FIELD OF THE INVENTION
The present invention generally relates to a Heat Recovery Steam Generator
(HRSG) installed behind a Gas Turbine (GT) for generating steam used in a
variety of industrial process applications. This invention particularly relates to an
apparatus for air addition to the GT exhaust gas for enhancing the steam output
from a duct enclosed burner mounted HRSG system for enhancing the steam
output of the HRSG without adopting a water cooled wall enclosed furnace
configuration in the HRSG.
BACKGROUND OF THE INVENTION
It is known that HRSGs comprise a fuel burning equipment and optimized
quantity of heating tubes for absorbing heat from products of combustion of
variety of fuels for converting cold water into hot water and steam. The cold
water is converted so as to meet the process requirements considering a defined
input and output condition. Whenever there are deviations from the defined
input conditions and/or output requirements, the parameters of hot water and
steam accordingly deviate from the designed values.
In order to maintain the properties of the output medium within an usable range,
at least one mechanical apparatus with computerised control is provided. One
such apparatus is used to fire fuel in the gas flow path of HRSG to further
enhance the temperature of the GT exhaust gas. This additional heating
increases the heat absorbed in the heat transfer tubes resulting in higher steam
quantity from the HRSG. The computerised apparatus controls the fuel flow to
maintain the steam flow and pressure required by the downstream process.
One of the prior arts (Type-1) of HRSG system utilises solely the GT exhaust gas
(No fresh air supply) as the medium to supply oxygen to the burner system. The
burner(2) is located in an internally insulated duct enclosure (4) in view of cost
optimization. In this type of prior art system, the HRSG utilises a small quantity
of fresh air ( < 5-10% of GT Exhaust flow) which is supplied in the burner area
as seal / scanner or augmenting air for oil firing. In this type of HRSG, the upper
limit of steam output is limited by the burner outlet flue gas temperature
(typically less than 950°C) depending on the materials of insulation or liner or
other components in contact with the flue gas.
In another variant of prior art system, the HRSG utilises solely the fresh air (of
around 75% to 120% of GT Exhaust flow) supplied by a Forced draft (FD) fan
(15) when the Gas Turbine is not in service. This fresh air supply is stopped once
GT becomes operational. In this type of HRSG, all the heat transfer sections viz
Superheater (5), Evaporator(6), Economiser(7), Condensate/Makeup water
Preheater (8) shall be located downstream of the burner (2) and fresh air from
the FD fan (15) is supplied only on the upstream side of the burner(2). Since the
FD fan (15) is rarely operated, it is difficult to maintain and operate for short
periods.
US 2005/0034445 Al describes a method and apparatus for utilising hot exhaust
gases from a hot gas generator such as an auxiliary gas turbine , burner or other
type of hot gas source connected to the inlet of HRSG for controlling the
combined cycle plant during startup, part loads, for keeping the drum
pressurized during shut downs. Another objective of this patent is to provide
additional flow of hot exhaust gases to the HRSG on hot days. Yet another
objective of this patent is to provide reliable steam supply from the HRSG when
the associated GTG is unavailable. The prior art does not discuss mixing the
fresh air at ambient temperature supplied by the FD fan, with GT exhaust gas.
Another prior art (Type-2) of HRSG system utilises a water or steam cooled
membrane (4A) wall (made of plate fin welded tubes) enclosure around the
burner in order to withstand flue gas temperature higher than 950°C. However,
the system is uneconomical due to membrane wall construction. Also, since the
flue gas temperature drops to a lower value as it passes through the furnace
(4A) and hence achieving rated steam temperature at Superheater(5) outlet is
not possible, thus restricting the low load capability of HRSG. In this type of
HRSG, even if the steam demand is lower, the HRSG shall generate more steam
at rated temperature and some quantity of steam, which is excess of the
demand, shall be vented or dumped in a condenser.
US patent Number 5881551 describes a method of utilizing a known package
boiler to act as a HRSG. The package boiler (which would have only a single gas
flow path) with burner, waterwall chamber, a superheater bank, boiler bank is
modified with a division waterwall to divide the boiler bank into two parts and
economiser with suitable ducting and damper arrangement to divert the flue
gases through three different parallel paths (first path consisting of burner,
furnace, superheater and part of boiler bank and economiser in the same order,
second path through part of boiler bank and then to economiser, third path
directly to economiser), also having a forced draft blower for supplying ambient
air to burners located in the inlet of waterwall chamber. The amount of flue gas
apportioned through the three parallel paths is decided as a function of
combined boiler outlet gas temperature, to avoid steaming of the economiser,
and maintain the amount of oxygen needed to support combustion and the
allowable gas side pressure losses. This patent does not have a task to achieve
rated steam temperature at unfired mode of operation and at lower steam loads.
This patent envisages only switching over operation mode from GT Exhaust gas
mode to fresh air supply to burner inlet, and does not teach use of ambient air
from the blower or mixing of ambient air with CT exhaust at multiple locations in
the HRSG.
As shown in figure 1, the hot exhaust gas from a gas turbine is connected to a
Heat Recovery Steam Generator (HRSG) system through an inlet transition duct
(1) and the temperature of the GT exhaust gas is raised to a higher level in
aburner (2) consisting of several levels of burner elements each receiving fuel
from an external fuel header (10) and taking oxygen from the GT exhaust gas
thus resulting in combustion. A first combustion zone (3) has a duct enclosure
(4) insulated internally or externally, lined with suitable thin liner to hold the
insulation together. The first combustion zone (3), due to presence of flame,
does not internally house any heat transfer or structural section. In the zones
downstream of the first combustion zone (3), various heat transfer sections viz a
Superheater (5), an Evaporator(6), an Economiser(7), a Condensate/Makeup
water Preheater (8) are positioned for absorbing the heat available in the flue
gas before exhausting the flue gas through a chimney (9).
Generally the first section to receive the GT exhaust gas is the burner (2)
followed by the super heater system (5). There may be one or more
superheaters formed of a plurality of tubes, a de superheater system consisting
of single or plurality de superheaters, one or more evaporator sections (6) and
one or more Economisers (7) and a plurality of water heating sections (8). Feed
water for the HRSG is supplied from a feed water manifold (12) to an inlet of
the Economiser (7) and the steam from an outlet of the superheater (5) is
supplied to a main steam manifold (11). Similarly, the water preheater (8)
receives water from a cold water manifold (14) and supplies to a hot water
manifold (13).
The prior art of fuel burning control modulates the fuel (10) flow rate to the
burner (2) in order to increase or reduce the steam generation in order to
maintain the steam header (11) pressure. The possibility of achieving or
maintaining the superheater (5) outlet steam temperature depends upon the flue
gas temperatures in a superheater area, heating surface area provided in the
superheater (5) and the extent of desuperheater spray water flow rate. The flue
gas temperature in the superheater (5) depends on the fuel (10) firing rate
which in turn is dictated by the steam demand. The heating surface area and the
capability of the desuperheater system can be adjusted to some extent to ensure
rated steam temperature for a particular range of steam flow.
If the desired steam flow is lower than the above range, venting or dumping of
excess steam shall be done. If the desired steam flow is higher than the above
range, there may be a need for one or more independent steam generators.
Both these disadvantages of prior art need a technical solution.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose an air receiving and
distribution apparatus for regulating the steam flow including temperature of
water/steam at different stages of a duet fired heat recovery steam generator in
a gas turbine, which eliminates the disadvantages of prior art.
Another object of the present invention is to propose an air receiving and
distribution apparatus for regulating the steam flow including temperature of
water/steam at different stages of a duct fired heat recovery steam generator in
a gas turbine, which enhances the steam output of the HRSG.
A further object of the present invention is to propose an air receiving and
distribution apparatus for regulating the steam flow including temperature of
water/steam at different stages of a duet fired heat recovery steam generator in
a gas turbine, which reduces the operational cost of the HRSG.
SUMMARY OF THE INVENTION
According to the invention, there is provided a separate air receiving and
distribution apparatus for regulating the steam flow and / or for regulating the
steam or water temperature at different stages of a duct fired heat recovery
steam generator (HRSG) behind a gas turbine (GT), the apparatus receiving air
from a Forced draft fan and distributing to single or multiple zones in the flue gas
path of a heat recovery steam generator (HRSG) for increasing or decreasing the
air supply rate including uniform mixing of air with the GT exhaust gas coming
out from an inlet transition duct connecting the GT and the HRSG, the apparatus
comprising; air receiving and supply manifold with at least one set of incoming
single air input pipe from the draft fan the input pipe branching out into a single
or plurality of air supply pipes having integral dampers to divert or isolate the
flow to one or more branches; a plurality of air regulation devices positioned in
each of the outlet branch from the air receiving and supply manifold to adjust
the air flow supply to each zone in the gas path; a plurality of air distribution
devices consisting of a grid of multiple air openings on an interconnected pipe
network covering the entire cross section of the flue gas path for uniform mixing
of air and GT exhaust gas within a short length of the gas path zone; means for
actuating the integral dampers of air receiving and supply manifold (16) and the
air regulation devices (17 A to D) based on the dynamic operational steam
parameters so as to maintain safe operating conditions of the HRSG through a
computerized control system.
In accordance with the present invention, an apparatus and a method are
envisaged for increasing the flue gas flow through HRSG by mixing fresh ambient
air with the hot GT exhaust gas on the upstream side of burner (2) to increase
the flue gas mass flow rate thus creating an opportunity to increase the fuel
firing rate without crossing the limiting flue gas temperature and enhance the
steam output whenever required.
The proposed apparatus can also be used for reducing/ regulating the flue gas
temperatures by regulating the mixing air flow rate upstream / downstream of
the Evaporator(6) or Economiser(7) or water preheater (8) to reduce the steam
flow or avoid steaming of Economiser (8) / Water Preheater (8), thus improving
the low load capability or Turn down of the HRSG.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure-1 is a schematic view of Prior Art Type-1 HRSG System without Forced
draft fan for fresh air supply
Figure-2 is a schematic view of Prior Art Type-1 HRSG System with Forced draft
fan for fresh air supply
Figure-3 is a schematic view of Prior Art Type-2 HRSG System with Membrane
wall Furnace
Figure-4 is a schematic view of the HRSG incorporating the apparatus proposed
in the present invention
DETAILED DESCRIPTION OF A PREFERRREP EMBODIMENT OF THE
INVENTION
Figure 4 shows proposed arrangement representing the present invention
consisting of an air receiving and supply manifold (16) to receive fresh air from a
FD fan (15) and supply of air supply to a plurality of air regulation devices (17)
and then to a plurality of air distribution devices (18) provided at various
locations on the upstream of a Burner (2), a Superheater (5), an Evaporator (6),
an Economiser(7)/ a Condensate/Makeup water Preheater (8) respectively.
In accordance with the present invention, an apparatus is provided to increase
the flue gas flow through a HRSG by mixing fresh ambient air with hot GT
exhaust gas upstream of the burner though a first air regulation device (17A)
and a first air distribution device (18A). This allows ef increase of the fuel firing
rate without exceeding the temperature limit of the flue gas at burner outlet, and
enhance the steam output whenever required.
The proposed apparatus is further enabled to reduce/regulate the flue gas
temperatures by controlling the mixing air flow rate upstream of the
Evaporator(6) though a second air regulation device (17B) and a second air
distribution device (18B). This measure reduces the flue gas temperature
entering the evaporator (6) which leads to a reduced steam flow in a controlled
way without reducing the flue gas temperature in the superheater (5) region and
hence rated steam temperature is maintained even at lower steam generation
levels and thus improving the low load capability or Turn down of the HRSG.
The proposed method and apparatus is also capable to prevent a non-steaming
condition in the economizer (7) which is a serious issue, generally encountered
during startup and low load operation of the GT & HRSG. This can be
accomplished by reducing/ regulating the flue gas temperatures by regulating
the mixing air flow rate upstream of the Economiser though a third air regulation
device (17C) and a third air distribution device (18C). This measure eliminates
steaming in the economiser without resorting to increased blowdown or
economiser recirculation at low loads.
The proposed apparatus is enabled to maintain the water preheater (8) in
service even at very low loads in non-steaming condition, and further maintain a
minimum of 5-8°C subcooling before entering the deaerator. This is achieved by
reducing/ regulating the flue gas temperatures by regulating the mixing air flow
rate upstream of the water preheater (8) though a fourth air regulation device
(17D) and a fourth air distribution device (18D). This measure eliminates
bypassing of the Water Preheater at low loads, and in turn reduces the pegging
steam consumption in the deaerator.
ADVANTAGES OF THE INVENTION
The apparatus of the invention provides a cost effective solution for
enhancement of steam output from a duct fired HRSG beyond the normal
capability prior art duct fired HRSG without the need for water or steam cooled
enclosure or a costly refractory or block insulation in the combustion region, that
is the regions immediately downstream of the burner.
The inventive apparatus provides improved low load capability i.e. operation of
HRSG at reduced steam generation level, with rated steam temperature and also
without encountering steaming in economiser and water preheater, without any
venting of steam or blow down of water or part / full bypassing of
economiser/water preheater heat transfer sections on water side or gas side.
The apparatus allows operation of the HRSG with relatively lower flue gas
temperature due to dilution with fresh air and hence contributes to increase life
or reliability. Accordingly, the apparatus enables the HRSG to operate at a
high level of operational flexibility & reliability.
The air distribution device of the apparatus disposed on the upstream of the
burner can also function as a flue gas distribution grid and can be of lower grade
material since the same is air cooled and hence can act as a cost effective
substitute for uncooled gas distribution grid plates made of Stainless steel used
in prior arts.
WE CLAIM
1. An air receiving and distribution apparatus for regulating the steam flow and/
or for regulating the steam or water temperature at different stages of a duct
fired heat recovery steam generator (HRSG) in a gas turbine (GT), the
apparatus receiving air from a Forced draft fan (15) and distributing to single
or multiple zones in the flue gas path of a heat recovery steam generator
(HRSG) for increasing or decreasing the air supply rate including uniform
mixing of air with the GT exhaust gas coming out from an inlet transition duct
(1) connecting the GT and the HRSG, the apparatus comprising;
• air receiving and supply manifold (16) with at least one set of incoming
single air input pipe from the draft fan (15) the input pipe branching out
into a single or plurality of air supply pipes having integral dampers to
divert or isolate the flow to one or more branches;
• a plurality of air regulation devices (17 A to D) positioned in each of the
outlet branch from the air receiving and supply manifold (16) to adjust
the air flow supply to each zone in the gas path;
• a plurality of air distribution devices (18 A to D) consisting of a grid of
multiple air openings on an interconnected pipe network covering the
entire cross section of the flue gas path for uniform mixing of air and GT
exhaust gas within a short length of the gas path zone;
• means for actuating the integral dampers of air receiving and supply
manifold (16) and the air regulation devices (17 A to D) based on the
dynamic operational parameters so as to maintain safe operating
conditions of the HRSG through a computerized control system.
2. The devices as claimed in claim 1 provide a cost effective solution for
enhancement of steam output from a duct fired HRSG beyond the normal
capability of prior art duct fired HRSG without the need for water or steam
cooled enclosure or a costly refractory or block insulation in the
combustion region (4) regions immediately downstream of the burner(2).
3. The devices as claimed in claim 1 provide improved low load capability i.e.
operation of HRSG at reduced steam generation level, with rated steam
temperature and also without encountering steaming in economiser and
water preheater, without any venting of steam or blow down of water or
part / full bypassing of economiser/water preheater heat transfer sections
on water side or gas side.
4. The devices as claimed in claim 1 result in relatively lower flue gas
temperature due to dilution with fresh air and hence contribute to
increased life or reliability.
5. The devices as claimed in claim 1 provide in combination both enhanced
steam output as claimed in claim-2 and improved low load capability as
claimed in claim-3 in a single HRSG configuration thus providing very high
level of operational flexibility & reliability.
6. The air distribution device (18A) on the upstream of burner can also
function as a flue gas distribution grid and can be of lower grade material
since the same is air cooled and hence can act as a cost effective
substitute for uncooled gas distribution grid plates made of Stainless steel
used in prior arts.
7. The apparatus as claimed in claim 1, wherein the air distribution devices
(18 B to D) positioned at various stages are enabled to act as additional
flow distribution devices.

ABSTRACT

The invention relaters to an air receiving and distribution apparatus for
regulating the steam flow and/ or for regulating the steam or water
temperature at different stages of a duct fired heat recovery steam generator
(HRSG) in a gas turbine (GT), the apparatus receiving air from a Forced draft
fan (15) and distributing to single or multiple zones in the flue gas path of a
heat recovery steam generator (HRSG) for increasing or decreasing the air
supply rate including uniform mixing of air with the GT exhaust gas coming
out from an inlet transition duct (1) connecting the GT and the HRSG, the
apparatus comprising; air receiving and supply manifold (16) with at least
one set of incoming single air input pipe from the draft fan (15) the input
pipe branching out into a single or plurality of air supply pipes having integral
dampers to divert or isolate the flow to one or more branches; a plurality of
air regulation devices (17 A to D) positioned in each of the outlet branch
from the air receiving and supply manifold (16) to adjust the air flow supply
to each zone in the gas path; a plurality of air distribution devices (18 A to
D) consisting of a grid of multiple air openings on an interconnected pipe
network covering the entire cross section of the flue gas path for uniform
mixing of air and GT exhaust gas within a short length of the gas path zone;
means for actuating the integral dampers of air receiving and supply manifold
(16) and the air regulation devices (17 A to D) based on the dynamic
operational parameters so as to maintain safe operating conditions of the
HRSG through a computerized control system.