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Chapter 2 Fuels and Combustion. Fuels. Any material that can be burned to release thermal energy is called a fuel. Fuels are the materials which ignite in presence of oxygen and produce heat. The heat energy is then converted into various forms of energy.
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Fuels • Any material that can be burned to release thermal energy is called a fuel. • Fuels are the materials which ignite in presence of oxygen and produce heat. The heat energy is then converted into various forms of energy. • The selection of right type of fuels depends on various factors such as availability, storage, handling, pollution, and landed cost of fuel. • Fossil fuels take millions of years to form. Fossil fuels originate from the earth as a result of the slow decomposition and chemical conversion of organic material. They exist in nature in three basic forms—solid (coal), liquid (oil) and natural gas.
Coal • Coal is classified into three major classes, namely, anthracite, bituminous, and lignite.
Analysis of Coal • Ultimate Analysis • The ultimate analysis indicates the various chemical constituents such as carbon, hydrogen, oxygen, sulphur, etc. • It is useful in determining the quantity of air required for combustion and the volume and composition of the combustion gases. • Proximate Analysis • Proximate analysis indicates the percentage by weight of the fixed carbon, volatiles, ash, and moisture content in coal. • The amounts of fixed carbon and volatile combustible matter directly contribute to the heating value of coal. • Fixed carbon acts as a main heat generator. High volatile matter content indicates easy ignition of fuel.
Liquid Fuels • Petroleum • Petroleum means ‘rock oil’ as it is found underground in porous rocks. It is often used synonymously for ‘crude oil’, ‘crude petroleum‘ or simply ‘oil’. • Its composition varies with geological location, a general indication of the chemical composition (by weight) of petroleum would be as follows: C(84%), H(14%), S(1–3%), N(<1%), O(<1%), metals and salts (<1%). • The value of petroleum is high due to its ease of storage, transportation, utilization, high stored-energy density and relative ease of conversion to thermal energy.
Kerosene • Kerosene is a refined petroleum distillate consisting of a homogeneous mixture of hydrocarbons. • It is used mainly in wick-fed illuminating lamps and kerosene burners. • Oil for illumination and for domestic stoves must be high in paraffin to give low smoke. • The presence of naphthenic and especially aromatic hydrocarbons increases the smoking tendency. • A ‘smoke point’ specification is a measure of flame height at which the tip becomes smoky. The ‘smoke point’ is about 73 mm for paraffin, 34 mm for naphthalene and 7.5 mm for aromatics and mixtures.
Diesel • Diesel engines, developed by Rudolf Diesel, rely on the heat of combustion of the fuel. • Fuel is injected into the combustion chamber in an atomized spray at the end of the compression stroke after air has been compressed to 450–650 psi and has reached a self-ignition temperature due to compression of at least 500°C. • This temperature ignites the fuel and initiates the piston’s power stroke. • The fuel is injected at about 2,000 psi to ensure good mixing. • Diesels are expensively used in truck transport, rail trains, and marine engines. • Fuels for diesels vary from kerosene to medium residual oils.
Gaseous Fuels • LPG • LPG is a predominant mixture of propane and butane with a small percentage of unsaturates(propylene and butylene) and some lighter C2 as well as heavier C5 fractions. Included in the LPG range are propane (C3H8), propylene (C3H6), normal and iso-butane (C4H10) and butylene (C4H8). • LPG may be defined as those hydrocarbons, which are gaseous at normal atmospheric pressure, but may be condensed to the liquid state at normal temperature by the application of moderate pressures. • LPG vapour is denser than air; butane is about twice as heavy as air and propane about one and a half time as heavy as air.
Gasoline • Gasoline, or petrol, is made of a mixture of hydrocarbons, which are molecules composed of carbon and hydrogen atoms. • Typically, in standard gasoline, the hydrocarbons consist of carbons chains that are 5–10 carbon atoms long. • The exact mixture of hydrocarbons depends entirely on the specific sample of gasoline (type of oil that it was made from, company that refined it, additives that were added, etc.). • Gasoline is more volatile than diesel oil, or kerosene, not only because of the base constituents but also because of additives. • Volatility is often controlled by blending with butane, which boils at –0.5°C. The volatility of petrol is determined by the Reid Vapour Pressure (RVP) test.
Compressed Natural Gas (CNG) • Methane is the main constituent of natural gas and accounting for about 95 per cent of the total volume. • Other components are ethane, propane, butane, pentane, nitrogen, carbon dioxide and traces of other gases. • Natural gas is a high calorific value fuel requiring no storage facilities. • CNG is made by compressing natural gas [which is mainly composed of methane (CH4)] to less than 1 per cent of the volume it occupies at standard atmospheric pressure. • It is stored and distributed in hard containers at a pressure of 200–248 bar usually in cylindrical or spherical shapes.
Biofuels • A biofuel is derived from biological carbon fixation. Biofuels include fuels derived from biomass conversion as well as solid biomass, liquid fuels, and various biogases. • Bioethanolis an alcohol made by fermentation from carbohydrates produced in sugar or starch crops such as corn or sugarcane. • Ethanol can be used as a fuel for vehicles in its pure form but it is usually used as a gasoline additive to increase octane and improve vehicle emissions. • Biodiesel is produced from oils or fats using transesterification. Biodiesel can be used as a fuel for vehicles in its pure form, but it is usually used as a diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles.
Green diesel, also known as renewable diesel, is a form of diesel fuel which is derived from renewable feedstock rather than the fossil feedstock used in most diesel fuels. Green diesel feedstock can be sourced from a variety of oils including canola, algae, jatropha, and salicornia in addition to tallow. • Green diesel uses traditional fractional distillation to process the oils, not to be confused with biodiesel which is chemically quite different and processed using transesterification. • Vegetable oil is increasingly being processed into biodiesel or (more rarely) cleaned of water and particulates and used as a fuel. • Oils and fats can be hydrogenated to give a diesel substitute. • Bioethers (oxygenated fuels) are cost-effective compounds that act as octane rating enhancers. • They also enhance engine performance, whilst significantly reducing engine wear and toxic exhaust emissions.
Biogas is methane produced by the process of anaerobic digestion of organic material by anaerobes. • It can be produced either from biodegradable waste materials or by the use of energy crops fed into anaerobic digesters to supplement gas yields. • The solid by-product, digestate, can be used as a biofuel or a fertilizer. • Syngasis a mixture of carbon monoxide, hydrogen, and other hydrocarbons and produced by partial combustion of biomass. Before partial combustion, the biomass is dried and sometimes pyrolysed. • The resulting gas mixture, syngas, is more efficient than direct combustion of the original biofuel; more of the energy contained in the fuel is extracted.
Solid biofuels are wood, sawdust, grass trimmings, domestic refuse, charcoal, agricultural waste, non-food energy crops and dried manure. • When raw biomass is already in a suitable form, it can burn directly in a stove or furnace to provide heat or raise steam. • When raw biomass is in an inconvenient form, the typical process is used to densify the biomass. • This process includes grinding the raw biomass to an appropriate particulate size (known as hogfuel).
Combustion • Combustion refers to the rapid oxidation of fuel accompanied by the production of heat or heat and light. • Complete combustion of a fuel is possible only in the presence of an adequate supply of oxygen.
