Steam boilers

There is always a danger of slagging on and/or corrosion of the tubes of the highest temperature superheater bank and, if present, reheater bank when burning heavy bunker fuel oil. Therefore a clean fuel burning furnace is provided which is separate from the heavy bunker fuel oil burning furnace and only the combustion gases from burning the clean fuel are used to heat the highest temperature superheater bank and, if present, reheater bank. The clean fuel is preferably provided by a gasifier in which a portion of the heavy bunker fuel oil is pre-combusted to give a clean combustible fuel gas.

This invention relates to steam boilers and in particular marine steam 
boilers used to supply steam to the steam turbines powering a ship. 
BACKGROUND TO THE INVENTION 
In marine steam boilers providing high temperature and pressure superheat 
and reheat steam there is a danger of slagging and/or corrosion of the 
tubes forming at least the highest temperature superheater and reheater 
bank when burning heavy bunker fuel oil which has a high ash and sulphur 
content. It is therefore an object of this invention to reduce this 
slagging and/or corrosion in such steam boilers. 
BRIEF SUMMARY OF THE INVENTION 
According to the invention there is provided a steam boiler having a heavy 
oil burning furnace and a separate clean fuel burning furnace, and a 
number of tube banks providing one or more stages of superheated steam and 
optionally one or more stages of reheated steam, the tube banks being 
arranged so that the tubes of the highest temperature superheater tube 
bank and of the or the highest temperature reheater tube bank if provided 
are heated by the combustion products from the clean fuel burning furnace. 
In such a boiler the slagging on and/or corrosion of the tubes of the 
highest temperature superheater tube bank and reheater tube bank is larged 
avoided because those tubes are contacted solely by the clean combustion 
gases from the clean fuel burning furnace. The other tubes of any lower 
temperature superheater and reheater tube ganks can be swept by the 
combustion gases from the heavy fuel oil burning furnace but for these 
tubes which operate at lower temperatures the risks of slagging and/or 
corrosion are less. 
The clean fuel for the clean fuel burning furnace can be provided by the 
pre-combustion of heavy fuel in a gasifier which can provide a supply of 
clean product gas for burning whilst retaining the impurities in the heavy 
fuel. These gasifiers are known and can take the form of a fluidized bed 
gasifier. 
Under no-steam flow conditions in the reheater, cooling air must be passed 
over the tubes of the reheater tube bank or banks. This can be achieved by 
positioning the bank or banks in gas passes whose outlets under such 
conditions can be closed by dampers and cooling air can then be fed in 
reverse flow through the pass or passes directly from an air supply fan 
such as the forced draught fan for the combustion air. 
Separate forced draught fans and air pre-heaters can be provided for each 
furnace if required to provide air for the furnaces and/or the gasifier. 
This may serve to avoid the back-feed of contaminants from the air 
preheating system for the clean fuel burning furnace by the air supply to 
the heavy oil burning furnace so as to ensure that the combustion products 
from the clean fuel burning furnace remain clean.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The boiler 10 shown in the drawing has a heavy fuel oil burning furnace 12 
and a separate clean fuel gas burning furnace 14. The combustion gases 
from the furnace 12 pass up through two parallel passes 16 and 18 while 
the combustion gases from the furnace 14 pass up through a pass 20. The 
flow of gases through the passes 16, 18 and 20 are controlled by dampers 
22, 24 and 26, respectively. The gases from all three passes are 
thereafter combined in a common flue 28 containing an economiser tube bank 
30. The walls of the furnaces 12 and 14 and passes 16, 18 and 20 are 
defined by conventional water tube walls which are not shown in detail. 
The furnace 12 is fired by heavy fuel oil from a supply 32. The heavy fuel 
oil is burned by burners 34 and combustion air is supplied from a windbox 
36. Air for this windbox is supplied from a common duct 38 and its rate of 
supply is controlled by a damper 40. 
The furnace 14 is fired by clean gas from a gasifier 44. This gasifier 
receives heavy fuel oil from the supply 32 and air from the common duct 38 
via a passage 39 at a rate determined by a damper 46 and converts the oil 
to a clean combustible gas which is fed to a burner 48 in the furnace 14 
via a duct 50. Combustion air for the furnace 14 is supplied to a windbox 
52 from the common duct 38 and the rate of supply is controlled by a 
damper 54. 
The air supply to the common duct 38 is made from a forced draught fan 56, 
the air being preheated in a regenerative air heater 58 heated by the 
combined combustion gases in the flue 28. 
The pass 16 contains a first or low temperature superheater 60 and a first 
or low temperature reheater 62 while the pass 18 contains a second 
superheater 64 and a by-pass economiser 66. Dirty combustion gases from 
the furnace 12 flow over these items. The pass 20 contains a third or 
final superheater 68 and a second or final reheater 70, this superheater 
68 and reheater 70 being the highest temperature superheater and reheater 
passes, respectively. Clean combustion gases from the furnace 14 pass over 
these items and to ensure this the damper 26 associated with the pass 20 
is controlled to maintain a slight positive pressure differential between 
that pass and pass 18 at the level of intermediate open screens 80 and 82 
provided respectively between the passes 16 and 18 and passes 18 and 20. 
These screens 80 and 82 are provided so that the combustion gases can 
deviate from one pass to another after flow over the first, second or 
third superheater in dependence upon the positioning of the dampers 22, 24 
and 26, the damper 26 being arranged as noted above to ensure that dirty 
combustion gases do not flow from the pass 18 to the pass 20. 
Under no-steam flow conditions through the first and second reheaters 62 
and 70, these have to be cooled. Therefore cooling air ducts 90 and 92 
lead from the fan 56 to the tops of the passes 16 and 20, respectively. 
The passage of the cooling air through its ducts 90 and 92 is controlled 
by dampers 94 and 96, respectively. Thus to prevent flow of hot combustion 
gases over the reheaters 62 and 70 when they contain no steam, the dampers 
22 and 26 are closed, all of the combustion gases then passing over the 
economiser 66, and cooling air is allowed to flow down over the reheaters 
62 and 70 by opening the dampers 94 and 96 and this cooling air then 
passes through the open screens 80 and 82 to join the combustion products. 
If desired a separate forced draught fan can be provided to supply air to 
the furnace 14 and gasifier 44 and dirt packed up by such air from any air 
preheater can be fed back into the combustion air for the furnace 12 so as 
to keep the combustion gases from the furnace 14 clean. 
The operation of the boiler 10 should be clear from the above description. 
As will be appreciated, the final or third superheater 68 and the final or 
second reheater 70 both operate at the highest temperatures and so are 
most liable to be affected by slag and corrosion problems. However, both 
can only receive clean combustion gases and so problems of slagging and 
corrosion are reduced as compared with a situation in which they receive 
combustion gases from the furnace 12. 
Although the words water and steam have been used herein those words are to 
be construed as including any liquid and its vapour unless the context 
specifically requires otherwise. 
A latitude of modification, change and substitution is intended in the 
foregoing disclosure and in some instances some features of the invention 
will be employed without a corresponding use of other features. 
Accordingly it is appropriate that the appended claims be construed 
broadly and in a manner consistent with the spirit and scope of the 
invention herein.