As methanol (also true for LNG) have different properties than traditional marine diesel fuel it is not possible to go from one type of fuel to the next without modifications of either the engine or the fuel. Several possible alternatives are available, which one that will become the norm yet to early to tell.

Upgrade methanol to DME

One possible way to use methanol without large modifications to the engine is to convert it to dimethyl ether (DME) which is an excellent diesel fuel. The process is a catalytic reaction where two molecules of methanol are transformed into one molecule of DME and one molecule of water.

2 CH3OH → H20 + CH3OCH3

Approximately 90% of the methanol is converted to DME which result in a mixture of 10 % methanol, 45 % DME and 45 % water on molar bases. In industrial applications the substances is separated by distillation.

For marine applications Haldor Topsoe has proposed to use the mixture as it is. They call the mixture OBATE (on board alcohol to ether).

By doing the conversion of methanol to DME on board bunkering and storage is made more efficient as DME is gaseous in normal temperatures and pressures.

Tests on small diesel engines have shown that this fuel mixture will ignite properly from the compression heat. In the SPIRETH project the fuel will be tested in 300 kW engines on board of a Swedish RoPax vessel owned by Stena Line.

The benefit with the use of OBATE fuel is that the required engine modifications are limited to the fuel system. A new fuel feed pump needs to be fitted, modifications to the injectors might be required, and replacement of incompatible sealing material must be done.

The main drawback with methanol-to-DME conversion is that the OBATE-unit requires space.

An extrapolation from the design for the SPIRET project to a larger engine size indicate that the upgrader will be somewhat larger than de space required for the separator room.

 

Based on: B. Ramne, “EffShip WP8: Demonstration and findings,” 2013.