of leaching from a test road built with MSWI bottom ash from Dåva incineration plant in Umeå, Sweden. Results were compared with leaching from natural gravel. 3. (Lidelöw, Lagerkvist, 2007) A three-year study leaching from the test road built with MSWI bottom ash from Dåva incineration plant in Umeå, Sweden. Results were compared to a reference section build with crushed rock. Additionally, results of the field leaching tests were compared to laboratory two-stage batch leaching experiments. 4. (Izquerdo et al., 2008) 15 months of leaching study from a test road build with bottom ash from the Mataro incineration plant (NE Spain). Results were compared to laboratory two-stage batch leaching experiments. 5. (Lind et al., 2008) A three-year study of leaching from the test road made of bottom ash from Umeå Energi heat and power plant (Sweden). Results were compared to leachates from a reference road made with natural gravel, from surroundings of the road and to the results of long-term batch leaching tests modeling (up to 1000 years period).

Using the information from those sources the following aspects of leaching from bottom ash layers in real-life conditions can be reviewed: 1. Leaching as a function of L/S ratio and time measured through the test period. In general it can be concluded that there is a period of intense leaching from all the studied types of bottom ash. Duration of that “first-flash pulse” period was defined by (Lind et al., 2008) as the time until the L/S ratio of the percolating water reached a value of 0,3 (about 3 years for (Lind et al., 2008)). After that “first-flash pulse” heavy metal leaching rate decreased. According to that it can be assumed that highly soluble heavy metal containing phases are presented in all the types of MSWI bottom ash. Study of time-dependent leaching patterns of heavy metals (Bruder-Hubscher et al., 2001), (Åberg et al., 2006), (Lidelöw, Lagerkvist, 2007) leads to the following conclusions: - Differences in time-dependent leaching for the same elements from MSWI bottom ash produced at different incineration facilities point to different mineralogy caused by waste composition or different conditions of incineration process. - Clear correlation between leached amount of Cu and DOC content in leachates was found (Lidelöw, Lagerkvist, 2007). Cu leaching has a tendency to decrease monotonously with time. Laboratory modeling of long-term leaching behavior of Cu have shown that the cumulative amount of Cu leached from MSWI bottom ash for a period of 1000 years would be even lower than the amount of Cu leached from natural gravel and surroundings (Lind et al., 2008). - Zn leaching rate decreases during the first year of leaching becoming lower than leaching from natural gravel (Åberg et al., 2006), (Lidelöw, Lagerkvist, 2007), but at the third year 23

an increase in leaching was detected (Lidelöw, Lagerkvist, 2007). Modeling has shown higher cumulative leaching of Zn on a timescale of 1000 years in comparison to leaching from natural gravel and surroundings (Lind et al., 2008). - Leaching of Cr from MSWI bottom ash is much higher than from natural gravel (Åberg et al., 2006) and it may have peak values after 1,5 year of leaching exceeding leaching at the beginning of the test period (Lidelöw, Lagerkvist, 2007). 2. Consistency between laboratory leaching test and results of field leaching tests was studied by (Lidelöw, Lagerkvist, 2007), (Lind et al., 2008) and (Izquerdo et al., 2008). The following batch tests were used: - Two-stage batch test according to European standard (EN 12457-3) – first step is performed at L/S 2 for 6 h and the second step – at L/S 8 for 18 h (Lidelöw, Lagerkvist, 2007), (Lind et al., 2008) and (Izquerdo et al., 2008). - Single step batch leaching test (EN 12457-2) performed at L/S 10 for 24 h (Izquerdo et al., 2008). Both tests were performed on material grinded to 4 mm. Results of the observed works lead to the conclusion that batch leaching tests were acceptable for a rough estimation of leaching at some certain conditions and leaching trends at changing L/S ratio, but they could not predict exact quantitative leaching of the elements. Overestimation and underestimation of leaching from bottom ash at real-life conditions were common (Izquerdo et al., 2008), (Lind et al., 2008). That may be caused by differences in water percolation processes occurring in batch and “flow- through” leaching tests described by (Poon and Chen, 1999). Thus, leaching tests utilizing “flow- through” process, like column leaching tests, may be more consistent to the field experiments. (Hjelmar et al., 2007) studied consistency of laboratory column leaching test (CEN/TS 14405) with leaching data from field experiment. A good consistency in leaching dependence from L/S was found for sulphate, chloride and Ni, while Sb leaching was overestimated.

As it was mentioned before column leaching tests is promising technique to model heavy metal leaching from MSWI bottom ash. In (Hjelmar et al., 2007) leaching results for only 3 elements (Ni, Cu and Mo) from the range of 13 were found inadequate due to the specificity of these metals leaching control mechanisms. Ni and Cu leaching is controlled by DOC, while for Mo it is controlled by availability of mobile 2- MoO4 (Hyks et al., 2009). Meanwhile column leaching still needs further comparison with field leaching tests.

7. The influence of thermal treatment on slag parameters

Thermal treatment as a technique which can significantly reduce the heavy metals leaching from the MSWI bottom ash is studied through the past decade. In the work by Nüßlein et al. (Nüßlein et al., 1994) it was shown that leaching of heavy metals from MSWI bottom ash decreases with increasing incinerator temperature. The same year results of thermal treatment of bottom ash were published in (Schneider et al., 1994) where favorable changes in heavy metals leaching occurred through thermal treatment. From that time a number of studies have been performed to study the changes in MSWI bottom ash through the thermal treatment and the dependence of different ash chemical and physical parameters of the treatment conditions. Information on the experimental procedures used to study MSWI bottom ash thermal 24

treatment are given in the table 6.1 while results of those experiments are discussed in the following text. Works concerning thermal treatment of pure bottom ash were selected.

Table 6.1 Summary of the bottom ash thermal treatment experiments Source Short description Sample preparation Details of the experiment (Schneider et al., sintering of bottom Metal pieces and particles with 2 kg subsamples of 1994) ash samples in a size over 40 mm were removed bottom ash were muffle furnace manually; samples were dried muffled for 30 min at at 105 oC. No other 850, 1000 and 1300 oC pretreatment reported (Bethanis et al., 2004) sintering in <8mm bottom ash fraction Sintering at the cylindrical samples from the South-East London temperatures between Combined Heat and Power was 1020 and 1100 oC using used. Different portions were a ramp rate of 6 oC/min wet milled for 2, 8 and 16 and a dwell time of 1 h hours, dried at 105 oC and at maximum grounded. Cylindrical samples temperature. were produced by pressing at 32 MPa. (Cheeseman et al., sintering in Six batches of <8mm bottom Sintering at the 2003a) cylindrical samples ash fraction from the South- temperatures between East England EfW plant were 1080 and 1120 oC using used. Samples were wet milled a ramp rate of 6 oC/min for 8 hours, dried at 105 oC and and a dwell time of 1 h grounded. Cylindrical samples at maximum were produced by pressing at temperature. 32 MPa. (Cheeseman et al., sintering in spherical <8mm bottom ash fraction Pellets were sintered at 2003b) pellets, rotary from the South-East England the temperatures electric tube furnace EfW plant was used. Samples between 1000 and 1050 were milled, the controlled oC in an electric rotary amount of water was added tube furnace. The and the pellets (8-10 mm rotating tube internal diameter) were formed. Pellets diameter – 7,7 cm, were dried at 105 oC. length – 150 cm, length of a heated zone – 90 cm. Rotation speed – 2,8 rev./min, angle of a tube – 2o. The average pellets traverse time was 10 min and 35 s. (Pfrang-Stotz et al., sintering of bottom The bottom ash fractions 0- Samples were heated in 2002) ash samples in a 0,063 mm and 0,063-0,09 mm a muffle furnace at 900, muffle furnace were used. No pretreatment 1000, 1100, 1150, 1200 was made. and 1300 oC for 45 min.

(Wang et al., 2003) sintering in The samples were made by The samples were cylindrical samples compaction at 35 MPa from sintered at 400, 500, the bottom ash fractions <1,41 600, 700 oC for 60, 120, mm and 1,41-4,76 mm 180, 240 min. (Selinger et al., 1997), rotary kiln Two types of bottom ash from Thermal treatment at (Bergfeldt et al., 1997) experiments German MSWI plants were the temperatures used: “wet ash” from standard between 700 and 1065 wet ash discharge system and oC. The laboratory rotary “dry ash” from InRecTM process kiln with an inner (Simon, Andersson, 1995). No diameter of 0,4 m and pretreatment was made except volume of 0,11 m3 was removing large (>32 mm) used. Ash was kept for particles. about 30 min at the experimental temperature. “Dry ash” was thermally treated under special oxidizing atmosphere (60% air, 30% H2O and 10% CO2) (Arickx et al., 2007) thermal treatment in 0,1-2 mm fraction from the Heating for 2 h at 100, muffle furnace MSWI plant in Flanders was 200, 300, 400 and 500 used. Ash was mixed for 9 days oC and heating at 400 oC at 50 oC for 15, 30, 60, 90 and 120 min

Applying high temperature treatment to a substance like MSWI bottom ash, which cannot be treated as neutral or stable (Piantone et al., 2004), (Speiser et al., 2000), it is obvious to expect some changes of its chemical and physical appearance. Chemistry of treated bottom ash can be changed both in terms of elemental composition and mineralogy (changing content of chemical substances and mineral phases), thermal treatment may also affect some of the general chemical parameters of bottom ash like pH. Physical changes are mostly caused by melting and sintering processes and affect the density and porosity of bottom ash. The obtained information from the thermal treatment experiments can be described as follows: 1. Elemental composition of bottom ash may be changed only trough volatilization of some

compounds. First, it is release of carbon as CO2 due to combustion of organic matter and decomposition of carbonates which has been reported in some papers (Selinger et al., 1997),(Bergfeldt et al., 1997), (Arickx et al., 2007).In (Arickx et al., 2007) the decrement of DOC content in bottom ash leachates was detected even for the temperatures below 500 oC . DOC changed from 41,5 mg/l for the untreated bottom ash to the 15,0 mg/l for the bottom ash treated at 400 oC (Arickx et al., 2007). In the work by Arickx et al. the decreasing amount of DOC is considered as the cause of decreasing leaching ability of copper. It may be possible to reduce carbon content of MSWI bottom ash significantly, even to complete removal of carbon at some degree of treatment. For other elements than carbon no such certainty in published results can be found. There were two contradictory sets of results. In (Schneider et al., 1994) it was reported that no change in 26

chemical structure of MSWI bottom ash thermally treated up to 1300 oC occurred: “No other analytical parameter, neither the heavy metal concentration, nor those of chloride were changed significantly by any thermal treatment” (Schneider et al., 1994, p. 610). Similar facts were published in (Bethanis et al., 2004), in that paper no decrease of elements’ content through thermal treatment was detected. The fact that no volatilization of alkali metals’ salts was observed, while such volatilization had been expected by the authors, was explained by salts removal during bottom ash washing which preceded thermal treatment. (Selinger et al., 1997) reported about the changes in elemental compositions occurring through thermal treatment of MSWI bottom ash. Pb, Ca and K contents were decreased by some 20%. Similar 20% decrease of availability as incinerator temperature increased was reported for Pb and Zn in (Nüßlein et al., 1994). Some explanations can be given to this contradiction. First, it can be caused by different mineralogy of bottom ash used in experiments. Another explanation is difference in treatment techniques applied to bottom ash in those works: in (Schneider et al., 1994) it has been reported that samples were muffled in crucibles and in (Bethanis et al., 2004) bottom ash was sintered in pellets while (Selinger et al., 1997 and Bergfeldt et al., 1997) performed experiments using laboratory rotary kiln and in (Nüßlein et al., 1994) incineration at waste-to-energy power plant was studied. Motion of material under treatment in rotary kiln or waste incinerator makes volatilization of salts easier than it is in static conditions of crucible or pellet. 2. Mineralogy of bottom ash changes significantly through the treatment. It is shown in (Bethanis et al., 2004), (Cheeseman et al., 2003a), (Cheeseman et al., 2003b) and (Pfrang-Stotz et al., 2002) that the quartz and hematite content in bottom ash decreases as the temperature increases. It is noticed by Pfrang-Stotz et al. that quartz cannot be detected at the temperatures higher than 1150°C, in Bethanis et al., 2004) quartz was not detected at 1080°C. The formation of new minerals

as wollastonite (Ca[SiO3]) and diopside(CaMgSi2O6) are detected. In (Bergfeldt et al., 1997) no decreasing content of quartz was noticed, but the formation of diopside at higher temperatures is detected too. Formation of glassy phase was shown in (Pfrang-Stotz et al., 2002) for all the studied samples. For the samples from different sources a noticeable formation of glass begins at different temperatures, but for the temperatures higher than 1200oC the newly formed glass becomes dominant phase. There is no detailed study of influence of mineralogy alteration occurring within bottom ash under high temperatures on leaching. Only some facts mentioned in different studies: a. Calcium which is presented in untreated bottom ash as oxide, hydroxide and carbonate (Krzanowski et al., 1998; Johnson et al., 1995; Bergfeldt et al., 1997) associated with particle surfaces (Krzanowski et al., 1998; Bendz et al., 2007) after thermal treatment is more incorporated within mineral phases forming Ca-Si minerals (Johnson et al., 1995; Bethanis et al., 2004; Bergfeldt et al., 1997). So, calcium becomes less available to leaching. These processes also may affect pH and ANC (Acid Neutralizing Capacity) of the samples. b. Glass phases of MSWI bottom ash were described as “hydrophobic” in (Johnson et al., 1995). So, formation of glass may change the way water interacts with bottom ash. c. It is suggested (Bethanis et al., 2004) that heavy metals incorporation within crystalline phases is preferable to presence in amorphous phases because heavy metals bounded within glasses are more available to leaching. 27

d. As the result of artificial mixtures thermal treatment study (Kuo et al., 2009) it was found that Al may substitute silica and this process lead to increase leaching of heavy metals. e. It was found (Kuo et al., 2009) that amorphous phases were more stable at acidic conditions in comparison to crystalline ones. Even at conditions when crystalline phases of studied samples were highly corroded no evidence of decomposition of the amorphous part was seen. It may be suggested that the situation may be quite opposite under basic conditions. 3. Changes in pH and ANC (Acid Neutralizing Capacity) of bottom ash occurring through thermal treatment were reported in many papers (Johnson et al., 1995), (Bergfeldt et al., 1997), (Bethanis et al., 2004), (Selinger et al., 1997), (Wang et al., 2003), (Schneider et al., 1994). As it was considered pH level was controlled mainly by chemical forms of alkali and alkali earth metals, aluminum and iron (Johnson et al., 1995), (Bergfeldt et al., 1997), (Bethanis et al., 2004). As high temperature treatment leads to decomposition of organic acids and carbonates it may be suggested that pH level have to increase. Meanwhile, pH value can rise or going down with the rising temperature (Selinger et al., 1997), (Wang et al., 2003), (Schneider et al., 1994) being controlled by the changing mineralogical composition. Some of elements with high influence on pH may be incorporated within mineral phases. So, pH changes occuring through thermal treatment should be studied individually for each region. Influence of pH on heavy metal leaching was discussed in previous section. 4. Porosity of bottom ash particles is a physical property which has big influence on leaching. Changes in porosity due to thermal treatment were described in (Bethanis et al., 2004). Porosity of sintered samples was studied through bottom ash density and water absorption data supported with SEM observations. It was found that at temperatures below 1080 oC samples of milled bottom ash had higher density and lower water absorption. SEM micrographs shown a reduced amount of pores in the samples treated at higher temperatures. But at temperatures higher than 1080 oC density of the samples began to decrease while water absorption continued its decreasing tendency. SEM had shown formation of spherical pores within the samples at temperatures higher than 1080 oC. Thus, there are two processes occurring during thermal treatment: closing of pores due to glass formation and melting and formation of new spherical gas pores due to decomposition of some compounds. In (Bethanis et al., 2004) it was suggested that gases comes from decomposition of sulfates.

Most of the studies concerning thermal treatment of bottom ash provided only results of natural pH leaching tests making only the overview in terms of “increasing” or “decreasing” leaching of heavy metals possible. Those results can be represented the following way: (Wang et al., 2003) The leaching test was provided according to the US EPA SW846-1311 procedure. Cu concentration in the leachates from the sintered samples was significantly lower or not detected. Concentration of Cu in leachates decreased with rising temperature or increasing sintering time. For Zn and Cd leaching abilities were decreased significantly with rising temperature and increasing sintering time. For Pb the increase of leaching ability at low temperatures was detected, for the high temperatures concentrations of Pb in leachates were decreased. (Selinger et al., 1997) The DIN 31414-S4 test was used. Concentrations of Pb, Zn and Cu were decreased significantly after thermal treatment. For Cr the increase of leaching ability was shown for the temperatures below 900 oC. 28

(Schneider et al., 1994) Column leaching test with L/S ratio of 20 was provided. For Ni, Cu, Zn, Pb a decreasing leaching ability with rising treatment temperature was found.

In contrary to the general results (there are applied no pH control) pH-static batch leaching experiments were provided by (Cheeseman et al., 2003a) and (Bethanis et al., 2004). Those references make a general overview of the processes occurring in bottom ash during thermal treatment possible. Results of pH-static leaching experiments from (Cheeseman et al., 2003a) and (Bethanis et al., 2004) are presented on fig. 6.1 – 6.4. In (Cheeseman et al., 2003a) a study was performed to six bottom ash samples from one incineration plant untreated (hollow points (figures 6,1 and 6.2)) and treated at 1110 oC (filled points (figures 6,1 and 6.2)). (Bethanis et al., 2004) representing data on untreated “milled” bottom ash (hollow circles) and samples treated at different temperatures. Conclusions from the presented data are the following: 1. Data provided by (Cheeseman et al., 2003a) shows the clear tendency of heavy metal leaching from bottom ash to decrease with thermal treatment (fig. 6.1, 6.2). For Zn, Cu, Al and Pb leaching at natural pH was reduced to almost zero level. Meanwhile, (Bethanis et al., 2004) have not shown such clear tendency. For some elements thermal treatment at high temperatures may lead to slight increase of leachability at some pH range (fig. 6.3, 6.4). 2. There are clear evidences of phase alteration processes occurring through the thermal treatment. For most studied elements their leaching curves change shapes with changing treatment temperatures. Meanwhile, nature of those changes is still uncertain. Studies including geochemical modeling and analysis of microstructure are needed to clarify this subject.

Fig. 6.1. Leaching of Ca (a), Na (b), Mg (c) and K (d) as functions of pH. Results of pH-static batch leaching test (Cheeseman et al., 2003a, p. 914) for untreated (hollow shapes) and treated (filled shapes) bottom ash. Sintering at 1110°C.

Fig. 6.2. Leaching of Zn (a), Pb (b), Al (c) and Cu (d) as functions of pH. Results of pH-static batch leaching test (Cheeseman et al., 2003a, p. 915) for untreated (hollow shapes) and treated (filled shapes) bottom ash. Sintering at 1110°C. 31

Fig. 6.3. Leaching of Cr, Zn, Cu, Cd, Ni and Pb as functions of pH and treatment temperature. Results of pH-static batch leaching test (Bethanis et al., 2004, p. 263).

Fig. 6.4. Leaching of Ca, Na, Mg and K as functions of pH and treatment temperature. Results of pH- static batch leaching test (Bethanis et al., 2004, p. 261)

8. Resume and conclusion

A large fraction of Danish municipal waste is combusted in waste incineration plants which generates different ash residues. The largest residue amount is bottom ash of which approximately 600 thousand tones is produced yearly in Denmark. The bottom ash is often utilized as construction material in roads, ramps, noise barriers etc. The determination of which materials that can be utilized and to which purposes is determined by the composition and the heavy metal leaching properties of the bottom ash. A low release of heavy metals makes it possible to utilize the bottom ash, while a high release means that the ash has to be deposed in a controlled expensive landfill. One of the objectives of this research project is to investigate the possibilities to reduce the leaching of heavy metals from the incineration bottom ash by a thermal treatment of the slag in a rotary kiln. This literature review was done to provide a background for the experiments done on a pilot scale rotary kiln.

The bottom ash from Municipal solid waste incineration (MSWI) plants are very inhomogeneous and is often dominated by the elements Si, Ca, Al, Fe, Na, Mg and K. However a very broad range of heavy metals also appear in the ash. 33

A specification of incineration residues into three categories is specified in a Danish statutory order.

Category 1: To fulfill category 1 an ash must both be limited with respect to content of some heavy metal species and regarding the leaching of heavy metals from the ash. Category 1 residuals can be used without any limitation. However, in practice all municipal waste incineration bottom ashes exceed some of the specified heavy metal content concentrations.

Category 2: Maximum leaching of different heavy metals in the range of 0.1 µg/l (Hg) up to 300 µg/l (Ba) is specified. The bottom ash can be utilized for roads, noise barriers, foundations etc.

Category 3: Maximum leaching of different heavy metals in the range of 1 µg/l (Hg) up to 4000 µg/l (Ba) is specified. Some of the same utilizations as category 2, but without use in squares, cable shuts and noise barriers.

Outside of category: Material has to be deposed in a carefully controlled landfill

The statutory order regarding category 2 and 3 specifies limits for the leaching of the heavy metals As, Ba, Pb, Cd, Cr, Cu, Hg, Mn, Ni, Zn and somewhat higher limits for the leaching of Na, chloride and sulphate. In practice it is not possible to determine the leaching from an actual site, so the leaching properties of a given bottom ash residue is determined by a laboratory test. Batch leaching tests are presently used as part of a standard compliance test that attributes material into category 2, 3 and out of category. There is used a water to solid ratio of 2 and a leaching time of 6 h.

Typically fresh municipal incineration bottom ash exceeds even category 3 with respect to leaching limits of several metals (Pb, Zn, Cd Cu, Cr). However, after the ash has been weathered (stored in open air) for typically 3 month most ashes comply with the category 3 limitations. The natural pH of fresh bottom ash is typically 10.5 – 11.0, while after weathering values of 8.5 – 9.5 is observed.

After weathering most bottom ashes comply with the category 3 rules. Exceeding of leaching limits of some ashes are mainly seen for the elements Cu and Pb. Ashes rarely comply with category 2 specifications. The largest deviations are seen for Cu, Cr and Pb, but some ashes do also exceeds the leaching limits of Cl, Na, As, Ni and Zn.

The control of the leaching of heavy metals is a complicated process that cannot be modeled accurately today. Leaching is controlled by phase precipitation/dissolution, molecular species sorption/desorption and transport processes occurring through interaction of the material with percolating water. pH of the interacting system is often considered as a main parameter which controls the leaching. According to present knowledge some of the controlling mechanisms for leaching of different elements can somewhat simplified be summarized by the following statements: - The leaching of Al, Fe, Zn, Cu and Pb follows typically a u-shaped curve as a function of pH. Minimum leaching is observed at intermediate pH. However, for Zn, Cu and Pb there are only observed a moderate increase in leaching at high pH levels; while at low pH the leaching increases significantly. - The leaching of Ca, Mg, Cr, Na, Cd and Ni generally decreases with increased pH level 34

- The slag organic matter content influences the leaching of Cu, Zn and Pb. Oxidation of slag organic matter can lead to decreased leaching of Cu, Zn and Pb. Cu leaching ability is mostly controlled by organic acids like fulvic acid

Thermal treatment of bottom ashes has for several years been known to have the capability to reduce the leaching of some elements from the ashes. The bulk of the thermal treatment studies have used treatment temperatures from 900 to 1100°C and the results can be summarized in the following statements:

- A decreased leaching with thermal treatment has been observed for most studies for the following elements Cu, Ca, Mg, K, Pb, Zn and Ni. In some cases the reduced leaching of Zn and Pb has only been observed at low pH (bellow 6). In a single case Cu leaching was increased by high temperature treatment (1100°C).

- Cd and Cr have mainly been reduced by high temperature treatment (1100°C).

- A reduced leaching of Al is mainly seen at high pH (above 5).

Because of a high variability and a very broad range of elements in bottom ashes precise quantification of leaching behavior is a complicated task. Overestimation and underestimation of leaching in comparison to field tests is usual for the current batch leaching tests used for categorization of bottom ash. Thermal treatment of MSWI bottom ash may be a possible method to reduce leaching. Provision of further information on the processes occurring within bottom ash during thermal treatment would be advantageous. It is not known how the treatment operation conditions and the highly variable composition and structure of raw bottom ash affect the quality of the treatment product. Properties as treatment atmosphere and residence time, addition of additives, influence of reactor type (thermal treatment in static conditions or in the rotary kiln) should be investigated.

References

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Dijkstra J.J., van Zomeren A., Meeussen J.C.L., Comans R,.N. J., 2006b, Effect of accelerated aging of MSWI bottom ash on the leaching mechanisms of copper and molibdenium. Environmental science and technology, 40, pp. 4481-4487 Eusden J.D., Eighmy T.T., Hockert K., Holland E., Marsella K., 1999, Petrogenesis of municipal solid waste combustion bottom ash. Applied Geochemistry, 14, pp. 1073-1091 Faulstich M., 1993, Smellzen von Ruekstaenden aus der Muellverbrennung –Integrieren oder Nachshalten?- , Reaktoren zur thermischen Abfallbehandlung, K.J. Thome-Kozmiensky (Editor), EF-Verlag fuer Energie und Umwelttechnik GmbH, Berlin, pp. 175-188 Ferrari S., Belevi H., Baccini P., 2002, Chemical speciation of carbon in municipal solid waste incineration residues, Waste Management, 22, pp. 303-314 Faelman A-M., Hartlen J., 1994, Leaching of slags and ashes – controlling factors in field experiments ersus in laboratory tests – Environmental Aspects of construction with Waste Materials, Goumans J.J.J. et al. (Editors) , Elsevier Amsterdam, pp. 39-54 Hjelmar O., 1996, Disposal strategies for municipal solid waste incineration residues. Journal of Hazardous Materials 47, pp. 345-368 Hjelmar O., 2004, Regulation of utilisation of residues in Denmark, Workshop on Existing National Assesment Methods for Reuse of Residues, Swedish Environmental Protection Agency, Stockholm, 15 October Hjelmar O., Holm J., Crillesen K., 2007, Utilization of MSWI bottom ash as sub-base in road construction: First results from a large-scale test site. Journal of Hazardous Materials, A139, pp. 471-480 Hyks J., Astrup T., Christensen T.H., 2009, Leaching from MSWI bottom ash: Evaluation of non-equilibrium in column percolation experiments. Waste Management, 29, pp. 522-529 Izquierdo M., Lopez-Soler A., Ramonich E.V., Barra M., 2002, Characterization of bottom ash from municipal solid waste incineration in Catalonia. Journal of Chemical Technology and Biotechnology, 22, pp. 576-583 Izquierdo M., Querol X., Josa A., Vazquez E., Lopez-Soler A., 2008, Comparison between laboratory and field leachability of MSWI bottom ash as a road material. Science of the Total Environment, 389, pp. 10-19 Johnson A.C., Brandenberg S., Baccini P., 1995, Acid neutralizing capacity of municipal waste incineration bottom ash, Environmental Science and Technology, 29, pp. 142-147 Johnson C.A., Kersten M., Ziegler F., Moor H.C., 1996, Leaching behaviour and solubility – controlling solid phases of heavy metals in municipal solid waste incinerator ash. Waste Management, 16, pp. 129-134 Kirby C.S., Rimstidt J.D., 1993, Mineralogy and surface properties of municipal solid waste ash. Environmental Science and Technology, 27, pp. 652-660 Krzanowski J.E., Eighmy T.T., Crannell B.S., Eusden J.D. Jr., 1998, An analytical electron microscopy investigation of municipal solid waste incineration bottom ash. Journal of Materials Research, 13, pp. 28-36 Kuo Y-M., Wang C-T., Wang J-W., Huang K-L., 2009, Effect of Al2O3 mole fraction and cooling method on vitrification of an artificial hazardous material. Part 2: Encapsulation of metals and resistance to acid. Journal of Hazardous Materials, 169, pp. 635-642 Lidelöw S., Lagerkvist A., 2007, Evaluation of leachate emissions from crushed rock and municipal solid waste incineration bottom ash used in road construction. Waste Management, 27, pp. 1356-1365 Lind B.B., Norrman J., Larsson L.B., Ohlson S-Å., Bristav H., 2008, Geochemical anomalies from bottom ash in a road construction – Comparison of the leaching potential between an ash road and surroundings. Waste Management, 28, pp. 170-180. Liu Y., Li Y., Li X., Jiang Y., 2008, Leaching of heavy metals and PAHs from MSWI bottom ash in a long-term static immersing experiment. Waste Management, 28, pp. 1126-1136 Meima J.A., Comans R.N., 1997, Geochemical modeling of weathering reactions in municipal solid waste incinerator bottom ash. Environmental Science and Technology, 31, pp. 1269-1276 Meima J.A., Comans R.N., 1998, Application of surface complexation/precipitation modeling to contaminant leaching from weathered municipal solid waste incinerator bottom ash. Environmental Science and Technology, 32, pp. 688-693 37

Meima J.A., Comans R.N., 1999, The leaching of trace elements from municipal solid waste incinerator bottom ash at different stages of weathering. Applied Geochemistry, 14, pp. 159-171 Nüßlein F., Wunsch P., Rampp F., Kettrup A., 1994, Influence of combustion bed temperature on concentration and leachability of metals in slags from an incineration plant. Chemosphere, 28, pp. 349-356 Olsson S., van Schaik J.W.J., Gustafsson J.P., Kleja D.B., van Hees P.A.W., 2007, Copper (II) binding to dissolved organic matter fractions in municipal solid waste incinerator bottom ash leachate. Environmental Science and Technology, 41, pp. 4286-4291 Pfrang-Stotz G., Schneider J., 1995, Comparative studies of waste incineration bottom ashes from various grate and firing systems, conducted with respect to mineralogical and geochemical methods of examination. Waste Management and Research, 13, pp. 273-292 Pfrang-Stotz G., Reichelt J., Roos R., Seifert H., 2002, Evaluation of incineration processes on the basis of mineralogical phase transformations in particular consideration of fuel bed temperature and slag quality 6th European Conference on Industrial Furnances and Boilers, Lisboa, P, April 2-5 Piantone P., Bodenan F., Chatelet-Snidaro L., 2004, Mineralogical study of secondary mineral phases from weathered MSWI bottom ash: implication for the modeling and trapping of heavy metals. Applied Geochemistry, 19, pp. 1891-1904 Poon C.S., Clark A.I., Peters C.J., Perry R., 1985, Mechanisms of metal fixation and leaching by cement based fixation processes. Waste Management and Research, 3, pp. 127-142 Poon C.S., Chen Z.Q., 1999, Comparison of the characteristics of flow-through and flow-around leaching tests of solidified heavy metal wastes. Chemosphere, 38, pp. 663-680 Rand T., Haukohl J., Marxen U., 2000, Municipal solid waste incineration. A decision making guide., The RenoSam, 2006, The most efficient waste management system in Europe. Waste-to-energy in Denmark, RenoSam and Rambøll, Denmark Rendek E., Ducom G., Germain P., 2007, Assesment of MSWI bottom ash organic carbon behavior: A biophysicochemical approach. Chemosphere, 67, pp. 1582-1587 Rogheck J., Hartlen J., 1996, Ash gravel – a material for recycling. Waste Management, 16, pp. 109-112 Rosende M., Miro M., Cerda V., 2008, The potential of downscaled dynamic column extraction for fast and reliable assessment of natural weathering effects of municipal solid waste incineration bottom ashes. Analytica Chimica Acta, 619, pp. 192-201 Rubli S., Medilanski E., Belevi H., 2000,Characterization of total organic carbon in solid residues provides insight into sludge incineration processes, Environmental Science and Technology, 34, pp. 1772-1777 Saikia N., Cornelis G., Mertens G., Elsen J., Van Balen K., Van Gerven T., Vandecasteele C., 2008, Assesment of Pb-Slag, MSWI bottom ash and boiler and fly ash for using as a fine aggregate in cement mortar. Journal of hazardous materials, 154, pp. 766-777 Sabbas T., Polettini A., Pomi R., Astrup T., Hjelmar O., Mostbauer P., Cappai G., Magel G., Salhofer S., Schneider J., Vehlow J., Vogg H., 1994, Improving the MSWI bottom ash quality by simple in-plant measures – Environmental Aspects of Constructing with Waste Materials , Goumans J.J.J. et al. (Editors) , Elsevier Amsterdam, pp. 605-620 Selinger A., Schmidt V., Bergfeldt B., Vehlow J., Simon F.-G., 1997, Investigation of Sintering Processes in Bottom Ash to Promote the Reuse in Civil Construction (Part 1) – Element Balance and Leaching – Waste Materials in Construction: Putting Theory in Practice, Goumans J.J.J. et al. (Editors) , Elsevier Amsterdam, pp. 41-49 Simon F.G., Andersson K.H., 1996, InRec process for recovering materials from solid waste incineration residues. Fuel and Energy Abstracts, 37, p. 70 Skovhaug H., 2007, Energioptimering I/S Reno-Nord, Aalborg ovnlinie 4, DAKOFA konfernce ”Forbedret energieffektivitet i affaldsforbrændingen”, København, 4 december (in Danish) Smidt E., Meissl K., Tintner J., Ottner F., 2009, Interferences of carbonate quantification in municipal solid waste incinerator bottom ash: evaluation of different methods. Environmental Chemistry Letters, online publication, DOI 10.1007/s10311-009-0209-y 38

Speiser C., Baumann T., Niessner R., 2000, Morphological and chemical characterization of calcium-hydrate phases formed in alteration processes of deposited municipal solid waste incinerator bottom ash. Environmental Science and Technology, 34, pp. 5030-5037 Speiser C., Baumann T., Niessner R., 2001, Characterization of municipal solid waste incineration (MSWI) bottom ash by scanning electron microscopy and quantitative energy dispersive X-ray microanalysis (SEM/EDX). Fresenius Journal of Analytical Chemistry, 370, pp. 752-759 Speiser C., Heuss-Assbichler S., Klein R., Lechner P., 2003, Management of municipal solid waste residues, Waste Management, 23, pp. 61-88 Stumm W., Morgan J.J., 1996. Aquatic Chemistry (3rd edition), John Wiley, New York Suhr N.H., Ingamells C.O., 1966, Solution technicue for analysis of silicates. Analytical Chemistry, 38, pp. 730-734 Tatzber M., Stemmer M., Spiegel H., Katzberger C., Haberhauer G., Gerzabek M.H., An alternative method to measure carbonate in soils by FT-IR spectroscopy. Environmental Chemistry Letters, 5, pp. 9-12 van Zomeren A., Comans R.N.J., 2009, Carbon speciation in municipal solid waste incinerator (MSWI) bottom ash in relation to facilitated metal leaching. Waste Management, 29, pp. 2059-2064 Wang K., Tsai C., lin K., Chiang K., 2003, The recycling of MSW incinerator bottom ash by sintering. Waste Management and Research, 21, pp. 318-329 Yousuf M., Mollah A., Vempati R.K., Lin T-C., Cocke D.L., 1995, The interfacial chemistry of solidification/stabilization of metals in cement and pozzolanic material systems. Waste Management, 15, pp. 137-148 Ziegler F., Scheidegger A.M., Johnson C.A., Daehn R., Wieland E., 2001a, Sorption mechanisms of zinc to calcium silicate hydrate: X-ray absorption fine structure (XAFS) Investigation. Environmental Science and Technology, 35, pp. 1550-1555 Ziegler F., Giere R., Johnson C.A., 2001b, Sorption mechanisms of zinc to calcium silicate hydrate: Sorption and microscopic investigations. Environmental Science and Technology, 35, pp. 4556-4561 Åberg A., Kumpiene J., Ecke H., 2006, Evaluation and prediction of emissions from a road built with bottom ash from municipal solid waste incineration (MSWI). Science of the Total Environment, 355, pp. 1-12

September 2010. DTU no. 50443

Appendix A2 Report: Design, construction and commissioning of rotary kiln facility

Energinet.dk project no. 2006 1 6368

Improved electrical efficiency and bottom ash quality on waste combustion plants

Erhardt Mogensen1, Peter A. Jensen2, Henrik Kløft2, Martin Bøjer2.

1) Babcock & Wilcox Vølund A/S, Falkevej 2, DK-6705 Esbjerg Ø, Denmark

2) Department of Environmental Engineering, Technical University of Denmark, Building 115, DK-2800 Kgs. Lyngby, Denmark

CHEC no. R1003 2

Content

1.0 Introduction 3

2.0 Design of rotary kiln 3

3.0 Kiln facility description 4

4.0 Commissioning 5

4.0 Enclosures (Documentation on the kiln system not included in the report) 6

1.0 Introduction

To test the patented concept of integration of a rotary kiln with a grate waste combustion plant it a bench scale rotary kiln reactor was constructed. The kiln should use the already generated bottom ash from a grate waste combustion plant. It was the objective to make a plant were the influence of rotary kiln heat treatment on bottom ash leaching could be investigated, and that the concept of using the exit flue gas from the kiln to provide extra superheating could be tested. To make experiments a grate slag is treated in the kiln and leaching tests are then performed on the product. The corrosivety of the kiln exit flue gas can be tested by collection on a small deposit probe that simulates a superheater. By chemical analysis of the probe deposit the corrosivety of the flue gas can be estimated. The changes in slag leaching properties can be done by comparing treated and non treated bottom ashes.

It was decided to try to simulate the conditions in a full size rotary kiln as well as possible. Therefore a continuous kiln process was used and slag particles up to a size of 15-20 mm can be treated in the kiln. The particle size meant that a minimum inner kiln diameter of 200 mm had to be used.

The main components in the system is a combined preheater and slag feeder that provides the bottom ash to the kiln, and thereby simulate the grate bottom ash output process, and a gas heated rotary kiln that can adjust the inclination angel and the rotation speed. The complete system contains several different support systems including:

- Burner including gas and air supply - Control, safety and data registration system - Slag feeding and collection containers - Water cooling system - Flue gas cooling and suction system - Air cooled deposit probe unit

The design and construction of the rotary kiln facility was performed as a collaboration of Vølund Babcock & Wilcox Vølund and Department of Chemical Engineering at DTU. Vølund delivered the rotary kiln, the feeder system and the control system DTU delivered the other support systems.

2.0 Design of rotary kiln

A rotary kiln tube length of 2 meters and an inner diameter of 20cm were found appropriate. The kiln is isolated and refractory lined to ensure sufficiently high temperatures. The refractory can accept a maximum temperature of 1200°C. With a kiln inner size as specified bottom ash residence times in the rotary kiln chamber of 15min to 3 hours the following adjustment of the kiln is needed:

Kiln angel: 2 - 6°

Kiln rotation speed: 0.2 – 1.3 rpm

An example of possible operation points are: 4

- Filling degree 5%, residence time 15 minutes, then the following has to be used: feeding rate 17 kg/h, kiln angel 6°, rotation speed 1.2 rpm. - Filling degree 5%, residence time 3 hours, then the following has to be used: feeding rate 1.1 kg/h, kiln angel 2°, rotation speed 0.23 rpm.

Further calculations can be seen in enclosure X1.

3.0 Kiln facility description

The main components of the rotary kiln facility are shown on Figure 1. A vibration feeder preheats and provides the slag to the rotary kiln through an intermediate chamber. The rotary kiln angel and rotation speed can be changed and the kiln is heated by a gas burner with an input power of 5 to 30 kW. The flue gas flows in the opposite direction of the slag in the kiln. Heat treated bottom ash is collected in a 70 liter water cooled closed slag silo. The outer diameter of the kiln is approximately 700 mm and the inner size is a diameter of 200 mm and a length of 2 meters. The feeder can provide from 1 to 18 kg/h of bottom ash to the rotary kiln. There is placed five thermocouples in the kiln refractory to measure the temperature of the inner surface of the kiln. The inner temperatures of the kiln can be changed from 800 to 1200°C.

Figure 1. Sketch of rotary kiln experimental facility 5

The flue gas leaves the oven system through the intermediate chamber. Her is also measured the flue gas temperature, an air cooled deposit probe is inserted and flue gas samples used for gas analysis is extracted. The facility is mounted with windows so slag and gas burner operation can be monitored.

The rotary kiln angel can be adjusted in steps of 0.1° in the range of 2° - 6°. The kiln rotation is driven by a 0.55 kW electrical engine and the rotation velocity can be changed from 0.08 rpm – 1.24 rpm. Slag residence times of 15 to 240 minutes in the kiln can be obtained by adjustment of rotation velocity and kiln angel. The kiln system is reasonably air tight and sub pressure in the system is obtained by a use of an induced draft fan.

The slag feeder and preheater is constructed around a central 180 mm steel tube. The solid feeding is done by a controlled vibration of this tube leading the slag from the inlet silo to the rotary kiln. The steel tube is surrounded by three khantal heating elements that heat the slag while it is transported in the tube. It is possible to preheat the slag to 700°C. The complete unit is placed on three weight cells whereby the amount of slag feed to the kiln can be measured.

A detailed description/manual on how to conduct experiments on the rotary kiln facility is described in a separate report (See Appendix B2)

4.0 Commissioning

The commissioning process included modification of several parts of the facility:

- The flue gas cooler was modified to make continuous removal of condense water - The control system was corrected - The exit of the feeder tube was modified so it did not hit the intermediate chamber and the weight measurements worked correctly - The rollers supporting the kiln was modified to run more smoothly - The fixation of the heating elements in the preheater were improve to make them more resistant towards vibrations - The feeding equipment was mounted on a large steel plate to prevent movements induced by the vibrations - Modification of the kiln seals to obtain sufficient limited air leak to the furnace

5.0 Enclosures (Documentation on the kiln system not included in the report)

X1: Basic rotary kiln design. Martin Bøjer & Peter Arendt Jensen X2: Design grundlag for pilot skala roterovn 13/4 2007 X3: Beskrivelse af roterovn. Vølund 13/7 2007 X4: Tegninger - placering af roterovn i hal X5: Kravspecifikation til styring, 19/8 2007. X6: Automatic syd – Funktionsbeskrivelse 11/10 2007. X7: Hazop og PI diagrammer X8: Styre og dataopsamling + automatic syd X9: Køle og røggaskølesystem X10: billeder af anlæg under produktion X11: Belægningsprobe X12: Detaljerede maskintegninger af roterovn X13: samlingstegninger X14: Maksinttegninger af fødeanlæg X15: Dokumentation af gasrampe og gasbrænder

September 2010. DTU no. 50443

Appendix A3

Report: Manual to rotary kiln experiments

Energinet.dk project no. 2006 1 6368

Improved electrical efficiency and bottom ash quality on waste combustion plants

Igor Nesterov, Peter A. Jensen, Kim Dam-Johansen

Department of Chemical Engineering, Technical University of Denmark, Building 229, DK-2800 Kgs. Lyngby, Denmark

CHEC no. R1003 2

Content

1.0 Introduction 3

2.0 Plant description 3

2.1 Control system 3

2.2 Mechanical systems 10

2.3 Slag and flue gas cooling systems 13

2. 4 Gas supply system 16

2.5 Flue gas analysis and sampling 16

3.0 Material mean residence time (MRT) and feeding rate (FR) estimation 18

4.0 Step by step kiln experiment manual 18

Appendix A. Feeding rate estimation 21

Appendix B. Residence time estimation 23

1.0 Introduction

This operation manual shall always be followed when experiments are conducted on the rotary kiln facility.

2.0 Plant description

A general schematic view of the rotary kiln system is presented in figure 1 (see next page). It can be seen from that the whole setup consists of:

- A refractory lined rotary kiln tube, with a gas burner and a receiving slag bunker (“slag pot”) mounted on a movable frame which allows to adjust the inclination angle by the motor M1; - A feeding silo containing the material to be treated combined in one system with a vibrating feeder and an electrical preheater. The whole system is mounted on a steady frame with three weight- measuring sensors so the amount of feed material can be determined; - An intermediate chamber which connects the feeder and rotary kiln itself.

The description for some kiln subsystems is given in the following sections.

2.1 Automatic control system

The kiln control system was designed to provide full automatic control and measurement of operation data for the rotary kiln. Sensors of different kind are used to measure temperature (thermocouples), pressure (pressure sensors) and weight (piezoelectric sensors). Signals from the sensors are registered and processed by electronics within two racks behind the kiln. Most operation data is controlled from a computer, but some machinery is started by using devices on the racks as shown on Figure 2.

Fig. 2. Set of controls of automatic control system (see explanation of the numbers in the text) 4

Fig. 1. Rotary kiln, schematic view.

The controls on the rack of the controlling system provide the following functionality (fig. 2):

- 1, 2, 3 – controls the slag dosing and weight measuring system “SKAKO”. Switch (1) “SKAKO MAN-AUTO” turns between automatic and manual mode of the dosing system; status is indicated by the lamp (2) – “SKAKO STYRESPÆNDING OK”. Dosing can be controlled manually by the “SKAKO MANUEL” control (3) if switch (1) is in the “MAN” position, but it is strongly recommended to keep switch (1) in “AUTO” position all the time and controlling the dosing from the computer. - In the case of emergency button (4) “NOT AUS/NØDSTOP/EMERGENCY STOP” should be used to stop all the systems of the kiln. Button (5) “RESET NØDSTOP” is used to restart kiln systems after an emergency stop or a power-off and makes the setup operable again. Button (5) is lit when reset is needed. - Switch (6) turns on and off the kiln inclination adjustment system. When switch (6) is on (turned to the right and lit) inclination of the kiln can be adjusted by the buttons (7) “NED” – increasing inclination and (8) “OP” which decreases inclination of the kiln. - Switch (9) “HOVEDAFBRYDER” turns the whole setup on and off.

Four main automatic control loops are used on the kiln and is described below:

- To control the kiln temperature a signal from the thermocouple measuring flue gas temperature (T14) is used. According to a set point level for T14 a controlling signal is sent to the gas burner control system and the gas burner load is changed. - Temperature in the preheater is controlled by regulating the power to the three electrical heating sections; temperature signals from thermocouples (T1, T2, T3) installed at the preheater is used. - Pressure within the kiln is controlled by the automatic valve MV 1 (fig. 1) which is installed between the flue gas exit tube and the chimney. Signal is taken from pressure sensor P2 (fig. 1) - Kiln rotation is controlled by the parameters of the motor rotating the kiln.

All the parameters of the kiln (except kiln inclination) are controlled from the computer connected to the automatic controlling system through the network interface. The software used to control and record measuring data is the Eurotherm iTools. Files needed to operate the kiln are listed in the table 1. Detailed description of them is given below.

Table 1. Eurotherm iTools files needed to operate the kiln

Name Where to find Explanation Start.UIU Desktop\Start Roterovn Tool to control and set parameters of the kiln (in Danish) Start_UK.UIU Desktop\Start Roterovn Tool to control and set parameters of the kiln (in English) Temperaturer og Desktop\Start Roterovn Tool to observe and record signals received from Tryk.UIX the kiln.

Start_UK.UIU (Start.UIU for the version in Danish) is the iTools Runner program which allows the user to observe and control all the kiln parameters. Main screen of the Start_UK.UIU program is shown in fig. 3. It contains the following sections:

- Start/Stop buttons for all the kiln processes and the set of buttons to control the MV1 valve (upper-left corner) - Alarms from the kiln systems (upper-right corner) - Schematic representation of the kiln with the measured values of signals taken from all the kiln sensors (rest of the screen)

Fig. 3. Start.UIU main screen.

Some of the set point parameters on the Start_UK.UIU main screen can be changed and a list is given in table 2, the rest of parameters are the measured values.

Table 2. Rotary kiln set point parameters which can be changed on the iTools screen

Parameter Explanation T1, T2, T3 Temperature (oC) of preheater heating sections, can be set separately T18 SP Temperature (oC) of the deposit probe % Amplitude Amplitude of vibrating dozer, it is better to keep this parameter at 70% as this value provides maximal effectiveness at acceptable level of noise Feeding Time (%) Percentage of vibration of the dozer from the whole “vibration-pause” working cycle. 100% means that dozer vibrates continuously Minimum on time (s) Minimal time of dozer vibration set in seconds Setpoint, % Rotation speed of the kiln set in %. 100% corresponds to 1,263 rpm tSP, oC Setpoint for the flue gas temperature (T14 thermocouple) controlling temperature in the kiln by changing burner load SPrr, oC/h Ramp rate for the working setpoint for the temperature in the kiln to reach the tSP value Lambda, % Parameter to set the air to gas ratio at the burner. SP, mbar Setpoint for under pressure in the oven.

Buttons in the upper left corner (fig. 3) start and stop processes of the kiln system. Each button has two checkboxes on the right, the first of them indicates that a signal to start the corresponding process is sent to the actuator, and the next indicates start of the process. List of the processes is given in the table 3.

Table 3. On/off operations conducted by using the iTools programcomputer system

Button set Description Cooling Controls the pump which moves the water through the flue gas cooling system. Probe Starts electrical heating on deposit probe. Rotation Rotation of the kiln. Starts the motor which rotates the kiln. Burner Burner. Can be started only if the under pressure at the kiln is kept within the working range, flue gas cooling is turned on and rotation of the kiln is started Preheater Preheater. Can be started only if the under pressure in the kiln is kept within the working range. Feeder Vibrating feeder. Can be started only if the under pressure in the kiln is kept within the working range.

Kiln pressure set of controls consists of five buttons and two indication fields. Normally pressure is regulated automatically according to a set PV value. If manual control of pressure is needed, then the button “Valve MAN.” should be pressed. Manual pressure regulation state is indicated by checked checkbox on this button. Further regulation is provided by the buttons “OPEN” and “CLOSE” to send a command to open or close valve MV1 correspondingly. Opening and closing processes are indicated by “True” parameter shown in “Opening” or “Closing” field respectively. For example, fig. 1 shows a situation when the kiln pressure is controlled manually and valve MV1 is closing. To stop MV1 in some position button “STOP” is used. To return the system to the automatic pressure control the button “Valve AUT” shall be pressed.

Table 4. Critical safety alarms of the kiln (relevant measuring devices and sensors can be seen on Figure 1 and 9)

Name of Alarm Explanation Reaction of kiln systems Burner Preheater Feeder Rotation Temperature High Zone 1/Pre- Temperature in Stops Stops Stops Continues heater preheater section 1 is too high Temperature High Zone 2/Pre- Temperature in Stops Stops Stops Continues heater preheater section 2 is too high Temperature High Zone 3/Pre- Temperature in Stops Stops Stops Continues heater preheater section 3 is too high High Temp. Cooling Water (T23, Temperature of water Stops Stops Stops Continues 28, 26) at the collected slag cooling system is too high No signal from flow switch 3 No water circulation Stops Stops Stops Continues through the cooling system No signal from flow switch 1, 2 No water circulation Stops Stops Stops Continues or 4 through the cooling system High Temp. Slag In (T16) Temperature at the Stops Stops Stops Continues kiln inlet (T16) is too high Sub-pressure to Small Underpressure is too Stops Stops Stops Continues low Temp. in Flue Gas Cannel to Temperature at flue Stops Stops Stops Continues High/T27 gas outlet (T27) is too high Error on VLT Stops Stops Stops Stops Rotation Stop Error Mechanical problems Stops Stops Stops Stops at the kiln driving system Temperature to High in Kiln Temperature in the Stops Stops Stops Continues (T14) oven is too high

Errors occurring through the kiln functioning are indicated on the “Alarms” panel on the computer screen where critical alarms are shown in red. The system cannot start or continue working if the status of any critical alarm is set to “True”. Two fields marked with “Critical Error” show the overall critical alarm state of the systems in rack 2 and 1 respectively. The full set of critical error alarms including explanations and reactions of the kiln systems is shown in table 4.

Observation and recording of the kiln measuring parameters are done by activating the program “Temperaturer og tryk.UIX”, and the screen looks as shown in Figure 4. By Starting the program one can 9

access the charts representing real-time values of measurements by the kiln sensors. An internal notation is used by the control system for signals representation, so in table 5 explanation of some signals is done in terms of sensors notation (fig. 1) and kiln parameters (fig. 3, table 1).

A Chart control panel to be used to change set of parameters recorded, scale time and value axes is called using a right-click on the chart or by using the button “1” (fig. 4). Button “2” (fig. 4) is used to start the recording of the selected parameters to a .CSV file which can be read and processed using MS Excel.

Table 5. Rotary kiln measuring signals

Internal notation Parameter/Signal E1.192-168-111-11-502-ID001-2584.IO.Module04.M04_C1.Val T1 E1.192-168-111-11-502-ID001-2584.IO.Module04.M04_C2.Val T2 E1.192-168-111-11-502-ID001-2584.IO.Module04.M04_C3.Val T3 E1.192-168-111-11-502-ID001-2584.IO.Module06.M06_C1.Val T16 E1.192-168-111-11-502-ID001-2584.IO.Module06.M06_C2.Val T17 E1.192-168-111-11-502-ID001-2584.IO.Module06.M06_C3.Val T23 E1.192-168-111-11-502-ID001-2584.IO.Module07.M07_C2.Val Pressure E1.192-168-111-11-502-ID001-2584.IO.Module08.M08_C1.Val Amplitude E1.192-168-111-11-502-ID001-2584.IO.Module12.M12_C1.Val T26 E1.192-168-111-11-502-ID001-2584.IO.Module12.M12_C2.Val T27 E1.192-168-111-11-502-ID001-2584.IO.Module12.M12_C3.Val T28 E1.192-168-111-11-502-ID001-2584.Toolkit_Blocks.USRVAL.Usr10 Feeding Time E2.192-168-111-12-502-ID002-2584.IO.Module05.M05_C1.Val T12 E2.192-168-111-12-502-ID002-2584.IO.Module05.M05_C3.Val T14 E2.192-168-111-12-502-ID002-2584.IO.Module07.M07_C2.Val Air Valve E2.192-168-111-12-502-ID002-2584.IO.Module08.M08_C1.Val Rotation setpoint COM1.ID001-Mini8.IO.Mod.5.PV T11 COM1.ID001-Mini8.IO.Mod.4.PV T10 COM1.ID001-Mini8.IO.Mod.3.PV T9 E1.192-168-111-11-502-ID001-2584.IO.Module07.M07_C1.Val Weight

Fig. 4. Representation of kiln parameters by iTools system (Temperaturer og tryk.UIX)

2.2 Mechanical systems

A mechanical system is designed to provide rotation of the kiln tube and change the inclination angle of the kiln. All components of this system are mounted on a metal frame. One end of the frame is fixed while the other is put on poles which length can be changed by a mechanism driven by motor M2 (fig). M2 is operated by “Ned” and “Op” buttons from the set of controls on the automatic controlling system (fig. 2) as it was described in the previous section.

The kiln tube sits on four wheels, and one pair of the wheels (plastic ones) is driven by the motor M1 (fig. 5). Each end of the rotating kiln tube has a Teflon sealing to protect it from contact with parts of metal frame and to keep the kiln isolated.

Moving parts of mechanical system have to be lubricated in the beginning of an experiment. Lubricant is added using a special syringe (fig. 6) to the points shown on fig. 7.

M 1

M 2

Fig. 5. Motors of mechanical system.

Fig. 6. Syringe used to lubricate kiln mechanisms. 12

1 Points to add 2 lubricant

2 1

Fig. 7. lubrication of the kiln (front) 13

Fig. 9. PI diagrams of the flue gas (left) and treated slag (right) cooling systems

2.3 Slag and Flue gas cooling systems

PI diagram of the water cooling system is shown in Figure 9. The flue gas cooling system is combined with a flue gas tube cooling water jacket as it is shown at fig. 10. There is no automatic control of this system, although signals from the thermocouples are registered by the computer control system. The cooling water flows are controlled by needle valves and should be adjusted manually during pre-experimental runs. 14

Before the start of an experiment it is only needed to check if all the ball valves of the system are switched to the “on” position.

The treated slag cooling system is presented on fig. 11. Water circulates in a closed loop through the two slag pots as shown on fig 9. Filling of this loop is done by valve V3. It should be opened until the expansion tank is filled and water runs from the overflow line.

Fig. 10. Part of the rotary kiln setup with flue gas cooling system.

Expansion Tank

Heat Exchanger

T29 V10 V11

V3 Pump/Fs3

VR4 VR5

V8 V6 V7 V9

Fig. 11. Treated slag cooling system: main part (left) and slag pot (right).

Fig. 12. Gas dosing system. Valves in “Closed” position.

2.4 Gas supply system

A gas dosing system provides a gas-air mixture for the gas burner. Components of this system are mounted on the rack behind the kiln tube (fig. 12). This system is fully automatic and designed to operate the burner by providing air-gas mixture and controlling the burner ignition. As system it is fully automatic the only thing needed is to keep both valves marked on fig. 12 open.

2.5 Flue gas sampling and analysis

A flue gas sampling system provides continuous gas sampling and preparation to analysis. Gas is taken from the flue gas line after the heat exchanger. Gas is cleaned from aerosols and dust going through the system of filters and freezer. Sampling is started by turning on the pump (“Pump” button) (fig. 13). The whole gas sampling system should be checked for water during sampling and accumulated water should be removed by emptying the filter flask highlighted on fig.13. 17

Fig. 13. Gas sampling system rack.

3.0 Material mean residence time (MRT) and feeding rate (FR) estimation

MRT (Mean Residence Time) of material in the rotary kiln can be estimated through known rotation speed and kiln inclination using Sullivan’s formula (Sullivan et al., 1927)

퐿 Θ 푀푅푇 = 1.77 ∙ ∙ 퐷 푛 ∙ 훽 L – length of the kiln (L=2m) D – diameter of the kiln (D=0,2m) Θ – dynamic angle of repose n – rotation speed, rpm (varied) β – inclination angle (varied)

The dynamic angle of repose is a value which cannot be measured directly during the experiment and it changes with changing amount of material in the kiln and particle size distribution of material. Thus, it can be estimated by measurement of MRT at some known rotation speed and inclination angle values. Estimated value of Θ can be used then to calculate MRT using Sullivan’s formula (value used 30°). This method was found to be reliable during series of 10 experiments with MRT of 60 and 120 min (see appendix B). Deviations of experimental MRT from calculated values did not exceed 7 min. Generally the experiments gave somewhat higher MRT times than the calculations.

4.0 Step by step kiln experiment manual

To insure safe and reliable operation of the kiln the following step by step procedure shall always be followed when kiln experiments are conducted.

When conducting experiments all parameters and events shall be written in a logbook.

Before an experiment is started some parameters should be decided:

- Desired temperature in the kiln. As it is not possible to set the same temperature through the whole kiln length, then the decision should be made which signal is set to the desired value. - Rotation speed and inclination angle to provide a desired material motion speed through the kiln. - Feeding rate as the function of ”Feeding time” and “Minimum on time” parameters (described at table 2).

1. Pre-start check. These have to be done before start of the experiment whether it is a first start or not. 1.1. Check if the computer connected to the kiln automatic controlling system is on and the Start_UK.UIU and Temperaturer og tryk.UIX are running. If not, then turn on the computer (no password is needed to log on) and run applications (“Start Roterovn” folder at Desktop). 19

1.2. Check the water in the receiving bunker cooling system by opening valve V13 until the water from the expansion tank runs out the overflow line (fig. 11). 1.3. Add lubricant to the kiln rotation system (fig. 7) using syringe (fig. 6). 1.4. Check the rotary kiln inclination angle (controls at fig. 2) and adjust it to the desired value if necessary. 1.5. Set rotation speed of the kiln it to the value needed for experiment using “Setpoint” parameter from Start_UK. UIU (fig. 3). 1.6. Set feeder parameters to the desired values using ”Feeding time” and “Minimum on time” parameters from Start_UK. UIU (fig. 3, table 1 for description of parameters). 1.7. Check the state of the valve MV1, if it is in manual mode – press “Valve AUT” button. 2. First start. All the steps from “Pre-start check” have to be done before the start of following sequence. 2.1. Check the valves on gas and air lines of the burner. They have to be turned to the “open” position. 2.2. Check pressure signal using Temperaturer og tryk.UIX. It should be regulated to be maintained negative 2.3. Start water flow through the heat exchanger on the exit of flue gas line (“Køling Start” button of Start. UIU). 2.4. Start preheater (“Preheater Start” button of Start_UK. UIU). 2.5. Start rotation of the kiln (“Rotation Start” button of Start_UK. UIU). 2.6. Set “tSP” parameter of Start_UK. UIU (setpoint for flue gas temperature (T14)) to the value about 200 oC lower than the needed temperature. 2.7. Start burner (“Burner Start” button of Start. UIU). 3. Experiment. 3.1. Wait until temperature on thermocouples T10 and T11 reaches desired values. During this period some corrections of tSP (T14 setpoint) value have to be made to set the needed temperature. Trends of signals at all thermocouples have to be noticed through Temperaturer og tryk.UIX charts. Goal – stable temperature on at least T9, T10 and T11 which can be controlled by changed tSP. Typically it takes about 3-4 hours for the kiln to reach the stable state. 3.2. Stability of the temperature has to be proved through Temperaturer og tryk.UIX charts for at least 30 min. Temperature can change spontaneously after short period of stability. 3.3. If temperature in the kiln is stable – start dosing (“Feeder Start” button of Start. UIU). 3.4. During the slag feeding period the amount of material coming into kiln has to be observed through the viewport and controlled, if necessary, by changing dosing parameters. 3.5. At the temperatures higher than 1080 oC bottom ash flow through the kiln has to be controlled due to melting of bottom ash which can stick to the walls of the kiln. In the case of such sticking the kiln temperature must be decreased. At lower temperatures no problems with slag motion have been observed. 3.6. When an sufficient amount of slag is fed - stop bottom ash dosing (“Feeder Stop” button of Start_UK. UIU). 3.7. Wait until all slag run through the kiln. Residual quantities of fine fraction may remain in the kiln. 3.8. Turn off the burner (“Burner Stop” button of Start_UK. UIU). 3.9. Wait until highest temperature at the kiln decreases to about 750 oC. 3.10. Turn on manual mode for pressure control by pressing “Valve MAN” button. 20

3.11. Close the valve MV1 by pressing “Close” button, waiting until the valve is closed and then pressing “STOP” button 3.12. Turn off rotation (“Rotation Stop” button of Start_UK. UIU). 3.13. Dismount slag pot. It is safe to keep it dismounted for about 7-10 minutes, that time is enough to collect treated bottom ash and clean the bunker. To dismount the slag pot the following sequence has to be used: - It is better to change inclination angle to 3,8o. - Lift cart should be moved to the kiln and put down the slag pot. It should be lifted up to hold the pot. - There are four bolts holding the rails on which slag pot lies. Two of those bolts are placed near the kiln tube; these bolts should be loosed to the half of their length. - Two bolts at the end of the kiln should be loosed to make the fastening open. - After the fastening is open the cart should be lifted down to free the pot. If it is impossible to transport the pot off the kiln, then two bolts near the kiln tube have to be loosed more. - Transport the pot on the cart off the kiln. 3.14. Mount slag pot to the kiln. - Place the pot under the end of the kiln being sure that it is placed right under the sealing groove. Sometimes it is needed to loose the bolts near the kiln tube more to make this. - Lift the pot up to place its’ top to the sealing groove. - Close the fastening. - Tight the bolts. First – the bolts near the kiln tube, then – bolts at the fastening. 3.15. If another experiment is planned then do steps 2.6 and 2.7 3.16. If the kiln should be turned down then after the slag is removed and slag pot is fitted on its place – turn on the rotation and automatic control of the pressure. The kiln should be left rotating for about 7 hours to cool down. After that period all systems of the kiln can be turned off.

Appendix A. Feeding Rate Estimation Experiments

It was the goal to estimate which parameters should be used to obtain feeding rates of 1, 2, 4 and 8 kg/h and reproduce them. It was the objective to determine appropriate values of the relative puls time (%) parameter to obtain the desired feeding rates. For all the experiments the Amplitude value was fixed at 70% and the minimum time on time at 3 seconds. A series of cold flow feeding experiments were conducted.

At high feeding rates and low kiln angels there were a risk of back feeding into the intermediate chamber.

The results of the FR (Feeding Rate) measurements at different Puls values are shown at fig. B1 There is observed a linear dependence between Puls and FR.

8 y = 0,2425x + 0,1605

7 R² = 0,9891

3 Feeding Rate, kg/h Rate, Feeding 2

0 0 5 10 15 20 25 30 35 Puls, %

Fig. B1. The dependence of feeding rate on Puls dosing parameter obtained from experiments performed 19.03.09-20.03.09. Amplitude value was fixed at 70% and the minimum time on was set at 3 seconds.

A new experiment with a set point puls of 24.3% should give a feeding of 6Kg/h but 9.5 kg/h was obtained. The estimated FR is about 3 kg higher than the expected value. The measurements indicate that a predominance of coarser fraction particles in the feed ash leads to higher FR values. This was confirmed by further experiments.

There is a dependence of feeding rate on the feeding ash particle size distribution. Prevalence of coarser fraction leads to the higher feeding rates. It seems that grain size of feeding ash cannot be predicted for the experiment, so feeding rate values cannot be obtained from the dosing parameters precisely. Experiments show that ranges of feeding rates at the same dosing parameters can be about 3 kg/h wide. 22

Fig. B2. Ash dosed during the 20.03.09, 10:07-10:37 run. Picture has been taken at 10:31

Appendix B. Mean Residence Time Estimation Experiments

The objective of the cold test experiments were the following: 1. To provide direct measurements of the mean residence time (MRT) of the bottom ash in the kiln. 2. To obtain data on the MRT dependence on the rotation speed and rotary kiln inclination angle. 3. To compare the results of the measurements with values obtained from calculations.

The experiments were performed from 26.03.09 to 20.04.09 The rotary kiln inclination angle was varied from 2.0o to 5.0o The rotation speed was varied from 0.378 to 1.200 rpm

The dozing was provided mostly as moderate portions (2.5 s of dosing), for some experiments small and extremely small volumes of ash were dozed (1 and 0.5 s correspondingly) and also the large amounts (three sequential portions of 2.5 s dosing each) were used. The variations of particle size distribution from one doze to another were observed. The collected data have shown a reproduction of the timeline for all the experiments provided at the same parameters (oven inclination, rotation speed).

As the injected portion of slag spreads over the rotary kiln five zones can be defined: 1. Individual particles moving separately (Fig. B1). This zone usually contains a small amount of particles having near-spherical shape and diameter of 1.5-2 mm, sometimes (more frequently at high values of inclination angle) some particles of about 10 mm in diameter present. The total amount of the particles in this zone is no greater than 10. The time for the first particle of this zone to reach the end of rotary kiln varies for the same values of rotation/inclination. In the laboratory notebook it is marked as “First stones” 2. In this zone first rare interactions among the particles occur (Fig. B2). The formation of the groups containing 2-5 independently moving particles with the same motion speed is observed. This zone contains total amount of 15-20 particles with the size of 1-10 mm (1-3 mm mostly), shape of the particles is still near-spherical. The time of the first particles from this zone to reach the end reproduces for the experiments with the same rotation/inclination parameters. “Multiple stones” mark in the laboratory notebook. 3. Interactions among the particles become common, but particles still move individually between the interactions (Fig. B3). Only the first evidences of the flow formation are observed. In this zone the fine particles with the diameter less than 1 mm appear first. “Mass flow” mark in the laboratory notebook. 4. From this time material forms the dense flow where no independent motion of the particles can be detected (Fig. B4). The zone contains particles of all shapes and sizes presented in the portion. Within the zone size of the prevailing particles decreases from the beginning to the end until the dust-size is reached. There is no special mark for this zone in the laboratory notebook; times were taken from the study of the pictures made during the experiment. 5. The residual dust and some individual particles of about 5-7 mm having a low motion speed (Fig. B5). It was decided to consider this zone as the residual as it contains only a small amount of portion by both weight and volume, and the detection of its end because small amounts of dust 24

present at the end of the kiln for quite a long time. Usually this part of the portion was removed using a higher rotation speed. “Dust and individual particles” mark in the laboratory notebook.

Recording of the experiments were made by the marks for the beginning of each zone to run out of the kiln and by photographing of the material moving along the kiln and reaching the end. Pictures were taken with an average interval of 1.5 min.

After studying all the collected material a decision was made to determine the MRT as the mean time between the beginning and the end of the zone 4 flow. Arguments were the following: 1. Visually this zone contains most of the dozed portion by mass. 2. This zone presents the continuous flow of material where speeds of the particles are averaged due to the interactions. Thus the flow of the material during the continuous dozing probably can be modeled by the motion of this zone.

Calculations of residence time were made using the equation developed by Sullivan et al. (1927) 퐿 Θ 푀푅푇 = 1.77 ∙ ∙ 퐷 푛 ∙ 훽 L – length of the kiln (L=2m) D – diameter of the kiln (D=0,2m) Θ – dynamic angle of repose (Θ=30o, measured during the experiments) n – rotation speed, rpm (varied) β – inclination angle (varied)

The results of the experimental measurements of residence time compared to the results of calculations are presented at the table B1 and figures B6 and B7. The approximating equations for the collected data are also given on those charts. It is seen that calculation provides higher MRT values than the ones obtained experimentally. The difference between calculated and measured results grows with the MRT value while the dependence of MRT from the rotary kiln parameters for both the experimental and calculated values is described by the equation of the same type.

Further study of the real flow of the bottom ash through the rotary kiln leaded to the following conclusions: 1. The MRT measured by the single portion dozing shows the value for the fraction with the particle size larger than 0.5 mm. No significant interactions were detected between the particles of this fraction and the fine <0,5mm fraction which easily separates from the flow. 2. During the continuous dozing no separation of the dust flow was detected. Dust can form a sort of “dust bed” if it is dozed in larger quantities or move as the series of portions divided by the portions of coarser fraction (figures B8 and B9). 3. The following means that the measured MRT values should define the lower border of the MRT range for the bottom ash – the case when almost no fine fraction is dozed. The pure dust flow or “dust bed” type of the flow could define the higher border of the range. Speeds of the coarser particles and “dust” should be averaged at the real flow. 4. The equation given for the MRT could be considered to provide the MRT for the dust flow. This assumption was proved with one measurement at the rotation speed of 0,885 rpm and inclination 25

angle - 2,9o. The attempt to determine the beginning and the end of the dust flow was made. Measured residence time for the dust was 37 min. The calculated value was 38 min.

It was also obtained during the experiments that an kiln inclination angle of 2o was too low to provide effective feeding. Even at a feeding rate of 3 kg/h and rotation speed of 0,778 rpm there was serious risk of the ash dropping from the feeding end of the kiln.

Thus, the conclusion to the experiments provided is the following: The experiments can be started with the MRT being calculated as the mean value between the calculated from the experimental data and the given equation. Inclination angle should be of about 3-4o; at these values there is almost no risk of damage due to the backflow of ash particles and the quantity of the particles running forward of the main flow is not as large as for the higher values (5o). Observations made during the experiments shown that no segregation by particle size or shape occurred within the continuous material flow.

Table B1. Results of the mean residence time measurements compared to the MRT calculation

Angle, Rotation speed, Beginning End of zone Measured Calculated MRTcalc.- degrees rpm of zone 4, 4, min MRT, min MRT, min MRTmeas., min min

2,0 0,378 80 107 94 128 35 2,0 0,630 45 78 62 77 15 2,0 0,630 49 71 60 77 17 2,0 0,778 51 66 59 62 4 2,0 0,883 35 50 43 55 12 2,0 1,200 24 37 31 40 10

2,9 0,630 34 52 43 53 10 2,9 0,885 24 38 31 38 7 2,9 1,200 19 26 23 28 5

3,9 0,378 40 62 51 66 15 3,9 0,378 45 61 53 66 13 3,9 0,378 44 59 52 66 14 3,9 0,378 42 59 51 66 15 3,9 0,505 37 43 40 49 9 3,9 0,505 32 43 38 49 12 3,9 0,505 34 43 39 49 11 3,9 0,505 32 44 38 49 11 3,9 0,630 22 35 29 39 11 3,9 0,630 24 35 30 39 10 3,9 0,884 17 25 21 28 7 3,9 0,884 21 26 24 28 5

Table B1. Continued

Angle, Rotation speed, Beginning End of zone Measured Calculated MRTcalc.- degrees rpm of zone 4, 4, min MRT, min MRT, min MRTmeas., min min

3,9 0,884 17 26 22 28 7 3,9 1,200 12 21 17 21 4 3,9 1,200 10 19 15 21 6

5,0 0,378 34 48 41 51 10 5,0 0,630 22 28 25 31 6 5,0 0,884 16 20 18 22 4 5,0 1,200 11 16 14 16 3

Fig. A1. Example of zone 1 flow type.

Fig. B2. Example of zone 2 flow type.

Fig. B3. Example of zone 3 flow type. 28

Fig. B4. Example of zone 4 flow type. Dense flow.

Fig. B5. Example of zone 5 flow type. Residual dust. 29

Fig. B6. Experimental and calculated values for MRT of the bottom ash at the inclination angles of 2,9 and 5,0 degrees.

Fig. B7.Experimental and calculated values for MRT of the bottom ash at the inclination angles of 2,0 and 3,9 degrees. 30

Fig. B8. “Dust bed”

Fig. B9. “Wave-like” type of the flow

Bibliography

Sullivan J.D., Maiter C.G., Ralston O.C. Passage of solid particles through rotary cylindrical kilns. United States Department of the Interior – Bureau of Mines – Technical Papers, 1927