The Missing Link in Clean Coal Technology: The Proper Use of Waste Coal as an Environmental Measure
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Dr. Mike Clarke, CPEng, FIEAust, MAusIMM, RPEQ
(Formerly) Senior Lecturer, Environmental Engineering, Griffith University, QLD
CEO, M.E.T.T.S. Pty. Ltd.
Email: metts[at]metts.com.au
Published in: The Bulletin, AusIMM, No. 2, March/April, 2003.
Synopsis
Mine rehabilitation that includes the reduction in carbonaceous material going into overburden and pit reclamation material should be looked as a serious environmental activity and one that has major local and regional environmental consequences. Un-recovered and wasted coal, carbonaceous pit wastes, coal washery rejects and carbonaceous shale have energy values that in most cases can be utilised given the right technology and economic incentives. The carbonaceous material contained in overburden can have significant environmental consequences that include, the leaching of sulphuric and sulphurous acid from the overburden, the emission of toxic VOCs that include aromatic compounds from spontaneous combustion in the wastes, the physical destabilisation of rehabilitated land through spontaneous combustion coupled with the loss of vegetation and mineral/rock decomposition, and the emission of CO2 from slowly oxidising coal and organics without the creation of useful energy.
Mine mouth coal waste fired power stations provide an environmentally friendly solution as to how a coal mine can reduce the fuel value of its wastes to make them more benign regarding spontaneous combustion and acid leaching. An example of such an operation already exists at Redbank New South Wales. There are however many more opportunities that exist for such power stations around Australia. The engineering, environmental and economic aspects of such power stations are examined in this paper.
1. Introduction
The United States Government through its Clean Coal Technology Programme as propagated through it Department of Energy [1] and the Australian Government's Clean Coal Initiative as funded through the Co-operative Research Centre for Coal in Sustainable Development [2] has considered numerous ways of allowing for the use of coal with decreasing environmental impact. The campaign in the US has a long history, whilst in Australia has been subject to stops and starts, but now seems to be truly underway.
Clean Coal Technology can mean many things to many people and groups. It includes:
· propagation of technologies that produce more useful electrical output per unit input of coal (by increasing conversion efficiencies),
· the introduction of end of pipe solutions to clean up emissions,
· the introduction of carbon sinks to remove CO2 produced during coal combustion,
· mitigation measures for acid produced during coal mining and storage and
· management systems for mine ash.
All of the above technologies and activities will help to continue the public's acceptance of coal as an important fuel and energy source. It will in time allow for the introduction of technologies that will convert coal to liquid fuels, where these fuels will be needed to replace the dwindling traditional petroleum resources.
2. The Missing Clean Coal Technology
One area of coal utilisation that could be further 'cleaned-up', and usually does not appear on the Clean Coal Technology horizon, is improved pit and mine rehabilitation. In particular the rehabilitation that includes the reduction in carbonaceous material going into overburden and pit reclamation material should be looked as a serious environmental activity and one that has major local and regional environmental consequences.
Un-recovered and wasted coal, carbonaceous pit wastes, coal washery rejects and carbonaceous shale have energy values that in most cases can be utilised given the right technology and incentives. The carbonaceous material contained in overburden can have significant environmental consequences that include:
· the leaching of sulphuric and sulphurous acid from the overburden,
· the emission of toxic VOCs that include aromatic compounds from spontaneous combustion in the wastes,
· the physical destabilisation of rehabilitated land through spontaneous combustion coupled with the loss of vegetation and mineral/rock decay, and
· the emission of CO2 from slowly oxidising coal and organics without the creation of useful energy.
Coal is not toxic to the environment in the sense that it or its constituents are not directly absorbed into organic systems to any major degree. Coal that is mined or exposed during mining but not utilised can give toxic products where is subjected to slow and air-starved combustion. Coal wastes can be seen burning around Ipswich, Queensland, the Lower Hunter, New South Wales, the Collie, Western Australia and Leigh Creek, South Australia. In Ipswich attempts to mix coal wastes that are subject to spontaneous combustion with sewage sludge to produce a 'bio-solids' for garden use have resulted in a steaming cocktail of tars, liberated sulphur and other ooze, as observed by the author.
Coal washeries, that are part of the clean coal technology scenario, in that they provide a 'cleaner' product for use in power generation or steel making, produce a carbonaceous waste that often has a very useful energy content. The washery wastes have in some cases been reworked (rewashed) to recover lost coal values, but again a carbonaceous waste is still produced. At the Redbank operation [3], Hunter Valley, New South Wales, washery rejects are used for power generation. Redbank has installed a version of the missing link in CCT.
3. Management and Technology Considerations for Coal Wastes uUsage and Mine (Pit) Clean-Up.
The collection, use and management of coal wastes to produce useful products are the key to this CCT. The products will be electrical power and an environmentally benign ash. The carbonaceous material produced during mining and coal washing that cannot be marketed as product can be used in mine-mouth power stations, as with Redbank. The concept is not new and has provided coal mines with a cash flow and a destination for marginal coal and washery middlings in the past. What is new is the use of a mine-mouth power station as an environmental tool.
The use of coal wastes can further be seen as a Risk Management activity. Where there is the possibility of spontaneous combustion or leaching from wastes there is an environmental hazard. If this hazard comes to the notice of the public or the regulatory authorities then there is a corporate risk.
The technology has to be flexible, in that the fuel to be burnt will vary in energy content, hardness characteristics, water content, handling performance and flame properties in the combustors. At Redbank the choice of combustion technology was Circulating Fluidised Bed, as provided by Alstom Power. Circulating Fluidised Bed Combustors (CFBC) are a forgiving and user friendly technology that has found a place in many waste firing systems. It burns at a significantly lower temperature than Pulverised Fuel (pf) systems, typically 850°C as against 1200°C. It produces virtually no NOx since the flame temperature is too low for NOx formation, and can capture sulphur given the addition of limestone absorbent.
CFBC's disadvantage is that it typically has a lower conversion efficiency that pf stations, and hence will produce greater CO2 per unit of fuel, with around 900 kg CO2 /MWh being produced by modern pf stations and 1100 kg CO2 /MWh being produced CFBC stations. This disadvantage is however offset by considering that much of the coal wastes that would have been included in the overburden would have slowly oxidised to produce CO2 without the production of any useful energy.
The delineation of what is marketable coal, as against pit-waste, carbonaceous shale and other possible fuels that could be aimed at the mine-mouth power station, needs to be a management priority to make the system economically viable and environmentally useful. A full geological survey of the mined measures with energy content being measured over the entire strata would be needed.
5. Some Costs and Economics
The cost of fuel to the combustors should be as low as possible to make the systems viable. The cost should include:
· the cost of delineation and separation of non-marketable combustible material contained in the mined measures,
· the cost of picking up and transporting the non-marketable combustible material and pit wastes to the combustors,
· the cost of separating washery rejects that cannot be reprocessed/rewashed and need to be directed to the power station,
· the cost of storage of the wastes before combustion and
· the management required to achieve the above.
The size of the power stations will not be large. Redbank is 151 MWe , whilst a project that is being proposed for a Queensland pit by the author is 125 MWe, for a mine saleable output of 2 m tonnes. For the Queensland project, the combustors would require around 700 kT of wastes per year, at around an SE of 12- 15 MJ/kg. The capital cost of plant and the running costs (exclusive of fuel) will be greater than a large base load pf plant, with the margin being budgeted at around +20%.
The CFBC technology as stated is user friendly, and is tolerant of small glitches in fuel feed. It can be brought back on line after a stoppage of say an hour or more quickly, say 30 minutes. It is however a technology that is essentially a base or intermediate load system, and as such will be offering power to the grid as a base or intermediate contribution.
Under the present rules for 'green power' mine mouth coal waste fired power stations do not qualify for pricing concessions or support (as would a wind turbine for instance). To make mine mouth coal waste fired power stations a common site some concession to their environmental benefit will be required. That concession may simply be that their output is given a higher priority for taking than other producers that do not have an environmental advantage. It may also be that coal mines that install such units are given a taxation holiday during power station construction and commissioning, that is equal to the interest cost during that period.
Another product of the power stations will be a biologically benign ash. This ash could be used for mine site rehabilitation or as a sterile agricultural material. It may further find uses in the manufacture of 'earth' based goods such as bricks and pavers [4]. Marketing opportunities for the ash and its products should be examined in the overall economic analysis of the power station's operation.
6. Establishing Mine Mouth Coal Waste Fired Power Stations into the Future.
Mines and groups of mines that have reasons, such as spontaneous combustion in overburden or acid mine drainage, for introducing mine waste collection and use, should consider approaching local utilities for incorporation in their future power production programmes. Given that power planning is now largely decentralised, utilities will need to compete on the market with their product from the mine mouth coal waste fired power stations. They will need long term guarantees of availability of fuel (coal wastes), site provision and management stability.
The size of power station will firstly be dependant on the availability and type of fuel. The second factor that will determine the size of plant will be opportunity (need) for power supply into the grid or to captive users. The economics of power supply must favour larger plant, in that management and infrastructure costs will be similar for a 50 MWe or a 150 MWe plant. Foster-Wheeler [5] report that they have supplied CFBC power plants ranging 6 - 110 MWe into China that burn coal, coal wastes and assorted other waste fuels. Alstoms [6] state that they have 'sizes (of CFBCs) from 10 to 400 MWe. Other suppliers can likewise produce plant of practically any size for any fuel.
The economic size in Australia however is likely to begin at 100 MWe, and perhaps go as high as 300 MWe (2 x 150) for a very large mining operation. A large mining operation that could be producing around 1.5 million tonnes of coal wastes, marginal coal and washery rejects, with an average SE of 15 MJ/kg would be able to maintain supply a 300 MWe power station.
Governments can be approached to provide favourable conditions for mine mouth coal waste fired power stations. The argument for asking for such conditions should be that the stations are an environmental asset and that their introduction will extend the life of the total fossil fuel resource, in that they will reduce the necessity for further generation monsters such as the 840-MWe Millmerran power station, Queensland [7]) and its dedicated coal mine in the future.
7. Other Approaches to Minimising Environmental Damage from Coal Mining.
Graham Taylor [8], the Co-ordinator Mining Environmental Research, CSIRO Environmental Projects Office, Australia, came to the conclusion that the best way to reduce impacts from coal mining was to reduce the scale of coal mining and make it less mechanised. The reduction in mechanisation may well allow the better recovery of carbonaceous pit wastes but it would not allow for coal mining and hence carbonaceous waste production to be on such a scale that the use of the wastes for power generation would be worthwhile.
The use of small scale mining with low levels of mechanisation produced the smouldering coal waste piles of the Ipswich and Hunter Valley regions. The use of the 150 MWe coal waste fired station that has been installed at Redbank and the 100 MWe (Foster Wheeler) coal waste fired station that has been installed in Sichuan China rely on the availability of around a minimum of 400 kT of coal wastes per year. Such wastes will not be produced by small scale mining without messy aggregation of the waste product of small holdings, which is not a environmentally or economically sensible procedure.
8. Best Practice Environmental Management (BPEM)for Mining.
Environmental Australia (EA) has produced considerable documentation for good environmental management practices for the mining industry, under the title, Landform Design for Rehabilitation. In case studies reported by Environment Australia, the Drayton/Shell [9] open cut coal mine gives the following Best Practice Principles for avoiding spontaneous combustion:
· Define all fuel sources, ensuring the correct placement of carbonaceous materials,
· Minimise the quantity of fuel (carbonaceous materials) going to spoil,
· Reduce oxygen pathways in spoil piles.
· Avoid dumping carbonaceous or hot materials over dump batters and
· Prevention is better than cure.
In another EA publication Landform Design for Rehabilitation [10], the principles for the design of final voids includes the following statement, 'In open-cut coal mines, it will be necessary to cover exposed coal seams with several metres of inert material to prevent ignition either from spontaneous combustion, bushfires or human interference.'
The two references to Best Practice Environmental Management strongly support the introduction of mine mouth waste fired power stations, for the best way to reduce the quantity of fuel (carbonaceous materials) going to spoil is to remove it from the mine and use it. Further by defining all fuel sources, carbonaceous material will be less likely to find its way into material used for rehabilitation.
The necessity to cover exposed coal seams with several metres of inert material, could best be achieved by using the ash from the combustors as the inert material. Further compacted fine ash will help reduce oxygen pathways, and thus further limit the opportunity for spontaneous combustion.
9. Conclusion
Mine mouth coal waste fired power stations provide an environmentally friendly solution as to how a coal mine can reduce the fuel value of its wastes to make them more benign regarding spontaneous combustion and leaching, and thus making the overburden 'safer' for use in mine site rehabilitation. The introduction of such power plants would thus be another means for the coal industry to improve its environmental image and in effect be a Clean Coal Technology.
Mine mouth waste fired power stations can provide a secondary cash flow from power sales as well as a stable ash product for use in mine site rehabilitation. Further by utilising carbonaceous material that would otherwise be going to waste, coal mines with mine mouth generation plants would be contributing to the better utilisation of the total fossil fuel resource, and thus would be effectively extending the fossil fuel resource that is available for human use in the future.
10. References
1. US Dept of Energy. Clean Coal Technology Program-The Investment Pays Off.
A Report by the Assistant Secretary for Fossil Energy, November 1999, Office of Fossil Fuels Publication.
2. CRC for Coal in Sustainable Development. http://www.crc.gov.au/centres/mining/coal.htm
3. Redbank Power Station. http://www.redbankpower.com.au/
4. The Beneficiation and Utilisation of Rejects from Coal Preparation plants. Norm Stockton. Unpublished PhD Thesis, UNSW 1982
5. Foster Wheeler/China CFBC. http://www.fwc.com/publications/tech_papers/powgen/china.cfm
6. Alstoms CFBC. http://www.power.alstom.com/
7. Millmerran Power Station. http://www.intergen.com/australia.html
8. Pollution Control and Mine Site Rehabilitation in Surface Coal Mining. Graham Taylor, 3rd APEC Coal Trade and Investment Liberalization and Facilitation (TILF) Workshop, 7-9 November, 2000, Hanoi, Vietnam
9. BPEM/Mining Drayton/Shell, Case Studies http://www.ea.gov.au/industry/sustainable/mining/booklets/landform/cs.html
10. BPEM/Mining Part 6, Final Voids http://www.ea.gov.au/industry/sustainable/mining/booklets/landform/cs.html
Appendix: The Redbank II Controversy
Newcastle Herald, General News, page 14, Monday 27 October 2003
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