Liquid Biofuels

Ethanol

Ethanol is the main liquid biofuel and can be produced from switchgrass through the two major processes described below.

Biochemical Conversion: In cases of corn grain and sweet sorghum stalks, sugars are directly fermented to ethanol by adding enzymes and yeast. Cellulosic feedstocks are more difficult to break down into fermentable sugars than starch- or sugar-based feedstocks. The cellulosic biochemical conversion process requires additional steps. Two key steps are biomass pretreatment and cellulose hydrolysis. During pretreatment, the hemicellulose part of the biomass is broken down into simple sugars and removed for fermentation. During cellulose hydrolysis, the cellulose part of the biomass is broken down into the simple sugar, glucose.

Xylose is the most prevalent pentose sugar released by the hemicellulose hydrolysis reaction. In this step, xylose is fermented, using Zymomonas mobilis or other genetically engineered bacteria. The glucose fermentation reaction is caused by yeast or bacteria, which feed on the sugars. As the sugars are consumed, ethanol and carbon dioxide are produced. Lignin and other byproducts of the biomass-to-ethanol process can be burned to produce electricity required for the ethanol production process. Burning lignin actually creates more energy than is needed, and selling electricity may help the processes economics.

Schematic of a Biochemical Cellulosic Ethanol Production Process
(Source:http://www.afdc/ethanol/production_cellulosic.html)

Thermochemical Conversion: Switchgrass also can be used to produce ethanol using thermochemical processes. In this approach, heat and chemicals are used to break biomass into syngas (a mixture of carbon monoxide and hydrogen) and reassemble it into products such as ethanol.

Schematic of a Biochemical Cellulosic Ethanol Production Process.
(Source:http://www.afdc/ethanol/production_cellulosic.html)

Methanol

As an engine fuel, methanol has similar chemical and physical characteristics as ethanol. It is predominantly produced by steam reforming natural gas to create a synthesis gas, which is then fed into a reactor vessel in the presence of a catalyst. This process then produces methanol and water vapor. Although a variety of feedstocks can be used to create methanol, today’s economics favor the use of natural gas.

Biobutanol

Like ethanol, biobutanol is an alcohol that can be produced through processing of domestically grown crops such as corn, sugar beets, and other biomass such as fast-growing grasses, and agricultural waste products. The ability to produce butanol from biomass sources via fermentation has existed since the early 1900s. However, these older biobutanol processes are more expensive than today’s petrochemical production processes. Today, butanol is produced almost entirely from petroleum. Renewed interest in butanol as a sustainable vehicle fuel has led to the development of improved biobutanol production processes. DuPont® and BP® are making biobutanol the first product of their joint effort to develop, produce, and market next-generation biofuels.

Biodiesel

Another important liquid fuel worth mentioning is biodiesel, even though it is not derived from cellulosic feedstocks. It will play a major role in reducing foreign oil dependency as it can be used in trucks and freight vehicles. In the U.S., most biodiesel is made from soybean oil or recycled cooking oils. Animal fats, vegetable oils, and other recycled oils can also be used to produce biodiesel, depending on their costs and availability. In the future, blends of all kinds of fats and oils may be used to produce biodiesel. The main reaction for converting oil to biodiesel is called transesterification. This transesterification process reacts an alcohol (like methanol) with the triglyceride oils contained in vegetable oils, animal fats, or recycled greases forming fatty acid alkyl esters (biodiesel) and glycerin. The reaction requires heat and a strong base catalyst, such as sodium hydroxide or potassium hydroxide. The byproducts include methanol and the base catalyst that can be reused in the process. Another byproduct is glycerin that can be for sale to the pharmaceutical and cosmetic industries.