CHEMICAL LOOPING GASIFICATION OF BIOMASS FOR HYDROGEN-ENRICHED GAS PRODUCTION
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Environmental concerns and energy security are two major forces driving the fossil fuel based energy system towards renewable energy. In this context, hydrogen is gaining more and more attention in this 21st century. Presently, hydrogen is produced from reformation of fossil fuels, a process that could not address above two problems. For this it needs to be produced from a renewable carbon neutral energy source. Biomass has been identified as such a renewable energy source. Conversion of biomass through thermo-chemical gasification process in the presence of steam could provide a viable renewable source of hydrogen. This thesis presents an innovative system based on chemical looping gasification for producing hydrogen-enriched gas from biomass. The other merit of this system is that it produces a pure stream of carbon dioxide by conducting in-process capture and regeneration of sorbent. A laboratory scale chemical looping gasification (CLG) system based on a circulating fluidized bed (CFB) is developed and tested. Experiments conducted to gasify sawdust in CFB-CLG system shows that it could produce a gas with as much as 80% hydrogen and as little as 5% carbon dioxide. A kinetic model is developed to predict the performance of the gasifier of a CFB-CLG system, and is validated against experimental results. To understand the science of biomass gasification in the presence of steam and CaO, a number of additional studies are conducted. It show that for higher hydrogen and lower carbon dioxide concentration in the product gas, the optimum values of steam to biomass ratio, sorbent to biomass ratio, and operating temperature are 0.83, 2.0 and 670oC respectively. In CFB-CLG system the sorbent goes through a series of successive calcination-carbonation cycles. Calcination studies in presence of three alternate media, nitrogen, carbon dioxide and steam show, that steam calcination is best among them. An empirical relation for calcination in presence of three media is developed. Owing to the sintering, irrespective of medium used for calcination, the conversion of CaO reduces progressively as it goes through alternate calcination-carbonation cycles. An additional empirical equation is developed to predict the loss in sorbent’s ability during carbonation.