A Review of Fixed Bed Gasification Systems for Biomass

Sangeeta Chopra, Anil K Jain


The gasification of biomass into useful fuel enhances its potential as a renewable energy
resource. The fixed bed gasification systems are classified as updraft, Imbert downdraft,
throatless downdraft, crossdraft and two stage gasifiers. Updraft gasifiers are suitable for
gasification of biomass containing high ash (up to 15 %) and high moisture content (up to 50
%) and generate producer gas having high tar content (50–100 g/Nm3). The high temperature
(830 oC) air gasification of biomass in updraft gasifiers increase the lower calorific value of
producer gas and reduce the tar content. The updraft gasifiers have been used for gasification
of bark, wood blocks, chips and pellets, straw, maize cobs, refuse derived fuel (RDF), and
waste pellets with air and O2 as the gasifying media. The Imbert downdraft gasifiers are
suitable to handle biomass fuel having ash and moisture content less than five per cent and
20% respectively. Modifications in the design of grate and hopper of Imbert downdraft
gasifiers have been suggested to gasify non-woody biomass such as coir dust, cotton stalks,
wheat straw, hazelnut shells, leather residues, sludge etc. Downdraft gasifiers yield producer
gas with lower tar content (1-2 g/Nm3) than updraft gasifiers. Throatless downdraft gasifiers
have been developed to overcome the problems of bridging and channelling in Imbert
downdraft gasifiers. The throatless gasifiers have been successfully used for gasification of
rice husk, wood chips, bagasse, sugarcane leaves, coconut shells etc. Improving the insulation
of the gasifier, re-circulation of producer gas and varying the air distribution have been
reported to enhance the performance of the throatless gasifiers and reduce the tar content to
50–250 mg/Nm3. In two stage gasifiers, pyrolysis and gasification of biomass takes place in
separate chambers resulting in low tar (15–50 mg/Nm3) producer gas. Some aspects of the
research and development in fixed bed gasification of biomass and their commercial
applications are reviewed and cited in this paper.

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