Biomass Gasification with Steam in Fluidized Bed - American

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Ind. Eng. Chem. Res. 1997, 36, 1535-1543

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Biomass Gasification with Steam in Fluidized Bed: Effectiveness of CaO, MgO, and CaO-MgO for Hot Raw Gas Cleaning Jesu ´ s Delgado and Marı´a P. Aznar Department of Chemical and Environmental Engineering, University of Saragossa, 50009 Saragossa, Spain

Jose´ Corella* Department of Chemical Engineering, University “Complutense” of Madrid, 28040 Madrid, Spain

The upgrading of the raw hot gas from a bubbling fluidized bed biomass gasifier is studied using cheap calcined minerals or rocks downstream from the gasifier. Biomass gasification is made with steam (not air) at 750-780 °C and about 0.5-1.0 kg of biomass/h. Calcined solids used are dolomite (MgO-CaO), pure calcite (CaO), and pure magnesite (MgO). Variables studied have been temperature of the secondary bed (780-910 °C), time of contact or space-time of the gas (0.08-0.32 kg‚h/m3n), and particle diameter (1-4 mm) and type of mineral. Their effects on tar conversion, tar amount in the exit gas, product distribution, and gas composition are presented. Using a macrokinetic model for the tar disappearance network, the activities of the stones are expressed by their apparent kinetic constant. Apparent energies of activation for tar elimination (42-47 kJ/mol) and preexponential and effectiveness factors are given for all tested solids of which the most active is the calcined dolomite. Introduction Biomass is a renewable energy not very competitive today but with some future in certain areas or scenarios. For this reason, both the United States and the European Union are supporting some research and development activities, projects, and processes about it. Among the different thermochemical processes for biomass, gasification is one of them. Its main interest is to obtain a useful gas. For commercial applications, the gasifying agent can be air, steam, or steam-oxygen mixtures. Most gasification activities and published data in the world are on gasification with air which produces a gas with a low heating value (4-7 MJ/m3n) and an 8-14 vol % H2 content only. Gasification with steam (with or without O2 added) is another process which produces a medium heating (10-16 MJ/m3n) value gas with a 30-60 vol % H2 content. In all gasification processes there is always formation of dangerous, carcinogenic, and undesirable tars which have to be eliminated from the raw gas for most applications. The tar content in the raw gas (at the gasifier exit) varies very much from one process to another one, from about 1 to 180 g/m3n. This concentration has to be lowered to only 50-500 mg/m3n, depending on applications (Rensfelt and Ekstro¨m, 1989), or even to a total tar elimination for downstream fuel cells. To clean the hot raw gas, a good solution is to use catalysts, which, in turn, has different alternatives. The first one is the downstream use of steam reforming nickel-based catalysts (Aznar et al., 1993, 1996), but these nickel catalysts, and processes based on them, are now beyond the scope of this paper. The second alternative is the use of calcined dolomites (or related materials like calcites or magnesites). These dolomites can be located in (or fed to) the same gasifier bed with good results (Corella et al., 1988a; Karlsson et al., 1995; Narvaez et al., 1996), but calcined dolomites are quite soft and they erode a lot under the gasifier conditions. Good cyclones and filter systems have thus to be used then. Not rejecting this in-bed use of the dolomite for the future, only the use of dolomites in a downstream reactor has been studied here. This secondary reactor can be designed (dimensioned) and S0888-5885(96)00273-4 CCC: $14.00

operated at appropriate conditions for the soft dolomites. These conditions can be different from the ones existing in the gasifier. The work and data here presented concern thus only (i) biomass gasification (in bubbling fluidized bed) with steam and (ii) dolomites (and other related materials) located in a downstream reactor. Among the pioneering works in this field are the papers of Saito et al. (1971), Morita (1978), Yeboah et al. (1980), Wen and Cain (1984), Ellig et al. (1985), van Heek and Mu¨hlen (1985), Baker and Mudge (1985, 1987), Huang and Zhang (1989), and Garcı´a and Hu¨ttinger (1989). Donnot et al. (1985, 1988) and Magne et al. (1990) used two reactors in series: a pyrolyzer followed by a bed of dolomite. They found an activation energy of 46 kJ/mol for tar cracking over a calcined dolomite and 77 kJ/mol over an inert bed of silica sand. Nowadays, most of the known work in Europe on the use of dolomites for hot gas cleaning is being made in Sweden (TPS AB and KTH), in Finland (VTT Energy), in Spain (Universities of Madrid and of Saragossa), and in The Netherlands (BTG B.V.). TPS AB has developed a well-known process using dolomites in a secondary reactor (Alden et al., 1988; Rensfelt and Ekstro¨m, 1989). Their work is, nevertheless, made on gas coming from a gasifier with air or with pure and targeted molecules like naphthalene (synthetic atmospheres), and not with gas coming from a biomass gasifier with steam (objective of this paper). Sjo¨stro¨m and co-workers at KTH are studying the (steam) cracking of pure targeted compounds (like n-heptane) and of tars coming from a pyrolyzer, not from a gasifier (i.e., Taralas et al., 1991; Sjo¨stro¨m et al., 1988). The work being made at VTT Energy (i.e., Simell and Bredenberg, 1990; Leppa¨lahti and Kurkela, 1991; Simell et al., 1992, 1993) is significant for catalytic hot gas cleaning, but also for a gas produced in gasifiers with air, or for synthetic gas mixtures. Nothing has been published to date and to our knowledge about the specific use of calcined dolomites for the cleaning of hot raw gas produced in fluidized bed gasifiers of biomass with steam, which is just the objective of this paper. © 1997 American Chemical Society

1536 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997

Steam gasification could technically be used for production of synthesis gas or of an H2-rich gas. When using the synthesis gas so produced, the methane concentration has to be well below 4%. Therefore, the problem may be not only tar decomposition but also methane breakdown (Liinanki et al., 1982). A steam reforming Ni-based catalyst is then needed, but this catalyst will deactivate if the throughput of tar coming with the gas to the catalytic bed is high (Aznar et al., 1993). So, using dolomites (in bed or downstream) is needed to reduce the tar amount in the raw gas before the nickel catalyst. Equipment The gasification tests were made in a bench or very small pilot scale facility with two reactors connected in series as previously described (Corella et al., 1991; Aznar et al., 1993). The first reactor is the biomass gasifier, of 6 cm i.d. in the bed zone. A silica sand (from Arija, Spain) of -0.200 + 0.125 mm was used as stationary bed or fluidizing solid in all experiments. The gas superficial velocity (u1,0) was between 12 and 18 cm/ s. The gasifier was continuously fed by a screw at the top of the vessel at 6-12 g of biomass/min. The biomass fell thus to the upper part of the gasifier bed, where there was some degree of segregation (Aznar et al., 1989a). It produced a higher tar yield or tar amount in the flue gas than when feeding of biomass was made near the bed bottom (Corella et al., 1988b). The temperature at the freeboard was always measured, and it was relatively low (500-700 °C). These two facts meant that the tar content in the raw gas at the exit of this gasifier (Ctar,o, Table 1, or tar loaded to the dolomite bed) was quite high in this work. After the gasifier there was a hot metallic filter (at 500-550 °C) and then the second vessel with the calcined stone. The temperature of the second bed was measured at three points, two along the central axis and one near the wall. The temperature (T2) here used was the temperature in the center of the bed, at 5 cm from the distributor plate. Two vessels were used for this secondary bed. They had 4.0 and 6.0 cm i.d. The superficial gas velocity at the inlet (u2,0) was able to be varied, controlled, and designed before each experiment. On the other hand, the umf of the calcined stone, for the temperatures and gas compositions existing in this secondary reactor, was calculated and known beforehand (Delgado et al., 1991). The u2,0/umf ratio was then always between 0.4 and 1.3 (this last value corresponds to a very smooth fluidization). It avoided erosion of the soft calcined dolomite and, on the other hand, allowed use of the plug-flow model for the gas. Some experimental conditions for the secondary bed are shown in Table 1. The product distribution, gas composition, and tar yield at the gasifier exit for the same installation, but with an inert bed (instead dolomite) downstream from the gasifier, were previously determined and known in detail (Corella et al., 1991). They are the basis for calculating the “catalytic conversion” of tars over calcined minerals, which is presented in this paper. Tar Sampling and Analysis Regarding the tar and CO amounts formed, smoking is a gasification process (and not a combustion one) and tobacco companies around the world use standardized methods for tar collection and analysis (in the two gas

Table 1. Experimental Conditions in the Secondary or Catalytic Downstream Bed T2, °C 912 782 840 846 797 807 842 801 795 800 802 800 840 853 803 804 879 847 803 882 801 880 840

τ2,e, dp,mean, u2,s/ kg‚h/ Ctar,o, Ctar, Xtar, run mm umf m3n g/m3n g/m3n % no. 2.05 1.75 1.75 1.75 1.80 1.80 1.50 1.80 2.25 2.75 3.25 1.80 1.50 1.50 1.30 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50

1.3 1.1 1.2 0.9 1.1 1.0 1.3 1.0 0.7 0.5 0.4 0.5 0.6 0.6 1.0 0.8 0.9 0.6 0.7 0.8 0.7 0.9 0.8

0.204 0.197 0.204 0.147 0.145 0.079 0.103 0.128 0.132 0.130 0.122 0.320 0.169 0.205 0.114 0.210 0.195 0.239 0.204 0.207 0.184 0.230 0.239

207.5 182.5 178.2 124.7 53.2 63.8 85.3 48.3 28.5 46.2 60.6 324.6 276.2 47.8 23.0 31.2 37.4 24.5 16.9 45.8 10.8 28.1 16.3

0.7 5.3 1.4 4.0 3.4 16.2 7.3 5.5 4.1 9.4 16.4 2.3 5.5 0.8 2.1 1.0 0.4 0.6 1.4 0.8 1.5 0.5 0.7

99.7 97.1 99.2 96.8 93.6 74.6 91.5 88.6 85.6 79.6 73.0 99.3 98.0 98.3 90.9 96.8 99.0 97.4 91.6 98.3 85.9 98.1 96.0

42 46 47 59 61 62 65 74 75 76 81 90 91 94 96 98 100 93 97 102 99 101 103

calcined stone dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite dolomite magnesite magnesite magnesite calcite calcite calcite

Table 2. Some Properties of the Biomass Used particle diameter, mm ultimate analysis, % dry basis carbon hydrogen oxygen proximate analysis, % total ash moisture lower heating value, MJ/kg daf

-2.0 + 0.63 41-45 6-7 49-53 0.5-1.2 8.5-12.9 18.0-18.4

flows from smoking cigarettes). This standardization has not been agreed on yet by institutions around the world working on biomass gasification. Nowadays each institution uses its own and different tar sampling or analysis methods (Corella, 1995). There is no equivalence yet between the numbers and figures for tar yields or contents given by different authors. Tar is a lump which is even defined in different ways by different institutions. Thus, our numbers for tar yields, tar contents, or tar concentrations could be different if obtained in other research centers but we are confident of our numbers. The condensates (water and tars) of the gas produced were sampled periodically during every run. Every sample was first filtered; thus the nonsoluble fraction remained on the cake. This cake was treated with acetone for tar extraction. The obtained acetone solution was distilled at 80 °C, giving a residue named nonsoluble tars. Liquid chromatography of the nonsoluble tar fraction was obtained for some runs; examples of these results were published in a previous paper (Aznar et al., 1989b). On the other hand, the filtered water was analyzed to obtain the soluble organic compounds. For this purpose, a high-temperature total organic carbon (TOC) analyzer, a Dohrmann DC-90 model, was used. This analysis (TOC) was also used by Sjo¨stro¨m et al. (1988). Vassilatos et al. (1992) demonstrated that naphthalene is the dominant compound of tars (60-80%) when the gas is treated over a hot bed of dolomite. Thus, naphthalene will be the standard compound to calculate the tar content from TOC results. The sum of soluble and nonsoluble tars will give the total tar content of the gas, which will be the one presented in this paper.

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1537 Table 3. Some Physical and Chemical Properties of the Used Stones catalyst origin composition, % CaO MgO CO2 SiO2 Fe2O3 Al2O3 Fpacked bed, g/cm3 before calcining calcined Fparticle, g/cm3 before calcining calcined BET surface, m2/g before calcining calcined

Dolomite Norte Dolomitas del Norte (Cantabria, Spain)

Calcite Morata Cementos Portland Morata de Jalo´n (Zaragoza, Spain)

Magnesite Navarra Magnesitas de Navarra (Navarra, Spain)

30.9 20.9 45.4 1.7 0.5 0.6

53.0 0.6 41.9 2.7 0.8 1.0

0.7 47.1 52.0

1.57 0.90

1.51 0.89

1.69 0.83

2.76 1.35

2.91 1.75

3.31 1.58

0.2 11.2

0.1 5.4

0.1 9.5

Biomass, Dolomite, and Stones Used As biomass feedstock, pine (Pinus pinaster) sawdust has been used. Its properties are shown in Table 2. The origin, composition, and some physical properties of the stones, rocks, or solids used here are indicated in Table 3. The detailed pore structure characterization of these solids is given by Delgado et al. (1995) and Orio et al. (1997). We indicate here that these calcined solids have only big macropores in parallel, with a mean pore diameter of around 900 Å (90 nm), and not much microand mesoporosity. The effect of these physical properties on the tar elimination activity has been analyzed by Orio et al. (1997). Some Previous Concepts for the Secondary Bed of Dolomite Time of Contact. To compare dolomites and results obtained in this area by different authors is very difficult nowadays. Each one uses a different term to identify the activity and the time of contact of the gas with the dolomite. The concept of space-time or space-velocity seems to be the best one for comparison purposes. The space-time for the secondary bed can be defined in many different ways, as Corella et al. (1996a,b) have recently indicated. In this paper the following definitions were used:

τ2,e ) τ2,e′ )

kg of dolomite (calcined) in the bed m3n exit, dry gas (without steam)/h

kg of dolomite (calcined) in the bed 3

m exit, wet gas (reactor temperature)/h

(1)

(2)

Kinetic Index Regarding the Activity of the Solid for Its Tar Elimination Capacity. Working in the secondary, downstream or catalytic reactor in experimental conditions in which plug flow can be accepted, and using a simple first order kinetic equation for the overall tar elimination reaction, the following equation is obtained:

-ln(1 - Xtar) ) kapp′ητ2,e ) kappτ2,e

(3)

kapp, calculated from this equation, can be used to compare the solids regarding their tar elimination activity. Although tar is a complex lump, a simple first order equation seems to fit the data, as will be shown in this paper.

Most European institutions working in this subject have recently agreed in considering first order for the overall tar elimination over dolomites (Corella, 1995). A first order reaction was also used for thermal cracking of tars from steam gasification of biomass (Corella et al., 1991) and for conversion of tars from pyrolysis of biomass (Donnot et al., 1985; Stiles and Kandiyoti, 1989; Boronson et al., 1989). Index for Estimating the Quality of the Gas Produced in the Gasifier. In order to study the secondary or catalytic bed, the raw flue gas entering it has to be well-known. Some gasification variables such as temperature, space-time, or superficial gas velocity in the gasifier bed were studied previously (Corella et al., 1991). Nevertheless, deviations in the quality of the raw gas from the gasifier were observed due, for instance, to bad functioning of the steam generator, gas distributor plate plugging, differences of temperature and space-time in the freeboard, etc. A rigorous evaluation of all these variables is too complex. A reasonable goal is the ability to predict the gas product by using a simplified index, in a way analogous to the one used by Dellinger (1990) and Rei et al. (1987). The experimental data obtained by Corella et al. (1991) were employed. The most important gasification parameters were evaluated by means of a correlation matrix using the coefficients of Pearson, Spearman, and Kendall (examples of this procedure could be consulted in Walpole and Myers (1992) or Wadsworth and Harrison (1990)). Two parameters become significant with this method: the steam/biomass ratio (FRdaf) and the char yield in the gasifier (Ychar). The steam/biomass ratio has some similitude with the equivalence ratio (ER) used in gasification with air (Narva´ez et al., 1996). Taking into account the kinetic study of Gonza´lez (1988) and by a multiple regression with respect to tar concentration, the quality of the gasification (in this gasifier) was well described by the following empirical index, which we call the “gasification index”: 1.1 FRdaf

GI ) 10

1.36 Ychar

(4)

The tar concentration in the raw gas, the ratio between tar and gas yields, and the gas composition are associated with this index, as Figures 1 and 2, respectively show. The product distribution (from our gasifier) can be estimated by this index. Of course, the relationships shown in Figures 1 and 2 have to be calculated for every process, biomass, gasifier, or installation.

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Figure 1. Dependence of the tar and gas yield at gasifier exit with the gasification index, GI (experimental data obtained by Corella et al. (1991)).

Figure 3. Effect of particle diameter of dolomite on tar conversion and on tar concentration in the gas produced (for the calcined dolomite in the second bed, T1 ) 780 °C, T2 ) 800 °C, and τ2 ) 0.12 kg‚h/m3n).

Results and Analysis

Figure 2. Gas composition (the CO closes the balance of dry gas) at gasifier exit vs gasification index, GI (experimental data obtained by Corella et al. (1991)).

A high GI value means a good biomass gasification. A low GI value indicates a high production of tar or high tar loaded to the second reactor. Cluster analysis (Bisquerra, 1989) of the experiments, using the product distribution and gas composition, gives the same ranking (for the quality of the experiments) as the one obtained using GI. By using EDA methods (“Experimental Data Analysis”; Freixa et al., 1992), for example box-and-whiskers plots, anomalous tests can also be discarded. The quality of the raw gas, fed to the secondary reactor, can be thus estimated by GI. Experimental Conditions. Two set of experiments were made for this paper, at two different gasification temperatures: 750 ( 7 °C (GI ) 0.23-0.71) and 780 ( 5 °C (GI ) 0.71-5.00). At T1 ) 750 °C the averaged GI value is 0.36, equivalent to a tar concentration at the inlet of the secondary or catalytic reactor of 180 g/m3n. At 780 °C GI had a mean value of 1.67, equivalent to a tar concentration at the inlet of the secondary reactor of 40 g/m3n. The operation variables studied in detail have been the following: particle diameter (dp), between 1.0 and 4.0 mm; temperature of the secondary bed (T2), between 782 and 912 °C; space-time (τ2,e), between 0.08 and 0.32 kg‚h/m3n; type of alkaline-earth oxide, i.e., calcined dolomite, magnesite, and calcite.

The tar conversion in the bed (Xtar) and the tar concentration in the gas at the exit of the second bed (Ctar) here shown are the ones obtained when experimental data are extrapolated to zero time-on-stream (t ) 0). The life and deactivation of the calcined stone has been shown in a previous paper (Delgado et al., 1996). Effect of the Particle Diameter of the Dolomite. The effect of the particle diameter has been studied at 794 ( 9 °C (T2) with sizes between 1.0 and 4.0 mm (dp,mean ) 1.30-3.25 mm). Other experimental conditions were T1 ) 782 ( 4 °C, FRdaf ) 1.1 ( 0.2, and GI ) 1.75 ( 0.75. Conditions for the secondary bed were Ctar,o ) 42 ( 19 g/m3n, τ2,e ) 0.12 ( 0.01 kg‚h/m3n, and u2,e/umf ) 0.4-1.0. The smallest particle size used was -1.6 + 1.0 mm. Smaller particle sizes (smaller umf) were not used to avoid their fluidization, which would produce some erosion of the soft calcined dolomite. a. Effect on Xtar and Ctar. The tar conversion and the tar content in the gas at the exit of the bed depend on the particle size used for the dolomite as Figure 3 shows. A clear effect of dp can be seen, indicating some internal diffusion control. The kapp values obtained from the Xtar-τ2,e data and eq 3 were 21.0, 16.9, 14.7, 12.3, and 10.7 m3n/(kg‚h) for dp,mean of 1.3, 1.8, 2.3, 2.8, and 3.3 mm, respectively. With these values and the iterative method indicated by Satterfield (Satterfield, 1970), the values of the Thiele modulus, φs, at 800 °C for the different dp were calculated, and this relationship appears:

φs ) dp(mm)0.804

(5)

With this equation (valid for T2 ) 800 °C and for the calcined dolomite here used), the effectiveness factors were calculated, Figure 4. To get η > 0.99 (for tars generated at T1 ) 780 °C and destroyed at T2 ) 800 °C), a dp lower than 0.3 mm should be used. In working with bigger particles sizes, as the ones here used, internal diffusion is important and will have an influence on all kinetic values obtained, including the apparent energy of activation.

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1539

Figure 4. Effectiveness factor vs particle diameter of calcined dolomite in the second bed (for T1 ) 780 °C, T2 ) 800 °C, and τ2 ) 0.12 kg‚h/m3n).

Figure 6. Tar conversion at t ) 0 over our calcined dolomite for different temperatures of the secondary bed (T2), for T1 ) 750 and 780 °C.

Figure 5. Composition of the gas obtained vs particle diameter, with calcined dolomite in the second bed (for T1 ) 780 °C, T2 ) 800 °C, and τ2 ) 0.12 kg‚h/m3n).

Figure 7. Tar concentration in the exit gas, at t ) 0 over our calcined dolomite for different temperatures of the secondary bed (T2), for T1 ) 750 and 780 °C.

b. Effect on Gas Composition. In a complex reacting network, the internal diffusion affects all reactions in the network and, therefore, the final product distribution. The gas composition at the exit of the second bed will depend thus on dp of dolomite. This is confirmed in Figure 5. The main components in the flue gas are H2, CO, CO2, CH4, and light hydrocarbons C2H2, C2H4, and C2H6 (here lumped together and given as C2). Decreasing dp, the tar conversion and the direct Shift reaction (CO + H2O T CO2 + H2) are favored, increasing thus the H2 concentration at the bed exit. Effect of the Temperature of the Bed of Dolomite. Two sets of experiments were made under the following conditions: (a) T1 ) 752 ( 3 °C (FRdaf ) 0.84 ( 0.06, GI ) 0.33 ( 0.03) and T2 ) 782-912 °C (Ctar,o ) 193 ( 15 g/m3n, τ2,e ) 0.20 ( 0,01 kg‚h/ m3n, dp,mean ) 2.0 ( 0.2 mm); (b) T1 ) 780 ( 2 °C (FRdaf ) 1.21 ( 0.03; GI ) 1.67 ( 0.37) and T2 ) 804-879 °C (Ctar,o ) 40 ( 9 g/m3n, τ2,e ) 0.20 ( 0.01 kg‚h/m3n; dp,mean ) 1.5 mm). Xtar, Ctar, and gas composition were thus measured at different T2 values. a. Effect of T2 on Xtar and Ctar. Tar conversion (elimination) at t ) 0, in the bed of dolomite, Xtar, is shown in Figure 6 at different T2 values. Xtar is the highest at the lowest T1 because the tars so produced can be easily destroyed. To get Xtar ) 99%, with τ2,e ) 0.20 kg‚h/m3n, T2 has to be 840 °C (when T1 ) 750 °C)

or 880 °C (when T1 ) 780 °C). To get such a high conversion at a lower T2, higher space-times should have to be used. The tar content in the flue gas at the exit of the secondary bed is shown in Figure 7. The tar concentration is lower when T1 ) 780 °C than when T1 ) 750 °C because the loading of tars to this secondary reactor is only 40 g/m3n at 780 °C instead of 180 g/m3n at 750 °C. Therefore, 780 °C for T1 instead of 750 °C should be better, although tars are more stables, as Figure 6 indicates. Notice also how, according to Figure 7, after a bed of dolomite, it is very difficult to get tar contents in the gas less than 500 mg/m3n. kapp values were then calculated, with eq 3 at several T2 values (from data of figure 6). The kapp values obtained are shown in Figure 8 according to the Arrhenius law. If all the data (750 and 780 °C for T1) are included, the values obtained for the apparent activation energy and preexponential factor are kapp,o ) 1960 m3n/ (kg‚h) and Eapp ) 42 kJ/mol. Some comments can be made: (i) Eapp here obtained is similar to the value of 46 kJ/ mol obtained by Donnot et al. (1988) and Magne et al. (1990), both obtained under diffusion control. (ii) Etrue would be near twice this value. That is, Etrue ≈ 84 kJ/mol. (iii) The apparent kinetic constants for tar destruction, kapp, are somewhat higher when T1 ) 750 °C than

1540 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997

Figure 8. Arrhenius representation for apparent kinetic constant for tar elimination over our calcined dolomite for tars produced at two different gasification temperatures.

Figure 9. Tar conversion (at t ) 0) over our calcined dolomite vs τ2′ for the two sets of experiments.

when T1 ) 780 °C, Figure 8. This clearly indicates that tars obtained at lower gasification temperatures are easier to destroy than the ones obtained at higher temperatures (higher aromaticity, more stable or refractory tars). On increasing T1 the proportion of hard tars in the lump tar is bigger. (iv) By the same reason, the activation energy, Eapp or Etrue, increases somewhat (Figure 8) with the gasification temperature (T1). b. Effect on the Gas Composition. An unimportant effect of T2 is detected between the intervals here used. Effect of the Time of Contact (Given as τ2,e) in the Catalytic Bed. The effect of τ2,e has been studied in two different sets of T1 and T2 at the following experimental conditions: (a) T1 ) 754 ( 3 °C (FRdaf ) 1.1 ( 0.3, GI ) 0.54 ( 0.17) and T2 ) 843 ( 3 °C (Ctar,o ) 132 ( 47 g/m3n, dp,mean ) 1.6 ( 0.2 mm), τ2,e ) 0.100.20 kg‚h/m3n; (b) T1 ) 779 ( 4 °C (FRdaf ) 1.1 ( 0.2, GI ) 1.46 ( 0.54) and T2 ) 802 ( 5 °C (Ctar,o ) 58 ( 17 g/m3n, dp,mean ) 1.6 ( 0.2 mm), τ2,e ) 0.08-0.32 kg‚h/ m3n. a. Effect of τ2,e on Xtar and Ctar. The Xtar-τ2,e and Xtar-τ2,e′ curves for the two sets of experiments are shown in Figures 9 and 10, respectively. A 99% tar destruction can be obtained when (i) τ2,e ) 0.20 kg‚h/ m3n, for T1 ) 750 °C and T2 ) 840 °C, or (ii) τ2,e ) 0.30 kg‚h/m3n, for T1 ) 780 °C and T2 ) 800 °C. To get a

Figure 10. Tar conversion (at t ) 0) over our calcined dolomite vs τ2′ for the two sets of experiments.

Figure 11. Fitting tar conversion (over calcined dolomite in the second bed) to eq 3.

tar content in the exit gas of 500 mg/m3n, a space-time of 0.30 kg‚h/m3n is required in both sets of experiments. The Xtar-τ2,e curves fit eq 3 well as Figure 11 shows. This figure confirms the previous assumption of a first order reaction for the kinetics of tar conversion over a calcined dolomite. The slope of the lines in Figure 11 is the apparent kinetic constant for tar destruction (kapp) at T2. The kapp value increases when T1 decreases (such fresh tar is more reactive) or T2 increases. b. Effect of τ2,e on Gas Composition. The effect of τ2,e on the gas composition at the exit of the bed of dolomite is similar on the two set of experiments. The set at higher gasification temperature is shown in Figure 12. There is only a small effect. The use of higher τ2,e produces more H2 and CO2, because of tar conversion reactions and the direct Shift reaction (similar trends have been seen when the particle diameter is decreased). Effect of the Type of the Calcined Stone/Rock. Not only calcined dolomites (CaO-MgO) can be active for tar elimination, but also calcined pure calcites (CaO) or magnesites (MgO). These three solids have been tested for tar elimination at T2 ) 800-880 °C. The gasifier operated at T1 ) 780 ( 4 °C, FRdaf ) 1.2 ( 0.2, and GI ) 5 ( 3. The secondary bed conditions were Ctar,o ) 29 ( 19 g/m3n, τ2,e ) 0.22 ( 0.04 kg‚h/m3n, and dp,mean ) 1.5 mm. a. Effect on Xtar. The tar conversions (at t ) 0) for the three calcined solids at different temperatures are

Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1541

Figure 12. Composition of gas produced vs space-time (τ2), in the bed of the calcined dolomite, for T1 ) 780 and °C, T2 ) 800 °C.

Figure 13. Tar conversion (at t ) 0) over different calcined stones (alkaline-earth oxides), for different temperatures of the second bed (T1 ) 780 °C).

shown in Figure 13. From it, the order of activity found for tar elimination is calcined dolomite (CaO-MgO) > calcined magnesite (MgO) > calcined calcite (CaO). This trend is similar to the one obtained by Taralas et al. (1991) for steam reforming of n-heptane. This fact could indicate that the mixture of oxides would create some degree of distortion in the array of the Ca or Mg atoms, generating thus more active catalytic sites. Of course, the low BET surface of our calcined calcite could also influence the low tar conversion with this solid. The values of kapp for the three solids calculated with eq 3 are represented in Figure 14 according to the Arrhenius law. The apparent activation energy for the three solids seems to be the same: 42 kJ/mol (Etrue ≈ 84 kJ/mol). The preexponential factors, indicating the differences between the activity of the calcined stones, are 1960, 1460, and 1280 m3n/(kg‚h) for calcined dolomite, magnesite, and calcite, respectively. b. Effect on Gas Yield and Composition. Gas yield varies with temperature (T2) with the same trend for the three solids. It increases significantly above 840 °C but there are differences for the three solids studied, as Figure 15 indicates. The gas yield obtained follows this order: calcined dolomite > calcined magnesite > calcined calcite. The gas compositions at the bed exit for the calcined magnesite and calcite are similar to the one obtained with dolomite.

Figure 14. Arrhenius representation for apparent kinetic constant obtained for different calcined stones (T1 ) 780 °C, τ2,e ) 0.22 kg‚h/m3n, dp,mean ) 1.5 mm). (Also included are the values obtained when an inert is used in the secondary bed, from Corella et al. (1991).)

Figure 15. Gas yield obtained with different alkaline-earth oxides used, for different temperatures in the second bed (T1 ) 780 °C).

Acknowledgment This work was supported by the EU Agroindustry R&D Programme under Project No. AIR2-CT93-1436 and by the DGICYT Project No. PB-91-0375. The authors thank Dr. I. Narva´ez and A. Orio very much for their help and collaboration, and Dr. K. Sjo¨stro¨m, K. Ho¨rnell, and G. Taralas for their valuable comments. Nomenclature Ctar ) tar concentration in the gas, mg/m3n Ctar,o ) tar concentration at the inlet of the second bed, mg/m3n daf ) dry, ash free (for the biomass) De ) Effective global diffusion coefficient, cm2/s dp ) particle diameter of calcined stone, by sieving, mm dp,mean ) mean particle diameter of calcined stone, mm Eapp ) apparent energy of activation for the tar conversion, kJ/mol FRdaf ) ratio of steam to biomass fed to the gasifier, in weight, dimensionless GI ) gasification index, defined by eq 4 kapp′ ) apparent kinetic constant defined by eq 3, m3n(dry)/ (kg(calcined stone)‚h) m3n ) m3 in “normal” conditions (0 °C, 1 atm) kapp ) apparent kinetic constant defined by eq 3 including η (kapp ) kapp′η), m3n(dry)/kg(calcined stone)‚h) T1, T2 ) temperatures in the gasifier bed and in center of the bed of dolomite (5 cm from the gas distributor), respectively, °C u2,e ) superficial gas velocity at exit of second reactor, cm/s

1542 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 umf ) minimum fluidization velocity (at the temperature and for gas composition existing in the reactor) of the solid in the bed, cm/s W ) weight of the calcined stone in the secondary bed, kg Xtar ) tar conversion, dimensionless (the basis is Ctar,o) Ygas, Ytar ) gas and tar yields, respectively, % daf biomass fed Ychar ) char yield, recovered in the gasifier and in the filter, % daf biomass bed yH2, yH2O ) weight fraction of H2 or H2O, respectively, dimensionless Greek Symbols η ) effectiveness factor, averaged for all reactions in the network given in eq 3 Fpacked bed ) packed bed density of the stone, g/cm3 Fp ) particle density of the stone, g/cm3 τ2,e ) space-time for the secondary bed, defined by eq 1 τ2,e′ ) space-time for the secondary bed, defined by eq 2 φs ) Thiele modulus, defined by (dp/6)(kapp′Fp/De)1/2

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Received for review May 16, 1996 Revised manuscript received January 23, 1997 Accepted January 26, 1997X IE960273W

X Abstract published in Advance ACS Abstracts, March 15, 1997.