Posted on behalf of Samuel Lickiss.
In 1850, at the height of the Industrial Revolution, Britain employed 1.2 million people to mine 56.7 million tonnes of coal per year. By 2018, coal production had plummeted to 2.6 million tonnes. Britain’s coal-powered past may still have an important role to play in its green future where water collecting in the scars left by historic mining activity may provide a valuable source of heat with minimal carbon emissions. This week, a paper published in the Quarterly Journal of Engineering Geology and Hydrogeology by a team led by Gareth Farr from the British Geological Survey addresses the issue of what to do with Britain’s coal mines.
Figure 1: Llwynpia, South Wales, a colliery village built up around a coal mine.
Coal now only contributes a small proportion of Britain’s electricity production. In August 2020, a coal power station was turned on the first time in 55 days as a prolonged heatwave brought wind turbines to a standstill. Nevertheless, the urgency to move electricity production away from hydrocarbons and towards renewable sources in an effort to tackle climate change is the priority in domestic energy production.
Most of Britain’s coal mines no longer contain any coal; yet, below the towns and cities that sprung up around the coalfields, a labyrinth of tunnels excavated by generations of miners now lie abandoned and have been allowed to flood. Still, they could prove to be valuable, not because of the coal, but because geological forces have heated floodwater to rather balmy temperatures, and British Geological Survey researchers have concluded an ambitious project to map these temperatures across the country.
Figure 2: An exposed mine shaft in Leeds city centre. Many British settlements grew around coal mines during the Industrial Revolution. Often seen as a liability in modern age, they could prove to be a valuable source of energy. © L. Donnelly.
As we descend into the earth, the temperature increases at a constant rate called the geothermal gradient. Mapping these temperatures is a complicated task as researchers had to standardise data gathered from over 170 years of recordings yielded from a variety of methodologies.
Figure 3: Methods of measuring temperature in coal mines. 1 and 2, modern method of measuring the temperature of flooded shafts from the surface using geophysics; 3, historical measurements from dry, operational mines. © BGS, UKRI.
Data came from three main sources: 1) the Coal Authority’s borehole temperature profiles, 2) the British Geological Survey’s UK Geothermal Catalogue and 3) historical data from operational mines. Geothermal gradients across Britain vary where factors like the presence of radioactive elements in rocks affect temperatures in the crust. Researchers have found that, on average, temperature increases by 24.1oC per kilometre of depth with northern England enjoying geothermal gradients of more than 30oC per kilometre.
Figure 4: Predicted temperatures at different depths in Britain’s coalfields. Equilibrium temperatures are from unpumped mines. Researchers have found that pumping mines increases the temperature further. © Coal Authority 2020.
The water may be warm, but it’s probably not a good idea booking a spa weekend to your nearest coal mine. Mine water is typically a toxic cocktail of chemicals that poses a significant environmental risk. In January 1992, 50 million litres of highly acidic water discharged from the Wheel Jane mine in Cornwall causing environmental devastation to the surrounding fisheries and agriculture. The clean-up operation cost an estimated £20 million.
Bathing, no, but what about geothermal heat recovery? Among the many complications of standardising these data is whether the mine is being actively pumped of water or not. Researchers found that pumped temperatures are typically higher than in mines that are not being pumped, perhaps because water in pumped mines is being drawn up from much deeper in the Earth’s crust. This is supported by data from projects already established.
At the Dawdon colliery in County Durham, a scheme to heat 1,500 homes in the new South Seaham Garden Village is underway. Since the project began pumping water in 2010, temperatures are higher than temperature profiles measured between 2000 and 2008.
The Dawdon project is an open system, meaning the toxic water pumped to the surface is treated before being discharged into the environment. Closed-loop systems where the water is recycled may prove to be more productive.
With these new temperature maps and real-world data from Dawdon and a handful of similar schemes in the UK, researchers have produced a valuable tool that can be used to inform feasibility studies and policy decisions of generating energy from disused coal mines. As Britain continues its transition away from hydrocarbons and towards renewable sources, the contribution of the millions of people who toiled in Britain’s coal mines may continue to power the nation.