Green plants, their products and animals stand as an alternative to the energy crisis in the present context. Conventionally, a fraction of this resource is used, while a major part is thrown as waste. This huge waste has been referred to as biomass.
The energy contained in the biomass is harnessed and used as an alternative to the conventional energy resources. There are five methods of generating energy from biomass: (a) solid fuel combustion; (b) gasification; (c) pyrolysis; (d) digestion; and (e) fermentation.
(a) Solid fuel combustion
It is perhaps the simplest and most common way of extracting energy from biomass. In developing countries, the major part of their energy need is fulfilled by this method. Electricity is generated in thermal power plants by burning coal in most countries.
The solid fuel is exposed to high temperature and limited oxygen to produce a gaseous fuel. This fuel is a mixture of several gases such as carbon monoxide, carbon dioxide, nitrogen, hydrogen and methane.
In conventional practice, the solid biomass is heated to generate volatile substances. The remaining is a black colour residue, known as charcoal. The volatile matter is collected, which contains hydrogen and carbon monoxide. These may combine in chemical reactions to produce methane, methanol and other hydrocarbons.
The complex molecules present in the biomass are broken down into simpler molecules by anaerobic bacteria. These microorganisms usually live at the bottom of swamps, where there is no air. These consume dead organic matter and digest them to methane and hydrogen.
Landfill technology is a related technology to digestion. A large proportion of household biomass waste, such as kitchen scraps, lawn clippings and pruning are thrown in pits. Over a period of several years, anaerobic bacteria at the bottom of such pits, steadily decompose the organic matter and emit methane.
The gas can be extracted and used by ‘capping’ a landfill site with an impervious layer of clay and then inserting perforated pipes that collect the gas and bring it to the surface.
It is an anaerobic conversion of simple sugars into ethanol. This practice is as old as human-civilization. The same principle is applied to the production of ethanol from biomass.
In addition, food products; such as cheese, yoghurt, sauerkraut, pickle, soya sauce and other oriental products; enzymes and antibiotics are commercially produced by this technology. It is also known as bioprocess technology.
Fermentation is achieved in a self containment vessel, designed for optimal microbial growth and metabolic activity. This vessel is known as a bioreactor or fermented.
Biotechnologists have turned to this biomass as an alternate energy source. Many energy rich molecules (fuel), such as ethanol, methanol, methane and hydrogen have been synthesized from the biomass by applying the principles of recombinant DNA technology.
These fuels have been collectively referred to as biofuels. Simultaneously, the photosynthetic ability of green plants has been enhanced by gene manipulation, such that they trap and transform more solar energy per unit area and unit time. There are three major classes of biofuels: (1) bioalcohols (ethanol and methanol), (2) lipid biofuels [straight vegetable oil (SVO) and biodiesel and (3) biogas (methane).
Alcohol from biomass (Bioalcohols)
Alcohol produced from biomass is known as Bioalcohols. It includes ethanol and methanol. Ethanol has been used as an abundant automotive fuel in Brazil since 1980s.
It is cheaper in comparison to fossil fuel (petrol or diesel). Several countries like USA, EU (European Union), Zambia, Zimbabwe, Nicaragua and Paraguay have followed the foot steps of Brazil. In USA, gasohol (10% ethanol + 90% petrol) is also available as an automotive fuel. Bio-production of ethanol is based on a biochemical reaction known as fermentation.
Microorganisms, especially yeast cells undertake fermentation reaction as a source of their energy requirement. The process is anaerobic i.e. operates in the absence of molecular oxygen. Biomass derived methanol is formed through gasification. The biomass is converted into a synthesis gas (syngas) that is later processed into methanol. However, currently, methanol is produced from natural gas, since, biomass derived methanol is not economically viable at anywhere near the current price level. It is not used as abundantly as ethanol in automotive engines.
It is only used as a blend with petrol or diesel. The principles of recombinant DNA technology are applied to the commercial production of ethanol from biomass, such that the price will be competitive and bioethanol will be economically viable.
While commercializing its production, the attention is focused on three different aspects, namely, (1) improving upon the yeast host; (2) availing cheap and abundant raw materials; and (3) providing best fermentation conditions so that as much ethanol is produced from raw materials.
Improving upon the yeast host
ATP is required by the yeast host to drive the fermentation reaction. It does so from simple sugars such as glucose and fructose. Ethanol is formed by anaerobic degradation known as fermentation of simple sugars.
When the ethanol reaches 12-15 %, yeasts stop growing and even die due to toxicity of the ethanol. Now yeast cells are genetically engineered, which may withstand the concentration up to 24 %. This reduces the cost of purification of ethanol by distillation from time to time.
Availing cheap and abundant raw materials
Sucrose present in the sugarcane is used in Brazil, while in USA, it is corn. These are important crops, which can be used as food instead of being used for fuel generation. Possible alternate sources are cassava, potato, woody trees and grass.
These sources have abundant starch and cellulose. If the source is starch, it is to be made available to yeast in simple form. Starch is a polysaccharide having many monosaccharides joined by glycosidic bonds, a amylase and glucoamylase (a- 1,6-glycosidase) are the enzymes, which do the work.
In some industrial processes, a -amylase is used. Yeasts do not contain amylase. However, some yeast possesses amylase, but are poor fermenters. Bacillus amyloliquefciciens contains a gene coding for a -amylase and Aspergillus niger for glucoamylase. These genes are genetically engineered into the genome of a high yeilding yeast host.
The end product of this treatment is glucose, which may either be fermented to ethanol or be transformed into fructose by the enzyme, glucose isomerase.
This method has led to the replacement of sucrose by fructose, which is much cheaper. Most commercial conversion processes use a temperature of above 60° C. The thermophilic bacterium, Tliermus tliermophilus is the most suitable source of this enzyme. This enzyme with stands a temperature of above 60°C used in this conversion process.
Alternately, if the resource is cellulose, the requirement of the host changes Cellulose is a polysaccharide consisting of a linear chain of glucose units linked by (3-1, 4-glycosidic bonds. A specific enzyme, known as cellulase ((3-1, 4-glycosidase) is necessary to break the glycosidic bonds and release the glucose units for utilization by the yeast.
Some fungi, such as dry rot, Trichoderma reesei, Coriolus hirsuitus and Polyporus anceps are known to synthesize cellulase. The cellulase gene is genetically engineered into the yeast genome so that the yeast performs both the functions. Cellulose resource encounters another problem. In woody plants, cellulose is associated with lignin. Some fungi are known to break down lignin. Yeasts are genetically engineered by inserting the lignin digesting
Gene into its genome, In this case, the fermentation task is carried out in two steps: (1) digestion of lignin; and (2) digestion of cellulose into glucose molecules.
Providing best fermentation condition
The fermentation process is anaerobic. As a eukaryotic cell, yeast possesses mitochondria. In the presence of molecular oxygen, the normal fermentation path way is switched over to Krebs cycle and then to electron transport system.
In this case, the entire purpose is defeated. A mutant strain of yeast known as petite is available, which lacks mitochondria. This strain is used for effective fermentation.
Alternative to yeast
Free floating yeast cells, in the large ferentation vessel, reduces the fermentation ability. Zymomonas mobilis, a bacterium is known to carry out the same work as does the yeast. It has an advantage in that it is immobilized to cotton fibers, thereby increasing the efficiency of fermentation.
The ethanol fermented in the fermentation vessel needs to be purified from time to time. This is done by distillation process and distillation process requires high temperature.
In this case, a normal yeast strain would not work. Some thermophilic bacteria such as Clostridium thermocellum and Thennoanaerohacter ethanolicus also can ferment sugars even at a higher temperature. Thus these bacteria will carry out fermentation, while the distillation process is on.
Lipid biofuels are of two types: (1) Straight vegetable oils (SVO) and (2) Biodiesel.
Straight vegetable oil (SVO): SVOs are relatively more viscous and pure vegetable oils or cleaned waste vegetable oils. These are used in diese) engines. There are two fuel tanks in a vehicle using SVO as a fuel. The first tank contains the SVO, while the second, petrodiesel or biodiesel. Diesel is used for ignition and switching on the engine.
It is produced from animal fat and vegetable oils by lipid trans- esterification, which removes glycerol. It is not an explosive nor is it inflammable. It is biodegradable and non-toxic.
Biogas is a complex mixture of several gases such as methane, carbon dioxide, etc. Methane is the predominant gas. It is an inflammable gas produced by bacteria in the intestine of ruminant cattle and natural wetlands by the anaerobic digestion of complex organic molecules.
The methane producing bacteria are known as methanogenic bacteria and the process as methanogenesis. Man has been using biogas for over 200 years. People’s Republic of China has over 5 million biogas installations in its rural areas.
The Government of India has also embarked upon an ambitious plan of installing biogas (Gobar gas) plants, mainly in rural areas, where the cattle population is at its maximum. Economically viable biogas is produced in large vessels known as digesters or bioreactors.
The anaerobic digestion of complex organic molecules is achieved in three steps. In the first step, cellulose, fats and proteins are made soluble.
In the second step, the low molecular weight products are converted into organic acids, mainly acetate, by microbial action and in the third step, acetate is specifically converted by methanogenic bactcria to methane and carbon dioxide
In recent years, biogas is obtained from municipal acwagc, agricultural and urns waste. Municipalities have installed community biogas reactors for treating the city sewage.
It serves two purposes. Firstly, the threat of pollution posed by the sewage is minimized and secondly, the energy requirement of the city is partial Fulfilled However, this process has one inherent problem. At present, the cost collection of the organic matter is expensive and methane production is low.
The major obstacle is the presence of lignin in most agricultural and urban wastes Lignin is not easily digested in anaerobic digesters. Therefore, pretreatments a required for making the process cost-effective. The waste is pretreated with lignin digesting enzyme and then fermented.