FUELS AND COMBUSTIONPrimary fuels used are fossil fuels in the form of coal, fuel oil and natural gas.

Besides these, industrial wastes like blast furnace gas, coke oven gas, refinery gas, sugar factory refuse, saw mill wood dust, rice husk etc. are also used as boiler fuels.

If more than one fuel is simultaneously burned, the boiler is said to have a combination firing.

COALCoal is the principal form of energy.According to geological order of formation, coal may be of the following types:PeatLigniteSubbituminousBituminousSubanthraciteAnthraciteWith increasing percentages of carbon.Anthracite contains more than 86% fixed carbon and less volatile matter.Volatile matter helps in the ignition of coal.Bituminous coal is the largest group containing 46-86% of fixed carbon and 20-40% of volatile matter.

Low, medium and high volatile.Lower the volatility, higher the heating value.Lignite is the lowest grade of coal containing moisture 30% and high volatile matter.Peat not regarded as a rank of coal by ASTM (American Society of Testing and Materials) as it contains 90% moisture.Rank means degree of maturation (carbonization) and is a measure of carbon content in coal.Lignite-low rank and Anthracite-high rank.

COAL

Proximate and Ultimate (mass percent basis)Both may be based on:as-received basis, useful for combustion calculation.dry or moisture free basisdry mineral-matter-free or combustible basis.

COAL ANALYSIS

PROXIMATE ANALYSISIndicates the behavior of coal when it is heated.1 g of coal is heated at 1050C for 1 hour, weight loss gives the moisture content of the coal.1 g of coal is placed in a covered platinum crucible and heated at 9500C for 7 min, loss in weight due to elimination of moisture and volatile matter.1 g of coal is heated in an uncovered crucible at 7200C, until a constant weight is reached i.e. only ash is remaining.Complete combustion is determined by repeated weighing of sample.So, the proximate analysis of coal gives:FC + VM + M + A = 100% by mass.Some carbon may also be in the form of hydrocarbons which may have been distilled off while determining the volatile matter.VM indicates whether the coal will burn with a short or long flame and whether it will tend to produce smoke.More volatile means more smoke.

ULTIMATE ANALYSIS

COMBUSTION REACTIONCombustion is the high temperature oxidation of the combustible elements of a fuel with heat release.The combustible elements in coal and fuel oil are carbon, hydrogen and sulphur. C + O2 CO22H2 + O2 2H2O S + O2 SO2When insufficient oxygen is present, the carbon will burn incompletely and form carbon monoxide. 2C + O2 2CO

COMBUSTION REACTIONIn order to burn a fuel completely, four basic condition must be fulfilled:Supply enough air for complete combustion of fuel.Secure enough turbulence for thorough mixing of fuel and air.Maintain a furnace temperature high enough to ignite the incoming fuel air mixture.Provide a furnace volume large enough to allow time for combustion to be completed.Since complete mixing of fuel and air is virtually impossible, excess air must be supplied to ensure complete combustion.The greater is the rate of mixing or turbulence, the lower would be the excess air required.

Theory of Orsat analyserFollow P.K Nag chapter 4(Fuel and combustion) [4.1,4.2 ,4.11.2]4.11.2- Description of orsat analyser is given