Pretreatment costs
Each raw material requires a different processing and pretreatment strategy which has to be tailored, taking into consideration their composition and susceptibility to such treatments. Physical/ chemical/physico-chemical pretreatment, or combinations thereof, needs to be optimized for pretreatment of each feedstock. The cost of pretreatment is a major factor in the saccharification-fermentation route and considerable developments in pretreatment technologies can be expected in coming years. The pretreatment must be advanced and appropriately integrated with the rest of the process to achieve the full potential of lignocellulosic ethanol. Another challenge in the pretreatment and enzymatic hydrolysis areas will be to reduce chemical usage and severity in pretreatment, while maintaining good enzyme digestibility of the pretreated biomass. Reduction in pretreatment severity in temperature and pressure can significantly cut down on capital costs of plants. The formation of undesired inhibitory compounds during the pretreatment is a loss in sugars, and adds complex, expensive detoxification steps to the process. Therefore, minimization of inhibitory compound formation is a high priority. Pretreatment research in the future should focus on processes consuming less energy and chemicals for the development of simpler, less expensive technologies.
Although enzyme costs have decreased in the last few years, the cost of cellulases can be as high as 15-30% of the minimum ethanol selling price (MESP) [2] in ethanol produced via cellulolysis process. This is another major challenging area of research in cellulosic ethanol and the development of low-cost and highly-effective enzymes for lignocellulose saccharification is a high priority. The high cost of enzyme production and the requirement of a higher enzyme dosage for hydrolysis of biomass are considered to be main hurdles for the economic viability of lignocellulosic bioethanol. Synthetic biology and metabolic engineering approaches have to be utilized more aggressively in the development of efficient and robust microbes for SHF, SSF and CBP processes. Integration of processes for reducing the number of process steps, and reuse of process streams to make the conversion processes more economical is another hopeful direction.