This methodology represents carbon dioxide (CO2) and methane (CH4) emissions associated with the flaring of methane. The data and calculation methodology are sourced from the IPCC, as published in Volume 2, Chapter 4 - Fugitive Emissions of their 2006 IPCC Guidelines for National Greenhouse Gas Inventories.
The methodology represents a mass-balance approach which considers the quantity of carbon (C) which enters the combustion process and calculates the resulting associated emissions of CO2 and CH4 using conversion factors describing the physical processes involved.
When methane (CH4) is burned in the presence of oxygen (O2) the following sequence of reactions occurs:
1. CH4 + O2 => CO + H2 + H2O
2. 2H2 + O2 => 2H2O
3. 2CO + O2 => 2CO2
with the net reaction occuring as:
4. CH4 + 2O2 => CO2 + 2H2O
As such, the burning of methane produces CO2. Indeed, as can be seen from the final (net) reaction, for each molecule of CH4 burned, 1 molecule of CO2 is produced. Since the molecular masses of CH4 (16) and CO2 (44) are constant, the quantity (mass) of CO2 produced can be calculated as 2.75 (44/16) times the mass of CH4 burned.
However, the above reactions assume full combustion, i.e. 100% of the methane carbon (C) is oxidised according to reaction 1. In the case where incomplete combustion occurs, a residual emission of methane remains according to the efficiency (%) of the combustion process.
Emissions are therefore calculated on the basis of the quantity of methane combusted, and the efficiency with which combustion is considered to have taken place.
Several data values are used in this methodology:
- density of CH4: This enables the conversion of of volumetric quantities of CH4 into a mass-based measure, which, in turn, enables the calculation of emissions by mass. This takes the value of 0.67 kg / m3 by default.
- efficiency of combustion: Specifies the proportion of CH4 which undergoes combustion. Default, 98%.
- ratio of molecular masses of reactant CH4 and product CO2 under oxidation: 2.75
- global warming potential of CH4: Enables the conversion of CH4 emissions into a CO2e quantity - i.e. the quantity of CO2 which would exert the same atmospheric warming effect.
Activity data required
Greenhouse gas emissions are directly proportionate to the volume of CH4 flared, which therefore must be provided in order to calculate. In addition, the specific values for combustion efficiency and CH4 density can be provided if available, rather than using the default values.
Calculation and results
CO2 emissions are calculated by multiplying the quantity of CH4 flared, by the combustion efficiency factor (%) and the molecular mass ratio for CO2:CH4.
CH4 emissions are calculated by multiplying the quantity of CH4 flared by the unburned fraction (1 - the combustion efficiency factor (%)).
Three emissions quantities are ultimately provided, representing: CO2, CH4, total CO2e emissions. All emissions calculated by this methodology represent those attributable to the specified quantity of methane flared.