Carrie 

I could not find emission factors for stationary gas engines burning digester gas.  However, there are U.S. EPA emission factors for natural gas and for digester gas burned in stationary gas turbines. Perhaps, you could apply the same percentage differences from those data to the natural gas factors you have for natural gas in your gas engines.  For example, the EPA emission factors for NOx  from burning digester gas in turbines are 50% of the EPA emission factors for NOx from burning natural gas in turbines.  Then perhaps you could assume that NOx emissions from burning digester gas in an engine would be 50% of the NOx emissions from burning natural gas in an engine.

If you are interested the EPA emission factors for for gas turbine fuels are available online at:

http://www.epa.gov/ttn/chief/ap42/ch03/final/c03s01.pdf

I hope this is helpful.

Milton Beychok
(Visit me at www.air-dispersion.com)

I could not find emission factors either and have not to this date found anything in my research.  I think your suggestion for estimating the emissions is a good one and I will give it a try.  I am working with a state regulatory agency so I will see what they think.

Thanks for the suggestion.

Carrie

Now back to the sulfur.  You need to know what the sulfur content of the gas is, I used Bendix gas tubes and found that our hydrogen sulfide content was running between 200 ppm and 2000 ppm depending on the compostion of the feed sludge (Primary/Waste Activated).  More primary, the higher the H2S readings. An average was 665 ppm.  

1. Look for an emissions factor in USEPA's AP-42 guidance document. We settled on Section 13.5 Industrial Flares from the January 1995 edition as the most realistic representation of what we were doing (supplemental fuel in an Incinerator). Engineers seem to think that was reasonable at the time.  Don't know if they've changed their minds as the factors listed change with each edition. We were looking at Organic compounds particulates etc for emissions reporting. Couldn't find one.  Later I think we were steered toward landfill gas flares... Anyway this is what we came up with.

2.  Heating value of digester gas as stated above = 640 Btu/cf

3. Emissions factor units are in lbs./MMcf (million cubic feet)

4. The sulfur content of digester gas as stated above = 665 ppm

5. Mass emitted = (PV/RT) MW

P = 1

V = Million cubic feet digester gas burned

R = 0.732 (Gas Constant)

T = Temperature assumed 72 F or 532 R

MW = 64 for SO2

6. Emission factor = (665 cf SO2/MMcf digester gas)*(1)*(64)/(0.732)*(532) = 109.29 lbs SO2/MMcf burned.

7. (Emissions factor * MMcf burned per year) /2000 lbs. ton = SO2 Emissions in tons per year.

based on stack testing of many of our engines, engines burning natural gas and sludge digester yield about the same NOx emissions on bhp-hr basis.  in other word, because its lower heating value, digester gas emits a little more NOx than natural gas.    if you want to derive your digester gas mode NOx emission factor from natural gas emission factor, you got to make sure that you start with same emission factor on mmbtu basis, not same emission factor on cubic feet basis.  because the source of NOx is from oxidation of the nitrogen in the air by the engine combustion process.  with regarding SOx, no question that digester gas mode emits more than burning natural gas.  you can't use natural gas emission factor to calculate digester gas emission factor.  i use mass balance calculation and sulfur concentrations from digester gas analysis results.  hope i can be a little help.

Wayne

The NOx, VOC, PM, and CO emissions will be proportional to Btu content.  If your digestor gas btu/cf is 60% that of natural gas, then the emission factors for natural gas  multiplied by 0.6 will be the approximate factors for your digestor gas.  This is true for emission factors in the form of lb of pollutant/cubic feet natural gas combusted. 

The commentator is correct that stated that the emission factors for digestor gas would be approximately equal to those of natural gas is correct for factors in the form of lb of pollutant/hp-hr (operating load times operating hours).  If you run your engines at a contant load for the majority of their up time you can use the same emission factors as natural gas.  You will burn more cubic feet of DG per horsepower-hour becuase it has less Btu per cubic feet, but you burn the same number of Btus per hour with either gas.

SOx emissions are directly proportional to sulfur content.  If your digestor gas has twice the percentage sulfur content that your natural gas supply has, then the emission factor for SOx for DG will be twice the EF for NG.  This is true whether you are useing EFs in the form of lb. SOx/cubic foot NG or lbs. SOx/hp-hr NG.  You should use the average Btu/CF and percent sulfur content from analysis of your digestor gas to make the appropriate adjustments to emission factors for combustion of natural gas in and internal combustion engine.

Do you have source test (stack test) data that you can compare to the defualt emission values?  If they are in ppm, they can be converted into lbs/cubic foot.

Ron in LA