The fuel

DICE requires cost-effective production of ultra-low ash coals. Although the cleaner the better, detailed coal specifications for large diesel engines remain unclear. The earlier USDOE work concluded that coal with 2–3% ash was suitable for DICE. After collaborating with MAN, the ash target is currently 1–2%, but will be a trade-off between processing cost, and engine and maintenance costs. Depending on engine speed, MRC should have a top size of around 50 μm, a coal concentration of at least 55%, and enable effective pressure atomisation. However, it is likely that, as DICE develops, coarser and higher solids MRC will be preferred – reducing processing and transportation costs, and improving engine efficiency and output.

Bituminous coals

For black coal, the MRC process can use a variety of technologies all commercially available and well known to the industry. All involve micronising to increase mineral liberation, followed by flotation/selective agglomeration or dense medium separation.

These processes have been used in a number of studies since 1990. While in the past there was no ready market for ultra-fine wet coal (dewatering to product normal moisture specifications being uneconomic), with DICE, ultrafine wet coal is now alright.

Note that micronising before de-ashing also avoids needing to micronise clean product MRC before the engine, thereby avoiding fuel contamination by the grinding media. Overall, this approach gives an improvement over the processes used for the USDOE program.

In practice, there will be many options for producing MRC, including starting with washed coals through to scavenging MRC from tailings streams, and blending of biochars, for example as shown below.

Options for black coal-MRC production
Options for black coal-MRC production

MRC is somewhat similar to the coal water fuel produced for boilers and gasifiers in China, where conversion of combustion equipment to use cleaner and more efficient coal water fuel receives a range of subsidies, leading to rapid growth of the industry over the last few years to over 40 Mtpa. Although similar, MRC has a finer grind, lower ash specification, and slightly lower solids content. The Chinese boiler fuel is significantly more viscous than that specified for DICE – at least with conventional pressure atomisation; see images below.

coal water fuel for a boiler by JGC MRC for DICE
Left, coal water fuel for a boiler by JGC; right, MRC for DICE

Low rank coals

Low rank coals have received far less attention for DICE. It is generally assumed that processing will need to reduce the porosity of the coal (eg by hydrothermal processing, roller/press compaction, or both). As it is unlikely that surface selective separation techniques (eg flotation or selective agglomeration) will be suitable for low rank coals, especially Victorian brown coals, size/density separation would be used for a de-sanding step.

These processes have been successfully tested by Exergen, Ignite Energy Resources, JGC Corporation and in the current Brown Coal Innovation Australia (BCIA)-CSIRO R&D project. It is noted that most of the ash formed from Victorian coals is from organically bound elements in the coal, and not mineral ash (this can be as low as 0.3%), and that the de-sanding step is mostly to remove relatively coarse sand entrained during mining operations.

After de-ashing/de-sanding and hydrothermal treatment, it is necessary for partial dewatering of the fuel to achieve the required solids concentration, followed by micronisation.

For all coals, formulation may then be required with small amounts of dispersant/stabiliser to obtain the required rheological properties (low viscosity, high stability). The stability specification will depend on the transportation and storage requirements, and end application (for example, for captive MRC-DICE plants, fuel stability is unlikely to be an issue). The amount of dispersant required varies greatly with the coal and dispersant used, and is typically 0.05-0.5wt%.

The production of MRC fuels from biomass, including algal soups, is also being investigated.