Low NOX and high organic compound emissions from oilfield pumpjack engines

We measured a comprehensive suite of pollutants emitted from 58 natural gas-fueled pumpjack engines in Utah’s Uinta Basin. Air–fuel equivalence ratio (the ratio of air taken in by the engine to the amount of air needed for combustion of the fuel) was a strong predictor of emissions. Higher air–fuel equivalence ratios led to lower oxides of nitrogen (NOX) emissions and higher emissions of organic compounds. For engines with air–fuel equivalence ratios greater than 3 (34% of 58 total engines tested), a median of 57% of the fuel gas passed through the engine uncombusted, and exhaust gas contained a median of only 3 ppm NOX. Lower air–fuel equivalence ratios were associated with less fuel slip, higher NOX, and the formation of more reactive organic compounds, including alkenes and carbonyls. Average NOX emissions measured in this study were only 9% of average emissions from natural gas-fueled pumpjack engines in a regulatory oil and gas emissions inventory. In contrast, volatile organic compound emissions in the study were 15 times higher than in the inventory. We hypothesize that these discrepancies are due to changes in emissions as engines operate at lower loads and as they age in field conditions. In addition to improving emissions inventories and the effectiveness of related regulatory efforts, this work will improve the ability of photochemical models to simulate the atmospheric impacts of oil and gas development.