Tapping the methane
People have been pumping methane from Lake Kivu on a small scale for decades to make use of it for energy. But efforts ramped up seriously when KivuWatt, run by London-based ContourGlobal, began operation in 2016. The $200-million project is currently providing 26 MW of electrical power, and it has a contract to increase that to 100 MW. This will add considerably to Rwanda’s baseline installed grid capacity of about 200 MW.
For now, KivuWatt’s withdrawals are minor in terms of the lake’s stock: at the current rate of extraction, the company will remove less than 5% of the methane in the lake in 25 years. “For sure, this speed cannot be considered sufficient to really decrease the risk of limnic eruption,” says Francois Darchambeau, a limnologist at KivuWatt. “So, we need to expand to more capacity.” But expansion plans are on hold until electricity demand catches up with supply, the company says. KivuWatt is also considering options for removing CO2from the lake and selling it as a commercial product.
Meanwhile, Rwandan company Shema Power Lake Kivu has bought a tiny pilot plant, KP-1, that started pulling methane from the lake in 2006. The firm is currently constructing a facility planned to deliver 56 MW. The company’s website says it expects to have construction finished in early 2022, but Shema Power’s project director Tony de la Motte declined to answerNature’s questions about the plant’s schedule or details of its operation.
The general principle of all such projects is to pull up deep water so the methane bubbles out and can be purified and pumped to a power plant. The degassed water is then returned to the lake. Questions surround how best to do this; plans vary, depending on the company and the proposal.
The degassed water still contains high levels of nutrients and toxic hydrogen sulfide, so returning it too near the surface could kill fish and lead to harmful algal blooms, say some researchers. It is also salty and laden with CO2, making it relatively dense. So, if released into the lake at too shallow a depth, the degassed water would sink, potentially disturbing the main density gradient, 260 metres deep, that keeps the gassy waters of the resource zone trapped below. “It wouldn’t necessarily blow up, but it would be more prone to blow up,” says Morkel.
Pushing the main gradient upwards could also be problematic, because it would reduce the pressure on the gassy waters. And diluting the resource layer with degassed water might lower gas concentrations enough that commercial extraction would no longer be possible. If that happened, it would leave a lot of dangerous gas in the lake, with no good way to remove it other than venting it to the surface — an approach that could both release potent greenhouse gases and contaminate surface waters.
In 2009, an international group of researchers, including Morkel, Wüest and Schmid, published‘management prescriptions’(MPs) outlining best practices for extracting the lake’s methane. The majority of the experts favoured a strategy called the density zone preservation method, which involves controlling the density of degassed waters by managing the amount of CO2they contain, so they can be carefully returned to the lake without causing mixing. This is technically difficult to do, but would largely maintain the current structure of the lake.
KivuWatt opted for an alternative strategy, in which degassed waters are released just above the main gradient. This is simpler to accomplish and should avoid diluting the resource layer, but is expected to alter the structure of the lake.
Darchambeau says KivuWatt monitors the surface waters daily, and does weekly profiling to get a robust data set regarding the lake’s stability. He says that after five years of operation, the firm did start to see, as expected, a weakening of lake stability — but not by much. “If we pursue the gas extraction as we do, during 50 years we will reduce the lake stability by 1%,” he says. This is well below the MPs’ guideline, which is that the stability — expressed in terms of the energy needed to completely mix the lake — must not be reduced by more than 25%.
Some argue, however, that KivuWatt’s approach is problematic. “That is the way to disaster,” says Finn Hirslund, an engineer with consultancy firm COWI, based in Lyngby, Denmark, who was part of the group that wrote the MPs and who has published peer-reviewed papers about Lake Kivu. Hirslund argues that the project will “destroy the main gradient”, and worries that continuing and scaled-up extraction from the lake using similar methodologies might have long-term consequences that only become apparent after decades6.
Morkel, too, is critical of KivuWatt’s approach. He argues that the company’s degassed water has too much CO2and is too dense, which he thinks will punch a hole through the main gradient. Morkel advocates taking water and returning it to different depths from those chosen by KivuWatt. He thinks that would better preserve the lake’s layering while extracting gas for energy. He continues to try to raise funding for his approach.
Others are not concerned, however. “In terms of safety, I’m absolutely confident,” says Wüest, who also serves on KivuWatt’s independent expert advisory group. “I have a really positive view on the whole thing,” says Bertram Boehrer, a physicist at the Helmholtz Centre for Environmental Research in Magdeburg, Germany, who has worked on the lake. “If something goes in an unexpected way, there’s enough time to act.”
Future Forecasts
Perhaps the only way to resolve debate about how these operations might affect the lake is to track whether and how the density layers are changing. The Rwanda’s lake-monitoring division surveys the depths and inspects the gas-extraction companies, and Mudakikwa says its weekly profiling shows the lake remains stable for now. “The main gradient is not changing,” he says. “If there is a lake instability, we will be the first ones to be concerned.”
