Thermo chemistry of combustion P M V Subbarao Professor Mechanical Engineering Department Selection of Sufficient Air to use the Entropy Vehicles…..

Fuel Models A gravimetric analysis of fuels Dry Basis As Received Basis Proximate Analysis Ultimate Analysis Proximate Analysis Ultimate Analysis C, M, VM & A C, M=0, VM & A C, H,O, S, & A C, H,O, S, & A

Proximate Analyses Seeds S. No. Scientific Name Total Solids (%) M Volatile Solids (% of TS) Non-volatile solids 1 Acacia nilotica  88.48 11.52 91.59 8.41 2 Prosopis julifora 89.95 10.05 90.43 9.57 3 Albizia lebbeck 89.38 10.62 94.58 5.42 4 Leucaena 89.4 10.6 93.99 6.01 5 Cattle manure 18.43 81.57 77.97 22.03

Ultimate analysis Seeds S. No. Scientific Name Carbon (%) Hydrogen Nitrogen 1 Acacia nilotica 42.17 6.93 5.63 2 Prosopis julifora 40.73 6.94 5.97 3 Albizia lebbeck 47.31 6.81 7.08 4 Leucaena leucocephala 44.87 6.44 5.34 5 Cattle manure 35.54 4.51 1.71

Equivalent Chemical Formula Ultimate Analysis of dry (moisture free) fuel: Gravimetric Percentage of carbon : x --- Number of moles, X = x/12 Percentage of hydrogen : y --- Number of atomic moles, Y = y/1 Percentage of oxygen: k --- Number of atomic moles, K = k/16 Percentage of sulfur: z – Number of atomic moles, Z = z/32 Equivalent chemical formula : CXHYSZOK Equivalent Molecular weight : 100 kgs.

Equivalent Chemical Formulae S. No. Scientific name Chemical formula 1 Acacia nilotica C3.51H6.93O2.22N0.40 2 Prosopis julifora C3.39H6.94O2.26N0.43 3 Albizia lebbeck C3.94H6.81O2.05N0.50 4 Leucaena leucocephala C3.74H6.45O2.30N0.38 5 Cattle manure C2.96H4.51O2.26N0.12

Ideal Combustion Ideal combustion CXHYSZOK + 4.76(X+Y/4+Z-K/2) AIR → P CO2 +Q H2O + R N2 + G SO2 Air- Fuel Ratio: Mass of fuel = one kilo mole = 100 kg : Equivalent chemical formula. Chemically exact amount of air for ideal combustion of one kilo morel air. Stoichiometric air fuel ratio is the ratio of exact mass of air required to mass of fuel.

Stoichiometric Ideal Combustion

Philosophy of Combustion (Reaction) It is spontaneous Combination of species, known as reactants to become products and release heat. The first and foremost molecules of reactants react in infinitesimal (~zero) time. It requires infinite time for last set of molecules of reactants to become products. Humans depend on combustion, in spite of knowing that they generate pollutants.

Classification of Engineering Combustion Systems External Combustion Systems: Only combustion of fuel with air occurs in these systems. These systems transfer the thermal energy liberated due to combustion to surroundings thru various modes of heat transfer. Process Heat Utilization Surroundings. Power generating Water-steam Surroundings. Air is just a source of oxygen. Internal Combustion Systems: Thermal energy liberated due to combustion is used generate Mechanical Power. Air is both working fluid and source of oxygen.

Furnace in A Modern Coal Fired Steam Power Plant

First Law Analysis of External Combustion System: SSSF Many of the thermal power plants running on Ranke Cycles use an external combustions system known as Coal (fuel) Fired Steam generator. First Law Analysis of a Combustion System (SSSF) in molar form :

First Law Analysis of A Furnace First Law Analysis of a Furnace (SSSF) in molar form :

Model Testing for Determination of important species Furnace of a Steam Generator in À Modern Thermal Power Plant Water Flow Rate Air Flow Rate Flue gas Analysis Fuel Flow Rate

Results of Model Testing For a given fuel and required steam conditions. Optimum air flow rate. Optimum fuel flow rate. Optimum steam flow rate. Optimum combustion configuration!!!

Stoichiometry of Actual Combustion at Site For every 100 kg of Dry Coal. Moisture in fuel 4.76

Stoichiometry of Actual Combustion Conservation species: Conservation of Carbon: X = P+V+W Conservation of Hydrogen: Y = 2 (Q-MA) Conservation of Oxygen : K + 2 e (X+Y/2+Z-K/2) = 2P +Q +2R +2U+V Conservation of Nitrogen: 2 e 3.76 (X+Y/2+Z-K/2) = T Conservation of Sulfur: Z = R

Solid Residue & Unburnt Carbon Solid fuels contain large amounts of non-combustible solid residue. This is called as Ash. In modern power plants this is lost as fly ash and bottom ash. Unburnt carbon is lost with ash. Ash sample is generally collected to assess the amount of carbon loss. Combustible Solid Residue is defined as:

Actual Air-Fuel Ratio For 100 kg of coal: Mass of air: e*4.76* (X+Y/2+Z-K/2) *28.96 kg. Mass of Coal: 100 kg. Extra/deficient Air: (e-1)*4.76* (X+Y/2+Z-K/2) *28.96 kg.

Recognition of Actual Air Fuel Ratio Define equivalence Ratio as the ratio of the actual fuel/air ratio to the stoichiometric fuel/air ratio.

Optimization of Furnace Air

Danger of Deficient Air

Influence Unnecessarily Excess Air

Theoretical (100%) Air for Combustion Perfect or stoichiometric combustion is the complete oxidation of all the combustible constituents of a fuel. The oxygen consumed by a Perfect Combustion is known as 100 percent theoretical oxygen. The air required by a perfect combustion is 100% theoretical air. Excess air is any amount above that theoretical quantity.

Optimal Requirement of Excess Air for Combustion Commercial fuels can be burned satisfactorily only when the amount of air supplied to them exceeds that which is theoretically calculated. The quantity of excess air required in any system depends on : The physical State of the fuel in the combustion chamber. For complete combustion, solid fuels require the greatest, and gaseous fuels the least, quantity of excess air. Fuel particle (drop) size, or viscosity. The proportion of inert matter (ASH) present in the fuel. The design of furnace and fuel burning equipment. Excess air requirement can be decreased: By finely subdividing the fuel. By producing high degree of turbulence and mixing.

Typical values of Excess Air vs Fuel Fuels % Excess air Solid Coal(P) 15 -- 30 Coke 20 -- 40 Wood 25 -- 50 Bagasse 25 – 45 Liquid Oil 3 – 15 Gas Natural Gas 5 – 10 Refinery gas 8 – 15 Blast-furnace gas 15 – 25 Coke-oven gas 5 - 10