2.5 HFCs, PFCs and SF₆

The UK emissions of HFCs are shown in Table 2.7. HFCs had limited usage prior to the phase out of CFCs in the production of semiconductors and as refrigerants blended with CFCs. They are being used increasingly as:

• substitutes for CFCs and HCFCs in domestic, commercial and industrial refrigeration and air conditioning
  
• substitutes for CFCs in plastic foam blowing
  
• substitutes for CFCs for some medical aerosols
  
• fire fighting fluids
  

Fugitive emissions from the manufacture of HCFCs and HFCs accounted for 56% of the total emission in 1996. Refrigeration is the next largest source and contributed 22% of the total. Here emissions arise due to leakage from refrigeration and air conditioning equipment during its life time and from losses during the recovery of the refrigerants on decommissioning. In the case of closed foams where the fluid is retained within the foam there will be some leakage from the foam during its lifetime and on disposal but with open foams all losses occur during manufacture. The remaining emission sources, namely, aerosols, electronics & electrical and fire fighting are fairly small and only comprise 4% of total emissions. The total emission has approximately doubled since 1990 due to the increasing use of HFCs and increased production of halocarbons.

Table 2.7 UK Emissions of HFCs (tonnes)

Table 2.8 shows the UK emissions of PFCs. PFCs had limited usage prior to the phase out of CFCs in the electronics and electrical industry in:

• etching processes in the semiconductor industry
  
• chemical vapour deposition in the electronics industry
  
• soldering processes
  
• leak testing of electrical components
  
• cooling electrical components , for example in supercomputers and radar systems.
  

Other minor uses were in cosmetics and as a tracer gas. PFCs are formed as a by-product of aluminium smelting and this is the major emission source contributing around 63% of the UK total in 1996. The emissions are caused by the anode effect which occurs when alumina concentrations become too low in the smelter. This can cause very high electrical current and decomposition of the salt - fluorine bath. The fluorine released reacts with the carbon anode, creating PFC compounds CF₄ and C₂F₆ . Total emissions have declined by 74% since 1990 reflecting steps taken by the industry to reduce emissions.

Table 2.8 UK Emissions of PFCs (tonnes)

Table 2.9 shows the UK emissions of SF₆. It has the following applications:

• insulation medium in high voltage applications such as switchgear and circuit breakers
  
• cover gas in magnesium foundries to protect the molten magnesium from re-oxidising when it is cast
  
• degasser in aluminium casting applications, though its use in the UK is rather limited
  
• insulating gas in double glazing applications, replacing vacuum as an insulation technique
  
• plasma etching of polysilicon and nitrite surfaces
  
• atmospheric tracer for scientific studies
  

The largest source is from magnesium manufacture which accounted for 86% of the UK total in 1996. It is not possible to recover the SF₆ so the total annual consumption is emitted to atmosphere. The other main source is electrical & electronics which is dominated by emissions from electrical equipment. These arise during the manufacture and filling of the devices and from leakage and maintenance during the equipment's lifetime. This application has only been in use for the last 20 to 30 years and little of the equipment has been decommissioned. It is expected that users will take great care over future fluid recovery so that emissions will be minimised. The other emission sources mentioned are very small and since SF₆ is not used as a substitute for CFCs the total emissions show little change over the period considered.

Table 2.9 UK Emissions of Sulphur Hexafluoride (tonnes)

Due to the complex nature of estimating emissions inventories, no simple quantitative treatment can be given for the accuracy of emission estimates. However, a qualitative indication can be made of the systematic errors. Some of the more important factors resulting in uncertainties in greenhouse gas emission estimates are:

• Lack of measurements of emission factors for UK plant and conditions;
  
• Limited samples of plant and vehicles where UK measurements have been made;
  
• Short period measurements being used to estimate annual averages;
  
• Uncertainties in fuel consumption data, for example data may be deliveries rather than consumption;
  
• Uncertainties in secondary data which are used to estimate emissions. For example, vehicle-kilometres used in calculating road transport emissions are often derived from sample surveys which are themselves subject to uncertainties.
  

Table 2.10 Uncertainty of the Emission Inventories