TITLE
A NOx removal catalyst based on a combination of transition metal oxides, poor metal oxide and a semi metal oxide for thermal Power plants and the method of production thereof

FIELD OF INVENTION

The invention relates to a process for production of selective catalytic reduction (SCR) catalyst powder suitable for NOx reduction in thermal Power plants.

BACKGROUND OF INVENTION

The selective catalytic reduction (SCR) of nitric oxide based pollutants in the thermal power plants is very important to curb air pollution. Among the various forms of Nitrogen oxides such as : N2O ( Nitrous oxide). NO ( Nitric oxide), N2O3 ( dinitrogen trioxide), NO2 ( Nitrogen dioxide) and N2O5 ( Nitrogen pentoxide), pollutant NOx, a major factor for causing smog and acid rain comprises of mainly nitric oxide ( NO) and relatively small amount of Nitrogen di-oxide ( NO2) with other forms of such oxides in a trace quantity. Automobiles and other mobile sources contribute about half of the NOx that is emitted. Electric power plant boilers produce about 40% of the NOx emissions from stationary sources.
The reduction of NOx selectively plays an important role to reduce NOx to atmospheric benign N2 gas over a catalyst. In a coal fired plant, the SCR catalyst can be used in three different places, most prominent being after the economizer and before the air preheater with flue gas temperature in the range of 300-350 deg C and with high dust concentration typically found in Indian thermal power plants. While the reductant ammonia concentration has to be optimized to curtail the ammonia slip, the

active catalyst component vanadia also needs to be optimized to abate SO2 to SO3 formation; together resulting in forming another pollutant ammonium bisulphate ( ABS). Therefore, the most important aspect of such catalyst preparation is the effective distribution of active catalyst component in a small quantity in the catalyst carrier typically in a porous configuration. In addition to this, other functional additives are required in appropriate quantities and the mixing of all such components forms an important role for achieving high catalytic efficiency. Another important parameter for SCR catalyst is the erosion resistance of SCR catalyst at high temperature operation in the presence of dust concentration.
While Titaniainits anatase form is the basic catalyst support used by most of the inventors across the world, there are reports of alumina and diatomaceous earth either independently or mixed with titania are used. Titania in its anatase form can be obtained by proper heat treatment during synthesis from its ore. Therefore instead of anatase titania as the main support material, many inventors have utilized hydrated titania titanyl sulphate and metatitanic acid as the main ingredient to build the porous support so as to retain the high surface area which can support the active catalyst components on its base. The active catalyst component is vanadia along with promoters like molybdenum and tungsten in most cases. Though vanadium pentoxide is most widely used as active catalyst component, many inventors have also made catalyst using vandium in its ammonium vanadate form. Further , there are many other additives have been used in such compositions to improve the catalytic efficiency in a wide temperature range. There are many inventus on zeolite based catalysts for high
temperature catalyzing applications.
Typically the catalyst composition for DeNOx applications in thermal power plants mainly consists of the porous ceramic support which is mixed with many ceramic additives in smaller quantities. Homogenous mixing of such components play an important role in imparting the required catalytic properties with high NOx

conversion efficiency and over a long period of time. Impregnation method is most widely used in infiltrating the active catalyst component like vanadia and other additives to porous titania support. Another most important method is coating of such active catalyst components onto porous support substrates. One other widely used method of making SCR catalyst is kneading the mixture of all the components together alongwith binding agents like silica and glass fibers. Other methods of making such catalysts are hydrothermal synthesis. Though, the invention on SCR catalyst development was reported as early as 1966, there are still inventions on such catalyst development in progress in different countries around the world. In India, the activity on catalyst preparation was not very significant as there was no environmental norms in place for use of such catalyst for NOx conversion. With the 2015 environmental norm in force from 1st January 2017, there are renewed interest on this subject.
Zeolite based thermally stable ammonia-SCR catalyst composition was disclosed in WO 2016046129 Al and in WO 2004002611 A and in many other disclosures. In such cases, zeolite is considered as porous catalytic support in place of analase titania and other catalytically active compounds are either coated or impregnated to it. As the zeolite based catalysts are operated at high temperature , they are most suitable for stationary engines and not much use in thermal power plants where the flue gas temperature lies in the range of 300-350 deg C. Therefore, emphasis was given in developing Vanadia based catalyst on titania based catalyst support for power plant deNOx application.
In the invention, US 6380 128 BI , the inventors have used barium oxide as suppressant for SO2 conversion alongwith ammonium vanadate and other materials in a metatitanate slurry dehydrated using a vacuum evaporator to get the catalyst powder. In a similar type of invention reported in US 8 685 882, the inventors have used ammonium

vanadate as a source for vanadia and wet impregnated with liatnia in the form of a slurry and used an rotary evaporator to dry the catalyst powder. In an invention in US 7393 511, the inventors have described a secondary catalyst of a precious metal and vanadia on titania to catalyze the oxidation of both ammonia and carbon monoxide while maintaining sulphur dioxide levels. In this invention, vanadyl oxalate solution in water was impregnated to titania followed by drying and calcination. In another invention US 2005/0085383, the erosion resistance of SCR catalyst was improved by combining anatase titania in the form of tetra isopropyl titanate in butanol of surface area > 40 m2/g with diatomacious earth and formed a slurry with grinding followed by calcination. The catalyst support material was then impregnated with solutions made from ammonium vanadate and ammonium tungstale.
The US patent 6054408 describes a method of kneading the compounds of titanium, molybdenum, vanadium with water and coating the supporting bodies with catalyst mass by extruding in the form of honeycombs following drying and calcination in the range of 500- 600 deg C. In an invention reported in EP 2 719 454, the aqueous slurry of TiCh/ V powder was combined with bohemite or pseudo bohemite as an inorganic binder and coating the mixed solution to a cordierite substrate and drying with hot air stream to obtain the SCR catalyst. In another related invention applicable to plate type catalysts US 7 713 901 , a silicone based polymer was dissolved in anorganic solvent and added the glass fiber as a bonding agent and finally adding SCR catalyst powders to the resulting mixed solution. The impregnation of a porous plate type glass fiber support into the mixed solution followed by suitably baking the same.
In an invention EP 2 213 371 Al, a coating process was described on the monolithic support containing a bilayer : lower layer with cerium oxide and upper layer comprising atleast one kind of metal selected from a group consisting of copper, manganese, iron, cobalt nickel or a compound thereof and also zeolites. The water

based slurries made using the constituents are coated on the monolithic support by dipping followed by drying and calcination. In a similar type of invention reported in US 2003/0231997A1, a catalytically active metal oxide was deposited on catalysl support substrates of metallic titanium or alloys thereof by a known method of wash coating or applying as a paste or slurry. In this invention, a plate type catalyst is made using titanium metal as a support instead of commonly used stainless steel to reduce the weight of catalyst support by at least 40 %. In another invention as described in WO 2013/182255 A1, a method of wash coating is disclosed by coating the corrugated glass fiber substrate with iron containing zeolite followed by impregnation of cerium oxide method followed by calcination. The iron promoted beta-zeolile was used alongwith cerium oxide, titania, compounds of tungsten, neodymium and silicon in addition to the diatomaceous earth additive.
In an invention US 6475 944 Bl, a process was disclosed for a vanadia impregnation onto titania pillared interlayer clay . As the precursor of titania, titanium tetrachloride and titanium alkoxides were employed. The process involves the addition of titania precursor to clay water suspension in presence of hydrochloric acid followed by filtering and washing and subsequent drying and calcination. In an invention disclosed in CN 104226336 A, a sulphur containing oxide was added into anatase type porous titanium dioxide to obtain a porous mixed oxide by impregnation, vacuum degassing, drying and calcination. The disclosure has used titanyl sulphate as the source for titania and vanadaies were used alongwith ammonium tungstate .
Use of nanomelric Ti02-Sn02 with high surface area and high thermal stability as a catalyst support material has been disclosed in CN 102205240 A. Titanium tetrachloride and tin tetrachloride were slowly added dropwise to ethanol, followed by ammonia addition, filtering, drying and calcination. The laboratory level denitration catalyst evaluation was carried out at a lower reaction temperature of 150-300 deg C to achieve NOx conversion efficiency of 74 to 97 % . In another disclosure US 6 419 889B1 , a commercially available titania powder was peptized in presence of hydrogen

peroxide followed by compaction in a kneader and then extruded as trilobes. The extrudates were then dried and calcined prior to testing for NOx conversion efficiency. In this invention, vanadium was sourced as vanadium ammonium oxalate which was separately prepared by adding ammonia and oxalic acid to ammonium polyvanadate suspended in water. The catalyst obtained had a surface area in the range of 70-99 m2/g. The invention in US 8 975 206 B2, it was disclosed that two separate slurries were prepared : one containing vanadium compound and other containing titanium and other additive compounds. Both the slurries were mixed together with stirring at 90 deg C and dried followed by calcination in a temperature range of 500 to 850 deg C to obtain the catalyst powder. The catalyst composition comprised a vanadate represented by the formula XV04/S , wherein XV04 stands for a Bi-, Sb-, Ga- and AI- vanadate and with other rare earth materials.
In an invention US 8 569 199 B2 , a homogenous cerium-zirconium mixed oxide was suspended in water, ground and applied to a ceramic honeycomb followed by drying and calcination. This catalyst is vanadium free and contains 0.01-5 wt. % sulphur resulting in > 80 % NOx conversion efficiency at a temperature of 300 deg C . In an invention Us US 8 524 185 B2, a two step coating process was disclosed comprising first intermediate layer of gamma alumina wash coated on multichannel honeycombs followed by wash coating a second layer of SCR catalyst. The flue gases on passing through the channel comes in contact with the top active catalyst layer and facilitates NOx conversion. This type of coating method is generally used in the automobile catalytic converter applications.
The above prior art indicates that coating, impregnation and mixing are the methods used by various inventors to produce SCR catalyst powders. Further, expensive chemicals like nano materials are used which are difficult to produce in commercial scale without anticipating significant increase in NOx conversion efficiency.

Further, However, these methods are not foolproof in ensuring homogenous mixing for different additives in a very small quantity in the main catalyst support. Though titania is added either in the form of anatase titania or metatitanic acid or hydrated titania, the effect of an intermediate untreated titania from the titania manufacturing plant can be an ideal source for anatase titania which has not been studied. Further, though surface area measurement has been carried out for most of the previous studies, a systematic study on the effect of processing on the surface area is not studied which is important for catalytic activity determination. Therefore, in the present invention, a simple but practical approach was used to select suitable titania precursor and a method to produce a homogenously mixed catalyst compound prior to fabrication in the form a plate or honeycomb followed by testing .
OBJECTS OF THE INVENTION:
An object of the present invention is to optimally design a SCR catalyst composition using a combination of transition metals, poor metal and semi metal for NOx conversion in thermal power plants.
Another object of the present invention is to propose a method of preparing SCR catalyst by using combined method of dry mixing, wet mixing and paste mixing to produce homogenous SCR catalyst composition with both inorganic and organic binders embedded in it.
Yet another object of the present invention is to use an intermediate titania raw material with very high surface area as the catalyst support material.
Yet another object of the present invention is to select optimum calcination temperature to maintain surface area to get the catalytic activities of the SCR catalyst.
Further object of the present invention is to demonstrate the NOx conversion efficiency of the catalysis suitable for thermal power plants.

DESCRIPTION OF THE INVENTION:
The embodiment of the present invention is based on an approach for generating a homogenous SCR catalyst composition using a high surface area intermediate anatase titania with all the required additives such as promoters, stabilizing agents and reinforcing agents and a combination of inorganic binders for further use of fabricating honeycombs or plates. In the present invention , an intermediate titania was obtained from the titania making process from the illemenite ore . The intermediate titania was in slip form, which was dried at 200 deg C to remove moisture without distrurbing the particle size growth leading to high surface area of the support material in the range of 220-250 m2/g. The intermediate titania was used for batch calculation by taking into account its moisture content as these are hydroxy! based titania, which otherwise reduces effective catalyst support component for long term conversion efficiency. The process involves cone blending the intermediate titania material alongwith transition metal oxide additives like vanadium pentoxide, Molybdenum oxide, tungstic acid and manganese oxide in different proportions etc. and poor metal oxide like tin dioxide all in commercial purity. The blending process mixes the powders of different constituents and in very small to large quantities uniformly. The blended powder is then added with appropriate amount of ammonia solution alongwith part of the inorganic colloidal silica binder in its nano form for effective binding and wet milled for further homogenous mixing of the constituents. The water based slurry is then dried in a gas fired/ electrical fired oven and the mixed composition is calcined at a temperature range of 400-500 deg C to convert ammonium tungstate to tungsten oxide with uniform mixing at molecular level. In the third step, the mixed and calcined catalyst powder is mixed in a sigma mixer with water, organic binder like methyl cellulose, inorganic binder like colloidal silica and reinforcing agent like chopped e -glass fiber strands, lubricating agent like low molecular weight polyethylene glycol etc. resulting in a triple step mixed

homogenous SCR catalyst paste which are kept in a cold room at a temperature of 4 degC for 24 h prior to use in fabrication of either honeycombs or plates by the known methods. The fabricated components are used for testing in a test rig in the laboratory to understand the NOx conversion efficiency.
Transition metals play an important role in the formulation of selective catalytic reduction catalyst. Among metal oxides, analase Ti02 with excellent dispersity and good resistance to SO2 poisoning is regarded as the best support material for NO abatement. Vanadium oxide is the main source of active catalyst component in this series and is an oxidation catalyst. M0O3 acts as a promoter additive with no catalytic properties of its own but enhances the activity of catalyst. WO3 on V^O/TiOz increases the temperature window of SCR and significantly increases the poison resistance of the catalyst by increasing the surface acidity. It also is responsible for stabilization of anatase structure. The Mn/TiO: anatase catalyst is very active for low temperature SCR of NO with NHj in comparison to other titanias supported Mn catalysts. Other important transition metal oxides used in SCR catalyst formulations are the oxides of Fe, Cu, Cr, Co, Ni etc. The poor metal oxide like Sn02 also is essential in SCR catalyst composition. Sn02-decorated T1O2 interpenetrating networks with highly branched structure and large surface area are responsible for the excellent activity of selective catalytic reduction of NO with ammonia in the presence of oxygen. Silicon Metal is known as a semi-metallic or metalloid, having several of the characteristics of metals. The silica derived from this metal is an important constituent in its nano form to impart strength to the SCR catalyst without deteriorating the catalytic activity. Therefore, a combination of transition metals, poor metal and semi metal is required to suitably formulate the SCR catalyst composition for NOx conversion in thermal power plants and in other similar applications like in stationary source and in automobiles.

The present invention focuses on using an intermediate titania compound free from all alkali impurities. This intermediate titanium hydrogen oxide compound is removed prior to its treatment with the potassium sulphate in the production process. The titanium compound used in this invention had <0.005 % of potassium oxide and no trace of sodium oxide which are essential for improving catalytic conversion as the excess amount typically present in anatase titania or in treated titania are poisonous to the SCR catalyst in the long run. This intermediate titania is different from the hydrated titania and metatitanic acid which have been experimented by many inventers earlier. The untreated intermediate titanium compound from the production line in the form of a slurry is filter pressed for dewatering followed by drying at200degC and grinding to achieve powders with no alkali and sulphate impurities and with moisture content in the range of 5-8 % which are taken into account during batch calculation.
The process for making the catalyst composition is in three steps . In the first step, dry mixing is carried out in a cone blender to homogenously mix all the powders such as catalyst support, transition metal oxides like V, Mn and Mo, tungstic acid and poor metal compound like tin oxide for 4h. The cone blended powder ensures that the additives uniformly mixed in a solid slate mixing method with the porous catalysl support material having very high surface area. In the second step, the cone blended powder is mixed with ammonia solution of appropriate volume to neutralize the tungstic acid to form ammonium tungstate. Alongwith such reaction, inorganic binder like colloidal silica in nano form is also added preferably in half the desired quantity and ball milled with water for 4h . The wet mixing ensures that all the additives which are coated with porous titania support powder in the dry mix condition now can be separated, mixed in water and again get coated uniformly onto the porous support. The slurry of the catalyst powder can be dewatered only by drying in an oven without disturbing the catalyst composition. The nano silica in the composition binds the particles together by forming a temporary network among all particles with titania as

the support material. The dried powder forms the effective SCR catalyst composition, which undergoes a third stage homogenization by mixing with non-active catalyst component which are required for fabrication purpose. In the third step, the active catalyst component in the range of 85-90 % are mixed with remaining amount of colloidal silica , and other inorganic binders like ball clays, bentonite, and organic binder like cellulosic material, reinforcing agent like glass fiber strands and lubricating agents like polyethylene glycol. All these materials are required for providing binding action to the catalyst material without deteriorating the catalytic properties. The mixture of the active and non active constituents are mixed in a sigma mixer with water to form a paste of appropriate rheology. The third step mixing ensures the final homogenous mixing of the catalyst material with the binders of different types and the dough thus produced is kept in the cold room at 4 deg C for 24h prior to use in fabrication of honeycombs or plates,
The fabricated honeycombs or plates which is not the scope of this invention is carried out using the known and improved methods, The honeycombs or plates are thus dried in a microwave oven prior to calcination in the temperature range of 600 - 700 deg C for 3-5 h. The fired honeycombs are tested for conversion of NOx to N2 using a indigenously developed test rig and operated in the temperature range of 300 to 375 deg C. A conversion efficiency of > 90 % could be achieved in the laboratory without any dust loading at a temperature of 350 deg C. To check the homogeneity of the powder mixture, the conversion efficiency of the powder in the form of noodles was tested and found to deliver > 80 % conversion efficiency. This is an indirect proof of homogenous mixing of powder and fabricated component using the present invention.
The surface area plays an important role in the indirect determination of catalytic efficiency of a material. A systematic study has been made in the present invention to understand the effect of processing condition to the change in surface area of the catalyst.

The following examples will elaborate the invention in detail :
Example 1
The choice of catalyst support raw material plays an important role in achieving long term deNOx conversion efficiency of a SCR catalyst material. The titania compound chosen for this work is an untreated intermediate compound with surface area typically 230-240 m2/g. The material is devoid of any trace alkali compound and also no sulphur compound. The material was having moisture in the range of 8-10 % which was taken into account for developing the composition. The typical effective anatase titania value in the active catalyst composition lies in the range of 55-75 %, remaining being other inorganic additives. The choice of transition metal oxides and poor metal oxide was chosen such a way that none of them are poisonous to the final SCR catalyst at the operating temperature.
Example 2
The BET method of surface area measurement is an indirect indication of loss of catalyst activity during material processing. In a typical example, the BET surface area of raw material intermediate titania was 244 m2/g. The value reduced toll4m:/g by heat treatment of the catalyst material at 500 deg C. This has further reduced to 72 m2/g by heat treatment at 600 C followed by 40 m2/g at 650 deg C and to 27 m3/g at 700 deg C and finally to 15 m2/g at 750 deg C heat treatment. During this experiment, it was noted that the catalyst samples heat treated below 600 deg C are not strong enough to be used for testing and the samples fired above 650 deg C lost enough surface area not to be used as a catalyst for long term conversion. The optimum NOx conversion efficiency and strength the catalyst lie in the BET range of 65-70 m2/gm. Therefore, an ideal situation for maintaining the strength and the conversion efficiency is to heat treat in an intermediate temperature to achieve the best result.


Example 3
In a typical batch , commercial grade transition metal oxides like vanadium pentoxide < I % , molybdenum oxide < 0.5 % , tungstic acid with effective tungsten oxide < 8 % , MnCh < 4 %, SnOi < 4 % , colloidal silica with effective silica content < 8 % , glass fiberof<5% and other additives like different types of clay type materials were used in making the batch. Catalyst compositions were formulated using anatase titania as base material in the range of 55 -75 % , promoters in the range of 5.5 -7.5%, along with catalyst oxide, activators, stablisers, reiforceing agents and binders to enhance mechanical properties The powders were mixed in the three step method as described above ; converting first to a uniform solid state mixture, then to homogenous mixing at molecular level by suspending them in a solvent like water and finally to a paste form along with the binders to form an extrudable mass with appropriate rheological characteristics. The mixed binder content is processed in s such a manner so that the nano silica does not get filter out during dewatering and takes part in binding the active catalyst component to the porous support along with other inorganic binders These compositions were calcined at temperature preferably at 600- 625 degree C to get required BET surface area. The NOx reduction efficiency of the catalyst prepared by this method showed NOx reduction efficiency of 82 to 98 % when tested in the temperature range of 300 to 350 degree C suitable for SCR-DeNOx applications in thermal power plants.

We claim :

1. A process for SCR catalyst based on a combination of transition metal oxides, poor metal oxide and a semi metal oxide for NOx conversion in thermal power plant comprising of:
- an intermediate untreated titanium compound as the catalyst support material
- a mixture of transition metal oxides such as V, Mo, W and Mn, a poor metal oxide such as Sn and a semi metal oxide such as Si besides other additives
-a process comprising of three different types of mixing steps for homogenization -and a process to optimize the catalytic activity by optimizing the heat treatment
2. A process as defined in 1, wherein the catalyst support material is an intermediate derivative of transition metal oxide of Titanium in the anatase form with surface area of > 200 m2/g.
3. A process as defined in 2, where in the intermediate titania source has < 0.005 % K2O and no Na20 and sulphur impurities
4. A process as defined in I, wherein, the processing involves a combination of dry blending , ball milling and finally paste making with a sigma mixer prior to fabrication of shapes
5. A process as defined in 4, wherein the ball milling stage also simultaneously involves ammonization for conversion of tungstic acid to tungstates.
6. A process as defined in 1, wherein, BET surface area of the catalyst was maintained above 50 m:/gm by choosing the right calcination temperature
7. A process as defined in 1, wherein the catalyst developed in this study resulted in more than 80% NOx conversion efficiency using a simulated NOx in N2 gas.