Methane Extraction in Lake Kivu

Lake Kivu, on the border of the Democratic Republic of the Congo (DRC) and Rwanda, is unique in the fact that it is one of only three known ‘exploding lakes’ in the world, along with Lake Nyos and Lake Monoun, both in Cameroon. These lakes are so named due to the violent limnic eruptions they occasionally experience, which are caused by the sudden expulsion of significant quantities of carbon dioxide (CO2) and methane (CH4) from the lake depths into the surrounding atmosphere. By displacing the surrounding lighter, oxygenated air the heavier released gasses form a mazuku (Swahili for ‘evil wind’); an anoxic air pocket that proves lethal to wildlife and humans. The deadliness of the mazuku was brought abruptly to the attention of the world’s media on the 21st of August 1986 after a limnic eruption of 0.3 – 1 km3 of CO2 from Lake Nyos resulted in the asphyxiation of 1700 people and thousands of cattle. A similar, smaller eruption occurred late in the evening of the 15th of August 1984 and resulted in the deaths of 37 people in the low-lying regions around Lake Monoun.

Figure 1: Map of the Republic of Rwanda showing Lake Kivu on the western border with the Democratic Republic of the Congo (CIA World Fact book, 2011)

The exact cause of the cataclysmic release of CO2 from either of these lakes remains unknown, despite significant scientific interest in the aftermath of the disaster. The most widely accepted theory is that of the limnic eruption hypothesis; an as yet unknown trigger results in the local supersaturation and subsequent release of CO2 that has been accumulating the in water column. Under normal conditions, a vertical difference in the density of the water column confines much of the trapped gas to the deeper reaches of the permanently stratified lake. Beyond this, CO2 and CH4 concentrations increase with depth. A baroclinic disturbance in the local pycnocline, caused for example by a landslide, earthquake or volcanic eruption, could result in the creation of an area of intense local supersaturation and the eruption of these gases into the atmosphere. Although the limnic eruption theory is supported by observations of slow CO2 recharge after the Lake Nyos disaster, the possibility that the outgassing was caused directly by volcanic activity in the very tectonically active Great Rift Valley cannot be ruled out.

A report in Nature outlined the theory that magmatic CO2 diffusing into the benthic deposits at the bottom of Lake Kivu is fuelling the further production of CH4 by methanogenic bacteria in the sediment. It is now thought that Lake Kivu holds 300 km3 of CO2 and 60 km3 of CH4 at depths below 50 – 80 m, a respective increase of 10 and 15% since the 1970s. This gas reservoir is up to 350 times greater than that of Lake Nyos, the eruption of which would be devastating to the 2 million people that live along the shore of Lake Kivu. However, unlike Lake Nyos, concentrations of dissolved gases in Lake Kivu are thought to be below supersaturation at present and there are no plans to artificially degas the lake to reduce the probability of an eruption event.

Due to economic importance of the CH4 dissolved in Lake Kivu, and the uncertainties involved in its abstraction, recharge and volatility, the governments of the DRC and Rwanda recently jointly commissioned a technical appraisal of the viability of methane abstraction for power generation. Based on a 50 year economic yield, and using current estimates of gas recharge, the working group estimates that depending on the rate and efficiency of the extraction the methane reservoir in the lake could provide between 160 and 960 MWe (megawatts of electrical power), worth between US$7 and $42 billion at $100/MWh. This is a substantial estimate that neither government is likely to ignore, especially given the deteriorating economic and political climate in the DRC, and the relative scarcity of natural resources in Rwanda. It is therefore important that the science behind the extraction process is sound and the technology well monitored and maintained.

References:
Evans, W.C., Kling, G.W., Tuttle, M.L., Tanyileke, G., and White, L.D. (1993). Gas build-up in Lake Nyos, Cameroon: The recharge process and its consequences. Applied Geochemistry. 8 pp 207 – 221

Nayar, A. (2009). A Lakeful of Trouble. Nature. 460 pp. 321 – 323

CVO (Cascades Volcano Observatory) (2001). Volcanic Lakes and Gas Releases [Online] Available at: http://vulcan.wr.usgs.gov/Glossary/Lakes/description_volcanic_lakes_gas_release.html (Last accessed: 6th February 2011).

Scmid, M., Lorke, A., Wüest, A., Halbwachs, M. and Tanyileke, G. (2003). Development and sensitivity analysis of a model for assessing stratification and safety of Lake Nyos during artificial degassing. Ocean Dynamics. 53 pp. 288 – 301

Tietze, K., Hirslund, F., Morkel, P., Boyle, J., Wüest, A., Schmid, M. (2007). Management Prescriptions for the Development of Lake Kivu Gas Resources. Report to the Ministry of Infrastructure (Republic of Rwanda) and Ministry of Hydrocarbons (Democratic Republic of Congo).

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