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Home , Incineration, Sewage, Sewage sludge

Sludge : good practice and environmental aspects C M. Braguglia*, G. Mininni*, D. Marani* and V. Lotito** *Cnr - Istituto di Ricerca sulle Acque - Via Reno, 1 - 00198 Roma * Cnr - Istituto di Ricerca sulle Acque - via F. De Blasio 5 - 70123 Bari (E-mail: mininnifcbirsa. rm.cnr.it)

Abstract. Growing difficulties in the utilization in agriculture or make incineration an attractive alternative for sludge disposal. Capital and operating costs and concern about gaseous emissions may however limit convenience and acceptance. In this paper a model is presented for optimisation of the cake concentration before the , allowing an autogenous operation with a minimization of production. As far as emissions of heavy and organic micropollutants at the stack is concerned, results of tests on a demonstrative plant, including a fluidised bed and a rotary kiln furnace, are presented. The tests were carried out in different feeding (sludge alone or spiked with chlorinated hydrocarbons) and operating conditions (temperature of the afterburning chamber). Key words recovery; ; incineration; PAH; PCDD/F; thermal .

Introduction sludge after mechanical dewatering still contains high moisture content (60-80 %) with a relevant low heat value of 6,000-3,000 kJ/kg. Sludge incineration is therefore quite expensive due to high auxiliary consumption. Consequently, optimisation of is of predominant importance to minimize capital and operating costs of a sludge incineration plant and to render attractive such option. Typical design approach for energy recovery in sludge incineration performed by fluidised bed furnace considers the useof a sludge drying system, usually performed by an indirect dryer utilizing produced in the heat recovery section. Design of dryer, furnace, and boiler strictly depends on solids concentration before and after drying and on the temperature of exhaust gases after boiler. Energy conversion and recovery in a incineration process are discussed enlightening dryer integration in the process. In addition to fuel consumption, the key factor determining feasibility and acceptability of sludge incineration plants is the potential emission of at the stack, with emphasis on metals and organic micropollutants. In this framework the results of a research activity on sludge carried out by the Research Institute of the Italian National Research Council, are presented. Tests were carried out on a demonstrative plant equipped with a fluidised bed furnace (FBF) and a rotary kiln furnace (RKF) with the aim to investigate factors affecting emissions at the stack.

Sludge drying and combustion modelling Basic assumptions for mass and enthalpic balances of a drying/incineration process by fluidised bed furnace are reported in Table 1. Enthalpy balances were developed considering specific heat data reported by Perry & Green (1984). An algorithm was developed to calculate cake concentration for autogenous combustion. consumption (Nm3 /kg wet sludge) decreases proportionally vs. feed cake concentration to the furnace. The minimum cake concentration for autogenous combustion is 45.9 %. Incineration of a more concentrated cake requires additional fresh air to keep furnace temperature at 850 °C with consequent increasing production of exhaust gas. Composition of exhaust gases from the furnace is reported in Figure 1. Increasing cake concentration up to the point of autogenous combustion, humidity shows a linear decrease from 28.2 to 24.6 %, while oxygen remains constant at 6 %. After this point the decrease of humidity is exponential due to air dilution, whereas oxygen increases to a maximum value of 12.1 % at a 90 % cake concentration.

523 Table 1. Assumptions for enthalpic and mass balances in sludge drying and incineration (Task Force on Thermal Destruction, 1992) Parameter Values Elemental analysis of loss on ignition of dry solids C 53 %, H 7.7%, O 32.7 %, N 5.6 %, S 1 % Outlet temperature from the dryer 100 °C Extraction of non condensable gas from the dryer 1.5 kg/kg evaporated moisture Gross heat value of volatile solids (VS) 22,486 kJ/kg Gross heat value of methane 38,083 kJ/Nnf Heat losses (% of inlet and/or developed) 5 % in the furnace and the boiler; 7 % in the dryer Loss on ignition with respect to dry solids 70% Excessof air for methane combustion 20% Excessof air for VS combustion What needed to attain at least 6 % of oxygen in the gas Exhaust gas temperature 850°C Conversion of in VS to NO 10%

— C02 —H20 — N2 —02

Sludge solids concentration (weight fraction)

Figure 1. Composition of exhaust gas from fluidised bed furnace

As far as thermal drying is concerned, the theoretical model was developed considering the use of an indirect dryer with a recycle of dried product to agglomerate sludge, thus increasing specific exposure surface and avoiding clogging. Normally, it is necessary to feed the dryer with sludge at a solid concentration higher than 60 %. It is also useful to by-pass part of feed cake and mix it with the dried product to reach the pre-set cake concentration before incineration. The scheme of the dryer is shown in Figure2. The theoretical model of the incineration furnace implies the following sequential calculations: 1) minimum critical concentration (xu)cr allowing to operate the furnace in autogenous conditions; 2) maximum quantity of heat which can be recovered in the boiler downstream the furnace when the furnace is fed at the (xu)cr concentration, considering that gases exit the boiler at 250 °C; 3) feed concentration (xo)cr to the dryer producing a dried cake at (xu)CT recovering the heat calculated at point 2); 4) if x0<(xo)cr it is possible to determine the function xu=f(x0) by imposing the balance of the recoverable heat in the boiler with that needed in the dryer; 5) if xo>(x0)cr it is possible to determine the excess heat which is produced in the boiler not needed for drying which is available for electric energy production with an overall efficiency of 31.8 %; 6) the sequential procedure can be repeated with xu-0.9 (feeding the cake to the furnace at the highest practicable concentration).

524 Vapours and incondensable

Re cycle 0,9-0.6

By - pass stream k _ (0.9; Xu) 1

Figure 2. Scheme of an indirect dryer with internal recycle and by-pass

Figure 3 shows the function xu=fi(xo): when the cake concentration is lower than 21.1 % heat produced in the boiler is not sufficient to dry the sludge at the concentration needed for autogenous combustion. With cake concentration higher than 21.1 % excess heat is available if dried sludge is produced at 45.9 % or otherwise it is possible to obtain dried sludge at higher concentration.

0,9 -*

0.8 -

Dried sludge limit concentration 0.5 - for autogenous combustion=45.9 %

1 0.25 Cake concentration to the dryer (weight fraction)

Figure 3. Dried sludge concentration vs. cake concentration

Electric energy production from surplus steam for a plant serving one million of inhabitants was evaluated for two levels of sludge drying (45.9 and 90 %). The amount of electric energy obtained in the former case is 0.362 - 0.378 MW higher than in case of drying at 90 %. This can be explained considering that the enthalpy of the extra production of steam (about 1,830 kJ/kg dry solids) with a 90 % cake does not balance the heat required for sludge drying from 45.9 to 90 % (3,180-3,260 kJ/kg dry solids). Environmental aspects of sludge incineration Incineration to a great reduction of sludge and theoretically should result in the total conversion of the hazardous organic compounds to inorganic end products. Unfortunately, burning conditions are not ideal and generally they cause inefficiencies, mainly due to non-uniform temperature and mixing conditions inside the furnace. Cooling of hot gases in the combustion zone

525 due to the contact with the cool walls or locally reducing conditions can induce partial destruction and reactions. These inefficiencies to the formation of toxic organic compounds, referred to as products of incomplete combustion. Among them, special attention is devoted to PAHs for their relative abundance in the emissions and potential impact on human health (Akimoto et al., 1997, Ana et at, 1999, Eiceman et aL, 1979, Mascolo et al, 1999). Some authors showed that they are the starting compounds for formation, which is suspected to be the active site for solid - gas reactions leading to PCDD/F formation (lino et al., 1999 a and b, Schoonenboom and Olie, 1995, Stieglietz et al. 1991). On the contrary, others observed that the conditions favouring PAH production generally suppress PCDD/F formation (Benestad et al., 1990, Oehme et al. 1987). Metals regulated in incineration systems include Ag, As, Ba, Be, Cd, Co, Cr, Cu, Hg, Pb, Mn, Ni, Sb, Se, Sn, Tl, V and Zn (The European Parliament and the Council of the , 1994, van der Vlies and te Marvelde, 1992, Italian Ministry of the Environment, 1998, Linak, 1997). Among these metals, typically presents the highest concentrations in sewage sludge. Lead, , , , , , and may also be found in significant concentrations in sewage sludge, whereas is typically present only at low concentrations. Some research has specifically addressed the fate of metals in sewage sludge incineration (Dewling et al., 1980, Gerstle and Albrinck, 1982, Parrish et al., 1991, Balogh, 1996). These works have shown that several metals are enriched in with respect to their concentration in the , suggesting that volatile species are formed in the combustion furnace and carried away in the gas. Combustion temperature, excess air, feed composition (chlorine, sulfur, and water content) are the parameters that may affect metal speciation in the combustion chamber and consequently metal partitioning among the incineration residues (Fournier et al. 1990, Verhulst et al., 1996). Interactions with the solid substrate is also an important factor which regulates metal partitioning (Holbert and Eighty, 1998). Reactor configuration and feeding conditions are other important factors affecting homogeneity of the combustion chamber and oxygen availability (Verhulst et al., 1996, Marani et al. 1998, Mininni et al. 2000).

Results of pilot incineration tests Experiments were carried out on a pilot incineration plant, including a fluidised bed furnace (FBF) and a rotary kiln furnace (RKF), with the aim to investigate the influence of chlorinated hydrocarbons addition to sewage sludge on PCDD/F and PAH formation and on metal partitioning. A second objective of the study was to verify the feasibility of co-combustion of sludge and chlorinated hydrocarbons with the underlying idea that sludge incineration plants could be used also for co-disposal of hazardous , being the sludge basically the same as those used for hazardous . The demonstrative plant is located in the area of a treatment plant (Bari West) where sewage sludge was kept from the dewatering section performed by centrifuges. The flow sheet of the plant is reported in Figure 4. Main units are: an indirect dryer, a 590 mm I D. circulating fluidised bed furnace, a 1200 mm I D. rotating drum furnace, an afterburning chamber, a heat recovery system and a cleaning system (bag filter and wet ). Plant capacity is about 250 kg/h of sewage sludge at 20 % solids concentration or 160 kg/h of dried sludge at 75 % solids. Heat is recovered by a heat exchanger where diathermic is warmed up to 260 °C by the hot gases. The oil is used both for sludge drying and for air pre-heating up to 200 °C. The pilot plant is equipped with analytical process instrumentation for continuous monitoring of O2, and macropollutants (CO, CO2, NOx, SOx, HC1, total hydrocarbons and particulate). The pilot plant tests were performed using sewage sludge either alone or spiked, before entering the furnace, with tetrachloroethylene (TCE) or a “Surrogate Organic Mixture” (SOM), consisting of chlorobenzene (25 %), tetrachloroethylene (55 %) and toluene (20 %). In some tests copper nitrate was also added together with SOM to sewage sludge to check the hypothesis that copper catalyses dioxin formation. Methane was used in start-up phase of the plant and as auxiliary fiiel. Incineration tests

526 lasted a minimum of 30 hours after reaching steady state conditions. During each test parameters were recorded every each hour.

COVSUSTKW A3R FAN

j*-aut>ceuc *

£>rv*d

Figure 4. Flow sheet of the pilot plant

An afterburning chamber was used in different temperature conditions, up to 1.150°C; in some tests (3r, 6r, 7r, 13r, and 17 by FBF and 24, 30, and 34 by RKF) the burner was kept off to investigate the efficiency of this equipment. Grab samples of feed sludge, and ash were collected three times per day. The samples were then dried and finely ground (not needed for fly ashes) in a ball mill for homogenisation. About 30 tests were carried out with sludge cake at solid concentration around 15 %. Chlorinated compounds were added in all the runs (except for tests lr and 3r with FBF and tests 22 and 24 with RKF) to reach total chlorine concentration in the feed up to 5.2 %. Carbon monoxide, unbumed hydrocarbons, oxygen and carbon dioxide were monitored in the afterburning chamber, while nitrogen oxides, sulphur oxides, oxygen, and particulate were measured at the stack. Organic and inorganic micropollutants were sampled in three locations: before and after the bag filter and at the stack. Polycyclic Aromatic Hydrocarbons (PAH) and dioxins and furans concentrations are shown in Figure 5, where feed composition and temperature of the afterburning chamber for each test are also reported. European limit for PCDD/F emissions from waste incineration is at present 0.1 ng/Nm3 (ITEQ). No limit is fixed for PAH emissions in the European Directive 2000/76 on incineration of wastes, whereas a value of 10 pg/Nm3 is set in the Italian legislation. It clearly appears that dioxin and furan concentrations are much more variable than PAH concentrations, which poorly correlate with PCDD/F. PAH concentrations are always below 10 pg/m3 whereas PCDD/F concentrations are sometimes above the limit of 0.1 ng/m3 . Figure 5 does not show any dependence of concentrations on feed composition, afterburning temperature and furnace type.

527 Figure 5. PAH and PCDD/F emissions in FBF and RKF tests

Stack concentrations of heavy metals were always well below the concentration limits set by the European Directive (Cd+Tl=0.05 mg/Nm3 , Hg=0.05 mg/Nm3 , Sb + As + Pb + Cr + Co + Cu + Mn + Ni + V=0.5 mg/Nm3 ). In addition to monitoring metal emissions through the stack, partitioning of each metal in the main solid (bottom, cyclone and filter ash) was assessed to understand metal behavior in the combustion chamber. This study was focused on seven metals, which are typically present in significant concentrations in the feed sludge: Cd, Cr, Cu, Mn, Ni, Pb, and Zn. An enrichment factor was defined as the ratio of metal concentration in bag filter ash over the corresponding concentration in cyclone ash. This ratio may indicate the condensation of volatile metal species onto the filter ash upon cooling of the flue gas. For FBF tests the enrichment factor was corrected taking into account the dilution of metal concentration in cyclone ash due to carry over (Marani et al., 2002). Table 2 shows metal enrichment factors determined for FBF and RKF tests, which are reported with increasing chlorine concentration in the feed stream. As discussed by Marani et al. (2002) only enrichment factors above 1.9 are deemed as significant indication of metal volatilisation. A first group of metals, including Cr, Mn and Ni, behave as typical non volatile elements showing non significant enrichment in the fly ash even in the presence of high chlorine concentration. In contrast, a clear volatile behavior is presented by Cd and Pb in both furnaces. Cd and Pb enrichment in fly ash is enhanced by high Cl concentration in the feed stream and depends on the type of furnace (enrichment factors in RKF tests almost of one order of magnitude higher than those in FBF tests). Cu presents a somewhat ambiguous behavior, showing some significant enrichment factors at high Cl concentration in the feed stream. Finally, it is worth noting the peculiar behavior of Zn, which presents significant enrichment factors in all RKF tests and in a few FBF tests at high Cl concentration. Apparently, Zn volatilization in the RKF is not affected by Cl concentration in the feed stream. The comparison between FBF and RKF data in Table 4 suggests that rotary kiln technology presents potential of metals emissions into the if the fine fraction of fly ash is not efficiently entrapped with air control devices.

Conclusions The model presented in the paper showed that a concentration of 45.9 % of dry solids is the minimum required for autogenous combustion of sewage sludge in a fluidised bed furnace, which also minimizes the exhaust gas production. Such concentration can be reached by thermal drying using steam produced in the recovery boiler downstream the furnace. The evaporation capacity of a thermal dryer, with internal recycle and a by-pass of the dewatered sludge, should be determined to obtain a product at the above concentration. When cake concentration before drying is higher than

528 21.1 % excess heat could be available to produce electric energy. The maximum energy conversion can be reached limiting drying at 45.9 %. Table 2. Metal enrichment factors determined in the FBF and RKF pilot plant tests listed with increasing chlorine concentration

Cl cone. Metal Test Cl cone. Metal Test in the no. feed (%) Cd Cr Cu Mn Ni Pb Zn Cd Cr Cu Mn Ni Pb Zn FBF tests RKF tests Ir 0.03 1.7 1.2 1.3 0.9 1.5 2.2 1.7 22 0.03 2.7 1.2 1.6 0.8 1.2 5.9 10.5 3r 0.03 1.4 1.1 1.2 1 1.3 1.6 1.4 24 0.03 2.2 1.0 1.4 1.0 1.0 3.7 5.9 4r 1.71 2.8 1.1 1.6 1 1.3 3.5 1.6 31A 2.62 13.7 0.8 1.7 0.5 1.1 29 5 6r 1.71 1.1 1.4 1 1.2 1.4 28 4.07 17.7 1.2 2.7 0.9 1.4 50 6.9 10A 4.24 0.6 0.9 2 0.7 1.7 3 0.8 30 4.07 17.2 1.1 2.5 0.8 1.4 18.8 7.2 lOr 4.34 1 4.4 1 1.7 1.7 35A 4.99 13.7 1.0 2.7 1.1 1.4 33.4 5.4 9r 4.39 6.2 1.2 2.5 1.1 1.2 2.1 32 5.05 18.1 1.1 3.1 1.2 1.8 20.9 5.3 7r 4.41 3.7 1 1.8 1.1 1.3 4.5 1.6 34 5.05 26.9 0.9 2.6 0.8 0.9 26 5.9 18 4.45 0.9 1.1 1 1.9 2 35 5.05 20.6 0.9 2.9 1.1 1.2 22.2 5.8 18A 4.56 5.7 1.1 1.9 1.3 1.4 11.1 2.2 17 4.59 6.2 1.2 2.4 1.2 1.4 10.7 2.2 15 4.76 5.7 1 1.3 1 0.8 7.9 1.8 13r 4.82 3.9 1.1 1.8 1 1.2 6.3 1.8 14r 4.84 3.7 1.1 1.6 1 1.3 5.8 1.5 14A 5.05 1.2 1.8 1.1 1.7 Hr 5.17 1.1 2.3 1.1 2

The results of the tests performed on the demonstrative plant show that PCDD/F and PAH concentrations are not significantly correlated with any operating conditions (feed composition, afterburning chamber temperature, type of furnace). Moreover, the above micropollutant concentrations did not show any mutual correlation. PAH concentrations were well below the Italian limit of 10 pg/m3 , whereas in few tests PCDD/F concentrations (ITEQ) overcame the European limit of 0.1 ng/m3 . Heavy metals at the stack were always below the limits. Cr, Mn and Ni showed a non volatile behavior, partitioning equally among the different solid residues, whereas Cd and Pb showed the tendency to enrich onto the fly ash, mainly in RKF tests and at high chlorine concentration. Zn showed a clear volatile behavior in RKF tests independently on chlorine presence. The tests showed that RKF produced fly ashes more enriched in heavy metals (Cd, Pb, and Zn) than those produced by FBF. Co-incineration of sewage sludge and hazardous organic wastes may be considered a practicable option, considering that equipment used is basically the same and that no significant effect on the emissions was detected when sludge was spiked with chlorinated hydrocarbons up to a chlorine concentration in the feed of 5.2 %.

Acknowledgements The research project was funded by the Structural Funds, managed by the Italian Ministry of the Research and Science. The authors would like to thank the kind assistance of Rocco Antonacci in the experimental tests on the demonstrative plant and of Giuseppe Bagnuolo and Giuseppe Labellarte for micropollutants analysis.

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