Text: Sean Wood. Article from for the November 2011 issue of Popular Mechanics.
Converting landfill gas into electricity boosts our renewable energy mix
As much as landfill sites are potential environmental hazards and a blight on our landscape, not everything about them is doom and gloom. Here’s why: once captured, their harmful gas emissions can be converted into much-needed “green” electricity. And it gets better… South Africa is embracing this new technology, and we’re getting up to speed, fast.
Living a cosy suburban life makes it all too easy for many of us to ignore modern society’s dirty not-so-little secret – mountains of human-generated waste. Ugly municipal landfill sites, largely a product of our wanton consumerist ways, are here to stay, at least for the foreseeable future. However, Ener-G Systems – formed out of a partnership between local energy management company General Energy Systems and UK-based renewable energy giant Ener-G – has found a way for us to benefit, albeit modestly, from our messy reality.
What they do, and rather efficiently, is capture the methane emissions naturally produced by decomposing landfill waste, then convert it into electricity as well as that new lucrative global commodity, tradable carbon credits.
Carbon dioxide (CO²) may grab the headlines for its contribution to global warming, but methane (CH⁴) is a much bigger villain. Twenty-one times more efficient at trapping heat than CO² it’s a significantly harmful greenhouse gas. It’s also a serious health hazard. “It migrates underground and, in the right conditions, becomes highly explosive,” explains Ener-G Systems’ Greg Midlane. “If you can manage the methane gas emissions as well as generate power from them, that’s a big plus.”
Legally does it
Internationally, utilising alternative energy sources has become serious business. “Renewable energy is like the new industrial revolution,” says Midlane. “The amount of interest it is generating globally is huge.” On the local front, though, the main constraint on independent power producers (IPP) is a lack of legislation and tariff structure.
Fortunately, that’s all about to change.
The first round of the government’s IPP procurement programme (part of its Integrated Resource Plan 2010) was launched at the beginning of August. It set a benchmark of 3 725 MW of electricity to be generated from renewable sources – wind, solar, hydro, biomass and landfill gas – by 2014. By 2030, the Department of Energy wants that figure increased to 7 200 MW.
At last, IPPs have something constructive to work with. The bidding process is currently under way and is expected to be finalised some time during November. Once the successful bidders have been chosen, they’ll be able to roll up their sleeves and get down to serious business by June next year.
Getting down to business
But just because the tariff structure and legislation are not yet in place doesn’t mean that the alternative energy guys have been sitting around twiddling their thumbs. The Ener-G Systems Richards Bay operation has been selling power to BHP Billiton’s aluminium smelter for the past two years. “Because we’ve been dealing with a private company rather than a municipality on this project, it has made things much easier,” says Midlane. “And, although it’s a medium to small site with only a 400 kW generator, it shows what can be done.”
Five landfill projects are being developed in the Johannesburg area: Robinson Deep, Linbro Park, Goudkoppies, Marie Louise and Ennerdale. Midlane hopes to have all these sites fully operational during 2012, “depending on how the negotiations with the Department of Energy go”. Once completed, they are expected to generate 19 MW for around 15 years.
“That’s enough to electrify about 15 000 homes,” Midlane says. “A significant amount, considering that the methane would otherwise have been pumped into the atmosphere.”
The potential production of Ener-G Systems’ projects is a mere drop in the ocean compared with Eskom’s 37 500 MW output. But consider this: it’s not only a localised source of power; it’s also there to be exploited. Midlane explains: “We have crunched the numbers and have worked out that, if we could capture the methane from all the landfill sites in South Africa, we would generate about 100 MW. That figure doesn’t include waste water treatment works, anaerobic digestion in commercial agriculture or emissions from coal mines – areas where we have the expertise.”
Of the five projects, the Robinson Deep site, located in Turffontein, south of the city, is the best developed. So far, 60 wells have been sunk (with about 100 to go as the site expands), and the captured gas is being flared into the atmosphere. At the moment, it is producing 2 000 cubic metres of gas per hour, and that is expected to increase to 3 000 cubic metres per hour once the site is fully developed.
Robinson Deep might not be producing any electricity yet (the generator module will be installed only when the official process has been finalised), but it is busy racking up some lucrative carbon credits.
This is how credits get calculated: when burned in the presence of oxygen, methane splits into carbon dioxide and water vapour. Take the methane being combusted in tons, multiply that amount by 21, then subtract the amount of CO² created. This gives you the CO² equivalent in tons destroyed during the flaring process that can be converted into carbon credits.
Setting up shop
Once all the relevant permits are in place, Midlane reckons it takes only about nine months to complete a landfill project from scratch. First job is to conduct a “gas resource assessment” to determine how much methane can be generated from that specific site.
This entails looking at the amount of waste in place, future site expansion, waste composition, moisture content and climatic conditions. “We also look at the quality of the engineering of the landfill site itself and its collection efficiency. We have to ask ourselves, how much of the gas being produced are we going to be able to capture?”
All data is then captured in a computer model to produce a “gas yield curve” (against time). Only then can the potential generation capacity of the site be determined.
Once the size and number of generators to be installed are known, an electrical load study of the local distribution network is conducted. This indicates connections that can be made to the network, as well as the capacity it can handle.
Says Midlane: “Once these two studies have been completed and we can see that the project is commercially viable, we can start the full design and construction programme.”
This covers four areas – the design of the high-voltage power connection, well location and design, design of the gas collection system, and development of the equipment compound (where the gas blower, flare unit and generators are located).
Another factor to consider is that temperatures deep inside the landfill are around 60 degrees. As a consequence, gas brought to the surface is saturated with water vapour. Surface temperatures are much lower (typically around 20 degrees), so the water vapour condenses and, in a worst-case scenario, can even block the pipes. To prevent this from happening, Midlane ensures that most of the pipes run downhill with at least a 3 per cent fall, and installs a condensation knock-out point. “It’s just a big sump, really,” he says. “Once the water’s inside, it is pumped to the site’s leachate management facility, where it either evaporates or is taken away for treatment.”
Drilling through rubbish isn’t like sinking your average borehole. “Drilling into a landfill mass is a highly specialised process. You don’t know what you’re going to hit – it could be an old mattress, wire or concrete. All of these obstructions could damage the auger or block the well.”
A polyethylene pipe, anywhere from 300 to 450 mm in diameter and perforated from its base up to 6 metres from the surface, is then inserted into the well.
The perforated section of the pipe is surrounded by stone to facilitate the transfer of the gas, and its top 6 metres are wrapped in bentonite clay to seal off the well from the atmosphere.
A large blower then sucks on the pipes to create a mild vacuum at each well.
“This creates a bell-shaped vacuum underground, allowing us to extract the gas from the surrounding area,” Midlane explains. “We leave the top 6 metres of the pipe solid because we want the vacuum to be as low down as possible. As it works in anaerobic conditions, we don’t want air sucked into the system.”
Well pipes are fed into conveniently located manifolds dotted around the landfill site, each accommodating about 10 pipes. To help balance the gas flow, each well can be isolated should it not generate the correct volume of methane. The gas is piped from the manifolds into the main line that encircles the entire site and then enters the equipment compound via the condensation knock-out point.
Ener-G Systems finances all its projects, conducts all the research, puts in the required infrastructure, and manages every site for its entire lifespan. It even designs, develops and manufactures its own modular generator units to ensure everything’s a perfect match. “It’s a perfect win-win situation,” says Midlane. “We carry all the costs, derive an income out of selling the electricity and carbon credits, and the clients receive a royalty. For them it’s a big swingaround because it turns a cost into a revenue stream.”
Ener-G Systems intends expanding its operations countrywide, focusing first on the major centres. Once that’s done, all the smaller cities with medium-sized landfill sites will be in its sights. “South Africa’s a new market,” says Midlane.
“There’s plenty of opportunity here for councils and independent waste companies to manage their methane gas emissions.”
If you would like more information, contact Ener-G Systems on 031-564 0222. Alternatively, visit http://energ.co.uk/energsystems
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