WO1998039599A1

WO1998039599A1 - Staged supplemental firing of high vanadium content fuel oils

Info

Publication number: WO1998039599A1
Application number: PCT/US1998/004115
Authority: WO
Inventors: Michael Scott Briesch; Joseph A. Rumancik
Original assignee: Siemens Westinghouse Power Corporation
Priority date: 1997-03-06
Filing date: 1998-03-03
Publication date: 1998-09-11

Abstract

A heat exchanger for burning fuel oil having vanadium and for preventing melting of vanadium complexes formed from burning fuel oil includes a first (28) and a second (30) stage. Contained within each stage is a duct burner (18) that heats gas flowing through the heat exchanger by burning fuel having vanadium and a cooling component that cools gas heated by the duct burner. Within each stage the temperature of the gas is maintained about below the melting point of vanadium complexes. Consequently, the vanadium complexes do not achieve their corrosive molten state. This invention also includes a method for burning fuel oil containing vanadium and preventing the melting of vanadium complexes by heating and cooling the gas in a plurality of stages.

Description

STAGED SUPPLEMENTAL FIRING OF HIGH VANADIUM CONTENT FUEL OILS

BACKGROUND OF THE INVENTION This invention relates to a high efficiency, low maintenance heat exchanger of the type that burns fuel containing vanadium, and particularly to such a heat exchanger that prevents melting of vanadium complexes therein.

Power plants typically employ heat exchangers that use either fuel oil or crude oil (fuel) with a relatively high vanadium content. One heat exchanger of this type is known as a "heat recovery steam generator" or a "HRSG." A heat recovery steam generator typically intakes gas exhausted from a gas turbine and heats it by burning fuel oil. After being heated, the gas then flows through the steam generator and transfers its heat to steam or water.

In order to maximize the amount of energy that can be extracted from the fuel, the fuel is typically heated to relatively high temperatures. When the fuel is heated to these relatively high temperatures, the vanadium in the fuel forms combustion products. Additionally, at these temperatures these combustion products melt . In their molten state, these vanadium byproducts are very corrosive and may corrode the metal components of a heat exchanger and its associated systems. An example of a highly corrosive combustion product of vanadium is vanadium pentoxide .

Traditionally, the amount of corrosion due to vanadium combustion products was minimized by adding a chemical inhibitor, for instance magnesium, to the fuel. Chemical inhibitors prevent the formation of the corrosive vanadium products by producing an alternative compound. For example, magnesium reacts with vanadium to produce magnesium vanadate in lieu of the corrosive vanadium pentoxide . Magnesium vanadate is also corrosive in its molten state. However, since magnesium vanadate has a relatively high melting point, about 2175°F, the fuel can be burned at much higher temperatures without it melting. Thus, by forming vanadium complexes with higher melting points, higher temperatures can be achieved without melting the complexes and the fuel cycle is more efficient.

Although inhibitors prevent the formation of vanadium combustion products with lower melting points, they have a significant disadvantage. Specifically, the inhibitor may react with vanadium to form a substance, such as magnesium vanadate, that has undesirable properties. For example, magnesium vanadate tends to adhere to and form thick deposits on surfaces of power plant components. By fouling the heat transfer surfaces of the system components, the heat transferred in the system can be reduced. This may result in a decrease in the efficiency of the plant. To combat this problem, extra maintenance is required. More particularly, components of the system may have to be cleaned, as often as weekly, to remove the deposits. This additional maintenance is costly.

It is desirable to provide a high efficiency heat exchanger that can employ fuels containing vanadium, but which prevents melting of vanadium complexes and fouling of the heat exchanger. The present invention satisfies these goals. SUMMARY OF THE INVENTION

According to the present invention, a heat exchanger for burning fuel oil having vanadium and for preventing melting of vanadium complexes formed from burning fuel oil, includes a first and a second stage. Contained within each stage is a duct burner that heats gas flowing through the heat exchanger by burning the fuel, and a cooling component that cools gas heated by the duct burner. Within each stage the temperature of the gas is maintained below the melting point of the vanadium complexes. Consequently, the vanadium complexes do not achieve their corrosive molten state. Although the temperature is maintained below the melting point of the vanadium complexes, approximately the same amount of heat is extracted from the gas, as in a single stage heat exchanger, because a plurality of stages are employed within the heat exchanger.

The heat exchanger described above may be employed with a gas turbine having a compressor, a combustor and a turbine section. Such a turbine exhausts a hot gas that flows to the heat exchanger and through the first and the second stages .

This invention also encompasses a method of preventing melting of vanadium complexes while burning fuel containing vanadium. Such a method can be employed in a heat exchanger or similar apparatus that employs a plurality of duct burners and cooling components as described above. According to this method, fuel is burned in a duct burner and gas is heated by the burning of the fuel as it flows through the burner. The gas is then cooled as it flows through a cooling component. After it has been cooled, the gas is then reheated in another duct burner and again cooled in another cooling component. As mentioned above, by heating the gas in several stages the temperature of the gas, according to this method, can be maintained below the melting point of vanadium complexes .

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of a combined-cycle power plant according to the prior art;

Figure 2 is a diagrammatical view of a heat exchanger according to a preferred embodiment of the present invention; and

Figure 3 is an isometric view of a duct burner according to the prior art employed in the preferred embodiment of this invention depicted in Figure 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to Figure 1, there is shown a combined-cycle power plant comprising a heat exchanger 12 and a gas turbine 10. As illustrated, the gas turbine 10 includes a compressor 13, a combustor 14 and a turbine section 15. As is well known in the art, the compressor 13 produces pressurized air. Most of this air mixes with fuel in the combustor 14 and burns to produce a hot gas. From the combustor 14 the hot gas flows to the turbine section 15 where the energy of the hot gas is converted into useful work by causing the rotation of a rotor 17. The rotation of the rotor 17 drives a load 19 such as an electrical generator.

Exhaust gas 21 from the turbine section 15 is directed to the heat exchanger 12. In the heat exchanger 12 the exhaust gas 21 is then reheated and the energy in the gas 21 is used to produce a high energy steam 32. This steam 32 is then used to drive a steam turbine 34 that also drives a load 19 such as an electrical generator. As mentioned above, the heat exchanger employed in such a system is commonly referred to as a heat recovery steam generator.

The combined-cycle power plant described above is well known and the invention described herein may be employed in a power plant of this type . As depicted in Figure 2 , according to the present invention a heat exchanger 12 that can burn vanadium fuel without exceeding the melting point of extremely corrosive vanadium complexes includes an inlet 16, a plurality of heat transfer stages 28, 29 and an exhaust stack 26. Within each heat transfer stage 28 is a duct burner 18 of known type and a cooling component 30 such as a superheater, a boiler or an economizer. In the most preferred embodiment of this invention, the heat exchanger 12 employs two heat transfer stages 28, 29. One of the stages 28 includes a superheater while the other 29 has a boiler and an economizer. Depicted in Figure 3 is a typical duct burner 18. This invention does not relate to the details of the duct burner 18, which are well known, but rather to the arrangement and use of the duct burner 18 in the heat exchanger 12. As shown a duct burner 18 generally includes a plurality of small burners 36 spaced across the duct. The burners 36 are spaced so that a relatively even temperature distribution can be achieved. In operation, fuel is sent into the burners 36 where it is burned and heat is transferred to the gas flowing through the heat exchanger 12. The amount of fuel sent into a duct burner 18 can be varied with a controller 50, in a conventional manner, to control the temperature achieved by burning the fuel. For instance, burning a higher amount of fuel will produce a higher temperature. Since more heat is produced in the burner 18, the gas flowing over the burner will also achieve a higher temperature. Conversely, the amount of fuel burned can be decreased, in order to heat the gas passing through the burner to a relatively lower temperature. Thus, the temperature of the gas can be controlled by varying the amount of fuel burned in the duct burner 18. The controller 50 can be either automatically or manually operated. For example, the controller 50 could measure the temperature of the gas and vary the amount of fuel inputted to the burner 18 with valves 52 or similar flow control devices in order to either raise or lower the temperature.

The cooling components 30, including the superheater, the boiler and the economizer are all well known in the art. Each of these components includes a heat transfer surface 38 and a medium flowing through their interior. In the superheater, the medium is steam; in the boiler the medium is a mixture of water and steam and in the economizer the medium is water. Each of these cooling components may be in fluid communication with each other so that water from the economizer flows to the boiler and steam from the boiler flows to the superheater. Hot gas flows across the exterior of the heat transfer surface 38 of these components and transfers heat to the medium flowing in their interior. The heated cooling medium then is exhausted and its energy is extracted to produce useful work. For example, steam produced from the superheater may be used to drive a steam turbine. 5 As depicted in Figure 2, the heat exchanger 12 has an inlet 16 and an exhaust stack 26. The inlet 16 is disposed between the turbine 10 and the first stage 28. The ^* inlet forms a receiving area which functions to collect the gas exhausted from the turbine 10 and direct it to the first stage

10 28. Situated after the last heat transfer stage, which in the most preferred embodiment is the second heat transfer stage 29, is the exhaust stack 26. The exhaust stack 26 is of known type and functions to collect the gas after passing through the second heat transfer stage 29 and exhausts it to the

15 atmosphere or to another receiving area.

In operation, the heat exchanger 12 receives an exhaust gas from a turbine 10 and the gas flows into the inlet 16. After flowing through the inlet 16, the gas flows to the first stage 28. In the first stage 28 the exhaust gas flows

20 through a duct burner 18 where heat is transferred to the exhaust gas. From the duct burner 18, the heated exhaust gas flows over the exterior of the superheater, where heat is transferred from the gas to the steam flowing in the interior of the superheater. The exhaust gas then exists the first

25 stage 28 and enters the second stage 29. In the second stage, the gas flows through another duct burner 18 and is again elevated in temperature. From the burner 18, the gas flows to the boiler and the economizer where most of its energy is transferred to either steam or water. The cooled gas then

30 flows to the exhaust stack 26.

As described above, in the prior art, a single stage heat exchanger heats the exhaust gas with a single duct burner. Since only one stage is used, the gas has to be heated to a relatively high temperature so that it can provide

35 adequate heating for the other heat exchanger components such as a superheater, an economizer and a boiler. In order to heat the gas to high temperatures, the vanadium containing fuel oil is burned at relatively high temperatures. As the fuel is burned, vanadium combustion products are formed. These combustion products melt and are corrosive in their molten state. Melting of these vanadium complexes occurs at about 1250°F.

In contrast to the prior art, in this invention, the gas is heated in two stages. Since two stages are used, the heat transferred to the gas in each stage is significantly less than if a single stage is employed. Consequently, the temperature of the gas at any point in a multi-staged heat exchanger is significantly less than in a single stage heat exchanger. More specifically, in the present invention the temperature of the gas is kept below about 1250°F, the temperature at which vanadium complexes melt . The temperature of the gas is maintained below about 1250°F by limiting the fuel inputted into each duct burner in a conventional manner as discussed above. However, about the same amount of heat is transferred from the gas in these two stages as in a single stage in a prior art heat exchanger that employs one duct burner for the entire heat exchanger.

In further detail, in this invention the gas is heated in the first stage to a temperature below about 1250°F and the gas is then cooled by transferring its heat to steam in the superheater. In the second stage, the gas is then reheated to a temperature below about 1250°F and again cooled. Thus, at no time while flowing through the heat exchanger does the temperature of the gas exceed the melting point of the vanadium complexes .

As mentioned above, in the prior art chemical inhibitors are added in order to form a combustion product that has a much higher melting point. Although this permits the gas to achieve relatively high temperatures without forming corrosive molten substances, the inhibitor causes fouling of the heat transfer surfaces. Since in this invention the requisite energy can be extracted from the gas without heating the gas to above about the melting point, 1250°F, of the vanadium combustion products, chemical inhibitors are not needed.

As described in its most preferred embodiment, the heat exchanger 12 is a heat recovery steam generator that is employed in a combined cycle power plant. Although this is the most preferred embodiment, this invention is not limited to this embodiment. For example, this invention encompasses other heat exchangers that burn fuel containing vanadium in a plurality of stages. It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims

CLAIMS :

1. A heat exchanger for burning a fuel having vanadium and for preventing melting of vanadium complexes formed from the burning of the fuel, comprising: a first stage comprising a first duct burner for heating a gas flowing through the heat exchanger and a first cooling component for cooling the gas heated by the first duct burner; and a second stage that receives the gas that has flowed through the first stage and that comprises a second duct burner for heating the gas and a second cooling component for cooling the gas heated by the second duct burner.

2. The heat exchanger of claim 1, wherein the first and the second duct burners comprise a controller for maintaining the temperature of the gas in each stage below about a melting point of vanadium complexes formed from the burning of the fuel by controlling the amount of the fuel burned in the burners.

3. The heat exchanger of claim 1, further comprising a combustion turbine in fluid communication with the first stage.

4. The heat exchanger of claim 1, wherein the heat exchanger comprises a heat recovery steam generator.

5. The heat exchanger of claim 1, wherein the first and the second duct burners comprise a controller for maintaining the temperature of the gas in each stage below about 1250┬░F by controlling the amount of the fuel burned in the burners .

6. The heat exchanger of claim 1, wherein the first cooling component comprises a superheater.

7. The heat exchanger of claim 1, wherein the second cooling component comprises a boiler.

8. The heat exchanger of claim 1, wherein each duct burner comprises a plurality of burners.

9. A system for burning fuel having vanadium and for preventing melting of vanadium complexes formed from the burning of the fuel, comprising: a gas turbine comprising a compressor, a combustor and a turbine section; and a heat exchanger in fluid communication with the gas turbine, comprising: a first stage comprising a first duct burner for heating a gas flowing through the heat exchanger and a first cooling component for cooling the gas heated by the first duct burner; and a second stage that receives the gas that has flowed through the first stage and that comprises a second duct burner for heating the gas and a second cooling component for cooling the gas heated by the second duct burner.

10. The system of claim 9, wherein the first and the second duct burners comprise a controller for maintaining the temperature of the gas in each stage below about a melting point of vanadium complexes formed from the burning of the fuel by controlling the amount of the fuel burned in the burners .

11. The system of claim 9, further comprising an exhaust stack in fluid communication with the second stage.

12. The system of claim 9, wherein the stages are contained within a heat recovery steam generator.

13. The system of claim 9, wherein the first and the second duct burners comprise a controller for maintaining the temperature of the gas in each stage below about 1250┬░F by controlling the amount of the fuel burned in the burners.

14. The system of claim 9, wherein the first cooling component comprises a superheater.

15. The system of claim 9, wherein the second cooling component comprises a boiler.

16. A method of burning fuel oil having vanadium and of preventing melting of vanadium complexes, comprising, burning fuel in a first duct burner; heating a gas by directing the gas through the burner; cooling the gas by directing it over a first cooling component; burning the fuel in a second duct burner; reheating the gas by directing the gas through the second burner; and transferring heat from the gas by directing it over a second cooling component.

17. The method of claim 16, wherein the steps of heating the gas and reheating the gas each further comprise the step of maintaining the temperature of the gas below about 1250┬░F.

18. The method of claim 16, wherein the first cooling component comprises a superheater.

19. The method of claim 16, wherein the second cooling component comprises a boiler.

20. The method of claim 16, wherein the steps of burning, heating, cooling, reheating and transferring are performed within a heat recovery steam generator.