CA1196323A

CA1196323A - Combustion catalyst bed

Info

Publication number: CA1196323A
Application number: CA000419407A
Authority: CA
Inventors: Donald R. Mcvay; Herbert J. Setzer
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1982-02-01
Filing date: 1983-01-13
Publication date: 1985-11-05
Also published as: DK13383A; GB2114018A; DK159127B; GB2114018B; JPS58133510A; GB8301824D0; FI830262L; SE453121B; FI71410B; FI71410C; US4400356A; DK13383D0; SE8300382L; FI830262A0; NO830259L; JPS6235003B2; NO157488B; DK159127C; NO157488C; SE8300382D0

Abstract

Abstract A catalytic combustor bed configuration particularly adapted for use in wood and coal burning stoves is described comprising a support screen having a layer of fully catalyst impregnated particulate material next to the screen, a layer of particulate material catalyzed on its outer surface next to the layer of fully impregnated material and a layer of uncatalyzed particulate material next to the outer surface catalyzed material. Optionally, a layer of uncatalyzed particulate material may be used between the screen and the fully impregnated particulate material. Such an arrangement, in addition to making maximum utilization of the catalyst material, also mini-mizes pressure drop across the catalytic combustor.

Description

3~3 ~escription Combustion Catalyst sed Technical Field The field of art to which this invention pertains is combustion catalysts, and particularly bed configurations for such catalysts.

Background Art In view of the ever increasing concern over the availabilitv and cost of energy resources such as oil and natural gas, many people have turned to solid fuels such as wood and coal ~or heating their dwellings. ~uch of this solid fuel comhustion technology as represented by wood and coal burning stoves is 40-50 years old or older.
However, recently newer stove designs aime~ at cleaner burning and improvement of the efficiency have been developed. Note, for example, U. S. Patent 4,221,207.
The latest so-called "second generation" stoves have sought additional and substantial improvements in burning efficiency and reduced emissions of pollutants by including in the design such things as the insertion of a catalytic combustor in the exhaust portion of the stoves to cause additional combustion of the exhaust or smoke exiting from the stove. This "afterburning" or secondary burning of combustibles in the exhaust decreases pollutants leaving the stove and reduces such things as creosote build-up in chimneys. Such combustors also im~rove the combustion efficiency of the stove and thus provide greater heat per amount of fuel combusted. How-ever, because of the precious nature and costs of the catalyst material used in such combustors, there is a constant search for maximizingthe efficiency of such combustoxs per amount of catalyst utilized.

6;323 Accordingly, even though advances have been made in this area to date, there is still a need for improving the performance and efficiency of such combustion systems.

Disclosure of Invention The present invention is directed to a combustion catalyst bed configuration, especially adaPted for use in wood and coal burning stoves which maximizes catalyst effi-ciency per amount of catalyst utilized and additionall~
minimizes pressure drop across the catalyst bed for improved performance in natural draft solid fuel combustion devices such as wood and coal burning stoves. The com-bustion catalyst according to the present invention responsible for such results includes a support screen having an inlet layer of combustion catalyst support particles substantially fully impregnated with a metallic combustion catalyst. Next to this section of fully impregnated particles is a layer of catalyst support particles surface catalyzed with combustion catalyst material. The exit layer or section of the bed comprises catalyst support particles unimpregnated with catalyst material.
Another aspect of the invention includes a combus-tion catalyst bed as above described including a support screen having a section or layer of relatively large particles which are uncatalyzed and inert to the combus-tion gases next to the screen. The remaining layers next to the layer of uncatalyzed pellets are ~ully impregnated, surface catalyzed, and uncatalyzed particles as described above.
Another aspect of the invention includes a solid fuel burning stove containing such catalytic combustor material.
The foregoing~ and other features and advantages of the present invention, will become more apparent from the following description and accompanying drawings.

Brief Desc~iption of the Drawings Fi~s. 1 and ? show typical catalytic combustor con-~igurations according to the present invention.
Fig. 3 is a perspective view, partly in section and partly broken away, of a typical catalytic combustor according to the present invention.
Fig. ~ is a comparison of combustion activity of a commercially available catalyst and a catalyst accordin~
to the present invention.

Best Mode for Carryin~ Out the Invention An inspection of the Figures demonstrates typical catalyst bed configurations according to the present invention. In Fig. 1, support screen 1 can comprise stainless steel or any other material which is stable in an exhaust gas channel such as an internal exhaust gas manifold or an external flue pipe or chimney. While it can be any size required by the particular stove design in which it will be used, it is typically 0.5 to 2 feet2 (15.24 to 60.96 cm)2 with openi~gs sufficiently large so as not to interfere with the natural draft of the exhaust gas channel, but sufficiently small to support the layers of pellets above. Stainless steel screens with substan-tially square openings of approximately 0.0625 inch (0.159 cm) diagonal measure are typically used. The depth of partlcles loaded onto the screen generally range from 0.375 inch to 2 inches (0.935 cm to 5.08 cm) depending on the exhaust channel in which they will be used. In a natural draft environment, a lower pressure drop is required and catalyst bed depth must be kept thin. In a forced draft environment in which a gxeater pressure drop can be tolerated, the depth of the catalyst layers can be greater.
In Fig. 2, the first layer of particles 2 next to the screen is shown as uncatalyzed pellets which act as a radiation shield in the exhaust environment. This 32;~
:.
--'1~
material can be any material inert to the exhaust gas environment and has been made of the same material as the catalyzed paraticles, that is a lanthanum stabilized alumina or magnesium promoted lanthanum stabilized alumina.
These particles can be of any shape desired such as cylin-drical, spherical, irregular, or any shape to allow free passa~e of gas and capable of supporting the catalyst material. Spherical pellets are preferred because they tend to promote a uniform gas flow pattern across the bed.
While any size particles can be used as the first layer, they are typically 0.0625 to 0.625 inch (0.159 to 1.59 cm) in diameter and preferably 0.125 in. to 0.25 in. (0.318 cm to 0.635 cm) in diameter, and if other than spherical particles are used, they should be of similar dimensions.
The subsequent layers or sections in the combustor of Fig. 1 are similar to the layers making up the com-bustor shown in Fig. 2. This first layer of pellets 3 next tothe screen in ~ig. 2 and one layer removed from the screen in Fig. 1, comprises a substrate material similar to that uncatalyzed in Fig. 1 fully impregnated with the combustion catalyst material. In Fig. 1, the particles are shown as substantially the same size as those in the first layer, and in Fig. 2, the particles are shown relatively smaller. In both instances, the 25 particles are the same 0.125 in. to 0.625 in. (0.318 cm to 1.59 cm) in diameter, preferably 0.25 inch (0.635 cm) in the embodiment shown in Fig. 1 and 0.125 inch (0.318 cm) in the embodiment shown in Fig. 2. Next to this layer of fully catalyzed particles 3 is a layer of cata-lyst particles 4 which are "ring catalyzed". By ring catalyzed is meant that unlike the fully impregnated pellets of the former layer, only the outer portion of the pellet nearest the surface is impregnated with cata-lytic material. Typically, twenty five percent or less 35 (i.e. penetration of 0.001 in. to 0.100 in., 0.0025 cm to 0.254 cm) of the outermost portion of the pellet is i3~3 catalyzed. The innermost portion of the pellet remains uncatalyzed.
The exit layer of pellets 5 comprise pellets similar to the fully impregnated catalyst pellets only being uncatalyzed. This exit layer is added as a radiation shield to prevent heat loss from the catalyst bed which should be kept hot for best combustion efficiency. The term layer is meant to include not only a single layer of particlès, i.e. a layer one particle diameter thick but also a plurality of particulate layers, i.e. a layer several particle diameters thick. The layer thickness is determined by the function of the particular layer and in general would be about one to about five particle diameters thick.
While the relative proportions of the respective layers is dictated by such things as the particular combustion products passing therethrough and their rate of flow, coupled with the specific temperature they will - experience, typïcally, in the embodiment shown in Fig. 1 the relative amounts of the uncatalyzed, fully catalyzed, and ring catalyzed layers will be the same with about half as much uncatalyzed pellets being used as the upper-most layer and in the embodiment in Fig. ~, all layers being relatively the same thickness with the fully cata-lyst impregnated layer next tothe screen being about halfthe thickness of the other layer. The exact amount of catalyst required will be determined by the burning rates of the wood and coal in the stove. The combustion rate of the fuels determines the quantity of exhaust or flue gases generated which in turn sets the quantity of com-bustion catalyst required. Furthermore, in a wood or coal burning stove environment, the inlet temperature seen generally ranges from 400F to 900F (204C to 482C) and the exit temperature ranges from 1100F to 1600F (593C to 871C).
The function of the metal screen is obvious, i.e. as i3;~3 --6--a support material for the various particle layers. The function of the first ~ayer in the embodiment shown by Fig. 1, the uncatalyzed particles, is as a radiation/heat shield which functions to prevent the first layer of catalyzed pellets from cooling ~y radiation to incoming gases or to the cooler exhaust channel walls. The second layer in the embodiment shown by Fig. 1 and the first layer next to the screen in the embodiment shown by Fig. 2, the fully catalyst impregnated pellets function to reduce the ignition temperatures of the hydrocarbon and carbon monoxide material in the exhaust gas stream to burn in the range of 400F to 600~ (204C to 316C). At this temperature, it has been calculated that the combustion rate of the gases is kinetically relatively slow and it lS is this rate which limits the total burning of the exhaust gases. This is referred to as kinetically limited burn-ing. In this condition, fully impregnated pellets are preferred because the combustion rate is slow enough to permit diffusion of gases and air to the innermost sec-tions of the pellets where combustion occurs. In otherwords, in the kinetically limited mode the catalyst in the entire pellet is fully utilized to promote combustion.
The heat generated from this burning further raises the temperature of the catalyst and support material, which in turn, further increases its catalytic activity. Fur-thermore, the high temperatures produced crack the heavy materials in the smoke or exhaust stream and leads to further combustion. As the carbon-monoxide and heavy hydrocarbon material in the exhaust gas stream continue to burn as a result of contact with the first layer oî
fully catalyzed pellets, the temperature will rise to in the order of 1200F to 1400F (649C to 760C). At this temperature, the reaction rate is very fast~ and the combustion rate is limited by diffusion of the reactants to the surface of the pellets and diffusion within the outer layers of the pellets. The use of ring catalyzed 11~6~23 pellets in this zone is an ef~icient way to avoid loading the pellets with useless catal~st material in the inner portion of the pellet where it would not be utilized.
And finally, the outermost uncatalyzed particulate material functions in a manner similar to the first layer next to the screen in the embodiment shown by Fi~. 1, i.e. to prevent heat loss from the co~bustion catalyst particles to the co~ler walls of the exhaust channel.
The merits of using a layer of fully impregnated particle folIowed by ring catalyzed particle can be fur-ther demonstxated by observation of the Table ~here the effectiveness factors (Neff) are demonstrated for differ-ent diameter (D) pellets at different temperatures. From the Table, it can be seen that the entire pellet is being utilized at 500F (260C) test temperature (effectiveness factor of 1.0). The rate limiting step is primarily a function of the reaction itself. At 1000F (538C), however, the effectiveness factor falls significantly.
The calculations show that at 1000F (538C~, the outer 19% of 0.125 in. (0.318 cm) pellets and the outer 4% of 0.25 in. (0.635 cm) pellets are effective in catalyzing the combustion reaction. The rate of the combustion reaction is governed substantially by diffusion of the combustible gases into and out of the pores in the porous pellet material. And at 1600F ~871C)/ there is a further significant drop-off in the effectiveness factor.
At this temperature, the burning rate is limited by bulk diffusion of remaining unburned material and air to the surface of the catalyst pellets.

3~3 TA~LE
D= 0.125 in. D = 0.25 in~ Rate Temp F (C) (0.318 cm) (0.635 cm) Limiting Step Kinetically 500 (260) 1.0 1.0 controlled Pore Diffusion 1000 1538) .19 ~19/~).04 (4%~ Controlled Bulk Diffusion 1600 (871) 0.01 0.01 Controlled As the substrate material, either a lantbanum stabilized alumina or a magnesium promoted lanthanum stabilized alumina can be used~ The lanthanum stabi-lized alumina substrate is a commercially available catalyst support material available from W.R. Grace &
Co. (e.g. GRACE SMR 1449, trade mark). The magnesium promoted lanthanum stabilized alumina is prepared by impregnating the lanthanum stabilized alumina with a solution (preferably aqueous) of a magnesium salt (preferably magnesium nitrate) followed by drying to remove the sol-vent, and calcining in air to oxidize the deposited salt to magnesium oxide. Calcining tem-peratures may vary depending on the particular salt used, but generally temperatures in the range of about 1800F (982C) are used, e.g. for magnesium nitrate.
Enough magnesium salt is deposited on the support mate-rial such that after calcining, about 3% to about 15%
magnesium is present in the support material, and pre~
ferably about 5% by weight. Attention is directed to copending, commonly assigned Canadian Patent Application Serial No. 419,431, filed January 13, 1983 entitled "Catalytic Con~ustor" by R. Vine, J.C. Trocciola and H. Setzer.
The use of such substrate material is impor-tant because of its particular stability at elevated temperatures in such a gas combusting environment.
Such substrate material has been found to maintain a high B.E.T.

, . ~, 3~3 g (Bruinauer-Emmet-Teller) surface area, the substrate material maintains its dimensional st~bility (e.g. lack of shrinkagel especially in the preferred pellet form), and has an acceptable crush strength (e.g. when packed into canisters) especially when magnesium promoted. This substrate material has also been found to allow formation of small metal crystallites on its surface which is necessary for catalytic performance according to the present invention. The material also has improved toler-ance to carbon formation over, for example, unmodifiedalumina.
~ he active catalyst material according to the ~resent invention is deposited on the substrate material by any conventional method in this art, and preferably out of aqueous solution. Metal salts and typically the nitrates are dissolved in either aqueous or organic solvents and dried on the substrate. The deposited salts are then treated with hydrogen to form metal crystallites. Rhodium has been found to be a particularly suitable catalyst because of its sulfur tolerance in this environment.
Note the above cited commonly assigned application. It should be noted that any acceptable route may be used to go from the salt to the metal such as going from the salt form directly to the metal crystallites by hydrogen reduction or oxidation of the salt in air followed by reduction in hydrogen so long as the metal crystallites are formed on the substrate material ultimately. Amounts of rhodium used may vary over a wide range, but are generally used in amounts based on catalyst plus support material of about 0.01% to about 6% rhodium, and typically in amounts o~ about 0.1% to about 1% rhodium.

Example A lanthanum stabilized alumina catalyst support material was purchased from W. R. Grace ~ Co. in pellet form having dimensions of about 0.25 in. (0.636 cm) .

diameter and about 0.250 in. (0.638 cm) len~th. A batch of these pellets were immersed in an aqueous solution of Mg (NO3)3 6H2O having a concentration of about 57~ by weight. After immersion for approximately 5 minutes with ultrasonic vibration and 30 minutes without, the pellets were removed from the solution. The pellets were then oven dried in air for 3 hours at about 110C and calcined at 1300F (982C) for 16 hours and cooled. The magnesium promoted lanthanum stabilized alumina pellets were then immersed in an aqueous solution of Rh (NO3)3 having a concentration of about 11.1% by volume. After immersion for approximately 5 minutes under ultrasonic vibration and 30 minutes without vibration, the pellets were removed from the solution and dried in air for 3 hours at 230F
(110C), followed by heating in a hydrogen atmosphere to form the metal crystallites on the substrate material.
This procedure deposits a surface layer of catalyst about 0.050 in. (0.127 cm) on the particulate material.
If full impregnation is d~sired, immersion time in the Rh (NO3)3 should be extended, e.g. doubled.
The hydrogen reduction step wa~ performed as follows:
the above-treated pellets were placed on a tray in an oven which was first flushed with nitrogen. The oven tempera-ture was raised to approximately 600F (316C) and the atmosphere over the pellets changed according to the followirg schedule:
~N2 %H2Time in Hours 100 0 0.25 0.25 0.25 0.50 0 100 2.00 After cooling to 200F (93C), the atmosphere over the pellets is changed to 100% N2. The pellets are then cooled to ro~m temperature and the atmosphere over the 3~3 .

pellets adjusted as follows:
~N ~2 Time in Hours 95 5 0.5 90 10 0.5 80 20 0.5 To further demonstrate the improved performance of the combustion catal~st according to the present invention, the following testing was performed. Utilizing a micro-reactor 0.375 in. (0.953 cm) inn~r diameter containing 1 inch (2.54 cm) length or approximately 0.5 gram of cata-lyst material, reaction rate constants (synonymous with activity) were plotted as a function of test temperature.
Testing was performed for 30 hours combusting a mixture of methane containing approximately 2200 parts per million (by weight) H2S. The reaction rate constant (k) is defined by the pseudc first order rate equation:
k = (space velocity) x ln( 1 % conversion ) In Fig. 3, data for commercially available catalysts (15~ nickel by weight on alpha alumina - curve A) and a catalyst according to the Example (curve B) are plotted on a conventional Arrhenius graph. As can be seen from the curves, the catalyst of curve B provides much greater activity at lower temperature.
To prepare a catalytic combustor according to the present invention, it is preferred to utilize a canister type container having a stainless steel support screen utilizing wire with a .032 in. (0.081 cm) diameter and 256 holes per inch 2 or 40 holes per cm2 (i.e. 45% open).
The walls of the canister are typically 300 series stain-less steel such as 304 stainless steel. One or two layersof uncatalyzed 0.25 inch (0.635 cm) diameter pellets are dropped onto the support screen. Following this, one or two layers of the fully catalyzed pellets of the same size are deposited. One or two layers of ring catalyzed pellets 3~3 -~2-of the same si~e are poured on top of the fully catalyzed pellets followed by pouring one or two layers of uncata-ly7ed pelletsr again of ~he same size, as an exit radia-tion shield as the final layer. The canister can then be covered with a temporar~ plastic cover to prevent excess movement during shipping. Note Fig. 4 where the canister is shown as 6 and support screen 1 and particle layers 2, 3, 4 and 5 are as defined in Figs. 1 and 2.
The thickness of the bed should be kept to a minimum to minimize the pressure drop across the bed, especially for a natural draft device such as a coal or wood burning stove. Industrial burners which utilize blowers for pri-mary air can afford a deeper bed of catalyst and the accompanying higher pressure drop. If the pressure drop in either the natural draft or blower assisted environment is too great, i.e. the Flow is restricted, the co~bustion rate of the wood or other solid fuel is adversely affected.
However, there should be sufficient catalyst to provide - a gas residence time which permits the catalyst to affect combustion of the gases on the catalyst. Since the draft on typical residential chimneys is in the order of 0.05 in. to 0.10 in. (0.127 cm to 0.254 cm) water as can be determined from the Standard Handbook for Mechanical Engineers, 7th Edition, McGraw Hill Book Co., the catalyst bed in the natural gas environment is sized in frontal area and bed depth to have a pressure drop in the order of 0.01 in. (0.0254 cm) water to minimize flow restriction.
The pressure drop can be measured with a sensitive delta pressure guage. Another way of detecting if the pressure drop is low enough and is not restricting is by deter-mining if the fuel combustion rate, in pounds fuel burned per hour, is satisfactory. If the pressure drop is too low, i.e. the bed is too thin, bypassing and incomplete combustion of the smoke can occur. This can be detected by observing smok~ in the stove's exhaust.

,3 .

It should also be noted that while the invention has been described in terms of a rhodium catalyst, other cata-lysts such as ruthenium, nickel, palladium, iron oxicle, or conventional combustion catalysts are useful. Further, if it is desired, the exit layer of uncatalyzed particles can be eliminated and other heat retaining means to limit radiation losses from the bed can be designed into the system, such as a radiation shielding, plate of ceramic or other high temperature stable material.
Although the invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed inven-tion.

Z

Claims

The embodiments ot the invention, in which an exclusive property or privilege is claimed, are defined as follows_

1. In a catalytic combustor comprising combustion cata-lyst material so selected and constituted so as to combust uncombusted gases from a previous combustion reaction, wherein the improvement comprises utilizing such catalysts in a bed configuration comprising a support screen, a layer of substantially fully catalyst impregnated particu-late material next to the support screen, a layer of sur-face catalyst impregnated particulate material next to the fully impregnated material, and a layer of uncatalyzed particulate material next to the layer of surface impreg-nated material, the thickness of the layers and particu-late size so constituted as to substantially fully combust the uncombusted gases passing therethrough with a minimal pressure drop across the bed.

2. The combustor of claim 1 additionally containing a layer of uncatalyzed particulate material between the support screen and layer of fully impregnated particulate material.

3. The combustor of claims 1 or 2 wherein the catalyst is rhodium and the particulate material is lanthanum stabilized alumina or magnesium promoted lanthanum stabilized alumina.

4. The combustor of claim 1 wherein all of the particu-late material is substantially the same size excluding the fully impregnated particulate material which is about 0.25 to about 0.5 times the volumetric size of such particulate material.

5. A solid fuel burning stove comprising an air inlet section, a combustion section, a combusted and uncombusted gas exhaust section and a catalytic combustor in the exhaust section wherein the improvement comprises utiliz-ing a catalytic combustor comprising a support screen, a layer of substantially fully catalyst impregnated particulate material next to the support screen, a layer of surface catalyst impregnated particulate material next to the fully impregnated material, and a layer of uncata-lyzed particulate material next to the layer of surface impregnated material, the thickness of the layers and particulate size so constituted as to substantially fully combust the uncombusted gases passing therethrough with a minimal pressure drop across the bed.

6. The stove of claim 5 wherein the combustor addition-ally contains a layer of uncatalyzed particulate material between the support screen and a layer of fully impregnated particulate material.

7. The stove of claims 5 or 6 wherein the catalyst is rhodium and the particulate material is lanthanum stabi-lized alumina or magnesium promoted lanthanum stabilized alumina.

8. The stove of claim 5 wherein all of the particulate material in the combustor is substantially the same size excluding the fully impregnated particulate material which is about 0.25 to about 0.5 times the volumetric size of such particulate material.