Simulations and modeling of biomass gasification processes

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, June 2005.Includes bibliographical references (leaves 80-81).Available, low-cost, energy supplies are vital for the world's economy and stability. The current sources of energy harm our environment and are not renewable. Therefore, technology must accommodate new sustainable sources of energy to provide for the high-energy consumption. Biomass is a sustainable energy source that could ease the current reliance on fossil fuels. Gasification of biomass is a promising technology being researched by the National Renewable Energy Laboratory. An Aspen Plus® model was developed for the Thermochemical Process Development Unit (gasification unit) at the National Renewable Energy Laboratory. The model was designed for a feed of poultry litter and was also run with a feed of wood. The Aspen Plus® model is capable of trying various test conditions for the solids removal and scrubbing (condensation) systems. The model as it is currently formulated is not capable of predicting gasification output mixture compositions. It is desirable to decrease the amount of carbon dioxide and tars (defined for this study as compounds with a molecular weight equal to or greater than benzene) in the product gas of the TCPDU. Therefore, the model was run at temperatures for the scrubbing fluid varying from 15⁰C to 60⁰C (for wood) and from 10⁰C to 50⁰C (for poultry litter) and found that the total mole fraction of tars in the product gas for poultry litter feed to decrease by 4% by increasing the scrubbing fluid temperature from 40⁰C to 50⁰C and to increase by 4% by decreasing the temperature to 10⁰C and for wood feed to decrease by 7% by decreasing the temperature from 26⁰C to 15⁰C and to decrease by 10% by increasing the temperature from 26⁰C to 60⁰C.(cont.) The model was run for mole fractions of tars between 0 and 1, in increments of approximately 0.2, in the scrubbing fluid (with water as the remaining fluid). When the amount of tars in the scrubbing fluid increases to approximately 0.2, the amount of tars in the exit stream increases 58-fold for wood and 50-fold for poultry litter. As a secondary effect, by increasing the tar mole fraction from 0 to 1 in the scrubbing fluid, the model predicts a decrease in mole fractions of carbon dioxide in the product gas of 66% and 36% for poultry litter and wood feeds respectively.by Joan Tisdale.S.M