Boiler or furnace for combustion of fuel in an air staged combustion mode

Abstract

 The invention relates to a boiler or furnace (10) for combustion of fuel in an air staged combustion mode, comprising an enclosure (12) defining a combustion chamber (14), of which at least a part is provided with heat exchanging tubes (18) for generating a heated fluid, at least one burner (22) for combustion of fuel provided with a fuel supply (26) and a primary air supply (24), and a secondary air supply (32), and an outlet (28) for discharge of flue gas, wherein the secondary air supply (32) is positioned internally of the enclosure (12) downstream of the at least one burner (22).

Description

[0001] The invention relates to a boiler or furnace for combustion of fuel in an air staged combustion mode.

[0002] Air staging (also known as overfire air) is a well-known design feature for NOx control in boilers and furnaces, e.g. power plant boilers, industrial boilers for generating steam and/or hot water. Air staging involves a separation of combustion air into a primary air flow and a secondary air flow. The primary air - the majority of the total amount of combustion air - is mixed with the respective fuel in a burner, where combustion occurs at a relative low temperature producing a flue gas that is rich in fuel and deficient regarding oxygen, and moderate amounts of NOx are formed. The secondary air - the remaining combustion air - is introduced into the flue gas downstream of the burner(s) in a secondary combustion zone. There complete combustion and burnout are achieved again at a relatively low temperature minimizing the production of NOx.

[0003] In the prior art systems the secondary air is introduced from the outside via ports or nozzles extending through the housing of the boiler or furnace at the required position in the secondary combustion zone. The required position depends inter alia on the design of the boiler or furnace including the type of burner(s). The housing typically comprises numerous pipes for relatively cooling the housing usually made from refractory materials, as well as generating a heated fluid like hot water and/or steam. At the locations of the secondary air ports these closely spaced apart pipes need to be bended in order to create the necessary accessible area for introducing these ports between the pipes. This makes the design complex, cumbersome and contributes to the manufacturing costs. Also in retrofitting operations overfire air involves tube modifications to create the ports for the secondary air nozzles and the addition of ducts, dampers and a wind-box. In addition there may be objects in the vicinity of the boiler or furnace obstructing access to the side walls. See e.g. http://www.iea-coal.org.uk/site/ieacoal/databases/ccts/air-staging-for-nox-control-overfire-air-and-two-stage-combustion.

[0004] The invention aims at reducing the manufacturing costs of building or retrofitting a boiler or furnace by simplifying the design and/or reduction of required equipment.

[0005] According to the invention a boiler or furnace for combustion of fuel in an air staged combustion mode comprises an enclosure defining a combustion chamber, of which at least a part is provided with heat exchanging tubes for generating a heated fluid, at least one burner for combustion of fuel provided with a fuel supply and a primary air supply, a secondary air supply and an outlet for discharging flue gas, wherein the secondary air supply is positioned internally of the enclosure and the heat exchanging tubes downstream of the at least one burner.

[0006] The boiler or furnace for combustion of fuel, like oil, gas, or solid fuel such as coal or biomass, in an air staged combustion mode according to the invention comprises an enclosure. The walls of the enclosure define a combustion chamber. At least part of the enclosure is provided with heat exchanging tubes, wherein a heat exchange fluid flows, that is heated by the heat resulting from the combustion and simultaneously maintains the enclosure at a relatively low temperature. In operation the inlets of the heat exchanging tubes are connected, usually via an appropriate header system, to a source of heat exchange fluid, while the outlets, usually also by means of an appropriate header system, are connected to a discharge for passing the heated fluid to further processes, like electricity generation or heating process streams and/or buildings in an industrial plant Typically water is heated to hot water and/or steam. A so called waterwall comprising closely set tubes is a suitable example. Membrane tubes are another examples The enclosure itself is typically made from refractory materials and/or steel, such as refractory cladded steel. At least one burner for combustion of a fuel is provided in one of the walls of the enclosure. The fuel, typically a gaseous or liquid fuel, is fed to the burner via the fuel supply from a fuel source. Primary combustion air, typically 60 - 90 vol.% of the total amount of combustion air, is also fed to the burner via a primary air supply, suitably a wind-box. The enclosure also has an outlet for discharging flue gas resulting from the combustion process. According to the invention a secondary air supply is provided internally of the enclosure and the heat exchanging tubes at a position downstream of the at least one burner. Providing an internal supply for the secondary air allows to have as few positions as possible where the pattern of the heat exchanging tubes needs to be adapted around an entry duct, thus reducing the number of bends or other curves therein compared to the external secondary air supply in the prior art and thereby also reducing design and manufacturing costs. Typically the inlet of the secondary air supply through the enclosure and between heat exchanging tubes will be at the enclosure wall where the burner is situated, or at the wall opposite the burner. In a preferred embodiment the burner opening in the wall itself is used as a hole for this secondary air inlet, as will be explained hereinafter.

[0007] During operation a primary combustion zone and a secondary combustion zone can be distinguished in the combustion chamber. In the primary combustion zone fuel is combusted with the bulk of the total combustion air, typically 60-90%, while the secondary combustion air (the remaining volume 10-40% of the total combustion air) is fed to the secondary combustion zone, downstream the primary combustion zone.

[0008] In a preferred embodiment the secondary air supply comprises an inlet duct through the wall of the enclosure in fluid communication with a distribution channel extending along the inner periphery of the enclosure in a direction transverse to the flow direction of the flue gas in operation, wherein the distribution channel is provided with nozzles for injection of secondary air into the flue gas. In this embodiment the secondary air supply is a manifold system comprising at least an inlet duct for secondary air in fluid communication with a distribution channel. The distribution channel is provided with a plurality of injection nozzles for delivering the secondary air to the flue gas. E.g. the injection nozzles are spaced apart, such as at predetermined intervals. The injection nozzles can be grouped together forming subgroups of closely set nozzles, while the distance between subgroups might vary. The size of the nozzle openings might vary as well. The injection nozzles are preferably designed, including number, position, dimensions and direction, such that an intimate mixing with the flue gas occurs in the secondary combustion zone. The peripheral distribution channel such as an annular passageway, e.g. made from metal, refractory materials and/or concrete, extends in a direction perpendicular to the main flow of gas, and as it projects away from the enclosure wall to the centre, it forms a flow restriction for the flue gas flow. This restriction contributes to internal recirculation of flue gas, if any depending on the type of burner and design of boiler or furnace, which is beneficial in suppressing NOx generation This restriction also acts as a venturi thereby enhancing the draw of flue gas to the outlet and promoting the thorough mixing of the secondary air with the flue gas, thereby on its turn improving complete combustion and burnout.

[0009] As said the inlet of the secondary air supply through the enclosure is preferably positioned in the wall thereof, which wall also is provided with the at least one burner. Another preferred position of the inlet is in the wall opposite the burner. Typically these walls are more accessible than the side walls. Furthermore usually the impact of positioning the inlet in one of these walls on the arrangement of the heat exchanging tubes is less than in a situation, where the inlet would be positioned in a side wall extending between the wall having the burner and the opposite wall. The secondary air supply may be provided with its own dedicated equipment like a fan, wind-box and control devices in order to control the flow of secondary air, in particular the amount thereof.

[0010] In a preferred embodiment the at least one burner is a throat burner provided in a burner opening in a wall of the enclosure, and the inlet of the secondary air supply is provided in the burner opening. This arrangement allows as few as possible openings in the enclosure, simplifying the design and reducing manufacturing costs. In a further preferred embodiment thereof the inlet duct is integrated in the throat burner.

[0011] Advantageously the boiler or furnace comprises a wind-box for supplying air to the primary air supply and the secondary air supply. Again having a common wind-box for both of the primary air and secondary is beneficial in view of costs. Optionally the wind-box is provided with control means for controlling the secondary air flow. A damper is a suitable control means.

[0012] The boiler or furnace can be used in power plant facilities, where produced steam is fed to a steam turbine in order to generate electricity in an associated generator. The boiler or furnace may also be used in industrial facilities for producing (low pressure) steam or heated fluid, which is processed further for example for heating purposes.

[0013] The invention will be illustrated by means of the attached drawing, wherein

Fig. 1 is a diagrammatic cross section of a basic layout of an embodiment of a furnace according to the invention; and

Fig. 2 is a diagrammatic cross section of a preferred embodiment of a furnace according to the invention.

[0014] Fig. 1 is a diagrammatic view of a basic design of an embodiment of a furnace 10 according to the invention. The furnace 10 comprises a box like enclosure 12 defining a combustion chamber 14. The walls 16 of the enclosure 12 are provided with closely set heat exchanging tubes 18 as a so called water wall, wherein e.g. water and/or steam flows during operation. In the embodiment as shown, water is fed to the tubes 18 and converted into steam, which is discharge, as indicated by arrows. In the front wall 20 a number of burners 22, in this case two, are positioned, which are fed with primary air and fuel, like gas or oil, via primary air supply line 24 and fuel supply line 26 respectively. Flue gas is discharged from the combustion chamber14 via exhaust 28. In the front wall 20 two through-bores or holes 30 are provided for a secondary air supply 32, connected to the secondary air supply line 34, which is provided with a control valve 36. The internal secondary air supply line 34 comprises inlet channels 38 at the top and bottom of the enclosure 12, which channels 38 extend from their inlets 40 connected to the secondary air supply 32, along the walls 16 in the direction of the main direction of flue gas (indicated by arrow 42). At the opposite outlet ends 44 the channels 38 are connected to a distribution channel 46, which extends along the periphery of the enclosure 12 at the inside thereof. The distribution channel 46 is transverse to the main flow direction of flue gas in the combustion chamber 14 as indicated by an arrow. The distribution channel 34 comprises a number of nozzles 48.

[0015] Fig. 2 shows an another embodiment. Parts similar to those shown in Fig. 1 are indicated by the same reference numeral. The furnace or boiler 10 comprises an enclosure 12 defining a combustion chamber 14. In the walls 16 of the enclosure 12 heat exchanging tubes 18 are provided. A throat burner 22 is mounted in a burner opening 49 in the wall 20 of the enclosure 12. The burner 22 receives fuel from fuel supply line 26. As an air supply for both the primary air and the secondary air a wind-box 50 is provided, which passes the primary air via channel 52 to the burner and the secondary air via channels 54 each provided with a damper 56 for controlling the secondary air flow, into channels 38 and further into distribution channel 46 having nozzles 48. Reference numeral 58 indicates the primary flame, while the internal flue gas recirculation, if any, bears reference numeral 60.

Claims

1. Boiler or furnace (10) for combustion of fuel in an air staged combustion mode, comprising an enclosure (12) defining a combustion chamber (14), of which at least a part is provided with heat exchanging tubes (18) for generating a heated fluid, at least one burner (22) for combustion of fuel provided with a fuel supply (26) and a primary air supply (24), and a secondary air supply (32), and an outlet (28) for discharge of flue gas, wherein the secondary air supply (32) is positioned internally of the enclosure (12) downstream of the at least one burner (22).

2. Boiler or furnace (10) according to claim 1, wherein the secondary air supply (32) comprises an inlet (38; 54) through a wall of the enclosure (12) in fluid communication with a distribution channel (46) extending along the inner periphery of the enclosure (12) in a direction transverse to the main flow of flue gas during operation, wherein the distribution channel (46) is provided with a plurality of nozzles (48) for injection of secondary air into the flue gas.

3. Boiler or furnace (10) according to any one of the preceding claims, wherein the secondary air supply (32) comprises an inlet (38) into the enclosure (12) in the wall (20), wherein the at least one burner (22) is positioned.

4. Boiler or furnace (10) according to any one of the preceding claims, wherein the at least one burner (22) is a throat burner provided in a burner opening (30) in a wall (20) of the enclosure (12) and wherein the inlet (54) of the secondary air supply (32) is provided in the burner opening (30).

5. Boiler or furnace (10) according to any one of the preceding claims, wherein the inlet (54) is an integral part of the throat burner.

6. Boiler or furnace (10) according to any one of the preceding claims, comprising a wind-box (50) for supplying air to the primary air supply and the secondary air supply, wherein the wind-box (50) is provided with control means (56) for controlling the secondary air.

Drawing