| OCR Text |
Show Figure 3 shows the predicted NO in both volume (ppmv) and mass (lb/MMBtu) concentration units. Both curves show a steep rise in NO as X increases above that in air (21 %). This initial rise coincides with the steep rise in adiabatic flame temperature that increases thermal NOx which is exponentially dependent on temperature. However, as X increases, N2 concentration decreases which reduces the amount of N2 available to form NOx. This causes the curves to reach a maximum in the middle ranges of X and then decrease to zero at X = 1 as shown in equation (1). Since the flue gas volume decreases as X increases, the peak in the mass concentration curve occurs at a lower X than the volume concentration curve. 4 S_ ' 3 CD :E :-E .tl ~ 2 a z 12,000 10,000 8,000 > E C4 C4 2,000 o z o 0 20 30 40 50 60 70 80 90 100 02 IN OXIDIZER (%) Fig. 3 Adiabatic equilibrium NO in mass and volume concentration units for the stoichiometric combustion of CH4 with an oxidizer containing variable concentrations of 02 and N2. Some regulations, like those for many utility boilers, are written based on the volume concentration of NOx corrected to some 02 level in the flue and assume that air is the oxidizer. If the oxidizer has a higher concentration of 02 than normal air (21 %), it will have a lower flue gas volume flow (as shown in Fig. 2) for a given 02 concentration in the exhaust stack due to the reduction in N2. The regulations should be adjusted accordingly although this is difficult to do in practice. Other regulations, like those for glass furnaces in California, are based on the mass of NOx generated per unit mass of glass produced (lb NOx/ton of glass). The latter is a better method for specifying regulations since the ultimate objective is to limit the total mass of NOx emitted into the atmosphere. A set of calculations was done for X = 0.95 with ~ ranging from 0.5 to 4.0. Figure 4 shows the predicted NO increases with ~ while the adiabatic flame temperature decreases rapidly below \.f' = 1.5 and slowly decreases as \.f' goes from 2 to 4. Figure-3 showed that NO was strongly dependent on \.f' while this graph shows that NO is also |
