GB2024646A - Catalytic oxidation of smoke in exhaust gases - Google Patents

Abstract

Smoke forming particles entrained in a gas stream containing oxygen is passed through a catalyst unit in which a supported catalytic material is disposed to inhibit straight through flow of the said particles so that, firstly, the probability of contact between the particles and the catalytic material is increased and, secondly, turbulence is imparted to the gas stream to increase the probability of contact between the particles and the catalytic material. The support may be randomly arranged ceramic pellets, interwoven or randomly oriented wire as mesh or perforated corrugated sheet or foil which has been stretched.

Description

SPECIFICATION Control of pollutants in gases This invention relates to the reduction of smoke contained in gases and, in particular, in waste gases. The invention is especially concerned with at least the reduction of the quantity of smoke in such waste gases and those emitted by coal fired boilers and the exhaust gases from internal combustion engines.

In this specification, the words "gas" and "gases" are, as the context requires, to be taken as meaning a gas or gasesperse, a vapour orvapours or a mixture containing one or more gases and one or more vapours. Thus, the word "gas" in the statement "the exhaust gas from an internal combustion engine" means the mixture of gases and vapours (which may include some liquid droplets) which issue from an internal combustion engine when running.

Gases from boilers and internal combustion engines often contain finely divided particles of hydrocarbons and/or carbon and/or other solid matter which emerge in the form of smoke. The smoke from a diesel engine may include solid/liquid particles, solid chain aggregates in which spherical particles of between 100-800"A diameter link up together, liquid sulphates, liquid hydrocarbons and gaseous hydrocarbons. The solid/liquid particles generally comprise carbon particles with adsorbed liquid hydrocarbons, sometimes referred to as polynuclear aromatics, and the solid chain aggregates are generally composed of high molecular weight organic compounds and/or inorganic sulphates.

Three different types of smoke are commonly observed issuing from a diesel engine exhaust pipe, namely, "white smoke", "black smoke" and "blue smoke". White smoke is produced during engine run-up and results from the condensation of water vapour on to particles (e.g. hydrocarbons etc. as mentioned previously) contained in the exhaust gas so that a fine mist is formed. Black smoke is produced when the engine has warmed up and contains a relatively high proportion of carbon particles. In blue smoke there is some carbon with a relatively high proportion of gaseous hydrocarbons such as aldehydes.

Throughout the remainder of this specification the particles referred to above will be described as "smoke forming particles". About 90% of these smoke forming particles have maximum dimensions of less than one micron which is within the respirable particle size and the maximum dimension of the remaining 10% of these smoke forming particles are less than four microns.

An object of the present invention is at least to reduce the quantity of smoke contained in waste gases by effecting the catalytic oxidation of smoke forming particles in these gases.

Catalytic oxidation of carbon particles takes place at about 4000C whereas the normal temperature of combustion of these particles is 700-800 C. For hydrocarbon particles catalytic oxidation will take place at temperatures above 200 C. Since the presence of a catalyst enables oxidation of the smoke forming particles in a gas to take place at a lower temperature than the normal temperature at which combustion takes place, little or no heating of the exhaust gas from an internal combustion engine should be required, when it is desired to effect the catalytic oxidation of any smoke forming particles in the gas.For example, a diesel engine runs at about 400 C when operating at medium to full power so that no preheating of the exhaust gas issuing from the diesel engine would be required before passing the said exhaust gas over a catalyst to remove the smoke forming particles from the gas by catalytic oxidation, provided the catalyst is close to the engine. Preferably, the catalyst should be in the exhaust manifold or a plurality of such catalysts should be provided, one in each port.

A catalyst to effect the oxidation of the smoke forming particles in a gas, preferaby comprises a catalytic material applied to a support. This supported catalyst is contained in a catalytic purification unit through which the gas will pass to contact the catalyst. When using such a catalytic purification a catalytic purification unit through wich the gas will pass to contact the catalyst. When using such a catalytic purification unit, we have found that its effectiveness in oxidising particles in a gas stream passing through it is markedly increased if turbulence is created in the gas stream at least when it passes through or over the catalyst.

According to one aspect of the present invention, therefore, any smoke forming or other particles present in a gas are partially or wholly removed by effecting the catalytic oxidation of the said particles.

According to a second aspect of the present invention the smoke forming particles are oxidised in a catalytic purification unit wherein turbulence is created in the gas at least whilst it is passing through or over the catalyst contained in the said unit.

According to still further aspects of the present invention: (a) the catalytic material in the catalytic purification unit is applied to a support which is such as to create turbulence in the gas stream at least during contact with the catalyst; (b) the catalyst comprises one or more of the platinum group metals Pt, Pd, Ru, Ir and Os or one or more alloys containing one or more of these metals or an intermetallic material comprising platinum group metal.

and base metal; (c) a refractory coating, a so-called "washcoat", is provided between the support and the catalyst; (d) the washcoat material contains one or more oxides selected from the group consisting of the oxides of transition group metals and metals from groups la, Ila and lila and Illb (including the rare earth elements) and Va of the periodic table (using Fisher Scientific Co. Periodic Table 5-70210); (e) preferably, the washcoat material comprises one or more members of the group consisting of alumina, beryllia, zirconia, magnesia, tantalum oxide, silica, titania, hafnia, thoria, and a rare earth oxide as ceria and combinations thereof such as boria-alumina and silica-alumina; (f) the gas containing the smoke forming particles is the exhaust gas from an internal combustion engine.

(g) the internal combustion engine to which the invention is particularly applicable is a diesel engine.

We have found that an effective way of creating turbulence in a gas stream is to use as a support for the catalyst, porous ceramic, interwoven or randomly orientated wire or mesh or perforated corrugated sheet or foil which has been stretched. We have also found that turbulence is created or at least assisted when the catalyst is supported on pellets which are arranged randomly or other catalyst support. In addition to supporting the catalyst per Se, the support also acts as a filter to filter the actual "smoke forming particles" which are then subsequently catalytically oxidised. If desired, the exhaust may be passed through a device to initiate turbulence prior to entering the catalyst. The said device may be a conventional swirl chamber.It is believed that in practice, the "smoke forming particles" are forced into contact with and adhere to the catalyst surface where they are catalytically oxidised together with any adsorbed hydrocarbons and other oxidisable matter. Further, the catalyst is preferably chosen so that during oxidation, a minimum of SOB is produced from oxidation of SO2 present in the exhaust stream.

The metal support may be made from a base metal or a platinum group metal or an alloy containing a platinum group metal. Alternatively, Kanthal or an iron and chromium containing alloy such as Fecralloy may be used.

The pellets may comprise ceramic or metallic material. The ceramic and other materials suitable for the pellet supports may be any one or more of the following: porous silica, for example, that sold under the trade mark "Silocel"; granular charcoal; alpha or gamma alumina; naturally occurring or synthetic alumino silicates; magnesia, diatomaceous earth, bauxite, titania, zirconia, limestone, magnesium silicate, silicon carbide, activated and in-activated carbons. The pellets may be of regular or irregular form such as capillary tubes, rods, balls, broken pieces or tiles, etc.

Preferably, the ceramic and metallic supports should have a refractory oxide coating, commonly known as a "washcoat" interposed between the surface of the support and the catalyst. Preferred refractory oxide layers comprise members of the gamma or activated alumina family.

The washcoat may be prepared by precipitating a hydrous alumina gel and, thereafter, drying and calcining to expel hydrated water and provide active gamma alumina. A particularly preferred active refractory metal oxide is obtained by drying and calcining at temperatures of 4000 to 8000C a precursor mixture of hydrous alumina phases predominating in crystalline trihydrate, that is, containing in excess of 50 per cent by weight of the total alumina hydrate composition, preferably from 65 to 95 per cent by weight of one or more of the trihydrate forms of gibbsite, bayerite and norstrandite by X-ray diffraction. We prefer to use British Aluminium Co. grade MH180 alumina hydrate and convert itto activated alumina by drying and firing as described above.

Other methods of preparing and applying a washcoat are described in our co-pending application No.

32920/77. (JC 645).

The washcoat has a porous structure which gives it a large surface area between 50-500 square metres per gram of alumina on to which the catalyst may be deposited in the form of a continuous or discontinuous coating.

A preferred arrangement is one in which the catalyst is palladium and the washcoat contains tantalum oxide or cerium oxide. Alternatively, the catalyst may be an alloy of palladium and platinum containing up to 75 wt. % of platinum.

The tests described in the following example illustrate the effectiveness of the present invention in reducing the quantity of smoke in the exhaust gas from a diesel engine.

A Lister single cylinder engine coupled to a 3-Kw generator, to act as the load was used for the tests. With a 2.75 Kw load coupled to the engine, smoke emitted from the engine registered 4.5 on the Bosch scale of smoke density. A container loaded with pellets was placed in the diesel engine manifold such that the exhaust gas from the engine passed over the pellets. These comprised alpha alumina with 50/50 platinum/palladium alloy catalyst present as a coating on the pellets. The pellets were about 1/8 inch in diameter and approximately inch in length. With the container loaded with the pellets the smoke emitted from the engine registered 1.9 on the Bosch scale. On increasing the loading of the pellets in the container by 10% the smoke emitted by the engine was reduced further to a reading of 1.5 on the Bosch scale.When the engine was running with a load of 2.7 Kw couple to it the space velocity was 80,000 volumes of catalyst/hour and the catalyst volume was 1 litre.

With the diesel engine idling, the temperature of the exhaust gas was below 400 C. Under these circumstances, it was found that the smoke forming particles were collected on the pellets. It was also found that when the Lister engine, with the container loaded with pellets located in the manifold, was allowed to idle, it was 1 hour before the density of smoke emitted from the engine increased markedly. When the engine was then allowed to run at full power, all the smoke forming particles which had collected on the pellets were removed by catalytic oxidation.

Asecond test was conducted with a different catalyst placed in the Lister diesel engine manifold. A structure of 10 thou Fecralloy wire 4 inches in diameter and 6 inches in length was used as a support. A washcoat consisting essentially of alumina was coated onto the support with a catalytic layer comprising 7.5% Rh/Pt by weight. The catalyst loading was approximately 60g / cu ft of support.

The Lister engine was operated at 3,000 rpm with a space velocity of 800 columes of catalyst/hour. The temperature of the exhaust gas passing over the catalyst, the weight of the particulates present in the exhaust gas before and after passing over the catalyst were all measured. The results are shown in Table 1.

TABLE 1 Particulates Temperature Inlet Outlet % Reduction ( C) g/hr. g/hr.

250 19.8 16.8 15 300 21.6 19.9 8 400 25.4 22.1 13 450 35.8 28.4 21 500 19.4 14.2 27 550 30.4 24.1 21 600 35.3 27.5 22 650 25.6 14.8 42 Another test was conducted to study the effect of volume on the efficiency of particulates control and the results are shown in Table 2.

The temperature of the gas was kept a 3600C and the amount of particulates present in the gas was 19.5 g/hr. Catalysts were coated in the manner described in the second test but with two different aspect ratios.

The face areas were kept constant at 82 mm in diameter and 106 mm in diameter.

TABLE 2 Volume % Reduction % Reduction S.V./hr.

(litres) 82mm. dia. 106mm. dia.

0.27 9.4 12.4 250,000 0.38 11.6 16.9 178,000 0.48 18.9 23.8 141,000 0.56 24.8 28.4 120,500 0.74 35.6 43.8 91,216 0.84 48.2 53.7 80,350 0.93 52.1 61.4 72,500 1.21 62.1 68.5 55,785 It is preferred that, when a diesel engine is running at full power, the back pressure, which is the difference in the pressure of the exhaust gas stream before it passes through a catalytic purification unit and after it has been passed through such a unit, should be less than 4 inches of mercury.

Similar tests were carried out using a Perkins 4236 diesel engine driven at a speed of 2200 r.p.m. with a catalyst placed in the exhaust manifold and the results are set out in Table 3 below. The catalyst comprised 1600 grams of Fecralloy (12TM) wire washcoated with gamma alumma stabilised with barium and applied to the wire at 0.12 grams per gram of wire. The catalytic material was 7.5 % Rh/Pt and the total weight thereof was 3 grams.

TABLE 3 %offull 100% 50% 20% output power Hydrocarbons adsorbed on particles entering 20 15 160 catalyst unit measured in grams per hour Hydrocarbons adsorbed on particles emerging from 3 0 3 catalyst measured in grams per hour The quantity of hydrocarbon adsorbed was measured by collecting the particulates in the exhaust on a filter pad for a predetermined time. Thereafter, the filter pad was heated on thermogrammetric balance until weight loss ceased signifying that all volatile constituents had been burnt-off. Finally, the residue was analysed and the results obtained are indicated in Table 3 above.

A further test was carried out on the Perkins 4236 diesel engine running at 1400 r.p.m. and using the same catalyst and the results are shown in Table 4.

TABLE 4 % offull 100% 50% 20% output power Hydrocarbons adsorbed on particles entering 10 15 105 catalyst unit measured in grams per hour Hydrocarbons adsorbed on particles emerging from 4 4 5 catalyst measured in grams per hour Our tests indicate that practice of the present invention results in removal of up to 80% of the adsorbed hydrocarbons (polynuclear aromatics) and up to 40% of hydrocarbon particles from the exhaust gas of a diesel engine.

Although the invention has been described with reference to the reduction in the quantity of smoke forming particles from a diesel engine, it is by no means so limited and may also be applied to petrol engines, gas engines and turbines. It may, for example, be used for the reduction of the quantity of smoke forming and other particles in other gases.

Claims (10)

1. A method for the catalytic oxidation of smoke forming particles as herein before defined wherein particles entrained in a gas stream containing oxygen are passed through a catalyst unit in which a catalytic material is randomly disposed to inhibit the straight through flow of the said particles so that, firstly, the probability of contact between the particles and the catalytic material is increased and, secondly, turbulence is imparted to the gas stream to increase the probability of contact between the particles and the catalytic material.

2. A method according to claim 2 wherein the catalytic material is supported on a support made from a porous ceramic; interwoven or randomly oriented metallic wire or mesh, perforated metallic or ceramic material or corrugai ed metallic sheet or foil which has been stretched.

3. A method according to claim 2 wherein the ceramic material is in the form of pellets.

4. A method according to claim 2 or claim 3 wherein the ceramic material is porous silica or granular charcoal; alpha or c amma alumina; naturally occurring or synthetic alumino silicates; magnesia, diatomaceous earth, bauxite, titania, zirconia, limestone, magnesium silicate, silicon carbide, activated or in-activated carbon 3.

5. A method according to any of claims 1 to 4 wherein the catalyst support is coated with a refractory oxide coating prior to application thereto of the catalytic material.

6. A method ace ording to claim 5 whereas the refractory oxide coating is gamma alumina.

7. A method according to any preceding claim wherein the catalytic material is a platinum group metal, a mixture or alloy containing a platinum group metal, or an intermetallic material comprising a platinum group metal and a base metal.

8. A method according to claim 7 wherein the catalytic material is a 7.5% Rh/Pt alloy.

9. A method according to any preceding claim with any means for placing the gas stream in a turbulent condition prior to passage through the catalyst unit.

10. A method a ,cording to any preceding claim wherein the gas stream is exhaust gas from an internal combustion engine