Energy Fuels 2010, 24, 6223–6232 Published on Web 11/04/2010
: DOI:10.1021/ef101036c
Decoupling Gasification: Approach Principle and Technology Justification Juwei Zhang, Yin Wang, Li Dong, Shiqiu Gao, and Guangwen Xu* State Key Laboratory of Multiphase Complex System, Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), Beijing 100190, China Received August 6, 2010. Revised Manuscript Received September 25, 2010
Explicitly, the fuel gasification process refers to a reaction converting a solid fuel into gaseous products, but intrinsically, it involves a series of reactions, including fuel pyrolysis, char gasification, tar reforming/ cracking, combustible matter combustion, etc. All of these reactions are mutually interactive and fully coupled in a single gasification reactor (i.e., gasifier) for the major commercial gasification technologies. The decoupling gasification (DCG) mentioned in this paper is based on separating and, in turn, reorganizing at least one of the involved reactions to facilitate or suppress the interactive effects between the separated and other reactions. From this decoupling approach, there is potential to allow for the resulting gasification technology to realize the effects of polygeneration, low emission, high efficiency, good product quality, and wide fuel adaptability. This paper generalizes the decoupling approach into two types: isolating and synergizing. Through correlating technical features with these decoupling approaches, re-analysis of the technology principles is made for a few newly developed gasification technologies based on decoupling of the involved gasification reactions. The typical results obtained in research and development of these technologies at bench or pilot scales were recalled to justify the implicated decoupling principle and consequent benefits. As a consequence, the paper concludes that the decoupling of reactions provides a prospective approach to innovate technologies that enable high-efficiency clean conversion of solid fuels into high-quality products.
All of the above-mentioned reactions and their products are mutually interactive. The steam generated in fuel drying and pyrolysis can act as a reagent of gas/tar reforming and char gasification. Gasification of char may also be through direct interaction with the air or O2-steam mixed gas supplied for combustion. When all reactions occur in the same vessel, the combusted fuel for generating the gasification-required endothermic heat may be all of the combustible gases and solid matters presented in the vessel. CO2 generated in combustion and nitrogen fed into the system via an air reagent (if using air), on the other hand, seriously dilute the produced gasification gas. Pyrolytic char, containing some metals, is not only the reactant for gasification and combustion but also a catalyst for cracking/reforming tars and hydrocarbons. For most of the gasification technologies commercialized or under development,3 the above-mentioned various reactions are arranged to occur in a single space, making it impossible to manipulate any individual reaction to control its engaged interactions with the other reactions. Controlling an individual reaction and its engaged interactions, on the other hand, has the potential to optimize the reaction process to benefit the gasification performance, including possibly the realization of the high-value use of the hierarchical composition of the fuel, rise of fuel conversion efficiency, decrease of pollutant emission, upgrading of produced gas quality (e.g., raising heating value), and even allowing polygeneration and good fuel adaptability. This requires us to separate and reorganize one or some of the reactions in turn to strengthen the beneficial interactions, inhibiting the undesired interactions or isolating the products of different reactions. In
1. Conception Highlight Gasification, as a core technology for the production of chemicals and clean power, refers to a process converting solid fuels, including coal, biomass, and wastes, into either fuel gas (containing CH4 and some N2 usually) or syngas (containing mainly H2 and CO). It represents chemically a reaction complex that combines pyrolysis (including drying) of fuel, gasification of char, reforming and cracking of tar and hydrocarbons, combustion of combustible matters, etc. These reactions, with their independent functions, are closely related to each other to form a complicated reaction network. Figure 1 highlights this network by showing and intercorrelating the major reactions involved. The chemical process involved in gasifying a fuel is highly endothermic, and the entailed endothermic heat is generally supplied by a simultaneous combustion inside the gasifier. When this heat is applied to the treated fuel, what happened first is drying and pyrolysis, which convert the fuel into steam, noncondensable pyrolysis gas, tars (condensable pyrolysis gas), and char. A series of sequential reactions then occur in turn to these pyrolysis products to generate the final gasification product, either fuel gas or syngas. Both tars and pyrolysis gas may further crack/decompose, polymerize, or be reformed,1,2 while char is gasified by reacting with a gasification reagent. Meanwhile, water-gas shift (WGS, not mentioned in Figure 1) occurs to adjust the concentrations of H2, CO, CO2, and steam in the final gasification product gas according to the thermodynamic equilibrium among these gas species. *To whom correspondence should be addressed. Telephone/Fax: þ86-10-62550075. E-mail:
[email protected]. (1) Rath, J.; Staudinger, G. Fuel 2001, 80 (10), 1379–1389. (2) Gil, J.; Caballero, M. A.; Martin, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1999, 38 (11), 4226–4235. r 2010 American Chemical Society
(3) Higman, C.; Burgt, M. V. D. Gasification; Elsevier Science: Amsterdam, The Netherlands, 2003.
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: DOI:10.1021/ef101036c technologies. The third to fifth decoupling effects and consequent DCG technologies (TSG, POG, and PHG) shown in Table 1 are formed by decoupling pyrolysis reaction according to the synergizing approach. As will be detailed herein for every individual technology, these different decoupling effects are generated by reorganizing the pyrolysis and gasification reactions in different ways. The sixth decoupling effect in the table is in fact a combination of an isolating effect from separating combustion and gasification reactions and a synergizing effect generated by DCG and tar cracking/reforming reactions. These exemplified decoupling effects demonstrate that, through the so-called “synergizing” decoupling, there is potential to lower tar generation, enhance fuel adaptability, raise product quality in polygeneration and, thereby, innovate new gasification technologies. Hybrid decoupling through combining “isolating” and “synergizing” may further enhance these decoupling effects to lead to rather advanced technology design. Among these DCG technologies, the DBG, POG, and PHG are in the demonstration stage, while the others are in the pilot test stage. The following sections of this paper are devoted to describing in details the principle and experimental demonstration of the decoupling effects for those DCG technologies shown in Table 1. For this, a comprehensive review and summarization are presented herein for the related literature reports.
Figure 1. Reaction network in the gasification process of carbonaceous solid fuels.
contrast with the conventional gasification technologies, this paper generalizes the technologies with the feature of “control of the reaction and its related interactions” as “decoupling gasification (DCG)”. To build up a deep understanding of the principle and necessity of “decoupling”, the paper is devoted to a high-level interpretation of the approach of decoupling and then a comprehensive review on the design principle and realized advantages of a few typical DCG technologies developed worldwide in recent years. Through these, it is expected to demonstrate that the approach of “decoupling (reactions)” provides a necessary powerful tool for innovating new gasification technologies that lead to advanced performances in efficiency, product quality, clean conversion, and process intensification.
3. DCG with an Isolating Method Following the summary of Table 1, this section highlights the principle and technology features of the DBG and PG technologies. 3.1. DBG. The process principle of the DBG technology, as schematically shown in Figure 3a, is based on isolating the gas production reactions, mainly pyrolysis and gasification reactions, and the combustion reaction of unreacted char. The process mainly consists of a gasifier, a combustor, and the flow channels for heat carrier particles (HCPs). Solid fuel is fed into the gasifier, and the chars coming from the gasifier are combusted in the combustor. Between the two reactors, HCPs are circulated to carry the combustion heat from the combustor to the gasifier. The sensible heat of the circulating particles sustains partially or completely the highly endothermic fuel pyrolysis and gasification reactions occurring inside the gasifier. The gases from the two reactors are treated as two independent streams to avoid mutual mixing and dilution. Consequently, the technology enables the production of high- or middle-caloric fuel gas without using oxygen or oxygen-rich reagent. Generally, the gasifier and combustor are both fluidized-bed reactors, thus also calling the technology dual-fluidized-bed gasification (DFBG). The study on DFBG has attracted great interests in the world, especially with extensive research and development work in China,4-12 Japan,13-17 and Europe.18-20 Comprehensive reviews of the DFBG technologies have been reported in refs 13 and 21.
2. Approach Principle The main reactions involved in solid fuel gasification include pyrolysis, char gasification, combustion, reforming and cracking, or decomposition. Figure 2 replots how these reactions are interrelated from the aspect of occurrence sequence. In the figure, each of the solid line arrows points at a downstream reaction whose occurrence relies on the products from the upstream reaction. This implicates a kind of linkage between each pair of the neighboring reactions to indicate the coupling relationship of the linked two reactions. Theoretically, one can find a suitable way to break any of the linkages in the figure to realize the decoupling of the arrow-linked reactions, as illustrated by the star between pyrolysis and gasification. After breakage of the reaction linkage, there are two different methods to reorganize the decoupled reactions, and Figure 2 shows them schematically via the two branches stretched to both sides. The decoupled reactions can be arranged into two isolated reactors to separate their products to realize polygeneration and also suppress the intereffects between the products of the decoupled reactions. This kind of decoupling approach is called “isolating” decoupling, and the resulting technology integrates usually two reactors, thus calling it “dual-bed” technology. Another approach of decoupling is termed as “synergizing” decoupling, as illustrated in the right branch of Figure 2. In this method, the decoupled reactions are rearranged to facilitate the beneficial interactions or to suppress the undesired interactions between the linked reactions (for example, the effect of the products from one reaction on the other reaction). Via this kind of decoupling, it is expected to lower pollutant formation, enable high product quality and high conversion efficiency, and/or enhance the fuel adaptability of the technology. Table 1 summarizes a few typical decoupling effects and their consequent DCG technologies realized through implementing the “isolating” or “synergizing” decoupling among the major reactions mentioned in Figure 2. The first two effects are realized through isolating the combustion and pyrolysis reactions, respectively, resulting in the so-called dual-bed gasification (DBG) and pyrolysis gasification or topping gasification (PG or TG)
(4) Yao, J. Z.; Wang, F. M. New Energy Sources 1998, 20 (5), 14–18 (in Chinese). (5) Yao, J. Z.; Wang, F. M.; Li, Y. C.; Liu, S. J.; Han, K.; Wang, F.; Li, G. L.; Mi, W. S. Chinese Patent 96209381.5, 1996. (6) Yao, J. Z.; Wang, F. M.; Li, Y. C.; Liu, S. J.; Han, K.; Wang, F.; Mi, W. S.; Li, G. Proceedings of the 5th International Conference on Circulating Fluidized Beds (CFB); Beijing, China, 1996. (7) Wei, L. G.; Xu, S. P.; Liu, J. G.; Lu, C. L.; Liu, S. Q.; Liu, C. H. Energy Fuels 2006, 20 (5), 2266–2273. (8) Wei, L. G.; Xu, S. P.; Liu, J. G.; Liu, C. H.; Liu, S. Q. Proceedings of the 9th China-Japan Symposium on Coal and C1 Chemistry; Chengdu, Sichuan, China, 2006. (9) Fang, M. X.; Shi, Z. L.; Wang, Q. H. Power Eng. 2003, 23 (4), 2524–2529 (in Chinese).
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Figure 2. Two decoupling approaches applied in developing DCG technologies. Table 1. Summary of Typical DCG Technologies and Corresponding Decoupling Effects number
decoupling method
1
isolating 2
3 4 synergizing 5
6
isolating and synergizing
main decoupled reaction(s)
decoupling effects avoiding dilution of produced gas by N2 and combustion-generated CO2 to produce middle-caloric fuel gas using air as a gasification reagent co-producing pyrolyis oil and fuel gas (or syngas) to implement hierarchical use of solid fuel (such as coal) reforming/cracking of tar and pyrolysis gas through catalysis of char to produce fuel gas or syngas with little tar avoiding caking of softened coal or char to improve the fuel adaptability of the technology realizing partial hydropyrolysis of fuel (coal) to raise pyrolysis oil quality and increase energy efficiency increasing efficiency and enhancing in-bed tar elimination to produce middle-caloric fuel gas with less tar and more H2
typical technology
application condition
dual-bed gasification (DBG)
demonstration
pyrolysis gasification (PG)
pilot
two-stage gasification (TSG)
pilot
preoxidized gasification (POG)
demonstration
gasification
partial hydropyrolysis gasification (PHG)
demonstration
combustion and reforming
two-stage dual-bed gasification (T-DBG)
pilot
combustion
pyrolysis
(PR), Xu et al.13 proposed that the DFBG technology should have at least four different technical options varying with the bed combinations of (1) two BFBs, (2) two PRs, (3) a BFB char combustor plus a PR fuel gasifier, and (4) a PR char combustor plus a BFB fuel gasifier. By comparing the performances of these optional processes in gasifying the same biomass fuel, it was found that option 4 represents the best choice among all of those bed combinations for DFBG technology in terms of achieving high gasification efficiency and low tar generation. When combustion for endothermic heat was isolated from the gas production reactions, including mainly pyrolysis and gasification, the realized decoupling effect via DFBG is the avoidance of dilution of the produced gas via the combustion flue gas, as highlighted in Table 1. This enables DFBG to produce middle-caloric fuel gas without the use of oxygenrich reagent. This benefit of decoupling has been widely verified by many pilot tests performed worldwide for biomass fuels (see review in refs 13 and 21). The process principle, on the other hand, is also applicable to coal gasification. Currently, IHI Corporation, Ltd. in Japan and Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS) in China are engaged in the development of coal DFBG technologies. While a report about the IHI gasifier is
The fluidized bed has different types depending upon the gas velocity inside the bed. When the fluidized beds were grossly classified under different conditions into bubbling fluidized bed (BFB) and pneumatic transport bed or riser (10) Fang, M.; Wang, Q.; Yu, C.; Shi, Z.; Luo, Z.; Cen, K. Proceedings of the 18th International Conference on Fluidized Bed Combustion; Toronto, Ontario, Canada, 2005. (11) Zhang, X. F.; Jin, L.; Liu, Y. Y.; Xu, G. W. Chem. React. Eng. Technol. 2008, 24 (3), 193–198 (in Chinese). (12) Lv, Q. G.; Na, Y. J.; Bao, S. L.; Gao, M.; Sun, Y. K.; He, J.; Yuan, X. Y. Chinese Patent 200410081018.5, 2004. (13) Xu, G. W.; Murakami, T.; Suda, T.; Matsuzawa, Y.; Tani, H. Ind. Eng. Chem. Res. 2006, 45 (7), 2281–2286. (14) Xu, G. W.; Murakami, T.; Suda, T.; Matsuzawa, Y.; Tani, H. Energy Fuels 2006, 20 (6), 2695–2704. (15) Xu, G. W.; Murakami, T.; Suda, T.; Tani, H.; Mito, Y. Particuology 2008, 6 (5), 376–382. (16) Murakami, T.; Xu, G. W.; Suda, T.; Matsuzawa, Y.; Tani, H.; Fujimori, T. Fuel 2007, 86 (1-2), 244–255. (17) Asadullah, M.; Miyazawa, T.; Ito, S. I. Appl. Catal., A 2004, 267 (1-2), 95–102. (18) Pfeifer, C.; Rauch, C.; Hofbauer, H. Ind. Eng. Chem. Res. 2004, 43 (7), 1634–1640. (19) Kaiser, S.; Loeffler, G.; Bosch, K.; Hofbauer, H. Chem. Eng. Sci. 2003, 58 (18), 4215–4223. (20) Sudiro, M.; Bertucco, A.; Ruggeri, F.; Fontana, M. Energy Fuels 2008, 22 (6), 3894–3901. (21) Corella, J.; Toledo, J. M.; Molina, G. Ind. Eng. Chem. Res. 2007, 46 (21), 6831–6839.
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available in refs 13-16, Figure 4 shows the results from a pilot coal DFBG test performed recently in China to further justify the realized decoupling effects. Figure 3b presents a process schematic diagram of the Chinese pilot coal DFBG plant, which had a riser combustor of 240 mm in inner diameter and 10 m in height and a rectangular fluidized-bed gasifier of 1200 800 mm2 in cross-sectional area and 3 m high. A bituminous coal below 10 mm, as characterized in Table 2, was gasified in this plant at a feed rate of 230 kg/h (see testing conditions in Figure 4). Quartz sand of 0.25 mm in the Sauter mean diameter was employed as the HCPs, and a mixture of air and steam was fed into the gasifier as the gasification reagent. As shown in Figure 4, the pilot plant exhibited steady performances in a continuous operation running for about 15 h at a coal feed of 230 kg/h. The temperatures in the presented test were about 1173 K for the fluidized-bed gasifier and up to 1253 K in the riser combustor. The composition (analyzed with a micro GC, Agilent GC3000) and higher heating value (HHV) of the produced fuel gas measured at the gasifier exit steadily varied in the tested time. The averaged volume fractions of H2, CO, CH4, and CO2 were
22.3, 23.3, 2.66, and 10.3%, respectively. This corresponded to a mean HHV of about 1620 kcal/Nm3. The above results demonstrate not only the feasibility of DFBG for gasifying coal but also the merit of DFBG from decoupling char combustion reaction from the fuel overall gasification reaction (including pyrolysis). Although air was used, the realized HHV of the produced fuel gas reached
Figure 4. Operation temperature, HHV, and compositions of the produced gas during a continuous operation of a pilot coal DFBG plant for 15 h.
Figure 3. Principle and process schematic diagrams of the DFBG plant.
Table 2. Properties of the Fuels Adopted in Testing the DCG Technologies Referred to in This Paper proximate (wt %, ad)
ultimate (wt %, daf)
DCG technologies
involved figure/table
fuel type
moisture
volatiles
ash
C
H
N
S
HHV (MJ/kg, ad)
DBG PG TSG POG PHG T-DBG
Figure 4 Table3 and Figure 7 Table4 Table5 Figure 11 Figure 13
bituminite lignite wood chip several coals sub-bituminite coffee grounds
5.0 16.5 0.0
15.4 32.5 N/A
17.0 38.9 0.91
71.7 69.2 48.9
3.6 4.2 6.2
1.5 0.9 0.2
1.0 4.6 0.02
31.69 19.35 19.60
3.8 10.5
45.4 71.8
2.7 1.0
76.4 53.5
5.6 6.6
1.8 2.8
0.1 0.1
22.09
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: DOI:10.1021/ef101036c Table 3. Typical Composition and HHV of Produced Pyrolysis and Gasification Gases via PG composition (vol %)
pyrolysis gas including N2a pryolysis gas excluding N2 gasification gasb
H2
CH4
CO
CnHm
HHV (kcal/Nm3)
14.4 49.4 7.7
3.3 10.4 2.0
4.2 12.2 4.3
1.0 12.4 1.2
1053.0 4781.0 745.3
a The fluidizing gas was N2, and the pyrolysis temperature was 873 K. Gasifying reagent, air; gasification temperature, 1123 K; equivalence ratio, 0.5.
b
a riser gasifier, and in between a particle flow partition valve. A quenching and liquid-capturing system was used to collect tar, a condensable pyrolysis gas product. In the pilot test, N2 was used as the fluidizing gas for the pyrolyzer, although this may be the circulated pyrolysis gas in the commercial PG process. A Chinese coal was tested in this pilot plant, and the major properties are listed in Table 2. Table 3 shows the composition and corresponding HHV of the resulting pyrolysis gas (from the pyrolyzer) and the produced gas from the gasifier. In this test, the fluidizing gas for the pyrolyzer was N2, so that the generated pyrolysis gas had a high content of N2 in Table 3. Removing N2 from the gas found that the resulting pyrolysis gas has a heating value above 4700 kcal/Nm3. This heating value may be achieved using the pyrolysis gas as the fluidizing gas. In this test, the produced gas in the riser gasifier had a heating value of about 750 kcal/Nm3 only, because too much volatiles were released in the pyrolyzer, resulting in great differences of HHV between the gasification and pyrolysis gases. When the compositions of N2-free gasification and pyrolysis gases are compared, it can be seen that the pyrolysis gas contained much more CH4 and H2, indicating that the PG enables production of CH4-rich gas that is more suitable for production of SNG. The realized oil (tar) yield in displayed test was about 3% of dry coal, which complies with the condition that the treated coal was lignite. The obtained tar was analyzed using gas chromatographymass spectrometry (GC-MS), and the obtained typical chromatogram is presented in Figure 6, whose overhead table shows the main compounds corresponding to the peaks in the figure. It is obvious that the produced tar contained a considerable amount of high-value chemicals, such as phenol (7) and phenanthrene (12). This validates the technology possibility of co-producing chemicals with fuel gas or syngas via PG technology. Figure 7 further shows the process simulation results for the PG technology, aiming at demonstrating its technical merits and possible gas qualities. The simulation adopted air as a gasification reagent, and the pyrolysis gas is free of N2. Theoretically, the riser gasifier can generate fuel gas or syngas containing less CH4 and H2 but with a heating value of above 1000 kcal/Nm3, if we increase the C and H remaining in char by decreasing the residence time of coal in the pyrolyzer to reduce the extent of the pyrolysis reaction. For example, when the fractions of C and H remained in char are 84 and 38%, the HHVs of gasification and pyrolysis gases are 1078 and 2924 kcal/Nm3, respectively. Because the temperature of pyrolysis gas is definitely lower than that of the gasification-produced gas, the overall oxygen consumption is thus lower than the traditional one-vessel gasification technology, similar to Lurgi. It should be mentioned that the mixture gas HHV of the gasification and pyrolysis gases in
Figure 5. Principle and process schematic diagrams of the PG plant.
about 1600 kcal/Nm3, higher than 800-1200 kcal/Nm3, which is the heating value of the fuel gas generated via traditional air gasification (e.g., fixed-bed air gasification) that couples all of the involved reactions together in a single reactor. According to Xu et al.,14 rather higher HHVs, such as over 2000 kcal/Nm3, are possible for this coal DFBG through, for example, increasing the steam/air ratio of the reagent into the gasifier and, meanwhile, raising the circulation rate of HCPs from combustor to gasifier. This shows just the principle and advantages of the DFBG technology resulting from decoupling. 3.2. PG. The process principle of the PG technology is based on isolating pyrolysis and char gasification, as schematically shown in Figure 5a. Raw coal/biomass is first fed into a pyrolyzer and pyrolyzed quickly to produce tar and pyrolysis gas. The residual char is in turn conveyed into a gasifier and gasified there to produce fuel gas or syngas. Via a partition valve for the flow of circulated particles, a part of the high-temperature ash coming from the gasifier (containing also unreacted char) is conveyed into the pyrolyzer to supply the reaction heat required by fuel pyrolysis and then recycled into the gasifier with the resultant char. In comparison to the other conventional gasification technologies, in which the solid fuel is entirely gasified without considering the hierarchical composition characteristics of fuel (especially of coal), the PG technology makes full use of the high-value aromatic compound contained in the fuel (coal), so that it can co-produce tar and fuel gas or syngas to realize polygeneration. Figure 5b presents a process schematic diagram, which is drawn according to a pilot PG plant built recently at IPE, CAS. This plant consisted mainly of a fluidized-bed pyrolyzer, 6227
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Figure 8. Principle and schematic diagram of the two-stage gasifier. 27
for powder coal that cannot be treated by the two-stage fixed gasifier. Meanwhile, this process implements actually the hierarchical conversion of volatile-rich coal to increase the value of coal use. The PG technology has great potential to replace the conventional two-stage fixed gasifiers to meet the allocation requirement at large-scale production of industrial fuel gas with low cost.
Figure 6. GC-MS chromatograms of tar produced in the PG test corresponding to the gas composition data in Table 3.
4. DCG with a Synergizing Method According to the Table 1, the technologies of two-stage gasification (TSG), preoxidized gasification (POG), and partial hydropyrolysis gasification (PHG) will be reviewed in this section. 4.1. TSG. The reduction of tar in product gas is a key point in solid fuel gasification because the presence of tar can influence the application of product gas in downstream devices, such as the gas turbine and engine, and incur environmental pollutions. Thus far, gas cleanup (hot cracking/ reforming or scrubbing) after gasification and the elimination inside the gasifier are considered to be the two effective approaches to remove tar. Although gas cleanup methods are proven to be effective for tar reduction, they are generally not economically viable and cause possibly serious water pollution (tarry water).22 Suppressing or eliminating tar inside the gasifier is evidently more economically and environmentally competitive. The so-called TSG refers to a kind of gasification process that consists of fuel pyrolysis and removal of pyrolysisgenerated tars inside the process through tar cracking/ reforming over char particles that are gasified simultaneously. The TSG technology is based on decoupling pyrolysis reaction from the other gasification-involved reactions
Figure 7. HHV of the gasification and pyrolysis gases calculated by ASPEN at different pyrolysis extents in the pyrolyzer of the PG plant (the mixture gas HHV of the gasfication and pyrolysis gases in every condition is 1325 kcal/Nm3).
every condition is 1325 kcal/Nm 3, which is also a merit brought by the lower pyrolysis temperature and oxygen consumption. In addition, the PG even reduces the gas volume contained in tars to lower the burden for tar recovery and gas cleanup. In principle, the PG takes the same reaction procedure with the traditional two-stage fixed gasifier for lump coal that is widely used to produce industrial fuel gas. Hence, the PG shown above realizes in fact the two-stage gasification
(22) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24 (2), 125–140.
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Table 4. Summary of Operation Conditions and Experimental Results of the Viking Gasification Plant27,30 items experimental conditions fuel supply (kg/h) air flow rate to gasifier (Nm3 h-1) air flow rate to engine (Nm3 h-1) gas composition (vol %, db) CO CO2 H2 CH4 N2 LHVgasa (MJ/kg, db) tar content (mg/Nm3) efficiency (%) fuel to gas gas to electricity net electricity
values 22.0 16.8 110.2 19.6 15.4 30.5 1.16 33.3 6.19 littleb 93.2 29.1 25.1
a The fuel gas composition and heating value were measured before the gas enters the intake system of the engine. b Only around 0.1 mg/Nm3 naphthalene was detected.
and makes use of the catalysis effect of char for cracking and reforming tarry matters. Figure 8a shows the principle of this decoupling method. Fuel is pyrolyzed in a pyrolysis zone (the first stage), and the produced pyrolysis gas, tar, and char are transported in turn into a tar-reforming and char gasification zone (the second stage). This allows the char to be the bed material of the second-stage gasification zone and enables tar to be reformed/cracked in the gasifier over the char bed during the process of gasifying char. Benefited by the catalysis effect of char, the tar content in the produced gas can be reduced to a very low level to realize consequently the decoupling effect mentioned in Table 1. Several different types of TSG technologies have been proposed. Among them, some use external materials (e.g., char,23 activated carbon,24 and catalyst2,25) as the bed material of the reduction zone, while the others use the chars produced in situ in the gasification process.26,27 I this paper, only the latter type ofn technologies are introduced herein. In addition, there are some lab-scale two-stage gasifiers that are just designed to study the pyrolysis and gasification fundamentals.28,29 They are not included in this paper, although the nascent chars are also used as the bed material in these gasifiers. On the basis of the conception of TSG, Henriksen et al.27,30 built a 75 kWth TSG plant (named “Viking”), which used partial oxidation of pyrolysis gas and char bed to reduce tar in product gas. A schematic diagram of the Viking gasification plant is shown in Figure 8b. Wood chip, whose major properties are listed in Table 2, was dried and pyrolyzed in a screw pyrolyzer that was externally heated,
Figure 9. Principle and process schematic diagram of the KRW gasifier.
and the generated pyrolysis gas and residual char passed through an air partial oxidation zone and a char bed (fixed bed) in sequence. The char bed was formed via the char particles coming from the pyrolyzer and was, in fact, a downdraft fixed gasifier for char. The temperatures in the partial oxidation zone and the char gasification bed were 13731573 and 1073-1273 K, respectively. This Viking gasification plant was built to fuel a continuous heat and power co-production system via using a gas engine to produce electricity. In this demonstration plant, the high-temperature exhaust gas from the engine was used to heat the screw pyrolyzer to sustain the pyrolysis reaction. The plant has been totally operated for more than 3337 h and demonstrated that the tar content in the produced gas can be reduced to considerably low levels. Table 4 summarizes the experimental conditions and the results of a stable running for 24 h for the plant. It is noteworthy that only about 0.1 mg/Nm3 naphthalene could be detected in the product gas and the gas was completely suitable for gas engine operation (the upper limit of tar content for running an engine is 50 mg/Nm3). The realized net electrical efficiency (25.1%) was also satisfactory, being possibly a result of the use of the engine-exhaust heat for fuel pyrolysis. Evidently, these results verified just the anticipated decoupling effects for TSG highlighted in Table 1, thus indicating the necessity and effectiveness of the decoupling implicated in the TSG technology. 4.2. POG. The POG technology is based on the decoupling of the pyrolysis reaction from the other gasification-involved reactions and the suppressing effect of coal preoxidation on char caking during heating. Figure 9a shows schematically
(23) Bhattacharya, S. C.; Siddique, A. H. M. M. R.; Pham, H. L. Energy 1999, 24 (4), 285–296. (24) Mun, T. Y.; Kang, B. S.; Kim, J. S. Energy Fuels 2009, 23 (3), 3268–3276. (25) Myren, C.; H€ ornell, C.; Bj€ ornbom, E.; Sj€ ostr€ om, K. Biomass Bioenergy 2002, 23 (3), 217–227. (26) Soni, C. G.; Wang, Z.; Dalai, A. K.; Pugsley, T.; Fonstad, T. Fuel 2009, 88 (5), 920–925. (27) Henriksen, U.; Ahrenfeldt, J.; Jensen, T. K.; Gobel, B.; Bentzen, J. D.; Hindsgaul., C.; Sorensen, L. H. Energy 2006, 31 (10-11), 1542– 1553. (28) Li, X. J.; Hayashi, J. I.; Li, C. Z. Fuel 2006, 85 (10-11), 1509– 1517. (29) Li, C. Z. Fuel 2007, 86 (12-13), 1664–1683. (30) Ahrenfeldt, J.; Henriksen, U.; Jensen, T. K.; Gobel, B.; Wiese, L.; Kather, A.; Egsgaard, H. Energy Fuels 2006, 20 (6), 2672–2680.
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: DOI:10.1021/ef101036c Table 5. Typical Test Results from the KRW Gasifiera 32 gas composition (vol %, db)
fuel type
gasifying reagent
fuel supply (tons/day)
CO
CO2
CH4
N2
H2
HHV (kJ/Nm3)
bituminous coal bituminous coal sub-bituminous coal sub-bituminous coal lignite
air O2 air O2 O2
12.0 14.0 18.7 24.0 24.0
21.6 42.5 16.2 35.0 40.0
13.5 36.4 16.9 34.2 30.9
1.2 1.9 2.2 5.3 4.3
51.8 0.4 53.3 0.3 0.4
11.9 17.9 11.4 25.1 24.2
4375 7580 4185 8835 8920
a
The gasfication pressure is 1.57-1.67 MPa.
the technology principle. Coal particles conveyed by oxygencontaining gas are injected into a preoxidation zone, where the oxygen in the conveying gas ensures the occurrence of the pyrolysis and oxygen-lean combustion of the coal to form a jet flame. The char particles produced via this oxygen-lean combustion have a much reduced possibility to cake when they are thermally heated together in some dense particle reactor, such as a fluidized bed. There is also no need to worry about the pyrolysis tar, which should be cracked in the high-temperature flame. According to Figure 9a, the char produced in the jet flame is gasified in a char gasification zone, while the produced hot fuel gas can be arranged in turn to supply a part of the heat required by the coal pryolysis and preoxidation in the preoxidation flame. The so-called Kellogg-Rust-Westinghouse (KRW) gasifier sketched in Figure 9b is a typical POG technology. The gasifier is basically a fluidized-bed gasifier, and its preoxidation flame is built through a coaxial central jet of coal with air just above the fluidized particle bed. The formed char particles fall into the fluidized bed in turn to finish their gasification process. As a consequence, the operation temperature of the KRW gasifier has to be between the softening and slagging temperatures of coal ash. The operation temperature in the fluidized bed of the KRW gasifier should be low enough to avoid slagging and high enough to ensure high carbon conversion. The oxygen with the coal jet feed is adjustable according to the needs for forming the jet flame. Oxygen-lean coal combustion in the jet flame and its accompanied heat release cause the coal particles in the jet feed to devolatize quickly. The temperature of the central jet is sufficiently high to crack any tars or oils that might be produced. These make the char particles falling into the fluidized bed beneath the jet flame have a greatly reduced propensity to cake together and destroy the fluidization. Consequently, the coal with a certain caking possibility could also be gasified in the KRW gasifier. Another process feature in the KRW gasifier is that coal ash is removed in the form of agglomeration. This agglomeration occurs because of coal ash softening in the gasifier at 1423-1533 K around the jet flame,31 and the agglomerated ash particles, with some unreacted char particles as well, are removed from the bottom of the gasifier. On the other hand, the fine coal particles flowing upward through the jet flame zone complete their gasification in the freeboard of the gasifier to form the fly ash of the gasifier finally. The typical operational results of the KRW gasifier are listed in Table 5.32 As we know, a fluidized bed is generally applicable for the gasification of lignite and sub-bituminous coal because these types of coal have not only low-caking
Figure 10. Principle and process schematic diagram of the PHG plant.33
propensity but also high gasification reactivity. Table 5 reveals that the KRW gasification technology can accept bituminous coal as well as lignite and sub-bituminous coal. This just verifies the decoupling effect shown in Table 1, which is that the POG technology had greatly improved fuel adaptability via the benefit from preoxidizing coal before it is fed into the fluidization zone. The table also shows that the KRW gasification is suitable for both air and oxygen-rich gasification reagents and the produced gas is rich in H2 and CO and with low hydrocarbon (e.g., CH4) content. Because of the high temperature of the preoxidation flame, the produced gas was reported to contain little tar as well.32 As indicated by the heating value of the produced gas, it can be believed that the POG technology is highly suitable for the production of fuel gas from coal of different types. 4.3. PHG. The process principle of PHG technology is based on separating char gasification from coal pyrolysis reactions, as schematically illustrated in Figure 10a. The pyrolysis atmosphere is the gas from char gasification and the hydrogen obtained possibly from separating the final product gas. Accompanying coal pyrolysis, the upgrading of
(31) Collot, A. G. Int. J. Coal Geol. 2006, 65 (3-4), 191–212. (32) He, Y. D. Technical Manual of Modern Coal Chemical Engineering; Chemical Industry Press: Beijing, China, 2004; pp 472-473 (in Chinese).
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: DOI:10.1021/ef101036c
the pyrolysis-generated tar and gas must occur simultaneously, which refers to the synergizing effect in PHG through the interaction between coal pyrolysis products and the gas components (H2, CO, CH4, etc.) in the gasification product gas. Because of the presence of H2 in the pyrolysis atmosphere, the involved pyrolysis process is named partial hydropyrolysis. As shown in Figure 10b, the PHG system consists of a gasification zone (or partial oxidizer) and a hydropyrolysis zone. In the hydropyrolysis zone, coal is pyrolyzed and the generated tar and gas are further upgraded through their interaction with gasification gas (mainly H2). The involved reactions include mainly H2 stabilization of free radicals and hydrogenation of heavy tar. Then, the pyrolyzed char can be returned to the gasifier to undergo gasification. Therefore, both product gas and light oil are produced through this process. The desired decoupling effect via PHG is the production of good-quality liquid and the achievement of high energy efficiency through using the sensible heat of gasification gas to pyrolyze coal in the reforming zone. Figure 10b presents the schematic diagram of a 1 ton/h pilot PHG plant developed in Japan.33 The plant consists of two parts, a gasification bottom and a pyrolysis top section, both of which are operated as an entrained flow reactor. A part of coal is injected with oxygen into the gasifier to be gasified at 1823-1923 K, and the formed gasification gas flows upward and passes through the pyrolyzer/reformer. The coal ash is melted to withdraw from the plant bottom. Another part of coal is injected with hydrogen into the pyrolyzer/reformer to undergo hydropyrolysis and reforming at 700-800 °C using the sensible heat of gasification gas. The product from the plant top includes noncondensable gas, tar, and char. Some tests in this PHG plant were carried out by Yabe et al.33 A sub-bituminous coal (the analysis results are listed in Table 2) was used as fuel. Figure 11 presents the experimental conditions, the product yields, and HHV of the gas from the pyrolyzer/reformer. The product yields shown in Figure 10 were determined on the basis of the C content of coal, which were 36.9, 24.6, 12.5, and 16.9% for char, CO, H2, and BTX, respectively. In comparison to the other pyrolysis technologies, the realized BTX content was much higher, showing in fact one of the expected decoupling effects via PHG. The produced gas contained CH4 of about 12.5%, resulting in high HHV of fuel gas/syngas (above 2400 kcal/Nm3) and showing another merit from decoupling the gasification and pyrolysis. The overall efficiency estimated from the results was as high as 88%, if the H2 in the produced gas is partly recycled into the pyrolyzer/reformer,33 although the cold gas efficiency for gasification was only around 60% because of the production of tar. Therefore, high-efficiency coproduction of gas and tar with high BTX yield in the tar denotes in fact the realized major decoupling effects via PHG, validating the theoretical anticipation shown in Table 1.
Figure 11. Product yields and gas HHV from testing the 1 ton/h pilot PHG plant.33
Figure 12. Principle and schematic diagram of the T-DBG system.34
5. Hybrid Process effect from DCG and tar cracking or reforming reactions. Hence, the T-DBG takes the decoupling methods of both isolating and synergizing, as schematically illustrated in Figure 12a. It can be seen that the process of T-DBG adopts a two-stage bed to replace the single-stage bed gasifier employed in a conventional DBG process sketched in Figure 3a. When fuel is fed into the lower stage of the bed (calling this stage a gasifier), the solid fuel is pyrolyzed and
Two-Stage Dual-Bed Gasification (T-DBG). As mentioned in section 2, the process principle of T-DBG is based on isolating the gas products of gasification and char combustion reactions, meanwhile, making use of the synergizing (33) Yabe, H.; Kawamura, T.; Kozuru, H.; Goto, K.; Namiki, Y.; Yatoh, S.; Kosuge, K.; Itonaga, M.; Takeda, S. Nippon Steel Technical Report; Nippon Steel Corporation: Tokyo, Japan, 2005; Vol. 92, pp 8-15.
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: DOI:10.1021/ef101036c produced gas was decreased because of the occurrence of hydrocarbons reforming and WGS reactions. The reported data in ref 34 also showed that the produced gas avoided the dilution via flue gas and had higher H2 content. Consequently, the T-DFBG not only maintained the merits of the normal DFBG but also lowered the tar content in the product gas as well, justifying the decoupling effects mentioned in Table 1. 6. Concluding Remarks A conception called the DCG of solid fuel, such as coal and biomass, is proposed in this paper. This type of gasification technology is based on the decoupling of the major reactions, including pyrolysis, char gasification, combustion, reforming and cracking, and decomposition, involved in the fuel gasification process and makes use of the interactions between/ among the decoupled reactions or the products of these reactions to enhance the beneficial interactions but suppressing the unexpected ones. Isolating and synergizing, two different approaches of reaction decoupling, were generalized, and the realized effects through facilitating or suppressing the related interactions between/among the decoupled reactions and their products are termed as the decoupling effects. Six typical decoupling examples and their resulting DCG technologies were highlighted in this paper to explain the conception of the decoupling and justify its necessity in terms of the realized improvements on gasification performances. The results from some bench- or pilot-scale tests conducted worldwide demonstrated that the DCG technologies based on either isolating or synergizing decoupling or both of them can lower emission, raise efficiency, increase product quality, and/or allow good fuel adaptability. As a consequence, the idea of decoupling reactions actually provides a powerful approach/ tool to manipulate some complicated reaction networks to optimize the reaction behaviors and performances. As for gasification, the DCG technologies in fact have a very promising future because of their outstanding merits for clean and high-efficiency co-production of high-quality gas and liquids (tar). In fact, many of the DCG technologies mentioned in this paper are in the process of industrial demonstration and making progress toward commercialization.
Figure 13. Performance comparison between T-DFBG and DFBG for coffee ground mixed with 5% CaO (XC, C conversion; XH, H conversion; η, cold gas efficiency).34
gasified by contacting the HCPs from the upper stage of the bed. The produced gas, mixed with overfed gasification reagent, flows up and passes through the upper stage in turn, which works as a reformer for the gas. In this reformer, gasupgrading reactions, including tar/hydrocarbon reforming or cracking and WGS reaction, are expected to occur through its interactions with the circulated HCPs. These gas-upgrading reactions facilitate the conversion of tars into noncondensable gas, hopefully increasing the gasification efficiency and making the produced gas contain less tar and more H2. Figure 12b shows a 5 kg/h two-stage dual-fluidized-bed gasification (T-DFBG) experimental system reported by Xu et al.34 An overflow pipe was used to connect the upper and lower stages of the two-stage fluidized bed. This overflow pipe and the downcomer of the system were immersed into the particle beds of the lower and upper stage beds (gasifier and reformer), respectively. This allowed for the HCPs to enter the reformer first and then move into the gasifier via overflow. Gasification tests of CaO-blended coffee grounds with steam were performed in both the above-mentioned T-DFBG plant and a normal DFBG testing system. The analysis results of coffee grounds are listed in Table 2. The detailed settings of experimental conditions can be found in ref 34. Figure 13 compares the realized C conversion (XC), H conversion (XH), cold gas efficiency (η), tar content, and HHV in the product gas from the tests. In comparison to the results from testing the DFBG, the values of X C, XH , and η for T-DFBG were increased by about 4.5, 14.0, and 6.0% points and tar content was decreased by 7.0 g/m3, while HHV of
Acknowledgment. The authors are grateful for the financial support of the National High Technology Research and Development of China (Contract 2009AA02Z209), the Natural Science Foundation of China (Contracts 20776144 and 20606034), and the Key Projects in the National Science and Technology Pillar Program (Contract 2009BAC64B05). Note Added after ASAP Publication. The third paragraph of the Hybrid Process section was modified in the version of this paper published November 4, 2010. The correct version published November 9, 2010.
(34) Xu, G. W.; Murakami, T.; Suda, T.; Matsuzawa, T.; Tani, H. Fuel Process. Technol. 2009, 90 (1), 137–144.
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