Bio-alcohol is the most widely used liquid biofuel, most often as a gasoline additive. Alcohols are usually made using fermentation by yeasts, but that is just one step in the process. The typical path is as follows: First, aerobic photosynthesis is used to grow plants. Then some portion of the plant matter is pre-digested using enzymes to make sugars (a family of carbohydrates) if it isn’t already in the form of sugars. Next the sugars are fermented by anaerobic yeasts to make alcohols. Finally the alcohol is distilled out of the aqueous solutions to make nearly pure fuel.

Intentional fermentation to make a particular alcohol for human consumption – ethanol – was practiced at least since the time of the Sumerian civilization if not earlier. A receipe for beer in the form of a prayer known as the “Hymn to Ninkasi” has survived from around 1800 BC from the Sumerian city of Nippur.[1]

Ethanol happens to have chemical properties somewhat similar to gasoline, so that internal combustion gasoline engines can use it without very much modification. Ethanol tends to attack certain polymers (rubber and plastic compounds) that are gasoline-safe, so vehicles designed to run on fuel containing alcohol (“flex-fuel” cars) have to be careful with their choice of non-metallic fittings. A disadvantage of ethanol is that it has one-third lower specific energy (30 MJ/kg) and energy density (24 MJ/litre) than gasoline, resulting in proportionately poorer fuel economy.

Although bio-fuel alcohols are mostly the same chemical – ethanol – there are different methods for producing the stuff. Cane and corn ethanols are the most popular today but cellulosic ethanol is also promising.

The simplest process is ethanol production from sugar cane. This tropical plant is so sugar-rich that the rest of its molecules don’t need to be broken down by enzymes before fermentation becomes viable. This is a favorite method in Brazil which has an appropriately warm climate and produced 24.5 gigalitres of cane ethanol in 2008, representing approximately 18% of the country’s transportation energy. It is used mostly in the form of 20 to 25% ethanol blends in gasoline.[2] In that year, Brazil used over 7.7 million hectares of farmland for raising cane, and of that crop ethanol production accounted for 3.6 million hectares. Biofuel yield was in the range 6,800 to 8,000 litres/hectare/year.[3] It would have taken about 25 million hectares to produce enough ethanol to meet the nation’s transportation sector’s entire energy demand.

Corn ethanol is made mostly in the United States of America. In this process, it is the starch-rich grain that is cooked then enzymatically digested to convert starch to a sugar (dextrose), and that in turn is fermented. Most of the plant is discarded. Ethanol, mostly from corn, is used as a gasoline additive and amounted to almost 8 percent of the US gasoline supply in 2009, and US domestic production capacity has increased tenfold in two decades, from 3.4 gigalitres in 1990 to 40 gigalitres in 2009. Corn ethanol activity consumed 10 million hectares of arable land. Yield was 3,800 to 4,000 litres/hectare/year.[3] The production of ethanol fuel from corn is dogged by controversy concerning its actual costs. Critics point out that the true cost of corn ethanol should include the energy consumed in producing and using fertilizers and pesticides and their environmental impact including pollution from runoff, the energy consumed in the operation of agricultural machinery, the energy required to ship and process the grains, the consumption of massive quantities of water during all stages of the process, the consumption of large amounts of electricity most of which comes from coal plants, and the impact on global food availability and prices.[4] The large subsidies received by US farmers further distort the picture.

One may compare the cane and corn ethanol industries by examining the energy balance. Brazilian cane ethanol production has an energy balance of about 8 to 10, while US corn ethanol production is claimed to achieve an energy balance of only 1.3 to 1.6, and even that is contested by critics who fear corn ethanol may be below unity. The related greenhouse gas reductions resulting from the use of these fuels are 61% for Brazilian cane ethanol and 21% for US corn ethanol according to the US Environmental Protection Agency.[3]

Advocates of cellulosic ethanol point out that their proposed biofuel can improve on the ecological performance of corn ethanol and even on cane ethanol, but this technology has yet to scale up to market needs. The basic idea behind cellulosic ethanol is to maximize the use of the plant once you’ve gone to the trouble of growing it. Most of a plant is composed of “woody” material dominated by the molecules cellulose and lignin. These cannot be digested by yeasts, nor are they broken down by the enzymes used to break down starch. Over a century ago, the earliest processes for making cellulosic ethanol used sulphuric acid hydrolysis, but more recently enzymatic methods have become the dominant method of breaking down the woody molecules into sugars. Once this hurdle is overcome, the feedstock for fermentation can be any plant matter – wood, grasses, and agricultural waste such as straw or corn stover. Thus corn kernels can be used for food, feed, and vegetable oil, while the rest of the plant is converted to ethanol fuel. This sounds great, but the chemistry is not without its problems, and that is why the production and use of cellulosic ethanol is not yet widespread. It is more of a research and development activity. One problem is optimizing the enzymatic decomposition of lignin and cellulose into sugars. The search is still on for effective and inexpensive-to-produce enzymes to support this process at the lowest possible temperatures. The result of the enzymatic hydrolysis usually includes a significant proportion of large sugar molecules, which conventional yeasts cannot digest. As a result, there is a need for special yeasts that can digest these sugars. Rest assured, these problems are keeping a lot of genetic engineers and other researchers very busy. Subsidized pilot plants have been built and are in operation, but so far cellulosic ethanol costs more than twice as much as corn ethanol, despite the much lower cost of the feedstocks.[5] Scaling up to commercial levels is now underway by a technology leader in this field, Iogen Corp. of Canada.[6] The energy balance of cellulosic ethanol is not clear because it depends heavily on the feedstock and enzyme technology used, but it should at least match corn ethanol already, and should perform much better eventually.

[1] (and many other links)

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