WO2010100321A1

WO2010100321A1 - Method and catalyst arrangement for controlling emissions from a combustion source

Info

Publication number: WO2010100321A1
Application number: PCT/FI2010/050068
Authority: WO
Inventors: Jan Torrkulla
Original assignee: Wärtsilä Finland Oy
Priority date: 2009-03-02
Filing date: 2010-02-05
Publication date: 2010-09-10
Also published as: FI20095205A0; FI20095205A

Abstract

The invention relates to a method controlling emissions from a combustion source, in which method a flue gas flow (2) is discharged from the combustion source (1), a re- agent is provided, and the flue gas flow is further led to a catalyst arrangement after which the flue gas flow is discharged into the atmosphere. The catalyst arrangement comprises at least one oxidation catalyst element (4) and at least one selective catalytic reduction unit (6). The flue gas flow (2) is divided into a first part-flow (21) and a second part-flow (22), whereby the first part-flow (21) is led through the oxidation catalyst element (4) and the second part-flow (22) is led past the oxidation element. The first part-flow (21) and the second part-flow (22) are subsequently led through a selective catalyst unit (6) before discharge into the atmosphere.

Description

Method and catalyst arrangement for controlling emissions from a combustion source

Technical field

The invention relates to a method for controlling emissions, particularly NH₃ emis- sions, from a combustion source, in which method a flue gas flow is discharged from the combustion source, a reagent is provided, and the flue gas flow is further led to a catalyst arrangement after which the flue gas flow is discharged into the atmosphere according to the preamble of claim 1. The invention also relates to a catalyst arrangement according to the preamble of claim 7.

Background art

NO_x emissions from combustion sources form a major pollutant. Smog is a problem in many urban areas. NO_x plays a major role in the formation of smog by forming ozone (O₃) through chemical reactions in the atmosphere. NO2 is also a very strong irritant and has been shown to have detrimental effects on human health.

Selective catalytic reduction (SCR) is a known technology for reducing NO_x emissions from a flue gas flow from a combustion source. In an SCR process NO_x formed in a combustion process will react with ammonia (NH₃) to form atmospheric nitrogen and water. This can be exemplified as follows:

4 NO + 4 NH₃ + O₂ -> 4 N₂ + 6 H₂O (1 )

6 NO₂ + 8 NH₃ -> 7 N₂ + 12 H₂O (2)

Such a reaction may take place on a catalyst, which typically is a V₂O₅-TiO₂ catalyst.

An SCR system can be arranged in many ways, but typically a reagent, normally NH₃, is injected into the flue gas flow from the combustion source, whereby the reagent mixes with the flue gas and preferably forms a homogenous mixture before reaching one or more catalyst elements. On the catalyst elements NO_x and reagent react according to the above disclosed reactions. The reagent is consumed in the reactions according to the above stoichiometry. The required reduction rate is controlled by adjusting the dosing of the reagent. If less than a stoichiometric amount of NH₃ is injected, all NO_x will not react. To ensure efficient removal of NO_x it would be desirable to overdose NH₃. However, NH₃ emissions are also not desirable, whereby it is not possible to overdose reagent in order to achieve low NO_x emissions in a normal arrangement.

Summary of invention

An object of the invention is to achieve a method for controlling emissions, particularly NH₃ emissions, from a combustion source, which method avoids the above mentioned problems and provides for an efficient control of said emissions. This object is attained by a method according to claim 1.

The basic idea of the invention is provide a catalyst arrangement which comprises an oxidation catalyst element and a selective catalytic reduction unit. The flue gas flow from the combustion unit is split into at least two parts, whereby a first part-flow is led through the oxidation catalyst element and a second part-flow is led past the oxidation catalyst element. Thus a desired reaction ratio can be achieved by defining the fraction of the flue gas flow led through the oxidation element. The first part-flow and the second part-flow are then led through a selective catalytic reduction unit before discharge into the atmosphere.

Normally it is preferable that NH₃ is provided as reagent, whereby NH₃ is substantially completely oxidized to form NO_x in the first part-flow that is led through the oxi- dation catalyst element and NH₃ is not oxidized in the second part-flow.

Advantageously the first part-flow and the second part-flow are mixed into a mixed flow, whereby NO_x formed in the first part-flow and NH₃ present in the second part- flow react with each other, whereby the mixed flow is led through the selective catalytic reduction unit before discharge to the atmosphere.

The reagent NH₃ can be provided by introducing it into the combustion process of the combustion source or by introducing it into the flue gas flow.

If the reagent is introduced into the flue gas flow, it is advantageous that the flue gas flow is firstly led through at least one selective catalytic reduction unit, after which a resulting flow is divided into the first part-flow and the second part-flow. The advantageous features of the method are given in claim 2-6 and the main and advantageous features of the catalyst arrangement are given in claims 7-18.

Brief description of drawings

In the following the invention will be described, by way of example only, with refer- ence to the accompanying schematic drawings, in which

Figure 1 illustrates the principle of the present invention,

Figure 2 illustrates an alternative version of Figure 1 ,

Figure 3 illustrates a first embodiment of the present invention,

Figures 4a and 4b illustrate a second embodiment of the present invention, and

Figure 5 illustrates a third embodiment of the present invention.

Detailed description

Figure 1 illustrates the main principle of the present invention. A flue gas flow 2 is discharged from a combustion source 1. A means for introducing a reagent, NH₃, into the combustion process, normally directly into the combustion chamber, is schemati- cally indicated by reference numeral 3. An arrangement like this is generally called a SNCR Non-Catalytic NO_x removal. The flue gas flow 2 from the combustion source 1 with reagent is subsequently divided into a first part-flow 21 and a second part-flow 22.

The first part-flow 21 is led through an oxidation catalyst element 4 where practically a complete oxidation of NH₃ is achieved, i.e. NH₃ is substantially completely oxidized. The second part-flow 22 is led past the oxidation catalyst element, whereby NH₃ is not oxidized. This stage of the process can be achieved by an element structure as discussed more in detail below in connection with Figures 3 to 5.

The first part-flow 21 , where practically a complete oxidation has taken place, and the second part-flow 22, where no oxidation has taken place, are then mixed forming a mixed flow as indicated with reference numeral 23. When the first part-flow 21 and the second part-flow 22 are mixed at a ratio of preferably 50% to 50% (thus providing a mixture of 50% of NO_x and 50% of NH₃) the formed NO_x and the NH₃ will react with each other, whereby the desired stoichiometry between NO_x and NH₃ can be achieved.

The mixed flow 23 formed of the first part-flow 21 and the second part-flow 22 is led to a selective catalytic reduction unit 6. By providing adequate selective catalytic reduction volume the result will be zero NH₃ and NO_x emissions in a treated flue gas flow, indicated by reference numeral 24, discharged into the atmosphere.

The number of selective catalytic reduction units after said element structure, may be varied as appropriate and depending on the composition of the flue gas.

Figure 2 illustrates an alternative version of the main principle of the present invention as shown in Figure 1 , in which a selective catalytic reduction unit 6 is provided directly after the combustion source 1. A flue gas flow 2 is discharged from a combustion source 1. A means for introducing a reagent, NH₃, into the flue gas flow 2 is indicated by reference numeral 3. The flue gas flow 2 from the combustion source 1 with reagent is then led through a selective catalytic reduction unit 6, in which most of the NO_x from the combustion source reacts with the injected NH₃. The resulting flow 20, a so-called flow with NH₃ slip, is then divided into a first part-flow 21 and a second part-flow 22 as discussed above.

The first part-flow 21 is led through an oxidation catalyst element 4 where practically a complete oxidation of NH₃ is achieved, i.e. NH₃ is substantially completely oxidized. The second part-flow 22 is led past the oxidation catalyst element, whereby NH₃ is not oxidized. This stage of the process can be achieved by an element structure as discussed more in detail below in connection with Figures 3 to 6.

The first part-flow 21 , where practically a complete oxidation has taken place, and the second part-flow 22, where no oxidation has taken place, are then in this alternative embodiment led to a mixing device 5. When the first part-flow 21 and the second part-flow 22 are mixed at a ratio of preferably 50% to 50% (thus providing a mixture of 50% of NO_x and 50% of NH₃) the formed NO_x and the NH₃ will react with each other, whereby the desired stoichiometry between NO_x and NH₃ can be achieved. From the mixing device 5 the mixed flow 23, i.e. the mixed flow of the first part-flow 21 and the second part-flow 22, is led to a selective catalytic reduction unit 6. By providing adequate selective catalytic reduction volume the result will be zero NH₃ and NO_x emissions in a treated flue gas flow, indicated by reference numeral 24, dis- charged into the atmosphere.

The use of a separate mixing device 5 in the process is advantageous for treating exhaust gas in order to achieve a better control of emissions from a combustion source.

The number of selective catalytic reduction units, before and/or after said element structure, may be varied as appropriate and depending on the composition of the flue gas.

In theory, a flue gas flow with excess NH₃ slip from a selective catalytic reduction unit could be led as such to an oxidation catalyst element and then further to a selective catalytic reduction unit, whereby a 50% oxidation of the NH₃ could ideally be achieved in the oxidation element. However, dimensioning the oxidation process is extremely difficult since combustion processes are not stable depending largely on process parameters such as temperature and flue gas flow. If oxidation is too complete, the result would be a surplus of NO_x which would cause emissions of NO_x. On the other hand, if the oxidation process would not be efficient enough, the result would be a surplus of NH₃ which would cause emissions of NH₃.

The present invention avoids this problem by a controlled split of the flue gas flow and a controlled treatment of the first and second part-flows as discussed above.

The present invention is particularly applicable to combustion sources like gas engines, diesel engines, gas turbines and automotive and heavy duty diesel engines.

Figure 3 illustrates a first embodiment of the present invention. In this embodiment the catalyst arrangement comprises an element structure where every second element has oxidation properties, i.e. provides an oxidation catalyst element 4. The flue gas flow 2, in this embodiment, is firstly led through a selective catalyst reduction unit 6, whereby the resulting flow, a so-called flow with NH₃ slip, is subsequently led through said element structure, whereby a first part-flow 21 is led through an oxida- tion catalyst element 4 and a second part-flow 22 is led through a non-oxidation element 40. The catalyst arrangement thus comprises a number of oxidation catalyst elements 4, which are provided with a catalytic coating 43, and a number of non- oxidation elements 40 (without catalytic coating), which are arranged in a rectangular configuration side by side in an alternating manner forming the element structure. In a corresponding manner there are a number of first part-flows 21 and a number of second part-flows 22. The configuration of the element structure can be other than rectangular.

After said element structure the part-flows are mixed providing a mixed flow 23 which is led through a selective catalyst reduction unit 6.

The number of the selective catalytic reduction units, before and/or after said element structure, may be varied as appropriate and depending on the composition of the flue gas.

Figures 4a and 4b illustrate a second embodiment of the present invention. In this embodiment the catalyst arrangement comprises an element structure formed by a flat metallic foil 41 and a corrugated metallic foil 42 which are arranged against each other. The surfaces of the flat metallic foil 41 and the corrugated metallic foil 42 which face each other are provided with a catalytic coating 43 (shown with dotted lines).

This element structure thus forms channels (provided with an inner coating of cata- lytic coating 43) which provide oxidation catalyst elements 4.

The channels, i.e. uncoated channels, formed between uncoated sides of the flat metallic foil 41 and the uncoated sides of the corrugated metallic foil 42 form non- oxidation elements 40 (Figure 3). Arranging the flat metallic foil 41 and the corrugated metallic foil 42 in pairs on top of each other in a rectangular configuration pro- vides an element structure in which every second channel is provided with a catalytic coating and every second channel is without catalytic coating.

This embodiment shows three pairs of flat metallic foil and corrugated metallic foil placed on top of each other forming the element structure (Fig. 4b). The number of said pairs may be varied according to the desired reaction capacity and depending on the composition of the flue gas. This element structure can be deployed in a catalyst arrangement in a corresponding manner as the element structure discussed in connection with Figures 1 to 3 above. The configuration of the element structure can be other than rectangular.

Figure 5 illustrates a third embodiment of the present invention. In this embodiment the catalyst arrangement comprises a flat metallic foil 41 and a corrugated metallic foil 42 which are arranged against each other. The surfaces of the flat metallic foil 41 and the corrugated metallic foil 42 which face each other are provided with a catalytic coating 43 (shown with dotted lines). The flat metallic foil 41 and the corrugated metallic foil 42 thus form channels (provided with an inner coating of catalytic coating 43) which provide oxidation catalyst elements 4. The channels, i.e. uncoated chan- nels, formed between uncoated sides of the flat metallic foil 41 and the uncoated sides of the corrugated metallic foil 42 form non-oxidation elements 40 (Figure 3).

In this third embodiment the element structure discussed in connection with Figure 4a is rolled into an element structure with a round configuration. The size of the structure may be varied according to the desired reaction capacity and depending on the composition of the flue gas. This element structure can be deployed in a catalyst arrangement in a corresponding manner as the element structure discussed in connection with Figures 1 to 3 above.

The description and thereto related drawings are only intended to clarify the basic idea of the invention. The invention may vary according to the ensuing claims.

Claims

1. Method for controlling emissions, particularly NH₃ emissions, from a combustion source, in which method a flue gas flow (2) is discharged from the combustion source (1 ), a NH₃ reagent is provided, and the flue gas flow is further led to a cata- lyst arrangement after which the flue gas flow is discharged into the atmosphere, the catalyst arrangement comprising at least one oxidation catalyst element (4) and at least one selective catalytic reduction unit (6) is provided, said oxidation catalyst element (4) dividing the flue gas flow (2) with excess NH₃ slip into a first part-flow (21 ) and a second part-flow (22) with element structure having channels, in which every second channel is provided with a catalytic coating and every second channel is without catalytic coating and the first part-flow (21 ) is led through the at least one oxidation catalyst element (4) to form NO_x from divided excess NH₃ slip and the second part-flow (22) is led past the at least one oxidation catalyst element, and in that the first part-flow (21 ) and the second part-flow (22) are subsequently led through the at least one selective catalytic reduction unit (6) before discharge into the atmosphere.

2. Method according to claim 1 , characterised in that, that the first part-flow (21 ) and the second part-flow (22) are mixed into a mixed flow (23), whereby NO_x formed in the first part-flow and NH₃ present in the second part-flow react with each other, and in that the mixed flow (23) is led through the selective catalytic reduction unit (6) before discharge to the atmosphere.

3. Method according to claim 1 , characterised in that NH₃ is introduced into the combustion process of the combustion source (1 ).

4. Method according to claim 1 , characterised in that NH₃ is introduced into the flue gas flow (2).

5. Method according to claim 4, characterised in that the flue gas flow (2) is firstly led through at least one selective catalytic reduction unit (6), after which a resulting flow (20) with excess NH₃ slip is divided into the first part-flow (21 ) and the second part-flow (22).

6. Catalyst arrangement for controlling emissions, particularly NH₃ emissions, from a combustion source, which catalyst arrangement is arranged to receive a flue gas flow (2) from the combustion source (1 ), the catalyst arrangement comprises at least one oxidation catalyst element (4) and at least one selective catalytic reduction unit (6) arranged after the at least one oxidation catalyst element (4), characterised in that the oxidation catalyst element (4) is configured to divide the flue gas flow (2) with excess NH₃ slip into a first part-flow (21 ) and a second part-flow (22) with element structure having channels, in which every second channel is provided with a catalytic coating and every second channel is without catalytic coating and the first part-flow (21 ) is led through the at least one oxidation catalyst element (4) to form NO_x from divided flue gas flow (2) with excess NH₃ slip.

7. Catalyst arrangement according to claim 6, characterised in that the catalyst arrangement comprises a means (3) for introducing NH₃ reagent into the combustion process of the combustion source (1 ).

8. Catalyst arrangement according to claim 6, characterised in that the catalyst arrangement comprises a means (3) for introducing NH₃ reagent into the flue gas flow (2).

9. Catalyst arrangement according to claim 6, characterised in that the catalyst arrangement comprises at least one selective catalytic reduction unit (6) arranged before the oxidation catalyst element (4).

10. Catalyst arrangement according to claim 6, characterised in that the catalyst arrangement comprises a mixing element (5) for mixing the first part-flow (21 ) and the second part-flow (22) before the selective catalytic reduction unit (6) arranged after the oxidation catalyst element (4).

11. Catalyst arrangement according to claim 6, characterised in that the catalyst arrangement comprises an element structure, which provides at least one oxidation catalyst element (4) and at least one non-oxidation element (40).

12. Catalyst arrangement according to claim 11 , characterised in that the catalyst arrangement comprises a number of oxidation catalyst elements (4) and a num- ber of non-oxidation elements (40), and in that the oxidation catalyst elements (4) and non-oxidation elements (40) are arranged side by side in an alternating manner forming the element structure.

13. Catalyst arrangement according to claim 12, characterised in that the oxidation catalyst element (4) is provided by a channel formed between a flat metallic foil (41 ) and a corrugated metallic foil (42) arranged against each other.

14. Catalyst arrangement according to claim 12, characterised in that a catalytic coating (43) is provided on the side of the flat metallic foil (41 ) and on the side of the corrugated metallic foil (42) which face against each other.

15. Catalyst arrangement according to claim 12, characterised in that the oxida- tion catalyst element (4) is provided by a channel formed between a flat metallic foil

(41 ) and a corrugated metallic foil (42) arranged against each other, that pairs of flat metallic foil (41 ) and corrugated metallic foil (42) arranged against each other are arranged on top of each other into a into an element structure, whereby non- oxidation elements (40) are formed by uncoated channels between each of said pairs.

16. Catalyst arrangement according to claim 12, characterised in that the oxidation catalyst element (4) is provided by a channel formed between a flat metallic foil (41 ) and a corrugated metallic foil (42) arranged against each other, and in that the flat metallic foil (41 ) and the corrugated metallic foil (42) arranged against each other are rolled into an element structure with a round configuration, whereby non- oxidation elements (40) are formed by uncoated channels formed in the element structure.