Using Multi-Dimensional Combustion Simulations of a Natural Gas/Diesel Dual Fuel Engine to Investigate NO_x Trends With Air-Fuel Ratio

Natural gas/diesel dual fuel engines used in oil and gas drilling operations must be able to meet NO_x emissions limits across a wide range of substitution percentage, which affects the air to natural gas ratio or gas lambda. In a dual fuel engine operating at high substitution, premixed, propagating natural gas flames occur and the NO_x formed in such premixed flames is known to be a strong function of gas lambda. Consequently there is interest in understanding how NO_x formation in a dual fuel engine is affected by gas lambda. However, NO_x formation in a dual fuel engine is complicated by the interaction with the non-premixed diesel jet flame. As a result, previous studies have shown that enriching the air-fuel ratio can either increase or decrease NO_x emissions depending on the operating conditions investigated. This study presents multi-dimensional combustion simulations of an air-fuel ratio sweep from gas lambda 2.0 to 1.5 at 80% substitution, which exhibited a minimum in NO_x emissions at a natural gas lambda of 1.75. Images from the simulations are used to provide detailed explanations of the physical processes responsible for the minimum NO_x trend with natural gas lambda.