Choice of emission factors

TIER 1 METHOD

It is good practice to use the default emission factor shown in Table 3.4 and to assume that there is no abatement of N2O emissions.

TIER 2 METHOD

If plant-level factors are not available, it is good practice to use default factors. The Tier 2 method is based on default emission factors. These default values often represent midpoint or mean values of data sets (as determined by expert analysis). The extent to which they represent a specific plant's emission rate is unknown. Default factors in Table 3.4 should be used only in cases where plant-specific measurements are not available.

Also included in Table 3.4 are default N2O destruction factors for commonly used abatement technologies, and associated uncertainties. To use these factors, inventory compilers should verify that the abatement technology is installed at individual plants and operated throughout the year. Failure to determine if abatement technologies are being used can result in overestimation of emissions. Determination of the appropriate values requires that plants be classified according to the abatement technology implemented..

TIER 3 METHOD

The Tier 3 method requires plant measurements. If the N2O abatement system is in use, plant measurements provide the most rigorous data for calculating net emissions (i.e., N2O generation and destruction factors). Monitoring N2O emissions from adipic acid production is practical because these are point sources and there are a finite number of production plants. Given currently available technology, instrumentation for sampling and monitoring emission rates do not limit precision or accuracy of the overall measurement. Usually sampling frequency and timing is sufficient to avoid systematic errors and to achieve the desired level of accuracy.

Where the N2O abatement system is not in use, a plant-specific emission factor can be obtained from periodic monitoring of emissions which is multiplied by the production level to estimate plant-level emissions.

As a general rule, it is good practice to conduct sampling and analysis whenever a plant makes any significant process changes that would affect the generation rate of N2O, and sufficiently often otherwise to ensure that operating conditions are constant. In addition, plant operators should be consulted annually to determine the specific destruction technologies employed and confirm their use, since technologies may change over time. Precise measurement of the emissions rate and abatement efficiencies requires measurement of both the exit stream and the uncontrolled stream. Where measurement data are available only on the exit stream, good practice is to base emissions on these data. In this case, any available estimates of abatement efficiency should be provided only for information purposes and not used to calculate emissions.

Figure 3.3 Decision tree for estimation of N2O emissions from adipic acid production

Figure 3.3 Decision tree for estimation of N2O emissions from adipic acid production

Note:

1. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

Note:

1. See Volume 1 Chapter 4, Methodological Choice and Identification of Key Categories (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

Table 3.4

Default factors for adipic acid production

Production Process

N2O Generation Factor"'"

Uncertainty Estimate

Nitric Acid Oxidation

300 kg/tonne adipic acid (uncontrolled)

± 10% (based on expert judgement). The range of 300 kg ± 10% encompasses the variability from pure ketone to pure alcohol feedstocks, with most manufacturers somewhere in the middle.a

Abatement Technology

N2O Destruction Factorb

Uncertainty Estimate

Catalytic Destruction

92.5%

90-95% (based on expert judgement). Manufacturers known to employ this technology include: BASF (Scott, 1998), and DuPont (Reimer, 1999b).

Thermal Destruction

98.5%

98-99% (based on expert judgement). Manufacturers known to employ this technology include: Asahi, DuPont, Bayer, and Solutia (Scott, 1998).

Recycle to Nitric Acid

98.5%

98-99% (based on expert judgement). Manufacturers known to employ this technology include: Alsachemie (Scott, 1998).

Recycle to feedstock for Adipic Acid

94%

90-98% (based on expert judgement). Solutia implemented this technology around 2002.

Abatement System

Utilisation Factor"

Uncertainty Estimate

Catalytic Destruction

89%

80-98% (based on expert judgement)0.

Thermal Destruction

97%

95-99% (based on expert judgement)c.

Recycle to Nitric Acid

94%

90-98% (based on expert judgement)c.

Recycle to Adipic Acid

89%

80-98% (based on expert judgement/.

a With regard to a value from the Japan Environment Agency (1995) (282 kg N2O/tonne adipic acid), it is believed that this manufacturer uses oxidation of pure cyclohexanol (alcohol), instead of a ketone-alcohol mixture (Reimer et al., 1999). This is the only plant known to use this method.

b The destruction factor (that represents the technology abatement efficiency) should be multiplied by an abatement system utility factor.

c Note that these default values are based on expert judgement and not industry-supplied data or plant-specific measurements. In the first 1-5 years of the abatement technology implementation, the utilisation factor tends to be at the lower end of the range. Lower utility of the equipment typically results because of the need to learn how to operate the abatement system and because more maintenance problems occur during the initial phase. After 1-5 years, the operating experience improves and the utilisation factor would tend to be at the high end of the range.

Source:

d Thiemans and Trogler (1991).

e Reimer (1999b).

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