Landfill gas utilization

Landfill gas utilization is a process of gathering, processing, and treating the methane or another gas emitted from decomposing garbage to produce electricity, heat, fuels, and various chemical compounds. After fossil fuel and agriculture, landfill gas is the third largest human generated source of methane. Compared to CO2, methane is 27 times more potent as a greenhouse gas. Since methane is a combustible gas, the captured landfill gas can be used to make energy. This means that landfill gas utilization avoids the emission of methane and can substitute fossil fuels, reducing two sources of greenhouse gases that contribute to climate change. The number of landfill gas projects, which convert the gas into power, went from 399 in 2005 to 519 in 2009 in the United States, according to the U.S. Environmental Protection Agency. These projects are popular because they control energy costs and reduce greenhouse gas emissions. These projects collect the methane gas and treat it, so it can be used for electricity or upgraded to pipeline-grade gas to power homes, buildings, and vehicles.

Generation

Landfill gas (LFG) is generated through the degradation of municipal solid waste (MSW) and other biodegradable waste, by microorganisms. Aerobic conditions (presence of oxygen) leads to predominately CO2 emissions. In anaerobic conditions, as is typical of landfills, methane and CO2 are produced in a ratio of 60:40. Methane (CH4) is the important component of landfill gas as it has a higher heating value of 37.7 MJ/Sm3 which gives rise to energy generation benefits. The amount of methane that is produced varies significantly based on composition of the waste. Most of the methane produced in MSW landfills is derived from food waste, composite paper, and corrugated cardboard which comprise 19.4 ± 5.5%, 21.9 ± 5.2%, and 20.9 ± 7.1% respectively on average of MSW landfills in the United States. The rate of landfill gas production varies with the age of the landfill. There are 4 common phases that a section of a MSW landfill undergoes after placement. Typically, in a large landfill, different areas of the site will be at different stages at the same time. The landfill gas production rate will reach a maximum at around 5 years and start to decline. Landfill gas follows first-order kinetic decay after decline begins with a k-value ranging 0.02 yr-1 for arid conditions and 0.065 yr-1 for wet conditions. The Landfill Methane Outreach Program (LMOP) provides the Landfill Gas Emissions Model (LandGEM), a first-order decay model which aids in the determination of landfill gas production for an individual landfill. Typically, gas extraction rates from a municipal solid waste (MSW) landfill range from 25 to 10,000 m3/h where Landfill sites typically range from 100,000 m3 to 10 million m3 of waste in place. MSW landfill gas typically has roughly 45 to 60% methane and 30 to 40% carbon dioxide. Air can also permeate in the landfill and be captured in landfill gas collection pipes. Oxygen is often consumed by microbes in the landfill meaning that air infiltration increases the composition of nitrogen gas. The amount of air infiltration depends on weather, landfill cover and suction controls in the landfill gas collection system. Depending on the composition of the waste in place, there are many other minor components that comprises roughly 1% which includes H2S, NOx, SO2, CO, non-methane volatile organic compounds (NMVOCs), polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), etc. All of these gases are harmful to human health at high doses.

LFG collection systems

Landfill gas collection is typically accomplished through the installation of wells – vertically and/or horizontally – in the waste mass. Design heuristics for vertical wells call for about one well per acre of landfill surface, whereas horizontal wells are normally spaced about 50 to 200 feet apart on center. Efficient gas collection can be accomplished at both open and closed landfills, but closed landfills have systems that are more efficient, owing to greater deployment of collection infrastructure since active filling is not occurring. On average, closed landfills have gas collection systems that capture about 84% of produced gas, compared to about 67% for open landfills. Landfill gas can also be extracted through horizontal trenches instead of vertical wells. Both systems are effective at collecting. Landfill gas is extracted and piped to a main collection header, where it is sent to be treated or flared. The main collection header can be connected to the leachate collection system to collect condensate forming in the pipes. A blower is needed to pull the gas from the collection wells to the collection header and further downstream. A 40-acre (160,000 m2) landfill gas collection system with a flare designed for a 600 ft3/min extraction rate is estimated to cost $991,000 (approximately $24,000 per acre) with annual operation and maintenance costs of $166,000 per year at $2,250 per well, $4,500 per flare and $44,500 per year to operate the blower (2008). LMOP provides a software model to predict collection system costs.

Flaring

If gas extraction rates do not warrant direct use or electricity generation, the gas can be flared off in order to avoid uncontrolled release to the atmosphere. One hundred m3/h is a practical threshold for flaring in the U.S. In the U.K, gas engines are used with a capacity of less than 100m3/h. Flares are useful in all landfill gas systems as they can help control excess gas extraction spikes and maintenance down periods. In the U.K. and EU enclosed flares, from which the flame is not visible are mandatory at modern landfill sites. Flares can be either open or enclosed, but the latter are typically more expensive as they provide high combustion temperatures and specific residence times as well as limit noise and light pollution. Some US states require the use of enclosed flares over open flares. Higher combustion temperatures and residence times destroy unwanted constituents such as un-burnt hydrocarbons. General accepted values are an exhaust gas temperature of 1000 °C with a retention time of 0.3 seconds which is said to result in greater than 98% destruction efficiency. The combustion temperature is an important controlling factor as if greater than 1100 °C, there is a danger of the exponential formation of thermal NOx.

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