Combustion calculations were performed for typical
natural gas and #2 fuel oil. Table below shows the flue gas analysis,molecular
weight and flue gas generated per MM Btu fired at different excess air
levels.
Excess air,%  0  10  20  30  0  10  20  30  
Fuel  <natural gas>  <# 2 fuel oil>  
% vol CO_{2}  9.47  8.68  8.02  7.45  13.49  12.33  11.35  10.51  
H_{2}O  19.91  18.38  17.08  15.96  12.88  11.90  11.07  10.36  
N2  70.62  71.22  71.73  72.16  73.63  74.02  74.34  74.62  
O_{2}  0  1.72  3.18  4.43  0  1.76  3.24  4.50  
Mol wt  27.52  27.62  27.68  27.77  28.87  28.85  28.84  28.82  
Wgm  768  841  914  986  790  864  938  1011 
Converting mass of NOx to volumetric units we have: V_{n}=10^{6}xYx (N/46)x(MW/W_{gm})x(213)/(21O_{2}xY) where MW=molecular weight of wet flue gas N=lb of NOx per MM Btu fired(HHV basis) O_{2}=% volume of oxygen in wet flue gases V_{n}=ppmv dry NOx W_{gm}=flue gas produced per MM Btu fired Y=100/(100% H_{2}O),where %H_{2}O=volume of water vapor in wet flue gases From Table at zero excess air: Y=100/(10019.91)=1.248,MW=27.53,W_{gm}=768,O2=0 V_{n}=10^{6 }x1.248xNx27.52x18/(46x768x21)=832 N Similarly,to obtain CO conversion,use 28 instead of 46(molecular weight of CO instead of NO_{2}) V_{c}=1367 CO where CO=lb of CO per MM Btu fired Repeat at 30 % excess air: W_{gm}=986,Y=100/(10015.96)=1.189,MW=27.77,O_{2}=4.43 V_{n}=106x1.189xNx27.77/[46x986x(214.43x1.189)]=832N Thus independent of excess air,we obtain 832 as the conversion factor for Nox and 1367 for CO. Similarly for # 2 oil,it can be shown: V_{n}=783 N and V_{c}=1286 CO Example:If a natural gas fired boiler generates 0.1 lb/MM Btu of NOx,the equivalent is 0.1x832=83 ppmvd If oil is the fuel,then 0.1 lb/MM Btu of CO=129 ppmvd 
The procedure shows how to convert between lb/h of NOx or CO to ppmvd. The pollutants could be in the exhaust gas or could also be added by the burner.The main difference between steam generators and gas turbine exhaust gas is how the NOx or CO is reported. It is on 15 % dry oxygen versus 3 % in the case of steam generators. If
w lb/h of NOx in a total exhaust flow of W lb/h,the following expression
gives the NOx in volumetric units.
MW=.01x(3.5x44+10x18+75x28+11.5x32)=28
In
incineration systems,7 % oxygen dry is used as the basis and one can similarly
use the above method to perform the conversions.
