AU1553297A

AU1553297A - Desulphurisation method and apparatus

Info

Publication number: AU1553297A
Application number: AU15532/97A
Authority: AU
Inventors: Noel Alfred Warner
Original assignee: University of Birmingham
Current assignee: University of Birmingham
Priority date: 1996-02-01
Filing date: 1997-01-30
Publication date: 1997-08-22
Also published as: EP0956143A2; GB9602037D0; WO1997027930A3; WO1997027930A2

Description

DESULPHURISATION METHOD AND APPARATUS

This invention relates to the complete or partial removal of sulphur containing species, hereinafter referred to as "desulphurisation" from a gas stream, and is more particularly but not exclusively concerned with the desulphurisation of raw gas streams produced in high pressure coal gasification and with the novel use of a desulphurisation procedure in reduced or zero gas emission non-ferrous metal (e.g. copper) sulphide smelting.

It has been proposed to perform raw gas desulphurisation using regenerable sorbents such as iron oxides, zinc ferrite and zinc titanate. However, the use of zinc ferrite is limited to a maximum of 650°C during desulphurisation because of the potential reduction of Fe₂0₃ to FeO or Fe, which results in catalysed carbon formation and severe decrepitation of the sorbent and because of reduction of zinc oxide followed by evaporation of zinc. The same applies to sorbents based on iron oxides. In the case of zinc titanate sorbents, regeneration of the sorbent results in increasing reduction in crush strength, pore volume and surface area with severe cracking and spalling.

It is an object of first and second aspects of the present invention to obviate or mitigate the above disadvantages.

According to said first aspect of the present invention, there is provided a method of desulphurising a gas stream, comprising the steps of reacting the gas stream at an elevated temperature with a gaseous reactive material which is capable of reacting with a sulphur-containing species in the gas stream to form a sulphurated reaction product which is solid at said elevated temperature; and separating the solid sulphurated reaction product from the resultant desulphurised gas stream.

According to said second aspect of the present invention, there is provided apparatus for performing the method as defined in the last preceding paragraph, said apparatus comprising a reactor having (i) a first inlet for the gas stream to be desulphurised, (ii) a second inlet for introducing reactive material into the reactor, and (iii) a reaction zone arranged so that, in use, the gas stream is brought into contact with the reactive material which is in gaseous form so as to form the solid sulphurated reaction product; and a separator for separating the solid sulphurated reaction product from the desulphurised gas stream.

In a first embodiment, the apparatus further includes an evaporator in which, in use, the reactive material is evaporated to produce the gaseous reactive material, and wherein the second inlet is connected with an outlet of the evaporator so that, in use, the gaseous reactive material is introduced into the reaction zone.

The gaseous reactive material may be introduced alone or in admixture with a gaseous, substantially non-reactive diluent, e.g. nitrogen.

In a second embodiment, the second inlet is arranged to introduce the reactive material in liquid form into the reactor so that vaporisation occurs in the reactor to form the gaseous reactive material.

The liquid reactive material may introduced in admixture with a substantially non-reactive diluent. For example, the reactive material may be introduced in the form of an alloy of the reactive material with at least one substantially non-reactive diluent element.

In the first embodiment, the separator may contain a packing upon which the solid sulphurated reaction product from the reactor is deposited.

In the second embodiment, the reactor may contain a packing through which the liquid reactive material is arranged to pass without flooding of the packing

If desired, the gas stream to be desulphurised may be subjected to desulphurisation using apparatus according to both of said first and second embodiments. Thus, in use, the gas stream may first be subjected to desulphurisation by use of gaseous reactive material introduced into a first reactor, and then subjected to desulphurisation by passing liquid reactive material into a second reactor.

In either of the first and second embodiments, the packing may be circulated in a path which also includes a station where the solid sulphurated reaction product adhering to the packing is removed, e.g. by mechanically agitating the packing. Alternatively, the packing may be a fixed bed packing which is periodically removed to permit removal of the adherent solid sulphurated reaction product.

Most preferably, the reacting step is conducted in the presence of solid particles whereby to form the solid sulphurated reaction product by a heterogeneous reaction of the sulphur-containing species with the gaseous reaction material on the surface of the solid particles. Such solid particles may be constituted by the above-mentioned packing in the case of the second embodiment. Alternatively, the solid particles may be added specifically for the purpose of promoting nucleation to give a final particle size range suitable for simple gas/solid separation, e.g. by means of a high temperature cyclone or similar device. As a further alternative, the heterogeneous reaction may be effected on solid particles which are already present in the gas stream to be desulphurised.

Since particle growth is by deposition of all reacting species directly from the gas phase, the rate of reaction is not inhibited by protective layers which might be formed on a solid or liquid reactive material.

Conveniently, the gas stream to be desulphurised and the gaseous reactive material are present in the reaction zone in proportions such that there is a small stoichiometric excess of the reactive material relative to the sulphur to be removed.

In a convenient embodiment, the reactive material is a metal, preferably zinc. In such a case, the metal vapour is reacted with the sulphur- containing species to form metal sulphide particles, preferably zinc sulphide particles, which remain dispersed in the gas stream. If a small stoichiometric excess of metal vapour is present, this excess is quickly oxidised with the relatively high levels of water vapour and carbon dioxide which are present in the desulphurised gas stream. The gaseous metal, e.g. zinc, vapour reacts preferentially with H₂S and COS rather than with carbon dioxide and water vapour in the raw gas. The heterogeneous reaction, being controlled almost exclusively by gaseous diffusion, can be controlled accurately to optimise the entrainment and growth of the solid sulphurated reaction product so as to ensure efficient reaction and to enable effective separation of the solid sulphurated reaction product from the desulphurised gas stream.

In one example of the above-mentioned first embodiment, desulphurisation is effected in a reactor where the gas to be desulphurised is introduced laterally of a jet of the gaseous reactive material in such a way as to prevent deposition of the solid sulphurated product on surfaces in the reactor. This can be achieved by preventing the reaction to form the solid sulphurated product from taking place at such surfaces. To this end, it is preferred that the gas to be desulphurised is introduced so as to pass inwardly from a multiplicity of inlet orifices around the jet. In this way, the reaction can take place within the reaction zone at a location away from the surfaces. The inlet orifices may extend over a major proportion of the length of the jet in the vessel.

Separation of the solid sulphurated reaction product from the desulphurised gas stream may be effected by ceramic candle filters or similar devices either alone or in combination with cyclone separators or the like depending upon the particle size distribution of the dust being removed and the specification for gas cleanliness for the gas stream. In the case of a gas stream produced in high pressure coal gasification for generation of electricity, the desulphurised gas stream will ultimately be passed to a gas turbine.

In the case where the gas stream contains unacceptably high concentrations of HCl, it may be advantageous to subject the gas stream to be desulphurised to a treatment with solid sodium carbonate followed by filtration of the resultant gas stream at a suitable temperature, before the gas stream is desulphurised.

In the case where the gas stream to be desulphurised contains unacceptably high concentrations of sodium compounds, the filtration step recommended above for removal of HCl or gaseous zinc chloride can also be effective for removing alkali metal salt vapours if gas filtration is conducted between 350°C and 500°C with a cleanable filter system. Instead of using a wet scrubbing gas filtration system, dry gas cleaning at 350°C may be more advantageous in terms of overall energy conversion efficiency, and also avoids water treatment and possible liquid effluent problems. In other circumstances, it may be preferable to remove the HCl (or halides) at higher temperatures before admission of zinc into the system. A polishing to remove any residual halides may be effected after cooling following desulphurisation.

After having cleaned the gas stream of its chloride and alkali metal contents, it may also be desirable to re-heat it to about 1000 K by radiant heat exchange with the gases initially leaving the gasifier used to produce the gas stream to be desulphurised. Re-heating is particularly advantageous to minimise the risk of formation of a mist of liquid reactive material, e.g. liquid zinc mist, and subsequent fog formation which may result in a decrease in the efficiency of the desulphurising reaction because of the formation of a sulphide coating on the droplets of liquid reactive material. The alternative is to remove most of the HCl at higher temperature prior to zinc admission as referred to above, and then conduct the zinc-based desulphurisation at this higher temperature. A preferred method of forming the gaseous reactive material is to heat the liquid reactive material in a closed evaporator so as to generate a pressure whereby the liquid reactive material becomes equilibrated with its vapour at a temperature above its normal boiling point. Thus, in the case of zinc, a gas pressure of about 50 atmospheres is generated if liquid zinc is heated to 1800 K, whilst a pressure of 20 atmospheres is generated when the liquid zinc is heated to a temperature of about 1600 K. It is preferred to cause the molten reactive material to be admitted to the evaporator only as fast as it is vaporised so that the evaporator never contains more than a small amount of liquid reactor material at elevated temperature.

In the case where zinc is used and a graphite evaporator is employed, it is particularly convenient to pass a heavy electrical current through both the liquid zinc and the hearth of the evaporator to satisfy all the thermal requirements for evaporation at the desired rate by direct resistive electrical heating. In the case where the gas stream to be desulphurised is at a pressure in the region of 25 atmospheres, this requires a liquid zinc temperature in the evaporator above 1600 K but less than 1800 K. Thus, allowing for pressure losses in the transport of the zinc gas to the gas stream to be desulphurised as well as velocity head losses associated with injection of the zinc gas, the evaporator hearth temperature may typically be controlled at 1700 K, at which temperature the saturation vapour pressure of liquid zinc is about 32.5 atmospheres.

Gas temperatures in the zinc gas circuit are preferably maintained above the dewpoint within graphite and refractory components, whilst all exposed metal surfaces are preferably cooled to below the melting point of zinc to ensure that metallic corrosion is not permitted to occur. The solid sulphurated reaction product separated from the desulphurised gas stream may be converted directly back to zinc metal using any desired process, but preferably the process described in US Patent 5358544 and W094/21832. In US Patent 5358544 and W094/21832, zinc is recovered from zinc sulphide material by introducing the latter at a feed station into a molten copper sulphide matte which is circulated in a closed loop path through the feed station, a zinc recovery station and an oxidising station. The matte is heated electrically directly by resistive heating. Oxygen in the absence of other gases is introduced at the oxidising station. Sulphur dioxide may be removed in a sulphuric acid plant. The method is operated to keep the activity of copper in the total matte at less than unit activity and virtually zero gas emission is possible. As an alternative to converting the zinc sulphide directly back to zinc metal, the material may alternatively be subjected to straightforward mineral processing techniques to yield a high grade zinc sulphide concentrate which can be handled with relative safety and converted to zinc off-site.

In the case where the zinc sulphide is converted to zinc metal either on- site or off-site by a process involving the generation of sulphur dioxide, the sulphur dioxide may be removed in a sulphuric acid plant, as noted above. Alternatively, the sulphur dioxide may be reacted with a calcined alkali rare earth carbonate material such as dolomite or limestone, preferably the former.

In the case where there is full on-site integration of hot desulphurisation with zinc and reaction of high concentration S0₂ with dolomite, very little external fuel is needed to transform the zinc sulphide back to metal ready for immediate re-cycling back to the gasification circuit. This is because of the highly exothermic overall reaction between dolomite and S0₂ conducted at relatively high intensity compared with conventional flue gas desulphurisation. Normal flue gas desulphurisation with limestone operates with an S0₂ concentration of around 0.3 vol. percent, whilst in the present proposal, the gas is typically 67% S0₂ and 33% 0₂ at a temperature in the range of 1300 to 1400 K. Gas distribution is unimportant in the sulphation reactor, which can be regarded as merely sequestering SO, and 0₂ from the gas in the oxidising branch of the zinc regeneration system.

The thermal requirements for calcining pre-heated dolomite can be supplied entirely by re-circulation of reaction-exhausted solids around a closed loop, which separates the overall gas-solid reaction into its two fundamental components, calcination and sulphation, effected at separate calcination and sulphation stations. In such a case, the closed loop system preferably comprises an upper sulphation vessel containing a moving bed feeding into a moving bed in a separate lower calcination vessel via a conduit, e.g a downcomer or stalk, with means for keeping the C0₂ atmosphere in the calcination vessel quite separate from the SOj/O, involved in sulphation. Such means preferably includes a downward gas bleed (e.g. an oxygen gas bleed) into the conduit. The moving bed system is facilitated if the initial dolomite feed is relatively coarse, free-flowing and highly permeable to gas flow. Typically, the dolomite feed stone has a size of at least about 5 mm and more preferably is about 8 to 10 mm or more, provided that it is flowable. Dolomite when first added to the system may be arranged to flow by gravity through a column preheater counter-current to hot gases coming from a cooler used to cool directly the calcined product. In a preferred arrangement, the pre-heated feed stone meets re-circulated stone which is typically converted to anhydrite in the sulphation reactor before reaction virtually ceases as a result of the well-known pore plugging phenomenon for limestone, but proceeds effectively to completion with dolomite. Typically, retention times are in the region of 30 to 40 minutes and 40 to 60 minutes for calcination and sulphation, respectively. The dry, cooled sulphated dolomite stone from the cooler is then ready for immediate transportation away from the power generation site or, alternatively, can be processed downstream for by¬ product and recycled calcium carbonate recovery. Such a procedure eliminates the need for extensive water treatment and solid-liquid separation systems incorporating centrifuges etc, such as are required to be used for conventional flue gas desulphurisation.

In situations where there is a sustainable local market for sulphuric acid, then it is more preferable to manufacture acid from the high strength S0₂ gas stream containing excess oxygen rather than employing the sulphur fixation option using dolomite mentioned above. Because of the virtual zero gas emission mode of operation, a new approach for manufacturing sulphuric acid has been developed, wherein:-

1. The exclusive use of tonnage oxygen and zero fossil fuel requirement means that, provided air ingress is prevented, the whole gas circuit is moisture-free so that the various air and gas drying steps normally associated with acid manufacture can be eliminated.

2. There is no absolute requirement for high conversion of S0₂ to S0₃ and therefore no need for expensive catalysts and elaborate gas cleaning, but rather excess 0₂ and somewhat higher temperatures than normal are all that is needed to cause oxidation of S0₂ to S0₃

3. The gas volumes being handled are almost of an order of magnitude smaller than normal and, if a small amount of acid mist is formed, it is merely recycled along with excess 0₂ and unconverted S0₂ back to the top blow region of the zinc smelter by a gas eductor using the motive force of the pressurised 0₂ make-up supplied from the air separation unit.

4. The only gas exhausted to the surrounding atmosphere from the circuit is the small amount of nitrogen impurity in the tonnage oxygen supplied to the process, with the result that there is virtually zero gas emission. The small nitrogen bleed that is vented off can be chemically scrubbed to ensure that even the tightest environmental standards can be reached with ease.

5. The exothermic heat of S0₂ conversion reaction is recovered at a higher temperature than is normally the practice for catalytic conversion so that waste heat recovery and efficiency of steam-to- electricity generation are enhanced. The electricity so generated makes a major contribution to the total electrical requirement of the zinc sulphide smelting furnace powered by direct resistive heating involving passage of heavy current through the melt itself.

Unlike all other existing or previously proposed desulphurisation systems, the addition of gaseous zinc increases the calorific value of the raw coal gas as well as raising the temperature and thus the overall gas enthalpy. The above-described desulphurisation procedure using circulation of stone between separate sulphation and calcination stations also finds application in fields other than in the first and second aspects of the present invention. For example, desulphurisation of a gas stream containing sulphur dioxide by capturing the sulphur dioxide as calcium sulphate can be applied to the smelting of non-ferrous sulphides such as copper-bearing sulphides.

According to a third aspect of the present invention, there is provided a method of desulphurising a gas stream containing sulphur dioxide, comprising the steps of feeding solid alkaline earth metal carbonate material to a calcination station, calcining the material in the calcination station, passing the calcined material to a sulphation station, contacting the calcined mateπal in the sulphation station with the gas stream to be desulphurised so as to sulphate the calcined material, and removing the sulphated material from the sulphation station, wherein the calcination station and the sulphation are separate and carbon dioxide produced as a result of calcination of the alkaline earth metal carbonate material in the calcination station is withdrawn from the latter and prevented from entering the sulphation station.

To facilitate gas permeation therethrough, the solid material is preferably in the form of flowable particles having a size of at least 5 mm and more preferably around 8 to 10 mm or perhaps even greater.

In a preferred embodiment, the solid material is circulated through the sulphation station and the calcination station in a circulation path, with a proportion of the material being removed after sulphation. In a preferred embodiment, the sulphation station is disposed at a higher level than the calcination station and is connected therewith via a conduit through which the solid material which has not been removed from the circulation path is recycled back to the calcination station.

Preferably, carbon dioxide is prevented from passing into the sulphation station from the calcination station by introducing a minor upward and downward flow of gas (preferably oxygen) into the conduit between the sulphation station and the calcination station.

In a preferred embodiment, the sulphated material which has been removed from the sulphation station is passed to a cooling station where it is cooled with gas (e.g. inert gas and/or carbon dioxide) which is thereby heated and used to pre-heat the solid alkaline earth metal carbonate material being fed to the calcination station. The cooling station is preferably connected to the sulphation station via a further conduit provided with a gas bleed to prevent the cooling gas from the cooling station entering the sulphation reactor and to prevent sulphur dioxide from the sulphation reactor entering the cooling station.

It is preferred for the circulation path to be completed by a transfer system (for example a moveable insulated skip) which is arranged to transfer the calcined material from the calcination station to the sulphation station.

In accordance with a fourth aspect of the present invention, there is provided apparatus for performing the method according to said third aspect of the present invention, said apparatus comprising a calcination station, a sulphation station, means for feeding solid alkaline earth metal carbonate material to the calcination station, means for passing the calcined material to the sulphation station, means for removing sulphated material from the sulphation station, means for supplying gaseous material to be desulphurised to the sulphation station, and means for preventing carbon dioxide generated in the calcination station from entering the sulphation station.

The present invention in its third and fourth aspects can be used as a way of fixing sulphur dioxide produced in a non-ferrous metal (e.g. copper) sulphide smelting process, and is particularly useful in a copper sulphide smelting process of the recirculatory type such as is disclosed in GB-A-2048309 ( and corresponding EP-A-0016595 and US-A-4334918), the disclosure of which is incorporated herein by reference. The invention in its third and fourth aspects can also be used in a modification of the smelting procedures described in W093/24666 which is concerned with oxygen smelting of copper sulphide ore concentrates, nickel sulphide ore concentrates or bulk copper and nickel sulphide ore concentrates (hereinafter referred to simply as "copper/nickel sulphide ore concentrate") of high intrinsic energy value. W093/24666 discloses a procedure where such copper/nickel sulphide ore concentrate of high intrinsic energy value is oxygen smelted by forcibly circulating a molten sulphide carrier composition through a closed loop extraction circuit from which at least one product selected from copper, nickel and sulphides thereof can be continuously extracted at an elevated temperature, introducing the ore concentrate into the molten carrier composition at an ore receiving station so that the ore is dissolved in or melted by the composition, contacting the molten carrier composition containing said ore with an oxidising gas containing at least 30 volume percent oxygen at an oxidation station so as to oxidise at least part of the ore and/or the molten carrier composition, and utilising heat generated during the oxidation step as a result of oxidation of the ore concentrate. In such procedure, the oxidising gas is preferably technically pure oxygen and the heat generated during the oxidation step is utilised either by smelting the copper/nickel sulphide ore concentrate of high intrinsic energy value with another mineral concentrate of low or negative intrinsic energy value or by reducing iron oxide in slag produced in the method to a liquid iron product, and recovering such liquid iron product.

In accordance with a fifth aspect of the present invention, there is provided a method of smelting a non-ferrous metal sulphide comprising the steps of reacting the sulphide with oxygen to form the non-ferrous metal and a gas stream containing sulphur dioxide, and desulphurising the gas stream by means of the method according to said third aspect of the present invention.

The reacting step is preferably effected by contacting the oxygen with a stream of molten non-ferrous metal sulphide which is forcibly circulated in a closed loop path through a heating station and an oxidation station, preferably as disclosed in GB-A-2048309 or W093/24666 whose subject matter is incorporated herein by reference.

The invention in its fifth aspect is particularly suited to zero gas emission copper smelting using technically pure oxygen as the oxidising gas. Thus, in the fifth aspect of the present invention, fixation of sulphur as alkaline earth metal sulphate ( preferably anhydrite) is effected. This can be more advantageous than the procedures disclosed in W093/24666 in locations where there is a ready supply of alkaline earth metal carbonate such as dolomite or other reactive limestone and/or where it is not particularly economical to use the sulphur dioxide to form sulphuric acid. The anydrite or other alkaline earth metal sulphate is formed at a high temperature above the stability temperatures of other deleterious non-ferrous sulphates, when in the presence of excess alkaline earth metal oxide, so there is no risk of adventitious leaching of harmful metals from the sulphated material if such material is to be disposed of by dumping.

Oxidation of sulphidic sulphur to S0₂ and then its capture at elevated temperature as CaS0₄ gives considerably more energy to the process than oxidation to S0₂ by itself.

Provided that a supply of dolomite, dolomitic limestone or reactive limestone is available for a remote location copper smelter, probably located at the minesite, then a single step copper smelting based on melt circulation technology coupled with sulphur fixation at high temperature can be effected, resulting in unique advantages over all other existing copper smelting technology.

Smelting at the minesite is attractive because of the relatively lower non- ferrous metal content of copper concentrates (typically around 25%) compared with, say, zinc concentrates containing typically in excess of 50% zinc, means major savings in transport costs, allowing blister copper then to become the product for the international market place whilst ensuring greater value-added to the local mine operator.

Whilst the method according to said third aspect of the present invention with recirculation of the solid material may be employed for this purpose, in its simplest form, there may alternatively be employed a high temperature sulphation moving bed reactor having a once-through solids flow configuration under gravity with upper and lower sections separated with a solids flow valve (e.g. a star feeder) or downcomer filled with solids so that gas flow between the top and bottom sections is virtually prevented.

In the upper section, the hot S0₂-containing gases are contacted with calcined dolomite which has been calcined higher up in the upper section by countercurrent gas flow, whilst in the top of the upper region the crushed dolomite stone is preheated by the effluent gases as they pass up through the moving bed before leaving at the top to go to standard gas cleaning equipment before discharge to the atmosphere. Such an arrangement has an inherently high thermal efficiency.

In the lower section of the moving bed contactor, the hot stone, now comprised of anhydrous calcium sulphate (anhydrite), unreacted magnesia and rock components, is contacted countercurrently with air admitted at the base whilst leaving as preheated air ready for delivery to the top blow region of the melt circulation reactor, for example. This is a simple method of preheating the air for smelting to around 1 100K.

Incorporation of these features into the single step copper smelting process permits autogeneous operation without external fuel input for chalcopyritic copper concentrates and this is achieved without oxygen enrichment. The requirement for air separation or tonnage oxygen is eliminated in this approach to autothermal operation. At remote locations, this cooled anhydrite containing calcine emanating from the moving bed reactor system is likely not to have any significant commercial value. However, because it is environmentally benign it could be disposed of as back-fill to the limestone/dolomite quarry or to the mine itself.

Direct smelting with high level oxygen enrichment leads to a significant reduction in furnace and gas train dimensions and in general simplifies gas clean-up with consequent savings in capital cost. Also, if technically pure oxygen is used in sulphide smelting and no fuel combustion or air leakage is permitted, then by coupling the exhaust gas with high temperature sulphur fixation and recycling excess oxygen and sulphur dioxide back to the smelter, virtually zero gas emission can be achieved with all its attendant environmental benefits. Only a small gas discharge would be required from the smelter circuit to bleed-off accumulated nitrogen, associated with the input of technically pure oxygen, in an autothermal process fuelled solely by sulphide oxidation reactions.

Virtually zero gas emission smelting of non-ferrous metal sulphides is a highly desirable goal from an environmental viewpoint. Even after double contact sulphuric acid production, the normal gas effluents of current technology contain small quantities of acid mist and sulphur oxides which in the longer term may not be environmentally acceptable. These effluent gases are comprised normally of nitrogen and combustion gases which act as carriers for the contaminants already referred to. Clearly minimisation of gaseous emission or its virtual elimination permits even the most stringent environmental requirements to be met. Also, if in the very long term it becomes necessary to eliminate C0₂ emissions altogether because of global warming concerns, the carbon dioxide produced is uncontaminated with S0₂ or permanent gases and is therefore ideally suited to liquefaction, opening up the possibilities of its economic utilisation for enhanced oil recovery (EOR) or other environmentally friendly disposal options.

An air blown, single step copper smelter with integrated S0₂/dolomite sulphur fixation to supply the energy requirements in an environmentally acceptable way provides a low cost smelting unit of particular significance for remote locations and for relatively small scale operations at all locations.

The present invention will now be described in greater detail with reference to the accompanying drawings, in which :-

Fig 1 is a block flow diagram of an integrated gasification combined cycle (IGCC) for producing gas from coal for power generation, incorporating a zinc vapour desulphurisation process according to the present invention,

Fig 2 is an overall flow sheet showing part of the IGCC installation of Fig 1 in relation to a plant for conversion of zinc sulphide back to zinc metal whilst producing sulphur dioxide at high concentration and at elevated temperature,

Fig 3 shows schematically the non-catalytic conversion of S0₂ to S0₃ with enhanced heat recovery followed by sulphuric acid production with zero gas emission secured by virtually total gas recirculation back to the smelting furnace, Fig 4 illustrates schematically a plant for high intensity sulphur dioxide reaction with oxygen and dolomite (or limestone) to recover exothermic reaction heat and generate an environmentally benign calcined product for disposal,

Fig 5 shows schematically the configuration and approximate sizes of a desulphurisation module for a 250 MW_e IGCC incorporating a high momentum jet of zinc gas into which raw gas is entrained and where zinc sulphide particle growth occurs without contacting internal surfaces,

Fig 6 shows schematically the desulphurisation vessel of Fig 5 and alternative means for removing solid zinc compounds,

Fig 7 shows schematically an alternative desulphurisation module incorporating an irrigated moving bed desorber tower and a fixed bed absorber tower,

Fig 8 is a schematic view of another system according to the invention, employing fixed bed desorber tower,

Fig 9 shows schematically a two stage desulphurisation unit, incorporating the desulphurisation vessel of Fig 5 with sub-stoichiometric zinc addition connected in series with the desulphurisation module of Fig 7, and

Fig 10 shows schematically a desulphurisation vessel similar to that of Fig 5 connected directly to a moving bed absorber tower to remove the excess zinc added. Fig 1 is self-explanatory in terms of the various steps involved in the generation of desulphurised product gas from as-received coal. Briefly, as-received coal in storage region 100 is milled and dried at 102 and fed via a coal feed system 104 including lockhoppers to a gasifier 106 where it is gasified to form a gas which is partially cooled in cooler 108 before being passed into a hot gas clean-up region 1 10. In the hot gas clean-up region 1 10, the gas is passed through a cyclone 1 12 for removal of fly slag, a soda ash treatment unit 1 14 for sodium and halide removal, and finally through a desulphurisation reactor/separator unit 1 16 in which the process of the present invention is conducted. Although Fig 1 indicates gas/solid separation taking place at three different locations in the hot gas clean-up region 1 10, an alternative approach employing single gas/solid filtration can be employed as described previously.

Fig 2 shows in more detail the preferred way of regenerating zinc gas from the zinc sulphide generated during H₂S and COS removal. In Fig 2, zinc gas is shown entering the hot gas clean-up region 1 10 rather than making use of the enthalpy available in the hot gas to effect vaporisation of liquid zinc injected into the high pressure gas circuit. However, admission of zinc gas at a high pressure, self-generated by temperature control in an electrically heated system, is relatively straight forward and robust and is least likely to cause operational problems in the high pressure gasification circuit. In Fig 2, the gas to be desulphurised is passed into the reactor/separator unit 1 16 via a first inlet 1 18 to leave, after desulphurisation, via an outlet 120, whilst a jet of zinc gas is injected through a second inlet 122, and solid zinc sulphide separated from the gas stream is removed from the unit 1 16 via outlet 124. The inlets 1 18 and 122 are mutually perpendicular to promote effective intermixing of these materials without zinc gas and hydrogen sulphide having the opportunity to react on the walls of the unit 1 16. The zinc sulphide from the unit 1 16 is passed into an electric zinc smelter 126 of the type disclosed in US Patent No. 5358544 and W094/21832, wherein the zinc sulphide is oxidised to zinc and sulphur dioxide. The resultant zinc is cast into ingots and passed via a feeder 128 to a closed evaporator 130 to produce the zinc gas feed to the unit 1 16.

Referring now to Fig 3, the procedure illustrated therein relates to one which can be effected off-site using zinc sulphide solids resulting from the H₂S and COS removal step illustrated in Fig 1. In this procedure, the zinc sulphite solids are passed to melt circulation electric zinc smelter 126 of the type described and illustrated in the above-mentioned US Patent 5358544 and W094/21832 whose disclosure is incorporated herein by reference, to produce liquid zinc which is cast to form the zinc ingots referred to above. The sulphur dioxide produced in the smelter 126 together with excess oxygen from the top blow is passed into radiative oxygen preheater 136 used for preheating oxygen supplied for top blowing in the smelter 126. The sulphur dioxide then passes through a hot gas filter 138 and into a hot gas eductor 140 where it is reacted with excess oxygen to form sulphur trioxide. Waste heat from the resultant sulphur trioxide/oxygen mixture is recovered in boiler 142. The sulphur trioxide is dissolved in a recycled stream of sulphuric acid in absorber 144, whilst the now S0₃-free oxygen is passed into preheater 136 before being used for top blowing.

The zinc ingots are then transported back to the high pressure coal gasification plant to be melted and evaporated for re-use. The oxidation of sulphur dioxide produced in the zinc smelter to S0₃ and the subsequent conversion of the S0₃ to sulphuric acid are conducted as illustrated in Fig 3 and as described previously.

Referring now to Fig 4, dolomite is added to the top of a cylindrical pre¬ heating vessel 200 containing a packed bed of dolomite stone. Hot inert gas or C0₂ is passed via a line 201 from a cooler 202, in counter current fashion to the downward flow of stone as it moves under the action of gravity towards a transfer chute 204. The cooled inert gas or CO, is removed from the top of the preheater 200 via line 205. The flow of stone through the pre-heating vessel 200 is regulated by a feeding mechanism 206 which admits a controlled amount of the pre¬ heated stone into a calcination reactor 208. A moving packed bed of stone particles is established in the calcination reactor 208 with very hot sulphated stone via a downcomer or transfer chute 210 from a sulphation reactor 212 being uniformly dispersed throughout the cooler material so that sensible heat is transmitted from the material leaving sulphation reactor 212 to the endothermic sites of dolomite calcination reaction in reactor 208 to yield calcium and magnesium oxides and liberating hot C0₂. The hot C0₂ evolved is passed via a line 213 either to a waste heat boiler (not shown) for raising steam to supply some of the electricity for converting zinc sulphide back to zinc metal, or to line 201 to augment heat supplied to the preheater 200.

To sustain the overall thermal requirements of calcination, hot stone from the exothermic sulphation reactor 212 is circulated in a closed loop path through the sulphation reactor 212, the downcomer 210 and the calcination reactor 208. This is preferably done mechanically by use of a transfer system employing an insulated skip 214 movable between top and bottom positions which are both illustrated in Fig 4. Such transfer system can be operated automatically to accept stone from the calcination reactor 208 and then transport this vertically up to a charging location above the sulphation reactor 212 so that the cooled stone can be added back to the exothermic reaction regions in the sulphation reactor 212. S0₂ and 0₂ from the top blow section of the zinc smelter 126 illustrated in Fig 2 are added via line 213 to the bottom of the sulphation reactor 212 so that calcium oxide is reacted with the S0₂ to form the anhydrous calcium sulphate (anhydrite). The heat of reaction is captured by the circulating cooler stone added from the top. Excess S0₂/0₂ (plus any S0₃) are removed from the top of the sulphation reactor 212 via line 215 and are recycled back to the top blow region of the zinc smelter 126.

To ensure that the S0₂-0₂ gases do not mix with the C0_2/ the downcomer 210 is provided so as to ensure that very little gas exchange is allowed to occur between the calcination reactor 208 and the sulphation reactor 212. A small positive bleed of oxygen into the downcomer 210 is made via line 217 to prevent C0₂ from reaching the sulphation reactor 212, whilst a small escape of 0₂ to the C0₂ generated in the calcination reactor 208 is of little consequence. If necessary or desirable, to limit C0₂ emissions to atmosphere, the virtually pure C0₂ discharged is ideally suited to liquefaction or other techniques to eliminate gaseous C0₂ discharge. By keeping C0₂ away from the SO^O_j gases, excess S0₂/0₂ can be routed directly back via the line 215 to the top blow region of the zinc smelter 126 so that all of the positive benefits of virtually zero gas emission can be secured. The reaction-depleted stone passes via a transfer chute 216 into cooler 202 from where it is mechanically withdrawn via a gas-tight sealing mechanism attached to base 218 at such a rate that the solids in the sulphation reactor 212 are maintained at a relatively high level, whilst transfer chute 212 is kept full at all times. To prevent escape of S0₂- containing gases into the cooler 202 and to prevent hot inert gas or C0₂ from the cooler 202 entering the sulphation reactor 212, a small bleed of 0₂ is added via line 219 to the base of the transfer chute 216 so that it flows upwardly and downwardly in the transfer chute 216. This small 0₂ bleed, and the previously mentioned small 0₂ bleed via line 21 7 into the downcomer 210, contribute to the total oxygen requirement of the overall zinc smelting and sulphur fixation systems.

The above described system for the fixation of sulphur as anhydrite is also useful for fixing sulphur dioxide generated in a copper sulphide smelter as described previously.

Referring now to Fig 5 of the drawings, coal gas containing H₂S and COS enters a cylindrical, vertically disposed desulphurisation vessel 300 through a circumferential manifold 302 so that gas flow is distributed uniformly around the space between inner and outer walls 304 and 306 of the vessel 300. The inner wall 304 has a porous, perforated or mesh¬ like structure so as to permit radially inward flow of gas whereby to cause entrainment of the gas into a high momentum turbulent jet of zinc gas 308 which is ejected downwardly from a top inlet 310 symmetrically of the axis of the vessel 300. To effect desulphurisation for 250 MW_e generating capacity based on 1 % sulphur in coal, the throat of inlet 310 which ejects zinc gas at a rate of about 0.42 kg/s at 1700 K and just over 25 atmospheres total pressure needs to be around 1 cm in diameter. The discharge velocity is approximately 438m/s, corresponding to a velocity head loss of 1 1.6 atmospheres and a momentum of 184N. The radial inlet velocity over the entire inner porous wall 306 is about 0.22m/s with a momentum of around 9N, which is insignificant in comparison with the high momentum of the jet 308 of zinc gas. Desulphurized gas and zinc sulphide particles leave the vessel 300 via bottom outlet 31 1 to pass to a solid/gas separator (not shown).

A 500 MW_e desulphurisation module would require a throat diameter of 1.5 cm to admit 0.84 kg/s zinc gas at 1700 K and 25 atmospheres having a momentum of 327N, a velocity of 390 m/s and a velocity head loss of 9.1 atmospheres. The zinc addition rate in this case would be 3.02 t/h and, if all the zinc leaves the desulphurisation vessel 300 as zinc sulphide solid particles 10μm in diameter this corresponds to 5.91 x 10ⁿ particles per second which, for a one second residence time, is a total surface area of 185 m² at the end of the growth process. Particles of this size are suitable for collection by a hot gas cyclone (not shown) followed optionally by filtration as a final polishing step.

An even larger surface area for particle growth would be attained if, for example, the partially cleaned coal gas is seeded with, say, 2 μm zinc sulphide particles or perhaps another solid material readily available at the power generation site. If the seed particles subsequently grow to 5 μm diameter, the initial seed comprises only 6.4% of the final volume and, in this case, the number of particles for 500 MW_e from 1 % S in coal is 4.73 x 10¹² per second. Assuming that the solid particles move at the centre line velocity of the jet 308, the total residence time over a 6 m length is about 600 ms, during which time it can be predicted that desulphurisation would be approaching completion. However, to attain an even higher level of sulphur removal, it may be desirable to entrain some nitrogen from the air separation unit of the generating station into a lower zone 308' of the gas jet 308 illustrated in Fig 5 by having a separate compartment 312 for nitrogen addition downstream of the manifold 302 supplying the principal gas entrainment into the upstream jet. This cools the fuel gas and causes further precipitation of zinc sulphide particles and excess zinc.

For the first one metre of the jet 308, the residence time is less than 5 ms, during which time entrainment of coal gas increases the mass flow from 0.84 kg/s to about 14.3 kg/s. From heat balance considerations, this means that, if the entrained gas is at a temperature of 1000 K with the jet originally at 1700 K, the average bulk temperature will be about 1070 K after an initial 1 m distance travelled. The sudden quench thereby occurring can result in the possible risk of liquid zinc mist formation but since the time under supersaturation is very short, this is not considered serious. For each 1 m travelled downstream, the amount of H₂S and COS entrained into the jet 308 is approximately constant, but as the solid particles grow, the surface area thereof increases. As the gas velocity continues to decrease as the jet fluid and its entrained components travel away from the inlet 310, the residence time increases reaching about 150 ms for the region between 5 and 6 m downstream of the inlet 310. These two effects combined give progressively increasing extents of reaction for H₂S and COS within each 1 m section provided, of course, that there is an adequate excess of gaseous zinc available even though its concentration progressively decreases because of the combined effects of dilution and removal from the gas phase by reaction. To avoid problems of zinc aerosol formation, it may be necessary to modify the composition of the inlet gas by diluting the gaseous zinc with inert gas at high pressure, upstream of the inlet 310, so that under the conditions that prevail in the vessel 300, supersaturation of the gas phase with zinc does not occur. For example, calculations indicate that for a particular integrated gasification combined cycle plant, it may be necessary to dilute the zinc gas jet so that the mole ratio of nitrogen to gaseous zinc at the inlet 310 is 2:1.

Referring now to Fig 6, solid zinc sulphide which collects in the bottom of the vessel 300 passes out of the outlet 31 1a. Outlet 31 1 of the vessel 300 leads to a high-temperature cyclone 320 in which zinc sulphide particles carried by the desulphurised gas stream are deposited and subsequently removed. The cleaned fuel gas is at a relatively high temperature and contains a small but significant partial pressure of gaseous zinc which must be reduced to ensure that permissible levels of zinc at a gas turbine inlet (not shown) after combustion are not exceeded. This is achieved by passing the fuel gas through a waste heat boiler 322 which generates high pressure steam and cools the fuel gas to between 300 and 400°C, which is then suitable for hot gas filtration to remove the condensed zinc. This is more efficient thermally than quenching the lower region 308' of the gas stream with nitrogen as described with reference to Fig 5.

Referring now to Fig 7, a molten lead/zinc solution trickles down a packed desorber tower 324 under conditions safely removed from what is known as the flooding point and under temperature and composition conditions that preclude oxidation of the molten lead/zinc solution by the water vapour and carbon dioxide contents of the fuel gas stream, which flows upwards from near the base of the desorber tower 324. Gaseous zinc evaporates (desorbs) from the molten lead/zinc solution and reacts heterogeneously with (H₂S + COS) on the solid surfaces of packing elements 326 such as raschig rings, solid spheres other suitable material in a moving bed configuration. The solid packing elements 326 become progressively more and more accreted with zinc sulphide deposited from the gas phase but because gaseous zinc is being released throughout the entire height of the packed bed, the deposited solids themselves are distributed throughout the whole mass of packing rather than having an excessively large build-up of solids at the gas inlet 302 causing blockage and premature plant shutdown. The packing elements 326 are withdrawn continuously from the base of the desorber tower 324 into a recovery area 328 where the zinc sulphide is separated by vibratory means or tumbling action as in a ball mill by attrition followed by recovery of zinc sulphide using conventional dry mineral processing techniques. The cleaned packing is returned to the top of the desorber tower 324 to be re-used in the moving bed. The lead/zinc solution falls off the accreted packing in the recovery area 328 into a sump 330, from where a pump 332 recirculates it to the top of the desorber tower 324. It is highly desirable to conduct the removal, cleaning, solids separation and return of packing elements 326 within the high pressure region of the gasification circuit with only the zinc sulphide solids being discharged as a product at atmospheric pressure via appropriate pressure lock hopper devices. To facilitate the above it is desirable to base the packing elements 326 on a ferromagnetic material with a relatively high Curie temperature so that magnetic transport becomes a practical proposition after only moderate cooling, if corrosion conditions permit. The desulphurised gas stream leaving the desorber tower 324, still at a relatively high temperature and containing a small but significant partial pressure of gaseous zinc, next has its zinc content reduced. As described above, this can be achieved simply by cooling the gases using a waste heat boiler 322 followed by hot gas filtration. However, overall efficiency of combined cycle power generation, for example, can be enhanced by routing more of the enthalpy contained in the hot fuel gases to the gas turbine rather than to the steam circuit. Accordingly, a preferred approach is to cool the gas and remove the zinc content to a very low level under non-aerosol forming conditions by countercurrent absorption using a liquid metal solvent in a cooler/absorber tower 334. This is the physically reverse process of desorption of gaseous zinc in the desorber tower 324.

The cooler/absorber tower 334 is essentially a vertical tower with stationary packing elements, which may be of the same general design as employed in the desorber tower 324, but now not having such a high solids deposition load. As with the desorber tower 324, molten lead is circulated through the packing but in this case, gaseous zinc is absorbed into the molten lead. If an ultra-low residual zinc content in the gases proceeding to the gas turbine (not shown) is required, in-line refining the molten lead (now containing zinc) to very low zinc levels is effected by preferential oxidation, before a pump 336 returns it to the top of the absorber tower 334. This is readily achieved by oxygen injection into the zinc enriched molten lead in a sump 338. Dry zinc oxide solids are allowed to accumulate to such a depth above the molten lead surface that their removal to a storage hopper via a screw conveyor 340 is facilitated. The zinc oxide product is removed periodically via a pressure lock discharge system (not shown). At the temperature of the absorber tower, sodium chloride and other halide salts which may be present in the gas stream become accreted to the packing. The purity of the gas leaving the absorber tower may be further increased by addition to the packing of sodium carbonate, which reacts with any HCl gas which may be present in the fuel gas stream to form sodium chloride which in turn becomes accreted to the packing.

Referring now to Fig 8, an alternative approach to collecting deposited zinc sulphide, which does not rely on smooth operation of a slowly moving packed bed in the desorber tower 324, is to use a fixed bed of material which is sacrificial and at the end of its useful life is transported away from the power station site along with the adhering zinc sulphide solids for eventual recovery of metallic zinc. A preferred packing material comprises clean steel scrap in a shredded or fragmented state. Such a material can be arranged to form a packed bed having initially a very high void fraction, if appropriately selected and sized. The packing needs to be installed inside the desorber tower 324 in such a fashion that the bed of accreted material at the end of its service life, which could be a week or longer, can be readily removed. For example, the packing may be contained in removable baskets to ensure that individual packing elements are not "pinned' or cemented to the walls of the desorber tower making removal difficult and expensive. At the end of an operating campaign, the baskets and their contents are removed, preferably individually, and replaced with fresh baskets.

For fixed bed operation it is preferred to have a duplicate tower and its associated plant in parallel with that on-line and, for high temperature/high pressure gasification, this means that switching valves are provided downstream of the highest temperature regions, where conditions are less severe. After flow through the on-line desorber tower 324 has been stopped, and after its temperature has dropped to a predetermined sufficiently low level, the gas inlet can then be isolated from the high pressure gasification circuit and removal of the zinc sulphide accreted packing can then proceed. Prior to this the desorber tower 324 and its contents are blanketed with inert gas after safely displacing the fuel gas with inert gas and establishing, as an added precaution, a labyrinth-type seal with positive flow of inert gas under pressure back into the gasification circuit to ensure no gas leakage into the tower when it is being dismantled.

Recovering zinc metal from the zinc sulphide/steel scrap material is not a straightforward procedure for established zinc smelting plants which can normally only operate with zinc/iron input levels greater than about 5. At best the zinc/iron ratio in the accreted packing from desulphurisation is expected to be no higher than about 3. However, the direct zinc smelting process described in US Patent No 5,358,544 does not suffer from this limitation. It is in fact a positive advantage to have metallic iron in the feedstock for both in situ slag cleaning and energy conservation reasons.

In embodiments using the absorber tower 334, some zinc oxide dross will be deposited within the solid packing of the absorber tower 334, so the bed will require cleaning and possible replacement over a period of time. This can be undertaken conveniently at the same time as the upstream desorber tower is taken off-line, if the latter is of the fixed-bed type. It will be appreciated that the necessary primary control valves for switching the gas flow between the parallel streams comprising desorber and absorption tower assemblies will be located in the absorber tower gas offtake line, where conditions are very much less severe than upstream of the desorber tower itself.

The in situ generation of gaseous zinc as described above is particularly useful if the (H₂S + COS) content of the raw gas is relatively low such that the admission of zinc gas as a jet to entrain all the gas being treated is no longer a feasible proposition. On theoretical grounds it can be shown that there is a minimum amount of zinc required per unit mass of gas being treated and at low sulphur levels this zinc addition rate is not needed.

In Fig 9, the moving bed desorber tower 324 is sited downstream of the desulphurisation vessel 300 and high-temperature cyclone 320. This combined approach can enable problems stemming from uncontrolled solids deposition in the vessel 300 to be minimised. This is achieved by injecting into desulphurisation vessel 300 sub-stoichiometric levels of zinc so that the entrained gases achieve exceedingly low levels of zinc metal within the jet 308 before impacting on containment walls or surfaces. This, of course, is at the expense of residual levels of (H₂S + COS) being above specification limits after the jet contact. By operating in this mode, concerns about solids deposition on the downstream walls of the vessel 300 and within the hot gas cyclone 320 are completely eliminated. High levels of desulphurisation are then attained in the desorber tower 324, followed by removal of residual gaseous zinc in the absorber tower 334 as previously described.

Embodiments of the present invention not only achieve high levels of desulphurisation but also secure the added benefits of retention of harmful trace elements, such as selenium, tellurium, cadmium and arsenic, all of which either react with elemental zinc or are absorbed into the irrigating molten lead in the absorber tower 334. Both cadmium and arsenic can be extracted from the molten lead, for example, by selective oxidation after the zinc content is reduced to very low levels and in the case of arsenic, oxygen plus caustic soda addition is used commercially to remove arsenic to very low levels in refined market lead (< 0.0001 %). It follows therefore that absorption of arsenic from the gas phase into molten lead can be expected to be a very effective means of scrubbing out arsenic from fuel gases.

To secure low levels of mercury emissions is more difficult and would probably require further cooling of the fuel gas down to around 130°C. This can be achieved by absorbing mercury into a low melting point solvent such as the lead bismuth eutectic (MP = 128°C).

In ail embodiments involving the irrigated absorber tower 334, the heat removed can be effectively transferred back to the fuel gas stream in a direct contact liquid-metal irrigated packed bed unit functioning as a fuel gas reheater. Reheating to about 550 - 600°C can be achieved before the volatility of molten lead and bismuth or their alloys becomes a matter for possible concern at presently accepted emission or operational target specifications.

Very importantly, the liquid metal irrigated configurations shown in Figures 7 to 9 can be designed to virtually eliminate concerns about the possibility of aerosol formation when zinc containing gases are cooled so that the gas phase is supersaturated with zinc. If zinc aerosols are formed the very real danger exists that they could pass straight through downstream filters and find their way, for example, into power generating gas turbines.

In Fig 10, outlet 31 1 from desulphurisation vessel 300 discharges directly into an irrigated moving bed absorber tower 334. Zinc sulphide is separated from the packing in the same manner as was described for the desorber tower of Fig 7. The molten lead/zinc solution passes through a waste heat boiler 322 to a sump 336 where the zinc is preferentially oxidised and removed as previously described. The molten lead is then returned to the top of the absorber tower 334.

Claims

CLAIMS:

1 . A method of desulphurising a gas stream in a reactor, comprising the steps of reacting the gas stream at an elevated temperature with a gaseous reactive material which is capable of reacting with a sulphur-containing species in the gas stream to form a sulphurated reaction product which is solid at said elevated temperature; and separating the solid sulphurated reaction product from the resultant desulphurised gas stream.

2. A method according to claim 1 , wherein a reactive material is evaporated to produce the gaseous reactive material before it is introduced into the reactor.

3. A method according to claim 2, wherein the gaseous reactive material is introduced into the reactor in essentially pure form without dilution, or as an admixture with a gaseous, substantially non-reactive diluent.

4 A method according to any preceding claim, wherein the sulphurated reaction product is separated by deposition onto a solid packing.

5. A method according to claim 1 , wherein the reactive material is introduced into the reactor in liquid form, so that vaporisation occurs in the reactor to form the gaseous reactive material.

6. A method according to claim 5, wherein, the liquid reactive material is introduced into the reactor in admixture with a substantially non-reactive diluent.

7. A method according to claim 5 or 6, wherein the reacting step is conducted in the presence of a solid packing, through which the reactive material is arranged to pass, so as to deposit the solid sulphurated reaction product onto the solid packing.

8. A method according to claim 7, wherein the solid packing is circulated in a closed path which includes a station where the solid sulphurated reaction product adhering to the solid packing is removed.

9. A method as claimed in any of claims 2 to 4, wherein the desulphurised gas stream is further treated with gaseous reactive material produced by introducing reactive material in liquid form into a further reactor so that vaporisation occurs in the further reactor to form the gaseous reactive material which is reacted with residual-sulphur containing species in the desulphurised gas stream.

10. An apparatus for performing the method of any preceding claim, comprising a reactor having (i) a first inlet for a gas stream to be desulphurised, (ii) a second inlet for introducing a reactive material into the reactor, and (iii) a reaction zone arranged so that, in use, the gas stream is brought into contact with the gaseous reactive material so as to form a solid sulphurated reaction product; and a separator for separating the solid sulphurated reaction product from the desulphurised gas stream.

1 1. A method of desulphurising a gas stream containing sulphur dioxide, comprising the steps of feeding solid alkaline earth metal carbonate material to a calcination station, calcining the material in the calcination station, passing the calcined material to a sulphation station, contacting the calcined material in the sulphation station with the gas stream to be desulphurised so as to sulphate the calcined material, and removing the sulphated material from the sulphation station, wherein the calcination station and the sulphation are separate and carbon dioxide produced as a result of calcination of the alkaline earth metal carbonate material in the calcination station is withdrawn from the latter and prevented from entering the sulphation station.

12. A method according to claim 1 1 , wherein, the solid material is circulated through the sulphation station and the calcination station in a circulation path, with a proportion of the material being removed after sulphation.

13. A method according to claim 12, wherein the sulphated material which has been removed from the sulphation station is passed to a cooling station where it is cooled with gas which is thereby heated and used to pre-heat the solid alkaline earth metal carbonate material being fed to the calcination station.

14. A method of smelting a non-ferrous metal sulphide, comprising the steps of reacting the sulphide with oxygen to form the non-ferrous metal and a gas stream containing sulphur dioxide, and desulphurising the gas stream by means of the method according to any of claims 1 1 to 13.

15. A method according to claim 14, wherein the reacting step is effected by contacting the oxygen with a stream of molten non-ferrous metal sulphide which is forcibly circulated in a closed loop path through a heating station and an oxidation station.