Greenhouse Gas Protocol methodology for flaring

Contents

Summary

This methodology represents carbon dioxide (CO2), methane (CH4) and total carbon dioxide-equivalent (CO2e) emissions associated with the flaring of hydrocarbon-containing gases. The data and calculation methodology are sourced from the Greenhouse Gas Protocol, as published in the preadsheet tool CO2 emissions from the production of iron and steel, version 2.0.


The methodology

Emissions model

The methodology represents a mass-balance approach which considers the quantity of hydrocarbon (HC) based carbon (C) which enters the flaring process and calculates the resulting associated emissions of CO2 and CH4 using data on the hydrocarbon content of the gas and other conversion factors describing relevant chemical properties and the physical processes involved.

The volume of gas is converted into a molar quantity using the molar volume. This quantity represents the actual number of molecules within the gas. The number of hydrocarbon-bound carbon (C) atoms within the gas are estimated from this quantity using data describing the fraction of the whole gas which represents combustible hydrocarbons, and the fraction of these hydrocarbons which are carbon atoms. CO2 emissions are estimated based on the fact that each combusted carbon atom produces one molecule of CO2, together with the molecular mass of CO2 which enables a conversion of molar quantities to mass quantities.

The methodology assumes that the combustion efficiency of hydrocarbons in the gas is 98%. This means that 2% of hydrocarbons escape combustion (and therefore conversion into CO2) and are emitted to the atmosphere directly, in their original form. From a greenhouse gas perspective, such hydrocarbon emissions can be ignored - except in the case of methane (CH4), which is itself a greenhouse gas. Therefore, the methodology accounts for the quantity of non-combusted CH4 additionally, on the basis of the estimated combustion efficiency and data describing the quantity of CH4 as a fraction of the whole gas.

Model data

Several data values are used in this methodology:

  • molar volume: This enables conversion of volumetric quantities of gases into a molecular-based measures, which, in turn, enables the calculation of molar fractions for hydrocarbons and carbon. This takes the value of 379.3 ft3 / lb-mole.
  • efficiency of combustion: Specifies the proportion of hydrocarbons which undergo combustion. Default, 98%.
  • Molecular mass of CO2: 44
  • Molecular mass of CH4: 16
  • Stoichiometric ratio of C to CO2: 1
  • 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 gas flared, and also the fraction which comprises hydrocarbons (and specifically CH4), and the sub-fraction of hydrocarbons which comprises carbon. Each of these must therefore be provided in order to calculate emissions. In addition, the facility-specific values for combustion efficiency and gas molar volume can be used if available, rather than using the default values.

Calculation and results

Three emissions quantities are ultimately provided, representing: CO2, CH4, and total CO2e emissions.


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