FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13)
HEAT RECOVERY SYSTEM FOR FLUE GAS CONTAINING SULFUR POLLUTANT AND METHOD THEREOF
THERMAX LIMITED
an Indian Company
of D-13, MIDC Industrial Area,
R.D. Aga Road, Chinchwad,
Pune - 411 019, Maharashtra, India.
The following specification particularly describes the invention and the manner in which it is to be
performed.

FIELD OF DISCLOSURE
The present disclosure relates to an improved system for flue gas heat recovery for a flue gas containing sulfur pollutant and method thereof.
BACKGROUND
Residual fuel oil contains approximately 2.5 - 4.5 % sulfur. This sulfur is converted to sulfur dioxide (SO2) through combustion. Depending on flame temperature and combustion environment, 1 - 5 % of the sulfur dioxide is converted to sulfur trioxide (SO3). The sulfur trioxide reacts with water vapor to form sulfuric acid. The dew point temperature of the sulfuric acid is in the range of 120 °C to 155 °C. Dew point temperature is a function of the partial pressure of sulfur trioxide and water vapor. Normally, flue gases leaving the boiler are available at considerably high temperature with a good potential for heat recovery. This heat can be used to preheat combustion air and/or feed water. As the combustion air and the feed water are present at low temperature, they can result in lower tube metal temperature. If the tube metal temperature is lower than the dew point temperature of the sulfuric acid, the sulfuric acid condenses over the tube metal wall resulting in corrosion. This type of corrosion is called low end corrosion or dew point corrosion.
A conventional flue gas heat recovery system having a shell and tube type configuration is illustrated in the FIG. 1 of the accompanying drawings, in which flue gases are conveyed through metal tubes 110 via a tube-side inlet 102 and feed water to be heated is conveyed through a shell-side inlet 104 of the heat exchanger 100. The heated feed water is discharged at a shell-side outlet 108 and the heat extracted flue gases are discharged through a tube-side outlet 106. As the water side heat transfer coefficient is very high, the metal tube temperature is very near to the water temperature. Typically, the water temperature is less than 110 °C. This can lead to the condensation of the sulfuric

acid present in the flue gases, resulting in corrosion of the metal tubes 110. As the temperature in the heat exchanger 100 is higher than 100 °C, the heat exchanger 100 is to be maintained under pressure and placed subsequent to a feed water pump. The pressurized shell and tube heat exchanger 100 requires significant attention for the safety due to very high water hold-up.
Another conventional flue gas heat recovery system having a cross flow heat exchanger 200 is illustrated in the FIG. 2 of the accompanying drawings. The heat exchanger 200 comprises a serpentine metal tube bank 210 through which feed water to be heated is conveyed for extracting heat from hot flue gases. The hot flue gases enter at a flue gas inlet 202 and traverse over the tube bank 210. The feed water to be heated enters at a feed water inlet 204. The heated feed water is discharged at a feed water outlet 208 and the heat extracted flue gases are discharged through a flue gas outlet 206. As the metal tube temperature is dependent on the feed water temperature, dew point corrosion due to the condensation of the sulfuric acid present in the flue gases cannot be eliminated. The heat exchanger 200 has less water hold-up and is relatively safer compared to the shell and tube heat exchanger 100. However, due to the serpentine tubes 210, the heat exchanger 200 is comparatively more expensive.
Yet another conventional flue gas heat recovery system, using a thermosyphon heat exchanger 300 with water as a heat transfer medium, is illustrated in the FIG. 3 of the accompanying drawings. The thermosyphon heat exchanger 300 comprises two sections; viz., a primary heat exchanger and a secondary heat exchanger. In the primary heat exchanger, heat from the flue gases is extracted by water. The primary heat exchanger comprises a serpentine metal tube bank 310 through which water to be heated is conveyed for extracting heat from hot flue gases. The hot flue gases enter at a flue gas inlet 302 and traverse over the tube bank 310. The heat extracted flue gases are discharged through a flue gas

outlet 306. The water and vapor mixture from the primary heat exchanger is conveyed to the secondary heat exchanger 312, where steam is condensed by transferring heat to cold feed water entering at feed water inlet 304. The heated feed water is discharged at feed water outlet 308 and the cooled condensed water is returned to the primary heat exchanger. The secondary heat exchanger 312 comprises a heat transfer coil to transfer the heat to the feed water. In the heat exchanger 300, water acts as a heat transfer medium and circulates between the two heat exchangers by natural circulation. Thus, the feed water is not in direct contact with the flue gases. Therefore, the tube temperature is controlled by the temperature of the circulating water, which is considerably higher than the feed water temperature. The temperature of the circulating water can be controlled by controlling pressure in the secondary heat exchanger 312 by regulating the heat transfer in the heat transfer coil.
Several attempts have been made in the past to solve the afore-mentioned drawbacks of the conventional flue gas heat recovery systems.
IN Patent No. 252946 by the applicant discloses a boiler economizer, wherein feed water is not directly heated by flue gases but circulating water is used as a secondary fluid to heat the feed water. The system comprises risers, downcomers, a primary heat exchanger coil, a secondary heat exchanger and a drum, forming a closed loop for natural circulation of the water. The primary heat exchanger coil is used to transfer heat from the flue gases to the secondary fluid and the boiler feed water is passed through said drum where it is heated in the secondary heat exchanger by heated secondary fluid. The hot feed water is then fed to the boiler. The economizer is provided with a control valve for controlling the water flow thereby controlling the metal temperature.

US Patent No. 5878675 discloses a flue gas desulftirizer having an absorption . tower for bringing untreated flue gas into gas-liquid contact with an absorbent slurry, wherein there is provided heat recovery means for recovering heat from the flue gas passing through the flue gas inlet section of the absorption tower prior to gas-liquid contact, and to boiler equipment including heat release means for releasing the recovered heat to heat utilization equipment. The disclosure relates to a thermal electric power generation equipment including extraction feed water heaters for heating boiler feed water with steam from steam turbines, a flue gas desulftirizer using an absorbent slurry, and means for recovering heat from the flue gas passing through the flue gas desulftirizer and/or the absorbent slurry within the flue gas desulftirizer, whereby boiler feed water is preheated by the recovered heat and then introduced into the extraction feed water heaters.
US Patent No. 4799461 discloses a waste heat recovery boiler comprising heat exchange rate switching means for controlling the rate of heat exchange between an exhaust gas and feed water in a heat exchanger by changing the state of the feed water in accordance with the concentration of sulfur oxides in the exhaust gas, thereby maintaining the temperature at which low temperature corrosion due to the exhaust gas is prevented in a downstream portion of the heat exchanger in the direction in which the exhaust gas flows. The phenomenon of steaming in the heat exchanger is eliminated irrespective of whether the kind of exhaust gas is a dirty gas or a clean gas.
The drawback of the prior art document EST Patent No. 252946 is that the heat recovery system is complex and requires an external circulation loop consisting of risers, downcomers, and drum. The drawback of the prior art document US Patent No. 5878675 is that the system uses an expensive and complex flue gas desulfurization technique. The other prior art document, US Patent No.

4799461, has a complex control and is difficult to operate. The known systems therefore are expensive, difficult to manufacture, and difficult to operate.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
It is an object of the present disclosure to overcome the above listed drawbacks of the known flue gas heat recovery systems. Accordingly, an object of the present disclosure is to provide a simple and compact flue gas heat recovery system, particularly suitable for extracting heat from flue gases containing sulfur pollutant, wherein the flue gas heat recovery system is adapted to prevent the dew point corrosion.
It is another object of the present disclosure to provide a flue gas heat recovery system which is easy-to-construct, easy-to-maintain and economical.
These objects and other advantages of the present disclosure will be more
apparent from the following description.
SUMMARY
In accordance with the present disclosure, there is provided a flue gas heat
recovery system for extracting heat from flue gases containing sulfur pollutant,
said flue gas heat recovery system comprising:
■ a shell for receiving a first heat exchange fluid, said shell having disposed therein a plurality of heat transfer tubes for circulating hot flue gas;
■ a heat transfer coil for circulating a second heat exchange fluid, said heat transfer coil disposed in said shell in heat exchange relation with the first heat exchange fluid; and

■ inlet means and outlet means for the flue gas and the second heat
exchange fluid;
■ wherein, the first heat exchange fluid temperature is a function of said
shell pressure and said shell pressure is a function of the heat transfer area of
said plurality of heat transfer tubes and said heat transfer coil, and, wherein, for
eliminating the dew point corrosion, said shell pressure is maintained such that
the first heat exchange fluid temperature is higher than the flue gas dew point
temperature.
Typically, in accordance with the present disclosure, a set of tube plates are provided for supporting said plurality of heat transfer tubes.
Preferably, in accordance with the present disclosure, the first heat exchange fluid is selected from water, organic fluid and mixture thereof, and the second heat exchange fluid is selected from boiler feed water, make-up water, combustion air, process fluid, and fluid required for process heating application.
Typically, in accordance with the present disclosure, said shell is only partially filled with the first heat exchange fluid to provide space for thermal expansion of the first heat exchange fluid.
Alternatively, in accordance with the present disclosure, a control loop comprising a bypass line having a control valve is provided for selectively controlling the flow rate of the second heat exchange fluid through said heat transfer coil to control at least one parameter selected from the temperature and the pressure of the first heat exchange fluid by controlling the heat transfer to the secondary heat exchange fluid.

In accordance with the present disclosure, there is provided a method for operating a flue gas heat recovery system for extracting heat from flue gases containing sulfur pollutant, said method comprising the following steps:
■ receiving hot flue gas through a plurality of heat transfer tubes disposed in a shell;
■ filling said shell with a first heat exchange fluid;
■ receiving a second heat exchange fluid through a heat transfer coil disposed within said shell in heat exchange relation with the first heat exchange fluid;
■ extracting heat from the hot flue gas in the first heat exchange fluid to obtain a heated first heat exchange fluid; and
■ extracting heat from the heated first heat exchange fluid in the second heat exchange fluid to obtain a heated second heat exchange fluid;
■ wherein, the first heat exchange fluid temperature is a function of said shell pressure and said shell pressure is a function of the heat transfer area of said plurality of heat transfer tubes and said heat transfer coil, and, wherein, for eliminating the dew point corrosion, said shell pressure is maintained such that the heated first heat exchange fluid temperature is higher than the dew point temperature of the flue gas.
Typically, in accordance with the present disclosure, the method comprises selecting the first heat exchange fluid from water, organic fluid and mixtures thereof, and the second heat exchange fluid from boiler feed water, make-up water, combustion air, process fluid, and fluid required for process heating application.
Preferably, in accordance with the present disclosure, the method comprises filling said shell only partially with the first heat exchange fluid to provide space for the thermal expansion of the first heat exchange fluid.

Alternatively, in accordance with the present disclosure, the method comprises selectively controlling the flow rate of the second heat exchange fluid through said heat transfer coil by means of a bypass line having a control valve to control at least one parameter selected from the temperature and the pressure of the first heat exchange fluid by controlling the heat transfer to the secondary heat exchange fluid.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The disclosure will now be described with the help of the accompanying drawings, in which,
FIGURES 1 illustrates a conventional flue gas heat recovery system having a shell and tube type heat exchanger;
FIGURE 2 illustrates another conventional flue gas heat recovery system having a cross flow heat exchanger;
FIGURE 3 illustrates yet another conventional flue gas heat recovery system having a thermosyphon heat exchanger;
FIGURE 4 illustrates a generalized scheme of the flue gas heat recovery system in accordance with the present disclosure;
FIGURE 5 illustrates a preferred embodiment of the flue gas heat recovery system in accordance with the present disclosure; and
FIGURE 6 illustrates another preferred embodiment of the flue gas heat recovery system in accordance with the present disclosure.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The disclosure will now be described with reference to the accompanying drawings which do not limit the scope and ambit of the disclosure. The description provided is purely by way of example and illustration.
The present disclosure envisages a flue gas heat recovery system, particularly suitable for extracting heat from flue gases containing sulfur pollutant. The flue gas heat recovery system uses a shell and tube type configuration comprising an exterior housing defining a shell filled with first heat exchange fluid, and comprising a plurality of heat transfer tubes for circulating hot flue gas, a heat transfer coil for receiving and circulating a second heat exchange fluid, and inlet means and outlet means for the flue gas and the second heat exchange fluid. The first heat exchange fluid temperature is a function of the shell pressure and the shell pressure is a function of the heat transfer area of the plurality of heat transfer tubes and the heat transfer coil, wherein, for eliminating the dew point corrosion, the shell pressure is maintained such that the first heat exchange fluid temperature is higher than the flue gas dew point temperature.
The system of the present disclosure is a shell and tube heat exchanger, in which hot flue gas are received through the plurality of heat transfer tubes, the first heat exchange fluid is filled in the lower part of the shell around the plurality of heat transfer tubes, leaving space for thermal expansion and steam, and the second heat exchange fluid is received through the heat transfer coil disposed in heat exchange relation with the first heat exchange fluid, such that the hot flue gas is not in direct contact with the second heat exchange fluid. Heat from the hot flue gas is extracted in the first heat exchange fluid to obtain a heated first heat exchange fluid. The heated first heat exchange fluid then heats the second heat exchange fluid. The first heat exchange fluid is typically water and the second heat exchange fluid is typically boiler feed water or combustion air. By

the method of the present disclosure, direct contact between the flue gas and the boiler feed water is avoided.
As the first heat exchange fluid is at lower temperature and the heat transfer from the hot flue gas is higher than the heat transfer to the second heat exchange fluid, the temperature of the first heat exchange fluid keeps on increasing until the temperature exceeds 100 °C and steam generation occurs causing the first heat exchange fluid to be pressurized. The temperature of the first heat exchange fluid is the function of the shell pressure. A steady state condition is attained, when the heat transfer from the hot flue gas is equal to the heat transfer to the second heat exchange fluid. In the steady state condition, the heat transfer tube wall temperature is dependent on the temperature of the first heat exchange fluid, which in turn is the function of shell pressure. Typically, the first heat exchanger fluid will be at a considerably higher temperature than the second heat exchange fluid temperature. This helps to eliminate the sulfur dew point corrosion problem. The steady state pressure is the function of the area of the flue gas heat transfer tubes and the heat transfer coil. If the flue gas heat transfer tube area is less than required, steady state is attained at a lower pressure. If the area of the heat transfer coil is less than required, steady state is attained at a higher pressure. Therefore, the shell pressure can be selectively controlled by careful selection and distribution of the heat transfer areas of the plurality of heat transfer tubes and the heat transfer coil.
There is still a possibility that the pressure is less than desired, which can lead to dew point corrosion. To avoid such a situation, a control loop is provided which is adapted to control the shell pressure. The control loop has a bypass line which is provided operatively parallel to the heat transfer coil. The bypass line comprises a control valve to regulate the flow rate to the heat transfer coil. If the shell pressure is less than desired, a portion of the second heat exchange fluid is

bypassed. This leads to the reduction in the heat transfer in the heat transfer coil, which in turn leads to increase in the shell pressure and the first heat exchange fluid temperature.
A similar arrangement of the flue gas heat recovery system can be used for preheating combustion air or a process fluid. In this arrangement, the hot flue gas is passed through the plurality of heat transfer tubes surrounded by the first heat exchange fluid of the shell. The combustion air/process fluid is passed through another set of heat transfer tubes arranged in heat exchange fashion with the first heat exchange fluid. The first heat exchange fluid acts as a heating medium for transferring heat from the hot flue gas to the combustion air/process fluid. A control loop, adapted to regulate the shell pressure, can be provided for this air/fluid heating system. In this control loop, the combustion air/process fluid is controlled by using by pass regulator to raise the shell pressure to the desirable level for the elimination of corrosion problem. The flue gas heat recovery system of the present disclosure is most suitable for small boilers using residual fuel oil.
FIGURE 4 illustrates a generalized scheme for the heat recovery system for recovering heat from flue gas containing sulfur pollutant in accordance with the present disclosure. The flue gas heat recovery system 100 comprises an exterior housing defining a shell 116. The shell 116 is partially filled with a first heat exchange fluid, leaving space for thermal expansion of the first heat exchange fluid. The first heat exchange fluid is typically selected from water, organic fluid or mixture of organic fluids thereof. The shell 116 comprises a plurality of heat transfer tubes 118 adapted to receive hot flue gas and extract heat therefrom into the first heat exchange fluid. The plurality of heat transfer tubes 118 can be referred as a primary heat exchanger for transferring heat from the hot flue gas to the first heat exchange fluid. The hot flue gas enters the primary

heat exchanger 118 through the flue gas inlet 104. The hot flue gas rejects the heat to the first heat exchange fluid and leaves the primary heat exchanger 118 through the flue gas outlet 108. The shell 116 further comprises a heat transfer coil 112 to transfer heat from the first heat exchange fluid to a second heat exchange fluid. The heat transfer coil 112 can be referred as secondary heat exchanger. The second heat exchange fluid enters the secondary heat exchanger through the inlet 106 and exits from 110 after receiving the heat from the first heat exchange fluid. Both primary and secondary heat exchangers are placed inside the shell 116 and are in heat exchange relation with the first heat exchange fluid. Initially the first heat exchange fluid is at lower temperature so the heat transfer from the flue gas is higher than the heat transfer to the second heat exchange fluid. Due to this reason, the temperature of the first heat exchange fluid starts increasing. If the temperature reaches beyond the boiling point of the first heat exchange fluid, a portion of the first heat exchange fluid is converted to steam by pressurizing the shell. Thus, the temperature of the first heat exchange fluid is the function of the shell pressure. The pressurization can be avoided by selecting organic fluid or mixture of water and organic fluid, as their boiling temperature is higher. The temperature of the first heat exchange fluid is also the function of heat transfer area of the primary and secondary heat exchanger. The temperature of the first heat exchange fluid can be increased by increasing primary heat exchanger area and reducing secondary heat exchanger area. With optimal selection of primary and secondary heat exchanger surface area, the temperature of first heat exchange fluid can be maintained beyond the dew point temperature of the flue gas. A bypass control valve 130 is provided to regulate first heat exchange fluid temperature in off-design condition. If the first heat exchange fluid temperature decreases due to lower load or lower temperature of second heat exchange fluid, a portion of second heat exchange fluid is bypassed. This reduces heat transfer from the first heat exchange fluid to

the second heat exchange fluid and results in increase in the first heat exchange fluid temperature. This helps to eliminate corrosion in off-design condition.
Referring to FIG. 5 of the accompanying drawings, therein is illustrated a preferred embodiment of the flue gas heat recovery system, referenced by the numeral 200, showing a preferred design of a heat transfer coil 212 of a secondary heat exchanger inserted in a conduit 220. The flue gas heat recovery system 200 comprises an exterior housing defining a shell 216 with an insulation 214 placed around the shell 216. The shell 216 comprises a plurality of heat transfer tubes 218 supported by means of a set of tube plates 222; wherein the plurality of heat transfer tubes 218 represent a primary heat exchanger. The conduit 220 in a circular flue gas duct 224 is used to facilitate the insertion of the heat exchanger coil 212 in the shell 216 and to isolate the heat transfer coil 212 from the direct contact with the flue gas. The conduit 220 comprising the heat transfer coil 212, which is a set of linearly arranged heat transfer tubes, are provided in heat exchange relation with the first heat exchange fluid filled in the shell 216. Hot flue gas enters the system 200 at inlet means 204, where the inlet means 204 are provided in operative communication with the plurality of the heat transfer tubes 218 to provide the hot flue gases. A first heat exchange fluid is provided in the shell 216 in heat exchange relation with the hot flue gas to extract heat from the hot flue gas. The first heat exchange fluid, which is typically water, organic fluid or mixtures thereof, gets heated and forms a steam mixture. A second heat exchange fluid, which is typically boiler feed water or combustion air, is received in the heat transfer coil 212 via the inlet means 206. The second heat exchange fluid extracts heat from the heated first heat exchange fluid to obtain a heated second heat exchange fluid, which is discharged at the outlet means 210. The second heat exchange fluid does not come in direct contact with the hot flue gases conveyed through the heat transfer tubes. The heat extracted flue gases are discharged through

outlet means 208. The flue gas inlet means 204 and the flue gas outlet means 208 are provided near the operative top of the tube plate 222 in a circular flue gas duct 224 of the system 200. The circular flue gas duct 224 is divided in two sections by a vertical plate to isolate entry and exit side of the flue gas. The second heat exchange fluid inlet means 206 and outlet means 210 are provided at the operative top of the circular flue gas duct the exterior shell 224 of the system 200.
Referring to FIG. 6 of the accompanying drawings, therein is illustrated another preferred embodiment of the flue gas heat recovery system, referenced by the numeral 300, showing another preferred design of a heat transfer coil 312 of a secondary heat exchanger. The flue gas heat recovery system 300 comprises an exterior housing defining a shell 316 with an insulation 314 placed around the shell 316. The shell 316 comprises a plurality of heat transfer tubes 318 supported by means of a set of tube plates 322. The heat transfer coil 312 is provided in heat exchange relation with the first heat exchanged fluid stored in the shell 316. Hot flue gas enters the system 300 at inlet means 304, where the inlet means 304 are provided in operative communication with the plurality of the heat transfer tubes 318 to provide the hot flue gases. A first heat exchange fluid is provided in the shell 316 in heat exchange relation with the hot flue gas to extract heat from the hot flue gas. The first heat exchange fluid, which is typically water, organic fluid or mixtures thereof, gets heated and forms a steam mixture. A second heat exchange fluid, which is typically boiler feed water or combustion air, is received in the heat transfer coil 312 via the inlet means 306. The second heat exchange fluid extracts heat from the heated first heat exchange fluid to obtain a heated second heat exchange fluid, which is discharged at the outlet means 310. The second heat exchange fluid does not come in direct contact with the hot flue gases conveyed through the heat transfer tubes. The heat extracted flue gases are discharged through outlet means 308.

The flue gas inlet means 304 is provided near the operative top of the shell 316 of the system 300 and the flue gas outlet means 308 is provided at the operative bottom of the shell 316 of the system 300. The second heat exchange fluid inlet means 306 and outlet means 310 are provided near the operative bottom of the shell 316 of the system 300.
TECHNICAL ADVANTAGES
A flue gas heat recovery system, particularly suitable for extracting heat from flue gases containing sulfur pollutant, as described in the present disclosure has several technical advantages including but not limited to the realization of: the system is adapted to prevent the dew point corrosion, and is simple, compact, easy-to-construct, easy-to-maintain, economical.
Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression "at least" or "at least one" suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the invention as it existed anywhere before the priority date of this application.

The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.
In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principle of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

WE CLAIM
1. A flue gas heat recovery system (100) for extracting heat from flue gases containing sulfur pollutant, said flue gas heat recovery system comprising:
■ a shell (116) for receiving a first heat exchange fluid, said shell (116) having disposed therein a plurality of heat transfer tubes (118) for circulating hot flue gas;
■ a heat transfer coil (112) for circulating a second heat exchange fluid, said heat transfer coil (112) disposed in said shell (116) in heat exchange relation with the first heat exchange fluid; and
■ inlet means and outlet means for the flue gas and the second heat exchange fluid;
■ wherein, the first heat exchange fluid temperature is a function of said shell (116) pressure and said shell (116) pressure is a function of the heat transfer area of said plurality of heat transfer tubes (118) and said heat transfer coil (112), and, wherein, for eliminating the dew point corrosion, said shell (116) pressure is maintained such that the first heat exchange fluid temperature is higher than the flue gas dew point temperature.
2. The flue gas heat recovery system as claimed in claim 1, wherein a set of tube plates are provided for supporting said plurality of heat transfer tubes (118).

3. The flue gas heat recovery system as claimed in claim 1, wherein the first heat exchange fluid is selected from water, organic fluid and mixture thereof.
4. The flue gas heat recovery system as claimed in claim 1, wherein the second heat exchange fluid is selected from boiler feed water, make-up water, combustion air, process fluid, and fluid required for process heating application.
5. The flue gas heat recovery system as claimed in claim 1, wherein said shell (116) is only partially filled with the first heat exchange fluid to provide space for fluid expansion.
6. The flue gas heat recovery system as claimed in claim 1, wherein a control loop comprising a bypass line having a control valve (130) is provided for selectively controlling the flow rate of the second heat exchange fluid through said heat transfer coil (112) to control at least one parameter selected from the temperature and the pressure of the first heat exchange fluid by controlling the heat transfer to the secondary heat exchange fluid.
7. A method for operating a flue gas heat recovery system for extracting heat from flue gases containing sulfur pollutant, said method comprising the following steps:

■ receiving hot flue gas through a plurality of heat transfer tubes disposed in a shell;
■ filling said shell with a first heat exchange fluid;

■ receiving a second heat exchange fluid through a heat transfer coil disposed within said shell in heat exchange relation with the first heat exchange fluid;
■ extracting heat from the hot flue gas in the first heat exchange fluid to obtain a heated first heat exchange fluid; and
■ extracting heat from the heated first heat exchange fluid in the second heat exchange fluid to obtain a heated second heat exchange fluid;
■ wherein, the first heat exchange fluid temperature is a function of said shell pressure and said shell pressure is a function of the heat transfer area of said plurality of heat transfer tubes and said heat transfer coil, and, wherein, for eliminating the dew point corrosion, said shell pressure is maintained such that the heated first heat exchange fluid temperature is higher than the dew point temperature of the flue gas.

8. The method as claimed in claim 7, in which the first heat exchange fluid is selected from water, organic fluid and mixtures thereof.
9. The method as claimed in claim 7, in which the second heat exchange fluid is selected from boiler feed water, make-up water, combustion air, process fluid, and fluid required for process heating application.
10. The method as claimed in claim 7, in which said shell is only
partially filled with the first heat exchange fluid to provide space for the
thermal expansion of the first heat exchange fluid.

11. The method as claimed as claim 7, in which flow rate of the second heat exchange fluid through said heat transfer coil is selectively controlled by means of a bypass line having a control valve to control at least one parameter selected from the temperature and the pressure of the first heat exchange fluid by controlling the heat transfer to the secondary heat exchange fluid.