2011+combustion

2011+combustion - Fundamentals of Combustion Thermodynamic...

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Fundamentals of Combustion Thermodynamic of Reacting Mixtures Normally we would do full examination of non-reacting mixtures – Part of non-reacting mixtures will be handled here, partly later with cooling towers Interested in combustion of fossil fuels – hydrocarbons – The process of combustion is a high speed, high temperature chemical reaction. 1
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2 General – Very little of the released carbon is actually “consumed” in the combustion. Combustion takes energy from recombination of molecules Nuclear energy comes from the recombination of atoms!!! – Example - Combustion of Methane
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3 – When considering air requirements, two roles must be considered: The air must be present for combustion to take place. The air required to remove the products of combustion. – Oxygen for combustion is normally obtained from the atmosphere, which is about 21% oxygen by volume or 23% by weight. Role of Air
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4 Role of Air – Role of Nitrogen Most of the 79% of air that is not oxygen is nitrogen, with traces of other elements. Nitrogen is an unwanted “parasite” that must be accepted in order to obtain the oxygen. – It contributes nothing to combustion, – it increases the volume of combustion products to be vented, – it steals heat from the reaction, – creates a growing environmental problem.
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5 Role of Air Water vapor Comes from ambient air and is a by-product of oxidizing hydrogen. Ambient water takes heat from the flame, passing out of the vent as vapor mixed with the combustion products. Altitude Differences Burning of fuel requires the same amount of oxygen whether at sea level or at great altitude Combustion at great altitude needs a higher “air” volume to maintain the same energy release
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6 Review chemical equations --> conservation of atoms! => Coefficients of chemical equation = stoichiometric coefficients => Moles = # of molecules, can change if atoms combine to make more complex molecules. 2H 2 + O 2 × 2H 2 O reactants products
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7 Definition of mole fraction : Air consists of 21% O 2 79% N 2 by mole fraction or volume fraction 1 mole of air has 0.21 moles of O 2 and 0.79 moles of N 2 χ i = N i N i = N i Ν
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8 Stoichiometric mixture: mixture of reactants where molal proportions are such that there are no excesses. In combustion equations we want the stoichiometric coefficient of O 2 to be an integer – With air, if there is 1 mole of O 2 than there must be (1/.21)* 0.79 = 3.76 moles of N 2 – Or, there are 4.76 moles of air to supply 1 mole of O 2 .
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9 Stoichiometric mixture: stoichiometric combustion: all oxygen available is used in chemical reactions
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10 Stoichiometric Combustion of Methane CH 4 + 2(O 2 + 3.76N 2 ) CO 2 + 2H 2 O + 7.52N 2 Methane is CH 4 Stoichiometric products are CO 2 , water and N 2 Balance carbon first, then hydrogen and then oxygen Studies have shown that the actual combustion of methane has 81 intermediate states!
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This note was uploaded on 01/17/2012 for the course ME 462 taught by Professor Muller during the Fall '11 term at Rutgers.

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2011+combustion - Fundamentals of Combustion Thermodynamic...

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