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REVIEWS A Review on Dual Fluidized-Bed Biomass Gasifiers Jose´ Corella,* Jose´ M. Toledo, and Gregorio Molina Department of Chemical Engineering (Faculty of Chemistry), UniVersity Complutense of Madrid, 28040 Madrid, Spain
Biomass gasification with pure steam in a fluidized bed is a highly endothermal process that has been connected in several ways to a fluidized-bed combustor to burn the char that is generated in the gasifier. This resulted in what currently is called dual fluidized-bed (DFB) biomass gasifiers. This review starts by describing the pioneering DFB biomass gasifiers that were operated during the period of 1975-1990 by Kunii’s group in Japan, Battelle-Columbus and FERCO in the United States, TNEE in France, AVSA in Belgium, etc., ... and Corella and Herguido’s gasifier, which was operated during the period of 1989-1991. A description of the gasifiers operated today in Europe (TU Wien and Gu¨ssing in Austria and ECN in The Netherlands), Japan (IHI Co., EBARA, AIST-Tsukuba), and the People’s Republic of China (Dalian, Hangzhou, and Beijing) then is given. Their most-relevant operation data, and the results from these gasifiers (mainly, the gaseous hydrogen (H2) and tar contents in the raw produced gas), are finally presented briefly. Introduction How an endothermal reactor that is connected to an exothermal reactor can generate an autothermal process is well-known. The best and most well-known example of this system is the fluidized catalytic cracking (FCC) process, which reached its commercial scale 50 years ago. Other similar examples are Exxon’s fluid and flexicoking processes and the Japanese Kunii-Kunugi (K-K) process. This K-K process was further simplified using sand, instead of coke particles, and, under Kunii’s guidance, Tsukishima Machinery Co. developed a new gasification process, called Pyrox, which also reached a commercial scale. Three plants with a capacity of 150 tons of MSW/ day were operated under steady state for 8 years in Funabashi City. The product gas there had a heating value of 5 MJ/Nm3. The heat needed for the highly endothermic gasification with pure steam may be provided by (i) transfer of the char generated in the gasifier to the combustor, (ii) combustion of the char generated in the gasifier, and then (iii) the use of a circulating solid to transfer the heat generated in the combustor to the gasifier. Some plants for gasification of biomass with pure steam based on this promising concept started to appear worldwide ∼25 years ago. They were, in some way, similar to the FCC units. Nevertheless, years later, all those pioneer gasification plants disappeared, because of different technical and economical problems. Biomass gasification with pure steam in a fluidized bed may generate a gasification gas with 60 vol % H2 (dry basis). This content may be further increased to 70-75 vol % H2 if a circulating system is used.1 The raw gasification gas can still be improved, using an in-bed CO2-adsorbent2 and additives and/ or catalysts to eliminate most of the tar generated in the pyrolysis step.3 Thus, a gasification gas that is very rich in H2 and with a very low tar content currently can be obtained, via biomass gasification with pure steam. This requires the use of a concept * To whom correspondence should be addressed. Fax: +34-91-394 4164. E-mail address:
[email protected].
similar to that used in FCC units or in the Pyrox process. For this reason, a renewed interest in these gasification systems has reappeared in the last 10 years. This communication reviews the history and the existing plants worldwide of this type of gasification process. The promising gasification process, first, must be named, which is really not easy, because it is not yet fully defined, and it has received (and is still receiving) different names from different institutions. It has been called, for example, the Battelle-Columbus gasification process, EBARA’s Internally Circulating Fluidized-bed Gasifier (ICFG), Hayashi’s dual-gasflow two-stage (DGF-TS) gasifier, Corella and Herguido’s Multisolid and Catalytic Double Fluidized Bed Circulating (MSCDFBC) gasifier, Hofbauer et al.’s Fast Internally Circulating Fluidized Bed (FICFB) gasifier, TNEE’s dual fluidized bed, and the Oxygen Donor Process, among other terms. Among all these names, the authors select and will hereafter use the name “dual (or double) fluidized bed” (DFB) biomass gasifier. The reason for this choice is that this is the name currently used by most workers in the field of biomass gasification. The FCC acronym comes from fluidized catalytic cracking, and it is still in use, although the oil cracking in all FCC units is performed today in risers or even perhaps in downers, rather than in fluidized beds. In the same manner, in a DFB system, the gasification may not be conducted in a fluidized bed but in a riser or another type of reactor. DFBs are composed basically of two interconnected reactors: an endothermal gasifier and an exothermal combustor. Each reactor, and in particular the biomass gasifier, may be one of the following types: (i) bubbling fluidized bed (BFB), (ii) two-in-series BFBs, (iii) revolving (EBARA’s concept) bed, (iv) circulating fluidized bed (CFB), (v) riser with a BFB at the bottom, (vi) moving bed or downer, or (vii) others. All these types of reactors are, or have been, used as gasifiers in DFBs. Therefore, the DFB biomass gasifiers reviewed here are quite different among themselves.
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Remarks on the Data and Review Presented Below In addition to the authors’ 20-25 years of knowledge on DFB gasifiers, which has been obtained in many different ways, an extra effort has been made to collect the data presented in this review. Besides some personal or in situ visits to several DFB biomass gasifiers, and a detailed lecture and analysis of most of the publications on this topic, a comprehensive worldwide survey was also conducted. A questionnaire was sent to those organizations who have (or have had) a DFB biomass gasifier, and the questionnaire was addressed to those who were thought to be responsible for the gasifier. Some owners of plants did not reply. Hence, the information/ data included here is not as complete as the authors would like it to be. Furthermore, several owners of DFB biomass gasifiers never published the real reasons or causes of any failures, stoppage, and/or dismantling of their CFB-based gasification plants. Probably some errors existed in their conception, design, operation, and/or economic circumstances. The fact is that several people never stated why their gasifiers failed. It is a pity, because many important conclusions could be determined from these failures, and many errors could be avoided in future designs. Therefore, there are several blind spots and unknown reasons why some projects failed or were stopped and later disappeared. The authors also wish to note that this review does not intend to provide all the data published concerning all the existing DFB biomass gasifiers. Details and results from some DFB gasifiers are already being extensively disseminated and, for more details, the reader is referred to the references cited in this review. The Pioneers: DFB Biomass Gasifiers To the knowledge of these authors, the oldest publication on a DFB applied to biomass (refuse-derived fuel (RDF) in this case, which may be considered as a type of biomass) is that of the distinguished Prof. D. Kunii,4-6 who, in 1975, had already set up a demo plant in Japan (in the Miyagi Prefecture) for the gasification of RDF in a fluidized bed. [This particular, and promising, research motivated J. Corella in 1982 to start working on biomass gasification in a fluidized bed.] Some results from Kunii’s developments are shown in Table 1. When the K-K and Pyrox processes were designed by Kunii, his great concern was the complete sealing of the gasification and the combustion zones.6 Leakage of both gases is dangerous and, because it also occurs in the FCC units, it must be avoided. Deep legs in the K-K and Pyrox Units were then used to prevent the leakage.6 Unfortunately, much of that work in Japan remains unpublished. The so-called “Oxygen Donor Process”7 has sometimes been referred to as another DFB-based pioneering development. Nevertheless, it is not known by present authors if it was also applied to the gasification of biomass, because very little was published by the authors of that process. The John Brown/ Wellman British Company could have used or tested this process with biomass; however, no published information has been found on this topic. In France, LERMAB (University of Nancy 1), the TNEE Company, and Saint Gobain developed a DFB biomass gasifier.8,9 It operated between 1984 and 1985 in “La Cellulose du Pin”, at Facture (Dept. 33, France). It had a capacity of 500 kg wood/h and the exit gas had a lower heating value (LHV) of 16 MJ/Nm3. The gasifier was a BFB fluidized with its own exit gas (pyrolysis gas in this case) and operated at ∼800 °C. The combustor was a riser operated at 950 °C. Some data from this plant are shown in Table 1, and other details in refs 8 and 9.
In the United States, Batlelle-Columbus Laboratories (BCL), in Ohio, developed a DFB biomass gasifier that was similar to the Japanese Pyrox system. Several reports or publications can easily be found about that DFB gasifier (see, e.g., ref 10). In 1992, that technology was purchased and patented by FERCO11 and later reached the demonstration scale in Burlington, VT. Some results from that plant are shown in Table 1. Although that development and plant was presented and discussed in several international conferences (see, e.g., refs 12-14), the following data, because of their relevance, are mentioned here: (1) The raw gasification gas had a relatively low H2 content (17.3 vol %).13 (2) The raw gasification gas had a high tar content (23-32 g tar/Nm3).14 (3) That demo plant was promising and even quite important to all people working worldwide on biomass gasification in fluidized beds. Nevertheless, the reasons why it ceased operation have not been published. The gasification technology would be assisted if the technical problems, if any, in that plant, were known. The two reactors that exist in a DFB may be located in a single vessel. There are three known examples of this type of DFB biomass gasifier in a single vessel or reactor: (1) The AVSA Biomass Gasification Process: The AVSA Biomass Gasification Process was developed by Masson and Faniel at Institut Nat’l des Industries Extractives (INIEX) in Liege, Belgium.15 They used a so-called “communicating fluid bed system”, in which there was a circulation of the solids between two fluidized beds located in the same vessel and separated by a common wall. The gasification with steam was conducted at ∼700 °C and the combustor was operated at ∼850 °C. Unfortunately, the few publications on this work are very difficult to locate. The reasons why that research was halted are not known by the authors of this review. (2) Kunii’s Process: Two fluidized beds in the same vessel were also used in 1986 by Kunii et al.40 That system proved good operation at a small scale.6 Nevertheless, no further development for practical or commercial applications was attempted.6 (3) The Process DeVeloped by Italenergie and AGIP: In Sulmona, Italy, Italenergie and AGIP set up and operated an interesting pilot or demo DFB-based gasification unit.16,17 It was based on two concentric fluidized beds. The fluidized-bed gasifier was inside of a fluidized-bed combustor. The heat was transferred to the gasification zone through a curious “margaritashaped” common wall. Some results from that DFB biomass gasifier are shown in Table 1. Several attempts have been made to obtain news on the reasons why that demo plant ceased operation after a very short time. Nevertheless, no information is available on the problems that led to the end of that plant’s operations. In the opinion of these authors, these plants, with the concept of circulation system within a single reactor, have the following problems: (a) The common wall (of the two reactors) did not have enough surface to be able to transfer to the gasifier the (very high) amount of heat required there. Therefore, the heat must be mainly transferred by the circulating solid but, with this design, some gas leakage between the two reactors may occur. The seal between the gasifier and the combustor, and between the combustor and the gasifier, might not have been good enough. There are no nonmechanical (U- or L-type) valves between the gasifier and the combustor to seal both zones.
a
4 760 atmospheric
MSW 6250 classified classified 720-750 ambient
feedstock capacity (kga.r./h) internal diameter (at the bottom zone) (cm) effective (internal) total height (m) temperature (°C) pressure (at the top of the gasifier)
a.r. ) as received. b Dry basis.
H2 content (vol %)b remarks
tar content (g/Nm3)b LHV gas (MJ/Nm3)b carbon conversion (%)
type internal diameter (cm) total height (m) temperature (°C) pressure (at the top of the regenerator) sorbent or catalyst or in-bed or in-system circulating material, C circulation, cycling, or recycle ratio, C/F (kg C/h)/(kg feedstocka.r./h)
BFB recycled product gas (0.76 Nm3/kg pine barks) pine barks (38% water content) 500 55
deep BFB steam
type gasification agent
20
40-50
14.6-16.5 commercial operation for 8 yrs
∼ 21
sand
sand
22
23-32 17.3
?
sand
riser
660-820
several types of biomass ∼ 9000
riser steam
Burlington, VT
Paisley and co-workers11-14
Value/Comment
Results 38 15.0 83% in gas [17% of C used for the entire process] 22.7 pilot was operated by TNEE in la Cellulose du Pin Chemical Pulp Mill, at Facture, near Bordeaux
Regenerator/Combustor riser 50 8 980 atmospheric
deep BFB classified classified 820-850 ambient
Gasifier
Rueil-Malmaison, Nancy, France
Lelan and co-workers7,8
Funabashi, Japan
Kunii and co-workers4-6
location of the authors
parameter
Table 1. Pioneering DFB Biomass Gasifiers
33.5 the combustor was an annular chamber surrounding the gasifier (they were “two concentric BFBs”)
unavailable 14.6 74
?
“inert material” (R-Al2O3)
830-880
BFB
0.70 (bed) 700-750
wood 100-300 120
BFB recycled pyrolysis gas + steam
Sulmona, Italy
Fonzi and Italenergie S.p.A.16,17
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Figure 1. Schematic of Corella and Herguido’s dual fluidized-bed (DFB)-based plant for biomass gasification with pure steam.
(b) The steel wall that separated the gasifier and the combustor was under a reducing atmosphere at the gasifier side but under an oxidizing atmosphere at the combustor side. Therefore, that steel would have operated under a high redox and/or chemical stress. Its lifetime would probably not have been long enough. Corella and Herguido’s DFB-Based Gasification Plant As shown previously, by 1986, there were several promising DFB biomass gasifiers. During that time, Corella and co-workers were operating a small FCC pilot plant at the Universidad de Zaragoza (Spain) and, therefore, had experience with these types of circulating units (see, for example, ref 18). They were previously gasifying biomass with pure steam in a BFB.19 Consequently, they decided to design, set up, and use a DFBbased pilot plant for biomass gasification with pure steam. A cold model of that circulating system was also set up, to learn about its hydrodynamics. That DFB biomass gasifier was presented at the Fifth EC Conference on Biomass, which was held in Lisbon, Portugal, in October 1989.20 The gasifier was a BFB (inner diameter (ID) of 15 cm), and pure steam was used as the fluidizing/gasifying gas. The combustor was a BFB (ID ) 30 cm). A diagram of such a DFB gasification system is shown in Figure 1. The overall plant was not autothermal: both fluidized bed reactors had external ovens. Some results from that DFB gasifier are shown in Table 2. A novelty of that facility and research was that, in addition to the biomass, its char, and the silica sand that was used as the main fluidizing agent, there was also a fourth solid added to the bed,20 to catalyze the elimination of the tar produced in the gasifier. The catalyst used was an “in-equilibrium” (previously discarded by oil refineries) FCC commercial catalyst (average particle diameter of 66 µm). There were no hydrodynamic problems (no obstructions) in the gasification plant. The overall circulating system operated well. Nevertheless, the research with that DFB-based gasifier was stopped in mid-1991, when Corella left the Universidad de Zaragoza and moved to the Universidad Complutense de Madrid. With the experience gained in biomass gasification after the operation of that plant was stopped, and after an analysis of the data obtained in that plant, today, it is known that that facility
had some drawbacks, and that its design and operation were not good enough. These errors are described here to avoid their repetition in future DFB-based gasification plants: (1) The residence time of the gas in the gasifier was low. The height of the gasifier (and that of combustor too) should have been much higher. (2) The temperature in the gasifier (∼750 °C) should have been higher. (3) The relevant recycle ratio was very low (1.0); it also should have been much greater. (4) A better in-bed catalyst should have been used. (5) The aforementioned facts (points 1-4) made the tar content in the (raw) gasification gas relatively high (9 g/Nm3, on average). (6) The temperature in the combustor was low because very little char was being generated in the gasifier, because of the low (4-10 kg biomassa.r./h) flow rate of the biomass fed to the gasifier. The mass flow rate of the biomass also should have been much greater. In addition, to keep the steam-to-biomass ratio constant, the steam flow rate, which determines the superficial gas velocity of the steam in the gasifier, should have been greater. To keep the gas residence time (or space time in the gasifier) constant, the height of the gasifier should have been higher (which reiterates point 1 previously discussed). DFB-Based Gasifiers in Europe Today According to ref 22, by 1994, Hofbauer and his co-workers at TU Wien started to work on a DFB biomass gasifier. Years later, that concept reached the demo scale in Gu¨ssing (Austria). Some results from those gasifiers are shown in Table 2. This Austrian team published much information. For example, some hydrodynamic studies on these gasifiers can be found in ref 23. At the 15th European Biomass Conference, which was held in Berlin, Germany, May 7-11, 2007, ∼10 different papers were directly related to the two DFB gasifiers in Austria; therefore, no more details are provided here, because they can be found very easily (see, e.g., refs 3, 23, and 24). Among the interesting data published over the past few years by Hofbauer and co-workers, we draw your attention to the H2O (steam) conversion or consumption in the gasifier. This team has obtained and published several times (i.e., ref 24) that, at a
BFB steam pine sawdust (dp < 2.5 mm) 4-10 15 3b 750 890 mm Hg BFB 30 0.9c 490 780 mm Hg, as an average value Commercial “in equilibrium” FCC catalyst: mean dp ) 66 µm Circulating catalyst: 3.0 kg (overloaded in the regenerator unit) 1.0 (kg C/h)/(kg feedstocka.r./h) [9 kg h-1/9 kg h-1]
type gasification agent feedstock capacity (kga.r./h) internal diameter (at the bottom zone) (cm) effective (internal) total height (m) temperature (°C) pressure (at the top of the gasifier)
type internal diameter (cm) total height (m) temperature (°C) pressure (at the top of the regenerator) sorbent or catalyst or in-bed or in-system circulating material, C
30.1
H2 content (vol %)d remarks
50 kg/kg dry biomass
0.5-2 (hydrocarbons larger than naphthalene) 12-13 100%; not useful in this application ∼40
Results 0.5-1 (hydrocarbons larger than naphthalene) 12-13 100%; not useful in this application ∼40
riser 10 4.25 930 atmospheric (-5 mbar) silica sand, olivine, catalysts
BFB steam wood chips, wood pellets 20-25 30 (freeboard) 4 850 atmospheric (-5 mbar)
Vienna, Austria
Hofbauer and Rauch24
Value/Comment
50 kg/kg dry biomass
Regenerator/Combustor riser 85 9.7 930 atmospheric (-5 mbar) olivine
BFB steam wood chips 2,000 220 (freeboard) 6 850 atmospheric (-5 mbar)
Gasifier
Gu¨ssing, Austria
Kaiser et
al.23
a.r. ) as received. b Bottom bed: 40 cm (9.7 kg) of silica sand. c Fluidized bed: 20-40 cm in height; total catalyst load ) 14.6-23.5 kg. d Dry basis.
15.8
LHV gas (MJ/Nm3)d carbon conversion (%)
a
9.0
tar content (g/Nm3)d
circulation, cycling or recycle ratio, C/F
Zaragoza, Spain
Herguido and
co-workers20,21
location of the authors
parameter
Table 2. DFB-Based Biomass Gasifiers in Europe
∼20 Milena pilot plant will be erected in 2007 (capacity ) 180 kg/h biomass)
17 100
∼40 (when using silica sand)
∼40 (kg C/h)/(kg feedstocka.r./h)
BFB ∼25 ∼1.0 ∼50 °C above gasifier riser temperature near atmospheric generally silica sand, but also tested olivine
riser steam or CO2 wood, grass, sewage sludge ∼5 ∼4 ∼1 800-850 near atmospheric
Petten, The Netherlands
van der Drift and co-workers26,27
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steam-to-biomass ratio of 0.60-0.80 (kg/h)/(kg/h), the conversion of the H2O in the gasifier is as low as 10%-15%. This conversion is still 10%. This means that 90% of the H2O leaves the gasifier unreacted. The H2O itself may be inexpensive, but the heat that is associated with the evaporation of this H2O, and the heat required to bring it up to the gasifier temperature (∼850 °C) is not inexpensive at all. Therefore, the unreacted H2Osca. 90% of the total H2O usedsrepresents an important waste of energy and money. This fact has not been fully quantified to date in the publications on DFB biomass gasifiers. (3) It is “believed” (and only “believed” because these figures or amounts were never published) that all the DFB biomass gasifiers have received, or are receiving, significant amounts of money to support them. This financial aid/assistance comes from the company itself, from the State in which the plant is located, and/or from international organizations such as the Commission of the European Union in Brussels. Most of the discussed gasification plants have operated or may be operating using this important financial aid. It is “believed” (and only “believed” because the authors cannot fully demonstrate it) that when that external financial aid was cut, the plant stopped operating. It is the economical feasibility of these gasifiers, and not their hydrodynamics, that is, for these authors, the key problem or weakness in the DFB-based biomass gasification plants. Acknowledgment This work was conducted under Project Reference No. ENE2006-15425 of the Spanish Ministry of “Educacio´n y Ciencia”. The authors are grateful for the financial aid received for this work. Literature Cited (1) Corella, J.; Toledo, J. M.; Molina, G. Biomass gasification with pure steam in fluidized bed. Revisited. Presented at the 15th European Conference on “Biomass for Energy Industry and Climate Protection”, Berlin, Germany, May 7-11, 2007, Ref. No. V2.1.I.61. (2) Corella, J.; Toledo, J. M.; Molina, G. Steam gasification at lowmedium (600-800°C) temperature with simultaneous CO2 capture in fluidized bed at atmospheric pressure: The effect of inorganic species. 1. Literature review and comments. Ind. Eng. Chem. Res. 2006, 45 (18), 61376146. (3) Pfeifer, C.; Rauch, R.; Hofbauer, H. In-bed catalytic tar reduction in a dual fluidized bed biomass steam gasifier. Ind. Eng. Chem. Res. 2004, 43 (7), 1634-1640. (4) Kagayama, M.; Igarashi, M.; Hasegawa, M.; Fukuda, J.; Kunii, D. Gasification in a dual fluidized bed reactor (Chapter 38). In Thermal ConVersion of Solid Waste and Biomass; Jones, J. L., Radding, S. B., Eds.; ACS Symposium Series 130; American Chemical Society: Washington, DC, 1980. (5) Igarashi, M.; Hayafune, Y.; Sugamiya, R.; Nakagawa, Y.; Makishima, K.; Kunii, D. Pyrolysis of municipal solid waste in Japan. J. Energy Resour. Technol. 1984, 106, 377-382. (6) Kunii, D. Private communication, November 2006.
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ReceiVed for reView April 19, 2007 ReVised manuscript receiVed May 23, 2007 Accepted July 20, 2007 IE0705507
