Co-gasification of coal and biomass

Semester

Spring

Date of Graduation

2012

Document Type

Dissertation/Thesis

Degree Type

MS

Committee Chair

Kaushlendra Singh.

Abstract

Gasification is thermo-chemical conversion technology in which complex organic polymers present in feedstock react with limited oxygen supplied by steam or air. The gasification reaction produces CO2 (Carbon-dioxide), H2 (Hydrogen), CO (Carbon-monoxide), CH4 (Methane) and other lower hydrocarbons. The gas produced from a gasifier, known as syngas, can used to produce electricity and liquid fuels. Coal and biomass are used in a gasfier as feedstock to produce syngas but problems like high carbon footprints and GHG emissions are associated with coal gasification. On the other hand, high preprocessing cost and tar formation are the problems related with biomass gasification. Recently, there has been significant research interest in co-gasification of coal and various types of biomass blends to improve biomass gasification by reducing the tar content in the product gas. In addition, ash present in biomass catalyzes the gasification of coal. However, due to the fibrous nature of biomass and the large difference in gasification temperature of coal and biomass, co-gasification in existing systems presents technical challenges.;In this study, feedstock properties like gross heating value, proximate analysis, ultimate analysis, heat capacity, chemical composition and mineral composition were determined for coal, red oak, yellow poplar and coal-biomass mixtures. Also, thermo-gravimetric analyses (TGA) were performed in air, carbon-dioxide and nitrogen environments representing oxidation, gasification and pyrolysis conditions, respectively. Results showed that gross heating value, ash content and sulfur content significantly decreased when wood was mixed with coal. It was found that the reduction reactions for the gasification of char formed started at 800°C.Moreover, mixtures of hardwood pellets and bituminous coal (Pittsburgh seam) were gasified in a down-draft fixed bed gasifier to quantify the effect of feedstock on the syngas composition, syngas carbon conversion efficiency and syngas energy conversion efficiency. Also, a mixture of air-mist was injected into the system to its effect on biomass gasification. It was found that syngas with 20.95% carbon-monoxide and 16.05% hydrogen was produced from the gasification of hardwood pellets. The hydrogen and carbon-monoxide content in the product gas decreased with an increase of the coal ratio of the mixtures due to low temperature of combustion zone. A significant increase in carbon efficiency was observed with an increase of the coal ratio in the mixtures as the carbon-dioxide content of syngas increased. Syngas carbon and syngas energy efficiency increased with injection of the mist along with air in the system for the biomass gasification.

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