FIELD OF INVENTION
The present invention relates to a system and a method for achieving coal flexibility employing flue gas recirculation in coal fixed thermal power plants. More particularly the present invention relates to a system and a method to improve the flexibility to operate efficiently with wider quality of coal property ranges than for which the boiler is designed for coal fired thermal power plants and maintain the Reheater temperature and Superheater temperature designed values and reduce the Slagging and Fouling that might take place due to firing coals of various qualities either in 100% same type quality coal or blend of coals with various qualities.
BACKGROUND OF THE INVENTION
Current scenario of availability of coal for any of the thermal power plants drives towards using blend of coals or use entirely different coal type to achieve the targeted load. In this process, achieving temperatures and other important parameters that decides the performance of the power plant and as well as the overall cost per unit power produced would become crucial factors. The scenario is not only true for new power plants that are being planned but also true for existing power plants. It may be possible to redesign the new power plants that are coming-up to suit for various coal qualities. But it is not easy task in terms of cost and efforts that are needed to modify the design of existing power plants as most of the power plants are designed to a linked coal base that is assumed to be supplying

the amount of coal demanded by that power plant. In current circumstances the excavation from the linked coal-mines is not meeting We power plant demand; due which power plants are going for importing the coal which are of different coal property ranges and load the power plant partially or completely with imported coal blend with coal from linked coal mine. This situation leads to problems like Slagging, fouling, inability to achieve designed temperatures etc. To address this issue an innovative method is proposed in this invention.
While using extreme coals and coal blends there is a possibility of Slagging and fouling in furnace walls. Also, it has been noticed that with the existing control operations approach, in case of power plant being loaded with extreme coal quality, it is highly difficulty to achieve the designed temperature. Figure 3 shows one scenario where considered power plant was designed for high-ash and low-calorific value coals and loaded with same coal to operate the power plant, which show the control strategies are achieving desired values on temperature and power plant performance. Figure 4 shows extremely opposite scenario where considered power plant was designed for high-ash and low-calorific
value coals in which low ash content and high calorific value coal is being loaded to operate. It can be clearly seen that the Reheater temperature achieved is much less than the design temperature value even after executing the control parameter like burner tilt and excess air and are set at their respective extreme values that are possible.

PRIOR ART
There are variety of methods and processes of exhaust gas recirculation in Internal Combustion Engine, Gas Turbines etc. Some of the prior art describes those methods and processes.
In abstract, US patent #4147141, discloses a method and a process related
improvement to recirculation of exhaust gas in internal combustion engine, where the
improved recirculation of exhaust gas system helps in carbon accumulation and
condensation of the acid water in the system.
In abstract, US patent #8720179B2 discloses a method and a process for injecting recirculated exhaust gases in the fuel and compresses air of gas turbine engine in power plants.
In abstract, US patent # 4271664 discloses a method and apparatus for improving the efficiency of an integrated combined cycle Brayton-Rankine engine. The engine has a main power turbine operating on an open-loop Brayton cycle. Its air supply is furnished by a compressor independently driven by the turbine of a closed-loop Rankine cycle which derives heat energy from the exhaust of the Brayton turbine. A portion of the exhaust gas is recirculated into the compressor inlet during part-load operation. The recirculation of exhaust gas improves the efficiency of the engine at part-load over that which would occur with only ambient temperature air entering the compressor.

In abstract, WO2014071089A1 discloses one embodiment, a system includes at least one sensor configured to communicate a signal representative of a gas turbine operations. The system further includes a controller communicatively coupled to the sensor. The system additionally includes a stoichiometric model configured to receive one or more inputs representative of the gas turbine operations and a measured equivalence ratio, wherein the controller is configured to transform the signal into the one or more inputs and to use the stoichiometric model to derive an actuation signal based on a target equivalence ratio.
In abstract, US patent application #20140216364A1 discloses methods and systems for reheat outlet steam temperature control in a steam generating oxy fuel f.red boiler. More specifically, the present disclosure relates to methods and systems for controlling multiple control handles for reheat steam temperature control in a steam generating boiler.
In abstract, US patent #4335660A discloses a method of utilizing flue gas recirculation on solid fuel f.red boilers and furnaces. By providing this mixture under the burning fuel bed, the temperature of the fuel bed is lowered and excess oxygen is reduced. A more reactive fuel bed results providing various advantages including improved boiler operation and a clinker-free fuel bed.
In abstract, US patent # US 8329125B2 discloses an invention relates to air quality control systems, the principal purpose of which is to remove pollutants from the flue gas generated during combustion of solid fuels such as coal, wood products and municipal solid

waste. More specifically, this invention relates to dry, circulating dry, semi-dry or spray dryer type flue gas treatment systems, also commonly referred to as dry scrubber systems.
In abstract, US patent #8521333B2 discloses an invention to provide an improved method and system for using the existing Induced Draft Fan (ID Fan) of a boiler draft control system to drive flue gas through a recirculation duct to the inlet side of a scrubber-baghouse or other Air Quality Control System (AQC System) in order to maintain an optimal flue gas flow rate through the system during all load conditions, thereby eliminating the need for a separate recirculation/booster fan.
Unlike in the invention disclosed current specifications, none of the above mentioned prior art methods or processes or systems that address the drawbacks mentioned herein, or used method or process or system for recirculation of flue gas from Electrostatic Precipitator (ESP) outlet duct to mix with secondary air system before entering into combustion chamber of boiler in the power plant to achieve better controllability on achieving designed temperatures, minimizing Slagging and fouling. Overall the prior art is related to Oxy-fuel boilers, Circulation Fluidized Bed boilers, IC engines, Gas Turbines etc.
in addition, a review on the existing methods of handling when extreme coal qualities
are fired as follows:
1. Existing approach to handle the coal quality/type variation is mainly handled by
reducing the load on the power plant leading to less power output from the power plant.

2. With existing designs, to maintain the load for any new coal that was not meant for that design, is to face the Slagging and fouling problems.
3. Further, with the existing design, to maintain the load for any new coal that was not meant for that design, is to face lower performance of the power plant.
Figure 3, Figure 4 and Figure 5 show the control strategy based on the prior art approach. Figure 3 shows the control strategy when 100% designed coal is fired and response of both Reheater Temperature (34) and Superheater Temperature (32) and amount of superheater spray (33) needed for the changes carried out on control parameters i.e. Burner Tilt, Excess Air, Reheater spray and Superheater spray. Figure 4 shows the control strategy when blend of 50% of designed coal and 50% of coal with properties outside the design coal are fired and response of both Reheater Temperature and Superheater Temperature for the changes carried out on control parameters i.e. Burner Tilt, Excess Air, Reheater spray and Superheater spray. Similarly, Figure 5 shows the control strategy when 100% of coal with property outside the design coal is fired and response of both Reheater Temperature and Superheater Temperature for the changes carried out on control parameters i.e. Burner Tilt, Excess Air, Reheater spray and Superheater spray. Further, Figure 3, Figure 4 and Figure 5 also show the amount of Reheater spray and Superheater spray needed when the existing control strategy is followed.

OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a system and a method for achieving coal flexibility employing flue gas recirculation in coal fixed thermal power plants which is capable of dealing with the variation in the coal properties and achieve efficient combustion leading to best performance of the boiler and thereby power plant.
Another object of the invention is to propose a system and a method for achieving coal flexibility employing flue gas recirculation in coal fixed thermal power plants which is able to reduce furnace absorption and heat transfer to furnace due to radiation and to increase convective heat transfer in super heater and Reheater.
A further object of the invention is to propose a system and a method for achieving coal flexibility employing flue gas recirculation in coal fixed thermal power plants which is capable of eliminating the problems of slagging and fouling in furnace walls.
SUMMARY OF THE INVENTION
When the coal with properties outside the design ranges, the existing control parameters are not sufficient to operate the power plant efficiently with full load. In this invention, flue gas recirculation is proposed to address this drawback.

The flue gas recirculation proposed apportions the heat absorption in the furnace, Superheater, Reheater to meet the optimization level to get the desired performance of reheater and Superheater. Convection heat transfer is increased in Superheater and reheater by increasing the flue gas recirculation quantity. The flue gas recirculation serves as an additional, control parameters to deal with the variation in the coal properties and achieve efflcient combustion leading to best performance of the boiler and thereby power plant.
fouling in furnace walls if the cold gas is mixed with the hot secondary air and introduced into the furnace windbox, peak flame temperatures are avoided. Thus Sagging and fouling problems are avoided.
As discussed in the prior art, existing control strategy with existing control parameters will be only suitable for coals with properties that are with the designed coal properties When the coal with properties outside the design ranges, the existing control parameters are not sufflcient to operate the power plant efflciently with full load and it is not possible to achieve targeted temperature with part load operations in current invention, flue gas recirculation is proposed to address this drawback figure6 andfigure 7
show the control strategy proposed in this invention using flue gas recirculation mixing with
secondary air and allowed into combustion chamber (18) through windbox (16). figure 6
shows the control strategy proposed in the current invention when 50% design coal(e.g.

high ash coal) and 50% coal (e.g. low ash coal) with outside the ranges of the designed coal are fired and response of reheater temperature (34), superheater temperature (32) and amount of superheater spray (33) needed to achieve the targeted temperature (35). Typical High ash and low ash coal properties are listed in Table 1 for better understanding. in figure 6, it can be clearly seen that the existing control parameters reach their extreme values by the time power plant reaches lower loads (below 50% loads) leaving the reheater temperature (34) and superheater temperature (32) below their designed values (target values) (35). Further, Rgure 6 (A) depict the response of temperatures and have achieved their designed values even at lower loads (i.e. less than 50% loads) with the new control parameter i.e. recirculation of amount of flue gas shown in figure 6(C). Rgure 7 shows
s.ategy proposed In the invention will achieve control on temperatures to attain the designed value for both reheater temperature and superheater temperature. Further, Table 3 lists values of reheater temperature (34) and superheater temperature (32) achieved their designed targeted (35) values with higher efficiency of boiler based proposed method and
apparatus.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
1 Figure 1- Shows a schematic of power plant with depicting boiler, combustion chamber,ESP outlet flue gas recirculation arrangement according to the invention

2. Figure 2: Shows mixing chamber for Flue Gas and Secondary Air before windbox inlet (A) Front View and (B) Side View (C) Top view according to the invention.
3. Figure 3: Shows Prior art Steam Temperature control strategy for achieving Reheater and Superheater temperatures when 100% design coal (i.e. high ash coal) is fired and their response curves with respect to (A) Reheater Spray and Superheater Spray (B) Angle of burner tilt, (C) percentage Excess Air.
4. Figure 4: Shows Prior art Steam Temperature control strategy for achieving Reheater and Superheater temperatures when 50% design coal (i.e. high ash coal) and 50% coal with outside the ranges of the designed coal (i.e. low ash coal) are fired and their response curves with respect to (A) Reheater Spray and Superheater Spray (B) Angle of burner tilt, (C) percentage Excess Air.

5. Figure 5: Shows Prior art Steam Temperature control strategy for achieving Reheater and Superheater temperatures when 100% coal with outside the ranges of the designed coal (i.e. low ash coal) is fired and their response curves with respect to (A) Reheater Spray and Superheater Spray (B) Angle of burner tilt, (C) percentage Excess Air.
6. Figure 6: Shows Proposed Steam Temperature control strategy for achieving Reheater and Superheater temperatures when 50% design coal (i.e. high ash coal) and 50% coal with outside the ranges of the designed coal (i.e. low ash coal) are fired and their response curves with respect to (A) Reheater Spray and Superheater Spray (B) Angle of burner tilt, (C) percentage Excess Air and (D) Percentage of Flue Gas Recirculation.
7. Figure 7: Shows Proposed Steam Temperature control strategy for achieving
Reheater and Superheater temperatures when 100% coal with outside the ranges of
the designed coal (i.e. low ash coal) is fired and their response curves with respect to

(A) Reheater Spray and Superheater Spray (B) Angle of burner tilt, (C) percentage Excess Air and (D) Percentage of Flue Gas Recirculation.
8. Table 1: Shows List of coal properties for both typical high-ash and low-ash coals.
9. Table 2: Shows Prior Art based strategy: Values of Reheater and superheater Temperatures achieved based on prior, art control strategy and boiler efficiency, where targeted temperature (540°C) is achieved only between 100% to 60% load.
10. Table 3: Shows Invention based strategy: Values of Reheater and superheater Temperatures achieved based on proposed method and apparatus and boiler efficiency, where targeted temperature (540°C) is achieved between 100% to 40% load with additional control parameter i.e. Flue Gas Recirculation.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure 1 shows the schematic diagram of boiler portion of the power plant displaying along with the essential embodiment of invention herein. The power plant is monitored and controlled from a control room (1) from where all the state variables of the power plant i.e. pressures, temperatures, flow rates etc. at various locations consisting hundreds of parameters are measured, collected through sensor signal connection (4) and stored into a database (2) for various purposes including modelling, monitoring, diagnostics and for devising a better control strategy to encounter odds. Visualizing device (3) displays all the collected information in database (2) as needed by operator for the ease of operations. In power plant operations, as per the prior art, primary air generated by the primary air fan (5) passes through primary air duct (14) to lift the coal from the coal mills (8) and enters into

the windbox (16).Similarly, secondary air generated by the Force-draft (FD)-Fan (6) passes through secondary air duct (7) and enters into the windbox (16). Further, from windbox (16) the primary air with coal particles and secondary air supplied into the boiler furnace combustion area (18). As per the prior art the combustion is control through varying burner tilt angle (17), percentage of excess air measured prior to the air-preheater (13), and supplement with reheater-spray and reheater spray to fine tuning the temperature of
steam.
figure 3,figure 4, figure 5 and table2 show the control strategy based on the prior
art approach . figure 3 shows the control strategy when 100% coal is fired for which the
boiler was designed, the response of both reheater Temperature (34) and Superheater Temperature (32), and amount of superheater spray (33) needed for the changes carried out on control parameters i.e. Burner Tilt, Excess Air, reheater spray and Superheater spray
to achieve targeted temperature (35). It is clearly shown in figure 3 that at the higher loads
i e 100% to 60% loads, with spray (33) targeted reheater and superheater temperature
(35) i e 540oC is achieved; and whereas at lower loads i.e. 60% to 40% loads it is not
possible to achieve the targeted temperature (35) instead 500oc is achieved by controlling
Burner angle to maximum possible ranges and 520oC is achieved by exercising excess air supply to maximum allowed values. Same information at gross level is shown inTable 2 for
clarity.

similarly , figure 4 shows the prior art control strategy when blend of 50% of
designed coal and 50% of coal with properties outside the design coal are fired and
response of both Reheater Temperature (34) and Superheater Temperature (32) for the
changes carried out on control parameters i.e burner tilt excess air reheater spray and
superheater spray in this case also it is clearly noticed that higher (100% to 50%) loads
targeted temprature is achieved by supplementing with reheater and superheater sprays and at lower (50% to 40%) loads targeted temperatures are not achieved.
Further, figure 5 shows the prior art control strategy when 100% of coal with
properties out side the design coal is fired and responce of both reheater temperature(34)
and super heater temperature (32) for the changes carried out on control parameters ie
burner tilt excess air reheater spray and superheater spray,in this case also it is clearly
noticed that higher (100% to 60%) loads targeted temperature is achieved by
supplementing with reheater and superheater sprays and at lower (60% to40%) loads
targeted temperatures are not achieved. Further, figure 3, figure 4 and figure 5 also show
the amount of Reheater spray and Superheater spray needed when the prior art control
strategy is followed.
The exhaust gases of the combustion i.e. flue gas is passed through Electrostatic
precipitator [ESP] (21) to remove the suspended particulates in the flue gas and moves into
ESP outlet duct (22) this dust free flue gas is then driven out by induced draft (id) fan
(23) to release out to the environment through the chimney (24). In the current invention

a portion of the dust free flue gas is fed back into the combustion chamber (18) for improving the coal flexibility of the power plant to be able to fire wide range of coals in the combustion chamber (18). As the quantity of flue gas recirculation increases it reduces furnace absorption and reduces heat transfer to furnace due to radiation and increases convective heat transfer in super heater and reheater Thus flue Gas recirculation becomes an additional control parameter and provides flexibility to control steam temperatures with wide range of coals as fuels in the boiler.
Further, figure 1 also shows the essential embodiment of invention proposed herein i e flue Gas Recirculation system for coal flexibility of power plants. flue Gas Recirculation
(FGR) system consists of FGR duct (15),FGR Fan (20), FGR damper (26), Thermocouples
Figure 2 fgr duct (15), which connects between eSP outlet duct (22) and secondary
and FGR duct (15) are joined through mixing chamber (12).
The FGR duct (15) is mounted with an automated damper (26) to adjust the amount of flue gas to be allowed into SA system (7) for the recirculation. To maintain positive pressure at the FGR header (28) inlet when flue gas enters into the mixing chamber (12), a FGR Fan (20) is attached to FGR duct (15) which drives the Flue Gas into mixing chamber (12) with positive pressure. To monitor the temperature of the flue gas, thermocouples (10, 19) are mounted on FGR duct (15) one Thermocouple (19) at ESP outlet duct (22) and

other thermocouple (10) at FGR mixing chamber (12) inlet. Similarly, to monitor pressure variations in the FGR system two pressure sensors are mounted; one pressure sensor (25) at the FGR system inlet and other pressure sensor (27) at the FGR mixing chamber (28) inlet. Additionally, to monitor temperature and pressure in the SA system one thermocouple (9) and one pressure sensor (11) are mounted on SA system duct (7). By monitoring and controlling temperatures and pressure changes of both flue gas in the FGR system and secondary air in the SA system in addition to the coal property variations that are being fired, the amount of flue gas to be allowed for recirculation is estimated.
Figure 2 consists of (A) Front view. (B) Side view and (C) Top view of the FGR mixing chamber assembly. Figure 2 shows more details of assembly of the FGR mixing chamber (12) along with the FGR header (28), where the flue gas is accumulated and passed trough three distributed pipes With holes (30) inside the FGR chamber (12) to help in uniform mixing of flue gas with secondary air and release into windbox through FGR chamber outlet duct (31). To achieve uniform distribution of the received flue gas in the FGR chamber (12), baffles (29) are arranged in the FGR header (28).
As discussed in the prior art, existing control strategy with existing control parameters will be only suitable for coals with properties that are with the designed coal properties. When the coal with properties outside the design ranges, the existing control parameters are not sufficient to operate the power plant efficiently with full load and it is

not possible to achieve targeted temperature with part load operations. In current invention, flue gas recirculation is proposed to address this drawback. Figure 6 and Figure 7 show the control strategy proposed in this invention using flue gas recirculation mixing with secondary air and allowed into combustion chamber (18) through windbox (16). Figure 6 shows the control strategy proposed in the current invention when 50% design coal (e.g. . high ash coal) and 50% coal (e.g. low ash coal) with outside the ranges of the designed coal are fired and response of reheater temperature (34), superheater temperature (32) and amount of superheater spray (33) needed to achieve the targeted temperature (35).
In figure 6, it can be clearly seen that the existing control parameters reach their extreme values by the time power plant reaches lower loads (below 50% loads) leaving the reheater temperature(34) and superheater temperature (32) below their designed values (target values) (35). Further, figure 6 (A) depict the response of temperatures and have achieved their designed values even at lower loads (i.e. less than 50% loads) with the new control parameter i.e. recirculation of amount of flue gas shown in Figure 6(C). Figure 7 shows
another extreme case of firing different coal other than the designed coal assuming the
boiler was designed for high ash coal and when 100% low ash is fired how the control
strategy proposed in the invention will achieve control on temperatures to attain the designed value for both reheater temperature and superheater temperature. Further, Table 3 .ists values of reheater temperature (34) and superheater temperature (32) achieved their

designed targeted (35) values with higher efficiency of boiler based proposed method and
apparatus.
While the invention has been described in detail in connection with only a limited
number of embodiments, it should be readily understood that the invention is not limited to
such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. additionally while various embodiments of the invention have been described, it is to be under stood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

WE CLAIM
1. A system and a method for achieving coal flexibility employing flue gas recirculation in coal fixed thermal power plants, the said system comprising;
a reheater (34) for providing reheater spray;
a superheater (33) for providing superheater spray;
a burner (17) providing heat with predetermined tilt angle;
an electrostatic precipitator (ESP) (21) disposed for removing the suspended
particulates in the flue gas released out in the environment;
characterized in that,
a flue gas recirculations (fgr) system consisting of fgr duct (15) connected between electrostatic percipotator (21) and a secondary air (SA) sytem duct (7) through a
mixing chamber (12);
a FGR fan (20) attached to FGR duct (15) for driving the flue has into mixing
chambers (12) with positive pressure;
a plurality of thermocouples (10'19) mounted on the said fgr duct for monitoring the temperature of flue gas where one of the said thermocouples (19) disposed at
electrostatic precipitator (ESP) (21) outlet duct (22) and other thermocouple (10) disposed at FGR mixing chamber (12) inlet;

a pressure sensor (25) disposed at the FGR system inlet and another pressure sensor (27) disposed at the FGR mixing chamber (28) inlet for monitoring pressure variation in the
FGR system;
a thermocouple (9) and a pressure sensor mounted on secondary air (SA) system
duct (7) disposed for monitoring temperature and pressure in SA system;
a windbox (16);
a secondary air system (SA) duct (7) joined with FGR duct (15) through mixing
chamber (12);
an automated dumper (26) mounted on FGR duct (15) for adjusting the amount of
dust free flue gas to be allowed into SA system for the recirculation; wherein
a FGR header (28) assembled with FGR mixing chamber (12) and is disposed for allowing the dust free flue gas to accumulate, when baffles (29) are arranged in the FGR header (28) for making uniform distribution of the received flue gas in the FGR chamber (12) wherein the accumulated flue gas passes through three distributed pipes with holes (30) inside the FGR chamber (12) to help in uniform mixing of flue gas with secondary air and for releasing into windbox (16) through FGR chamber outlet Duct (31) and allows the mixed flue gas to enter the combustion chamber that results in maintaining the targeted temperature for any type of coal fired, when Flue gas recirculation being an additional control parameter provides flexibility to control steam temperature with wide range of coals as fuels in the boiler.

2. A system as claimed in claim 1, wherein the secondary air is generated by force-draft (FD) fan (6).
3. A method for achieving coal flexibility employing flue gas recirculation in coal fired
thermal power plants comprising;
allowing flue gas to pass through an electrostatic precipitator (ESP) (21) to remove the suspended particulates in the flue gas and releasing to the environment through ESP
outlet duct (22) and chimney (24);
feeding back a portion of the dust free flue gas in the Flue gas recirculation (FGR)
system;
generating secondary air by a Force-draft (FD) fan (6) and allowing the said air to
pass through secondary air duct (7) entering into a windbox (16);
adjusting the amount of flue gas to be allowed in SA system for recirculation by an
automated damper;
driving the dust free flue gas into a mixing chamber (12) by a FGR Fan (20) attached
to FGR duct (15);
monitoring the temperature of flue gas by thermocouples (10,19); monitoring the pressure variation in FGR system by pressure sensors (25, 27); accumulating the flue gas and allowing the said flue gas to pass through three distributed pipes with holes (30) disposed inside the FGR chamber (12) to have uniform

mixing of flue gas with secondary air and the said mix releasing into windbox (16) through FGR chamber outlet duct (31) when uniform distribution is made in the FGR chamber (12) by arranged baffles in a FGR header (28) fixed to the said chamber (12); wherein the flue gas recirculation mixing with secondary air is allowed to enter into combustion chamber (18) through windbox (16) resulting in maintaining the targeted temperature for any type of
coal fired.
4 A method as claimed in Claim 1, wherein the controlled recirculated flue gas acts as
temperature at their set points.