FIELD OF THE INVENTION
The present invention relates to an improved gas fired duct burner for efficient
combustion of Heat Recovery Steam Generator (HRSG).
BACKGROUND OF THE INVENTION
Heat recovery steam generators (HRSG) are employed to recover heat available
in exhaust gas of gas turbine/diesel engine/rotary kiln, in order to improve over-
all thermal efficiency of a system. The temperature of the exhaust gas of gas
turbine is typically between 500°C and 600°C which passes through a duct to
HRSG to improve overall thermal efficiency of the system. The exhaust gas
contains oxygen composition of the order of 13% to 16% balance being carbon
dioxide and nitrogen. The steam generated in the HRSG is either used for
process or in the steam turbine to generate power. Uninterrupted steam for
operation of steam-turbines or process industries are preferred. However, when
a gas turbine operated at part load, the temperature of the exit flue gas drops
and consequently, generation of steam reduces in the HRSG. Hence, to
overcome the shortfall of steam generation in the HRSG, a supplementary fuel
(either gas or oil) is fired through a duct burner which is installed in the inlet
duct of HRSG. The combustion of fuel gas takes place using the oxygen rich gas
turbine exhaust gas and the air for combustion of oil supplied through a
supplementary fan.

The duct burner assembly consists of an array of fuel gas inlet headers which are
mounted across the flue gas inlet duct through which fuel gas enters and leave
out through a plurality of orifice opening for combustion. Baffles installed in the
burner assembly direct a portion of oxygen rich exhaust gas from the turbine to
the burner and the remaining portion along the downstream of flame. But, many
duct burners lack efficient combustion and leave out higher composition of,
carbon monoxide, unburned hydrocarbons and, high concentration of
greenhouse gas nitrogen oxides (NOx).
Prior art teaches various design of duct burner with fuel gas headers which
contains baffle plates of different configuration to distribute the oxygen rich
turbine exit flue gas for effective combustion in burner zone and for after
combustion. The distance between the fuel gas headers and between edge of
baffle plates are designed based on velocity of turbine exhaust gas and the
capacity of duct burner.
There are yet another design of duct burner assembly which consists of dummy
headers with baffles placed in between fuel inlet headers. The dummy headers
also contain baffle plate to achieve better combustion of fuel gas in the duct
burner assembly. The fuel gas header contains a plurality of orifices for out let of

gas along its length. The fuel gas headers and the dummy headers contain
baffles of different configuration so as to spread the burner flame across entire
section of the inlet duct. The baffles plates, welded in the dummy header,
contain holes to distribute the oxygen rich turbine exhaust gas for combustion
and the remaining portion mix with the flame emanate from individual burners.
The distribution of gas leads to efficient combustion of gas and help to reduce
nitrogen oxides formation.
US Patent No. 4,767,319 teaches duct burner with baffles, wherein the baffles
are so placed to bisect the oxygen rich flue gas for combustion and the rest for
post combustion.
US Patent No. 5,461,853 describes duct burner with staged turbine exhaust gas
admission for reduction of NO emission and reduction of CO.
Design of the baffle plate mounted in the fuel gas inlet header and a dummy
header plays a vital role for orienting the oxygen rich turbine exit flue gas for
combustion and for mixing with burner flame so as to achieve better combustion
of fuel gas.

OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide an improved gas
fired duct burner for efficient combustion in Heat recovery steam generator.
Another object of the present invention is to propose an improved gas fired duct
burner for efficient combustion in Heat recovery steam generator which achieves
optimum mixing of turbine exhaust bas from the dummy gas header assisted by
combustion through the flame originate from gas burner.
Yet another object of the invention is to propose an improved gas fired duct
burner for efficient combustion in Heat recovery steam generator in which
shorter length of flame from the gas burner is developed due to mixing of
turbine exhaust gas from dummy header baffles. A still another object of the
present invention is to propose an improved gas fired duct burner for efficient
combustion in Heat recovery steam generator in which diameter ad number of
holes in each baffle plate is optimized for mixing of turbine exhaust gas with the
help of the burner flame with higher turbulence for complete combustion.

A further object of the present invention is to propose an improved gas fired duct
burner for efficient combustion in Heat recovery steam generator in which
positions holes, dummy headers and distance between the fuel headers, are
decided to achieve higher combustion of gas in the duct burner zone.
A still further object of the invention is to propose an improved gas fired duct
burner for efficient combustion in Heat recovery steam generator which is easy
to manufacture.
SUMMARY OF THE INVENTION
Accordingly, there is provided an improved gas fired duct burner for efficient
combustion in Heat recovery steam generator, the improvement is characterized
in that at least one reconfigured baffle plate is rigidly fixed on a dummy header
of the duct burner assembly disposed in the heat recovery steam generator
(HRSG) to enhance combustion of fuel gas by mixing with the exhaust gas from
the turbine, a plurality of reconfigured baffle plates is welded perpendicular to
the dummy headers which allows orientation of turbine exhaust gas to generate
a short flame length by he burner; the baffle plate is configured with a plurality

of holes of variable diameter between 5 mm and 10 mm; each of the plurality
baffle plates is welded to maintain a distance of 10 mm to 15 mm between the
edges of the baffle plates; and each of the plurality of holes provided in the
baffle plates as located in triangular pitch between 75 mm to 100 mm across the
plates.
The baffle plates are connected either through welded or bolted construction in
perpendicular to the dummy header for mixing of turbine exhaust gas with flame
leave out individually from burners of gas header.
The baffle plate of the dummy header consists of holes of variable diameter
between 5 mm and 10 mm in baffle plate to mix turbine exhaust flow gas with
flame, leave out from gas header, so as to achieve shorter length of flame in
duct burner assembly. The baffle plates each to maintain a distance of 10 mm to
15 mm between edges of the baffle plate so as to mix turbine exhaust gas
turbulently with flame. The holes provided in the baffle plat are located in
triangular pitch between 75 mm and 150 mm across the plate to distribute the
air for better combustion induct burner zone.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIGURE 1 - Schematic view of a conventional Heat Recovery Steam Generator.
FIGURE 2 - is across sectional view of a conventional duct burner installed in the
HRSG of Figure 1.
FIGURE 3 - various design of baffle plates used in gas fired duct burners of prior
Art.
FIGURE 4 - is a cross-sectional view of a duct burner according to the present
Invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
A conventional horizontal natural circulation HRSG is shown schematically in
figure 1. Flue gas leave from gas turbine connect through a duct (1) with HRSG
(2). A duct burner assembly (3) is placed in between gas turbine exit duct and
HRSG. Heat recovered from the flue gas leave out through chimney (4). The duct
burner assembly as shown in Figure 2 contains a plurality of fuel gas inlet header
(5) through which gas enter for combustion. The gas inlet header (5) is made up
of multiple baffle plates (7) for effectively orienting the flame and splitting the
turbine exhaust flue gas for effective combustion. The duct burner assembly of

figure 3 further contains a dummy header (8) with a multiple type of baffle
plates (9, 10) placed in between the fuel gas headers (5) for orienting the
oxygen rich turbine exhaust flue gas for combustion. The configuration of the
baffle plate (10) fixed in the dummy header (8) has plurality of holes (11) for
creating a high velocity so as to admit the flue gas partially to the burners (3)
for combustion and the rest of the flue gas for post combustion, Figure 3 explain
various design of baffle plates used in prior art gas fired duct burners.
Duct burner assembly as shown in figure 4 consists of several fuel gas headers
and dummy headers placed alternatively. The fuel gas headers and dummy
headers are welded from profiled baffles containing holes for entry of turbine exit
gas. The invented dummy headers with their baffle plate are placed in a pitch to
admit turbine exhaust flue gas for combustion as well as for post combustion of
gas burners. The diameter of holes available in the baffle plate of the dummy
headers is of variable diameter and various pitch. The fuel gas header contains a
plurality of gas burners for combustion of fuel gas and the oxygen rich turbine
exhaust flue gas oriented from its baffle plate, wherein flue gas from dummy
header baffle plate assists to achieve better combustion of fuel gas. The invented
baffle plate of dummy header installed in the duct burner assembly for fuel gas
combustion is shown in figure 4.

WE CLAIM :
An improved gas fired duct burner for efficient combustion in Heat
recovery steam generator, the improvement is characterized in that :-
at least one reconfigured baffle plate is rigidly fixed on a dummy header
of the duct burner assembly disposed in the heat recovery steam
generator (HRSG) to enhance combustion of fuel gas by mixing with the
exhaust gas from the turbine,
a plurality of reconfigured baffle plates is welded perpendicular to the
dummy headers which allows orientation of turbine exhaust gas to
generate a short flame length by he burner; the baffle plate is configured
with a plurality of holes of variable diameter between 5 mm and 10 mm;
each of the plurality baffle plates is welded to maintain a distance of 10
mm to 15 mm between the edges of the baffle plates; and

each of the plurality of holes provided in the baffle plates as located in
triangular pitch between 75 mm to 100 mm across the plates.

ABSTRACT

The invention relates to an improved gas fired duct burner for efficient
combustion in Heat recovery steam generator, the improvement is characterized
in that at least one reconfigured baffle plate is rigidly fixed on a dummy header
of the duct burner assembly disposed in the heat recovery steam generator
(HRSG) to enhance combustion of fuel gas by mixing with the exhaust gas from
the turbine, a plurality of reconfigured baffle plates is welded perpendicular to
the dummy headers which allows orientation of turbine exhaust gas to generate
a short flame length by he burner; the baffle plate is configured with a plurality
of holes of variable diameter between 5 mm and 10 mm; each of the plurality
baffle plates is welded to maintain a distance of 10 mm to 15 mm between the
edges of the baffle plates; and each of the plurality of holes provided in the
baffle plates as located in triangular pitch between 75 mm to 100 mm across the
plates.