Alcohols : The Liquid Fuel

General Account

Historical background and methods of alcohol production from various substrates are discussed in Primary Metabolites. In this section, use of alcohols as liquid fuel is described. During World War II, mixing of methanol and ethanol in different amount with petrol was carried out in Europe. Thereafter, blending of alcohol with petrol to be used as transport fuel started after the oil crisis in 1973. Use of 5% alcohol (ethanol) in petrol was made compulsory in Brazil by 1931. This country has been the second World sugar producer, therefore, large quantity of alcohol is produced from sugar cane and cassava. In 1985, Brazil launched a programme of blending 20% ethanol (produced from sugar cane and cassava) with petrol and thus saved about 40% of its petrol consumption. In 1990, it produced about 20.5 billion liters of alcohol from molasses and saved 11 billion dollars of foreign exchange. This country marketed about one million cars running on alcohol alone (an additional heat exchanger is required to start cars as the ignition point of alcohol is higher than that of gasoline) (Lones, 1980). In 1986, Brazil gave employment to 3.5 million people from 14 billion liters of tipsy fuel.

In addition to an alternative of petrol, another advantage of ethanol is that there is safety for cars if used as fuel. The flashpoint (i.e. the temperature at which a substance ignites) of ethanol is three times higher than that of petrol (Table 20.2). Also it emits less hydrocarbon. But the disadvantage is that in cold engine starting will be difficult. It may also pick up water from air. Therefore, mixture of enthanol-water will not burn readily and cause corrosion problems in tanks and engines. Ethanol also reacts with metals such as alloy of magnesium and aluminum. An alternative to using pure ethanol as a fuel is to mix it with petrol. In mixed condition it becomes a part of energy. Such a mixture containing 20 per cent ethanol is currently marketed in the USA as 'gasohol. The drawback with this approach is that the ethanol must be distilled to 100 per cent purity before mixing with petrol, otherwise it would be separated from the petrol.

India is fortunate enough for having many sources of biomaterials to be used in ethanol production. The Government is facing a crisis in case of molasses. Every year, potatoes have rotten for lack of buyers. Cassava is grown on large scale in Kerala and some parts of Tamil Nadu (see Sources of wastes). A large number of distilleries are in operation and many more to be set up. Utilization of sugary and starchy materials for the production of ethanol would be a good step to cut down the oil price and meet the fuel demand in country.

Conversion of coal to oil also represents an important facet of the general energy problem. The Central Fuel Research Institute suggested that replacement of oil by coal will help in saving 50 per cent cost of fuels. In our country, coal is a major source of energy. It is gasified and converted to methanol with a high degree of efficiency. Methanol can be directly converted to gasoline. Using this method, New Zealand has set up commercial plants and supplying one-third of national requirements of gasoline.

Table 20.2. Some of the physical and chemical properties of petrol and ethanol.
Ethanol Production
Ethanol is produced by chemical as well as biological routes. Through the chemical route, synthetic alcohol is produced by catalytic hydration of ethylene (C₂H₂) with water, using phosphoric acid at 70 atmosphere pressure and 300°C. This process involves the use of petroleum as fuel to generate high pressure and temperature for producing alcohol. Ethylene is derived from both natural and coke oven gases, and the waste gases released in refining petroleum to produce gasoline.

Biological route is an alternative way to produce alcohol. The biomass to be converted into liquid fuel can be derived from agriculture, municipal or forestry wastes (see Sources of wastes). The agricultural crops contain sugar (e.g. sugar cane, sugar beet), starch (maize, tapioca) or cellulose (cotton, species of populus) in high amount. Dr. T.K. Ghosh and coworkers of IIT, New Delhi are making effort to use lignocellulosic materials for alcohol production in addition to starch based substrates.

Fermentable Substrates
Following are the types of substrates used for alcohol production :
Sugary Materials: Examples of sugary materials are sugarcane and its by products/wastes (molasses, bagasse) and sugar beet, tapioca, sweet potatoes, fruit juice, sweet sorghum, etc. Sugar cane molasses is largely being used in many country for alcohol production.

Starchy Materials:Starchy materials used in ethanol production are tapioca, maize, wheat, barley, oat, sorghum, rice and potatoes. But tapioca and corns are the two major substrates of the interest (see Sources of wastes). It has been estimated that 11.7 kg of corn starch can be converted into about 7 liters of ethanol (Chahaf and Overend, 1982).

Lignocellulosic Materials: The sources of cellulosic and lignocellulosic materials are the agricultural wastes and wood. However, yield of ethanol from lignocellulose is low because of lack of suitable technology and failure of conversion of pentoses into ethanol. On the basis of technology available today about 409 liters of ethanol can be produced from one tonne of lignocelluloses (Chahal and Overend, 1982). Structure of cellulose, hemicellulose and lignin is given in Figs. 19.1-19.4 and anaerobic hydrolysis of these is discussed in the preceding section. Production of ethanol from lignocelluloses follows the following steps: (i) hydrolysis, (ii) fermentation, and (iii) recovery.

Hydrolysis of Lignocellulosic Materials
Hydrolysis of lignocelluloses are done to release the monomer sugars. These materials are hydrolyzed by acids and/or enzymes. Acid hydrolysis is performed by using dilute sulphuric acid (0.1 -0.2%) through layers of saw dust or wood chips under pressure and a high temperature (180-121°C). Sugars produced during hydrolysis are destroyed by acid. Therefore, it must be quickly removed. Acid portion should be neutralized with lime before the solution is fermented (Chahal and Overend, 1982). The other chemicals used in hydrolysis are hydrochloric acid, sodium hydroxide, ammonia, etc. Substrate hydrolysis coupled with high pressure and temperature for varying times, depending on nature of substrate, has shown a good result.

Enzymatic hydrolysis of cellulosic and lignocellulosic materials is performed by using the respective enzymes i.e. cellulases, hemicellulases, pectinases, lignases, etc. These enzymes are produced on a large scale from microorganisms (see Enzyme Technology). Enzymatic hydrolysis of cellulose, hemi-cellulose and lignin is discussed elsewhere (see Enzymatic digestion).

Upon hydrolysis of carbohydrates of plant materials the following sugars are liberated in the solution which in turn are fermented into ethanol :



Effect of Substrate Composition on Hydrolysis
Composition of substrates directly governs the hydrolysis, such as insolubility, high crystallinity and lignin coating (over the cellulose microfibrils in cell wall).
Crystallinity of cellulose directly affects its hydrolysis by cellulases. Cellulases will readily degrade the amorphous portion of cellulose, but not the crystalline portion, until the cellulase complex is rich in C_x enzyme. Lignin hinders substrate hydrolysis. As the amount of lignin is high in substrate, the yield of ethanol would be low. Treatment of substrate with alkali (2% NaOH at 70°C for 90 minutes, washed and sterilized) would be more effective than without treatment.

Moreover, steam explosion (i.e. steam treatment at 190°C) of lignocellulosic materials is another pre-treatment which has proved to be the best for hydrolysis (Chahal and Overend, 1982).

Fermentation
The soluble hexoses and pentoses obtained in solution after hydrolysis of sugary-starchy-cellulose are subjected to anaerobic fermentation by using bacteria, yeast and filamentatous fungi (see Alcohols). Theoretically, maximum conversion of glucose to ethanol is 51 per cent by weight. Yeast ferments glucose to 10-20 per cent ethanol. An outline of ethanol production is given in Fig. 20.1. The micro-organisms associated with fermentation utilize various metabolic process, depending on substrates i.e. glucose, xylose, etc. (see Metabolic pathways in microorganisms).

Recovery of Ethanol
When fermentation is over, the microbial mass (cells, mycelia and conidia) is separated from the fluid. The fluid contains the mixture of ethanol, water and small amount of alcohols and ether. Ethanol is recovered by distillation process i.e. vaporization of ethanol/water mixture.