On 20 January 2007, the plant produced LNG at the rate of 1,000 gallons per day. As the commissioning process continues, the company expects the plant’s operations to approach its nameplate capacity of 5,000 gallons per day.

At a production level of 5,000 gallons per day, the project will reduce carbon dioxide output by the equivalent of 10,000 tons per year.

Prometheus LFG LNG system. Click to enlarge.

While the basic technologies and processes for the production of LNG are well established, Prometheus has focused on integrating and packaging them into a compact format for small-scale liquefaction. Prometheus’ process design incorporates a systematic approach to purification of LFG prior to and while cooling and liquefying the feed gas mixture. The process also uses a novel method to reject N₂ from the LNG. The optimal recapture of work and thermal energy increases the overall thermal efficiency of the system while minimizing its capital costs, according to the company.

To compress, purify and cool the biomethane, Prometheus uses a modular approach. The Prometheus landfill gas system consists of the following:

• Pre-Purification Module to remove corrosive sulfur compounds, low concentrations (parts per million) of non-methane-organic compounds (NMOCs) including siloxanes, and water from the LFG process stream and also compresses the LFG from ~2 psig to ~50 psig or more as required.

• Bulk Carbon Dioxide (CO2) Purification Module to further purify the LFG process stream by removing CO₂ using a proprietary cryogenic freezing technique that simultaneously pre-cools the methane (CH₄) and any nitrogen (N₂) while freezing out the CO₂.

• Combined Liquefaction and Post-Purification Module to liquefy the purified LFG and enhance the concentration of CH₄ in the LNG by dynamic flash evaporation of the N₂. The N₂ rejection steps in the process reduce the N₂ concentration to less than 3% in the LNG.

• Refrigeration Module to provide the cooling to the process stream in the purification and liquefaction modules. The refrigeration system is a closed loop system that uses a separate refrigerant and therefore remains immune from process stream variations. Careful use of the cryogenic refrigerant allows maximum pre-cooling of the LFG process stream in stages—an important feature for increasing the thermodynamic efficiency of the overall purification and liquefaction.

• Instrumentation and Controls Module to manage the entire LFG-to-LNG system. Once installed and configured, the system can be operated and managed remotely.

Together with three other projects, two in California and one in Poland, the company is working on developments with an estimated capacity of 65,000 gallons of LNG per day. In Poland, Prometheus has joined with the Polish company CETUS Energetyka Gazowa to form LNG Silesia to take advantage of coal mine methane opportunities in southern Poland and the emerging alternative energy markets present there.

The Company is also in the design phase of its landfill gas-to-LNG project at the Kiefer Road Landfill in Sacramento County, California, and in the build phase of a stranded gas well to LNG project in San Joaquin County, California.

(A hat-tip to John!)