Biomass Gasification in Fluidized Bed: Where To Locate the Dolomite

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Biomass Gasification in Fluidized Bed: Where To Locate the Dolomite To Improve Gasification? Jose´ Corella,† Marıá-Pilar Aznar,*,‡ Javier Gil,‡ and Miguel A. Caballero‡ Chemical and Environmental Engineering Department, University of Saragossa, 50009 Saragossa, Spain, and Chemical Engineering Department, University ‘Complutense’ of Madrid, 28040 Madrid, Spain Received February 8, 1999

Calcined dolomites (OCa.OMg) are often used for hot gas cleanup in biomass gasification processes in a fluidized bed. Two locations are envisaged for such dolomite: in the same gasifier bed (mixed with silica sand) or in a secondary bed, downstream from the gasifier. Previous results obtained under very similar circumstances are here analyzed to determine which is the best location (regarding its chemical usefulness for tar elimination) of the dolomite. Gas and tar yields and gas compositions are here given for the two locations of the dolomite, and it is made both for gasification with air and with steam-O2 mixtures. It is demonstrated how the effectiveness of the dolomite in the second reactor is only a little bit higher than for the in-bed location. This small increase in effectiveness is mainly found in gasification with H2O-O2 mixtures, there no having noticeable chemical differences (between the two locations of the dolomite) in gasification with air.

Introduction Practitioners in the field know well how the gas produced in biomass gasification in a fluidized bed has to be cleaned of tars (and of sulfur, particulates, etc.) for several advanced applications as its use in gas engines and turbines. One method and/or process for such hot gas clean up is by using calcined dolomites (OCa‚OMg) or related materials such as calcite, limestone, magnesite, etc. These dolomites are or have been studied for this cleanup by several institutions whose work is well-known and has been reviewed recently.1-3 There are two, at least, different approaches or ways of using dolomites: in the same gasifier bed, mixed with the fluidizing medium (usually silica sand),4-6 or downstream from the gasifier in a secondary reactor.1-3,7 These calcined dolomites are usually referred to as cracking catalysts, although tar elimination on these basic oxides occurs mainly by steam and dry (CO2) reforming reactions. After many years of studying the usefulness of dolomites for gas cleanup, the question which arises now is where to locate the dolomite? In the gasifier bed or downstream? Which is the best approach/process/location? With the enormous amount of data collected under similar conditions, the authors can now shed some light on this concrete problem. And this is the main objective of this paper. * Author to whom correspondence should be addressed. Fax: + 34976 76 21 42. E-mail: [email protected]. † University ‘Complutense’ of Madrid. Fax: + 34-91 394 41 64. E-mail: [email protected]. ‡ University of Saragossa. (1) Delgado, J.; Aznar, M. P.; Corella, J. Calcined Dolomite Magnesite and Calcite for Cleaning Hot Gas from a Fluidized Bed Biomass Gasifier with Steam: Life and Usefulness. Ind. Eng. Chem. Res. 1996, 35, 3637-3643. (2) Delgado, J.; Aznar, M. P.; Corella, J. Biomass Gasification with Steam in Fluidized Bed: Effectiveness of CaO, MgO, and CaO-MgO for Hot Raw Gas Cleaning. Ind. Eng. Chem. Res. 1997, 36, 1535-1543.

To compare results obtained by different people is always dangerous. It must be made carefully. There are so many operation parameters which can affect tar conversion that one parameter omitted or forgotten can lead to a wrong conclusion. For instance, in biomass gasification (in fluidized bed) with pure steam, Corella et al.4 and Delgado et al.1,2 used the same dolomite in the same gasifier bed and in a reactor downstream of the gasifier, respectively. Although the biomass used and the heading of both works were the same, such works (on gasification with steam) are not going to be here used/compared because gasifiers used were quite different. Tar yields or tar contents in the produced or raw gas in the work of Delgado et al.1,2 were very high. It was due to the fact that biomass feeding to their gasifier was made at its top instead of at the bottom of the gasifier as in the work of Corella et al.4 Because of the difference in the feeding point, these two works cannot be considered for comparison in this paper. The best location of the dolomite (in-bed or in a secondary bed) will be here analyzed for two different gasification processes: in gasification with air and in gasification with H2O-O2 mixtures. The basic data are found in the following previous papers/works: location of the dolomite gasifying agent

in-bed

downstream

no dolomite

air H2O + O2 mixtures

Gil et al.6 Olivares et al.5

Orio et al.3 Perez et al.7

Narvaez et al.8 Gil et al.9

The possible differences in the effectiveness of the dolomite, depending on its location, will now be studied in this paper using the above indicated previous studies. Basis of Comparison. To make the following comparison useful, the authors have carefully checked that most operation parameters were the same. Such operation parameters are

10.1021/ef990019r CCC: $18.00 © 1999 American Chemical Society Published on Web 10/28/1999

Biomass Gasification in Fluidized Bed

gasifier type location of the biomass feeding point biomass type gasifier bed temperature, in gasification with air in gasification with H2O-O2 temperature in the secondary bed of dolomite (downstream from the gasifier): tar sampling and analysis methods gas analysis dolomite type dolomite status

data collection

Energy & Fuels, Vol. 13, No. 6, 1999 1123

bubbling and atmospheric fluidized bed near the gas distributor plate (at the bottom of the bed) small pine (Pinus Pinaster) wood chips (described in ref 8) 800-850 °C 820-840 °C

{

820-850 °C (in the same range as the temperatures in the gasifier bed) the same (described in ref 8) the same (described in ref 8) the same (Ma´laga one, described in ref 3) near to the incipient fluidization, uo/umf ) 2 to 4 (when in-bed) uo/umf ) 1 to 4 (when downstream/2nd bed) under stationary state (after 1 h on-stream)

There are a few differences between the data/experiments to be compared: (1) Location (type) of the second reactor. In ref 7 dolomite was placed in the reactor located in a “slip flow” (or by-pass) after the gasifier. In ref 3 dolomite was placed in a reactor which received all the flow from the gasifier (and which will be called a “full-flow” reactor). In both cases the diameter of the reactor was similar (around 6 cm), the bed was smoothly fluidized, and the space-time was similar too [in the range of 0.038 to 0.070 kg calcined dolomite h/m3 of gas flow (T reactor and wet conditions)]. It is believed that the location (“full flow” or “slip flow”) of the second reactor will not influence then the conclusions got in this paper. (2) Particle size of the dolomite. In-bed dolomite had a particle size of 0.40-0.63 mm. Dolomite in the second reactor (downstream from the gasifier) had a bigger diameter: 1.0-1.6 mm in gasification with air, 0.6-1.0 mm in gasification with H2O-O2. The particle diameter (dp), and the Thiele modulus [(dp/6)xkF/De] thus, of the “downstream dolomite” is bigger than the “in-bed dolomite” making its effectiveness factor being somewhat smaller.10 This fact (which can be different in another gasification plants/situations, which is specific of the work here used for comparisons purposes) will have to be taken into account in the further analysis of results. (3) Orıó, A.; Corella, J.; Narvaéz, I. Performance of Different Dolomites on Hot Raw Gas Cleaning from Biomass Gasification with Air. Ind. Eng. Chem Res. 1997, 36, 3800-3808. (4) Corella, J.; Herguido, J.; Gonza´lez-Saiz, J.; Alday, F. J.; Rodrı´guez-Trujillo, J. L. Fluidized bed steam gasification of biomass with dolomite and with a commercial FCC catalyst in the same gasifier bed. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Appl. Sci.: London, 1988; pp 754765. (5) Olivares, A.; Aznar, M. P.; Caballero, M. A.; Gil, J.; France´s, E.; Corella, J. Improving the Product Distribution and Gas Quality in Biomass Gasification by In-bed Use of Dolomite. Ind. Eng. Chem. Res. 1997, 36, 5220-5226. (6) Gil, J.; Martıń, J. A.; Aznar, M. P.; Caballero, M. A.; Corella, J. Biomass Gasification with Air in Fluidized Bed: Effect of the In-bed Use of Dolomite under Different Operation Conditions. Ind. Eng. Chem. Res. 1999, in press. (7) Pe´rez, P.; Aznar, M. P.; Gil, J.; Caballero, M. A.; Martıń, J. A.; Corella, J. Hot Gas Cleaning and Upgrading with Calcined Dolomite Located Downstream a Biomass Fluidized Bed Gasifier Operating with Steam-Oxygen Mixtures. Energy Fuels 1997, 11, 1194-1203. (8) Narvaéz, I.; Orıó, A.; Aznar M. P.; Corella, J. Biomass Gasification with Air in an Atmospheric Bubbling Fluidized Bed. Effect of Six Operational Variables on the Quality of the Produced Raw Gas. Ind. Eng. Chem. Res. 1996, 35, 2110-2120. (9) Gil, J.; Aznar, M. P.; Caballero, M. A.; France´s, E.; Corella, J. Biomass Gasification in Fluidized Bed at Pilot Scale with SteamOxygen Mixtures. Product Distribution for Very Different Operating Conditions. Energy Fuels 1997, 11, 1109-1118.

Effectiveness of the dolomite location will be checked/ compared by main product distribution (gas and tar yields) and by gas composition (H2, CO, CO2, CH4, and C2Hn contents in the flue gas after the bed or reactor where the dolomite is located). All these measures are going to be modified, of course, by the catalytic activity of the dolomite. Concerning tar, tars cannot be the same in both vessels or reactors. There can be secondary reactions for tars in the connecting pipes, making more refractory to destroy the tars in the second vessel/reactor but when these work were made, tars were not characterized in detail and only one whole lump “tar” can be compared. As it is well-known, tar conversions depend on the gas residence time or space-time (τ). This variable (space-time) has to be considered thus when comparisons be made. Units which can be used for τ in the secondary bed have been discussed previously.11 Nevertheless, two different types of bed are going to be compared now. The first one (the gasifier bed) contains not only dolomite but also silica sand and char. Units to be used for τ in the first reactor (gasifier) can be discussed, thus. Analyzing such possible units, it was decided to use (to compare the dolomite in both beds, gasifier and second bed) the following parameter:

(calcined) dolomite used kg calcined dolomite biomass feed kg biomass daf/h

(

)

It is a measure of the space-time. Since gas yields from biomass gasification are well-known [5,6,7,8,...], (kg biomass fed /h) can be easily converted to gas flow (m3n/ h) when other units be required for this parameter (τ). If the above said ratio is 1.0 it means that a small commercial biomass gasifier feeding 1 Ton biomass daf/h will need 1 Ton calcined dolomite in the gasifier bed or downstream from the gasifier, this is just the problem to be analyzed in this paper. If a part of such dolomite is lost afterward by erosion and carry over/elutriation, it will need to be periodically or continuously replaced, but this fact is another problem. (10) Corella, J.; Narvaéz, I.; Orio, A. Effectiveness Factor for a Commercial Steam Reforming Catalyst and for a Calcined Dolomite Used Downstream from Biomass Gasifiers. In “VTT Symposium 163”, Espoo (Finland), January 1996. Ed. by VTT (Espoo, Finland), 1996; pp 185-191. (11) Corella, J.; Narvaéz, I.; Orio, A. Criteria for Selection of Dolomites and Catalysts for Tar Elimination from Biomass Gasification. In “VTT Symposium 163”, Espoo (Finland), January 1996. Ed. by VTT (Espoo, Finland), 1996; pp 177-183.

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Figure 2. CH4 and C2Hn contents in the flue gas vs relative amount of dolomite used for two different locations of the dolomite (gasification with H2O + O2 mixtures; GR ) 0.861.16; T ) 820-840 °C).

Figure 1. H2, CO, and CO2 contents in the flue gas vs relative amount of dolomite used for two different locations of the dolomite (gasification with H2O + O2 mixtures; GR ) 0.861.16; T ) 820-840 °C).

Comparison of Data Gas Composition. H2, CO, CO2, CH4, and C2Hn contents in the flue gas after the bed where the dolomite is located are shown in Figures 1 and 2 for gasification with H2O-O2 mixtures and in Figures 3 and 4 for gasification with air. Gasification and equivalence ratios (GR and ER) for points in such figures were 0.86-1.16 and 0.22-0.26, respectively. Points shown in such figures are (1) for three different cases: no dolomite used (only silica sand), in-bed dolomite and “downstreamdolomite”, and (2) for different dolomite-to-biomass ratios. Several facts already appear from these figures. H2 and CO2 contents are somewhat higher for “downstreamdolomite” than for in-bed dolomite (both in gasification with air and with steam-O2 mixtures). CO content is somewhat smaller for “downstream-dolomite” (being this decrease higher in gasification with steam-O2 than in gasification with air). CH4 and C2Hn contents are very similar for both locations. Steam content in the flue gas

(after the bed where dolomite is located) in gasification with H2O-O2 mixtures is somewhat higher for the case of “in-bed dolomite” than for “downstream-dolomite” as it is shown in Figure 5. It can be understood in the following way: For the same GR and (H2O/O2) ratio in the feeding, steam is consumed to a greater extent in “downstream-dolomite” case than in ”in-bed dolomite” one. A detailed statistic analysis of data in figures concerning H2, CO, CO2, and steam contents provides a definitive difference between the “in-bed” and “downstream” cases/locations but, simultaneously, this difference is small and sometimes (CO, CH4 and C2 contents) it is in the interval of the experimental error. All these findings could be already interpreted but let us first see what happens with other results. Lower Heating Value of the Gas (after the bed where dolomite is located), LHV. Once the gas composition is known, LHV is easily calculated. Their values, dry basis, are shown in Figure 6 for the two locations (“in-bed dolomite” and “downstream dolomite”) and for the two gasifying agents used. No important differences are found between the two locations of the dolomite (for each one of the two different gasifying agents used). This finding is due to the fact that the increase in the H2 amount in the flue gas is compensated with the decrease in the CO amount (Figures 1 and 3) and by the not-important variation in the CH4 and C2Hn amounts in the flue gas (Figures 2 and 4). So, regarding the LHV of the gas there is no difference



Figure 4. CH4 and C2Hn contents in the flue gas vs relative amount of dolomite used for two different locations of the dolomite (gasification air; ER ) 0.22-0.26; T ) 800-850 °C).

Figure 3. H2, CO, and CO2 contents in the flue gas vs relative amount of dolomite used for two different locations of the dolomite (gasification with air; ER ) 0.22-0.26; T ) 800850 °C).

between locating the dolomite in the same gasifier bed or in a downstream reactor. Gas Yield. Gas yield is somewhat higher for the “downstream-dolomite” case than for the “in-bed” one as Figure 7a shows for gasification with steam-O2 mixtures. Using air as a gasifying agent, no important differences appear in gas yield between the two locations of the dolomite, Figure 7b. With respect to gas yield, the “downstream-dolomite” solution/process/ approach/case would be somewhat better than the “inbed” one, but only in gasification with steam-O2 mixtures. Tar Content in the Flue Gas after the Bed Where Dolomite Is Located. Tar is a lump whose amount and composition depend on how it is sampled and analyzed. Nowadays, a “tar protocol” is being discussed by a lot of institutions worldwide. Meanwhile, tar measurements have a subjective character and must be related to the procedure used for their collection and analysis.

Tar amounts here mentioned were determined according to the method indicated in ref 8 and it will be called tar*. In gasification with steam-O2 mixtures, tar* content after the bed with dolomite (gasifier or second reactor, depending the case) is somewhat smaller for “downstream-dolomite” than for “in-bed dolomite”, Figure 8a. For (dolomite/biomass) ) 0.50, tar* contents are 1.3 and 2.5 g tars*/m3n, respectively. When air is the gasifying agent, tar* contents in the flue gas are near the same for the two locations of the dolomite, Figure 8b. The small differences observed in Figure 8b on tar contents are between the interval of experimental error. The decrease in tar* content for the location in a second bed instead of in-bed for gasification with steam-O2 mixtures is attributed to the higher steam content in the flue gas. Gasifying with steam-O2 mixtures the steam content in the flue gas is very high (Figure 5a). On the other hand, steam is a main reactant in steam-reforming reactions of tars catalyzed by the calcined dolomite (OCa.OMg) (remember that these oxides are usually added to and found in most of the commercial steam-reforming catalysts for natural gas and for naphthas). These two facts would explain how steam would react with tars more in gasification with steam-O2 mixtures than in gasification with air. Final Remarks and Conclusions Some years ago, and perhaps due to the good impression to this authors of the Swedish TPS AB process,

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Figure 5. Steam content in the flue gas vs relative amount of dolomite used for two different locations of the dolomite and for two gasifying agents; (a) gasification with H2O + O2 mixtures, GR ) 0.86-1.16, T ) 820-840 °C; (b) gasification with air, ER ) 0.22-0.26, T ) 800-850 °C.

these authors believed that a secondary bed, downstream from the biomass gasifier, with calcined dolomite was a much better solution /approach /process for hot gas clean up than using “in-bed dolomite”. Nevertheless, the overall consideration of all results shown in this paper leads to the main conclusion that “downstream-dolomite” has only a little bit higher chemical effectiveness for tar removal than “in-bed-dolomite”. No much better results with a second bed dolomite than with “in-bed dolomite” (if it is designed and used properly, of course!). It has to be remembered that the dolomite particle size in the second reactor was slightly larger than in the gasifier. If both particle sizes be the same (in both locations) the difference in the overall efficiency of the “downstream dolomite” could be clear then. The above-said main conclusion leads to another one: if the gasifier bed is well designed and operated, a downstream or second bed of dolomite would not be necessary; there would not be a big improvement of the gas quality. The second reactor could be omitted (not used in the overall process) with a clear saving of money and an improvement of the economical feasibility of the overall gasification process. This conclusion has nothing to do with the fact that for a further polishing of the flue gas (to decrease tar content well below 1 g/m3n) another catalytic bed with a nickel-

Corella et al.

Figure 6. Low heating value of the flue gas for two locations of the dolomite and for two gasifying agents; (a) gasification with H2O + O2 mixtures, GR ) 0.86-1.16, T ) 820-840 °C; (b) gasification with air, ER ) 0.22-0.26, T ) 800-850 °C.

based catalyst should be used12 but it is out the scope of this paper. The authors believe that the gas (tar)-dolomite contact (for the same type and dp of dolomite and fluidization conditions) is better in a secondary reactor than in the gasifier bed in which there are also char and relatively big particles of biomass, both interfering with the gas (tar)-dolomite contact (and maybe blocking some pores of the dolomite too). So, dolomite should be more active in a second reactor than in the gasifier, which has been proved in this paper not to be true. Why is the dolomite so (relatively) highly active in the gasifier bed? The authors believe that it could be explained considering that in the gasifier bed there are the “nascent tars”, while in the freeboard, connecting pipes, and second reactor inlet some tars could have been polimerized and could be harder or more refractory to destroy. Acting in-bed, dolomite would destroy softer (nascent) tars but, to demonstrate it, a detailed tar composition analysis would be needed in both vessels (gasifier and second reactor). As final and minor conclusion, it can also be noticed that a second reactor/vessel provides a new volume at (12) Corella, J.; Orio, A.; Aznar, P. Biomass Gasification with Air in Fluidized Bed: Reforming of the Gas Composition with Commercial Steam reforming Catalysts. Ind. Eng. Chem. Res. 1998, 37, 4617-4624.



Figure 7. Gas yield for two locations of the dolomite and for two gasifying agents; (a) gasification with H2O + O2 mixtures; GR ) 0.86-1.16, T ) 820-840 °C; (b) gasification with air, ER ) 0.22-0.26, T ) 800-850 °C.

Figure 8. Tar content in the flue gas vs relative amount of dolomite used for two locations of the dolomite and for two gasifying agents; (a) gasification with H2O + O2 mixtures, GR ) 0.86-1.16, T ) 820-840 °C; (b) gasification with air, ER ) 0.22-0.26, T ) 800-850 °C.

high-temperature with respect to the case of “in-bed dolomite. Some gas-phase reactions, like the CO-shift one (CO + H2O ) CO2 + H2) will proceed to a bigger extent when there is a second reactor/vessel than when there is not such a volume (as it is the case here considered for “in-bed dolomite” in which gas sampling and analysis is made just after the gasifier). For this reason the CO contents shown in Figures 1 and 3 are smaller for the “downstream dolomite” than for the “inbed” case. In the first case the reaction volume for gasphase reactions is much bigger than in the gasifier alone (“in-bed” case). Exit gas composition will depend then not only on the catalyzed (by dolomite) tar elimination reactions but also on the simultaneous high-temperature gas-phase reactions. When these reactions are also taken into account, all results here shown are well understood.

Nomenclature

Acknowledgment. This work has been carried out under the JOULE III Program of the EU DG-XII, Project no. JOR3-CT98-0306. The authors thank the European Commission for its financial support. The work also has been done under the Spanish DGES financed project No. PB96-0743.

daf dp De ER

GR

k LHV T uo umf

dry, ash-free particle diameter of the calcined dolomite (mm) effective coefficient of diffusion in the particle (cm2/s) equivalence ratio, defined as the air-to-fuel ratio used in the gasifier divided by the air-to-fuel ratio for the stoichiometric combustion (dimensionless) gasification ratio (in gasification with steam), defined as (kg of (H2O + O2) fed to the gasifier/h)/(kg biomass daf fed/h) (dimensionless) apparent kinetic constant for a first-order tar removal reaction (s-1) lowest heating value of the produced gas (MJ/m3n, dry basis) temperature in the bed where dolomite is located (gasifier or second bed) (°C) superficial gas velocity at the inlet of the bed where dolomite is located (gasifier or second bed) (cm/s) minimum fluidization velocity of the (mixture of, if so) solids (cm/s)

Greek symbols τ F

space-time of the gas in the bed of dolomite, defined as kg calcined dolomite h [m3 (T, wet) flue gas]-1 particle density (g/cm3) EF990019R

Biomass Gasification in Fluidized Bed: Where To Locate the Dolomite

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