Transcript Combustion
Combustion Combustion ► Combustion is a chemical reaction accompanied by the evolution of light and heat. ► Combustion in the furnace of a boiler is the rapid chemical union of the combustible elements in a fuel and the oxygen in the air, with the resultant liberation of heat. Combustion ► Fuel oil is burned in a boiler by pumping oil into a furnace in a fine atomized spray. ► The air for combustion is admitted at high velocity through openings around the burner and mixes thoroughly with the oxidation of oil and the liberation of heat. ► The finer the oil particles the more rapid and efficient the combustion Combustion ► The oil must be under pressure and it must be reduced to a viscosity by preheating before it is sent to the burners. ► The complete and efficient combustion of the fuel oil depends of the following factors: Proper mixing of the air and fuel to be burned. Air in proper quantity. Temperature that is high enough to cause combustion reactions. Time to complete the combustion reaction. Combustion ► More air is actually admitted than that amount which is theoretically needed for complete combustion. ► The amount of air which is added above the theoretical amount needed is called “Excess Air”. ► The excess air is necessary in order that all the atomized fuel oil particles will come in contact with the oxygen in the air to ensure complete combustion Combustion ► If only the theoretical amount of air is used Incomplete Combustion will occur. ► If to much excess air is added at too high velocity, some of the atomized fuel oil might be swept out of the furnace and up into the stack before it has time to undergo combustion in the furnace. Combustion ► The three main combustible elements in fuel oil that produce heat during combustion are carbon, hydrogen, and sulfur. ► Air is the medium which provides the oxygen for the combustion of these elements. ► The percentage of oxygen in the air by weight is nearly 23 %. Combustion Possible combustion reactions ► C + O2 = CO2 14,600 BTU/lb C ► 2C + O2 =2CO 4,500 BTU/lbm C ► 2CO + O2 =2CO2 10,100 BTU/lbm C ► 2H2 + O2 =2H2O 61,950 BTU/lbm H2 ► S + O2 = SO2 4,000 BTU/lbm S Combustion ► If CO2 is formed, the combustion is said to be complete. ► If CO is formed, the combustion is said to be incomplete. ► If CO is burned to CO2 before it leaves the furnace the energy would be recovered. Combustion ► In the total process of combustion of released 4,500 + 10,100 BTU of heat or the same amount heat as when the carbon is burned to carbon dioxide directly. ► When carbon monoxide is formed, only 4,500 BTU are liberated per pound of carbon versus 14,600 BTU per pound of carbon when carbon dioxide is formed. Fuel Oil Composition Specific Gravity 0.86 0.90 0.94 0.99 Carbon 84.0 % 85.0 % 86.0 % 87.0 % Hydrogen 13.0 % 12.0 % 11.0 % 9.5 % Sulfur 0.3 % 0.5 % 0.8 % 1.1 % Nitrogen 0.2 % 0.2 % 0.2 % 0.2 % Oxygen 1.0 % 1.0 % 1.0 % 1.0 % Higher Heating Value ► The Higher Heating Value (HHV) of a fuel can be calculate using Dulong’s Formula. 14100BTU 61100BTU 1 4000BTU * C * S HHV * H 2 * O2 lbm C lbm H 2 8 lbm S Lower Heating Value ► The Lower Heating Value (LHV in BTU/lbm fuel), which does not include the latent heat of the water vapor in the exhaust gas formed from hydrogen in the fuel 1040BTU 9 lbm H 2O * H 2 LHV HHV lbm H 2O lbm fuel Combustion Calculate the Higher Heating Value (HHV) and Lower Heating Value (LHV) of a Heavy Fuel Oil with the below composition. ► Carbon – 85% ► Hydrogen – 13% ► Sulfur – 1% ► Oxygen – 0.5% ► Nitrogen – 0.5% Higher Heating Value ► The Higher Heating Value (HHV) of a fuel can be calculate using Dulong’s Formula. 14100BTU 61100BTU 1 4000BTU * C * S HHV * H 2 * O2 lbm C lbm H 2 8 lbm S Higher Heating Value Btu Btu 0.005 Btu 0.01 HHV 14100 0.85lbC 61100 0.13lbH 2 4000 lbC lbH 2 8 lbS HHV 11985 7904.8 40 Btu HHV 19930 lbFuel Lower Heating Value Btu lbWater 9 H 2 LHV HHV 1040 lbWater lbFuel LHV 19930 104090.13 Btu LHV 18713 lbFuel Adiabatic Flame Temperature LHV AFT TIn m Exhaust C PExhaust Adiabatic Flame Temperature ► AFT = Adiabatic Flame Temperature ► LHV = Lower Heating Value ► ṁExhaust = Fuel + Air ► CP = Specific Heat of the Exhaust Gas ► Tin = Temperature of the air entering the boiler. Adiabatic Flame Temperature ► Calculate the AFT of the heavy fuel if it is used in a boiler which requires 15% excess air for complete combustion. The inlet air to the furnace is 220 °F. Assume the specific heat of the exhaust gas is 0.31 Btu/lbm-°F. ► Theoretically each lbm of fuel requires 13.75 lbm of air for complete combustion. Adiabatic Flame Temperature ► AFT ► AFT ► AFT ► AFT ► AFT with theoretical air requirement = LHV/(ṁExhaust X CP Exhaust) + Tin = (18713/((1 + 13.75) X .31)) + 220 = 18713/4.57 + 220 = 4315 °F Adiabatic Flame Temperature ► AFT with 15% excess air ► AFT = LHV/(ṁExhaust X CP Exhaust) + Tin ► AFT = (18713/((1+(13.75 X 1.15)) X .31) + 220 ► AFT = 18713/((1 + 15.81) X .31) + 220 ► AFT = 18713/5.21 + 220 ► AFT = 3812 °F