Third IMO Greenhouse Gas Study 2014

124 Third IMO GHG Study 2014
Table 1-1). The 2009 study allocated significant discussion in sections describing potential reductions in GHGs
to characterizing natural gas methane emissions, and identified efforts to achieve reductions in methane
emissions from marine engines. The Third IMO GHG Study 2014 explicitly applied current knowledge of
methane slip in marine engines to those vessels fuelled by natural gas in our bottom-up inventories, thereby
better characterizing CH 4 emissions.
However, more detailed characterization of fleet technology can result in different technology mixes. For
example, the Second IMO GHG Study 2009 documented auxiliary boilers for crude oil tankers only, whereas
the Third IMO GHG Study 2014 identified boiler technology on some bulk carriers, chemical tankers,
container ships, general cargo ships, cruise passenger ships, refrigerated bulk, ro-ro and vehicle carriers. The
Third IMO GHG Study 2014 assigned engine-specific EFs at the individual ship level where possible, including
differentiating between MSD and SSD engines, and residual versus distillate fuel types. These differences can
help explain inventory differences between the two studies.
For CO 2 , NO x and PM, the Third IMO GHG Study 2014 values for 2007 closely match the results reported in
the Second IMO GHG Study 2009. The differences in these EFs are 1% for CO 2 , 3%–9% for NO x and 2%–13%
for PM respectively (approximate values). (The two values for NO x represent SSD and MSD respectively;
similarly, the two values for PM represent HFO and MDO typical values respectively.) The match is best where
vessel activity comparisons are similar, where observed fleet technology matches and where the emissions
factors have changed little. This again confirms that the general impact of the updated methodology is greater
precision and ability to update year-on-year variation in technology or activity among individual vessels in
the fleet. Major differences in emissions results for other relevant substances, therefore, can be explained by
the different EFs used in the Second IMO GHG Study 2009 compared with the more detailed assignment of
EFs in the Third IMO GHG Study 2014. This mainly relates to the emissions of CH 4 , N 2 O, CO and NMVOC.
These EF differences are 80%, 100% and 63% lower in the current study for CH 4 , N 2 O and CO respectively,
and 30% higher for NMVOC (approximate values). These emissions represent combustion emissions of fuels
and do not include evaporative losses from the transport of cargos; the Second IMO GHG Study 2009
estimated the CH 4 losses from the transport of crude oil to be 140,000 tonnes. Table 1-1 of that study added
direct emissions from engine combustion with the estimated losses of CH 4 from the transport of crude oil; no
equivalent calculation is performed here.
Differences in sulphur (SO x ) emissions are similarly attributed to different fuel sulphur contents, using updated
IMO sulphur reports. In this study, the bottom-up model allocation of fuel types for auxiliaries and some main
engine technologies enables more detailed delineation of heavy residual and distillate fuel use; this accounts
for most of the difference in sulphur emissions inventories between the studies. Moreover, the use of updated
fuel sulphur contents can account for about 12% difference in the heavy residual fuel sulphur contents in
2007.