A Method for Estimating the Rate of Landfill Gas Generation By Measurement and Analysis of Barometric Pressure Waves

Estimation of the rate of landfill gas (LFG) generation is needed (1) to satisfy USEPA and state regulatory requirements associated with estimating non-methane organic carbon emissions; (2) to assess the impact of landfill-generated methane on global warming; (3) as part of the design of LFG and methane control systems; and (4) to provide information necessary to evaluate and design LFG-to-energy projects. Current methods for estimating LFG generation are not sufficiently accurate to reliably meet these needs. In the first case, inaccurate estimation of LFG generation rate can lead to installation of a costly and unnecessary LFG collection system. For the second, third, and fourth cases, the reliability of the estimated LFG generation rate is critical to technical and/or financial project success. This paper presents an improved methodology for estimating the LFG generation rate called the “baro-pneumatic method”. The baro-pneumatic method is based on the recognition that the rate of landfill gas generation can be determined with reasonable engineering accuracy by analysis of the transient pressure responses at depth in the landfill to natural variations in barometric pressure. The method consists of monitoring atmospheric pressures at landfill surface and subsurface pressures in the landfill (and, where appropriate, in surrounding soils) over a period of 2-5 days. The pressure monitoring data are then used to calibrate a site-specific, distributed-parameter gas flow model of the landfill based on Darcy’s Law and the continuity equation. LFG generation rates and gas permeabilities of the refuse, cover, and soils are varied to match the observed atmospheric pressures 2 at the landfill boundary to the pressures observed in the refuse and soils. Examples of baropneumatic testing at several landfills suggest that baro-pneumatic estimates of LFG generation rate more reliably predict LFG recovery rates actually achieved by LFG collection systems than estimates by other methods.