COAL NOZZLE ASSEMBLY FOR A STEAM GENERATION APPARATUS

Abstract

A steam generating system includes a nozzle assembly for pulverized coal and air, the coal nozzle assembly comprises an inner housing (3) for conveying primary air and coal and an outer housing (5) for conveying secondary air to an exit face (13) of a nozzle tip (1), wherein the outer housing (3) and the inner housing (5) are arranged coaxially and limit a channel (15) for the secondary air, wherein the cross-sectional area (A.sub.IH) of the inner housing (3) increases towards the exit face (13) of the nozzle tip (1), wherein the cross-sectional area (A.sub.OH) of the outer housing (5) decreases towards the exit face (13), and wherein bars (11) are located in the inner housing (3) near the exit face (13) that accelerate the velocity of the primary air and coal particles.

Claims

1. Coal nozzle assembly for a steam generation apparatus comprising an inner housing (3) for conveying primary air and coal through a nozzle tip (1) toward an exit face (13) and an outer housing (5) for conveying secondary air through the nozzle tip (1), wherein the outer housing (5) and the inner housing (3) are arranged coaxially and limit a channel (15) for the secondary air, characterized in that the cross-sectional area (A.sub.IH) of the inner housing (3) increases towards an exit face (13) of the nozzle tip (1), that the cross-sectional area (A.sub.OH) of the outer housing (5) decreases towards the exit face (13), and in that at least one bar (11) is located in the inner housing (3) near the exit face (13).

2. Coal nozzle assembly according to claim 1, characterized in that the inner housing (3) has a square or rectangular cross-section.

3. Coal nozzle assembly according to claim 1, characterized in that the outer housing (5) has a square or rectangular cross-section.

4. Coal nozzle assembly according to claim 1 characterized in that the at least one bar (11) extends between two opposite walls of the inner housing (3).

5. Coal nozzle assembly according to claim 1 characterized in that it comprises two or more bars (11) extending between two opposite walls of the inner housing (3) and being arranged parallel to each other.

6. Coal nozzle assembly according to claim 1 characterized in that the bars (11) are arranged as a grid.

7. Coal nozzle assembly according to claim 1 characterized in that downstream of the at least one bar (11) a cover plate (9) is located to prevent abrasion of the trailing edges of the at least one bar (11).

8. Coal nozzle assembly according to claim 1 characterized in that the relation between the cross section Area (A.sub.IH) of the inner housing (3) at the entrance of the primary air and the exit face (9) is within a range of 1.2 to 1.5, preferably 1.3.

9. Coal nozzle assembly according to claim 1 characterized in that the at least one bar (11) reduces the cross section Area (A.sub.IH) of the inner housing (3) at the exit face (13) by a factor within the range of 0.2 to 0.5, by a factor 0.25.

10. Coal nozzle assembly according to claim 1 characterized in that the relation between the cross section Area (A.sub.OH) of the outer housing (5) at the entrance of the primary air and the exit face (13) is within a range of 0.3 to 0.5, preferably 0.4.

11. Coal nozzle assembly according to claim 1 characterized in that a catalyst (21) is applied to the internal walls of the nozzle tip (5), namely the inner surface of the inner housing (3) and/or to the at least one bar (11).

12. Coal nozzle assembly according to claim 1 characterized in that a catalyst (21) is applied to a cover plate (9) of the nozzle tip (1).

13. Coal nozzle assembly according to claim 11 characterized in that the catalyst (21) is Lanthanum Strontium Titanate doped with metals.

14. Steam generating system which comprises a furnace and at least one coal nozzle assembly according to claim 1.

15. A method of operating a coal nozzle assembly comprising an inner housing (3) for conveying primary air and coal through a nozzle tip (1) toward an exit face (13) and an outer housing (5) for conveying secondary air through the nozzle tip (1), comprising the steps of decelerating the flow of primary air and coal particles, subsequently accelerating the flow of primary air and coal particles and mixing flow of primary air and coal particles by inducing a stall when exiting the inner housing and enclosing the primary air and coal particles by a perimeter flow of secondary air.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1: A perspective view of an embodiment of a nozzle tip according to the invention;

[0023] FIG. 2: A longitudinal section of the nozzle of FIG. 1 and

[0024] FIG. 3: A longitudinal section of the nozzle of FIG. 1 illustrating the flow of the primary and the secondary air.

DETAILED DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 illustrates a perspective view of a nozzle tip 1 according to the invention. An inner housing 3 of the nozzle tip 1 is surrounded by an outer housing 5. The space between the outer housing 5 and the inner housing 3 forms a channel for transporting secondary air into a furnace (not shown). The secondary air exits the nozzle tip 1 via a square or rectangular gap between the inner housing 3 and the outer housing 5, thus building a perimeter flow of secondary air. This gap between the inner housing 3 and the outer housing 5 is the exit area of the a.m. channel for transporting secondary air.

[0026] The primary air and the entrained coal particles exit the nozzle tip 1 through openings 7 in a cover plate 9. For reasons of clarity, not all openings 7 have reference numerals. Altogether, there are sixteen (square) openings 7 visible in FIG. 1.

[0027] As can be seen from FIG. 1, the cover plate 9 has a grid-like design dividing an exit face of the nozzle tip 1 in sixteen openings 7.

[0028] FIG. 2 shows a longitudinal section through the nozzle tip 1 according to FIG. 1.

[0029] In FIG. 2 the inner housing 3 can be seen more clearly than in FIG. 1. The cross sectional area of the inner housing on the right side of FIG. 1 (this is where the primary air and the coal enter the nozzle tip 1) is smaller than the cross sectional area of the inner housing at the cover plate 9. In this view the cross sectional area cannot be seen. Only the height H can be seen. Of course, the cross section area A depends from the height H; in case of a square the cross section area A=HH.

[0030] The difference in the height of the inner housing 3 can be used to illustrate this fact. In FIG. 2 the height H1 at the entrance of the primary air into the inner housing 3 is smaller than the height H2 near the cover plate 9 of the inner housing 3. The different heights H1, H2 indicate the growth of the cross sectional area A of the inner housing 3 from the entry towards the cover plate 9.

[0031] This increasing cross sectional area A of the inner housing 3 reduces the velocity of the flow of the primary air which promotes mixing of the coal particles and the primary air.

[0032] This mixing takes place inside the nozzle tip 1. To avoid that the flame is pulled inside the nozzle tip 1 at least one bar 11 is arranged near an exit face of the nozzle tip 1. The downstream and blunt end of the at least one bar 11 may be protected against abrasion by an optional cover plate 9.

[0033] In this embodiment the bars 11 have a triangular cross section and are arranged in a grid-like manner. A tip of this triangular cross section of the bars 11 has the reference numeral 13 and is the most upstream part of the bars 11.

[0034] As a result, the primary air flowing through the inner housing 3 is accelerated just before exiting the nozzle tip via the openings 7 between the bars 11 and in the optional cover plate 9. This prevents pulling the ignition point of the flame inside the inner housing 3.

[0035] It is obvious, that the bars 11 not necessarily have a triangular cross section. Other cross sections resulting in an acceleration of the velocity of the primary air without raising the pressure drop more than necessary are possible, too.

[0036] The cover plate 9 is an optional feature to prevent the downstream and blunt end of the bars 11 from abrasion. Either the blunt end of the bars or the cover plate 9 induce stall to the primary air which initializes further mixing of coal particles and the primary air when entering the furnace.

[0037] As can be seen from FIG. 2, the outer housing 5 and the inner housing 3 limit a channel 15 through which the secondary air (cf. the arrows 17) flows. The primary air that flows through the inner housing 3 is illustrated by arrows 19.

[0038] As further can be seen from FIG. 2, the cross sectional area of the channel 15 near the cover plate 9 or the blunt ends of the bars 11 is smaller than at the entrance of the secondary air (on the right side of FIG. 2).

[0039] Further, the outer housing 5 is formed as a truncated pyramid near the cover plate 9, thus directing the secondary air exiting the gap 20 between the outer housing 5 and the inner housing 3 inwardly to keep the primary air focused and directed to the flame inside the furnace (not visible).

[0040] The claimed nozzle tip results in an efficient combustion and low NOx emissions.

[0041] To further reduce the NOx emissions of the claimed Ultra-Low NOx burner nozzles a catalyst 21 may be applied to the internal walls of the nozzle tip 1, namely the inner surfaces of the inner housing 3, the bars 11 and the cover plate 9 that are in contact with the primary air and the entrained coal particles. The catalyst 21 is more effective in the regions of decelerated flow, i. e. the inner surface of the inner housing 3 just upstream of the bars 11.

[0042] Catalytic combustion of the volatile matter in the injected fuel is achieved at temperatures favorable for the reduction of NOx species originating from the volatile matter or partial combustion of solid fuels. Catalytic combustion inside the nozzle tip also improves the quality of the flame downstream and corresponding reduced NOX emission within the furnace.

[0043] Catalytic combustion of the volatile matter in the injected fuel is achieved at temperatures favorable for the reduction of NOx species originating from the volatile matter or partial combustion of solid fuels. Catalytic combustion on the nozzle cover plate also improves the quality of the flame and corresponding reduced NOX emission within the furnace.

[0044] The catalyst may be of the perovskite-type with catalytic activity in the preferred temperature range, but not limited to, of 500 C. to 900 C.

[0045] FIG. 3 shows the cross section of FIG. 2 without reference numerals but with the arrows 17 and 19 to illustrate the flow and the mixing of the primary air and the coal particles behind the cover plat e 9 in the furnace.

[0046] Further, the velocity of the primary air and the secondary air is shown in two diagrams. The respective deceleration and the subsequent acceleration of the primary air are illustrated as well as the acceleration of the secondary air.

LIST OF REFERENCE NUMERALS

[0047] 1 nozzle tip [0048] 3 inner housing [0049] 5 outer housing [0050] 7 opening [0051] 9 cover plate [0052] 11 bar [0053] 13 tip of the bar [0054] 13 exit face [0055] 15 channel [0056] 17 arrows (secondary air) [0057] 19 arrows (primary air) [0058] 20 gap [0059] 21 catalyst