Renewable Sources of Energy : Bio mass

(ii). The energy obtained from biomass is known as biomass energy. Animals feed on plants and plants grow through the photosynthesis process using solar energy. Thus, the photosynthesis process is primarily responsible for the generation of biomass energy. A small portion of the solar radiation is captured and stored in the plants during the photosynthesis process. Therefore, it is an indirect form of solar energy.

(iii). The average efficiency of photosynthetic conversion of solar energy into biomass energy is estimated to be 0.58–1.0 per cent.

(iv). To use biomass energy, the initial biomass may be transformed by chemical or biological processes to produce more convenient intermediate biofuels such as methane, producer gas, ethanol, and charcoal etc.

(v). On combustion, it reacts with oxygen to release heat, but the elements of the material should be available for recycling in natural ecological or agricultural processes. Thus, the use of industrial biofuels, when linked carefully to the natural ecological cycle may be nonpolluting and sustainable.

(vi). For biomass to be considered as renewable, growth must at least keep pace with its use.

(vii). Although fossil fuels have their origin in ancient biomass, they are not considered biomass by the generally accepted definition because they contain carbon that has been “out” of the carbon cycle for a very long time. Their combustion, therefore, disturbs the carbon dioxide content in the atmosphere.

(viii). It still plays a significant role in the supply of primary energy supply of the industrialized countries is not more than 3 per cent, a number of developing counties still use a substantial amount of it mostly in the form of noncommercial energy.

Advantages and Disadvantages of Biomass:

Main advantages of biomass energy are:

• It is a renewable source.
• The energy storage is its inbuilt feature.
• It is an indigenous source requiring little or no foreign exchange.
• The forestry and agricultural industries that supply fees stocks also provide substantial economic development opportunities in rural areas.
• The pollutant emissions from the combustion of biomass are usually lower than those from fossil fuels.
• Commercial use of biomass may avoid or reduce the problems of waste disposal in other industries, particularly municipal solid waste in urban centers.
• Use of biogas plants apart from supplying clean gas, also leads to improved sanitation, batter hygienic conditions in rural areas as the harmful decaying biomass gets stabilized.
• The nitrogen–rich bio–digested slurry and sludge from biogas plant serves as a very good soil conditioner and improves the fertility of the soil.
• Varying capacity can be installed; any capacity can be operated, even at lower loads;
• No seasonality

  Its main disadvantages are:

• it is a dispersed and land-intensive source
• it is often of low energy density and
• it is also labour-intensive and the cost of collecting large quantities for commercial application is significant. Most current commercial applications of biomass energy use material that has been collected for other reasons, such as timber and food processing residues and urban waste.
• capacity is determined by the availability of biomass and not suitable for varying loads
• not feasible to set up at all locations

USABLE FORMS OF BIOMASS, THEIR COMPOSITION AND FUEL PROPERTIES

Biomass is organic material that reacts with oxygen in combustion and natural metabolic process to release heat. Sometimes it is used as such in its original form but more often it is transformed into modern energy forms such as liquid and gaseous fuels, electricity, and process heat to provide energy services needed by rural and urban populations and also by industry. Some of its forms available to users are discussed below:

(a). Fuel Wood (Virgin Wood)

(i). Wood is the most obvious and oldest source of biomass energy. This was the main sources of energy used by mankind for centuries.

(ii). Direct combustion is the simplest way to obtain heat energy. Its energy density is 16–20 MJ/kg. It can also be converted to more useful forms such as charcoal or producer gas.

(iii). About half of the world population depends on fuelwood or other biomass such as cow dung, crop residue etc. for cooking and other domestic uses. Because of inefficient use of fuelwood in the conventional household stoves (Chulhas), only 5 per cent heat is utilized. The remainder is lost due to wind, incomplete combustion, radiation losses and other losses resulting from a mismatch of fire and pot size. Considerable energy is also wasted in evaporation from uncovered pot and from use of wet fuel.

(iv). Smoke, which is in fact unborn tar and carbon, is a health hazard. There is little control over the rate at which wood is burnt. Improved household stoves (Chulhas) and use of pressure cooker is being encouraged for better fuel utilization.

(b). Charcoal

(i). Charcoal is a clean (smokeless), dry, solid fuel of black color.

(ii). It has 75–80 per cent carbon content and has energy density of about 30 MJ/kg.

(iii). It is obtained by carbonization process of woody biomass to achieve higher energy density per unit mass, thus making it more economical to transport. It can be used as fuel in domestic environment as it burns without smoke.

(iv). In the industrial sector it is used in specialized applications where specific fuel characteristics are required, such as high carbon and low sulphur content.

(v). Chemical grade charcoal has many uses in laboratory and industrial chemical processes. (vi). It is also used for making high quality steel.

(c). Fuel Pellets and Briquettes:

(i). Crop residues such as straw, rice husk etc. and waste wood are pressed to form lumps, known as fuel pellets or briquettes and used as solid fuel.

(ii). The purpose is to reduce moisture content and increase the energy density of biomass making it more feasible for long distance transportation.

(d). Biodiesel:

(i). Some vegetable oils, edible as well as non–edible can be use (after some chemical processing) in pure form or its blend with petroleum diesel as fuel in a compression–ignition (diesel) engine, are known as biodiesels.

(ii). Biodiesel is simple to use, biodegradable, nontoxic, and essentially free of sulfur and aromatics.

(iii). The raw vegetable oil is upgraded as biodiesel through a chemical process called trans–esterification whereby the glycerin is separated from the animal fat or vegetable oil. The process leaves behind two products–methyl (or ethyl) esters (the chemical name for biodiesel) and glycerin (a valuable by product usually sold to be used in soaps and other products.).

(iv). Biodiesel can be produced from vegetable oils, animal its or recycled restaurant greases.

(v). Fuel–grade biodiesel must be produced to strict industry specifications (such as ASTM D6751) in order to insure proper performance.

(vi). Biodiesel refers to the pure fuel before blending with diesel fuel. Bio–diesel blends are denoted as, “BXX” with “XX” representing the percentage of biodiesel contained in the blend (i.e., B20 is 20 per cent biodiesel and 80 per cent petroleum diesel, B100 is pure biodiesel).

(vii). Jatropha and Karanj are the most promising biodiesel resources.

(e). Bioethanol:

(i). Ethanol (C2H5OH) is a colorless liquid biofuel.

(ii). Its boiling point is 78 ° and energy density is 26.9 MJ/kg.

(iii). It can be derived from wet biomass containing sugars (e.g. sugarcane, sugar beet, sweet sorghum, etc.), starches (grains, tubers such as are potato, cassava, etc.) or cellulose (woody matter).

(iv). Main constituents of woody matter are lignin (fibrous part) and cellulose (juicy part). Ethanol is largely produced from sugar cane.

(v). Commercial ethanol is used in specially designed IC engines with 25 per cent mileage penalty compared to conventional vehicles. Blend of up to 22 per cent anhydrous ethanol with gasoline requires no engine modification and incurs no mileage penalty. It is being used by a large number of automobiles in the world.

(f). Biogas:

(i). Energy obtained from the organic wastes such as plants, animals and humans wastes known as the biomass energy.

(ii). Aquatic biomass can also be used. Biogas is produced in a biogas fermenter of digester. Nitrogen rich sludge (fertilizer) is also produced as a byproduct with improved sanitation as an added bonus.

(iii). If raw material is cow manure the output biogas will contain about 50–60 per cent CH4 30–40 per cent CO2 5–10 per cent H2, 0.5–0.7 per cent N2 with trace amounts of O2 and H2S. (iv). Its energy density is about 23 MJ/M3. It is used for cooking lighting (using mantle lamps), heating and operating small IC engines, etc.

(v). It is unlikely to be used for mobile vehicles on large scale because of low pressure and high inert fraction.

(g). Producer Gas

(i). Woody matter such as crop residue, wood chips, bagasse (fibrous residue of sugar cane after juice extraction), rice husk, coconut shell etc. can be transformed to producer gas (also known as synthesis gas, syn gas wood gas and water gas or blue gas) by a method known as thermal gasification of solid fuel.

(ii). The composition of gas produced depends upon the type of biomass and the design of gasifier. For wood chips as input the typical gas composition:

• 19% CO.
• 18% H2.
• 1% CH4.
• 11% CO2 and the rest N2.

(iii). It has a typical energy density of 4–8 MJ/m3 (5–10 MJ/kg). This can be used to fuel IC engines (diesel, dual fuel mode engines) for irrigation pumps, motor vehicles, and small–scale power generation or to produce process heat.

(iv). The commercial diesel engine has to be modified to a duel–fuel mode engine. The air intake manifold is modified to such (air + producer gas) mixture.

(v). Limited quantity of diesel is required to initiate the ignition. The engine is started with diesel fuel only and subsequently the quantity of diesel is reduced as producer gas is mixed with air. About 85–87 per cent diesel replacement can be obtained  

URBAN WASTE TO ENERGY CONVERSION

   MSW Incineration Plant

(i). Municipal solid waste is the solid waste generated by households, commercial and institutional operations and some industries.

(ii). Disposal of MSW is major problem in big cities where large quantities of waste is to be disposed of far away from the city centers. The emerging solution is to use this waste biomass as an energy resource in a waste–to–energy conversion plant near city center. The energy thus generated is used within city itself and only a relatively small residue of used biomass (ash etc.) is disposed away in landfills.

(iii). Through incineration or gasification, electrical energy may be generated along with thermal energy for process heat.

A block diagram of MSW–to–energy incineration plant showing the sequence of various steps is shown in Fig.3. The dry biomass is shredded to pieces of about 2.5 cm diameter. An air stream segregates refuse derived fuel (RDF), which is lighter from heavier metal and glass pieces. The heavier part is reclaimed and recycled. About 30 per cent of US waste stream is recycled. The RDF thus obtained is burnt in the furnace at about 1000 °C to produce steam in the boiler. Combustion process may be assisted by required amount of auxiliary fuel when RDF does not burn properly by itself. The superheated steam obtained from boiler is used in a steam turbine coupled with an alternator to produce electrical output in the same way as in a conventional thermal plant. The flue gases are discharged to atmosphere through stack after removal of pollutants such as particulate matter, SOx and NOx, etc. Heat recovery steam generator extracts maximum possible heat from flue gases to form thermal output. The ash is removed and disposed of to landfills.

 MSW to an energy incineration plant

Strong concerns exist about the pollutants emitted during combustion such as sulfur, chlorine, fluorine, nitrogen, chlorinated hydrocarbons and heavy metals. One particular important group of pollutants, polychlorinated dibenzo–p dioxins (PCDD) and the dibenzofurans (PCDF) are possible among the most acutely carcinogenic compounds known so far. It has been proven to cause cancer in animals and is linked with a variety of other health problems, including genetic defects. The residual ash, which has high metal concentration, is also of concern. The ash is usually disposed of in landfills; the leachate (garbage juice; liquid that results when water percolates through the landfill) must be well contained to prevent groundwater contamination.

   Sewage to Energy Conversion   

(i). Sewage produced by human settlements also has some energy potential and it can also be anaerobically digested (after some processing) to produce methane in the same way as with animal manure.

(ii). The gas produced can be used to heat the digester. Though the gas production is not much and may not be sufficient for heating, it can reduce the conventional furl requirement.

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