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Influence of Fuel Feeding Positions on Gasification in Dual Fluidized Bed

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Influence of Fuel Feeding Positions on Gasification in Dual Fluidized Bed biomass and bioenergy 54 (2013) 46e58 Available online at www.sciencedirect.com http://www.elsevier.com/locate/biombioe Influence of fuel feeding positions on gasification in dual fluidized bed gasifiers V. Wilk a,*, J.C. Schmid b, H. Hofbauer b a Bioenergy2020þ GmbH, Wienerstraße 49, 7540 Gu¨ssing, Burgenland, Austria b Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, 1060 Vienna, Austria article info abstract Article history: An in-bed and an on-bed feeding system are implemented in a dual fluidized bed gasifier in Received 19 June 2012 order to investigate the influence of the fuel feeding position on the gasification process. Received in revised form Two bed materials, fresh and used olivine, are used because of their varying catalytic ac- 11 March 2013 tivity. The comparison of in-bed and on-bed feeding of wood pellets shows that in-bed Accepted 12 March 2013 feeding is more favorable, because lower tar concentrations are achieved and the gas Available online composition is closer to wateregas shift equilibrium. Better mixing of bed material and fuel particles occurs with in-bed feeding. The residence time of the gas phase in the fluidized Keywords: bed is longer in the case of in-bed feeding, and therefore better performance of the gasifier Biomass is achieved. Sufficient residence time of the fuel in the bubbling bed is important when a Gasification less active bed material is used. More active bed material is capable of compensating for Gasesolid contact the shorter residence time of the gas phase in contact with bed material during on-bed Feeding position feeding. Reforming ª 2013 Elsevier Ltd. All rights reserved. À 1. Introduction of the DFB gasifier has a high calorific value of 12e14 MJ m 3. Since the 1990s, steam blown DFB gasification technology has Gasification is a promising technology for future energy sup- been the subject of scientific studies at the Vienna University ply, as it converts carbonaceous solids into valuable producer of Technology [1,2]. The basic principle of this technology is gas. For gasification processes, fluidized bed reactors are displayed in Fig. 1. applied by preference. The good mixing conditions of fuel In the field of biomass gasification, extensive research has particles, bed material, and gas phase and an excellent heat been conducted at Vienna University of Technology with transfer promote the conversion of the feedstock. Air blown several generations of 100 kW pilot plant gasifiers. This has fluidized bed concepts were proposed for the gasification of led to commercially available DFB gasifiers. The gasification biomass. However, conventional air gasification yields a pro- process was demonstrated successfully in 2001 with the ducer gas which is highly diluted with nitrogen and as a first industrial sized plant in Gu¨ ssing (Austria). The high À consequence has a low calorific value of 4e6MJm3.By quality producer gas yielded with high hydrogen content 3 À3 contrast, a dual fluidized bed (DFB) steam gasifier system al- (>0.40 m m ) and high calorific value is used to produce lows the generation of a nitrogen free producer gas without electricity and heat for the local district heating grid. Several the use of pure oxygen as gasification agent. The producer gas industrial gasifiers based on this technology are under * Corresponding author. Tel.: þ43 158801166387; fax: þ43 15880116699. E-mail addresses: [email protected] (V. Wilk), [email protected] (J.C. Schmid), hermann.hofbauer@ tuwien.ac.at (H. Hofbauer). 0961-9534/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2013.03.018 biomass and bioenergy 54 (2013) 46e58 47 more reliable, and more economical compared to in-bed sys- tems. Biomass is well suited for on-bed feeding, since biomass is a very reactive fuel compared to bituminous coal or lignite. Thus, less unburned carbon is elutriated, which constitutes the majority of combustion losses. Examples of on-bed feeding systems are gravity chutes, where the biomass drops on the fluidized bed, or spreader-stoker systems, which throw the biomass on a large bed area [13]. The influences of in- or on-bed feeding on gasification have been investigated by several researchers. Corella et al. [14],in 1988, were among the first; they stated that biomass should be fed directly into the bed and not from the top of the fluidized Fig. 1 e The basic principle of the dual fluidized bed bed gasifier. They used two different lab-scale bubbling bed gasification technology. gasifiers for their investigation, one with on-bed and the other with in-bed feeding. They observed poorer gas quality and increased tar formation when feedstock was thrown on the fluidized bed because fuel particles accumulated on the sur- construction or have been in operation ever since, for example face of the bed. in Oberwart (Austria), Villach (Austria), Senden/Neu-Ulm Fuel accumulation on the surface of the fluidized bed is (Germany), and Gothenburg (Sweden). called segregation. Fiorentino et al. [15] measured the segre- In addition to electricity and heat production, the nitrogen gation time of a fuel particle in a bubbling fluidized bed. This is free producer gas from a DFB steam gasifier is well suited for the time that it takes a particle to reach the surface of the chemical syntheses of interesting products. Various research fluidized bed after being released at the bottom. They activities focus on the production of FischereTropsch diesel observed that volatile matter of the fuel is released immedi- [3], synthetic natural gas [4], and other chemicals [5] at the ately when the particle is injected into the bed. Volatile matter gasification plants in Gu¨ ssing and Oberwart. Biomass-derived creates an endogenous bubble which is able to lift the fuel producer gas can be a valuable feedstock for the chemical particle to the surface. This segregation mechanism is inde- industry, and thus might increase the share of renewable pendent from the atmosphere in the fluidized bed. The fuel materials in this sector. particle does not come into contact with the fluidization There is increasing interest in industry in substituting agent, because volatiles are released from the fuel particle. natural gas with renewable energy carriers. This demand can Bruni et al. [16] extended the work of Fiorentino et al. and be satisfied by gasification and subsequent methanation. Be- showed that the segregation time is shorter than the devola- sides wood chips, other types of biomass or residues and tilization time of the fuel. This means that the majority of waste are also main focuses. Therefore, extensive research on volatile matter is released on the bed surface even though the the DFB gasification technology is being conducted at Vienna particles are inserted at the bottom of the bubbling bed. University of Technology because it is a promising technology Wang et al. [17] studied fluidized bed pyrolysis of single fuel for such gas production from solid fuels. particles which were forced to stay either in the dense zone or Gasification process parameters like temperature, steam- in the splash zone of the fluidized bed. They fixed the position to-carbon ratio, and type of feedstock influence the compo- of the particles by the fluidization settings and also with sition of the producer gas [6]. It has been demonstrated that weight loads on the particles. Although they measured dif- various fuels such as sawdust, bark, waste wood chips, ferences in heating time, they did not find differences in gas strawewood blends, coalewood blends, municipal waste yield as a function of the particle position. They also stated fractions, reed, sugar cane bagasse, waste plastics, wheat that these results are not valid for excessive fuel accumulation bran, sewage sludge, and other alternative feedstocks can be on the bed surface, because then the gas yield will be affected. processed with the DFB technology [7e11]. All these fuels have Ross et al. [18] used a lab-scale bubbling fluidized bed different physical and chemical properties. gasifier to analyze the influence of two different feeding po- Therefore, the feeding system is a crucial part of the gasi- sitions in the fluidized bed. They measured the producer gas fication plant which requires careful consideration [12]. Most composition at various heights of the bed and in the free- commonly, a screw feeding system is implemented in an in- board. When particles are inserted closer to the bed surface, e dustrial DFB gasifier. This is an in-bed system with a plug higher concentrations of C1 C3 hydrocarbons are measured, screw, which is purged with nitrogen. The plug screw inserts which might be an indicator of lower conversion. the solid fuel directly into the fluidized bed of the gasification Vriesman et al. [19] investigated the influence of the reactor. In-bed feeding systems provide intensive mixing of feeding position on the formation of ammonia during gasifi- the fuel and the bed material and therefore increase the cation. But there was no significant difference in char and tar conversion efficiency. As the end of the screw enters the flu- yields when biomass was fed into or onto the bubbling fluid- idized bed, mechanical and thermal stress and abrasion are ized bed. very likely and can cause damage. Normally such in-bed One of the most recent works has been accomplished by feeding screws are water cooled. Rapagna` and di Celso [20]. They used a lab-scale bubbling However, on-bed feeding systems are usually applied in fluidized bed of olivine, a catalytically active bed material, and fluidized bed combustion plants. These systems are simpler, tested in- and on-bed feeding. Rapagna` and di Celso found 48 biomass and bioenergy 54 (2013) 46e58 that the gas yield increases when the fuel is fed into the flu- idized bed.
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