What exactly does clean coal mean? We know that mining coal creates dust and releases methane gas. We know that burning it liberates CO2 and emits particulates including SO2 and NOx. And we know that it creates wastes that are often stored in unsightly and potentially dangerous slag heaps and lagoons. When clean coal technology is discussed, it sometimes seems that creating modern or “ultra super critical” coal-fired power stations is presented as the main or even the only answer. I argue for a more holistic approach.
Using the energy potential of coal is always going to have environmental impacts. Some of these will be negative. But the need to secure future energy resources demands that we tackle these negative issues, and find ways in which they can be minimised and offset.
Worldwide experience from other industries tells us that the development of clean coal technologies needs to embrace the entire coal supply chain. And in order for coal to remain an acceptable component of any nation’s energy mix, we need to make more progress in all areas — from mining and transportation to infrastructure, energy distribution, and the controlled utilisation of abandoned coal mine space.
Innovations such as automated longwall mining equipment, modern coal washing facilities to reduce waste transportation and industrial coal gasifiers can all play their part. Coal mine rehabilitation after closure is also important — including environmental levies and funds covering closure and post-closure use of land.
In addition, we are at last seeing the emergence of new clean coal technologies and approaches that can tackle head on some of the biggest issues facing the international coal mining industry. This article looks at three of these development areas:
- Methane control and utilisation
- Underground coal gasification
- Carbon dioxide sequestration
Methane: Deadly enemy or untapped resource?
Methane has been the deadly enemy of coal miners for generations. Colourless, tasteless and odourless, it can be hard to detect. But it’s explosive at a concentration of between 5 and 15 per cent in a normal air mixture — and through gas outburst and explosion, methane remains the number one killer of coal miners in countries such as China.
Effective methane drainage can improve safety by reducing seepage and preventing the accumulation of explosive concentrations into the underground working environment. However, at higher concentrations, more than 30 per cent, it can be burned safely as a direct fuel. Coal mine methane gas is chemically the same as the gas that we burn every day in our kitchens and that we otherwise know as natural gas.
In addition to its immediate human danger, methane is between 20 and 30 times more potent as a greenhouse gas than CO2, and is therefore a significant contributor to global warming. Currently, some 90 per cent of coal mines exhaust methane directly to the atmosphere. An average sized mine (producing one megatonne of coal per year) circulates about 10,000 cubic metres per minute of ventilation air with a methane concentration of about 0.3 per cent. That’s some 30 cubic metres of methane per minute, or almost 16M cubic metres of methane per year, or between 300 and 500M cubic metres of CO2 equivalent.
As well as posing a hazard to mining operations and contributing to harmful emissions of greenhouse gases, this is also a wasted energy resource. Effectively harnessed, coal mine methane can be used to generate clean energy by capturing and utilising it in power generation units located at the mine.
So there are very clear safety, environmental and financial reasons to deal more effectively with methane — and they apply in a number of different areas:
- Coal mine methane capture (CMM): This most usually relates to the capture of methane from operating underground coal mines
- Coal bed methane capture (CBM): The collection of methane from unworked coal seams, usually from the ground surface
- Abandoned coal mine methane capture (ACMM): The collection of methane from abandoned coal mine workings
- Ventilation air methane (VAM): The capture and destruction of the low-concentration methane in mine ventilation air
Let’s look first at coal mine methane capture. A well designed CMM collection system can capture between 40 and 60 per cent of methane generated by underground coal mining operations. Drainage from in-seam mining of coal prepared in advance of mining is often used, and improvements in technology (especially in China) include large-diameter, long-hole (600-1000-metre long) gas drainage wells from the back of the panel. These are usually located 20 to 30 metres above the roof of the coal seam to intercept the relaxed zone as the coal face retreats. Gas drainage drifts excavated from a gas drainage gate road have also shown very good results.
Specially designed water ring vacuum pumps are used at the ground surface to draw the CMM from underground boreholes via gas ranges. Storage tanks at the ground surface help to supply CMM to small (0.5 to 2.8-megawatt) portable gas engines at a consistent rate, pressure and methane concentration. Alternatively, steam boilers can use the methane to generate hot water for use in the mine or to supply district heating.
The cost of CMM mainly involves drainage costs and investment for utilisation. Where CMM drainage is a compulsory requirement for mine safety, investment in drainage equipment can be regarded as a mining cost. But there are further revenue opportunities too — because the United Nations Framework Convention on Climate Change (UNFCCC) under the Kyoto Protocol allows developing countries to benefit financially from the introduction of technologies that reduce greenhouse gas emissions through the Carbon Credits Scheme. CERs are sold in Europe at between 10 and 15 Euros per tonne of CO2 equivalent that’s destroyed or utilised for power generation. Given the huge quantities of emissions we looked at earlier, the math is easy.
Unlike CMM, coal bed methane (CBM) is the process where methane is extracted from unworked seams, where it’s either adsorbed onto the surface of the coal or held in minute pore spaces within the coal. The amount that can be extracted depends on many factors including the age and rank of the coal, its permeability and the orientation of the extraction borehole relative to the fractures or cleats in the coal. In some countries, the CBM reservoirs are much larger than similar sized conventional gas reservoirs and therefore represent highly valuable reserves. It has the potential to apply to vast areas of coal which have been closed down or are unworked because they’re too deep or uneconomical, or lacking in infrastructure.
Conventional boreholes are drilled vertically from the ground surface, although when they intersect the target seams, the holes can be directionally drilled using advanced surface to in-seam drilling to increase the contact area between the CBM borehole and the coal. Gas capture from relatively low-permeability coal is enhanced by hydrofracturing techniques. The high-pressure water used to fracture the coal is often supplemented with sand to keep pathways open to facilitate the capture of the gas. A fully commissioned headwork typically includes a “nodding donkey” to pump out groundwater within the CBM borehole. This reduces groundwater pressures and stimulates gas movement towards the well, allowing the CBM itself to be collected from its own gas pressure. It can then be processed for direct input to domestic and commercial gas supply, or used to generate electricity, or used to generate liquefied natural gas.
Similarly, abandoned coal mines can be a rich source of high-quality methane (ACMM). Relatively few boreholes can tap into large sealed reservoirs of methane, allowing extraction from existing gas vents, vent wells, gob wells or CBM wells.
And yet even when active coal mines use advanced CMM drainage systems, they can only collect about half of the methane generated by mining operations, with the other half still released through the mine ventilation system. This results in large volumes of methane still being emitted as greenhouse gases. Ventilation air methane (VAM) is therefore gaining great interest for environmental reasons, with VAM destruction achieved through high-temperature oxidation technology.
Although VAM is available in large volumes, methane concentration in ventilation air (typically between 0.2 and 0.5 per cent) is too low for use in conventional gas engines or turbines, with the energy output from the methane oxidation at best sustaining the process once in operation. However, it’s still eligible for financial support through the generation and sale of CERs. Some processes (EesTech) supplement the VAM with other low-grade fuels such as coal waste and biomass to achieve low levels of power generation.
Underground coal gasification
With many remaining coal seams lying at depths not considered recoverable by underground mining, or under the sea, or in locations that are difficult to access, it’s estimated that perhaps as much as 90 per cent of the world’s coal resources are currently uneconomical to mine conventionally. So perhaps it’s not surprising that there’s been a very noticeable recent upsurge in interest in this emerging technology in places as far apart as Australia, China, Russia and CIS, Bulgaria, South Africa, the UK, Hungary and North America.
As a means of producing gas via controlled underground combustion without extraction, and converting coal deposits to energy in a clean and environmentally efficient way, the prize is potentially vast. The product of the process — synthesis gas or “syngas” — is also very flexible. In addition to being burned in a gas turbine to produce electricity for power generation it can be used for a wide variety of processes including the production of hydrogen, methanol, ultra-clean synthetic transport fuels, and ammonia for fertilisers. This gives operators the flexibility to target different processes for the best commercial return.
The environmental and economical advantages of UCG compared to conventional mining are many. Overall emissions are estimated to be much lower, the upfront capital requirement can be up to 90 per cent lower, and the overall cost only around half as much. It occupies a much smaller footprint, has fewer safety risks with little or no need for people to work underground, and can be increased in capacity simply by drilling additional bore holes and adding modules to the surface plant.
If it all sounds too good to be true, UCG does have risks — and while there have been many successful pilot projects, there have also been a number of failures. These have mainly resulted from losing control of the underground combustion process, and/or from contamination of groundwater by the byproducts of the underground combustion. The improvements in drilling technology and greater understanding of the UCG process are reducing the first risk, while detailed studies of the regional hydrogeology are needed to mitigate the second.
While low natural gas prices during the 1990s restricted development in Europe and America, successful commercial scale UCG projects in the former Soviet Union have been in operation for many years, using conventional vertical boreholes in relatively shallow coal seams of 200 to 300 metres deep. Today, the adoption of advanced drilling technologies from the oil and gas industry (notably steerable surface-to-horizontal in-seam drilling) combined with higher natural gas prices is creating a resurgence of interest.
In simple terms, UCG requires the development of two boreholes to intersect within a target coal seam. The coal is then ignited by pumping air and steam (sometimes with additional oxygen) into the coal seam. The products of this combustion are hydrogen, carbon monoxide and carbon dioxide, which are collected at the second, recovery borehole.
But where UCG gets really interesting is in connection with other coal technologies. In particular, the relatively simple processing of the syngas that’s produced enables the CO2 to be completely removed — and sequestered.
Carbon sequestration captures the carbon dioxide formed by converting carbon-based fossil fuels to energy — and then stores it in a way that can cause no further harm to the environment instead of releasing it to the atmosphere as a greenhouse gas.
The UCG process lends itself well to CO2 capture, as do other clean coal technologies such as surface coal gasification and coal-to-liquid technologies. Modern coal-fired power stations are also looking to incorporate CO2 capture technologies.
Finding locations for the long-term sequestration of CO2 is now a major issue. Paradoxically, fossil fuel resources and reservoirs can be amongst the best sequestration sites ––a fact recognised for many years by the oil and gas industry which has pumped CO2 into depleting underground reservoirs to stimulate production.
CO2 has a great affinity to attach itself to fossil fuels, greater even than methane gas. In fact, the sequestration of CO2 into coal seams is seen as a means of stimulating the recovery of coal bed methane. The cavities formed by commercial UCG following completion of UCG panels also have the potential to serve as a CO2 sequestration site immediately adjacent to the CO2 source.
Abandoned coal mines could play a similar role in storing CO2 and harvesting methane at the same time. Since CO2 has an affinity for coal that’s greater than methane, pumping it into abandoned mine workings has been shown to displace methane molecules for capture as ACMM whilst remaining underground.
Despite the race for sustainable energy, the world still relies heavily on coal. And even as the climate change agenda intensifies, the continued use of coal is inevitable for the foreseeable future. In our search for more benign and cost-effective clean coal technologies, it will pay us to take a holistic approach — and take advantage of everything we can.
Editor’s note: This was a guest commentary by Phil Shelton, an international mining specialist and technical director of Wardell Armstrong, an independent engineering consultancy specialising in mineral resource development and management. The firm helps clients to deal responsibly with issues like corporate and social responsibility, planning policy and environmental protection legislation, while also successfully maximising commercial and economic benefits. Its projects range from exploration and resource assessment of mineral reserves in central Asia to mineral estate management for UK landowners.