FIELD OF THE INVENTION:
The present invention relates to premixed type mixed gas injection burners for
combustion system in reheat furnaces adapted to utilize mixed gases of high calorific
value. The injection type burner of the invention is further adapted to be easily installed
for/conversion of existing burners for combustion of low CV mixed gas as fuel into
burners adapted for using fuel having higher CV. The premixed type injection burner of
the present invention is further capable of firing a broad range of CV based mixed gas
fuel with the in-built configuration to prevent the risk of back-fire or flame flash-back,
caused due to reduced gas flow rate or high gas-air mixture temperature, whereby the
gas flow velocity is increased with matching air injection. The present system of injection
burner is capable of using mixed gas fuel of CV 1800-2000 Kcal/Nm3 and a flow velocity
of 90-95 m/s. Importantly, there is further provision for sucking in cold atmospheric air
in case of burner backfiring is caused due to increased gas air mixture temperature and
the same can be brought down to a desired level. The present injection type mixed gas
burner thus have a firing capacity in the range of 1400-1500 Nm3/hr for heating zone
and 800-1000 Nm3/hr for soaking zone burners of the reheat furnace, the mixed gas
being supplied at a pressure range of 1400-1500 mmWC. The premixed type injection
burner, according to the present invention is thus capable of injecting required amount of
combustion air by the increased momentum of mixed gas, having simple configuration to
construct/install, uniform annulus area for uniform distribution of gas near the tip,
providing high efficiency.
BACKGROUND ART
It is well known in the art of combustion system of premixed type injection burners,
which are used for heating and soaking of the blooms in the reheat furnace such as in
the rail and structural mills of steel plants. In injection type combustion system, there
remain no provision of combustion air blower and the required amount of air sufficient for
complete combustion of fuel is injected by the injection effect created by the momentum
of fuel gas. It is basically premixed type burner, where gas and fuel are mixed in the
mixer body before being injected into the furnace. The turn down ratio of such burners
are very less about 1:2 and any major deviation from design parameters affects the
burner performance. In pre-mixed type burners, backfiring/flame flash-back frequently
takes place when gas air mixture velocity becomes less than the flame propagation
2

velocity, which depends on many factors like, temperature of gas air mixture, gas
composition, mixture velocity, pressure etc. These burners are normally designed for a
particular calorific value (CV) of mixed fuel gas and its composition and the basic burner
dimensions are selected accordingly. Once a burner is installed and the CV of mixed fuel
gas available is increased subsequently, there arises the need to reduce the gas flow rate
and maintain air injection rate to maintain equivalent heat load or otherwise resulting
incomplete combustion in absence of desired quantity of air injection lead to secondary
combustion in the flue passage raising the temperature of gas-air mixture.
Presently, the calorific value of mixed gas has increased to 2000 Kcal/Nm3. Increase in
gas CV resulted in reduced gas flow rate through each injection burner for the same heat
load requirement. Reduced gas flow rate resulted in reduced injection effect caused by
low gas velocity in place of mixed gas of CV 1400 Kcal/Nm3 for which the injection
burners in Rail & Structural Mills were originally designed for firing the mixed gas. Such
increase in gas CV resulted in reduced gas flow rate through each injection burner for the
same heat load requirement. Reduced gas flow rate resulted in reduced injection effect
caused by low gas velocity in the range of 50-60 m/s against the designed velocity of
90-95 m/s. Reduced availability of combustion air because of reduced injection effect,
resulted in incomplete combustion of fuel in the furnace and leading to secondary
combustion in the flue passage raising its temperature before entering into metallic
recuperator causing its damage. As a short term measure to combat the problem of high
gas temperature and the flame flash-back because of low gas velocity, reducing the
number of burners actually in operation, keeping the other burners off to avoid
backfiring, is adopted which resulted non-uniformity in temperature of the blooms.
There has been therefore a continuing need in the art of firing of burners in the premixed
injection type combustion system for different sections of reheat furnace using mixed gas
as fuel and creating air injection of desires volume and flow rate based on flow rate of
mixed gas, that the system shall be efficiently working even in the event of a rise in the
CV of mixed gas fuel, such that the proportionate reduction in flow passage inherently
takes care of the requirement of reduced flow rate for maintaining desired heat load at
individual burners and uniform temperature of steel product in respective zones e.g.
heating and soaking in reheat furnace, eliminating the risk of backfire or flame flash-back
and resultant high gas-air mixture temperature which in turn damage the metallic
recuperator on entering at a later stage and thus increasing the overall performance and
efficiency of the mixed gas injection burner based combustion systems in reheat furnaces
of steel plants.
3

OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for a mixed gas injection
burner adapted to utilize higher CV of mixed gas fuel with simple construction of injection
type premixed mixed gas based combustion systems for reheat furnace which are usually
configured for lesser CV of mixed gas and thereby eliminate the problems of
backfire/flame flash-back on one hand maintaining desired flow rate, air injection, gas
temperature and on the other hand ensure highly efficient burning of mixed gas to
maintain thereby the desired heat load and temperature at respective zones in the reheat
furnace.
Another object of the present invention is directed to providing a simple and cost-
effective manner of achieving reduction in the flow area by about 40% and higher gas
flow velocity and proportionate air injection, even when the system involves a higher CV
mixed gas as fuel and complete combustion takes place with higher efficiency.
A still further object of the present invention directed to a premixed type mixed gas
injection burner adapted to utilize fuel having CV of higher value than the installed
handling capacity by way of a simple modification of existing burner system.
A still further object of the present invention is directed to a high efficiency mixed gas
injection burner system which is adapted to facilitate suction of cold atmospheric air in
the event of burner backfiring due to increased gas air mixture temperature by opening
the cap and bring down the gas-air mixture temperature at desired level.
A still further object of the present invention directed to a high efficiency mixed gas
injection burner system wherein during use of mixed gas fuel having a CV of 1800-2000
Kcal/Nm3, high flow velocity could still be achieved in the range of 90-95 m/s as
compared to existing reduced range of 50-60 m/s.
A still further object of the present invention directed to said modified high efficiency
mixed gas injection burner system adapted such as to facilitate achieving the firing
capacity of the modified injection burners at about 1400-1500 Nm3/nr for heating zone
and 800-1000 Nm3/hr for soaking zone burners of the reheat furnace, said mixed gases
being supplied at a pressure in the range of 1400-1500 mmWC.
4

A still further object of the present invention is directed to a modified high efficiency
mixed gas injection burner system wherein said burner system is adapted to inject
required amount of air for complete combustion at reduced flow rate of mixed gas,
effecting said air injection by increased velocity and momentum of mixed gases.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a mixed gas
injection burner for reheating furnaces adapted for reduced gas flow rate and gas velocity
involving mixed gas of increased calorific value in the range of 1800 to 2000Kcal/Nm3 CV
comprising:
an air inlet;
a gas nozzle for supply of the fuel gas;
a mixer body ahead of said fuel gas and said air inlet for premixing of the gas and
air; and
burner,
wherein the gas nozzle comprises dummy pipe of suitable dimensions adapted for
reducing the nozzle area and increasing the gas velocity.
A further aspect of the present invention directed to said mixed gas injection burner for
reheating furnaces wherein a spacer is provided at the tip of the dummy pipe which is
inserted through a flange provided at the back of the nozzle.
5

A still further aspect of the present invention directed to said mixed gas injection burner
for reheating furnaces comprising a threaded cap at the back end of the dummy pipe
adapted to facilitate suction of cold atmospheric air in the event of burner backfiring due
to increased gas air mixture temperature.
According to an important aspect of the present invention directed to said mixed gas
injection burners for reheating furnaces wherein the firing capacity of the burner is in the
range of 1400 to 1500 and 800 to 1000 Nm3/hr for heating and soaking zone burners
respectively.
A still further aspect of the present invention directed to said mixed gas injection burner
for reheating furnaces wherein the mixed gas is supplied to the burner at a pressure in
the range of 1400 to 1500 mmWC.
According to another aspect of the present invention directed to said mixed gas injection
burner for reheating furnaces wherein the cross section of the nozzle is reduced by up to
40%.
The present invention and its objects and advantages are described in further details with
reference to the accompanying non-limiting illustrative figures.
BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1: is the illustration of the schematic diagram of the existing mixed gas injection
burner.
Figure 2: is the illustration of the schematic arrangement of the modified high efficiency
mixed gas injection burner according to the present invention showing the insertion of
the dummy pipe through the burner nozzle.
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DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING
DRAWINGS
The present invention relates to a system of premixed type injection gas burners of
reheat furnaces adapted to utilize mixed gases of a higher range of calorific value, while
ensuring complete combustion of fuel with required quantity of air injection eliminating
backfire/flame flash back. It is known in the art of heating and soaking of blooms in
reheat furnaces, the premixed type mixed gas injection burners are used to
attain/maintain the desired temperatures.
Reference is first invited to the accompanying Figure 1 that illustrates the conventional
configuration of the mixed gas injection type burner system. As already described, the
combustion system for premix type injection burners are used for heating and soaking of
the blooms in the reheat furnace such as in the rail and structural mills of steel plants. In
mixed gas injection type combustion system, there remain no provision of combustion air
blower and the required amount of air sufficient for complete combustion of fuel is
injected by the injection effect created by the momentum of fuel gas. The burners for
combustion of mixed gas are basically premixed type burner, where gas and fuel are
mixed in the mixer body before being injected into the furnace. The turn down ratio of
such burners are very less about 1:2 and any major deviation from design parameters
affects the burner performance. In these pre-mixed type burners, backfiring/flame flash-
back frequently takes place when gas air mixture velocity becomes less than the flame
propagation velocity, which depends on a number of factors comprising temperature of
gas air mixture, gas composition, mixture velocity, pressure etc. These burners are
normally designed for a particular calorific value (CV) such as in the order of 1400
Kcal/Nm3, of the mixed fuel gas based on its composition and the basic burner
dimensions are selected accordingly. Once such burners are installed and the CV of
mixed fuel gas available is increased subsequently, there arises the need to reduce the
gas flow rate and maintain air injection to maintain equivalent heat load or otherwise
resulting incomplete combustion in absence of desired quantity of air injection lead to
secondary combustion in the flue passage raising the temperature of gas-air mixture.
Reference is now invited to the accompanying Figure 2 that illustrates the schematic
arrangement of the mixed gas injection burner of the present invention with modification
to be adapted to mixed gas having higher CV in the range of 1800-2000 Kcal/Nm3.
As already described, that the calorific value of mixed gas is changed due to its
composition and character to a higher value in the range of 2000 Kcal/Nm3 in place of
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mixed gas of CV 1400 Kcal/Nm3 for which the injection burners in Rail & Structural Mills
were originally designed for firing the mixed gas. Increase in gas CV resulted in
controlled/reduced gas flow rate through each injection burner for the same heat load
requirement. Reduced gas flow rate in turn, resulted in reduced injection effect caused by
low gas velocity. Reduced gas flow rate resulted in reduced air injection effect caused by
low gas velocity in the range of 50-60 m/s against the designed velocity of 90-95 m/s.
Reduced availability of combustion air because of reduced injection effect, resulted in
incomplete combustion of fuel in the furnace and leading to secondary combustion in the
flue passage raising its temperature before entering into metallic recuperator causing its
damage. As a short term measure to combat the problem of high gas temperature and
the flame flash-back because of low gas velocity, reducing the number of burners
actually in operation, keeping the other burners off to avoid backfiring, is adopted which
resulted undesirable non-uniformity in temperature of the blooms leading to poor quality
of finished product.
To overcome the limitation and disadvantages of the existing system of mixed gas
injection burners, they are suitably modified to enable complete combustion of fuel
having said higher CV, with high efficiency and ensuring safety eliminating occurrence of
backfire/flashback or damage of recuperators and the like.
In the modified injection burners according to the invention as evident in accompanying
Figure 2, a dummy pipe of selective dimension has been inserted in the existing gas
nozzle of injection burner. The injection burners of the present invention comprise a
dummy pipe (1) inserted in existing gas nozzle for reducing the nozzle area & increasing
the gas velocity. A spacer (2) has been provided near the tip of dummy pipe to keep the
annulus area uniform, at exit of nozzle. A threaded cap (3) at the back end of dummy
pipe has been provided to facilitate suction of cold atmospheric air in the event of burner
backfiring due to increased gas air mixture temperature. This dummy pipe has been
inserted through the flange provided at the back end of gas nozzle, such that the
installation or servicing in operation is easy and simple. The modified injection type
burner of the invention is further adapted to easy conversion of existing burners for firing
fuel having higher CV with improved efficiency, maintaining operational safety. The
premixed type injection burner of the present invention is further capable of firing a
broad range of CV based mixed gas fuel with the in-built configuration to prevent the risk
of back-fire or flame flash-back, caused due to reduced gas flow rate or high gas
temperature, by selective use of a concentric dummy pipe mounted within the gas nozzle
such as to reduce the effective cross-sectional flow area by upto 40% and thereby
increasing the gas flow velocity and matching quantity of air injection.
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The modified premixed type mixed gas injection burners are thus adapted to use mixed
gas having calorific value of about 1800 - 2000 Kcal/Nm3. The firing capacity of modified
mixed gas injection burners are 1400 - 1500 Nm3/hr for heating zone and 800 - 1000
Nm3/hr in the soaking zone burners respectively for the reheating furnace. The mixed
gas is supplied to burner at a pressure in the range of 1400 - 1500 mmWC. The selective
gas air mixture pressure and temperature and maintaining the selective air-fuel ratio
through desired level of air injection at enhanced flow velocity of gas, ensure complete
combustion with higher efficiency. Moreover, elimination of backfire or flash back in the
mixer body by restricting unwanted temperature rise of air-gas mixture. Importantly,
provision of a threaded cap at the back end of the dummy pipe inserted in the burner
nozzle further facilitate suction of cold atmospheric air, in the event of burner backfiring
due to increased gas air mixture temperature, ensuring safety and efficient operation. A
spacer provided at the nozzle tip to mount the dummy pipe for retaining concentricity
with the nozzle, ensure a symmetric uniform annulus for the gas/flame exit from the
nozzle tip and uniform heating.
It is thus possible by way of this invention to obtain mixed gas injection burner based
combustion system adapted to firing of higher CV of gas, for operation in the heating and
soaking zone of reheat furnace, by suitable modification of the existing burner
configuration to make them capable of using fuel gas having higher Calorific Value and
also to ensure complete combustion through proper air injection and thereby obtaining
higher operational efficiency. The invention further enable said high efficiency mixed gas
injection type premixed burners, by inserting a dummy pipe inside the existing burner
nozzle which is simple in construction and installation. The modified burner of the
invention further provide means for limiting the tendency of increase in temperature of
mixed gas-air mixture causing backfire or flame flash back depending on flow rate
variation, by simple provisioning of a threaded end cap at the backside of said dummy
pipe such as to enable suction of cold atmospheric air by opening said cap, to reduce the
mixture temperature to desired level. The invention thus involve simple inexpensive
gadgets and devices to modify the existing mixed gas injection type burner system to
highly efficient combustion system eliminating the need for installation of new burners to
accommodate variation in CV and composition of mixed fuel gas, and thus having wide
industrial application in similar situation with advantage and economy.
9

WE CLAIM:
1. A mixed gas injection burner for reheating furnaces adapted for reduced gas flow
rate and gas velocity involving mixed gas of increased calorific value in the range
of 1800 to 2000Kcal/Nm3 CV comprising:
an air inlet;
a gas nozzle for supply of the fuel gas;
a mixer body ahead of said fuel gas and said air inlet for premixing of the gas and
air; and
burner,
wherein the gas nozzle comprises dummy pipe of suitable dimensions adapted for
reducing the nozzle area and increasing the gas velocity.
2. A mixed gas injection burner for reheating furnaces as claimed in claim 1 wherein
a spacer is provided at the tip of the dummy pipe which is inserted through a
flange provided at the back of the nozzle.
3. A mixed gas injection burner for reheating furnaces as claimed in anyone of
claims 1 or 2 comprising a threaded cap at the back end of the dummy pipe
adapted to facilitate suction of cold atmospheric air in the event of burner
backfiring due to increased gas air mixture temperature.
10

11
4. A mixed gas injection burner for reheating furnaces as claimed in anyone of
claims 1 to 3 wherein the firing capacity of the burner is in the range of 1400 to
1500 and 800 to 1000 Nm3/rir for heating and soaking zone burners respectively.
5. A mixed gas injection burner for reheating furnaces as claimed in anyone of
claims 1 to 4 wherein the mixed gas is supplied to the burner at a pressure in the
range of 1400 to 1500 mmWC.
6. A mixed gas injection burner for reheating furnaces as claimed in anyone of
claims 1 to 5 wherein the cross section of the nozzle is reduced by upto 40%.
7. A mixed gas injection burner for reheating furnaces substantially as
hereindescribed and illustrated with reference to the accompanying figures.

The present invention relates to premixed type mixed gas injection burners for
combustion system in reheat furnaces adapted to utilize mixed gases of high calorific
value. A concentric dummy pipe is inserted within the gas nozzle of existing injection
burner to reduce the effective cross-sectional flow area by about 40% with simple
construction/installation, and thereby increasing the gas flow velocity and eliminating
backfire or flame flash back. The present system of injection burner is capable of using
mixed gas fuel of CV 1800-2000 Kcal/Nm3 and a flow velocity of 90-95 m/s. Importantly,
the provision of a threaded cap at the back end of the dummy pipe is utilized to suck in
cold atmospheric air to reduce the gas-air mixture temperature to desired level. The
present injection gas burners having the firing capacity in the range of 1400-1500
Nm3/hr for heating zone and 800-1000 Nm3/hr for soaking zone, supplied at a pressure
range of 1400-1500 mmWC. Provision of a spacer ensure uniform annulus area for exit
near the tip of burner nozzle, providing high efficiency with complete combustion.