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  First-Order Kinetic Gas Generation Model Parameters for Wet Landfills (66 pp, 1.12 MB) (EPA/600/R-05/072) June 2005

First-Order Kinetic Gas Generation Model Paramters for Wet Landfills Report cover
Landfill gas is produced as a result of a sequence of physical, chemical, and biological processes occurring within an anaerobic landfill. Landfill operators, energy recovery project owners, regulators, and energy users need to be able to project the volume of gas produced and recovered over time from a landfill. Mathematical and computer models for predicting gas yields are widely available.

EPA developed a methodology for determining landfill gas generation based on a first-order degradation model and has provided default values for model input parameters. However, these values are based on data obtained from conventional landfills. Waste stabilization can be enhanced and accelerated to occur more rapidly if the landfill is designed and operated as a bioreactor, primarily involving moisture addition. Enhanced waste stabilization will result in increased gas production; therefore, the rate constant (k) and methane generation potential (L0) values will be different from those of conventional landfills.

The objective of this report is to investigate landfill gas collection from wet cells and to estimate first-order gas generation model parameters. The task was accomplished by conducting a literature review regarding landfill gas generation and modeling. Case studies of gas collection from wet landfills were identified. Parameters were determined through statistical comparison of predicted and actual gas collection.

The terms k and L0 were estimated for a set of landfills with short-term waste placement and long-term gas collection data. Mean and 95 percent confidence parameter estimates for these data sets were found using mixed-effects model regression followed by bootstrap analysis. The mean values for the Vst0, L0, and k were 33 m3/Mg, 76 m3/Mg, and 0.28 yr-1, respectively.

Parameters were also estimated for three full-scale wet landfills where waste was placed
over many years. The k and L0 estimated for these landfills were 0.21 yr-1 and 115 m3/Mg; 0.11 yr-1 and 95 m3/Mg; and 0.12 yr-1 and 87 m3/Mg. A conservative set of parameter estimates is suggested based on the upper 95 percent confidence interval parameters as a
k of 0.3 yr-1 and an L0 of 100 m3/Mg, with a negligible Vst0 if the design is optimized and the lag is minimized.

Wet cells were observed to produce more gas at a faster rate than conventional landfills, particularly after they are closed and when more effective leachate recirculation was practiced. To better quantify the parameters for a larger sample of landfill, more data from full-scale landfills are needed with complete data sets that provide descriptions of gas collection systems, gas quality and quantity, waste placement rates, and moisture conditions.

It is recommended that a time step of 0.1 yr be used in the model to avoid inaccurate estimation of flow rates, especially when using a k value greater than 0.1 yr-1. A LandGEM form, based on the cumulative volume of gas generated, can be amended to achieve accurate estimates of gas generation.


Susan Thorneloe

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