Modelling of CH4 Formation Diffusion and Oxidation in Landfills and Cover Soils

Bogner et al. (1997) developed a simulation model describing diffusion and oxidation of CH4 in a landfill cover soil in terms of collisions of CH4 molecules with soil particles and biomass. The model was successfully validated using field data, but the approach is unconventional in gas transport modelling, and the conceptual validity of the model assumptions remains an issue. Simulation models based on more conventional concepts were developed by Hilger et al. (1999), Stein et al. (2001) and De Visscher and Van Cleemput (2003b).

The model of Hilger et al. (1999) is based on the Stefan-Maxwell equations for gas flow and diffusion. Here, a model applicable to thick biofilms was used. Oxygen is assumed to be the only limiting substrate. This is an acceptable assumption because the depth of the methanotrophically active zone is limited by oxygen penetration, but it might lead to overestimates of the CH4 oxidation close to the soil surface, where CH4 concentration is low.

The model of Stein et al. (2001) incorporates flow and Fickian diffusion with concentration-dependent diffusion coefficients. This approach is well established in chemical engineering because it combines high accuracy with ease of implementation (Froment and Bischoff, 1990). A dualsubstrate CH4 oxidation model was used to describe both CH4 and oxygen limitation. Perera et al. (2002a) developed a variant with the aim of calculating the relationship between local CH4 production and CH4 emission. Recently, this model was combined with a geographic information systems (GIS) approach to obtain a 'pseudo-3D'

model (Perera et al., 2004). A two-dimensional (2D) model was developed by Perera et al. (2002b) with the aim of optimizing closed-box measurements on landfills.

The model of De Visscher and Van Cleemput (2003b) incorporates StefanMaxwell mass transport equations. They used the same dual-substrate model for CH4 oxidation as Stein et al. (2001). The model also incorporates a growth model, which makes possible the calculation of a meth-anotrophic activity profile from minimal information. A sensitivity analysis was also performed, which indicated that the parameter ^max>max, describing the maximum value of Vmax that can be supported by the soil, has the largest influence on the model predictions.

Figure 12.6 shows model predictions of the influence of environmental factors on CH4 oxidation efficiencies at conditions given in Table 12.2. Temperature and moisture content are the main governing factors of the process. The temperature results suggest a Q 10 value below 2, which is less than the physiological Q10. This is due to diffusion limitation effects, as discussed in Section 12.2.3. Mahieu et al. (2005) extended the model of De Visscher and Van Cleemput (2003b) to include isotope frac-tionation effects.

In conclusion, artificial CH4 sinks - such as those that form in landfill cover soils -represent an important component of net CH4 exchange globally. The efficiency at which CH4 is removed before emission to the atmosphere is highly dependent on soil water content, temperature and CH4 supply.

Table 12.2. Environmental factors for the simulation of laboratory conditions and field conditions in Fig. 12.6. (From De Visscher and Van Cleemput, 2003b.)

Factor

Laboratory value

Field value

T (°C)

22

10

e (m3gas/m3soil)

0.412

0.25

Gas flux (mol/m2/s)

3.1 x 10-4

1 x 10-4

CH4/CO2 (mol/mol)

50/50

50/50

f (m3void/m3soil)

0.5878

0.5

PDB (kgsoil DW/m3)

1039

1300

100 90 807060504030 20 10 0

Field

y _ab

Temperature (°C)

40 50 CH4 mole fraction (%)

CH 40

0L 0

CH 40

800 1000 1200 1400 1600 1800 Soil bulk density (kgsoilDW/m)

Fig. 12.6. Influence of environmental factors on methane (CH4) oxidation predicted by the model for laboratory soil column (solid lines) and field conditions (dotted lines): (a) temperature; (b) water-filled pore space (as percentage of total void space); (c) CH4 mole fraction in the landfill gas (at constant total flux and at constant CH4 flux); (d) soil bulk density. (From De Visscher and Van Cleemput, 2003b.)

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