Lignocellulosic Biomass: A Review of Conversion Technologies and Fuel Products

The increasing consumption of energy, increasing demand for fossil fuels, escalating fuel prices, and rising levels of CO2 and other greenhouse gases are some of the factors contributing to the search for alternative sources of energy. As solar, wind, hydro and geothermal sources are attractive in providing electricity, they lack the ability to support the production of transportation fuels. This brings lignocellulosic biomass into limelight to sustain the supply of solid, liquid and gaseous biofuels. Lignocellulosic biomass is chemically composed of cellulose, hemicellulose and lignin that can be transformed into energy-dense components for use as fuels or chemicals. This review focuses on the various conversion technologies available for biomass conversion to biofuels. The two basic conversion routes discussed are thermochemical (pyrolysis, liquefaction, torrefaction and gasification) and biochemical (fermentative pathways). Compared to biochemical conversion that requires ambient conditions, thermochemical routes require the involvement of high temperatures and pressures for biomass conversion. The fuel products comprehensively discussed here are bio-oil, syngas, ethanol and butanol. Syngas fermentation has been presented as an advanced biomass conversion technology that converts syngas to higher alcohols in the presence of mesophilic and thermophilic microorganisms. Butanol, a progressive fuel than ethanol, is highlighted in terms of its biochemical production and fuel properties. Nevertheless, the review summarises the integration of thermochemical and biochemical conversion routes to efficiently produce the primary fuel product and utilize the co-products.