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Introduction..... Composition of Air..... Combustion Notes..... Combustion Equations.....
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Introduction The notes below relate to the combustion process. Combustion is a rapid reaction between a fuel and oxygen that produces heat (the chemical energy content of a fuel is converted to heat energy). Composition of Dry Air Oxygen is involved in the majority of combustion reactions and this is present in the air. The composition of dry air as mole fractions is 0,7809 N2, 0,2095 O2, 0,0093Ar, and 0,003 CO2. For combustion equations it is convenient and practical to treat Ar and CO2 as Nitrogen. The composition of molar air can then be taken as (approx.) 0,79kmol N2 and 0,21 kmol O2 per kmol of Air. Note: I am using kmols (The molar volume is 22,414 m³/kmol at 0 °C and 101.325 kPa absolute pressure ) The equivalent mass fractions of air are 0,768N2 and 0,232 O2 per unit of air. These values are conveniently represented as 1 kmol O2 + 3,76 kmol N2 = 4,76 kmol Air Combustion Notes When considering the combustion of fuels with air equations are used to determine the proportions of the various chemicals involved. The fuels generally are composed of carbon, hydrogen and sulphur with other substances including oxygen and ash. The carbon, hydrogen and sulphur combine with oxygen in the air and the nitrogen and other gases in air are assumed to take no part in the combustion process..... The proportion of air for complete combustion is called the stoichiometric air/fuel ratio. Normally an excess of air is available and the mixture is weak or lean. When insufficient air is available for complete combustion the mixture is rich... Some of the terms used in combustion are identified below.....
Rich mixture M s greater than 100% It is important to note that the total combustion of a fuel requires ideal conditions. The fuel must be intimately mixed with the oxygen, the temperature must be appropriate, the ignition cannot start without a source of activation energy ( a spark, or flame, or local high temperature). Once the ignition has commence the combustion will generally spread spontaneously. For an car engine it may be desireable to have a rich mixture to allow for maximum power or at start up. A boiler requires an excess or air to ensure complete combustion of the fuel for efficient operation.... Combustion Equations The following typical equations are used to determine the combustion process
C H4 + 2 O 2 -> CO 2 + 2 H2 - Δ H The enthalpy term is negative if the reaction is exothermic and positive if the reaction is endothermic.
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Last Updated 16/07/2006