JPH0257808A - Method for fuel combustion with oxidization catalyst - Google Patents

Abstract

PURPOSE:To prevent any loss in action of catalyst by a method wherein one catalyst having as its major constituent rhodium having a relative small particle diameter is stored at a front stage and another catalyst having as its major constituent rhodium having a relative large particle diameter is stored at a rear stage. CONSTITUTION:Catalysts having as their major constituents rhodium are applied at front and rear stages of a combustion device. Rhodium at the front stage is of a high active catalyst with a relative small particle diameter under its high dispersed state and rhodium at the rear stage is of a catalyst having a large particle diameter under a relative low dispersed state. A combustion device having these catalysts is applied. Since the rhodium catalyst at the front stage has no problem of losing any active state, this catalyst is dispersed as highly as possible and in order to increase its active characteristic, a particle diameter of each of rhodium particles is 5 to 20nm. At the rear stage, a combustion of fuel is continued to reach a substantial high temperature. The catalysts may easily be condensed and diluted if a degree of dispersion is high. Accordingly, a particle diameter of rhodium particle is 0.05 to 2mum in order to reduce a degree of dispersion and decreased a degree of active state as compared with that of the former stage. With this arrangement, a stable combustion can be carried out even at a high temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a fuel combustion method using a catalyst that does not become deactivated over a long period of time.

(Prior art and its problems) Traditionally, as a system for combusting various fuels such as city gas, the system has two or more stages, and by changing the catalyst in each stage, it is possible to effectively deactivate each catalyst. A method has been proposed to prevent this and burn fuel stably over a long period of time (for example, Japanese Patent Application Laid-Open No. 1983-237905).

Conventional systems are often two-stage systems in which a palladium catalyst is used in the first stage and a rhodium catalyst is used in the second stage. The catalyst on the front stage side is used for ignition and requires high activity, so a catalyst with highly dispersed palladium is used, but in this system, city gas is used as fuel like a cogeneration gas turbine. In some cases, sulfur compounds (such as mercaptans) contained as odorants in the city gas poison the palladium and reduce its activity.

On the other hand, as the catalyst for the latter stage, in addition to the rhodium catalyst mentioned above, platinum-based catalysts, palladium-based catalysts, base metal oxides such as cobalt and chromium, and composite oxide-based catalysts have been proposed; has the drawback of poor high-temperature durability and being easily oxidized and volatilized. Furthermore, the base metal catalyst is inferior to the noble metal catalyst in low temperature ignition performance and high temperature combustion completion performance. The rhodium catalyst has higher high-temperature durability than platinum and palladium, and has better high-temperature combustion completion than base metal catalysts. However, the rhodium catalyst has the disadvantage that it is oxidized and deactivated when the temperature is too high.

(Objective of the Invention) An object of the present invention is to provide a method that eliminates the drawbacks of the prior art described above, effectively prevents catalyst deactivation, and enables stable combustion over a long period of time.

(Means for Solving the Problems) The present invention provides a method for supplying fuel to a combustion device divided into a front stage side and a rear stage side for combustion, in which rhodium with a relatively small particle size is used as a main component in the front stage side. This method is characterized in that a catalyst having a relatively large particle size and containing rhodium as a main component is housed in the latter stage.

The present invention will be explained in more detail below.

The present invention uses a catalyst mainly containing rhodium in both the front and rear stages of the combustion device, and uses a highly active catalyst with a relatively small particle size with highly dispersed rhodium in the former stage, and compares the rhodium in the latter stage. We are trying to use a combustion device that uses a catalyst with a large particle size and low dispersion.

Rhodium catalysts are less susceptible to poisoning by sulfur components than conventional palladium catalysts, and the catalyst does not deactivate over a long period of time. Moreover, since rhodium has the second highest methane oxidation activity after palladium, it is also convenient as an ignition catalyst. Also, rhodium may be oxidized and deactivated when it reaches a fairly high temperature, but since the first stage is used for ignition and the temperature does not rise that much, no problem occurs. Since the rhodium catalyst in the first stage does not have the problem of deactivation, it is preferable to disperse it as highly as possible to increase its activity, and it is desirable that the particle size of each rhodium particle is 5 to 20 nm.

When the particle size is less than 5 nm, the particle size is too small and agglomeration begins to occur at the ignition temperature, and when it exceeds 20 nm, the activity decreases slightly.

The role of the catalyst in the latter stage is to complete the combustion of the fuel ignited in the former stage without deactivation, but in the present invention, the catalyst in the latter stage is a base metal that has high temperature durability compared to platinum and palladium. Rhodium is used because it has a higher combustion completion rate than other catalysts. In the latter stage, fuel continues to burn and reaches a fairly high temperature, but if the degree of dispersion of the catalyst in the latter stage is too high, the high temperature tends to cause it to coagulate and volatilize. Therefore, the rhodium has a lower degree of dispersion and lowers its activity than in the previous stage to prevent deactivation due to aggregation and volatilization. The particle size of the rhodium particles is desirably about 0.05 to 2 μm; if it exceeds 2 μm, the activity becomes too low and stable combustion at high temperatures may not be ensured.

As mentioned above, the rhodium catalyst is easily oxidized when the temperature is too high, and if it is insufficient to reduce the degree of dispersion in advance, it is preferable to add cerium to prevent the oxidation of rhodium. By adding cerium, oxidation of rhodium is almost completely prevented, and stable combustion can be achieved even at fairly high temperatures.

(Examples) The present invention will be explained in more detail based on Examples below.
The examples are not intended to limit the invention.

Catalyst A: 3.5% by weight lanthanum oxide on a coppillite support, 3
．． Alumina containing 5% by weight of neodymium oxide and 3% by weight of barium oxide was coated, then immersed in dinitrodiammine palladium nitric acid solution, dried and treated in a stream of hydrogen at 800°C to obtain 20g of palladium per carrier 11. A supported catalyst A was obtained.

Catalyst B 3.5% by weight of lanthanum oxide on a copeillite support, 3
．． Alumina containing 5% by weight of neodymium oxide and 3% by weight of barium oxide was coated, then immersed in a rhodium nitrate solution, dried, and treated in a hydrogen stream at 800°C to support 20g of rhodium per carrier 11. Rhodium catalyst B
I got it. The rhodium particle size at this time was L2 nm.

Catalyst C 3.5% by weight of lanthanum oxide on a copierite support, 3
．． Alumina containing 5% neodymium oxide and 3% barium oxide by weight was coated, then immersed in a rhodium nitrate solution containing 5% cerium to rhodium, dried, and heated at 1300°C in a hydrogen stream. Carrier 1
A rhodium-cerium oxide catalyst C was obtained in which 20 g of rhodium and 1 g of cerium as its oxide were supported per catalyst. The rhodium particle size at this time was 0.8 μm.

The catalyst prepared in this way was set in a combustion device with catalyst B in the front stage and catalyst C in the rear stage, and the methane was reduced to 90%.
Catalytic combustion of city gas containing gas under normal pressure with an air volume of 6ONm”
7 o'clock, catalyst entrance gas flow rate 15 m/sec, combustion temperature 130
When a catalyst performance test was conducted at 0° C., combustion gas containing no nitrogen oxides, unburned hydrocarbons* (UHC), and carbon oxides was obtained for 1000 hours. Further, as a comparative example, a catalyst performance test was conducted in the same manner using catalyst A on the front stage side and catalyst B on the rear stage side, and ignition became impossible after about 200 hours.

(Effects of the Invention) The present invention is characterized in that both the front and rear stages of a combustion device are filled with catalysts containing rhodium as a main component, and the degree of dispersion of the rhodium catalyst on the front stage is greater than the degree of dispersion of the rhodium catalyst on the rear stage. In other words, this is a method of reducing the particle size and supplying fuel to the combustion device to enable stable combustion even at high temperatures.

In the present invention, the catalyst in the front stage, which is used for ignition and does not reach high temperatures, is made of rhodium, which has excellent ignition ability and is not susceptible to poisoning by sulfur components, thereby greatly improving the ignition performance of the combustion device. By keeping the degree of dispersion of the rhodium particle size low, the agglomeration of rhodium is suppressed to ensure stable combustion over a long period of time.

Even if a combustion device with such a configuration is used, if the combustion temperature becomes too high, rhodium will be oxidized and deactivated, but in that case, cerium may be added to the rhodium catalyst to prevent the oxidation of the rhodium. If this is prevented, deactivation of rhodium can be more reliably avoided and stable combustion can be reliably performed.

Claims (6)

[Claims] (1) In a method of supplying fuel to a combustion device divided into a front stage side and a rear stage side for combustion, a catalyst mainly composed of rhodium with a relatively small particle size is placed on the front side, and a catalyst with a relatively small particle size is placed on the rear side. A method characterized in that a relatively large rhodium-based catalyst is contained. (2) The method according to claim 1, wherein the rhodium in the first stage has a particle size of 5 to 50 nm, and the rhodium in the second stage has a particle size of 0.05 to 2 μm. (3) The method according to claim 1 or 2, wherein cerium is added to the rhodium in the latter stage. (4) The method according to claim 3, wherein the amount of cerium relative to rhodium is 0.1 to 10% by weight. (5) The method according to any one of claims 1 to 4, wherein the catalyst is dispersed and supported on a carrier coated with at least one oxide selected from the group consisting of alumina, magnesia, and zirconia. (6) Any one of claims 1 to 5, wherein the oxide coated on the carrier is stabilized with an oxide of at least one element selected from the group consisting of lanthanum, cerium, neodymium, praseodymium, and barium. Method described in Crab.