In an ideal world, the need to store coal at various points along the coal supply chain would be kept to a minimum. Rather it would move efficiently from mine to combustion with the least number of stops in between. But back in the real world and the need to store coal – whether in a silo, stockpile or storage dome – is a day-to-day reality that arises for two principal reasons:1
- To serve as a buffer between material delivery and processing, acting as a strategic stock against short- and long-term disruption.
- To provide opportunity to blend different coals to meet specific end-user requirements.
Coal storage can occur along the whole coal supply chain from the mine to the preparation plant, transhipment facility and end user site (such as a power plant, coking plant or cement plant). The vast majority of these coal storage facilities are open stockpiles, which can vary in size from a few thousand tonnes to the 4 million t plus capacities at the large coal export terminals at Richards Bay in South Africa and Port Waratah in Australia.
Where open coal storage is problematic, such as in regions that experience heavy tropical rainfall or near to population centres, stockpiles can be covered – for example in a storage dome or silo. But this is not without its challenges, explained Francisco Castano, CEO of storage dome designer, Geometrica: “one, coal stockpiles are often very large; two, coal is stacked in amorphous and changing piles with moveable stackers; and three, interior particulate matter is hazardous to respiration and conflagration.”
Beyond the practical reasons for storing coal, the amount of coal at storage facilities is also a good indicator of demand with higher stocks a potential indicator of weaker demand. For example, in March 2015 – the latest month for which data was available at the time of writing – coal stocks in the US increased 3% on February’s figures to reach 156 million t, according to the US Energy Information Administration (EIA). This bucked the usual winter trend, which would expect a drawdown of coal stocks on higher demand for electricity.
“This increase in coal stockpiles occurred because the country experienced significantly above-average temperatures in March 2015, which led to a decreased demand for heating and thus, a decreased need for electricity generation,” explained the EIA.2 And more critically for the industry: “Coal has also lost market share to natural gas in all regions of the country.”
Similarly, it was recently reported that stockpiles of coal at Indian ports had reached record levels on the back of weak buying by Indian utilities. “Coal stocks have soared not just in India but also in international ports,” Dipesh Dipu of energy and resources consultancy, Jenissi Management Consultants, told The Economic Times.3 “There is huge oversupply in the overseas markets, resulting in a lot of coal finding its way to the Indian market. However, the demand in India has not grown commensurately. Industrial growth is not as high as we would expect it to be and also supply in domestic market has picked up well.”
Coal storage and blending
“With the opportunity for electric utilities to buy coals of different qualities from a wider range of suppliers and to take advantage of cheaper prices on the spot market, there is a greater need for coal blending capability in stockyards,” said an IEA Clean Coal Centre report on coal transport, storage and handling back in 2003.1 Over a decade on and this remains the case – perhaps even more so in regions such as Europe, where coal-fired power will need to become increasingly flexible to cope with the challenge posed by a boom in renewables generation.
“It’s expected that the current [coal-fired power] market will consolidate and more blends will be required,” agreed Kai Winkelman of Rietlanden Terminals at the port of Amsterdam. “Terminals will be more and more used as back up storage facilities with an on-demand logistics model. Flexibility will be a key factor. With a throughput of approximately 8 million tpa, Rietlanden Terminals has become one of the leading blending terminals in ARA [and] we expect that our activities will continue with potential growth on the blending operations.”
It was a message echoed by Sander van der Veeke at OVET dry bulk terminal: “OVET is a niche market player and offers a wide range of value-adding activities, such as screening, crushing, blending and washing. We believe that these activities will become more and more important and have therefore invested in related equipment over the last few years.”
What advantages does blending provide? The IEA CCC report notes three broad areas in which it can offer advantages to utilities:
- It can help to reduce costs by allowing the use of cheaper and low-quality coals.
- It can help to meet emissions limits e.g. by allowing the blending of high- and low- sulfur coals to meet tightening sulfur emissions regulations.
- It can help improve combustion performance of coals by maintaining a uniform feed into the coal boiler.
With cost and environmental pressures increasingly at the forefront of debates about the future of coal-fired power, such advantages are never more relevant.
Challenges in coal storage
Moving to the more technical aspects of coal storage and there are a number of key challenges that face coal storage facilities, including:
- Spontaneous combustion (self-heating).
- Equipment maintenance.
Spontaneous combustion results from the self-heating properties of coal during oxidation – and it is a particular risk where large quantities are stored for an extended period. It not only has obvious impacts on safety but also poses significant environmental, economic and handling problems if it becomes established in stockpiles.1 For example, even without the risk of fire, heat-affected coal may become unsuitable for its intended purpose, making prevention and early detection of spontaneous combustion of huge importance.
“The safe management of coal piles and silos is the key challenge that faces coal storage facilities,” explained Derek Stuart of AMETEK Land. “Top priority is operator safety, but protecting a plant is also very important. Preventing and detecting spontaneous combustion can help with both goals”. There is a range of equipment to help detect coal heating – AMETEK Land offers thermal imagers, infrared line scanners and carbon monoxide (CO) detectors, for example. But it is CO detectors that are often preferred as they provide the most advanced warning periods4 – and in the case of coal fires, detection is definitely better than cure.
According to the IEA CCC: “Stockpile fires are difficult to extinguish […] The cost and effort involved in monitoring stockpiles is generally small in relation to the value of the coal within it. Where there is an indication that coal heating is underway, the hot coal should be removed and cooled down. A rapid response in the event of a fire will prevent it from spreading.”
Coal silos and bunkers present particular challenges when it comes to spontaneous combustion. Long-term storage in these enclosed structures allows the possibility of air movement – providing ideal conditions for accelerated self-heating.
To avoid this, coal silos from Eurosilo, for example, include not only various sensors – including CO sensors – but also active airflow prevention to form a key part of the company’s fire system. All Eurosilos are also equipped with a nitrogen-purging system, enabling operators to purge either a specific section or the entire silo. Finally, a gel fire-retardant, FIRESORB®, can be sprayed to cover the coal surface in case of a coal-heating emergency.4
Beyond spontaneous combustion and the creation of dust is another key challenge of coal handling and storage – particularly in areas close to population centres, as is the case at Port of Amsterdam, which recently took part in extensive talks with stakeholders regarding the current coal lightering facility. “In December 2014, an agreement was reached, which will allow an increase in lightering to 4.5 million tpa,” explained Femke Brenninkmeijer, a spokesperson for the port. “Part of the agreement is the improvement of cranes in term of environmental performance and working on other ways to reduce noise and dust emissions during lightering operations.”
A similar point was made by Rietlanden’s Winkelman: “We operate under the strictest environmental regulations due to recent permitting and proximity to the city [of Amsterdam]. Our terminals are surrounded by units that measure fine dust concentrations in the air, while software has been specifically developed for mapping dust concentrations and additions in the terminal area, taking into account wind direction and speed.”
Operational measures Rietlanden takes to minimise dust creation at the terminal includes restricted speed on machinery, closed grabs, a sprinkler system on the hoppers and conveyor belts and continuous water spraying across the terminal.
Ultimately, a coal stockpile can be covered to prevent release of excess dust, such as with Geometrica’s freeform domes, which can span distances of 300 m in circular or free-style formations. But even then, the issue of dust must still be dealt with.
“The Geometrica structural system comprises round tubular members, which present almost no ledges or areas on which coal dust can accumulate,” said Castano. “And cladding may be attached to the inside of the structure to eliminate coal dust accumulation on the structure. Forced or gravity ventilation is available for full compliance with occupational health and safety requirements.”
“Structures that have been run in the harsh conditions, as found in a coal handling facility, cannot be expected to perform forever” noted Gerry Lynskey, Principal at SSI Consulting. “Conveyor support structures and trusses, transfer towers, preparation plant structures, equipment support platforms and storage silos all deteriorate with time. These structures are durable but not permanent.”
Lynskey is a strong proponent of proactive maintenance – including regular inspection routines – for equipment at coal handling facilities to help to mitigate the impact the harsh environment has on machines: “Some in the coal industry took a proactive stance towards maintenance and reliability and many silos are still in use due to the decision early on to implement a silo inspection programme. Many of these companies are still around today partly because of this mentality to make informed decisions for the future.
Such regular inspection routines can also help when regulators come calling, as Dennis Blauser of Marietta Silo pointed out: “A silo inspection programme, which may lead to silo repair, can assist in keeping [regulators] from issuing decisions to limit or stop the use of a silo, especially where inspectors, who may have an untrained eye, believe a safety problem is present. In this case, a recent inspection or repair may prove the silo is usable.” “When faced with an enquiring government inspector, showing that the plant owner already knows the condition of their facility can go a long way towards reducing possible orders, citations and fines,” agreed Lynskey.
“Many in the US talk about the nation’s infrastructure problem – crumbling bridges, overextended water systems, antiquated electrical grids – but an often overlooked issue is the problem with the world’s aging industrial infrastructure,” concluded Lynskey. “Several facilities have experienced failures and many facilities are in need of repair. The coal industry should, once more, take the lead in implementing maintenance and reliability programmes, including the inspection of existing facilities and planning for the future. Without self regulation, it is possible that government agencies will hand down a mandate requiring specific inspections within rigid timelines.”
The demands of end-users, regulators and local residents – as well as the challenges inherent with the coal itself – make coal storage a far more complicated process than it may seem on the surface. And these challenges are likely to increase over time, as the quality of coal gets poorer, while the environmental restrictions on its transportation and use get tighter.
This means coal storage facilities must keep moving forward, adopting the best technologies and approaches. As Port of Amsterdam’s Hoolwerf said: “We put great effort into expanding the coal handling capacity of the port. The completion of the new sea locks in 2019 will allow the port to handle bigger ships; however, we do not sit around waiting for construction to finish. We will not stop investing in facilities and partnerships, all in the most sustainable and efficient way.”
- CARPENTER, A., PORTER, D., SCOTT, D.H., and WALKER, S., ‘Transport, Storage and Handling of Coal’ (IEA Clean Coal Centre; 2003).
- ‘Electricity Monthly Update’ (US Energy Information Administration; 28 May 2015).
- SINGH, S. ‘Imported coal stocks pile up at a record high of 16 MT as power firms stop spot buy’, The Economic Times (15 June 2015).
- GEIJS, C. and RUIJGROK, J.P.J., ‘Safe Storage’, World Coal (July 2014), pp. 44 – 48.
For sharing their expertise and experience with me, my thanks go to: Derek Stuart, AMETEK Land; Francisco Castano, Geometrica; Dennis Blauser, Marietta Silos; Sander van der Veeke, OVET; Mark Hoolwerf, Port of Amsterdam; Kai Winkelman, Rietlanden Terminals; and Gerry Lynskey, SSI Consulting.
Written by Jonathan Rowland. This article first appeared in the September issue of World Coal.
Read the article online at: https://www.worldcoal.com/handling/10092015/under-the-storage-dome-2838c/