The massive utilization of fuel ethanol in the world requires that its production technology be cost-effective and environmentally sustainable. In particular, ethanol production costs should be lowered. For current technologies employed at commercial level, the main share in the cost structure corresponds to the feedstocks (above 60%) followed by the processing expenditures. In general, the use of sucrose-containing materials as cane molasses allows producing ethanol with the lowest costs compared to the starchy materials (mostly grains). Particularly, although the ethanol yield from corn is higher than that from sugar cane, the lower annual yield of corn per cultivated hectare makes it necessary to use larger cropping areas. On the other hand, the lignocellulosic biomass represents the most prospective feedstock for ethanol production. The availability and low cost of a wide range of lignocellulosic materials offer many possibilities for the development of bioindustries that could support the growth of the international biofuel market and contribute to the reduction of greenhouse gas emissions worldwide.
The current research tendencies for improving fuel ethanol production are linked to the nature of employed raw materials, processing steps, and related process engineering issues. The trends regarding the latter aspect were discussed in a previous work (Cardona and Sánchez, 2007). In the present paper, main trends in the conversion of different feedstock into ethanol were discussed. Some of them are summarized in Table 8, which complements the Table 4 of the above-mentioned previous work concerning the research priorities for improving ethanol production by means of process engineering. For the three main types of feedstocks, the development of effective continuous fermentation technologies with near to 100% yields and elevated volumetric productivities is one of the main research subjects in the ethanol industry. To this end, many of newly proposed technologies for reducing the product inhibition effect on the cell growth rate should be scaled-up at industrial level. This progress should complement the intense efforts oriented to the selection and development of microbial strains with particular traits like specific flocculating properties or increased tolerance to ethanol, inhibitors and salts.
The three kinds of feedstock analyzed in this paper correspond to resources that are present in almost all the countries. In particular, all populated regions in the world account for vast amounts of lignocellulosic waste materials that eventually can be converted into ethanol. Tropical countries like Colombia exhibit comparative advantages in the availability of feedstocks for ethanol production in comparison to European or North American countries. In fact, the dynamics of the global ethanol market could require these countries to supply the growing demand of those countries that have implemented or will implement ambitious programs for the partial substitution of fossil fuels with renewable liquid fuels. These programs may have dissimilar motivations different to environmental concerns, but humankind and global climate will be benefited in any case.