DOWNSHOT BURNER

Abstract

A burner outlet set for a downshot firing burner is described comprising a first outlet array having at least one primary outlet, and at least one vent air outlet disposed either side of the primary outlet in an array direction of the first outlet array; second and third outlet arrays each comprising an array of secondary air outlets, respectively disposed either side of the a first outlet array. A burner system with a plurality of such burner outlet sets, a burner arch configured for downshot firing and having one or more such burner sets, and a combustion furnace with one or more such arches are also described.

Claims

1. A burner outlet set for a downshot firing burner comprising: a first outlet array comprising at least one primary outlet, and at least one vent air outlet disposed either side of the primary outlet in an array direction of the first outlet array; second and third outlet arrays each comprising an array of secondary air outlets, respectively disposed laterally either side of the a first outlet array,

2. A burner outlet set thus comprises a first outlet array disposed in an elongate first array direction and comprising at least one primary outlet with at least one vent air outlet disposed either side of the primary outlet, and paired second and third outlet arrays each comprising a plurality of secondary air outlets respectively disposed in elongate second and third array directions that are laterally spaced either side of the first outlet array. Typically the second and third outlet arrays are even laterally spaced either side of the first outlet array. Typically the second and third outlet arrays and arrayed in elongate array directions generally parallel to the first array direction. Typically each of the first, second and third outlet arrays extend in elongate array directions to the same longitudinal extent.

3. A burner outlet set in accordance with claim 1 wherein each primary outlet is a square or rectangular outlet disposed with a median line direction parallel to and for example coincident with the array direction of the first outlet array.

4. A burner outlet set in accordance with claim 1 or claim 2 wherein each vent air outlet is a square or rectangular outlet disposed with a median line direction parallel to and for example coincident with the array direction of the first outlet array.

5. A burner outlet set in accordance with any preceding claim wherein each secondary air outlet is a square or rectangular outlet disposed with a median line direction parallel to and laterally spaced from the array direction of the first outlet array.

6. A burner outlet set in accordance with any preceding claim wherein each primary outlet is associated with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array.

7. A burner outlet set in accordance with claim 6 wherein each vent air outlet is associated with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array.

8. A burner outlet set in accordance with claim 7 wherein each outlet in the first array is transversely aligned with exactly one secondary outlet in the second array and exactly one secondary outlet in the third array.

9. A burner outlet set in accordance with any preceding claim wherein a first outlet array comprises at least one primary/vent outlet module consisting exactly of a single primary outlet and a single vent air outlet disposed either side of the primary outlet.

10. A burner outlet set in accordance with any preceding claim wherein a first outlet array comprises at least one primary/vent outlet module consisting exactly of a pair of primary outlets and a single vent air outlet disposed either side of the primary outlet.

11. A burner outlet set in accordance with claim 9 or 10 wherein a first outlet array comprises exactly one said primary/vent outlet module.

12. A burlier outlet set in accordance with claim 9 or 10 wherein a first outlet array comprises exactly two of the said primary/vent outlet modules disposed adjacently.

13. A burner outlet set in accordance with any preceding claim wherein each primary outlet is provided with a wedge shaped bluff-body stabilizer.

14. A burner outlet set in accordance with claim 13 wherein the bluff body stabilizer is slitted and located in a position withdrawn into its outlet.

15. A burner outlet set in accordance with any preceding claim wherein each primary outlet is a square or rectangular slot provided with four ignition teeth located at the corners of the square or rectangular slot.

16. A burner system comprising a plurality of burner outlet sets in accordance with any preceding claim.

17. A fuel combustion system comprising: a pulverous solid fuel supply source; a transport gas supply source to supply comburant gas as a transport gas; a mixing module configured to entrain fuel from the fuel supply within the transport gas; a transport conduit to transport the fuel entrained in the transport gas; a separator configured to separate the mixture of fuel and transport gas into a relatively fuel rich primary stream and a relatively fuel depleted vent air stream; a primary conduit to convey the primary stream to primary outlets of a burner outlet set in accordance with any preceding claim; a vent air conduit to convey vent air to vent air outlets of a burner outlet set in accordance with any preceding claim; a secondary air supply system including a secondary conduit to convey secondary air to the secondary air outlets of a burner set of a burner outlet set in accordance with any preceding claim.

18. A burner arch configured for downshot firing of fuel in a combustion chamber of a combustion apparatus having at least one burner outlet set in accordance with one of claims 1 to 15 disposed on the burner arch whereby combustion of the fuel is supported in the vicinity of the burner set during use,

19. A combustion apparatus comprising: a combustion chamber; at least one burner arch configured for downshot firing of fuel in the combustion chamber; at least one burner outlet set in accordance with one of claims 1 to 15 disposed on the burner arch whereby combustion of the fuel is supported in the vicinity of the burner set during use.

20. A combustion apparatus in accordance with claim 19 further configured for the supply of overfire air to the combustion chamber in that each burner set is provided in association with at least one outlet or series of outlets for overfire air.

21. A combustion apparatus in accordance with claim 20 wherein each burner set is provided in association with at least one outlet or series of outlets for overfire air respectively provided at each of first and second levels respectively below and above a flame zone within the combustion chamber.

22. A combustion apparatus in accordance with claim 21 wherein a first overfire air outlet or series of outlets is provided above and closely adjacent to a hopper knuckle of the combustion chamber, and wherein a second overfire air outlet or series of outlets is above and closely adjacent to the burner arch.

23. A combustion apparatus in accordance with claims 19 to 22 further comprising: a pulverous solid fuel supply source; a transport gas supply source to supply comburant gas as a transport gas configured to entrain fuel from the fuel supply within the transport gas; a transport conduit to transport the fuel entrained in the transport gas; a separator configured to separate the mixture of fuel and transport gas into a relatively fuel rich primary stream and a relatively fuel depleted vent air stream; a primary conduit to convey the primary stream to the primary outlets; a vent air conduit to convey vent air to the vent air outlets; a secondary air supply system including a secondary conduit to convey secondary air to the secondary air outlets.

Description

[0079] The invention will now be described by way of example only with reference to FIGS. 1 to 8 of the accompanying drawings in which:

[0080] FIG. 1 is a cut away side elevation of a prior art downshot fired furnace,/steam generator arrangement for a thermal power plant;

[0081] FIG. 2 is a plan view of a prior art burner outlet set of a burner of the downshot fired furnace/steam generator arrangement of FIG. 1;

[0082] FIG. 3 shows a possible arrangement for overfire air which may be applied generally not only to the prior art furnace illustrated in FIG. 1 but to a furnace modified to include burner sets in accordance with the invention such as might be exemplified in FIGS. 4-8;

[0083] FIGS. 4-8 show alternative example arrangements of burner sets on half burner arches embodying the principles of the first aspect of the invention.

[0084] An example prior art downshot firing arrangement is shown in FIGS. 1 (furnace) and 2 (burner).

[0085] In a downshot firing arrangement for a steam generator 1 such as that illustrated, fuel and combustion air is introduced into the furnace via burners 2 situated on the firing arches 3. The inlet flows are directed generally downwards to the hopper 4. Paired firing arches with equivalent burner arrangements are provided so that in the idealised situation a symmetrical W shaped flame pattern is developed. The flame geometry serves to give a flame with a long residence time for more effective combustion of low volatile fuels.

[0086] Because of the low volatile content of the coals typically fired in downshot plant, flame stabilization cannot rely upon the rapid release from volatiles combustion in the early part of the flame (as practised in conventional swirl stabilized circular burners for bituminous and low rank coals). Instead stabilization is achieved by the recycling of heat from the hot products of combustion through the following mechanisms:

[0087] radiation from the hot upward flowing combustion products into the cold downward flowing inlet coal and air (6);

[0088] radiation from hot refractory surfaces in the lower furnace into the cold inlet stream (7); refractory is typically installed on the firing arches and the front/rear walls below the arches;

[0089] entrainment of hot products of combustion from the upward flowing gases into the cold inlet stream (8).

[0090] The firing arrangement of FIG. 1 is shown in a furnace of a thermal electric utility power plant. The thermal energy is used to heat a process fluid and thereby drive a series of turbines in a generator in familiar manner, and the skilled person will readily infer the features of such a more complete thermal electric utility power plant system. The invention is applicable to the general furnace principles illustrated by FIG. 1, and is embodied in the detail of the burner set arrangements such as illustrated by example in FIGS. 4 to 8.

[0091] The fuel source is a low volatile carbonaceous fuel in pulverous form such as a low volatile coal in pulverous form, for example anthracite or lean coal.

[0092] Coal and transport air from the pulverizing plant is separated into two streams in a cyclone (10); a vent air stream 11 containing typically 60% of the air and 10% of the coal, and a primary stream 12 containing typically 40% of the air and 90% of the coal. Following separation, the coal and air are introduced into the furnace via an arrangement of slots in the firing arch and described in more detail with reference to FIG. 2, a series of slots each primary air/coal slot being located between a pair of longer secondary (windbox) air slots.

[0093] Separation of the coal and transport air into a fuel lean and a fuel rich stream aims to further improve ignition and flame stability. Removal of air from the coal reduces the total mass of fuel stream, and therefore reduces the time for this stream to be heated to ignition temperature. Furthermore the higher concentration of the coal creates a mixture that is more conducive to combustion. It is generally considered that for low volatile coals a particle concentration in the range 1.01.5 kg/m.sup.3 gives the highest propagation velocities (i.e. best combustibility) (M Tarniguchi et al; Comparison of flame propagation properties of petroleum coke ad coals of different rank; Fuel 88 (2009), 1478-1484). In the described downshot system the concentration of coal in the primary stream is typically 1.3 kg/m.sup.3.

[0094] A tertiary air stream 14 is introduced on the front and rear walls just above the hopper knuckle.

[0095] Wall air, supplied from the secondary air windboxes, is introduced at the outside edge of the furnace arch via slots created by the removal of the membrane strip,

[0096] The various streams are introduced via a series of slots, as shown in FIG. 2. Following separation, the coal and air are introduced into the furnace via a series of slots, each primary air/coal slot being located between a pair of longer secondary (windbox) air slots. Vent air slots are additionally provided.

[0097] The slot dimensions are chosen to deliver the required inlet velocities; in particular the velocity differential between the primary and secondary streams facilitates the mixing between them.

[0098] In the particular arrangement of FIG. 2, part of a burner arch 20 is shown in simple schematic view carrying three burner sets 21 per half arch, and a central outlet 22 for thermal biasing air. This simple schematic is shown to the right of the figure. An inlet showing greater detail of the slotted burner set 21 is shown to the left of the figure.

[0099] In the illustrated burner set, four identical sets of primary/vent/secondary slots are shown, each comprising an elongate primary slot 25 and adjacent vent slot 26 with secondary slots 27 longer than and disposed either side of the primary slot 25. Wall air is supplied through the wall air slots 28. This is an example arrangement only. Similar arrangements with the primary and vent slots swapped can be considered so that either the vent slots or the primary slots may be next to the wall.

[0100] Although the general desire is to produce a symmetrical W shaped flame pattern in practice there is often significant asymmetry.

[0101] Typically the flames from one of the firing arches will extend down to the hopper region before turning up (the long flame path), while the flames on the opposite firing arch will turn up almost immediately and bypass the lower furnace (the short flame path). This behaviour will have an adverse effect on combustion efficiency as the short flame has a reduced residence time for burnout. It also raises concerns with in-furnace NOx reduction technology based on staging of the combustion air, where the reducing zone residence time is not well defined, and NOx reduction performance is likely to be compromised for the short flames. Finally the asymmetric flame pattern leads to significant imbalances in the pattern of heat absorption in the furnace and pendants, causing operational problems.

[0102] Some or all of these problems may be mitigated by arrangements of primary/vent/secondary outlets in accordance with the principles of the invention such as exemplified by the illustrated embodiments of FIGS. 4 to 8. In particular in the preferred case some or all of these problems may be mitigated by provision of a system in which such burner sets are further provided with overfire air (OFA), most preferably directed at first and second levels respectively below and above a flame zone within the furnace. The arrangement of level 1 OFA and level 2 OFA described with reference to FIG. 3 also aims to mitigate flame asymmetry.

[0103] FIG. 3 shows a possible arrangement for overfire air which is applicable in general principle to burner sets in accordance with the invention and may be viewed in the context of the general furnace design of FIG. 1. Overfire air nozzles are provided at two levels respectively below and above the flame.

[0104] Overfire air outlets (level 1) to produce the stream shown will be located across the front and rear wall width below the flame, for example just above the hopper knuckle. The air is angled downwards (in the embodiment notionally 3045 below horizontal, although overfire air directed at any angle from horizontal to 45 below horizontal might be considered), and turning vanes may be installed in the approach duct to reinforce the downward direction. The air is typically introduced via a series of nozzles. An example notional 15% of the air may be supplied as level 1 OFA (equivalent to a stoichiometry of 0.2). As well as air staging, the intention of the level 1 OFA is to draw the flames down towards the hopper, and to turn the flames before they impinge in the hopper slope.

[0105] Overfire air outlets (level 2) to produce the stream shown will be located across the front and rear wall width just above the firing arch. The air is angled downwards (notionally 3045 below horizontal), and turning vanes may be installed in the approach duct to reinforce the downward direction. The air will be introduced via a series of nozzles. Notionally 15% of the air will be supplied as level 2 OFA (equivalent to a stoichiometry of 0.2). As well as air staging, the intention of level 2 OFA ports is to prevent the premature turning of the flames to reinforce the downward direction.

[0106] Air staging using conventional overfire air has not been widely adopted for the firing of lower volatile coals in downshot fired systems as it has been seen to have a detrimental effect on flame stability. However, in preferred embodiments of the invention, these overfire principles are effectively applicable to burner sets in accordance with the principles of the invention such as are exemplified in FIGS. 4 to 8.

[0107] All the embodiments exemplified in FIGS. 4 to 8 show a half burner arch 40 with three burner nozzle sets 41 each embodying the principles of the invention. Each example burner nozzle set has in common a central array of primary 45 and vent 46 air slots with inner 47 and outer 48 arrays of secondary air slots. The slots are square or rectangular, but need not be identically configured, although a burner set typically at least exhibits mirror symmetry about both a longitudinal and a transverse mid line as regards the disposition of these primary, vent air and secondary air slots. A central thermal biasing air outlet 42 is optionally provided.

[0108] Other outlets (not shown) for example to deliver wall air, may optionally be included.

[0109] The system comprises in familiar manner (not expressly shown) a supply of pulverised low volatile coal which is for example an on site pulverising plant and a supply of transport air. The transport air entrains the pulverised coal in familiar manner and transports the coal from the pulverising plant or other supply.

[0110] The supply of transport air and coal is then split into primary and vent air streams in accordance with suitable principles such as will for example be familiar from the prior art. For example the split between a primary stream and a vent air stream is effected in a cyclonic separator.

[0111] Coal and transport air from the pulverizing plant is separated into two streams in the cyclonic separator; a vent air stream containing typically 60% of the air and 10% of the coal, and a primary stream containing typically 40% of the air and 90% of the coal. Following separation, the coat and air are introduced into the furnace via an arrangement of slots in the firing arch via primary air (PA), vent air (VA) and secondary air (SA) slots in generally familiar manner, the invention being characterised by the particular arrangement/configuration of slots in accordance with the invention.

[0112] The invention is particularly characterised by the provision of burner sets in accordance with the principles of the first aspect of the invention, and such as might be exemplified by the embodiments of burner set illustrated in FIGS. 4-8. Other aspects of a more complete combustion system or furnace embodying the principles of the invention not specifically shown in the drawings or described herein will readily be inferred by the skilled person from general knowledge of prior art downshot fired systems, whether as generally exemplified by the system illustrated in figure or otherwise. The invention encompasses all downshot fired systems provided with burner shots embodying the principles of the first aspect of the invention such as are illustrated in the following figures.

[0113] In the embodiments shown a nozzle set provides a primary outlet via a single primary air (PA) slot 45 or a pair of primary slots 45 as the case may be. Each such primary slot or pair might for example be referred to as a burner outlet, Each such primary slot or pair (or burner outlet) has at least one vent air (VA) slot 46 at either side. Each primary or vent air slot 47, 48 in the central row is aligned with exactly one secondary air (SA) slot in a row transversely spaced on either side, not necessarily the same shape as the central slot but in all the example cases of the same width. Each burner outlet is thus completely surrounded by clean air or vent air.

[0114] The following short description applies to each proposed burner set configuration given by way of example.

[0115] The burner sets may first be categorised as grouped into one of two options as follows.

[0116] Option A. Two PA slots per burner.

[0117] Option B: Single PA slot per burner. In the suggested configurations of the embodiments such a PA slot can be made wider than the VA slots. SA slots are made with the same widths as the respective PA/VA slots to which they are aligned.

[0118] Each of the above options can then be sub-categorised based on the manner in which the burner sets are defined. One possibility is to assume each burner set has only one burner (ie only one primary slot or adjacent pair or other adjacent plural series of primary slots) while the other is to assume each burner set is composed of two burners (two separate primary slots, or pairs or series, in the context of the invention defined by and separated by vent air slots). Dual burner sets are the current established practice. Embodiments of the invention comprising two burners in a set (and hence in accordance with the principles of the invention two separate single primary slots, or adjacent pairs other adjacent plural series of primary slots, separated by vent air slots) are likely to be preferred.

[0119] Another important consideration for arranging burners within the firing arch is to ensure they are better distributed as it is beneficial for the NOx performance (plant operational experience indicates that uniform distribution of burners within the firing arch, hence uniform heat release, gives lower NOx emissions than grouped and separated burner sets).

[0120] FIGS. 4 to 8 illustrate examples only of burner sets which embody these principles.

[0121] In FIG. 4, a burner set comprising dual burners is shown. Each burner comprises a pair of primary slots 45 disposed (with reference to a direction longitudinal to the arch) between a pair of vent air slots 46. Both the primary and the vent air slots are rectangular and elongate in a direction transverse to the arch, with the primary slots elongate to a greater extent. The primary and vent air slots form a central array defining an array direction coincident with the respective median lines of the rectangles.

[0122] Each primary and vent air slot is paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots. The secondary slots make up respectively the second and third rows of slots so that each slot in the central row is disposed between secondary slots on either side in a transverse direction. The secondary slots are also rectangular, identical to each other, and identical in longitudinal extent along the arch to the longitudinal extent of the respective slots within the central row with which they are paired.

[0123] The arrangement produces a situation in which each burner outlet, in this case made up of a pair of adjacent primary slots, is entirely surrounded by vent air or secondary slots. Two such burners are provided in the illustrated embodiment.

[0124] Having regard to operational requirements and plant geometrical constraints, such an arrangement might be too crowded for many operational scenarios, and accordingly alternative embodiments such as those exemplified in FIGS. 5 to 8 might be explored instead.

[0125] In FIG. 5, a burner set comprising a single burner is shown. The burner comprises a single primary slots 45 disposed between a pair of vent air slots 46. Both the primary and the vent air slots are rectangular and elongate in a transverse direction, with the primary slots elongate to a greater extent. The primary and vent air slots form a central array defining an array direction coincident with the respective median lines of the rectangles.

[0126] Each primary and vent air slot is paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots. The secondary slots make up respectively the second and third rows of slots so that each slot in the central row is disposed between secondary slots on either side in a transverse direction. The secondary slots are also rectangular, and identical in longitudinal extent along the arch to the longitudinal extent of the respective slots within the central row with which they are paired, but in this case are of different lengths as shown. The arrangement still maintains the principle however that the primary slot is entirely surrounded by vent air or secondary slots.

[0127] In FIG. 6, a burner set comprising dual burners is shown. Each burner comprises a single primary slot 45 disposed between a pair of vent air slots 46. Each primary and vent air slot is again paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots.

[0128] In FIG. 7, a burner set comprising a single burner is shown. The burner comprises a single primary slot 45 disposed between a pair of vent air slots 46. Each primary and vent air slot is again paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots. It differs from the FIG. 5 embodiment in that the outlet slots have a different aspect ratio.

[0129] In FIG. 8, a burner set comprising dual burners is shown. Each burner comprises a single primary slot 45 disposed between a pair of vent air slots 46. Each primary and vent air slot is again paired on either side in a transverse direction by respective inner 47 and outer 48 secondary slots. It differs from the FIG. 6 embodiment in that the outlet slots have a different aspect ratio.

[0130] All these alternative arrangements maintain the principle that the primary slot is entirely surrounded by vent air or secondary slots. Although the invention is not limited by particular theory, it is generally suggested that such an arrangement can assist in supporting a flame whose ignition point is clearly defined in a robustly repeatable manner and encouraging a symmetrical flame path.