Biomass Gasification: Produced Gas Upgrading by In-Bed Use of

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Ind. Eng. Chem. Res. 1997, 36, 5220-5226

Biomass Gasification: Produced Gas Upgrading by In-Bed Use of Dolomite Ana Olivares, Marıá P. Aznar,* Miguel A. Caballero, Javier Gil, Eva France´ s, 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

When some calcined dolomite (OCa‚OMg) is used in the bed of a biomass gasifier of fluidized bed type the raw gas produced is cleaner than when only silica sand is used in it as fluidizing medium. In-bed dolomite changes the product distribution at the gasifier exit because of insitu catalytic reactions promoted by the calcined dolomite. Gasifying with steam-O2 mixtures causes the tar content in the exit gas to decrease from 12 to 2-3 g tar/m3n, the H2 content to increase from 25-28 to 43 vol %, and the CO content to decrease from 45 to 27 vol % when the gas and char yields, heating value of the gas, and other main variables also undergo important changes because of the in-bed dolomite. The experimental work here reported is carried out at small pilot plant scale in a 15 cm i.d. atmospheric and bubbling fluidized bed gasifier fed by 10 kg biomass/h. Dolomite is continuously fed to the gasifier, mixed with the biomass in percentages of 2-3 wt % of the total mass flow fed. A 10 wt % of calcined dolomite in the gasifier bed is enough to significantly improve the product distribution and gas quality. Introduction Biomass is a renewable energy of interest in some scenarios. Its thermochemical gasification produces a gas that can find some end-uses such as electricity production, but such produced gas usually contains tars, particles, ammonia, H2S, etc., and has to be cleaned for most of its applications. This cleaning in biomass gasification usually entails elimination of tars and dust from the raw flue gas. Hot cleaning of this flue gas is recommended (instead of wet cleaning) because it destroys the tars (instead of sending or transferring them to a secondary flow of condensates) and because high biomass-to-electricity efficiencies can be obtained. Two lines or ways of hot gas cleaning are now emerging in biomass gasification: the use of dolomites and steam reforming (nickel-based) catalysts. Under the same experimental conditions, commercial nickel-based catalysts are 8-10 times more active than calcined dolomites (Corella, 1997) but they are also more expensive and can also become deactivated. Deactivation of nickel catalysts in biomass gasification can be due to, i.e., sulfur, dust, and coke. Coke is formed in tar elimination reactions but it can simultaneously be removed from the catalyst surface by steam and dry (CO2) gasification reactions. If the rate of coke formation is higher than the simultaneous coke removal by gasification, a coke buildup occurs with a subsequent deactivation. To avoid this situation, very little coke has to be formed on the catalyst surface and this happens when the tar content in the flue gas is low. We consider this limit to be 2 g of tar/m3n (Narvaéz et al., 1997). So, to use nickelbased catalysts for hot gas cleaning, the flue gas does not to have a high tar content. A dirty gas soon deactivates the nickel catalyst (Aznar et al., 1993). So, both for using nickel catalysts downstream of the gasifier and for most applications of the produced gas, the gas has to be “quite” clean. To get a “quite” (not enough for some applications) clean flue gas the solid most used to date is dolomite. * To whom correspondence should be addressed. Fax: +3476-76 21 42; +34-1-394 41 64. † University “Complutense” of Madrid. S0888-5885(97)00379-5 CCC: $14.00

Calcined dolomites (OCa‚OMg) are cheap solids that have proved their catalytic activity and effectiveness for tar elimination in several laboratories and pilot plants (i.e., Donnot et al. 1988; Sjo¨strom et al. 1988; Rensfelt and Ekstro¨m, 1988; Aznar et al., 1989; Simell et al., 1990, 1992, 1994, 1997; Taralas et al., 1991, 1994; Taralas, 1996; Vassilatos et al., 1992; Delgado et al., 1996, 1997; Orio et al., 1997a,b; etc...). Tar elimination over these calcined dolomites is mostly due to steam and dry (CO2) reforming reactions (Pe´rez et al., 1997; Orio et al., 1997b), although they are commonly and incorrectly referred to as tar cracking reactions. As has just been shown, there is a great deal of literature about the use of these dolomites downstream of biomass gasifiers. To set up and use a secondary bed of dolomite downstream of a biomass gasifier is thus useful for hot raw gas cleaning, but the cost of the overall gasification process clearly increases. This cost is even more increased if a tertiary bed with a nickel catalyst is used for polishing the flue gas (needed for some applications). A three-step (gasifier + guard bed with dolomite + catalytic (Ni) bed) gasification process then appears. It produces a very clean and useful gas (Narvaéz et al., 1996, 1997) but at a high cost. A non uneconomically feasible gasification process can appear then, but such a process can be simplified if one step (the bed of dolomite) is eliminated without producing major problems. This fact can be achieved by a very optimized operation of the gasifier because the tar content in the raw gas at the gasifier exit depends on the operating conditions in the gasifier (Narvaéz et al., 1996). An optimized operation of a fluidized bed biomass gasifier includes in-bed use of dolomite. Using in-bed dolomite instead of downstream of the gasifier could simplify the overall gasification process, making it economically feasible. In-bed use of dolomite is not a new process. Corella et al. (1988) already did it at small pilot plant scale, and in Scandinavia it is often used in the VTT pilot plant (Leppa¨lahti and Kurkela, 1991) and in the Carbona plant at Va¨rna¨mo, Sweden (Karlsson et al., 1995; Ståhl, 1996). Narvaéz et al. (1996) have also described the improvement of the quality of the raw gas when © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5221 Table 1. Operating Conditions of the Gasifier run no. condns

17

29

30

31

time on stream

h

11.2

9.2

8.8

4.0

feeding biomass feed rate throughput biomass moisture biomass dp dolomite (not calc.) feed rate dolomite (not calc.)/biomass d.a.f. dolomite (not calc.) dp steam oxygen (steam + oxygen)/biomass d.a.f. steam/oxygen

kg a.r./h kg/(h m2) wt % mm kg/h g/kg d.a.f. mm kg/h kg/h kg/kg d.a.f. mol/mol

6.6 374 10 -5.0 + 1.0 0.126 21 -1.0 + 0.63 3.6 2.1 0.97 3

7.3 413 10 -5.0 + 1.0 0.126 19 -1.0 + 0.63 4.71 2.79 1.16 3

9.9 560 10 -5.0 + 1.0 0.255 29 -0.63 + 0.4 5.08 3.01 0.91 3

10.0 566 13 -5.0 + 1.0 0.259 30 -0.63 + 0.4 5.08 3.01 0.95 3

silica sand + dolomite 11.8 11.0 -0.63 + 0.4 0.8 7 mm 50 816 601 2.29 1.35 37.2 12.0 3.1 23 1.6

silica sand + dolomite 13.9 7.5 -0.63 + 0.4 6.4 46 -1.0 + 0.63 56 835 671 1.46 0.95 49.0 12.0 4.1 17.0 2.9

silica sand + dolomite 10.9 5.4 -0.63 + 0.4 5.5 51 -1.0 + 0.63 51 835 660 1.46 0.95 46.0 12.0 4.5 17.0 3.2

silica sand + dolomite 14.8 10.5 -0.63 + 0.4 4.3 29 -1.0 + 0.4 59 833 671 1.78 1.10 54 12.0 4.5 17.0 3.1

32

33

gasifier solid total wt in the bed silica sand wt silica sand dp dolomite (calc.) wt dolomite (calc.) percentage dolomite Hbed Tbed Tfreeboard τ′ τ u0 umf (silica sand) u0/umf (silica sand) umf (dolomite) u0/umf (dolomite)

kg kg mm kg wt % dp cm °C °C kg sol/(m3n/h) s cm/s cm/s cm/s

run no. condns

34

35

time on stream

h

7.8

10.6

9.2

10.0

feeding biomass feed rate throughput biomass moisture biomass dp dolomite (not calc.) feed rate dolomite (not calc.)/biomass d.a.f. dolomite (not calc.) dp steam oxygen (steam + oxygen)/biomass d.a.f. steam/oxygen

kg a.r./h kg/(h m2) wt % mm kg/h g/kg d.a.f. mm kg/h kg/h kg/kg d.a.f. mol/mol

9.4 532 13 -5.0 + 1.0 0.253 30 -0.63 + 0.4 5.08 3.01 0.96 3

11.3 640 8 -5.0 + 1.0 0.253 30 -0.63 + 0.4 5.08 3.01 0.78 3

12.8 724 10 -5.0 + 1.0 0.253 30 -0.63 + 0.4 5.08 3.01 0.70 3

11.1 628 10 -5.0 + 1.0 0.300 30 -0.63 + 0.4 5.08 3.01 0.81 3





gasifier solid total wt in the bed silica sand wt silica sand dp dolomite (calc.) wt dolomite (calc.) percentage dolomite (calc.) dp Hbed Tbed Tfreeboard τ′ τ u0 umf (silica sand) u0/umf (silica sand) umf (dolomite) u0/umf (dolomite)

kg kg mm kg wt % mm cm °C °C kg sol/(Nm3/h) s cm/s cm/s cm/s

dolomite is fed to an air-blown gasifier mixed with the biomass. So, there are at least four institutions in the world who have found this in-bed use of dolomite in biomass gasifiers of fluidized bed type useful. Never-

theless, there are no details about the optimal content of dolomite in the bed or in the feedstock and about the changes in the gas composition when in-bed dolomite is used instead of pure silica sand. This paper intends

5222 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

to solve the lack of knowledge in this specific or concrete point: how beneficial is the use of in-bed dolomite instead of the pure silica sand typically used as the fluidized medium. The existence of calcined dolomite (OCa‚OMg) in the gasifier bed acts as an in-situ catalyst promoting several chemical reactions in the same gasifier. Thus, the product distribution at the gasifier exit is going to change, thus. But, simultaneously, calcined dolomite is softer than silica sand and becomes more erosioned than silica sand, and in the gasifier there is going to be more carryover of fines and/or dust. New fluid dynamic problems will thus appear when dolomite is used. These fluid dynamic aspects will be analyzed in a forthcoming paper (Olivares et al., 1998). This paper will focus only on the changes in product distribution at the gasifier exit and in the quality of the raw gas produced. Installation, Gasifier, and Biomass Used The installation used is a small pilot plant based on an atmospheric and bubbling fluidized bed of 15 cm i.d. and 3.2 m height previously described (Aznar et al., 1995). The biomass is fed near the bed bottom at flow rates of around 10 kg biomass/h. The gasifying agent (fluidizing medium) in steam-O2 mixtures with H2O/ O2 ratios between 2 and 3 and GR [(H2O + O2)/biomass] ratios between 0.7 and 1.2. Product distribution and gas quality when dolomite is not used in the gasifier bed have been reported previously (Aznar et al., 1997; Gil et al., 1997). They will be the basis of comparison with results obtained when dolomite is used in the gasifier bed. Biomass. The biomass used in all experiments reported here was small chips of pine (Pinus pinaster). It was previously sieved at an interval of -5.0 + 1.0 mm. The moisture of the pine wood chips fed was 1012 wt %. A detailed chemical and physical characterization of these pine wood chips has been also previously reported (Aznar et al., 1997; Gil et al., 1997). Dolomite. The dolomite used was “Ma´laga”, coming from the quarries in Coıń, South of Spain. Its chemical analysis and physical characterization are given in the work of Orio et al. (Orio et al., 1997a,b). Tar Sampling and Analysis Methods have been also previously reported (Narvaéz et al., 1996, 1997; Aznar et al., 1997; Gil et al., 1997). Criticisms as those of Milne et al., 1997, were already known and taken into account. Dolomite Location. A prefixed amount of calcined dolomite was located in the gasifier bed, mixed with the silica sand, when the gasifier was prepared for one test (Table 1). Since dolomite is soft, it erodes during the test, and it is eluted out of the bed with the flue exit gas. A batch of in-bed dolomite disappeared quickly, and the bed would not operate under stationary conditions. To get results under stationary conditions (concerning the amount of in-bed dolomite), some dolomite had to be continuously fed to replace the eroded portion. For this reason, and from results gained in a fluid dynamic study simultaneously carried out in the same gasifier (Olivares et al., 1997), some (noncalcined or fresh, now) dolomite of -0.63 + 0.40 mm particle diameter was fed, mixed with the biomass, to the gasifier. Depending on experimental conditions and on the prefixed amount of dolomite in the gasifier bed under stationary conditions, the amount of dolomite fed was 2 or 3 wt % of the overall mass flow fed. Table 1 shows details of the main experimental conditions in

Figure 1. H2 and CO contents in the gas at the exit of the gasifier vs time-averaged content of calcined dolomite in the bed (0.95 < τ < 1.35 s).

some representative tests made for this study. A detailed picture of the fluid dynamic conditions in the gasifier bed in these tests is presented elsewhere (Olivares et al., 1997b). The presence of in-bed calcined dolomite can also increase the melting point of the gasifier bed under operation, thus avoiding formation of cakes (by sintering) when high potassium content biomass (such a cereal straw) is used as feedstock. Results Two series of tests have been performed. The main difference in both series is the gasifying ratio (GR) used. At “high” values of GR (between 0.90 and 1.20) the bed temperature was relatively high (795-835 °C). Since a great deal of O2 is spent at high GR values, the GR value was lowered (till 0.70-0.90) in the second series of experiments. Since tar yield increases at low GR values (Aznar et al., 1997) a higher bed height (Hbed) was used at low GR values. It increases the gas residence time in the bed and decreases the tar content in the raw gas at the gasifier exit. The two series of tests gave always the same trends and conclusions but with different relationships, as will be shown. Gas Composition. H2 and CO Contents. The H2 and CO contents in the raw gas at the gasifier exit are shown in Figure 1 for different percentages of (calcined) dolomite in the gasifier bed. These results were obtained (as the ones indicated in following figures) under a stationary state in the gasifier, that is, with a fixed amount of dolomite in the bed. The H2 content in the flue gas increased from 25 to 28 vol %, dry basis, to 43 vol % when in-bed dolomite

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5223

Figure 3. CO2 content in the gas at the gasifier exit vs timeaveraged content of calcined dolomite in the bed (0.95 < τ < 1.35 s).

Figure 2. CH4 and C2Hn contents in the gas at the exit of the gasifier vs time-average content of calcined dolomite in the bed (0.95 < τ < 1.35 s).

was used (for 0.90 < GR < 1.15). This result is clear, important, and well demonstrated with these tests. This “extra” H2 could come from the in-situ tar-destruction (by reforming, cracking, ...) reactions and also by the CO-shift reaction (CO + H2O ) CO2 + H2), which would be promoted by the OCa‚OMg now existing in the bed. In fact, with in-bed dolomite the CO-content in the raw gas clearly decreases (Figure 1b), and it could be attributed to an increase in the CO-shift reaction being promoted by the OCa‚OMg. The CO in the exit gas decreases (when 0.90 < GR < 1.15) from 45 to 27 vol %, dry basis, Figure 1b. Light Hydrocarbons. CH4-content in the flue exit gas clearly decreases when in-bed dolomite is used, as Figure 2a shows. C2 hydrocarbons also decrease as shown in Figure 2b, from 3.2 to 1.5 vol % in this case. The net result shown in Figure 2 would come from two simultaneous reactions: in-bed tar cracking, which generates some light hydrocarbons, and in-bed steam and dry (CO2) reforming of light hydrocarbons, which would be catalyzed by OCa‚OMg and which makes some of these hydrocarbons disappear from the flue gas. CO2 Content. CO2 intervenes in a lot of reactions (dry (CO2) reforming of tars and of light hydrocarbons, CO-shift, combustions (there is some O2 fed to the gasifier), etc.). The overall or net effect is shown in Figure 3. No variation in the CO2 content in the exit gas was detected when in-bed dolomite was used, as Figure 3 shows. H2/CO and CO/CO2 Ratios. These two interesting ratios in the flue gas vary when in-bed dolomite is used,

Figure 4. H2/CO and CO/CO2 ratios in the gas at the exit of the gasifier vs time-averaged content of calcined dolomite in the bed (0.95 < τ < 1.35 s).

as Figure 4 clearly shows. The important H2/CO ratio increases from about 0.6 (when no dolomite is used) to about 1.5 when the bed has got a 25 wt % of dolomite. The authors consider that this is an important finding because this now high H2/CO ratio could generate some new applications for the raw gas so produced. Heating Value of the Gas. The low heating value (LHV) of the gas decreases somewhat (from 15 to 12.3 MJ/m3n, dry gas) when dolomite is used in the gasifier bed, Figure 5. This decrease can be due to the decrease of the amount of light hydrocarbons in the flue gas

5224 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

Figure 5. Heating value of the gas when there is some dolomite in the bed (0.95 < τ < 1.35 s).

Figure 7. Steam content in the gasifier vs time-averaged content of calcined dolomite in the bed (0.95 < τ < 1.35 s).

Figure 6. Steam content in the gas at the gasifier exit vs timeaveraged content of calcined dolomite in the bed (0.95 < τ < 1.35 s).

(Figure 2), which are a very important contribution to the heating value of the gas. Steam in the Gas. Steam is a major reactant in the steam-reforming reactions of tars and light hydrocarbons and in the CO-shift reaction. Such reactions, carried out in the gasifier bed, are increased by the presence of OCa‚OMg as previous figures have shown. Steam content in the gas at the gasifier exit is going to decrease, by these reactions, when dolomite is present in the bed. This fact is confirmed by the results shown in Figure 6. When there is some in-bed dolomite, steam reacts more in the gasifier bed and its content at the gasifier exit is lower than when there is not dolomite. Since the steam feed is known and the steam at the gasifier exit is measured, the conversion of the steam (XH2O) in the gasifier bed can easily be calculated. Such an XH2O value is shown in Figure 7 for different contents of in-bed dolomite. The result seems clear: the OCa‚OMg enhances somewhat the conversion of the steam (useful fact in this process). Gas and Char Yields. The yield to gas, Ygas (product of both primary and secondary reactions), increases when in-bed dolomite is used, as Figure 8a shows. This useful fact is surely due to the conversion of the tar molecules into smaller (H2, CO, CH4, ...) ones, thus generating an increase in the gas flow. The yield to char, Ychar, also increases when dolomite is used, Figure 8b. This fact is not so easy to understand. Perhaps on increasing the tar removal reactions

Figure 8. Gas yield at the exit of the gasifier (a) and char yield in the gasifier (b) vs time-averaged content of calcined dolomite in the bed (0.95 < τ < 1.35 s).

the char formed also increases. Nevertheless, the char yield increase (from 1.2 to 2.8 wt %, by average, at 0.90 < GR < 1.15) is not big, and no further technical problems derived from this fact have been detected. Apparent Thermal Efficiency. Once gas yield (m3/ kg biomass d.a.f.) and LHV (MJ/m3) are known, their product (divided by the heating value of the biomass) gives an apparent thermal efficiency (heating content in the gas phase/heating content of the biomass d.a.f.). Such thermal efficiency is shown in Figure 9. It is again verified how some dolomite in the bed has a positive effect: it increases the thermal efficiency from 86% to

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5225

Conclusions In-bed use of dolomite in a fluidized bed biomass gasifier has a net or overall positive effect in the gasification process. A 10 wt % of (calcined) dolomite in the bed is enough to change and significantly improve the product distribution. Under the experimental conditions described in this paper, the following averaged changes are noticed in the flue gas: H2 content inc from 25-28 to 43 vol % dry gas CO content dec from 45 to 27 vol % dry gas CH4 content dec from 6.4 to 4.8 vol % dry gas H2/CO ratio inc from 0.6 to 1.5 LHV dec from 15 to 12.3 MJ/m3n, dry gas steam convn inc from 30 to 47 vol % gas yield inc from 1.1 to 1.35 m3n dry gas/kg biomass d.a.f. tar content dec from 12 to 2-3 g tars/m3n Figure 9. Apparent thermal efficiency of the gas at the exit of the gasifier vs time-averaged content of calcined dolomite in the bed (0.95 < τ < 1.35 s).

No problems were detected in the feeding system when some dolomite was fed mixed with biomass. An addition of 2 wt % dolomite in the overall mass flow fed to the gasifier is thus considered to be enough and very positive. Under the experimental conditions used in this work, 2 wt % of dolomite in the feedstock can lead to 20 wt % of (calcined) dolomite in the bed. This percentage is our choice as the best content of dolomite in the gasifier bed. Acknowledgment This work has been carried out under the JOULE III Programme of the EU, DG-XII, Project No. JOR3-CT950053. The authors thank to the European Commission for its financial support. Nomenclature

Figure 10. Tar content in the gas at the exit of the gasifier vs time-averaged content of calcined dolomite in the bed (0.95 < τ < 1.35 s; Tfreeboard ) 650-675 °C).

about 96%. Note that this thermal efficiency is only apparent: the heat supplied by external ovens and the energy spent by engines connected to the screw feeders were not taken into account when the ordinate of Figure 9 was calculated. Tar Content in the Raw Gas. Tar content in the raw gas may be the most important objective of this research: the main end of using in-bed dolomite. Remember that the main objective was to obtain a “quite” clean exit gas. Results are shown in Figure 10 for several contents (percentages) of dolomite in the bed. Working with 0.90 < GR < 1.15 (and Tfreeboard ) 650675 °C), the tar content in the raw gas (gasifier exit) is, by average, 12 g/m3n. This value is decreased to only 2-3 g tars/m3n when there is in-bed dolomite, as Figure 10 shows. The main decrease in tar content occurs when the dolomite in the bed passes from 0 to 10 wt %. On increasing the in-bed dolomite content the tar content in flue exit gas decreases only a little. Perhaps a 20 wt % dolomite in bed is the best content. Higher contents of dolomite in the bed do not significantly decrease the tar content in the exit gas, Figure 10. Thus, 20 wt% dolomite would be our choice. Other authors could select another content, but, always, the results shown in Figure 10 demonstrate the usefulness of in-bed use of dolomite.

a.r. ) as received Ctar ) tar concentration in the gas at the exit of the gasifier, mg/m3n (here and following units, dry gas, normal conditions, 1 atm, and 273 °C) d.a.f. ) dry, ash free dp ) particle diameter of the calcinated stone, by sieving, mm GR ) gasifying ratio, defined as kg (H2O + O2) fed/h/kg biomass d.a.f. fed/h, dimensionless Hbed ) height of the gasifier bed, bulk fixed bed conditions, cm LHV ) low heating value of the gas, MJ/m3n, dry basis Tbed, Tfreeboard ) temperature in the gasifier bed and in its freeboard, respectively, °C u0 ) superficial gas velocity at the inlet of the gasifier bed (at the temperature of the gasifier), cm/s umf ) minimum fluidization gas velocity (gasifier bed conditions), cm/s XH2O ) degree of conversion of the steam in the gasifier, % Ygas ) yield to gas in the gasification process, m3n dry gas/ kg biomass fed d.a.f. Ychar ) char yield (char recovered in the gasifier and in the cyclons), % d.a.f. biomass fed Greek Symbols τ ) space-time for the gas in the gasifier bed, defined as Hbed/u0, s τ′ ) pseudo-space-time for the gas in the gasifier bed, defined as kg of calcined solid in the bed, h/m3n (dry, exit gas)

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Received for review May 29, 1997 Revised manuscript received August 25, 1997 Accepted September 26, 1997X IE9703797

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

Biomass Gasification: Produced Gas Upgrading by In-Bed Use of

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