Reducing Methane Emissions From Lean Burn Natural Gas Engines With Prechamber Ignited Mixing-Controlled Combustion

Abstract

The oil and gas industry heavily relies on lean burn spark ignited natural gas reciprocating engines. These engines are used in many applications, such as oil and gas exploration, production, processing, compression, transmission, and power generation. These engines produce criteria pollutants, such as nitrogen oxides (NOx) and carbon monoxide (CO), but due to their premixed nature, also produce relatively large amounts of unburned methane (CH4) emissions. These pollutant emissions not only impact air quality, but methane is a very powerful greenhouse gas, with a global warming potential ∼25–84 times higher than that of CO2. Therefore, it is essential to decrease these emissions for the protection of our environment and public health. The primary source of methane emissions in lean burn engines is the crevices and near wall quench layers. Thus, one method to dramatically reduce methane emissions is to alter the combustion process to be nonpremixed, mixing-controlled combustion. High auto-ignition resistance fuels, like natural gas, are not conducive to mixing-controlled combustion due to very long ignition delays. This study investigates using an active prechamber ignition source and a direct injector to achieve mixing-controlled combustion of natural gas with very short ignition delays. In this concept, the active prechamber acts as a reliable ignition source for the direct injected natural gas, which is referred to as prechamber ignited mixing-controlled combustion (PC-MCC). The PC-MCC concept enables a ∼10× reduction in methane emissions, making it a promising technology for reducing the environmental impact of reciprocating engines. In this study, computational fluid dynamics (CFD) simulations have been used to compare two modeling approaches for PC-MCC: a pure Eulerian gaseous injection approach and a gas-parcels injection method. Using the parcel method to model the gas injection enables an engineering approach to study and design the PC-MCC concept in a timely manner with coarser computational grids. This study also investigated the impact of several variables that may contribute to the performance and emissions of the PC-MCC strategy. The parameters that were examined include prechamber passageway characteristics like nozzle diameter, number of nozzles, and the orientation of nozzle orifices. Understanding the effects of each of these parameters will allow PC-MCC with natural gas to be optimized for low methane emissions.