Gas injector for fluidized bed furnace

A fluidized bed furnace employed for depositing pyrolytic carbon or silicon carbide on nuclear fuel particles is supplied with an inert dilution gas and a decomposable coating gas. These are supplied by means of a gas injector having at least three pipes 4 for coating gas extending within a surrounding duct for a stream of dilution gas 6. This leads to a partition 2 at the base of the bed, having at least three apertures 3 for through passage of the gases.

The invention relates to a gas injector for a fluidised bed furnace. 
Nuclear fuel particles for use in high-temperature reactors are customarily 
coated with pyrolytic carbon or silicon carbide. This is effected by 
thermal decomposition of suitable substances in the gas phase in a 
fluidised bed containing the particles to be coated. The substances to be 
decomposed may be hydrocarbons such as acetylene, propylene or methane, or 
methyl trichlorosilane, with decomposition taking place at temperatures of 
1000.degree. to 2000.degree. C. To produce a predetermined/desired 
movement of the bed, and to reduce the concentration of the coating gas, 
an inert dilution gas--generally argon--is supplied to the bed in addition 
to the coating gas. 
These gases are fed into the fluidised bed furnace by means of suitable gas 
injectors or nozzles. Such a nozzle is described in our German Patent 
Specification No. 26 11 844 and corresponding U.K. patent specification 
No. 1555732. 
The present invention is concerned with improving and further developing 
these known gas injectors. 
Broadly this invention provides a gas injector having three or more supply 
pipes to convey coating gas, to form central or inner streams, housed in a 
common duct which serves to convey dilution gas. Accordingly the invention 
provides a gas injector for a fluidised bed furnace having at least three 
apertures for the passage of dilution gas and coating gas into the bed. 
The advantages of this gas injector are: 
The provision of a plurality of supply pipes makes it possible to increase 
the supply of coating gas by aggregrating the individual inner streams 
formed by the coating gas. In this way the batch size of the particles to 
be coated can be correspondingly increased, which results in a reduction 
of the cost of the coating. 
By division of the coating gas among a number of central streams, and thus 
among a number of gas entry locations a more uniform distribution of the 
coating gas in the fluidised bed is achieved. This can improve the coating 
operation and the quantity of the coating materials. For example it can 
give more uniform particle shape, more uniform coating thickness and 
density from particle to particle, and a lower anisotropy factor. In 
addition, the deposition efficiency is increased due to the more uniform 
distribution of the coating gas in the fluidised bed.

Referring first to FIG. 1, the fuel particles to be coated are situated in 
the conical holder 1, which is separated from the space beneath by 
partition member 2. 
In the partition member 2 there are an appropriate number of apertures 3 
which open conically towards the top. Beneath the partition member 2, 
there is a duct with circular cross section, through which dilution gas is 
supplied to the particle bed. Within this duct are several supply pipes 4 
which convey coating gas. 
The coating gas leaves the supply pipes 4 at a distance h from the 
apertures 3 in the partition member 2, forms laminar central jets 5 
and--more or less contracted by the surrounding dilution gas stream 
6--passes through the apertures 3 into the fluidised bed. In the circular 
duct which guides the somewhat annular dilution gas stream 6 there is a 
porous plate 7 which ensures a radially uniform velocity profile of this 
gas stream 6. 
The supply pipes 4 are axially displaceable in order to set the required 
distance h. These pipes 4 may each consist of a single pipe or a bundle of 
pipes as illustrated in FIGS. 2 and 3. In either case there is a porous 
plate 8 at the lower end of each supply pipe 4 extending across the bore 
of the pipe. All of these plates give a particular equal pressure drop 
.DELTA.p so that the coating gas is admitted equally to all the feed pipes 
4 from only one duct 9. The dilution gas enters by way of the duct 10. 
In order that the gas streams are not affected by the movements of the 
fluidised bed, the apertures 3 in the partition member 2 should be so 
dimensioned that the pressure drop which they produce is greater than the 
maximum fluctuation of pressure in the fluidised bed. In the case in 
question this is achieved with diameters d.sub.B of the apertures 3 not 
greater than 7 mm. 
It is advantageous for the arrangement of the supply pipes 4 in the 
circular duct which surrounds them to be as diagrammatically illustrated 
in the examples of FIG. 3. It has been found preferably for the 
longitudinal axis of the supply pipes to extend through the centre of 
gravity S.sub.F of the equal-sized, symmetrically arranged, 
cross-sectional areas of the surrounding circular duct which are defined 
by the number of the supply ducts.