Convert ppm NOx to lb/MMBtu NOx

[clsengr]

In search of a thougough explanation (I have the calc, need to understand it) to convert measured flue gas ppm NOx (from natural fired heater) to lb/MMBtu NOx

 

[25362]

This is one way to do it. The volume of flue gases in relation to the volume of natural gas burnt depends on excess air, the natural gas composition, and the combustion characteristics. Assume, as an example, this ratio is 13:1.

Now [Θ] ppm NOx on flue gases would be 13[×][Θ] ppm on natural gas, e.g. 13 [Θ] cf/10⁶ cf natural gas.

Convert 13 [Θ] cf NOx into lb, using a molecular weight, and 10⁶ cf natural gas into Btu using its net calorific value.

It is assumed stack-emited NOx is about 90% NO and 10% NO₂.

 

[athomas236]

ppm NOx times 1.29E-3 = lb/million BTU based on 3%O2 in dry flue gas and 270Nm^3/GJ

 

[AMT23]

If you want to know the derivation of the formula, try this:

ppmvd = (lbmol NOx/10^6 lbmol FG_dry) * (1 FG_dry/(1-water fraction)* FG_wet) * MW_NOx / MW_FG * (lb FG/MM BTU_HHV)

The last factor lb FG/MM Btu depends on excess air and fuel type. You can have this info handy if you use any simuation program (PRO II, FRNC5, etc.) Athomas236 is a great estimate.

I have done a study of this before, and I come to a conclusion that about 750 ppmvd NOx for each 1 lb NOx/MM Btu for refinery fuel gas at 3%O2.