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Efficient gasification of wet biomass residue to produce middle caloric gas
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Particuology 6 (2008) 376–382
Efficient gasification of wet biomass residue to produce middle caloric gas Guangwen Xu a,b,∗ , Takahiro Murakami a , Toshiyuki Suda a , Hidehisa Tani a , Yutaka Mito c a
b
Research Laboratory, IHI Co., Ltd. (IHI), Isogo-Ku, Yokohama 235-8501, Japan State Key Laboratory of Multiphase Phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China c Energy Research Division, Kobe steel Co., Ltd., Kobe, Hyogo 651-2271, Japan Received 30 October 2007; accepted 14 July 2008
Abstract Various process residues represent a kind of biomass resource already concentrated but containing water as much as 60 wt.%. These materials are generally treated as waste or simply combusted directly to generate heat. Recently, we attempted to convert them into middle caloric gas to substitute for natural gas, as a chemical or a high-rank gaseous fuel for advanced combustion utilities. Such conversion is implemented through dual fluidized bed gasification (DFBG). Concerning the high water content of the fuels, DFBG was suggested to accomplish either with high-efficiency fuel drying in advance or direct decoupling of fuel drying/pyrolysis from char gasification and tar/hydrocarbon reforming. Along with fuel drying, calcium-based catalyst can be impregnated into the fuel, without much additional cost, to increase the fuel’s gasification reactivity and to reduce tar formation. This article reports the Ca impregnation method and its resulting effects on gasification reactivity and tar suppression ability. Meanwhile, the principle of directly gasifying wet fuel with decoupled dual fluidized bed gasification (D-DFBG) is also highlighted. © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: High-water content biomass; Catalytic gasification; Dual fluidized bed gasification; Decoupled gasification; Calcium
1. Introduction Fig. 1 shows the biomass resources available on the earth’s surface (IEA Bioenergy). The wet biomass referred to in this article includes both “process residues” and “organic wastes”, the latter referring to not only “sewage sludge” but also “animal excrements”. These materials represent a kind of readily collectable biomass resources which are suitable for energy production at scales from tens to hundreds of MW. In order to fully take advantage of the CO2 neutralization function of the fuels, we have recently worked on converting the wet biomass into middle caloric gas (NEDO, 2004), which is free from serious dilution by combustion air and can be either used as gaseous fuels for various high-efficiency combustion utilities, such as gas engines and gas turbines, or converted into synthetic natural gas (SNG) to replace fossil natural gas. The technology involved is the dual fluidized bed gasification (DFBG) conceptualized in Fig. 2, which consists of two fluidized bed (FB) reactors: an ∗ Corresponding author at: State Key Laboratory of Multiphase Phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China. Tel.: +86 10 62550075; fax: +86 10 62550075. E-mail address: [email protected] (G. Xu).
FB gasifier and an FB combustor. The fuel combusted in the FB combustor is exclusively the unreacted char coming from the FB gasifier. Between the two reactors heat carrier particles (HCPs) are circulated to carry the combustion heat from the combustor to the gasifier. The sensible heat of the circulating particles sustains the highly endothermic fuel gasification and pyrolysis reactions occurring inside the gasifier. A distinctive feature of DFBG is the isolation of gas production reactions (pyrolysis and gasification) from unreacted char combustion, making it possible to produce a gas not diluted by N2 from combustion air. Compared to other oxygen-blown gasification processes, DFBG is lower in cost because it does not require oxygen supply. DFBG is characterized by a dense bubbling/turbulent fluidized bed gasifier coupled with a pneumatic riser char combustor (Xu, Murakami, Suda, Matsuzawa, & Tani, 2006a). Nonetheless, the prototype DFBG is difficult to treat wet biomass fuels with water of about 60 wt.%. Heat and mass calculation with ASPEN demonstrated that the gasification efficiency for DFBG decreases perceptibly with increasing fuel’s water content, and in order to realize cold gas efficiencies above 75% the water content of feedstock should be lower than 20 wt.% (Murakami et al., 2007). A straightforward remedy for fuel’s high water content is to dry and burn the virgin fuel in
1674-2001/$ – see inside back cover © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V.All rights reserved.
doi:10.1016/j.partic.2008.07.004
G. Xu et al. / Particuology 6 (2008) 376–382
377
Fig. 1. Biomass types and sources pertinent to the present work.
advance. This requires a drier in addition to the gasification plant. The technology incorporating fuel gasification and fuel drying should thus be more economic and prospective, resulting in our proposal of the “decoupled dual fluidized bed gasification (D-DFBG)” to gasify wet fuel directly (Xu, Liu, Suda, & Fujimori, 2006). In a companion technique of fuel drying, called “slurry dewatering in kerosene” (Mito, Komatsu, Hasegawa, & Mae, 2005), the fuel’s water content could be reduced to about 10 wt.%, as exemplified by drying wet coffee grounds with 65 wt.% water (Mito et al., 2005). Gasifying the dried coffee grounds in a 5.0 kg/h pilot DFBG facility demonstrated that the gas produced had HHVs of over 3500 kcal/N m3 (Xu et al., 2006a; Xu et al., 2006c), though the gas was rich in tar that could reach 45 g/N m3 for gasification at 1173 K. The gasification efficiency, defined as the ratio of total HHV of product gas to total HHV of supplied fuel, was hardly 75% at this temperature. Advanced technologies are therefore needed to increase the fuel’s reactivity and suppress the tar formation during gasification. Catalytic gasification with impregnated Ca catalyst was suggested. Impregnating coal with Ca through ion-exchange (Ohtsuka & Tomita, 1986; Salinas-Martínez de Lecea, Almela-Alarcónh, & Linares-Solano, 1990) or kneading (Salinas-Martínez de Lecea et al., 1990; Clemens, Damiano, & Matheson, 1998) has long been recognized to produce catalytic effect on coal gasification. The tediousness and extra cost incurred by such impregnation, however, precluded its practical application.
Fig. 2. Working principle of dual fluidized bed gasification.
The present article is devoted to demonstrating the prospect of Ca impregnation in fuel drying for converting wet biomass efficiently because this method of impregnation may overcome the cost barrier of catalytic gasification. Succeeding our previous report (Xu et al., 2006c), the article will clarify how impregnated Ca catalyzes fuel gasification. As an efficient alternative to gasify biomass with high water content, the concept and working principle of the so-called decoupled dual fluidized bed gasification (D-DFBG) will be elucidated. 2. Catalytic gasification of predried biomass with DFBG 2.1. Predrying and calcium impregnation Fig. 3 outlines the process of slurry dewatering in kerosene and Ca impregnation process. Without Ca impregnation, the wet coffee grounds containing 65 wt.% water were first mixed with kerosene to make a fuel-oil slurry. After pressurizing to 0.3 MPa, preheating, dewatering and separation, the slurry was split into three streams: dried fuel for gasification, separated kerosene for recirculation and water to be drained. In Fig. 3, slurry was dewatered via flash evaporation under reduced pressure at 443 K, while the steam generated in the flash evaporation was re-compressed for heating the fuel-oil slurry to recover the energy with the steam. For Ca impregnation, Ca(OH)2 or CaO was first mixed with kerosene to make an oil-based slurry, followed by, mixing the wet fuel with the Ca-containing slurry and drying of the fuel in the slurry in the same way as for the case without Ca impregnation. Fig. 4 compares the water content in the dried coffee grounds as a function of the slurry temperature in the dewatering tank for the cases with and without Ca impregnation. The dosed Ca was Ca(OH)2 with 7% impurities and the dosing ratio was about 4 wt.% of the dry fuel. The results show that after drying the water contents were reduced from 65% to about 10 wt.% and the presence of the impregnated Ca in fuel hardly affected the fuel dewatering characteristics. Table 1 summarizes the properties of the dried coffee grounds, showing that after drying the Caimpregnated fuel had higher ash content because of of the Ca species added. Consequently, C and H contents as well as HHVs
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Fig. 3. Drying fuel via slurry dewatering in kerosene. Steps highlighted with broken lines apply to Ca impregnation.
nated Ca(OH)2 , respectively. The CaO adopted for mechanical mixing was calcined limestone, whose properties are also outlined in Table 1. X-ray fluorescence (XRF) analysis confirmed that both fuels have similar Ca content (Xu et al., 2006c), about 4 wt.% of dry-base fuel. Nevertheless, the dispersion of Ca shown as red dots in Fig. 5 over the fuel matrix is evidently different from each other: far more even and discretely dispersed for impregnation in comparison with mechanical mixing.
Fig. 4. Dewatering characteristics of coffee grounds with and without Ca impregnation.
of the Ca-impregnated fuel were all lower than the fuel without Ca impregnation. In both cases the coffee grounds were rich in volatile matters (up to 70 wt.%) and oxygen. Measurement showed also that the kerosene remaining in the dried fuel was less than 0.2% of the oil used. Fig. 5a and b show the Ca distributions over fuel matrix measured by energy dispersive X-ray detector (EDX) for the dried coffee grounds with mechanically mixed CaO and impregTable 1 Properties of fuels and CaO adopted No Ca
Ca-impregnated
Proximate (wt.%) Moisture VM FC Ash
10.5 71.8 16.7 1.0
14.8 64.9 15.8 4.5
Ultimate (wt.%-db) C H N S O
52.97 6.51 2.80 0.05 36.62
50.17 6.07 2.78 0.08 35.78
HHV (MJ/kg-db) Sizes Bulk density (kg/m3 )
21,980
20,457 <2.0 mm About 350
CaO for physical mixing: lightly calcined CaCO3 ore, bulk density = 1050 kg/m3 , sizes <1.0 mm (Sauter mean: 70 m), specific surface area = 2.0–5.0 m2 /kg, pore volume = 0.01 L/kg.
Fig. 5. Distributions (1000×) of Ca over fuel matrix: (a) With mechanically mixed CaO; (b) with impregnated Ca(OH)2 .
G. Xu et al. / Particuology 6 (2008) 376–382
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Table 2 Molar compositions of rude product gases from gasifier corresponding to efficiency data of Fig. 6 Items
Pure coffee grounds Mixed with CaO Impregnated with Ca(OH)2
H2 CO CO2 CH4 C2 H4 C2 H6 C3 H6 Ar H2 /COa
18.65 29.36 10.54 13.24 4.80 2.48 0.03 18.77 0.65
a
21.75 26.28 11.56 13.19 4.90 2.50 0.03 17.77 0.83
31.86 23.97 14.48 10.05 3.51 1.84 0.02 12.57 1.33
The mentioned H2 /CO refers to molar ratio.
2.2. Catalytic gasification of predried biomass Gasification of the predried bare coffee grounds and Caimpregnated as well as Ca-mixed coffee grounds was carried out in a 5.0 kg/h pilot DFBG facility. The facility consists of a bubbling fluidized bed (BFB) gasifier with a rectangular cross section of 370 mm × 80 mm and a height of 2.0 m and a pneumatic riser char combustor of 52.7 mm i.d. and 6.4 m high. Fuel was supplied into the gasifier from a location slightly above the dense bed surface (generally 0.6 m high). The heat carrier particles (HCPs) circulating between the gasifier and combustor were silica sand of 190 m in Sauter mean diameter. Details about this facility and related experiments were already published (Xu et al., 2006a, 2006c; Xu, Murakami, Suda, Matsuzawa, & Tani, 2006b). The major operating parameters for the gasifier are listed in Fig. 6, while the riser combustor ran generally at about 1093 K at a superficial gas velocity of 2.5 m/s. Methods for recovering tar and measuring product gas composition and volume were detailed in Xu et al. (2006a,b,c). The tar-recovery system trapped 90% of the total tar in the product gas. Both Fig. 6 and Table 2 show quasi-steady-state gasification results at a temperature (Tb ) of about 1083 K. The typical time series of product gas composition is available in Xu et al. (2006c). Compared to bare coffee grounds (open bars in Fig. 6), the grounds with impregnated Ca (black bars) greatly increased fuel C and H conversions (Xc and Xh ) and cold gas efficiency ηe (Fig. 6a), while the tar content in the product gas was decreased by 75%, from about 40 to 10 g/N m3 (Fig. 6b). The reported ηe refers to an explicit value against the HHV of the treated fuel (i.e., without considering the running energy consumption). The Ca-impregnated coffee grounds exhibited a particularly high increase in H conversion Xh , from 88% to 140%, as compared to bare fuel. This,shows that a considerable amount of H in the product gas was derived from H2 O through, for example, steam gasification, reforming and watergas-shift reactions. Under the same gasification conditions, the mechanically mixed CaO yielded merely weak effects (shaded bars in Fig. 6), though with similar amount of Ca in fuel. Table 2 demonstrates that the product gas from the Ca-impregnated fuel has much more H2 and CO2 and much less CO and hydrocarbons in comparison with that from bare fuel. The mechanically mixed CaO affected the composition of product gas in the same trend
Fig. 6. (a and b) Gasification results of bare coffee grounds and coffee grounds with mechanically mixed CaO and impregnated Ca(OH)2 , with tar content and HHV calculated on the basis of Ar-free gas. Xc Xh ηe F S Tb Ub ter
C conversion H conversion Cold gas efficiency Fuel feed rate excluding Ca Steam feed rate Gasifier temperature Superficial gas velocity in gasifier Explicit residence time of fuel in gasifier
% % % kg/h kg/h K m/s s
but to a lesser degree. Corresponding to these differences in gas composition, the HHV of the product gas is much lower for the Ca-impregnated fuel but only slighty lower for the CaO-mixed fuel (Fig. 6b). Nonetheless, for the Ca-impregnated fuel the HHV of product gas is still as high as 4000 kcal/N m3 (free Artracer), thus demonstrating the merit of the DFBG technology conceptualized in Fig. 2. 2.3. Analysis on calcium catalysis Biomass gasification involves both fuel pyrolysis and char gasification. Though Ca is known to catalyze biomass and coal
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Fig. 7. TG data in steam circumstance for the fuels without Ca and with mechanically mixed CaO and impregnated Ca(OH)2 . All TG data are against the base excluding their residues held in TG cell.
gasification, it is not clear as to which step the calcium catalysis contributes. To unravel the catalytic mechanism of impregnated Ca in biomass gasification, all the fuels tested in Fig. 6 were further examined with TG in steam. Fig. 7 shows the TG results, where the plotted TG variation was estimated against the sample weight removing the post-reaction residue in the cell. For each TG diagram in Fig. 7, three inflection points can be identified: A, B and C. Before point A fuels are dried, between points A and B the major reaction is pyrolysis, followed by a stagnation period until point C is reached at which onset of char gasification begins. Although little difference exists for pyrolysis of the three fuels, the sample weight variation around the inflection point C is quite obvious for the Ca-impregnated fuel, while for the fuel without Ca additive it is not evident. Hence, we suggest that point C denotes the onset of obvious catalysis of Ca for char gasification. The data of Fig. 7 demonstrate that the impregnated Ca catalyzed char gasification exclusively and that this catalytic effect was very low for mechanically mixed CaO. The status of Ca dispersion on char particles taken from the gasification tests was further examined with EDX, as shown in Fig. 8. For mechanically mixed CaO the Ca species are poorly dispersed (Fig. 8a), while the Ca dispersion is highly uniform for the impregnated Ca(OH)2 (Fig. 8b). To catalyze char gasification requires close contact between Ca species and char particles (Clemens et al., 1998). Adequate dispersion of Ca in the char-C matrix is consequently an indispensable prerequisite for high catalytic activity of the Ca species added into the fuel. 3. Direct gasification of wet biomass with D-DFBG The proposed decoupled dual fluidized bed gasification (D-DFBG) for direct gasification of wet biomass is conceptualized in Fig. 9 (Xu, Liu, et al., 2006). The D-DFBG is characterized with the use of a two-compartment fluidized bed as the fuel gasifier. The bed may have various configurations or simply a BFB modified by partitioning its reaction space into two compartments with a partition plate (see the
Fig. 8. Calcium dispersion maps on char particles from the fuels with (a) mechanically mixed CaO and (b) impregnated Ca(OH)2 by EDX analysis.
broken-line boxed part). The partition plate is immersed in the fluidized particles but leaves a passage between the lower end of the plate and the bed distributor to allow both particles and gas to flow from one compartment to the other. Hence, this bed has a U-shaped configuration and may be named U-type fluidized bed (UFB). As illustrated in Fig. 9, wet fuel and re-circulated hot HCPs are fed into the right-side compartment of the bed. Steam, possibly mixed with an oxidant (such as air), is also fed into this compartment to fluidize the particles. Residence time of the fuel particles is controlled appropriately to allow the fuel to undergo predominantly drying and pyrolysis in this compartment. The required endothermic reaction heat comes mainly from the hightemperature HCPs. Steam, pyrolysis gas and char generated in this compartment all move into the left-side compartment via the passage below the partition plate. In the left-side compartment char gasification and tar/hydrocarbon reforming take place to produce a product gas. Both sensible heat of the HCPs from the right-side compartment and the heat released by combustion of fuel with oxidant, either oxygen or air, fed into the left-side compartment can provide the required endothermic reaction heat in
G. Xu et al. / Particuology 6 (2008) 376–382
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Fig. 10. Potential benefit from using fuel water to replace part of steam reagent (treated fuel: coffee grounds containing 50 wt.% water; base conditions: ratio of fed-steam to wet-base fuel is 1.0 kg/kg, heat lose is 5% of fuel HHV).
Fig. 9. Process concept of the decoupled dual fluidized gasification (D-DFBG).
this compartment. The steam from the right-side compartment becomes then part of the steam reagent required for char gasification and tar/hydrocarbon reforming, leading to a decrease of the steam fed into the left-side compartment. Meanwhile, particles in the left-side compartment are actually a mixture of HCPs and char so that the reforming of tars and hydrocarbon takes place in a bed of char particles. This enables consequently char to catalyze such reactions. As in normal DFBG, HCPs with lowered temperatures, together with unreacted char, in the left-side compartment of the UFB are forwarded to the riser combustor of the D-DFBG to allow the unreacted char to burn out there with air and to reheat the HCPs. The distinctive feature of the devised D-DFBG is the use of a UFB to substitute for the BFB used for normal DFBG. This allows the decoupling of fuel drying and pyrolysis from char gasification and tar/hydrocarbon reforming to enable separate controls for these reactions and to make it possible to take advantage of the interactions between these reactions. In fact, the D-DFBG enables (1) char to catalyze reforming of tars and hydrocarbon, (2) the use of water vapor generated in fuel drying/pyrolysis as part of steam reagent, and (3) the adoption of different gasification reagents for fuel pyrolysis and char gasification reactions. With all of these merits, high gasification efficiency and low tar evolution are hopefully available with D-DFBG. The high efficiency of D-DFBG is associated with its use of vaporized fuel water as part of the required steam as reagent and thereby the reduction of steam supply. This would make the DDFBG particularly suitable for wet fuels. Fig. 10 demonstrates how the use of fuel water as part of steam reagent affects the gasification efficiency. The data were from a thermodynamic calculation (without considering kinetics) using the model and approach for normal DFBG system reported in Murakami et al.
(2007). The model was based on Aspen computation, capable of estimating gasification efficiencies for steam gasification of different fuels under different steam feeds. The treated fuel was coffee grounds containing 50 wt.% water, and the base condition at zero abscissa is for a feed ratio of 1.0 kg/kg of steam reagent to wet-base fuel and a system heat loss of 5% of fuel’s HHV. Fig. 10 shows that using 50% and 80% of the fuel water to reduce an equal amount of steam reagent supply may raise the cold gas efficiency by 5% and 8%, respectively. Although decreasing steam feed lowers the steam to dry fuel ratio in the reactor, for the estimated case in Fig. 10 the lowest ratio is still higher than 2.0 kg/kg. This ratio is sufficiently high to keep the reaction kinetics to vary little with the steam/dry-fuel ratios. Consequently, the prediction revealed, to a great degree, what actually occurs to the D-DFBG and demonstrated essentially its inherent technical advantage. 4. Conclusions Different from many biomass resources that are widely scattered for convenient gathering, various kinds of process residues and sewage sludge represent a different kind of biomass resource, already concentrated, though with water contents as high as 60 wt.%. These biomass matters are therefore readily available for industrial-scale exploitation to take advantage of their CO2 -neutralization function. The present study devised a process to convert such biomass into middle-caloric gas with heating values of 3000–5000 kcal/N m3 via dual fluidized bed gasification (DFBG). Because of the high water content of the fuel, gasification needs to follow fuel drying in advance. In this study, such fuel drying is taken with impregnation of Ca-base catalyst in the fuel, which, with the high dispersion of Ca in the char-C matrix, enhances the reactivity of char during gasification and reduces tar formation during gasification. Calcium impregnation was implemented by simply mixing CaO or Ca(OH)2 into the high-water-content biomass at the beginning. Experiments showed that mixing CaO or Ca(OH)2 did not adversely affect the fuel’s dewatering characteristics. On the other hand, the so-called decoupled dual fluidized bed gasification (D-DFBG), with the use of a U-type fluidized bed (UFB), appeared highly
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efficient in gasifying biomass with high water content. The DDFBG decouples fuel drying/pyrolysis from char gasification and tar/hydrocarbon reforming to enable the vaporized water from the wet fuel to be part of the required steam for gasification. Acknowledgements All experimental work presented in this article was conducted in IHI Co. Ltd., Japan, under a financial support from The New Energy and Industrial Technology Development Organization (NEDO), Japan. The Natural Science Foundation of China (NSFC) financed the first period of research on the decoupled dual fluidized bed gasification (20606034, 20776144). The authors are grateful for all of these financial assistances. References Clemens, A. H., Damiano, L. F., & Matheson, T. W. (1998). The effect of calcium on the rate and products of steam gasification of char from low rank coal. Fuel, 77, 1017–1020. IEA Bioenergy. (2006). What Is Biomass? From IEA bioenergy education website on biomass and bioenergy: http://www.aboutbioenergy. info/definition.html.
Mito, Y., Komatsu, N., Hasegawa, I., & Mae, K. (2005). Slurry dewatering process for biomass. In Proceedings of the international conference on coal science and technology (Paper 2E01). London: IEA-Clean Coal Center. Murakami, T., Xu, G., Suda, T., Matsuzawa, Y., Tani, H., & Fujimori, T. (2007). Some process fundamentals of biomass gasification in dual fluidized bed. Fuel, 86, 244–255. NEDO. (2004). Research & development of advanced upgrading & pyrolytic gasification system for biomass with high moisture. In High efficiency bioenergy conversion project, NEDO printed: Tokyo, Japan, pp. 22–23, http://www.nedo.go.jp/shinene/baio/baio2.pdf. Ohtsuka, Y., & Tomita, A. (1986). Calcium catalysed steam gasification of Yallourn brown coal. Fuel, 65, 1653–1657. Salinas-Martínez de Lecea, C., Almela-Alarcónh, M., & Linares-Solano, A. (1990). Calcium-catalysed carbon gasification in CO2 and steam. Fuel, 69, 21–27. Xu, G., Liu, X., Suda, T., & Fujimori, T. (2006). Method and apparatus of decoupled fluidized bed gasification for solid fuels. China Patent Appl. No. 200610113063.3. Xu, G., Murakami, T., Suda, T., Matsuzawa, Y., & Tani, H. (2006a). The superior technical choice for dual fluidized bed gasification. Industrial and Engineering Chemistry Research, 45, 2281–2286. Xu, G., Murakami, T., Suda, T., Matsuzawa, Y., & Tani, H. (2006b). Gasification of coffee grounds in dual fluidized bed: Performance evaluation and parametric investigation. Energy & Fuels, 20, 2695–2704. Xu, G., Murakami, T., Suda, T., Matsuzawa, Y., Tani, H., Mito, Y., et al. (2006c). Enhancing high water content biomass gasification with impregnated Ca in fuel drying. AIChE Journal, 52, 3555–3561.
Particuology 6 (2008) 376–382
Efficient gasification of wet biomass residue to produce middle caloric gas Guangwen Xu a,b,∗ , Takahiro Murakami a , Toshiyuki Suda a , Hidehisa Tani a , Yutaka Mito c a
b
Research Laboratory, IHI Co., Ltd. (IHI), Isogo-Ku, Yokohama 235-8501, Japan State Key Laboratory of Multiphase Phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China c Energy Research Division, Kobe steel Co., Ltd., Kobe, Hyogo 651-2271, Japan Received 30 October 2007; accepted 14 July 2008
Abstract Various process residues represent a kind of biomass resource already concentrated but containing water as much as 60 wt.%. These materials are generally treated as waste or simply combusted directly to generate heat. Recently, we attempted to convert them into middle caloric gas to substitute for natural gas, as a chemical or a high-rank gaseous fuel for advanced combustion utilities. Such conversion is implemented through dual fluidized bed gasification (DFBG). Concerning the high water content of the fuels, DFBG was suggested to accomplish either with high-efficiency fuel drying in advance or direct decoupling of fuel drying/pyrolysis from char gasification and tar/hydrocarbon reforming. Along with fuel drying, calcium-based catalyst can be impregnated into the fuel, without much additional cost, to increase the fuel’s gasification reactivity and to reduce tar formation. This article reports the Ca impregnation method and its resulting effects on gasification reactivity and tar suppression ability. Meanwhile, the principle of directly gasifying wet fuel with decoupled dual fluidized bed gasification (D-DFBG) is also highlighted. © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved. Keywords: High-water content biomass; Catalytic gasification; Dual fluidized bed gasification; Decoupled gasification; Calcium
1. Introduction Fig. 1 shows the biomass resources available on the earth’s surface (IEA Bioenergy). The wet biomass referred to in this article includes both “process residues” and “organic wastes”, the latter referring to not only “sewage sludge” but also “animal excrements”. These materials represent a kind of readily collectable biomass resources which are suitable for energy production at scales from tens to hundreds of MW. In order to fully take advantage of the CO2 neutralization function of the fuels, we have recently worked on converting the wet biomass into middle caloric gas (NEDO, 2004), which is free from serious dilution by combustion air and can be either used as gaseous fuels for various high-efficiency combustion utilities, such as gas engines and gas turbines, or converted into synthetic natural gas (SNG) to replace fossil natural gas. The technology involved is the dual fluidized bed gasification (DFBG) conceptualized in Fig. 2, which consists of two fluidized bed (FB) reactors: an ∗ Corresponding author at: State Key Laboratory of Multiphase Phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100080, China. Tel.: +86 10 62550075; fax: +86 10 62550075. E-mail address: [email protected] (G. Xu).
FB gasifier and an FB combustor. The fuel combusted in the FB combustor is exclusively the unreacted char coming from the FB gasifier. Between the two reactors heat carrier particles (HCPs) are circulated to carry the combustion heat from the combustor to the gasifier. The sensible heat of the circulating particles sustains the highly endothermic fuel gasification and pyrolysis reactions occurring inside the gasifier. A distinctive feature of DFBG is the isolation of gas production reactions (pyrolysis and gasification) from unreacted char combustion, making it possible to produce a gas not diluted by N2 from combustion air. Compared to other oxygen-blown gasification processes, DFBG is lower in cost because it does not require oxygen supply. DFBG is characterized by a dense bubbling/turbulent fluidized bed gasifier coupled with a pneumatic riser char combustor (Xu, Murakami, Suda, Matsuzawa, & Tani, 2006a). Nonetheless, the prototype DFBG is difficult to treat wet biomass fuels with water of about 60 wt.%. Heat and mass calculation with ASPEN demonstrated that the gasification efficiency for DFBG decreases perceptibly with increasing fuel’s water content, and in order to realize cold gas efficiencies above 75% the water content of feedstock should be lower than 20 wt.% (Murakami et al., 2007). A straightforward remedy for fuel’s high water content is to dry and burn the virgin fuel in
1674-2001/$ – see inside back cover © 2008 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V.All rights reserved.
doi:10.1016/j.partic.2008.07.004
G. Xu et al. / Particuology 6 (2008) 376–382
377
Fig. 1. Biomass types and sources pertinent to the present work.
advance. This requires a drier in addition to the gasification plant. The technology incorporating fuel gasification and fuel drying should thus be more economic and prospective, resulting in our proposal of the “decoupled dual fluidized bed gasification (D-DFBG)” to gasify wet fuel directly (Xu, Liu, Suda, & Fujimori, 2006). In a companion technique of fuel drying, called “slurry dewatering in kerosene” (Mito, Komatsu, Hasegawa, & Mae, 2005), the fuel’s water content could be reduced to about 10 wt.%, as exemplified by drying wet coffee grounds with 65 wt.% water (Mito et al., 2005). Gasifying the dried coffee grounds in a 5.0 kg/h pilot DFBG facility demonstrated that the gas produced had HHVs of over 3500 kcal/N m3 (Xu et al., 2006a; Xu et al., 2006c), though the gas was rich in tar that could reach 45 g/N m3 for gasification at 1173 K. The gasification efficiency, defined as the ratio of total HHV of product gas to total HHV of supplied fuel, was hardly 75% at this temperature. Advanced technologies are therefore needed to increase the fuel’s reactivity and suppress the tar formation during gasification. Catalytic gasification with impregnated Ca catalyst was suggested. Impregnating coal with Ca through ion-exchange (Ohtsuka & Tomita, 1986; Salinas-Martínez de Lecea, Almela-Alarcónh, & Linares-Solano, 1990) or kneading (Salinas-Martínez de Lecea et al., 1990; Clemens, Damiano, & Matheson, 1998) has long been recognized to produce catalytic effect on coal gasification. The tediousness and extra cost incurred by such impregnation, however, precluded its practical application.
Fig. 2. Working principle of dual fluidized bed gasification.
The present article is devoted to demonstrating the prospect of Ca impregnation in fuel drying for converting wet biomass efficiently because this method of impregnation may overcome the cost barrier of catalytic gasification. Succeeding our previous report (Xu et al., 2006c), the article will clarify how impregnated Ca catalyzes fuel gasification. As an efficient alternative to gasify biomass with high water content, the concept and working principle of the so-called decoupled dual fluidized bed gasification (D-DFBG) will be elucidated. 2. Catalytic gasification of predried biomass with DFBG 2.1. Predrying and calcium impregnation Fig. 3 outlines the process of slurry dewatering in kerosene and Ca impregnation process. Without Ca impregnation, the wet coffee grounds containing 65 wt.% water were first mixed with kerosene to make a fuel-oil slurry. After pressurizing to 0.3 MPa, preheating, dewatering and separation, the slurry was split into three streams: dried fuel for gasification, separated kerosene for recirculation and water to be drained. In Fig. 3, slurry was dewatered via flash evaporation under reduced pressure at 443 K, while the steam generated in the flash evaporation was re-compressed for heating the fuel-oil slurry to recover the energy with the steam. For Ca impregnation, Ca(OH)2 or CaO was first mixed with kerosene to make an oil-based slurry, followed by, mixing the wet fuel with the Ca-containing slurry and drying of the fuel in the slurry in the same way as for the case without Ca impregnation. Fig. 4 compares the water content in the dried coffee grounds as a function of the slurry temperature in the dewatering tank for the cases with and without Ca impregnation. The dosed Ca was Ca(OH)2 with 7% impurities and the dosing ratio was about 4 wt.% of the dry fuel. The results show that after drying the water contents were reduced from 65% to about 10 wt.% and the presence of the impregnated Ca in fuel hardly affected the fuel dewatering characteristics. Table 1 summarizes the properties of the dried coffee grounds, showing that after drying the Caimpregnated fuel had higher ash content because of of the Ca species added. Consequently, C and H contents as well as HHVs
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Fig. 3. Drying fuel via slurry dewatering in kerosene. Steps highlighted with broken lines apply to Ca impregnation.
nated Ca(OH)2 , respectively. The CaO adopted for mechanical mixing was calcined limestone, whose properties are also outlined in Table 1. X-ray fluorescence (XRF) analysis confirmed that both fuels have similar Ca content (Xu et al., 2006c), about 4 wt.% of dry-base fuel. Nevertheless, the dispersion of Ca shown as red dots in Fig. 5 over the fuel matrix is evidently different from each other: far more even and discretely dispersed for impregnation in comparison with mechanical mixing.
Fig. 4. Dewatering characteristics of coffee grounds with and without Ca impregnation.
of the Ca-impregnated fuel were all lower than the fuel without Ca impregnation. In both cases the coffee grounds were rich in volatile matters (up to 70 wt.%) and oxygen. Measurement showed also that the kerosene remaining in the dried fuel was less than 0.2% of the oil used. Fig. 5a and b show the Ca distributions over fuel matrix measured by energy dispersive X-ray detector (EDX) for the dried coffee grounds with mechanically mixed CaO and impregTable 1 Properties of fuels and CaO adopted No Ca
Ca-impregnated
Proximate (wt.%) Moisture VM FC Ash
10.5 71.8 16.7 1.0
14.8 64.9 15.8 4.5
Ultimate (wt.%-db) C H N S O
52.97 6.51 2.80 0.05 36.62
50.17 6.07 2.78 0.08 35.78
HHV (MJ/kg-db) Sizes Bulk density (kg/m3 )
21,980
20,457 <2.0 mm About 350
CaO for physical mixing: lightly calcined CaCO3 ore, bulk density = 1050 kg/m3 , sizes <1.0 mm (Sauter mean: 70 m), specific surface area = 2.0–5.0 m2 /kg, pore volume = 0.01 L/kg.
Fig. 5. Distributions (1000×) of Ca over fuel matrix: (a) With mechanically mixed CaO; (b) with impregnated Ca(OH)2 .
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Table 2 Molar compositions of rude product gases from gasifier corresponding to efficiency data of Fig. 6 Items
Pure coffee grounds Mixed with CaO Impregnated with Ca(OH)2
H2 CO CO2 CH4 C2 H4 C2 H6 C3 H6 Ar H2 /COa
18.65 29.36 10.54 13.24 4.80 2.48 0.03 18.77 0.65
a
21.75 26.28 11.56 13.19 4.90 2.50 0.03 17.77 0.83
31.86 23.97 14.48 10.05 3.51 1.84 0.02 12.57 1.33
The mentioned H2 /CO refers to molar ratio.
2.2. Catalytic gasification of predried biomass Gasification of the predried bare coffee grounds and Caimpregnated as well as Ca-mixed coffee grounds was carried out in a 5.0 kg/h pilot DFBG facility. The facility consists of a bubbling fluidized bed (BFB) gasifier with a rectangular cross section of 370 mm × 80 mm and a height of 2.0 m and a pneumatic riser char combustor of 52.7 mm i.d. and 6.4 m high. Fuel was supplied into the gasifier from a location slightly above the dense bed surface (generally 0.6 m high). The heat carrier particles (HCPs) circulating between the gasifier and combustor were silica sand of 190 m in Sauter mean diameter. Details about this facility and related experiments were already published (Xu et al., 2006a, 2006c; Xu, Murakami, Suda, Matsuzawa, & Tani, 2006b). The major operating parameters for the gasifier are listed in Fig. 6, while the riser combustor ran generally at about 1093 K at a superficial gas velocity of 2.5 m/s. Methods for recovering tar and measuring product gas composition and volume were detailed in Xu et al. (2006a,b,c). The tar-recovery system trapped 90% of the total tar in the product gas. Both Fig. 6 and Table 2 show quasi-steady-state gasification results at a temperature (Tb ) of about 1083 K. The typical time series of product gas composition is available in Xu et al. (2006c). Compared to bare coffee grounds (open bars in Fig. 6), the grounds with impregnated Ca (black bars) greatly increased fuel C and H conversions (Xc and Xh ) and cold gas efficiency ηe (Fig. 6a), while the tar content in the product gas was decreased by 75%, from about 40 to 10 g/N m3 (Fig. 6b). The reported ηe refers to an explicit value against the HHV of the treated fuel (i.e., without considering the running energy consumption). The Ca-impregnated coffee grounds exhibited a particularly high increase in H conversion Xh , from 88% to 140%, as compared to bare fuel. This,shows that a considerable amount of H in the product gas was derived from H2 O through, for example, steam gasification, reforming and watergas-shift reactions. Under the same gasification conditions, the mechanically mixed CaO yielded merely weak effects (shaded bars in Fig. 6), though with similar amount of Ca in fuel. Table 2 demonstrates that the product gas from the Ca-impregnated fuel has much more H2 and CO2 and much less CO and hydrocarbons in comparison with that from bare fuel. The mechanically mixed CaO affected the composition of product gas in the same trend
Fig. 6. (a and b) Gasification results of bare coffee grounds and coffee grounds with mechanically mixed CaO and impregnated Ca(OH)2 , with tar content and HHV calculated on the basis of Ar-free gas. Xc Xh ηe F S Tb Ub ter
C conversion H conversion Cold gas efficiency Fuel feed rate excluding Ca Steam feed rate Gasifier temperature Superficial gas velocity in gasifier Explicit residence time of fuel in gasifier
% % % kg/h kg/h K m/s s
but to a lesser degree. Corresponding to these differences in gas composition, the HHV of the product gas is much lower for the Ca-impregnated fuel but only slighty lower for the CaO-mixed fuel (Fig. 6b). Nonetheless, for the Ca-impregnated fuel the HHV of product gas is still as high as 4000 kcal/N m3 (free Artracer), thus demonstrating the merit of the DFBG technology conceptualized in Fig. 2. 2.3. Analysis on calcium catalysis Biomass gasification involves both fuel pyrolysis and char gasification. Though Ca is known to catalyze biomass and coal
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Fig. 7. TG data in steam circumstance for the fuels without Ca and with mechanically mixed CaO and impregnated Ca(OH)2 . All TG data are against the base excluding their residues held in TG cell.
gasification, it is not clear as to which step the calcium catalysis contributes. To unravel the catalytic mechanism of impregnated Ca in biomass gasification, all the fuels tested in Fig. 6 were further examined with TG in steam. Fig. 7 shows the TG results, where the plotted TG variation was estimated against the sample weight removing the post-reaction residue in the cell. For each TG diagram in Fig. 7, three inflection points can be identified: A, B and C. Before point A fuels are dried, between points A and B the major reaction is pyrolysis, followed by a stagnation period until point C is reached at which onset of char gasification begins. Although little difference exists for pyrolysis of the three fuels, the sample weight variation around the inflection point C is quite obvious for the Ca-impregnated fuel, while for the fuel without Ca additive it is not evident. Hence, we suggest that point C denotes the onset of obvious catalysis of Ca for char gasification. The data of Fig. 7 demonstrate that the impregnated Ca catalyzed char gasification exclusively and that this catalytic effect was very low for mechanically mixed CaO. The status of Ca dispersion on char particles taken from the gasification tests was further examined with EDX, as shown in Fig. 8. For mechanically mixed CaO the Ca species are poorly dispersed (Fig. 8a), while the Ca dispersion is highly uniform for the impregnated Ca(OH)2 (Fig. 8b). To catalyze char gasification requires close contact between Ca species and char particles (Clemens et al., 1998). Adequate dispersion of Ca in the char-C matrix is consequently an indispensable prerequisite for high catalytic activity of the Ca species added into the fuel. 3. Direct gasification of wet biomass with D-DFBG The proposed decoupled dual fluidized bed gasification (D-DFBG) for direct gasification of wet biomass is conceptualized in Fig. 9 (Xu, Liu, et al., 2006). The D-DFBG is characterized with the use of a two-compartment fluidized bed as the fuel gasifier. The bed may have various configurations or simply a BFB modified by partitioning its reaction space into two compartments with a partition plate (see the
Fig. 8. Calcium dispersion maps on char particles from the fuels with (a) mechanically mixed CaO and (b) impregnated Ca(OH)2 by EDX analysis.
broken-line boxed part). The partition plate is immersed in the fluidized particles but leaves a passage between the lower end of the plate and the bed distributor to allow both particles and gas to flow from one compartment to the other. Hence, this bed has a U-shaped configuration and may be named U-type fluidized bed (UFB). As illustrated in Fig. 9, wet fuel and re-circulated hot HCPs are fed into the right-side compartment of the bed. Steam, possibly mixed with an oxidant (such as air), is also fed into this compartment to fluidize the particles. Residence time of the fuel particles is controlled appropriately to allow the fuel to undergo predominantly drying and pyrolysis in this compartment. The required endothermic reaction heat comes mainly from the hightemperature HCPs. Steam, pyrolysis gas and char generated in this compartment all move into the left-side compartment via the passage below the partition plate. In the left-side compartment char gasification and tar/hydrocarbon reforming take place to produce a product gas. Both sensible heat of the HCPs from the right-side compartment and the heat released by combustion of fuel with oxidant, either oxygen or air, fed into the left-side compartment can provide the required endothermic reaction heat in
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Fig. 10. Potential benefit from using fuel water to replace part of steam reagent (treated fuel: coffee grounds containing 50 wt.% water; base conditions: ratio of fed-steam to wet-base fuel is 1.0 kg/kg, heat lose is 5% of fuel HHV).
Fig. 9. Process concept of the decoupled dual fluidized gasification (D-DFBG).
this compartment. The steam from the right-side compartment becomes then part of the steam reagent required for char gasification and tar/hydrocarbon reforming, leading to a decrease of the steam fed into the left-side compartment. Meanwhile, particles in the left-side compartment are actually a mixture of HCPs and char so that the reforming of tars and hydrocarbon takes place in a bed of char particles. This enables consequently char to catalyze such reactions. As in normal DFBG, HCPs with lowered temperatures, together with unreacted char, in the left-side compartment of the UFB are forwarded to the riser combustor of the D-DFBG to allow the unreacted char to burn out there with air and to reheat the HCPs. The distinctive feature of the devised D-DFBG is the use of a UFB to substitute for the BFB used for normal DFBG. This allows the decoupling of fuel drying and pyrolysis from char gasification and tar/hydrocarbon reforming to enable separate controls for these reactions and to make it possible to take advantage of the interactions between these reactions. In fact, the D-DFBG enables (1) char to catalyze reforming of tars and hydrocarbon, (2) the use of water vapor generated in fuel drying/pyrolysis as part of steam reagent, and (3) the adoption of different gasification reagents for fuel pyrolysis and char gasification reactions. With all of these merits, high gasification efficiency and low tar evolution are hopefully available with D-DFBG. The high efficiency of D-DFBG is associated with its use of vaporized fuel water as part of the required steam as reagent and thereby the reduction of steam supply. This would make the DDFBG particularly suitable for wet fuels. Fig. 10 demonstrates how the use of fuel water as part of steam reagent affects the gasification efficiency. The data were from a thermodynamic calculation (without considering kinetics) using the model and approach for normal DFBG system reported in Murakami et al.
(2007). The model was based on Aspen computation, capable of estimating gasification efficiencies for steam gasification of different fuels under different steam feeds. The treated fuel was coffee grounds containing 50 wt.% water, and the base condition at zero abscissa is for a feed ratio of 1.0 kg/kg of steam reagent to wet-base fuel and a system heat loss of 5% of fuel’s HHV. Fig. 10 shows that using 50% and 80% of the fuel water to reduce an equal amount of steam reagent supply may raise the cold gas efficiency by 5% and 8%, respectively. Although decreasing steam feed lowers the steam to dry fuel ratio in the reactor, for the estimated case in Fig. 10 the lowest ratio is still higher than 2.0 kg/kg. This ratio is sufficiently high to keep the reaction kinetics to vary little with the steam/dry-fuel ratios. Consequently, the prediction revealed, to a great degree, what actually occurs to the D-DFBG and demonstrated essentially its inherent technical advantage. 4. Conclusions Different from many biomass resources that are widely scattered for convenient gathering, various kinds of process residues and sewage sludge represent a different kind of biomass resource, already concentrated, though with water contents as high as 60 wt.%. These biomass matters are therefore readily available for industrial-scale exploitation to take advantage of their CO2 -neutralization function. The present study devised a process to convert such biomass into middle-caloric gas with heating values of 3000–5000 kcal/N m3 via dual fluidized bed gasification (DFBG). Because of the high water content of the fuel, gasification needs to follow fuel drying in advance. In this study, such fuel drying is taken with impregnation of Ca-base catalyst in the fuel, which, with the high dispersion of Ca in the char-C matrix, enhances the reactivity of char during gasification and reduces tar formation during gasification. Calcium impregnation was implemented by simply mixing CaO or Ca(OH)2 into the high-water-content biomass at the beginning. Experiments showed that mixing CaO or Ca(OH)2 did not adversely affect the fuel’s dewatering characteristics. On the other hand, the so-called decoupled dual fluidized bed gasification (D-DFBG), with the use of a U-type fluidized bed (UFB), appeared highly
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efficient in gasifying biomass with high water content. The DDFBG decouples fuel drying/pyrolysis from char gasification and tar/hydrocarbon reforming to enable the vaporized water from the wet fuel to be part of the required steam for gasification. Acknowledgements All experimental work presented in this article was conducted in IHI Co. Ltd., Japan, under a financial support from The New Energy and Industrial Technology Development Organization (NEDO), Japan. The Natural Science Foundation of China (NSFC) financed the first period of research on the decoupled dual fluidized bed gasification (20606034, 20776144). The authors are grateful for all of these financial assistances. References Clemens, A. H., Damiano, L. F., & Matheson, T. W. (1998). The effect of calcium on the rate and products of steam gasification of char from low rank coal. Fuel, 77, 1017–1020. IEA Bioenergy. (2006). What Is Biomass? From IEA bioenergy education website on biomass and bioenergy: http://www.aboutbioenergy. info/definition.html.
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