Parametric Study of a Commercial-Scale Biomass Downdraft Gasifier: Experiments and Equilibrium Modeling

Abstract

Biomass has already emerged in the renewable energy arena as one of the promising candidates for the future. Biomass has been a major source of fuel for human from the existence of mankind. Rapid urbanization and widespread use of fossil fuels in the industrial world has relegated it to the status of a minor source of energy. The rejuvenation, however, started with increasing concerns over reducing carbon footprints and also due to strong causative connections between non-renewable fossil fuels and “global warming”. Biomass gasification is a thermochemical process of converting biomass into the producer gas or syngas (used interchangeably) which can be subsequently used for heat, power and liquid fuels production through various synthesis processes. The major objective of this study was to better understand the effect of various parameters on the syngas composition from a stratified downdraft gasifier. The study is primarily experimental but supplemented by the mathematical modeling that explains various steps in terms of existing scientific principles. Chapter 1 provides basic literature review on the gasification process, various types of gasifiers and elaborated discussion about the effect of various parameters on syngas composition for different types of gasifiers. The effects were primarily discussed for updraft, downdraft and fluidized bed gasifiers which currently cover more than 98% of the total biomass gasification market. Chapter 2 presents a thermodynamic model of the process inside the gasifier. Syngas composition is predicted assuming thermodynamic equilibrium condition inside the gasifier. The thermodynamic equilibrium can be assumed because residence time is high in the fixed bed iii gasification process. The effect of moisture content as well as temperature was determined. The model was run for nearly 100 samples. Based upon the results of the simulations, using linear regression analysis, equations were derived to predict the syngas composition of the biomass based on their elemental composition and moisture. Chapter 3 is an experimental study on the effect of selected process parameters such as moisture content and biomass flow rate on syngas composition in the downdraft gasifier. Parameters studied are moisture content and biomass flow rate inside the gasifier. A mass, energy and exergy analysis is also done to corroborate the experimental results as well as to visualize the carbon, heat, and availability loss inside the gasifier in the process. Chapter 4 discusses tar downdraft gasifier. Although the amount of tar from a downdraft gasifier is always assumed to be small, it is more stable and might adversely affect when used for power generation. Significant amount of toluene, o/p-xylene, naphthalene, phenol, styrene and indene was observed. Tar concentration in the syngas from the gasifier was found to be 0.34-0.68 mg/Nm3.