Ethanol is the most widely used liquid biofuel. It is an alcohol and is fermented from sugars, starches, or cellulosic biomass. Most commercial production of ethanol is from sugar cane or sugar beet, as starches and cellulosic biomass usually require expensive pretreatment.

Carbohydrates (hemicelluloses and cellulose) in plant materials can be converted into sugars by hydrolysis. Fermentation is an anaerobic biological process in which sugars are converted into alcohol by the action of microorganisms, usually yeast. The resulting alcohol is ethanol. The value of any particular type of biomass as feedstock for fermentation depends on the ease with which it can be converted into sugars.

Bioethanol is a fuel derived from renewable sources of feedstock, typically plants such as wheat, sugar beet, corn, straw, and wood. Bioethanol is a petrol addi- tive/substitute. It is possible that wood, straw, and even household wastes may be economically converted into bioethanol.

Bioethanol can be used as a 5% blend with petrol under EU quality standard EN 228. This blend requires no engine modification and is covered by vehicle war­ranties. With engine modification, bioethanol can be used at higher levels, for exam­ple, E85 (85% bioethanol). Figure 5.2 shows world ethanol production from 1980 to 2008 (RFA 2009).

Bioethanol can be produced from a large variety of carbohydrates with a general formula of (CH2O)n. Fermentation of sucrose is performed using commercial yeast such as Saccharomyces ceveresiae. The chemical reaction is composed of enzymatic hydrolysis of sucrose followed by fermentation of simple sugars. First, invertase enzyme in the yeast catalyzes the hydrolysis of sucrose to convert it into glucose and fructose. Second, zymase, another enzyme also present in yeast, converts the glucose and the fructose into ethanol.

Glucoamylase enzyme converts the starch into D-glucose. The enzymatic hy­drolysis is then followed by fermentation, distillation, and dehydration to yield an­hydrous bioethanol. Corn (60 to 70% starch) is the dominant feedstock in the starch- to-bioethanol industry worldwide.

Carbohydrates (hemicelluloses and cellulose) in lignocellulosic materials can be converted into bioethanol. The lignocellulose is subjected to delignification, steam explosion, and dilute acid prehydrolysis, which is followed by enzymatic hydrolysis and fermentation into bioethanol. A maj or processing step in an ethanol plant is enzy­matic saccharification of cellulose to sugars through treatment by enzymes; this step requires lengthy processing and normally follows a short-term pretreatment step.

Hydrolysis breaks down the hydrogen bonds in the hemicellulose and cellulose fractions into their sugar components: pentoses and hexoses. These sugars can then

be fermented into bioethanol. The most commonly applied methods can be classified into two groups: chemical hydrolysis (dilute and concentrated acid hydrolysis) and enzymatic hydrolysis. In chemical hydrolysis, pretreatment and hydrolysis may be carried out in a single step. There are two basic types of acid hydrolysis processes commonly used: dilute acid and concentrated acid.

The biggest advantage of dilute acid processes is their fast rate of reaction, which facilitates continuous processing. Since 5-carbon sugars degrade more rapidly than 6-carbon sugars, one way to decrease sugar degradation is to have a two-stage pro­cess. The first stage is conducted under mild process conditions to recover the 5-carbon sugars, while the second stage is conducted under harsher conditions to recover the 6-carbon sugars.

Methanol, also known as “wood alcohol,” is generally easier to find than ethanol. Sustainable methods of methanol production are currently not economically viable. Methanol is produced from synthetic gas or biogas and evaluated as a fuel for in­ternal combustion engines. The production of methanol is a cost-intensive chemical process. Therefore, in current conditions, only waste biomass such as old wood or biowaste is used to produce methanol.

Before modern production technologies were developed in the 1920s, methanol was obtained from wood as a coproduct of charcoal production and, for this reason, was commonly known as wood alcohol. Methanol is currently manufactured world­wide by conversion or derived from syngas, natural gas, refinery off-gas, coal, or petroleum:

2H2 C CO! CH3OH (5.1)

The chemical composition of syngas from coal and then from natural gas can be identical with the same H2/CO ratio. A variety of catalysts are capable of causing the conversion, including reduced NiO-based preparations, reduced Cu/ZnO shift preparations, Cu/SiO2 and Pd/SiO2, and Pd/ZnO (Takezawa et al. 1987; Iwasa et al.

1993).