Statement
Find the mass of water and carbon dioxide exhausted from a combustion engine, relative to the mass of fuel burnt (take octane for the fuel).
Calcular la masa de vapor de agua y de dióxido de carbono producidas en un motor de combustión, por unidad de masa de fuel (supóngase que se quema octano)
Solution.
Fig. 1. Sketch of the combustion chamber: 1, air; 2, fuel; 3, products.
For complete combustion (let assume the engine is in good conditions), for each mole of octane, C8H18, at least 25/2 moles of oxygen are required, as the stoichiometry shows:
C8H18+aO2 = 8CO2+9H2O ® a=8+9/2=25/2
We check for the mass balance per mole of fuel: 114 g of fuel (8·12+18), plus 400 g of oxygen ((25/2)·32), equals 352 g of CO2 (8·44) plus 162 g of H2O (9·18), what matches perfectly (514 g of reactives produce 514 g of products).
Of course, in most cases the oxidant would be air and not pure oxygen, and the amount of air taken may be larger than the minimum requirement for complete combustion (it is nearly double in diesel engines, and much more in gas turbine engines), but neither the extra oxygen, nor the nitrogen, generate more H2Oor CO2.
Thus, for every 114 g of fuel, 352 g of CO2 and 162 g of H2O are produced, so that, in relative terms (emission index, EI):
EICO2=3.09, i.e. 3.09 kg of CO2 per kg of burnt fuel.
EIH2O=1.42, i.e. 1.42 kg of H2O per kg of burnt fuel.
Comments. The above values would be EICO2=3.14 and EIH2O=1.29 for a jet-fuel (assimilated to C13H26), or EICO2=3.67 and EIH2O=2.25 for a natural gas (assimilated to CH4).
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