2. NATURAL GAS FOR MARINE APPLICATIONS
The alternative concepts of reciprocating engines for marine applications are:
・Gas diesels
・Dual-fuel engines
・Lean-burn gas engines
The gas diesel (Figure 2) with common rail injection of the gas is a mature and well established technology. Operational experience up to 65,000 h exists, and some 50 engines are in operation in the offshore field. 50 more are in power plant use. The gas diesel is the safest of all gas engine concepts, because
・There is no unburnt gas in the exhaust gas.
・There is no gas penetrating into the crankcase.
・In practical terms no risk of misfiring.
・No risk of gas in the air receiver.
Other benefits of the gas diesel concept are:
・High efficiency
・Very high reliability and fuel flexibility, because the engine can run on any liquid fuel without hardware changes.
Since the processes are a pure diesel process, the combustion temperatures are pretty high and the NOx emissions are only some 30% lower than for a corresponding engine running on liquid fuel.
The gas diesel also needs compression of the gas to a minimum 250 bar pressure. In a case when liquified natural gas would be the fuel the pressurizing system for the gas is quite simple. The gas diesel would therefore be pretty feasible for shuttle-type service, like ferries and ro-ro vessels.
The dual-fuel engine concept developed by the author's company (Figure 3) differs from traditional dual-fuel engines, because the ignition of the gas is performed by a micro pilot, which means a common rail injection in a separate system, which allows for less than one per cent pilot injection. In gas mode this engine can therefore operate at very low NOx emissions, for example TA-Luft or half TA-Luft. A liquid back-up fuel can be used in a conventional injection system. The safety concept for a marine application of this engine concept is seen in Figure 4, where there is an oxidation catalyst removing unburnt methane from the exhaust gas. Special attention has also been paid to the crankcase ventilation. This concept is specially developed for the typical gas fuel ships, i.e. the LNG carders, where boil-off gas can readily be used. This concept would also be suitable on short routes, where for example pressurized gas is utilized.
