Fixed-bed gasification of biomass

This work investigates the performance of a downdraft-throated gasifier with tangential air injectors using a range of biomass feed-stocks with particular reference to hazelnut shells and also sewage sludge, leather waste and wood chips in order to generate clean produced gas for direct use in power generation engines. The experimental work included the design, fabrication and commissioning of a 5kWe/33kWt downdraft-throated gasifier with a throat size ranging from 7.2 to 13.5 cm and associated gas clean-up line and flaring facility. Experiments were carried out on this unit to investigate the effect of dry fuel capacity on gasifier performance, the influence of reactor insulation and preheated air effects, different air-feed nozzle types and numbers and also throat diameters. The effect of these variables on the gas quality and tar cracking ability was the main concern of this work. In parallel, a laboratory scale large sample thermogravimetric analyser is also used which allows the data on burnout characteristics of different fuel particles to be measured under agitated conditions. The conditions chosen simulate the combustion and gasification behaviour in a gasifier for a range of biofuel pellets and the burnout data obtained is linked to the operating parameters of the gasifier. Around 50 gasifier runs have been performed and full mass and energy balances reported. Good mass and energy balances closures were achieved and the overall energy efficiency from fuel to cold clean gas averaged 60.4%. The gasifier was found to operate successfully for a wide range of fuels without much evidence of bridging, but this was linked to fuel size and preparation, and it had a maximum turndown ratio of 3:1. The highest quality gas for stable operation was found at 70-80% of the maximum capacity.