CH629679A5 - PROCESS FOR THE CATALYTIC OXIDATION OF PARTICLES CONSTITUTING THE SMOKE OF COMBUSTION GASES. - Google Patents

Description

The present invention relates to a process for the catalytic oxidation of particles constituting the smoke from gases emitted for example by coal-fired boilers or combustion engines.

The gases from boilers and combustion engines often contain finely divided particles of hydrocarbons and / or carbon and / or other solid matter which escape in the form of smoke. Diesel engine smoke can include solid / liquid particles, solid chain aggregates where spherical particles between 100 and 800 Å in diameter bind to liquid sulfates, liquid hydrocarbons and gaseous hydrocarbons. Solid / liquid particles generally include carbon particles which adsorb liquid hydrocarbons, sometimes called polynuclear aromatics, and solid chain aggregates are generally composed of high molecular weight organic compounds and / or inorganic sulfates.

Three different types of smoke are usually observed coming out of the exhaust pipe of a Diesel engine, including white smoke, black smoke and blue smoke. White smoke is produced during engine acceleration and comes from the condensation of water vapor on the particles (i.e., hydrocarbons, etc., noted above) found in the exhaust gases , so that a light mist is formed. Black smoke is obtained when the engine has warmed up and contains a relatively high proportion of carbon particles. In the blue smoke is a little carbon, with a relatively high proportion of gaseous hydrocarbons such as aldehydes. About 90% of the particles constituting the smoke have maximum interior dimensions at 1 jx, which is within the limits of the dimensions of the respirable particles, and the maximum dimension of the remaining 10% of the particles constituting the smoke is less than 4 (jl.

The object of the present invention is to reduce the quantity of smoke present in the gaseous waste by implementing a catalytic oxidation of the particles constituting the smoke in these gases.

To this end, the method according to the invention is characterized in that the entrained particles are passed through a gaseous stream containing oxygen through a catalyst arranged so as to hinder the passage in a straight line of the particles of so that, firstly, the probability of contact between the particles and the catalyst is increased and, secondly, turbulence is imparted to the gas to increase the probability of contact between the particles and the catalyst.

The catalytic oxidation of carbon particles occurs at around 400 ° C, while the normal combustion temperature of these particles is 700-800 ° C. For hydrocarbon particles, the catalytic oxidation will occur at temperatures above 200 ° C. Since the presence of a catalyst allows the oxidation of the particles constituting the smoke in a gas at a temperature lower than the temperature normal at which combustion occurs, little or no heating of the exhaust gases from a combustion engine should be required when catalytic oxidation of any particles constituting smoke in the gas is desired. For example, a diesel engine operates at around 400 ° C when operating at medium power or at full power, so that no preheating of the exhaust gases exiting the diesel engine would be necessary before passing them over a catalyst. to remove the particles constituting the smoke from the gas by catalytic oxidation, provided that this catalyst is located near the engine. Preferably, the catalyst should be in the exhaust manifold, where a number of these catalysts should be provided, one at each port.

A catalyst for carrying out the oxidation of the particles constituting the smoke in a gas preferably comprises a catalytic material applied to a support. This supported catalyst is located in a catalytic purification unit through which the gas will pass to come into contact with the catalyst. When using such a catalytic purification unit, it has been found that its effectiveness in oxidizing the particles of a gas stream passing therethrough is remarkably increased if turbulence is created in the gas stream at least when it passes through or above the catalyst.

The catalytic material in the catalytic purification unit can be applied to a support such that it creates turbulence in the gas stream at least during contact with the catalyst.

The catalyst may comprise one or more platinum group metals Pt, Pd, Ru, Ir and Os or one or more alloys containing one or more of these metals, where an intermetallic compound comprising a platinum group metal is an ordinary base metal.

A refractory lining, called a base layer, can be provided between the support and the catalyst.

The material of the base layer may contain one or more oxides chosen from the group comprising the oxides of transition metals and of metals of groups la, Ila and Illa and Illb (including the rare earth elements) and Va of the periodic table (using the Fisher Scientific Co. periodic table 5-70210).

Preferably, the material of the base layer comprises one or more elements from the group comprising alumina, beryllium oxide, zirconia, magnesia, tantalum oxide, silica, titanium oxide, hafnium oxide, thorium oxide and an oxide of

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629,679

rare earths such as cerium oxide and combinations of these compounds.

It has been found that an effective way to create turbulence in a gas stream is to use as a carrier for the catalyst a randomly interlaced or oriented porous refractory wire or a mesh or perforated corrugated sheet which has been stretched. It has also been discovered that turbulence is created or at least promoted when the catalyst is carried by pellets which are randomly arranged or by any other catalyst support. Besides supporting the catalyst itself, the support also acts as a filter to filter the particles constituting the real smoke which are then subsequently oxidized by catalysis. If desired, the exhaust gas can pass through a device to initiate turbulence before entering the catalyst. This device can be a usual turbulence chamber. It is accepted that in practice the particles constituting the smoke are forced to come into contact with the surfaces of the catalyst - and to adhere to it - where they are catalytically oxidized with all the adsorbed hydrocarbons and other oxidizable materials. Furthermore, the catalyst is preferably chosen so that, during the oxidation, a minimum of SO 3 is produced by oxidation of the SO 2 present in the exhaust gas.

The metal support can be made of any ordinary metal or a platinum group metal or an alloy containing a platinum group metal. Alternatively, Kanthal or an alloy containing iron and chromium such as Fecralloy can be used.

The pellets can be made of a refractory or metallic material. The refractory or other materials suitable for the support pellets can be any of the following materials: porous silica, for example that sold under the trade name Silocel, granular charcoal, alumina "or y, natural or synthetic aluminosilicate, magnesia , diatomaceous earth, bauxite, titanium oxide, zirconium oxide, limestone, magnesium silicate, silicon carbide, activated and inactivated carbons. The pellets can have a regular or irregular shape such as capillary tubes, rods, beads, broken pieces or tiles, etc.

Preferably, a refractory oxide coating must be applied to a refractory and metallic support, this coating being called a base layer, sandwiched between the surface of the support and the catalyst. The preferred refractory oxide layers are the members of the y or activated alumina family.

The base layer can be prepared by precipitating a hydrated alumina gel and then drying and calcining to express the water of hydration and obtain active alumina therein. A particularly preferred active refractory metal oxide is obtained by drying and calcining at temperatures between 400 and 800 ° C. a precursor mixture of hydrated alumina phases, predominant in crystalline trihydrate form, that is to say containing more than 50 % by weight of the total hydrated alumina composition, preferably 65 to 95% by weight of one or more trihydrated forms of gibbsite, bayerite and norstrandite determined by X-ray diffraction. It is preferred to use the quality of hydrated alumina MH170 from British Aluminum Co. and transform it into activated alumina by drying and baking as described above.

Other methods of preparing and applying a base coating are described in UK patent application No. 32920/77.

The base layer comprises a porous structure which gives it a large surface area between 50 and 500 m2 / g of alumina on which the catalyst can be deposited in the form of a continuous or discontinuous coating.

A preferred arrangement is that where the catalyst is palladium and the base layer contains tantalum oxide or cerium oxide. According to another possibility, the catalyst can be an alloy of palladium and platinum containing up to 75% by weight of platinum.

The tests described in the following example show the efficiency of the process of the present invention for reducing the amount of smoke in the exhaust gases of a diesel engine.

A Lister type cylinder engine coupled to a 3.5 kW generator acting as a load was used for the tests. With a 2.75 kW load coupled to the engine, the smoke emitted from the engine indicated 4.5 on the Bosch smoke density scale. A container filled with pellets was placed in the manifold of the diesel engine so that the engine exhaust passed over the pellets. These consist of alumina a with a 50/50 platinum / palladium alloy catalyst present in the form of a coating on the pellets. The pellets had a diameter of 0.31 cm and a length of about 0.31 cm. With the container filled with pellets, the smoke emitted by the engine indicated 1.9 on the Bosch scale. By increasing the load of pellets in the container by 10%, the smoke emitted by the engine was further reduced to 1.5 on the Bosch scale. When the engine was operated with a 2.7 kW coupled load, the space speed was 80,000 volumes of catalyst per hour and the volume of catalyst was 11.

With the diesel engine running at idle, the exhaust gas temperature was below 400 ° C. Under these conditions, it was found that the smoke-forming particles were collected on the pellets. It was also found that when the Lister engine, with the pellet container disposed in its tubing, idled, it took 1 hour before the density of the smoke emitted by the engine increased significantly. When the engine was then allowed to operate at full power, all of the particles constituting the smoke which had been collected on the pellets was eliminated by catalytic oxidation.

A second test was carried out with a different catalyst placed in the manifold of the Lister type engine. A wire structure of 2.54 (a of Fecralloy of 10.16 cm in diameter and 15.25 cm in length was used as a support. A base layer consisting essentially of alumina was applied to the support with a layer of catalyst comprising 7.5% Rh / Pt by weight The catalyst charge was approximately 60 g / 28.13 dm3 of support.

The Lister engine operated at 3000 rpm with a space speed of 8000 volumes of catalyst per hour. The temperature of the exhaust gas passing over the catalyst, the weight of the particles present in the exhaust gas before and after passing over the catalyst were measured. The results are shown in Table 1.

Table 1

Particles

QC temperature

At the entrance

(g / h)

At exit (g / h)

% reduction

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 carried out to study the influence of volume on the yield of particle limitation and the results are given in Table 2.

The gas temperature was maintained at 360 ° C and the amount of particles present in the gas was 19.5 g / h. The catalysts were coated as described in the second test, but with two different ratios. The surfaces were kept constant at 82 mm in diameter and 106 mm in diameter.

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Table 2

Volume

(I)

% reduction diameter 82 mm

% reduction in diameter 106 mm

Spatial speed per hour

0.27

9.4

12.4

250,000

0.38

11.6

16.9

178000

0.48

18.9

23.8

141000

0.56

24.8

28.4

120500

0.74

35.6

43.8

91216

0.84

48.2

53.7

80350

0.93

52.1

61.4

72500

1.21

62.1

68.5

55785

It is preferred that, when a diesel engine is running at full power, the pressure difference, which is the difference of the pressure of the exhaust gas stream before passing through a catalytic purification unit and after it has passed through such a unit, ie less than 10.16 mm of mercury.

Similar tests were carried out using a Perkins 4236 type diesel engine operating at a speed of 2200 rpm with a catalyst placed in the exhaust manifold, and the results are shown in Table 3. The catalyst is consisting of 1600 g of Fecralloy wire (12TM) on which an alumina base coating has been applied, stabilized therein with barium and applied to the wire at a density of 0.12 g / g of wire. The catalytic material was constituted by 7.5% Rh / Pt and the total weight was 3 g-

Table 3

% of total power

100%

50%

20%

Hydrocarbons adsorbed on

particles entering

catalyst unit, measured

in grams per hour

20

15

160

Hydrocarbons adsorbed on

particles coming out of the cat

lyser, measured in grams

per hour

3

0

3

The quantity of hydrocarbons adsorbed was measured by collecting the particles of the exhaust gas on a filter for a predetermined period of time. Then the filter was heated on a thermogram scale until the weight loss stopped, which meant that all of the volatile constituents had been burned off.

Finally, the residue was analyzed and the results obtained are shown in Table 3.

Another test was carried out on the Perkins 4236 diesel engine operating at 1400 rpm using the same catalyst and the results are given in Table 4.

Table 4

% of total power

100%

50%

20%

Hydrocarbons adsorbed on

particles entering

catalyst unit, measured

in grams per hour

10

15

105

Hydrocarbons adsorbed on

particles coming out of the cat

lyser, measured in grams

per hour

4

4

5

Tests have shown that the present process eliminates up to 80% of the adsorbed hydrocarbons (polynuclear aromatics) and up to 40% of the hydrocarbon particles from the exhaust gases of a diesel engine.

55 Although the method has been described with reference to the reduction in the quantity of particles constituting the smoke of a diesel engine, it is not limited to these and can also be applied to petrol engines, engines gas and turbines.

R

Claims (10)

629679 1. A method of catalytic oxidation of particles constituting the smoke of combustion gases, characterized in that the entrained particles are passed through a gaseous stream containing oxygen through a catalyst arranged so as to impede the passage in a straight line of the particles so that, firstly, the probability of contact between the particles and the catalyst is increased and, secondly, turbulence is imparted to the gas to increase the probability of contact between the particles and the catalyst. 2. Method according to claim 1, characterized in that the catalyst is carried by a support made of a porous refractory material, an interlaced metal wire, randomly oriented or a trellis; a sheet of metal or perforated refractory material or a corrugated metal sheet which has been stretched. 2 3. Method according to claim 2, characterized in that the refractory material is in the form of pellets. 4. Method according to one of claims 2 or 3, characterized in that the refractory material is porous silica or granular charcoal, alumina a or y, natural or synthetic aluminosilicates, magnesia, diatomaceous earth, bauxite, titanium oxide, zirconia, limestone, magnesium silicate, silicon carbide, activated or inactivated carbons. 5. Method according to one of claims 1 to 4, characterized in that the catalyst support is coated with a coating of refractory oxide. 6. Method according to claim 5, characterized in that the coating of refractory oxide is alumina y. 7. Method according to one of claims 1 to 6, characterized in that the catalyst is a platinum group metal, a mixture or an alloy containing a platinum group metal, or an intermetallic compound comprising a platinum group metal and a ordinary metal. 8. Method according to claim 7, characterized in that the catalyst is an alloy of 7.5% Rh / Pt. 9. Method according to one of claims 1 to 8, characterized in that a device is used to put the gas stream in a state of turbulence before passing through the catalyst unit. 10. Method according to one of claims 1 to 9, characterized in that the gas stream is an exhaust gas from an internal combustion engine.