Burning natural gas releases less heat-trapping carbon dioxide than burning coal does because it has only about half as much carbon per unit of energy. But it can exacerbate global warming if it escapes unburned into the atmosphere as methane; in a century, a methane molecule will trap as much heat as 21 carbon dioxide molecules would.
The easy solution is simply to burn the methane. But some sources emit methane at concentrations too low to burn. What then?
And even when the concentration is high enough for conventional burning, burning methane creates temperatures high enough to break up nitrogen molecules in the atmosphere and marry them to oxygen, creating nitrogen oxides, a precursor of smog.
FlexEnergy, a company in Irvine, Calif., has developed a new way to use gas at low methane concentrations, a process it has been trying out on landfill gas at Fort Benning in Georgia.
The landfill produces methane at a concentration of 20 percent, too low for conventional combustion. (The Pentagon is under orders to reduce its carbon footprint.) Later this year FlexEnergy plans to open a commercial-scale system at the Santiago Canyon landfill in Orange County, Calif.
The company doesn’t really burn the gas; it consumes it in slow motion. While a typical power plant burns a methane molecule in a thousandth of a second, give or take, FlexEnergy takes its time, combining it with oxygen to make heat in a tenth of a second or a hundredth of a second.
Instead of burning the fuel, “I like to use the phrase, we chemically cook it,’’ said Joseph Perry, the company’s chief executive. “People who are used to burning things, it takes them a while to get their head around what we’re doing.’’
The process runs at a relatively low temperature, 1,750 degrees Fahrenheit, far too low for nitrogen oxides to form. It destroys other stray but troublesome pollutants that may be present in the landfill gas, like volatile organics, and it produces electricity. And company executives say that it does so at low concentrations of methane, 1.5 percent. (Ordinary pipeline gas is about 80 percent methane.)
The electricity output is small, 250 kilowatts. (A big commercial power plant is about 2,000 times larger.) But some landfills could support eight or so such units, company executives say. And in the case of the Orange County landfill, pipeline gas could potentially be added to make the mixture rich enough to burn, reducing gas consumption and displacing some fossil fuel burning at a power plant somewhere else on the grid.
FlexEnergy will invest about $9 million in the plant at the Santiago Canyon landfill.
The process is not as efficient at converting pipeline gas into electricity as a big generator would be, but company executives say it can be highly effective in places where both electricity and hot water are needed, because the waste heat can be reused.
And in California, the system is viewed as having one other virtue: it qualifies as renewable energy. The state has a 33 percent renewable energy portfolio standard, but relying on the wind and sun has its limits because those sources produce on their own schedule, independent of human control. FlexEnergy’s system could be modulated to compensate for fluctuations in wind or sun.
On Tuesday, I’ll describe an entirely different route for making electricity from renewable biomass.
The New York Times