Methane flaring

Summary

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

Emissions model

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.

Model data

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.