Characterization of particulate matter emitted by a marine engine operated with liquefied natural gas and diesel fuels

Liquefied natural gas (LNG) is becoming increasingly popular as a marine fuel as emission regulations become more stringent. However, very little data are available on the particulate matter (PM) emissions of modern marine natural gas engines. In this study, we present a first detailed characterization of the composition of the PM emitted by a modern, in-use, natural-gas-powered vessel. The vessel engines use compression-ignition and only a small amount of diesel fuel as pilot. These engines drive electrical generators, providing propulsion as well as auxiliary power for the vessel. Our emissions characterization includes six different techniques to measure black carbon (BC), including all methods determined as appropriate for measuring BC emissions from ships by the International Maritime Organization, as well as particle size distributions, metal concentrations, and organic particulate emissions. PM emissions differed significantly between idle and at-sea operating conditions. At idle, PM emission factors were primarily organic (approximately 1500 mg/kWh), with BC emission factors over two orders of magnitude lower (5.6 +/- 0.4 mg /kWh). At engine loads above 25%, all emissions were independent of load and substantially lower than at idle, at 4.4 +/- 1.7 mg/kWh for organics and 0.8 +/- 0.2 mg/kWh for black carbon. When operated only on diesel fuel, this engine emitted 8-fold more organic PM (38 +/- 15 mg/kWh) and 37-fold more BC (30 +/- 11 mg/kWh) at loads above 25%. At idle loads, the diesel-fuel emissions were comparable to the natural-gas emissions. In addition to organics and BC, a third category of non-volatile sub-10-nm particles was identified. A detailed consideration of our measurements indicated that the sources of the organic, BC, and sub-10-nm particles were lubrication oil, diesel pilot fuel, and lubrication-oil metals, respectively. Future studies should seek to quantify the emissions of other dual-fuel engines that will be entering the market.