On-board conversion of alcohols to ethers for fumigation in compression ignition engines

Fumigation of dimethyl ether (DME) is an interesting option for using methanol as a fuel in compression ignition engines. In this concept, a fraction of the methanol used as a fuel is catalytically converted on-board to DME and water, and the products of the conversion are introduced into the engine via the combustion air. With an optimized engine the performance as well as emissions are comparable with those obtained when running the engine on alcohol with polyethylene glycol as ignition improver. The methanol conversion has been tested with different catalysts under various conditions. Because of its superior thermal stability and the low costs, gamma-Al2O3 has been selected as the most promising catalyst for converting methanol to DME in sufficient rates for an on-board application. The chemical kinetics and the mass transfer limitations of the gamma-Al2O3 catalyst used for the methanol dehydration were evaluated. The rate-determining step of the catalytic reaction is found to be the reaction of adsorbed intermediates (the Langmuir-Hinshelwood mechanism); mass transfer is limited by Knudsen diffusivity. The kinetic data were used to design a catalytic converter for fuel processing on-board. Providing DME for fumigation in a 180 kW engine will require approximately 0.7 kg of catalyst. The compact catalyst is necessary for an efficient and fast start-up of the process. The transient behaviour (cold/warm start-up; load changes) of a fixed-bed reactor with gamma-Al2O3 has been estimated using simplified models, which show that the cold start problem should be manageable in less than 1 min. With the hot gas of a methanol burner in front of the fixed bed or a bifunctional catalyst, the catalyst bed can be heated to 250degreesC and the reaction of methanol to DME started within 25 s. This is an acceptable time for cold-starting an engine in heavy-duty vehicles.