Claims:WE CLAIM:
1. Secondary air distribution system of circulating fluidized bed boiler comprising of a plurality of secondary air nozzle ducts branching from a secondary air common header duct connected to respective nozzle in a combustor at various elevations thereof wherein each of said secondary air nozzle ducts is provided with a flow orifice plate with a hole.

2. Secondary air distribution system of circulating fluidized bed boiler as claimed in claim 1, wherein said orifice plate is having profile corresponding to that of the nozzle duct provided with the hole at centre thereof, in which diameter of the hole is determined relying on the flow area required and the pressure drop at the orifice for each secondary air nozzle.

3. Secondary air distribution system of circulating fluidized bed boiler as claimed in claim 1 or 2, wherein the outer diameter of the orifice plate is equivalent to the secondary air duct inner diameter to remain intact and have optimum flow to said each secondary air nozzle.

4. Secondary air distribution system of circulating fluidized bed boiler as claimed in any of the preceding claims, wherein secondary air duct connecting secondary air fan to said secondary air nozzle ducts forms secondary air common header duct.

5. Secondary air distribution system of circulating fluidized bed boiler as claimed in any of the preceding claims, wherein the secondary air common header pressure and total air flow is controlled by Secondary air fan and by regulating dampers provided in the secondary air common header duct.

6. Secondary air distribution system of circulating fluidized bed boiler as claimed in any of the preceding claims, wherein aero foil arrangement is provided so as to monitor the flow through said duct.

7. Secondary air distribution system of circulating fluidized bed boiler as claimed in any of the preceding claims, wherein dimensioning of secondary air seal boxes and refractory thickness at secondary air injection nozzle varies according to the slopes of combustor wall and angle of injection of each secondary air nozzle.

8. Secondary air distribution system of circulating fluidized bed boiler as claimed in any of the preceding claims, wherein lower portion of combustor water walls are refractory lined and secondary air injection nozzles are placed in sloped wall of said combustor where refractor is thicker than in the secondary air injection nozzle connecting to straight wall of the combustor.
, Description:Secondary air distribution system in circulating fluidized bed boilers
FIELD OF INVENTION
[001] The present invention is related to secondary air distribution system in circulating fluidized bed boilers (CFBC). More particularly the invention is dedicated to the secondary air system design factors considered to prevent refractory failure owing to excessive expansion in the Secondary air nozzle area. It is also related to a common control system for secondary air distribution across different combustor elevation to reduce Nox emission.

BACKGROUND OF THE INVENTION

[002] In large capacity circulating fluidized bed boilers, the size of combustor is bigger which requires a secondary air distribution system at various elevations across the combustor so as to provide staged combustion for better Nox control. To have multiple secondary air injection nozzles circulating fluidized bed boiler combustors are selected with Pant leg type designs. In Pant leg type design, combustor walls at the bottom portion are devised with different slopes forming a trapezoidal shape in order to have better fluidization. Secondary air nozzles are located at two elevations at the trapezoidal portion of the combustor. The combustor pressure level varies across the elevation due to the quantity of bed material circulating.

[003] According to an existing art available in the market, which is best seen in figure 1 the arrangement of secondary air distribution system consists of secondary air injection nozzles (3), flow control dampers (4), secondary air duct forming common header (8), aero foils for flow measurement (5) and secondary air fan (6). These secondary air nozzles are directed to combustor through seal boxes (13) filled with refractory (12) and forming the secondary air opening in combustor (1).

[004] As per the existing art (Figure 1) secondary air to upper elevation (9) and lower elevation (10) of the combustor (1) is controlled by a flow control damper (4). During the course of operation, insufficient secondary air through lower elevation nozzles (10) causes refractory erosion in the lower elevation SA nozzles and backflow of bed material into secondary air common header (8) causing blockage of flow area.

[005] Now, reference may be made to the following publication.

[006] US patent 5378443 A “Method of reducing emissions when burning nitrogen containing fuels” teaches the combustion with over-stoichiometric conditions is effected in the combustion chamber at a relatively low temperature of about 700°-1000° C. Combusting coal at about 850° C in circulating fluidized bed boilers. At this temperature a low NO2 formation is achieved and maximum Sulphur capture with lime occurs. Air may be introduced at different levels in order to achieve staged combustion and minimal NOx formation. However, this invention is only related to Nox control in circulating fluidized bed boiler by providing staged secondary air for combustion.

[007] Thus, none of the aforesaid prior arts can fulfill the requirements of the present invention, for which it is designed.

[008] Hence there is a need to develop a secondary air distribution system to deliver required flow though each secondary air nozzles with variation in combustor pressure across the elevation.

OBJECTS OF THE INVENTION

[009] An object of the invention is to propose secondary air distribution system in circulating fluidized bed boilers which overcomes disadvantages of prior art(s).

[0010] Another object of the invention is to propose secondary air distribution system in circulating fluidized bed boilers which is simple in construction.

[0011] Still another object of the present invention is to propose secondary air distribution system in circulating fluidized bed boilers which enables to vary the flow through individual secondary air Nozzles.

[0012] Yet another object of the invention is to propose secondary air distribution system in circulating fluidized bed boilers having a common control system varying the required flow through individual secondary air nozzles.

[0013] Further object of the invention is to propose secondary air distribution system in circulating fluidized bed boilers which reduces NOx, prevents refractory erosion and back flow of bed material into secondary air common header duct.

[0014] Another object of the invention is to provide staged combustion at combustor bottom portion considering Nox reduction.

SUMMARY OF THE INVENTION

[0015] Secondary air distribution system of circulating fluidized bed boiler comprising of a plurality of secondary air nozzle ducts branching from a secondary air common header duct connected to respective nozzle in a combustor at various elevations thereof wherein each of said secondary air nozzle ducts is provided with a flow orifice plate with a hole.

[0016] Said orifice plate is having profile corresponding to that of the nozzle duct provided with the hole at centre thereof, in which diameter of the hole is determined relying on the flow area required and the pressure drop at the orifice for each secondary air nozzle.

[0017] The outer diameter of the orifice plate is equivalent to the secondary air duct inner diameter to remain intact and have optimum flow to said each secondary air nozzle.

[0018] Secondary air duct connecting secondary air fan to said secondary air nozzle ducts forms secondary air common header duct.

[0019] The secondary air common header pressure and total air flow is controlled by Secondary air fan and by regulating dampers provided in the secondary air common header duct.

[0020] Aero foil arrangement is provided so as to monitor the flow through said duct.

[0021] Dimensioning of secondary air seal boxes and refractory thickness at secondary air injection nozzle varies according to the slopes of combustor wall and angle of injection of each secondary air nozzle.

[0022] Lower portion of combustor water walls are refractory lined and secondary air injection nozzles are placed in sloped wall of said combustor where refractor is thicker than in the secondary air injection nozzle connecting to straight wall of the combustor.

[0023] Accordingly, the present invention with flow orifice plate in the individual branch ducts of the secondary air distribution system ensures that required flow through each secondary air nozzles reduces NOx and prevents refractory erosion and back flow of bed material into secondary air common header duct. The shape and dimensions of the flow orifice plates are determined based on the flow requirement according to the pressure difference across the elevation and on the requirement for cooling the refractory in secondary nozzle area.

[0024] Sizing of secondary air seal boxes and refractory thickness at secondary air injection nozzle varies according to the slopes of combustor wall and angle of injection of each secondary air nozzle. Failure rate of refractory increases with increase in refractory thickness in secondary air nozzle due to lack of cooling. Secondary air flow rate through each secondary air injection nozzle is also to supplement cooling of refractory in each secondary air nozzle controlling excessive expansions.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0025] Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings of the exemplary embodiments and wherein:

Fig 1 shows: Conventional secondary air distribution system according to prior art.
Fig 2 shows: Secondary air distribution system in circulating fluidized bed boilers in accordance with present invention.
Fig 3 shows: Orifice plate in secondary air nozzle duct of present invention.
Fig 4 shows: Thickness of refractory at seal box of secondary air injection to sloped wall of combustor of invention.
Fig 5 shows: Thickness of refractory at seal box of secondary air injection to straight wall of combustor of invention.

DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

[0026] The present invention makes a disclosure regarding secondary air distribution system in circulating fluidized bed boilers. The principle of circulating fluidized bed boiler is to fluidize the bed material in the combustor by primary air supplied through bottom nozzles and secondary air is injected at various locations over the bed by secondary air nozzles connected at the lower portion of the combustor. Since better distribution and penetration of secondary air into the combustor improves the combustion efficiency and lowers Nox emission level, Design optimization of secondary air distribution system is required, for which the instant invention has been introduced.

[0027] Now, reference may be made to fig. 2 illustrating secondary air distribution system of the present invention. Said system includes secondary air common header duct (8) constituting two different sizes of secondary air ducts. A plurality of secondary air nozzle ducts (2) branching from the secondary air common header duct (8) are connected to respective secondary air nozzle (3) in the combustor (1). Secondary air nozzle ducts and secondary air nozzles can be of same sizes. Each individual branch duct (2) is provided with a flow orifice plate (11).

[0028] Said orifice plates are having profile corresponding to that of the secondary air nozzle duct with a hole at the centre, wherein diameter of the hole is determined relying on the flow area required and the pressure drop at the orifice for each secondary air nozzle. The outer diameter of the orifice plate is equivalent to the secondary air duct inner diameter to remain intact.

[0029] The provision of orifice plate is made to avail optimum flow to each secondary air nozzle. The material of the plate is preferably carbon steel without restricting ambit of the invention to the same. Thus, other materials readily apparent to a person skilled in the art are understood to be within scope of the invention.

[0030] In circulating fluidized bed boilers, combustion of fuel takes place in combustor 1 formed by waterfall panel on all four sides, where Primary air, secondary air and fuel are supplied. Primary air is supplied through the primary air wind box and bottom grate (floor) air nozzles 7 of the combustor. The primary function is to fluidize the bed material inventory in the combustor. The pressure inside the circulating fluidized bed combustor decreases across the combustor height, with maximum pressure measured at bottom and decreases at combustor top. This pressure difference is due to the circulating bed inventory.

[0031] The total air required for complete combustion of the fuel is provided by Primary and secondary air, in which the secondary air ensures proper combustion of the fuel. The total secondary air required is distributed across the combustor walls at different elevations.

[0032] The secondary air is supplied to the combustor through Secondary air nozzles 3 connecting to the side walls of the combustor at different elevations, secondary air nozzle connecting at two different elevations 9, 10 are shown in the figure 2.

[0033] The total secondary air required for combustion is supplied by the Secondary fan 6. The secondary air duct connecting the secondary air fan (6) to all secondary air nozzle ducts 2 forms secondary air common header duct 8. The flow through each secondary air nozzle is due to pressure difference between secondary air common header (8) (higher pressure) & the pressure inside the combustor at secondary air nozzle location (lower pressure) which is decided by the flow area of the Secondary air nozzle ducts. The secondary air common header pressure and total air flow is controlled by Secondary air fan 6 and by regulating dampers 4 provided in the secondary air common header duct. The dampers can be operated to vary the flow area of the secondary air common duct. In order to monitor the flow through the duct aero foil 5 arrangement is provided. The air flow is calculated based on the pressure drop measured across the aero foil. The orifice plates 11 are selected to vary the flow area of the secondary nozzle ducts in order to compensate the pressure difference across different elevations of the combustor and maintaining the required flow through each Secondary nozzle.

[0034] In circulating fluidized bed boiler, combustor is positively pressurized and the pressure level decreases across the height of the combustor. As can be seen in figure 2, secondary air injection takes place through multiple nozzles (3) connecting to combustor (1) at two different elevations E1 (10) and E2 (9). The combustor pressure at elevations E1 and E2 are P1 and P2 respectively. The pressure P1 can be calculated from length L1, breadth B1, height H1 of the combustor at E1 and density of fluidizing medium p. The pressure P2 can be calculated from length L2, breadth B2, height H2 of the combustor at E2 and density of fluidizing medium p. The operating pressure of secondary air common header is P3. Since the pressure P1 is greater than P2, the secondary air ducts branching from common header are required to be sized accordingly to have uniform flow.

[0035] The circulating fluidized bed boiler combustor has a trapezoidal cross section at bottom portion where the secondary air nozzles are connected. To form a trapezoidal shape, combustor walls have different slope. In the combustor water wall (membrane wall) opening for secondary air injection are providing bends in the straight tubes forming the Water wall. The secondary air nozzles sloping downwards are connected to the openings. For sealing the combustor in the secondary air nozzle area, there is provision of seal boxes filled with a refractory layer inside. In the figure, the thickness of the refractory 12 provided in the seal boxes 13 of secondary air Nozzles at combustor straight wall and sloped wall are shown.

[0036] In circulating fluidized bed boilers lower portion of combustor water walls are refractory lined due to higher velocities. Secondary air injection nozzles are connected in the lower part of combustor in the opening provided. Now referring to fig. 4, secondary air injection nozzles are placed in sloped wall of combustor where the thickness of refractory (12) is more in comparison to (figure 5) refractory provided in the secondary air injection nozzle connecting to straight wall of the combustor.

[0037] It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-