the chemical engineer|issue 852|june 2012

KIVURENEWABLE ENERGY | PROCESS SIMULATION | EMISSION CONTROL | PETROCHEMICALS

ENERGY

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32 www.tcetoday.com june 2012

tce RENEWABLE ENERGY

Lake Kivu: turning a threat into prosperity

june 2012 www.tcetoday.com 33

CAREERS tceRENEWABLE ENERGY

Figure 1: Vertical density profile and structure of Lake KivuThe water column of Lake Kivu is roughly divided into four major zones which are characterised by distinct conductivity, density and gas concentrations. Below 260 m, large quantities of CO2 and CH4 with traces of sulphur are trapped. The homogenous and stable zones keep the lake permanently stratified, but an earthquake or a volcanic eruption could suddenly cause instability of the lake and result in an eruption of the trapped gases.

Removing dangerous gases from Lake Kivu will not only make the area safer but also provide clean energy for East Africa. Henk Oosterdijk and Taco Hoencamp explain

BENEATH the calm waters of Lake Kivu in Africa lies a dangerous natural phenomenon. Large concentrations of

CO2 and methane gas (CH4) are trapped in its

deep waters, presenting a constant threat of a gas eruption. Here we look at a project which aims to extract that gas from the lake and put it to good use in providing a large part of East Africa with sustainable energy. Lake Kivu is located on the East African Rift Valley and shared by Republic of Rwanda and the Democratic Republic of Congo (DRC), an area susceptible to volcanic and tectonic activity. The lake has a maximum water depth of 485 m, making it one of the deepest lakes in the world. However, its stratification traps gasses. The water column of Lake Kivu is roughly divided into four major zones which are characterised by distinct conductivity, density and gas concentrations. Below 260 m, large quantities of CO

2 and CH

4 with traces of sulphur

are trapped (see Figure 1). The homogenous and stable zones keep the lake permanently stratified, but an earthquake or a volcanic eruption could cause instability and result in an eruption of the trapped gases. The large concentrations of CH

4 were first

discovered in the 1930s. Numerous studies of the lake and its potential resource followed. Due to anaerobic processes and the stratification of the water column the quantities of CO

2 and CH

4 are

still increasing, with current estimates that the deep waters are 50% saturated. Without a controlled reduction of the gas, we risk a disastrous eruption within the next 100 years. For many of the millions of people living in the Lake Kivu basin this would cause large-scale asphyxiation by CO

2. One way of

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reducing the concentration is to vent the gas – a solution used at Lake Nyos in Cameroon, where large quantities of (only) CO

2 are

trapped, and de-gassing is carried out via syphoning water from the bottom layers to the surface, allowing the CO

2 to escape to the

atmosphere. In 1986 a landslide at Lake Nyos triggered the release of a large cloud of CO

2,

which suffocated more than 1,700 people in nearby villages. At Lake Kivu it’s highly undesirable to vent the methane, which is a potent greenhouse gas contributing to global warming, but can also be a source for much-needed power generation. Venting or flaring the methane would thus not only be polluting but also wasteful. Extracting the gases and using them for energy production reduces the catastrophic risk of release, but in an environmentally responsible manner. The extraction of gases could affect the lake’s stability, ecology and public safety. An expert working group (EWG) was established to guide any gas extraction activities with scientific-technical expertise. The governments of Rwanda and the DRC asked this working group to prepare a guiding document for the gas extraction in Lake Kivu – its findings were published in 2009. In this context public safety was the primary priority of both governments when developing gas extraction activities, with protection of the lake environment and maximising the social benefits for the local population second and third. The EWG formulated mandatory technical requirements (MTRs), specifically designed for Lake Kivu, which should prevail over international codes and standards.

the KivuWatt projectThe KivuWatt project aims to extract Lake Kivu’s methane gas to generate electrical power which will be used in Rwanda and neighbouring countries. The project comprises:• construction and operation of four offshore gas extraction facility barges to extract methane from the deeper layers of the lake;• an onshore marine landing site that will be used for constructing and launching the barges; and• an onshore 100 MW power plant based on reciprocating gas engines that will use the methane to produce power to be sold to the national power utility EWSA. The extraction component of the project is located some 13 km offshore from the village of Kibuye in Rwanda some 85 km south-west of the capital Kigali; the power plant is located just outside Kibuye (see Figure 2). The project will be carried out in two phases: phase I consists of one barge and a net power production of 25 MW; phase II consists of three additional barges with 75 MW net power production. Phase I, costing

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34 www.tcetoday.com june 2012

tce RENEWABLE ENERGY

Rwanda is a developing country and is faced with an increasing energy demand. Project KivuWatt will be used to boost the development of this country.

around US$142m, is being supported by the African Development Bank, the Dutch development bank for developing countries FMO, the Belgium Investment Company for developing countries BIO, and the London-based Emerging African Infrastructure Fund, with a loan of US$91m. Rwanda is a developing country and is faced with an increasing energy demand. The additional energy source of Project KivuWatt is more than welcome and will be used to boost the development of the country. The additional 100 MW per year from the project would triple the power generation capacity of Rwanda. Phase I is expected to start delivering power by the end of 2012. The power will be sold to EWSA based on a 25-year power purchase agreement.

ensuring stabilityGas has never been extracted from a lake on this scale before, making Project KivuWatt a world-first. The floating gas extraction facility consists of a production system, a gas treatment system (scrubbers) and a submerged gas export pipeline to the shore. Secondary systems such as the flare system, drains and power supply are also present. The gas extraction facility is situated on a barge of approximately 64 m x 25 m (see Figure 3). The gas production consists of four HDPE production risers, main raw water/gas separators with start-up pumps and four degassed water injection risers. The separators are situated about 20 m below the water surface. At this pressure maximum separation is obtained. Once started-up by a pump the water flow will become self-sustainable due to the ‘auto-siphon’ principle. The auto siphon is started as the deep waters from -355 m become saturated when lifted as the pressure decreases. At around -120 m bubbles arise, creating the natural gas lift. The control of such large scale auto-siphoning is critical and rather new. In

general, pumps are used to keep the water flow going. The downside of the auto-siphon system is that it requires a start-up procedure and a balancing of the outflow. The control of the production rate is fully determined by the operating pressure in the main water/gas separator which is controlled to a certain extent by a pressure control valve. The priority during all operations is ensuring the lake’s stability by not disturbing the stratification significantly. A critical point is therefore the extraction at -355 m and re-injection at -280 m of the water before and after the gases have been separated from it. The intake and outlet of the risers are therefore equipped with horizontal diffusers to prevent vertical mixing. The effects of the extraction and re-injection of the production water have been extensively analysed by US-based consultancy Exponent. In a series of continuous flow dynamics (CFD) simulations, the stratification in the injection zones and the uplift (‘Bernoulli effect’) has been evaluated. These simulations show no adverse effects on the stability of the production zones. The gas treatment plant consists of compressors, coolers, refrigerator units, four large washwater towers with washwater supply and injection/re-injection risers. The extracted gas, a mixture of CH

4 (around 25%)

and CO2 with some traces of H

2S, is upgraded

to around 85% methane in the wash towers. Lake water from -40 m is pumped up and under a pressure of almost 7 bar the CO

2 and

some H2S is absorbed in the water. The CO

2-

rich water is re-injected at -60m below the biozone, which is inhabited by fish. The influence of stratification and acidification of the washwater (in particular of H

2S) on the biozone have been analysed

using dedicated software and methodologies. The washwater plume stratification occurs at a depth of -70 m just below the biozone. In the absence of horizontal currents, the plume will

Figure 2: Rwanda: The extraction component of the project is located some 13 km offshore from the village of Kibuye in Rwanda, some 85 km south-west of the capital Kigali; the power plant is located just outside Kibuye. Above: Lake Kivu

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june 2012 www.tcetoday.com 35

CAREERS tceRENEWABLE ENERGY

Project KivuWatt is opening up a long-term sustainable energy source and at the same time increasing public safety, giving a boost to the future of East & Central Africa.

influence a zone of about 30 m around the diffusers. The gases are quickly diluted below saturation levels and very small, only short-lived bubbles are likely to form and disappear in the immediate vicinity of the diffusers. Although the plume stratifies at around -70 m, H

2S is transported upward by wind-induced

mixing. Along the way it is oxidised to SO2

which consumes O2. The results of a transport

and chemical reaction model show that the zone of H

2S oxidation and O

2 consumption

within a radius of 10 m of the diffusers does not exceed above -40 m. Although no large impacts were simulated, it is acknowledged that the simulations do not fully describe the lake behaviour and a dedicated lake monitoring system will be implemented.

minimising the impactsAs part of the project preparation the social and environmental impacts of Project KivuWatt have been carefully examined in accordance with the international IFC Performance Standards and guidelines. Among others the impacts on the air quality, noise, traffic, flora and fauna, the lake surface water and land use were assessed – a process which actively involved the local community through public consultation sessions. The lake surface waters are used for many purposes such as fishing, washing and cooking, so the the upper layer of the lake must be protected. Water re-injection might potentially influence the water quality of this upper layer but the impacts remain acceptable and the simulations show that the stratification will stay intact. On a local as well as national level there is a high level of support, and expectations are high. Project KivuWatt will not only boost

Export pipeline

Control building

Generators

Compressor units

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surge tank

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Washwater towers

Figure 3: The gas extraction facility is situated on a barge of approximately 64 m x 25 m

the development of Rwanda by delivering electrical power to the national grid thus reducing expensive diesel imports, but also by employing local staff. To ensure safety during all activities an occupational health and safety (OHS) management system has been prepared, with particular attention paid to emergency preparedness and response for working on the 13 km offshore gas extraction facility. Gas detection and shutdown systems are foreseen at the barge. This project will generate sustainable energy and play its part in reducing CO

2

emissions, as diesel-powered facilities would otherwise have been built for power supply. The project therefore qualifies under the clean development mechanism of the UNFCCC and will boost revenues by selling carbon credits.

a promising futureWith production of 25 MW of sustainable electrical power almost a reality, phase II will start soon and increase capacity to 100 MW/y for supply in Rwanda. East & Central Africa needs energy for its development and besides hydropower, methane has now proven to be a viable source for sustainable energy production. Rwanda and the DRC can use the unique characteristics of Lake Kivu to increase their energy production in a sustainable way. Project KivuWatt is opening up a long-term sustainable energy source and at the same time increasing public safety, giving a boost to the future of East & Central Africa. tce

Henk Oosterdijk (h.oosterdijk@royalhaskoning.com) is senior energy consultant, and Taco Hoencamp is senior oil & gas consultant, both at Royal Haskoning