Fluidized bed reactor system and method having a heat exchanger

Abstract: A fluidized bed reactor (12) in which a heat exchanger (36) is located adjacent the reactor with each enclosing a fluidized bed and sharing a common wall including a plurality of water tubes. A mixture of flue gases and entrained particulate materials from the fluidized bed in the reactor are separated in the cyclone (28) and the separated particulate material is passed to the fluidized bed in the heat exchanger (36). Coolant is passed in a heat exchange relation with the separated materials in the heat exchanger (36) to remove heat from the materials after which they are passed via (44) to the fluidized bed in the reactor (12). Auxiliary fuel is supplied to the heat exchanger (36) for combustion to control the temperature of the coolant. When the system of the present invention is utilized to generate steam the coolant can be controlled to match the requirements of a steam turbine.

Description: This invention relates to fluidized bed reactors, and more particularly, to a system and method in which a heat exchanger is provided adjacent a fluidized bed reactor. Often in circulating fluidized bed reactors, a heat exchanger is located in the return solids-stream from the cyclone separator which utilizes water cooled surfaces for the extraction of thermal energy at a high heat transfer rate. Examples of this are shown in US-A-5133943 and EP-A-461 847. In steam generation applications this additional thermal energy can be utilized to regulate the exit temperature of the steam to better match the turbine requirements. Typically, at relatively high demand loads, the heat exchanger supplies only a relatively small percentage of the total thermal load to the reactor, while at relatively low demand loads, the heat exchanger could supply up to approximately 20% of the total thermal load. Unfortunately, while the heat exchanger could thus supply a significant percentage of the total thermal load of a fluidized bed reactor under low demand loads and start-up conditions, the heat exchanger typically has limited capacity for thermal regulation. More particularly, during these low demand loads and start-up conditions, the exit temperature of the water/steam is less than optimum due to the reactor conditions taking precedence. This results in a decrease in the overall efficiency of the system and in an increase in mechanical stress on the external equipment that receives the mismatched coolant. It is therefore an object of the present invention to provide a fluidised bed reactor system and method in which a heat exchanger is provided adjacent the reactor section which provides additional capacity for thermal regulation. According to the invention there is provided a fluidised bed reactor system comprising a reactor means for supporting a fluidised bed of combustible particulate material in the reactor, heat exchange means disposed adjacent to the reactor, separating means for receiving a mixture of flue gases and entrained particulate material from the fluidised bed and separating the particulate material from the flue gases, means for passing the separated particulate material to the heat exchange means, means for passing air through the separated particulate material in the heat exchange means to fluidise the separated material, means disposed in the heat exchange means for passing a coolant in heat exchange relation to the separated material to cool the separated material, characterised in that means are provided for heating the separated material whilst in the heat exchange means (36) to control the temperature of the coolant. Also according to the invention there is provided a method of operating a fluidised bed reactor system comprising supporting a fluidised bed of combustible particulate material in a reactor receiving a mixture of flue gases and entrained particulate material from the fluidised bed and separating the particulate material from the flue gases, passing air through the separated particulate material to fluidise the separated material, and passing a coolant in a heat exchange relation to the separated material to cool the separated material, characterised by heating the separated material to control the temperature of the coolant. In one embodiment the superficial fluidising velocity of the fluidised bed in the heat exchanger can be varied according to the reactor's thermal demand requirement. Also, the size of the fluidised bed in the heat exchanger can be varied according to the reactor's thermal demand requirement. External fuel is supplied to the heat exchanger according to the reactor's thermal demand requirement. Thus, burners may be disposed within the heat exchanger for supplying additional heat energy in the event of low demand loads and start-up conditions. The invention will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic view depicting a fluidized bed reactor of the present invention; Fig. 2 is a cross sectional view taken along line 2-2 in Fig. 1; and Fig. 3 is a cross sectional view taken along line 3-3 in Fig. 1. The system and method of the present invention will be described in connection with a fluidized bed reactor forming a portion of a natural water circulating steam generator shown in general by the reference numeral 10 in Fig. 1 of the drawings. The steam generator 10 includes a fluidized bed reactor 12, a separating section 14, and a heat recovery area 16. The reactor 12 includes an upright enclosure 18 and a perforated air distributor plate 20 disposed in the lower portion of the reactor and suitably attached to the walls of the enclosure for supporting a bed of particulate material including coal and relatively fine particles of sorbent material, such as limestone, for absorbing the sulfur oxides generated during the combustion of the coal. A plenum 22 is defined below the plate 20 for receiving air which is supplied from a suitable source (not shown), such as a forced draft blower, and appropriately regulated to fluidize the bed of particulate material, and according to a preferred embodiment, the velocity of the air is of a magnitude to create a circulating fluidized. One or more distributors 24 are provided through the walls of the enclosure 18 for introducing the particulate material onto the bed and a drain pipe 26 registers with an opening in the distributor plate 20 for discharging relatively-coarse spent particulate material from the enclosure 18. It is understood that the walls of the enclosure 18 include a plurality of water tubes disposed in a vertically extending relationship and that flow circuitry (not shown) is provided to pass water through the tubes to convert the water to steam. Since the construction of the walls of the enclosure 18 is conventional, the walls will not be described in any further detail. The separating section 14 includes one or more cyclone separators 28 provided adjacent the enclosure 18 and connected thereto by a duct 30 which extends from an opening formed in the upper portion of the rear wall of the enclosure 18 to an inlet opening formed in the upper portion of the separator 28. The separator 28 receives the flue gases and entrained relatively fine particulate material from the fluidized bed in the enclosure 18 and operates in a conventional manner to separate the relatively fine particulate material from the flue gases by the centrifugal forces created in the separator. The relatively-clean flue gases rise in the separator 28 and pass into and through the heat recovery area 16 via a duct 32. The heat recovery area 16 operates to extract heat from the clean flue gases in a conventional manner after which the gases are discharged, via outlet duct 16a. The separated solids from the separator 28 pass into a hopper 28a connected to the lower end of the separator and then into a dipleg 34 connected to the outlet of the hopper. The dipleg 34 is connected to a heat exchanger 36 which includes a substantially rectangular enclosure 38 disposed adjacent to, and sharing the lower portion of the rear wall of, the enclosure 18. An air distributor plate 40 is disposed at the lower portion of the enclosure 38 and defines an air plenum 42 to introduce air received from an external source (not shown) through the distribution plate 40 and into the interior of the enclosure 38. Three drain pipes, one of which is shown by reference numeral 43 in Fig. 1, register with openings in the plate 40 for discharging relatively fine spent particulate material from the interior of the enclosure 38, as will be discussed. Openings, shown generally by the reference numeral 44 in Fig. 1, are formed through the common wall between the enclosures 38 and 18 for communicating solids and gases from the heat exchanger 36 to the reactor 12, as will be discussed. A partition wall 45 is formed over the opening 44 and extends downwardly to define a passage to allow solid material from the heat exchanger 36 to pass into the interior of the reactor 12. A small trough enclosure 46 is formed adjacent to, and shares, the middle portion of the rear wall of the enclosure 38 for receiving relatively fine particulate material received from the dipleg 34 and distributing the particulate material to the enclosure 38. An air distributor plate 48 is disposed in the lower portion of the enclosure 46 and defines an air plenum 50 to introduce air received from an external source through the distributor plate 48 and into the interior of the enclosure 46. An opening 52 is formed in the common wall between the enclosure 46 and the enclosure 38 for communicating the solids and the fluidizing air from the enclosure 46 to the enclosure 38. As shown in Figs. 2 and 3, two partition walls 58a and 58b are contained in the enclosure 38 and extend from the base of the enclosure, through the plate 40 to the roof of the enclosure to divide the plenum 42 and the enclosure 38 into three portions 42a, 42b and 42c, and 38a, 38b and 38c, respectively. As shown in Fig. 2, two partition walls 60a and 60b extend from the base of the enclosure 46, through the plate 48 (Fig.1) and midway up the walls of the enclosure to divide the enclosure 46 into three portions 46a, 46b and 46c. It is understood that the two partition walls 60a and 60b also divide the plenum 50 (Fig.1) into three portions. Referring to Fig. 1, it is understood that three burners, one of which is shown by the reference numeral 62, are disposed in the enclosure portions 38a, 38b, 38c, respectively, to combust fuel, such as gas or oil, in an ordinary fashion to supply additional heat. Further, three heat exchanger tube bundles, one of which is shown by reference numeral 64, are disposed in the enclosure portions 38a, 38b, 38c, respectively, to receive cooling fluid, such as water, for extracting heat from the relatively fine particulate material in the enclosure portions. In addition, three openings 44a, 44b, 44c (Fig. 2) are formed in the common wall between the enclosures 38 and 18, and three drain pipes 43a, 43b, 43c (Fig. 3) register with openings formed in the distributor plate 40 for the discharge of the particulate material from the interior of the enclosure portions 38a, 38b, 38c, respectively, as will be described. In operation, particulate fuel and adsorbent material from the distributor 24 are introduced into the enclosure 18, as needed. Pressurized air from an external source passes into the air plenum 22, through the distributor plate 20 and into the bed of particulate material in the enclosure 18 to fluidize the material. A lightoff burner (not shown), or the like, is disposed in the enclosure 18 and is fired to ignite the particulate fuel material. When the temperature of the material reaches a relatively high level, additional fuel from the distributor 24 is discharged into the reactor 12. The material in the reactor 12 is self-combusted by the heat generated by the combusting fuel material and the mixture of air and gaseous products of combustion (hereinafter referred to as "flue gases") passes upwardly through the reactor 12 and entrain relatively fine particulate material from the bed in the enclosure 18. The velocity of the air introduced, via the air plenum 22, through the distributor plate 20 and into the interior of the reactor 12 is established in accordance with the size of the particulate material in the reactor 12 so that a circulating fluidized bed is formed, that is the particulate material is fluidized to an extent that substantial entrainment of the particulate material in the bed is achieved. Thus the flue gases passing into the upper portion of the reactor 12 are substantially saturated with the relatively fine particulate material. The balance of the air required for complete combustion is introduced as secondary air, in a conventional manner. The saturated flue gases pass to the upper portion of the reactor 12, exit through the duct 30 and pass into the cyclone separator 28. In the separator 28, the relatively fine particulate material is separated from the flue gases and the former passes through the hoppers 28a and is injected, via the dipleg 34, into the enclosure portion 46b. The cleaned flue gases from the separator 28 exit, via the duct 32, to the heat recovery area 16 for passage through the recovery area 16 before exiting to external equipment. Cooling fluid, such as water, is passed through conventional water flow circuitry, including a superheater, a reheater and an economizer (not shown), disposed in the heat recovery area 16 to extract heat from the flue gases. The enclosure portion 46b receives the relatively fine particulate material from the dipleg 34. The particulate material is fluidized by air supplied to the portion of the plenum 50 disposed below the enclosure portion 46b, overflows the enclosure portion 46b and fills the enclosure portions 46a, 46c and the enclosure portion 38b. It is understood that the flow of relatively fine particulate material from the enclosure portion 46b to the enclosure portions 46a, 46c and to the enclosure portion 38b is regulated by the fluidization velocity of the air supplied to the portion of the plenum 50 disposed below the enclosure portion 46b. Similarly, the flow of relatively fine particulate material from the enclosure portions 46a, 46c to the enclosure portions 38a, 38c, respectively, is regulated by the fluidization velocity of the air supplied to the portion of the plenum 50 disposed below the enclosure portions 46a, 46c. In general, the air supplied to the portion of the plenums disposed below the enclosure portions 46a, 46b, 46c is regulated so as to enable the build up of relatively fine particulate material in the enclosure portions 46a, 46c, 46c to a level at least sufficient to cover the heat exchanger tubes 64. The relatively fine particulate material is then either returned, via the openings 44a, 44b, 44c, to the reactor 12 or discharged, via the drain pipes 43a, 43b, 43c, from the enclosure portions 38a, 38b, 38c, respectively, which enables the regulation of the inventory of the relatively fine particulate material in the reactor 12. The fluidization of the particulate material in the enclosure portions 38a, 38b, and 38c is independently regulated by the fluidization velocity of the air supplied to the plenums 42a, 42b, and 42c (Fig. 3), respectively. Cool fluid, such as water, is passed through the tubes forming the walls of the reactor 12, and the heat exchanger tube bundles 64 in the heat exchanger 36 to extract heat from the beds of particulate material in the reactor and the enclosure portions 38a, 38b and 38c, respectively, to provide temperature control of the later beds. Also, the burners 62 (Fig. 1) provide heat to the beds of particulate material in the enclosure portions 38a, 38b and 38 during start-up and low load operation, as necessary to provide additional temperature control of the beds. As a result of the foregoing, substantial regulation of the final exit temperature of the cooling fluid passing through the heat exchanger tube bundles 64 can be obtained to better match the turbine requirements. For example, the flow of fine particulate material to the enclosure portions 38a, 38b, 38c and consequentially, coming in contact with the heat exchange tube bundles 64, can be regulated by the fluidization velocity of the air supplied to the plenums 50, thus regulating the transfer of heat to the cooling fluid flowing through the heat exchange tube bundles 64. In addition, the individual beds disposed in the enclosure portions 38a, 38b, 38c can be independently fluidized or drained by the plenums 42a, 42b, 42c, and the drain pipes 43a, 43b, 43c, respectively, thus further regulating the transfer of heat to the cooling fluid flowing through the heat exchange tube bundles 64. Further, the burners 62 provide substantial heat to the cooling fluid flowing through the heat exchange tube bundles 64 during start-up and low load operation, thus resulting in an increase in the overall system efficiency and in a decrease in mechanical stress on the external equipment that receives the coolant. It is understood that for example, at least part of the additional regulated heat provided to the enclosures 38 may be supplied by a burner heating the air directed towards the plenums 42.

Claims: A fluidised bed reactor system comprising a reactor (12) means (20) for supporting a fluidised bed of combustible particulate material in the reactor (12), heat exchange means (36) disposed adjacent to the reactor (12), separating means (28) for receiving a mixture of flue gases and entrained particulate material from the fluidised bed and separating the particulate material from the flue gases, means (34) for passing the separated particulate material to the heat exchange means (36), means (40) for passing air through the separated particulate material in the heat exchange means (36) to fluidise the separated material, means (64) disposed in the heat exchange means (36) for passing a coolant in heat exchange relation to the separated material to cool the Separated material, characterised in that means (62) are provided for heating the separated material whilst in the heat exchange means (36) to control the temperature of the coolant. The system of Claim 1 in which the means (62) for heating the separated material comprise burner means disposed in the heat exchange means (36) to add heat to the separated material. The system of any preceding claim in which the heat exchange means (36) comprise a housing (38), and partition means (58a, 58b) disposed in the housing (38) to divide the fluidised separated material in the heat exchange means (36) into at least two fluidised beds (38a, 38b, 38c). The system of Claim 3 further comprising means for regulating the fluidising air to the said at least two fluidising beds in the heat exchanger to individually control the fluidisation of these fluidised beds and the temperature of the coolant. The system of Claim 3 or Claim 4 further comprising drain means for individually draining the said at least two fluidised beds in the heat exchanger for controlling the temperature of the coolant. A method of operating a fluidised bed reactor system comprising supporting a fluidised bed of combustible particulate material in a reactor (12) receiving a mixture of flue gases and entrained particulate material from the fluidised bed and separating the particulate material from the flue gases, passing air through the separated particulate material to fluidise the separated material, and passing a coolant in a heat exchange relation to the separated material to cool the separated material, characterised by heating the separated material to control the temperature of the coolant. The method of Claim 6 in which the separated particulate material is divided into at least two beds through which the fluidising air is individually controlled to control the fluidisation of the said at least two beds. The method of Claim 6 or Claim 7 further comprising individually draining the said at least two fluidised beds in the heat exchanger (36). The method of any of claims 6 to 8 in which the coolant is water and further comprising passing water in heat exchange relationship to the fluidised bed to convert the water to steam, the steam being used to drive a steam turbine, and in which the heating of the separated material controls the temperature of the coolant to match requirements of the turbine.

Inventors: GARCIA-MALLOL JUAN ANTONIO, GARCIA-MALLOL, JUAN ANTONIO
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1993
Main classification: F23C11
IPC codes: F22B21, F28D13, F23C10, F22B31, F23C99