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 12740454 cu m/yr (449924899 cu ft/yr)
- Flow at the peak 856 cfm of methane, 1712 cfm of landfill gas
- Landfill gas (CFM) 1712.04
- Heat rate 46.8 MMBtu/h
- Gross generation if all of it arrived 4114 kW
- Honest peak after collection 3086 kW -- 1029 kW escapes through the cover
- Ten years after closure 2068 kW (67% of the peak)
- Twenty years after closure 1386.41
Details, formula, and sources
Sums 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.
Q = 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).
First-order decay, with the Clean Air Act default methane yield of 100 cubic metres per megagram and a methane generation rate constant of roughly 0.02 arid, 0.04 conventional, and 0.05 to 0.07 wet or bioreactor.
The model and its defaults are published by the EPA and are freely available.
Estimate. AHJ and licensed professional govern.
Field names used by the API: annual_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