Opportunity In The Air: Rethinking Landfill Gas

Stuart G.Tays

A wealth of potential energy - once considered just a smelly nuisance - lies buried among tons of trash in our nation's landfills.

Landfill gas (LFG), a normal by-product of decomposing organic refuse in sanitary landfills, consists of methane, carbon dioxide, nitrogen, oxygen, moisture and trace organic components. The gas is generated under both aerobic and anaerobic conditions.

The aerobic (composting) phase begins soon after refuse enters a landfill and continues until all the trapped oxygen has been depleted from voids in the refuse and the organic material itself. The gaseous product consists of a high amount of carbon dioxide, a low amount of methane, and water, residual lug and heat.

The anaerobic phase of decomposition, which occurs in the absence of oxygen, produces significantly higher concentrations of methane (which is a usable energy source) and lower carbon dioxide concentrations in a 55:45 mixture. In contrast, commercial pipeline natural gas typically contains about 95 percent methane.

In order to produce landfill gas with maximum methane content, it is important to reduce the aerobic decomposition process and maximize the anaerobic process.

Controlling aerobic activity requires a relatively impermeable landfill cover, a well-designed and maintained gas collection system and care not to pull an excessive vacuum on the existing landfill gas collection system.

An operating landfill will increase methane production during its lifetime, assuming that the aerobic activity is maintained at a manageable level and the anaerobic activity is maximized. Upon closure of the landfill, the half-life of methane production is approximately 15 years; it will remain a viable energy source for up to 45 years.

Alternative Energy Clean air regulations are driving the use of alternative clean fuels by heavy-duty fleet owners. Concurrent regulations are forcing landfill operators to manage their landfills, whether closed or still operating, in an environmentally sensitive manner by controlling and managing landfill gas as a useful energy resource.

Converting landfill gas into a clean fuel for vehicles has shown mixed results in the past few years. However, the Los Angeles County Sanitation Districts (LACSD) has unleashed the potential of landfill-derived fuel to reduce vehicle emissions while maintaining normal performance (see sidebar on page 64).

The landfill gas collection system prevents odors and gaseous releases by maintaining a negative pressure within the landfill and its collection system. To maintain the low surface concentrations of methane meeting South Coast Air Quality Management District (SCAQMD) standards, the gas contains air drawn in from the landfill surface.

The complete system cost, including design, construction and initial startup, was $970,000, exclusive of piping from the dedicated wells. Since capital recovery of construction costs represents a majority of the fuel production cost, fuel usage was key to low-cost production.

The 1992 National Energy Policy Act established the federal government's commitment to use alternate fuels. It required federal fleets to begin purchasing alternative fuel vehicles (AFVs) in 1993 and will require state fleets and alternate fuel providers to begin buying AFVs in the 1996 model year. The new law may require private and municipal fleets to acquire AFVs starting as early as the 1999 model year.

To implement these mandates, Title XIX of the act began providing tax incentives for AFVs and alternative fuel dispensing stations in June 1993. These tax credits can amount to $50,000 for heavy-duty trucks and $100,000 for a refueling station.

Historically, California leads the federal vehicle emissions regulations by about three years because state authority pre-empts the EPA. In general, the California Air Resources Board (CARB) sets emission standards based on test procedures specified by the EPA, but has traditionally set more stringent numerical standards for nitrous oxide and hydrocarbons.

In California, one of the most important changes in bus operations in 30 years will occur in 1996 - heavy-duty transit bus operators will be required to convert to alternate fuels. Other segments of the heavy-duty vehicle market may be required to meet similar technology forcing standards by the end of the decade.

In the past, landfill gas has been used for high Btu (1000 Btu/scf), pipeline-quality gas for sale to various utility companies; medium Btu (550 Btu/scf) fuel gas for direct sale to industrial customers; and on-site co-generation of electric power using LFG as a fuel.

Producing high-Btu, pipeline-quality gas from raw LFG requires extensive processing to remove all moisture, trace components and carbon dioxide, leaving virtually pure methane. The methane gas must meet strict quality standards before it can be injected into utility company pipelines where it is mixed with natural gas, distributed and sold to customers.

Many gas utilities worry about the presence of halogenated compounds, which are not destroyed by combustion like other contaminants, and which can be dangerous if released in the home through a gas stove or heater. Because high-Btu gas production is the most expensive LFG processing option, its use is not likely to become widespread.

Producing medium-Btu gas from landfill gas requires minimal processing, and usually consists of removing any existing moisture, hydrocarbons and contaminants. The final product, which is half methane and half carbon dioxide with a typical heating value of 500 to 550 Btu/scf, can be used as a fuel for furnaces, boilers or other large burner-tip processes.

The most common energy application for LFG is the on-site generation of electricity as a fuel for engines or turbines driving electrical power generators or as boiler fuel for a steam-turbine generating facility. As power companies have reduced the cost of producing power, however, they have often shunned the price of using supplemental energy.

Landfill Gas Standards LFG is nonthreatening as long as it is contained within a landfill. Regulations developed since the mid-1980s govern either the lateral movement of the gas outside the landfill boundary or its vertical movement into the air.

Subtitle D regulations of the Resource Conservation and Recovery Act (RCRA) presently restrict only the off-site lateral migration aspects of LFG, but newly proposed regulations address collecting and monitoring LFG and demonstrating financial capability to maintain a closed landfill in an environmentally safe manner for up to 30 years.

In addition to Subtitle D, the Environmental Protection Agency will issue its New Source Performance Standards (NSPS) for municipal solid waste (MSW) landfills as part of the EPA's Clean Air Act Amendments.

The standards are intended to "limit emissions from certain new and modified MSW landfills" that "cause or contribute significantly to air pollution and which may reasonably be anticipated to endanger public health or welfare." The design standards for this new regulation will also require installing gas collection, monitoring and treatment/disposal systems.

A small number of states have already developed, or are developing, their own specific set of LFG standards. For example, California regulations are designed primarily to reduce air pollution while New York's regulations aim to eliminate the danger of explosion from migrating methane.

Imminent regulatory and technological forces can make the alternate fuel vehicle an economical and significant element in the nation's transportation system.

The profitable conversion of landfill gas into a clean fuel for medium and heavy-duty vehicle fleets has the capability to reduce vehicle and landfill emissions and lower transportation operating costs - a beneficial use for a natural gas.