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Exhaust NO2/NOx Ratios from Lean-Burn Natural Gas Engines
Of the many obscure phenomena that can affect compressor station operators, one of the least examined is the NO2/NOx ratio from reciprocating engines. This matters because of the manner in which NOx emissions are determined by conventional emissions testing equipment, and because of existing and pending NO2 emissions regulations. While recip engines generally have low NO2/NOx ratios (NO + NO2 equaling NOx), gas turbines usually have high NO2/NOx ratios due to their leaner combustion mode.
Testing at Colorado State University on their laboratory Cooper-Bessemer GMV-4 lean-burn engine confirmed that some of the configurations that will be employed in the future, namely ultra-lean operation and oxidation catalysts, carry implications for NO2 emissions and for NOx measurement accuracy. Specifically, the work found that:
- Ultra lean combustion led to very high NO2/NOx ratios at very low NOx levels
- The NO2/NOx ratio is generally temperature-dependent, with high exhaust temperatures favoring NO formation, and low exhaust temperatures favoring high NO2 levels.
- Oxidation catalysts can significantly alter the ratio by reducing NO2 to NO, or by oxidizing NO to NO2 across the catalyst element. The pattern is not known and appears to be catalyst-specific, as eleven different catalyst elements were examined with no consistent trend.
- The portable emissions analyzer proved to be the most robust and consistent method for determining the stack emissions – more than the chemi-luminescent method or the FTIR measurements.
With the pending revision to the NO2 NAAQS, understanding the split between these species will become quite important for assessing compliance strategies at the fenceline. Likewise, identifying the factors that can confound traditional NOx measurement methods is valuable for cases where the apparent exceedence is borderline, and the measurement method itself can be a factor in determining compliance. With the possible expansion of formaldehyde control programs that would require most large pipeline engines to install oxidation catalyst, it is important to understand that the effect of these catalysts are on the NO2 vs. NO split, given the array of regulatory initiatives involving these species.