Scope Type: Direct Emissions

Description of Emissions

Direct GHG emissions are principally the result of the following types of activities occurring at companies or institutions:

• Generation of electricity, heat, or steam:  These emissions result from combustion of fuels in stationary sources, such as, boilers, furnaces, turbines, etc.

• Physical or chemical processing:  Most of these emissions result from manufacturing or processing chemicals and materials, such as, cement, aluminum, adipic acid, ammonia, and waste.

• Transportation of materials, products, waste, and employees:  These emissions result from the combustion of fuels in company owned/controlled mobile combustion sources (e.g., trucks, trains, ships, airplanes, buses, and cars).

• Fugitive emissions:  These emissions result from intentional or unintentional releases, e.g., equipment leaks from joints, seals, packing, and gaskets; methane emissions from coal mines and venting; HFC emissions during the use of refrigeration and air conditioning equipment; and methane leakages from gas transport.

Emissions from Combustion

The combustion of fuels produce emissions of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O).  Carbon dioxide accounts for the majority of greenhouse gas emissions from most combustion units.  When weighted by their GWPs, CO₂ typically represents over 99 percent of the greenhouse gas emissions from the combustion of fossil fuels.  Therefore, only basic guidance is necessary on the estimation of CH₄ and N₂O emissions from the combustion.

The energy content of a fuel is an inherent chemical property that is a function of the number and types of chemical bonds in the fuel.  The carbon content of a fuel (i.e., the fraction or mass of carbon atoms relative to the total mass or number of atoms in the fuel) is also an inherent chemical property.  The vast majority of the energy released during combustion results from the breaking of chemical bonds between carbon and hydrogen atoms and the formation of a double bond between those same carbon atoms and oxygen atoms in the ambient air.  Therefore,both the amount of heat released from the combustion process and the amount of CO₂ produced are functions of the amount of carbon in the fuel.  A small fraction of the carbon in the fuel can escape oxidation and remain as a solid after combustion in the form of soot or ash.  The nature of the combustion process allows CO₂ emissions to be estimated based on a simple mass balance approach that accounts for the mass of carbon entering the combustion process in the form of fuel and the amount of carbon exiting the process in the form of CO₂, other carbon containing gases, soot, or ash.

The approach used to estimate CO₂ emissions varies significantly from that required to estimate CH₄ and N₂O emissions.  Methane and N₂O emissions depend not only upon fuel characteristics, but also on the combustion technology type, conditions within the combustion chamber, usage of pollution control equipment, and ambient environmental conditions.  However, since the amount of CH₄ and N₂O emissions is a small fraction of the CO₂ emissions, an approximation of these two emissions may be sufficient.  Users have the ability to enter their own custom values based on flue gas lab tests, but the default coefficients should suffice for the majority of users.
 
Calculation based methods typically entail the collection of :

• Activity data:  in the form of the quantity of fuel consumed for combustion purposes

• Emission factor data:  in the form of information on the characteristics of the fuel combusted and the efficiency of the oxidation process

Calculation of CO₂ Emissions

To calculate CO₂ emissions using fuel consumption and emission factor data, the following equations can be applied:

        where:
        E = Mass emissions of CO₂ (short tons or metric tons)
        A_f,v = Volume of fuel consumed (e.g., L, gallons, ft³, m³)
        A_f,m = Mass of fuel consumed (e.g., short tons or metric tons)
        A_f,h = Heat content of fuel consumed (GJ or million Btu)
        F_c,v = Carbon content of fuel on a volume basis (e.g., short tons C/gallon or metric tons C/m³)
        F_c,m = Carbon content of fuel on a mass basis (e.g., short tons C/short ton or metric tons C/metric ton)
        F_c,h = Carbon content of fuel on a heating value basis (e.g., short tons C/million Btu or metric tons C/GJ)
        F_ox = Oxidation factor to account for fraction of carbon in fuel that remains as soot or ash
        (44/12) = The ratio of the molecular weight of CO₂ to that of carbon

Depending on the type of fuel, the volume, mass, or heat content can be converted directly from EnergyCAP bills and converted to one of the usable units required for the equation above.  Appendices B, C, D, and E list the appropriate factors.

Determining coal, and to a lesser extent oil and natural gas, consumption can problematic.  It may be necessary to estimate the amount of coal in storage piles (or oil and natural gas in storage tanks) at the beginning of the year and at the end of the year to determine stock changes.  Using purchase and delivery records can also be problematic, as fuel from one delivery may not be used up before the end of the year.  For these reasons, a combination of fuel metering and purchase and delivery records may be needed to ensure the estimation of the amount of fuel consumed each year is complete and consistent over time.

In cases where combustion units are co-fired with a mix of fossil and biomass fuels, the biomass and fossil fuel components of the CO₂ emissions should be calculated separately, using carbon content and oxidation factors related to the specific fuels.  The resulting emissions from biogenic carbon should be reported separately.

Calculation of CH₄ and N₂O Emissions  

Fuel characteristics (e.g., calorific value), the type of technology (e.g., combustion, operating and maintenance regime, the size and the vintage of the equipment), and emission controls, are major factors in determining rates of emissions of CH₄ and N₂O gases from stationary sources.  Specifically, N₂O emissions are closely related to air-fuel mixes and combustion temperatures, as well as the composition and operating temperature of any catalytic emission control equipment employed.  Methane emissions from stationary combustion are primarily a function of the CH4 content of the fuel and combustion efficiency.

Unlike for CO₂, the emissions of CH₄ and N₂O from the combustion of biomass fuels should be included with emissions from the combustion of fossil fuels.

Given the dependence on specific combustion conditions and other characteristics, the preferred approach for estimating CH₄ and N₂O emissions is to use a method based on combustion unit-specific data.  This method generally calls for the use of detailed activity data and emissions factors that account for these characteristics.  Facility specific emission factors may also be based on direct measurements.  Additional information on more technologic and fuel-specific emission factors can be found in the IPCC Emission Factor Database, U.S. EPA’s AP-42, and the European Environment Agency’s EMEP/Corinair Emission Inventory Guidebook.

The IPCC Guidelines also provide default stationary combustion emission factors (which assume no emission controls are in place) for five sectors :

1. Energy Industry
2. Manufacturing Industry
3. The Commercial/Institutional Sector
4. The Residential Sector
5. Agriculture/Forestry/Fishing Sectors

The default emission factors are listed in Appendix F.  Emissions estimated using these factors have a degree of uncertainty associated with them.  Users can also select emission factors from one of the databases listed above or develop and input their own factors.

To calculate CH₄ emissions using fuel consumption and emission factor data, the following equations can be applied:

    where:
    E = Mass emissions of CH₄ (short tons or metric tons)
    A_f,h = Heat content of fuel consumed (GJ or million Btu)
    EFm,h = Emissions Factor for Methane from Appendix F on a heating value basis (lb/million Btu or Kg/GJ)
    C = control efficiency/utilization of any emission control equipment (percent)
    CF = Conversion Factor to correct to short tons or metric tons

Similarly, to calculate N₂O emissions using fuel consumption and emission factor data, the following equations can be applied:

    where:
    E = Mass emissions of N₂O (short tons or metric tons)
    Af,h = Heat content of fuel consumed (GJ or million Btu)
    EFn,h = Emissions Factor for Nitrous Oxide from Appendix F on a heating value basis (lb/million Btu or Kg/GJ)
    C = control efficiency/utilization of any emission control equipment (percent)
    CF = Conversion Factor to correct to short tons or metric tons

Calculation of Fluorinated Gas and Non Combustion Emissions 

 Calculating emissions from any of the three classes of fluorinated gases (HFCs, PFCs, and SF₆) are done approximately the same way. Users must know the quantity of each gas and multiply it by a corresponding GWP Emission Factor.  Also, additional emissions of CO₂, CH₄, and N₂O may occur as a consequence of a manufacturing or industrial process.  Those subsequent emissions would be calculated in a similar fashion.