Metallurgical apparatus – Means for treating ores or for extracting metals – By means applying heat to work – e.g. – furnace
Reexamination Certificate
2000-02-07
2001-10-09
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Means for treating ores or for extracting metals
By means applying heat to work, e.g., furnace
C432S058000
Reexamination Certificate
active
06299826
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to fluidised bed apparatus and is concerned in particular with an improved gas inlet plenum assembly for such apparatus. The invention has been found to be particularly useful in the fluidised bed reactors employed for direct reduction of iron ore (DRI) but has general application in the improvement of gas flow distribution in the plenum chamber of fluidised bed reactors and furnaces.
BACKGROUND ART
In a known process for the direct reduction of iron ore, hereinafter referred to as the subject fluidised bed DRI process, iron ore fines are passed downwardly in succession through a series of fluidised bed reactors against an upward counterflow of reducing gas. The process is practised commercially in Venezuela and is known there as the FIOR process. In that plant, the product of the lowermost reactor (the first reactor for the gas flow) is passed to a briquetting facility to produce a stable metallised iron product which is, inter alia, a suitable feed for electric arc steel furnaces, so-called mini-mills. The reducing gas delivered to the lowermost reactor is typically a mix of a reformed natural gas with compressed scrubbed top gas recycled from the uppermost reducing reactor. In each reactor, this gas serves as the fluidising medium as well as the reductant and is therefore delivered to the bottom of each reactor housing via a plenum chamber, for admission to the main reaction chamber through multiple nozzles of a distributor plate which supports the fluidised bed. Within each reactor, internal cyclones extract fine ore dust from the top gas before it is delivered to the next upper reactor or, in the case of the uppermost reactor, recycled to the reducing gas feed stream. The dust extracted in the cyclones is returned to the fluidised bed via respective diplegs.
Particular forms or modifications of this process are described, for example, in U.S. Pat. Nos. 5,082,251, 5,370,727 and 5,439,504, international patent publication WO 96/10094 and Australian patent application no. 15007/95.
In the current commercial practice of the FIOR DRI process, the gradual build up of accretions at various points in the reactor configuration steadily diminishes the performance of the process and eventually reduces gas flows to a point where the process must be stopped and the accretions removed. Experience has shown that accreted material in nozzles and in the reactor lower plenum chamber tends to be deposited in regions of stagnation in the fluid flow. Accretions occur in stagnant regions resulting from direct gas impingement as well as in regions of flow separation.
The problem was addressed in Australian patent 490892, which proposed a modified shape of nozzle for the distributor plate. Although not directly suggested as relevant to the reduction of the rate of accretion formation, this reference also illustrates a dual baffle plate arrangement in the plenum chamber under the distributor plate rather than the single baffle plate used in the FIOR reactors. The upper baffle plate is an annulus or donut, and the other is a smaller disc.
More complex distributor plate configurations are described in British patent 1119250, Swiss patent 583066, and European patent publication 421506. The first two of these disclose a double-plate arrangement with offset apertures. EP 421506 depicts two spaced perforated plates or screens sandwiching a bed of balls.
It is an object of the present invention, in a preferred application, to reduce the rate at which accretions form in the subject fluidised bed DRI process at and adjacent the distributor plate nozzles of the inlet plenum assemblies.
DISCLOSURE OF THE INVENTION
In accordance with the invention, it has been realised that the rate of accretion formation can be reduced materially by modifying the gas flow distribution in the plenum chamber, and that one way of achieving this in an advantageous manner is by means of a selected perforated baffle arrangement within the plenum chamber, in place of the impervious baffle plate traditionally employed in the FIOR DRI process to substantially reduce cross-flow at the nozzle inlet openings.
The invention accordingly provides, in one aspect, in a gas plenum assembly for a fluidised bed apparatus, including:
structure defining one or more side and base walls about a plenum
chamber;
means defining an inlet for gas to the chamber, arranged so that the gas flows generally upwardly into said chamber from the inlet;
distributor means overlying said chamber and having multiple openings through which the gas exits the chamber for forming a fluidised bed above the distributor means; and
means disposed between said inlet and said distributor means for spreading the gas flow among the openings;
the improvement wherein the gas flow spreader means has a plurality of apertures for said gas flow therethrough, and wherein said gas flow spreader means and said flow apertures are sized and arranged to substantially reduce cross-flow velocity of said gas at said openings.
Preferably, the gas spreader means includes one or more perforated baffle plates.
In another aspect, the invention provides, in a fluidised bed reactor for reducing particulate iron oxide, including:
a gas inlet plenum assembly having structure defining one or more side and base walls about a plenum chamber:
means defining an inlet for gas to the chamber, arranged so that the gas flows generally upwardly into said chamber from the inlet;
distributor means overlying said chamber and having multiple openings defined by upwardly convergent nozzles through which said gas exits the chamber for forming a fluidised bed above the distributor means; and
means disposed between said inlet and said distributor means for spreading the gas flow among said openings;
the improvement comprising means in said gas inlet plenum assembly arranged to substantially reduce gas cross-flow velocity at said nozzles and thereby to minimise the rate of formation of accretions on or adjacent to the nozzles which diminish flow from the plenum chamber.
Preferably, the gas inlet plenum assembly is substantially axially symmetrical about a central vertical axis. The distributor means preferably includes a distributor plate and the openings therein may include nozzles which, at least for application to the reduction of iron oxides, are upwardly convergent cones.
The baffle plate means advantageously includes between one and three baffle plates extending generally parallel to the distributor plate and generally midway between the distributor plate and the gas inlet.
Preferably, where there is more than one baffle plate, the spacing between the plates is no greater than about ten times the average centre-to-centre spacing of the apertures in the baffle plate. These apertures are preferably circular and may be arranged in any convenient array eg a square or triangular array.
REFERENCES:
patent: 3915657 (1975-10-01), Staffin et al.
patent: 4257171 (1981-03-01), Johnson et al.
patent: 5538544 (1996-07-01), Nowobilski et al.
patent: 5904119 (1999-05-01), Gomez
patent: 490892 (1976-12-01), None
patent: 775393 (1957-05-01), None
patent: 1069729 (1967-05-01), None
Brent Allon Dudley
Caffery Grant
Haywood Ross Jeffrey
Warnica William David
Baker & Botts L.L.P.
BHP Direct Reduced Iron Pty Ltd.
Kastler Scott
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