Emissions Scenarios - IPCC

268 Emission Scénarios
assumes a 30% substkution by HFCs. However, in the SRES
scénarios this value is reduced to 10% to remain consistent
with the above assumption that HFCs ultimately will substitute
for about 25% of the CFCs.
As well as using non-halocarbon substitutes, HFC emissions
can be avoided by better housekeeping, for instance by reduced
spilling of cooling agents. Leakage control equipment can also
serve this purpose. Finally, halocarbons can be recovered for
recycling or destruction when equipment is discarded. Some of
this emission reduction potential is likely to be implemented as
a result of technological changes introduced to control ODSs.
In the SRES scenarios, reduction rates were varied over time
and between industrialized and developing countries to reflect
the definitive features of the underlying storylines (Chapter 4).
Generally, the reduction rates are assumed highest in scenarios
that emphasize sustainability and environmental policies (Bl
family). These reductions, however, were not associated with
any explicit GHG reduction policies, as required by the SRES
Terms of Reference (see Appendix I). In one scenario family,
A2, no reductions were assumed, whereas in the Al and B2
families reduction rates were set at intermediate levels.
In addition to consumption-related emissions of HFCs, HFC-
23 is emitted as an undesired by-product from the HCFC-22
production process. As a result of the Montreal Protocol, the
direct use of HCFC-22, and hence the related HFC-23
emissions, will come to a halt in 2050. To calculate the HFC-
23 emissions, information from Oram et al. (1998) was used
(estimated emissions of HFC-23 at 6.4 kt in 1990). By relating
this value to 178.1 kt HCFC-22 emitted in 1990 (WMO/UNEP,
1999), an emissions factor of 0.036 tons of HFC-23 per ton of
HCFC-22 was calculated and applied to estimate future
emissions. Since this estimation procedure does not take into
account any pollution control regulations (that are not driven
by climate considerations), it may result in an overestimation
of HFC-23 during the early decades of the 2P' century, until
HCFC production is phased out under the Montreal Protocol.
After the phase-out of HCFC-22 consumption, some HFC-23
emissions will still occur because of the continued HCFC-22
feedstock production allowed under the Montreal Protocol.
The resultant projections are shown by individual HFC in Table
5-8.
In general, the SRES scenaiios might underestimate HFC
emissions if the substitution of CFCs with altematives that
have no radiative forcing effect and with more efficient HFCsbased
technologies does not penetrate as quickly as is assumed,
especially in developing countries. However, more effective
technologies and/or suitable non-HFC altematives may be
developed, which would lead to even lower emissions.
5.4.3.2. Perfluorocarbons
PFCs, fully fluorinated hydrocarbons, have extremely long
atmospheric lifetimes (2600 to 50,000 years) and particularly
high radiative forcing (Table 5-7). The production of aluminum
is thought to be the largest source of PFCs (CF^, and CoFg )
emissions. These emissions are generated, primarily, by the
anode effect, which occurs during the reduction of alumina
(aluminum oxide) in the primary smelting process as alumina
concentrations become too low in the smelter. Under these
conditions, the electrolysis cell voltage increases sharply to a
level sufficient for bath electrolysis to replace alumina
electrolysis. This causes substantial energy loss and the release
of fluorine, which reacts with carbon to form CF^ and CjFg.
In 1990, the total annual global primary aluminum production
was 19.4 Mt. Secondary aluminum production from recycling
accounted for 21.5% of the total consumption in 1990. The
production statistics from the World Bureau of Metal Statistics
(1997) show that the total aluminum production was 27.5 Mt,
and recycling has increased to 25.6%, or by about 3.5
percentage points, in 10 years.
The scenarios developed by Fenhann (2000) adopt a
methodology of projecting future aluminum demand based on:
• Aluminum consumption elasticity with respect to GDP.
• Use of altemative assumptions conceming recycling
rates.
• Varying emission factors to reflect future technological
change.
These assumptions are altered to be in consistent with the four
SRES scenario storylines described in Chapter 4.
For instance, in Fenhann (2000) the aluminum consumption
elasticity varies between 0.8 and 0.96, and the range of
increases in aluminum recycling rates varies between 1.5 and
3.5 percentage points per decade. The RFC emission factor
varies according to the aluminum production technology used.
The default emission factor from the Revised IPCC Guidelines
(IPCC, 1997) is 1.4 kgCF/ aluminum. However, Hamisch
(1999) gives evidence that the average specific emissions of
CF4 per ton of aluminum has decreased from about 1.0 kg to
0.5 kg between 1985 and 1995. Accordingly, an emissions
factor of 0.8 kgCF^/t was used for 1990 and this was assumed
to decrease to 0.5 kg CFyt in the future. This is also in
agreement with the value of 0.51 kgCF^/t recommended by the
IPCC Expert Meeting on Good Practices in Inventory
Preparation for Industrial Processes and the New Gases
(January 1999, Washington, DC). The same sources also agree
on an emission factor for C^Fg that is 10 times lower than that
for CF4. This assumption was also used in the calculations
presented here (Table 5-8).
Aluminum production is being upgraded from highly
inefficient smelters and practices to reduce the frequency and
duration of the anode effect. Since aluminum smelters are large
consumers of energy, the costs of these modifications are offset
by savings in energy costs and are therefore assumed to occur
in all scenarios. The ultimate reduction of the anode eiïect
frequency and duration was assumed to reach the same level in
all the SRES scenarios. However, scenarios vary with respect
to the rate of introducing the underlying modifications. It is