In the past decade, energy has become one of the most topical issues across the globe, sparking a massive increase in researching ways to produce energy more efficiently.
A former non-conforming nation, Australia recently signed the Kyoto Protocol.
For Australia to achieve new emission targets it will need to implement new technologies that reduce the carbon dioxide rate produced from coal or look to other sources for energy production.
The Australian government is allocating substantial funding for research into cleaner coal technologies, so now is the time to use these funds to develop these technologies.
Generally, traded coking and thermal coal contains mineral contents between 8 and 30% (by weight).
If this content is cost effectively reduced or removed industries that use the material would benefit, in addition to other industries that don’t currently use coal as a feed source.
There are many potential uses for clean coal including the direct firing of coal into gas turbines, increasing the efficiency of power generation and eliminating the need for ash dams
Clean coal can also produce carbon anodes for the aluminium industry, which in turn, could replace petroleum coke.
Other uses for clean coal include carbon steel alloy production, wood charcoal alternative for silicon smelting and replacing oil in the production of fuels and chemicals
In terms of energy usage, using Ultra Clean Coal (UCC) in these applications is competitive with oil, as the carbon content of coal is much higher than that of oil, and is predominantly present as aromatic structures (Steel et al 2001a).
What is UCC?
UCC contains less than 1% by weight ash.
It is created via chemical leaching, as conventional, physical separating techniques cannot achieve this extremely low mineral content.
Clean coal is possible, but further studies are required to gain a deeper understanding of the impacts on the carbonaceous matrix, energy requirements for reagent recycling and the final product’s performance.
Typical minerals found in coal include, but are not limited to Kaolinite (Al2Si2O5(OH)4), Montmorillonite ((Na,Ca)(Al,Mg)6(Si4O10)3(OH)6, Quartz (SiO2), Calcite (CaCO3), Siderite (FeCO3), Dolomite (CaMg(CO3)2), Ankerite (CaFe,Mg,Mn)(CO3) and Pyrite (FeS2).
The underlying theory to the production of UCC product is that when the minerals come into contact with particular chemical reagents they dissociate into soluble products, which are then removed through filtration. The coarse material can be removed by conventional separation methods, while the fine particles can be removed by chemical leaching.
In some cases, leaching can remove the microscopic material, depending on the particle size fraction used. For sufficient contact to occur during leaching, the coal must be ground to a fine size (maximum particle size of 500 µm).
The first UCC research was conducted in Germany before World War II.
According to Brooks, the process used physically cleaned coal product.
The coal was combined with aqueous alkali solution to form a paste which was heated, separated from the liquid, washed in acid then rinsed in water.
Steel et al (2001a) showed that both aqueous sodium hydroxide and aqueous hydrofluoric acid were investigated by the Germans to construct a plant that used hydrofluoric acid.
The plant was capable of producing 70,000 t/y of low ash product to be used as carbon anodes in the aluminium industry. However, production stopped after World War II when Germany decided to follow the worldwide trend of using petroleum coke for its carbon anodes.
Since then, while various studies have been undertaken, there are no full-scale commercial facilities for the production of UCC.
According to Clark and Langley (2005), the thermal efficiency for combustion of coal can be increased from 38% (conventional coal-fired power station) to about 53% (gas turbine being directly fired), which would reduce greenhouse gas emissions by about 24%.
When analysed over the whole life cycle, the reduction in total greenhouse gas emissions would be about 10%.
UCC Energy owns the patent rights to a process that describes the leaching of coal which is capable of producing coal with a mineral content as low as 0.2%.
The patent describes three main processes that the coal is subject to — caustic digestion, acid soaking and hydrothermal washing.
The patent describes a process which involves sequential leaching with NaOH and H2SO4 or HCl as the second leachant, followed by ethanol or C6H8O7.
As this information is from a patent, test results are not available to confirm the reasoning behind these reagents and sequence.
The claim in mineral content reduction is by 93% down to 1.0% or less.
Using a similar process to that described above for the production of UCC, including the regeneration of reagents, UCC Energy constructed a pilot plant in Cessnock, New South Wales, Australia which is capable of processing 350 kg/h of coal.
Trials performed by Mitsubishi Heavy Industries in Japan gave results that proved the use of this coal to be very beneficial. Similar testing was undertaken by Idemitsu Kosan in Japan although no mention was given to the outcomes.
After modification of their gas turbine, which had, to date, predominantly used natural gas as the fuel source, Mitsubishi directly injected the UCC which presented efficient combustion.
Analytical tests of the same sample revealed an ash value of less than 0.2%. This impurity level should not cause major problems in the gas turbine.
Ideal Process Characteristics
Having examined relevant studies, it is clear that the best results for dissociating mineral content in coal are achieved using a combination of reagents sequentially.
The studies showed performance trends and similar dissociation results across the range of chemicals tested.
This suggests the results are valid and capable of being reproduced.
Depending on predominant minerals present in the coal, particular chemicals can be selected for the leaching process.
Closely linked and affecting the performance of these reagents are the process conditions under which each treatment is performed.
Specifically, those factors affecting the environment are chemical concentration, residence time, temperature and pressure.
The final key influence on leaching performance is the particle size.
Grinding the sample to a fine fraction is necessary for increasing the surface exposed to the leachant. Steel et al (2002) discussed the effects of having a smaller particle size in their paper, Ultra Clean Coal Part III.
As the sample is ground to finer fractions, the minerals scattered throughout the matrix become exposed to the reagent and therefore are able to react into soluble compounds.
The drawbacks however, of grinding to a fine size are the amount of extra energy required for grinding at the front end of the process and the increase in energy for filtration and handling throughout the demineralisation process.
Although some of these studies discuss the influence of reagents on the carbonaceous matrix, further solid evidence is required to determine, where appropriate, the extent of dissolution of the matrix for those leachants that appear to be most favourable.
When there is a reduction in the sample’s calorific value after demineralisation, this indicates that more organic material has been dissociated than inorganic and negates most intentions for creating UCC.
Therefore any reagent exploiting this reaction should be well avoided.
The general process for producing UCC is a viable energy option as it enables energy reuse in methods that require high temperatures.
For example, in the process patented by Brooks et al (2004), where 250°C is required for the slurry, heat exchangers and pressure letdown vessels can be used to recycle the heat, which makes the principal UCC process an attractive option.
UCC Energy regenerate and reconcentrate spent reagents in the Cessnock plant.
Depending on the amount of wash water used, the energy required for reconcentrating can be extremely large, and may be more than what is required by the main process operations.
Alternatives for reconcentration include evaporation by steam, which consumes a lot of energy, solar evaporation which demands a large surface area or possibly membrane technology.
Using UCC and current status
Without details published it is difficult to analyse and compare the statement made by UCC Energy claiming to have achieved positive results for trials directly firing UCC into gas turbines.
Opportunities have been identified that show significant benefits from using UCC, however further published research is required to confirm this. Production costs and environmental implications are two areas that need further evaluation.
Studies show that sequential leaching using specific reagents predominantly dissociate all minerals typically found in coal. Although a maximum particle size of 500 µm is adequate for mineral dissolution, further research is required to confirm this. Studies investigating negative and positive effects on the carbonaceous matrix are also vital for the development of UCC. Further investigation on the applications of UCC is also required to persuade companies that UCC is a superior substitute.