Choice of method

The decision tree in Figure 3.3 describes good practice in adapting the methods to national circumstances. Emissions can be estimated from continuous emissions monitoring (CEM) where emissions are directly measured at all times, periodic emissions monitoring that is undertaken over a period(s) that is reflective of the usual pattern of operation of the plant to derive an emission factor that is multiplied by output to derive emissions, irregular sampling to derive an emission factor that is multiplied by output to derive emissions, or by multiplying a default emission factor by output.

Methods are classified according to the extent of plant-level data that are available. Both Tier 2 and Tier 3 require plant-level activity data.

TIER 1 METHOD

Emissions are estimated as follows:

Where:

EN2O = N2O emissions, kg

EF = N2O emission factor (default), kg N2O/tonne adipic acid produced

AAP = adipic acid production, tonnes

When applying the Tier 1 method it is good practice to assume that there is no abatement of N2O emissions and to use the highest default emission factor shown in Table 3.4.

TIER 2 METHOD

The Tier 2 method uses plant-level production data and default emission factors. Where default values are used to estimate emissions, it is good practice to categorise plants according to the abatement technology type employed and the utilisation factor of the abatement technology. Emissions are calculated as follows:

Equation 3.8

N2O emissions from adipic acid production - Tier 2

Where:

EN2O = emissions of N2O, kg

EFi = N2O emission factor for technology type i, kg N2O/tonne adipic acid produced

AAPi = adipic acid production from technology type i, tonnes

DFj = destruction factor for abatement technology type j, fraction

ASUFj = abatement system utilisation factor for abatement technology type j, fraction

The basic equation for estimating N2O emissions includes additional terms that recognise the use of N2O abatement technologies. The N2O destruction factor has to be multiplied by an abatement system utilisation factor in order to account for any down-time of the emission abatement equipment (i.e., time the equipment is not operating).

To achieve the highest accuracy, good practice is to apply this equation at the plant-level using N2O generation and destruction factors developed from plant-specific measurement data. In this case, the national total is equal to the sum of plant totals. Where plant-level information is not available, good practice provides default N2O generation factors and destruction factors as shown in Table 3.4, Default Factors for Adipic Acid Production, based on abatement technologies implemented. To use these factors, inventory compilers should verify that the abatement technology is installed at individual plants and operated throughout the year.

TIER 3 METHOD - DIRECT MEASUREMENT

The Tier 3 method uses plant level production data and plant-level emission factors obtained from direct measurement of emissions. These may be derived from irregular sampling of emissions of N2O or periodic emissions monitoring of N2O undertaken over a period(s) that reflects the usual pattern of operation of the plant. Emissions can be derived using Equation 3.8. To achieve the highest accuracy, good practice is to apply this equation at the plant-level using N2O generation and destruction factors developed from plant-specific measurement data where relevant. Given the relatively small number of adipic acid plants (about 23 globally, Choe et al., 1993), obtaining plant-specific information requires few additional resources.

Alternatively, the Tier 3 method uses the results of continuous emissions monitoring (CEM), although it is noted that most plants are unlikely to employ CEM due to the resource costs. Where CEM is employed, emissions can be estimated based on the sum of measured N2O emissions derived from the concentration of N2O in monitored emissions for each recorded monitoring interval.

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