A natural byproduct that is generated when organic material decomposes in a landfill, landfill gas (LFG) has attracted significant attention in recent years due to its potential as an alternative energy source. In part one of this exploration, you will learn more about what LFG is and the various ways it can be converted into energy.
What is LFG and how is it generated?
Municipal solid waste landfills typically contain a certain proportion of organic material, generally comprised of things like food scraps, yard waste, and wood and paper products. As this material sits in the landfill, bacteria gradually begin to break it down, producing a mixture of many different types of gases. It is this mixture that the umbrella term “landfill gas” refers to. Other processes that can also yield LFG include chemical reactions and volatilization (the term for a liquid or solid changing into a vaporous state).
The quantity of LFG produced by any given landfill, as well as how quickly the LFG is produced, depends on a number of different factors, including the composition of the waste (generally speaking, the more organic waste is present in a landfill, the more LFG will be generated), how long the waste has been buried in the landfill, how much oxygen is present in the landfill, the moisture content of the waste, and the interior temperature of the landfill.
What are the components of LFG?
By volume, LFG breaks down into roughly equal parts methane and carbon dioxide, with small or trace amounts of numerous other gases and compounds, including water vapor, oxygen, hydrogen, nitrogen, and inorganic compounds such as hydrogen sulfide. LFG also contains small amounts (less than 1%) of what are known as non-methane organic compounds (NMOCs). Often contained in municipal solid waste items like household cleaners, paints, and adhesives, NMOCs consist of certain volatile organic compounds and hazardous air pollutants that can produce a number of adverse side effects, including health hazards and smog formation.
How is LFG converted to energy?
Instead of simply allowing LFG to escape into the air—which can have serious consequences given that both methane and carbon dioxide are potent greenhouse gases—more landfills are leveraging new technologies to capture, convert, and use LFG as an energy source. At present, according to the EPA, there are over 650 operational LFG energy projects across nearly all 50 US states, with an additional 415 candidate landfills awaiting project approval.
A typical system to extract LFG from landfills before it can escape involves a series of wells and a blower, flare, or vacuum. The collected gas is then directed to a central point for processing and treatment. The method in which the gas is treated will depends on how it will eventually be used to produce energy. The main energy conversion options for LFG include:
A majority (about 75%) of LFG energy projects in the US use LFG to generate electricity. Internal combustion engines or turbines are the most popular technologies used for electricity production. Microturbine technology is sometimes used at smaller landfills, and fuel cells are occasionally used for niche applications. Electricity generated from LFG can either be used onsite at the landfill or sold to the grid.
Another popular method of converting LFG to energy, used by about 25% of currently operational LFG energy projects, is simply to use LFG directly in place of another source of fuel. For example, LFG can be used to replace natural gas, coal, or fuel oil in a number of different thermal applications that include firing kilns used by potters and glass-blowers, and providing power and heat to greenhouses. LFG can also be used directly for applications such as evaporating landfill leachate or waste paint. A wide range of industries make direct use of LFG, including chemical production, auto manufacturing, food processing, cement and brick manufacturing, pharmaceutical production, wastewater treatment, paper and steel production, and consumer electronics.
Also referred to as “combined heat and power” or CHP, cogeneration is an emerging area of energy production in which electricity and thermal energy are generated simultaneously, typically in the form of either steam or hot water. While only a few cogeneration projects—mainly those using engines or turbines—are currently installed at industrial LFG operations, the efficiency gains offered when thermal energy is captured at the same time that electricity is generated make cogeneration a very attractive option for LFG use.
Another emerging area in terms of the conversion of LFG to energy focuses on exploring the possibility of using LFG as an alternative fuel. To date, LFG has been successfully pumped through natural gas pipeline systems, meaning that applications that currently use natural gas could potentially be adapted to run on LFG. In addition, various experiments with LFG have successfully produced the equivalent of both compressed natural gas and liquefied natural gas, which could be used to run motor vehicles.