Micro-Turbine: Maximum Flexibility

Around the world thousands of historic landfill sites are silently producing millions of tonnes of methane gas, which either makes its way into the atmosphere where it adds to the cocktail of greenhouse gases potentially heating the planet, or it is flared, and the energy it contains is wasted.

With the ability to run on ultra-low quality methane, the combination of micro-turbine and thermo oxidiser technology being installed at the closed Santiago Canyon Landfill site in California will be the biggest landfill gas project delivered by FlexEnergy so far. Ben Messenger takes a closer look at the development and the technology behind it.

Around the world thousands of historic landfill sites are silently producing millions of tonnes of methane gas, which either makes its way into the atmosphere where it adds to the cocktail of greenhouse gases potentially heating the planet, or it is flared, and the energy it contains is wasted. However, Irvine, California based FlexEnergy has been honing a micro-turbine based solution to the problem, and following on the heels of two previous installations, has recently been granted approval in a binding agreement with the Board of Supervisors in Orange County, California to install and operate a landfill gas to energy system at the Santiago Canyon Landfill in Orange.

The site was closed in 1988 and until now has flared the methane being generated by the decomposing waste. According to FlexEnergy an assessment of the available landfill gas at the site suggests that between 1.5 MW and 2 MW of power will be produced by an installation of eight Flex Powerstation FP250s.

The FP250 integrates two technologies that have been around for some time - micro-turbines and thermo oxidisers, which have historically been used to destroy solvents and chemicals as a post treatment system.

However, according to Mike Levin, vice president of government affairs at the company, the difference with the FlexEnergy system is that it uses the thermo oxidiser not just to destroy chemicals, but to create the heat energy necessary to power the turbine.

FlexEnergy's second installation at a landfill was at the Department of Defence's Fort Benning base in Georgia Credit: Fort Benning

The Santiago Canyon project is not the company's first installation of a system to utilise low BTU methane from a landfill site. In 2010 FlexEnergy installed an "alpha" system in collaboration with the County of Riverside Waste Management Department, at Lamb Canyon Landfill in California. The system proved the concept and passed testing by the Air Quality Management District in Southern California with results showing less than one part per million of NOx, zero CO2 and extremely high destruction efficiency of Volatile Organic Compounds (VOCs).

However, the "alpha" system used a different turbine from the one installed in the FP250s destined for the latest project, and according to Levin: "It became very clear that in order to scale the technology we needed to own the turbine as well as the oxidiser technology in order to be able to properly integrate the controls and the software. But none the less we got the system to function and we proved the concept."

Shortly after, in late 2010, the company announced an agreement to acquire the Energy Systems business of Ingersoll Rand, headquartered in Portsmouth, New Hampshire - a deal that was completed in January this year. The acquisition of the turbine systems and recuperator specialist bought FlexEnergy the MT250 turbine product line, which it says will allow it to accelerate the role out of its FP250 Flex Powerstation.

The second significant landfill project for the company came when it was awarded a contract to install one FP250 system at a closed Department of Defence landfill at a base in Fort Benning, Georgia. Levin calls this installation the "beta" site, and says that the company is still operating this facility successfully.

How it works

The FP250 units to be installed at Santiago Canyon are an integration of the company's thermal oxidiser and a customised gas turbine. Thanks to its recent acquisition it has been able to tailor the application with its own system architecture and software control algorithms. The system is able to use the existing gas collection system at the site, but how does it operate on 15 BTU methane?

According to Levin, at the inlet to the system the methane is diluted down to 15 BTU, or just 1.5% methane and 98.5% air. The dilute mixture is then pressurised to four atmospheres and recuperated waste heat is used to preheat the mix to 1100°F (593°C), at which point it is transferred into the oxidiser, a large cylindrical packed bed pressure vessel about 23 feet (7 metres) in height, where it is further heated to around 1800°F (982°C). The residence time at this stage is between two and three seconds.

"Whereas combustion is extremely hot and extremely fast, we're a little less hot and a little less fast. But that 1800°F is really important because it's hot enough to destroy carbon dioxide and hot enough to destroy volatile organic compounds, but not so hot as to create NOx. And of course NOx, especially in air pollution sensitive areas is a precursor to ozone and smog," explains Levin.

The company says that the Flex system tolerates both moisture and siloxanes in the landfill gas. Additionally, the process of mixing fuel with atmospheric air to achieve a uniform gas mixture of 15 BTU allows the system to operate on a wide range of gaseous, and potentially liquid, fuels. However, each fuel is different, and requires its own compressor, clean-up, and unique combustor. The fuel conditioning consumes about 20% of the power generated.

For a landfill to be a viable site for the system it requires around 3.5 million BTUs of methane to be available. At older sites with lower concentrations of methane, that means that a higher volume of gas needs to be available. According to Levin for most closed landfill sites being reviewed by the company, there should easily be enough energy being released as gas to fuel the system for 20 years - the expected lifespan of the equipment.

Finance and permitting

The project is being built under a Build Own Operate model, with the County of Orange owning the gas, and giving FlexEnergy a lease to install its facility and access to the gas. The company will then enter into a power purchase agreement with a local utility and pay a percentage of its revenue from that agreement to the county. The remainder of the revenue is used to pay off the capital expenditure necessary to install and maintain the equipment at the site. Levin expects a payback period of five to six years, assuming that everything is operational and that the company secures a reasonable power purchase agreement.

When it comes to air pollution and emissions regulations, California is renowned for being one step ahead with its requirements. Back in 1971 the State's Air Resources Board adopted the first automobile NOx standards in the nation, and in 2009 it introduced the Landfill Methane Control Measure that is intended to reduce 1.5 million tonnes of greenhouse gas emissions. The regulation, which became effective in June 2010 primarily requires owners and operators of certain uncontrolled MSW landfills to install gas collection and control systems, and also requires existing and newly installed gas collection and control systems to operate in an optimal manner. But for those looking to install a landfill gas to energy project, it also means meeting tough emission targets.

To get the Santiago Canyon project off the ground, Levin explains that the first hurdle is the California Environmental Quality Act, which requires a full environmental evaluation, including site studies, surveys, soil samples and reports on perceived environmental impact. In addition, the project needs to meet local permitting requirements for Air District.

"There's that Frank Sinatra song New York New York which says if you can make it there you can make it anywhere," says Levin. "That's how doing a project in California is from a regulatory and permitting perspective. Our experience doing Santiago Canyon will serve us well to be able to efficiently deploy a number of these projects throughout California. And if we can do them in California, doing them elsewhere, with very few exceptions will be much easier."

Ben Messenger is associate editor of Waste Management World magazine.
email: benm@pennwell.com

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