Landfill Gas Generation (First-Order Decay)
Sums first-order decay across placement years to the peak at closure.
Example
You enter
- Waste placed per year (tons) 250000
- Years of placement 20
- Methane yield L0 (cu m per Mg) 100
- Decay constant k (per year) 0.04
- Methane fraction of the gas (%) 50
- Collection efficiency (%) 75
- Methane heating value (Btu/cu ft) 911
- Generator efficiency (%) 30
You get
- Annual megagrams 226796
- Decay sum 14.0439
- Methane generated at the peak 12,740,454 cu m/yr (449,924,899 cu ft/yr)
- Flow at the peak 856.021
- Landfill gas (CFM) 1712.04
- Heat rate 46.8 MMBtu/h
- Gross generation if all of it arrived 4,114 kW
- Honest peak after collection 3085.52
- Ten years after closure 2,068 kW (67% of the peak)
- Twenty years after closure 1386.41
Checked against published worked examples from: US EPA Landfill Methane Outreach Program.
Details, formula, and sources
ScopeSums first-order decay across placement years to the peak at closure, then converts methane flow to heat rate and generating capacity and decays it forward. The worked site peaks at 3.1 MW collected and falls to 45% of that twenty years on. Generation is not collection, and the difference is the site's own emission.
Formula or tableQ = sum over placement years of k x L0 x M x e^(-k t); capacity = methane flow x heating value x generator efficiency, decayed forward as e^(-k t).
Source and editionFirst-order decay, with the AP-42 inventory defaults of a methane yield of 100 cubic metres per megagram and a methane generation rate constant of 0.04 (roughly 0.02 arid, 0.05 to 0.07 wet or bioreactor). The Clean Air Act regulatory defaults for the NMOC calculation, 40 CFR 60.764, are higher: 170 cubic metres per megagram and 0.05 (0.02 under 25 in of annual precipitation).
Free accessThe model and its defaults are published by the EPA and are freely available.
What governsEstimate. AHJ and licensed professional govern.
API field namesannual_tons, placement_years, methane_yield_m3_per_mg, decay_constant_per_year, methane_fraction_pct, collection_efficiency_pct, methane_heating_value_btu_per_cf, generator_efficiency_pct, annual_megagrams, decay_sum, methane_m3_per_year, methane_cfm, landfill_gas_cfm, heat_rate_mmbtu_per_hr, gross_capacity_kw, collected_capacity_kw, capacity_10yr_kw, capacity_20yr_kw
- Decay constant where the uncertainty lives; it depends overwhelmingly on moisture and a factor of two or three between an arid and a wet site is ordinarysite gas data
- Peak modelled at closure, when every placement year contributes and none has decayed farspec-v1790 scope
- Time step whole years, the newest counted at age zero; EPA's LandGEM steps in tenths of a year and reads 2.2% lower at closure at k = 0.04, 3.8% at 0.07, and 31% at its wet-inventory 0.7LandGEM v3.02 equation