Method and system for controlling the backflow sealing efficiency and recycle rate in fluidized bed reactors

Abstract: A system in which air is introduced into two chambers (38a, 38b) formed in a sealing vessel (30) for receiving the separated solids from the separator (18). The air is introduced into two chambers in the sealing vessel in a direction opposite to that of the flow of the separated solids through the vessel. One of the chambers is located below the separator dipleg (28) and in alignment therewith and the other chamber surrounds the first chamber. The air flow through each path can be separately adjusted as necessary.

Description: This invention relates to a fluidized bed reactor and, more particularly, to a method and system for eliminating gas backflow in the solids recycle system of a fluidized bed reactor. Fluidized bed reactors, such as gasifiers, steam generators, combustors, and the like, are well known. In these arrangements, air is passed through a bed of particulate material, including a fossil fuel such as coal and an adsorbent for the sulphur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. The entrained particulate solids are separated externally of the bed and recycled back into the bed. The head produced by the fluidized bed is utilized in various applications which results in an attractive combination of high heat release, high sulphur absorption, low nitrogen oxides emissions and fuel flexibility. The most typical fluidized bed reactor is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density an a well-defined, or discrete, upper surface. In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turndown afforded by the bubbling bed, a fluidized bed reactor has been developed utilizing a "circulating" fluidized bed process. According to this process, the fluidized bed density is well below that of a typical bubbling fluidized bed, the air velocity is greater than that of a bubbling bed and the flue gases passing through the bed entrain a substantial amount of particulate solids and are substantially saturated therewith. Also, the circulating fluidized bed is characterized by relatively high solids recycling which makes it insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore, decreasing the nitrogen oxides formation. Also, the high solids recycling improves the efficiency of the mechanical device used to separate the gas from the solids for ensuring high solids recycle. The resulting increase in sulfur adsorbent and fuel residence times reduces the adsorbent and fuel consumption. To accomplish this it is necessary to have a loop seal device that does not restrict the flow of solids or reduce collection efficiency. In other situations or designs, such as for high ash fuels, it may be necessary to effect the collection efficiency by passing air/gas flow upwards to a dip leg extending from the separator device. Most of the circulating bed designs currently being utilized control load by regulating the solids recycle rate and some approach this by reducing the solids inventory from the loop seal, i.e., from the sealing system located between the outlet of the external separating devices and the recycle inlet to the fluidized bed. However, this normally has to be accomplished with a metering cooler, such as a water cooled screw to remove solids from the recycle system, which adds mechanical complexity and costs penalties in addition to requiring downstream handling equipment. In U.S. Patent No. 4,781,574 issued November 1, 1988, and assigned to the assignee of the present invention, the above problems were addressed by disposing an air source at the separated solids outlet of a cyclone separator and discharging air into the separator in a direction opposite the direction of flow of the separated solids. The air entrains a portion of the solids and is passed back through the separator and to the heat recovery area. Although this technique enabled the solids inventory to be controlled without incurring significant additional costs, it interfered with the operation of the separator. It is therefore an object of the present invention to provide a method and system for controlling the collection efficiency and the recycle rate in a fluidised bed system. According to the present invention there is provided a fluidised bed combustion system including separating means for receiving a mixture of flue gases and entrained particulate material from a fluidised bed and separating the entrained particulate material from the flue gases, means for passing the separated flue gases from the separating means to a heat recovery unit, a dipleg for passing the separated material from the separating means to a seal vessel, means for passing the separated material from the seal vessel back to the fluidised bed, and means for introducing air or gas to the seal vessel for fluidising the separated material in the seal vessel to seal against the backflow of the separated material from the fluidised bed through the dipleg and back to the separating means, characterised in that the air introducing means comprise a first conduit for introducing air or gas into the seal vessel in a path aligned with the flow path of the separated material from the dipleg, and a second conduit for introducing air or gas into an area of the seal vessel surrounding the flow path of air or gas from the said first conduit, and in that means are provided for independently regulating the air or gas flow into the first and second conduits to control the rate of separated material passed back to the fluidised bed. Also according to the invention in another aspect there is provided a method of controlling a fluidised bed combustion system comprising the steps of receiving a mixture of flue gases and entrained particulate material from a fluidised bed, separating the entrained particulate material from the flue gases, passing the separated flue gases to a heat recovery unit, passing the separated material to a seal vessel, passing the separated material from the seal vessel back to the fluidised bed, and introducing air or gas to the seal vessel for fluidising the separated material in the seal vessel to seal against the backflow of the separated material, characterised in that air or gas is introduced into the seal vessel in a path aligned with the flow path of the separated material into the seal vessel, air-or gas is also introduced into an area of the seal vessel surrounding the said path aligned with the flow path of the separated material into the seal vessel, and independently regulating the two air or gas flows into the seal vessel to control the rate of passage of the separated material back to the fluidised bed. In a system and method according to the invention recycle rate can be increased to a relatively high value and decreased to a relatively low value as desired. Also a metering device and downstream handling equipment is avoided and solids inventory control is accomplished internally to the fluidised bed system. Further the operation of the cyclone separator under certain selected operating parameters is not materially affected. The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of the system of the present invention; and Figure 2 is an enlarged cross-section taken along the line 2-2 of Figure 1. Referring specifically to the drawing, a fluidised bed reactor vessel 10 has a plenum 12 disposed at its lower end portion for receiving air or gas from a source (not shown) such as a source located externally of the vessel. A perforated plate or grate 14 is provided at the interface between the lower end of the vessel 10 and the air plenum 12 for allowing the pressurized air or gas from the plenum to pass upwardly through the vessel 10. An inlet, or feeder, 16 is provided through one of the sidewalls of the vessel 10 for introducing a particulate material into the bed which can include relatively fine particles of fuel, such as coal, and an adsorbent material, such as limestone, for adsorbing the sulphur generated during the combustion of the fuel in a known manner. The air or gas from the plenum 12 fluidizes the particulate material in the vessel 10 and combines with the gaseous products of combustion to form flue gases as will be described in detail later. A cyclone separator 18 is provided adjacent the vessel 10 and is connected thereto by a gas channel 23 which extends from an opening formed in the upper portion of the vessel rear wall to an inlet opening formed in the upper portion of the separator 18. The separator 18 receives the flue gases and entrained particulate material from the fluidized bed in the vessel 10 and operates in a conventional manner to disengage the particulate material from the flue gases due to the forces created in the separator. The separated flue gases pass, via a duct 22, into a heat recovery area shown in general by the reference numeral 24 under the action of an induced draft fan 26 connected to the downstream end of the heat recovery area. The separated solids from the separator 18 pass into a dipleg 28 connected to the outlet of the separator. A seal vessel 30 is disposed below the separator 18 and adjacent the vessel 10 and receives the dipleg 28. A grate 34 is disposed in the lower portion of the vessel 30 to divide same into an upper chamber 36 for receiving the separated solids from the dipleg and a lower chamber 38. As shown in Figures 1 and 2, a cylindrical partition 40 is disposed in the chamber 38 to divide it into a central plenum 38a extending just below the dipleg 28, and a plenum 38b surrounding the central plenum. Conduits 42 and 44 are connected to the plenums 38a and 38b, respectively, for introducing air or gas from a source (not shown), such as a source located externally of the vessel 10. The air or gas passes upwardly through the grate 34 to fluidize the separated material in the upper chamber as will be described. Valves 46 and 48 are provided in the conduits, 42 and 44, respectively, for controlling the flow rate of the air or gas passing therethrough. A duct 50 connects the vessel 30 to the vessel 10 so that the particulate material from the vessel 30 passes into the interior of the vessel 10 and into the fluidized bed formed therein. In operation, particulate fuel material from the inlet 16 is introduced into the vessel 10 and adsorbent material can also be introduced in a similar manner as needed. Pressurized air or gas passes into and through the air plenum 12, through the grate 14 and into the particulate material in the vessel 10 to fluidize the material. A lightoff burner (not shown) or the like is disposed in the vessel 10 and is fired to ignite the particulate fuel material. When the temperature of the material reaches a relatively high level, additional fuel from the inlet 16 is discharged into the vessel 10. The material in the vessel 10 is self-combusting by the heat in the vessel and the mixture of air and gaseous products of combustion (hereinafter referred to as "flue gases") pass upwardly through the vessel 10 and entrain, or elutriate, the relatively fine particulate material in the vessel. The velocity of the air or gas introduced, via the air plenum 12, through the grate 14 and into the interior of the vessel 10 is established in accordance with the size of the particulate material in the vessel so that a circulating fluidized bed is formed in each chamber, i.e., the particulate material is fluidized to an extent that substantial entrainment or elutriation of the particulate material in the bed is achieved. Thus, the flue gases passing into the upper portion of the furnace are substantially saturated with the particulate material. The saturated flue gases pass to the upper portion of the vessel and exit into the duct 20 and pass into the cyclone separator 18. In the separator 18, the solid particulate material is separated from the flue gases and the former is injected, via the dipleg 28, into the sealing vessel 30. The cleaned flue gases from the separator 18 exit, via the duct 22, to the heat recovery section 24 for further treatment. Air or gas is injected, via the conduits 42 and 44, into the chambers 38a and 38b and passes through the grate 34 to slightly fluidize the separated solids in the vessel 30. From the vessel 30, the separated fluidized solids normally pass, via the duct 50, through the rear wall of the vessel 10 and into the other fluidized bed in the vessel where they mix with the other solids in the vessel. The flow of air or gas into the chambers 38a and 38b can be regulated by the valves 46 and 48, respectively, to regulate the flow of separated solids from the cyclone 18, through the vessel 30 and into the vessel 10 while sealing against the backflow of flue gases from the vessel 10 through the conduit and into the separator 18 in a direction opposite from the normal system flow described above. The air or gas flow into the plenum 38a can be regulated by the valve 42 independently of the flow into the plenum 38b so that an increase in the air or gas flow into the plenum 38a relative to that into the plenum 38b will decrease the recycle rate and, similarily, a decrease in the air or gas flow into the plenum 38a relative to that in the 38b will increase the recycle rate. Also, the air or gas flow into the chamber 38a can be carefully controlled so as not to interfere with or affect the operation of the separator 18. Thus, by varying the opening of the valves 46 and 48, the solids injection into the vessel 10 from the separator 18 can be indirectly controlled as needed to regulate the recycle rate according to the particular load desired. This enables the load of the reactor to be varied without the use of expensive complex metering devices and downstream handling equipment and thus considerably reduces the costs of the system. It is understood that the foregoing description relates to a balanced draft system but that forced draft operation could be utilized by pressurizing the air or gases entering the conduits 42 and 44.

Claims: A fluidised bed combustion system including separating means (18) for receiving a mixture of flue gases and entrained particulate material from a fluidised bed and separating the entrained particulate material from the flue gases, means (22) for passing the separated flue gases from the separating means (18) to a heat recovery unit (24), a dipleg (28) for passing the separated material from the separating means (18) to a seal vessel (30), means for passing the separated material from the seal vessel (30) back to the fluidised bed, and means (42,44) for introducing air or gas to the seal vessel (30) for fluidising the separated material in the seal vessel (30) to seal against the backflow of the separated material from the fluidised bed through the dipleg (28) and back to the separating means (18), characterised in that the air introducing means (42,44) comprise a first conduit (42) for introducing air or gas into the seal vessel (30) in a path aligned with the flow path of the separated material from the dipleg (28), and a second conduit (44) for introducing air or gas into an area of the seal vessel (30) surrounding the flow path of air or gas from the said first conduit (42), and in that means (46,48) are provided for independently regulating the air or gas flow into the first and second conduits (42,44) to control the rate of separated material passed back to the fluidised bed (10). A system as claimed in claim 1 further comprising means (12,14) for introducing fluidising air or gas to the bed, at a velocity sufficient to form a circulating bed. A system as claimed in Claim 1 or Claim 2 further comprising an upstanding cylindrical partition (40) in the seal vessel (30) aligned with the dipleg (28), to provide defined portions of the seal vessel (30) into which the first and second conduits (42,44) introduce air or gas. A method of controlling a fluidised bed combustion system comprising the steps of receiving a mixture of flue gases and entrained particulate material from a fluidised bed, separating the entrained particulate material from the flue gases, passing the separated flue gases to a heat recovery unit, passing the separated material to a seal vessel (30), passing the separated material from the seal vessel (30) back to the fluidised bed, and introducing air or gas to the seal vessel (30) for fluidising the separated material in the seal vessel (30) to seal against the backflow of the separated material, characterised in that air or gas is introduced into the seal vessel (30) in a path aligned with the flow path of the separated material into the seal vessel, air or gas is also introduced into an area of the seal vessel (30) surrounding the said path aligned with the flow path of the separated material into the seal vessel, and independently regulating the two air or gas flows into the seal vessel (30) to control the rate of passage of the separated material back to the fluidised bed. A method as claimed in claim 4 in which fluidising air or gas is introduced into the fluidised bed at a velocity sufficient to form a circulating bed.

Inventors: ABDULALLY IQBAL FAZALEABAS, ABDULALLY, IQBAL FAZALEABAS
Applicants: FOSTER WHEELER ENERGY CORP, FOSTER WHEELER ENERGY CORPORATION
Application year: 1990
Main classification: F23C11
IPC codes: B01J8, F23C10