SF6 emissions from industrial and medical particle

ACCELERATORS

SF6 is used as an insulating gas in two types of industrial particle accelerators (low and high voltage) and also in medical (cancer therapy) particle accelerators, as is the case for university and research particle accelerators. However, the emission and charge factors for industrial and medical particle accelerators are different from those of university and research accelerators, as discussed below.

Global banked capacity for industrial particle accelerators is roughly estimated to be 500 tonnes with annual SF6 emissions of 35 tonnes. Global banked capacity for medical (radiotherapy) particle accelerators is roughly estimated to be less than 5 tonnes with annual SF6 emissions of less than 5 tonnes. (Schwarz, 2005).

Figure 8.4 Decision tree for industrial and medical particle accelerators

Figure 8.4 Decision tree for industrial and medical particle accelerators

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.

Tier 1 method - country-level method

In cases where individual user accelerator charge data is unavailable, one extremely rough method involves determining the total number of particle accelerators by process description in the country and using factors to determine the country level annual emission rate as noted in Equation 8.18. For this Tier 1 method, the only data that requires collection is the total number of particle accelerators which contain SF6 by process description in the given country.

Equation 8.18

Industrial/medical accelerator emissions (country-level)

Emissions = (Number of particle accelerators that use SF6 by process description in the country) • (SF6 Charge Factor, kg) • (SF6 applicable particle accelerator Emission Factor)

Where:

Number of particle accelerators by type in the country = The total number of particle accelerators by type (industrial high voltage, industrial low voltage and radiotherapy) that use SF6 in the country, 1, 2, etc. (Only count particle accelerators that use SF6. This differs for the Tier 1 calculation for university and research particle accelerators)

SF6 Charge Factor = The average SF6 charge in a particle accelerator by process description as noted below.

SF6 particle accelerator Emission Factor = The average annual SF6 particle accelerator emission rate as a fraction of the total charge by process description.

Table 8.9

Average SF6 charge in a particle accelerator by process description

Process Description

SF6 Charge Factor, kg

Industrial Particle Accelerators - high voltage (0.3-23 MV)

1300

Industrial Particle Accelerators -low voltage (<0.3 MV)

115

Medical (Radiotherapy)

0.5a

a This is the average of values ranging from 0.05 kg to over 0.8 kg, depending on model and manufacturer. Source: Schwarz (2005)

Tier 2 method - user-level emission-factor approach

If data on the quantity of SF6 contained within each industry and medical accelerator are available, use the Tier 2 method for university and research facilities; however, multiply the emission factor for each process description provided below by the total, country-specific SF6 charge for that process description.

Table 8.10

Emission factor for each process description, (SF6 emissions from industrial and medical particle accelerators)

Process Description

Emission Factor, kg /kg SF6 charge

Industrial Particle Accelerators - high voltage (0.3-23 MV)

0.07

Industrial Particle Accelerators - low voltage (<0.3 MV)

0.013

Medical (Radiotherapy)

2.0a

a This emission factor is the average of values ranging from 1 kg to 10 kg per kg charge, depending on model, manufacturer, and service intervals.

Source: Schwarz (2005)

Tier 3 method - user-level mass-balance method

To calculate SF6 emissions from industrial and medical particle accelerators, use the same Tier 3 method as the university and research facilities. The customer service organisations for manufacturers and distributors of the equipment are likely to have information on equipment stocks, imports, and exports, and on the quantities of SF6 used to fill and refill the equipment.

EMISSIONS FROM OTHER APPLICATIONS OF SF6 AND PFCs

It is good practice to contact all gas producers/distributors to identify SF6 and PFC users and to investigate the gas consumption of source categories other than those mentioned above. The key difference among the applications discussed below is the typical delay between the purchase of the SF6 or PFC and the release of the chemical. In some cases (e.g., SF6 used in sound-proof glazing, PFCs used as heat transfer fluids), the chemical is fairly well contained during the life of the equipment or product, and most emissions are associated with the manufacture and disposal of the product. In these cases, the delay between the purchase of the chemical and its final emission depends on the lifetime of the product, ranging from three years for tyres and sport-shoes to 25 years for sound-proof glazing. In other cases (e.g., use of SF6 and PFCs as tracers or in medical applications), the chemical is fully emitted within a year of its purchase. If, as a result of an initial survey, applications with distinctive delayed emissions appear significant, then good practice is to use a source category-specific emission calculation, taking into account the delay in emissions.

Adiabatic uses

Adiabatic uses of SF6 and some PFCs exploit the low permeability of these gases through rubber. Historically, SF6 has been the dominant gas in these applications; however, PFCs with similar molecular weights (such as C3F8) have recently been used as well. Applications with a delay period of 3 years include or car tyres, sport shoe soles and tennis balls (Schwarz et al., 1996). For applications with emissions that are delayed by three years, the following formula can be used.

Equation 8.19 Adiabatic property applications

Sound-proof glazing

Double-glazed sound-proof windows: Approximately one-third of the total amount of SF6 purchased is released during assembly (i.e., filling of the double glass window) (Schwarz/Leisewitz, 1999). For the stock of gas remaining inside the window (capacity), an annual leakage rate of 1 percent is assumed (including glass breakage). Thus, about 75 percent of initial stock remains at the end of its 25-year lifetime. The application of SF6 in windows began in 1975, so disposal is only beginning to occur. Emissions from this source sub-category should be calculated using Equations 8.20 to 8.22:

Equation 8.20 Double-glazed windows: assembly

Assembly Emissions in year t = 0.33 • SF6 purchased to fill windows assembled in year t

Equation 8.21 Double-glazed windows: use

Leakage Emissions in year t = 0.01 • Capacity of Existing Windows in year t

Equation 8.22 Double-glazed windows: disposal

Disposal Emissions in year t = Amount Left in Window at End of Lifetime in year t • (1 -

Recovery Factor)

Unless country-specific data are available, a default recovery factor value of zero should be assumed in Equation 8.22. If no specific information is available for these sub-source categories, good practice is to treat them as prompt emissions.

PFCs used as heat transfer fluids in consumer and commercial applications

PFCs are used as heat transfer fluids in a number of high-power-density commercial and consumer electronic applications. Commercial applications include cooling for supercomputer, telecommunication, and airport radar systems, as well as drive units (rectifiers) on high-speed trains (Burton, 2006). These applications consume much smaller volumes of liquid PFCs than electronics manufacturing, but are believed to be significant among 'niche' applications. Consumer applications include cooling kits for desktop computers that are operated at high voltages to increase their processing speed. The specific PFCs used in these applications are believed to be similar to those identified as heat transfer fluids in electronics manufacturing in Chapter 6. In all of these applications, the liquid PFCs are used in closed modules, indicating that most emissions occur during the manufacture, maintenance, and disposal of the product or equipment. Thus, if inventory compilers can acquire information on emission rates during the manufacture, maintenance, and disposal of the equipment, along with the quantities of equipment manufactured, used, and disposed each year, they can use the Tier 2 or Tier 3 method for electrical equipment to estimate emissions. For applications with different emissions profiles (e.g., prompt emissions), the appropriate equation from Section 8.2 may be used.

PFCs used in cosmetic and medical applications

PFCs with relatively large molecular weights (e.g., CioFi8) are used in cosmetic and medical applications, exploiting their ability to carry oxygen to living tissue (May, 2006). Cosmetic applications include anti-wrinkle creams and are estimated to consume fairly small amounts. Current and potential medical applications include storage of pancreatic tissue for transplants (using the 'two-layer method'), eye surgery (to repair retinal tears), pneumonectomy (lung therapy and diagnosis), use as a contrast agent in ultrasonic and MRI examinations, blood extension, wound healing, and treatment of diseases of the middle ear. All but the first two medical applications involve only small quantities and/or are at the research stage. Storage of pancreatic tissue is a small but growing application. Emissions from medical uses are uncertain but are believed to be small.

In all of these applications, the PFC is believed to be emitted into the atmosphere within one year of its purchase. Thus, emissions from these sources can be estimated using Equation 8.23 for prompt emissions.

Any other uses of SF6 and PFCs

Other applications for SF6 and PFCs that are not specifically addressed above include their use as tracers (in leak detection, indoor and outdoor tracking of air-masses, and oil recovery6) and use of SF6 in the production of optical cables (for fluorodoping of glass fibres7). Often the gases or liquids are emitted within one year of purchase. In this case, good practice in calculating SF6 and PFC emissions from these 'prompt' emissive applications is to use the following formula:

Equation 8.23 Prompt emissions

Emissions in year t = (0.5 • Amount Sold in year t) + (0.5 • Amount Sold in year t - 1)

This equation is similar to the equation for prompt ODS Substitute applications (e.g., aerosols and solvents) addressed in Chapter 7 of this volume. The equation covers more than one year because both sales and emissions are assumed to be continuous over the year; that is, chemical sold in the middle of year t-1 is not fully emitted until the middle of year t.

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