​How legacy underground coal mines could become part of the world's alternative energy infrastructure

Converting legacy underground mines into large-scale, low-cost bioreactors or for thermal energy storage has been a subject of two large-scale Canadian research projects in recent years.

One of these projects was a multimillion-dollar Carbon Management Canada project called Bioconversion of coal by enhanced engineering pathways into fuel products . It explored extending the life of coalbed methane wells by injecting nutrients to feed the microbes already present in the coal seam as a way to produce more methane.

“The conclusion to that research was that it gives you an initial effect, but it still doesn't access much of the coal, so I was part of a team looking at how we could get some value from this [research],” says Bruce Peachey, president of New Paradigm Engineering.

The problem was that as the microbes consumed the nutrients—sewage, fish waste or other biomass—they grew in number and plugged the flow channels between the coal seam and the wells. Peachey says that what was needed was much more surface area in which the microbes could grow without plugging off the means to harvesting the methane. This led to the concept of using underground coal mines.

“We went to a bioreactor conference in Sydney, Nova Scotia, and started talking to them about the 4,000 kilometres of [legacy] mine tunnels that they have under the ocean and all the coal that has been left behind while, in the meantime, they're burning coal from Colombia and the U.S. in a power plant sitting on top of the former coal mine,” Peachey says.

Sydney's coal mines are an ideal candidate for use as a bioreactor by sending sewage or other bio waste into the mine shafts. This would accelerate the conversion of coal to methane—a process that is happening there anyway by virtue of the microbes that live in the coal.

On March 23, Peachey will be presenting a roadmap for the development of bioreactors at a Petroleum Technology Alliance Canada information session.

He says this roadmap will address what technologies will still be needed to make coal mine bioreactors viable alternative energy sources, and what research still needs to happen to address some of the obstacles, such as concerns over groundwater contamination if sewage is pumped into underground mines (Peachey describes the coal to methane bio-conversion process as “self sealing”: as the microbe colonies expand, they close the seepage routes out of the formation).

But perhaps the biggest challenge is that many people think that since this technology has to do with coal, it shouldn't be developed.

“But we're basically converting coal in situ into methane, which doesn't give you the problems that coal has. We can also inject the CO2 and the waste heat, which actually helps the bacteria grow more,” he says.

Peachey has already talked to various communities in Alberta, all of which have legacy coal mines within their jurisdictions—Lethbridge, Drumheller, Edmonton and Medicine Hat—about potential bioreactors.

“Right now, they can’t justify direct involvement but want us to keep them advised,” he says.

“But Drumheller was the most interested as they will lose local jobs when coal power is shutdown.”

In Canada, a bioreactors will have to compete with cheap natural gas, but other parts of the world don't always have that luxury. This technology could work in a small mine in Africa just as it could a big mine like in Sydney, Nova Scotia or across the prairies, Peachey says.

“You can go to just about any small town in Saskatchewan and find a coal mine, turn them into a bioreactor and provide that town natural gas while treating its municipal waste.”

The other dimension that Peachey will be discussing at the PTAC event is the use of legacy mines for thermal storage. Lethbridge has coal mines beneath it and he says those could be used to store water heated by the sun in the summer for use in the winter for space-heating of municipal buildings.

“Springhill, Nova Scotia, is using an old coal mine as a geothermal energy source to run a few businesses and an arena among other things,” he says.

“These old mines are all over the place, but people see them more as a liability than an asset. The mine in Springhill goes down deep enough that the temperature is 65° C. You can actually draw quite a bit of heat up from it.

“That's also the ideal temperature for a bio-reactor. So if you could also pump some of this town's sewage down there, or waste biomass, or fish waste from the fishing industry, then we could supply energy to the whole town.”

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