The company says that originally, LNG was used in ships to reduce emissions of NOx, sulphur oxides and particulates such as smoke or soot. LNG engines have also successfully reduced CO2 emissions. But any gas that is not combusted in LNG engines is a highly potent greenhouse gas, with an effect that until now has offset the gain from reduced CO2, Rolls-Royce says. Methane emissions, in particular, have proven to be a problem.
Despite this “methane slip,” the company says its engines’ contribution to total greenhouse gas emissions is minute, especially when compared with direct emission of methane from agriculture, mining, shale oil and gas production, industry and natural seepage.
The greenhouse effect of methane is estimated differently by various government authorities and environmental organizations as between 21 and 25 times that of CO2 for the same quantity. Rolls-Royce uses a figure of 23 in its calculations, and presents the engine emissions figure as the net reduction of CO2 emissions after accounting for the negative effect of what it calls a very low methane slip. The engines’ total GHG emissions are about 22 percent lower than that of a comparable diesel engine, Rolls-Royce says.
The Rolls-Royce marine gas engine ranges fulfill the requirements for operation in Emission Control Areas (ECA) and the strict IMO Tier III rules that come into force in 2016, the company says. The engines’ emissions of NOx are about 92 percent less than that of liquid fuel engines, and SOx and particulates are negligible, Rolls-Royce says.
The thermal efficiency of these engines is also very high, in the range of 49 percent to 50.3 percent depending on the engine type. Other advantages of these gas engines include greatly reduced risk of oil spills, a cleaner engine room and the absence of smoke, the company ads.
Currently Rolls-Royce Bergen gas engines are in service in, or on order for, ferries, roro vessels, tankers, coast guard ships and offshore support vessels. To date, 35 marine engines have been sold, as well as about 500 land engines, with cumulative running time of more than 20 million running hours.