BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a schematic drawing showing the predicted reverse flow IRZ streamlines for a low-NOx pulverized coal fired burner having the conventional air separation cone;
FIG. 2 is a schematic drawing of the mid-zone air separation cone of the present invention at the end of a burner;
FIG. 3 is a graph plotting reverse volumetric flow rate versus axial distance for both a conventional air separation cone and the mid-zone air separation cone of the present invention;
FIG. 4 is a schematic drawing of the low NOx DRB-XCL® pulverized coal burner incorporating the mid-zone air separation cone of the present invention;
FIG. 5 is a schematic drawing of the low NOx DRB-4® burner incorporating the mid-zone air separation cone of the present invention; and
FIG. 6 is a schematic drawing of the low NOx central air jet pulverized coal burner incorporating the mid-zone air separation cone of the present invention.
FIG. 7 is a schematic drawing of the low NOx XCL-S pulverized coal burner incorporating the mid-zone air separator cone of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, in which like reference numerals are used to refer to the same or similar elements, FIG. 2 shows the end of a burner 2 which is adjacent or near a furnace. The end of the burner 2 includes a large diameter mid-adjacent air separation cone 1 with a short cylindrical leading edge that fits in the middle of an outer secondary air zone 4. The device is supported by standoffs (not shown) inside the outer secondary air zone 4 and is not directly connected to any conduits in the burner. It essentially splits the outer air zone 4 secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone 1 is farther from the burner centerline than the radial position of the conventional air separation cone shown in FIG. 1, it expands the IRZ size and with that, the NOx emissions are minimized.
The diverging angle of the mid-zone air separation cone can be between 25 to 45° from the horizontal axis (50 to 90° included angle). Although the embodiment in FIG. 2 shows that mid-zone air separation cone fits at approximately the middle of the outer air zone annulus, the cone may also be fitted anywhere within the outer air zone annulus to divide the secondary air stream in any desired proportion. The length of the cone 1 can vary depending on the air zone gap and burner size. The mid-zone air separation cone 1 can also be used in burners designed for firing pulverized coal, fuel oil, and natural gas.
FIG. 3 shows the computer modeling predictions of reverse (recirculating) flow rates in the near-burner region of the flame at different axial distances up to 2.5 burner diameters (x/D=2.5). The plots clearly indicate a larger IRZ (more reverse flow) for the case with the mid-zone air separation cone relative to conventional air separation cone. It is noted that the calculations correspond to staged combustion of an eastern bituminous coal at 0.85 burner stoichiometry.
FIGS. 4 through 7 show four possible installations of the mid-zone air separation cone 1 in four different types burners. Although four different embodiments of the invention are shown, the invention is not limited to these embodiments. The mid-zone air separation cone of the present invention can also be installed in other burners not shown here, where there is at least one air zone surrounding a fuel delivery nozzle or annulus.
FIG. 4 shows installation of the mid-zone air separation cone 1 in a low NOx DRB-XCL® pulverized coal burner 10, which is described in more detail as prior art (FIG. 2) in U.S. Pat. No. 5,829,369, which is incorporated by reference. The burner 10 includes a conical diffuser 12 and deflector 34 situated within the central conduit of the burner 10 which is supplied with pulverized coal and air by way of a fuel and primary air (transport air) inlet 14. A windbox 16 is defined between the inner and outer walls 18, 20 respectively. The windbox 16 contains the burner conduit which is concentrically surrounded by walls which contain an outer array of fixed spin vanes 22 and adjustable angle spin vanes 24 within an outer air zone 26. An inner air zone 27 is provided concentrically within the outer air zone 26. The burner 10 is provided with a flame stabilizer 30 and a slide damper 32 for controlling the amount of secondary air 28.
A mid-zone air separation cone 1 of the present invention is provided for increasing the IRZ zone and decreasing NOx. The air separation cone 1 has a larger diameter than the air separation cone shown in FIG. 1. The mid-zone air separation cone 1 also has a short cylindrical leading edge that fits in the middle of the outer air zone 26. The mid-zone air separation cone 1 is supported by standoffs (not shown) inside the outer air zone 26. The mid-zone air separation cone 1 splits the outer air zone 26 secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone 1 is farther from the burner centerline than the conventional air separation cone shown in FIG. 1, it expands the IRZ size and accordingly, NOx emissions are minimized.
FIG. 5 shows a burner generally depicted 40 in accordance with the present invention. Burner 40, which is also referred to as the DRB-4Z® burner, comprises a series of zones created by concentrically surrounding walls in the burner conduit which deliver a fuel such as pulverized coal with a limited stream of transport air (primary air), and additional combustion air (secondary air) 28 provided from the burner windbox 16. The central zone 42 of the burner 40 is a circular cross-section primary zone, or fuel nozzle, that delivers the primary air and pulverized coal by way of inlet 44 from a supply (not shown). Surrounding the central or primary zone 42 is an annular concentric wall 45 that forms the primary-secondary transition zone 46 which is constructed either to introduce secondary combustion air or to divert secondary air to the remaining outer air zones. The transition zone 46 acts as a buffer between the primary and secondary streams to provide improved control of near-burner mixing and flame stability. The transition zone 46 is configured to introduce air with or without swirl, or to enhance turbulence levels to improve combustion control. The remaining annular zones of burner 40 consist of the inner air zone 48 and the outer air zone 50 formed by concentrically surrounding walls which deliver the majority of the combustion air.
The burner 40 includes a mid-zone air separation cone 1 having a short cylindrical leading edge that fits in the middle of the outer air zone 50. The mid-zone air separation cone 1 is supported by standoffs (not shown) inside the outer secondary air zone annulus. The mid-zone air separation cone 1 splits the outer air zone 50 secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone 1 is farther from the burner centerline than the conventional air separation cone shown in FIG. 1, it expands the IRZ size and accordingly, NOx emissions are minimized.
Structurally, the design of the burner 40 (DRB-4Z®) according to the present invention is based largely on that for the DRB-XCL® burner shown in FIG. 4. A detailed explanation of the differences between the two types of burners is provided in U.S. Pat. No. 5,829,369.
FIG. 6 shows a low NOx central air jet pulverized coal burner 60 in which pulverized coal and primary air (PA/PC) 61 enter at an inlet and pass through a burner elbow 62. The pulverized coal mostly travels along the outer radius of the elbow 62 and concentrates into a stream along the outer radius at the elbow exit. The pulverized coal enters a coal pipe 63 and encounters a deflector 64 which redirects the coal stream into a conical member 65, dispersing the coal. A core or central pipe 66 is attached to the downstream side of conical member 65. The coal pipe 63 expands in section 63A to form a larger diameter section 63B. The dispersed coal travels into an annulus 71 formed between central pipe 66 and the coal pipe 63A and then 63B. The PA/PC 61 then exits the coal annulus 71 into the burner throat 68, and then out into the furnace (not shown). The core or central pipe 66 and the annulus 71 form a fuel nozzle.
Secondary air 78 is supplied by forced draft fans or the like, preheated in air heaters, and supplied under pressure. Feeder duct 69 supplies core air to central zone 66. Wedged shaped pieces 69A and 69B provide a more contoured flow path for the PA/PC 61 as it travels past the core air supply feeder duct 69. The core air proceeds down central zone 66 until it exits. Some secondary air flows into transition zone 76 or outer air zone 77. Secondary air can be throttled to one zone or the other, or to supply lesser quantities of air to both zones to cool the burner when the burner is out of service. The transition zone 76 is separated from the outer air zone 77. The transition zone 76 is constructed to provide air for near-burner mixing and stability. Adjustable angle spin vanes 81 are situated in the transition zone 76 to provide swirl to transition air. Outer air proceeds through fixed spin vanes 80 and adjustable angle spin vanes 82 which impart swirl to the outer air.
A large diameter mid-zone air separation cone 1 with a short cylindrical leading edge fits in the middle of the outer air zone 77. The cone 1 is supported by standoffs (not shown) inside the outer air zone 77 and is not directly connected to any conduits in the burner. The cone 1 splits the outer air zone 77 secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone 1 is farther from the burner centerline than the conventional air separation cone shown in FIG. 1, it expands the IRZ size and with that, the NOx emissions are minimized.
Performance of the mid-zone air separation cone was further tested with low NOx central air jet pulverized coal burner at 100 million Btu/hr while firing a pulverized eastern bituminous coal. At 17% overall excess air level, and 0.80 burner stoichiometry, NOx emissions were 0.276 lb/million Btu with the conventional air separation cone installed on the end of the cylindrical sleeve 5 separating the transition zone 76 from outer air zone 77, and 0.238 lb/million Btu with the mid-zone air separation cone, shown in FIG. 6, while maintaining low CO and unburned carbon levels.
FIG. 7 show another low NOx burner embodiment according to the present invention. A fossil fuel, such as pulverized coal, and primary air enter burner 100 via burner inlet 102, and pass through burner elbow 104. Secondary air 106 is provided to outer air zone 108, wherein swirl may be added via adjustable vanes 110.
Mid-zone air separation cone 1 is provided within outer air zone 108. Air separation cone 1 is supported by standoffs (not shown) inside the outer air zone 108. Air separation cone 1 splits the outer air zone 108 secondary air flow into two streams and deflects a portion of the secondary air flow radially outward. Since the radial position of the air separation cone 1 is farther from the burner centerline than the conventional air separation cone shown in FIG. 1, it expands the IRZ size and provided a means for minimizing NOx emissions.
While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
Patent Application
20070281265
