Theoretical and experimental investigation of hydrogen production by gasification of biomass.

摘要

A detailed theoretical and experimental investigation of hydrogen production by thermochemical gasification of biomass was conducted. The thermodynamics of biomass gasification was first studied to determine the hydrogen yield at equilibrium. The gasification process is characterized by a number of endothermic and exothermic reactions. A combination of these reactions enables internal energy transfer, and therefore improved process efficiency. The maximum hydrogen yield is limited by thermodynamic equilibrium. One solution to this problem is to remove one of the co-products (CO2) that governs the equilibrium hydrogen yield. In recent times, sorbents (such as calcium oxide) have been used for CO2 removal from fossil fuel exhaust. The same principle was applied here to drive the reactions in favor of hydrogen. In the process the sorbent gets saturated and has to be regenerated for further use.; Process simulations were conducted using an ASPEN simulator with the end objective of determining the hydrogen yield in presence of a CO2 sorbent. Ethanol was used as the model biomass compound and calcium oxide was the representative sorbent. The simulations showed 19% increase in hydrogen yield and about 50% reduction in product gas CO2 while using the sorbent. The hydrogen yield in the presence of sorbent at a gasification temperature of 600°C was comparable to the hydrogen yield without the sorbent at 750°C. Hence there is a potential to reduce the gasifier operating temperature by about 100-150°C while still getting the same amount of hydrogen. The in-situ heat transfer (CO2 absorption is exothermic) reduced the gasifier heat duty by almost 42%. Based on the encouraging results obtained from simulations, experiments were conducted using Southern pine bark as the model biomass and calcium oxide as the representative sorbent. Hydrogen yield increased substantially (from 320 ml/g to 719 ml/g) by using sorbents at gasification temperature as low as 500°C. The product gas had much less tars and particulate matter as compared to conventional gasification. The carbon conversion efficiency (a way of quantifying the effectiveness of gasification) increased from a mere 23% to 63% while using sorbent.; Sorbent enhanced biomass gasification has the potential to produce a hydrogen rich, CO2 free and possibly tar free gas that can be sent to a fuel cell or gas turbine with minimal cleaning. Hence there is a potential to reduce the equipment needed for hydrogen production. This will lead to reduced capital and operating costs. Hence sorbent enhanced biomass gasification has the potential to become a cost effective technology for producing renewable hydrogen.