LFG recovery: A road map to success

LFG has been recovered and put to use since the mid-1970s. Today, at least 550 projects are in 49 states and territories, according to the Environmental Protection Agency (EPA). Projects include electricity generation, direct use of LFG’s thermal capacity and RNG, where LFG is cleaned and used in common natural gas applications.

Several key factors must be considered when setting up and maintaining a recovery system for environmental and economic success: landfill decomposition, LFG generation rates and recovery system design and management, the EPA says.

Landfill decomposition

In a LFG recovery project, it is essential to understand how LFG is generated in a typical municipal solid waste (MSW) landfill. When it comes to landfill decomposition, the quality and quantity of landfill gas is a function of material composition; conditions in the landfill, such as temperature, moisture content, routing and depth of landfilled material; and the dominant decomposition processes.

Water is the most significant factor affecting decomposition because increases in moisture content promote decomposition. From this, estimates of LFG generation rates can then be developed, and a recovery system can be designed, managed and maintained.

Decomposition proceeds according to three processes:

• Biological processes are the most significant factors and affect chemical and physical processes. Biological decomposition occurs in three primary phases: aerobic, anaerobic, acid and methanogenic; but, depending on the detail desired, it can be divided into four or five phases.
• Chemical processes are changes that arise from chemical reactions within the MSW, such as oxidation, reduction, a change in pH, dissolution, precipitation or complexation.
• Physical processes have to do with changes in the placement and structure of the materials within a landfill during decomposition, such as settling and physical movement. Physical changes are heavily dependent on the amount and rate of water flow.

Over the life of a landfill, most decomposition occurs within the anaerobic-methanogenic phases. Yet, in some cases where landfills are too dry or wet, the anaerobic acid system could be dominant. In most landfills that are not limited by the presence of water, however, these growth conditions will govern, and methane will be produced.

The age of materials in landfills can vary. Therefore, all types and phases of decomposition occur simultaneously. This is one of the reasons why gas recovery systems are phased in over time to coincide with landfill sequencing and the age of the material placed. Yet, one phase usually will dominate and control the quality of gas and leachate produced.

Landfill gas generation rates

The most important biological decomposition phase for gas generation is the methanogenic phase. Under these conditions, materials are converted to methane (CH4) and carbon dioxide (CO2) according to the chemical makeup of the decomposing material. Provided that the material decomposes over a known time and its general characteristics are understood, a theoretical estimate of the total quantity and quality of the LFG produced can be calculated. This can be applied to specific materials, such as cellulose, food waste and general MSW with known composition. Gas composition and the amount and rate of generation are functions of the chemical composition and degradation of materials at a given time. The EPA’s Landfill Gas Emissions Model (LandGEM) is the foremost software model used to calculate estimates of LFG generation in the U.S. for regulatory and nonregulatory applications.

LandGEM uses a first-order decay equation to estimate methane generation, according to the LFG Energy Project Development Handbook. In my experience, the model provides reasonable estimates for annual methane emissions reporting and is the best available software application to estimate for nonregulatory purposes such as:

• determining design requirements for a gas recovery and control project;
• establishing the feasibility of an LFG energy project; and
• evaluating actual performance over the project life.

Its first-order decay equation uses two constants for the type of material composition (potential methane generation capacity) and the methane generation rate. With the model output being sensitive to both selected constants, these are key characteristics to consider given the landfill’s location and general material composition.

In using this model, primary concerns in project evaluation include the model’s inability to account for material composition changes and changes in moisture content or applications of liquids to existing waste over time.

Additional runs of the model in an iterative process could help mitigate inaccuracies. However, even with ideal criteria, the model relies on the third major factor— recovery system design and management.

The likelihood that a project achieves economic success can be subjective. Many projects have succeeded despite conditions and criteria that are not ideal.

To determine whether a landfill is likely to produce enough methane to support an energy recovery project, the EPA’s Landfill Methane Outreach Program LFG Energy Project Development Handbook suggests these initial screening questions:

• Is the landfill open or recently closed?
• Does it have a gas collection system?
• Does the site receive at least 25 inches of precipitation annually?
• Does the landfill contain at least 1 million tons of MSW?
• Is the landfill 50 feet or more deep?
• Does the landfill contain enough organic content to generate sufficient LFG?

Recovery system design and management

LFG systems are designed to control the migration of gas off-site by collecting and treating it thermally, such as through combustion. Further systems, for economic benefits, are augmented with treatment units, combustion engines and generators for the sale of LFG to a direct gas pipeline utility, third-party developer or a local power utility generating electricity.

System design components include:

• vertical or horizontal wells with a radius of influence of roughly 150 feet placed 150 to 300 feet apart (These well-field networks usually laid out as branch or loop systems to maximize LFG recovery.);
• blowers or compressors to collect gas from the well field;
• condensate collection;
• an open or enclosed flare to treat gas through thermal combustion; and
• gas monitoring equipment.

Gas monitoring equipment includes supervisory control and data acquisition for remote monitoring, flow and velocity meters, gas quality monitors, temperature sensors and well-field valving to manage balancing and well tuning.

The larger the system, the more complex the controls, gas recirculation and gas quality clean-up treatment. With proper design, a collection efficiency of 90 percent to 95 percent is possible. Management considerations include best practices to achieve contractual requirements for gas quality and quantity.

To take advantage of current economic trends and incentives in renewable energy production, many landfills are abandoning electricity generation in favor of RNG collection and processing. These projects generate more revenue than electric generation, provided gas cleanup is not a significant cost, especially when the landfill is near a natural gas pipeline.

Chris Lund is a senior vice president with Gershman, Brickner & Bratton, based in Vienna, Virginia. He can be reached at clund@gbbinc.com.