Following the successive oil crises of the 1970's, renewable alternatives to petroleum as an energy source, have been intensively investigated worldwide. The Research Association for Petroleum Alternatives Development (RAPAD) was established in Japan, in May 1980, by 23 private companies with the support of the Ministry of International Trade and Industry (MITI). One of RAPAD's main tasks was to investigate the development of technologies for biomass conversion and utilization, in particular, the production of ethanol from cellulosic biomass. As a part of this project, studies were conducted in our laboratory, on the production of fuel ethanol from cellulosic biomass.
Various forms of biomass resources exist (Fig. 3-1). Among these, sugar and starch crops are inappropriate for use as energy sources since they are primary food sources, and are unstable from the viewpoints of long-term supply and cost. Cellulosic resources, on the other hand, represent the most abundant global source of biomass, and have been largely unutilized. In our study on fuel ethanol production processes, our efforts were directed toward the use of agricultural waste materials such as bagasse or sugar cane molasses, rice straw, and forestry waste materials such as wood chips from thinning.
Cellulose, a main component of plant cell walls, can be solubilized by either enzymatic or acid hydrolysis. Enzymatic processes are however preferable owing to drawbacks of the acid hydrolysis process. The development of cellulose-decomposing enzymes, i.e. high-titer cellulases, is however a problem that needs to be addressed prior to implementation of the enzymatic hydrolytic process. The commercial feasibility of ethanol production from cellulosic biomass is dependent on the availability of a cheap source of cellulase. Extensive work conducted in our laboratory resulted in the development a high titer cellulase from Trichoderma reesei which can be produced at a low cost.
In order to enhance the efficiency of the use of cellulase enzymes, immobilized yeast cells were used as the enzyme source. This resulted in the development of a continuous process for the production of ethanol from cellulosic biomass (rice, straw, bagasse, and wood) and led to the construction of a pilot plant. While this plant includes unit processes which have been studied by various research organizations (1, 2, 3), it was the first such total system to be constructed worldwide.
Figure 3-1 - Biomass resources of the world