Hot Gas Cleaning and Upgrading with a Calcined Dolomite Located

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Energy & Fuels 1997, 11, 1194-1203

Hot Gas Cleaning and Upgrading with a Calcined Dolomite Located Downstream a Biomass Fluidized Bed Gasifier Operating with Steam-Oxygen Mixtures P. Pe´rez,† P. M. Aznar,*,†,‡ M. A. Caballero,† J. Gil,† J. A. Martıń,† and J. Corella§,| Department of Chemical and Environmental Engineering, University of Saragossa, 50009 Saragossa, Spain, and Department of Chemical Engineering, University “Complutense” of Madrid, 28040 Madrid, Spain Received March 24, 1997X

Cleaning and upgrading of the hot raw gas from a biomass gasifier, bubbling fluidized bed type, is studied at small pilot plant scale (10 kg biomass fed/h) using a calcined dolomite located downstream from the gasifier. Gasification is made with steam-oxygen mixtures at 800-850 °C and atmospheric pressure. Main variables studied are the gas residence time in the bed of dolomite and the gas atmosphere composition, which depends on the gasifying agent (H2O + O2)-to-biomass and H2O/O2 ratios. H2 and CO content in the flue gas increases by 16-23 vol % and decreases by 15-22 vol % (dry basis), respectively. Although CH4 conversion (elimination) higher than 30 vol % has never been reached, tar conversion (elimination) of 90-95 vol % are obtained with space times of 0.06-0.15 kg calcined dolomite h-1 m-3. A detailed study is here presented on how the calcined dolomite significantly cleans and upgrades the flue gas, increasing also the gas yield by 0.15-0.40 m3(STP)/kg daf biomass fed.

Introduction It is well-known how gasification of biomass produces an useful gas from a renewable solid. For most further applications the raw gas so produced has to be cleaned for tars and dust (which can include char, coke, and fine particles of silica sand and/or dolomite elutriaded from the gasifier bed). This work is focused on the cleaning and upgrading of the hot raw gas from a biomass gasifier. Biomass gasifiers are classified depending on their type (bubbling or circulating fluidized bed, moving bed, ...), operating pressure (atmospheric or high pressure), gasifying agent used (air, steam, steam-oxygen mixtures, ...), scale of work (laboratory, pilot, demonstration, ...), etc. Other details of the gasifier, such as the location of the feeding point for the biomass and its throughput (kg biomass fed h-1 m-2 of cross sectional area), are also important for characterizing the gasifier used. This paper mainly concerns biomass gasification in an atmospheric and bubbling fluidized bed at small pilot plant scale, gasifying with steam-oxygen mixtures. The operation or “concept” of hot gas cleaning usually includes the use of ceramic filters for fine particle removal and of catalytic reactors for removal of tar, NH3, SH2, etc., but the key component in such flue gas is tar. Its elimination to very low content is not easy work. Two ways are emerging for such an objective: use of calcined dolomites (or related solids) and use of steam reforming (nickel-based) catalysts.1 The work presented here is focused only on the use of calcined dolomites for cleaning and upgrading of the raw gas. Calcined dolomites †

University of Saragossa. Fax: +34-76-76 21 42. University “Complutense” of Madrid. | Fax: +34-1-394 41 64. X Abstract published in Advance ACS Abstracts, October 1, 1997. ‡ §

S0887-0624(97)00046-7 CCC: $14.00

(OCa‚OMg) are very cheap and abundant on the earth. They have already proved their usefulness in biomass gasification because of their catalytic activity for tar destruction. Till very recently, the typical term used for this activity was “tar cracking”, but nowadays, it is thought that their activity in this process is not cracking but of steam (and dry) reforming of tars, as it will be demonstrated in this paper. The OCa‚OMg would dissociate the molecule of steam, generating some reactive radicals7,8 that would further react with the “hard-to-destroy” molecules of tar mainly composed of phenol derivatives and of polyaromatics (PAHs) such as naphthalene. (1) Narvaéz, I.; Corella, J.; Orio, A. Fresh Tar (from a Biomass Gasifier) Elimination over a Commercial Steam-Reforming Catalyst. Kinetics and Effect of Different Variables of Operation. Ing. Eng. Chem. Res. 1997, 36 (2), 317-327. (2) Delgado, J.; Aznar, M. P.; Corella, J. Calcined Dolomite, Magnesite, and Calcite for Cleaning Hot Gas from a Fluidized Bed Biomass Gasifier with Steam: Life and Usefulness. Ind. Eng. Chem. Res. 1996, 35 (10), 3637-3643. (3) Delgado, J.; Aznar, M. P.; Corella, J. Biomass Gasification with Steam in Fluidized Bed: Effectiveness of CaO, MgO and CaO-MgO for Hot Raw Gas Cleaning. Ind. Eng. Chem. Res. 1997, 36 (5), 15351544. (4) Orio, A.; Corella, J.; Narvaéz, I. Characterization and Activity of Different Dolomites for Hot Gas Cleaning in Biomass Gasification. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1997; Vol. 2, pp 1144-1157. (5) Orio, A.; Corella, J.; Narvaéz, I. New Developments on the Effectiveness of Dolomites of Different Origin for Hot Raw Gas Cleaning in Biomass Gasification with Air. Ind. Eng. Chem. Res. 1997, 36, 3800-3808. (6) Simell, P.; Hakala, N.; Haario, H. Catalytic Decomposition of Gasification Gas Tar with Benzene as the Model Compound. Ind. Eng. Chem. Res. 1997, 36 (1), 42-51. (7) Taralas, G.; Vassilatos, V.; Sjo¨stro¨m, K.; Delgado, J. Thermal and Catalytic Cracking of n-Heptane in Presence of CaO, MgO and Calcined Dolomites. Can. J. Chem. Eng. 1991, 69, 1413-1419. (8) Taralas, G.; Sjo¨stro¨m, K.; Bjo¨rnbom, E. Dolomite Catalysed Cracking of n-Heptane in Presence of Steam. In Advances in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackie Academic: Glasgow, U.K., 1994; Vol. 1, pp 233-245.

© 1997 American Chemical Society

Hot Gas Cleaning and Upgrading

Energy & Fuels, Vol. 11, No. 6, 1997 1195

Calcined dolomites can be located in the same gasifier bed (feeding them together with the biomass into the gasifier) or downstream from the gasifier. The first situation is positive and easy to do. It decreases in situ the tar content in the gas, but it generates much more dust in the exit gas. Although we are not discarding this possibility for further study and application, the work here presented is only devoted to the use of the calcined dolomite in a vessel (reactor) downstream from the gasifier. This type of application, process, and/or concept is similar to the one of the well-known (in this area) Swedish company TPS (formerly Studsvik) AB, but there is one big difference between its application and the one presented in this paper. The TPS AB process is of gasification with air, whereas the work here presented is of gasification with steam-oxygen mixtures. These two processes generate a raw gas with two very different compositions, and under such very different gaseous atmospheres, calcined dolomites can have different behavior and activity. Use of calcined dolomites (and related materials) in biomass gasification for hot gas cleaning and upgrading is being studied in several European countries. The European Commission (DG-XII) has recently financed quite a big project on just this subject (Project AIR2CT93-1436) in which five organizations have participated: KTH and TPS AB from Sweden, BTG B.V. and RUG from The Netherlands, and UCM from Spain who acted as coordinator. That project was not only for raw gas cleaning and upgrading but also for gas coming both from biomass gasifiers with air and for synthetic gas atmospheres, which is again the main difference with the work presented in this paper. In Spain, for some reason, there has been a big effort on the study of calcined dolomites for hot raw gas cleaning and there have been three different teams or groups of people devoted to this subject each one using a different facility or setup. Delgado et al.2,3 studied the use of calcined dolomites in biomass gasification with steam. Orio et al.4,5 studied the usefulness of dolomites for gasification with air. These authors2-5 and also Simell et al.6 and Taralas et al.7,8 have very recently made an exhaustive study on the state-of-the-art in this subject, identifying the laboratories and people working or having worked in this matter. It is addressed to these recent publications for details about previous research on dolomites for hot gas cleaning. A bed of a calcined dolomite placed after a biomass fluidized bed gasifier in which gasification is made with steam-oxygen mixtures is going to clean and upgrade the raw gas. The gas composition and dirtiness of this raw gas, obtained under very different operating conditions in the gasifier (gasifying with steam-oxygen mixtures), have been published beforehand.9,10 Gas composition and its tar content are thus now known at the inlet of the bed of dolomite. They are shown in Table 1. Main Properties of the Gas at the Gasifier Exit or at the Bed of Dolomite Inlet (from Refs 9 and 10) gas composition at the gasifier exit(vol %, dry basis) H2 CO CO2 CH4 H2O (vol %, wet basis) Ctar (mg (dry m3(STP))-1) LHV (MJ (dry m3(STP))-1) gas yield (dry m3(STP)/kg daf)

13-31 32-48 14-31 5.3-7.4 39-60 2250-42000 13-16 0.87-1.2

Table 1. The downstream bed of dolomite is going to change these gas properties. Apparatus, Solids, and Methods Used Installation Used. The present work has been carried out in a small pilot plant previously described.9-11 In short, it is based in a bubbling fluidized bed of 15 cm internal diameter and 3.2 m total height, continuously fed near the bed bottom with two screws at rates of 5-20 kg biomass/h. The gasifier had an external oven for preheating, for a starting up period, and for keeping the gasifier bed temperature at a prefixed level (of around 780-850 °C). The gasifier had a stationary bed of about 15 kg of silica sand of 0.60 ( 0.43 mm (umf ≈ 17 cm/s), equivalent to a height (bulk, fixed bed conditions) of about 60 cm. The superficial gas velocity at inlet was around 60 cm/s. After the gasifier there were three high-efficiency cyclones to remove most particles in the flue gas. From the main flow, which is cooled, measured, and then burnt, a slip flow was taken and directed to two reactors connected in series. The first one was for the calcined dolomite, and the second one was for a steam reforming (nickel-based) catalyst. Bed of Dolomite. The reactor for dolomite, Figure 1, had an internal diameter of 41.4 mm and the flow in it was upward. At the bottom there was a bed of particles of silica sand of 5 ( 1 mm diameter. It improves the gas distribution along the whole cross-sectional area of the reactor, and it heats the gas to the desired temperature. The temperature was measured at the center of the bed of dolomite and at its wall (inner side), as Figure 1 shows. In some runs the central thermocouple was moved along the bed axis for determination of the longitudinal temperature gradient. Both radial and axial temperature gradients were not much important (always lesser than 10-15 °C) because the bed was always smoothly fluidized. The uo/umf ratios used were 1.0-1.5 for the biggest particle size and 2.5-3.5 for the smallest. These radial and longitudinal temperature gradients were thus not as important as the ones of 5-30 °C found and described by Orio et al.4,5 using a fixed bed for the dolomite. Dolomite was first calcined (2 h at 900 °C) in an external oven and then carefully handled (to avoid its recarbonation) and weighed before its introduction into the reactor. The slip flow rate through this reactor was able to be varied and was prefixed in each test to get (uo/umf) values of 1.0-1.5 or 2.53.5, depending on the particle size used. The bed was thus always only smoothly fluidized to avoid important (radial and longitudinal) temperature gradients in the bed and erosion of the particles of the calcined dolomite. In fact, calcined dolomite is quite soft and it might erode very much if the bed were fully fluidized (high uo/umf ratios), deactivating thus the dolomite2 and even losing the bed by the carryover of the fine particles so produced. Dolomite Used. The dolomite used in all tests had the same origin. It was delivered by Prodomasa Co. It came from its quarry in Coin (Malaga, Spain). Its chemical analysis is shown in Table 2. Note the very low content of iron in this dolomite, having thus a white color. Once calcined for 2 h at 900 °C in an oven, it was characterized for its pore structure (9) Aznar, M. P.; Corella, J.; Gil, J.; Martıń, J. A.; Caballero, M. A.; Olivares, A.; Pe´rez, P.; France´s, E. Biomass Gasification with Steam and Oxygen Mixtures at Pilot Scale and with Catalytic Gas Upgrading. Part I: Performance of the Gasifier. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1996; Vol. 2, pp 1114-1128. (10) Gil, J.; Aznar, M. P.; Caballero, M. A.; France´s, Eva; Corella, J. Biomass Gasification in Fluidized Bed at Pilot Scale with SteamOxygen Mixtures. Product Distribution for Very Different Operating Conditions. Energy Fuels 1997, 11, 1109-1118. (11) Aznar, M. P.; Borque, J. A.; Campos, I. J.; Martıń, J. A.; France´s, E.; Corella, J. New Pilot Plant for Biomass Gasification in Fluidized Bed and for Testing of Downstream Catalysts. In Biomass for Energy Environment Agriculture and Industry (8th EC Conference on Biomass in Vienna, Austria, 1995); Chartier, P. H., Beenackers, A. A. C. M., Eds.; Pergamon Press: London, 1995; Vol. 2, pp 1520-1527.

1196 Energy & Fuels, Vol. 11, No. 6, 1997

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Figure 1. Scheme of the bed of dolomite used in this work. by nitrogen adsorption (in an ASAP 2000 apparatus) and mercury porosimetry (in an 9320 pore-sizer apparatus). Main results about its pore structure and surface area are shown in Table 2. Its BET surface area is 12 m2/g, the total pore volume is 0.34 cm3/g, and its averaged pore diameter is 850 Å. It has thus big macropores with only a very small amount of micropores or microporosity.5 To get the bed under a smooth fluidization, only two particle sizes were used: 1.0 ( 0.63 and 1.6 ( 1.0 mm. Their umf values (at reactor temperature and for a gas composition close to the realistic one), calculated according to the study of Delgado et al.12 are shown in Table 3. These particle sizes, selected thus by fluid-dynamic considerations, have effectiveness factors (η) lesser than unity, according to the calculations (12) Delgado, J.; Corella, J.; Aznar, M. P.; Araguës, J. L. Minimum Fluidization Velocities at Elevated Temperatures with Gas from a Steam Fluid Bed Gasifier of Biomass/Coal for Dolomite, Silica Sand and a Steam Reforming Catalyst. In La Fluidisation; Laguerie, C., Guigon, P., Eds.; Lavoisier Technique et Documentation: Paris, 1991; Vol. 5, pp 62-69.

made by Delgado et al.3 for the same reaction network and under gas atmospheres and temperatures close to the ones used here. Such values of η are shown in the third column of Table 3, and they will be important for further understanding some results in this paper. Gas and Tar Sampling and Analysis. At the inlet and at the exit of the bed of dolomite there were sampling points. In each run or test, samples of gas and of condensates were taken periodically along time-on-stream. Samples of gas were taken every half hour and samples of condensates every hour. The length of a test/run under stationary state was several (4-10) hours. Several samples of gas and of condensates were thus taken in every run. No deactivation was observed for the dolomite. Results did not change with time-on-stream in these tests. No long-term deactivation tests have been made, and the results presented here should thus be considered as initial activity or activity of a “fresh” calcined dolomite. Some deactivation curves for these dolomites were previously studied by Delgado et al.2



Table 2. Characterization of the Dolomite Used from PRODOMASA of Coin-Malaga (Spain) Chemical Characterization composition (wt %) CaO 30.6 MgO 21.2 Fe2O3 0.01 CO2 47.3 Al2O3 0.40 Averaged Parameters for the Pore Structure of the Dolomite Once Calcined at 900 °C N2 adsorption BET surface area, m2/g 12 micropore area, m2/g 0.91 area 17-3000 Å, m2/g 18-20 pore volume, 17-3000 Å, 0.07 cm3/g micropore volume, cm3/g 4.9 × 10-4 average micropore diameter, 144 Å Hg porosimetry pore area, m2/g 16 total pore volume, cm3/g 0.34 average pore diameter 850 (4 V/A), Å bulk density, g/cm3 1.52 apparent (skeletal) density, 3.16 g/cm3 Table 3. Effectiveness Factors and Minimum Fluidization Superficial Gas Velocities (Bed Conditions) of the Two Particles Sizes Used for the Dolomite dp (mm)

umf (cm/s) (from ref 12)

η (from ref 3)

1.0 ( 0.63 1.6 ( 1.0

20 40

0.74-0.85 0.58-0.74

The methods used for gas and tar sampling and analysis have been previously described in detail1,9,13 except that gas composition was determined by off-line gas chromatography under well-known, established, and checked conditions. Tar analysis was made by total organic carbon (TOC) analysis according to a method13 that is not yet standardized but that falls well among the most advanced ones and that can be accepted worldwide.14 Other tar sampling and analysis methods would provide somewhat different values for the tar content in the gas phase, but the authors are convinced that the main trends and conclusions obtained in this work would remain the same. Tar reactivity in a bed of dolomite depends on the tar composition, which in turn can vary with the operating conditions in the gasifier. For this reason the authors tried to determine the tar composition at the inlet of the bed of dolomite, but these attempts failed. Some samples of condensates were sent to two laboratories (NREL and KTH) expert on tar analysis, but no definitive and good results for this variable were obtained.

Results The activity of a calcined dolomite for tar elimination has been demonstrated to be high enough in the interval 800-900 °C.3-5 Since the effect of the temperature is known in biomass gasification with air4,5 and with steam,3 this variable was maintained constant in the present work and a temperature of 840 ( 10 °C was selected for all experiments. (13) Narvaéz, I.; Orio, A.; Aznar, M. P.; Corella, J. Biomass Gasification with Air in an Atmospheric Bubbling Fluidized Bed. Effect of Six Operational Variables on the Produced Raw Gas. Ind. Eng. Chem. Res. 1996, 35 (7), 2110-2120. (14) Development of standard procedures for gas quality in biomass gasifier/power generation systems. Novem Report 9608 to EC/EU for the JOU2-CT93-0408 EC Contract; BTG B. V.: Enschede, The Netherlands, June 1995.

Figure 2. Increase of the hydrogen content in the gas in the bed of dolomite at different space times (T2,c ) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Mainly two variables have been studied: space time and gas residence time in the bed of dolomite and composition of the gas atmosphere. The space time was varied in the interval 0.03-0.25 (kg calcined dolomite h-1 m-3 of flue gas in the bed of dolomite, i.e., calculated at the temperature of the bed and including the steam content in the gas). These space times are equivalent to gas hourly space velocity (GHSV) of 800-5900 (m3(STP) m-3 h-1). The composition of the reacting atmosphere in the bed of dolomite depends on the conditions in the upstream gasifier. The gas composition at the gasifier exit (bed of dolomite inlet) depends mainly on two variables: gasifying ratioo GR, or (H2O + O2)/biomass fed, and H2O/O2 ratio.9,10 The GR value was varied from 0.7 to 1.6; H2O/O2 was 2.0 and 3.0. Note that for future commercial applications, the values that these authors recommend nowadays for these ratios are GR < 1.0 and H2O/O2 ) 3.0. Since gas composition and properties were measured before and after the bed of dolomite, let us show how this bed influences the most important properties of the flue gas. Modification of the Gas Composition. H2 Content. Figures 2 and 3 show how the H2 content in the gas increases by 16-23 vol % (dry basis), which is an important increase. This ∆H2 increases with the gas residence time in the bed (Figure 2) and decreases with GR (Figure 3). The first fact is easy to understand because H2 is a product of several cracking and reforming reactions in the bed of dolomite. The second fact (Figure 3) is produced because with high GR, the flue gas at the gasifier exit contains less tar than with low GR.9,10 So if the tar content is low, the H2 produced by tar elimination reactions (cracking and reforming) will be low, and the H2 increase will be less than if the gasifier operates at higher GR values. CO Content. The CO has just the opposite behavior of H2. CO content always decreases in the bed of dolomite (Figures 4 and 5). It is thought that it is due to the CO-shift reaction (CO + H2O ) H2 + CO2). The decrease in the CO content is higher on increasing the gas residence time (Figure 4), which is easy to understand, and on increasing the GR values (Figure 5). The results shown in this last figure could be explained if


Figure 3. Increase of the hydrogen content in the gas in the bed of dolomite for different values of GR in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Pe´ rez et al.

Figure 5. Decrease of the CO content in the gas in the bed of dolomite for different GR in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Figure 4. Decrease of the CO content in the gas in the bed of dolomite at different space times (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

tar reforming (in the bed of dolomite) produces CO. With low GR values in the gasifier, the tar content at the bed of dolomite inlet is high,9,10 and CO production in the bed of dolomite (due to tar reforming) would be higher than with high GR values in the gasifier, which produces a low tar content at the gasifier exit. This fact could explain why with low GR values the CO decrease in the bed of dolomite is less than with high GR values. CO2 Content. The CO2 content in the gas can increase or decrease in the bed of dolomite, as Figure 6 shows. It is due to at least three simultaneous reactions: (1) the CO shift, which produces CO2; (2) the combustion of some gases with the O2 remaining in the flue gas (at the gasifier exit there is still some unreacted O2); (3) the CO2 reforming of tars and light hydrocarbons, which are CO2 consuming. On average, the CO2 content increases a little in the bed of dolomite. It would indicate that the CO shift and combustion reactions are more important than the CO2 reforming ones. CH4 Content. The CH4 always decreases a little (0.8 to 2.0 vol %, dry basis) in the bed of dolomite, as Figure 7 shows. The methane disappears in the bed of dolomite by steam and CO2 reforming reactions and the amount

Figure 6. Variation of the CO2 content in the gas in the bed of dolomite at different (a) space times and (b) GR values in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

of CH4 that disappeared is higher with increasing gas residence time of the gas, as Figure 7a shows. The effect of GR shown in Figure 7b is somewhat difficult to explain. The reason for this effect is similar to the reason for H2.



Figure 7. Decrease of the CH4 content in the gas in the bed of dolomite at different (a) space times and (b) GR values in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Figure 8. Methane conversion in the bed of dolomite at different (a) space times and (b) GR values in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

From data shown in Figure 7 and knowledge of the amount of CH4 at the bed inlet, the degree of conversion of methane has been calculated (Figure 8). CH4 conversions (eliminations) are always less than 30 vol %, which would indicate a low activity of the dolomite for the CH4disappearance reactions. As another conclusion, Figure 8 also indicates how the CH4 conversion increases with space time and decreases when the value of GR increases. For further comparison purposes, an overall kinetic constant for CH4 elimination (k2,app,CH4) has been calculated assuming a first-order reaction with respect to CH4 and piston flow in the bed of dolomite (which is close to reality). With these two considerations it is easily obtained that

-ln(1 - XCH4) ) k2,app,CH4 τ

(1)

From the XCH4-τ relationships shown in Figure 8, the values of k2,app,CH4 have been calculated with eq 1. Such values are shown in Figures 9 and 10 for the two particle diameters (dp) used here. For dp ) 0.84 mm, the average value for k2,app,CH4 (kCH4) is around 5 m3 kg-1 h-1), and for dp ) 1.3 mm, kCH4 is around 3. If the internal diffusion is dominant, as it is deduced from a previous work for this same pro-

Figure 9. Apparent kinetic constant for methane elimination in the bed of dolomite at different space times (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/ mol).

cess and solid,3 then

k2,app,CH4 ηk2,intrinsic,CH4 η φ′ dp′ ) ) ) ) k′2,app,CH4 η′k′2,intrinsic,CH4 η′ φ dp

(2)


Figure 10. Apparent kinetic constant for methane elimination in the bed of dolomite for different values of GR in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

kCH4/kCH4′ ) 5/3 ) 1.7 should be thus equal to dp′/dp ) 1.3/0.84 ) 1.5 (in this case). It does not happen here. The reason for this small discrepancy could be due to the experimental error and also to the fact that with the smallest dp used, the bed was fully fluidized (uo/ umf ) 2.5-3.5) and was thus isothermal, whereas for dp ) 1.3 mm the bed was not fully fluidized (uo/umf ) 1-1.5) and there were temperature gradients of 10-15 °C, which would make the averaged temperature in the bed of dolomite somewhat higher than the 840 °C measured at its center (remember that this overall process is endothermic, and since the reactor is heated externally, the temperatures at the center would be the smallest ones). So, kCH4 values for dp ) 1.3 mm would correspond to temperatures 5-10 °C higher than 840 °C, which is the true temperature for dp ) 0.84 mm. Heating Value of the Gas. Once the gas composition is known before and after the bed of dolomite, the low heating value (LHV) of the gas can be easily calculated at these two points. The result is shown in Figures 11 and 12 for several values of τ and GR, respectively. The LHV always decreases a little, by 1.8 to 3.5 MJ m-3(STP) under the conditions of the present work. The decrease of LHV is mainly due to the decrease of the CH4 content in the flue gas. This decrease of LHV increases with both τ and GR (Figures 11 and 12, respectively). Tar Content in the Gas. The tar content in the flue gas is one of the key issues in biomass gasification. Tar content always decreases in the bed of dolomite, as it was foreseen, by 3-35 g m-3(STP) (in the present setup), as Figures 13 and 14 show. The tar content decrease increases with space time (Figure 13), as it is easy to understand, and decreases with GR (Figure 14) because on increasing GR from 0.7 to 1.6, the tar content in the raw gas (which corresponds with the dolomite bed inlet) decreases from aproximately 35 to 4 g m-3(STP).9,10 So, the high decrease in tar content at low values of GR is due to the fact that for such GR values the tar content at the dolomite bed inlet is quite high.

Pe´ rez et al.

Figure 11. Decrease of the low heating value of the gas in the bed of dolomite at different space times (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Figure 12. Decrease of the low heating value in the gas in the bed of dolomite for different values of GR in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/ O2 ) 2 mol/mol).

From knowledge of the tar content at the bed inlet and its decrease in the bed, tar conversions (Xtar) were calculated. Tar conversion increases with space time, as Figure 15 indicates, and decreases with GR, as it is shown in Figure 16. Note how (i) tar conversions higher than 96% are very difficult to reach and even never reached under the experimental conditions used in this work, (ii) tar conversions of 90-95% are obtained for space times of 0.06-0.15 kg h m3(T2,c, wet), and (iii) as it was observed in previous figures, the particle diameter of the dolomite (among the two used here) clearly affects the tar conversion values. By comparison of these results, with the same units, with similar results previously obtained by Delgado et al.,3 it is observed how tar conversions of Delgado’s work are higher than in the present conversions. It is attributed to the fact that Delgado et al.3 worked at low temperatures in the gasifier bed and operated with steam only. Low temperatures in the gasifier produce



Figure 13. Decrease of the tar content in the gas in the bed of dolomite at different space times (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Figure 15. Tar conversion in the bed of dolomite at different space times (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Figure 14. Decrease of the tar content in the gas in the bed of dolomite for different values of GR in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Figure 16. Tar conversion in the bed of dolomite for different values of GR in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

a high tar yield and/or tar content in the gas. By gasification with pure steam, more phenolic tars are also produced and shown in the paper of Aznar et al.,10 which would be easier to destroy than the ones produced with steam-oxygen mixtures. These two facts explain why tar conversions obtained by Delgado et al.3 were higher than the ones obtained here. For instance, to get 90% tar conversion, Delgado et al.3 needed τ ) 0.015-0.020 kg h m-3(STP), whereas in this work (with dolomite from Coin-Malaga only a little less active, according to Orio et al.,4,5 than the “Norte” one used by Delgado et al.) values needed are (from Figure 15) 0.05-0.10 kg h m-3; i.e., 3-4 times higher values for τ are needed here to get the same tar conversion. It is in fact an important difference regarding tar reactivity. Apparent Kinetic Constant for the Overall Tar Disappearance Reactor. When results on the activity of different solids used by different European institutions in the previously mentioned European project were compared, the following was agreed: (i) use of an apparent and overall kinetic constant for tar disappearance; ii) use of a kinetic order of 1 to calculate such

a kinetic constant. So, under experimental conditions in which a piston flow can be accepted in the bed of dolomite,

-ln(1 - Xtar) ) k2,app,tar τ

(3)

From the Xtar-τ values shown in Figure 15, k2,app,tar was calculated for all experiments. These values for ktar are shown in Figures 17 and 18 for different values of τ, GR, and particle sizes. If averaged values should be selected, to remember them for further comparison purposes, for 840 °C the authors select these. For dp ) 0.84 mm, k2,app,tar ) 45 m3 kg-1 h-1. For dp ) 1.3 mm, k2,app,tar ) 30 m3 kg-1 h-1. Several conclusions that easily emerge are the following. (1) The work has been performed under internal diffusion control, and the dp value then has a clear effect on k2,app,tar. (2) By comparison of k2,app values for tar and methane, Table 4 is obtained. This table clearly shows that the calcined dolomite used here is 9-10 times more active


Pe´ rez et al.

Figure 17. Apparent kinetic constant for tar elimination in the bed of dolomite at different space times (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/O2 ) 2 mol/mol).

Figure 19. Decrease of the steam content in the gas in the bed of dolomite for different (a) space times and (b) GR values in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4 ( 0.8 mm and H2O/O2 ) 2 mol/mol). Figure 18. Apparent kinetic constant for tar elimination in the bed of dolomite for different values of GR in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4-0.8 mm and H2O/ O2 ) 2 mol/mol). Table 4. Averaged Values for the Apparent Kinetic Constants for Methane and Tar Disappearance for the Two Different Particle Diameters of Dolomite k2,app (m3/kg‚h) dp (mm)

for CH4 disappearance

for tar disappearance

0.84 1.30

5 3

45 30

for tar than for CH4 elimination, which is an interesting conclusion. (3) The kinetic constant k2,app,tar decreases on increasing the gas residence time (Figure 17). This fact is because tar is a lump and the “easy-to-destroy” tars react at the reactor inlet (low space times) and the “hard-to-destroy” tars remain at the exit bed zone (high space times). Other Properties of the Gas. Steam Content in the Flue Gas. The steam content in the flue gas always decreases when the gas passes the bed of dolomite, as Figure 19 shows. This decrease is of 3-8 vol % (wet basis) under the conditions used in this work. It is easy

to understand this decrease of the steam content: most of reactions in the bed of dolomite (CO shift, steam reforming, steam cracking, ...) are steam-consuming. The decrease in the steam content in the flue gas increases with the time of residence of the gas in the bed (Figure 19a) and decreases when the GR used in the gasifier increases (Figure 19b). Thus, it appears clear that not all steam in the gasification process has to be consumed in the same gasifier by the steam gasification itself. Some steam must be present in the raw gas (at the gasifier exit) because it is needed as reactant (to be consumed) in the downstream bed of dolomite. Gas Yield Variation. Some (not all) reactions in the bed of dolomite, such as the steam reforming of tars, produce an increase in the number of moles and of gas flow and gas yield. The gas yield, measured as m3(STP) flue gas produced/kg biomass daf, increases somewhat by the bed of dolomite, as Figure 20 shows. The increases of gas yield by the bed of dolomite is 0.150.40 m3(STP) (dry basis)/kg biomass daf. This increase (in the gas yield) increases in turn with the space time (Figure 20a) and decreases with the GR value used in the gasifier (Figure 20b).



values are higher than the ones needed in the same process except gasifying with pure steam (without O2 added). These results would indicate that, in the bed of dolomite, reactions that are very important are

CO + H2O ) CO2 + H2 tar + H2O ) H2 + ... tar + CO2 ) H2 + ... and the ones that are not as important are

CH4 + H2O ) H2 + CO + ... CH4 + CO2 ) H2 + CO + ... The particle diameter of the dolomite (in the interval 0.63-1.6 mm) is a parameter that has to be taken into account. For these particle sizes, the internal diffusion controls the overall process. Finally, the calcined dolomite used here is 9-10 times more active for tar elimination than for CH4 elimination. CH4 conversions (eliminations) higher than 30 vol % have never been reached. Overall, all these data indicate that the calcined dolomite easily and significatively cleans and upgrades the flue gas from a biomass gasifier with steam-oxygen mixtures. Glossary Ctar dp GHSV GR Figure 20. Increase of the gas yield in the gas in the bed of dolomite at different (a) space times and (b) GR values in the gasifier (T2,c) 840 °C; 0, silica sand with dp ) +0.4 ( 0.8 mm and H2O/O2 ) 2 mol/mol).

Conclusions In biomass gasification with steam-oxygen mixtures carried out in an atmospheric and bubbling fluidized bed gasifier at small pilot plant scale, a calcined dolomite placed downstream from the gasifier clearly cleans and upgrades the raw flue gas. By use of a dolomite from Coin-Malaga (Spain), which does not contain iron oxide, at 840 °C, the gas composition of the flue gas is modified in the following ways. (1) H2 content increases by 16-23 vol %, dry basis. (2) CO content decreases by 15-22 vol %, dry basis. (3) CO2 content slightly varies by -2 (decrease) to +6 (increase) vol %, dry basis. (4) CH4 content decreases by 0.8-2.0 vol %, dry basis. (5) steam content decreases by 3-8 vol %, wet basis. The variation of some main properties of the flue gas are the following. (1) LHV decreases by 1.8-3.5 MJ/m3(STP). (2) Gas yield increases by 0.15-0.40 m3(STP) (dry basis)/kg biomass daf. (3) Tar content decreases by 3-35 g tar m-3(STP). Tar conversions higher than 96% have never been reached in this work. Tar conversion of 90-95% has been obtained with space times of 0.05-0.15 kg h-1 m-3 (T2,c, wet). These

k2,app,CH4

k2,app,tar

k′ LHV T2,c umf uo XCH4 Xtar η τ

φ

tar concentration, mg/dry m3(STP). particle diameter of the calcined dolomite, mm. gas hourly space velocity, m3(STP) m-3 h-1. gasification ratio, defined as (kg of (H2O + O2) fed to the gasifier/h)/(kg biomass daf fed/h), dimensionless. apparent kinetic constant for the overall disapperance of methane, defined by eq 1, m3 (T2,c, wet) (kg calcined dolomite)-1 h-1. apparent kinetic constant for the overall disapperance of tar defined by eq 3, m3 (T2,c, wet) (kg calcined dolomite)-1 h-1. same as definition for k2,app,tar except for another particle size of the dolomite. low heating value of the gas, MJ/m3(STP). temperature at the center of the bed of dolomite, °C. minimum fluidization velocity, cm/s. superficial gas velocity at inlet of the bed of dolomite, cm/s. methane conversion, dimensionless. tar conversion, dimensionless. effectiveness factor, dimensionless. space time of the gas in the bed of dolomite, defined as kg calcined dolomite‚h (m3 flue gas)-1 (T2,c, wet). Thiele modulus, dimensionless.

Acknowledgment. The present work was carried out under Contracts JOU2-CT93-0399 and JOR3-CT950053 of the JOULE 2 and 3 programs of the European Commission (DG-XII) to whom the authors thank for financial support. PRODOMASA from Coin-Malaga (Spain) is also acknowledged for sending the samples of the dolomite used here. EF970046M

Hot Gas Cleaning and Upgrading with a Calcined Dolomite Located

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