(258f) A Parametric Optimization Study of Downdraft Biomass Gasification Using a Comprehensive Transport and Kinetic Model

Biomass gasification is one of the cleaner and cheaper processes for energy production. Although it is a proven technology for power production still it is not being commercially used in large parts of the world. Much of the problems lie with reactor design and understanding the complex phenomena occurring inside the reactor during the gasification process. Hence it is necessary to develop a comprehensive model for the gasification process which takes into account all the Physico-chemical process such as drying, pyrolysis, combustion and reduction occurring in a gasifier. In our previous study, such a model was being developed, which incorporates reaction kinetics as well as transport of mass and energy for both solid and gaseous phase species in the gasifier. The model was also validated with our experimental data. In the present study, the previously developed model is used for the parametric analysis of different important operating parameters such as biomass moisture content, equivalence ratio, reduction zone length over a wide range of variation. The biomass moisture content is varied over a range of 4 % to 15 % wet basis. Similarly, the equivalence ratio and reduction zone length are varied over a range of 0.15 to 0.45 and 5 cm to 45 cm respectively. The effect of these parameters on the producer gas composition and its calorific value and also the tar composition are reported. It has been found from the results that higher moisture content in biomass leads to lowering of the temperature inside the gasifier, which in turn dilutes producer gas composition and lowers its calorific value. In the case of equivalence ratio, the initial increase from 0.15 up to 0.25, improves the producer gas composition but beyond this value further increase in equivalence ratio till 0.45 results in deterioration of the producer gas quality and decreases its calorific value. Similarly higher reduction zone length implies higher residence time for gasses coming out from the combustion zone in the charcoal bed, hence better composition of producer gas at the gasifier outlet. Apart from producer gas composition, the effect of reduction zone length on the amount of primary and secondary tar in the product gas is also analyzed. It is found from the results that, larger hot charcoal bed length reduces the primary tar vapors coming from pyrolysis zone to a large extent, which in turn reduces the amount of tar available for the high-temperature tar cracking reactions in the combustion zone. The results of the present study will be beneficial for finding the optimum operating conditions to obtain maximum calorific value and low tar producer gas at the gasifier outlet.