Tar Reduction by Primary Measures in an Autothermal Air-Blown

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Tar Reduction by Primary Measures in an Autothermal Air-Blown Fluidized Bed Biomass Gasifier Manuel Campoy,* Alberto Go´mez-Barea, Diego Fuentes-Cano, and Pedro Ollero Bioenergy Group, Chemical and EnVironmental Engineering Department, Escuela Superior de Ingenieros, UniVersity of SeVille, Camino de los Descubrimientos s/n, 41092 SeVille, Spain

An experimental investigation in a 100 kWth bubbling fluidized bed gasifier (FBG) was conducted to study the effect of primary measures on the tar composition in the gas. The measures tested included the variation of the stoichiometric ratio, SR, defined as the ratio of fed to stoichiometric air, secondary-air injection in the freeboard, and addition of lime and alumina to the reference bed material (ofite). The reactor was operated nearly adiabatic, with the aim of simulating the performance of industrial autothermal air-blown FBG units. An optimum value of SR was found at approximately 0.3, where the cold gasification efficiency was maximum (70%) and the tar concentration was reduced by a factor of 3 with respect to reference conditions. Addition of secondary air (keeping SR constant) reduced the gravimetric tar and the water-soluble tar compounds up to 20 and 30%, respectively; a fixed ratio of secondary to total air of 11% was investigated in this work for three valued of SR. The use of lime allowed for the reduction of the overall tar content up to 50%, whereas the decrease using alumina was rather low. None of the measures tested effectively reduced the heavy tar compounds, so the minimum value computed for the tar dew point was 170 °C. 1. Introduction Air-blown biomass fluidized bed gasification (FBG) is an important way of converting waste and biomass materials into useful fuel gas suitable for cofiring in existing boilers and to burn in gas engines and turbines for electricity generation.1 One of the most important concerns for electricity production by engines and turbines is to deal with the tar formed during the process. Tar is undesirable because it condenses even at relatively high temperatures, thus blocking and fouling process equipment.2 Two main classifications for tar components have been proposed.3,4 Milne et al.,3 on the basis of the severity of operation conditions (temperature and residence time), distinguished between primary, secondary, and tertiary tars. In most early studies,5-21 the focus was made on total tar production (gravimetric tar). This classification was useful for understanding tar generation and transformation during conversion. When looking at the impact of tar on downstream equipment, however, tar composition is the key factor to be considered.4,22-24 The presence of heavy tar compounds in the product gas was shown to be responsible for the fouling and soot formation in downstream processes, whereas phenols and other high-watersoluble tars may cause wastewater problems during water condensation before the end-use of gas. As a result, ECN4 classified families of tars on the basis of their effect on the condensing behavior of the gas, coming up with five tar classes: gas chromatography undetectable tars (class 1), heterocyclic components (class 2), aromatic components (class 3), light polyaromatic hydrocarbons (class 4), and heavy polyaromatic hydrocarbons (class 5). Tar removal technologies can be categorized according to the location where the tar is removed: primary methods are those applied in the gasifier itself, whereas, in secondary methods, the tar is removed/converted downstream of the gasifier. Secondary methods comprise chemical (thermal or catalytic cracking)5-7,25-29 or physical (mechanical separation or scrub* To whom correspondence should be addressed. Tel.: +34954481391. Fax: +34 -954461775. E-mail: [email protected].

bing) treatment.8,22 Although downstream gas cleaning methods are reported to be effective, they are usually expensive and economical feasibility is doubtful.8 In primary methods, design and operating type (composition and distribution of gasification agent, use of in-bed additives, etc.) are optimized in the gasifier to improve the gas quality as much as possible.2,23 If sufficiently low tar content gas is achieved, secondary cleaning is minimized or eliminated. Optimization of operating conditions involves adjusting the temperature, pressure, gas composition, and solid and gas residence times by manipulating operational variables such as oxygen and steam to biomass ratios and throughput (kilograms of biomass per cross-section of the gasifier and hour). Several works10,11 have studied the effect of these variables on tar content in the gas. A great deal of research has analyzed the use of in-bed additives such as dolomite, limestone, olivine, and alumina.5,6,12-18,30,31 A few works focused on the effects of gasifier design, such as two-stage gasifiers32-34 or secondaryair injection in the freeboard,10,11,19 on tar concentration of the gas produced have been published. Most studies have been made in laboratory apparatus or small FBG units. This has allowed study of the effect of different variables on the tar content and its composition with relatively minor effort/cost. However, in laboratory or pilot plant studies the tests are conducted allothermally; i.e., the temperature and oxygen-to-biomass ratio are varied independently, which is achievable because in small rigs the temperature of the gasifier is controlled by external heat addition using electric heaters. Supplying heat by an electrical oven around the vessel is, however, neither technically nor economically feasible for largescale autothermal air-blown FBG. Consequently, the results from “allothermal” laboratory-scale rigs must be interpreted with caution since the thermal level may greatly differ from that attainable in an autothermal gasifier. In previous works,35,36 we studied the gasification process under simulated adiabatic and autothermal conditions. With this method of operation, significant differences were found compared to tests carried out allothermally. Details on how this operation can be achieved at pilot scale by properly controlling the heat addition to the gasifier

10.1021/ie101267c  2010 American Chemical Society Published on Web 10/01/2010

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Table 1. Characterization of the Bed Materials

mean size (µm) density (kg/m3)

ofite

alumina

limestone

290 2650

370 3550

640 2500

Main Composition Expressed as Oxides (% w/w) SiO2 Al2O3 CaO Fe2O3 MgO loss at 750 °C (%)

53.93 13.61 11.15 9.15 7.90 0.64

0.04 99.50 0.02 0.08 0.03 -

0.30 0.10 55.40 0.20 43.50

were described.35 These previous works were focused on the improvement of the air gasification process by adding steam35 and enriched air.36 The present study is focused on the effects of various primary measures on tar reduction during air-blown operation. The tests were carried out in a 100 kWth pilot plant, varying the stoichiometric ratio, SR (defined as the ratio of actual to stoichiometric air flow rates), using two types of in-bed additives and injecting secondary air in the freeboard. Addition of secondary air was made by setting one ratio of secondary to total air keeping SR constant and analyzing various SR. The tar content of the produced gas is analyzed in terms of both the gravimetric tar concentration and the individual tar compounds, allowing for the characterization of the gas behavior for different applications. 2. Experimental Section Materials. The fuel used for the tests was wood pellets, whose physical and chemical properties were given in previous works.35,36 The main composition (% w/w, expressed in dry basis) was as follows: 49.5% C, 5.8% H, 2% N, 0.1% S, and 42.6% O. The water and ash contents were 6.3 and 0.5%, respectively, and the lower heating value (LHV) was 17.1 MJ/ kg (as received). The reference bed material used in the FBG was ofite, a silicate subvolcanic rock with mineralogical formula (Ca, Mg, Fe, Ti, Al)2(SiAl)2O6. In the tests with additive, tabular alumina (a sort of R-alumina), Al2O3, and limestone, CaCO3, were used. The main characteristics of these materials are shown in Table 1. Facility. Figure 1 shows the layout of the pilot plant. The rig has been described in detail in previous work;35-39 therefore only a brief summary is presented here. The reactor is a bubbling fluidized bed divided into two zones: the bed, with an internal diameter of 150 mm and height of 1.40 m, and the freeboard, with an internal diameter of 250 mm and height of 2.15 m. A 45 kWe electrical oven covers both the reactor and freeboard, allowing adjustment of the heat supply during the gasification process. The hot air can be preheated in a 7 kWe electrical heater before the windbox. Secondary air can also be fed to the reactor in the freeboard zone. The gas leaving the freeboard section passes through two cyclones, in series. The gas sampling point is located downstream of the cyclones. The sampling line is electrically heated to avoid the condensation of organic compounds within the probe. The composition of the produced gas is measured continuously (CO, CO2, CH4, H2, and O2) by an online analyzer. Tars, light hydrocarbons, particles, moisture, and other contaminants (NH3, HCN, H2S, and HCl) are sampled and measured discontinuously. After leaving the analysis section, the product gas enters a postcombustion chamber. To avoid tar condensation, the pipes between the gasifier and the combustion chamber are maintained at a sufficiently high temperature using heating elements and insulation blankets.

Figure 1. Layout of the FBG pilot plant. Table 2. Tar Species Quantified by the GC-MS in the Tests and the Corresponding Class from ECN Classification compound

formula

class4

benzene thiophene toluene p-/m-xylene o-xylene styrene phenol benzonitrile benzofuran indene m-cresol naphthalene 2-methylnaphthalene 1-methylnaphthalene biphenyl ethenylnaphthalene acenaphtylene acenaphtene dibenzofuran fluorene phenanthrene anthracene 4H-cyclopenta[def]phenanthrene fluoranthene pyrene benzo(a)anthracene chrysene benzo(k)fluoranthene benzo(a)pyrene perylene

C6H6 C4H4S C7H8 C8H10 C8H10 C8H8 C6H6O C6H5CN C8H6O C9H8 C7H7OH C10H8 C11H10 C11H10 C12H10 C12H10 C12H8 C12H10 C12H8O C13H10 C14H10 C14H10 C15H10 C16H10 C16H10 C18H12 C18H12 C20H12 C20H12 C20H12

3 2 3 3 3 3 2 2 2 3 2 4 4 4 4 4 4 4 2-4 4 4 4 4 4 5 5 5 5 5 5

Tar Characterization. Tar is sampled and measured according to the “tar protocol”.40 After each sampling, the solution is kept at -20 °C. The day after the test, two samples of several milliliters are taken from the solution captured. One of these samples is analyzed (the other sample is preserved) in a gas chromatograph with a mass spectrometer detector (GC-MS) for complete tar characterization. The remainder of the solution is distilled following the “tar protocol” for the determination of gravimetric tar. The species detected by the GC-MS during gas chromatography are presented in Table 2, where the corresponding class to which the tar compounds belong (according to ECN classification4) is also specified. The gas dew point was estimated

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Table 3. Test Results R1

R2

R3

S1

biomass flow rate (kg/h) total air flow rate (Nm3/h) secondary air flow rate (Nm3/h) bed mean temp(°C) freeboard mean temp (°C) gas flow ratea (Nm3 dry/h) gas resident time in bedb (s) gas resident time in freeboardc (s)

17.9 18 0 785 745 29.1 1.3 3.5

14.1 18 0 806 752 27.5 1.2 3.7

11.9 18 0 813 753 25.5 1.2 4.0

total SR secondary air (%) additive

0.24 0 -

0.30 0 -

0.36 0 -

gravimetric class 2 class 3 class 4 class 5 gas dew pointd (°C)

24.3 1.7 12.9 4.8 0.40 179.6

23.2 0.9 10.2 4.6 0.38 173.6

16.9 0.8 8.4 3.7 0.27 172.3

S2

S3

A1

A2

A3

L1

L2

L3

11.9 18 2 804 767 25.5 1.4 3.9

17.8 18 0 784 752 29.4 1.3 3.4

14.0 18 0 798 752 27.7 1.3 3.7

11.7 18 0 809 751 26.0 1.2 3.9

18.4 18 0 785 749 31.5 1.3 3.2

14.4 18 0 803 751 30.4 1.3 3.3

12.0 18 0 810 749 28.1 1.2 3.6

0.24 0 Al2O3

0.30 0 Al2O3

0.36 0 Al2O3

0.23 0 CaO

0.29 0 CaO

0.35 0 CaO

21.1 2.5 19.4 6.9 0.72 180.8

17.5 2.0 17.9 6.5 0.65 179.4

14.4 1.4 13.2 5.0 0.49 176.6

16.1 2.3 15.7 5.8 0.46 176.2

12.0 1.1 10.2 3.1 0.27 172.6

10.2 0.8 9.1 2.7 0.24 172.1

Operational Conditions 17.9 18 2 779 755 28.8 1.4 3.5

14.1 18 2 798 770 27.2 1.4 3.7

Main Variables for Analysis 0.24 11 -

0.30 11 -

0.36 11 -

Tar Content (g/(Nm3 dry)) 19.7 1.4 13.0 6.0 0.48 179.1

22.6 0.7 9.6 4.7 0.43 173.5

21.4 0.6 8.0 4.5 0.41 174.8

a Calculated by mass balance. b Based on the total air fed to the reactor and the bed temperature. temperature. d Calculated using the complete model of Thersites (ECN).41

by using the complete model which is available on the Web site “Thersites” published by ECN.41 Test Procedure. The operation procedure has been described in detail in previous publications.35-39 Only a short summary is presented below. At the beginning of each test a batch of selected bed material (around 8 kg) is added to the reactor. In tests using lime, previous in-bed calcination of limestone was carried out. To reach the target proportion of lime in the bed material, the loss of mass inventory at 750 °C presented in Table 1 was accounted for. The bed was heated with hot air and the electrical furnace. After 4 h, the bed and freeboard temperatures reached 750 °C and the temperature upstream of the combustion chamber was higher than 300 °C. Once these requirements were fulfilled, a small amount of biomass was fed. The transition from combustion to gasification was achieved by increasing the biomass flow rate. Once the plant reached steady-state conditions, secondary air was injected (in the specific tests of secondary-air injection). Once the new steady state was reached the system was operated between 3-4 hours and tar, particle, and water vapor content of the product gas were measured twice. Also, the cyclone ash was sampled for analysis. Operating Conditions. All the tests were conducted using ofite as the main bed material and air as a gasifying agent. The flow rate and inlet temperature of the air were 18 Nm3/h and 400 °C (the latter set to simulate inlet temperature that can be achieved by heat recovery from the product gas without tar condensation35). Table 3 summarizes the operational conditions and results of the 12 experiments conducted. A set of tests using ofite as bed material (without additives) and introducing the air through the distributor was conducted as a reference. These tests were carried out to study the effectiveness of the injection of secondary air in the freeboard and the use of alumina and lime, at various stoichiometric air ratios (SR). Three SR were tested within the range of 0.24-0.36 for each type of test [reference, Ri; secondary-air injection, Si; alumina addition to bed material, Ai; and lime addition to bed material, Li; with i ) low SR (SR ≈ 0.24), i ) 2 medium SR (SR ≈ 0.30), and i ) 3 high SR (SR ≈ 0.36)].

c

Based on the gas outlet and the freeboard

In the test with secondary air, 2 Nm3/h of air (11% of the total air fed to the gasifier) were injected in the freeboard at height 150 mm. This value was chosen on the basis of previous works found in the literature10,11,19 and on the results from a computer model developed in our group.42 The secondary air was injected at ambient temperature. Experiments with in-bed additives were made using 50% (w/w) additive (lime or alumina) mixed with ofite. More detailed data than that presented in Table 3 can be found in Tables S1 and S2 included in the Supporting Information. 3. Results and Discussion The effect of the various measures tested on the gravimetric tar produced is summarized in Figure 2. Figure 2a presents the total concentration in the gas (g/Nm3) and Figure 2b the tar yield (g of tar/kg of biomass fed dry and ash free, daf) as a function of SR for the various types of tests. The bed and freeboard temperatures for the tests of Figure 2 are shown in Figure 3, indicating that the bed temperature increases with SR, regardless of the type of test. As expected,3,4,23 the tar concentration decreases with SR for all types of tests, except for secondary-air tests (Figure 2a). However, the tar yield (Figure 2b) increased with SR for tests using secondary-air injection while staying roughly constant for the reference tests and for those using lime and alumina. When alumina or lime was added, the maximum reduction of the tar concentration in the gas was attained for all the SR tested (Figure 2a). The higher activity of lime compared to alumina is shown; however, the reduction of tar observed in this work is lower than that reported in previous work using similar additives such as dolomite,6,21 where reduction of up to 90% has been reported. Apart from the different nature of the additive, the difference may be due to the allothermal operation, under which most of the tests published were conducted, leading to a higher operating temperature than that investigated in this study. In the tests with secondary air, tar concentration slightly increased with SR, whereas the tar yield significantly increased. The reduction of bed temperature when secondary air was injected (compared to the reference test at equal SR), see Figure

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Figure 2. Tar content in the producer gas [(a) gravimetric tar concentration and (b) tar yield] as a function of the SR for different tests: (() reference, (9) secondary air, (2) alumina, and (b) lime.

Figure 3. Mean temperature in (a) the bed and (b) the freeboard as a function of the SR for different tests: (() reference, (9) secondary air, (2) alumina, and (b) lime.

3a, can explain these observations. Note that an increase in the freeboard temperature was observed during secondary-air tests (Figure 3b), caused by the partial combustion of part of the combustible gas flowing up from the bed. The temperature reduction in the bed as secondary air is injected increases the tar generated in the devolatilization step. In spite of the higher temperature of the freeboard, the temperature and gas residence time in this zone are not enough for significant secondary conversion of tar (by cracking and reforming), yielding an outlet gas with higher tar content (and composition, as it is shown below) than that obtained by operating with the same SR without air injection (reference test). The effect of SR on the gas yield, lower heating value of the gas (LHV), cold gasification efficiency, and carbon conversion is shown in Figure 4. The LHV of the gas (accounting for the measured hydrogen, carbon monoxide, and methane) decreases with SR (Figure 4b), whereas the carbon conversion (Figure 4d) and the gas yield (Figure 4a) (Nm3 of dry and nitrogen free gas/kg of biomass daf) rise with SR. The cold gasification efficiency reaches a maximum at intermediate SR (Figure 4c). In this work, the carbon conversion was calculated as the difference between the carbon flow rate in the fuel and cyclone ash, divided by the flow rate of carbon in the fuel. The flow rate of carbon for both streams is computed by the product of the flow rate of the stream (fuel and ash) and its total carbon content determined by ASTM analysis. Figure 4a shows that the highest gas yield was attained when lime was added to the bed. This result, together with the higher reduction in tar concentration (Figure 2a) and the increase in hydrogen concentration (Figure 5a), suggests the enhancement of tar reforming promoted by the lime. The lower CO

concentration measured when using additives suggests that cracking is not as effective as reforming. The water-gas-shift reaction (WGSR) seems to be enhanced by the presence of lime in the bed. For secondary-air injection, the CO and H2 concentrations were, respectively, slightly higher and lower than the reference test at the same SR, suggesting that cracking is the main mechanism for tar reduction in this case. The injection of secondary air, however, lowered the carbon conversion (Figure 4d) due to the reduced availability of oxygen in the bed, leading to a reduction of the char oxidation in the bed. The increase in carbon dioxide when using lime is also evident in Figure 5c, consistent with the enhancement of the WGSR as explained above. It could also be due to the decomposition of some remaining carbonate in the bed. Lime addition lead to higher cold gasification efficiency, reaching a maximum of 69% at SR around 0.3. This maximum is explained by the significant increase in the gas yield with SR (Figure 4a) and the relatively small decrease in LHV (Figure 4b) with SR. Overall, the results suggest that an optimum SR exists, where significant tar and char conversions are achieved, leading to maximum gasification efficiency.2 The effect of the primary measures on the tar nature is summarized in Figure 6, where the trends have been studied by grouping the tars in various classes following the ECN classification4 (see Table 2). The individual concentration of the species presented in Table 3 for all the tests conducted in this work is provided as supporting material in Table S1 (see Supporting Information). The main findings are summarized below as follows. (1) Tar class 2 (water-soluble tars), Figure 6a: Tar class 2 was effectively reduced by increasing SR as well as through

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Figure 4. Effect of the SR on (a) gas yield, (b) lower heating value, (c) cold gasification efficiency, and (d) carbon conversion for different tests: (() reference, (9) secondary air, (2) alumina, and (b) lime.

Figure 5. Concentration of the main compounds in the gas [(a) hydrogen, (b) carbon monoxide, (c) carbon dioxide, and (d) methane] as a function of the SR for different tests: (() reference, (9) secondary air, (2) alumina, and (b) lime.

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Figure 6. Tar content in the off-gas [(a) class 2 (soluble tars), (b) class 3, (c) class 4, and (d) class 5] as a function of the SR in the different tests: (() reference, (9) secondary air, (2) alumina, and (b) lime.

the injection of secondary air in the freeboard, this latter being more effective than the former, probably due to the enhancement of oxygen-tar contact, contributing to oxidation and reforming of these tar compounds.23 On the other hand, an increase in water-soluble tar was measured when using alumina, whereas the use of lime was ineffective for reduction of tar class 2. (2) Tar class 3 (light aromatics hydrocarbons, including benzene), Figure 6b: Increasing SR was effective for tar class 3 conversion. The use of lime and injecting secondary air did not significantly affect the tar class 3 concentration, whereas the use of alumina, significantly increased the concentration of these tar compounds. (3) Tar class 4 (2-3 rings polyaromatic hydrocarbons), Figure 6c: Rising SR led only to moderate reduction for the reference tests. The use of secondary air and the addition of alumina promoted a net generation of tar class 4, probably due to the enhancement of polymerization reactions;4 despite this, increasing SR for these tests reduced these compounds in a way similar to the reference tests. The use of lime is revealed to be an interesting option since significant reduction of tar class 4 was attained, especially when SR was varied from low to medium SR. (4) Tar class 5 (four- to five-ring polyaromatic hydrocarbons), Figure 6d: Increasing SR and the use of lime lead to significant reduction of these tars. Secondary-air injection and addition of alumina increased the concentration of these tar compounds in the gas. It should be noted that the concentration of tar class 5 is considerably lower than tar belonging to other classes. However, even low concentrations of these tar compounds may limit the final use of the gas when it has to be cooled before use. This is because tar class 5 determines, to a large extent, the dew point

Figure 7. Gas dew point as a function of the SR for different tests: (() reference, (9) secondary air, (2) alumina, and (b) lime.

of the product gas. The tar dew point calculated for the gas produced in the 12 tests investigated is represented in Figure 7. It is observed that increasing SR and adding lime contribute to lowering the gas dew point. However, a reduction of only a few degrees was achieved (Figure 7). The essential contribution of the class 5 tar to the gas dew point is illustrated in the following: the gas dew point was computed41 for the three reference tests (Ri), considering two cases: (1) accounting for only tar class 5, leading to the dew point Td1, and (2) accounting for all the tars measured except to those belonging to tar class 5, leading to the dew point Td2. The results showed that Td2 ranged between 103 and 108 °C, whereas Td1, between 172 and 180 °C. Therefore, class 5 tars give the actual gas dew point. It is worth emphasizing that tar class 1 (GC nondetectable tars) is not included in the dew point calculation. Therefore, the actual dew point of the gas is always higher than that calculated using the ECN complete model.41

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compounds measured were grouped. An optimum SR at around 0.3 was found, where both significant tar and char conversion were achieved. (2) The use of secondary air reduced the bed temperature, slightly decreasing the carbon conversion. The proper combination of SR and secondary air came up with a significant reduction of the water-soluble tars. Therefore, combining secondary-air injection with water scrubbing could be interesting since they could be optimized to minimize the contaminated water disposal. (3) The use of lime in the bed changed the composition of the product gas, increasing the gas yield and gasification efficiency. A small impact on these process parameters was found when using alumina. The use of lime was found to be much more effective than alumina with regard to tar reduction, being the most effective method tested in this work to reduce tar classes 4 and 5. The use of alumina was found to be efficient only for gravimetric tar reduction. The use of lime should be made in combination with proper adjustment of SR. Overall, none of the investigated measures reduced the heavier tar concentration to the extent needed for applications where the gas is cooled down before burning it, for instance for electricity production in engines. Therefore, for these applications, the primary measures presented in this work should be used in combination with secondary measures. Acknowledgment

Figure 8. Effect of the SR level (L, low; M, medium; H, high) on the tar weight distribution (a) and the tar yield (b): (dark blue) class 2, (light blue) class 3, (green) class 4, and (yellow) class 5.

Despite this limitation, the dew point calculated in this way provides an useful indication for studying the effect of the primary measures tested in this work. The effect of SR (low, medium, and high) on the tar distribution in the gas lumped in ECN classes4) is shown in Figure 8, grouped into four types of tests: reference, secondary air, alumina, and lime. Figure 8a presents the results in terms of tar composition in the gas, whereas Figure 8b, in terms of tar yields. It can be seen that tar class 3 is the prevalence species, reaching up to 70% tar content in the gas. By contrast, tar class 5 represents less than 3%. The use of additives reduces tar classes 4 and 5, whereas increases tar classes 2 and 3 slightly (Figure 8a). The trend is the opposite when secondary air is used: the concentration of tar classes 2 and 3 decreases while the polyaromatic compounds (tar classes 4 and 5) increase. In terms of tar yields (Figure 8b) the reduction measured for class 2 tars was noticeable, about 50%. Tar classes 3 and 4 yields were slightly reduced, and no decrease in tar class 5 yield was observed. The use of additives significantly increased the tar class 3 yield, up to 100% when alumina was used, whereas the use of lime reduced tar class 4 yield. Reduction was not observed for the yield of tar class 5. 4. Conclusions Tests were conducted in air-blown bubbling FBG to study the effectiveness of various primary measures on tar reduction. The tests were conducted by simulating autothermal and adiabatic operations in order to scale up the results to industrial air-blown FBG units. The following conclusions were made: (1) The increase in the SR reduced both the gravimetric tar as well as the various tar classes into which the 30 tar

This work was financed by the Junta de Andalusia under the project FLETGAS. The help of Susanna Nilsson and Jose Guillermo Claro in sampling the gas for tar measurements are acknowledged. Supporting Information Available: Tables showing more data about complete tar characterization and additional operation conditions. This information is available free of charge via the Internet at http://pubs.acs.org/. Nomenclature daf ) dry and ash free ECN ) Energy Center of The Netherlands FBG ) fluidized bed gasification (or gasifier) GC-MS ) gas chromatograph-mass spectrometer LH ) lower heating value SR ) stoichiometric (air) ratio (also called equivalence ratio) WGSR ) water-gas-shift reaction

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ReceiVed for reView June 11, 2010 ReVised manuscript receiVed August 20, 2010 Accepted September 1, 2010 IE101267C