Technological Challenges
There are weak points and challenges in all cellulosic ethanol paths. Some of the main challenges are in the following areas.
1. Carbon conversion efficiency
In today’s best available technologies only about half the carbon in biomass can be converted to carbon in ethanol. This is due to several reasons; in the saccharification fermentation route, lignin which makes 15-30% of biomass is not used. In the gasification route all three major components in biomass are used; however this technique generally suffers from inherently poor solubility of CO and H2 in aqueous phase resulting poor conversion efficiencies. Then in the cellulolysis process, CO2 is produced as a byproduct in fermentation, which is biomass-derived carbon not converted to ethanol. As discussed in Chapter 8, fermentation of a mole of six carbon sugars produces two moles of carbon dioxide as a byproduct. Utilization of CO2 for conversion to reduced products like alcohols or feedstock chemicals is a challenging research field and adds value to the cellulosic ethanol biorefinery. Improvement in carbon conversion efficiency is a direct measure of improvement in ethanol yield.
Even though in theory any form of lignocellulosic biomass can be used to make cellulosic ethanol, availability of feedstocks for biofuel production, their variability and sustainability are major criteria to be addressed in a global-scale substitution of fossil fuels by cellulosic biofuels. In the case of agricultural wastes like corn stover, removing all agricultural wastes for cellulosic ethanol production can have adverse effects on agriculture and the environment. Then there are land use constraints in growing dedicated biofuel crops. There are opportunities for improvements in biofuel crop yields and development of high-yielding varieties by genetic engineering methods.