Gasification of biomass in aqueous media

Abstract

The lignocellulosic biomass materials are abundant, cheap and renewable feedstocks suitable for biofuel and biochemical production. They can be derived from forestry wastes such as residues of the trees and shrubs, energy crops like maize, sorghum, miscanthus, kenaf, switchgrass, jatropha, corn, sugarcane and any agricultural residues such as corn stovers, wheat straw etc. The use of biofuels derived from lignocellulosic biomass does not cause additional increase in the carbon dioxide level in the earth's atmosphere. The release of carbon dioxide during biofuel utilization is balanced by the carbon dioxide consumed in biomass growth. In many cases, because of large water content and high drying cost, biomass is not a suitable feedstock for conventional thermochemical gasification technologies. Thermochemical gasification techniques such as biomass gasification and pyrolysis are energy intensive processes and produce relatively high amounts of char and tar with low conversion of biomass into gas. Among other various conversion methods, hydrothermal gasification, using super-or sub-critical water as the reaction medium, is seen as a promising way to produce hydrogen from biomass with high efficiency. These processes can be applied to the conversion of biomass with high moisture content without drying. While processes applied in sub-and super-critical water around 250-400°C, methane and carbondioxide are the major products in addition to the target gas hydrogen but the formation of these major side products can be minimized by using appropriate catalysts and adjusting processing temperature and pressure conditions. Aqueous phase reforming (APR) process is a rather new evolving technology involving decomposition of the oxygenated hydrocarbons to produce hydrogen-rich gas. The main advantage of APR is its relatively low gasification temperature where CO concentration within the hydrogen stream is rather low. The process produces high yield of hydrogen gas with low CO byproduct due to the water-gas shift reaction (CO + H2O - H2 + CO2) which is effective at the processing temperature. APR of carbohydrates take place at considerably lower temperatures compared to conventional alkane steam reforming process. A lower temperature reduces unwanted decomposition reactions that normally observed when carbohydrates are heated to elevated temperatures. Carbohydrates such as sugars (e.g., glucose) and polyols (e.g., ethylene glycol, glycerol) can be efficiently converted in the aqueous phase over appropriate heterogeneous catalysts at relatively mild processing conditions to produce hydrogen rich gas mixture. Lignocellulosic materials containing high level of polysaccharides are potential biomass sources for the APR gasification provided that, by using ecological pre-treatment techniques, the water-insoluble polysaccharides are hydrolyzed into relatively smaller carbohydrates which are soluble in water. This chapter will focus on APR and summarize the relevant research and development activities including the authors' work on conversion of lignocellulosics.© 2011 Nova Science Publishers, Inc. All rights reserved