Landfill Methane Estimator

Buried municipal waste decomposes anaerobically, and the resulting gas is roughly half methane and half carbon dioxide by volume. Generation is modelled as first-order decay: degradable organic carbon declines exponentially, so gas production falls with the age of the material. The two governing parameters are L0, the methane generation potential in cubic metres per megagram, reflecting degradable organic content, and k, the decay constant in reciprocal years, setting how fast that potential is released. EPA inventory defaults are k of 0.04 with L0 of 100, while Clean Air Act regulatory defaults are k of 0.05 with L0 of 170; arid sites below about 635 mm of annual precipitation use k of 0.02, since moisture limits microbial activity. A k of 0.04 corresponds to a half-life of roughly 17 years, which is why closed landfills continue producing gas for decades. The factor of two reflects gas being about half methane, and sources differ over whether it applies to total gas or methane alone, so confirm which basis your L0 assumes. Waste mass must be in megagrams, that is metric tonnes; entering kilograms overstates output a thousandfold. Methane density of 0.717 kg per cubic metre applies at 0 degrees C and one atmosphere, falling to about 0.668 at 20 degrees C by ideal-gas scaling with absolute temperature. The energy figure is fuel energy, not deliverable electricity, since reciprocating engines convert at roughly 30 to 40 percent efficiency. This single-lump form assumes all waste was placed at once, whereas real sites need year-by-year summation, and collection efficiency varies from about 50 to 95 percent with cover and wellfield density.

The first-order decay approach traces to the Scholl Canyon model, developed around 1980 by EMCON Associates in connection with the Scholl Canyon Landfill in Glendale, California. Its insight was to treat methane generation not as a fixed yield but as exponential decay of the degradable fraction, which matched gas well data far better than earlier mass-balance estimates. That formulation remains the mathematical core of essentially every landfill gas model in use today. The US Environmental Protection Agency built the Landfill Gas Emissions Model, LandGEM, around it to support regulation of municipal solid waste landfills. New Source Performance Standards and Emission Guidelines under 40 CFR Part 60 Subpart WWW were promulgated in March 1996, requiring larger landfills exceeding a non-methane organic compound threshold to install gas collection and control. LandGEM Version 3.02 was released in May 2005 and is still the version most commonly cited. In parallel, the IPCC moved national inventory practice from the simpler default mass-balance method of its 1996 guidelines to the first-order decay method set out in Volume 5, Chapter 3 of the 2006 Guidelines, which is now the expected approach for reporting under the UN climate framework. The EPA launched its Landfill Methane Outreach Program in 1994 to promote energy recovery rather than flaring, mandatory reporting for large landfills followed under 40 CFR Part 98 Subpart HH from 2010, and emission standards were tightened again with Subpart XXX in 2016.