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Published: Thursday, 09 January 2014 04:23

Written by Administrator

Landfill Methane Opportunities

Gregory DiCenzo

Landfills are areas that are filled, sometimes with rubble for renovation but, in the context of this document, with municipal waste material. How that waste is managed differs widely, from simple burial, to temporary holding for later recycling but, in the last 40 years, it is being managed more for biogas recovery.

Landfill biogas is a mixture of gasses, usually 40-60% CH₄ and most of the balance being CO₂[1]. The value of biogas is in the methane content, which is a product of microbial processes acting on the organic matter in the landfill waste.

When landfill gas was recognized as a problem (explosion hazard) methods were considered for remediation and flaring was the most common option chosen. Later, then, methane emissions to the atmosphere from landfills were considered an environmental hazard and landfills over 2.75 million tons were considered worthy of monitoring, if not ameliorating. The hazardous biogas migration can enter 1) atmosphere, 2) groundwater, and/or 3) close-by structures.

The anatomy of a landfill is simple: layering of a day’s waste often with compaction and a cover of soil or other material. In areas where preventing hydrologic contamination is paramount, rubber liners can define the bottom of the landfill to block leachate from leaving the system.

When managing a landfill for gas production, control and reuse of the leachate is critical for operation and lining is, therefore, required. The system is managed to be anaerobic as H₂S is often elevated in landfill biogas. However, the landfill is not closed until it is retired; in fact, mass is predicted to be biologically removed at some rate allowing for increased additions, necessitating a non-sealed structure until formal shut-in.

Water is required for methanogenesis to start-up; indeed, in a more temperate climate, methanogenesis will begin within a year or two; in an arid climate such as the desert Southwest 5 years is a better expectation.[2] 

In addition to moisture, nutrients, type of waste material, etc also determine the rate of gas generation[3]:

 “The value of [methane rate constant] is a function of (1) waste moisture content, (2) availability of nutrients for methane-generating bacteria, (3) pH, and (4) temperature.”

Disposition of methane from managed landfills is most commonly via flaring, thus converting it to CO₂. Flaring is used because it is the simplest method. One alternative to flaring is to provide a biological, oxidizing cover to the landfill when gas collection is inefficient and emissions containment is a top priority (see Appendix 1). Another alternative to flaring, if conservation is desired, is to upgrade the gas, compress it, and sell it to a pipeline.

“Landfill Gas to Electricity” (LFGTE) is yet another option and implies burning gas on-site to drive turbines and generate electricity; co-generation of heat may be part of the design. Microturbines to combust the low BTU biogas are already prevalent in the anaerobic digester biogas market and can be employed with landfill biogas generation systems.

Landfill gas capacity currently entails 621 operational sites in the U.S. [1] which produce 1978 MW and 311 mmscfd. An additional 450 candidate landfills suitable for landfill gas production have been identified which could produce an additional 805 MW electric and 470 mmscfd.

Infrastructure of a landfill gas operation is complex and can cost several millions of dollars per site. Figure 1 depicts a generalized system.

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