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CONVERSION OF NOx,CO from lb/MM BTU to ppmvd basis by V.Ganapathy
Burner suppliers usually state their emissions of NOx and CO in lb/MM Btu fired on HHV basis. However pollution regulation agencies use the units ppmvd(parts per million on dry volume basis adjusted to 3 % oxygen dry in the case of steam generators and 15 % oxygen dry in the case of gas turbine exhaust).The procedure describes how one can perform these conversions.

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 CO2 9.47 8.68 8.02 7.45 13.49 12.33 11.35 10.51
H2O 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
O2 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
Natural gas analysis used:% vol # 2 fuel oil analysis methane=97,ethane=2,propane=1
(HHV=23,759 LHV=21,462 Btu/lb)
#2 fuel oil analysis used: C=87 %,H2=12.5,deg API=32(HHV=19,727,LHV=18,512)
 
CALCULATIONS FOR STEAM GENERATOR
Converting mass of NOx to volumetric units we have:  
Vn=106xYx (N/46)x(MW/Wgm)x(21-3)/(21-O2xY) where 
MW=molecular weight of wet flue gas 
N=lb of NOx per MM Btu fired(HHV basis) 
O2=% volume of oxygen in wet flue gases 
Vn=ppmv dry NOx 
Wgm=flue gas produced per MM Btu fired 
Y=100/(100-% H2O),where %H2O=volume of water vapor in wet flue gases 
From Table at zero excess air: 
Y=100/(100-19.91)=1.248,MW=27.53,Wgm=768,O2=0 
Vn=106 x1.248xNx27.52x18/(46x768x21)=832 N 
Similarly,to obtain CO conversion,use 28 instead of 46(molecular weight of CO instead of NO2) 
Vc=1367 CO where CO=lb of CO per MM Btu fired 
Repeat at 30 % excess air: 
Wgm=986,Y=100/(100-15.96)=1.189,MW=27.77,O2=4.43 
Vn=106x1.189xNx27.77/[46x986x(21-4.43x1.189)]=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: 
Vn=783 N and Vc=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 
 
Calculation for Gas Turbine Exhaust 
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.
V=100x(w/46)/(W/MW)/(100-% H2O)
where % H2O=volume of water vapor in exhaust gas
MW=molecular weight of exhaust gas
Converting V to 15 % oxygen dry,we have:
Vn=Vx(21-15)x106/(21-%O2dry)= Vx F
%O2dry=100xO2/(100-O2)
Similarly CO can be converted:  Vc=Vnx1.642
Example: 550,000 lb/h of turbine exhaust having the analysis:% vol CO2=3.5,H2O=10,N2=75,O2=11.5 contains 25 lb/h of NOx and 15 lb/h of CO.

MW=.01x(3.5x44+10x18+75x28+11.5x32)=28
V=100x(25/46)/(550,000/28)/(100-10)=0.00003074
%O2dry=100x11.5/(100-10)=12.78
Vn=0.00003074x6x106/(21-12.78)=22.4 ppmvd
Similarly,Vc=(15/25)x22.4x1.642=22 ppmvd

In incineration systems,7 % oxygen dry is used as the basis and one can similarly use the above method to perform the conversions.
 


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