Waste Wise: Gauging Your Gas

Have you ever gazed out across a landfill gas field and wondered how the number of collection wells is determined, or how much gas a landfill can make? Even if you’re a businessman rather than an industry “techie,” the question is relevant since the gas volume that can be collected and converted to energy determines the economic and environmental value of the landfill-gas-to-energy (LFGTE) proposition.

The prevalence and widespread development of LFGTE projects observed today is based not only on industry drivers of revenue and regulated collection and control of gas, but also on scientific research. The Environmental Research and Education Foundation and many of its stakeholder consulting firms and universities have primarily led these research efforts.

Early on, LFGTE systems were installed on the largest landfills because they generated the most gas and thus had the highest revenue potential. But only recently have a significant number of medium and small landfills begun to consider LFGTE. This is due, in part, to a lack of understanding of how and when gas is produced in landfills, a need for more refined ways to measure or estimate gas collection system efficiency, and the need for more accurate predictions of gas yields prior to the implementation of a LFGTE system.

These issues can be addressed through research. The first step is to use a model to predict the volume of landfill gas generation. From the initial design of the well field and sizing of generators used to convert the gas to electricity, to the estimation of potential revenue, such models are invaluable tools in this process.

EPA’s LandGEM model is currently the industry standard when it comes to modeling landfill gas production. However, in order for a model to be predictive and useful, the inputs must be accurate. The LandGEM model uses two primary inputs: the methane yield and the decay rate, commonly referred to as “Lo” and “k,” respectively.

The methane yield refers to the total amount of gas that can come from a specific mass of refuse. In other words, for a typical waste composition there is only so much gas that can be made by a ton of refuse. The decay rate, or k value, determines how fast that gas is formed from that same ton of refuse. The methane yield is based on waste composition and, specifically, waste components that are biodegradable. The decay rate is based primarily on environmental factors such as moisture content of the waste, temperature, compaction and the availability of microbes to degrade the waste. The LandGEM model uses these two variables to predict the amount of gas production in a landfill during its lifetime.

Given the importance of these two variables, they are the subjects of considerable research. Initially, this work was conducted in a laboratory setting under ideal environmental conditions using simulated waste compositions. This created an obvious disconnect from conditions in the field. Thus, I have been told that when people use the LandGEM model, an experience-based “fudge factor” is applied to correlate modeled values to what actually happens in the field. While marginally effective, a model that is inaccurate loses its utility, and the fudge factor may not always fit a given situation. Nonetheless, liberal use of fudge is common.

Recent work conducted by Mort Barlaz, Ph.D., and his team at N.C. State, via a grant from EREF, is changing this. N.C. State’s work aims to define Lo and k based on field-derived data rather than lab data. Additionally, information is being collected at landfills across the country to account for the effect of different climates on the decay rate. The work is significant because it reduces uncertainty in predicting landfill gas production.

When a consultant or engineer for a landfill owner can say that there is an 85-percent chance that this landfill will produce a certain amount of gas over a given number of years, it not only helps the development of an accurate pro forma, but also helps define the risk when considering implementing LFGTE on small to medium landfills. This is a win for everyone.