Circulating fluidized bed boiler apparatus

Abstract: Apparatus for supplying fluidized bed material to a circulating fluidized bed boiler (50) having a primary zone (42) with a fuel feed point (68) and a furnace zone (48) above the primary zone (42), the apparatus including a mixing chamber (54) for receiving solid fuel and a portion of the total combustion air needed in the boiler (50). The combustion air and circulating fluidized bed solids are supplied tangentially into the chamber (54) which is advantageously in the form of a horizontally extending cylinder. The combustion air and solids are intimately mixed with each other before they are discharged into the primary zone (42) of the boiler (50). The primary zone (42) of the boiler (50) is shaped with an upwardly increasing cross sectional area so that the already mixed combustion air and solids diffuse evenly through the fluidized bed boiler (50).

Description: This invention relates to fluidized bed boilers and, in particular, to apparatus for supplying fluidized bed material to a circulating fluidized bed boiler. The combustion zone of a circulating fluidized bed (CFB) boiler 10, as shown in Figure 1 of the accompanying drawings, is divided into two parts, a primary zone 12 and a furnace or secondary zone 18. The primary zone 12, located below the furnace, is the area in which the circulating fluidized bed solids are re-injected back into the combustion zone by a particle separator 14 and a return line 16. A non-mechanical seal or valve 32 is provided in the return line 16. The primary zone 12 is also where fuel and sorbent for sulphur retention (if required) are introduced. The primary zone 12 acts both as a distribution zone for solids (CFB solids, fuel and sorbent) so that they are evenly distributed across the primary zone and furnace, and also as a preliminary combustion zone. About 50% to 100% of the total combustion air is fed into the bottom of the primary zone 12 at a windbox 20. Any remaining combustion air is fed in through wall ports 22. These wall ports 22 define the separation between the primary zone 12 and the furnace or secondary zone 18. Combustion is completed in the furnace. The primary zone 12 has a refractory lining 24 since it is exposed to a reducing atmosphere. The furnace or secondary zone 18 is refractory lined only in high erosion areas. The boiler also includes a convection pass 30 for the hot exhaust gases. A major problem area for CFB boilers is in firing highly volatile or highly reactive fuels such as wood. The usual means of feeding fuel into the primary zone 12 of a CFB boiler is with a screw conveyor 26 which pushes the fuel in through a wall port called the fuel feed point 28. A highly reactive fuel will devolitize in the area immediately around the fuel feed point 28. This results in a plume of combustible gases immediately over the fuel feed point 28. These concentrated combustible gases cannot readily mix with the combustion air because the air is evenly distributed across the cross section of the primary zone 12 and the furnace. The result is a temperature gradient across the unit due to the combustion being concentrated at the combustible gases plume above the fuel feed point 28. This high temperature zone encourages NOx formation. Also, the poor mixing of the combustible gas with the combustion air can lead to low combustion efficiency, high CO emissions and combustion occurring in the particle separator 14 and in the convection pass 30. What is needed is a close, intimate mixing of fuel, air and circulating bed solids in a way such that they are evenly distributed in the primary zone and that combustible gases, air and circulating bed solids are intimately mixed and evenly distributed in the furnace. US Patent No. US-A-4 552 203 (Chrysostome, et al) discloses a fluidized bed reactor having a particle return and supply mechanism which includes a feed screw and conduit that receives both cold and hot portions of the particles being returned to the fluidized bed. Gas is injected along the length of the return conduit for suspending and conveying the solid particles. A fluidized bed having an inlet zone which is positioned laterally of the combustion zone is disclosed in US Patent No. US-A-4 585 706 (Klaschka). A boiler with a fluidized bed which is divided into a deep part with walls inclined towards a lower outlet, and a shallow part above the deep part, is disclosed in US Patent No. US-A-4 528 945 (Virr, et al). In that arrangement, fuel is supplied by a feed screw near the top of the upper shallow part of the bed. Other arrangements of fluidized beds in which the fuel is supplied at a relatively high location in the bed area are disclosed in US Patent Nos. US-A-4 446 629 (Stewart, et al), US-A-4 539 939 (Johnson) and US-A-4 542 716 (Dreuilhe, et al). US Patent No. US-A-4 594 967 (Wolowodiuk) discloses a fluidized bed which is divided into separate bed portions. International Patent Application Publication No. WO-A-8 810 291 discloses a circulating fluidized bed boiler for combustion of fuel containing alkalines. Alkaline-containing fuel is supplied to a mixing chamber by a feed screw, where it is mixed with a reactant mixture. Solid particulates from above the furnace zone are also supplied to the mixing chamber. The material in the mixing chamber passes down an inclined conduit to the fuel feed point of the boiler. Ash is separated from the flue gas and may be blown into the mixing chamber for recirculation. "Proceedings Sixth International Symposium on Coal Slurry Combustion and Technology" 1984, Pittsburgh Energy Technology Center, Pittsburgh, Pennsylvania, pages 158-163, describes techniques for production of heat from residual coal-water mixture (CWM) fuels. In one arrangement, horizontal pre-burners are provided for tangential supply of fuel and air mixture to the bed of a fluidized bed boiler. The present invention seeks to avoid the major problem area for CFB boilers, wherein volatile or highly reactive components of the fuel(s) introduced at the fuel feed point do not mix sufficiently with the combustion air in the primary zone of the boiler. Accordingly the present invention provides a circulating fluidized bed boiler having a primary zone with a fuel feed point for fluidized bed material, and a furnace zone above the primary zone, the boiler comprising: a mixing chamber having a first inlet for receiving solid fuel, a second inlet for receiving recycled fluidized bed solids, and an outlet connected to the fuel feed point; fuel feed means connected to the mixing chamber first inlet for feeding solid fuel to the mixing chamber; first combustion air supply means connected to the primary zone of the boiler for supplying a portion of the total combustion air needed for combustion to the primary zone; second combustion air supply means for supplying another portion of the total combustion air needed for combustion; and a particle separator above the furnace zone with a return line connected between the particle separator and the second inlet of the mixing chamber for returning solid particles to the mixing chamber to form the recycled fluidized bed solids; characterised in that: the mixing chamber is in the form of a horizontally extending cylinder with the first inlet being at an axial end of the mixing chamber opposite from the fuel feed point, the second inlet being connected for tangential feed into the mixing chamber at a location near the first inlet and spaced from the fuel feed point; and the second combustion air supply means is connected for tangential feed into the mixing chamber at a location near the first inlet and spaced from the fuel feed point, for facilitating mixing between the combustion air, the recycled fluidized bed solids and the solid fuel in the mixing chamber which resulting mixture is supplied through the mixing chamber outlet to the primary zone of the boiler. The mixing chamber may advantageously be a cyclonic mixing and combustion chamber which is cylindrical in shape, extends horizontally and has a refractory lining. No heat absorbing surfaces are incorporated into the chamber unless required for structural strength and support. In the preferred embodiment of the invention, both the combustion air of the second combustion air supply means and the fluidized bed solids are supplied tangentially into the cylindrical mixing chamber to help facilitate mixing of the different components in the chamber. This arrangement is particularly suited to fuels which have high volatile contents or which themselves are highly reactive, such as wood particles or chips. The fluidized bed solids, which are supplied to the mixing chamber separately from the fuel, may include conventional CFB solids such as limestone or absorbent for sulphur retention. Preferably, approximately 25% to 45% of the total combustion air is supplied through the second combustion air supply means into the mixing chamber. A total of approximately 60% to 80% of the total combustion air is supplied through the first and second combustion air supply means jointly. The remainder of the combustion air is supplied through combustion air wall ports which are positioned between the primary zone and the secondary zone in the combustion zone of the boiler. The primary zone is preferably configured to have an upwardly increasing cross sectional area. This can be achieved by utilizing one or more inclined walls for the primary zone, so that the primary zone is in the form of a wedge or hopper. These walls diverge in an upward direction. The fuel feed point is also advantageously located near the bottom of the primary zone. This combination of features further enhances the dispersion effect of the well mixed combustion air, fluidized bed solids and solid fuel, into the resident fluidized bed and combustion air mass in the primary zone. As they rise in the primary zone, the CFB solids, combustible gases and combustion air will diffuse at a rate matching the expansion area of the primary zone. Since the CFB solids, combustible gases and combustion air are well mixed at the bottom of the primary zone, they will remain well mixed as they diffuse. In the preferred embodiment, the primary zone is refractory lined since it will run substoichiometrically. The primary zone ends at the combustion air wall ports. Since the height of the primary zone might become extremely large if a significant slope is used for its walls, the height of the primary zone can be reduced by placing the combustion air wall ports on the sloped walls so that the furnace zone starts in the wedge shaped lower portion of the boiler. Advantages of embodiments of the invention include the provision of a larger number of combustion air stages. As opposed to the conventional use of two stages for CFB boilers, one at the bottom of the primary zone and the other at the wall ports between the primary zone and the furnace zone, a CFB boiler with the cyclonic mixing and combustion chamber (CMCC) system uses these combustion air feed points plus the combustion air feed in the CMCC. With more stages of combustion air feed, there is better mixing of fuel and air, better burn-out, lower NOx and lower CO emissions, and a more even temperature profile in the boiler. The CMCC system provides better mixing of fuel, combustion air and CFB solids, and ensures that the resultant combustion gases are well mixed with the CFB solids when they enter the primary zone in which they mix with additional combustion air. The result is uniform combustion without any pockets of intense combustion. This effective mixing also ensures an even distribution of CFB solids and combustible gas in the furnace since they will diffuse simultaneously in the wedge shaped primary zone. The preferred embodiment also provides a simplified fuel feed system. In order to achieve equivalent fuel and air mixing in the primary zone offered by the CMCC system of the preferred embodiment, a conventional CFB boiler would have to utilize a complex, multipoint, underbed fuel feed system. The CMCC system offers excellent fuel and air mixing with a few simple parts. No auxiliary burner is required. For standard CFB boilers, a duct burner or auxiliary burner is required for warming the boiler and the circulating bed material. With a CMCC system, oil or gas may be fired in the CMCC for warming the boiler and the circulating bed. The oil or gas can be fed into the combustion air duct of the CMCC in a manner similar to that used to fire oil in a cyclone burner. The preferred embodiment of the present invention provides an apparatus for supplying fluidized bed material to a CFB boiler which is simple in design, rugged in construction and economical to manufacture. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic vertical sectional view of a circulating fluidized bed boiler of standard design; Figure 2a is a schematic vertical sectional view of a circulating fluidized bed boiler in accordance with an embodiment of the present invention; Figure 2b is a schematic sectional view taken along the line 2b - 2b in Figure 2a; Figure 3a is a sectional view taken transversely of the longitudinal axis through a cyclonic mixing and combustion chamber (CMCC) shown in Figures 2a and 2b; Figure 3b is a sectional view taken along the line 3b - 3b in Figure 3a; Figure 4a is a view similar to Figure 2a, on a reduced and simplified scale, showing an alternative embodiment of the invention; Figure 4b is a view similar to Figure 4a of a further embodiment of the invention; Figure 4c is a view similar to Figure 4a of a still further embodiment of the invention; Figure 5a is a view similar to Figure 3a showing a CMCC according to another embodiment of the invention; and Figure 5b is a sectional view taken along the line 5b - 5b in Figure 5a. Referring to the drawings and in particular to Figures 2a and 2b, there is shown an apparatus 40 for supplying fluidized bed material to a circulating fluidized bed (CFB) boiler 50. The CFB boiler 50 includes a primary zone 42 which has a refractory lining 43. A windbox 60 at the bottom of the primary zone 42 supplies a portion of the combustion air needed for burning fuel in the boiler 50. A secondary or furnace zone 48 is positioned above the primary zone 42. The primary and secondary zones 42, 48 are separated by a plurality of combustion air wall ports 52 for supplying an additional portion of combustion air. The boiler 50 also includes a particle separator 44 and a convection pass 70. Particles which escape from the primary and second zones 42, 48 are returned by the separator 44 through a return line 46 to a cyclonic mixing and combustion chamber (CMCC) 54. As shown in Figure 20, a plurality of the return lines 46 which are connected to a plurality of the cyclonic mixing and combustion chambers (CMCC's) 54 may service one boiler. Each of the mixing chambers 54 has a first inlet which is connected to a screw conveyor 56 for solid fuel. Each mixing chamber 54 also includes a second inlet connected to the return line 46 for receiving the CFB solid material returned by the separator 44. Each mixing chamber 54 also includes second combustion air supply means in the form of a combustion air duct 58. As best shown in Figures 3a and 3b, each cyclonic mixing and combustion chamber (CMCC) 54 is in the form of a horizontally extending cylinder. The return line 46 for the CFB solids, and the combustion air line 58, both connect tangentially to the chamber 54 at a location near the inlet of the screw conveyor 56 into the chamber 54. This enhances swirling and mixing of the solids and the gas components with each other as they move along the cylindrical chamber 54 and are discharged at a fuel feed point 68 into the primary zone 42. As shown in Figure 2a, the primary zone 42 has an upwardly increasing cross sectional area by virtue of the inclined, refractory lined walls 43. So as to avoid having a primary zone which is overly tall, the embodiment of Figure 4a shows an alternative version of the invention wherein the combustion air wall ports 52, which separate the primary zone 42 from the second zone 48, are positioned on the inclined walls of the primary zone. Figure 4b shows another version of the invention wherein only one side wall of the primary zone 42 is inclined. In Figure 4c, another version of the invention includes an inclining opposite side wall of the primary combustion zone 42. In operation, approximately 25% to 45% of the total combustion air is supplied through the combustion air conduits 58 into the CMCC 54. Approximately 60% to 80% of the total combustion air is supplied in a combined fashion through the conduits 58 and through a conduit 62 for supplying combustion air to the windbox 60. The remaining combustion air is supplied through the combustion air wall ports 52. For low loads, 100% of the combustion air can be supplied through the windbox 60 and the CMCC 54, leaving the combustion air wall ports 52 dormant. Figures 5a and 5b show a still further embodiment of the invention wherein the mixing chamber 54 receives combustion air not only along the conduit 58, but also through a conduit 78 which is connected to an annular chamber 74 around the outlet end of the screw conveyor 56. An annular port or circular row of bores 76 communicate with the annular chamber 74 and discharge a ring of combustion air into the mixing chamber 54. This can initiate combustion prior to the main CMCC 54 combustion air input via the conduit 58. The operating temperature of the CMCC 54 is controlled by varying the amount of combustion air fed to the mixing chamber to obtain the desired adiabatic equilibrium combustion temperature. Therefore, the outlet temperature of the CMCC 54 may be higher than the temperature of the CFB solids. The solid fuel may be fed into the CMCC 54 by means other than a screw conveyor. Pneumatic transport or gravity feed through the top of the CMCC 54 may instead be used. Sorbent feed, used to control the sulphur emissions, may be fed with the fuel into the CMCC 54 or fed into the primary zone 42 or the furnace zone 48 directly.

Claims: A circulating fluidized bed boiler having a primary zone (42) with a fuel feed point (68) for fluidized bed material, and a furnace zone (48) above the primary zone (42), the boiler (50) comprising: a mixing chamber (54) having a first inlet for receiving solid fuel, a second inlet for receiving recycled fluidized bed solids, and an outlet connected to the fuel feed point (68); fuel feed means (56) connected to the mixing chamber first inlet for feeding solid fuel to the mixing chamber (54); first combustion air supply means (60) connected to the primary zone (42) of the boiler (50) for supplying a portion of the total combustion air needed for combustion to the primary zone (42); second combustion air supply means (58) for supplying another portion of the total combustion air needed for combustion; and a particle separator (44) above the furnace zone (48) with a return line (46) connected between the particle separator (44) and the second inlet of the mixing chamber (54) for returning solid particles to the mixing chamber (54) to form the recycled fluidized bed solids; characterised in that: the mixing chamber (54) is in the form of a horizontally extending cylinder with the first inlet being at an axial end of the mixing chamber (54) opposite from the fuel feed point (68), the second inlet being connected for tangential feed into the mixing chamber (54) at a location near the first inlet and spaced from the fuel feed point (68); and the second combustion air supply means (58) is connected for tangential feed into the mixing chamber (54) at a location near the first inlet and spaced from the fuel feed point (68), for facilitating mixing between the combustion air, the recycled fluidized bed solids and the solid fuel in the mixing chamber (54) which resulting mixture is supplied through the mixing chamber outlet to the primary zone (42) of the boiler (50). A boiler according to claim 1, wherein the mixing chamber (54) includes a refractory lining. A boiler according to claim 1 or claim 2, wherein the second combustion air supply means comprises a conduit (58) feeding tangentially into the mixing chamber (54). A boiler according to any one of the preceding claims, wherein the primary zone (42) has an upwardly increasing cross sectional area from the first combustion air supply means (60) towards the furnace zone (48). A boiler according to claim 4, wherein the primary zone (42) includes at least one wall which is inclined. A boiler according to any one of the preceding claims, including third combustion air supply means (52) connected between the primary zone (42) and the furnace zone (48) for supplying additional combustion air into the boiler (50). A boiler according to claim 5 and claim 6, wherein the third combustion air supply means comprises a plurality of combustion air wall ports (52) with at least one of the wall ports positioned in the inclined wall. A boiler according to any one of the preceding claims, including supplemental combustion air supply means (74) connected to the mixing chamber (54) for supplying additional combustion air around the fuel feed first inlet into the mixing chamber (54) to initiate combustion in the mixing chamber. A boiler according to claim 8, wherein the supplemental combustion air supply means (74) includes a circular array of bores (76) around the first inlet for supplying a ring of additional combustion air.

Inventors: PERUSKI MARK EDWARD, PERUSKI, MARK EDWARD
Applicants: BABCOCK & WILCOX CO, THE BABCOCK & WILCOX COMPANY
Application year: 1992
Main classification: F23C11
IPC codes: F23C3, F23C10, F22B1, F23C6