Exhaust gas purification catalyst on which influence of iron compound has been suppressed

Abstract

An exhaust gas purification catalyst is provided which contains titanium oxide as a main component and an oxide of one element or two or more elements selected from the group consisting of tungsten (W), molybdenum (Mo), and vanadium (V) as an active component, wherein the exhaust gas purification catalyst contains phosphoric acid or a water soluble phosphoric acid compound so that the atomic ratio of phosphorus (P) to a catalytically active component represented by the following formula is more than 0 and 1.0 or less;P / catalytically active component (atomic ratio)=number of moles of P / (number of moles of W+number of moles of Mo+number of moles of V).

Description

TECHNICAL FIELD [0001]The present invention relates to an exhaust gas purification catalyst, and more particularly to, a catalyst used for oxidizing elemental mercury (Hg) as well as reducing nitrogen oxides (NOx) contained in coal combustion exhaust gas by ammonia, which can maintain to a very low level an activity of oxidizing SO2 contained in the exhaust gas to SO3 for a long period of time by suppressing an increase in the activity of oxidizing SO2 with the lapse of time by an increase in Fe compound, and a method of producing the same.BACKGROUND ART[0002]As the denitration catalyst for ammonia catalytic reduction, containing titanium oxide as a main component has high activity and favorable durability, it is generally used worldwide for the treatment of exhaust gas such as gas released from a boiler and constitutes the mainstream denitration catalyst (Patent Document 1).[0003]In recent years, there is a rapid increase in demand for energy and coals having high sulfur content (i...

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Benefits of technology

[0020]According to the invention, by having the atomic ratio of P to a catalytically active component in the catalyst to be within the range described above, formation of SO2 oxidation active site in the catalyst that is caused by the adhesion of a Fe component comprised in ash from gas to be treated is suppressed, and therefore SO2 oxidation rate can be maintained at a low level for a long period of time. In particular, for a catalyst using a metal substrate, formation of SO2 active site caused by corrosion product containing a Fe component, that is generated when the catalyst is used in harsh condition, is prevented so that even for the catalyst using a metal substrate as a carrier the SO2 oxidation rate can be maintained at a low level for a long period of time.

[0021]As the catalyst of the invention has not only high denitrating performance and Hg oxidizing performance but also low SO2 oxidation rate, when it is used for denitration of exhaust gas from a high S coal boiler used in the United States, etc., generation of SO3 can be suppressed to a low level. Furthermore, since it is difficult for SO2 oxidation rate to increase even when the Fe component contained in ash or the like migrates into the catalyst, problems such as generation of purple smoke due to SO3 resulting from oxidation of SO2 can be avoided when it is applied for exhaust gas of high S coals containing a great amount of a Fe component.

[0022]Inventors of the present invention intensively studied the increase in SO2 oxidation rate of the catalyst caused by a Fe component. As a result, it was found that the increase in SO2 oxidation rate proceeds through the following steps (1) to (4).

[0023](1) Fe component such as iron oxide or the like adheres on the surface of a catalyst or corrosion of a metal substrate occurs at the interface between the metal substrate and the catalyst component, but no increase in SO2 oxidation rate occurs during this step. (2) The Fe component in the catalyst is sulphated by SOx present in exhaust gas. (3) The sulphate is dissolved in water which is absorbed when operation of a combustion furnace is on hold, and as a result, it migrates as a Fe ion to the inside of the catalyst. (4) The migrated Fe ion is adsorbed onto titanium oxide to form SO2 oxidation active site.

[0024]The inventors of the present invention were curious about the possibility of preventing efficiently an increase in SO2 oxidation rate by blocking the formation of SO2 oxidation active site during step (4) among the four steps described above. In this connection, as a means therefor, a phosphorus compound is included in the catalyst so that the Fe component is reacted with the phosphorus compound to form an insoluble iron phosphate, and as a result the increase in SO2 oxidation rate is prevented.

[0025]According to the invention, part of the catalytically active component is present as a complex resulting from a qualitative reaction with phosphoric acid / phosphoric acid compound, and it is believed that Fe ion and the complex of phosphoric acid and the active component undergo the reaction as follows.Fe ion+P2O5-WO3 complex→FePO4+WO3  (Formula 1)Fe ion+P2O5-MoO3 complex→FePO4+MoO3  (Formula 2)Fe ion+P2O5-V2O5 complex→FePO4+V2O5  (Formula 3)

Problems solved by technology

In addition to this, a trouble caused by SO3 increases, for example, part of SO2 is oxidized to SO3 due to a SO2 oxidizing activity of a denitration catalyst so that visible stack plumes originating from SO3 is released from a stack or a downstream equipment in a denitration apparatus is corroded, etc.