DEPARTMENT FOR SCIENCE, INNOVATION AND TECHNOLOGY

E-LIEU : A unique Energy resource in the face of naturaL rIsks: LakE KivU (Rwanda)

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Description

Lake Kivu is located on the border of Rwanda and the Democratic Republic of Congo, along the western branch of the East African Rift, a region of active volcanism and high seismicity. The lake spans nearly 2400 km2 and contains nearly 60 km3 of methane (CH4) and over 300 km3 of carbon dioxide (CO2), dissolved in the deep and saline lake waters, from 250m- 485m below the surface. Upper waters of the lake are less saline and ventilated by inflow of cold but relatively fresh groundwater at 250-260m, leading to a strong, stable density stratification called the chemocline. The lake's unique stratification, combined with volcanic and tectonic activity, presents a natural risk of overturning known as a limnic eruption. Methane from Lake Kivu is extracted for electricity production in Rwanda by KivuWatt (owned by Contour Global, UK), who produce 26 MW, and Shema Power Lake Kivu (SPLK) (RW) who produce about 37 MW of power, with plans to increase the rate of extraction. This represents a very significant fraction of the energy needed for the 400 MW of power used in Rwanda: at present extraction rates, the lake could continue producing power for over 100 years. Gas is produced by extracting water from a depth of 260-270m. As it decompresses, the CH4 and CO2 come out of solution. The degassed water is then reinjected into the lake, near the chemocline. The CH4 and CO2 are separated by washing the gas in a stream of shallow lake water, extracted from a depth of 60-70m, at a pressure of about 6 atm. This water resorbs the CO2 and some H2S, and is then reinjected at a depth of 70-100m, while the CH4 remains as a gas and is transported to power plants on the lake shore. We plan to develop fundamental new understanding of the fate of the return water, both (A) injected deep in the lake, near the chemocline, to ensure minimal dilution of the methane rich deep water, and also (B) injected much shallower in the lake with the resorbed CO2, to ensure this does not degrade or stress the surface water ecosystem, especially the fish which are an important food resource. The research will involve running small-scale laboratory and theoretical models of the mixing produced by the plumes of reinjected water. This will enable accurate predictions of the evolving stratification and gas concentration in the lake over the next 50-75 years. We will use the models to explore different approaches for reinjection to identify the most effective approach. We will also develop new quantitative understanding of the risks and likelihood of a lake overturn event, leading to a major release of the dissolved gas, perhaps triggered by a fissure eruption of the nearby and active Nyiragongo Volcano. We will work with the University of Rwanda to build a cohort of students in Rwanda with specialist modelling capability on lake mixing; we will run workshops describing the research and demonstrating modelling tools which will emerge from the project, with KivuWatt and SPLK, as well as REMA the government environment agency and REG, which maintains and operates the energy infrastructure in Rwanda. This will help optimise the longevity of the power generation from Lake Kivu, minimise impact on the shallow lake ecosystem; and assess the evolving risks of a limnic eruption.

Objectives

Lake Kivu is located on the border of Rwanda and the Democratic Republic of Congo, along the western branch of the East African Rift, a region of active volcanism and high seismicity. The lake spans nearly 2400 km2 and contains nearly 60 km3 of methane (CH4) and over 300 km3 of carbon dioxide (CO2), dissolved in the deep and saline lake waters, from 250m- 485m below the surface. Upper waters of the lake are less saline and ventilated by inflow of cold but relatively fresh groundwater at 250-260m, leading to a strong, stable density stratification called the chemocline. The lake's unique stratification, combined with volcanic and tectonic activity, presents a natural risk of overturning known as a limnic eruption. Methane from Lake Kivu is extracted for electricity production in Rwanda by KivuWatt (owned by Contour Global, UK), who produce 26 MW, and Shema Power Lake Kivu (SPLK) (RW) who produce about 37 MW of power, with plans to increase the rate of extraction. This represents a very significant fraction of the energy needed for the 400 MW of power used in Rwanda: at present extraction rates, the lake could continue producing power for over 100 years. Gas is produced by extracting water from a depth of 260-270m. As it decompresses, the CH4 and CO2 come out of solution. The degassed water is then reinjected into the lake, near the chemocline. The CH4 and CO2 are separated by washing the gas in a stream of shallow lake water, extracted from a depth of 60-70m, at a pressure of about 6 atm. This water resorbs the CO2 and some H2S, and is then reinjected at a depth of 70-100m, while the CH4 remains as a gas and is transported to power plants on the lake shore. We plan to develop fundamental new understanding of the fate of the return water, both (A) injected deep in the lake, near the chemocline, to ensure minimal dilution of the methane rich deep water, and also (B) injected much shallower in the lake with the resorbed CO2, to ensure this does not degrade or stress the surface water ecosystem, especially the fish which are an important food resource. The research will involve running small-scale laboratory and theoretical models of the mixing produced by the plumes of reinjected water. This will enable accurate predictions of the evolving stratification and gas concentration in the lake over the next 50-75 years. We will use the models to explore different approaches for reinjection to identify the most effective approach. We will also develop new quantitative understanding of the risks and likelihood of a lake overturn event, leading to a major release of the dissolved gas, perhaps triggered by a fissure eruption of the nearby and active Nyiragongo Volcano. We will work with the University of Rwanda to build a cohort of students in Rwanda with specialist modelling capability on lake mixing; we will run workshops describing the research and demonstrating modelling tools which will emerge from the project, with KivuWatt and SPLK, as well as REMA the government environment agency and REG, which maintains and operates the energy infrastructure in Rwanda. This will help optimise the longevity of the power generation from Lake Kivu, minimise impact on the shallow lake ecosystem; and assess the evolving risks of a limnic eruption.

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Location

The country, countries or regions that benefit from this Programme.

Rwanda