What is billed as the world’s first plant to produce liquefied natural gas (LNG) from landfill gas is now being commissioned in Irvine, Calif., by Prometheus Energy Company (Seattle, Wash.; edlinks.chemengonline.com/6897-540). The first module is expected to reach its capacity of around 5,000 gal/d (about 750 scf/min of gas) by fall, says Dan Clarkson, vice-president. Other modules will be added later to use the landfill’s capacity of 50,000 gal/d.
The plant produces LNG of 97%+ methane concentration for transportation fuel, the first customer being the local Orange County Transit Authority. Clarkson notes that pipeline gas typically contains 95% CH4 or less, but higher-purity gas is required by fleet operators to reduce vehicle maintenance costs. While the operation is small compared to a conventional LNG plant, he says the advantages are that it uses an otherwise wasted biogas, and it reduces gas-transportation costs, since the nearest fuel-grade LNG plants are in Arizona, Wyoming and Texas.
The landfill gas contains about 50% methane, 40% carbon dioxide and 10% nitrogen. Minor contaminants, including sulfur compounds, are removed by conventional means, then most of the CO2 is removed by freezing and pressure-swing adsorption. Finally, the CO2 content is reduced to about 50 ppm by temperature-swing adsorption. All of the plant’s cooling, including liquefaction, is done by a closed-loop, Brayton cycle nitrogen refrigeration system. Electricity for the plant is produced by a 1-MWe, Jenbacher engine from GE Energy (Atlanta, Ga.). The 20-cylinder reciprocating engine uses a mixture of low-Btu landfill gas and N2/CH4 purge gases from plant processes.
Prometheus has obtained a $640,000 grant for the project from the California Air Resources Board (Sacramento) and receives a 50¢/gal federal tax credit from the sale of the fuel. For future plants, says Clarkson, the company plans to buy the rights to gas from landfill operators and expects to produce LNG for less than the cost of diesel fuel on an equivalent Btu basis.