A hydrocarbon fuel whose composition may be represented by CH2 burns with 30% excess air to reach a product temperature of 2200 K. At this temperature the mole fractions of the dissociation products OH, O, H, NO, and N are all very small; however, even at 10-100 parts per million NO is a serious environmental concern. Estimate the equilibrium concentration of NO present at the above temperature, assuming that the concentrations of N₂, O₂, H₂O, and CO₂ are the same as if no dissociation had taken place. The equilibrium constant for
N₂ + O₂ ⇄ 2NO at 2200 K is Kp = e⁻⁶.⁸⁶⁶ (at 1000 K it is Kp = e-18.776). = = The equilibrium concentration of NO at 1000 K can be shown to be quite small. The problem is that during rapid cooling of the combustion products-for example, in the turbine of a gas turbine- the NO formed at high temperatures does not have time to revert to N, and O₂ ; it tends to be "locked in" to the mixture and discharged from the engine in undesirably large concentrations at low temperature. The equilibrium calculation at high temperature serves therefore to estimate the worst possible case.