Landfill gas utilization

Landfill gas utilization is a process of gathering, processing, and treating the methane 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 25 times more effective as a greenhouse gas. It is important not only to control its emission but, where conditions allow, use it to generate energy, thus offsetting the contribution of two major sources of greenhouse gases towards 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 US 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. These projects power homes, buildings, and vehicles. Landfill gas utilization is a process of gathering, processing, and treating the methane 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 25 times more effective as a greenhouse gas. It is important not only to control its emission but, where conditions allow, use it to generate energy, thus offsetting the contribution of two major sources of greenhouse gases towards 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 US 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. These projects power homes, buildings, and vehicles. 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 calorific value of 33.95 MJ/Nm^3 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 simultaneously. 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 first order decay model to aid in the determination of landfill gas production named LandGEM (Landfill Gas Emissions Model). Typically, gas extraction rates from a municipal solid waste (MSW) landfill range from 25 to 10000 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 40 to 60% carbon dioxide, depending on the amount of air introduced to the site, either through active gas extraction or from inadequate sealing (capping) of the landfill site. 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 the aforementioned agents are harmful to human health at high doses. Landfill gas collection is typically accomplished through the installation of wells installed 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. If gas extraction rates do not warrant direct use or electricity generation and, in order to avoid uncontrolled release to the atmosphere, the gas can be flared off. One hundred m3/h is a practical threshold for flaring in the US. 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 1100ºC, there is a danger of the exponential formation of thermal NOx. Landfill gas must be treated to remove impurities, condensate, and particulates. The treatment system depends on the end use. Minimal treatment is needed for the direct use of gas in boiler, furnaces, or kilns. Using the gas in electricity generation typically requires more in-depth treatment. Treatment systems are divided into primary and secondary treatment processing. Primary processing systems remove moisture and particulates. Gas cooling and compression are common in primary processing. Secondary treatment systems employ multiple cleanup processes, physical and chemical, depending on the specifications of the end use. Two constituents that may need to be removed are siloxanes and sulfur compounds, which are damaging to equipment and significantly increase maintenance cost. Adsorption and absorption are the most common technologies used in secondary treatment processing. Pipelines transmit gas to boilers, dryers, or kilns, where it is used much in the same way as natural gas. Landfill gas is cheaper than natural gas and holds about half the heating value at 16,785 – 20,495 kJ/m3 (450 – 550 Btu/ft3) as compared to 35,406 kJ/m3 (950 Btu/ft3) of natural gas. Boilers, dryers, and kilns are used often because they maximize use of the gas, limited treatment is needed, and the gas can be mixed with other fuels. Boilers use the gas to transform water into steam for use in various applications. For boilers, about 8,000 to 10,000 pounds per hour of steam can be generated for every 1 million metric tons of waste-in-place at the landfill. Most direct use projects use boilers. General Motors saves $500,000 on energy costs per year at each of the four plants owned by General Motors that has implemented landfill gas boilers. Disadvantages of Boilers, dryers, and kilns are that they need to be retrofitted in order to accept the gas and the end user has to be nearby (within roughly 5 miles) as pipelines will need to be built. In situations with low gas extraction rates, the gas can go to power infrared heaters in buildings local to the landfill, provide heat and power to local greenhouses, and power the energy intensive activities of a studio engaged in pottery, metalworking or glass-blowing. Heat is fairly inexpensive to employ with the use of a boiler. A microturbine would be needed to provide power in low gas extraction rate situations. The gas coming from the landfill can be used to evaporate leachate in situations where leachate is fairly expensive to treat. The system to evaporate the leachate costs $300,000 to $500,000 to put in place with operations and maintenance costs of $70,000 to $95,000 per year. A 30,000 gallons per day evaporator costs $.05 - $.06 per gallon. The cost per gallon increases as the evaporator size decreases. A 10,000 gallons per day evaporator costs $.18 - $.20 per gallon. Estimates are in 2007 dollars.