Abstract

Production of renewable fuels, especially bio-ethanol from lignocellulosic biomass, holds remarkable potential to meet the current energy demand as well as to mitigate greenhouse gas emissions for a sustainable environment. Present technologies to produce bioethanol largely depend on sugarcane and/or starch based grains and tubers (mainly corn, potatoes). This is partly due to ease of substrate handling and processing. On the other hand, use of sugarcane and food grains to produce bio-ethanol has caused significant stress on food prices and food security. Accordingly, the recent focus has been on lignocellulosic materials as a source for bio-ethanol.  In fact, many countries are moving towards developing or have already developed technologies to exploit the potential of lignocellulosic materials for the production of bioethanol. This process of ethanol production generally involves hydrolysis of lignocellulosic biomass to fermentable sugars followed by fermentation of such sugars to ethanol.  Achieving fermentable levels of sugars from lignocellulosic biomass requires relatively harsh pretreatment processes. The pretreatment process has pervasive impact on the overall operation because the process depends on the choice of lignocellulosic source, the size reduction via grinding, chemical treatment, acid hydrolysis, neutralization and fermentation. Recent advances in the process technologies have made it possible to use simultaneous saccharification and fermentation.  In this process cellulase enzyme is the critical reagent as well as the cost determining factor. The advances in biotechnology as related to bioethanol have focused on engineering organisms that are capable of producing ethanol from cellulose, hemicellulose and lignocellulose.  Such organisms are expected to be capable of not only degrading cellulose, hemicellulose and lignocellulose to fermentable sugars, but also are able to utilize both pentose and hexose sugars to produce ethanol at a relatively high yield.  More recent and emerging approaches in bioethanol production are focused on reducing production costs. This approach uses consolidated bioprocessing schemes in which cellulase production, substrate hydrolysis, and fermentation are all accomplished in a single step. Countries, such as Nepal, that totally depend on the import of fossil fuels cannot ignore the potential of bioethanol derived from lignocellulosic biomass.  Nepal is rich in biodiversity and posses variety of energy crops.  Accordingly, developing policies and mechanisms that promote bioethanol will go a long-way in reducing the fuel crises in the countries lacking oil resources.

Key words: Lignocellulosic biomass, bioethanol, saccharification and fermentation (SSF), consolidated bioprocessing (CBP).