HK1001780A - Supplying heat to an externally fired power system - Google Patents

Description

Supplying heat to external combustion type power system

The invention relates to heat supply for an external combustion type power system.

In a direct-fired power plant, preheated combustion air, fuel, such as pulverized coal, is supplied to a combustion chamber where it is burned. The pipe surrounding the flame zone contains a working fluid (e.g., water) that is heated to boiling and then delivered to a power system (e.g., a steam turbine) for conversion to a useful form of energy, such as electrical energy. Us patent 5450821 describes a multi-stage combustion system employing separate combustion chambers and heat exchangers, the temperature of the heat released by each stage being controlled to match the thermal performance of the working fluid to maintain the temperature below that at which NOx gases are produced.

In general, the invention features a multi-stage system having two or more combustion zones for supplying heat to an externally fired power system. Each combustion zone has a respective heat exchanger that delivers a respective working fluid stream from an externally fired power system. Each combustion zone receives a portion of the total amount of fuel and the amount of fuel and air supplied to each combustion chamber is adjusted to control the temperature to a predetermined value. In this way, the temperature of the combustion zone can be controlled, thereby preventing the temperature of the tube metal from being too high and avoiding damage. In addition, two or more separate flow-cooled sections can be used to define the boundaries of the furnace combustion zone and help reduce the temperature of the tube metal, while the temperature of the individual working fluid flows can be made to match the needs of the power system to improve efficiency.

In a preferred embodiment, each combustion zone is located within the same furnace, the air supplied to one or more combustion zones is preheated by heat from the flue gases, and the tubes of the heat exchanger surround the combustion zones. There are also convection sections associated with receiving flue gas from the combustion section and also including heat exchangers for transferring heat from the flue gas to the respective working fluid streams within heat exchanger tubes of the convection sections. The working fluid stream from the heat exchanger in the combustion zone can be in communication with the working fluid stream in the convection zone.

Other advantages and features of the present invention will become apparent from the following description of a particular embodiment and from the claims.

FIG. 1 is a schematic representation of an embodiment of the apparatus and process of the present invention having two combustion zones and two separate process streams;

FIG. 2 is a profile view showing the arrangement of the burner combustion zone and convection tubes of the furnace of FIG. 1.

FIG. 1 shows a furnace system comprising an air preheater 100, two combustion zones 101 and 102 comprised of separate working fluid cooled heat exchangers HE1A and HE2A, respectively, two convection pass zones 103 and 104 comprised of working fluid cooled heat exchangers HE2B and HE1B, respectively, and an external power system 105. The amount of fuel in the fuel streams 5 and 6 and the amount of air in the air streams 3 and 4 are controlled by suitable control mechanisms, such as the mechanisms 203, 204, 205, 206 shown in fig. 1. Power system 105 may be any external direct-fired power conversion system. The combustion system of the present invention is particularly useful in power cycles and systems where most of the heat required for the energy conversion cycle is not used for vaporization of the working fluid, but rather for its superheating and reheating. Examples of such power systems are described, for example, in U.S. patent nos. 4732005 and 4889545, which are incorporated herein by reference. Further incorporated by reference are energy conversion systems disclosed in U.S. patents 3346561, 4489563, 5548043, 4586340, 4604867, 4732005, 4763480, 4899545, 4982568, 5029444, 5095708, 5450821, and 5440882. The working liquid stream may be a sub-chilled liquid, a saturated liquid, a two-phase liquid, saturated steam, or superheated steam.

Referring to FIG. 1, combustion air enters the air preheater 100 at location 1 where it is preheated to a temperature of 500 and 600 degrees Fahrenheit at location 2. The amount of fuel in the fuel stream 5 supplied to the combustion zone 101 is only a fraction of the total amount of fuel being combusted. Combustion zone 101 is within the working fluid cooling tubes of heat exchanger HE1A, and the first working fluid stream enters the heat exchanger at location 11 and exits the heat exchanger at location 12 after increasing in temperature. The heat of the flue gas stream is substantially converted as radiant energy. The amount of fuel and preheated air supplied to the combustion chamber is selected in accordance with the amount of heat to be absorbed by the surrounding furnace walls, and the temperature of the combustion chamber is controlled to a predetermined value. In particular, controlling the temperature within the first combustion zone 101 prevents excessive furnace wall temperatures within the heat exchanger HE1A to avoid damaging the heat exchanger.

The flue gas exiting the first combustion zone 101 flows into the second combustion zone 102 via location 7, where it is mixed with the combustion air stream 4 and the fuel stream 6. The temperature within combustion zone 102 is controlled to prevent excessive furnace wall temperatures within heat exchanger HE2A to avoid damaging the heat exchanger. The combustion zone 102 is within the working fluid cooling tubes of heat exchanger HE2A, and the second working fluid stream enters the heat exchanger HE2A at position 13 and exits the heat exchanger at position 14 after increasing in temperature.

The flue gas from the second combustion zone 102 is passed to the convection tubes of the first convection zone 103 of the furnace where it is cooled in heat exchanger HE 2B. In communication with the second working fluid stream, the third working fluid stream enters heat exchanger HE2B at 15, exits the heat exchanger HE2B at 16 after increasing in temperature, and then returns to power system 105. At position 9, the flue gas exits the convection zone 103 at a lower temperature than it did at position 8 and enters the second convection zone 104.

Similarly, the flue gas is further cooled within the second convection zone 104 due to the heat rejected in the heat exchanger HE 1B. In communication with the first working fluid stream, the fourth working fluid stream enters heat exchanger HE1B at position 17, exits the heat exchanger HE1B at position 18 after increasing in temperature, and then returns to power system 105. The flue gas exits the convection duct at location 10 and flows to the air preheater 100. In the air preheater 100, the flue gas is further cooled, releasing heat to the combustion air stream, which is passed to the flue at location 11 after the temperature has been reduced.

An important advantage of the multi-stage furnace arrangement is that the combustion temperature achieved in each flame zone can be controlled separately by adjusting the fuel and air flow. Sub-stoichiometric or over-stoichiometric combustion may be used to control the temperature in the first stage flame zone. Furthermore, by forming the periphery of the furnace with separate working fluid streams, cooling by the working fluid streams can be achieved in the highest temperature zone of the furnace. The working fluid stream is heated for the last time in the convection tubes of the furnace. The present invention provides heat to a furnace system in a manner that facilitates control of the temperature of the combustion zone to prevent excessive temperatures of the metal tube.

We have described a two-stage system having a combustion zone and a convection conduit cooled by two separate working fluids in series between the combustion zone and the convection conduit. In each case one flue gas stream contains all the flue gases from the preceding step. Other variations including tertiary and quaternary systems of similar nature may be made. Furthermore, separate streams of working fluid may be used to cool only some portions within the furnace or some portions within the convection tubes.

Claims (22)

1. A method of supplying heat to an external combustion power system, comprising the steps of:

feeding a first portion of the total amount of the first air stream and fuel into a first combustion zone;

combusting said first portion of fuel in said first combustion zone to form a first flue gas stream;

transferring heat from said first combustion zone to a first working fluid stream from an external combustion power system in conduits of a first heat exchanger exposed to said first combustion zone, adjusting the amount of fuel and air supplied to the first combustion zone to control the temperature of the first combustion zone at a first predetermined value;

supplying said first flue gas stream, said second air stream and a second portion of the total fuel to a second combustion zone;

combusting said second portion of said fuel in said second combustion zone to form a second flue gas stream; and

transferring heat from said second combustion zone to a second working fluid stream from an externally fired power system in conduits exposed to a second heat exchanger of said second combustion zone, and adjusting the amount of fuel and air supplied to the second combustion zone to control the temperature of the second combustion zone at a second predetermined value.

2. The method of claim 1, wherein said first and second zones are in the same furnace.

3. The method of claim 1 wherein said first air stream is preheated by heat from said second flue gas stream.

4. A method according to claim 3, wherein said second air stream is preheated by heat from said second flue gas stream.

5. The method of claim 2 wherein said first heat exchanger tubes surround said first combustion zone and said second heat exchanger tubes surround said second combustion zone.

6. The process of claim 1 further comprising flowing said second flue gas stream through a first convection zone and transferring heat from said first convection zone to a third working fluid stream from an external combustion power system in a third heat exchanger conduit exposed to said first convection zone.

7. The process of claim 6 further comprising passing said second stream of flue gas from said first convection zone through a second convection zone and transferring heat from said second convection zone to a fourth stream of working fluid from an external combustion power system in a fourth heat exchanger conduit exposed to said second convection zone.

8. The method of claim 6, wherein said third process stream is in series with one of said first and second process streams.

9. The method of claim 7 wherein said third process stream is in series with one of said first and second process streams and said fourth process stream is in series with the other of said first and second process streams.

10. The process of claim 7 wherein said first and second air streams are preheated by heat received from said second convection zone with said second flue gas stream.

11. The method of claim 1, further comprising,

providing one or more additional combustion zones in series to receive additional respective portions of the second stream of flue gas, additional respective streams of air, and the total amount of fuel;

combusting a respective further portion of said total amount of fuel in said further combustion zone to form a respective further flue gas stream;

transferring heat from said further combustion zone to a respective further working fluid stream from the external combustion power system in conduits of a further heat exchanger exposed to said further combustion zone, adjusting the amount of fuel and air supplied to the further combustion zone to control the temperature of the further combustion zone at respective predetermined values.

12. An apparatus for providing heat to an externally fired power system, comprising:

a first combustion zone in communication with receiving a first air stream and a first portion of a total amount of fuel and providing a first flue gas stream comprising products of combusting said first portion of fuel in said first combustion zone;

a first heat exchanger conduit exposed to said first combustion zone and carrying a first working fluid stream from an external combustion power system;

control means for controlling the amount of fuel and air supplied to said first combustion zone to control the temperature of the first combustion zone at a first predetermined value;

a second combustion zone receiving said first flue gas stream, a second air stream and a second portion of the total fuel and providing a second flue gas stream comprising products of combusting said second portion of the fuel in said second combustion zone;

a second heat exchanger conduit exposed to said second combustion zone and carrying a second working fluid stream from the externally fired power system;

control means for controlling the amount of fuel and air supplied to said second combustion zone to control the temperature of the second combustion zone at a second predetermined value;

13. the apparatus of claim 12 wherein said first and second zones are in the same furnace.

14. The apparatus of claim 12 further comprising a preheater for utilizing heat from said second flue gas stream to preheat said first air stream.

15. The apparatus of claim 14 wherein said preheater preheats said second air stream using heat from said second flue gas stream.

16. The apparatus of claim 13 wherein the tubes of said first heat exchanger surround said first combustion zone and the tubes of said second heat exchanger surround said second combustion zone.

17. The apparatus of claim 12, further comprising:

a first convection zone in communication with receiving said second flue gas stream from said second combustion zone, and

a third heat exchanger conduit exposed to said first convection zone and carrying a third working fluid stream from the external combustion power system.

18. The apparatus of claim 17, further comprising:

a second convection zone in communication with receiving said second flue gas stream from said first convection zone, and

a fourth heat exchanger conduit exposed to said second convection zone and carrying a fourth working fluid stream from the external combustion power system.

19. The apparatus of claim 17 wherein said third process stream is in series with one of said first and second process streams.

20. The apparatus of claim 18 wherein said third process stream is in series with one of said first and second process streams and said fourth process stream is in series with the other of said first and second process streams.

21. The apparatus of claim 18 further comprising a preheater for preheating said first and second air streams with heat received from said second convection zone by said second flue gas stream.

22. The apparatus of claim 12, further comprising:

one or more further combustion zones connected in series to receive a further respective portion of the second flue gas stream, a further respective air stream and the total amount of fuel;

additional heat exchanger tubes exposed to said additional combustion zone and carrying additional respective working fluid streams from the external combustion power system;

further control means for controlling the amount of fuel and air supplied to said further combustion zone to control the temperature of the further combustion zone to be at a further predetermined value.