Challenges

The use of coal in engines requires addressing a number of technological issues, many of which are critical to producing a commercial engine with acceptable longevity and R&M requirements. While all issues were considered in the comprehensive USDOE program from 1978-92, and should be easier for the larger engines and lower speed engines being targeted in the current initiatives, all remain as areas of consideration. This is because the measures required depend on trade-offs between fuel quality and engine modifications, and the technology solutions need to be reassessed in the context of developments in ultra-hard materials and manufacturing over the last 20 years. The following gives a summary of key issues (in decreasing order of significance), effects and measures:

MRC production cost

In past programs, the cost of coal processing has significantly reduced the cost advantage of DICE over the fuel oil engine, and was the main factor in terminating the USDOE program with falling oil prices in the early 1990s.  For most MRC, micronising is likely to represent ~60% of fuel production cost.  Recent initiatives have the advantage of large and efficient commercial scale bead mill technology (eg the Isamill) which has proved highly effective for coals, and improvements in fine coal cleaning. Both allow cost effective recovery of MRC from tailings, and provide a step reduction in MRC processing cost over that in the early programs.  Other improvements are likely from the expected increase in the allowable grind size for MRC for the larger engines and with developments in atomisation.

Atomiser nozzle wear

Consistent and effective atomisation is the most critical issue for coal fired diesel engines. Poor atomisation leads to a sequence of phenomena which can destroy an engine within hours: it increases the droplet size, resulting in slow ignition, late burning and incomplete combustion, and also increases fuel jet penetration.  All of these factors will result in raw or partially combusted coal depositing on the cylinder walls, resulting in ring jamming and rapid cylinder wear.  While MRC slurry can be effectively atomised by using high injection velocity, which with conventional nozzle materials causes rapid nozzle wear which reduces atomisation quality.

Solutions include diamond compact or sapphire nozzles, and the use of lower speed engines with increased time for combustion, and therefore a higher tolerance to coarser atomisation.

Air blast atomisation is also being considered by CSIRO for next generation paste fuelled engines.

Ring jamming

Particles of ash and coal deposited on the cylinder walls are continually scraped by the action of the piston rings. If the particle loading is sufficiently high, these particles accumulate and pack behind the piston rings causing ring scuffing and rapid wear.  Unburnt coal is expected to be particularly difficult in this regard, due to the presence of tars which exacerbate ring sticking.

To date, the most effective control measure has been to ensure effective atomisation and correct injection profile to avoid exposure of the cylinder walls to fuel spray.  However, it is likely that other design factors (eg ring contact profile) will also be important, and that solutions for 4-stroke engines will be different to those for 2-stroke engines employing total loss cylinder lubrication.

Abrasive wear

Abrasive wear of the cylinder, rings and ring grooves has been shown to be chronic for unhardened engines.  A range of technological solutions were demonstrated during the USDOE program using hard plasma/HVO sprayed coatings, in particular tungsten and chromium carbide.

Use of these coatings is now more wide spread for engines, and, whilst increasing the cost of engine components, is likely to be the key enabling technology for DICE. Other measures include the use of larger engines, say 1-5 MW/cylinder, compared to the 200 kW/cylinder capacity used for the USDOE program.  This will reduce wear due to increased clearances and a proportional reduction in cylinder area for a given engine capacity.

Ignition/Ignition delay

Rapid ignition after injection is critical to efficient diesel operation.  Fuels that exhibit a long ignition delay (say 20 ms for low speed engines) can produce shocks from excessively rapid pressure rise early in combustion, and a peak pressure that exceeds the mechanical limits of the engine.  Overheating and ring failure can also occur. For poor fuels, this requires engine derating, which increases engine cost per MW and reduces thermal efficiency.

An extreme case of failure can occur if ignition does not occur, say due to poor atomisation or failure of an atomiser nozzle. With coal, catastrophic engine failure from hydraulic lock would occur within seconds from the resulting deposits of fuel mud in the combustion chamber.

To avoid these issues, pilot injection with diesel fuel was used in many of the USDOE pilot engines.  Whilst acceptable for starting and warm-up, for normal operation this reduces the cost advantage of DICE, and it is expected that other measures will be developed to ensure consistent ignition, and to guard against ignition failure.

Recent CSIRO work has shown that the ignition delay for MRCs is around 5 ms which is better than most heavy fuel oils - providing that good atomisation is achieved.  Engine tests with MRC provided by Yancoal has shown despite this small delay, the combustion rate of MRC is roughly proportional to the rate of injection, and without appreciable premix combustion.  The CSIRO work has also shown the benefits of air blast atomisation for achieving a very short ignition delay and a step improvement of atomisation quality – even with highly viscous MRC.

Exhaust valve seat wear

Accelerated wear has been observed for conventional valve materials, however it is expected that ultra-hard materials will avoid this issue. These valves are already available as an option for many large engines.

Fuel system blockages

Due to the high solids loading of MRC, it can block orifices and other parts of the fuel system when the fuel is left to dry in hot atomiser nozzles.

Proven solutions include recirculation fuel delivery systems, flushing the fuel system with a water-based fluid during engine shut down, and avoiding rapid cross sectional area changes in the fuel lines (dead spots/eddies). It is generally assumed that flushing with diesel fuel is less desirable unless it can be achieved quickly and completely (diesel causes agglomeration of coal).

Some fuel formulations with unusual gel-sol behaviour are more prone to blocking, and this phenomenon requires more fundamental research to develop better dispersants.

Fuel stability

Coal water fuels are inherently unstable and given sufficient time will settle.  Stability and fuel handling equipment and procedures have received extensive consideration in the commercialisation of coal water fuels for boilers.  While this is generally applicable to MRC for DICE, the lower solids content of MRC makes stability a bigger challenge.  Solutions include correct formulation of dispersants (more is not necessarily better) and the use of paste-type fuels, with minor dilution with water immediately prior to use (using a fuel preconditioning module – as is employed for most heavy fuel oils).

Fouling

This has received scant attention, probably because ash fouling has not been reported as an issue. The lack of fouling from coal in diesel engines was the main reason for terminating development of the coal fired gas turbine in favour of the diesel engine in the late 1970s - which resulted in the comprehensive USDOE coal engine program.

A complete lack of fouling is unexpected, as coal cleaning processes only remove the extraneous mineral components in coal, leading to flyash with proportionally higher amounts of deleterious elements (Na, K, S, Cl, Ca, Mg) from the original coal – a condition certain to cause fouling in combustion devices.  However, past engine tests totalling several thousand hours, including continuous operation periods of over 100 hours, have not observed cylinder or turbocharger fouling.  Compared to boilers (and gas turbines), the lack of fouling has generally been attributed to the highly cyclic heat flux within reciprocating engines: cylinder surfaces are only exposed to gases (say above 800°C) for ~10% of the cycle, and the temperature of the metal surfaces is lower (mostly below 300°C). 

Fuel system corrosion

Although corrosion rates from sometimes low pH MRC can be reduced by pH control, it is expected that corrosion resistant fuel systems will be essential (as required for some biofuels).  This will avoid the need for complete corrosion protection by fuel additives/pH control – both would further complicate achieving the optimum fuel rheology.