DICEnet is the support group for the development and demonstration of the direct injection carbon engine (DICE) and associated fuel cycle for ultra-high efficiency power generation - from carbon fuels derived from coal and biomass.
DICE is a technology that could provide lowest cost energy security with a step reduction in carbon emissions.
DICE technology
DICE is a technology that could yield lowest cost energy security with a step reduction in carbon emissions, by providing:
- The highest thermal efficiency of any heat engine, especially at smaller unit scale.
- Most flexibility to underpin a high penetration of renewables, directly via efficient utilisation of biomass carbons, and indirectly via providing cost effective backup and grid security.
- Potential for the most energy efficient and cost effective implementation of CO2 capture to allow carbon storage as the last step in meeting emissions targets.
Yancoal UCC initial tests (courtesy of Yancoal Australia)
Alternative pathway to low carbon emissions
Alternative pathway to ultra-efficiency low CO2 intensity power generation
Overall, the DICE fuel cycle provides an alternative technology pathway to a 50% reduction in carbon intensity for power generation in the short to medium term. It is envisaged that niche, rather than major, CCS would be employed as the final step in meeting carbon emissions targets.
DICE fuel cycle
The current DICE fuel cycle is based on incremental improvements to the cycle developed during the extensive USDOE program over 1978-92, and more recent developments by the CSIRO and partners: Exergen, Ignite Energy Resources and the BCIA for brown coals, and Yancoal, Newcrest and Xstrata for black coals.
For black or bituminous coals, the preferred fuel cycle involves micronising the coal (fine milling to give a top size of around 50 µm), followed by deashing, trim dewatering, and homogenising with the aid of a dispersant to form MRC. The process steps are shown graphically below.
DICE fuel cycle with black coal
For brown coals, processing also includes a densification step (eg hydrothermal treatment) aimed at decreasing the water contained within the pores of the coal – thereby increasing the specific energy of brown coal MRC. The process steps are shown graphically below.
DICE fuel cycle with brown coal
Implications for coal
From a coal industry perspective, DICE would create new markets for domestic and export thermal coals. Opportunities are being pursued for both black and brown coals.
For black coals, there is the option of recovering MRC economically from coal washery rejects and higher ash bituminous coals – thereby increasing resource recovery and reducing disposal costs.
For brown coals, MRC provides the potential to export brown coals as MRC paste, noting that brown coals are not currently exported because unprocessed they contain too much water, and dried they are pyrophoric and difficult to ship – even as briquettes.
Another potential new market (interest by Maersk, D/S Norden and SEACO) is fuelling the global deepwater shipping fleet with coal - a market of around 250 Mtpa carbon equivalent. With only a small amount of biochar co-firing, a carbon footprint below that of fuel oil can be achieved, but at significantly reduced cost . This also applies for remote area power generation applications (ie alternative to fuel oil or diesel for isolated island communities or remote mine sites away from large power grids).
The cost and energy for MRC processing is more than offset by these benefits.
From the fuel production side, while the technologies involved are well known to the coal industry, the fuel cycle is new, and requires broad development – a key objective of DICEnet.
It is noted that, during the last five years, the production of coal water fuel for boilers in China has more than tripled to a +100 Mtpa industry, and, while lower quality than the MRC required for DICE, this expansion provides a good analogue and market entry for the DICE fuel cycle.
CO2 intensity
Size-for-size, the most efficient means of converting fuel energy to electricity is by using large compression ignition (diesel) engines. Coal and other sources of carbon could also be used to fuel these engines, if micronised and refined to remove most of the mineral matter (i.e. ash content), and dispersed in water to produce a slurry fuel. This fuel is called micronised refined carbon or MRC (to differentiate it from coarser grain size and lower quality coal water slurry fuels produced for boilers).
CO2 Intensity Reduction with DICE
The use of MRC would enable a delivered efficiency (from resource through to delivered electricity) of around 50% HHV, and with a CO2 emission rate of 670-700 kg/MWh – a level that is significantly lower than new conventional coal technologies (a 20-25% saving compared to new coal plants), and comparable to open cycle natural gas turbines (see figure above).
The CO2 savings from DICE are even greater when compared to most existing coal-fired power plants, and savings of 30-35% are likely when replacing existing black coal plants, and around 50% when used to replace current brown coal plants (especially those in Victoria).
Application of coal technologies in different thermal machines
Integrated carbon management
The efficiency and flexibility of DICE provides a range of options for carbon management. These include:
- the use of a range of biomass carbons as fuel and provide options for bio-CCS as soil carbon,
- reduced production of CO2 during generation from the higher efficiency and outstanding grid support to allow a high penetration of renewables - without inefficiencies from part load operation or storage losses, and
- energy efficient CO2 capture
These features give a DICE-based electricity grid the potential for net negative CO2 emissions, for example as shown below.
Victorian CO2 reduction with DICE
Key benefits
Since the USDOE program, there have been important changes to economic and industry drivers which increase the advantage of DICE: carbon penalties, the need to support a step increase in intermittent renewables, changes to the structure of the electricity supply industry, energy security issues, the ability to be capture ready and capture efficient, and the shortage of cooling water.
Overall, DICE now promises a considerably more efficient, nimble and adaptable generation technology than is possible with pulverised coal-fired steam plants.
Key benefits of DICE
- A step reduction in CO2 intensity of around 20–35% for black coal, and 30–50% for brown coals, and doubling of the CO2 benefit of biochars.
- High efficiency at small unit size, which allows smaller and easier investment steps.
- Lower capital cost at $1200–2000/kW (about half that of supercritical pulverised coal plants) which, together with tolerance to load changes, makes stop-start operation for peak and backup duty a practical and economic option for coal-based generation. DICE should be economic for base load, peaking and backup duties to support renewables, and changing electricity markets. Unlike natural gas turbines, this flexibility can be provided without losing efficiency or increased maintenance.
- Allowing ultra-efficient use of opportunity biofuels, particularly biochar, to further reduce the net carbon footprint. For example, MRC could be co-fuelled with char to provide a sweetener for char, by improving its ignition and combustion.
- Providing an enabling technology for CCS by enabling bolt-on integration of CO2 capture, with a substantially lower energy penalty, and without significant power output de-rating, compared to existing coal power plants.
- Maintaining rating under hot, arid and high altitude conditions, and with low overall water use.
A solution to the water dilemma
DICE uses no water at the power plant, and only a small amount of water for MRC preparation - about 1/10th of that of a conventional steam power plant. While dry cooling can be used by conventional steam plants, this increases the plant capital cost by around 15%, and decreases efficiency by around 2% points which increases CO2 emissions.
For dry cooled steam plants an additional 1 tonne of CO2 is emitted per 30 tonnes of water saved. Dry cooling also gives a 15% increase in plant capital cost.
The water issue is of urgent concern for India and China.
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
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.
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.
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)
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)
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.
DICE engine ©CSIRO, MURRAY McKEAN
Source: DiceNet
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