NOx ABATEMENT SYSTEM FOR A STATIONARY BURNING SYSTEM

Abstract

The present application relates to abatement processes and systems of nitrogen oxide (NO.sub.x) contained in exhaust gases, more in particular produced by stationary burner and combustion systems. The NO.sub.x are removed by reduction using a catalyst and a reducing agent that is introduced into the exhaust gases and being mixed therewith. The mixture is then conducted over the catalyst N resulting in conversion of the NO.sub.x in environmentally neutral N.sub.2 and H.sub.2O. The present application more in particular relates to a mixing system for such a NO.sub.x abatement system to mix the exhaust gases with the reducing agent.

Claims

1. A mixing system for a nitrogen oxide (NO.sub.x) abatement system, which comprises: a flow passage for conducting a NO.sub.x comprising exhaust gas stream in an exhaust gas stream flow direction; at least one injection lance positioned inside the flow passage and arranged with at least one injection point for introducing a NO.sub.x reducing agent into the exhaust gas stream; and flat deflector plates operatively associated with the at least one injection lance and positioned inside the flow passage downstream from the at least one injection point and in a transverse direction on the exhaust gas flow direction for mixing the NO.sub.x reducing agent with the exhaust gas stream by inducing turbulent flows, wherein the flat deflector plates are connected by means of at least one spacer to the at least one injection lance.

2. The mixing system according to claim 1, wherein the flat deflector plates are operatively associated with each of the injection lances.

3. The mixing system according to claim 1, wherein the mixing system further comprises an injection grid comprising a multitude of parallel injection lances for introducing the NO.sub.x reducing agent into the exhaust gas stream, and projecting in a plane perpendicular to the exhaust gas stream flow direction along a central NO.sub.x reducing agent feed pipe, wherein to each injection lance a flat deflector plate is connected by means of the at least one spacer.

4. The mixing system according to claim 3, wherein each of the injection lances of the injection grid penetrate through the central feed pipe, and each of the injection lances is provided with a hole where the injection lance crosses the feed pipe, through which hole the NO.sub.x reducing agent can flow.

5. The mixing system according to claim 1, wherein the at least one injection lance is provided with a multiplicity of openings, each serving as an injection point and each being individually supplied with NO.sub.x reducing agent and being arranged to inject evaporated NO.sub.x reducing agent, either directly, or carried by a carrier gas stream, into the exhaust gas flow channel.

6. The mixing system according to claim 5, wherein the carrier gas stream is recycled flue gas or air.

7. The mixing system according to claim 6, wherein the mixing system further comprises a heating system to evaporate liquid NO.sub.x reducing agent; or heat the carrier gas stream if it is too cold to evaporate liquid NO.sub.x reducing agent.

8. The mixing system according to claim 5, wherein the openings are nozzle apertures.

9. The mixing system according to claim 1, wherein the deflectors are made out of carbon or stainless steel.

10. The mixing system according to claim 1, comprising between 5 and 60 injection points per square meter.

11. The mixing system according to claim 1, wherein the flow passage for the exhaust gas stream has a cross-section of 10 to 400 m.sup.2 and a flow velocity between 1 m/s and 30 m/s.

12. A method for introducing a nitrogen oxide (NO.sub.x) reducing agent into and mixing the NO.sub.x reducing agent with an exhaust gas stream comprising NO.sub.x, the method comprising: conducting the exhaust gas stream comprising NO.sub.x through a flow channel; introducing the NO.sub.x reducing agent, through at least one injection lance, into the exhaust gas stream comprising NO.sub.x, wherein said at least one injection lance is arranged with at least one injection point for introducing the NO.sub.x reducing agent into the exhaust gas stream; mixing the NO.sub.x reducing agent with the exhaust gas stream comprising NO.sub.x by inducing turbulent flows with flat deflector plates operatively associated with the injection lances and positioned downstream from the injection points and transverse to the direction of the exhaust gas stream flow direction, wherein the flat deflector plates are connected by means of at least one spacer to an injection lance.

13. The method according to claim 12, wherein the NO.sub.x reducing agent is chosen from liquefied anhydrous ammonia (NH.sub.3), an aqueous ammonia solution, an aqueous urea (CO(NH.sub.2).sub.2) solution or solid urea.

14. (canceled)

15. A stationary burning system comprising an NO.sub.x abatement system for reducing nitrogen oxides (NO.sub.x) in exhaust gases produced by the stationary burning system, comprising an exhaust gas flow channel for conducting the exhaust gases comprising NO.sub.x in a given flow direction in the exhaust gas flow channel, the NO.sub.x abatement system comprising a mixing system according to claim 1 and a catalytic converter disposed downstream from the mixing system.

16. The mixing system of claim 7 wherein the heating system is selected from an electrical heating system, a steam heating system, a hot water heating system, a heat transfer fluid heating system or a spray evaporator.

17. The mixing system of claim 11 wherein the flow passage has a cross section of 20 to 300 m.sup.2.

18. The mixing system of claim 11 wherein the flow velocity is between 4 to 12 m/s.

Description

DESCRIPTION OF FIGURES

[0037] The following description of the figures of specific embodiments is only given by way of example and is not intended to limit the present explanation, its application or use. In the drawings, identical reference numerals refer to the same or similar parts and features.

[0038] FIG. 1 shows an embodiment of a mixing system according to the present application;

[0039] FIG. 2 shows an embodiment of an injection element of a mixing system according to the present application;

[0040] FIG. 3 shows a Computer Fluid Dynamics (CFD) representation of the turbulent flow profiles of an embodiment of a mixing system according to the present application.

[0041] FIG. 4 shows a detailed view of the position of an injection lance vis-a-vis a deflector plate of a mixing system as shown in FIGS. 1 and 2.

[0042] FIG. 5a shows a front view of an injection lances penetrating through the NOx reducing agent feed pipe of a mixing system as shown in FIGS. 1 and 2.

[0043] FIG. 5b shows the cross-section A-A as shown in FIG. 5a.

[0044] The following reference numerals are used in the description and figures: 100—mixing system; 101—injection point; 102—injection lance; 103—deflector plate; 104—central NO.sub.x reducing agent feed pipe; 105—spacer; 106—hole in each of the injection lances; 200—exhaust gas stream flow direction; 201—flow channel; 202—exhaust gas inlet; 202—exhaust gas outlet; 300—catalyst, X—axis parallel to the exhaust gas stream flow direction; Y—axis parallel to the frontal surface of the deflector plate (along the width of the deflector plate); Z—axis parallel to the frontal surface of the deflector plate (along the length of the deflector plate); α—angle between X-axis and Y-axis.

DETAILED DESCRIPTION

[0045] A mixing system (100) for a nitrogen oxide (NO.sub.x) abatement system as shown in FIG. 1, comprises a flow passage for conducting an exhaust gas stream comprising NO.sub.x in an exhaust gas stream flow direction (200). This flow passage (201) for the exhaust gas stream has an exhaust gas inlet (202) and an exhaust gas outlet (203). It furthermore typically has a cross-section of 10 to 400 m.sup.2, more specifically 20 to 300 m.sup.2. The flow velocity of the exhaust gas through the flow channel is more in particular smaller than 30 m/s.

[0046] Inside the flow passage, one or more injection lances (102) are positioned that are provided with one or more injection points for introducing a NO.sub.x reducing agent into the exhaust gas stream. The NO.sub.x reducing agent can be a liquefied anhydrous ammonia (NH.sub.3), an aqueous ammonia solution, an aqueous urea (CO(NH.sub.2).sub.2) solution or solid urea.

[0047] Flat deflector plates (103) are operatively associated with the injection lances (102) and positioned inside the flow passage for mixing the NO.sub.x reducing agent with the exhaust gas stream by inducing turbulent flows. More in particular, each of the injection lances (102) has a respective flat deflector plate (103) that operatively is associated with it. These deflector plates (103) are positioned downstream from the one or more injection lances (102), in particular downstream from the injection points (101), and in a transverse direction on the exhaust gas stream flow direction (200).

[0048] Each of the injection lances (102) may comprise one or more injection locations through which the NO.sub.x reducing agent is injected into the exhaust gas stream. Each of the multiplicity of openings is more in particular individually supplied with NO.sub.x reducing agent.

[0049] The injection points (101) may be any type of injection point known in the art such as simple openings or holes. The injections lances (102) may be a perforated tube, or the injection location may also comprise a more complex structure such as an injection nozzle or a nozzle aperture, for instance as terminating piece of the injection lance (102). The nozzle may atomize the NO.sub.x reducing agent such that it is supplied and distributed quickly and homogeneously in the exhaust gas. The injection lances (102) may comprise gas-liquid injectors such as atomizers, in particular dual-fluid atomizers that use air or steam as the atomizing medium, as well as suitably designed pressure atomizers.

[0050] The mixing system (100) may have between 5 and 60 injection points per square meter.

[0051] The deflector plates (103) may have a cuboid or rectangular prism shape. The rectangular prism shape is the most simple one. When the deflector plates (103) have a cuboid shape, the surface of the deflector plates (103) facing the exhaust gas stream flow have a surface area which is larger compared to the adjacent side surfaces of the cuboid shape. In particular, the spacer/deflector plate combination is T-shaped, as shown in FIG. 4. This shape provides a higher strength to the deflector, resulting in less risk of vibrations in the structure.

[0052] The deflector plates (103) can be made out of carbon or stainless steel.

[0053] The NO.sub.x reducing agent can be injected into the exhaust gas flow channel (201) as a liquid or as a vapor. In case the NO.sub.x reducing agent is injected as a liquid, the temperature of the exhaust gas must be sufficient high to evaporate the NO.sub.x reducing agent.

[0054] In case the NO.sub.x reducing agent is injected as a vapor, this can be done by directly injecting evaporated reducing agent into the exhaust gas flow channel (201), or via a carrier gas stream.

[0055] A heating system (not shown on the Figures), which is more in particular positioned outside the exhaust gas flow channel (201), can be arranged to evaporate liquid NO.sub.x reducing agent itself, or to heat the carrier gas when it is too cold to evaporate liquid NO.sub.x reducing agent and to carry this evaporated NO.sub.x reducing agent towards the mixing system (100).

[0056] As can be seen in FIG. 1, in the exhaust gas flow channel (100), different mixing systems (100) as shown in in FIGS. 2a to 2c can be arranged next to and/or above each other. The evaporated NOx reducing agent, with or without carrier gas, is then distributed to the different mixing systems (100) by means of adjustment dampers (not shown on the Figures). More in particular, during commissioning, these dampers are adjusted manually. This is more in particular used in case the NO.sub.x distribution in the exhaust gas stream is very inhomogeneous. The manual adjustment of the dampers during commissioning targets a good NOx outlet distribution of the catalyst, and taking care of a compensation of the geometrical deviation of the systems. Each of the different mixing systems (100) gets then a part of the NO.sub.x reducing agent vapour, with or without the carrier gas, and mixes it into the exhaust gas stream by using the injection lances (102) and the deflector plates (103). The central feed pipe (104) is connected to the adjustment dampers. After typically 1 meter to 20 meter, the mixture of exhaust gas and reducing agent vapour enters the catalyst (300), where finally the nitrogen oxides are reduced to nitrogen and water.

[0057] The carrier gas can be recycled exhaust gas or air. If recycled exhaust gas is used as carrier gas, this gas is extracted from the exhaust gas stream by means of fans/blowers (not shown on the Figures).

[0058] As can be seen in FIGS. 2a and 2b, the multitude of injection lances (102) that are positioned in parallel to each other and that are connected to the feed pipe (104) form a grid like structure.

[0059] As can be seen in FIGS. 5a and 5b, each of the injection lances (102) penetrate through the feed pipe (104). In order to enable NO.sub.x reducing agent to still flow through the feed pipe (104), each injection lance (102) is provided with a hole (106) where the injection lance (102) crosses the feed pipe (104).

[0060] While the deflector plates (103) could be connected to a structure that is positioned in parallel with the injection grid, the direct connection of the deflector plates (103) onto the injection lances (102) is considered more economical from a manufacturing perspective as well as for maintenance and reparation perspectives. The direct attachment of the deflector plates onto the injection lances can be made using spacers (105) (see FIG. 4). Combining the injection lances and the deflectors in a single structure also reduces the weight of the structure and reduces the space which is taken by the structure, thereby rendering the structure more economical. The direct connection between the deflector plates (103) and the injection lances (102), in particular via spacers (105), can be a fixed connection, such as via a welded connection, but can as well be a releasable connection through which the deflector plates (103) can be removed from the injection lances (102), for instance for maintenance reasons. The use of welds to attach the deflector plates (103) onto the injection lances (102) provides in a robust attachment that ensures the durability of the mixing system.

[0061] As can be deducted from the CFD as shown in FIG. 3, the specific position of the deflector plates (103) downstream from and in near vicinity of the injection locations (101) of the NO.sub.x reducing agent results in local turbulence near the injection locations sufficient for a good mixing of the NO.sub.x reducing agent with the exhaust gas stream. The zone after the deflector plates (103) until the entrance of the catalyst (300) (see FIG. 1), is arranged such as to provide both the necessary turbulence, and after that, a defined flow pattern at the inlet of the catalyst to ensure low pressure drop and good reduction performance of the catalyst.

[0062] As can be seen on FIGS. 1 and 4, the exhaust gas stream flows through the flow channel (201) along an axis X and the surface of the deflector plates (103) facing the exhaust gas stream flow is positioned in a plane defined by axes Y and Z. The angle α between axes X and Y is typically ranged between 30° and 150°, more in particular between 45° and 135°, more in particular between 60° and 120°, more in particular between 75° and 105°, more in particular between 80° and 100°, more in particular between 85° and 95°, and more in particular at an angle of about 90°. More in particular the plane of the face of the deflector plates (103) with the largest surface area stands perpendicular to the flow direction of the exhaust gas stream in the flow channel.

[0063] A method according to the present application for introducing a nitrogen oxide (NO.sub.x) reducing agent into and mixing the NO.sub.x reducing agent with an exhaust gas stream comprising NO.sub.x comprises the steps of:

[0064] conducting the exhaust gas stream comprising NO.sub.x through a flow channel;

[0065] introducing the NO.sub.x reducing agent, through one or more injection lances, into the exhaust gas stream comprising NO.sub.x;

[0066] mixing the NO.sub.x reducing agent with the exhaust gas stream comprising NO.sub.x by inducing turbulent flows with flat deflector plates operatively associated with the injection lances and positioned downstream from the injection lances and transverse to the direction of the exhaust gas stream flow direction.

[0067] The mixing system (100) as described above can be used in a NO.sub.x abatement system for a stationary burning system.

[0068] A stationary burning system more in particular comprises an abatement system for reducing nitrogen oxides (NO.sub.x) in exhaust gases produced by the stationary burning system comprising an exhaust gas flow channel for conducting the exhaust gases comprising NO.sub.x in a given flow direction in the exhaust gas flow channel and a NO.sub.x abatement system comprising a mixing system (100) as described above and a catalytic converter disposed downstream from the mixing system (100).