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Landfill Gas Recovery Process

In the past, methane emissions in the United States have been viewed as a problem. Landfill emissions can be collected to prevent spreading into nearby commercial or residential areas to reduce the risk of explosion. Then the landfill gas collected was disposed of by incineration which was wasteful. Demand for this gas was created due to natural gas shortages and escalating energy prices.

Landfill gas (LFG) recovery is the process by which methane gas is collected from solid waste deposited in a landfill. LFG is a natural byproduct of the decomposition of organic material in landfills. The composition of LFG is about 50% methane, 50% carbon dioxide, and a small amount of non-methane organic compounds.

When solid waste is deposited in a landfill, it undergoes an aerobic (with oxygen) decomposition stage when little methane is generated. Within one year, anaerobic conditions are established where methane-producing bacteria begin to decompose the waste and generate methane.

Instead of escaping into the air, LFG can be captured, converted, and used as a renewable energy resource. Using LFG helps to reduce odors and other hazards associated with LFG emissions. It also prevents methane from migrating into the atmosphere and contributing to local smog and global climate change. In addition, LFG energy projects generate revenue and create jobs in the community and beyond.

To make the gas usable, it first must be treated to remove offensive odors, water; and particulates. The gas can be used as low-energy fuel gas for gas engines, gas turbines, or gas-fired steam boilers. The gas can be sold to a commercial on-site power generation or cogeneration project. Additional processing of the gas by molecular sieves is performed to remove the carbon dioxide and enrich the Btu content. This additional processing makes the gas salable to a pipeline transmission company or utility.

Generally, reciprocating compressors are utilized in the compression of landfill gas. Rotary positive displacement units can be used in lower discharge pressure applications required in the recovery of landfill gas.

Economics of Landfill Gas Recovery

LFG energy projects generate revenue from the sale of the gas from the landfill gas recovery plant. Jobs are created as a result of using LFG. Positions need are with the design, construction, and operation of energy recovery systems and with the landfill gas recovery companies. LFG energy projects involve engineers, construction firms, equipment vendors, and utilities or end users of the power produced. Much of the project costs are spent locally for drilling, piping, construction, and operational personnel. This local spending helps communities to realize economic benefits from increased employment and local sales. Local businesses realize cost savings associated with using LFG as a replacement for more expensive fossil fuels. Some companies could save millions of dollars over the life of their LFG energy projects.

Basic LFG Collection and Processing System

Today many landfill sites around the country are collecting landfill gas emissions quite economically for commercial or industrial use. The expected life of most landfills is between 15 and 20 years. Landfill methane gas recovery reduces greenhouse gas emissions into the atmosphere and provides an additional energy source.  A typical landfill must decompose for 5 to 10 years before a commercially attractive quantity of methane gas is produced. A landfill gas recovery system collects the gas to a central location. This is accomplished using a series of wells and a blower/flare or vacuum system to collect the gas. Once at the central location point, the gas undergoes scrubbing, dehydration, compressing, and processing. From the central collection system, the gas can be flared or beneficially used landfill gas energy recovery project. 

Landfill Gas Energy Project Types

There are many options available for converting LFG into energy. There are three broad categories used for grouping different types of LFG energy projects. They are Generating Electricity, Medium-Btu Gas Direct Use, and Upgraded LFG. 

Generating Electricity

About three-fourths of currently operating projects in the United States generate electricity from LFG. A variety of technologies can generate electricity for onsite use or sale to the grid. Some of these technologies include reciprocating internal combustion engines, turbines, microturbines, and fuel cells. The reciprocating engine is the most used conversion technology for LFG electricity applications. Reciprocating engines offer relatively low cost, high efficiency and size ranges that complement the gas output of many landfills. Gas turbines are typically used in larger LFG energy projects. Microturbines are generally used for smaller LFG volumes and in niche applications.

Cogeneration projects use LFG to generate both electricity and thermal energy, usually in the form of steam or hot water. Several cogeneration projects have been installed at industrial operations, using engines or turbines. The efficiency gains of capturing the thermal energy in addition to electricity generation can make this project type very attractive.

Medium-Btu Gas Direct Use

LFG is used to offset the use of another fuel like natural gas, coal or fuel oil. This occurs in about one-fourth of current operational projects. LFG can be used directly in a boiler, dryer, kiln, greenhouse or other thermal application.

Upgraded LFG

LFG can be upgraded to a high-Btu gas through treatment processes. These processes increase its methane content and reduce its CO2, nitrogen and oxygen contents. Upgraded LFG can produce the equivalent of pipeline-quality natural gas, compressed natural gas or liquefied natural gas. Options for using upgraded LFG include injection into a natural gas pipeline for fueling stationary combustion equipment or creating vehicle fuel.

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