Reciprocating compressor vent gas capture and return to process for methane emission mitigation

Abstract

Methane emissions from natural gas compressor stations pose environmental and regulatory challenges for operators and the community, particularly regarding the possible reinstatement of the Environmental Protection Agency's Waste Emissions Charge. Gas leakage past compressor seals accounts for a portion of these methane emissions. These gases are traditionally vented to the atmosphere or flared and can equal % of the driving engine's fuel before legally mandated replacement. This report considers the effectiveness and mechanisms of compressor vent gas capture and use. Recirculation (i.e., directing vent gases to the intake of the driving engine) and recompression (i.e., directing vent gases back to the main product pipeline) systems to capture and redirect these vent gases are compared for techno-economic feasibility. The research involves laboratory simulation of gas recirculation using a Caterpillar G3516J lean-burn natural gas engine. Performance metrics, including engine efficiency, emissions reduction, and engine stability, are evaluated under varying operating conditions to determine the feasibility of integrating such a system at industrial sites. A techno-economic analysis is included to compare the recirculation and recompression systems, as well as conventional venting, considering potential compliance costs and fuel savings. This research demonstrates that simulated gas recirculation via fumigation to the driving engine intake can be conducted in a safe manner, while offsetting fuel demand from the primary fuel line. During steady state introduction, the engine was shown to handle up to 35% of its fuel flow fumigated to the air intake independent from the fuel system. This fuel was shown to directly offset fuel provided to the engine by the fuel line. Additionally, tests were conducted to demonstrate that the engine can handle rapid transients in fumigation flow while maintaining controlled combustion. The techno-economic analysis indicates that vent gas capture can provide an effective and financially viable emissions reduction strategy considering state regulations or possible federal penalties, mitigating methane release while utilizing otherwise wasted gas. Yearly emission charge savings of $95,000 and fuel savings of $7,900 could be realized per compressor during typical operation.