News & Events

Pipe Dreams in the U.S.

Waste Management World / December 1, 2011
Author: Ben Messenger

Prior to investing in a costly gas collection system the company used landfill gas generation models to analyse where to locate the wells and to forecast gas generation
In South Eastern U.S. the low price of electricity and poor grid interconnections mean that landfill gas to energy projects just aren't economic. But as one joint venture in Tennessee is demonstrating, that needn't mean the gas goes unharnessed. Ben Messenger investigates a recently completed landfill gas to grid upgrading project at the Meadow Branch Landfill.

It's common knowledge, particularly in the waste industry, that harnessing Green House Gases (GHG) emitted by decomposing wastes in landfill not only reduces the environmental impact of landfills, but also helps reduce the risk of explosions and threats to human health. Furthermore, with growing demand for 'green' energy, landfill gas offers a cost effective alternative to fossil fuels for baseload energy.

There are many hundreds of examples of landfill gas being utilised as a fuel for gas engine generators. The technology is mature, and the benefits well understood. But at many landfills it's simply not possible to fully exploit this potential due difficulties connecting to the grid, or financial and economic issues.

In the U.S. some of these issues are best demonstrated in the south east of the country, where electricity prices have historically been very low and grid interconnect standards are challenging. For the south east U.S. and other areas across the globe facing similar circumstances, that leaves two options for harnessing landfill gas. If a suitable long-term consumer can be found nearby the gas can be moderately cleaned and used for industrial purposes. The other option is to treat the gas to reach pipeline quality and inject it into the national gas grid.

At Meadow Branch Landfill in McMinn County, Tennessee, such a project has recently commenced operations. Developed by Renewco, LLC - a joint venture between natural gas distributor, AGL Resources and Keystone Renewable Energy - the $12.5 million processing plant, which received no tax payer funding - is producing pipeline quality gas for grid injection.

Analysis and planning

The project began over two years ago with a thorough analysis of the landfill characteristics to determine the viability of the project. Due to its small size, the Meadow Branch landfill didn't have a mandate obliging it to have a gas collection system, meaning that in order to collect the gas Renewco had to install the well field.

Before embarking on the full installation of 70 wells, Renewco first put in a test field of about a dozen wells 18 months ago, to determine if the results from the models held up against actual gas collection rates. Once the project was deemed to be suitable, gas wells and a gas collection system were installed on the landfill site. Simultaneously the company began looking for a long term gas purchaser to ensure that it had locked in its commodity value on a long-term basis.

A 9 mile pipeline delivers the landfill gas to Spectra Energy's East Tennessee Natural Gas pipeline
One of the selling points for upgraded landfill gas is that it commands a premium on the wholesale markets due to its 'green' credentials, while also being able to meet baseload energy requirements at a lower price than either solar or wind - neither of which can offer baseload supply.

Speaking to Waste Management World, Ira Pearl, chief executive officer of Renewco and vice president, AGL Resources explains that the company is careful to avoid naked commodity risks and highlights the importance of securing long-term contracts both for access to the gas, and with customers buying the gas. With long-term contracts the company can plan ahead and avoid the problems that pushed some developers out of business as prices fluctuated wildly in recent years, he says.

"We've signed a ten year contract to sell the gas to a wholesaler, who in turn has a customer that needs renewable gas to meet their renewable portfolio requirements."

Upgrading technology

Landfill gas is around 50% to 55% methane and 40% to 45% carbon dioxide, with traces of other constituents such as non-methane organics and hydrogen sulphide. Typically, most pipeline tariffs require the gas to be around 970 BTU per cubic foot, which equates to roughly 97% methane.

Pearl explains that there are three or four competing process technologies capable of upgrading landfill gas, such as chemical scrubbers, pressure swing absorption and membrane technologies. While landfill gas to grid projects are currently few and far between in the U.S., one of Renewco's founding partners, Keystone Renewable Energy, had previously developed just such a project using Air Liquide's MEDAL membrane technology, which is also utilised at the Meadow Branch facility. In order to upgrade the gas for pipeline injection, first of all a vacuum is carefully drawn around the gas well field to collect as much gas as possible, while trying to minimise the volume of air that is drawn. This gas is then slightly pressurised to a few pounds per square inch before being sent to a refrigeration skid. This reduces the dew point of the gas so that all the water condenses out, leaving a very dry gas - important for the rest of the process, and in preventing corrosion to the pipeline.

The dried gas is compressed using screw compressors to around 200 psi, which is necessary for the processing step. The processing step has three distinct components. The first is pressure swing absorption, where the gas alternates between two vessels, one of is which pressurised, the other not. This allows methane to pass through, but traps some of the undesirable constituents in a media.

The final stage of the system sees the gas passing to a two stage membrane filtration system. Pearl explains the process: "Picture a bank of canisters in parallel that each have what looks like spaghetti on the inside, but is actually tubes that the gas passes through, and the very tiny pores in the membrane allow the CO2 to leave and the methane to continue through." Once the gas is 97% - 98% methane it is transferred to a booster compressor which raises it to the pressure required for injection into the pipeline. For this project Renewco built a nine mile, four inch diameter pipeline to connect to the East Tennessee Natural Gas Pipeline which operates at 500 to 600 psi.

The future

In many ways high BTU projects that inject gas into the grid could be seen as the most efficient means of transmitting the energy contained within landfill gas. It allows 'green' gas to be delivered anywhere on the national gas grid for use by the highest value customer. However, it's not without its difficulties, requiring not only the gas to be upgraded to meet the pipeline specifications, but also the pipeline operator to be convinced that the product will reliably maintain that standard.

This was the first landfill gas to grid project that the company has done, and it partly served as a proving ground. According to the CEO, there's a lot of developers in the U.S. competing for gas rights at each landfill. However, Renewco prides itself on its risk management policies in terms of taking commodity risks. That means that long-term contracts need to be in place for both gas rights, and future sales.

Following this financially sound template, the company is planning to open up additional landfill gas. While such projects can be viewed with scepticism by pipeline operators in many parts of the world, the ability to bring the credibility of an established gas company to the table could prove the decisive factor for this joint venture, which has several other projects on the horizon. With landfill gas to electricity projects likely to remain financially unrewarding, and technically challenging in the region, gas to grid projects could be set to flourish here, and where circumstance permits, further afield.