Assessing the effects of compaction and saturation on diffusion and diffusive fractionation of methane

Abstract

A considerable contribution to the greenhouse effect originates from landfills. This contribution originates from the degeneration of waste in the landfill. The gas which is created during the degeneration consists of 40 to 60% methane. One mole of methane has a global warming potential which is 28 times higher than that of carbon dioxide. Therefore, emissions of methane should be reduced. One method which can be used to reduce this emission is the oxidization of methane to carbon dioxide. This can be done by the construction
of a cover soil with methanotrophic bacteria in the soil. These bacteria oxidize methane as their source of carbon and energy. This oxidation process discriminates against the heavier isotopes present in the gas, this shifts the isotope signature. A method to determine the oxidation efficiency of the methane is based on the evaluation of this change in isotope signature over the cover soil. Next to the fractionation of isotopes due to oxidation, fractionation also occurs due to diffusion. However, fractionation due to diffusion is generally
assumed to be negligible. Mostly, this assumption is valid, since the dominant part of transport is advective; which does not discriminate against an isotope.

In this thesis the effect of compaction and saturation on two sands are evaluated. Using this evaluation, the criteria during which diffusion becomes an important transport phenomena are determined. This is determined by a series of experiments, where the concentration of methane is monitored while the gas diffuses through a soil sample. In this thesis the effect of the soil matrix on the fractionation of methane is also evaluated. This is evaluated by performing isotope spectrometry on samples of gas, taken at different times during the experiments. These two aspects are combined to evaluate if the oxidation efficiency can still be determined using fractionation of stable methane isotopes when diffusion plays a considerable role in gas transport.

During the experiments the decrease of methane in the chamber was measured. Spread over two soils, for a total of 18 variations in compaction or saturation the decrease of methane over time was measured. The results of the experiments show that for diffusion no distinction between variation in compaction and saturation needs to be made. Compaction and saturation can be described together for both soils using the air-filled porosity. The relation between the air-filled porosity and the effective diffusion coefficient is linear for both soils. Due to the large sand fraction the share of coarse pores is high for the whole range of compaction, meaning that no significant increase of tortuosity is visible in the relation between air-filled porosity and the effective diffusion coefficient. When the effective diffusion coefficients are compared to effective permeability values of these soils, it showed that the diffusion is more important for drier and more compacted soils. Thus, diffusive transport becomes important for gas transport over a cover soil when the pressures are lower, the soil is drier, and the soil is more compacted.

The fractionation factors due to diffusion, which have been determined for a selection of experiments, showed no trend with any other measured parameters. The precision of the measured values from which the fractionation factors are determined is high. Thus, the confidence in the fractionation factors is high.
The lack of a trend present means that in sand the soil matrix has no effect on the fractionation factor. This means that the fractionation factor due to diffusion in soil is the same as in free air. For the calculation of the oxidation efficiency with a relevant diffusive flux the ratio of diffusive to advective flux needs to be
determined but the fractionation factor due to diffusion is constant. Before the oxidation efficiency can be calculated, first the load needs to be corrected for any loss through hot-spots. Next, the advective flux and diffusive flux are combined, this is done by adding the fluxes together. The fluxes can be added because there are no interdependent effects, each flux only increases the total flux. Thus, the oxidation efficiency can be calculated using the changes in isotope signatures, even when a significant diffusive flux is present.