A Review of Fixed Bed Gasification Systems for Biomass

Abstract

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.