Landfill Gas Wellfield Spacing: A Saturation Bound from First-Order Generation and Pressure Diffusion, and an Honest Bound on Powering a Town

Landfill gas collection is often described with two loose claims: that closer well spacing always captures more gas, and that a landfill can power its town. This paper makes both claims quantitative for a stated case and bounds them. SIMULATED, NOT MEASURED: all results are modelled. For a landfill receiving 600,000 Mg of waste over 20 years, with an ultimate methane yield of 100 cubic metres per Mg and a first-order decay constant of 0.04 per year, total methane generation over 30 years is 33.5 million cubic metres, peaking at 1.69 million cubic metres per year in year 20. Pressure diffusion through waste is fast enough that diffusion alone does not set the radius of influence; air intrusion at the cover caps the practical radius at 38 m in this model. Capture efficiency saturates as spacing tightens: 91.0 percent at 40 m spacing, 78.6 percent at the 50 m design spacing, and 49.6 percent at 75 m. Tightening from 50 m to 30 m raises modelled capture to the 92 percent ceiling but raises the well count on the 50,000 square metre footprint from 20 to about 56. At 50 m spacing, 26.4 million cubic metres are captured over 30 years, 78 percent of captured gas is utilised and 22 percent flared, producing 67,400 MWh of electricity in total, an average of 0.26 MW. Against a stated town demand of 100,000 MWh per year, that is 2.2 percent on average, equivalent to about 214 homes. The claim that this landfill can power the town is false for the sized case by a factor of about 45.