STEAM METHANE REFORMING UNIT FOR CARBON CAPTURE

Abstract

A molten carbonate fuel cell-powered system for capturing carbon dioxide produced by a steam methane reformer system. Tail gas from a pressure swing adsorption system is mixed with exhaust gas from the fuel cell anode, then pressurized and cooled to extract liquefied carbon dioxide. The residual low-CO.sub.2 gas is directed to an anode gas oxidizer, to the anode, to the reformer to be burned for fuel, and/or to the pressure swing adsorption system. Low-CO.sub.2 flue gas from the reformer can be vented to the atmosphere or directed to the anode gas oxidizer. Reduction in the amount of CO.sub.2 reaching the fuel cell allows the fuel cell to be sized according to the power demands of the system and eliminates the need to export additional power output.

Claims

1. A system for capturing carbon dioxide from a steam methane reformer system, the system for capturing carbon dioxide comprising: a molten carbonate fuel cell comprising an anode and a cathode; a compressor configured to compress a gas mixture, the gas mixture comprising exhaust gas from the anode and tail gas from the steam methane reformer system; a chiller configured to cool the gas mixture; and a carbon dioxide separator configured to separate the gas mixture into liquefied carbon dioxide and a residual gas mixture.

2. The system of claim 1, further comprising: an anode gas oxidizer; wherein a first portion of the residual gas mixture is directed from the carbon dioxide separator to the anode of the molten carbonate fuel cell and a second portion of the residual gas mixture is directed to the anode gas oxidizer.

3. The system of claim 2, wherein a third portion of the residual gas mixture is directed from the carbon dioxide separator to the steam methane reformer system to be burned as fuel.

4. The system of claim 2, wherein a third portion of the residual gas mixture is directed from the carbon dioxide separator to a pressure swing adsorption system in the steam methane reformer system.

5. The system of claim 3, wherein a fourth portion of the residual gas mixture is directed from the carbon dioxide separator to a pressure swing adsorption system in the steam methane reformer system.

6. The system of claim 1, wherein flue gas from a reformer in the steam methane reformer system is vented to the atmosphere.

7. The system of claim 2, wherein flue gas from a reformer in the steam methane reformer system is directed to the anode gas oxidizer.

8. The system of claim 7, wherein a third portion of the residual gas mixture is directed to a second pressure swing adsorption system outside the steam methane reformer system.

9. The system of claim 3, wherein the molten carbonate fuel cell is sized to power at least one of the system for capturing carbon dioxide or the steam methane reformer system.

10. The system of claim 1, wherein the steam methane reformer system comprises a pressure swing adsorption system configured to produce the tail gas.

11. The system of claim 4, wherein flue gas from a reformer in the steam methane reformer system is directed to the anode gas oxidizer.

12. A system for capturing carbon dioxide from a steam methane reformer system, the system for capturing carbon dioxide comprising: a compressor configured to compress tail gas from the steam methane reformer system; a chiller configured to cool the tail gas; and a carbon dioxide separator configured to separate the tail gas into liquefied carbon dioxide and residual tail gas.

13. The system of claim 12, wherein the residual tail gas is directed to the steam methane reformer system to be burned as fuel.

14. The system of claim 12, wherein the residual tail gas is directed to a pressure swing adsorption system in the steam methane reformer system.

15. The system of claim 12, wherein a first portion of the residual tail gas is directed to a pressure swing adsorption system in the steam methane reformer system and a second portion of the residual tail gas is directed to the steam methane reformer system to be burned as fuel.

16. A method of capturing carbon dioxide from a steam methane reformer system, the method comprising: mixing tail gas from the steam methane reformer system with anode exhaust gas from an anode of a molten carbonate fuel cell to form a gas mixture; compressing the gas mixture; cooling the gas mixture; and separating the gas mixture into liquid carbon dioxide and a residual gas mixture.

17. The method of claim 16, further comprising: directing a first portion of the residual gas mixture to an anode gas oxidizer; and directing a second portion of the residual gas mixture to the anode of the molten carbonate fuel cell.

18. The method of claim 17, further comprising: directing a third portion of the residual gas mixture to the steam methane reformer system to be burned as fuel.

19. The method of claim 17, further comprising: directing a third portion of the residual gas mixture to a pressure swing adsorption system in the steam methane reformer system.

20. The method of claim 18, further comprising: directing a fourth portion of the residual gas mixture to a pressure swing adsorption system in the steam methane reformer system.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0014] The disclosure will become more fully understood from the following detailed description, taken in conjunction with the accompanying figures, wherein like reference numerals refer to like elements, in which:

[0015] FIG. 1 shows a schematic view of a conventional SMR-CO.sub.2 capture system.

[0016] FIG. 2 shows a schematic view of a SMR-CO.sub.2 capture system, in accordance with a representative embodiment of the present disclosure.

[0017] FIG. 3 shows a schematic view of a SMR-CO.sub.2 capture system, in accordance with another representative embodiment.

[0018] FIG. 4 shows a schematic view of a SMR-CO.sub.2 capture system, in accordance with yet another representative embodiment.

[0019] FIG. 5 shows a schematic view of a SMR-CO.sub.2 capture system, in accordance with yet another representative embodiment.

[0020] FIG. 6 shows a method of capturing CO.sub.2 from a SMR, in accordance with a representative embodiment.

DETAILED DESCRIPTION

[0021] Referring generally to the figures, disclosed herein are various embodiments of an enhanced SMR-CO.sub.2 capture system capable of capturing CO.sub.2 in a more efficient and cost effective manner, as compared to some conventional CO.sub.2 capture systems. The various embodiments disclosed herein may be capable of increasing the amount of CO.sub.2 captured, improving efficiency of capturing CO.sub.2, increasing the amount of hydrogen produced, and/or reducing costs associated with capturing CO.sub.2. In the various embodiments disclosed herein, like reference numerals refer to like elements between Figures, but are increased by 200 from figure to figure (e.g., PSA 450 in FIG. 2 is the same as PSA 650 in FIG. 3, etc.).

[0022] Generally speaking, in a typical SMR unit of a SMR-CO.sub.2 capture system, natural gas is reacted with water to form hydrogen and CO.sub.2. Some methane is unconverted and some carbon monoxide is also generated in the process. These impurities, along with any water that is not separated out by condensation, is normally removed from the hydrogen using a PSA system, which can desorb these impurities at atmospheric pressure to generate a PSA tail gas that is generally high in CO.sub.2 and also contains CO, methane, and hydrogen. Typically, the PSA tail gas is recycled as fuel in the SMR unit, where the gases are combusted with air to provide the heat needed for the endothermic reforming reaction. This reaction produces a flue gas having relatively high CO.sub.2 content that can be directed to an MCFC for subsequent CO.sub.2 capture. In this type of system configuration, the size of the MCFC is dictated, in part, by the amount (or percentage) of CO.sub.2 to be transferred to the anode of the MCFC from the flue gas received from the SMR unit (e.g., 70%-90% of the CO.sub.2).

[0023] However, the composition of the PSA tail gas is very similar to the shifted anode exhaust gas of the MCFC in the CO.sub.2 capture system. Thus, Applicant advantageously determined that the PSA tail gas could be mixed directly with the anode exhaust gas of the MCFC before being compressed and cooled to separate the CO.sub.2 from the gas, instead of recycling the PSA tail gas in the SMR unit. In this manner, the size of the MCFC is not dictated by the PSA tail gas from the SMR unit, since the CO.sub.2 that is normally contained in the flue gas from the PSA tail gas is mixed with the anode exhaust gas downstream of the MCFC for subsequent CO.sub.2 capture instead. Accordingly, the overall cost of the system can be reduced by selecting a smaller sized MCFC that can, for example, produce just enough power for the chiller and other electrical loads of the CO.sub.2 capture system, the SMR system itself, or both, as discussed in greater detail below.

[0024] In some implementations, the system can also be configured to direct the flue gas from the SMR unit to the MCFC for additional CO.sub.2 capture. In some implementations, the unused fuel from the anode of the MCFC containing hydrogen and carbon monoxide can be directed from a CO.sub.2 capture tail gas to the PSA to increase the hydrogen production of the system. In some implementations, power is purchased from an external source instead of using an MCFC to power the CO.sub.2 capture system.

[0025] FIG. 1 shows a typical SMR-CO.sub.2 capture system. As shown in FIG. 1, steam supplied by a steam supply line 210 and natural gas supplied by a natural gas supply line 220 are mixed and fed to a reformer 230 of a reformer system 200 for converting the methane to hydrogen, CO.sub.2, and CO. The reformer effluent may be transported to a shifting assembly of the reformer system 200, where the effluent may be cooled and most of the CO can be shifted to hydrogen according to the reaction:

CO+H.sub.2O.Math.H.sub.2+CO.sub.2

The shifted gas is then sent via shift gas line 240 to a PSA system 250 where the hydrogen is separated from the residual methane and CO in the gas along with the CO.sub.2 produced from the reforming and shift reactions. The residual gases are recycled as fuel to the reformer 230 via a recycling line 260, where the gases are combusted with air supplied by an air supply line 270 to provide the heat needed for the endothermic reforming reaction. All of the CO.sub.2 generated in the production of the hydrogen is vented in the reformer flue gas as a mixture of N.sub.2, CO.sub.2, and H.sub.2O with some NOx.

[0026] Still referring to FIG. 1, the reformer flue gas containing the CO.sub.2 is sent to an AGO (anode gas oxidizer) 110 of a CO.sub.2 capture system 100 where the flue gas is optionally combined with air, if required to increase the flue gas oxygen content to the level required for MCFC operation, from an air supply line 112. The flue gas and air are heated and fed into a cathode 124 of an MCFC 120. Natural gas is provided to a pre-heater 115 before being fed to an anode 122 of the MCFC 120 via a natural gas supply line 114. Due to the unique properties of the MCFC, CO.sub.3.sup.= is transferred from the cathode 124 to the anode 122 of the MCFC 120 during normal power production of the MCFC. This transfer removes CO.sub.2 and O.sub.2 from the flue gas containing cathode feed and generates a cathode exhaust, which is relatively low in CO.sub.2, thereby reducing CO.sub.2 emissions. The CO.sub.3.sup.= transferred to the anode 122 reacts with hydrogen in the anode 122 to form H.sub.2O and CO.sub.2 while generating power. In the carbon capture process, the outlet stream from the anode 122 is cooled by, for example, an evaporator 125, and enters a shift reactor such that CO in the outlet stream is converted to hydrogen and CO.sub.2 using the following shift reaction:

CO+H.sub.2O.Math.H.sub.2+CO.sub.2

The outlet stream is then compressed by a compressor 130 and then cooled by, for example, a chiller 135. The compressed and cooled outlet stream is then transferred to a CO.sub.2 separator 140. In the compressed and cooled outlet stream, about 60% to about 90% of the CO.sub.2 is condensed as a liquid and separated from a residual CO.sub.2 capture tail gas containing hydrogen, any unconverted CO, the remaining uncondensed CO.sub.2, and methane. The residual CO.sub.2 capture tail gas is partially recycled to the anode 122 via a recycle line 142 to be used as fuel in the MCFC 120. The remainder of the residual CO.sub.2 capture tail gas is sent to the AGO 110 to help to prevent buildup of inert gases, such as nitrogen, and to heat up the gas in the AGO 110 by combusting the remaining hydrogen gas from the residual CO.sub.2 capture tail gas. This recycling has the advantage of increasing the amount of CO.sub.2 recovered from the anode exhaust.

[0027] In the system of FIG. 1, the MCFC 120 is sized, in part, based on the amount of CO.sub.2 in the flue gas to transfer to the anode 122 (e.g., about 70% to about 90% of the CO.sub.2 in the flue gas). Thus, the system configuration of FIG. 1 can be costly to implement and may generate more power than desirable.

[0028] Referring now to FIG. 2, a SMR-CO.sub.2 capture system including a CO.sub.2 capture system 300 and a SMR system 400 is shown according to an exemplary embodiment of the present disclosure. In contrast to the system of FIG. 1, the flue gas from the SMR system 400 is not directed to the MCFC for CO.sub.2 capture, but is instead vented since it has generally low CO.sub.2 content. Instead, the SMR system 400 is configured such that a PSA tail gas from a PSA 450 in the SMR system 400 is mixed directly with an anode exhaust gas from an anode 322 of an MCFC 320, where the mixture can be compressed and cooled, such that liquefied CO.sub.2 can be separated from the mixture to be captured. In this way, the size of the MCFC 320 is independent of the flue gas from the SMR reformer 430, such that the MCFC 320 may be sized to provide only the power needed by the CO.sub.2 capture system 300, the SMR system 400, or both, thereby reducing the overall cost of the system while still providing adequate CO.sub.2 capture.

[0029] Still referring to FIG. 2, steam supplied by a steam supply line 410 and natural gas supplied by a natural gas supply line 420 are mixed and fed to a SMR reformer 430 of a SMR system 400 for converting the methane to hydrogen, CO.sub.2, and CO. The reformer effluent may be transported to a shifting assembly of the SMR system 400, where the effluent may be cooled and most of the CO can be shifted to hydrogen. The shifted gas is then sent via shift gas line 440 to a PSA 450 where the hydrogen is separated from the residual methane and CO in the gas along with the CO.sub.2 produced from the reforming and shift reactions. Instead of recycling the residual gases in the PSA tail gas as fuel in the SMR reformer 430, as in the system of FIG. 1, the PSA tail gas is directed to the CO.sub.2 capture system 300 by a PSA tail gas supply line 460 to be mixed directly with the anode exhaust gas of the anode 322 of the MCFC 320 for subsequent CO.sub.2 capture. In this way, the size of the MCFC 320 is not dictated by the amount of CO.sub.2 in the PSA tail gas to be transferred to the anode 322. The MCFC 320 can therefore be sized to any specifications, for example, to produce sufficient power for the CO.sub.2 capture system 300, the SMR system 400, or both.

[0030] Still referring to FIG. 2, the mixture of anode exhaust gas and the PSA tail gas is then compressed by a compressor 330 and then cooled by, for example, a chiller 335. The compressed and cooled outlet stream is then transferred to a CO.sub.2 separator 340. In the compressed and cooled outlet stream, about 60% to about 90% of the CO.sub.2 is condensed as a liquid and separated from a residual CO.sub.2 capture tail gas containing hydrogen, any unconverted CO, the remaining uncondensed CO.sub.2, and methane. A portion of the CO.sub.2 capture tail gas stream is recycled to the anode 322 via a recycle line 342 to be used as fuel in the MCFC 320, with a small portion of the tail gas also being sent to the AGO 310 to help to prevent buildup of inert gases, such as nitrogen, and to heat up the gas in the AGO 310 by combusting the remaining hydrogen gas from the residual CO.sub.2 capture tail gas.

[0031] In the system configuration of FIG. 2, the PSA tail gas is no longer being directed back to the SMR Reformer 430 for subsequent burning. As such, natural gas may be used instead to provide fuel to SMR Reformer 430, and the resulting flue gas containing a relatively low amount of CO.sub.2 can be vented as a mixture of N.sub.2, CO.sub.2, and H.sub.2O with some NOx as allowed by regulation. In this configuration, about 50% to about 60% of the CO.sub.2 normally emitted from the SMR is captured.

[0032] According to another representative embodiment shown in FIG. 2, a portion of the residual CO.sub.2 capture tail gas may also be directed back to SMR Reformer 430 by a CO.sub.2 capture tail gas supply line 344 (represented by dashed lines/arrows). The portion of the residual CO.sub.2 capture tail gas can be used as fuel in SMR Reformer 430, so as to reduce the amount of natural gas required. The portion of the residual CO.sub.2 capture tail gas may also be recycled to the PSA 450, as shown in FIG. 2, to increase the H.sub.2 production of the PSA without increasing the size of SMR Reformer 430. According to another representative embodiment, the MCFC 320 may be sized to use only the residual CO.sub.2 capture tail gas as fuel, thereby eliminating the need for natural gas at the MCFC 320, except for startup and upset operations.

[0033] Referring now to FIG. 3, a SMR-CO.sub.2 capture system including a CO.sub.2 capture system 500 and a SMR system 600 is shown according to another exemplary embodiment of the present disclosure. In contrast to the system of FIG. 2, the flue gas from the SMR system 600 is directed to the MCFC along a flue gas supply line 635 for CO.sub.2 capture, instead of being vented. In addition, tail gas from a PSA 650 of the SMR system 600 is directed along a PSA tail gas supply line 660 to be mixed directly with an anode exhaust from the anode 522 of the MCFC 520 for capturing CO.sub.2. In this way, the MCFC 520 can be sized to be smaller than in a typical SMR-CO.sub.2 capture system (such as the system of FIG. 1), since the tail gas containing about 50-60% of the CO.sub.2 normally present in the flue gas to be captured by the MCFC 520 is directed to the anode exhaust of the MCFC instead. Thus, this exemplary system can provide relatively higher CO.sub.2 capture, as compared to the system of FIG. 2, while still reducing overall cost, as compared to the conventional system of FIG. 1.

[0034] According to another representative embodiment, the size of the MCFC 520 is configurable to offset the power consumed by the CO.sub.2 capture system 500 and the SMR system 600. In this configuration, a relatively large percentage of the normally emitted CO.sub.2 (e.g., about 60% to about 70%) would still be captured by the system, but the capital cost would be significantly reduced and the need to export power to a third party would be eliminated or reduced.

[0035] Referring now to FIG. 4, a SMR-CO.sub.2 capture system including a CO.sub.2 capture system 700 and a SMR system 800 is shown according to another exemplary embodiment of the present disclosure. As shown in FIG. 4, flue gas from the SMR system 800 is directed along a flue gas supply line 835 to the cathode 724 of an MCFC 720, and tail gas from a PSA 850 of the SMR system 800 is directed along a PSA tail gas supply line 860 to be mixed directly with an anode exhaust from the anode 722 of the MCFC 720 for capturing CO.sub.2. The unused fuel from the anode 722 containing hydrogen and carbon monoxide (e.g., about 30%) with small amounts of methane, can be directed from the CO.sub.2 capture tail gas to the PSA 850 along a CO.sub.2 capture tail gas supply line 744 to increase the hydrogen production of the system. In some embodiments, the CO.sub.2 capture tail gas could be sent to a separate PSA from the PSA 850 if, for example, the PSA 850 did not have the required capacity for the additional feed. In this manner, this exemplary system can provide for relatively high CO.sub.2 capture and increased hydrogen production, while reducing overall cost.

[0036] Referring now to FIG. 5, a SMR-CO.sub.2 capture system including a CO.sub.2 capture system 900 and a SMR system 1000 is shown according to another exemplary embodiment of the present disclosure. As shown in FIG. 5, power is received from an external source instead of using an MCFC to power the CO.sub.2 capture system 900. The external power source could be an existing power plant, utility grid, and/or renewable power such as solar or wind power, according to various exemplary embodiments. The tail gas from a PSA 1050 of the SMR system 1000 is compressed by the compressor 930 and cooled by the chiller 935, such that the liquefied CO.sub.2 can be separated from the gas for capturing CO.sub.2. The CO.sub.2 capture tail gas may be directed along a PSA tail gas supply line 942 to the PSA 1050 and/or to the reformer 1030 of the SMR system 1000, so as to increase the hydrogen production of the system and helping to prevent build-up of inert gases in the SMR system 1000. In this manner, this exemplary system can provide a lower cost option for capturing CO.sub.2, as compared to other CO.sub.2 capture systems. It should be appreciated, however, that the amount of potential CO.sub.2 that is released by the external power source should be taken into account when estimating the amount of CO.sub.2 reduction with this configuration.

[0037] Referring to FIG. 6, a method of implementing the systems described above is shown according to an exemplary embodiment. The exemplary method includes a mixing step 1101 in which tail gas from the PSA of the SMR system is mixed with anode exhaust gas from the anode of the MCFC; a compression step 1103 in which the mixed gasses are compressed by a compressor; a cooling step 1105 in which the gases are cooled by, for example, a chiller such that most of the CO.sub.2 is output as a liquid; a separation step 1107 in which the liquid CO.sub.2 is separated from the residual gases by a CO.sub.2 separator; a collection step 1109 in which the liquid CO.sub.2 is collected to be sequestered or used for other purposes; and a recycling step 1111, in which the residual gases are recycled for use in one of more of an anode gas oxidizer, the anode of the MCFC, the SMR as a fuel source for the reformer, and the PSA to produce hydrogen gas. In certain embodiments, the mixing step 1101 is omitted and SMR tail gas is processed without mixing with anode exhaust gas, particularly when an external power source is used in place of a MCFC.

[0038] According to a representative embodiment, tail gas from a PSA in a SMR system is mixed directly with an anode exhaust gas from the anode of an MCFC, where the mixture can be compressed and its temperature lowered by a chiller, such that liquefied CO.sub.2 can be separated from the mixture to be captured. The CO.sub.2 from the combustor of the reformer of the SMR system, i.e. the flue gas from the SMR system, is not directed to the MCFC for CO.sub.2 capture, as compared to some conventional SMR-CO.sub.2 capture systems. In this way, the size of the MCFC is independent of the flue gas from the SMR reformer, and instead is governed by the CO.sub.2 capture from the PSA tail gas, thereby reducing the overall cost of the system while still providing CO.sub.2 capture.

[0039] According to another representative embodiment, flue gas from a SMR system is sent to the cathode of an MCFC, and tail gas from a PSA of the SMR system is mixed directly with an anode exhaust from the anode of the MCFC for capturing CO.sub.2. In this way, the MCFC can be sized to be smaller than in a typical SMR-CO.sub.2 capture system, since the tail gas containing about 50-60% of the CO.sub.2 normally present in the flue gas to be captured by the MCFC is directed to the anode exhaust of the MCFC instead. Thus, this exemplary system can provide relatively high CO.sub.2 capture while reducing overall cost of the system.

[0040] According to another representative embodiment, flue gas from a SMR system is sent to the cathode of an MCFC, and tail gas from a PSA of the SMR system is mixed directly with an anode exhaust from the anode of the MCFC for capturing CO.sub.2. A portion of the unused fuel from the anode of the MCFC, following removal of CO.sub.2 and containing hydrogen and carbon monoxide (CO) can be directed from the CO.sub.2 capture tail gas to the PSA to increase the hydrogen production of the system. In this manner, this exemplary system can provide for relatively high CO.sub.2 capture and increased hydrogen production, while reducing overall cost of the system.

[0041] According to another representative embodiment, the tail gas from a PSA of the SMR system is compressed and its temperature lowered by a chiller using an external power source (and, in cases where an absorption chiller is used, an external heat source), such that the liquefied CO.sub.2 can be separated from the gas for capturing CO.sub.2. The unused fuel from the CO.sub.2 capture tail gas containing hydrogen, CO, residual CO.sub.2, and other non-condensable gases may be directed to the PSA so as to increase the hydrogen production of the system, and/or to the reformer of the SMR system, helping to prevent build-up of inert gases in the SMR system. In this manner, this exemplary system can provide a lower cost option for capturing CO.sub.2, as compared to some conventional SMR-CO.sub.2 capture systems.

[0042] Disclosed herein are various embodiments of an enhanced SMR-CO.sub.2 capture system capable of capturing CO.sub.2 in a more efficient and cost effective manner, as compared to some conventional CO.sub.2 capture systems using MCFC. The various embodiments disclosed herein may be capable of increasing the amount of CO.sub.2 captured, improving efficiency of capturing CO.sub.2, increasing the amount of hydrogen produced, and/or reducing costs associated with capturing CO.sub.2.

[0043] As utilized herein, the terms “approximately,” “about,” “substantially”, and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims.

[0044] The terms “coupled,” “connected,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another.

[0045] References herein to the positions of elements (e.g., “top,” “bottom,” “above,” “below,” etc.) are merely used to describe the orientation of various elements in the Figures. It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure.

[0046] It is important to note that the construction and arrangement of the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present invention. For example, the heat recovery heat exchangers may be further optimized.