GB2261225A

GB2261225A - Air separation, iron ore reduction and power generation plant

Info

Publication number: GB2261225A
Application number: GB9222993A
Authority: GB
Inventors: John Terence Lavin; Paul Michael Latham
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1991-11-04
Filing date: 1992-11-03
Publication date: 1993-05-12
Anticipated expiration: 2012-11-03
Also published as: GB9123381D0; GB2261225B; GB9222993D0; AU634144B3; ZA928508B

Abstract

Air is compressed in a compressor 32 and a first stream of the compressed air is separated in a plant 34 into oxygen and nitogren. A stream of the oxygen is reacted in a gasifier 22 with coal to form a reducing gas. The reducing gas is reacted with iron ore in a shaft furnace 24 to form molten iron and a residual fuel gas. The residual fuel gas is compressed in a compressor 40 and is combusted in a chamber 52. A second stream of the compressed air is used to support this combustion. The resulting combustion gases are expanded in a turbine 54 and useful work is recovered therefrom. A stream of nitrogen from the air separation plant 34 is heated in a heat exchanger 60 and expanded with the combustion gases in the turbine 54 or separately in a further turbine. The work recovered from the turbine (5) exceeds that expended in compressing the air. <IMAGE>

Description

AIR SEPARATION This invention relates to air separation.

It is known to produce a fuel gas typically including as its main components carbon monoxide, carbon dioxide and hydrogen by a process in which coal is gasified and the resulting gas mixture is used to reduce iron ore to iron. Examples of such processes are described in a paper entitled: "Coal-based iron-making", R B Smith and M J Corbett, Ironmaking and Steelmaking (1987), 14, pp 49 to 56. Even after reduction of the iron oxide, the resulting gas has an appreciable calorific value. It may be used for heating purposes, be recycled to the reactor in which the coal is gasified and the iron ore is reduced, or taken for combustion in the combustion chamber of a gas turbine with resultant expansion of the combustion products in the turbine which can accordingly be used to drive an alternator and thereby generate electricity.

The gasification process typically includes partial oxidation. An air separation plant is used to supply the necessary oxygen. Nitrogen is therefore produced as a by-product.

According to the present invention there is provided a process, comprising the steps of: a) compressing air; b) separating a first stream of the compressed air into oxygen and nitrogen; c) reacting a stream of said oxygen with coal under conditions such that a reducing gas is formed; d) reacting iron ore with the reducing gas to form molten iron and a residual fuel gas; e) compressing a stream of the residual fuel gas; f) passing the compressed residual fuel gas and a second stream of the compressed air into a combustion chamber and burning the fuel gas therein to form gaseous combustion products using the second compressed air stream to support combustion; g) expanding the combustion products in a turbine and thereby recovering useful work therefrom.

h) heating a stream of said nitrogen at elevated pressure; and i) expanding the heated stream of nitrogen in a turbine and thereby recovering work therefrom; whereby the work recovered in said steps (g) and (i) is in excess of the work expended in compressing the first stream of said air.

Typically, the stream of nitrogen is compressed upstream of its being heated.

Typically, the work recovered from said steps (g) and (i) is in excess of all the work of compression entailed in compressing the first stream of said air and in providing the oxygen at a suitable pressure from step (c) of said process and the nitrogen at a suitable pressure from step (i) of the process. Some of the work may be recovered by mounting compression equipment on the same shaft a expansion equipment.

Alternatively, or in addition, work may be recovered by employing the turbine or turbines to generate electricity.

Typically, at least some of the heated stream of nitrogen is expanded in a turbine separate from the one used in step (g).

If desired, hydrogen may be separated from the fuel gas upstream of other combustion chamber.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic flow diagram of a second plant which includes means for generating electrical power from a fuel gas, the fuel gas being produced by a reactor that reduces iron oxide to iron by reaction with gasified coal; and Figure 2 is a schematic flow diagram of an air separation plant for use in the plant shown in Figures 1.

The drawings are not to scale.

Referring to Figure 1, a reactor 20 for the direct reduction of iron oxide and for the production of a fuel gas comprises a gasifier 22 and a vertical shaft reduction furnace 24. Measured quantities of lump, pelletised or sinter iron oxide ore, lime and dolomite are charged directly into the top of the furnace 24. Simultaneously, a reduction gas comprising carbon monoxide and hydrogen is blown into the shaft furnace 24 at an intermediate region thereof. The reduction gas moves upwards against a descending flow of ore to the top where it is drawn off via a conduit 26. While descending through the hot gas, lime and dolomite are calcined and the ore is reduced to sponge iron.Screw conveyers (not shown) are employed to extract the sponge iron from the bottom of the shaft furnace 24 at a desired rate and the extracted sponge iron is allowed to fall under gravity directly into the gasifier 22. The gasifier 22 is of a kind having a hearth (not shown) at its bottom a fluidised bed into which coal is fed, and an uppermost free board zone. Oxygen is blown through tuyeres (not shown) into the fluidised bed region of the gasifier 22, and the coal is thereby gasified. The resulting gas is withdrawn through a conduit 28, is passed through a cyclone 30 and is then divided. Part of the flow provides the gas for the furnace 24 while the remainder is returned to the gasifier 22. Sponge iron falling under gravity into the fluidised bed region of the gasifier 22 is melted.Liquid iron and slag, comprising coal ash, lime and dolomite, drop into the hearth and separate naturally into two layers owing to the difference in density between the heavier iron and the lighter slag.

Liquid iron can thus be withdrawn from the bottom of the gasifier 22.

Operation of such reduction furnaces-cum-gasifiers are well known in the art and the above description is merely a brief summary of the way in which they operate. One example of process for operating such plant is the COREX process.

The oxygen for the reactor 20 is provided by taking an air stream and compressing it in a compressor 32. A minor portion of the compressed air stream is then passed into a cryogenic air separation plant 34 in which the air is separated into oxygen and nitrogen by rectification. An oxygen stream is withdrawn from the plant 34 is compressed in a compressor 36 to the operating pressure of the gasifier and is then passed into the gasifier 22 to provide its oxygen requirements.

The fuel gas passing out of the top of the furnace 24 typically has the following composition: carbon monoxide 40 to 45% by volume; carbon dioxide 30 to 37% by volume; hydrogen 15 to 18% by volume; water vapour 1.5 to 3% by volume; methane 0.5 by volume; nitrogen 3 to 4% by volume and a calorific value in the range of 7.5 to 8 MJ/Nm3. (lNm3 of gas is the quantity of gas that occupies 1 cubic metre at OOC and 1 atmosphere absolute). The gas typically also contains from 10 to 100 vpm of hydrogen sulphide and up to 10 mg/m3 of particulates. The fuel gas typically leaves the top of the furnace 24 at a temperature in the range of 250 to 3000C.

It is then subjected in plant 38 to cooling to approximately ambient temperature and clean-up by removal therefrom of particulates and such environmentally harmful constituents as hydrogen sulphide and oxides of nitrogen. The construction and operation of plants for this purposes are well known.

Downstream of the plant 38, the fuel gas stream is compressed in a compressor 40, typically to a pressure in the range of 10 to 20 atmospheres absolute and a little in excess of the pressure to which the air is compressed in the compressor 32. A slip stream of the compressed fuel gas is then taken and hydrogen is separated therefrom by operation of a membrane separation plant 42. Substantially pure hydrogen is produced as the permeate gas and if desired can be compressed by a further compressor or compressors (not shown) suitable for filling compressed hydrogen cylinders or for its liquefaction. The non-permeate gas is preferably returned to the compressed fuel gas at a region downstream of the location from which the slip stream is taken but upstream of a combustion chamber 52 into which the compressed fuel gas is introduced. If desired, carbon dioxide may be separated, for example, by pressure swing adsorption in means not shown in the drawing from the fuel gas stream or the non-permeate gas stream.

The combustion chamber 52 is associated with a turbine 54. Combustion of the fuel in the chamber 52 is supported by the major portion of the stream of compressed air produced by the compressor 32. Typically, the compressor 32, the combustion chamber 52 and the turbine 54 form a single piece of plant with the turbine 32 and the compressor 54 each having rotors (not shown) mounted on the same shaft, whereby the expansion turbine 54 is effective to drive the compressor 32. The compressor 32 is typically of a size that enables a chosen rate of combustion of fuel gas to be achieved in the chamber 52 and hence hot combustion products to be provided to the turbine 54 at a chosen rate. A stream of nitrogen is taken from the air separation plant 34 and is compressed in a compressor 56 to approximately the operating pressure of the chamber 52.The resulting stream of compressed nitrogen is then mixed with the combustion products produced in the chamber 52, and the resulting mixed gas stream expanded in the turbine 54. The nitrogen thereby introduced into the turbine 54 is able to help balance this machine, compensating for the air withdrawn from the compressor 32 for separation into oxygen and nitrogen. The turbine 54 as well as providing drive for the compressor 32 also drives an alternator 58 forming part of a power station 48.

If desired, a stream of combustion products exiting the turbine 54 may be employed to pre-heat the compressed nitrogen upstream of its being mixed with the combustion products produced in the chamber 52. The pre-heating may be effected by countercurrent heat exchange in a heat exchanger 60 with exhaust gases from the turbine 54. If necessary, an additional heat exchange stream may be employed in the heat exchanger 60 to heat the nitrogen to a desired temperature. The waste gases from the turbine 54 are typically vented to the atmosphere via a stack (not shown). In an alternative embodiment (not shown), the compressed nitrogen may be preheated by heat exchange with the air taken for separation.In such an embodiment the heat of compressor of the air taken for separation is recovered, so therefore the compressor 32 is provided with no aftercooler upstream of the heat exchange of the air stream with the nitrogen. If desired, heat from the exhaust gas that leaves the turbine may be recovered by using the exhaust gas to raise steam. The steam may then be expanded in a separate turbine (not shown).

Referring now to Figure 2, there is shown an air separation unit which may be used as part of the plant shown in Figure 1 of the accompanying drawings. A compressed air stream is passed through a purification apparatus 70 effective to remove water vapour and carbon dioxide from the compressed air. The apparatus 70 is of a kind which employs beds of adsorbent to adsorb water vapour and carbon dioxide from the incoming air.

The beds may be operated out of sequence with one another such that while one or more beds are being used to purify air, then the others are being regenerated, typically by means of a stream of nitrogen. The purified air stream is divided into major and minor streams.

The major stream passes through a heat exchanger 72 in which its temperature is reduced to a level suitable for the separation of air by rectification. Typically, therefore, the major air stream is cooled to its saturation temperature at the prevailing pressure. The major air stream is then introduced from the heat exchanger 72 through an inlet 74 into a higher pressure stage 78 of a double rectification column 76 having in addition to the stage 78, a lower pressure stage 80. Both rectification stages 78 and 80 contain liquid-vapour contact trays (not shown) and associated downcomers (not shown) (or other means for effecting intimate contact between the descending liquid phase and an ascending vapour phase) whereby a descending liquid phase is brought into intimate contact with an ascending vapour phase such that mass transfer occurs between the two phases.The descending liquid phase becomes progressively richer in oxygen and the ascending vapour phase progressively richer in nitrogen. The higher pressure rectification stage 78 operates at a pressure similar to that to which the incoming air is compressed and separates the air into an oxygen-enriched air fraction and a nitrogen fraction. The lower pressure stage 80 is preferably operated so as to give a substantially pure nitrogen fraction at its top but an oxygen fraction as its bottom which still contains an appreciable proportion of impurities (say, up to 5 by volume).

The stages 78 and 80 are linked by a condenser-reboiler 82. The condenser-reboiler 82 receives nitrogen vapour from the top of the higher pressure stage 78 and condenses it by heat exchange with boiling liquid oxygen in the stage 80. The resulting condensate is returned to the higher pressure stage 78. Part of the condensate provides reflux for the stage 78, while the remainder is collected, sub-cooled in a heat exchanger 84 and passed into the top of a lower pressure stage 80 through an expansion valve 86 and thereby provides reflux for the stage 80. The lower pressure rectification stage 8Q operates at a pressure lower than that of the stage 78 and receives oxygen-nitrogen mixture for separation from two sources.

The first source is the minor air stream formed by dividing the stream of air leaving the purification apparatus 70. Upstream of its introduction into the stage 80, the minor air stream is compressed in a compressor 88 having an aftercooler (not shown) associated therewith, is then cooled to a temperature of about 200K in the heat exchanger 72, is withdrawn from the heat exchanger 72, and is expanded in an expansion turbine 90 to the operating pressure of the stage 80, thereby providing refrigeration for the process. This air stream is then introduced into the lower pressure stage 80 through an inlet 92. If desired, the expansion turbine 90 may be employed to drive the compressor 88, alternatively the two machines, namely the compressor 88 and the turbine 90 may be independent of one another.If desired, the compressor 88 may be omitted, and the turbine 90 used to drive an electrical power generator (not shown).

The second source of oxygen-nitrogen mixture separation the lower pressure rectification stage 80 is a liquid stream of oxygen-enriched fraction taken from the bottom of the higher pressure stage 78. This stream is withdrawn through an outlet 94, is sub-cooled in a heat exchanger 96 and is then passed through a Joule Thomson valve 98 and flows into the stage 80 at intermediate level thereof.

The apparatus shown in Figure 2 of the drawings produces a product oxygen stream and a.product nitrogen stream. The product oxygen stream is withdrawn as vapour from the bottom of the lower pressure stage 80 through an outlet 100. This stream is then warmed to approximately ambient temperature in the heat exchanger 72 by countercurrent heat exchange with incoming air. A nitrogen product stream is taken directly from the top of the lower pressure rectification stage 80 through an outlet 102. This nitrogen stream flows through the heat exchanger 84 countercurrently to the liquid nitrogen stream withdrawn from the higher pressure stage 78 and effects the sub-cooling of the stream. The nitrogen product stream then flows through the heat exchanger 96 countercurrently to the liquid stream of oxygen-enriched fraction and effects the sub-cooling of this liquid stream. The nitrogen stream flows next through the heat exchanger 72 countercurrently to the major air stream and is thus warmed to approximately ambient temperature.

It is to be appreciated that the separation of the hydrogen is an optional step and if desired all the gas exiting the compressor 40 may be passed to the combustion chamber 52.

If desired, some or all of the nitrogen stream leaving the heat exchanger 60 may be expanded in a turbine (not shown) separate from the turbine 54.

If desired, this separate turbine may receive only nitrogen for expansion.

Claims

1. A process, comprising the steps of: a) compressing air; b) separating a first stream of the compressed air into oxygen and nitrogen; c) reacting a stream of said oxygen with coal under conditions such that a reducing gas is formed; d) reacting iron ore with the reducing gas to form molten iron and a residual fuel gas; e) compressing a stream of the residual fuel gas; f) passing the compressed residual fuel gas and a second stream of the compressed air into a combustion chamber and burning the fuel gas therein to form gaseous combustion products using the second compressed air stream to support combustion; g) expanding the combustion products in a turbine and thereby recovering useful work therefrom; h) heating a stream of said nitrogen at elevated pressure; and i) expanding the heated stream of nitrogen in a turbine ad thereby recovering work therefrom whereby the work recovered in said steps (g) and (i) is in excess of the work expended in compressing the first stream of said air.

2. A process for separating air, producing molten iron and recovering work substantially as herein described with reference to Figure 1 of the accompanying drawings.