Fuels and Combustion

Recommended Books Fuels and Combustion; M.L. Smith & K.W. Stinson Fuels and Fuel Technology ; W. Francis & M.C. Peters Fuel – Solid , Liquid and Gaseous; J.S.S. Brame & J.G. King Hydrocarbon Fuels; E.M. Goodger Coal Conversion Processes; Stanley & Lee Fuel Testing: Laboratory Methods in Fuel Technology; G.W. Himus Methods of Analysis of Fuels and Oils; J.R. Campbell

Fuel A substance which produce heat either by combustion or by nuclear fission / fusion

Classification of Fuels Fuels can be classified as solid, liquid and gaseous fuels. Solid fuels         : wood, coal, charcoal and coke Liquid fuels        : petrol, kerosene, diesel, alcohol etc Gaseous fuels   : methane, propane, butane, hydrogen, coal gas, gobar gas etc

Classification of Fuels Primary Fuels: Naturally occuing e.g. coal, wood, natural gas Secondary Fuels: Which are derived from primary fuels e.g. kerosene, coke etc Naturally occurring Artificially prepared

Combustion Combustion is the conversion of a substance called a fuel into chemical compounds known as products of combustion by combination with an oxidizer. The combustion process is an exothermic chemical reaction, i.e., a reaction that releases energy ???

Combustion Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel (usually a hydrocarbon) and an oxidant accompanied by the production of heat or both heat and light

Combustion Fuel ? Oxidizer ? Products of Combustion ? Fuel + Oxidizer => Products of combustion + Energy Fuel ? Oxidizer ? Products of Combustion ? Incomplete Combustion ?

Fundamental Definitions Basic Flame types Premixed: Fuel and oxidizer are mixed first and burned later Non-premixed: Combustion and mixing occur simultaneously

Combustion Air-Fuel Ratio; A/F Oxygen-Fuel Ratio; O/F Stoichiometric or Theoretical A/F Excess Air % excess air = 100[(A/F)actual - (A/F)theo ]/(A/F)theo 120% of theoretical air ? Fuel- Rich flame: If there is an excess of fuel Fuel - lean flame : if there is an excess of oxygen

Combustion 1 kg of C needs ? kg of O2 1 kg of H2 needs ? Kg of O2 1 kg of Sulphur needs ? kg of O2

Combustion Problem: A fuel contains by mass 88 % carbon, 8 % H2, 1% S and 3% ash. Calculate the stoichiometric air/fuel ratio. Ans: ?

Fundamental Definitions Calorific value Amount of heat librated by the combustion of unit quantity of fuel. kcal/ kg , kcal / m3 Gross Calorific Value (G.C.V) or HCV heating value measurement in which the product water vapour is allowed to condense Net Calorific Value (N.C.V) or LCV heating value in which the water remains a vapor and does not yield its heat of vaporization HHV = LHV + (mwater /mfuel)ʎwater

Fundamental Definitions Flash Point The lowest temperature at which a liquid fuel gives enough vapours in air which produce a momentary flash when exposed to a flame Firepoint The lowest temperature at which a liquid fuel vapours in air produces a continuous flame when exposed to a flame

Fundamental Definitions Density Specific gravity Viscosity Pour Point Carbon Residue

Fundamental Definitions Theoretical Flame Temperature: It is the temperature attained by the products of combustion of fuel when there is no loss of heat to the surroundings Flue Gas: It is the gaseous product of combustion of fuel

Coal Origin of Coal Coal has been formed by the partial decay of plant materials accumulated million of years ago and further altered by the action of heat and pressure In situ Theory: coal occupies the same site where the orignal palnts grew Drift Theory: plants were uprooted and drifted by rivers to get deposited

Coal classification Peat : Lignite: soft coal and the youngest sub-bituminous Bituminous: semi-bituminous: Anthracite: hard and geologically the oldest composed mainly of carbon

Coal Analysis Proximate analysis of coal Ultimate analysis of coal Determines only fixed carbon, volatile matter, moisture and ash Useful to find out heating value (GCV)‏ Simple analysis equipment Ultimate analysis of coal Determines all coal component elements: carbon, hydrogen, oxygen, sulphur, etc Useful for furnace design (e.g flame temperature, flue duct design)‏ Laboratory analysis

Proximate analysis Moisture Content : Moisture in coal must be transported, handled and stored Since it replaces combustible matter, it decreases the heat content per kg of coal Aids radiation heat transfer 1-2 gm 72 mesh coal at 105-110 C till constant weight

Proximate analysis Volatile Matter: Consist of CH4, hydrocarbons, H2 and CO, and incombustible gases like CO2 and N2 Proportionately increases flame length, and helps in easier ignition of coal Sets minimum limit on the furnace height and volume 72 mesh coal 900-950 C for 7 minutes

Proximate analysis Ash Content : Ash is an impurity that will not burn Reduces handling and burning capacity. Increases handling costs. Affects combustion efficiency and boiler efficiency Causes clinkering 1-2 gm 72 mesh 800 C (burned)

Proximate analysis Fixed carbon: Solid fuel left in the furnace after volatile matter is removed consists mostly of carbon may contains some H2, O2, S and N2 gives a rough estimate of heating value of coal