The term biofuel refers to solid, liquid, or gaseous fuels that are predominantly pro­duced from biorenewable or combustible renewable feedstocks. Liquid biofuels will be important in the future because they will replace petroleum fuels. The biggest dif­ference between biofuels and petroleum feedstocks is oxygen content. Biofuels are nonpolluting, locally available, accessible, sustainable, and reliable fuels obtained from renewable sources.

There are two global biorenewable liquid transportation fuels that might replace gasoline and diesel fuel. These are bioethanol and biodiesel. Bioethanol is a good alternative fuel that is produced almost entirely from food crops. Biodiesel has be­come more attractive recently because of its environmental benefits.

Transport is one of the main energy consuming sectors. It is assumed that biodiesel is used as a fossil diesel replacement and that bioethanol is used as a gaso­line replacement. Biomass-based energy sources for heat, electricity, and transporta­tion fuels are potentially carbon dioxide neutral and recycle the same carbon atoms. Due to the widespread availability of biofuels, opportunities in biorenewable fuel technology can potentially employ more people than fossil-fuel-based technology.

Renewable liquid biofuels for transportation have recently attracted considerable attention in different countries around the world because of their renewability, sus­tainability, widespread availability, and biodegradability and the benefits they bring with respect to regional development, rural manufacturing jobs, and reduction in greenhouse gas emissions. Table 5.2 shows the major benefits of biofuels.

Biofuels can be classified based on their production technologies: first-generation biofuels (FGBs), second-generation biofuels (SGBs), third-generation biofuels (TGBs), and fourth-generation biofuels.

The FGBs refer to biofuels made from sugar, starch, vegetable oils, or animal fats using conventional technology. The basic feedstocks for the production of FGBs are often seeds or grains such as wheat, which yields starch that is fermented into

Table 5.2 Major benefits of biofuels

Economic impacts Sustainability

Fuel diversity

Increased number of rural manufacturing jobs

Increased income taxes

Increased investments in plant and equipment

Agricultural development

International competitiveness

Reduced dependency on imported petroleum

Environmental impacts Greenhouse gas reductions

Reduced air pollution Biodegradability Higher combustion efficiency Improved land and water use Carbon sequestration

Energy security Domestic targets

Supply reliability Reduced use of fossil fuels Ready availability Domestic distribution Renewability

bioethanol, or sunflower seeds, which are pressed to yield vegetable oil that can be used in biodiesel. Table 5.3 shows the classification of renewable biofuels based on their production technologies.

SGBs and TGBs are also called advanced biofuels. SGBs are made from nonfood crops, wheat straw, corn, wood, and energy crops using advanced technology. Algae fuel, also called algal oil or TGB, is a biofuel from algae (Demirbas 2007). Algae are low-input/high-yield (30 times more energy per acre than land) feedstocks to produce biofuels using more advanced technology. On the other hand, an emerg­ing fourth-generation is based on the conversion of vegetable oil and biodiesel into biogasoline using the most advanced technology.

There are some barriers to the development of biofuel production. They are tech­nological, economical, supply, storage, safety, and policy barriers. Reducing these barriers is one of the driving factors in the government’s involvement in biofuel and biofuel research and development. Production costs are uncertain and vary with the feedstock available. The production of biofuels from lignocellulosic feedstocks can be achieved through two very different processing routes: biochemical and ther­mochemical. There is no clear candidate for “best technology pathway” between the competing biochemical and thermochemical routes. Technical barriers for enzy­matic hydrolysis include: low specific activity of current commercial enzymes, high cost of enzyme production, and lack of understanding of enzyme biochemistry and mechanistic fundamentals.

The major nontechnical barriers are restrictions or prior claims on use of land (food, energy, amenity use, housing, commerce, industry, leisure or designations as areas of natural beauty, special scientific interest, etc.), as well as the environmental and ecological effects of large areas of monoculture. For example, vegetable oils are a renewable and potentially inexhaustible source of energy with energy content close to that of diesel fuel. On the other hand, extensive use of vegetable oils may lead to other significant problems such as starvation in developing countries. The vegetable oil fuels were not acceptable because they were more expensive than petroleum fuels.

There are few technical barriers to building biomass-fired facilities at any scale, from domestic to around 50 MW, above which considerations of the availability and cost of providing fuel become significant. In general, however, the capacity and generating efficiency of biomass plants are considerably less than those of modern natural-gas-fired turbine systems. The main nontechnical limitations to investment in larger systems are economic, or in some countries reflect planning conditions and public opinion, where a clear distinction may not be made between modern effective biomass energy plants and older polluting incinerator designs.