Oxidation of Methane in Landfill Covers: A Strategy to Maximize Energy Revenue

Abstract

Yolo County, CA is planning to optimize landfill gas collection by collecting gas during times of peak power demand and storing gas in the landfill during off peak periods by reducing the vacuum on the gas collection system. The objective of this research was to evaluate the efficacy of this approach by evaluating whether gas emissions would increase at low vacuum. Biologically active covers composed of compost⁄wood chips (80⁄20), pure compost, or green waste at thicknesses of 0.31 to 0.9 m were tested using static chambers to measure methane emissions and oxidation potential during rainy and dry season. In addition, laboratory column tests were conducted with two year old green waste to evaluate the effects of pressure gradient, moisture addition to simulate the dry and rainy seasons in California, and continuous vs. intermittent methane feed.

The effect of climate was significant. In the field, the highest CH4 emission was 20.85 gm CH4⁄m2-d in 0.91 m compost + wood chips and 5.255 gm CH4⁄m2-d in 0.31 m soil during the rainy and dry seasons, respectively. When CH4 oxidation was measured using stable isotopes, it varied from 5 to over 99%. In the rainy season tests, there was a statistically significant decrease in CH4 emissions when the gas system vacuum was increased for the covers with the highest emissions (0.91 m compost + wood chips, 0.31 m compost, soil). In contrast, there was not an increase in CH4 emissions associated with reduced gas collection system vacuum during the dry season tests. The average flux rate in dry season was below 5.255 gm CH4⁄m2-d in all the covers both for low and high vacuum. While there were many confounding issues, emissions at low vacuum during the rainy season were the highest. Overall, all covers tested were effective in controlling gas release. The emissions data are consistent with the soil gas profile data where CH4 concentrations for the dry season were in the ppmv range while the corresponding concentrations were in the % range for the rainy season tests.

Laboratory columns were filled with 2 year old green waste that is used at the Yolo County landfill as a daily cover. In most cases, methane oxidation decreased as the flux or pressure gradient increased. The addition of moisture inhibited methane oxidation by restricting O2 diffusion in the green waste as evidenced by the reduced O2 concentrations at depth in the water addition columns. The inhibitory effect of water addition on methane oxidation was most apparent at the end of the experiment at the CH4 feed rate of 147.2 gm⁄m2-day. Moreover, the average CH4 uptake data do not suggest any effect associated with pulsed flow in CH4 oxidation. The maximum oxidation rate measured in this study was 664.2 gm CH4⁄m2-d but was only measured in one of the eight columns.

Both the field and laboratory results indicate that high moisture can inhibit methane oxidation. It is encouraging to note that during the dry season, there was no increase in emissions during periods when the gas collection system vacuum was reduced. There was an increase in CH4 emissions at low vacuum during the rainy season. However, this may not be critical as the hottest temperatures and associated highest demand for electricity are likely to occur during the dry season. Except for the soil and 0.31 m green waste, all covers of different thicknesses performed well. Therefore, the cover type to be used in the field should be based on availability of the material and cost.