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Beans, Baboons and Biogas

In March 2015 Tropical Power commissioned Gorge Farm, the largest biogas plant in sub-Saharan Africa. Located in Naivasha, Kenya, it is reputed to be the first biogas to grid-connect project on the Continent. By all accounts it is a pioneering showcase of how such a project is possible and anything but a monkey’s business. Except when it comes to risk assessment.

I suppose baboons are not top of mind

The coast is clear – a troop of baboons from neighbouring Hell’s Gap National Park take the opportunity to rummage around recently delivered vegetable waste for an afternoon meal. Whist the tend to avoid getting close to humans they are a risk factor as is the odd large stray cat.

– I suppose baboons are not top of mind for European biogas plants, but they are certainly a risk assessment factor to be reckoned with here in Kenya, commented Christopher Macharia, Project Engineer, Tropical Power Energy Group during a visit to the plant.

His work brief includes setting up operational health and safety procedures, which in Kenya includes dealing baboons and big stray cats. With subsidiaries in Ghana and Kenya, Tropical Power is a UK-headed engineering, procurement and construction (EPC) company specialised in developing and building utility scale renewable energy projects from biomass and solar. The recently commissioned 2.6 MWe Gorge Farm biogas facility is reputed to be the first biogas power to grid plant in Africa and the largest biogas plant in sub-Saharan Africa.

A first of several kinds, the Gorge Farm biogas plant in Kenya is a showcase installation and landmark project for EPC contractors Tropical Power. With farm residues as feedstock, it supplies heat, power and fertilser to the farm and excess power to grid as project engineer Christopher Macharia explained.

A first of several kinds, the Gorge Farm biogas plant in Kenya is a showcase installation and landmark project for EPC contractors Tropical Power. With farm residues as feedstock, it supplies heat, power and fertilser to the farm and excess power to grid as project engineer Christopher Macharia explained.

Horticultural major

The biogas plant is owned and managed by Biojoule Kenya, an Independent Power Producer (IPP) which in turn is jointly owned by Tropical Power and Kenya-headed VP Group, the largest fresh-produce exporter in East Africa. Gorge Farm is nestled on the slopes of the Rift Valley between Hells Gap National Park and the southern shore of Lake Naivasha. The farm is an 800 ha horticultural estate owned and operated by VP Group. Complete with its own on-site packing facilities, Gorge Farm is one of several large “flower, fruit ’n’ veg” farms in the Lake Naivasha region. Situated around 2 000 m above sea level the region has a warm, dry climate which together with fertile soils and access to water makes it suitable to grow flowers and vegetables. Lake Naivasha’s relative proximity to Nairobi’s airports, about 90 km, gives producers direct daily access to major export markets in Europe and the Middle East especially for roses and French beans.

Feedstock availability

Gorge Farm alone generates around 50 000 tonnes per annum of organic material in the form of harvest residues, pack-house rejects as well as tops and tailings.

– Through our partnership with VP Group, we are able to access abundant high-quality feedstock: around 150 tonnes of fresh organic matter every day. We only use farm residues as feedstock not purpose grown energy crops such as maize silage, stressed Macharia emphatically rebutting a rumour that the contrary could also be the case.

– However, we also carry out experimental biogas trials here on other feedstock that could be grown or used to produce biogas for power projects elsewhere in Kenya. For instance cacti that could be grown in the non-used semi-arid regions or invasive species like the Giant Water Hyacinth which is a problem here in Lake Naivasha, explained Macharia, adding that plenty of residues from neighbouring farms were available should the need arise.

The biogas is used to fuel two self-contained GE Jenbacher 420 gensets. The engine and exhaust heat is recovered to support the biogas process and to heat nearby plant propagation houses. Nighttime temperatures can be quite cool at 2 km above sea level and roses love a steady 22oC.

The biogas is used to fuel two self-contained GE Jenbacher 420 gensets. The engine and exhaust heat is recovered to support the biogas process and to heat nearby plant propagation houses. Nighttime temperatures can be quite cool at 2 km above sea level and roses love a steady 22oC.

Two-stage process

The biogas plant is of a two-stage process type, with the hydrolysis stage separated from the methanogenic stage enabling optimum temperature and pH conditions for the various microorganisms to work in. The plant is of German origin, from Snow Leopard Projects GmbH (SLP) in Bavaria. SLP have been involved in over 40 biogas installations in Europe and elsewhere though this is one of the largest.

An advantage with a two-stage plant, of particular importance to Gorge Farms, is  feedstock flexibility in terms of time and type.

– The farm grows a variety of seasonal products over the year. As you can see at the moment we’re getting in residues from baby sweetcorn, beans and cauliflower, said Macharia.

Mixing the feedstock and gradual progression from one substrate to another over a season avoids shocking the micro-organisms while keeping retention time and biogas yields on an even keel. The retention time is around 20 days, significantly shorter and with better biogas yields than what might have been achievable with a typical single stage plant.

– Plant start-up was a challenge though and took quite a long time, revealed Macharia.

Normally for a new plant startup one takes samples from a selection of biogas digesters using the same type of feedstock to get the microorganism base needed to start the biological process.

– Being the first such plant we had to feed cows and other herbivores with the different substrates and take samples from their digestive system to find the right mix of microorganisms, he explained.

Displacing artificial fertiliser

A key product for VP Group is the digestate that when dewatered is reminiscent of horse-manure in colour and texture. The plant gives rise to roughly 35 000 tonnes annually of nitrogen-rich digestate. Both the liquid and dewatered material are used as fertiliser on the farm and it is expected that this will help reduce the overall use of artificial fertiliser over time. How much remains to be seen.

– While fertile the soil lacks humus so the digestate is a very important product for the farm as it acts both as a fertiliser and a soil conditioner, Macharia explained.

Water is also a constraint and irrigation is monitored, all commercial farms around the lake need permits that regulate water usage.

– We don’t need to add water to the process other than the initial start-up. The feedstock is fresh plant material that contains a lot of water and the facility recirculates its process water with the excess used as a nutrient rich liquid fertiliser, Macharia pointed out.

First biogas power to grid

The two containerised genets, each with a GE Jenbacher 420 biogas engine and supplied by UK-headed Clarke Energy, also mark a first for GE Jenbacher in so far as they are the first biogas engines from the company in East Africa. They are configured for cogeneration with engine heat recovery supplying heat to the biogas process as well as heat to nearby plant propagation houses.

– This is also the first biogas plant in Kenya to be granted a Power Purchase Agreement (PPA)  to supply to the grid, said Macharia.

The PPA with Kenya Power Lighting Co (KPLC), the sole power distributor in the country, is for the excess power after supplying Gorge Farm. In theory this means that there is no minimum or maximum supply obligation. The current (2012) feed-in tariff (FiT) for a grid connected biogas plant in the 200 kW – 10 MW band is US$0.10 kWh. The value is fixed, except for the annually indexed operations and maintenance (O&M) component which in this case is 15 percent, from the signing the PPA and valid for 20 years.

– Obviously it is in our best interest to supply as much power as possible to the grid as it is our main source of revenue, said Macharia.

In essence the feedstock supplied by VP Group is free subject to receiving the digestate in return for free. The electricity supplied to VP Group is estimated to displace 6 million litres of diesel fuel previously used to run gensets.

Tech-transfer

The Gorge Farm biogas project investment was around KES 591 million (≈US$6.5 million) and includes the biogas plant and grid connection together with onsite workshop, office and laboratory.

– The plant has work. As you can see we have a fully equipped workshop with a  comprehensive spares stock along with a proper laboratory. Just getting OEM spares takes a long time in this part of the world so we need to be as proficient and self-sufficient as possible on-site, remarked Christopher Macharia.

The plant took less than 12 months to build, quite a remarkable achievement and a testament to the commitment of the project partners and the knowledge-transfer capabilities of the technology providers. Unlike countries with a well developed industrial biogas sector, there were no experienced engineers in Kenya at the start of the project to build or operate a facility of this scale.

Renewable hybrid ahead

With the PPA the biogas plant has, according to Tropical Power, an estimated payback of just under six years and a technical lifespan of 20 years. Furthermore the company can begin to leverage on its experience and expertise in developing other biogas projects in Africa as part of its overall target to build 130 MW of renewable power assets by 2018. For instance it has already announced plans for a 5 MW biogas project in Ghana where VP Group also have operations. The proposed setup is similar: Tropical Power the EPC, Biojoule the IPP owner operator and feedstock supplied by VP Group.

– Actually we’re renewable technology agnostic. We have a biomass boiler installation that uses residues from a VP Group rose farm to heat the farm at another location in Kenya,  said Mike Nolan, Operations Director, Tropical Power.

Apart from serving as a demonstrator and shopfront for other African biogas project developers, Tropical Power have additional development plans for Gorge Farm, a 10 MW grid-connected solar photovoltaic (PV) plant.

– We’ve secured the PPA and the PV plant will be set-up adjacent to the biogas facility. The idea is to integrate the two facilities to provide baseload and dispatchable peak load power to the grid. VP Group has a very clear agenda to reduce its climate impact and environmental footprint. Our job is to provide the best suited renewable energy solution adapted for the site and situation in question, ended Mike Nolan.

5395/AS

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