Fuel 90 (2011) 1340–1349 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel Experimental test on a novel dual fluidised bed biomass gasifier for synthetic fuel production K Göransson ⇑, U Söderlind, W Zhang Department of Natural Sciences, Engineering and Mathematics, Mid Sweden University, SE-871 88 Härnösand, Sweden a r t i c l e i n f o Article history: Received 15 September 2009 Received in revised form 22 December 2010 Accepted 29 December 2010 Available online 12 January 2011 Keywords: Allothermal gasification Biofuel Biomass Gasifier and syngas a b s t r a c t This article presents a preliminary test on the 150 kWth allothermal biomass gasifier at Mid Sweden University (MIUN) in Härnösand, Sweden The MIUN gasifier is a combination of a fluidised bed gasifier and a CFB riser as a combustor with a design suitable for in-built tar/CH4 catalytic reforming The test was carried out by two steps: (1) fluid-dynamic study; (2) measurements of gas composition and tar A novel solid circulation measurement system which works at high bed temperatures is developed in the presented work The results show the dependency of bed material circulation rate on the superficial gas velocity in the combustor, the bed material inventory and the aeration of solids flow between the bottoms of the gasifier and the combustor A strong influence of circulation rate on the temperature difference between the combustor and the gasifier was identified The syngas analysis showed that, as steam/ biomass (S/B) ratio increases, CH4 content decreases and H2/CO ratio increases Furthermore the total tar content decreases with increasing steam/biomass ratio and increasing temperature The biomass gasification technology at MIUN is simple, cheap, reliable, and can obtain a syngas of high CO + H2 concentration with sufficient high ratio of H2 to CO, which may be suitable for synthesis of methane, DME, FT-fuels or alcohol fuels The measurement results of MIUN gasifier have been compared with other gasifiers The main differences can be observed in the H2 and the CO content, as well as the tar content These can be explained by differences in the feed systems, operating temperature, S/B ratio or bed material catalytic effect, etc Ó 2011 Elsevier Ltd All rights reserved Introduction Synthetic fuel production from biomass is an important issue from the viewpoint of climatic conventions and energy shortage crises Synthetic fuels such as methane, DME, FT-fuels, and alcohol fuels, as the second generation bio-automotive fuels, can be produced via gasification and synthesis based on various forest and agricultural biomass residues For biomass, a S&M (small or medium) scale bio-automotive fuel plant is preferable as biomass feedstock is widely sparse, inhomogeneous in size and shape, difficult to be pulverized, and has relatively low density, low heating value, and high moisture content [1] Direct gasification with air produces a fuel gas of heating value 4–7 MJ/Nm3 – not suitable for synthesis of bio-automotive fuels Pure oxygen gasification generates a fuel heating value of 10–12 MJ/Nm3, but an oxygen plant is needed, which can be economic only for large-scale bio-automotive fuel production plants [2] However, indirect (or allothermal) gasification, in a dual fluidised bed gasifier (DFBG) with steam as the gasification agent, produces a syngas of 12–20 MJ/Nm3 heating va⇑ Corresponding author Tel.: +46 70 289 26 30, +46 611 862 13; fax: +46 611 861 60 E-mail address: kristina.goransson@miun.se (K Göransson) 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd All rights reserved doi:10.1016/j.fuel.2010.12.035 lue Thus, DFBG turns out to be a promising biomass gasification technology for synthetic fuel production There are a number of DFBGs or similar designs worldwide: the well known Güssing gasifier in Austria, e.g [3], MILENA gasifier in The Netherlands [3–5], Trisaia gasifier in Italy by ENEA’s research center [3,6], Battelle Columbus Laboratories (BCL) gasifier in USA [3] (now called The Rentech-SilvaGas Process [7]), and the CAPE FICFB Gasifier in New Zealand by the University of Canterbury [8] A DFBG of 2–4 MWth was recently built by Göteborg Energi at Chalmers University of Technology in Sweden [9] In Japan there is a DFBG in Yokohama, by Xu et al [10,11] and in China there are some examples in Beijing by the Chinese Academy of Sciences [3] and in Hangzhou by Fang and co-workers [12] DFBGs can be designed with different combinations of the BFB and the CFB So far, the most attractive design is supposed to have biomass gasification in the BFB and char combustion in the CFB Examples of this design are the Trisia gasifier, the Güssing gasifier, the CAPE FICFB gasifier, the MIUN gasifier, etc The principle of the Chalmers gasifier is similar to these gasifiers as well, as a BFB gasifier is integrated into the loop of an existing 12 MWth research CFB-boiler [13] The MILENA gasification process uses the riser for gasification and the BFB for combustion [4], and the Rentech-SilvaGas Process consists of two CFBs interconnected with each other [7] K Göransson et al / Fuel 90 (2011) 1340–1349 1341 Nomenclature BFB CFB db DFB DFBG FB GC bubbling fluidised bed circulating fluidised bed dry basis dual fluidised bed dual fluidised bed gasifier fluidised bed Gas Chromatograph In general, the biomass gasification process occurs through three steps: (1) pyrolysis, which produces volatile matter and char residue; (2) secondary reactions, involving the volatile products; (3) gasification reactions of the remaining carbonaceous residue with steam and carbon dioxide [14] The pyrolysis of biomass results in volatiles and char that subsequently participate in a series of complex and competing reactions The main conversion of the biomass to syngas takes place within the bed, but some conversion to syngas occurs in the freeboard section In DFBGs, biomass is gasified in a bed fluidised with steam The composition of the syngas is mainly dependent on the type of gasification agent used Steam encourages the shift reactions with carbon and carbon monoxide to increase the hydrogen content Shift reactions and steam reforming of methane reaction are common reactions used to predict the composition of syngas Synthetic fuel production requires a high quality syngas of high CO + H2 concentration with sufficient high ratio of H2 to CO and low tar content Steam-to-biomass (S/B) ratios, gasification temperature and bed material circulation rate, are important parameters in determining the syngas composition and tar content The required heat for biomass steam gasification is maintained by the char combustion and the bed material circulation The bed material circulation rate can control the temperature balance, and is an important parameter to be considered in the gasifier design and operation The operation of the gasifier needs to be optimised, and hence, the operational behaviour of the gasifier needs to be studied Attempt to measure the solid circulation rate have been performed both in old proven ways like ‘‘bucket-and-stopwatch’’ and in new imaginative ways, e.g by using signals from an optical mouse to PC [15] or a fibreglass spiral with a rotation electronics [16] Unfortunately, most of the existing techniques for measuring solid circulation rate are limited for various reasons A novel solid circulation measurement system is developed in the presented work, which is called here as Pressure-induced Measurement of Circulation (PIMC), a technique that also works at high bed temperatures under gasification conditions The 150 kW allothermal biomass MIUN gasifier has been built up in 2008 for the research on synthetic fuel production This paper presents a preliminary test of the operational behaviour of the MIUN gasifier The test was carried out by two steps: (1) fluid-dynamic study; (2) measurements of gas composition and tar The experimental results of MIUN gasifier have been compared with other DFBGs, which leads to a discussion on DFBG design and operation This paper shows a successful development of a pilot-scale DFBG with internal reforming of tar and methane for synthesis gas production Experimental facilities The BTL (biomass to liquids) system at MIUN is sketched in Fig It has been presented elsewhere [17] A description focusing on the gasifier is given below Gs MIUN PIMC S/B Uc Ug solid circulation rate (kg/m2 s) Mid Sweden University Pressure-induced Measurement of Circulation steam/biomass ratio (kg/kg dry biomass) superficial gas velocity in the combustor (m/s) superficial gas velocity in the gasifier (m/s) 2.1 The gasifier and test conditions The gasifier (see Fig 2) consists of an endothermic steam fluidised bed gasifier and an exothermal CFB riser combustor, and has a biomass treatment capacity of 150 kWth, i.e approx 30 kg wood pellet feed per hour The heat carrier between the reactors is silica sand of about 150 lm diameter The bed material circulation is controlled with the gas velocity in the combustor, the total solids inventory and aeration in the tube connecting the bottoms of the gasifier and the combustor The gas flow through the gasifier can also enhance the aeration in the abovementioned tube, so that the bed material circulation increases with gas velocity in the gasifier The PLC-operated (Process Logic Controller) feeding system is designed for constant biomass feeding The biomass is fed into a pneumatic oscillatory vane feeder to achieve high precision A coaxial left- and right-handed thread screw feeder thereafter makes a steady stream of biomass into the final screw feeder without plugging the pipe The final screw feeder is rapidly feeding biomass into the gasifier to avoid reaction in the screw feeder and particle congestion The feeding rate can be controlled by frequency-controlled motors of screw feeder Nearest to the gasifier, the final screw feeder is cooled by water flow through a cylindrical jacket The gas distributors in both the gasifier and the combustor are own-produced sintered metal plates The fluidisation agent in the gasifier is steam (12 bar and 150 °C) and the synthesis gas is drawn off from the top of the gasifier The residual biomass char is then transferred by bed material into the combustor through the lower pressure lock In the combustor, the fluidisation agent is air, which results in an oxidation of the char and produces heat at a temperature of 950–1050 °C The hot bed material separates from the flue gas in a cyclone to be recycled into the gasifier through the upper pressure lock (loop seal pot), which prevents gas leakage between the separate environments in the gasifier and the combustor The aeration medium in the upper pressure lock is steam The steam gasifier is surrounded by electrical heaters (total effect of 20 kW) and insulated There are no lining in the reactors The gasifier and the combustor have a height of 2.5 and 3.1 m, and inner diameters (i.d.) of 300 and 90 mm, respectively Temperatures and pressures, at a number of points from the distributor to the top of the gasifier and the combustor, at the upper pressure lock and at the cyclone, as well as all gas flows, are registered through a computer data collection system The test on the DFBG was carried out at the temperatures 750, 800 and 850 °C, in the fluidised bed gasifier The steam supply into the gasifier was held at kg/h The biomass feedstock was wood pellets from SCA BioNorr AB, and the fuel analysis is given in Table 1342 K Göransson et al / Fuel 90 (2011) 1340–1349 Fig The BTL system at MIUN with (A) biomass feeding system, (B) gasifier, (C) syngas cleaning system, and (D) catalysis reactor Table Fuel analysis of wood pellets from SCA BioNorr AB [18] Elementary analysis % db Ash cont Sulphur, S Chlorine, Cl Carbon, C Hydrogen, H Nitrogen, N Oxygen, O (calc.) Moisture Durability of pellets Bulk density Calorific value as rec Calorific value db 0.4