ARRANGEMENT OF A COMBUSTOR AND A DEVICE FOR SELECTIVE NON CATALYTIC REDUCTION AND PULSED INJECTION METHOD

TECHNICAL FIELD

The present invention relates to an arrangement of a combustor and a device for selective non catalytic reduction and injection method. In particular, the present invention refers to a combustor being a boiler, such as a boiler of a power plant for electric power generation; the combustor can anyhow be a different combustor, such as a rotary kiln for cement production. In the following reference to a boiler is made.

BACKGROUND

Boilers comprise tubed walls defining an inner space; a fuel such as coal, oil, waste is supplied into the inner space and is combusted with an oxidizer, such as air or oxygen. During the combustion flue gas is generated that is rich in NO _x , whose amount has to be reduced below given limits before the flue gas is vented into the atmosphere.

In order to remove NO _x from the flue gas, a regent such as ammonia (N¾ as liquid solution) or urea (CH ₄ N ₂ O as liquid solution) can be injected into the flue gas and the flue gas can be passed through a catalyzer, for the NO _x to be converted into N ₂ and H ₂ 0 (selective catalytic reduction, SCR) ; this reaction occurs at low temperatures, e.g. between 250-450 °C. The catalyst is expensive and can be damaged during operation, for this reason a method has been developed according to which ammonia or urea are injected into the flue gas having a high temperature, e.g. in the range 750- 1300°C, such that the NO _x conversion into 2 and ¾0 occurs without the need of a catalyst (selective non catalytic reduction, SNCR) .

WO2012/059 184 discloses a boiler for waste to energy applications (i.e. waste is used as a fuel in the boiler) . The boiler has an aperture at its top wall such that a device for selective non catalytic reduction is introduced into the inner space through this aperture. The device for selective non catalytic reduction has a tubular lance and a hose sliding into the lance. The hose has a nozzle at its end housed in the inner space. During operation the lance hangs from the top wall of the boiler and the hose extends from the lance; the reagent passes through the hose and is injected through the nozzles upwards.

The reagent after injection has to evaporate to mix with the flue gas, for the mixture to undergo the selective non catalytic reaction for NO _x removal. The inventors have found that evaporation occurs in a limited space around the nozzle; it would be beneficial to have a larger evaporation zone for a larger amount of the flue gas to mix with vapor reagent and thus treat a larger portion of flue gas by selective non catalytic reaction. SUMMARY

An aspect of the invention includes providing an arrangement and a method that allow reagent evaporation in a large area around the nozzle.

These and further aspects are attained by providing an arrangement and method in accordance with the accompanying claims . BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages will be more apparent from the description of a preferred but non ^¬ exclusive embodiment of boiler, illustrated by way of non- limiting example in the accompanying drawings, in which:

Figure 1 shows a schematic boiler according to the invention ;

Figures 2 through 4 show different embodiments of the lance ;

Figures 5 through 10 show different embodiments of the nozzle;

Figure 11 shows an isothermal curve within the inner space of the boiler (above the flame) ;

Figure 12 shows the valve opening-time relationship for a pulsing injection of reagent;

Figures 13 and 14 show the evaporation areas for different nozzles operated in different ways; Figure 15 shows a lance connected on a support beam; Figure 16 shows a chariot connected to a support beam; Figure 17 shows an embodiment of a support beam with side position regulation.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

With reference to the figures, these show a combustor being a boiler 1 comprising side tubed walls 2 enclosing an inner space 3 and a top wall 4. The boiler is supplied with fuel such as coal (but other kinds of fuel such as waste, oil, gas, etc. are possible) and an oxidizer such as air or oxygen via burners 5.

The boiler also has a device 7 for selective non catalytic reduction SNCR; the device 7 comprises a lance 8 carrying a hose 9 having at least a nozzle 10; as shown, preferably the nozzle is at the hose terminal part that is housed within the inner space 3. The device 7 further has a hose drive mechanism 11 for driving the hose within the lance 8.

The figures show an example of a 2-pass boiler and in this case the device 7 is associated to the first pass, it is anyhow clear that the boiler can be any type of boiler and the device 7 can be located anywhere, at locations where temperatures for correct selective non catalytic reaction exist.

Advantageously, the lance 8 protrudes into the inner space 3 from a side tubed wall 5 of the boiler. This way the length of the hose housed within the inner space 3 is shorter than in case the lance was hanging from the top wall 4 of the boiler, such that the reagent passing through the hose during operation does not evaporate or at least does not undergo evaporation to a large extent before it reaches the nozzle 10. Preferably the lance 8 protrudes substantially horizontally.

In order to further counteract the evaporation of the reagent, the lance 8 is preferably provided with a cooling system; figures 2 through 4 show lances 8 with different cooling systems.

The cooling system comprises one or more channels 15 for a cooling medium such as air and/or another gas and/or steam and/or water. The channels 15 preferably have an annular shape and are located between a lance wall 16 and the hose 9. For example, figure 2 shows a lance 8 with the hose 9 projecting from its open end; one annular channel 15 is defined between the wall 16 of the lance 8 and the hose 9. Figure 3 shows a lance 8 having an opening 19 and the hose 9 projecting from this opening 19; one annular channel 15 is defined between the wall 16 of the lance 8 and the hose 9. Figure 4 shows a lance 8 with the hose 9 projecting from its open end; two annular channels 15a, 15b are defined between the wall 16 of the lance 8 and the hose 9.

An inlet 20 into the channels 15, 15a, 15b for the cooling medium is located at a portion of the lance 8 outside the inner space 3 (figures 2-4); in contrast, an outlet from the channels 15, 15a, 15b can be located at a portion of the lance 8 inside the inner space 3 (figures 2 and 3) and can be defined by the opening 19 and/or the outlet can be located at a portion of the lance 8 outside the inner space 3 (figure 4) and can be defined by a dedicated opening 21.

In addition, the lance 8 can have an outer insulation layer 22 for thermally insulating the lance and protecting it against the flue gas. The outer insulation layer 22 is implemented if needed according to the heat removal capacity of the cooling system.

In case the cooling medium is air and/or another gas: - when the air/other gas has a low pressure (for example air/other gas are supplied into the channels 15, 15a, 15b via a blower) the insulation layer 22 is typically needed;

- when the air/other gas has a medium pressure (for example air/other gas are supplied into the channels 15,

15a, 15b via a blower) the insulation layer 22 is typically not needed because the increased pressure of the air/other gas allows a sufficient heat removal;

- when the air/other gas has a high pressure (for example air/other gas are supplied into the channels 15,

15a, 15b via a compressor) the insulation layer 22 is typically not needed.

When the cooling medium is air/other gas it can be discharged into the inner space 3 of the boiler (i.e. the lance 8 can have the structure shown in figures 2 or 3) ; in fact, the air can advantageously allow further reaction of the fuel (i.e. it can act like overfire air) .

In case the cooling medium is water it can be supplied to the channels 15, 15a, 15b at medium pressure typically without any need of outer insulating layer 22; in this case the water can be discharged either within the inner space 3 (like in figures 2 and 3) or outside of the inner space 3 (like in figure 4) .

In case the cooling medium is steam:

- it can be supplied into the channels 15, 15a, 15b at low pressure, typically without any need of the outer insulating layer 22; in this case the steam is discharged into the inner space 3 (like the lance shown in figures 2 and 3) ;

- it can be supplied to the channels 15, 15a, 15b at high pressure, typically without any need of the outer insulating layer 22; in this case the steam is discharged outside of the inner space 3 (like the lance shown in figure 4 ) .

Naturally the specific design of the lance, the presence of the outer insulating layer and the location of the inlet 20 and outlet, the configuration of the channels 15, 15a, 15b, the type of cooling means and the pressure of the cooling means can be any combination and will depend on the particular conditions.

The lance 8 is preferably provided with a lance positioning system for regulating the position of the hose 9 within the inner space 3. In particular, the lance positioning system is arranged for regulating the position of the lance over its longitudinal axis 25.

In one embodiment, the lance positioning system for regulating the position of the lance over its longitudinal axis comprises a lance support beam 26 and a chariot 27 movable on the support beam 26; the lance 8 is connected to the chariot 27.

A drum 29 is also provided for carrying the hose 9; the drum 29 is preferably not connected to the chariot 27; it can for example be connected to and supported by the support beam 26 or an external support.

The chariot 27 further supports the hose drive mechanism 11. The hose drive mechanism 11 has opposing wheels 31 through which the hose 9 passes through and a motor 32 for driving the wheels 31, such that during operation the wheels 31 drive the hose 9 and the drum 29 is only used to wind or unwind the hose 9. A drive system for example with a chain 36 and a motor 37 is used to drive the chariot 27 along the support beam 26. In this respect the chariot 27 is fixed to the chain 36 at a position 38. Additionally, the support beam could also have a hinge 40 and a side drive mechanism for example comprising a chain 41 having the ends connected to the support beam 26 and wheels 42; at least one of the wheels 42 is connected to a motor 43 for controlling and adjusting the side position over the arrows S.

The boiler 1 further has a control system 44 for controlling the flow from the nozzle 10. Advantageously the control system 44 can be arranged for generating a pulsed flow from the nozzle 10. For example, the boiler can have one or more valves 45 connected to and driven by the control system 44 for generating the pulsed flow. In addition or as an alternative the reagent can be supplied via a pump that is controlled in order to provide the pulsed flow. Other means for generating the pulsed flow are naturally possible.

The operation of the boiler is apparent from that described and illustrated and is substantially the following .

Fuel and oxidizer are supplied into the inner space 3 and the fuel is combusted generating flames F and flue gas G that passes through the boiler. The flue gas contains NO _x (mainly NO and lower amounts of O ₂ ) that has to be at least partly removed.

The lance 8 is introduced into the inner space 3, e.g. through a side opening of the side tubed wall 2 (but this is not mandatory and the lance can be introduced into the inner space from e.g. the top wall 4 of the boiler); the opening can be an opening for different scopes, such as an inspection opening or a dedicated opening.

From the lance 8 the hose 9 and the nozzle 10 hang in the inner space 3.

The nozzle position can be adjusted both horizontally and vertically.

The nozzle position is adjusted vertically for the nozzle 10 to inject the reagent in zones where the temperature is the best for selective non catalytic reaction. The vertical adjustment can be done by operating the motor 32 in order to drive the wheels 31 for pushing/pulling the hose 9 through the lance 8 as indicated by arrow Fl and regulating the nozzle vertical position as indicated by the arrow F2.

The nozzle position is adjusted along the axis 25 (for example horizontally) to place the nozzle 10 in zones where the flue gas velocity (and thus the mass flow) is higher, because the most of the NO _x passes through these zones. The nozzle position can be horizontally regulated along the support beam 26 by operating the motor 37 in order to drive the chariot 27 along the support beam 26 as indicated by the arrow F3. The movement of the chariot 27 causes the corresponding movement of the lance 8 along the support beam 26 as indicated by the arrow F4. Additionally, the nozzle position can also be pivotally adjusted. This is often not needed because in particular for large boilers more than one lance are provided; anyway side adjustment could render regulation more flexible. In this case the regulation can be done by operating the motor 43 in order to move the chain 41 as indicated by the arrows F5 and thus pivot the support beam 26 (and the lance 8 supported by it) as indicated by the arrows S .

The position of the nozzle can be adjusted before the boiler is started, according the foreseeable operating conditions, but can also be changed during operation of the boiler, in response to changed operating conditions or to betted adapt the nozzle position to the current operating conditions and/or temperature distribution and/or flue gas velocity distribution.

During operation of the boiler 1, the hose 9 is supplied with a reagent such as ammonia or urea from a tank 47; the reagent passes through the hose 9 and is injected via the nozzle 10.

Different possibilities are available for the injection. Preferably the reagent is injected over a conical surface 48 for example via a slot, but it can also be injected via a plurality of single jets each resting on a conical surface. Preferably the angle A between the hose 9 and the flow injected through the nozzle 10 is in the range 0° < A < 180°, more preferably it is in the range 90°

< A < 180° and more preferably it is in the range 135° < A

< 155°.

An injection over a conical surface 48 defining an angle A between 90° - 180° and preferably 135° - 155° is advantageous because it allows to spray the reagent over areas where the temperature is uniform. In fact, the temperature within the inner space 3 is lower close to the side tubed walls 2 and it is lower upwards (i.e. apart from the flame) ; therefore the isothermal curve has a bell shape with ends directed downwards and close to the tubed side walls 2 (figure 11) . Therefore injection over a conical surface with apex at the top and directed downwards allows the reagent to intersect the flue gas having a substantially uniform or more uniform temperature than with other configurations.

Figures 5 through 10 show different embodiments of the nozzle 10.

Figure 5 shows an embodiment in which the nozzle 10 has a body 50 (preferably tubular in shape) for example made out of steel or other metal with a support arm 51 connected in it. One end of the nozzle 10 is connected to the hose 9 and the other end has an occlusion 53 connected to the support arm 51. The occlusion 53 defines together with the body 50 a slit 54. The slit 54 can have (but this is not mandatory) a convergent passage area from the inside of the body 50 to the outside thereof. During operation the reagent passes through the hose 9 and enters the body 50; then the reagent is injected into the inner space 3 of the boiler via the slit 54.

Figures 6 through 8 show a different embodiment of the nozzle 10. Also in this embodiment the nozzle 10 has a body 50 with a support arm 51 that supports one or more (in the example shown only one) intermediate discs 56 and the occlusion 53. The discs 56 are provided with openings 57 for the passage of the reagent. The disc 56 defines with the body 50 a first slit 54a and with the occlusion 53 a second slit 54b; the slits 54a and 54b can have the same size SZ but preferably they have different size SZ, such that the reagent injected through them forms droplets of different diameter that evaporate in different parts of the boiler (smaller droplets evaporate faster) , in order to better distribute the reagent within the inner space 3. Naturally also more than one disc and thus more than two slits are possible.

Figures 9 and 10 show a further example of the nozzle; in this embodiment the disc 56 has an outer border 59 that is not straight (like in the previous examples) but has a zig-zag or curved shape; the size of the slit changes over the border 59 from a minimum size SZ1 to a maximum size SZ2. Naturally also embodiments with solutions combining those described above are possible, such that for example the zig-zag border can be provided only at the occlusion 53 or at both the disc 56 and occlusion 53 or instead of or in addition to the outer border 59, also the inner border 60 can have a zig-zag shape.

In addition or as an alternative to the nozzle features (e.g. numbers of slits and/or their configuration) the reagent distribution and evaporation can also be improved by the pulsed injection, controlled via the control system 44 and preferably valves 45.

Figure 12 shows a relationship between the valve opening and time; it is clear that the minimum valve opening 65, the maximum valve opening 66, the duration of maximum opening 67 and the ramp frequency can be set according to the needs. Preferably pulsation has a frequency between 0.1-10 Hz.

During operation, because of the varying volume flow, the exit velocity from the slits increases and/or decreases, this causes a change in the diameter of the reagent droplets. Droplets of different diameter evaporate differently such that with pulsing injection it is possible to achieve a larger evaporation zone than without it.

As a matter of example, figures 13 and 14 show a view from the top of the area where the reagent droplets evaporate within the inner space 3; figure 13 refers to a nozzle embodiment with different slits 54a, 54b (for example the nozzle can have a configuration as shown in figures 6-8) and figure 14 refers to a nozzle embodiment with pulsed reagent injection (for example the nozzle can have a configuration as shown in figure 5) .

From figure 13 it is apparent that the droplets having a smaller diameter (e.g. injected through the slit 54b) evaporate faster than the droplets having a larger diameter (e.g. injected through the slit 54a) . In this respect the annular area 68 indicates the zone where the droplets of smaller diameter evaporate and the area 69 indicate the zone where the droplet of larger diameter evaporate. From figure 13 it is apparent that the two (and possibly more than two) slits allow evaporation over a much larger area than in case only one slit is used.

Figure 14 shows the zone 70 where evaporation of the reagent droplets occurs; also in this case it is apparent that the evaporation with pulsed injection is much larger than without pulsed injection; as an example line 71 indicates the possible evaporation starting point in case no pulsation is used.

After injection and evaporation, the reagent mixes with the flue gas and carries out the known selective non catalytic reaction to at least partly remove NO _x from the flue gas . The present invention also refers to a method for injecting a reagent from a nozzle of a device for selective non catalytic reduction 7 into a combustor. The method comprises pulse injecting the flow from the nozzle 10.

The combustor has a control system 44 and one or more valves 45 intercepting the flow directed to the nozzle 10; preferably, according to the method, the control system 44 controls the valve 45 for pulse injecting the flow from the nozzle 10. In addition, the flow is advantageously injected with an angle A between the hose 9 and the injected flow in the range 0° < A < 180°, preferably in the range 90° < A < 180° and more preferably in the range 135° - 155°.

Naturally the features described may be independently provided from one another.

In practice the materials used and the dimensions can be chosen at will according to requirements and to the state of the art.

REFERENCE NUMBERS

1 boiler

2 side tubed wall

3 inner space

4 top wall

5 burner

7 device for selective non catalytic reduction

8 lance

9 hose

10 nozzle

11 hose drive mechanism

15, 15a, 15b cooling channel

16 lance wall

19 side opening

20 inlet

21 opening

22 insulation layer

25 longitudinal axis

26 support beam

27 chariot

29 drum

31 wheel

32 motor

36 chain

38 fixing position

37 motor 40 hinge

41 chain

42 wheel

43 motor

44 control system

45 valve

47 tank

48 conical surface

50 body

51 support arm

53 occlusion

54, 54a, 54b slit

56 intermediate disc

57 opening

59 outer border

60 inner border

65 minimum valve opening

66 maximum valve opening

67 duration of the maximum opening

68 evaporation zone of the droplets of smaller diameter

69 evaporation zone of the droplets of larger diameter

70 evaporation zone of the droplets

71 possible droplet evaporation starting point without pulsed injection A angle

F flame

Fl pushing/pulling of the hose

F2 regulation of the nozzle vertical position F3 chariot movement on the support beam

F4 lance movement

F5 movement of the chain 42

G flue gas

S arrow

SZ, SZ1, SZ2 size of the slit