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BIOLOGICAL FUEL GENERATION

The world depends for its energy requirement upon three major fossil fuels-coal, natural gas and oil, which would become exhausted completely in near future (coal may last for 200 years, natural gas and oil for 40 years). Realizing energy crisis during 1970s when the oil prices increased massively and due to environmental awareness, the scientists are trying to find the alternative non-conventional cleaner sources of energy, like wind, solar, tidal and wave, geothermal and bio-energy, Photosynthetic organisms convert dilute solar energy into more concentrated chemical energy (e.g., carbohydrates) with 3-4% efficiency, making the biomass (2 × 1011 tonnes C fixation/ year). Photosynthetically derived biomass (which does not have much calorific value), like forest, agricultural and animal residues and domestic organic wastes can be converted by fermentation into clean renewable fuels (like alcohol and CH4) possessing quite higher energy value. With time, continuous depletion of fossil fuels is making them increasingly expensive and in such situation biofuels may become more economic. Biofuels can be used for cooking, generation of electricity or in automobile engines. The CO2 released during their combustion is entrapped in the photosynthesis (which also produces O2) for biomass production and, thus, do not damage the environment as the fossil fuels do. Except for biogas, other biofuels are not economical.

The biomass for energy production can be obtained either by cultivating the energy crops (which would require considerable agricultural land and would compete for the food production) or by utilizing agricultural and other organic wastes (e.g., bagasse, molasses, straw, that would help in the waste management also). As there would be competition between biomass energy production and food production on limited agricultural land, particularly in developing countries, obtaining energy from the waste materials would be more promising. Alternatively, the energy crops can be better grown on waste lands unsuitable for food crops.

Forest biomass can also be used for fuel production but slow metabolism of lignocellulosic materials poses problem. ‘Steam explosion technology’ can be employed for partial breakdown of such complexes in which wood is first exposed to high pressure steam and subsequently pressure is released to cause explosive shredding of wood. Thus, wood gets converted into fermentable substrate.

Moreover, algae, cyanobacteria, aquatic plants can also be used as biomass for fuel production as they are not very commonly used ns food resources (though microalgae are difficult to harvest). In addition, productivity in natural habitats is generally limited by low nitrogen and phosphorus, and cyanobacteria are potentially attractive sources as many of them fix N2 gas. In open seas the nutrients are made available by natural or artificial upwelling forces to enhance biomass production.

The biomass can either be used directly (burning of dried fuel wood, cow dung cake, etc.) or can be converted to more usable fuels achieved by biological (e.g., fermentation) or chemical (chemical hydrolysis, destructive distillation, gasifiction) means or by a combination of both. Some of the important bioenergy are:

1. Bioethanol

Among biofuels, ethanol is the most promising alternative automobile fuel because of its good fuel properties (flash point 45°C and octane number is 89) and availability of huge amount of raw material that is needed for its production. It can be obtained by fermentation of either sugary (e.g., molasses, bagasse, sugarcane, sugarbeet) or starchy (e.g., corn, wheat, barley, sorghum, potatoes, cassava roots, etc.; but by saccharification they are required to be first converted to simple fermentable sugars) materials by the yeast Saccharomyces cerevisiae (Fig, 18.1) at 25°C and pH 4.5-4.7 under anaerobic condition or by the anaerobic bacterium Zymomonas mobilis (that gives higher yield of ethanol than the yeast). Co-culturing of amylolytic Aspergillus niger with S. cerevisiae converts starch to ethanol.

Fig.18.1 Cells of Saccharomyces cerevisiae

About 225 litres ethanol/ton of molasses can be obtained. To make the availability of necessary fermentable sugars, most raw materials require some degree of pretreatment. With sugarcane this treatment is minimal (milling), whereas cassava roots (25-38% starch) require saccharification (conversion of starch into simple sugars) by hydrolysis either by acids or by enzymes (amylases). Cellulosic raw materials (e.g., wood and straw) require more extensive pretreatment, which needs increased energy input e.g., treatment with cellulase (obtained from the fungus Trichoderma reesei) to convert cellulose into glucose.

The distilled ethanol (green petrol) burns completely and can be used as a partial or complete substitute for motor fuel, fossil petrol (gasoline, flash point 13°C). It is energy efficient, produces 57% lesser (toxic) CO, 64% lesser hydrocarbons and 13% less NO during combustion in vehicle engines than petrol. Historically, ethanol and methanol were used as motor fuels in Europe prior to World War II either in pure form or as mixed with petrol. Gasohol is 10-20% ethanol in gasoline that can be used in existing engines without any modification. Ethanol of very low moisture content is only suitable for blending with petrol, as the alcohol and gasoline get separated in the presence of water. About 95% ethanol is obtained by the distillation process (ethanol has boiling point 87°C as against 100°C for water helps in the distillation process) which is further distilled to about 100% ethanol by adding small amount of benzene. The proper distillation of ethanol makes the bioethanol costly. Since 1980s, it is extensively used in Brazil (in about 30% vehicles) and is obtained from sugarcane and cassava where the climate suits for the growth of these crops. Here, the ethanol is obtained mainly by batch fermentation of the substrate. in USA it is obtained from com. Though at present, the ethanol’s price is about twice to that of petrol, the oil prices would continue to increase, whereas, with technical advancement the production price of ethanol would decrease; thus a time will come when the prices of the two would become almost same and the bioethanol would substitute for some of the fossil petrol. Field trials and testing are continuing in many countries.

 

Biotechnology is involved in the production of more efficient microorganisms by genetic engineering technology for improved ethanol production, with extra necessary enzymes, resistance to high alcohol concentration (as ethanol produced limits the growth of yeast), etc. This would lead to the substantial reduction of cost of ethanol production.

 

2. Methanol

Methanol, which produces lesser hydrocarbon, NOx and CO than the fossil fuels in automobile engines, can be obtained from fermentation of organic wastes and can be used as fuel in vehicles
either in pure form or as 85% methanol blended in petrol or as 15-20% in diesel (diesohol).

3 Biodiesel

Considerable interest is being developed for utilization of biodiesel in Europe, where, plenty of agricultural land can be utilized for the energy plantation. Many plants yield oils (lipids) and many others hydrocarbons, which after processing can be used to substitute for the fossil diesel. Vegetable oil based fuels are most important to replace the diesel.

 

(i)     Edible Vegetable Oils: Diesel engine without any modification can run on many unblended edible oils like, peanut oil (used in Paris in 1990, flash point 237oC), sunflower, rapeseed (10% in diesel), corn, linseed, soybean (having flash point 219 oC, flash point of fossil diesel is 46 oC), coconut oils (scientists in Australia and Europe have conducted tests). It was found that fuel consumption in automobile was slightly higher for these oils in comparison to the fossil diesel. Since these edible oils are used in large scale for food preparation, more attention should be given for the use of non-edible oils for the purpose of energy use.

(ii)   Non-edible Vegetable Oils: Some of the important non-edible vegetable oils that can be used as fuel for diesel-vehicles are Jatropha, Jojoba, Azadirachta indica (neem), Croton, Garcinia, Salvadora, etc. In Thailand Jatropha seed oil has been successfully tested to run the diesel engine. This plant can grow on marginal nutrient poor soils. Trees are planted at a distance of about 2 m, which start producing nuts after 4-5 months and can survive for about 50 years. The seeds contain 30-60% oil to yield about 2,000 kg oil/ hectare land (Fig. 18.2).

Fig. 18.2 Jatropha plants

For the extraction of oils, the seeds (edible or non-edible) are subjected to a number of preliminary treatments to produce the meal from which the oil is extracted by solvent extraction using hexane or trichloroethylene. The crude oil thus obtained is then refined to be used as biodiesel,

(i)     Hydrocarbons: Many plants belonging to the families Euphorbiaceae (e.g., Euphorbia) and Asclepidaceae (e.g., Calotropis procera, Asclepias Copaifera Multijuga).
Copaifera multijuga,
a tropical legume tree with NT fixing root nodules and preferably grows in arid lands that are unsuitable for the growth of crop plants. It is most promising source of biodiesel. The latex produced contains mainly water and emulsified 30% hydrocarbons (mainly CsHs) that can be processed into biodiesel. After collection, the latex is dried and processed into diesel-like substance. Euphorbia lathyris is the most important species for this purpose.

Many algae (e.g. green alga Botryococcus braunii can accumulate 75% hydrocarbons and grows in abundance in Sambhar Salt Lake in Rajasthan) (Fig.18.3), fungi and bacteria (e.g., Micrococus leuteus) also 10-fold increase in hydrocarbon in M.leuteus has been achieved.

Fig.18.3 Bloom (top) and colonies of Borryococcus braunii as seen through microscope (bottom)

The exploitation of hydrocarbon as biodiesel is at the experimental stage.

4. Biogas

Methane is a valuable and economical energy source that is obtained by the fermentation of organic wastes from agriculture (including cattle dung) domestic, food processing industries sewage sludge in biogas reactors or landfill by the anaerobic digestion where CH4, is captured. It is obtained by a group or indigenous anaerobic microorganisms optimally at about 30°C and helps not only in generating energy and manure production but also in the management of solid wastes (has multiple benefits). At the first stage a group of microorganisms lead to the solubilization of complex organic molecules, like cellulose, fats, proteins, etc. to produce simple soluble products. These soluble products are then converted to organic acids (mainly acetic acid and formic acid) and H2 by another group of microorganisms. In this final stage, these acids are converted (or decomposed) by methanogenic bacteria (e.g. Methanobacterium, Methanobacillus, Methanosarcina, etc.) to CH4 and CO2 using H2 that is generated microbially.

The production of CH4, depends upon the nature of waste, e.g., the presence of lignin (in most agricultural and urban wastes), which is not easily biodegradable and affects the CH, production. When CH4, is obtained from cattle dung, it is called biogas (gobar gas) and is used for cooking purpose in rural areas of India, China (largest user having over 7 million biogas units) and Pakistan, It generates lesser smoke than cow dung cake burnt in conventional chulhas. For its production animal dung is mixed with water and allowed to ferment in underground airtight bioreactors of family-size or community-size that are made up of bricks or steel. Gas formed gets entrapped and raises the inverted metal drum covering the surface of waste from where it is withdrawn by pipelines. Biogas is a mixture of 50-80% CH4, 15-45% CO2, 5% H2O and traces of gases like N2, H2S. etc. (Fig, 18.4). About 30%-50% of urban waste can be converted to CH4. Purification of CH4, from biogas has more calorific value, but is expensive and unsafe.

Fig.18.4 Biogas Plant

The solid or liquid residue after CH4 production is rich in nutrients (N, P and K) and can be used as manure for crop plants. Methane gas, the main component of biogas, can be used for the generation of mechanical, electrical (used as fuel to generate steam for driving electricity in California. USA, UK. Timarpur in Delhi) and heal energy (cooking). Under favourable conditions,10 kg of dry organic matter can produce 3m3 or biogas which can provide 3 h of cooking or lighting.

Methane gas also exists in the atmosphere as greenhouse gas (next to CO2) and of this metal methane, 20% is derived from natural wetlands, 20% from rice fields and J 5% from enteric fermentations in animals by microbial action, which can be captured and used as fuel.

5. Biohydrogen

Phototrophic microorganisms are capable of significant amount of H2 production, Hydrogen is highly energetic and many photosynthetic (e.g., Rhodospirillium, Rhodopseudomonas, etc.) and non-photosynthetic bacteria (e.g., anaerobic Clostridium reduces H+ ions by fermentation), green algae (e.g., Scenedesmus, Chlorella, Chlamydomonas), cyanobacteria (e.g., Anabaena variabilis that can produce H2 and CO2 from fructose sugar in the presence of light and anaerobic condition created by Ar gas), etc. possess hydrogenase enzyme (which is sensitive to O2) and produce H2 efficiently. The photosynthetic organisms produce H2 from H20 in the presence of sunlight under certain conditions. In vitro, it can be obtained by isolated chloroplasts in the presence of water, light and the hydrogenase enzyme:

The anaerobic bacteria produce H2 as:

C6H12O6    →      Organic acids + CO2 + H2

Though, it is a pollution-free gas as during combustion it produces only H20, but its production at present is very costly and more research efforts are needed for its utilization as an as attractive and economical fuel. Currently, H2 is not a major fuel.

QUESTIONS

1. Write short notes on the following:

i.Biological fuel generation

ii. Biogas or ‘gobar gas’

iii.  Bioethanol

iv. Biodiese

v.Biohydrogen

2. Define biofuel. List the various biofuels, their sources and the status of their exploitation. Discuss the various features of biofuels and their limitations.

3. What are energy crops? Briefly describe the various methods of utilization of biomass as energy sources and discuss their limitations.

4. Explain the role of various microorganisms involved in biogas production and list the various advantages and disadvantages of biogas as a fuel.