Influence of Fuel Feeding Positions on Gasification in Dual Fluidized Bed

biomass and bioenergy 54 (2013) 46e58

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Influence of fuel feeding positions on gasification in dual fluidized bed gasifiers

V. Wilk a,*, J.C. Schmid b, H. Hofbauer b a Bioenergy2020þ GmbH, Wienerstraße 49, 7540 Gu¨ssing, Burgenland, Austria b Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, 1060 Vienna, Austria article info abstract

Article history: An in-bed and an on-bed feeding system are implemented in a dual fluidized bed gasifier in Received 19 June 2012 order to investigate the influence of the fuel feeding position on the gasification process. Received in revised form Two bed materials, fresh and used olivine, are used because of their varying catalytic ac- 11 March 2013 tivity. The comparison of in-bed and on-bed feeding of wood pellets shows that in-bed Accepted 12 March 2013 feeding is more favorable, because lower tar concentrations are achieved and the gas Available online composition is closer to wateregas shift equilibrium. Better mixing of bed material and fuel particles occurs with in-bed feeding. The residence time of the gas phase in the fluidized Keywords: bed is longer in the case of in-bed feeding, and therefore better performance of the gasifier Biomass is achieved. Sufficient residence time of the fuel in the bubbling bed is important when a Gasification less active bed material is used. More active bed material is capable of compensating for Gasesolid contact the shorter residence time of the gas phase in contact with bed material during on-bed Feeding position feeding. Reforming ª 2013 Elsevier Ltd. All rights reserved.

1. Introduction of the DFB gasifier has a high calorific value of 12e14 MJ m 3. Since the 1990s, steam blown DFB gasification technology has Gasification is a promising technology for future energy sup- been the subject of scientific studies at the Vienna University ply, as it converts carbonaceous solids into valuable producer of Technology [1,2]. The basic principle of this technology is gas. For gasification processes, fluidized bed reactors are displayed in Fig. 1. applied by preference. The good mixing conditions of fuel In the field of biomass gasification, extensive research has particles, bed material, and gas phase and an excellent heat been conducted at Vienna University of Technology with transfer promote the conversion of the feedstock. Air blown several generations of 100 kW pilot plant gasifiers. This has fluidized bed concepts were proposed for the gasification of led to commercially available DFB gasifiers. The gasification biomass. However, conventional air gasification yields a pro- process was demonstrated successfully in 2001 with the ducer gas which is highly diluted with nitrogen and as a first industrial sized plant in Gu¨ ssing (Austria). The high consequence has a low calorific value of 4e6MJm3.By quality producer gas yielded with high hydrogen content 3 3 contrast, a dual fluidized bed (DFB) steam gasifier system al- (>0.40 m m ) and high calorific value is used to produce lows the generation of a nitrogen free producer gas without electricity and heat for the local district heating grid. Several the use of pure oxygen as gasification agent. The producer gas industrial gasifiers based on this technology are under

* Corresponding author. Tel.: þ43 158801166387; fax: þ43 15880116699. E-mail addresses: [email protected] (V. Wilk), [email protected] (J.C. Schmid), hermann.hofbauer@ tuwien.ac.at (H. Hofbauer). 0961-9534/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2013.03.018 biomass and bioenergy 54 (2013) 46e58 47

more reliable, and more economical compared to in-bed systems. Biomass is well suited for on-bed feeding, since biomass is a very reactive fuel compared to bituminous coal or lignite. Thus, less unburned carbon is elutriated, which constitutes the majority of combustion losses. Examples of on-bed feeding systems are gravity chutes, where the biomass drops on the fluidized bed, or spreader-stoker systems, which throw the biomass on a large bed area [13]. The influences of in- or on-bed feeding on gasification have been investigated by several researchers. Corella et al. [14],in 1988, were among the first; they stated that biomass should be fed directly into the bed and not from the top of the fluidized Fig. 1 e The basic principle of the dual fluidized bed bed gasifier. They used two different lab-scale bubbling bed gasification technology. gasifiers for their investigation, one with on-bed and the other with in-bed feeding. They observed poorer gas quality and increased tar formation when feedstock was thrown on the fluidized bed because fuel particles accumulated on the sur- construction or have been in operation ever since, for example face of the bed. in Oberwart (Austria), Villach (Austria), Senden/Neu-Ulm Fuel accumulation on the surface of the fluidized bed is (Germany), and Gothenburg (Sweden). called segregation. Fiorentino et al. [15] measured the segre- In addition to electricity and heat production, the nitrogen gation time of a fuel particle in a bubbling fluidized bed. This is free producer gas from a DFB steam gasifier is well suited for the time that it takes a particle to reach the surface of the chemical syntheses of interesting products. Various research fluidized bed after being released at the bottom. They activities focus on the production of FischereTropsch diesel observed that volatile matter of the fuel is released immedi- [3], synthetic natural gas [4], and other chemicals [5] at the ately when the particle is injected into the bed. Volatile matter gasification plants in Gu¨ ssing and Oberwart. Biomass-derived creates an endogenous bubble which is able to lift the fuel producer gas can be a valuable feedstock for the chemical particle to the surface. This segregation mechanism is inde- industry, and thus might increase the share of renewable pendent from the atmosphere in the fluidized bed. The fuel materials in this sector. particle does not come into contact with the fluidization There is increasing interest in industry in substituting agent, because volatiles are released from the fuel particle. natural gas with renewable energy carriers. This demand can Bruni et al. [16] extended the work of Fiorentino et al. and be satisfied by gasification and subsequent methanation. Be- showed that the segregation time is shorter than the devola- sides wood chips, other types of biomass or residues and tilization time of the fuel. This means that the majority of waste are also main focuses. Therefore, extensive research on volatile matter is released on the bed surface even though the the DFB gasification technology is being conducted at Vienna particles are inserted at the bottom of the bubbling bed. University of Technology because it is a promising technology Wang et al. [17] studied fluidized bed pyrolysis of single fuel for such gas production from solid fuels. particles which were forced to stay either in the dense zone or Gasification process parameters like temperature, steam- in the splash zone of the fluidized bed. They fixed the position to-carbon ratio, and type of feedstock influence the compo- of the particles by the fluidization settings and also with sition of the producer gas [6]. It has been demonstrated that weight loads on the particles. Although they measured dif- various fuels such as sawdust, bark, waste wood chips, ferences in heating time, they did not find differences in gas strawewood blends, coalewood blends, municipal waste yield as a function of the particle position. They also stated fractions, reed, sugar cane bagasse, waste plastics, wheat that these results are not valid for excessive fuel accumulation bran, sewage sludge, and other alternative feedstocks can be on the bed surface, because then the gas yield will be affected. processed with the DFB technology [7e11]. All these fuels have Ross et al. [18] used a lab-scale bubbling fluidized bed different physical and chemical properties. gasifier to analyze the influence of two different feeding po- Therefore, the feeding system is a crucial part of the gasi- sitions in the fluidized bed. They measured the producer gas fication plant which requires careful consideration [12]. Most composition at various heights of the bed and in the free- commonly, a screw feeding system is implemented in an in- board. When particles are inserted closer to the bed surface, e dustrial DFB gasifier. This is an in-bed system with a plug higher concentrations of C1 C3 hydrocarbons are measured, screw, which is purged with nitrogen. The plug screw inserts which might be an indicator of lower conversion. the solid fuel directly into the fluidized bed of the gasification Vriesman et al. [19] investigated the influence of the reactor. In-bed feeding systems provide intensive mixing of feeding position on the formation of ammonia during gasifi- the fuel and the bed material and therefore increase the cation. But there was no significant difference in char and tar conversion efficiency. As the end of the screw enters the flu- yields when biomass was fed into or onto the bubbling fluid- idized bed, mechanical and thermal stress and abrasion are ized bed. very likely and can cause damage. Normally such in-bed One of the most recent works has been accomplished by feeding screws are water cooled. Rapagna` and di Celso [20]. They used a lab-scale bubbling However, on-bed feeding systems are usually applied in fluidized bed of olivine, a catalytically active bed material, and fluidized bed combustion plants. These systems are simpler, tested in- and on-bed feeding. Rapagna` and di Celso found 48 biomass and bioenergy 54 (2013) 46e58

that the gas yield increases when the fuel is fed into the fluidized bed. The effect is stronger for smaller fuel particles, as for them the segregation time is long enough for significant devolatilization to occur. Larger particles mostly react on the surface with less contact with the bed material. Sufficient contact with the bed material enhances the tar reforming reactions. Thus the tar and char yields increase when fuel is fed onto the fluidized bed. They stated that the effect of the bed material will be even stronger in industrial size gasifiers because of the enhanced exposure of the fuel particles to the bed material. The literature cited above only describes the influence of the fuel feeding position on bubbling fluidized beds, where segregation and fuel accumulation on the surface are possible. In circulating fluidized beds there is no distinct surface, as the particles are spread over the height of the reactor. Due to the higher lateral mixing capacity and deeper bed, considerably fewer feed points are necessary for circulating beds compared to bubbling beds. The number of feed points depends on the properties of the fuel, such as char reactivity and volatile concentration, and is found empirically [13]. No literature was found on the influence of the feeding position on dual fluidized bed reactor systems. As these systems are combinations of bubbling and circulating fluidized beds, the results from bubbling bed experiments with single reactors cannot be transferred to DFB systems without critical review. In the DFB gasifier there is constant circulation of bed material between the bubbling bed of the gasification reactor and the fast fluidized bed of the combustion reactor. Thus, regenerated hot bed material is fed into the gasification reactor, whereas devolatilized wood char and somewhat colder bed material are transported to the combustion reactor. This circulation cannot be considered in single bed gasifica- Fig. 2 e 100 kW DFB pilot plant at Vienna University of tion systems. Another point is that most of the mentioned Technology. experiments were carried out in very small reactors that were sometimes also operated in batch mode. The position of the fuel feeding system is closely related to segregation and such as loop seals, process media inputs, a solids separator, mixing phenomena, which are likely to be influenced by the and various feedstock hopper arrangements are visible in this size of the reactor. sketch. Table 1 lists the most important dimensions of the This paper presents extensive experimental investigations pilot plant; more details can be found in Fig. 6. on the fuel feeding position in a 100 kW steam blown DFB In the DFB gasifier, two separate gas streams are generated: gasifier. The study focuses on the influence of different loca- producer gas and flue gas. Feedstock is inserted into the tions of the fuel input: in-bed or on-bed feeding. Two bed gasification reactor, where a bubbling fluidized bed is formed. materials, fresh and used olivine, are employed, as they show There, the feedstock is devolatilized and gasified with steam. varying catalytic activity. Wood pellets and a mixture of wood Some ungasified char from the feedstock is transported to the pellets and shredder light fraction plastics (SLF-plastics) are combustion reactor together with the circulating bed material. gasified in the fluidized bed reactor system at the Vienna University of Technology. The gasification system and its fluid dynamic properties are explained in detail. Table 1 e Dimensions of the gasification system.

Gasiﬁcation reactor 2. Pilot plant Overall reactor height m 2.35 Average height of freeboard m 1.7 2.1. General description Average height of bubbling bed m 0.65 (lower end of cone to ﬂuidized bed surface) Square reactor cross section mm 270 270 Fig. 2 presents the 100 kW DFB pilot plant at Vienna University (upper bubbling bed and freeboard) of Technology, where the experiments are carried out. The Combustion reactor dashed line indicates the global solids circulation rate of the Overall reactor height m 3.9 bed material in the reactor system. Olivine with a mean par- Diameter mm 98 ticle diameter of 0.5 mm is used as bed material. Elements biomass and bioenergy 54 (2013) 46e58 49

Char is combusted with air in a fast fluidized bed of the combustion reactor. Due to combustion, the bed material is heated up. It is separated from the flue gas and returns to the gasification reactor via the upper loop seal. The bed material supplies the energy for the endothermic gasification reactions. The special arrangement of hoppers and screw feeders is illustrated in Fig. 2. It enables the comparison of gasification runs in which the feedstock is inserted at different locations. Feedstock hopper 1, hopper 2, and screw feeder A are used to obtain the experimental results for in-bed feeding. Therefore, the fuel is fed directly into the bubbling fluidized bed of the gasification reactor. By contrast, hopper 3 and screw feeder B, as well as hopper 4 and screw feeder C, throw the fuel onto the fluidized bed. This corresponds to on-bed feeding. Gasification of varying fuel mixtures during one experiment is also possible with this system. The energy input of specific fuels from the hoppers is calibrated by varying the screw rotation speed before each test run. In order to fulfill safety requirements, all hoppers of the 100 kW gasification plant are locked so they are gas-tight and are flushed constantly with a small stream of nitrogen.

2.2. Measurement equipment

The pilot plant is equipped with extensive measurement equipment and automatic data recording. Temperatures up to 1000 C inside the pilot plant are measured with high temperature thermocouples. Process media inputs, such as the ﬂuidization agents, steam and air, are measured with high quality ﬂow meters (Krohne). Pressures are measured along the height of the reactors with pressure sensors relative to atmosphere. Temperature and pressure measurements are the basis of an effective process control which guarantees the smooth operation of the pilot plant. Feedstock analysis is carried out according to international standards by the Test Laboratory for Combustion Systems at the Vienna University of Technology. The analysis usually comprises elemental composition and volatiles, water, and ash contents of the feedstock, as well as the ash melting Fig. 3 e Dust, char, and tar sampling scheme. behavior.

The main producer gas components, H2, CO, CO2, and CH4, are analyzed online with a Rosemount NGA2000 device. C2H4, shows the arrangement of the sampling equipment for dust, C2H6,C3H8, and N2 concentrations are measured with a Syn- char, water, and tars. tech Spectras GC 955 gas chromatograph. An impinger bottle method for tar measurement has been developed at the Vienna University of Technology. It is similar 2.3. Mass and energy balances with IPSEpro to the conventional tar protocol CEN BT/TF 15439, but has been adapted for producer gas from steam gasification. The process simulation tool IPSEpro is used for evaluation and Toluene is used as tar absorbent. Dust, entrained char, water, validation of the process data which are measured in the ex- and tar content can be analyzed from one sample. A minimum periments. IPSEpro is a software package which is frequently of six samples were taken during each series of the experi- used in the power plant sector and which offers stationary ments (two different operation points per series). The results process simulation based on flow sheets. The software uses are presented as average values of each operation point. There an equation-oriented solver. A comprehensive model library are two different methods of tar analysis for gravimetric and for gasification plants has been developed at Vienna Univer- GCMS tars. Gravimetric tars are weighed after vacuum evap- sity of Technology and is described in detail in Ref. [21]. oration of the solvent; they comprise mostly high molecular Mass and energy balances of the experimental runs are weight tars. A GCMS device is used to measure the content of also computed with IPSEpro. Therefore, measured data from many different tar species of medium molecular weight. The stationary operation of the pilot plant are the basis for the measurement range of the two techniques overlaps. Fig. 3 calculation. An over-determined equation system is formed 50 biomass and bioenergy 54 (2013) 46e58

which is solved by the Method of Least Squares. More infor- volatiles must pass through the char layer to reach the sur- mation about this procedure can be found in Ref. [22]. The face. Secondary reactions of volatiles and char may occur. The reconciled solution describes the actual operation of the pilot char layer can slow down further thermal decomposition plant best within the limits of the model. considerably [26]. SLF-plastic is a mixture of different plastic residues produced in a mechanical sorting plant with a mean particle size 3. Experimental work of 5 mm. It consists of plastics from end-of-life vehicles, films from commercial waste, and waste electrical equipment. 3.1. Bed materials Plastic materials behave differently from wood when heated. The polymer chain breaks into smaller fragments that In industrial scale DFB gasifiers, olivine is used as bed mate- vaporize rapidly. Depending on the type of polymer it either rial. It has proven to have enough resistance to attrition and breaks down completely without solid residues or some char shows moderate tar cracking activity [23]. Sufficient contact of or inorganic residues are formed [26]. gas and bed material is necessary for tar reforming reactions. In Table 2 the proximate and ultimate analysis of both The gas phase inside the fluidized bed mainly consists of materials is compiled. Although the volatile matter contents steam and volatiles from the fuel and their reaction products. of the two materials are not very different, SLF-plastics are When the feeding position is changed from in-bed to on-bed likely to decompose faster than wood. feeding, the residence time of the gas phase inside the bed is likely to be shorter. Two different bed materials with different 3.3. Overview of experiments catalytic activities are used in the experiments. Thus, the impact of different catalytic activity can be observed. Fresh Table 3 shows the experimental matrix with all series of ex- olivine and used olivine are employed as bed material. periments carried out in order to assess the influence of the Fresh olivine is provided by Magnolithe GmbH (Austria) feeding position. In series 1, wood pellets are fed into the and has never been used in the gasification process before. It bubbling bed by screw A and onto the bubbling bed by screw B is the standard bed material for the pilot plant. As different using fresh olivine as bed material. In series 2, used olivine is feedstock is tested in the pilot plant, the bed material is employed as bed material and wood pellets are also fed by disposed of after each experiment and fresh material is used. screws A and B. In series 3, SLF-plastics is gasified in a mixture Thus, it is ensured that all feedstocks are tested inside a bed with wood pellets. The mixture consists of 50% wood pellets material inventory with the same conditions and reproducible and 50% SLF-plastics in terms of fuel energy content. During experimental data are yielded. this series, only the feeding position for SLF-plastics is varied, Used olivine was taken from the biomass DFB gasifier in whereas wood pellets are constantly fed into the bed by screw Gu¨ ssing. Recent research by Kirnbauer et al. [24,25] has shown A. For in-bed feeding, SLF-plastics are also fed into the bed by that the properties of olivine change over time when it is used screw A, while for on-bed feeding SLF-plastics are thrown in biomass gasifiers. Due to reactions with additives and onto the bed by screw C. compounds from biomass ash, an active layer is built around In order to achieve meaningful comparisons, the fuel olivine particles. It enhances the reforming capacity, because input, temperatures, and steam-to-carbon ratio are kept in the the active layer is rich in calcium. Thus, used olivine is cata- same small ranges for the experimental test series. Only the lytically more active than fresh olivine. More details on used fuel feeding position is varied. The fuel input amounts to olivine from the Gu¨ ssing gasifier are available in Refs. [24,25]. 97e98 kW. This corresponds to 20 kg h 1 of wood pellets in Some data for the bed materials used in this investigation can series 1 and 2. In series 3, 10 kg h 1 of wood pellets and be found in Table 3. 5.6 kg h 1 of SLF-plastics are gasified. In all experiments, gasification is carried out at a fixed temperature of around 3.2. Feedstock 850 C, which is measured at a defined measurement point inside the bubbling bed of the gasification reactor. The It is reported in Refs. [15,16] that volatile matter forms endogenous bubbles that lift the fuel particles to the surface of the fluidized bed and that the segregation time is shorter than Table 2 e Fuel analysis. the devolatilization time of the fuel. Thus, not only the feeding position but also the devolatilization behavior of the fuel may Wood pellets SLF-plastics influence the gasification process. In the experiments two LCV MJ kg 1, wet 17.5 31.9 different fuels are used that have different thermal stabilities: Water kg kg 1 0.0611 0.0087 soft wood pellets and SLF-plastics. Ash kg kg 1, dry 0.0029 0.1067 1 Soft wood pellets are used in the pilot plant because they Volatiles kg kg , waf 0.867 0.8924 Ckgkg1, waf 0.5038 0.7276 are a standard fuel with good conveying properties for the Hkgkg1, waf 0.0606 0.0890 screw feeder. Experiments have shown that gasification of Okgkg1, waf 0.4351 0.1508 soft wood pellets in the pilot plant is in good agreement with Nkgkg1, waf 0.0005 0.0104 gasification of wood chips in industrial gasifiers. An example Skgkg1, waf 0.00005 0.0035 is given in Ref. [25]. Wood is a charring material, and thus Cl kg kg 1, waf 0.00003 0.0187 volatiles and char are formed during thermal decomposition. waf ¼ water and ash free. When deeper layers of the material are decomposed, the biomass and bioenergy 54 (2013) 46e58 51

Table 3 e Overview of experimental series. 4. Results Wood pellets SLF-plastics 4.1. Producer gas and equilibrium Fresh olivine A, B (series 1) A, C þ A (series 3) Used olivine A, B (series 2) Fig. 4 presents the producer gas composition measured during series 1, 2, and 3. It shows similar trends for series 1 and 2, gasification temperature amounts to 850 1 C for series 1 and where wood pellets are used as feedstock. H2 and CO2 increase 2 and 855 3 C for series 3. The fluidization settings for all and CO, CH4, and CxHy decrease when on-bed feeding is steam inputs are also kept constant within a series. However, changed to in-bed feeding. The most significant changes in devolatilization of the feedstock influences the fluidization of producer gas composition are found in series 1. Minor changes the bubbling bed in the gasification reactor. Therefore, the occur in series 2, because used olivine, which is the more mixing conditions inside the bubbling bed will change. In-bed reactive bed material, reduces the effect of the feeding posi- feeding should enhance the turbulence and the intermixing of tion. Plastic residues, which are gasified in series 3, show fuel particles in the bed in comparison to on-bed feeding. The almost no differences in producer gas composition when the steam-to-carbon ratio relates the mass flows of fluidization feeding position is changed. It seems that the constant stream steam and the fuel water to the mass flows of carbon of the of wood pellets into the bed in series 3 increases the turbu- fuel according to Eq. (1). The steam-to-carbon ratio is lence in the bed and reduces the impact of the different feed 1.65 kg kg 1 in series 1, 1.85 kg kg 1 in series 2, and 1.80 kg kg 1 points for SLF-plastics. in series 3. During each series, the steam-to-carbon ratio does In Fig. 4, the results of the two feed points are also not change. To indicate typical conditions of fluid dynamics, compared to the wateregas shift equilibrium gas composition. the ranges of operation parameters during the experiments It is the gas composition that would have been reached if the are displayed in Table 4. wateregas shift reaction had been in equilibrium. The wateregas shift reaction is given in Eq. (2). Several gasegas S m_ ; þ m_ ; ¼ H2O fluidization H2 O feedstock (1) reactions occur in the freeboard of the gasification reactor, C m_ ; C feedstock which also counteract the wateregas shift equilibrium. However, the wateregas shift reaction is the dominating reaction in the gas phase. The calculation of the equilibrium gas composition is based on the temperature of the gasification e Table 4 Operating conditions of the experiments in the reactor and the measured producer gas properties, especially 100 kW gasification pilot plant at Vienna University of the partial pressures of the gas compounds. The software tool Technology. IPSEpro is used for the calculation. General

Bed material particles Olivine CO þ H2O/CO2 þ H2 (2) (fresh and used) Bed material particle density kg m 3 2900 If the wateregas shift equilibrium gas compositions of series 1 m Mean particle size, dp50 m 510 and 2 are compared, slight differences are visible, although e m e Size distribution, dp10 dp90 m 400 660 the same fuel is gasified. This is due to different fluidization e Bed material inventory, total kg 99 101 settings: the steam-to-carbon ratio is higher in series 2. Heat losses of reactor parts kW 19e21 d WGS describes the deviation of the measured gas compo- Gasification reactor sition from wateregas shift equilibrium. It is calculated by Fluidization agent Steam d Eq. (3).If WGS is negative, the concentration of CO2 and H2 is (producer gas) lower than it would be in equilibrium state. If the reaction Mean Archimedes number, Ar e 450 1 continues, the concentration of CO2 and H2 will increase. Minimum fluidization velocity Umf ms 0.14 d 1 Positive values of WGS indicate that more reagents, CO and Terminal velocity Ut ms 5.5 1 d Steam-to-carbon mass ratio kg kg 1.68e1.85 H2O, will be present. If WGS becomes zero, equilibrium is Feedstock, fuel to gasification reactor kW 97e98 reached and the concentration of all gas species involved in e e d Temperature in gasification C 849 858 the water gas shift reaction will remain constant. WGS is reactor (defined height) compiled for all series in Table 5. 1 e Superficial gas velocity, U ms 0.52 0.62 ! e e Fluidization ratio, U/Umf 3.2 4.4 K p ; p ; p ; p ; d ¼ real ¼ CO2 real H2 real CO eq H2 O eq e e WGS log10 log10 (3) Fluidization ratio, U/Ut 0.09 0.11 ; ; ; ; Keq pCO realpH2O real pCO2 eqpH2 eq Combustion reactor The calculation of d confirms what is illustrated in Fig. 4. Fluidization agent Air (flue gas) WGS e Mean Archimedes number, Ar e 540 The producer gas composition is closer to the water gas shift 1 Minimum fluidization velocity, Umf ms 0.12 equilibrium in the case of in-bed feeding. This is observed in 1 Terminal velocity, Ut ms 4.7 series 1 and 2. When used olivine is employed as the bed Auxiliary fuel to combustion reactor kW 29e41 d material, WGS is less negative and gets only slightly closer to Temperature in combustion reactor C 878e931 e the water gas shift equilibrium when the feeding position is Superficial gas velocity, U ms 1 9.7e10.2 changed, due to its catalytic activity. During gasification of Fluidization ratio, U/U e 80e85 mf d e e plastic residues, values for WGS are highly negative for both Fluidization ratio, U/Ut 2.0 2.2 feeding positions. It seems that plastic residues react very 52 biomass and bioenergy 54 (2013) 46e58

Fig. 4 e Producer gas composition and wateregas shift equilibrium (WGS eq.) gas composition.

2. Series 3 differs considerably, as GCMS and gravimetric tars remain constant. The char content increases and dust Table 5 e Deviation from wateregas shift equilibrium gas composition. decreases. The dust content in the producer gas remains relatively Series 1 Series 2 Series 3 constant for both feed points in all series. Inorganic dust On-bed In-bed On-bed In-bed On-bed In-bed consists of ash from the feedstock and entrained bed material. d As the same feedstock is used in each series, dust in the form WGS 0.52 0.33 0.28 0.22 0.41 0.42 of ash is very likely to remain constant. The fluidization settings for the steam inputs are constant and the amounts of fast, because the polymer chains break into small molecules producer gas obtained are very similar during each series which vaporize rapidly and leave the fluidized bed. Thus, (more information is given in Table 6). Thus, comparable fewer fuel particles are gasified inside the fluidized bed while amounts of bed material should be entrained. in contact with bed material. The producer gas, which is Elutriated char decreases in series 1 and 2 but increases in streaming through the bed, mainly consists of volatile matter series 3. A decrease in entrained char is an indicator that less from plastics and from wood. For both feed points the contact char is present in the freeboard and that it is well mixed into time of gas and bed material is too short to reach the water- the bed. This is coherent for series 1 and 2. There is no clear egas shift equilibrium. explanation as to why the char content increases in series 3. But the very fast devolatilization of plastic fuel particles in the 4.2. Tars, dust, and char case of in-bed feeding influences the bubbling fluidized bed. SLF-plastics decompose into big and rapidly rising bubbles, Fig. 5 illustrates the tars, dust, and char in the producer gas. In which may lift up fine char particles inside the bed. Thus it is series 1, lower GCMS tars are found when wood pellets are fed possible that fine char particles from wood pellets are elutri- into the bed compared to on-bed feeding. The content of ated into the freeboard. The more active splash zone of the gravimetric tar and entrained char decreases slightly and the bubbling bed enhances the release of fine char particles into content of inorganic dust remains constant. In series 2, only the freeboard in the case of series 3 and in-bed feeding of the minor changes in GCMS tar, gravimetric tars, and dust are SLF-plastics. With in-bed feeding of wood pellets in series 1 observed. Due to the catalytic activity of used olivine only and 2 this phenomenon is not observed. This is probably due minor changes in tar concentration occur. A decrease in to the significantly slower devolatilization procedure of wood entrained char is measured with in-bed feeding in series 1 and pellets compared to plastic materials.

Fig. 5 e Tars, dust, and char in producer gas. biomass and bioenergy 54 (2013) 46e58 53

Tar measurement correlates with the considerations on the wateregas shift equilibrium. When the feed point is changed from on-bed to in-bed, tar concentration decreases. e d The deviation from the water gas shift equilibrium, WGS, also declines. In series 1 a signiﬁcant decrease in tar concentration d d and WGS is found; in series 2 tar concentrations and WGS d change only slightly. Similarly to the almost constant WGS, the tar concentration stays constant during series 3.

5. Discussion

5.1. Distribution of fuel particles in the gasiﬁcation system

Several studies have shown that fuel particles tend to float onto the surface of bubbling beds. During devolatilization endogenous bubbles are formed that lift the particles to the surface of the fluidized bed. In single bubbling bed gasification reactors, it is observed that fuel particles are likely to accumulate on the surface and are not mixed with the bed again. This phenomenon is enhanced by on-bed feeding [14,15,20]. Segregation is also influenced by the fluidization velocity. For example, Fiorentino et al. [15] and Bruni et al. [16] operated the fluidized bed at low fluidization ratios, where segregration is more likely. At higher fluidization velocities, fuel particles which float on the surface are mixed into the bed again, because there is a more active splash zone. As given in Table 4, the fluidization ratio was 3e4 during the experiments in the DFB gasifier. In addition to that, intermixture is enhanced in the DFB gasifier due to constant circulation of bed material between the gasification and combustion reactors. Bed material is transported out of the bubbling fluidized bed of the gasification reactor via the lower loop seal into the combustion reactor. Hot bed material returns to the gasification reactor via the upper loop seal. The height scale of the DFB pilot plant is illustrated in Fig. 6. As bed material is spread constantly on the surface of the bubbling bed, intermixture of fuel particles, char, and bed material is stronger in the DFB system. Fig. 7 provides temperature profiles of the gasification reactor for all experimental series. Hot and cold spots are Fig. 6 e Height scale of the DFB gasification reactor of the visible in all diagrams. Temperature decreases at the height pilot plant. where the fluidization steam is injected. In the upper freeboard region the temperature increases where the hot bed material returns from the combustion reactor and is thrown 5.2. Energy balance onto the fluidized bed. The reference gasification temperature is measured at the position of the in-bed feed point at a height The gasification reactor is an allothermal reactor. There, heat of 0.9 m from the base of the reactor. Fig. 7 also illustrates the is consumed due to endothermic gasification reactions. Heat differences in temperature between on-bed and in-bed is provided by the circulating bed material, which is heated up feeding. Series 1 shows that fuel particles are well mixed in the combustion reactor. Fig. 8 shows the in- and out-going into the fluidized bed during in-bed feeding and that gasifi- energy streams of the combustion reactor for an energy bal- cation also occurs deep in the bed. Heat is consumed there, ance. Bed material enters the combustion reactor together which results in lower temperatures. During on-bed feeding, a with ungasified char. Char and auxiliary fuel are combusted small increase in temperature of 5e10 C is measured in the with air. If no auxiliary fuel is added, the temperature in the gasification reactor. The increase occurs in the bed as well as gasification reactor is leveled according to the energy demand in the freeboard. In series 2 and 3, the temperatures remain of the gasification reactions, the impact of heat losses, and the quite constant except for minor deviations in the upper free- amount of char transported into the combustion reactor. In board region. order to control the gasification temperature and to keep it at 54 biomass and bioenergy 54 (2013) 46e58

Fig. 7 e Temperature proﬁles of the gasiﬁcation reactor.

850 C, auxiliary fuel is necessary for wood as well as for measured during the experiments. All other parameters listed plastics. In the pilot plant light fuel oil is used as auxiliary fuel. in Table 6 are calculated in IPSEpro. Flue gas and hot bed material with a temperature of around It is found that the gasification energy is not affected when 900 C leave the combustion reactor. on-bed feeding is changed to in-bed feeding. Less char is The thermal energy of the combustion air and the flue gas transported to the combustion reactor during on-bed feeding, are quite constant during a single series, as the fluidization and therefore more auxiliary fuel is injected. This is an indi- settings are constant and the flue gas temperature does not cation that there is less char in the lower parts of the bubbling change much. At nearly constant operating temperatures the bed in the gasification reactor and that the mixing quality thermal losses are constant, because they are mainly deter- decreases to some extent with on-bed feeding. mined by the quality of the insulation of the pilot plant. Thus, the consideration can be simplified: Energy consumed by the gasification reactions corresponds to the temperature differ- e ence of the bed material at the inlet and the exit of the com- Table 6 Key parameters of the gasification process. bustion reactor. Energy for the gasification reactions is Series 1 Series 2 Series 3 provided by combustion of residual char and auxiliary fuel. On- In- On- In- On- In- Table 6 lists several key parameters of the gasification bed bed bed bed bed bed process, such as the auxiliary fuel demand, the char content in the combustion reactor, and the gasification energy. Table 7 Auxiliary fuel kW 40.9 36.9 32.0 29.6 39.8 37.8 Char for kW 17.8 20.7 22.9 25.0 9.6 10.7 shows their changes from on-bed to in-bed feeding. During combustion the experiments the quantity of auxiliary fuel which is injec- Energy for kW 34.9 34.7 33.2 33.1 26.2 25.7 ted into the combustion reactor is measured. Based on gasification experimental data and the simulation equation system, the Carbon in % 77.2 75.6 72.1 70.9 81.1 79.2 mass and energy balances of the gasification process are producer gas 3 1 calculated in IPSEpro. The balances provide deeper insight Producer gas m h 160.3 160.2 163.1 162.2 137.1 134.9 (operation) into the process and determination of streams that cannot be Producer gas m3 h 1 22.7 23.5 22.0 21.9 19.1 18.5 (drya)

a At 273.15 K and 101.325 Pa.

Table 7 e Changes in key parameters of the gasiﬁcation process from in-bed to on-bed feeding. Changes Series 1 Series 2 Series 3

Auxiliary fuel % þ10.8 þ8.4 þ5.2 Char for combustion % 13.8 8.4 10.4 Energy for gasiﬁcation % þ0.7 þ0.2 þ1.9 Carbon in producer gas % þ2.1 þ1.8 þ2.4 Producer gas (operation) % 0.0 þ0.6 þ1.6 Producer gas (drya)%3.1 þ0.5 þ3.6 Fig. 8 e In- and out-going energy streams of the a At 273.15 K and 101.325 Pa. combustion reactor. biomass and bioenergy 54 (2013) 46e58 55

in-bed feeding, series 3 producer gas is very important. The contact time is the resi- 5.0 dence time of the producer gas in the bubbling fluidized bed and is mainly determined by the feeding position as well as by the formation and volume flow of the producer gas. 4.0 Pressure measurements along the reactor are an indicator gasification of the bed material distribution and the fluidization condi- reactor 3.0 tions in the gasification system. Fig. 9 shows the pressure combustion profile of the DFB pilot plant during in-bed feeding of series 3. reactor The pressure is highest in the bubbling bed of the gasification 2.0 upper/lower reactor because the majority of the bed material inventory is loop seal found there. The bed material is moving from the gasification reactor height , m 1.0 reactor toward the areas of lower pressure in the lower loop seal and the combustion reactor. In the fast fluidized bed of the combustion reactor, the bed material is distributed 0.0 sparsely over the height, and the pressure is lower there; 0 2000 4000 6000 8000 however, a denser zone is observed in the bottom area. The pressure relative to atmosphere, Pa pressure at the exit of the gasification reactor is slightly higher than atmospheric pressure, because the producer gas has to e Fig. 9 Pressure profile of the DFB pilot plant. overcome the pressure drop of the producer gas heat exchanger, which is located downstream of the gasification reactor. Carbon in the producer gas is the complement to char for In Fig. 10, changes in bed pressure are displayed when on- combustion. Carbon in the feedstock is either converted to bed feeding is changed to in-bed feeding. For these consider- carbonaceous producer gas compounds, char, and tar in the ations a mean value of two pressure measurements within producer gas or transported to the combustion reactor in the the bubbling fluidized bed is calculated. In series 1 and 2, the form of char. Carbon in the producer gas (Cgas) comprises all mean bed pressure increases with in-bed feeding. When the carbonaceous producer gas compounds according to Eq. (4) fuel is fed directly into the bubbling fluidized bed of the gasi- and it increases with on-bed feeding, because less char is fication reactor, intermixing of fuel particles into the fluidized combusted. bed is favored. Volatiles are released from the fuel particles in the bed and also act as the fluidization agent. Therefore, the C þ C þ C þ / C ¼ CO CO2 CH4 $100% particle hold-up increases and the pressures increase as well. gas C feedstock By contrast, pressures are lower during on-bed feeding. Then, 1 C ; þ C ; þ C ; ¼ tar PG char PG char combustion $100% (4) the overwhelming majority of volatiles are produced in the Cfeedstock upper region or above the bubbling fluidized bed. The location Although volatile matter is released at different heights in the of devolatilization and producer gas generation influences the fluidized bed and mixing of fuel particles is influenced by the conditions inside the bubbling bed. feed point, the producer gas volume flow stays approximately In series 3 (wood pellets plus SLF-plastics), no change is constant when the feed point is changed. detected. It is very likely that the continuous stream of wood pellets keeps the bed pressure drops constant. Wood pellets 5.3. Residence time of the gas phase in contact with the are fed directly into the fluidized bed during in- and on-bed bed material feeding of SLF-plastics. As SLF-plastics devolatilize rapidly, fewer fuel particles remain in the bed to be gasified there. The In order to benefit from the catalytic activity of the bed ma- gas phase reactions mostly take place in the freeboard section terial, the contact time of bed material, fuel particles, and with limited contact with bed material. Thus, they are not

Fig. 10 e Pressure measurement in the gasiﬁcation reactor. 56 biomass and bioenergy 54 (2013) 46e58

Fig. 11 e Illustration of producer gas generation during in- and on-bed feeding.

affected by the change in the feeding position of the SLF- position of the fuel feeding has a greater effect on the gasifi- plastics. The pressure diagram of series 3 is in line with tar cation process if a less active bed material is used. measurements and producer gas properties, where no sig- A recent publication of Saw and Pang [27] supports the nificant changes could be observed. presented experimental results. They varied the amount of Fig. 11 illustrates the gas production in the gasification bed material in a DFB gasifier. Thus, the position of the fuel reactor. The gas volume flow per height and its gradient are input was varied relative to the surface of the bubbling bed, plotted schematically for in- and on-bed feeding and for the and the residence time of the gas phase in the bed was also continuous stirred-tank reactor model (CSTR). In Fig. 11 varied. They showed similar influences on the gasesolid isothermal conditions are assumed, as all ingoing streams interaction: with an increase in residence time for the gas are heated immediately to the bed temperature. Thus, only phase in the fluidized bed, the tar concentration decreases. changes in volume due to gas production are considered. In a continuous stirred-tank reactor the gas volume flow increases linearly along the height of the fluidized bed as the fuel par- 6. Conclusion ticles are assumed to be evenly distributed over the whole bed volume. During in-bed feeding, volatiles are released mainly Extensive experimental investigations are carried out in order in the middle part of the fluidized bed. In-bed feeding favors a to assess the influence of the fuel feeding position in a dual longer contact time of producer gas and bed material, as the fluidized gasifier. The comparison of in-bed and on-bed volatiles are in contact with the bed material when they flow feeding shows that in-bed feeding is more favorable, through the bubbling bed. The gas phase reactions are cata- because lower tar concentrations are achieved and the gas lyzed by the bed material. Also, char, which remains after composition is closer to the wateregas shift equilibrium. devolatilization, is mixed within the bed. Additionally, gases Several parameters indicate better mixing of bed material and are generated from reactions with steam. They are also in fuel particles during in-bed feeding, such as the amount of contact with the bed material when they flow through the char which is transported to the combustion reactor and the bubbling bed. During on-bed feeding gas production mainly bed pressure drop. However, extensive fuel accumulation is occurs in the upper regions of the fluidized bed and the splash not observed on the surface on the bubbling bed, because zone. Thus, the contact with the bed material is significantly intermixture is enhanced by the circulating bed material, shorter and consequently the catalytic effects of the bed ma- which is constantly fed onto the surface of the bubbling flu- terial are lower. It is also possible that a defined amount of idized bed in the gasification reactor. Two different feedstock volatiles are produced in the freeboard if on-bed feeding is are tested because of their different devolatilization behav- used for very fast devolatilizing fuels. iors, wood pellets and SLF-plastics. In the experiments, a The benefit of catalytic activity can be described by the constant stream of wood pellets is fed into the fluidized bed extent of tar formation and by deviation from wateregas shift and only the feed point for SLF-plastics is varied. Changes in equilibrium. When fresh olivine is used as bed material in the feed point of SLF-plastics do not influence the gasification d series 1, the deviations in tar concentrations and WGS are process and the tar concentration also remains constant. As considerably higher than in series 2. The less active the bed devolatilization of plastics occurs rapidly, the gas phase re- material is, the more important the contact time is for suffi- actions take place mainly in the freeboard region with limited cient conversion. With on-bed feeding, the fuel is not mixed to contact with the bed material. Two different bed materials are the same extent within the bubbling bed. Therefore, the used in the experiments because of their different catalytic biomass and bioenergy 54 (2013) 46e58 57

activities. When a less active bed material such as fresh [6] Hofbauer H, Rauch R. Stoichiometric water consumption of olivine is used, sufficient residence time of the fuel in the steam gasification by the FICFB-gasification process. In: bubbling bed is important. With in-bed feeding, considerably Bridgwater AV, editor. Progress of thermochemical biomass conversion; September 17e22 2000; Innsbruck, better performance of the gasifier is achieved. More active bed Austria. Oxford, UK. Blackwell Science Ltd; 2000. p. material such as used olivine is capable of compensating for 199e208. the shorter residence time of the gas phase in contact with the [7] Pfeifer C, Koppatz S, Hofbauer H. Steam gasification of bed material during on-bed feeding. This underlines the various feedstocks at a dual fluidised bed gasifier: impacts of importance of catalytically active bed materials for the gasi- operation conditions and bed materials. Biomass Convers e fication process. It also shows the relevance of fluidization Biorefinery 2011;1:39 53.. http://dx.doi.org/10.1007/s13399- conditions in the fluidized bed, because they have a major 011-0007-1. [8] Wilk V, Kitzler H, Koppatz S, Pfeifer C, Hofbauer H. impact on the gasesolid contact. Gasification of waste wood and bark in a dual fluidized bed steam gasifier. Biomass Convers Biorefinery 2011;1:91e7.. http://dx.doi.org/10.1007/s13399-011-0009-z. [9] Schmid JC, Wolfesberger U, Koppatz S, Pfeifer C, Hofbauer H. Acknowledgment Variation of feedstock in a dual fluidized bed steam gasifier: influence on product gas, tar content, and composition. The authors would like to thank the team of Gasification and Environ Prog Sustain Energy 2012;31(2):205e15. Gas Cleaning and the Test Laboratory for Combustion Systems [10] Kitzler H, Pfeifer C, Hofbauer H. Gasification of reed in a at the Institute of Chemical Engineering at Vienna University 100 kW dual fluidized bed steam gasifier. 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