Landfill Gas Issues for Design of Monofill Alternative Covers, Horacio Ferriz

LANDFILL GAS ISSUES FOR DESIGN OF MONOFILL ALTERNATIVE COVERSHoracio Ferriz

HF Geologic Engineering, 14637 Claribel Rd., Waterford CA 95386 - Tel. (209) [email protected]

There seem to be two schools of thought regarding the need to address landfill gas issues in the design ofmonofill alternative covers:

The cover is not intended to control gas (gas is controlled by the extraction system), so gasmigration through the cover does not need to be addressed in comparing the performance of onetype of cover against another.

Exceptions:

- A porous cover (sand) would allow influx of air into the landfill gas extractionsystem, decreasing its performance. As far as the landfill gas engineer isconcerned, the use of monofill earth covers may lead to lower gas productionsbecause anaerobic conditions are destroyed by the influx of air!

- A clay cover will keep air out, but only as long as it does not crack.

1.

The cover is part of the gas control system (in fact may be the only gas control system in old,abandoned landfills with no gas extraction wells), so the potential diffusion or leakage of gasthrough the cover needs to be quantified. This is an issue of concern to toxicologists (for H2S andVOCs), air pollution regulators, and landfill landscapers.

2.

The California regulations take the following stand:

Section 21140(a), Title 27 CCR - CIWMB

"The final cover shall function with minimum maintenance and provide waste containment toprotect public health and safety by controlling, at a minimum, vectors, fire, odor, litter andlandfill gas migration. ..."

THE BASICS

The mathematics of gas flow through porous media get to be pretty horrendous (so I am told), and arecomplicated by heterogeneity of the porous medium, barometric pumping in response to diurnalvariations in barometric pressure, departures from ideal gas behavior (so you need to start working withfugacities), and spatial variations in landfill gas pressure.

Gas flows through refuse or soils either by convection or by diffusion.- Convection occurs when total gas pressure is not uniform throughout the system (i.e.,

when a total pressure gradient exists). Convective flow is in the direction in which totalpressure decreases, because gases tend to move from regions of high pressure to regionsof low pressure.

- Diffusive flow of a gas is in the direction in which its concentration (partial pressure)decreases.

Modeling gas flow through porous media requires a set of equations describing mass transport for each

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gas, including terms for convective and diffusive flow.

CONVECTIVE FLOW

For the flow of a single gas through a porous granular layer one could use Darcy's law (first, conservativeapproximation, because flow of gas through a cover could very well be turbulent). The problem is how toestimate the parameters, such as Kg and dh/dl.

Q = KgA(dh/dl)

Coefficient of permeability

The coefficient of permeability for a gas, Kg, can be expressed as

where

Table 1. Gas properties (0C and 1 atm)

CH4 CO2 Landfill gas50%CH450% CO2

Air

Viscosity (*10-5 Pasec) 1.03 1.39 1.21 1.71

Mass density (g/cm3) 0.72 1.97 1.35 1.29

Molecular mass (g) 16.0 44.0 30.0 28.9

Under unsaturated conditions Kg is a function of moisture content, since the volume of moisture in thepores determines the pore space available for gas migration. As moisture content increases, gaspermeability decreases.

Pressure gradient


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DIFFUSIVE FLOW

Gas emissions through a granular monofill cover can be treated as a diffusion-controlled process usingFick's Law for steady-state diffusion. For a given volatile compound i (e.g., methane), the emission ratecan be expressed as:

The effective diffusion coefficient for a volatile compound in soil, Dei, can be computed using theempirical relationship proposed by Currie (1961):

Assuming that the volatile compound exerts pure component vapor pressure, then the saturation vaporconcentration can be determined using the ideal gas law:


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Clearly, a significant amount of basic data needs to be collected to make a meaningful evaluation of theperformance of a landfill cover for control of landfill gas emissions.

FIELD STUDIES

Besides general statements such as "we don't have a gas problem", there is limited formal field data aboutgas migration through monofill covers. There is probably quite a large volume of data available, but Ionly know of one published study, by Carman and Vincent (1998), and even in this there was practicallyno documentation as to the nature of the landfill cover. Of interest to me was the fact that theirmeasurement devices had a lower detection limit for methane in soil gas of 0.2 percent (2,000 ppm) andof 0.1 percent (1,000 ppm) for atmospheric methane. If these values are representative of the monitoringbeing done on a regular basis, then I can see why "we don't have a gas problem".

Carman and Vincent (1998) summarized their results as follows:

Methane concentrations (by volume) in soil gas and in the atmosphere were measured overseveral days in February and March of 1996, at several onsite stations on three solid wastelandfills in Wood County, Ohio. The lower detection limit for methane in soil gas was 0.2percent (2,000 ppm) and in the atmosphere was 0.1 percent(1,000 ppm). The oldest site,Asman's Landfill (1962-1973), contained no atmospheric methane at or above the detectionlimit but had the second highest methane content in soil gas of the three landfills. (Note byHF: The cover seems to have been a monofill cover built without engineering control). WalesRoad Landfill (1950s-1994) had the highest soil gas methane concentration, as high as 96percent (960,000 ppm) methane, and had some detectible atmospheric methane (as much as4,000 ppm). (Note by HF: The cover seems to have been a monofill interim cover). WoodCounty Landfill (1972-present) had the lowest average soil gas methane content of the threelandfills, but the highest atmospheric methane (as much as 8,000 ppm) (Note by HF: Thecover seems to have been a monofill interim cover).

CONCLUSIONS

I believe there are two areas where some research effort might be fruitfully spent:

- Field studies where covers are instrumented to measure gas pressures and gasconcentrations at various levels in the cover. Hopefully a sensitivity better than 0.2% canbe achieved! Would this be possible in tandem with the rest of the instrumentation beingdeployed by ACAP?

- Theoretical studies to develop analogic or numerical models of gas migration through"beds" of porous media. I believe that there is a great deal of experience archived in thechemical engineering and industrial engineering literature.

REFERENCES

Boucher, D.F., Alves, G.E., 1973, Fluid and particle mechanics: in Chemical Engineer's Handbook,McGraw-Hill Book Company, New York, p.5-1 to 5-65.

Carman, R.E., Vincent, R.K., 1998, Measurements of soil gas and atmospheric methane content in oneactive and two inactive landfills in Wood County, Ohio.

Currie, J.A., 1961, Gaseous diffusion in porous media. Part 3 - Wet granular materials: British Journal ofApplied Physics, June 1961.


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Findikakis, A.N., Leckie, J.O., 1978, Numerical simulation of gas flow in sanitary landfills: Journal of theEnvironmental Division, ASCE

Leva, A., 1949, Pressure drop correlation for a single incompressible fluid: Chemical Engineering, v.56,p.115-117.

Leva, A., Grummer, Weintraub, Pollchick, 1948, Chemical Engineering Progress, v. 44, p.511-520.

Mohsen, F.N.M., Farquhar, G.J., Kowen, N., 1977, Modeling of methane migration in soil: Journal of theEnvironmental Division, ASCE

Moore, C.A., 1968, Theoretical approach to gas movement through soils: Progress report on EPA contractno. 68-03-0326.

Thibodeaux, L.J., 1982, Models of mechanisms for the vapor phase emission of hazardous chemicalsfrom landfills. Jour. of Hazardous Materials v. 7, p.63-74.

Thibodeaux, L.J., Springer, C., Hildebrand, G., 1986, Transport of chemical vapors through soil-- alandfill cover simulation experiment: presented at 1986 Summer National American Institute of ChemicalEngineers, August 24-27, 1986, Boston, Massachusetts.

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Landfill Gas Issues for Design of Monofill Alternative Covers, Horacio Ferriz

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