A local government authority in South Africa is developing a waste-to-energy project in South Africa under the Clean Development Mechanism of the Kyoto Protocol. Greg Midlane of Ener-G Natural Power describes how the generation plant at this scheme to create renewable electricity from greenhouse gases (GHG) will operate.

Greg Midlane, Ener-G Natural Power, South Africa

Ener-G Group of the United Kingdom and General Energy Systems (GES) of South Africa have finalized a joint venture company called Ener-G Systems to finance, design, build and operate landfill-gas-to-energy projects in South Africa under the Kyoto Protocol’s Clean Development Mechanism (CDM).

A consortium that Ener-G Systems has formed with several organizations has won a contract to develop Johannesburg City Council’s landfill sites for electricity generation under the CDM. The consortium’s other members are South Africa’s Central Energy Fund, waste management firm Waste Rite and Likusasa Energy Africa, which ENER-G’s is partnering under the South African Government’s Black Economic Empowerment policy that encourages technology and skills transfer to the local population. Also involved are leading emissions trading and GHG accounting company EcoSecurities and the Royal Danish Embassy, which, in association with the consortium, are buying a portion of the carbon credits generated by the project.

Landfill gas as fuel

Biodegradable waste deposited in landfill sites decomposes in the absence of oxygen to produce landfill gas, which is a mixture of methane (CH4), carbon dioxide (CO2) and trace amounts of volatile organic gases. Some 50 per cent of the landfill gas produced is methane, a GHG that is 21 times more potent than CO2 in its effect on climate change. Methane is a valuable fuel that can be used to run electricity generators. First, it is necessary to ascertain the amount of gas produced from each site to determine its generation capacity.

The project required a due diligence process to be undertaken, followed by the design and construction of the systems. During this process, a gas resource assessment was undertaken on Johannesburg City Council’s landfill sites, based on both historical and projected waste inputs. The underlying waste input figures are broken down into four categories: domestic, commercial, industrial and inert. Site construction, operating methods, moisture content and temperature are also assessed, and this information is fed into a gas model to produce a gas yield curve as shown in Figure 1, which is for Johannesburg Council’s Marie Louise landfill site.

Figure 1: The yield model that estimates landfill gas output for the Marie Louise landfill site in Johannesburg
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The primary gas model used by Ener-G is the Biogas Technology finite element landfill gas assessment (FELGA) model. However, for the purpose of comparison, the curves produced by the USEPA model, an internationally used and recognized standard, are also used. Both are empirical models built on similar underlying principles and incorporating a number of factors that can be varied based on experience to reflect particular site conditions.

The generated curves show the amount of gas that is going to be available from Johannesburg City Council’s sites over varying time periods. which are different for individual sites. Once the gas yield curve has been produced, the fuel consumption figures of each generator in Ener-G’s fleet is inputted in m3/h, as shown in Figure 1. The model is then used to determine the generating capacity of each site over the time period and the individual generator packages are specified to match the amount of fuel available.

The generators to be installed on the sites are built in Ener-G’s manufacturing facility. These are packaged systems installed in specially built containers complete with switchgear, control rooms and transformers for larger units. A particular feature of these systems is their remote management capability.

Each system has an intelligent onboard controller that constantly monitors key parameters within the generator and is capable of responding to problems that arise. The operation of the unit can also be remotely monitored and swift action taken if a problem occurs, minimizing downtime.

The large generators are based on Caterpillar 3516 spark-ignition gas engines with synchronous alternators, typically rated at 1150 kW. The smaller generators are based on the Perkins 4000 series spark-ignition gas engines, with synchronous alternators typically rated at 375 kW and 500 kW. Proven and reliable for landfill gas use, these generators form a highly flexible fleet. They are self-contained, simple to install, and can be moved easily from site to site to match gas availability. This allows Ener-G to optimize generation across the sites.

Landfill gas is extracted from the waste mass at an optimal rate to provide the generators with sufficiently high gas quality for smooth operation while maintaining a sufficient vacuum in the waste mass to provide environmental control of the gas. Consequently, excess gas not used for generation is spilled to an enclosed ground flare, provided as part of the gas extraction plant.

Gas is drawn from the site by exerting a vacuum on the waste mass that is created by a centrifugal blower, also part of the gas extraction plant, through a network of pipes, and out to the gas wells.

The Ener-G 1150 kW genset that will be used in the Johannesburg City Council CDM project
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Boreholes are sunk into the waste mass to a depth determined by the nature of individual sites. Perforated high density polyethylene (HDPE) pipes are placed in these bores within a distance from the surface that is dictated by site capping arrangements. From this point to the surface plain, HDPE pipe is used. The well is sealed by filling the bore with Bentonite to prevent air getting into the site as this would hamper methane production.

Piping away

Each gas well has a dedicated collection pipeline that leads from the well to a collection point or manifold, where all collector pipes are grouped for ease of control and monitoring. These collector pipes have control and sampling valves at the manifold end to regulate the gas flow. The manifolds are coupled to larger diameter HDPE pipework, which form what is called the carrier gas mains, which are sized according to the expected volume flow rate of the gas.

The carrier gas main runs back to where a central compound will be built on the edge of the landfill site. The compound encloses the generator sets, gas extraction blower, enclosed ground flare and various auxiliaries, including bunded oil and coolant tanks.

The output voltage of the generators will be stepped up by transformers fitted with ring main units to allow for multi-engine sites and the ease of installation and removal. The power is then fed into the grid via metered switches housed in dedicated substations. The sites will be installed as embedded generation stations in South Africa’s national electricity grid.

The projected power generation for the project is predicted to reach 25 MW, resulting in an output of 216 GWh per year depending on waste input. The calculation of this value allowed for scheduled service and average maintenance downtime.

This project was initiated by Johannesburg City Council as part of its ongoing commitment to providing environmental and cost-effective solutions to the growing challenge of waste disposal in the city.

A further benefit will be revenue generated for the council through the sale of carbon credits, as well as the electricity produced, with the majority going to improving services and to cover the long-term liability associated with maintaining the city’s waste disposal facilities.