Understanding fuel combustion process:

Understanding fuel combustion process:

(a) Fuels are chemical substances which may be burned in presence of oxygen to generate energy in the form of mostly heat. They mainly consist of carbon and hydrogen. Small quantity of sulfur is also present in fuel as contamination. Solid, liquid and gaseous fuels are used by various systems. Coke and coal are solid fuels, petrol, diesel, kerosene is liquid and LPG, CNG are the example of gaseous fuel.

C + O2 = CO2, (here C and O2 are reactants and CO2 is the product of burning fuel)

(b) Each fuel burn at a particular temperature, called ignition temperature. For starting the combustion process each fuel should be brought above its ignition temperature. An appropriate air-fuel ratio is also necessary to maintain in order to get desired result. The minimum ignition temperature at atmospheric pressure for some fuel is: (i) Carbon is 400 degree Celsius; (ii) Hydrogen is 580 degree Celsius; (iii) Carbon monoxide (CO) is 610 degree Celsius; (iv) Methane (CH4) is 630 degree Celsius; (v) Gasoline is 260 degree Celsius.

(c) As air is the major ingredient in any burning process, the air-fuel ratio is the term frequently used in the analysis of combustion process for any fuel. It is usually expressed on a mass basis, i.e., Mass of air required / mass of fuel burnt.

(d) Fuel combustion process is the process when a particular fuel is burnt completely, i.e., all carbon present in the fuel converts into carbon dioxide (CO2), all hydrogen converts into water (H2O) and all sulfur converts to sulfur dioxide (SO2). Theoretically, in completely burnt process, no un-burned residue of carbon, hydrogen should be present and process should not produce any carbon monoxide (CO). For any internal combustion engine, complete combustion is desirable, as energy conversion is maximized and exhaust gas characteristics is improved, thereby engine efficiency and less pollution.

(e) It is desirable to use more air than the actual requirement for the complete combustion of any fuel; to prevent chance of any incomplete combustion. Excess air is also needed to control rise in temperature of the combustion chamber.

(f) Energy is an inherent property of a system by which work is done. Any system at a given set of conditions has certain energy content. The concept of energy is derived to describe a number of processes such as conversion of work to heat. Joule (J) is the unit in SI system. Other units of energy / heat are: (i) 1 cal (calorie)= 4.1868 J; (ii) 1 kcal= 4186.8 J; (iii) 1 Btu (British thermal unit)= 1055.05 J; (iv) 1 ft.lbf= 1.35582 J; (v) 1 kJ= 1000 J; (vi) 1 hp.h (horsepower.hour)= 2,684,520 J; (vii) 1 kWh= 3,600,000 J.

(g) For any combustion energy is transferred to heat and heating value for any system is the amount of energy released when a fuel is burned completely.

Conservation of fossil fuels Fossil fuels take millions of years to make. We are using up the fuels that were made more than 300 million years ago. Once they are gone they are gone. It is best not to waste fossil fuels. They are not renewable; they can not be made again. We can save fossil fuels by conserving energy.

Properties of coal:

Properties of coal:

Physical and chemical properties of coal are very important factor to know before a particular coal sample is used. Coal as a fuel comes in various types or ranks, such as, (1) lignite or brown coal; (2) bituminous coal or black coal; and (3) anthracite. Several techniques have been developed for studying the physical and chemical properties of coal. The most commonly employed systems of classification are those based on analyses that can be performed relatively easily in the laboratory, e.g., determining the percentage of volatile matter lost upon heating to about 950 °C (about 1,750 °F), the amount of heat released during combustion of the coal under standard conditions etc. Each type of coal discussed above, has a certain set of physical parameters which are mostly controlled by moisture, volatile content and its carbon content.

(1) Moisture: It is an important property of coal. Moisture held within the coal itself is known as inherent moisture. Moisture in coal may be of (i) Surface moisture, i.e., water held on the surface of coal particles; (ii) Hydroscopic moisture, i.e., water held by capillary action within the micro-fissures of the coal; (iii) Decomposition moisture, i.e., water held within the coal's decomposed organic compounds; (iv) Mineral moisture, i.e., water which comprises part of the crystal structure of hydrous silicates such as clays. Total moisture is analyzed by loss of mass between an untreated sample and the sample once analyzed.

(2) Volatile matter: Volatile matter in coal is the presence of various components of coal (except for moisture), which are liberated at high temperature of about 950 °C (about 1,750 °F), in the absence of air. This is usually a mixture of short and long chain hydrocarbons, aromatic hydrocarbons and some sulphur. The volatile matter of coal is determined under rigidly controlled standards.

(3) Ash: Ash content of coal is the non-combustible residue left after coal is burnt. It is the bulk mineral residue left after carbon, oxygen, sulphur and water is driven off by combustion.

(4) Fixed carbon: The fixed carbon in a coal sample is the carbon found in the material which is left after volatile matters are driven off. This slightly differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon in coal sample is a very useful parameter, as it is used as an estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the moisture and the mass of volatile matters, above, from the original mass of the coal sample.

(5) Other physical and chemical criteria: Presence of oxygen, hydrogen, sulphur etc. is evaluated in a coal sample. Density, particle size distribution, abrasion etc., are some of the physical criteria of coal is also evaluated.

(6) Besides above physical or chemical tests and analysis made in order to determine the handling and pollutant profile of a coal, the energy output of a coal is the important factor, which also determined by using a bomb calorimeter. Energy output of a coal sample is the specific energy output of a coal during complete combustion. This is required particularly for coals used in boiler for in steam-raising in a coal-fired power plant.

(7) Spontaneous combustion of coal: Almost all types of coal ignite spontaneously in suitable environmental conditions. The spontaneous ignition of coal stockpiles is a serious economic and safety problem. Preventive measures may be adopted while coal stacking includes (i) periodic compaction, (ii) the use of a low angle slope, (iii) protection of the coal stockpiled with an artificial barrier and (iv) covering it with ‘ash–water’ slurry made with fly ash from the same power station.