Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Patent
1994-10-21
1996-10-22
Beck, Shrive
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
502303, 502349, 502355, B01J 2310
Patent
active
055676677
DESCRIPTION:
BRIEF SUMMARY
OXIDATION CATALYST
1. Field of the Invention
This invention relates to oxidation catalysts, and in particular to catalysts suitable for the combustion of a feedstock with an oxygen-containing gas, eg air, particularly with an excess of the oxygen-containing gas to effect complete combustion.
2. Background of the Invention
In order to reduce the formation of oxides of nitrogen (NOx) when a fuel, eg gaseous hydrocarbons such as natural gas and/or hydrogen, is combusted with air, it is desirable to employ fuel/air mixtures of such composition that the adiabatic flame temperature is relatively low, generally below 1500.degree. C., and desirably below about 1300.degree. C. For many applications this means using a composition that is so rich in air that normal combustion is unstable and may not be self-sustaining. Catalytic combustion wherein a mixture of the fuel and air is passed through a bed of a combustion catalyst, enables such problems to be overcome.
One application wherein catalytic combustion is desirable is in gas turbines. At initial start-up of a gas turbine, a mixture of the fuel and air, preheated, for example by a pilot burner, to a temperature typically of the order of 600.degree.-800.degree. C. when the fuel is methane or natural gas, is fed, normally at superatmospheric pressure, eg at a pressure in the range 2 to 20 bar abs., to the inlet of the combustion catalyst bed. Combustion is effected at the catalyst surface forming a gas stream at elevated temperature. There is a rapid rise in the temperature of the catalyst bed to about the adiabatic flame temperature, typically about 1200.degree. C., when the catalyst lights-off. The point at which this occurs is associated with the pre-heat temperature and the catalyst activity. Before light-off occurs, the solid temperature rises exponentially along the bed length. When the catalyst lights-off, the whole of the catalyst may be at essentially the adiabatic flame temperature. The average temperature of the gas mixture increases more gradually through heat transfer from the solid to the gas phase. The gas temperature rises gradually even when the catalyst is lit-off, ie the catalyst front face is at the adiabatic flame temperature. When the temperature of the gas mixture reaches a value, typically about 900.degree. C., at which homogeneous combustion commences, there is a rapid increase in the gas temperature to about the adiabatic flame temperature. Where, as is usual, the catalyst body is in the form of a monolith with through channels, eg a honeycomb configuration, a higher temperature may be required to achieve homogeneous, ie gas phase, reaction since there is some evidence to suggest that the monolith channels may quench gas phase reactions.
When operating a gas turbine with catalytic combustion, when combustion has been established, it is usually desirable to decrease the preheating of the feed, eg to the temperature, typically about 300.degree.-400.degree. C. corresponding LD the discharge temperature of the compressor compressing the air and fuel.
One important criteria is that the catalyst has to withstand heating to relatively high temperatures, considerably above 1000.degree. C., without loss of its low temperature activity. This means that catalysts containing Group VIII elements, or compounds thereof, are unsatisfactory in view of their volatility, and/or their tendency to sitter at the temperatures encountered.
Also, in gas turbine operation using catalytic combustion, the catalyst not only has to be able to withstand high temperatures, but also withstand the thermal shock of rapid temperature changes resulting from repeated stopping and starting of combustion. Also gas turbines are usually operated using high gas flow rates. These conditions impose severe restraints on the materials that can be utilized as the catalyst.
In EP-A-0472307 there are described catalysts based on combinations of certain oxides of elements of Groups IIIa and, possibly Group IVa, of the Periodic Table. Preferred catalysts contained a host oxide such as ceri
REFERENCES:
patent: 4656155 (1987-04-01), Josefowicz
patent: 5075277 (1991-12-01), Saiai et al.
patent: 5137862 (1992-08-01), Mackrodt et al.
Fowles Martin
Mackrodt William C.
Morris Michael A.
Beck Shrive
Imperial Chemical Industries plc
Meeks Timothy H.
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