Fuels and Combustion

1. Fuels and Combustion

2. 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

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

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

5. 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 Classification of Fuels

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

7. 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

8. Combustion • Fuel + Oxidizer => Products of combustion + Energy • Fuel ? • Oxidizer ? • Products of Combustion ? • IncompleteCombustion ?

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

10. 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

11. 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

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

13. 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

14. 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

15. Fundamental Definitions • Density • Specific gravity • Viscosity • Pour Point • Carbon Residue

16. 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

17. 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

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

19. Coal Analysis Proximate 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

20. 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

21. 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

22. 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)

23. 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