Data from the International Energy Association (IEA) suggests that global coal production has increased from less than 4 billion t in the 1980s to 7.2 billion t in 2010. In mass terms, the growth has been overwhelmingly from bituminous coal, although metallurgical coal and sub-bituminous coals have also seen significant growth. International trade has maintained a similar growth, accounting for some 15 – 20% of global supplies, at least for steam coal; metallurgical coal trade is even higher. More coal is being mined and transported across the world than ever before.
In a new report from the IEA Clean Coal Centre, Paul Baruya raises some of the issues that may affect a tonne of steam coal as it moves along the supply chain from the mine mouth to the customer, discussing some of the potential confusion that can arise when looking at this subject.
If a customer wants a tonne of coal, the reporting procedure must ensure that a tonne of coal passes all the way down the supply chain. If it does not, then coal will need to be found further down the chain, but quantifying this is not straightforward. The report examines he losses experienced along the coal supply chain, which may result in a reduction in the quality or value of a consignment of, coal are examined.
Few papers are published and relatively little importance is paid to the losses that might occur throughout the coal chain in terms of reports and published materials. While there are likely to be other losses along the coal supply chain, some industry analysts admit there are few attempts to quantify them. Losses start at the mine, with either incomplete extraction of the seam, leaving coal in the ground, or over mining at the peripheries, or incorrect blasting of overburden rock, diluting the coal with excess waste rock, causing a loss in heating value of each tonne of extracted material. Different areas of the coal seam can lead to varying losses.
Losses at the minesite and in the preparation plant
Intentional losses such as the coal left in the supports for room-and-pillar mining can be substantial: in extreme cases up to 90% of a coal reserve, but typically more like 40%.
Possibly the largest source of mass loss in the coal supply chain is the preparation plant, removing mineral matter and inert inorganic substances that cannot otherwise be burned. These waste extractions lead to a cleaner more mineral free coal product, so called washery yield. These yields vary widely across the world, with yields ranging between 40 – 90% depending on the coal and the need for processing.
Coal washeries can account for 20 – 30% mass loss through the separation processes of mineral matter from the coal. Depending on the coal, the separation of fine coal can be a large part of this loss. Fine coal can be utilised separately or stored in settlement ponds, but is usually not a desirable material to transport with the coarse and intermediate coal fractions. It is possible that some 0.43 – 1.09 billion t of waste could be rejected from the world’s coal washeries, some of which is coal.
The actual mass of material that would yield a heating value from the world’s washeries is unclear, but studies done on fine coal (<0.5mm) suggest in the US fine discards alone could be 70 – 90 million tpa, equivalent to roughly 10% of the country’s saleable coal production.
Losses from coal handling and transportation
Maintaining a check on the mass of coal passing through the supply chain from mine to end user is naturally fraught with error, but efforts to track coal tonnage are easily done using stockpile surveys, weighing mass on conveyor belts during transit, or draft surveys of ships for seaborne traded coal. The accuracy of such measurement systems is high but still gives a small error of ±0.5 – 5%.
Losses in mass and heating value of coal due to spontaneous/self-combustion are small, perhaps 0.5% in some examples, but this is dependent on the conditions of storage, the access to air flow, and the residence time of the coal stock in its static state. The priority for this type of combustion during transit and storage is the hazard that is posed by the production of heat and carbon monoxide that can be potentially lethal.
Losses from spillage and dust can be sizeable if not properly controlled. Fully covered conveyor systems and enclosed storage depots can minimise dust loss during windy conditions. Similarly, dust loss from coal wagons from rail haulage can also be considerable. Dust can either be windblown, or washed away with rainfall. At best it is a nuisance; at worst, it is a hazard for both respirable health and leaching into the environment.
Dust loss and spillage occurs anywhere along the chain where there is exposure to weather, and especially during transit and transfers between two modes of transit, whether it is conveyor to conveyor, conveyor to ship, hopper or dumper truck to rail wagon and so on. Simple and cost effective methods of minimising dust include water spray, but surface moisture can increase by up to 4%.
In some countries, theft of coal is a problem. This makes production data difficult to reconcile with the actual supply of coal to end users. As such considerable amounts of coal can be lost in the statistics as losses or merely “statistical differences”.
Losses at the power plant
Finally, it is at the power plant where perhaps the greatest losses occur. In the first instance minor losses might happen at the milling stage, with stockpile reclaiming and conveyor transportation experiencing the same losses seen in the coal mine, and where the milling equipment can generate some losses in pulverised fuel. The power plant fuel preparation stage can lead to further rejects of shale and iron pyrites.
The largest loss occurs during the conversion of fuel through the process of combustion and conversion to electricity. The efficiency of the power plant can lead to losses of coal of 55 – 70% (in energy terms). In terms of mass, the waste ash and byproducts of flue gas cleaning equipment can be reused, so in effect, the residues of coal and the emissions are all that are tangibly left of the coal at the end of its journey along the coal chain.
Understanding mass loss of coal at the power plant is as varied and complex as attempting to understand it further upstream. Quantifying these losses is a difficult task – and possibly too small to be of concern. However, identifying the locations and events that occur to a tonne of coal from the seam to the power plant is more straightforward. Ensuring best practice in coal mining, preparation, transportation and finally in combustion should ensure efficient use of a depleting resource and longevity of the world’s reserves.
Paul Baruya is a coal market analyst at the IEA Clean Coal Centre. His full report is available from the IEA Clean Coal Centre Bookshop.
Read the article online at: https://www.oilfieldtechnology.com/drilling-and-production/21022013/losses_in_the_coal_supply_chain_iea_clean_coal_centre/