Choice of method for estimating N2O emissions

Nitrous oxide is emitted in combustion processes at relatively low combustion temperatures between 500 and 950 °C. Other important factors affecting the emissions are the type of air pollution control device, type and nitrogen content of the waste and the fraction of excess air (BREF, 2005; Korhonen et al, 2001; Loffer et al, 2002; Kilpinen, 2002; Tsupari et al., 2005). N2O emissions from the combustion of fossil liquid waste can be considered negligible, unless country-specific data indicate otherwise.

Figure 5.2 provides a general decision tree for the estimation of N2O emissions from incineration and open burning of waste. The most accurate results will be obtained if N2O emissions are determined for each plant based on the plant-specific monitoring data, and then summed.

The calculation of N2O emissions is based on the waste input to the incinerators or the amount of waste open-burned and a default emission factor. This relationship is summarised in the following Equation 5.5:

Equation 5.5

N2O emission estimate based on the waste input to the incinerators

Where:

N2O Emissions = N2O emissions in inventory year, Gg/yr IWj = amount of incinerated/open-burned waste of type i , Gg/yr EFi = N2O emission factor (kg N2O/Gg of waste) for waste of type i 10-6 = conversion from kilogram to gigagram i = category or type of waste incinerated/open-burned, specified as follows:

MSW: municipal solid waste, ISW: industrial solid waste, HW: hazardous waste, CW: clinical waste, SS: sewage sludge, others (that must be specified)

The amount and composition of waste should be consistent with the activity data used for the calculation of CO2 and CH4 emissions.

Default emission factors are provided in Section 5.4.3. However, inventory compilers should be aware that default emission factors for N2O emissions from incineration and open burning of waste have a relatively high level of uncertainty. The use of country-specific data are preferable, if they meet quality assurance and quality control criteria outlined in Section 5.8 and in Chapter 6, QA/QC and Verification, in Volume 1. If N2O emissions from incineration or open burning of waste are key categories, it is good practice to use a higher tier.

Tier 2 uses the same method as for the Tier 1, however, country-specific data are used to obtain emission factors. Where practical, N2O emission factors should be derived from emission measurements. Where measured data are not available, other reliable means can be used to develop emission factors.

Emission factors for N2O differ with type of facility and type of waste. Emission factors for fluidised-bed plants are higher than those for plants with grate furnaces. Emission factors for MSW are usually lower than for sewage sludge. Ranges of N2O emission factors reflect abatement techniques, such as the injection of ammonia or urea used in some NOx abatement technologies that may increase emissions of N2O, temperature, and the residence time of the waste in the incinerator.

Tier 2 is applicable when country-specific emission factors are available but no detailed information on a plant-by-plant basis or further differentiated by management practices are available.

Tier 3 methods are based on site-specific data on flue gas concentrations. Equation 5.6 indicates the relevant factors of influence and enables to estimate N2O emissions.

Where:

N2O Emissions = N2O emissions in inventory year, Gg/yr IWi = amount of incinerated waste of type i, Gg/yr

EC! = N2O emission concentration in flue gas from the incineration of waste type i, mg N2O/m3

FGVj = flue gas volume by amount of incinerated waste type i, m3/Mg

10-9 = conversion to gigagram i = category or type of waste incinerated/open-burned, specified as follows:

MSW: municipal solid waste, ISW: industrial solid waste, HW: hazardous waste, CW: clinical waste, SS: sewage sludge, others (that must be specified)

Tier 3 outlines the most detailed and accurate approach, where data on a plant-by-plant basis or for various management practices are available. It requires data on the flue gas volume and concentration of N2O emissions in the flue gas. Continuous emission monitoring is technically feasible, but not necessarily cost-effective. Periodic measurements should be conducted sufficiently often to account for the variability of N2O generation (i.e., due to the nitrogen content in the waste), and different types of incinerator operating conditions (e.g., combustion temperature, with or without daily shut down).

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