Systems and methods for stack heat recovery

Abstract

A system and method are presented for recovering heat from flue gas produced by a recovery boiler in a pulp and paper mill, the recovery boiler having a flue gas stack. Flue gas is drawn from the flue gas stack and passed through first and second condensing heat exchangers before exiting through another stack. The first heat exchanger is used to heat boiler feed water and the second heat exchanger is used to generate hot process water for use in the pulp and paper mill. Steam previously used to heat the boiler feed water and generate hot process water can now be used to generate electricity that can be used to operate the pulp and paper mill or for transmission to the utility grid.

Description

[Technical Field]

[0001] This disclosure relates to the field of recovering heat from flue gases produced by recovery boilers used in paper and pulp mills. [Background technology]

[0002] In the production of chemical pulp, lignin and other organic non-cellulosic materials can be separated from the chemical pulp raw material by cooking using cooking chemicals. The cooking liquor, or spent liquor, used in chemical cooking is recovered. The spent liquor (also called "black liquor") that is mechanically separated from the chemical pulp has a high combustion value due to the carbonaceous and other organic combustible materials contained therein and separated from the chemical pulp. The spent liquor also contains inorganic chemicals that do not react in the chemical cooking. Several different methods have been developed to recover heat and chemicals from the spent liquor.

[0003] Black liquor, obtained during kraft pulp production, is burned in a recovery boiler. When the organic and carbonaceous materials contained in the black liquor are burned, the inorganic components in the black liquor are converted into chemicals that can be recycled and further utilized in the cooking process.

[0004] Hot flue gases, generated by the burning of black liquor, come into contact with various heat exchangers within the recovery boiler. The flue gases transfer heat to water, steam, or a mixture of water and steam flowing through the heat exchangers, which simultaneously cool the flue gases themselves. The flue gases typically contain a high ash content. The primary component of the ash is sodium sulfate, with the next largest component typically being sodium carbonate. The ash also contains other components. The ash entrained in the flue gas enters the furnace primarily in vaporized form and primarily begins to transform into fine dust or molten droplets in the boiler section downstream of the furnace. Salts contained in the ash melt can form sticky particles, even at relatively low temperatures. These sticky molten particles easily adhere to heat transfer surfaces and cause corrosion. The accumulation of sticky ash poses a risk of clogging the flue gas ducts and also causes corrosion and wear of the boiler's heating surfaces. Thus, a recovery boiler in a pulp and paper mill may produce hot flue gases that are released to the atmosphere through a flue gas stack, as is well known to those skilled in the art.

[0005] However, recovery boiler exhaust gas contains acids, particularly sulfuric acid vapor, which is known to cause acid corrosion. As a result, dry-type heat exchangers are typically used (where temperatures are above the sulfuric acid dew point), which is considered the conventional or standard type of heat exchanger when used in the pulp and paper industry. In addition, the exhaust gas particulates contain chlorides, which are known to cause stress cracking corrosion of ordinary stainless steels.

[0006] The typical material used for heat exchangers is carbon steel, because it is an inexpensive material with good heat exchange efficiency. Carbon steel can also be used in the construction of dry heat exchangers to avoid concerns about acid corrosion. In some cases, titanium can be used (as in the majority of Japanese mills), and the casing and ducts can be made from 316 stainless steel or carbon steel. However, titanium is very expensive, has low heat exchange efficiency, and is difficult to process. This is why, historically, pulp and paper manufacturers have used only dry heat exchangers, or no heat exchangers at all, to avoid corrosion problems caused by acid condensation.

[0007] Referring to Figure 1, a prior art recovery boiler system is shown. Exhaust gas from a recovery boiler 100 passes through an economizer 104 and then through a duct 106 into a precipitator unit 108. A fan unit 110 then draws the flue gas from the precipitator unit 108 and directs it through a flue gas duct 112 through the recovery boiler stack 102 and out to the atmosphere. In the example shown, the flue gas is split to pass through parallel paths to a pair of precipitator units 108 and fan unit 110, although the flue gas can be directed through a single path or through multiple paths through these units, as is well known to those skilled in the art. The recovery boiler stack 102 is typically constructed of carbon steel.

[0008] The exhaust gas contains sensible and latent heat that can be used to heat boiler feedwater to produce hot process water for use in plant operations. This can be accomplished by passing the exhaust gas through a condensing heat exchanger, but doing so causes condensation, and the acid condensation can cause corrosion in the heat exchanger, preventing the heat exchanger from being placed in the exhaust gas duct 112.

[0009] It would therefore be desirable to provide a system and method that can extract heat in the exhaust gases in a condensing heat exchanger without causing corrosion to both the heat exchanger and the recovery boiler stack. Summary of the Invention

[0010] Systems and methods for recovering heat from flue gas generated by a recovery boiler in a pulp and paper mill are presented. In some embodiments, the flue gas generated by the recovery boiler can be diverted or drawn from the recovery boiler stack and passed through one or more condensing heat exchangers, which can extract one or both of the sensible and latent heat in the flue gas and use it to heat boiler feedwater for the recovery boiler and other boilers used in the pulp and paper mill, and to generate process hot water for use in mill operations. In doing so, the low-pressure steam previously used to heat boiler feedwater and generate process hot water can be used to generate electricity using a steam turbine coupled to a generator. After passing through the heat exchangers, the heat-depleted flue gas can then be released to the atmosphere via a new stack. The new heat exchangers and new stacks are made of stainless steel to prevent acid corrosion and stress cracking corrosion caused by substances in the flue gas, including acids and chlorides, condensing and precipitating on the heat exchangers.

[0011] In some embodiments, the heat exchangers can be configured such that a first exchanger is positioned above a second heat exchanger, whereby the flue gas can be pulled from the recovery boiler stack using a variable frequency drive ("VFD") electric fan that can direct the flue gas downward through a duct and through the two heat exchangers. After passing through the heat exchangers, the flue gas can flow through a duct to a new stack for release to the atmosphere.

[0012] In some embodiments, as the exhaust gas flows through the heat exchanger, condensate that forms in the heat exchanger falls downward and is collected in a condensate collector, and the collected condensate (typically water) can be piped and stored in a holding tank or sump for use in plant operations or disposed of in a manner well known to those skilled in the art.

[0013] In some embodiments, a scrubbing system can be provided to scrub off precipitates and condensates that may accumulate in the heat exchanger. Exhaust gas from a recovery boiler can contain particles such as soot and inorganic matter. As the exhaust gas passes through a heat exchanger, the soot and inorganic matter can deposit or precipitate on the heat exchanger, forming a layer of accumulated precipitates that can reduce the heat transfer efficiency of the heat exchanger. This layer of precipitates must be periodically removed from the heat exchanger. A scrubbing system can be used to scrub off precipitates and condensates from the heat exchanger. In some embodiments, the scrubbing system can include spray nozzles disposed on the heat exchanger that can be used to direct pressurized fluid, such as water or collected exhaust gas condensate, onto the heat exchanger to break up the precipitates and wash them off along with the condensate so that the heat exchanger can operate at optimal efficiency. The scrubbing system can be configured to operate at predetermined times for predetermined periods of time to scrub off precipitates and condensates as needed.

[0014] Broadly speaking, in some embodiments, a heat recovery system for use with a recovery boiler system of a pulp and paper mill can be provided, the recovery boiler system having a flue gas stack operably coupled to it, the heat recovery system including: a first fan configured to draw flue gas from the flue gas stack; at least one first heat exchanger operably coupled to the first fan and configured to receive and pass the drawn flue gas; at least one second heat exchanger operably coupled to the at least one first heat exchanger and configured to receive and pass the drawn flue gas after passing through the at least one first heat exchanger; and a second flue gas stack operably coupled to the at least one second heat exchanger and configured to receive the drawn flue gas after passing through the at least one second heat exchanger.

[0015] Generally speaking, in some embodiments, the at least one first heat exchanger may be configured to heat boiler feedwater.

[0016] Broadly speaking, in some embodiments, the at least one first heat exchanger may comprise two or more boiler feedwater heat exchangers operatively coupled in series.

[0017] Broadly speaking, in some embodiments, the at least one second heat exchanger can be configured to heat hot process water for use in a pulp and paper mill.

[0018] Generally speaking, in some embodiments, the at least one second heat exchanger may comprise two or more process hot water heat exchangers operatively coupled in series.

[0019] Generally speaking, in some embodiments, at least one first heat exchanger is positioned above at least one second heat exchanger, and the drawn exhaust gas can be directed downwardly through the at least one first heat exchanger and the at least one second heat exchanger.

[0020] Generally speaking, in some embodiments, the heat recovery system may further comprise a washing system configured to wash away one or both of precipitates and condensates from one or both of the at least one first heat exchanger and the at least one second heat exchanger, where the precipitates and condensates are formed in one or both of the at least one first heat exchanger and the at least one second heat exchanger when the drawn exhaust gas passes through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0021] Generally speaking, in some embodiments, the cleaning system can include at least one spray nozzle disposed on one or both of the at least one first heat exchanger and the at least one second heat exchanger, wherein the at least one spray nozzle is configured to clean one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0022] Generally speaking, in some embodiments, one or both of the at least one first heat exchanger and the at least one second heat exchanger may comprise a condensing heat exchanger.

[0023] Broadly speaking, in some embodiments, the heat recovery system may further include a condensate collector disposed below one or both of the at least one first heat exchanger and the at least one second heat exchanger, and when the drawn exhaust gas passes through one or both of the at least one first heat exchanger and the at least one second heat exchanger, water vapor in the drawn exhaust gas condenses in one or both of the at least one first heat exchanger and the at least one second heat exchanger, thereby generating condensate.

[0024] Generally speaking, in some embodiments, the heat recovery system may further include a washing system configured to use at least a portion of the collected condensate to wash away one or both of precipitates and condensates from one or both of the at least one first heat exchanger and the at least one second heat exchanger, where precipitates and condensates form in one or both of the at least one first heat exchanger and the at least one second heat exchanger when the drawn exhaust gas passes through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0025] Generally speaking, in some embodiments, the cleaning system can include at least one spray nozzle disposed on one or both of the at least one first heat exchanger and the at least one second heat exchanger, and the cleaning system includes a first pump configured to send collected condensate through the at least one spray nozzle, thereby cleaning one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0026] Generally speaking, in some embodiments, one or both of the at least one first heat exchanger and the at least one second heat exchanger may be constructed from one or both of stainless steel and titanium.

[0027] Generally speaking, in some embodiments, one or both of the at least one first heat exchanger and the at least one second heat exchanger may be constructed from SAF2205™ stainless steel.

[0028] Broadly speaking, in some embodiments, a method can be provided for recovering heat from a recovery boiler system of a pulp and paper mill, the recovery boiler system having a flue gas stack operably coupled to it, the method including withdrawing flue gas from the flue gas stack, heating boiler feedwater using the withdrawn flue gas, then heating process hot water using the withdrawn flue gas, and then venting the withdrawn flue gas to the atmosphere.

[0029] Generally speaking, in some embodiments, the method may include heating boiler feedwater using at least one first heat exchanger.

[0030] Broadly speaking, in some embodiments, the at least one first heat exchanger may comprise two or more boiler feedwater heat exchangers operatively coupled in series.

[0031] Generally speaking, in some embodiments, the method can include heating the hot process water using at least one second heat exchanger.

[0032] Generally speaking, in some embodiments, the at least one second heat exchanger may comprise two or more process hot water heat exchangers operatively coupled in series.

[0033] Generally speaking, in some embodiments, the method may further include scrubbing one or both of the at least one first heat exchanger and the at least one second heat exchanger of precipitates and condensates formed in the at least one first heat exchanger and the at least one second heat exchanger after the drawn exhaust gas passes through the at least one first heat exchanger and the at least one second heat exchanger.

[0034] Generally speaking, in some embodiments, the method may further include using at least one spray nozzle to clean one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0035] Generally speaking, in some embodiments, the method may include sequentially cleaning at least one first heat exchanger and at least one second heat exchanger.

[0036] Generally speaking, in some embodiments, the method may further include collecting condensate produced by water vapor in the drawn exhaust gas condensing in one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn exhaust gas passes through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0037] Generally speaking, in some embodiments, the method may include using at least a portion of the collected condensate to wash away precipitates and condensates from one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn exhaust gas has passed through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0038] Generally speaking, in some embodiments, the method may further include pumping at least a portion of the collected condensate through at least one spray nozzle to clean one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0039] Generally speaking, in some embodiments, the method may include sequentially cleaning at least one first heat exchanger and at least one second heat exchanger.

[0040] Broadly speaking, in some embodiments, a heat recovery system can be provided for use with a recovery boiler system of a pulp and paper mill, the recovery boiler having a flue gas stack operatively coupled to it, the heat recovery system including: means for drawing flue gas from the flue gas stack; means for heating boiler feedwater using the drawn flue gas; means for heating process hot water using the drawn flue gas; and means for venting the drawn flue gas to the atmosphere.

[0041] Generally speaking, in some embodiments, the heat recovery system may further comprise means for scrubbing one or both of precipitates and condensates from one or both of the means for heating boiler feedwater and the means for heating process hot water, where precipitates and condensates form on one or both of the means for heating boiler feedwater and the means for heating process hot water after the drawn exhaust gas passes through one or both of the means for heating boiler feedwater and the means for heating process hot water.

[0042] Generally speaking, in some embodiments, the heat recovery system may further include means for collecting condensate produced by water vapor in the drawn exhaust gas.

[0043] Broadly speaking, in some embodiments, the heat recovery system may further comprise means for using the collected condensate to wash away one or both of precipitates and condensates from one or both of the means for heating boiler feedwater and the means for heating process hot water, where precipitates and condensates have formed in one or both of the means for heating boiler feedwater and the means for heating process hot water after the drawn exhaust gas has passed through one or both of the means for heating boiler feedwater and the means for heating process hot water. [Brief explanation of the drawings]

[0044] [Figure 1] FIG. 1 is a block diagram illustrating a prior art recovery boiler system for a pulp and paper mill.

[0045] [Figure 2] 1 is a block diagram illustrating a simplified first embodiment of a system for recovering heat from flue gases from a recovery boiler system of a pulp and paper mill.

[0046] [Figure 3] FIG. 1 is a block diagram illustrating a simplified second embodiment of a system for recovering heat from flue gases from a recovery boiler system of a pulp and paper mill.

[0047] [Figure 4] FIG. 10 is a block diagram illustrating a third embodiment of a system for recovering heat from flue gas from a recovery boiler system of a pulp and paper mill.

[0048] [Figure 5] FIG. 5 is a block diagram illustrating a heat exchanger cleaning system for the heat recovery system of FIG. 4.

[0049] [Figure 6] FIG. 10 is a block diagram illustrating a fourth embodiment of a system for recovering heat from flue gas from a recovery boiler system of a pulp and paper mill.

[0050] [Figure 7] FIG. 7 is a block diagram illustrating a heat exchanger cleaning system for the heat recovery system of FIG. 6.

[0051] [Figure 8] FIG. 8 is a block diagram showing a control system for the heat recovery system of FIGS. 4 to 7. DETAILED DESCRIPTION OF THE INVENTION

[0052] References herein to "one embodiment," "an embodiment," or "embodiments" mean that one or more referenced features are included in at least one embodiment of the technology. Separate references herein to "one embodiment," "an embodiment," or "embodiments" do not necessarily refer to the same embodiment, and are not mutually exclusive unless otherwise stated and / or readily apparent to one of ordinary skill in the art from this specification. For example, features, structures, acts, etc. described in one embodiment may, but are not necessarily, included in other embodiments. Thus, the technology may include various combinations and / or integrations of the embodiments described herein.

[0053] 2 and 3, a simplified embodiment of a stack heat recovery system 10 is shown. In its simplest configuration, in some embodiments, flue gas may be drawn from a recovery boiler stack 102 through a duct 12 by a fan 14, forcing the flue gas through a first heat exchanger 16 and then through a second heat exchanger 18. After passing through heat exchangers 16 and 18, the flue gas may exit to the atmosphere through a second stack 20.

[0054] In some embodiments, the first heat exchanger 16 can be constructed from stainless steel and can be used to convert sensible heat in the exhaust gas to heat boiler feedwater for use in the recovery boiler 100. In some embodiments, the first heat exchanger 16 can be constructed from SAF2205™ stainless steel, such as that manufactured by Sandvik AB of Stockholm, Sweden. SAF2205™ stainless steel is known to have high resistance to stress corrosion cracking in chloride-containing hydrogen sulfide environments, as well as high resistance to general corrosion and corrosion fatigue. As shown in FIG. 1 , in prior art systems, steam previously used to heat the boiler feedwater can be used to generate electricity by passing the steam through a steam turbine operably coupled to a generator (not shown), as is well known to those skilled in the art. In a typical example, the steam previously used to heat the boiler feedwater can generate approximately 2.7 megawatts of electricity.

[0055] In some embodiments, the second heat exchanger 18 can be constructed from stainless steel and can be used to convert sensible and latent heat in the flue gas to generate process hot water for use in pulp and paper mill operations. In some embodiments, the second heat exchanger 18 can be constructed from SAF2205™ stainless steel, such as that manufactured by Sandvik AB of Stockholm, Sweden. As shown in FIG. 1 , in prior art systems, the steam previously used to generate process hot water can then be used to generate electricity by passing the steam through a steam turbine operably coupled to a generator (not shown), as is well known to those skilled in the art. In a typical example, the steam previously used to generate process hot water can generate approximately 6.0 megawatts of power. Therefore, by implementing this heat recovery system, in this example, approximately 8.7 megawatts of power can be generated utilizing the steam previously used in the prior art system. It will be apparent to those skilled in the art that the amount of power generated can be increased or decreased depending on the size of the recovery boiler and the amount of flue gas generated therefrom.

[0056] 4 and 5, a third embodiment of a stack heat recovery system 10 is shown. In the embodiment illustrated in FIG. 4, the system 10 can draw flue gas from a recovery boiler stack 102 using a VFD-operated fan 14. In some embodiments, the system 10 can include a damper inlet guillotine valve 13 that can be used to open or close a flow path from the stack 102 to the fan 14 to perform operation and maintenance procedures for the system 10. The flue gas can flow from the fan 14 through a duct 15 into a stack structure 17, and heat exchangers 16 and 18 are arranged in a vertical configuration within the stack structure 17, with the first heat exchanger 16 positioned above the second heat exchanger 18, such that the flue gas flows from the top of the first heat exchanger 16 downward through the bottom of the second heat exchanger 18. From there, the flue gas can flow through a duct 19 and exit to the atmosphere 50 via the second stack 20.

[0057] In some embodiments, system 10 can include two sets of heat exchangers 16 and 18 operably configured in parallel vertical structures 17, as shown in Figures 4 and 5. Having multiple vertical structures 17 can be done to increase the amount of exhaust gas treated by system 10, or for practical reasons related to the logistics of transporting heat exchangers to a location where system 10 is implemented. This can also be done for redundancy, such that one vertical structure 17 can be taken down for maintenance or repair while the other vertical structure 17 remains operational.

[0058] In some embodiments, boiler feedwater may enter the first heat exchanger 16 via piping 22. The boiler feedwater to be heated may be pumped into inlet pipe 22a and into the heat exchanger inlet 16a, and the heated boiler feedwater may exit the heat exchanger outlet 16b and be pumped into the recovery boiler 100 via outlet pipe 22b.

[0059] In some embodiments, hot process water may enter the second heat exchanger 18 via piping 26. The hot process water to be heated may be pumped into inlet pipe 26a and enter the heat exchanger inlet 18in, and the heated hot process water may exit the heat exchanger outlet 18out and be pumped through outlet pipe 26b for use in the pulp and paper mill operation.

[0060] In the embodiment illustrated in FIG. 5 , the system 10 can include a heat exchanger cleaning system 30 that can be configured to wash the heat exchangers 16 and 18 clean of deposits and condensate that may accumulate therein over time as the exhaust gas passes through them. In some embodiments, the cleaning system 30 can include a liquid sump 34 that can be used to hold fluids, such as water or exhaust gas condensate, that may form in the heat exchangers 16 and 18. The exhaust gas condensate can accumulate by gravity in a condensate collector 32 located at the lower end of the vertical structure 17. The exhaust gas condensate can then be directed to the liquid sump 34 via piping 36. In some embodiments, a fluid pump 40 can be used to draw the exhaust gas condensate from the liquid sump 34 and pump it via piping 44 for dispensing through spray nozzles 38 a- 38 d located on the heat exchangers 16 and 18. Valves 45 a- 45 d can be opened and closed as needed to selectively operate one of the spray nozzles 38 a- 38 d according to a predetermined cleaning sequence or regime.

[0061] As the flue gas flows downward through heat exchangers 16 and 18, any deposits that may accumulate thereon may tend to accumulate more at the top of heat exchanger 16 because this is the first area the flue gas encounters and because of the drop in pressure. In some embodiments, the cleaning sequence may include first pumping a fluid, such as water or flue gas condensate, through spray nozzle 38a to first wash away the accumulated heavy condensate from the bottom of heat exchanger 18, thereby allowing it to circulate again through heat exchanger 18, then spraying fluid sequentially through spray nozzle 38d, then spray nozzle 38c, then spray nozzle 38b, and then once again through spray nozzle 38a. In some embodiments, the cleaning sequence may be performed as needed or on a predetermined time schedule, such as every 12 to 24 hours or longer, depending on the amount of soot and minerals suspended in the flue gas as it exits recovery boiler 100. Once the cleaning sequence is complete, drain valve 43 can be opened to allow the fluid in line 44 to return to sump 34 .

[0062] 6 and 7, a fourth embodiment of a stack heat recovery system 10 is shown. In the embodiment illustrated in FIG. 6, the system 10 can draw flue gas from a recovery boiler stack 102 using a VFD-operated fan 14. In some embodiments, the system 10 can include a damper inlet guillotine valve 13 that can be used to open or close a flow path from the stack 102 to the fan 14 to perform operation and maintenance procedures for the system 10. The flue gas can flow from the fan 14 through a duct 15 into a stack structure 17, and heat exchangers 16 and 18 are arranged in a vertical configuration within the stack structure, with the first heat exchanger 16 positioned above the second heat exchanger 18, such that the flue gas flows from the top of the first heat exchanger 16 downward through the bottom of the second heat exchanger 18. From there, the flue gas can flow through a duct 19 and exit to the atmosphere 50 via the second stack 20.

[0063] In some embodiments, the first heat exchanger 16 can comprise a condensing heat exchanger constructed from a corrosion-resistant material. In some embodiments, the first heat exchanger 16 can be constructed from stainless steel, titanium, or other corrosion-resistant materials as are well known to those skilled in the art. In some embodiments, the first heat exchanger 16 can be constructed from SAF2205™ stainless steel, such as that manufactured by Sandvik AB of Stockholm, Sweden. In the embodiment illustrated in FIG. 6 , the second heat exchanger 18 can be constructed from three separate, operatively coupled heat exchangers, labeled 18a, 18b, and 18c. In some embodiments, the second heat exchanger 18 can comprise a condensing heat exchanger constructed from a corrosion-resistant material. In some embodiments, the second heat exchanger 18 can be constructed from stainless steel, titanium, or other corrosion-resistant materials as are well known to those skilled in the art. In some embodiments, the second heat exchanger 18 may be constructed from SAF2205™ stainless steel, such as that manufactured by Sandvik AB of Stockholm, Sweden.

[0064] In some embodiments, system 10 can include two sets of heat exchangers 16 and 18 operably configured in parallel vertical structures 17, as shown in Figures 6 and 7. Having multiple vertical structures 17 can be done to increase the amount of exhaust gas treated by system 10, or for practical reasons related to the logistics of transporting heat exchangers to a location where system 10 is implemented. This can also be done for redundancy, such that one vertical structure 17 can be taken down for maintenance or repair while the other vertical structure 17 remains operational.

[0065] In some embodiments, boiler feedwater may enter the first heat exchanger 16 via piping 22. The boiler feedwater to be heated may be pumped into inlet pipe 22a and into the heat exchanger inlet 16a, and the heated boiler feedwater may exit the heat exchanger outlet 16b and be pumped into the recovery boiler 100 via outlet pipe 22b.

[0066] In some embodiments, hot process water can enter the second heat exchanger 18 via piping 26. The hot process water to be heated can be pumped into inlet pipe 26a and into heat exchanger inlet 18in of heat exchanger 18a, the hot process water is heated as it passes sequentially through heat exchangers 18a through 18c, and then exits through heat exchanger outlet 18out where it can be pumped through outlet pipe 26b for use in the pulp and paper mill operation.

[0067] In the embodiment illustrated in FIG. 7 , the system 10 can include a heat exchanger cleaning system 30 that can be configured to wash the heat exchangers 16 and 18a-18c clean of precipitates and condensate that may accumulate therein over time as the exhaust gas passes through them. In some embodiments, the cleaning system 30 can include a sump 34 that can be used to hold fluids, such as water or exhaust gas condensate, that may form in the heat exchangers 16 and 18a-18c. The exhaust gas condensate can accumulate by gravity in a condensate collector 32 located at the lower end of the vertical structure 17. The exhaust gas condensate can then be directed to the sump 34 via piping 36. In some embodiments, a fluid pump 40 can be used to draw the exhaust gas condensate from the sump 34 and pump it via piping 44 for dispensing through spray nozzles 38a-38d located on the heat exchangers 16 and 18a-18c. Valves 45a-45d can be opened and closed as needed to selectively operate one of spray nozzles 38a-38d according to a predetermined cleaning sequence or regimen.

[0068] As the exhaust gas flows downward through heat exchangers 16 and 18a-18c, any deposits that may accumulate thereon may tend to accumulate more at the top of heat exchanger 16 because this is the first area the exhaust gas encounters when it contacts heat exchanger 16 and because of the drop in pressure. In some embodiments, a cleaning sequence may include first pumping a fluid, such as water or exhaust gas condensate, through spray nozzle 38a to first wash away the accumulated heavy condensate from the bottom of heat exchanger 18a, thereby allowing it to circulate through heat exchanger 18a again, and then spraying the fluid sequentially through spray nozzle 38d, then spray nozzle 38c, then spray nozzle 38b, and then spray nozzle 38a again to circulate through all of heat exchangers 18a-18c. In some embodiments, the cleaning sequence can be performed as needed or on a predetermined time schedule, such as every 12-24 hours or longer, depending on the amount of soot and minerals suspended in the flue gas as it exits the recovery boiler 100. Once the cleaning sequence is complete, drain valve 43 can be opened to allow the fluid in line 44 to return to sump 34.

[0069] 8, a simplified block diagram of a control system 46 for controlling the operation of system 10 is shown. In some embodiments, control system 46 may include a computing controller 48, which may comprise one or more of a programmable logic controller (“PLC”), a general-purpose computer, a microcontroller, and a microprocessor-based computing device configured to control the subcomponents of system 10. In some embodiments, computing controller 48 may be operably coupled to one or more of guillotine valve 13, VFD-operated fan 14, fluid pump 40, drain valve 43, and nozzle control valves 45a-45d. In some embodiments, computing controller 48 may be used to control the operation of guillotine valve 13 and VFD-operated fan 14 to regulate and control the amount of exhaust gas flowing through system 10. In some embodiments, computing and control device 48 can be used to control cleaning system 40 through operation of fluid pump 40 and operation of spray nozzle valves 45a-45d according to a predetermined cleaning sequence of heat exchangers 16 and 18 to remove sediment or condensate from heat exchangers 16 and 18 on a predetermined time schedule for cleaning operations or depending on the amount of sediment or condensate accumulated in heat exchangers 16 and 18. In some embodiments, computing and control device 48 can be used to open drain valve 43 and drain piping 44 after cleaning operations of heat exchangers 16 and 18.

[0070] The various illustrative logic blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or a combination of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends on the particular application and design constraints imposed on the overall system. Those skilled in the art may implement the described functionality in various ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the embodiments described herein.

[0071] Computer software implemented embodiments may be implemented in software, firmware, middleware, microcode, hardware description language, or any combination thereof. A code segment or machine-executable instructions may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and / or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. can be passed, forwarded, or transmitted via any suitable means, such as memory sharing, message passing, token passing, network transmission, etc.

[0072] The actual software code or specialized control hardware used to implement these systems and methods does not limit the embodiments described herein, and thus the operation and behavior of the systems and methods have been described without reference to specific software code, with the understanding that software and control hardware can be designed to implement the systems and methods based on the description herein.

[0073] When implemented in software, the functions may be stored as one or more instructions or code on a non-transitory computer-readable or processor-readable storage medium. The steps of a method or algorithm disclosed herein may be embodied in a processor-executable software module, which may reside on a computer-readable or processor-readable storage medium. Non-transitory computer-readable or processor-readable media includes both computer storage media and tangible storage media that facilitate transfer of a computer program from one place to another. A non-transitory processor-readable storage medium may be any available medium that can be accessed by a computer. By way of example, and not limitation, such non-transitory processor-readable media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other tangible storage medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer or processor. As used herein, disk and disc include compact discs (CDs), laser discs, optical discs, digital versatile discs (DVDs), floppy disks, and Blu-ray discs, where disks typically reproduce data magnetically and discs reproduce data optically using a laser. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm may reside as one or any combination or set of code and / or instructions on a non-transitory processor-readable medium and / or computer-readable medium, which may be incorporated into a computer program product.

[0074] While several embodiments have been shown and described, it will be understood by those skilled in the art that various changes and modifications can be made to these embodiments without changing or departing from their scope, spirit, or function. The terms and expressions used in the foregoing specification are used herein as terms of description and not as terms of limitation, and there is no intention in the use of such terms and expressions to exclude equivalents of the features shown and described or portions thereof, recognizing that the present invention is defined and limited only by the claims that follow.

Claims

1. 1. A heat recovery system for use with a recovery boiler system of a pulp and paper mill, the recovery boiler system having a flue gas stack operatively coupled thereto, the heat recovery system comprising: a) a first fan configured to draw exhaust gases from the exhaust stack; b) at least one first heat exchanger operably coupled to the first fan and configured to receive and pass the drawn exhaust gas; c) at least one second heat exchanger operably coupled to the at least one first heat exchanger and configured to receive and pass the drawn exhaust gas after passing through the at least one first heat exchanger; d) a second exhaust gas stack operatively coupled to the at least one second heat exchanger and configured to receive the drawn exhaust gas after passing through the at least one second heat exchanger; Equipped with A heat recovery system, wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger are constructed from one or both of stainless steel and titanium.

2. The heat recovery system of claim 1 , wherein the at least one first heat exchanger is configured to heat boiler feedwater.

3. The heat recovery system of claim 2 , wherein the at least one first heat exchanger comprises two or more boiler feedwater heat exchangers operatively coupled in series.

4. 4. The heat recovery system of claim 1, wherein the at least one second heat exchanger is configured to heat hot process water for use in the pulp and paper mill.

5. The heat recovery system of claim 4 , wherein the at least one second heat exchanger comprises two or more process hot water heat exchangers operatively coupled in series.

6. 6. The heat recovery system of claim 1, wherein the at least one first heat exchanger is disposed above the at least one second heat exchanger, and the drawn exhaust gas is directed downwardly through the at least one first heat exchanger and the at least one second heat exchanger.

7. 7. The heat recovery system of claim 1, further comprising a washing system configured to wash off one or both of deposits and condensates from one or both of the at least one first heat exchanger and the at least one second heat exchanger, the deposits and condensates being formed in one or both of the at least one first heat exchanger and the at least one second heat exchanger when the drawn exhaust gas passes through the one or both of the at least one first heat exchanger and the at least one second heat exchanger.

8. 8. The heat recovery system of claim 7, wherein the cleaning system comprises at least one spray nozzle disposed on one or both of the at least one first heat exchanger and the at least one second heat exchanger, the at least one spray nozzle configured to clean one or both of the at least one first heat exchanger and the at least one second heat exchanger.

9. The heat recovery system of any one of claims 1 to 6, wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger comprise a condensing heat exchanger.

10. 10. The heat recovery system of claim 9, further comprising a condensate collector disposed below one or both of the at least one first heat exchanger and the at least one second heat exchanger, wherein the condensate is produced by water vapor in the drawn exhaust gas condensing in one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn exhaust gas passes through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

11. 11. The heat recovery system of claim 10, further comprising: a washing system configured to use at least a portion of the collected condensate to wash one or both of the at least one first heat exchanger and the at least one second heat exchanger of precipitates and condensates formed in one or both of the at least one first heat exchanger and the at least one second heat exchanger when the drawn exhaust gas passes through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

12. 12. The heat recovery system of claim 11, wherein the cleaning system comprises at least one spray nozzle disposed on one or both of the at least one first heat exchanger and the at least one second heat exchanger, and the cleaning system comprises a first pump configured to send the collected condensate through the at least one spray nozzle, thereby cleaning one or both of the at least one first heat exchanger and the at least one second heat exchanger.

13. The heat recovery system of claim 1 , wherein one or both of the at least one first heat exchanger and the at least one second heat exchanger are constructed from SAF2205™ stainless steel.

14. 1. A method for recovering heat from a recovery boiler system of a pulp and paper mill, the recovery boiler system having a flue gas stack operably coupled thereto, the method comprising: a) drawing exhaust gas from the exhaust gas stack; b) using the extracted flue gas to heat boiler feed water; c) then using the drawn off flue gas to heat process hot water; d) then venting the drawn exhaust gas to the atmosphere; A method comprising:

15. 15. The method of claim 14, including heating the boiler feedwater with at least one first heat exchanger.

16. The method of claim 15 , wherein the at least one first heat exchanger comprises two or more boiler feedwater heat exchangers operatively coupled in series.

17. 17. The method of claim 15 or 16, further comprising heating the hot process water using at least one second heat exchanger.

18. 20. The method of claim 17, wherein the at least one second heat exchanger comprises two or more process hot water heat exchangers operatively coupled in series.

19. 19. The method of claim 17 or 18, further comprising the step of scrubbing one or both of precipitates and condensates from one or both of the at least one first heat exchanger and the at least one second heat exchanger, wherein the precipitates and condensates are formed in one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn exhaust gas passes through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

20. 20. The method of claim 19, further comprising using at least one spray nozzle for cleaning one or both of the at least one first heat exchanger and the at least one second heat exchanger.

21. 21. The method of claim 19 or 20, further comprising the step of sequentially cleaning the at least one first heat exchanger and the at least one second heat exchanger.

22. 19. The method of claim 17, further comprising collecting condensate produced by water vapor in the drawn exhaust gas condensing in one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn exhaust gas passes through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

23. 22. The method of claim 21, further comprising using at least a portion of the collected condensate to wash away precipitates and condensates from one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitates and condensates having formed in one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn exhaust gas has passed through one or both of the at least one first heat exchanger and the at least one second heat exchanger.

24. 22. The method of claim 21, further comprising pumping the at least a portion of the collected condensate through at least one spray nozzle to clean one or both of the at least one first heat exchanger and the at least one second heat exchanger.

25. 23. The method of claim 21 or 22, further comprising the step of sequentially cleaning the at least one first heat exchanger and the at least one second heat exchanger.

26. 1. A heat recovery system for use with a recovery boiler system in a pulp and paper mill, the recovery boiler having a flue gas stack operatively coupled thereto, the heat recovery system comprising: a) means for withdrawing exhaust gas from said exhaust stack; b) means for heating boiler feedwater using the withdrawn exhaust gas; c) means for heating process hot water using the withdrawn exhaust gas; d) means for venting the drawn exhaust gas to the atmosphere; A heat recovery system comprising:

27. 27. The heat recovery system of claim 26, further comprising means for scrubbing one or both of said means for heating boiler feedwater and said means for heating process hot water of precipitates and condensates formed in one or both of said means for heating boiler feedwater and said means for heating process hot water after the drawn exhaust gas passes through one or both of said means for heating boiler feedwater and said means for heating process hot water.

28. 27. The heat recovery system of claim 26, further comprising means for collecting condensate produced by water vapor in the drawn exhaust gas.

29. 29. The heat recovery system of claim 28, further comprising means for using the collected condensate to wash away one or both of precipitates and condensates from one or both of the means for heating boiler feedwater and the means for heating process hot water, the precipitates and condensates being formed in one or both of the means for heating boiler feedwater and the means for heating process hot water after the drawn flue gas has passed through one or both of the means for heating boiler feedwater and the means for heating process hot water.

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