LECTURE UNIT NO. 7FUELS AND COMBUSTION I. Fuel – a compound containing carbon and hydrogen in elemental form or compounds which, in rapid chemical union with oxygen, produce combustionClassification of fuels 1. Solid fuels a.) coal (steam power plant fuel) b.) bagasse, wood barks and ipil – ipil (dendro thermal fuel) c.) fuel from garbage (biomass) d.) waste products from industrial and agricultural operations e.) coke (blast furnace fuel) 2. Liquid fuels a.) gasoline (C8H18) – octane b.) distilled fuel oil - diesoline (C16H32) c.) blended fuel oil - diesel fuel oil (C12H26) – dodecane d.) alcohols (CxHyOz) - ethyl and methyl alcohols e.) alco-gas (green gasoline) - blend: 70% gasoline + 30% anhydrous alcohol f.) light heating oils g.) kerosene h.) jet fuel i.) liquefied petroleum gas (LPG) - propane + butane + odorizer at high pressure 3. Gaseous fuel a.) natural gas - Methane (CH4) - Ethane (C2H6) - Propane (C3H8) - Butane (C4H10) b.) coke – oven gas c.) blast furnace gas d.) water gas e.) enriched water gas, carbureted water gas f.) producer gas g.) biogas (gas emitted from animal waste) 4. Nuclear fuels a.) natural – U238 (natural uranium) b.) prepared – U235 (enriched uranium)Basic desired fuel properties 1. It must have a high heating or calorific value 2. Easy handling of fuelFour basic conditions that must be considered to burn a fuel completely (complete combustion) 1. maintain a high flame temperature enough to ignite incoming air-fuel mixture 2. sufficient time to complete the burning process of the combustible elements in the fuel 3. correct air-fuel ratio 4. thorough mixing of air and fuel II. Combustion – a chemical reaction in which fuel combines wth oxygen liberating large quantities of heatMethods of reporting fuel analysis A.) Volumetric or Molar Analysis – represents the percent or fraction of its constituents by volume or moles with respect to the total volume of the mixture measured at the same temperature and pressureIllustration: consider 1 m3 of fuel
For Gas XNote: According to Avogadro’s Law in any mixture of gasses, the ratio of the volumes of each gas is proportional to their corresponding mol ratios at the same temperature and pressure. (% by volume of gas X) % VX = VX x 100%= nX x 100% VT nT For Gas Y (% by volume of gas Y) % VY = VY x 100%= nY x 100% VT nTIllustration: consider 1m3 of dry air For N2 (% by volume of N2) % VN2 = VN2 x 100% VT % nN2 = nN2 x 100% nT For O2 (% by volume of O2) % VO2 = VO2 x 100% VT % nO2 = nO2 x 100% nTFrom Avogadro’s Law (mol proportion of nN2 ): nO2Therefore: That for every mol of O2 there are 3.76 mols of N2 B.) Gravemetric or Ultimate Analysis – represent the percent or fraction of its constituents by weight or mass with respect to the total mass of the mixtureIllustration: consider 1 kgm of fuel For Gas X (% by weight of gas X) % GX = mX x 100% mT
For Gas Y (% by weight of gas Y) % GY = mY x 100% mTIllustration: consider 1 kg of dry air For N2 (% by weight of N2) % GN2 = mN2 x 100% mT For O2 (% by weight of O2) % GO2 = mO2 x 100% mTSeatwork1. Convert: Gravemetric or Ultimate Analysis to Volumetric or Molal Analysis of fuel %GC = 80% %GH2 = 20%2. Covert: Volumetric or Molal Analysis to Gravemetric or Ultimate Analysis of Fuel % VC = 40% % VH2 = 60%COMBUSTION EQUATIONFuels + Moist or Humid Air Products of Combustion Flue Gas for ECPP Exhaust Gas for ICPPFuels + Dry air + Water Vapor Products of CombustionNote: That for every mol of O2 there are 3.76 mols of N2Complete Combustion (100% air or Oxygen) or Stoichemetric AirWith Excess Air
Example 1: Combustion of Octane with 10% Excess Air Octane: C8H18Molal EquationBalancingBalanced Molal Equation:Converting to MassApplying Conservation of MassReactantsProducts1. Solving for Actual Air-Fuel ratio2. Solving for theoretical Air-Fuel ratio
3. Solving for the theoretical mass of air4. Solving for the amount of excess air5. Solving for the dilution coefficient6. Solving for the equivalence ratio7. Gravemetric or Ultimate analysis of the products of combustion8. Volumetric or Molal Analysis of the products of combustion
9. Gravemetric ot Ultimate Analysis of the dry products of combustion10. Volumetric or Molal Analysis of the dry products of combustion11. Solving for the Gas Constant of the products of Combustion12. Solving for the Dew point (condensation) temperature of water vapor (moisture) in the products ofcombustion
TERMS Anthracite – a clean, dense, hard coal, comparatively difficult to ignite, but which burns uniformly and smokelessly with a short flame Ash – noncombustible, solid mineral matter Bituminous coal – a coal that varies in composition but which generally has a higher volatile content than anthracite, starts easily, and burns freely with a long flame. Smoke and soot are possible if this coal is improperly fired Coke – the carbonaceous residue containing ash and sulfur created by burning coal with little or no air. The lighter coal constituents are vaporized, while heavier hydrocarbons crack and form carbon Fixed carbon – The combustible residue (not all of which is carbon) remaining in coal when all volatile matter has been driven off Flash point – the temperature at which an oil spontaneously ignites in the presence of sparks or flame Flue gas – the hot gases resulting from combustion, consisting primarily of nitrogen, carbon dioxide, water and small quantities of carbon monoxide and sulfur dioxide. If water vapor is removed, it is known as dry stack gas. Heating value – the heat generated during complete combustion of fuel Ignition temperature – the temperature at which more heat is generated by the combustion reaction then is lost to the surroundings, after which combustion becomes self-sustaining Lignite – a coal of woody structure, very high in moisture content with low heating value. It normally ignites slowly due to moisture content, breaks apart when burning, and burns with little smoke or soot Moisture – water content of fuels. Bed moisture of coal indicates moisture level when removed from earth. As received moisture indicates moisture level of a coal before dried or burned Primary air – air that is mixed with a fuel to initiate and sustain the combustion reaction Secondary air – air which combines with the flue gas SSU viscosity – kinematic viscosity determined on a Saybolt viscometer Stack gas – see “ flue gas” Stoichiometric combustion – a reaction in which all fuel is burned with the theoretically correct amount of air. Neither unburned fuel nor free oxygen is present in the stack gases. CO2 is at a maximum value in the combustion products Sulfur – a common constituent of lower grade fuels. Organic sulfur is combined with hydrogen and carbon in compounds. Pyritic sulfur is elemental. Volatile matter – that portion of fuel which is driven off as a gas or a vapor when the fuel is heated according to standard tests. It consists mainly of organic gasses such as methane.