As a result of EPA’s focus on Greenhouse Gases (GHG), many sites are assessing the most cost effective way to accurately measure CO2 mass emissions.  During the 2011 CiSCO CEMS Users Group Meeting, participants voiced an interest in a comparative analysis between using the 40 CFR 75 equation G-4 (which calculates CO2 mass emissions based on heat input from fuel) and using an actual CO2 monitor to determine CO2 GHG mass emissions for combustion turbines. The graphs below were presented at the 2012 CiSCO CEMS Users Group Meeting to provide a quantitative demonstration of the difference between the two methods.

The graphs below represent over eight thousand hours of data from a 40 CFR 75 combustion turbine site that determines its CO2 mass emissions (40 CFR 75 Eq. F-2) using an O2 analyzer and a stack flow monitor.  Percent CO2 is derived from the O2 analyzer (40 CFR 75 Eq. F-14A) and the resultant value along with stack flow is used to determine CO2 mass emissions.  In addition, the site provides the DAHS with a fuel flow signal, which provides CiSCO with the data that allows us to determine the CO2 mass emissions using the 40 CFR 75 equation G-4. It is important to note that 40 CFR 75 does not provide a method of measuring CO2 mass emission that uses CO2 analyzers without the need for a stack flow meter. Therefore, the only comparison available within the confines of Part 75 is mass emissions determined using either a fuel meter or a stack flow meter.

Based on the data analyzed, CiSCO has come to certain conclusions. First, using a fuel meter (40 CFR 75 Eq. G-4) with a reasonably conservative gas calorific value (GCV) provides a CO2 mass emission that is comparable to using the analyzer and flow monitor method. The advantage of the fuel flow method over the stack flow method is the reduced need for costly quality assurance testing like daily calibrations and multi-load flow RATAs.

Secondly, the data demonstrates that within typical CO2 levels for turbines (i.e. 45 – 65 tons/hr.), the fuel meter method allows for reduced CO2 mass emissions reporting on fuels that have a GCV value significantly less than the EPA default value of 1100 btu/scf. The lower emissions are especially realized when typical pipeline natural gas GCV values (e.g. ~ 1025 btu/scf) are used.

Finally, the data below shows (Weekly Data) that when the CO2 mass emissions drop below seven tons/hour, the analyzer method provides a lower CO2 mass emission rate.

In general, it appears that the fuel meter method provides the easiest and most cost effective means for determining CO2 mass emissions on combustion turbines.  Regardless, CiSCO will continue to research this issue and others as they come to our attention.

Comparison of CO2 emissions that were calculated by 40CFR75 Equation G-4 versus O2 and stack flow meters

Comparison of Weekly CO2 Emission Data calculated by 40FR75 Equation G-4 versus O2 and stack flow meters

· A=CO2 mass emission was determined using an O2 analyzer and a stack flow meter.

· B= CO2 mass emission was calculated using 40 CFR 75 Equation G-4. The HI was determined using the EPA default GCV of 1100 btu/scf.

· C= CO2 mass emission was calculated using 40 CFR 75 Equation G-4. The HI was determined using a conservative representative GCV of 1050 btu/scf.