Demand for energy, enough food, and a good environment is the most important concern in the entire world today. Then, the possibility of obtaining a renewable, available, safe, and effective source of energy is one of the challenges that human­ity should address. The biofuels, particularly bioethanol, are an environmentally clean source of energy. However, production costs of fuel ethanol are higher than production costs of gasoline. Nevertheless, many groups and research centers in different countries are continuously carrying out studies aimed at reducing etha­nol production costs for a profitable industrial operation. Diverse research trends and process improvements could be successful when trying to lower ethanol costs. These research tendencies are related to the nature of utilized feedstocks (looking for the most productive and cheap raw materials), tools of process engi­neering (mainly process synthesis, integration, and optimization), food security, and environmental impacts.

13.1 FEEDSTOCKS

The three kinds of feedstocks used for fuel ethanol production 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 exhibit compara­tive advantages in the availability of sucrose — and starch-containing 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 other than environmental concerns, but humankind and global climate will be ben­efited in any case.

For the three main types of feedstocks, the development of effective, continu­ous fermentation technologies with near 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 the industrial level. Additionally, past research tendencies for cell-free ethanol production, using
only the enzymes involved in the conversion of glucose to ethanol, may offer a practical and beneficial alternative. This progress should complement the intense efforts oriented to the selection and development of microbial strains with par­ticular traits, such as specific flocculating properties or increased tolerance to ethanol, inhibitors, and salts.

Consequently, an important part of the research trends on fuel ethanol produc­tion is geared to the reduction of feedstock costs, especially through the utiliza­tion of less expensive lignocellulosic biomass. In general, most of the research efforts are oriented to the conversion of lignocellulosics into fermentable sugars and then to ethanol. One of the key factors for enhancing the competitiveness of the biomass-to-ethanol process is the economic and concentrated access to large quantities of biomass distributed in rural areas of the world. Another important issue in transformation technologies for lignocellulosic feedstocks is the design of low-cost methods for its delignification. After that, the increase in the specific activity of cellulases and the decrease in their production costs play a crucial role in the process costs. The technology of recombinant DNA will provide impor­tant advances for the development of the fuel ethanol industry. The development of genetically modified microorganisms capable of converting starch or biomass directly into ethanol and with a proven stability under industrial conditions will allow the implementation of the consolidated bioprocessing of the feedstocks.

The massive utilization of fuel ethanol in the world requires that its production technology must be cost effective and environmentally sustainable. In particular, ethanol production costs should be lowered. For current technologies employed at the 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 such as cane molasses allows producing ethanol with the lowest costs compared to using the starchy materials (mostly grains).

Although the ethanol yield from corn is higher than that from sugarcane, 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 promising 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. A summary about research perspectives of feedstocks for ethanol production is presented in Table 13.1.