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.

· 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.