PR-457-16200-R01 Control of Vented Methane Emissions from Integral Compressor Engines

At natural gas pipeline compressor stations, methane emissions from compressor and engine crankcases are often vented directly into the atmosphere. There may be advantages to capturing these emissions and using them to offset fuel usage and / or decrease the overall carbon footprint of such a typical compressor station. This study evaluates the feasibility of directing such captured emissions into the intake of the compressor engine and subsequently combusting the methane emissions into carbon dioxide and water. Although seemingly an obvious or trivial solution, there are in fact several challenges and issues that must be considered to evaluate if such a scheme is a reasonable course of action for compressor station methane emissions reduction. The study relies on both extensive literature review and model-based engine simulations to determine the feasibility of rebreathing crankcase gases in pursuit of reducing compressor station methane emissions. The conclusions of the study are limited to the crankcase vent rebreathing into the air intake of large bore, natural gas, 2-stroke engines.

The model-based engine simulations require detailed physical geometry of the subject engines as well as knowledge of basic engine operating parameters (e.g., fuel flow, air flow, and fuel composition). The simulation is based on the experimental facility located at Colorado State University, which is a 4-cylinder Cooper-Bessemer GMV-4. One of the important parameters needing to be known is the gas composition of both compressor and engine crankcases. It is extremely difficult to determine the composition of the engine crankcase gases, since the composition depends on a large variety of parameters. For this study, the gases from the compressor crankcase are modeled as methane (i.e., pipeline gas), and the gases from the engine crankcase are modeled as products of combustion with two different levels methane concentration (1500 ppm and 3000 ppm). A sensitivity analysis is performed, and the observed pressure traces from the model-based engine simulation show that the engine performance is not affected by the addition of rebreathed gases. This insensitivity mainly results from the very small rebreathed flow rates compared to the air intake, and the adjustments made on engine parameters, boost pressure and fuel injection rate, to keep trapped equivalence ratio (TER) and the energy delivery rate the same.