Catalyst and process for nitric oxide reduction in a waste gas

Abstract

In order to improve the lifetime of an SCR catalyst in the waste gas purification by means of the SCR process of waste gas of a biomass combustion plant, the catalyst comprises a sacrificial component selected from a zeolite and/or a clay mineral, in particular halloysite. During operation, catalyst poisons contained in the waste gas, in particular alkali metals, are absorbed by the sacrificial component so that catalytically active centers of the catalyst are not blocked by the catalyst poisons.

Claims

1. A process for treating waste gas from a combustion plant, the method comprising contacting the waste gas with a reducing agent in the presence of a catalyst for nitric oxide reduction in a waste gas from a combustion plant, wherein the waste gas comprises nitric oxides, alkali metals, phosphorus, chromium, or mercury, the catalyst comprising: a first layer comprising a sacrificial component comprising halloysite, wherein the sacrificial component absorbs catalyst poison in the waste gas, and a second layer comprising a catalytically active component which comprises (1) a vanadium oxide and (2) tungsten oxide and/or molybdenum oxide, thereby reducing at least some of the nitric oxides to nitrogen and water; wherein the first and second layer are configured such that the first layer is positioned as an outer component; and optionally, the second layer is located on or in a support substrate and the first layer is located at least partially over the second layer.

2. The process of claim 1, wherein the sacrificial component reacts with catalyst poisons selected from alkali metals, phosphorus, chromium and mercury.

3. The process of claim 1, wherein the sacrificial component absorbs catalyst poisons which are bonded in aerosol form and/or to ash or sulphur.

4. The process of claim 1, wherein the weight ratio of the sacrificial component to the catalytically active component is in the range of from 1/10 to 1/3.

5. The process of claim 1, wherein the catalytically active component comprises vanadium oxide and the catalytically active component is positioned on a support substrate.

6. The process of claim 1, wherein a combination of (1) the catalytically active components together with (2) a support mass is in the range of from 0.1% by weight to 10% by weight of the total weight of the catalyst.

7. The process of claim 1, where the catalyst comprises a support substrate comprising TiO2 and the support substrate is present in the catalyst in an amount in the range of from 60 to 85% by weight.

8. The process of claim 1, wherein the first and second layer are configured such that the waste gas is subjected to the first layer before the second layer.

9. The process of claim 1, wherein the second layer is located on or in a support substrate and the first layer is located at least partially over the second layer.

10. The process of claim 1, wherein the first layer comprising the sacrificial component is located upstream of the second layer comprising the catalytically active component, when the catalyst is positioned in the direction of flow of the waste gas.

Description

(1) Embodiments of the invention will be described hereinbelow.

(2) The catalyst is either a plate catalyst or an extruded, in particular honeycomb catalyst. The basic formulation, that is to say the type and amount of the components of the catalyst mass, preferably corresponds to the basic formulation as is known from EP 0 762 925 B1, with the proviso that a sacrificial component is additionally added to the catalyst mass. The process for working up and producing the catalyst preferably also corresponds to the process which is to be found in EP 0 762 925 B1.

(3) The catalyst is a vanadium-based catalyst with titanium dioxide as the support mass and an amount of vanadium pentoxide in the range of from 0.01 to 5% by weight, preferably from 0.5 to 2.0% by weight, based on the weight of the catalyst mass. There is further provided as the catalytically active component preferably molybdenum trioxide MoO.sub.3 in an amount in the range of from 0.01 to less than 5% by weight and preferably in the range of from 1.5 to 4% by weight. Alternatively, a tungsten trioxide WO.sub.3 is used instead of the molybdenum trioxide.

(4) The catalyst mass further comprises a binder as well as fibres for improving the mechanical stability. The amount of binders, in particular clays, is, for example, in the range of from 2 to 7% by weight, as is the amount of fibres, in each case based on the total weight of the catalyst mass. Glass fibres are preferably used as the fibres.

(5) The catalyst mass further comprises as the sacrificial component an addition of a zeolite or a clay mineral. The clay mineral is preferably halloysite. The amount of the sacrificial component is in the range of from 10 to 30% by weight. The amount of the support mass varies, in dependence on the amount of the sacrificial component, from approximately 60 to 85% by weight. The support mass and the sacrificial component together form an amount in the range of approximately 90% by weight, in particular in the range of, for example, from 85% by weight to 93% by weight.

(6) Various compositions of a plate catalyst are given by way of example in Table 1 below.

(7) TABLE-US-00001 TABLE 1 Ex. 1 [% Ex. 2 [% Ex. 3 [% Component by weight] by weight] by weight] TiO.sub.2 69.6 60.9 78.3 Binder (bentonite) 3.6 3.2 4.1 Glass fibres 3.6 3.2 4.1 MoO.sub.3 2.2 1.9 2.4 V.sub.2O.sub.5 1.0 0.8 1.1 Halloysite 20.0 30.0 10.0

(8) Alternatively, it is also possible to use a zeolite, for example of the group A, X, Y, BEO, MOR, MFI, instead of the halloysite indicated in the table. However, the use is not limited to these zeolite types.

(9) The following compositions of the catalyst mass according to Table 2 are indicated by way of example for a fully extruded honeycomb catalyst:

(10) TABLE-US-00002 TABLE 2 Ex. 1 [% Ex. 2 [% Ex. 3 [% Component by weight] by weight] by weight] TiO.sub.2 67.4 59.0 75.9 Clay (bentonite) 4.8 4.2 5.4 Glass fibres 6.5 5.6 7.2 MoO.sub.3 0.7 0.6 0.8 V.sub.2O.sub.5 0.6 0.6 0.7 Halloysite 20.0 30.0 10.0

(11) Here too, the halloysite indicated in the table can be replaced by a suitable zeolite.

(12) The catalyst is preferably used generally in a combustion plant, in particular in a combustion plant for the generation of energy, for purifying the waste gas. In the combustion, biomass is used as the fuel or is at least added, so that the waste gas has a high dust content and also a high content of catalyst poisons, in particular alkali metals. Mention is to be made here of potassium as a particularly aggressive catalyst poison and of phosphorus, which has slightly lower aggressivity.

(13) The waste gas is passed through the catalyst and thereby comes into contact with the surface of the catalyst mass. A reducing agent, such as ammonia or a precursor substance, such as, for example, urea, is supplied to the waste gas stream before it enters the catalyst. The nitric oxides contained in the waste gas are reduced in the catalyst to nitrogen and water. Owing to the large amount of the sacrificial component, catalyst poisons contained in the waste gas are absorbed by the sacrificial component, so that the catalyst poisons do not become deposited at the catalytically active centres of the catalyst mass and block it. The lifetime of the catalyst is thereby increased significantly as compared with a catalyst without such a sacrificial component, as a result of which improved waste gas quality and in particular also reduced operating costs are achieved.