WO2010096028A1

WO2010096028A1 - Water treatment system and method for a fuel cell power plant

Info

Publication number: WO2010096028A1
Application number: PCT/US2009/000965
Authority: WO
Inventors: Bryan F. Dufner; Albert P. Grasso; John W. Kowalski; Lynn Reni
Original assignee: Utc Power Corporation
Priority date: 2009-02-17
Filing date: 2009-02-17
Publication date: 2010-08-26
Also published as: WO2010096028A9

Abstract

A system and method for treating water for use in fuel cell power plant 10 includes condensing exhaust E1, E2 into condensate C1 comprising carbonic acid and electrolyte, using filter 36 to selectively filter electrolyte from contaminated water W2 without removing carbonic acid, introducing the carbonic acid into coolant loop 44 and coolant channels 22 of fuel cell 16, and removing the carbonic acid from contaminated water using steam generator 42.

Description

WATER TREATMENT SYSTEM AND METHOD FOR A FUEL CELL POWER

PLANT

BACKGROUND

The present disclosure relates in general to fuel cell power plants, and more particularly, to a system and method for treating water used in fuel cell power plants.

Fuel cell power plants may utilize a steam reformer for converting a hydrocarbon fuel source such as methane, natural gas, gasoline or the like into a gaseous hydrogen rich reformate that is fed to the anode of a fuel cell, while oxidant such as air is fed to the cathode of a fuel cell to produce an electrochemical reaction. Reformed hydrocarbon fuels frequently contain significant quantities of carbon dioxide that tends to dissolve and dissociate into the water which is provided to, and created within, the fuel stack assembly. The resultant contaminated water supply may cause the conductivity of the water to increase to a point where shunt current corrosion occurs in fuel cell coolant channels and manifolds leading to degradation of materials, accumulation of dissolved solids that can lead to obstruction of flow fields, and other detrimental effects on performance.

Additionally, in passing the cathode, vapor is picked up by the air and transported away from the fuel cell in an exhaust stream that is typically fed to a condenser for recovery of water. A substantial amount of electrolyte is also dragged out of the fuel cell because of its high operating temperature which tends to vaporize the electrolyte. For example, phosphoric acid fuel cells are typically run at 400°F (204°C), producing phosphoric acid vapors. The recovery of the vapor from the cathode exhaust stream is desirable because condensed water can then be recycled for uses including, for example, humidifying the fuel cell inlet gases, performing evaporative cooling of inlet gases, or supplying water for a steam reformer. However, if the phosphoric acid or other electrolyte is recovered with the condensate, it can become unusable for steam reforming purposes and can be corrosive to condensing systems and other fuel cell components, thus shortening system life and requiring costly component replacement.

Therefore, most fuel cell power plants employ a water purification system to filter out both carbon dioxide and electrolyte from condensate to render it suitable for reuse. However, mechanisms for purifying water, such as filters and degasification columns, can be costly and troublesome to replace, and can add considerable weight and size to the fuel cell power plant. SUMMARY

A system and method includes condensing exhaust vapor from a fuel cell into a condensate comprising carbonic acid and electrolyte, filtering the electrolyte from the condensate without filtering the carbonic acid, and using a steam generator to remove the unfiltered carbonic acid from the condensate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell power plant showing a system and method for treating water for use in the power plant.

FIG. 2 is a schematic diagram of a fuel cell power plant showing a system and method for treating water for use in the power plant that includes a selective condenser.

FIG. 3 is a schematic diagram of a fuel cell power plant showing a system and method including a reverse osmosis unit for treating water for use in the power plant.

FIG. 4 is a prior art schematic diagram of a fuel cell power plant showing a system and method for treating water for use in the power plant. While the above-identified drawing figures set forth several embodiments of the invention, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the invention by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the invention. The figures may not be drawn to scale. Like reference numbers have been used throughout the figures to denote like parts.

DETAILED DESCRIPTION

Disclosed herein is a system and method for treating water in a fuel cell power plant that provides cost savings and lower maintenance, as well as package volume reduction when compared with prior art systems. The system and method includes filtering electrolyte from exhaust condensate and contaminated water without filtering carbon dioxide, allowing the carbon dioxide to enter the thermal management subsystem of the power plant, and using a steam generator to remove the carbon dioxide from the contaminated water. By using the steam generator for carbon dioxide removal, costly replacement of anion exchange resins and mixed ion exchange resins is eliminated as these exchange resins require frequent replacement due to the higher percentage of carbon dioxide than electrolyte in contaminated power plant water. Furthermore, package volume is reduced by eliminating the multiple exchange resins for a single, more compact electrolyte-specific filter, and by eliminating the need for a voluminous degasifier column and associated plumbing and valves. Further advantages will become apparent with the following disclosure.

FIG. 1 is a schematic diagram of a fuel cell power plant 10 including reformer 12; burner 14; fuel cell 16 having anode 18, cathode 20, and coolant channels 22; condenser 24; water filters 26 and 28; tank 30 having air filter 32; check valve 34; water filters 36, 37 and 38; flow restrictor 40; steam generator 42, mixed ion exchange resin 43; coolant loop 44; and blowdown 45. Further shown are air source Al and A2; air A3 and A4; fuel source Fl; fuel F2; water source Wl ; water W2, W3, W4, W5, W6, W7; blowers Bl and B2; reformate Rl and R2; exhaust El, E2, and E3; condensate Cl ; valves Vl and V2; pumps Pl and P2; and steam Sl and S2. Accumulated water is designated as hatched markings in tank 30 and steam generator 42. Furthermore, only one fuel cell 16 is shown for the sake of clarity, however, it is understood that multiple fuel cells 16 may be used in a stacked arrangement to form a cell stack assembly with various fuel, oxidant, and coolant channels and manifolds. During operation of fuel cell power plant 10, fuel F2 is provided from a hydrocarbon fuel source Fl to reformer 12 along with steam S2 to generate a hydrogen- rich reformate Rl . Reformate Rl is fed to anode 18 of fuel cell 16, and residual unreacted reformate R2 is directed to burner 14 for combustion. To assist in combusting reformate R2, blower Bl provides air A3 to burner 14 from air source Al . Burner 14 is shown thermally integrated with reformer 12 to assist in raising temperatures in reformer 12 to an adequate level for steam reformation to take place. Exhaust El is directed away from burner 14, and contains carbon dioxide from sources including carbon dioxide liberated from fuel F2 during the reformation process, as well as that existing in ambient air introduced from air source Al . Exhaust El will also contain some water vapor that was introduced by steam S2 in reformer 12. Exhaust El is directed to condenser 24 for recovery of the vapor, as is described in more detail below.

While reformate Rl is being provided to anode 18, blower B2 draws air A4 containing oxidant (e.g., oxygen) from air source A2, and provides it to cathode 20 of fuel cell 16. Exhaust E2 is directed away from fuel cell 16 to condenser 24, and will contain vapor from sources including product water generated by the electrochemical reaction of fuel cell 16, in addition to electrolyte that has been vaporized by the high operating temperatures of fuel cell 16.

Vapor present in exhaust El and exhaust E2 is recovered with condenser 24 to increase efficiency and operability of fuel cell power plant 10 by reducing the need for adding new water to the system from water source W. Uncondensed vapors and gases are directed away from condenser 24 in exhaust E3, while condensed vapors are directed as condensate Cl to tank 30 for accumulation and storage in water W2. Water present in fuel cell power plant 10 must be of sufficient quality to prevent corrosion, scaling, and other detrimental effects on system components. If new water from water source Wl is to be added to the system, valve Vl may be opened at a customer water interface to allow the new water to enter tank 30. However, because the quality of the water at water source Wl may be unknown or variable, filters 26 and 28 are provided. Water filter 26 may comprise activated carbon, and water filter 28 may be a demineralizing ion- exchange resin, both commercially available for standard water purification systems. FIG. 1 further shows air filter 32 on tank 30, which is provided so that oxygen may enter water W2 and prevent magnetite corrosion of components in the thermal management system. Prior art systems, such as that described with reference to FIG. 4, added oxygen to the system via a degasification column. When vapor contained in exhaust El and exhaust E2 condenses in condenser 24 to form condensate Cl, some of the carbon dioxide will hydrolyze in condensate Cl to form carbonic acid. However, carbon dioxide will not be very soluble in the hot water condensate present within condenser 24, and therefore the majority of it is exhausted away as gas in exhaust E3 into ambient air, for example. Condenser 24 further functions to condense electrolyte present in exhaust E2 to prevent it from being released into the environment via exhaust E3. Consequently, condensate Cl comprises condensed water, electrolyte and carbonic acid that is directed to water W2 accumulated in tank 30. In the case of a phosphoric acid fuel cell, the electrolyte will be phosphoric acid, thus making condensate Cl acidic and potentially corrosive to downstream components in the thermal management subsystem of fuel cell power plant 10, including coolant channels 22, coolant loop 44, and steam generator 42. Furthermore, excessive levels of carbonic acid can increase the conductivity of water and potentially cause unwanted shunt currents in coolant channels 22 if not properly reduced to safe levels.

FIG. 1 shows a fuel cell power plant 10 having a water treatment system and method for reducing electrolyte and carbonic acid in water to suitable levels, allowing the water to be used for purposes including providing coolant for cooling channels 22, as well as for providing steam Sl and S2 for fuel reformation in reformer 12. Valve V2 is used to control how much contaminated water W2 is allowed to be drawn by pump Pl and through filters 36 and 38 prior to passing into coolant loop 44 of the thermal management subsystem via pump P2. Check valve 34 prevents water from backing up into pump Pl, valve V2 and tank 30 when the pump is not operating, for example, and to maintain an elevated water pressure to force water W2 to travel through filters 36 and 38 to pump P2. Flow restrictor 40 is used to allow a small percentage of water W4 to recycle back into tank 30, and to help keep water pressure boosted for reaching transition pump P2 for the thermal management subsystem.

Filter 36 is used to selectively remove electrolyte from water W2 while allowing carbonic acid to pass through. A compound suitable for selective removal of electrolyte, such as phosphoric acid, is ferric oxide. Ferric oxide may be purchased from Severn Trent P.L.C. in granulated form, and may be contained in a fiberglass reinforced plastic container, for example. Due to the low levels of electrolyte vaporized in cathode 20 and subsequently condensed in condenser 24, carbonic acid will be much more prevalent than electrolyte in contaminated water W2 by as much as 98% or more carbonic acid relative to electrolyte by mass. Because ferric oxide filter 36 media will selectively remove electrolyte and not carbonic acid, the need to replace the media is significantly reduced when compared to media that removes both contaminants, for example, anion exchange resins (described with reference to FIG. 4).

After passing through filter 36, water W3 will contain some solubilized iron leached from the ferric oxide, and may also contain particulates including iron oxide or iron phosphate precipitate. A mechanical filter 37 is provided to filter out particulates and precipitated compounds, while filter 38 is provided to remove solubilized iron from water W3. Filter 38 may be a cation exchange resin, for example. By removing cations such as iron from water W3, scaling of downstream components is reduced.

Water W4 containing carbonic acid is then directed into coolant loop 44 of the thermal management subsystem by pump P2, with some of water W4 being redirected into tank 30 through flow restrictor 40 as discussed previously. Water W4 is directed through coolant channels 22 of fuel cell 16 where it will absorb heat. Heated water W5 is then directed to steam generator 42 and accumulates as water W6. Because of the high operating temperatures of fuel cell 16, and consequently the high temperature of water W5 when it enters steam generator 42, steam Sl will rise from water W6, and most of the hydrolyzed carbon dioxide (i.e., carbonic acid) will gasify and escape with steam Sl, rather than staying hydrolyzed in water W6. Thus, steam generator 42 removes carbon dioxide from water W6, leaving behind water W7 to recycle back into coolant loop 44 containing levels of carbonic acid low enough that when mixed with water W4 to form water W5, will not cause harmful shunt current corrosion in coolant channels 22. The rate of steam S2 generation is solely dependent on the amount of power (i.e., current) the customer demands from the fuel cell power plant. The rate of feedwater W4 into loop 44 is also primarily dependent on the amount of power the customer demands. Hence there is almost a balance of carbonic acid (carbon dioxide) entering via water W4 and leaving with steam S2 in loop 44. This combined with the fact that thermodynamic equilibrium drives most carbonic acid in steam generator 42 to gas phase carbon dioxide in steam Sl and S2 assures that there is no appreciable carbon dioxide build-up in loop 44. The only carbon dioxide present in loop 44 is that introduced by carbon dioxide present in water W4. But this concentration is small since it is proportioned to water W4 / (W4 + W7) in water stream W5 and since water flow W7 is much greater than the flow of water W4.

Carbon dioxide then travels with steam S2 to reformer 12, where it has no detrimental effect on reformation of fuel F2 or on anode 18 of fuel cell 16. The carbon dioxide eventually travels to burner 14 and combines with carbon dioxide produced by combustion of residual reformate R2. This carbon dioxide then enters condenser 24 via exhaust El, and as described previously, will be predominantly exhausted from fuel cell power plant 10 via exhaust E3 rather than condensed and put back into the system.

A mixed ion exchange resin 43 may also be included in coolant loop 44 to filter out any residual ionic species not specifically removed by filter 36, and is positioned just after steam generator 42 so that carbonic acid does not clog resin 43 and require frequent and costly replacement. Examples of ionic species may include trace chloride and sulfide ions introduced into the system via blower Bl and/or B2. A blowdown 45 transports a small fraction of water from coolant loop 44, at less then 10% for example, back to water tank 30, thereby removing any residual suspended solids generated in the coolant loop 44 and not removed by mixed ion exchange resin 43. Examples of suspended solids include iron phosphate from stainless steel corrosion, for example, that may take place in condenser 24.

FIG. 2 is a schematic diagram of fuel cell power plant 1OA including condenser 24A and condensate C2, C3, and C4. Condenser 24 A operates to separately condense electrolyte vapor into condensate C2 and water vapor into condensate C4. This may be accomplished, for example, based on the principle that electrolyte, such as phosphoric acid, has a higher dew point than water and therefore condenses before water. Condensate C2 containing predominantly phosphoric acid can then be drawn from a hotter region of condenser 24A closer to exhaust El and E2, while condensate C4 can be drawn from a cooler region further away from exhaust El and E2 and closer to the dew point of water. Another example of a condenser 24A capable of separately condensing electrolyte and water utilizes a selective porous membrane to force electrolyte to condense on one side of the membrane and water on the other side. By using condenser 24A capable of selectively condensing electrolyte vapor from water vapor, efficiency is improved as condensate C2 consisting predominantly of electrolyte will comprise only a small fraction of total exhaust El and exhaust E2 condensate, and will be directly routed to filter 36 in the absence of a large volume of condensed water. In the case of filter 36 comprising ferric oxide, less water will pass through filter 36 and therefore less iron will be solubilized. This will consequently lead to a lower demand on filter 38 to remove solubilized iron and result in lower filter maintenance for filter 38 when compared to the system and method described with reference to FIG. 1. Additionally, filter 36 may have a very small and constant leak rate of electrolyte, such as phosphoric acid, at the exit of filter 36. The advantage of the configuration shown in FIG. 2 is that less flow of electrolyte will travel through filter 36, thus the total amount of leakage will be lower.

After electrolyte contained in condensate C2 is removed by filter 36, condensate C3 comprising water and carbonic acid is directed to tank 30 and into water W2. Furthermore, condensate C4 comprising primarily water and some carbonic acid is also directed to tank 30 and into water W2. Water W2 is then directed through valve V2, pump Pl , and check valve 34 to reach filter 38, which may be a cation filter such as a cation exchange resin. Water W4 containing carbonic acid is then directed to coolant loop 44 of the thermal management system, where the carbonic acid is removed in the manner described with reference to FIG. 1. FIG. 3 is a schematic diagram of a fuel cell power plant 1OB including reverse osmosis filter 46 and receptacle 48. Instead of using filter 36 as described with reference to FIG. 1 and FIG. 2, reverse osmosis filter 46 may be used to selectively remove electrolyte from water W2 without removing carbonic acid. Electrolyte, such as phosphoric acid, is acidic and causes carbon dioxide present in water W2 to stay hydrolyzed as carbonic acid which cannot be rejected by a reverse osmosis filter 46 membrane. Typical reverse osmosis units have a high rejection rate, as much as 50% of input fluid, for example. To reduce the rejection volume such that only electrolyte is rejected into receptacle 48 and not a large volume of water, the reverse osmosis unit is run at a high cycle rate. A high cycle rate can reduce the volume of rejected fluid down to around 2% of the total water W2 flow, for example. This in turn eliminates the need for a designated drain to be provided for rejected water containing electrolyte. Furthermore, the presence of electrolyte such as phosphoric acid in water W2 makes it acidic (pH between about 3 to 5.5) and prevents build-up of divalent and trivalent cations that can cause hardness, scaling and damage to the reverse osmosis filter 46 membrane under high cycle rate conditions. Once electrolyte from water W2 is rejected into receptacle 48, it may be disposed of safely, for example, in an acid neutralizing compound held within receptacle 48. Additionally, residual ions and particulates transported from coolant loop 44 through blowdown 45 to tank 30 will be removed by reverse osmosis filter 46 and collected in receptacle 48, thus eliminating the need for mixed ion exchange resin 43 on coolant loop 44.

After filtration with reverse osmosis filter 46, water W4 containing carbonic acid is directed into coolant loop 44 of the thermal management system and is removed in the manner described with reference to FIG. 1. FIG. 4 is a schematic diagram of a fuel cell power plant 1 OC, showing a system and method for treating water according to the prior art that includes degasifier column 50, condensate C5, tank water loop 52, ejector 54, mixed ion exchange resin filters 56A and 56B, anion exchange resin filters 58A and 58B, and water W8. Condensate Cl is directed through degasifier column 50 for removal of carbon dioxide, and leaves as condensate C5 to ejector 54. Pump Pl pushes water W2 from tank 30 through ejector 54 in tank water loop 52 to create a negative pressure for drawing condensate Cl through degasifier column 50 and condensate C5 into tank water loop 52. Pump Pl further draws water W2 through valve 2 and check valve 34 and pushes it through mixed ion exchange resin 56A, anion exchange resin 58A, anion exchange resin 58B, and mixed ion exchange resin 56B. Anion exchange resin 58A and 58B filter both carbonic acid and electrolyte such as phosphoric acid from water W2, and mixed ion exchange resins contain both anion and cation selective resins for filtering electrolyte and carbonic acid, in addition to divalent and trivalent cations. Water W8, comprising carbonic acid, is then allowed to enter coolant loop 44 of the thermal management system. The system and method described with reference to FIGS. 1-3 represents an improvement over the prior art system described above with reference to FIG. 4 in terms of cost, efficiency, and package volume, among other advantages. Despite the use of degasifier column 50, water W2 will still contain a much larger quantity of carbon dioxide in the form of carbonic acid than electrolyte, consequently requiring more frequent replacement of the costly anion exchange resin filters 58A and 58B, as well as the mixed ion exchange resin filters 56A and 56B since they contain anion exchange resin as well. Each ion exchange resin is shown in duplicate because if anion exchange resins 58A and 58B were combined into one fiberglass reinforced plastic container, for example, the weight of the container containing a sufficient quantity of resin can by high enough to require a mechanical lifting device to install the container into fuel cell power plant 1OC. By using electrolyte selective filters and allowing carbon dioxide in the form of carbonic acid to reach the thermal management system where it can be removed via operation of steam generator, costly replacement and need for exchange resins 56A, 56B, 58A and 58B is eliminated. Although the system and method described with reference to FIG. 1 and FIG. 2 shows cation exchange resin 38, this resin is much cheaper and will need less frequent replacement than resin filters 56A, 56B, 58A and 58B.

Furthermore, electrolyte selective filters, such as filter 36 described with reference to FIG. 1 and FIG. 2, or reverse osmosis filter 46 described with reference to FIG. 3, occupy a much smaller overall space than multiple exchange resins. Additionally, degasifier column 50 occupies a large space requirement (for example, around 8 feet in length), and thus the system and method described with reference to FIGS. 1-3 represents additional space and cost savings by eliminating the need for degasifier column 50 as well as tank water loop 52, ejector 54, and associated plumbing. Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. A method comprising: exhausting vapor comprising carbon dioxide and electrolyte from a fuel cell; condensing the vapor to produce condensate comprising the carbon dioxide and the electrolyte; filtering the electrolyte from the condensate without filtering the carbon dioxide; and removing the carbon dioxide from the condensate using a steam generator.

2. The method of claim 1, wherein the electrolyte is phosphoric acid.

3. The method of claim 2, further comprising using ferric oxide to filter the electrolyte from the condensate without filtering the carbon dioxide.

4. The method of claim 3, wherein the ferric oxide leaches solubilized iron into the condensate, and further comprising filtering the solubilized iron from the condensate.

5. The method of claim 4, wherein a cation exchange resin is used to filter the solubilized iron from the condensate.

6. The method of claim 1, wherein a reverse osmosis filter is used to filter the electrolyte from the condensate without filtering the carbon dioxide.

7. The method of claim 6, further comprising directing filtered electrolyte from the reverse osmosis filter to a receptacle for acid neutralization of the electrolyte.

8. The method of claim 1, further comprising supplying steam from the steam generator along with removed carbon dioxide to a reformer.

9. The method of claim 1, further comprising allowing the carbon dioxide to enter a coolant channel of the fuel cell prior to removing the carbon dioxide from the condensate using the steam generator.

10. The method of claim 1, wherein condensing the vapor to produce the condensate comprising the carbon dioxide and the electrolyte further comprises using a condenser to produce a first condensate comprising primarily the electrolyte, and a second condensate comprising primarily water.

11. A system comprising: a fuel cell for producing an exhaust vapor comprising carbon dioxide and electrolyte; a condenser for condensing the exhaust vapor into condensate comprising the carbon dioxide and the electrolyte; a filter selective for removal of the electrolyte from the condensate but not the carbon dioxide; and a steam generator for removing the carbon dioxide from the condensate.

12. The system of claim 11, wherein the electrolyte is phosphoric acid.

13. The system of claim 12, wherein the filter comprises ferric oxide.

14. The system of claim 13, further comprising a cation filter for removal of iron leached into the condensate from the ferric oxide.

15. The system of claim 11 , wherein the filter is a reverse osmosis filter.

16. The system of claim 15, further comprising a receptacle for receiving electrolyte removed by the reverse osmosis filter.

17. The system of claim 16, wherein the receptacle comprises an acid neutralization compound.

18. The system of claim 11 further comprising a reformer for producing reformate for feeding to the fuel cell, and wherein steam from the steam generator is supplied with removed carbon dioxide to the reformer.

19. The system of claim 11, wherein the fuel cell further comprising a coolant channel, and wherein carbon dioxide not removed by the filter is allowed to enter the coolant channel.

20. The system of claim 11 , wherein the condenser condenses the exhaust vapor into a first condensate comprising primarily electrolyte, and a second condensate comprising primarily water.