Development and demonstration of strategies for GHG and methane slip reduction from dual-fuel natural gas coastal vessels

Abstract

Low Pressure Dual Fuel (LPDF) marine engines fueled with natural gas (NG) provide a cost-effective means for minimizing fuel sulfur content, and NOx and PM emissions for marine vessels. In addition, the use of NG can provide significant GHG reductions, if CH4 slip is mitigated. Alternative fueling strategies such as LPDF, are commonly assessed by life-cycle analyses using exhaust emission rates from generalized engine test cycles (e.g. IMO E2). As CH4 slip from LPDF NG engines is particularly sensitive to engine load, a greenhouse gas emission inventory based on generalized engine cycles may not accurately represent real-world vessel operation and emissions. The objectives of this work are to: (i) quantify the GHG emissions reductions possible using the current best available LPDF technology and, (ii) evaluate the attained real-world emissions reduction as a result of improved vessel operating strategies and state of the art engine technology for coastal LPDF vessels.Steady-state emission factors (g/kW-h) and emission rates (g/h) of CH4, CO, CO2, and NOx were measured during steady-state and dynamic operation for two roll-on-roll-off ferries with LPDF engines from two different manufacturers. During commercial operation, the combination of a revised LPDF engine calibration and best vessel operating strategy reduced GHG emissions by 57% and 24%, relative to stock LPDF and diesel operation on vessel #1, respectively. Compared to diesel operation, the annual GHG savings on vessel #1 as a result of this program equate to approximately 2.4 kT CO2e. The tank-to-wake CH4 emissions measured in the current and previous works were found to have poor agreement with the those based on the corresponding test cycle (E2). The E2 cycle overestimated the current CH4 emissions by 8–30% depending on the implemented operating strategy. Prior to implementation of these strategies, the E2 cycle underestimated the CH4 emissions by 74% (Peng et al., 2020). This underscores the value of in-use emission measurements for accurate evaluation of alternative fueling strategies, as well as the utility of emission-informed vessel operation strategies.