Performance of Different Dolomites on Hot Raw Gas Cleaning from

3800

Ind. Eng. Chem. Res. 1997, 36, 3800-3808

Performance of Different Dolomites on Hot Raw Gas Cleaning from Biomass Gasification with Air Alberto Orı´o, Jose´ Corella,* and Ian Narva´ ez Department of Chemical Engineering, University “Complutense” of Madrid, 28040 Madrid, Spain

Calcined dolomites (CaO-MgO) from four different quarries have been tested for the upgrading of the hot raw gas from a fluidized bed gasifier of biomass with air. These calcined dolomites have big macropores (900-4000 Å) and low (3.8-12 m2/g) BET surface areas. They have been tested in a fixed bed of 6 cm i.d. downstream from the air-blown biomass gasifier. The change in gas composition (contents in H2, CO, CO2, CH4, ...), tar content, gas heating value, etc., has been studied at different temperatures (780-920 °C) as well as space-times for the gas in the bed (0.03-0.10 kg‚h/m3) and the type of dolomite. Increasing the equivalence ratio used in the gasifier and decreasing the H/C ratio of the gas increases the refractoriness of the tars to be eliminated by the calcined dolomite. Activation energies (100 ( 20 kJ/mol) and preexponential factors for the overall tar elimination reaction have been calculated for the different dolomites under realistic conditions. The activity of the dolomite for tar elimination can increase by 20% on increasing its pore diameter or its Fe2O3 content. Comparison of results with similar ones obtained in biomass gasification with steam is also presented. Introduction Calcined dolomites (CaO-MgO) and related materials have proved their usefulness for the upgrading of the raw gas from biomass gasifiers. Besides the pioneering work of Peters et al. (Yeboah et al., 1980; Ellig et al., 1985; Boronson et al., 1989), during the last 10 years research has been made on this subject, at least, in the University of Nancy in France (Donnot et al., 1988; Magne et al., 1990), KTH (Sjo¨stro¨m et al., 1988; Taralas et al., 1991; Vassilatos et al., 1992) and TPS AB (Alde´n et al., 1988; Rensfelt and Ekstro¨m, 1989) in Sweden, VTT Energy in Finland (Simell and Bredenberg, 1990; Simell et al., 1992, 1993), and Universities of Zaragoza and “Complutense” of Madrid in Spain (Aznar et al., 1989; Corella et al., 1988; Orı´o et al., 1997; Delgado et al., 1996, 1997). A detailed review of the work carried out to date on this area has been made recently by Delgado et al. (1996, 1997) and will not be repeated here. Corella et al. (Orı´o et al., 1997) have lately studied the influence of the pore structure and chemical composition of the dolomite on its tar elimination capacity. Some physical properties of dolomites were related to their activity for tar elimination or gas upgrading in biomass gasification with air. But some aspects about the variation in the gas composition produced by the bed of calcined dolomite were not shown in detail, and these authors continued studying such areas. A more detailed, advanced, and full analysis on the variation of the raw gas composition by a bed of calcined dolomite downstream from the gasifier is now here presented. The reacting network for the overall tar elimination over a calcined dolomite is not exactly known yet, but at least it includes reactions of steam and dry (CO2) reforming and of steam and thermal cracking and hydrocracking of tars. For this reason, the activity of the calcined dolomite as a catalyst for tar elimination will depend on its physicochemical properties and on the composition of the reacting gas atmosphere. The content of steam, H2, and CO2 in the flue gas and the tar composition will affect the overall kinetics of its * Author to whom correspondence should be addressed. Fax: + 34-1-394 4164. S0888-5885(96)00810-X CCC: $14.00

elimination. But, simultaneously, both flue gas and tar compositions depend on how they were generated in the upstream gasifier (Narva´ez et al., 1996). The gasifier design and its operating conditions, like temperatures, equivalence ratio (ER), and gasifying agent used, affect very much the composition of the gas and tar there generated (Narva´ez et al., 1996). On the other hand, gasifying with air does not produce the same tar and gas compositions as gasifying with steam or with steam-oxygen mixtures. This paper will focus on biomass gasification with air. A study similar to this one which focused on biomass gasification with pure steam has appeared recently (Delgado et al., 1996, 1997). Installation Used The dolomites are located in a fixed bed of 6.0 cm i.d. which has a previous bed of an “inert” material (silica stones of 5-10 mm, previously calcined) for preheating purposes as Figure 1 shows. The bed of dolomite is situated downstream from an air-blown biomass gasifier. The gasifier is a bubbling fluidized bed of 6 cm i.d. continuously fed at 0.6-1.2 kg of biomass/h near the bed bottom. A full description of the gasifier and the product distribution (including gas composition and tar content) at the gasifier exit, under different operating conditions, has been given in a previous paper (Narva´ez et al., 1996). The gas composition and tar content at the inlet of the bed of dolomite are thus always well-known. Once all the raw gas generated in the biomass gasifier passes through a hot metallic filter at 500-600 °C, it goes across the bed of calcined dolomite. The bed was externally heated by an oven. To avoid its erosion if it is fluidized (Delgado et al., 1991), the calcined dolomite was used in most experiments as a fixed bed, which was not isothermal. So, their axial and longitudinal temperature profiles were carefully measured in each experiment with two mobile thermocouples located in the bed axis and in the wall (inside). Two examples for the axial temperature profile are shown in Figure 2, indicating temperature variations in the bed. The temperature of reference for each run, T2,c,av, was the © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3801

Figure 1. Scheme of the bed of dolomite used.

arithmetic mean between the maximum and minimum temperatures in the bed axis. Typical experimental conditions used in this work are given in Table 1. Samples of gas and condensates were taken before and after the bed of dolomite, in points shown in Figure 1. This sampling (with further analyses) was made several times in each run at different times-on-stream. No deactivation of the dolomite was observed in these tests of 5-10 h under stationary state. So, no further reference will be made about time-on-stream of the dolomite. Life and/or deactivation of the dolomite, under another gas atmosphere, was studied in a previous work (Delgado et al., 1996). The tar analyses method used here was also previously described (Narva´ez et al., 1996). Types of Dolomites Tested Chemical Composition. Dolomites from four different quarries and companies have been proven. The averaged chemical compositions of these dolomites are given in Table 2. The main difference upon comparison of their chemical compositions is in Fe2O3 content. Malaga and Sevilla dolomites do not contain a significant amount of Fe2O3, and they have a white color. Dolomite from Norte contains about 0.12 wt % Fe2O3 and has a light brown color. Chilches dolomite has a high content of iron (0.77 wt %) and, correspondly, has a dark brown-gray color. Physical Characterization. Pore Structure. Since raw dolomites are heterogeneous minerals, their correct sampling for further analysis is essential. Three samples of about 1 kg were taken for each dolomite. Once crushed and sieved to -2.0 + 1.0 mm, such samples were calcined in an oven near 0% CO2 for 1 h at 900 °C. The temperature was measured in the surrounding air and not on the dolomite bed. The (fixed) bed in the oven could have had some temperature gradients which could have produced larger differences among the

Figure 2. Temperature profiles in the axis of the bed of dolomite for (a) one experiment (run no. 39) with “relatively low” temperature gradient and (b) one experiment (run no. 30) with the highest temperature gradient.

dolomite particles. Once calcined, they were kept in closed vessels to avoid their hydration and carbonation with the steam and CO2 present in the air, respectively. A few grams from each of the three samples of each dolomite were taken for N2 adsorption analysis (in an ASAP 2000 apparatus) and for mercury porosimetry (in a 9320 pore-sizer apparatus). The remaining solid was used to test its chemical activity for fresh tar elimination. Sevilla and Malaga dolomites have the same pore size distribution (average pore diameter around 900 Å), but Chilches and Norte have still bigger macropores (pore diameters between 2000 and 4000 Å). Comparing pore size distributions between fresh calcined dolomites and ones used after few hours in a biomass gasification experiment, similar distributions but with pores 10% smaller in diameter are found for the used dolomite, indicating that some coke has been deposited on the pore walls, decreasing its diameter. The pore area distribution is also important for characterization of these solids. Again the Malaga and Sevilla dolomites are very similar. Macropores around 900 Å give the most important contribution to the pore area, but pores about 20-40 Å are also present in these solids; the micro- and “meso” porosities (up to 500-600 Å) have some importance: they contributed to about 50% of the overall pore area. Chilches and Norte dolomites are somewhat different from the other two. They have big macropores with two different sizes (diameters). Pores of 15-50 Å also exist,

3802 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 Table 1. Main Experimental Conditions and Results from the Bed of Dolomite Downstream of the Gasifier run number solid in bed W2 (calcined) ER2 T2,c dp2 H2,0 umf u2,0 u2,s τ2 τ′2 SV2′ (H2O/C)2

g °C mm cm cm/s cm/s cm/s s kg/(m3, wet/h) h-1 mol/mol

Main Experimental Conditions 3 6 12 Norte dol Norte dol Chilches dol 295 318 401 0 0 0 760 794 923 -2.0 +1.0 -1.0 + 0.8 -1.6 + 1.0 18 15 20 45 20 35 44 43 57 48 44 63 0.39 0.34 0.33 0.063 0.072 0.066 9200 10 400 10 800 0.32 0.39 0.22

13 Sevilla dol 348 0 844 -2.0 + 1.6 20 55 51 58 0.37 0.063 9700 0.23

23 Malaga dol 300 0 810 -2.0 + 1.6 15 55 51 62 0.27 0.052 13 500 0.62

Results gas composition at the exit (dry gas basis) H2 vol % CO vol % CO2 vol % CH4 vol % C2H4 vol % N2 vol % H2O (wet gas)a vol % (wet basis) Ctar,2 mg/Nm3 LHV2 MJ/Nm3 Ytar,2 g/kg of daf fuel Ygas,2 Nm3/kg of daf fuel Xtar %

14.0 13.7 14.5 3.3 1.3 53.2 6.6 1400 5.2 3.1 2.19 77.8

run number solid in bed W2 (calcined) ER2 T2,c dp2 H2,0 umf u2,0 u2,s τ2 τ′2 SV2′ (H2O/C)2

Main Experimental Conditions 25 28 Chilches dol Norte dol 300 300 0 0 814 850 -1.6 + 1.0 -2.0 + 1.6 15 14.5 35 55 51 44 64 54 0.26 0.30 0.052 0.061 13 700 12 100 0.51 0.69

g °C mm cm cm/s cm/s cm/s s kg/(m3, wet/h) h-1 mol/mol

11.9 14.9 15.5 2.8 1.7 53.2 10.9 1870 5.2 4.4 2.34 84.4

15.1 16.2 15.1 3.5 1.5 48.6 5.4 230 5.9 5.5 2.41 96.8

14.0 17.2 15.0 3.3 1.1 49.4 6.0 1200 5.6 28.8 2.40 91.4

19.0 13.7 14.3 3.1 1.4 48.5 12.6 160 5.7 5.4 3.38 92.0

36 Chilches dol 300 0 830 -2.0 + 1.6 15 60 49 53 0.29 0.058 12 200 0.74

38 Chilches dol 300 0 855 -1.6 + 1.0 15 35 52 67 0.25 0.049 14 300 0.49

41 Malaga dol 287 0 805 -1.6 + 1.0 14 35 57 64 0.23 0.047 15 600 0.34

13.0 15.0 13.0 4.0 1.2 53.8 17.8 2000 5.5 38.3 1.91 91.3

17.5 17.0 18.0 3.6 2.0 41.9 13.0 2650 6.5 59.4 2.24 87.4

17.9 17.5 15.5 5.4 2.2 41.5 7.2 6100 7.4 131.5 2.14 71.4

Results gas composition at the exit (dry gas basis) H2 vol % CO vol % CO2 vol % CH4 vol % C2H4 vol % N2 vol % H2O (wet gas)a vol % (wet basis) Ctar,2 mg/Nm3 LHV2 MJ/Nm3 Ytar,2 g/kg of daf fuel Ygas,2 Nm3/kg of daf fuel Xtar % a

19.3 14.1 18.5 4.5 2.1 41.5 12.1 560 6.8 14.3 2.55 94.4

15.0 19.0 15.7 3.3 0.7 46.3 18.8 250 5.7 9.4 3.75 91.1

From mass balances.

and micro- and “meso” porosities contributed about 50% of the total pore area. Nitrogen adsorption and mercury porosimetry tests were performed on each of the three samples for each dolomite. The main results from them are summarized in Table 3. Some important conclusions are as follows: (1) There can be a difference of about 50% in the results between one sample and another. This is due to the fact that these solids are minerals, with some heterogeneity, and the “true” calcination temperature can be different for each part of the sample. (2) In these dolomites, the macroporosity and the big pores of 800-3000 Å are the most important ones. (3) The microporosity (below 17 Å) is almost nonexisting.

(4). There is no major difference among the four dolomites apart from Chilches which has a slightly lower BET surface area (3.4-3.9 m2/g) instead of the 11-12 m2/g found in the Sevilla and Malaga ones. The Norte one has an intermediate value. (5) A low surface area is associated with large pores: the Chilches dolomite has the biggest mean pore diameter (∼3000 Å) instead of 800-1600 Å for the other samples. Effect of the Bed of Dolomite on the Product Distribution and Gas Quality Gas Yield. The gas yield increases when the gas passes through the bed of dolomite, as is shown in

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3803

Figure 3. Gas yield at the inlet and at the exit of the bed of dolomite for several types of dolomites and equivalence ratios in the gasifier.

Figure 4. Tar yield before and after the bed of dolomite for several biomass moistures and equivalence ratios in the gasifier.

Table 2. Chemical Characterization (wt %) of the Dolomites Used

CaO MgO Fe2O3 CO2 Al2O3 MnO Na2O K2O SiO2

Norte (Bueras, Cantabria)

Chilches (Pen˜a Negra, Castello´n)

Malaga (Coı´n)

Sevilla (Gilena)

32.2 18.7 0.12 45.5 0.06 0.09 0.01 0.01 3.3

29.7-31.3 17.5-19.0 0.74-0.80 47.4 1.19 0.04 0.05 0.24 3.2

30.6 21.2 0.01 47.3 0.40 n.d. n.d. n.d. n.d.

30.5 21.5 0.01 47.2 0.60 n.d. n.d. n.d. n.d.

Figure 3 for different equivalence ratios (ER) used in the gasifier. The yield after the biomass gasifier, indicated as a straight line in Figure 3, is increased by about 20% in the bed of dolomite, at all equivalence ratios used. This increase is clearly due to the conversion of the big molecules of tar into several smaller ones (H2, CO, CO2, CH4, ...) which produces an increase in the gas flow. The type of dolomite has no clear or significant influence on this variable, as can be seen in Figure 3. Some possible minor differences among dolomites used fall between the error interval. Tar Yield. The bed of dolomite decreases the tar content in the flue gas, as Figure 4 shows. This tarcontent decrease depends on the equivalence ratio (ER) in the gasifier (which in turn generates different tar yields (Narva´ez et al., 1996)) and on the biomass moisture (wt % of H2O in the biomass feed). This variable also affects the tar yield and, by modifying the steam content in the flue gas, the kinetics of the overall

Figure 5. Hydrogen content in the flue gas before and after the bed of dolomite.

tar elimination. Data shown in Figure 4 can be better understood with two examples: Gasifying with ER ) 0.30, the tar yield decreases in the bed of dolomite from 33 to 7 (g/kg of daf biomass) or from 17.5 to 2 (g/kg of daf biomass) when the moisture of the biomass feed is 15 or 22 wt %, respectively. Effect on Gas Composition H2 Content. The H2 content in the flue gas always increases in the bed of dolomite, as Figures 5 and 6 clearly show for all types of dolomites tested. On average, gasifying with ER ) 0.30, the H2 content in the gas, 6.2-9.8 vol % after the gasifier used in this work, increases to 12.2-19.0 vol % after this bed of dolomite. This increase, about 100%, is due to steamreforming and steam-cracking reactions of the tar and

Table 3. Averaged Parameters for the Pore Structure of the Four Types of the Calcined Dolomite (Calcination Conditions: 1 h at 900 °C) Norte N2 adsorption

BET surface area micropore area area 17-3000 Å pore volume 17-3000 Å micropore volume average micropore diameter

m2/g m2/g m2/g cm3/g cm3/g Å

Hg porosimetry

pore area total pore volume average pore diameter (4V/A) bulk density apparent (skeletal) density

m2/g cm3/g Å g/cm3 g/cm3

6.6 0.46 7.5-8.7 0.024 3.1 × 10-4 95 9.9 0.41 1630 1.35 2.98

Chilches 3.8 0.75 3.4-3.9 0.009 3.7 × 10-4 74 5.4 0.40 2940 1.38 3.03

Ma´laga

Sevilla

12 0.91 18-20 0.070 4.9 × 10-4 144

10.8 0.82 18-20 0.069 3.5 × 10-4 154

16.1 0.34 850 1.52 3.16

17.7 0.34 760 1.51 3.09

3804 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997

Figure 6. Hydrogen content in the gas before and after the bed of dolomite at different equivalence ratios.

Figure 7. Effect of the H/C ratio in the flue gas on the increase (in the bed of dolomite) of its H2 content (T2,c ) 850 °C, ER ) 0.30, τ2 ) 0.50 s).

light hydrocarbons present in the raw gas, both producing hydrogen (Zhang and Baerns, 1991), and also to the CO shift reaction (CO + H2O / CO2 + H2). Since steam is a reactant in most of the H2-producing reactions existing in this system, high H2O contents would facilitate the production of H2. Narva´ez et al. (1996) have found that the H2O/C and/or H/C ratios in the flue gas greatly affect the product distribution from the gasifier. The effect of this H/C ratio in the flue gas has been checked here for the bed of dolomite, and the result is shown in Figure 7. From this figure there is no doubt that the higher the H/C ratio in the flue gas is, the more H2 is produced in the bed of dolomite. CO Content. The CO content in the gas does not vary much by the bed of dolomite, as Figure 8 shows. At ER ) 0.30, the averaged CO value of 16.2% increases only to 15.8%. Sometimes, the CO content after the bed of dolomite is higher than that at its inlet. These results are due to at least two simultaneous and competing reactions: steam reforming which generates CO and a water-shift reaction which consumes CO. The net result is that CO may increase or decrease only a little in the bed of dolomite. CO2 Content. The variation of the CO2 content in the flue gas by the bed of dolomite is very small, as Figure 9 indicates. There are again two, at least, competing or simultaneous reactions: CO2 reforming which consumes CO2 and a shift reaction which generates CO2. The net result is the one shown in Figure 9. On average there is only a small increase of the CO2 content.

Figure 8. CO content in the flue gas before and after the bed of dolomite for gas obtained under different equivalence ratios.

Figure 9. Variation of the CO2 content in the gas in the bed of dolomite.

In conclusion, both CO and CO2 contents do not vary much. Their variation is minimal (only 5%). In each run the CO and CO2 contents show also a contrary behavior: if the CO content increases a little, the CO2 decreases. CH4 Content. The methane content also varies a little by the bed of dolomite (Figure 10). The net and averaged effect is quite a small decrease (from 3.7 to 3.4 vol % when ER ) 0.30). This result is again due to two simultaneous types of reactions: steam and thermal cracking of tars, which can generate CH4, and CH4 steam and CO2 reforming, which eliminates some CH4 from the flue gas. The net effect is shown in Figure 10. This result explains why some authors reported an increase in the CH4 content while others observed a decrease in this content. Heating Value. As a result of the net changes in the gas components, the heating value of the gas increases a little (14% by average at ER ) 0.30) by the bed of dolomite, as Figure 11 shows. This increase of the LHV is mainly due to the fact that most tars (not taken into account in the LHV of the gas at the bed inlet) are converted into H2 and CH4 (Figures 6 and 10), thus increasing the heating value of the gas at the bed exit. From Figure 11 it can be deduced that the type of dolomite does not have a significant effect on the increase in the LHV of the gas. Possible small differences among dolomites are probably masked by the experimental error.

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3805

Figure 10. Methane in the flue gas before and after the bed of dolomite.

Figure 12. Effect of the space-time on the conversion (elimination) over dolomites of tars produced in biomass gasification with air (this work) and with steam (values from Delgado et al., 1997).

its activity also increases. This fact is an important index for future process developments. Refractoriness of the Tars Obtained by Gasifying with Air vs the Ones Obtained with Steam

Figure 11. Variation of the heating value of the gas produced by the bed of dolomite for several experimental conditions.

Effect of Contact Time The flow rate along the bed of dolomite increases by about 20% due to tar cracking and reforming reactions. The superficial gas velocity thus increases something along the bed. For this reason, averaged along the bed height, values of the superficial gas velocity (u2) and of the flow rate (Q2) will be used here. For further comparisons with results reported by others, it is absolutely necessary to indicate if the gas flow is dry or wet and if the rate is calculated at normal (0 °C, 1 atm) or at reactor (T2,c) conditions (Corella et al., 1996a). A pseudo space-time (τ′2) defined as W2/Q2 will be used here as an index or reference for the contact time of the gas in the bed of dolomite. Both parameters, W2 and Q2, have been varied in this work to change τ′2. Tar conversions (Xtar) obtained with three different dolomites and at two levels of temperature are shown in Figure 12 at different space-times. From this figure it is deduced that to get Xtar > 90%, τ2′ > 0.053-0.087 (kg‚h/m3) is needed, depending on the dolomite and temperature used. As a reference, note that τ2′ ) 0.060 kg‚h/m3 is equivalent to a space velocity of 3350 h-1(n.c.). A careful observation of the results in Figure 12 indicates that not all of the dolomites have the same tar elimination activity. The ranking of dolomites, according to their activity at T > 820 °C, in the gasification plant used here, is Chilches > Norte > Ma´laga, which is also the order in Fe2O3 content in dolomites. As the iron content in the dolomite increases,

Delgado et al. (1997) give a Xtar-τ2′ relationship for tars produced in biomass gasification with steam. Biomass and the “Norte” dolomite used were the same in both works. The experimental facility was not the same (the feeding was at the top of the gasifier in the work of Delgado et al., instead of at the bottom, in this work) but both setups or installations were similar and had the same scale (PDUs or small pilot plants both based on bubbling fluidized beds). Delgado et al. preferred to use kg‚h/Nm3 as units for τ2′ instead of kg‚h/m3, but both units can be translated. Both Xtar-τ2′ results are plotted together in Figure 12. This figure is very important for the authors of this paper. It clearly shows how tars produced in steam gasification are much easier to convert, eliminate, destroy, crack, and/or reform than tars obtained by gasifying with air. A space-time 3 times higher is needed to destroy tars from air gasification compared to the one needed to destroy tars produced in steam gasification. Besides, high (97-99%) Xtar values are “easily” reached for tars from steam gasification (Figure 12) though it is difficult to get Xtar > 95% for tars from gasification with air. Two facts can explain the foregoing findings: (i) Tars from steam gasification contain more phenolic groups and C-O-C bonds (because of the reactant steam) than tars from gasification with air. Such phenolic and C-O-C containing tars would be easier to destroy over dolomites than the more aromatic tars obtained from gasification with air. (ii) In biomass gasification with steam, the steam content in the raw gas is of about 50 vol % instead of the 15-20 vol % content when gasifying with air. If steam reforming would be one of the most important mechanisms in the overall reacting network of tar elimination, a high steam content (steam is a main reactant) would produce a high rate of tar removal, which means a high tar conversion. Our results would suggest thus that steam reforming of tars plays an important role in the overall mechanism of tar removal. Depending on how the gasifier operates, it will generate different tars. For instance, the equivalence ratio in the gasifier influences the refractoriness of the tars

3806 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997

Figure 13. Effect of the equivalence ratio (ER) used in the gasifier on the refractoriness of the tars there produced to be converted/ destroyed in the bed of dolomite (T2,c ) 850 ( 50 °C; H/C ) 1.62.1; τ2 ) 0.60 ( 0.2 s; dp ) 2.0-1.0 mm).

Figure 15. Arrhenius graph for the Norte and Chilches calcined dolomites.

Figure 14. Effect of the temperature in the bed of dolomite on the tar conversion when T1,c ) 800 ( 10 °C, ER ) 0.30 ( 0.02 and τ′2 ) 0.060 ( 0.005 kg‚h/m3.

produced to be further destroyed, as Figure 13 clearly demonstrates. Under the same conditions in the bed of dolomite, as ER is increased (in the gasifier), the downstream tar conversion decreases, indicating such tars are more difficult to convert/destroy. Figure 13 indicates that harder tars are produced at higher values of ER (although simultaneously a lesser tar yield is also obtained, according to Narva´ez et al., 1996). Effect of the Temperature of the Bed of Dolomite Since most experiments were made with the dolomite in fixed bed, important longitudinal profiles of temperature were obtained. So, an averaged temperature (T2,c,av) had to be considered as a reference. Using this reference temperature, the tar conversion in the bed of dolomite under the same experimental conditions (T1,c ) 800 °C, ER ) 0.30, and τ2′ ) 0.060 kg‚h/m3) is shown in Figure 14. Data shown in this figure clearly indicates the effect of temperature on tar conversion. As a reference, and under the experimental conditions mentioned, to get Xtar ) 90%, T2,c,av ) 820 °C is needed, and to get Xtar ) 95%, T2,c,av ) 866 °C. With the simple kinetic model of Corella and coworkers (Corella et al., 1996a,b), an apparent kinetic constant (kapp) for tar elimination can easily be calculated supposing a constant and first-order (basis which could be modified in the future) overall tar elimination

reaction. Since the bed of dolomite worked under experimental conditions in which a piston flow model can be accepted, an easy mass balance in the bed of dolomite provides the following equation:

kapp )

-ln(1 - Xtar) τ′2

(1)

which allows the calculation of kapp for the dolomite (k2,app). k2,app values so calculated are shown in Figures 15 and 16 according to the Arrhenius law for the four dolomites tested. In these figures the error theory has been carefully taken into account. Each run, test, or experiment is given by a rectangle whose basis is the measured temperature interval in the bed of dolomite and whose ordinate comes from the interval in Xtar values found from the several samples of condensate taken in each experiment or run (which, in turn, also generates an interval for the k2,app values). When the bed (of dolomite) was smoothly fluidized, the temperature gradient is correspondingly smaller and the rectangle shown in Figures 15 and 16 then becomes smaller. Data shown in Figures 15 and 16 seem to present a broad interval of variation, implying a big experimental error. For some people having no experience in working in that process at this scale these figures could not even have sense or meaning. In order to help in their acceptance and to avoid misunderstandings, note that (i) it is as though personal errors are not big in this case. Ten years ago this group already was obtaining similar data on fresh tar elimination over dolomites, and (ii) “points” in Figures 15 and 16 correspond to a careful analysis together with an advanced data acquisition method.

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3807 Table 4. Apparent Activation Energies and Preexponential Factors for Tar Elimination over Four Calcined Dolomites dolomite

Eapp (kJ/mol)

k2,app,0 × 10-6 (m3, wet, T2,c,av/kg‚h)

Norte

100 ( 20

1.51 ( 0.80

Chilches

100 ( 20

1.45 ( 0.76

Ma´laga

100 ( 20

1.30 ( 0.54

Sevilla

100 ( 20

1.24 ( 0.77

increasing activity

or type of dolomite, in the interval used here, of course, does not affect much its activity for the tar elimination reaction. Maybe the tested dolomites did not have enough difference between themselves to show a different chemical activity. Only a minor or second-order effect by the type of dolomite can be noticed. It can come both from its Fe2O3 content (Table 2) or by its pore diameter (Table 3). On increasing these values, we found the k2,app,0 value increases somewhat (Table 4), but this small increase of k2,app,0 can be masked by the nonisothermicity of the fixed bed used. Acknowledgment

Figure 16. Arrhenius graph for the Ma´laga and Sevilla calcined dolomites.

This work has been made thanks to the financial support of the EU, DGXII, mainly through the project of the Agroindustry Programme Contract No. AIR2CT93-1436 but also, the latest part, under Contract No. JOR3-CT95-0053. We also express our gratitude to Dr. Sagrario Mendioroz for her pore structure analysis tests and to J. Talamantes of “Cales de la Plana, S. A.”, to H. Varona of “Dolomitas del Norte S. A.”, to J. Macias of “Prodomasa” in Coı´n (Ma´laga), and to “Minera del Santo Angel S. L.” de Gilena (Sevilla) for the provision of samples of their dolomites. Nomenclature

Figure 17. Comparison of the activity of the four dolomites with the Arrhenius graph.

With a careful handling of data shown in Figures 15 and 16, and using the averaged (in the bed) temperatures, a line can be drawn in each figure. Such ln k2,app1/T2,c,av relationships are shown in Figure 17 for the four dolomites. From it, the values for the apparent activation energy (Eapp) and preexponential factor (k2,app,0) obtained are shown in Table 4. Given the fact that the Eapp values are near the same for the four dolomites, the preexponential factor is thus a good index of the activity of the dolomite. Results shown in Figure 17 or in Table 4 indicate that the activity (for tar elimination) of all dolomites is similar but not the same. The difference in k2,app,0 values shown in Table 4 is 20%, the same order of magnitude of the experimental error (by the nonisothermicity, mainly). The possible difference in activity between dolomites has thus an order of magnitude similar to the possible experimental error. So, the origin

Ctar,1, Ctar,2 ) tar concentration in the gas at the exit of the gasifier and of the bed of dolomite, respectively, mg/m3(n.c.) dp1, dp2 ) particle diameter of the solid used in the gasifier as fluidizing material and of the dolomite used in the second bed, respectively, mm Eapp ) apparent activation energy for the overall tar elimination reaction, kJ/mol ER ) equivalence ratio (air-to-fuel weight ratio used in the gasifier divided by the air-to-fuel weight ratio for stoichiometric combustion), dimensionless ER1, ER2 ) equivalence ratio in the bottom part of the gasifier and in the freeboard (2nd air), respectively, dimensionless H1,0, H2,0 ) height of the bed (fixed, bulk) at t ) 0, in the gasifier and in the second reactor, respectively, cm H/C ) hydrogen-to-carbon ratio (atom g) in the gas throughout the installation, dimensionless (H2O/C)1 ) steam-to-carbon ratio at the inlet of the gasifier, mol/atom g (H2O/C)2 ) steam-to-carbon ratio at the inlet of the second reactor, mol/atom g k2,app, k′2,app ) apparent kinetic constant, m3wet(reactor temperature)/kg‚h and m3(n.c.) dry/kg‚h, respectively k2,app,0 ) preexponential factor of the Arrhenius eq, m3wet(reactor temperature)/kg‚h LHV1, LHV2 ) low heating value of the gas at the exit of the gasifier and of the bed of dolomite, MJ/m3(n.c.), dry basis m3(n.c.) ) cubic meter at normal conditions (1 atm, 0 °C)

3808 Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 O/C ) oxygen-to-carbon ratio (atom g) throughout the installation, dimensionless Q2 ) gas flow rate in the second reactor, averaged between its inlet and exit, m3wet(reactor temperature)/h SV ) space velocity, defined as m3(n.c.)/m3 of catalyst‚h, h-1 SV′ ) space velocity, defined as m3(reactor temperature)/ m3 of catalyst‚h, h-1 T1,c, T1,f ) temperature in the gasifier bed and in its freeboard, respectively, °C T2,c, T2,w ) temperature in the axis and at the wall (inside, 5 cm above the gas inlet to the bed), respectively, of the bed of dolomite, °C u1, u2 ) averaged superficial gas velocity in the gasifier and in the second bed, cm/s u1,o, u1,e ) superficial gas velocity at the inlet and at the exit of the gasifier, cm/s u2,o, u2,e ) the same, for the second bed, cm/s umf ) minimum fluidization velocity (at the temperature and gas composition inside the reactor) of the solid in the bed, cm/s W1, W2 ) weight of silica sand and of calcined dolomite in the gasifier bed and in the second bed, respectively, kg Xtar ) tar conversion, dimensionless Ytar,1, Ytar,2 ) tar yield at the exit of the gasifier and of the second reactor, gtar/kgdaf fuel Ygas,1, Ygas,2 ) gas yield at the exit of the gasifier and of the second reactor, m3(n.c.)/kgdaf fuel Greek Symbols τ1 ) pseudo space-time in the gasifier, defined as H1,0/u1, s τ2 ) pseudo space-time in the bed of dolomite, defined as H2,0/u2, s τ2′ ) the same, kg/m3,wet, T2,c,av/h

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Received for review December 18, 1996 Revised manuscript received April 29, 1997 Accepted May 2, 1997X IE960810C

X Abstract published in Advance ACS Abstracts, July 1, 1997.