LANDFILL: Washing Landfill Gas

Bill Brown

An emerging technology may potentially provide a new way to slice and dice your landfill gas (LFG) into more marketable products.

From Acrion Technologies Inc., Cleveland, the Carbon dioxide, (CO2) Wash removes landfill gas contaminants in a single-step process and produces clean, high-Btu methane (up to 800 Btu/ft[superscript]3) and non-combustible liquid CO2. Contaminated liquid CO2 solvent is not regenerated; it is vaporized and flared to eliminate solvent regeneration.

The process converts LFG to various products, including electricity, methanol, pipeline or liquefied methane, or clean, high-Btu fuel.

Merchant liquid CO2 can be produced along with the principal Btu product. CO2 recovered by this system is indistinguishable from "food-grade" CO2 sold in the United States. The methanol, liquefied methane and liquid CO2 can be trucked, offering LFG recovery options at landfills remote from the electric grid or a gas pipeline.

During the CO2 Wash process, raw landfill gas, typically containing a methane to CO2 ratio of 55-to-45 is filtered to remove particulates and liquid droplets. Compression boosts the LFG pressure to about 50 pounds per square inch gage (psig). Air cooling and refrigeration reduce the gas temperature to 40 degrees Fahrenheit, and condensate is removed.

To produce merchant liquid CO2, the gas is fed to an adsorbent bed that selectively removes hydrogen sulfide (H2S). Next, a desiccant system lowers the water dewpoint to prevent downstream icing. Dehydration also eliminates downstream corrosion. Further compression raises the gas pressure to between 400 and 600 psig. The high-pressure stream traverses a sequence of heat exchangers to recover cooling from returning process streams. To this point all processing is conventional, no different than any other high-pressure LFG processing system.

The cold, pressurized stream feeds an absorption column where LFG is scrubbed with in-situ liquid CO2 solvent. At the top of the column, partial condensation of CO2 produces liquid CO2 and clean, higher-Btu methane. Sufficient liquid CO2 to absorb contaminants is returned to the column, and the balance is CO2 product. Cooling is recovered from the overhead product gas and reduced to delivery pressure.

Clean product gas is suitable for feed to a fuel cell or a methanol-conversion skid. Production of pipeline methane requires further processing of the clean, methane-enriched stream to remove CO2. The gas contains no contaminants, and conventional high-pressure natural gas processing techniques can be used.

Liquid CO2 withdrawn from the top of the absorption column contains no volatile organic compounds (VOCs); it is flashed to a lower pressure to release methane and other dissolved light gases such as nitrogen. The resulting liquid CO2 is food-grade quality, equal to merchant liquid CO2 produced as byproduct at refineries, chemical plants or in natural gas purification. Liquid CO2 solvent containing absorbed VOCs exits the bottom of the absorption column, is vaporized to recover cooling and is thermally oxidized at the landfill flare.

In one pilot demonstration, a CO2 Wash unit (PDU) was placed next to a 700 kilowatt (kW) engine generator at the Al Turi Landfill in Goshen, N.Y., and fed by a slipstream of raw LFG engine fuel. Total VOCs such as vinyl chloride were removed below a detection limit of 20 parts per billion (ppb). The PDU produced clean, enriched methane from raw LFG that exceeded purity requirements for catalytic reforming of methane to hydrogen (e.g., fuel cells or methanol synthesis).

A second PDU designed to produce 2 tons per day of merchant liquid CO2 and fuel sufficient to power a 250kW fuel cell will become operational during summer 2000. This plant, a modular skid-mounted PDU, will operate continuously to supply local greenhouses and other CO2 consumers with test-market quantities of liquid CO2 recovered from LFG.

The Washington, D.C.-based U.S. Department of Energy's SBIR Program, overseen by National Energy Technology Laboratory, Morgantown, W. Va., is providing 63 percent of the funding.

Alcohol Solutions LLC, St. Louis, a joint venture of Acrion and methanol marketer/distributor Alcohol USA, manufactures methanol from LFG for sale to local customers.

Acrion and PMC Resources Inc., Bellevue, Wash., formed a joint venture, called Evergreen Eco Resources, to design, manufacture and operate facilities to recover and sell industrial gases from LFG with the CO2 Wash technology.