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.