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NOx Emissions From Large Stationary EngineHelpful Member!(2) 

TES59 (Mechanical)
25 Feb 08 15:40
Have measured NOx emissions from engine exhaust but analyzer units are in PPM. Need to convert to g/Bhp-Hr. Seeking formula for units conversion. Thank you!!!
Helpful Member!  JSteve2 (Automotive)
25 Feb 08 15:57
It's not ordinarily done the way you are proposing. I'll assume you mean ppm "by weight" b/c it could also be by mole.

First: you need to know the NO/NO2 ratios of your NOx. Then also realize that EPA assumes all the NO goes to NO2 in the atmosphere. E.g. if you produce NOx with 90% NO, you will be "credited" with .1 * x + .9 * (46/30) x, or 1.48 g NOx emissions per 1.0 g NOx measured.

So - you measure 100 g NOx, convert to 148 g NOx equivalent, and then you have to know how many hp-hr were used in generating that much NOx.

e.g. - let's say your engine runs 23,000 g/min exhaust flow at 200 hp, and you measure 200 ppm (weight) NOx; your g/hp-hr is: 23,000 * 60 * (200/1,000,000) * 1.48 / 200 = 2.04 g/hp-hr
Helpful Member!  ivymike (Mechanical)
25 Feb 08 16:08
take PPM, multiply by average molecular mass of NOx constituents (30 to 46), divide by average mass of exhaust constituents (including NOx, say 30-ish), you get lb NOx per lb exhaust.  

multiply the above by mass flow rate of exhaust (lb exh /hr) to get lb NOx per unit time (lb/hr)

divide by the corresponding horsepower to get brake-specific NOx emissions per unit time (lb/hp.hr)

in the end you'll get an answer that says 10ppm is about 0.03 gm/bhp.hr, depending on the above

JSteve2 (Automotive)
25 Feb 08 16:59
ivymike- there's nothing wrong with your math, but that method will underestimate the EPA-reported NOx due to the EPA correction as noted in my post above.

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