Biomass Gasification with Air in a Fluidized Bed - American Chemical

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Ind. Eng. Chem. Res. 1999, 38, 4226-4235

Biomass Gasification with Air in a Fluidized Bed: Effect of the In-Bed Use of Dolomite under Different Operation Conditions Javier Gil,† Miguel A. Caballero,† Juan A. Martıń,† Marıá-Pilar Aznar,*,† and Jose´ Corella‡ Chemical and Environmental Engineering Department, University of Saragossa, 50009 Saragossa, Spain, and Chemical Engineering Department, University “Complutense” of Madrid, 28040 Madrid, Spain

The performance of a biomass gasifier, fluidized-bed type, is improved by in-bed use of calcined dolomite. Tar contents in the raw flue gas below 1 g/mn3 are obtained by using a bed with a percentage between 15 and 30 wt % of dolomite (the rest being silica sand). The work is carried out at small pilot-plant scale (10 kg of biomass/h) with equivalence ratios (ER) between 0.20 and 0.35 and temperatures of 800-840 °C in the gasifier bed. To replace the eroded and elutriated dolomite (from the gasifier bed), an amount of dolomite (0.40-0.63 mm) is continuously fed, mixed with the biomass at 3 wt %. When the results obtained with in-bed dolomite are compared to the ones gained in a gasifier bed without dolomite, change of the following variables is reported: gas composition and its corresponding heating value, gas and char yields, apparent thermal efficiency, and tar contents. Once the usefulness of the in-bed use of dolomite is established, three main operation variables (ER and temperatures of the gasifier bed and freeboard) are studied in the improved gasifier. Carryover of solids from the gasifier also increases when calcined dolomite is used because of its softness. Elutriation rate constants are calculated for several operational parameters. Introduction It is well-known by practitioners in the field under study how biomass gasification in a fluidized bed generates a raw gas which has to be cleaned at least for tars and particulates for most of its advanced applications. The method most known and used for the cleaning up of tars is using a bed of calcined dolomite (OCa‚OMg) downstream from the gasifier. This specific process of hot-gas cleaning has been studied since a long time ago in at least TPS AB and KTH in Sweden,1-4 University of Nancy in France,5 VTT in Finland,6-9 and Universities of Saragossa and Madrid in Spain.10-15 Nevertheless, this method/approach is nowadays not good enough because, for instance, it is very difficult to get tar conversions/eliminations above 90-95% or, what is equivalent, to get tar contents in the flue gas, once cleaned, below 200 mg of tars/mn3.14,15 When the flue gas needs an exhaustive cleanup or polishing (below 200 mg of tars/mn3), another catalytic bed, usually with a nickel-based steam reforming catalyst, has to be added after the bed of dolomite.16 A very clean (below 20 mg of tars/mn3) exit gas can be obtained, then but the so-generated three-step process (gasifier + bed of dolomite + bed of a nickel-based catalyst) can be quite expensive and produce a not economically overall gasification process. Getting a very clean exit gas could also be possible with a not so-complicated (two steps instead of a three steps) process using not only downstream methods or improvements but also upstream ones. If the produced raw gas has a tar content higher than about 2 g/mn3, to * To whom correspondence should be addressed. Fax: + 34976 76 21 42. E-mail: [email protected]. † University of Saragossa. ‡ University “Complutense” of Madrid. Fax: +34-91 394 41 64. E-mail: [email protected].

get tar contents below 200 mg/mn3, a three-step process would be needed.16 Above 2 g/mn3 in the flue gas the nickel-based catalyst deactivates by coke formation and a bed of dolomite has to be used as a guard bed. However, if the gasifier performance is optimized, the raw gas at the gasifier exit could have a tar content below 1-2 g/mn3. With a “so-clean” gas, a nickel-based catalyst could be used downstream from the gasifier without a guard bed, getting thus a two-step (only) process and improving then the economical feasibility of the gasification process. This paper is focused then on the optimization itself of the biomass gasifier. To have an optimized gasifier, several “measures” have to be taken into account in both its design and operation. Some of them include a good feeding device, biomass feeding near the bed bottom, a good gas distribution plate, enough gas residence times in the gasifier bed and in its freeboard, high temperature in the freeboard, high H2O/C and/or H/C ratios,16 ..., and the use of an in-bed dolomite. Being that the other variables are quite well-known, the work presented in this paper is concentrated on the effectiveness of the in-bed use (instead of its use downstream from the gasifier) of dolomite under very different operation parameters. In-bed use of dolomite in biomass gasification is not a new fact again. Walawender et al. at Kansas State University already reported in 198117 some advantages of using dolomite or limestone as a bed additive in gasification with steam. Also Corella et al.18 reported in the 1980s interesting improvements by in-bed use of dolomite in gasification with steam too. More recently, Sta˚hl19 indicates how the commercial plant at Va¨rnamo (Sweden) uses in-bed dolomite, the gasification being in this case with air. Also Narvaéz et al.20 reported some improvements in gasification with air, but it was not until the deep study of Olivares et al.21 when it was

10.1021/ie980802r CCC: $18.00 © 1999 American Chemical Society Published on Web 10/09/1999

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clearly demonstrated how different percentages of dolomite in the bed improve the gasifier performance. Such authors target a 20 wt % of (calcined) dolomite in the bed as their choice for the best content of dolomite in the gasifier bed. They also show the variation of the gas composition in detail, gas yield, heating value of the gas and tar content with and without in-bed use of dolomite. Nevertheless, their work was made in biomass gasification with steam-O2 mixtures. Because their results were very promising, it was decided to continue their study but in biomass gasification with air which is another objective, task, and/or constraint of this paper. In-bed use of dolomite in biomass gasification with air in a fluidized bed thus generates a new approach and problematic. On the one hand, it could prevent the formation of solid agglomerates and subsequent choking of the bed. This aspect is being studied by other authors17,22 and is out of the scope of this paper. On the other hand, in-bed use of dolomite generates a higher carryover of solids from the gasifier bed with correspondingly higher content in particulates in the raw produced gas. This aspect is also studied in this paper. Besides, in-bed dolomite changes the composition of the generated tar which will be shown in a forthcoming paper. This paper is thus concentrated just in first the changes produced on gas composition, gas yield, tar content, etc, ... by the in-bed dolomite under very different operation conditions and second the increase of the carryover of solids from the bed. This study is carried out at pilot-plant scale. Because of its size and operation conditions, it is believed that most of the results presented here can be directly and easily extrapolated to a commercial scale. Installation, Gasifier, Biomass, and Dolomite Used. The installation used is a small pilot plant based on a bubbling fluidized-bed (BFB) gasifier of 15 cm i.d. and 3.2 m height, continuously fed by 5-20 kg of biomass/h near the bed bottom. The gasifier has a quite big (12 kW h) external oven to modify the bed temperature (Tbed) to different values, relatively independent of other operation variables. This gasifier and pilot plant has already been described in detail,23,24 and its performance is well-known under very different operating conditions. The main experimental conditions used in gasification tests designed and made for the purpose studied in this paper are shown in Table 1. Some important parameters are equivalence ratio (ER) between 0.20 and 0.35, biomass moisture between 7 and 15 wt % (average: 10 wt %), biomass flow rate between 7 and 12 kg/h, mean gas residence time in the bed between 1.3 and 1.6 s, freeboard temperature between 600 and 700 °C; u0/umf (of the silica sand) between 4 and 6. Gasifier Bed. Details about the bed itself merit a detailed description: The bed at the start of the gasification test already contains dolomite mixed with silica sand in percentages indicated in Table 1. The fresh (not calcined) dolomite placed initially in the gasifier bed has a particle diameter (dp) of 0.4-1.0 or 0.63-1.0 mm (see Table 1); and it is calcined in-situ (in-bed) before starting the biomass feeding. To get a constant (with time on stream) percentage of in-bed dolomite, some fresh (not calcined) dolomite had to be fed with the biomass. This percentage was calculated as 30 g of (not calcined) dolomite/kg of biomass daf. The dolomite to be fed was well-mixed with the biomass in the hoppers. The size of this dolomite (continuously fed

thus) is 0.40-0.63 mm. When we operate in such a manner, the (calcined) dolomite content in the bed is maintained constant along the time on stream. This content is varied from one test to another one, and it is always between 15 and 30 wt %. Biomass. A standard (for these authors) feedstock was always used: small pine wood chips. Its detailed proximate and ultimate analysis is given in ref 24. Dolomite. Between the lot of types of dolomite tested by Orio et al.14 for this application, the one called Ma´laga has been used in all tests. Its complete characterization can be found in such previous work.14 Tar Sampling, Analysis, and Content. Tar sampling was made just after the three high-efficiency cyclones located at the gasifier exit/top. Because the tar content in the flue gas is one of the major concerns of this paper, a big effort has been made in all aspects concerning tar sampling, analysis, and definition. When this work was carried out, there was not a standard method, worldwide accepted, for tar sampling and analysis and even for its definition (for instance, some people do not consider benzene as a constituent of tar). So, all measures concerning tar are somewhat relative and have to be related to the method followed by the institution who carried out such analyses. In this work two different methods of tar sampling and analysis have been used. Tar contents shown in this paper have been determined by the method indicated previously.16 It is similar to the one used by VTT in Finland.6-9 This tar will here be called tar*. Other tar samples were also taken in all tests by the Swedish SPA method25 and sent by courier just after finishing the gasification test to KTH in Sto¨ckholm to be there analyzed, after desorption, by gas chromatography.25 Tar was thus fully characterized in each gasification test, and such analyses will be shown in a forthcoming paper. When tar* -contents obtained by the first method (the ones reported here) were compared with the KTH ones, it was observed how most of the benzene and a part of toluene were not captured, and thus analyzed, in the first referred tar sampling system. The authors are confident with such a first sampling method, but they also indicate that, on average and after comparison of different tar sampling methods, tar* contents shown in this paper are 30% lesser than the ones obtained with the KTH (SPA) method or when dry (CO2) ice is located in the two first impinger flasks (VTT method) instead the disolvent used at 0 °C temperature in the authors’ method. Because tar* contents given in this paper were obtained always with the same tar sampling and analysis method, they are comparable among themselves and conclusions are valid. Tar* contents reported here should be increased by about a 30% if they are related to the VTT or KTH (SPA) sampling methods, which include or capture all benzene and toluene in the flue gas. Results The effect of the in-bed use of dolomite has been studied at different ER values and temperatures of the gasifier bed and freeboard. Results on gas composition, gas yield, tar content, etc., using in-bed dolomite have been compared with the previous values obtained by Narvaéz et al.20 which were obtained under the same operational parameters and the same tar sampling and analysis methods but without in-bed dolomite, with only a bed of silica sand.

total weight percentage of dolomite initial dp (dolomite) Hbed Tbed Tfreeboard τ0 u0 u0/umf sand u0/umf dolomite

ER biomass feeding rate moisture content (not calc.) dolomite/biomass dp fed dolomite

total weight percentage of dolomite initial dp (dolomite) Hbed Tbed Tfreeboard τ0 u0 u0/umf sand u0/umf dolomite

ER biomass feeding rate moisture content (not calc.) dolomite/biomass dp of fed dolomite

kg wt % mm cm °C °C s cm/s

kg/h %, wet basis g/kg of daf mm

0.24 11.4 10 30 -0.63 + 0.40

AG-1

15.3 28 -1.0 + 0.4 67 822 678 1.40 48 5.0 5.8

AG-2

14.3 24 -1.0 + 0.4 62 845 692 1.13 55 5.7 6.6

AG-3

AG-13 0.24 8.3 10 30 -0.63 + 0.40 15.1 26 -1.0 + 0.4 66 835 680 1.69 39 4.1 4.7

Gasifier 14.6 26 -1.0 + 0.4 64 800 661 1.52 42 4.4 5.1

AG-14a

14.5 31 -1.0 + 0.4 63 837 675 1.37 39 4.1 4.7

AG-15

12.7 29 -1.0 + 0.63 55 808 639 1.53 36 3.8 4.3

14.5 40 -1.0 + 0.4 63 839 672 1.62 39 4.0 4.7

0.24 6.9 10 30 -0.63 + 0.40

FW-1

14.0 21 -1.0 + 0.4 61 816 685 1.30 47 4.9 5.7

0.22 10.5 7 30 -0.63 + 0.40

AG-8

15.7 22 -1.0 + 0.4 69 830 610 2.09 33 3.4 4.0

0.20 9.1 7 30 -0.63 + 0.40

AG-7

0.27 7.6 10 30 -0.63 + 0.40

12.6 28 -1.0 + 0.63 55 798 601 1.49 37 3.9 4.5

0.19 10.0 11 30 -0.63 + 0.40

AG-6

0.24 8.2 10 30 -0.63 + 0.40

12.4 27 -1.0 + 0.4 54 801 642 1.50 36 3.8 4.3

Feeding 0.24 9.2 10 30 -0.63 + 0.40

Gasifier 14.5 15 -1.0 + 0.4 63 817 692 1.13 56 5.8 6.7

0.19 9.8 10 30 -0.63 + 0.40

AG-5

AG-12

15.9 22 -1.0 + 0.4 69 789 700 1.64 42 4.4 5.1

AG-4 0.27 10.7 10 30 -0.63 + 0.40

Feeding 0.21 10.9 10 30 -0.63 + 0.40

0.35 8.3 15 30 -0.63 + 0.40

AG-11

14.5 25 -1.0 + 0.4 63 802 690 1.15 55 5.7 6.6

0.26 10.8 10 30 -0.63 + 0.40

0.29 8.2 10 30 -0.63 + 0.40

AG-10

15.7 23 -1.0 + 0.4 69 820 686 1.28 54 5.6 6.5

gasification test

kg wt % mm cm °C °C s cm/s

kg/h %, wet basis g/kg of daf mm

gasification test

Table 1. Main Experimental Conditions in Gasification Tests with In-Bed Dolomite

16.5 31 -1.0 + 0.4 72 840 610 2.12 34 3.5 4.1

0.26 6.8 10 30 -0.63 + 0.40

FW-2

20.0 20 0.070 (FCC) 70 822 679 1.52 46 4.8 5.5

0.27 8.8 11 50 (FCC) 0.070 (FCC)

AG-9

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Figure 1. H2, CO, and CO2 contents in the produced or raw exit gas, with and without in-bed dolomite, at different equivalence ratios (790 °C < Tbed < 820 °C).

In each test, different gas and tar samples were taken at different times on stream. Under stationary state there were not noticeable variations (with time-onstream) in gas composition and tar content. Results here shown are the averaged values between the ones obtained at different times on stream. Gas Composition. Gas composition at the gasifier exit is shown in Figures 1 and 2 at different equivalence ratios with and without in-bed dolomite. In these figures it is observed how the H2, CO, CO2, CH4, C2Hn, and N2 contents in the raw produced gas clearly vary by the addition of dolomite. They vary in the following ways:

H2 content increases from 6-10 to 12-17 vol % CO content increases from 9-16 to 16-22 vol % CO2 content slightly decreases (it is in the range of 12-15 vol %) CH4- content increases from 2.5-3.5 to 4-5.2 vol % C2Hn- content increases from 1.2-1.8 to 1.5-2.5 vol % N2- content decreases from 52-62 to 42-50 vol % These results already indicate that, besides biomass gasification, the two important reactions

Figure 2. CH4, C2Hn, and N2 contents in the produced or raw exit gas, with and without in-bed dolomite, at different equivalence ratios (790 °C < Tbed < 820 °C).

{ }

H2 O tar + CO 2

f H2 + CO + CH4 + C2Hn + ...

CO + H2O + a CO2 + H2

(1) (2)

take place and that the first one is increased when dolomite is used in the bed. As a result of these changes in the gas composition, the low heating value (LHV) of the produced gas also increases when dolomite is used as an additive, as Figure 3 clearly shows. Gas and Char Yields. Apparent Thermal Efficiency. Gas yield is shown in Figure 4 with and without in-bed dolomite. Gas yield increases because of the reaction given by eq 1. Char yield is given also in Figure 4 at different ER values. Because the small size of the biomass fed and because of the operating conditions, most of the char generated is elutriated out from the bed. Such elutriated char is collected periodically (every 15 min) at the three hoppers located at the bottom of the three high-efficiency cyclones. So, char is calculated in this work as the amount of carbonaceous solid collected at the bottom part of the three cyclones together with the (small) amount of carbonaceous solids in the gasifier bed at the end of the test. The yield to char so calculated decreases on increasing ER and on increasing the superficial gas velocity at the top of the gasifier. Apparent thermal efficiency (gas yield × LHVgas/ LHVbiomass) seems to pass by a maximum in its variation with ER (Figure 4). It is due to the fact that gas yield

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Figure 3. Low heating value of the gas with and without in-bed use of dolomite (790 °C < Tbed < 820 °C).

Figure 5. Tar content, with and without in-bed dolomite, at different ER values (790 °C < Tbed < 820 °C).

Figure 6. Gas composition at the gasifier exit with in-bed dolomite, at different bed temperatures (ER ) 0.24; 20-30 wt % in-bed dolomite).

Figure 4. Gas yield (with and without in-bed dolomite), char yield, and apparent thermal efficiency with in-bed dolomite, at different ER values (790 °C < Tbed < 820 °C).

increases (Figure 4) and LHV decreases (Figure 3) on increasing ER. Tar* Content. The most significant result and/or fact is how the tar* content in the raw gas produced clearly decreases when in-bed dolomite is used, as is shown in Figure 5. Tar* contents of only 1.0 g/mn3 are now obtained at ER ) 0.30. It is not possible without using the in-bed additive. This result is one of the most important results or achievements of this paper/work. Effect of the Temperature of the Bed on the Gasifier Performance with In-Bed Dolomite Once the high usefulness of in-bed dolomite to improve the quality of the produced gas is proven, two

main operation parameters, besides ER, were studied using this bed additive: temperature of the gasifier bed (Tbed) and temperature of its freeboard, measured at its top (Tfreeboard). Tbed was varied, in different tests, between 800 and 840 °C. ER was maintained constant (0.24) in these tests. The percentage of calcined in-bed dolomite was also kept constant (20-30 wt %). Gas Compositions and Gas Yields. Gas composition at different Tbed values is shown in Figure 6. The H2 content was on average 14 vol % (dry basis) and the CO content 19 vol % (dry basis). The low heating value (LHV) of the gas decreases somewhat with Tbed, from 7.2 to 6.6 MJ/mn3. Notice how the LHV here obtained (with in-bed dolomite) is quite higher than the values usually reported in the literature (4-5 MJ/mn3). This is attributed to the in-bed tar elimination reactions which increase the H2, CO, and CH4 contents in the flue gas (Figures 1 and 2). For the just said reasons, gas yield increases with Tbed, as Figure 8 shows. Char yield decreases with Tbed because the rate of the in-bed char elimination reactions (partial oxidation, steam, and dry (CO2) gasification, etc.) increase on increasing Tbed. The apparent thermal efficiency (gas yield × LHVgas/ LHVbiomass) is shown in Figure 7. It is in the range of 70-73%. Tar* Content. The most important fact is again the low tar contents in the exit flue gas which decreases with Tbed. Above 830 °C tar* contents are e1 g/mn3. This low value is found in the literature very seldom. This low tar* content value could be decreased even more (increasing thus the quality of the produced gas) if a higher Tfreeboard were used. It would imply an optimized


Figure 7. Low heating value of the gas and apparent thermal efficiency in the gasifier, with in-bed use of dolomite, at different temperatures of the bed (ER ) 0.24; 20-30 wt % in-bed dolomite).

Figure 9. Gas composition at the gasifier exit with in-bed dolomite, with different temperatures at the exit of the freeboard (ER ) 0.19-0.22; Tbed ) 790-810 °C; 20-30 wt % dolomite).

Figure 10. Low heating value of the gas and apparent thermal efficiency in the gasifier, with in-bed use of dolomite, with different temperatures at the freeboard exit (ER ) 0.19-0.22; Tbed ) 790810 °C; 20-30 wt % dolomite).

low by these authors, but in this pilot plant no higher values of Tfreeboard could be obtained because of the relatively low flow rates of biomass and because no secondary air was used. ER in these tests was 0.190.22, and Tbed was between 790 and 810 °C. The main results are shown in the following figures, which do not need more comments: Gas composition LHV gas yield applied thermal efficiency Figure 8. Gas, char, and tar yields with in-bed use of dolomite at different temperatures of the bed (ER ) 0.24; 20-30 wt % inbed dolomite).

freeboard (higher residence times and temperatures there) which was not the case of the gasifier used in this work (the freeboard was too short because of space constraints of the building in which the plant was located). With an optimized bed and freeboard the authors believe that tar* contents as low as 0.3 g tars/ mn3 could be obtained, but it, of course, remains to be proved. Effect of Tfreeboard on the Gasifier Performance Tfreeboard was studied in the narrow range of 600-700 °C. The maximum temperature used is considered too

Figure 9 Figure 10 Figure 11 Figure 10

Tar* content in the exit gas is shown in Figure 11. Because these tar contents were obtained at relatively low values of Tbed (790-810 °C) and ER (0.19-0.22), the tar* contents are relatively high. Results in Figure 11 indicate that the tar* content under these circumstances were from 8 to 5 g/mn3, with this value decreasing on increasing Tfreeboard. A Commercial and “In-Equilibrium” FCC Catalyst as Another In-Bed Additive Another additive to the gasifier bed was also tested in this work. It was an “in-equilibrium” (once used, waste from oil refineries, with no economic value) FCC catalyst. It was the MZ-7P with 70 µm of averaged diameter. It is made of a zeolite embedded into an alumina matrix. This catalyst had already proved18 its

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than the calcined dolomite, as results (obtained under similar circumstances) shown in Figures 12 and 13 indicate. Such said low apparent activity of the FCC catalyst is mainly due to the following fact: because of its small particle size (70 µm), it was quickly elutriated out from the bed to the cyclones and to the exit pipes (which came close to being absolutely plugged at the end of the gasification test). The residence time of this solid in the gasifier bed is too short then. For this reason, it was decided not to continue working with such FCC catalysts. Elutriation of the Calcined Dolomite from the Gasifier Bed

Figure 11. Gas, char, and tar yields with in-bed use of dolomite with different temperatures at the freeboard exit (ER ) 0.190.22; Tbed ) 790-810 °C; 20-30 wt % dolomite).

Figure 12. Gas composition at the gasifier exit for three different types of beds (ER ) 0.27; Tbed ) 800-820 °C; 20 wt % in-bed dolomite; 5 wt % FCC in the feeding).

usefulness in biomass gasification as a tar cracking catalyst. Notice that the FCC catalyst is an acid solid, while a calcined dolomite (OCa‚OMg) is a basic catalyst. So, tar decomposition reactions are absolutely different with these two solids (cracking and steam reforming, respectively). Operating conditions using the FCC catalyst are indicated in Table 1 (test AG-9). Results using this bed additive are shown in Figures 12 and 13. From results in Figures 12 and 13 it is confirmed that the “inequilibrium” FCC catalyst is active for tar elimination. Under similar operation parameters (Tbed ) 800-820 °C and ER ) 0.27 in both cases), the tar* content in the raw gas is decreased from 20 to 8.5 g/mn3 by the addition of the FCC catalyst. The CO content increases from 15 to 23 vol %; the H2 content increases slightly, and the CO2 content decreases slightly too. All of these results would confirm the fact that in-bed tar decomposition reactions have been increased by cracking and possibly hydrocracking too. The FCC catalyst is thus useful for tar elimination, although its apparent activity is lower

In the above-described gasification tests, it was soon detected/observed how the amount of particulates in the flue gas increased quite a lot when in-bed dolomite was used. Calcined dolomite is very soft, and it suffers a fast process of erosion/abrasion in the bed by the hard silica sand particles. Besides, dolomite is fed (mixed with the biomass) as it is, as a mineral, and not calcined. When it arrives to the gasifier bed at 840 °C, it suffers a sudden calcination, which can break some particles. The small particles generated by the two said phenomena have a terminal velocity of less than the superficial gas velocity in the gasifier bed being thus elutriated out of the gasifier. This increased (by the in-bed dolomite) carryover of particles could plug exit pipes or deactivate a catalyst located downstream from the gasifier. So, besides the positive chemical function of the dolomite as a cheap tar-eliminating catalyst, there appears a negative physical function of the same, increasing the particulate content in the flue exit gas. It was decided thus to know/study the referred hydrodynamic behavior of the in-bed calcined dolomite. The feeding flow rate and particle size of the dolomite, as well as the amount (weight) and particle size of the dolomite located in the gasifier bed before starting the gasification test, are well-known in all tests. At the top of the gasifier there are three high-efficiency cyclones which have proved to collect about 99 wt % of the used dolomite. Each cyclone has, at its bottom, one hopper with two valves, which allows removal of the solid collected in it. Such a collection was made in the three cyclones every 15 min. The solid so collected was analyzed for char and for dolomite. The amount of dolomite elutriated was calculated by chemical analysis with HCl (after calcination to remove the char), taking into account that a part (67 wt %) of the collected and calcined char also reacts (dissolves) with HCl. The elutriation rate, Ed (kg of calcined dolomite elutriated from the bed/h), can be calculated then. Besides, an analysis of the elutriated dolomite was done by sieving to discover its size and its breaking. An example (test AG-12) of the size distribution of the dolomite elutriated from the bed and collected in the cyclones is shown in Figure 14. Elutriation rate (Ed) of the dolomite in one representative gasification test is shown in Figure 15. At the beginning of the gasification test the value of Ed is high because of the high relative amount of dolomite in the bed in such a period of time. Knowing the amount of dolomite fed and the amount being elutriated, an easy mass balance (for dolomite), allows one to discover the dolomite content in the gasifier bed. Such a content is shown in Figure 15 for gasification test AG-8. This mass


Figure 13. Tar content, gas yield, and low heating value of the gas for three different types of beds (ER ) 0.27; Tbed ) 800-820 °C; 20 wt % in-bed dolomite; 5 wt % FCC in the feeding).

Figure 14. Particle size distribution of the dolomite elutriated from the gasifier (test AG-12).

Figure 16. Dolomite elutriation rate vs fraction of dolomite in the bed.

Figure 15. Dolomite elutriation rate and dolomite content in the bed vs time on stream (test AG-8).

balance allows one, in turn, to calculate (to predict, in this case) the amount or percentage of dolomite which has to be fed in a gasification test in this pilot plant to keep constant (with time on stream) the dolomite content in the gasifier bed. Such a parameter was of around 3 wt % of the total fed flow rate or 30 g of dolomite/kg of biomass daf fed. Elutriation rates (of the dolomite and under stationary state) were calculated in all gasification tests. They were made under somewhat different operational parameters (Table 1) to discover their effect on the elutriation rate. Some results of this study are shown in Figures 16 and 17. The Elutriation rate (kg of calcined dolomite/h) is around 200 g/h when the super-

Figure 17. Dolomite elutriation rate vs superficial gas velocity at the gasifier bed exit.

ficial gas velocity at the exit of the gasifier bed (ue) was 1.0-1.2 m/s, and it increases only a little with the percentage of dolomite in the bed (Xd) when it is in the range of 0.20-0.40 (Figure 16). The elutriation rate of dolomite, when its percentage in the bed is in the range of 14-27 wt %, increases with the superficial gas velocity at the gasifier exit from 150 to 500 g/h, as shown in Figure 17. Elutriation rates shown in Figures 15-17 are considered high. They correspond to values of ue from 0.8 to 1.6 m/s. A circulating fluidized-bed biomass gasifier requires the use of higher values of ue with even higher

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values for Ed. Because the breaking of the dolomite and its carryover are intrinsic phenomena which cannot be avoided in a gasification process using a fluidized bed, it will be of key importance first to use downstream high-efficiency devices to eliminate particulates in the flue gas and second to use catalysts which can work with some particulates in the flue gas. Such catalysts should have a monolithic structure thus, and rings, pellets, or spheres would not be accepted. Nevertheless, a downstream bed of dolomite (or a related material) could be used because it would receive (with the flue gas) dust or particulates of the same type. Once used, this dolomite would not offer major problems for its disposal because it is not considered as toxic waste.

tar* ) tar referring to the first tar sampling and analysis methods used in this work and described in ref 20 u0 ) superficial gas velocity at the inlet of the gasifier bed (at the temperature of the gasifier), m/s ue ) superficial gas velocity at the exit of the gasifier bed (at the temperature of the gasifier), m/s umf ) minimum fluidization gas velocity (gasifier bed conditions), m/s utop ) superficial gas velocity at the top of the freeboard, m/s Xd ) weight fraction of dolomite in the gasifier bed Ygas ) gas yield in the gasification process, mn3 of dry gas/ kg of biomass-fed daf Wt ) total amount of solids in the gasifier bed, kg Greek Symbols

Conclusions Overall, in-bed use of dolomite is considered to be positive or beneficial in biomass gasification in a fluidized bed. Although its use significantly increases the carryover of solids elutriated from the bed, which is a negative fact, its use improves quite a lot the quality of the produced gas. With a 15-30 wt % of (calcined) dolomite in the bed, tar contents below 1 g/mn3 can be obtained. This in-bed tar elimination causes an increase in the H2 content (using ER in the range of 0.19-to 0.35) from 6-10 to 12-17 vol %, the CO content from 9-16 to 16-22 vol %, and the CH4 content from 2.5-3.5 to 4.0-5.2 vol %. The change in the gas composition causes the LHV of the produced gas (tar components excluded) to increase by an average of 1.5 MJ/mn3 when dolomite is used as a bed additive. The effect of three main operation parameters was studied with in-bed dolomite: ER, Tbed, and Tfreeboard. The tar content in the exit gas decreases when ER, Tbed, and Tfreeboard increase in a trend similar to that of the gasification process without in-bed dolomite (bed of silica sand only). The “maximum allowable” (in each gasification process) values for Tbed, and Tfreeboard should be thus used, besides the addition of dolomite, to get an optimized raw gas composition. Acknowledgment This work has been carried out under the JOULE III Program of the EU DG-XII (Project No. JOR3-CT950053). The authors thank the European Commission for its financial support. The work also was made under the Spanish DGES financed Project No. PB96-0743. Nomenclature daf ) dry, ash free dp ) particle diameter, by sieving, mm Ed ) elutriation rate of the dolomite from the bed, kg of calcined dolomite/h ER ) equivalence ratio, defined as the air-to-fuel ratio used in the gasifier divided by the air-to-fuel ratio for the stoichiometric combustion Hbed ) height of the gasifier bed, bulk fixed-bed conditions, cm LHV ) lowest heating value of the produced gas, MJ/mn3, dry basis mn3 ) cubic meter, normal conditions (0 °C, 1 atm or 101 kPa) Tbed, Tfreeboard ) temperature in the gasifier bed and in its freeboard, respectively, °C

τ0 ) space time for the gas in the gasifier bed, defined as Hbed/u0, s

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Received for review December 30, 1998 Revised manuscript received August 26, 1999 Accepted August 31, 1999 IE980802R

Biomass Gasification with Air in a Fluidized Bed - American Chemical

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