[0001] The present invention relates to a heat-recovery boiler for generating steam from
a flow of hot gases, of the type including a supply manifold for delivering the flow
to a chamber which houses a plurality of tube nests for water and/or steam, and at
least one burner in the supply manifold for the post-combustion of the gases.
[0002] Heat-recovery boilers of the type specified are known. They are used in so-called
combined cycle thermoelectric power plants which generally include one or more gas
turbines used to produce electricity and which generate a flow of hot gases which
is sent to the heat-recovery boiler.
[0003] The steam produced in the heat-recovery boiler is used in a steam cycle which uses
a steam turbine in turn connected to a generator unit.
[0004] It is known that the combustion upstream of the gas turbines occurs with a considerable
excess of air so that the gases produced, which contain a high percentage of oxygen,
may be used as the combustion-supporting gas for a gas burner.
[0005] Thus the gas flow entering the heat-recovery boiler is brought to a higher temperature
than that of the gases leaving the gas turbines. This temperature depends upon the
value of the combustible-gas flow rate which is delivered to the burner.
[0006] The excess oxygen available would allow the combustion of a much larger quantity
of additional gas, with a corresponding considerable raising of the temperature of
the combustion gases.
[0007] In practice, in the usual combined-cycle plants used in industry, the auxiliary combustion
is limited in order to contain the temperature within values which are compatible
with the nature of the heat-recovery boilers used, which are normally housed in ducts
which are not cooled and which use finned tubes for almost all the heat exchange surfaces.
[0008] From the point of view of physical chemistry, these temperatures, which are of the
order of 600-700°C, are sufficient to ensure that the combustion reaction occurs properly
when the auxiliary fuel is natural gas or gas from refineries with a high calorific
value.
[0009] When the auxiliary fuel is a so-called poor combustible gas, obtained as the by-product
of various petroleum and iron and steel processes, it is however necessary to use
a temperature of the order of 1000°C or more in order to sustain the combustion reaction.
[0010] Such reaction temperatures cannot normally be reached since the combustion of the
quantity of auxiliary fuel available in poor gas, carried out with a considerable
excess of oxygen, results in an insufficient output of heat, which is effectively
diluted by the exhaust gases discharged from the turbine.
[0011] The technical problem at the basis of the present invention consists of devising
a heat-recovery boiler which enables the problem mentioned above with reference to
the prior art to be overcome.
[0012] This problem is resolved by a heat-recovery boiler of the type specified which is
characterised in that it includes a branch in the supply manifold which defines a
first hot gas duct connected directly to the said chamber and a second hot gas duct
which houses the burner.
[0013] The main advantage of the heat-recovery boiler according to the invention lies in
the fact that it provides a zone suitable for the combustion of poor combustible gases
at an adequate reaction temperature with a flow of combustion-supporting gas to the
burner which is limited to that necessary for supplying the oxygen for the combustion
reaction.
[0014] Further characteristics and advantages of the heat-recovery boiler according to the
invention will become more apparent from the description of one preferred embodiment
thereof, given by way of non-limitative example with reference to the single appended
drawing which is a perspective view, in partial section, of a heat-recovery boiler
according to the invention.
[0015] In the drawing, a heat-recovery boiler for generating steam according to the invention
is indicated 1. It is used in a combined-cycle thermoelectric power plant, not shown,
which employs a series of gas turbines which produce a flow A of hot gases at a flow
rate which varies according to the operating conditions of the plant.
[0016] The heat-recovery boiler 1 according to the invention includes a rectangular-section
supply manifold 2 through which the hot gases are conveyed from the gas turbines and
a branch 3 in the manifold 2 which defines a first duct and a second duct, indicated
4 and 5 respectively, for the hot gases.
[0017] The first and second ducts 4, 5 together present a rectangular cross-section to the
hot gas flow which conserves the cross-sectional width of the supply manifold 2 while
the branch 3 extends upwardly, the first and second ducts 4, 5 being superposed and
having a common edge 6 which divides the hot gas flow A horizontally.
[0018] The section of the first duct 4 is larger than that of the second duct 5.
[0019] The first duct 4 has a first inclined portion 7 extending from the branch 3 and a
second horizontal portion 8.
[0020] The first inclined portion 7 of the first duct 4 includes a first adjustable baffle
shutter 9 comprising a plurality of flow-divider plates.
[0021] The second duct 5 which extends horizontally beneath the inclined portion 7 of the
first duct 4 has a second adjustable baffle shutter 18 similar to the first.
[0022] The boiler 1 includes a main chamber 10 which houses a plurality of tube nests 11
for water and/or steam arranged vertically and connected to a plurality of manifolds
12.
[0023] The main chamber 10, which is substantially box-shaped, is bounded by walls 10 and
has two open ends, an inlet end 14 and an outlet end 15 respectively, in opposite
sides.
[0024] The inlet end 14 of the main chamber 10 is divided into an upper inlet portion 16
and a lower inlet portion 17. The first duct 4 is connected directly to the upper
inlet portion 16 of the open inlet end 14 of the main chamber 10.
[0025] The boiler 1 further includes a pre-chamber 19 intermediate the second duct 5, to
which it is connected, and the lower inlet portion 17 of the open inlet end 14.
[0026] Thus the pre-chamber 19, which underlies the horizontal portion 8 of the first duct
4, opens to the main chamber 10.
[0027] The boiler 1 further includes a burner 20 for the post-combustion of the combustible
gas housed in the second duct 5 at the inlet to the pre-chamber 19. Thus the second
adjustable shutter 18 in the second duct 5 is located upstream of the burner 20 in
the direction of the hot gas flow A.
[0028] The combustible gas may be an industrial gas from the petroleum or iron and steel
industries.
[0029] The pre-chamber 19 houses some 21 of the plurality of tube nests 11. More particularly,
the tube nests 21 include a tube nest 22 for super-heating the steam produced in the
boiler 1 and a tube nest 23, downstream of the nest 22 in the direction of the flow
A, for re-heating steam coming from a stage of the steam turbine supplied by the heat-recovery
boiler 1.
[0030] The outlet end 15 of the main chamber 10 is connected to a chimney 24 through which
the exhaust gas flow is discharged into the atmosphere.
[0031] With reference to the appended drawing, the operation of the heat-recovery boiler
1 according to the invention will now be described.
[0032] The gas flow A coming from a series of gas turbines or a single gas turbine is conveyed
to the supply manifold 2 and from there to the branch 3 where it is divided into two
separate gas flows.
[0033] The magnitudes of the two flow rates which will pass through the first and second
ducts 4, 5 depend on the reciprocal opening of the adjustable shutters 9, 18 in the
ducts 4, 5 as well as on the flow cross-sections of the ducts 4, 5 which, with the
shutters 9, 18 completely open, are such that the flow through the second duct 5 is
less than the flow through the first duct 4.
[0034] The shutters are however mutually positioned so as to minimise resistance to the
flow.
[0035] The hot gases which flow through the first duct 4 flow directly into the main chamber
10 of the heat-recovery boiler 1 and impinge upon the tube nests 11 therein.
[0036] The hot gases which pass through the second duct 5 traverse the burner 20 supplied
with a regulable flow of combustible gas. Post-combustion of the hot gases thus occurs
in the burner 20 and the temperature of the hot gases themselves is thus raised before
they flow over the tube nests 21 in the pre-chamber 19.
[0037] The presence of the pre-chamber 19 prevents the immediate mixing of the post-combustion
gases with the unburnt gases in the first duct 4. Thus, in the pre-chamber 19, the
hot gases are kept at a high temperature and are preferably used for superheating
steam in the tube nest 22 and for heating steam coming from the turbine which processes
the steam from the boiler 1, this steam being withdrawn, for example, from the high-pressure
stages.
[0038] Only after the post-combustion gases have passed through the entire pre-chamber 19
and have been cooled by means of the surfaces 21 and 22 to a temperature close to
that of the unburnt gases, do they mix with these latter and are then allowed to impinge
upon the remaining proportion of the tube nests 11.
[0039] After passage through the main chamber 11,the hot gases are released through the
chimney 24.
[0040] In addition to the advantage indicated above, the heat recovery boiler 1 according
to the invention is also extremely flexible in use.
[0041] Indeed, if the adjustable shutters are adjusted as indicated above, it is possible
to make partial use of the gas turbines connected to the heat-recovery boiler and
a variation in the oxygen content and/or the temperature of the gases produced. Such
variations are inevitable in the working life of a gas turbine.
[0042] Such regulation is particularly useful when it is necessary to burn combustible support
gases of different qualities, that is, just as they are produced by the petrol or
iron and steel industries.
[0043] The variations in the flow of hot gases to the burner are accompanied by corresponding
variations in the supply of combustible support gas to the burner.
[0044] In addition, the boiler according to the invention is simple and economical to manufacture
with the use of conventional components in an innovative manner.
[0045] The components which are subject to high temperatures are also kept separate from
other components so as to facilitate maintenance of the boiler.
[0046] Furthermore, the more effective capacity for the temperature control within the heat-recovery
boiler according to the invention enables the quantity of toxic compounds discharged
through the chimney to be reduced.
[0047] In the case of a heat-recovery boiler which burns a valuable combustible support
gas, with the heat-recovery boiler according to the invention it is possible to achieve
a compromise between the flow rate of the expensive combustible support gas and the
acceptable thermal efficiency which depends on the maximum temperature in the boiler.
[0048] It is understood that an expert in the art may make numerous variations to the heat-recovery
boiler described above in order to satisfy various requirements all of which however
fall within the scope of protection of the invention as defined by the appended claims.
1. A heat-recovery boiler (1) for generating steam from a flow (A) of hot gases, including
a supply manifold (2) for the flow (A), a chamber (10) which houses a plurality of
tube nests (11) for water and/or steam and at least one burner (20) for the post-combustion
of the gases, characterised in that the supply manifold (2) includes a branch (3)
which defines a first hot gas duct (4) connected directly to the chamber (10) and
a second hot gas duct (5) which houses the burner (20).
2. A heat-recovery boiler (1) according to Claim 1, in which the first duct (4) presents
a flow cross-section to the hot gas flow (A) which is greater than that of the second
duct (5).
3. A heat-recovery boiler (1) according to Claim 1, in which the first duct (4) includes
an adjustable shutter (9).
4. A heat-recovery boiler (1) according to Claim 1, in which the second duct (5) has
an adjustable shutter (18) upstream of the burner (20) in the direction the hot gas
flow (A).
5. A heat-recovery boiler (1) according to any one of the preceding claims, which includes
a pre-chamber (20) connected to the second duct (5) and opening to the chamber (10),
which houses some (21) of the plurality of tube nests (11).
6. A heat-recovery boiler (1) according to Claim 5, in which the said some (21) of the
tube nests include at least one tube nest (22) for superheating steam.
7. A heat-recovery boiler (1) according to Claim 5, in which the said some (21) of the
tube nests include at least one tube nest (23) for re-heating steam from a steam turbine
supplied by the heat-recovery boiler (1).