WO2015088487A1

WO2015088487A1 - High efficiency heat exchange arrangement for an oxy-fuel combined cycle power plant

Info

Publication number: WO2015088487A1
Application number: PCT/US2013/073972
Authority: WO
Inventors: Theodore James Mallinson; Prasanth CHANDRAHASAN
Original assignee: Siemens Energy Inc
Current assignee: Siemens Energy Inc
Priority date: 2013-12-10
Filing date: 2013-12-10
Publication date: 2015-06-18
Anticipated expiration: 2016-06-10

Abstract

An oxy-fuel combined cycle plant (10) including: a HRSG (12) configured to transfer heat from exhaust (18) of a gas turbine engine to a working fluid (20); an exhaust stack (14) configured to receive the exhaust from the HRSG; and a heat exchange arrangement (30) configured to condense water vapor out from the exhaust as the exhaust flows through the exhaust stack, thereby improving the capture of latent heat energy present in the exhaust stream.

Description

HIGH EFFICIENCY HEAT EXCHANGE ARRANGEMENT FOR AN OXY-FUEL COMBINED CYCLE POWER PLANT

FIELD OF THE INVENTION

The invention relates to oxygen-fuel (oxy-fuel) gas turbine powered combined cycle power plants (CCPP). In particular, the invention relates to improved capture of latent heat energy present in the gas turbine engine's exhaust stream.

BACKGROUND OF THE INVENTION

Combined cycle plants using gas turbine engines often use a heat recovery steam generator (HRSG) to capture heat present in the exhaust from the gas turbine engine and transfer the heat to the working fluid. The working fluid is usually water and after being heated in the HRSG is used to drive at least one turbine used to drive a generator.

In conventional combined cycle plants that burn fuel and air the exhaust from the gas turbine engine that enters HRSG will be a mixture of nitrogen, carbon dioxide, unburned oxygen, and water vapor. After heat transfer occurs in the HRSG the exhaust is vented to the atmosphere. In response to a need to reduce nitrogen emissions a new type of combined cycle power plant is being developed that burns oxygen and fuel. This type of combined cycle power plant, known as an oxy-fuel combined cycle power plant, generally has a reduced amount of, or no nitrogen in the exhaust. In some instances the emissions from an oxy-fuel plant are essentially limited to carbon dioxide and water vapor. Both constituents may be considered valuable and often either or both are captured for other uses.

A HRSG removes sensible heat and latent heat from the exhaust. Sensible heat exchanged is that which brings about a change in temperature. Latent heat exchanged is that which occurs during a constant temperature process but with a change of phase. Water vapor is characterized by an anomalously high latent heat of vaporization when compared to other constituents in the exhaust. The fraction of water vapor present in the exhaust of a conventional CCPP is significantly smaller than the fraction of water vapor present in the exhaust of an oxy-fuel CCPP. Thus, while the exhaust that leaves the HRSG in a conventional plant may still be at a temperature that permits water vapor to exist, because there is so little water vapor the lost heat energy has been deemed tolerable. In contrast, the exhaust that leaves the HRSG in an oxy-fuel plant can be rich in water vapor and hence the heat worth capturing. This has conventionally been accomplished by positioning an exhaust condenser at the exit of the exhaust stack which receives the exhaust from the HRSG.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following description in view of the drawings that show:

FIG. 1 is a schematic view of a HRSG and an exemplary embodiment of a heat exchange arrangement.

FIG. 2 is a schematic view of an exemplary embodiment of coils of the heat exchange arrangement of FIG. 1 .

FIG. 3 is a schematic view of a HRSG and an alternate exemplary embodiment of a heat exchange arrangement.

FIG. 4 is a schematic view of a HRSG and another alternate exemplary embodiment of a heat exchange arrangement.

FIG. 5 is a schematic view of a HRSG and another alternate exemplary embodiment of a heat exchange arrangement.

FIG. 6 is a schematic view of a HRSG and another alternate exemplary embodiment of a heat exchange arrangement.

FIG. 7 is a table illustrating additional heat duty.

FIG. 8 is a table illustrating the effect of condensation on the overall heat transfer area of the exhaust condenser.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have recognized that the practices of conventional combined cycle power plants (CCPPs) that have so far been applied to the

development of oxy-fuel CCPPs can be improved upon to greatly increase the efficiency of the oxy-fuel CCPPs. In particular, the conventional practice of placing an exhaust condenser at the exit of an exhaust stack to capture latent heat in water vapor that remains in the exhaust at that point requires a large condenser to be elevated roughly seventy feet above the ground. The elevation is a result of a number of factors designed to extract as much energy from the exhaust as is possible. This separate heat transfer step may be required because the HRSG is often required to have a temperature profile in accordance with the requirements for the primary steam turbine. This means that there is a lowest allowable temperature within the HRSG and an inability to go below this threshold may limit the amount of heat that can be extracted from the exhaust 18 in the HRSG 12. This leaves the exhaust exiting the HRSG at a somewhat elevated temperature that permits water vapor to exist. Hence, a separate heat transfer step may be required, such as the exhaust condenser.

In oxy-fuel plants the latent heat of this remaining water vapor that has exited the HRSG, (which feeds the exhaust stack), represents a significant amount of the heat energy created by the CCPP and may even be greater than the amount of energy removed by the HRSG. For example, in a 500 MW reference CCPP the total heat present in the exhaust exiting the HRSG is about 349 MW. Out of this about 25 MW is superheat and 324 MW is latent heat. Thus, in an oxy-fuel CCPP the superheat is only about 7.7% of the total heat content of the exhaust leaving the HRSG, while the latent heat is about 92.3%. The 324 MW also represents a majority of the energy output of the 500 MW CCPP. Such an exhaust condenser may therefore have a duty of approximately 300 MW (approximately 1 ,000 MMBtu). The required exhaust condenser is thus very large, and it is difficult and expensive to lift such a large exhaust condenser to the required elevation.

In response, the inventors have recognized that a heat exchange arrangement could be disposed directly within the exhaust stack and used to capture the latent heat in the exhaust. Most exhaust stacks enclose a volume sufficiently large to permit placing a heat exchange arrangement therein with enough capacity to condense most, if not all of the water vapor. As a result, most, if not all of the latent heat present in the water vapor can be removed from the exhaust before the exhaust ever leaves the exhaust stack. If the heat exchange arrangement cannot condense all of the water vapor an exhaust condenser can still be placed at the exit of the exhaust stack to finish the condensation. Advantageously, this exhaust condenser can be substantially smaller, making is much less difficult and less expensive to install. The savings gained by eliminating or downsizing the exhaust condenser offsets the cost of the in-stack heat exchange arrangement and so a cost savings is realized. While the exemplary embodiments discussed herein include oxy-fuel turbines, the same principles can be applied to conventionally powered (air and fuel) turbines. The heat exchange arrangement may not be able to extract as much latent and sensible heat from the exhaust in the exhaust stack in a conventional CCPP. Nonetheless, some heat may be extracted and thus may be appropriate in certain circumstances.

FIG. 1 shows a portion of a combined cycle power plant 10 including a HRSG 12 and an exhaust stack 14 typically found in a conventional CCPP (not shown). In this exemplary embodiment a horizontal HRSG 12 is shown, but the concepts can be applied equally well to a vertical HRSG. The HRSG 12 includes heat exchanging coils 16 used to transfer heat present in exhaust 18 from a gas turbine engine (not shown) to a working fluid 20 used to drive at least one primary steam turbine (not shown). This working fluid 20 may be water and the heat transferred in the HRSG 12 may cause the working fluid 20 to boil and form steam that is used to drive the primary turbine. In a conventional CCPP, upon exiting the HRSG 12 the exhaust 18 enters the exhaust stack 14 and is vented to the atmosphere without any further processing. However, in the present exhaust processing arrangement 10 the exhaust 18 is further processed in the exhaust stack 14 via a heat exchange arrangement 30. The heat exchange

arrangement 30 may manifest itself in any of multiple exemplary embodiments and may condense any amount of the water vapor up to and including 100% of it. In practice it is believed that a heat exchange arrangement may be configured to condense

approximately seventy five to eighty percent of the water vapor entering the exhaust stack 14, although the inventors do not wish to be limited to this figure.

In the exemplary embodiment of FIG. 1 the heat exchange arrangement 30 includes coil arrangement 32 disposed in the exhaust stack 14. A cooling medium 34 is run through the coil arrangement 32 and draws the latent heat from the exhaust 18 in an area of enhanced heat exchange 36. The cooling medium 34 may run concurrent or counter-current to a direction of flow of the exhaust 18, depending on which is deemed more suitable for the specific CCPP. The cooling medium 34 may be working fluid 20 that is preheated before entering the HRSG 12. Such preheating may supplant any other external preheating arrangement and/or may enable additional heating flexibility throughout the CCPP. This will result in a higher log mean temperature difference (LMTD) value. The heat exchange arrangement 30 may further include one or more collection troughs 38 configured to collect condensate 40 as desired and direct the condensate 40 through a condensate outlet 42.

FIG. 2 shows an exemplary embodiment of an arrangement for the coil arrangement 32 disposed in the exhaust stack 14. Coils 44 of the coil arrangement 32 traverse the exhaust stack 14 from side to side and may change elevation into and out of the page from a heat exchange coil inlet 50 to a heat exchange coil outlet 52. In a counter current arrangement, if the exhaust 18 is traveling out of the page (toward the reader) then the heat exchange coil inlet 50 may be at a higher elevation (out of the page) than the heat exchange coil outlet 52 so the cooling medium 34 flows generally counter/against a direction of flow of the exhaust 18. In a concurrent arrangement, if the exhaust 18 is traveling out of the page (toward the reader) then the heat exchange coil inlet 50 may be at a lower elevation (into the page) than the heat exchange coil outlet 52 so the cooling medium 34 flows concurrent generally with a direction of flow of the exhaust 18. This staggered arrangement creates turbulence and this turbulence increases heat transfer efficiency.

FIG. 3 shows an alternate exemplary embodiment of the exhaust processing arrangement 10 that includes an exhaust condenser 60 disposed at an exhaust stack outlet 62. The exhaust condenser 60 may be used to supplement the heat exchange arrangement 30 when the heat exchange arrangement 30 within the exhaust stack 14 is unable to condense all of the water vapor in the exhaust 18. In an exemplary embodiment the heat exchange arrangement 30 and the exhaust condenser 60 together would condense up to 100% of the water vapor.

FIG. 4 shows an alternate exemplary embodiment of the exhaust processing arrangement 10 that includes the coil arrangement 32 in the area of enhanced heat exchange 36 that is part of a discrete, organic Rankine cycle 70. The use of low-boiling Rankine fluid enables heat extraction at lower temperatures which are typical for exhaust 18 exiting the HRSG 12. In this exemplary embodiment the organic Rankine fluid is boiled and used to drive a second turbine 72 which may, in turn, drive a second generator 74. After exiting the second turbine 72 the organic Rankine fluid is condensed in a condenser (not shown) and returned to the heat exchange coil inlet 50.

FIG. 5 shows an alternate exemplary embodiment of the exhaust processing arrangement 10 that includes a spraying arrangement 80 having injectors 82 configured to inject a medium 84 into the exhaust 18 in the exhaust stack 14. The medium 84 absorbs the latent heat from the exhaust 18 while remaining a liquid. This occurs without a wall separating the two, and hence the heat transfer can be more efficient. The condensate 40 and the medium 84 are collected by the collection trough 38 which is sized and configured to handle the combined volume of the two. The condensate 40 and the medium 84 exit the exhaust stack 14 where the extracted heat may be further utilized.

The spraying arrangement 80 may be locally tailored to maximize heat transfer efficiency. For example, the injectors 82 in a first row 86 may be configured to inject relatively larger droplets of the medium 84. The larger droplet may be able to absorb a relatively large amount of heat present in the relatively warmer exhaust 18 entering the spraying arrangement 80 without evaporating. This ensures that the captured heat remains captured and not vented to the atmosphere. The injectors 82 in a second row 88 may be configured to inject medium sized droplets, and the injectors 82 in a third row 90 may be configured to inject relatively smaller droplets. This arrangement considers that the exhaust 18 cools as it travels through the spraying arrangement 80. This arrangement may be altered when consideration is given to other aspects including, for example, the amount of time the droplets are in thermal communication with the exhaust 18. For example, depending on the configuration of the spraying arrangement 80, droplets injected by the injectors 82 of the third row 90 may spend more time in the exhaust 18 and thus be prone to absorb more heat, where the opposite may be true for the droplets injected by injectors 82 of the first row 86. Consequently, any amount of localization may be employed to optimize the heat transfer characteristics of the spraying arrangement 80.

FIG. 6 shows an alternate exemplary embodiment of the exhaust processing arrangement 10 that includes both the coil arrangement 32 and the spraying arrangement 80. Any combination of these arrangements and any number of each arrangement of any size may be used as necessary to accommodate the specific requirements encountered and desired.

The cases below calculate the advantages in terms of the increased thermal efficiency of the CCPP and the reduced exhaust condenser size.

CASE 1

LATENT HEAT CALCULATION

Turbine Exhaust Conditions (Entering HRSG)

Flow rate: 158 kg/s

Temperature: 555 °Celsius

Pressure: 150 millibar gauge

0.15 barg

1 .163 bara

16.87 psia

Fraction of water: 85 percent

HRSG Exhaust Conditions (Exiting HRSG Steam Coils)

Flow rate: 158 kg/s

348.3 Ib/s

Temperature: 97 °Celsius

Pressure: 1 .03 bar gauge

14.94 psia

Fraction of water: 85 percent

Properties of saturated steam at 97°C

Saturated liquid, h_f: 65

Saturated vapor, h_g: 1 103.5

Latent heat of vaporization, h_fg: 1038.5 Btu/lb

Total latent heat content: 307480 Btu/second (Please check

1 106.9 MMBtu/hr 324.3 MW

(If the entire latent heat is recovered in the LP steam cycle it will add 324 MW to the LP steam cycle.) Thermal Efficiency Calculation

Flow rate of fuel gas: 45 MMSCFD

LHV of pure methane: 915.5 Btu/ft³

Total heat content of fuel gas: 41 198 MMBtu/day

1717 MMBtu/hour (M above, MM here?)

503 MW

Oxy-fuel plant thermal efficiency: 28 percent

Heat recovered: 481 MMBtu/hour

141 MW

The table of FIG. 7 illustrates the additional heat duty.

CASE 2

LATENT HEAT CALCULATION

Let the area required for the heat exchanger to be A

Heat transfer area: A ft²

Overall heat transfer area: U W/m²C

Log mean temp, difference: ΔΤ °C

Heat transferred: Q

Qi : UAi ΔΤ

With the additional heat transferred to HRSG:

Q₂: UA₂ ΔΤ

Qi/ Q₂: (UAi ΔΤ) / (UA₂ ΔΤ)

Q1/ Q2: A1/ A2

Original duty of exhaust cond.: 324 MW The table of FIG. 8 illustrates the effect of condensation on the overall heat transfer area of the exhaust condenser. It can be seen that each additional 10% of heat recovered from the HRSG exhaust results in a reduction of overall heat transfer area of the exhaust condenser by 10%.

From the foregoing it is evident that the present inventors have devised a way to vastly improve the efficiency of an oxy-fuel CCPP at a lower capital cost and in a space- saving manner. Thus, this represents an improvement in the art.

While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.

Claims

The invention claimed is:

1 . An oxy-fuel combined cycle plant comprising:

a HRSG configured to transfer heat from exhaust of a gas turbine engine to a working fluid;

an exhaust stack configured to receive the exhaust from the HRSG; and a heat exchange arrangement configured to condense water vapor out from the exhaust as the exhaust flows through the exhaust stack.

2. The oxy-fuel combined cycle plant of claim 1 , wherein the heat exchange arrangement is configured to condense at least 75 percent the water vapor within the exhaust stack.

3. The oxy-fuel combined cycle plant of claim 1 , wherein the heat exchange arrangement further comprises heat exchange coils disposed in the exhaust stack and configured to capture heat released through the condensation.

4. The oxy-fuel combined cycle plant of claim 3, wherein the heat exchanger coils preheat the working fluid that the HRSG heats.

5. The oxy-fuel combined cycle plant of claim 3, wherein the heat exchanger coils heat a Rankine fluid disposed in a circuit separate from a circuit for working steam that flows through the HRSG.

6. The oxy-fuel combined cycle plant of claim 5, wherein the Rankine fluid is configured to boil via the transfer of the heat, and wherein the Rankine fluid drives a separate turbine.

7. The oxy-fuel combined cycle plant of claim 1 , wherein the heat exchange arrangement is configured to heat a medium sprayed into the exhaust in the exhaust stack with heat released though the condensation, and wherein the medium remains a liquid when heated.

8. The oxy-fuel combined cycle plant of claim 7, wherein the heat exchange arrangement comprises injectors configured to inject the sprayed medium in droplet form, and wherein at least one injector injects droplets of a different size than another injector, and wherein the droplet sizes vary in accord with an amount of heat energy to be absorbed.

9. The oxy-fuel combined cycle plant of claim 1 , further comprising heat exchanger coils and a medium spraying arrangement, each configured to capture heat released through the condensation.

10. The oxy-fuel combined cycle plant of claim 1 , further comprising a condenser configured to condense any remaining water vapor present in the exhaust flowing from the exhaust stack.

1 1 . The oxy-fuel combined cycle plant of claim 1 , wherein the heat exchange is configured to remove more heat energy from the exhaust than does the HRSG.

12. An oxy-fuel combined cycle plant comprising:

an exhaust stack configured to receive exhaust from a HRSG;

a heat exchange arrangement configured to remove latent heat energy from the exhaust within the exhaust stack.

13. The oxy-fuel combined cycle plant of claim 12, wherein the heat exchange arrangement further comprises heat exchanger coils disposed in the exhaust stack that are configured to capture the removed latent heat energy to preheat a same working fluid the HRSG heats.

14. The oxy-fuel combined cycle plant of claim 12, wherein the heat exchange arrangement further comprises heat exchange coils disposed in the exhaust stack that are configured to capture the removed latent heat energy to preheat boiler feed water.

15. The oxy-fuel combined cycle plant of claim 12, wherein the heat exchange arrangement further comprises heat exchanger coils disposed in the exhaust stack that are configured to capture the removed latent heat energy to heat a Rankine fluid that drives a second turbine.

16. The oxy-fuel combined cycle plant of claim 12, wherein the heat exchange arrangement comprises a medium sprayer that sprays a medium, wherein the medium is heated via the removed latent heat energy and remains a liquid after being heated.

17. The oxy-fuel combined cycle plant of claim 12, wherein the heat exchange arrangement removes more heat energy from the exhaust than does the HRSG.

18. The oxy-fuel combined cycle plant of claim 12, wherein the heat exchange arrangement is configured to condense at least 75 percent the water vapor within the exhaust stack.