CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This Application is related to Application No. 12/238,612, entitled "OXY/FUEL COMBUSTION SYSTEM WITH LITTLE OR NO EXCESS OXYGEN", Attorney Docket No. 07228 USA, filed contemporaneously with this Application on September 26, 2008, assigned to the assignee of the present disclosure and which is herein incorporated by reference in its entirety, Application No. 12/238,632, entitled "COMBUSTION SYSTEM WITH STEAM OR WATER INJECTION", Attorney Docket No. 07238 USA, filed contemporaneously with this Application on September 26, 2008, assigned to the assignee of the present disclosure and which is herein incorporated by reference in its entirety, Application No. 12/238,644, entitled "COMBUSTION SYSTEM WITH PRECOMBUSTOR", Attorney Docket No. 07255 USA, filed contemporaneously with this Application on September 26, 2008, assigned to the assignee of the present disclosure and which is herein incorporated by reference in its entirety, Application No. 12/238,657, entitled "OXY/FUEL COMBUSTION SYSTEM WITH MINIMIZED FLUE GAS RECIRCULATION", Attorney Docket No. 07257 USA, filed contemporaneously with this Application on September 26, 2008, assigned to the assignee of the present disclosure and which is herein incorporated by reference in its entirety, Application No. 12/238,671 , entitled "CONVECTIVE SECTION COMBUSTION", Attorney Docket No. 07254 USA, filed contemporaneously with this Application on September 26, 2008, assigned to the assignee of the present disclosure and which is herein incorporated by reference in its entirety, Application No. 12/238,731 , entitled "PROCESS TEMPERATURE CONTROL IN OXY/FUEL COMBUSTION SYSTEM", Attorney Docket No. 07239 USA, filed contemporaneously with this Application on September 26, 2008, assigned to the assignee of the present disclosure and which is herein incorporated by reference in its entirety, and Application No. 61/100,372, entitled "COMBUSTION SYSTEM WITH PRECOMBUSTOR", Attorney Docket No. 07262Z USA, filed contemporaneously with this Application on September 26, 2008, assigned to the assignee of the present disclosure and which is herein incorporated by reference in its entirety.

FIELD OF THE DISCLOSURE

[0002] The present disclosure is directed to a combustion system. In particular, the present disclosure is directed to a combustion system combining the convective section and the radiant section.

BACKGROUND OF THE DISCLOSURE

[0003] Water tubes are effective at accepting a large heat flux without damaging tubes fabricated from metal. Superheat steam tubes are important in efficient boiler operation but are not able to accept heat fluxes as high as water tubes due to decreased thermal heat capacity per unit volume. Known systems do not provide protection for superheat tubes from the high heat fluxes prevailing in the radiant section. Unprotected steam tubes exposed to high heat fluxes are subject to damage and/or failure. Therefore, known air/fuel boilers include a separate convection section to superheat steam where the heat flux is lower but the total flue gas enthalpy is still relatively high.

[0004] In oxy/fuel firing conditions, a larger percentage of the heat release occurs in the radiant section than for air/fuel combustion when the temperature of the gas exiting the boiler is held constant. To overcome this, known systems include flue gas recycle (FGR) to reduce flame temperatures by absorbing some of the heat in the radiant section thereby reducing the temperature at which the furnace gas exits and subsequently allow it to be absorbed in the convection section. Using FGR to transfer heat increases the complexity of the flue gas handling system, the size of the convection section and boiler and therefore increases the overall capital and operating cost of the system.

[0005] The heat release and flame temperature from coal combustion in oxygen (referred to as "oxy/coal combustion") is different than coal combustion in air (referred to as "air/coal combustion"). In air coal combustion the flame temperature is lower and the flue gas volume is higher than in oxy/coal combustion. The higher flame temperature and lower flue gas volume in oxy/coal combustion results in a decrease in the heat extraction available for the convection section and an increase in heat extraction available in the radiant section when compared to air/coal combustion. Another challenge for oxy/coal combustion is the furnace exit gas temperature. Desirable furnace exit gas temperatures are in the range of 1200-1400 ⁰ C (about 2200-2550 ⁰ F), primarily based on convective pass tube fouling considerations. Therefore it is necessary to remove sufficient heat in the furnace section so that the furnace exit gas temperature is reduced to acceptable limits.

[0006] In an oxy/coal combustion arrangement utilizing a convective heat exchange section, less heat is available for superheating the steam, due, in part, to a constraint on the temperature of the gases exiting the radiant section of the boiler, causing an imbalance in the heat available for transfer to feedwater, steam generation, and superheating. In an effort to reduce the temperature of the furnace gases prior to exiting the radiant section, large amounts of flue gas recycle (FGR) in the form of synthetic air (21-35%θ ₂ in CO ₂ ) have been used to approximate the combustion of coal in oxygen to be similar to air/coal combustion. Both in terms of furnace heat release, furnace exit gas temperature, and flame temperature. Recycling the flue gas adds overall capital and operating cost of the overall process.

[0007] Therefore, there is an unmet need to provide a heat exchange design that reduces or eliminated flue gas recycle to achieve the correct heat distribution to superheat and reheat steam, achieves lower overall cost, permits superheating of at least a portion of the steam in the radiant section of an oxy/coal boiler, and/or adequately protects the superheat tubes from the high heat fluxes prevailing in the radiant section.

SUMMARY OF THE DISCLOSURE

[0008] This disclosure provides a boiler design that does not require flue gas recycle to achieve the correct heat distribution to superheat and reheat steam, achieves lower overall cost, permits superheating of at least a portion of the steam in the radiant section of an oxy/solid fuel boiler, and/or adequately protects the superheat tubes from the high heat fluxes prevailing in the radiant section.

[0009] An embodiment of the disclosure includes a heat transfer system having a radiant source, a first heat exchanger configured to permit a first fluid to flow therethrough, and a thermal shield configured to provide controlled radiative heat from the radiant source to the first exchanger. The radiant source is a flame and the thermal shield is a second heat exchanger configured to permit a second fluid to flow therethrough.

[0010] Another embodiment of the disclosure includes an oxy/fuel combustion system. The system includes a furnace arranged and disposed to provide a flame radiant source. A first exchanger is disposed in the furnace and is arranged and disposed to exchange radiant heat from the radiant source and steam for use in a steam turbine. The system further includes a thermal shield configured to provide controlled radiative heat exposure from the radiant source to the first exchanger. The thermal shield is a second heat exchanger configured to permit a second fluid to flow therethrough.

[0011] Another embodiment of the disclosure includes an oxy/fuel combustion system. The system includes a furnace arranged and disposed to provide a flame radiant source a furnace having a chamber arranged and disposed to provide a flame radiant source and to circulate combustion fluid. A first heat exchanger configured to permit a first fluid to flow therethrough. The system further includes a non-contact thermal shield fabricated from a material arranged to provide controlled radiative heat exposure from the radiant source to the first exchanger. The radiant source is a flame and the first heat exchanger and the thermal shield are disposed within the chamber and in contact with the combustion fluid.

[0012] An advantage of the present disclosure is not requiring flue gas recycle to achieve the correct heat distribution to superheat and reheat steam. The reduction or elimination of recycle permits reduction in the size or elimination of the convection section and boiler and therefore decreases the overall capital and operating cost of the system.

[0013] Another advantage of the present disclosure is lower overall cost.

[0014] Another advantage of the present disclosure is the ability to superheat at least a portion of the steam in the radiant section of an oxy/solid fuel boiler.

[0015] Yet another advantage of the present disclosure is providing increased protection of the superheat tubes from the high heat fluxes prevailing in the radiant section.

[0016] Yet another advantage of the present disclosure is reduced flue gas recycle without increasing temperature at which gases exit the radiant section thereby reducing the propensity for convective pass fouling as well as maintaining the heat and mass balance for a given turbine cycle.

[0017] Further aspects of the method and system are disclosed herein. The features as discussed above, as well as other features and advantages of the present disclosure will be appreciated and understood by those skilled in the art from the following detailed description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 illustrates a perspective view of an exemplary embodiment of a combustion system.

[0019] FIG. 2 illustrates a sectioned view of an exemplary embodiment of a heat transfer system.

[0020] FIG. 3 illustrates the average tubewall temperatures for both the water and steam tubes as a function of Dχ/Dτ _U be at Dγ/Dτ _U be = 1 -5.

[0021] FIG. 4 illustrates calculated heat fluxes to water and steam as a function of Dχ/Dτ _U be at Dγ/Dτ _U be = 1 -5.

[0022] FIG. 5 illustrates the effect of changing tube row spacing (DY).

[0023] FIG. 6 illustrates a sectioned view of a heat exchanger system according to an embodiment.

[0024] FIG. 7 illustrates a sectioned view of a heat exchanger system according to an embodiment.

[0025] FIG. 8 illustrates a sectioned view of a heat exchanger system according to an embodiment.

[0026] FIG. 9 illustrates a sectioned view of a heat exchanger system according to an embodiment.

[0027] FIG. 10 illustrates a sectioned view of a heat exchanger system according to an embodiment.

[0028] FIG. 11 illustrates a sectioned view of a heat exchanger system according to an embodiment. [0029] FIG. 12 illustrates a sectioned view of a heat exchanger system according to another embodiment.

[0030] FIG. 13 illustrates a sectioned view of a heat exchanger system according to another embodiment.

[0031] FIG. 14 illustrates a sectioned view of a heat exchanger system according to another embodiment.

[0032] FIG. 15 illustrates a temperature plot for an exemplary heat exchanger system arrangement according to an embodiment.

[0033] Wherever possible, the same reference numbers will be used throughout the drawings to represent the same parts.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0034] The present disclosure now will be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the disclosure is shown. This disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the disclosure to those skilled in the art.

[0035] As used herein, the term "solid fuel" and grammatical variations thereof refers to any solid fuel suitable for combustion purposes. For example, the disclosure may be used with many types of carbon-containing solid fuels, including but not limited to: anthracite, bituminous, sub-bituminous, and lignite coals; tar; bitumen; petroleum coke; paper mill sludge solids and sewage sludge solids; wood; peat; grass; and combinations and mixtures of all of those fuels. As used herein, the term "oxygen" and grammatical variations thereof refers to an oxidizer having an O ₂ concentration greater than that of atmospheric or ambient conditions. As used herein, the term "oxy/coal combustion" and grammatical variations thereof refers to coal combustion in oxygen, the term "air/coal combustion" and grammatical variations thereof refers to coal combustion in air, the term "oxy/fuel combustion" and grammatical variations thereof refers to fuel combustion in oxygen _, and the term "air/fuel combustion" and grammatical variations thereof refers to fuel combustion in air. As used herein, the term "combustion fluid" and grammatical variations thereof refers to a fluid formed from and/or mixed with the products of combustion, which may be utilized for convective heat transfer. The term is not limited to the products of combustion and may include fluids mixed with or otherwise traveling through at least a portion of combustion system. Although not so limited, one such example is flue gas. As used herein, the term "recycled flue gas" and grammatical variations thereof refers to combustion fluid exiting the system that is recirculated to any portion of the system. As used herein, the term "flue gas recycle" and grammatical variations thereof refers to a configuration permitting the combustion fluid to be recirculated.

[0036] Referring to Figure 1 , an embodiment of the present disclosure includes a combustion system 102 with a furnace 104, a convective pass 106, a radiant source 108, a thermal shield 110, and a first heat exchanger 112. As illustrated in Figure 1 , the furnace 104 is depicted as a large enclosed space configured for fuel combustion and cooling of flue gas before the flue gas enters the convective pass 106. The convective pass 106 generally includes one or more of a superheater, a reheater, and an economizer. The superheater, the reheater, and the economizer surfaces are generally located in the path of the flue gas horizontal and vertical downflow sections of the boiler enclosure. The radiant source 108 includes a solid fuel flame. In one embodiment the radiant source 108 is oxy/coal flame.

[0037] As illustrated in Figure 1 , the first heat exchanger 1 12 is configured to permit a first fluid 114 to flow through it. As used herein, the term fluid includes, but is not limited to, movable solids, molten solids, gases, liquids, insoluble process components, colloids, and combinations thereof. A thermal shield 1 10 is arranged and disposed to partially insulate the first heat exchanger 112 from the radiant source 108. The thermal shield 110 and the first heat exchanger 112 are substantially within the furnace 104 of the combustion system 102. First heat exchanger 112 may, for example, include heat exchanger tubes or other structures integrated into the walls of the furnace 104 or otherwise disposed to receive radiant heat generated within the furnace 104. As depicted in Figure 1 , the thermal shield 1 10 is a heat exchanger. In addition, the position of the thermal shield 1 10 is such that the first heat exchanger 112 is controllably shielded from radiative heat exposure from the radiant source 108. "Radiative heat exposure", as utilized herein, includes a susceptibility or ability of a component to exchange heat from a radiation heat source. In other embodiments, a plurality of thermal shields 110 and/or a plurality of thermally shielded heat exchangers may be included. As described in Figure 11 , in another embodiment, the thermal shield 110 may be a structure other than a heat exchanger. In another embodiment, the thermally shielded heat exchanger comprises a plurality of steam tubes. In yet another embodiment, the thermally shielded heat exchanger (i.e. first heat exchanger 1 12) includes fins for dissipating heat. In still yet another embodiment, the thermal shield 110 is a mesh structure.

[0038] As illustrated in Figure 1 , the thermal shield 1 10 may be configured to permit a second fluid 116 to flow through it. The first fluid 114 differs from the second fluid 116 in at least one physical property. The physical property that differs may include one or more properties selected from the group of properties consisting of heat capacity, density, viscosity, thermal conductivity, pressure, phase, phase fraction, velocity, mass, mass-flow, and combinations thereof.

[0039] In one embodiment, the first fluid 1 14 is steam and the second fluid 1 16 is water. The reduced internal heat transfer coefficient for steam tubes (i.e. first heat exchanger 112) as compared to water tubes (i.e., thermal shield 110) leads to higher tube temperatures for steam tubes for a given heat flux. Typically, in one example, high temperature piping is designed for operational limits of about 1242 ⁰ F (672 ⁰ C). Therefore placing steam tubes within a high heat flux location, such as an unprotected portion of the furnace 104, can lead to excessive metal temperatures and subsequent failure.

[0040] In order to overcome these limitations while optionally still providing heating duty to the superheat tubes in the convective pass 106, in one embodiment, the thermal shield 110 is a plurality of tubes specifically configured to transport water. In this embodiment, the first heat exchanger 1 12 is a plurality of tubes specifically configured to transport steam. In this embodiment, the water-cooled tubes may reduce the heat flux to the steam tubes thereby reducing the metal temperature of the first heat exchanger 112 to a desired temperature. In other words the thermal shield 110 controllably reduces the radiative heat exposure of the first heat exchanger. With this arrangement, a portion (if not all) of the required superheat duty can be absorbed in the furnace 104 thereby taking advantage of the heat release profile for oxy/coal combustion. Any remaining superheat duty that is required may be obtained in the convective pass 106 as is known with air/coal combustion boilers. In one embodiment, including the water tubes (i.e. the thermal shield 110) and the superheat tubes (i.e. first heat exchanger 1 12) in the furnace 104 decreases the desire for including flue gas recycle. In another embodiment, including the thermal shield 110 and the first heat exchanger 1 12 in the furnace 104 permits the furnace 104 to serve the role of the convective pass 106 in known air/coal combustion boilers in addition to the role of the furnace 104 in known air/coal combustion boilers.

[0041] Figure 2 illustrates an embodiment of a heat transfer system 204. In the embodiment illustrated in Figure 2, the thermal shield 110 includes a plurality of transport structures 202. In the illustrated embodiment, the transport structures 202 are depicted as tubes filled with fluid, such as water or steam. In one embodiment, the transport structures 202 may be steel tubes. However, the transport structures 202 may be comprised of any material capable of withstanding the temperatures in the furnace 104 of the combustion system 102. In addition, other embodiments may include transport structures 202 with alternate geometries, such as, but not limited to, oval, square, triangular, or rectangular cross-sectional geometries. In the embodiment illustrated in Figure 2, the first heat exchanger 112 additionally includes a plurality of transport structures 202. In Figure 2, the transport structures 202 of the thermal shield 110 and the transport structures 202 of the first heat exchanger 112 are substantially the same. In other embodiments, the transport structures 202 of the thermal shield 110 and the first heat exchanger 112 differ in geometry, structure, orientation, or any other physical properties. In yet other embodiments, the transport structures 202 within the thermal shield 110 may differ from each other. Likewise, the transport structures 202 in the first heat exchanger 1 12 may differ from each other.

[0042] Referring again to Figure 2, the arrangement of the thermal shield 110 and the first heat exchanger 1 12 is specifically configured to permit the thermal shield 1 10 to insulate, or protect from radiation heat exposure, the first heat exchanger 112. In Figure 2, the plurality of the transport structures 202 in the first heat exchanger 112 are equally separated by a distance D _x i. Dχi is a distance corresponding to a distance between the centerpoints of transport structures 202 of the first heat exchanger 1 12. The transport structures 202 have a diameter d _Tube i In the embodiment illustrated in Figure 2, the transport structures 202 of the thermal shield 110 are separated by a distance D _X2 . D _x2 is a distance corresponding to a distance between the centerpoints of transport structures 202 of the thermal shield 110. In this embodiment, the transport structures 202 of the thermal shield 110 are arranged parallel to the transport structures 202 of the first heat exchanger 112. In other embodiments, other arrangements may be included. In the embodiment illustrated in Figure 2, the transport structures 202 of the thermal shield 110 are positioned intermediate between the radiant source 108 and the transport structures 202 of the first heat exchanger 112. D _y represents a distance between the thermal shield 110 and the first heat exchanger 1 12. More specifically, the distance D _y is the distance between a plane 208 passing through the center points of the transport structures 202 of the thermal shield and a plane 210 passing through the center points of the transports structures 202 of the first heat exchanger.

[0043] To control the heat transfer of the thermal shield 110 and the first heat exchanger 112, D _x i, D _x2 , dj _U bei _> dτ _U be2, and D _y may be modified. As illustrated in Figures 3-5, modifying D _x i, D _x2 , and D _y may provide reduced exposure of radiant heat to the first heat exchanger and reducing or eliminating damage or failure of the transport structures 202 due to overheating and/or exceeding maximum temperatures. Further, the overall heat exchange profile for the system including, in certain embodiments, the heating of water and superheating of steam in the furnace 104, provides improved heat transfer efficiency and increased component life within the furnace 104. In the embodiments represented by Figure 3, D _x i and D _X2 are depicted as being equal. In other embodiments, D _x i and D _X2 are not equal.

[0044] As indicated in the embodiments represented by Figure 4, modifying D _x1 and D _x2 (i.e., D _x ) when D _x i and D _x2 are equal, permits control of the heat fluxes for the transport structures 202. As illustrated, the average heat flux to both the transport structure 202 of the thermal shield 110 and the transport structure 202 for the first heat exchanger 1 12 increases with increasing D _x (i.e., D _x i and D _x2 ). The average heat flux per unit tube surface area for the transport structures 202 increases with increased spacing due to less shielding from neighboring tubes and because the hotter steam tubes (i.e. first heat exchanger 112) and subsequent hotter surroundings provide additional heat flux to the side of the water tubes (i.e., thermal shield 110) away from the radiation source 108. With increased spacing, the heat flux to the steam tubes increases due to a decrease in the shielding from the water tubes. [0045] As indicated in the embodiments represented by Figure 5, modifying D _y results in a less pronounced effect over the range studied except for when D _x1 and D _x2 , with D _x i and D _x2 being equal, are greater than three times djubei and djube2. with djubei and dτ _U be2 being equal.

[0046] Figures 6 through 10 illustrate other embodiments including various configurations modifying D _y . The embodiment illustrated by Figure 6 includes the plurality of the transport structures 202 of the thermal shield 1 10 arranged with the plurality of the transport structures 202 of the first heat exchanger 112 attached by means of a plurality of webbed connections 602. The webbed connections 602 provide additional surface area and connectivity between transports structures 202. In this embodiment, D _y is approximately three times dτubei and dτ _U be2, with dτ _U bei and dγ _U be2 being equal.

[0047] The embodiment illustrated by Figure 7 includes the plurality of the transport structures 202 of the thermal shield 1 10 arranged with the plurality of the transport structures 202 of the first heat exchanger 1 12 such that each of the transport structures 202 of the first heat exchanger 1 12 abuts a portion of one or more of the transport structures 202 of the thermal shield 110. In this embodiment, D _y is approximately equal to % D _x i or % D _x2 , (i.e., % D _x ) with D _x i and D _X2 being equal. As illustrated in this embodiment, a small gap may exist between some of the transport structures 202 of the thermal shield 1 10 and the transport structures 202 of the first heat exchanger 112. The embodiment illustrated by Figure 8 includes the plurality of the transport structures 202 of the thermal shield 110 arranged with the plurality of the transport structures 202 of the first heat exchanger 1 12 parallel to the transport structures 202 of the thermal shield 110. In this embodiment, D _y is approximately 3 to 4 times dτubei and d _Tu be2, with d _Tu bei and d _Tu be2 being equal.

[0048] In the embodiment illustrated by Figure 9 includes the plurality of the transport structures 202 of the thermal shield 110 arranged with the plurality of the transport structures 202 of the first heat exchanger 112 parallel to the transport structures 202 of the thermal shield 1 10 and at an equal distance from the radiant source 108 and attached by a plurality of the webbed connections 602. The webbed connections 602 provide additional surface area and connectivity between transports structures 202 to increase heat transfer. In this embodiment, D _y is approximately zero. The embodiment illustrated by Figure 10 includes the plurality of the transport structures 202 of the thermal shield 1 10 arranged with the plurality of the transport structures 202 of the first heat exchanger 112 parallel to the transport structures 202 of the thermal shield 110. The configuration of Figure 10 includes webbed connection 602 between transport structures 202 of the first heat exchanger 112. The webbed connections 602 provide additional surface area and connectivity between transports structures 202 to increase heat transfer. In this embodiment, D _y is approximately 3 to 4 times each of dj _U bei and dτ _U be2. with d _Tu bei and dτ _ubΘ 2 being equal.

[0049] In addition, as depicted in the embodiment illustrated by Figures 11 and 12, refractory 1101 may be included as the thermal shield 110 between the transport structures 202 of the first heat exchanger 112 and the radiant source 108. In other words, the thermal shield is a non-contact thermal shield 1 10 fabricated from a material arranged to provide controlled radiative heat exposure from the radiant source 108 to the first exchanger. In one embodiment, the refractory 1 101 may be comprised of materials such as, for example, alumina, silica, magnesia, and/or lime. In the embodiment shown in FIG 13, the refractory 1101 is shown in addition to the transport structures 202 as the thermal shield 110. Refractory 1 101 provides additional reduction in heat flux to the transport structures of the first heat exchanger 1 12. In yet another embodiment shown in FIG. 14, the refractory 1101 may be integrated into the thermal shield 110.

[0050] As would be appreciated by one of ordinary skill in the art, the following embodiment is described in relation to a sub-critical steam cycle consisting of at least a steam drum upstream of steam drying and superheat tubes that are upstream of a steam turbine. In this embodiment, the first heat exchanger 1 12 is configured to transport a near saturation wet fluid or low quality steam (i.e. a water-steam mixture), as opposed to pure steam. Using wet fluid may be desired for operational reasons. A water-steam mixture may be obtained by using a near saturation wet fluid (water/steam) mixture obtained from the bottom of the steam drum (or similar area with like conditions) rather than utilizing a saturated steam stream from the steam portion of a drum for a sub-critical plant. In this embodiment, the energy transferred into the first heat exchanger 112 boils the slightly sub-cooled fluid into a higher enthalpy wet steam which may be directed into the upper low pressure region of the steam drum. The wet steam containing greater enthalpy than the surrounding drum environment may exit the tube bundle in a slightly superheated state due to the lower pressure. This direct mixing serves to potentially deliver a marginally higher quality steam from the drum into the super heater.

[0051] In this embodiment the low quality steam in first heat exchanger 112 has a much higher heat transfer capacity than pure steam and will serve to protect tubes 202 in 1 12 during both steady state and transient conditions. With increased steam demand from the steam drum during escalation, the steam exiting 112 will be at a slightly more superheated condition relative to the bulk conditions within the steam drum. The increased steam demand from the steam drum, thereby causing a decrease in steam drum pressure, will result in increased flow through transport structures 202 in first heat exchanger 112, thereby rapidly increasing the heat removal from transport structures 202 in first heat exchanger 112 in anticipation of an automatic response of the furnace burners to increase firing rate.

[0052] When steam demand from the steam drum decreases, the steam exiting 1 12 will be at a slightly less superheated condition relative to the bulk conditions within the steam drum. The decreased steam demand from the steam drum, thereby causing a increase in steam drum pressure, will result in decreased flow through transport structures 202 in first heat exchanger 1 12, thereby rapidly suppressing the slightly superheated steam into a slightly sub- cooled fluid in transport structure 202 in first heat exchanger 112 in anticipation of an automatic response of the furnace burners to decrease firing rate.

[0053] An alternative option would be to direct the discharge from first heat exchanger 112 into the area immediately prior to initial steam drying and also just prior to any installed superheaters as this provides an even larger natural thermal driving head for the fluid while having little to no effect on thermal efficiency.

[0054] If additional sub-cooling for the heat transfer fluid in first heat exchanger 112 is desired, provisions can be made for utilizing a small sub- cooled stream of feed water as an eductive fluid supplying a jet pump to draw water from the steam drum resulting in continuous or additional flow into transport structures 202 of first heat exchanger 1 12. This may be accomplished during all modes of operation utilizing several methods.

[0055] Thus, the disclosure allows for reduced flue gas recycle rates while maintaining heat transfer to the steam/heat transfer fluid system by extracting additional heat in the furnace. Although the disclosure focuses on a power boiler using water as the heat transfer fluid, other embodiments may be applied to other process heaters where two dissimilar fluids (with regards to heat transfer properties) are heated. In addition, even though the disclosure focuses on additional heat duty in the furnace being taken from the superheat duty, economizer duty may also be taken from the furnace in place of the superheat duty. In addition, a third row (or more) of tubes may be added in the furnace. EXAMPLES

[0056] An example of the current disclosure was studied using computational fluid dynamics (CFD) simulating temperature distribution and heat transfer. The simulation region consists of a 2D rectangular area. One wall boundary was assumed to be at 3140 ⁰ F (2000 K) to mimic hot furnace gases. This boundary temperature was chosen to ensure that the maximum heat flux to the water tube was less than the critical heat flux for water. A mixed boundary condition (convection plus radiation) was applied at the opposite boundary which was assumed to be made of a 1 ft thick refractory brick. A periodic boundary condition was applied at the adjacent sides of the simulation area (see e.g., Figure 15). The water tubes (i.e. the thermal shield 110) and steam tubes (i.e., the first heat exchanger 112) were 2 inches in outer diameter with a wall thickness of 0.375 inches (0.95 cm). The conductivity of the metal tubes varied with temperature and was assumed to be the same as stainless steel. Both the water and steam tube walls were assumed to have an emissivity of 0.7. The convection boundary condition was applied at the inner walls of the water and steam tubes. The water and steam temperatures were kept constant at approximately 620 ⁰ F and 800 ⁰ F (327 ⁰ C and 427 ⁰ C), respectively. The internal heat transfer coefficients for the water and steam tubes were 50,000 W/m ² -K and 5,000 W/m ² -K, respectively.

[0057] The CFD simulations were performed for combinations of Dχ/dτ _Ube (where Dx is equal to D _x1 or D _x2 , with D _x1 and D _x2 being equal) and D _Y /d _Tub e (where d _Tu be is equal to d _Tu bei or d _Tube 2, with d _Tu bei and d _Tu be2 being equal).

[0058] Referring to Figure 3, the results for maximum and average tubewall temperatures for both the water and steam tubes are shown as a function of Dχ/Dτ _U be at D _Y /D _Tu be = 1 -5. In these embodiments the steam tubewall temperatures are effectively cooled for D _x /Dτ _U be of 3 or lower, but the actual limit of D _x /D _Tu be would depend on the specific material of construction and designed operating limits. [0059] Referring again to Figure 4, which was also calculated at Dγ/Dτ _U be = 1.5, calculated heat fluxes to the water and steam tubes are plotted. The average heat flux to both the water and steam tubes increases with increasing Dχ/Dτ _ube - The average heat flux for the water tubes increased with increased tube spacing. With increased spacing the heat flux to the steam tubes and water tubes increased.

[0060] Referring again to Figure 5, the effect of changing tube row spacing (DY) was also analyzed. The analysis revealed that there are two preferred operating regions. The analysis indicated that the heat transfer of the steam tubes should be maximized at DY = 4 and Dχ/Dτ _U be is between 2.5 and 3.5. Calculations indicated that this set of conditions provided for a steam heat transfer duty between about 60% and 70% of the water heat transfer duty while maintaining steam tube temperatures below the assumed design limit. The calculations were made with a limit set by the assumed maximum steam tubewall temperature (approximately 1242 °F/672 °C). Again, the temperature limit is a function of the steam tube material. To increase the heat transfer to the steam tubes even further, higher temperature limit materials could be used. Due to a resulting relatively flat temperature response to changes in Dχ/Dτube _> this set of conditions should also decrease the likelihood that the tubes will fail due to off-spec design or operation. A large design margin between steam tube operating temperature and the maximum steam tube temperature could be achieved by operating at a lower heat flux condition using a Dχ/Dτ _ube of less than 2.5.

[0061] Table 1 details the analysis of a high volatile bituminous coal. Table 2 shows the absorbed heat duty for the furnace (104) and the convection pass (106). Table 1

[0062] Table 2 shows the absorbed heat duty for two main boiler zones for a nominal 600 MW supercritical typical pulverized coal boiler. By shifting some of the duty from the connective pass to the furnace according to the present disclosure, the heat and material balance may be estimated for a power boiler.

Table 2: Coal fired boiler zones*

*For nominal 600 MW supercritical boiler.

[0063] Table 3 shows the firing duty and relative heat duty for the furnace and convection pass for different flue gas recycle (FGR) amounts based on the oxygen concentration in the oxidant for the process configurations and coal combustion defined in Tables 1 and 2. Table 3: Normalized Heat and Material Balance Results*

*For nominal 600 MW supercritical boiler

[0064] As represented in Table 3, example 1 is an air-fired combustion boiler for power generation using 650 ⁰ F (343 ⁰ C) preheated air. Example 2 is an FGR simulation using pure oxygen premixed with recycled particulate-free flue gas preheated to about 650 ⁰ F (343 ⁰ C) to closely approximate air combustion heat distribution. Examples 3 through 6 refer to configurations that decrease the amount of flue gas recycled while maintaining the oxidant preheat temperature at about 650 ⁰ F (343 °C). For all examples, the total duty transferred to the steam system at the turbine conditions defined in example 1 was kept constant. The configuration and the duty of the furnace and convection pass were kept constant for examples 1 and 2 as documented in Table 2. For the remaining examples, a portion of the duty required for producing steam was shifted from the convection pass to the furnace. The duty absorbed in the furnace beyond 2150 MMBtu/h was used to provide heat to the steam that was previously transferred in the convection pass. The amount of heat duty transferred from the convection pass to the radiant section was adjusted such that the furnace exit gas temperature was consistent with example 1. The remaining duty required for superheating the steam was transferred in the convection pass. The temperatures in the convection pass were checked to ensure that the temperatures in the exchangers did not cross. The overall firing duty was allowed to vary to close the energy balance. [0065] For example 2, the total required firing duty was slightly lower than in example 1 because the duty required for preheating incoming coal and oxidant was only about 84% of that required for example 1. The lower duty for the FGR was due to the lower oxidant mass flow required for preheating in example 2. As the amount of flue gas recycle was decreased, the radiant duty increased and the total firing duty decreased. At essentially no FGR, the radiant duty had increased about 57% above the air firing case. This result is consistent with the CFD studies that showed that about 60 to 70% of the duty transferred to the high density water heat transfer tubes could be transferred to the lower density steam heat transfer tubes in the furnace.

[0066] While the disclosure has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.