Design optimization of a fixed bed biomass gasifier of a two stage incinerator

Abstract

Waste wood, a renewable energy source is used as feedstock for Universiti Teknologi Malaysia’s newly-developed two-stage incinerator system. The research goals are to optimize the operation of the thermal system, to improve its combustion efficiency and to minimize its pollutants formation. The work focuses on one of its components, i.e. the primary chamber which comprises of a large updraft fixed-bed chamber, measuring 187.5cm in height and 194.5 cm in diameter respectively. During experimental work, the primary chamber is fed in batches of the processed feedstock while the air for combustion is metered through an eight air nozzle supply system, incorporated at the lower side of the tapered chamber. The feedstock will undergo four different processes; drying, devolatilisation, gasification and combustion. The combustion process is evaluated with the variation of fuel’s moisture content, i.e. set at 17%, 31%, and 40% respectively. The initial experimental work indicates that the temperature and oxide of nitrogen (NO) concentration are decreased with the increase of the moisture in the fuel. Furthermore, the concentration of carbon monoxide (CO) increases with the variation of this operating parameter. However the change for carbon dioxide (CO2) and oxygen (O2) concentrations are only around 1% with the variation of this operating parameter. For optimum operating condition, where the gasification efficiency is 95.53%, the moisture content of the fuel is best set at 17%; giving outlet operating temperature of 550oC and exhaust gas concentrations with 1213 ppm of CO, 6% of CO2, 66 ppm of NO and 14% of O2 respectively. In parallel to the experimental work, a computational fluid dynamics software is used to simulate the performance of the primary chamber at optimum operating condition. This technique provides detail insights on the dynamics of flow and the combustion behavior that occur in the reference chamber. A steady state model is formulated for the updraft fixed bed reactor. Here the predicted optimum gasification efficiency stands at 95.49% with CO, CO2, NO and O2 concentrations as 1301 ppm, 6.5%, 53.7 ppm and 13.5% respectively. The major amendment to the chamber design is proposed on the aspects of air-exit-velocity, i.e. by using smaller diameter of the air nozzles. In doing so, it will create high air-jet penetration in the combustion zone. Higher combustion temperature above 850oC is created for the gasification and combustion zones. This will also reduce NO formation from 54 ppm to 25 ppm at the exit point of the chamber.