Page 319 - Practical Ship Design

Muchiney Selection 277

9.4FUELS

9.4.1 General

All the propulsion machineries considered so far use oil fuels, albeit of a variety of
different grades. At times when the price of oil has peaked (see Fig. 9. l), consider-
able attention has been given to alternative fuels and in particular to the use of coal
and nuclear energy and the two sections which follow look at these alternatives
-although at today’s fuel prices neither is currently attractive.

9.4.2 Coal burning ships

When the cost of oil fuel increased by a factor of eight in the decade 1970-1980,
that of coal increased by a factor of about three. This led to a renewed interest in
coal as a marine fuel, particularly amongst Australian shipowners, who ordered a
number of ships when the ratio of the cost per tonne of oil/coal in Australian ports
was about 5:5. Since 1980, oil prices have fallen and there is at present no likeli-
hood of more coal-fired ships being ordered in the near future, however the design
problems involved are interesting and a brief look at them may not be out of place.
Recent coal-burning ships have had mechanical chain grate stokers serving their
boilers, but fluidised bed combustion seems likely to take over in any future ships.
Unfortunately the thermal efficiency of a boiledturbine combination is low: 25%
being typical of a medium-sized present day installation, although this should rise
to about 40% in a large next generation installation with reheat; but even this
compares poorly with the efficiency of a modern design of diesel which may attain
50%
Coal has a much lower calorific value (24 kJ/g) than oil fuel (40 kJ/g). When this
is taken along with the lower efficiency, the weight of fuel required for a coal
burning ship becomes 2 to 2.5 times that needed by a diesel ship. This is not the end
of the difficulties, however: coal requires more storage space since it stows at
1.15-1.35 m3/tonne, increasing, effectively to about 1.7 m3/tonne when allowance
is made for the space required for conveyors and the “self trim” empty space at the
top of the bunkers, as compared with about 1.05 m3/tonne (SG 0.96) for oil fuel.
Furthermore, coal cannot be stowed in double bottom and wing tanks that are so
conveniently used for oil fuel and instead requires space free of structure, which in
most ships means space that could have been used for cargo. Because of the large
consumable weight of coal, considerable care must be taken in its fore and aft
disposition or, alternatively, provision made for substantial water ballast capacity
if trim problems are to be avoided.
It can be readily seen why there must be a big cost differential between the price
of oil and that of coal before the latter becomes an attractive alternative.