Engines for DICE

Although a wide range of engines have been used to fire MRC, including up to 1900 rpm, it is generally accepted that the lower speed engines are most suitable: the low-speed two-stroke marine type engines (10–100 MW at 90–120 rpm) and largest four-stroke medium-speed engines (20 MW at 400-500 rpm). This is due to their longevity and tolerance to lower quality fuels (such as residual fuel oils which contain up to 0.15% of highly abrasive corundum-like catalyst fines), to allow easier MRC fuel specifications – higher mineral ash content, coarser coal top size, higher viscosity. The choice of engine will be site and application dependent: while the low-speed engine has slightly higher efficiency and lower maintenance costs, the cost of these engines is higher at around $1.8 M/MW compared to $1.2 M/MW for medium-speed engines.

Despite being a mature technology, these engines continue to undergo development that will further improve their suitability for MRC firing (eg higher firing pressure, electronic control, more efficient turbochargers, new materials and adaptations to enable the use of alternative fuels such as biofuels and bitumen water fuels). The new electronically controlled (ME) variants are being implemented as “intelligent engines” with auto-tune ability – perfect for maximising efficiency with MRC.

The use of bitumen water emulsions and slurries in diesel engines provides a good analogue for MRC. Over the last 20 years there have been a number of initiatives to produce bitumen water fuels to replace HFO in boilers, and these fuels have also been used in diesel engines. Such fuels include Orimulsion produced from natural bitumen, and MSAR (multiphase superfine atomised residue) produced from refinery residue (an extremely heavy tar).

A mid-size low speed engine with generator by MAN (55MW, 120rpm)
A mid-size low speed engine with generator
by MAN (55 MW, 120 rpm)

Wärtsilä has extensive experience with firing Orimulsion into medium-speed engines (including a 40 MW demonstration power plant at Vaasa and a 150 MW power plant in Guatemala). Wärtsilä expect that MRC will need similar adaptations.

MSAR was developed as an Orimulsion replacement, and is an MRC of solid bitumen particles in water. While it is a very difficult fuel, giving both poor atomisation and ignition, and contains highly abrasive catalyst fines, it is being used in adapted engines. It is of note that recent CSIRO work shows that, given reasonable atomisation, MRC from coal has superior combustion characteristics to MSAR (and also is superior to many heavy fuel oils).

A large medium speed engine by MAN (20MW, 500rpm)
A large medium speed engine
by MAN (20 MW, 500 rpm)

Another interesting possibility is the potential to adapt dual fuel low- and medium-speed gas reciprocating engines to future DICE operation, an adaptation that is not possible with gas turbines: the choice of appropriate reciprocating engines to burn gas now, may provide the future option to convert to MRC if higher gas prices eventuate.

A number of engine manufacturers are currently interested in DICE for applications ranging from new base load capacity, down to 5 MW backup capacity. MAN Diesel and Turbo have engaged with a number of MRC proponents and are the industry leaders. MAN has also established a staged program to assess DICE, complete with a specially adapted low-speed 1 MW single cylinder test engine. While all manufacturers have some previous negative experiences with coal fuelling of engines, all acknowledge that the previous work was undertaken without a high level of commitment, and none of the programs were completed because the expected scenario of oil shortages did not materialise or funding ceased. Future developments will clearly benefit from recent experience with Orimulsion and MSAR, the extensive experience from the USDOE program for black coals, and more recently by CSIRO’s R&D for both black and brown coals and chars.

Suitable adaptations have been considered by two large engine manufacturers, and examples are shown below, noting that several of these have already been developed for bio-oils. A fuel testing program is underway with an engine manufacturer to develop fuel specifications, and to identify a suitable engine for a demonstration plant.