TECHNICAL FIELD:

[0001 ] The present disclosure is related to the field of recovering heat from flue gas produced from a recovery boiler used in a pulp and paper mill.

BACKGROUND:

[0002] In the production of chemical pulp, lignin and other organic non-cellulosic material can be separated from the raw material of chemical pulp by cooking using cooking chemicals. Cooking liquor used in chemical digestion, that is, waste liquor is recovered. The waste liquor (also referred to as “black liquor”), which is separated mechanically from the chemical pulp, has a high combustion value due to carbonaceous and other organic, combustible material contained therein and separated from the chemical pulp. The waste liquor also contains inorganic chemicals, which do not react in chemical digestion. Several different methods have been developed for recovering heat and chemicals from waste liquor.

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

[0004] Hot flue gases are generated in black liquor combustion, which are led into contact with various heat exchangers within the recovery boiler. Flue gas conveys heat into water or vapor, or a mixture of water and vapor, flowing inside the heat exchangers, simultaneously cooling itself. Usually, flue gases contain abundantly of ash. Main part of the ash is sodium sulfate, and the next largest part is usually sodium-carbonate. Ash contains other components, too. The ash entrained in flue gases is in the furnace mainly in vaporized form and starts to convert into fine dust or smelt droplets mainly in the parts of the boiler downstream of the furnace. The salts contained in the ash melt can be sticky particles even at relatively low temperatures. Molten and sticky particles stick easily onto heat transfer surfaces and even corrode them. Deposits of sticky ash have caused a clogging risk of the flue gas ducts, and also corrosion and wearing of the heat surfaces in the boiler. A recovery boiler in a pulp and paper mill, thus, can produce hot flue gas that is released to the atmosphere through a flue gas stack, as well known to those skilled in the art.

[0005] Recovery boiler flue gas, however, contains acid, especially sulfuric acid vapour inside of flue gas that has been known to cause acid corrosion. As a result, dry heat exchangers are typically used (where temperatures are above the sulfuric acid dew point) and considered to be a common sense or standard type of heat exchanger as used in pulp and paper mill industries. In addition, flue gas particulates contain chloride and this is known to cause stress crack corrosion in normal stainless steel

[0006] A typical material used for heat exchangers is carbon steel because it is a low-cost material and provides better heat exchange efficiency. Carbon steel can also be used in the construction of dry heat exchangers to avoid concerns of acid corrosion therein. In some cases, titanium can be used (as is used in a majority of the Japanese mills) as well as the casing and ducts can be made off 316 stainless steel or carbon steel. However, titanium is very expensive, has low heat exchange efficiency and is difficult to fabricate. This is why, historically, pulp and paper mills have only used dry heat exchanges, or no heat exchangers at all, to avoid problem of corrosion caused by acid condensation. [0007] Referring to Figure 1 , a prior art recovery boiler system is shown. Flue gas in recovery boiler 100 passes through economizers 104 and then through ducts 106 into precipitator units 108. Fan units 110 then draw flue gas from precipitator units 108 and direct the flue gas through flue gas ducts 112 to exit to the atmosphere via recovery boiler stack 102. In the illustrated example, the flue gas is split through parallel paths to a pair of precipitator units 108 and fan units 110 although the flue gas can be directed through a single path or through multiple paths of these units, as well known to those skilled in the art. Recovery boiler stack 102 is, typically, constructed of carbon steel.

[0008] The flue gas contains sensible and latent heat that can be used to heat boiler feedwater and to produce process hot water used in the mill operations. This could be accomplished by passing the flue gas through condensing heat exchangers, however, the condensation produced in doing so prevents the heat exchangers from being placed in flue gas ducts 112 due to the acid condensation that causes corrosion therein.

[0009] It is, therefore, desirable to provide a system and method that can extract the heat in the flue gas with condensing heat exchangers without causing corrosion to both the heat exchanger and the recovery boiler stack.

SUMMARY:

[0010] A system and method for recovering heat from the flue gas produced by a recovery boiler in a pulp and paper mill is presented. In some embodiments, the flue gas produced by the recovery boiler can be diverted and drawn from the recovery boiler stack and passed through one or more condensing heat exchangers that can be used to extract one or both of the sensible and the latent heat in the flue gas to heat the boiler feedwater used for the recovery boiler and other boilers used in the pulp and paper mill, and to produce process hot water for use in the mill operations. In doing so, the low pressure steam previously used to heat boiler feedwater and to produce process hot water can be used to generate electrical power with steam turbines coupled to electrical generators. 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 the new stack can be made of stainless steel to prevent acid corrosion and stress cracking corrosion due to the substances in flue gas, which contains acid and chloride condensing and precipitating on the heat exchangers.

[0011 ] In some embodiments, the heat exchangers can be arranged such that the first exchanger is positioned above the second heat exchanger, whereby the flue gas can be drawn from the recovery boiler stack with variable frequency drive (“VFD”) electrically- operated fan that can direct the flue gas, through ducting, downward through the two heat exchangers. After passing through the heat exchangers, the flue gas can flow through ducting to the new stack for release into the atmosphere.

[0012] In some embodiments, the condensation that forms on the heat exchangers, when the flue gas flows therethrough, can fall downward and be collected in a condensation collector where the collected condensate, typically water, can be piped and stored in a holding tank or sump for use in mill operations, or disposed of in a manner as well known to those skilled in the art.

[0013] In some embodiments, a wash system can be provided to wash off precipitate and condensate that can build up on the heat exchangers. Flue gas from a recovery boiler can contain particles such as soot and minerals. As flue gas passes through the heat exchangers, the soot and minerals can deposit or precipitate onto the heat exchangers forming an accumulated precipitate layer that can degrade the heat transfer efficiency of the heat exchangers. The precipitate layer needs to be removed from the heat exchangers on a periodic basis. To wash the precipitate and condensate off of the heat exchangers, a wash system can be used. In some embodiments, the wash system can comprise spray nozzles disposed on the heat exchangers that can be used to direct pressurized fluid, such as water or collected flue gas condensate, onto the heat exchangers to break up the precipitate and wash it off along with condensate so that the heat exchangers can operate at optimum efficiency. The wash system can be configured to operate at predetermined times for predetermined periods, as necessary, to wash off the precipitate and the condensate.

[0014] Broadly stated, in some embodiments, a heat recovery system can be provided for use with a recovery boiler system in a pulp and paper mill, the recovery boiler system comprising a flue gas stack operatively coupled thereto, the heat recovery system comprising: a first fan configured to draw flue gas away from the flue gas stack; at least one first heat exchanger operatively coupled to the first fan and configured to receive the drawn flue gas to pass therethrough; at least one second heat exchanger operatively coupled to the at least one first heat exchanger and configured to receive the drawn flue gas to pass therethrough after passing through the at least one first heat exchanger; and a second flue gas stack operatively 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] Broadly stated, in some embodiments, the at least one first heat exchanger can be configured to heat boiler feedwater. [0016] Broadly stated, in some embodiments, the at least one first heat exchanger can comprise two or more boiler feedwater heat exchangers operatively coupled together sequentially.

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

[0018] Broadly stated, in some embodiments, the at least one second heat exchanger can comprise two or more process hot water heat exchangers operatively coupled together sequentially.

[0019] Broadly stated, in some embodiments, the at least one first exchanger can be disposed above the at least one second heat exchanger, and wherein the drawn flue gas can be directed downward through the at least one first heat exchanger and the at least one second heat exchanger.

[0020] Broadly stated, in some embodiments, the heat recovery system can further comprise a wash system configured to wash off one or both of precipitate and condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate forming on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough.

[0021 ] Broadly stated, in some embodiments, the wash system can comprise 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 wash off one or both of the at least one first heat exchanger and the at least one second heat exchanger. [0022] Broadly stated, in some embodiments, one or both of the at least one first heat exchanger and the at least one second heat exchanger can comprise a condensing heat exchanger.

[0023] Broadly stated, in some embodiments, the heat recovery system can further comprise a condensation collector disposed beneath one or both of the at least one first heat exchanger and the at least one second heat exchanger, the condensation produced by water vapour disposed in the drawn flue gas condensing on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough.

[0024] Broadly stated, in some embodiments, the heat recovery system can further comprise a wash system configured to use at least some of the collected condensation to wash off one or both of precipitate and condensate off one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate forming on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough.

[0025] Broadly stated, in some embodiments, the wash system can comprise 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 wash system comprising a first pump configured to pump the collected condensation through the at least one spray nozzle, thereby washing one or both of the at least one first heat exchanger and the at least one second heat exchanger. [0026] Broadly stated, in some embodiments, one or both of the at least one first heat exchanger and the at least one second heat exchanger can be comprised of one or both of stainless steel and titanium.

[0027] Broadly stated, in some embodiments, one or both of the at least one first heat exchanger and the at least one second heat exchanger can be comprised of SAF 2205™ stainless steel.

[0028] Broadly stated, in some embodiments, a method can be provided for recovering heat from a recovery boiler system in a pulp and paper mill, the recovery boiler system comprising a flue gas stack operatively coupled thereto, the method comprising: drawing flue gas away from the flue gas stack; heating boiler feedwater with the drawn flue gas; then heating process hot water with the drawn flue gas; and then exiting the drawn flue gas to the atmosphere.

[0029] Broadly stated, in some embodiments, the method can comprise heating the boiler feedwater with at least one first heat exchanger.

[0030] Broadly stated, in some embodiments, the at least one first heat exchanger can comprise two or more boiler feedwater heat exchangers operatively coupled together sequentially.

[0031 ] Broadly stated, in some embodiments, the method can comprise heating the process hot water with at least one second heat exchanger.

[0032] Broadly stated, in some embodiments, the at least one second heat exchanger can comprise two or more process hot water heat exchangers operatively coupled together sequentially. [0033] Broadly stated, in some embodiments, the method can further comprise washing one or both of precipitate and condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger, the precipitate and the condensate having formed on one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn flue gas has passed therethrough.

[0034] Broadly stated, in some embodiments, the method can further comprise using at least one spray nozzle for washing one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0035] Broadly stated, in some embodiments, the method can comprise washing the at least one first heat exchanger and the at least one second heat exchanger sequentially.

[0036] Broadly stated, in some embodiments, the method can further comprise collecting condensation produced by water vapour disposed in the drawn flue gas condensing on one or both of the at least one first heat exchanger and the at least one second heat exchanger as the drawn flue gas passes therethrough.

[0037] Broadly stated, in some embodiments, the method can comprise washing one or both of precipitate and condensate off of one or both of the at least one first heat exchanger and the at least one second heat exchanger with at least some of the collected condensation, the precipitate and the condensate having formed on one or both of the at least one first heat exchanger and the at least one second heat exchanger after the drawn flue gas has passed therethrough.

[0038] Broadly stated, in some embodiments, the method can further comprise pumping the at least some of the collected condensation through at least one spray nozzle to wash one or both of the at least one first heat exchanger and the at least one second heat exchanger.

[0039] Broadly stated, in some embodiments, the method can comprise washing the at least one first heat exchanger and the at least one second heat exchanger sequentially.

[0040] Broadly stated, in some embodiments, a heat recovery system can be provided for use with a recovery boiler system in a pulp and paper mill, the recovery boiler comprising a flue gas stack operatively coupled thereto, the heat recovery system comprising: means for drawing flue gas away from the flue gas stack; means for heating boiler feedwater with the drawn flue gas; means for heating process hot water with the drawn flue gas; and means for exiting the drawn flue gas to the atmosphere.

[0041 ] Broadly stated, in some embodiments, the heat recovery system can further comprise means for washing one or both of precipitate and condensate off of one or both of the means for heating boiler feedwater and the means for heating process hot water, the precipitate and the condensate having formed thereon after the drawn flue gas has passed therethrough.

[0042] Broadly stated, in some embodiments, the heat recovery system can further comprise means for collecting condensation produced by water vapour disposed in the drawn flue gas.

[0043] Broadly stated, in some embodiments, the heat recovery system can further comprise means for washing one or both of precipitate and condensate off of one or both of the means for heating boiler feedwater and the means for heating process hot water with the collected condensation, the precipitate and the condensate having formed thereon after the drawn flue gas has passed therethrough. BRIEF DESCRIPTION OF THE DRAWINGS:

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

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

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

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

[0048] Figure 5 is a block diagram depicting a heat exchanger wash system for the heat recovery system of Figure 4.

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

[0050] Figure 7 is a block diagram depicting a heat exchanger wash system for the heat recovery system of Figure 6.

[0051 ] Figure 8 is a block diagram depicting a control system for the heat recovery systems of Figures 4 to 7.

DETAILED DESCRIPTION OF EMBODIMENTS:

[0052] In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment can also be included in other embodiments but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein.

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

[0054] In some embodiments, first heat exchanger 16 can be comprised of stainless steel and can be used to convert the sensible heat in the flue gas to heat boiler feedwater for use in recovery boiler 100. In some embodiments, first heat exchanger 16 can be comprised of SAF 2205™ stainless steel, as manufactured by Sandvik AB of Stockholm, Sweden. SAF 2205™ stainless steel is known to have high resistance to stress corrosion cracking chloride-bearing and hydrogen sulphide environments, and high resistance to general corrosion and corrosion fatigue. The steam previously used to heat the boiler feedwater in the prior art system, as shown in Figure 1 , can then be used to generate electrical power by running the steam through a steam turbine operatively coupled to an electrical generator (not shown), as well known to those skilled in the art. In a representative example, the steam previously used to heat the boiler feedwater can produce approximately 2.7 megawatts of power. [0055] In some embodiments, second heat exchanger 18 can be comprised of stainless steel can be used to convert the sensible and latent heat in the flue gas to produce process hot water for use in operations in the pulp and paper mill. In some embodiments, second heat exchanger 18 can be comprised of SAF 2205™ stainless steel, as manufactured by Sandvik AB of Stockholm, Sweden. The steam previously used to produce the process hot water in the prior art system, as shown in Figure 1 , can then be used to generate electrical power by running the steam through a steam turbine operatively coupled to an electrical generator (not shown), as well known to those skilled in the art. In a representative example, the steam previously used to produce the process hot water can produce approximately 6.0 megawatts of power. Thus, the implementation of this heat recovery system can then free up the steam previously used in the prior art system to produce approximately 8.7 megawatts of power, in the representative example. It would be obvious to those skilled in the art that amount of power produced can be scaled up or down depending on the size the recovery boiler and the amount of flue gas produced therefrom.

[0056] Referring to Figures 4 and 5, a third embodiment of stack heat recovery system 10 is shown. In the illustrated embodiment shown in Figure 4, system 10 can draw flue gas away from recovery boiler stack 102 with VFD-operated fan 14. In some embodiments, system 10 can comprise damper inlet guillotine valve 13, which can be used to open and close the flow path from stack 102 to fan 14 to allow for operational and maintenance procedures on system 10. From fan 14, the flue gas can flow through ducting 15 to enter into stack structure 17 where heat exchangers 16 and 18 are disposed therein in a vertical configuration wherein first heat exchanger 16 is disposed above second heat exchanger 18 such that the flue gas flows downward from the top of first heat exchanger 16 through to the bottom of second heat exchanger 18. From there, the flue gas can flow through ducting 19 and exit to atmosphere 50 via second stack 20.

[0057] In some embodiments, system 10 can comprise two sets of heat exchangers 16 and 18 operatively configured in parallel vertical structures 17, as shown in Figures 4 and 5. Having multiple vertical structures 17 can be done to scale up the volume of flue gas processed by system 10 or can be done for practical reasons in terms of the logistics of shipping heat exchangers to a site where system 10 will be implemented. It can also be done for redundancy wherein one vertical structure 17 can be shut down for maintenance or repaid while the other vertical structure 17 remains in operation.

[0058] In some embodiments, boiler feedwater can enter first heat exchangers 16 via piping 22. Boiler feedwater to be heated can be pumped into inlet pipe 22a to enter heat exchanger inlets 16a, wherein heated boiler feedwater can exit via heat exchanger outlets 16b to be pumped via outlet pipe 22b to recovery boiler 100.

[0059] In some embodiments, process hot water can enter second heat exchangers 18 via piping 26. Process hot water to be heated can be pumped into inlet pipe 26a to enter heat exchanger inlets 1 Sin , wherein heated process hot water can exit via heat exchanger outlets 18out to be pumped via outlet pipe 26b for use in pulp and paper mill operations. [0060] In the illustrated embodiment shown in Figure 5, system 10 can comprise heat exchanger wash system 30 that can be configured to clean precipitate and condensate off of heat exchangers 16 and 18 that can accumulate thereon over time as flue gas passes therethrough. In some embodiments, wash system 30 can comprise of sump 34 that can be used to hold fluid, such as water or flue gas condensate that can form on heat exchangers 16 and 18. The flue gas condensate can accumulate in condensate collectors 32 disposed on the lower ends of vertical structures 17 due to gravity. The flue gas condensate can then be directed to sump 34 via piping 36. In some embodiments, fluid pump 40 can be used to draw flue gas condensate from sump 34 and pump it through piping 44 to be dispensed through spray nozzles 38a-38b disposed in heat exchangers 16 and 18. Valves 45a-45d can be opened and closed as needed to selectively operate one of spray nozzles 38a-38d in accordance with a predetermined wash sequence or protocol.

[0061 ] As flue gas flows downward through heat exchangers 16 and 18, the precipitate that can accumulate thereon can tend to accumulate more towards the top of heat exchangers 16 because of the first contact area and pressure drop as the flue gas contacts heat exchangers 16. In some embodiments, a wash sequence can include first pumping fluid, such as water or flue gas condensate, through spray nozzles 38a to first clean off heavy accumulation of condensate from the bottom of heat exchangers 18 so as to enable circulation therethrough again, and then spraying fluid sequentially through spray nozzles 38d, then spray nozzles 38c, then spray nozzles 38b and then spray nozzles 38a once again. In some embodiments, the wash sequence can occur as needed or on a pre-determined time schedule such as every 12 to 24 hours, or more, depending on how much soot and minerals are suspended in the flue gas as it exits from recovery boiler 100. Once a wash sequence is completed, drain valve 43 can be opened to allow fluid within piping 44 to drain back to sump 34.

[0062] Referring to Figures 6 and 7, a fourth embodiment of stack heat recovery system 10 is shown. In the illustrated embodiment shown in Figure 5, system 10 can draw flue gas away from recovery boiler stack 102 with VFD-operated fan 14. In some embodiments, system 10 can comprise damper inlet guillotine valve 13, which can be used to open and close the flow path from stack 102 to fan 14 to allow for operational and maintenance procedures on system 10. From fan 14, the flue gas can flow through ducting 15 to enter into stack structure 17 where heat exchangers 16 and 18 are disposed therein in a vertical configuration wherein first heat exchanger 16 is disposed above second heat exchanger 18 such that the flue gas flows downward from the top of first heat exchangers 16 through to the bottom of second heat exchangers 18. From there, the flue gas can flow through ducting 19 and exit to atmosphere 50 via second stack 20. [0063] In some embodiments, first heat exchangers 16 can comprise condensing heat exchangers comprised of material that is resistant to corrosion. In some embodiments, first heat exchangers 16 can be comprised of stainless steel, titanium or other corrosion- resistant materials as well known to those skilled in the art. In some embodiments, first heat exchangers 16 can be comprised of SAF 2205™ stainless steel, as manufactured by Sandvik AB of Stockholm, Sweden. In the illustrated embodiment shown in Figure 6, second heat exchanger 18 can comprise of three separate heat exchangers operatively coupled together sequentially, labelled as 18a, 18b and 18c. In some embodiments, second heat exchangers 18 can comprise condensing heat exchangers comprised of material that is resistant to corrosion. In some embodiments, second heat exchangers 18 can be comprised of stainless steel, titanium or other corrosion-resistant materials as well known to those skilled in the art. In some embodiments, second heat exchangers 18 can be comprised of SAF 2205™ stainless steel, as manufactured by Sandvik AB, of Stockholm, Sweden. [0064] In some embodiments, system 10 can comprise two sets of heat exchangers 16 and 18 operatively configured in parallel vertical structures 17, as shown in Figures 6 and 7. Having multiple vertical structures 17 can be done to scale up the volume of flue gas processed by system 10 or can be done for practical reasons in terms of the logistics of shipping heat exchangers to a site where system 10 will be implemented. It can also be done for redundancy wherein one vertical structure 17 can be shut down for maintenance or repaid while the other vertical structure 17 remains in operation.

[0065] In some embodiments, boiler feedwater can enter first heat exchangers 16 via piping 22. Boiler feedwater to be heated can be pumped into inlet pipe 22a to enter heat exchanger inlets 16a, wherein heated boiler feedwater can exit via heat exchanger outlets 16b to be pumped via outlet pipe 22b to recovery boiler 100.

[0066] In some embodiments, process hot water can enter second heat exchangers 18 via piping 26. Process hot water to be heated can be pumped into inlet pipe 26a to enter heat exchanger inlets 1 Sin of heat exchangers 18a, wherein process hot process can flow sequentially through heat exchangers 18a to 18c to be heated and can then exit via heat exchanger outlets 18out to be pumped via outlet pipe 26b for use in pulp and paper mill operations.

[0067] In the illustrated embodiment shown in Figure 7, system 10 can comprise heat exchanger wash system 30 that can be configured to clean precipitate and condensate off of heat exchangers 16 and 18a to 18c that can accumulate thereon over time as flue gas passes therethrough. In some embodiments, wash system 30 can comprise of sump 34 that can be used to hold fluid, such as water or flue gas condensate that can form on heat exchangers 16 and 18a to 18c. The flue gas condensate can accumulate in condensate collectors 32 disposed on the lower ends of vertical structures 17 due to gravity. The flue gas condensate can then be directed to sump 34 via piping 36. In some embodiments, fluid pump 40 can be used to draw flue gas condensate from sump 34 and pump it through piping 44 to be dispensed through spray nozzles 38a-38b disposed in heat exchangers 16 and 18a to 18c, one set of spray nozzles per heat exchanger as an example. Valves 45a-45d can be opened and closed as needed to selectively operate one of spray nozzles 38a-38d in accordance with a predetermined wash sequence or protocol.

[0068] As flue gas flows downward through heat exchangers 16 and 18a to 18c, the precipitate that can accumulate thereon can tend to accumulate more towards the top of heat exchangers 16 because of the first contact area and pressure drop as the flue gas contacts heat exchangers 16. In some embodiments, a wash sequence can include first pumping fluid, such as water or flue gas condensate, through spray nozzles 38a to first clean off heavy accumulation of condensate from the bottom of heat exchangers 18a so as to enable circulation therethrough again, and then spraying fluid sequentially through spray nozzles 38d, then spray nozzles 38c, then spray nozzles 38b and then spray nozzles 38a once again to cycle through all of heat exchanges 18a to 18c. In some embodiments, the wash sequence can occur as needed or on a pre-determined time schedule such as every 12 to 24 hours, or more, depending on how much soot and minerals are suspended in the flue gas as it exits from recovery boiler 100. Once a wash sequence is completed, drain valve 43 can be opened to allow fluid within piping 44 to drain back to sump 34. [0069] Referring to Figure 8, a simplified block diagram for control system 46 is shown for controlling the operation of system 10. In some embodiments, control system 46 can comprise computing control device 48 that can comprise of one or more of a programmable logic controller (“PLC”), a general purpose computer, a microcontroller and a microprocessor-based computing device configured for controlling the subcomponents let of system 10. In some embodiments, computing control device 48 can be operatively to one or more of guillotine valve 13, VFD-operated fan 14, fluid pump 40, drain valve 43 and nozzle control valves 45a to 45d. In some embodiments, computing control device 48 can be used to control the operation of guillotine valve 13 and VFD-operated fan 14 to regulate and control the volume of the flue gas flowing through system 10. In some embodiments, computing control device 48 can be used to control wash system 40 through the operation of fluid pump 40 and the operation of spray nozzle valve 45a to 45d in accordance with a pre-determined wash sequence of heat exchangers 16 and 18 to remove precipitate or condensate therefrom on a pre-determined time schedule for wash operations or in response to how much precipitate or condensate has accumulated on heat exchangers 16 and 18. In some embodiments, computing control device 48 can be used to open drain valve 43 to drain piping 44 after a wash operation of heat exchangers and 18.

[0070] The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or combinations 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 upon the particular application and design constraints imposed on the overall system. Skilled artisans can implement the described functionality in varying 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 ] Embodiments implemented in computer software can be implemented in software, firmware, middleware, microcode, hardware description languages, or any combination thereof. A code segment or machine-executable instructions can 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 can 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 including 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 is not limiting of the embodiments described herein. Thus, the operation and behavior of the systems and methods were described without reference to the specific software code being understood 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 can 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 can be embodied in a processor-executable software module, which can reside on a computer-readable or processor-readable storage medium. A 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 media can be any available media that can be accessed by a computer. By way of example, and not limitation, such non-transitory processor- readable media can comprise 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. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm can reside as one or any combination or set of codes and/or instructions on a non-transitory processor-readable medium and/or computer-readable medium, which can be incorporated into a computer program product.

[0074] Although a few embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications can be made to these embodiments without changing or departing from their scope, intent or functionality. The terms and expressions used in the preceding specification have been used herein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the invention is defined and limited only by the claims that follow.