These are all oxygen blown and from heat balance considerations there is no difficulty in producing a liquid slag at temperatures in the region of 1500°C. For air-blown gasifiers on the other hand single-pass slagging gasification systems are not yet commercially available.

Because air separation to generate oxygen for gasification is energy and capital intensive, there is still considerable interest in the development of improved air blown processes, particularly for use in advanced power generation. Also even with oxygen blowing the interface between liquid slag and the in-situ slag quench is known to be a source of operational problems. One object of the present invention is to overcome the shortcomings of both air-blown and oxygen-blown systems with respect to slagging.

It is well known that feeding a gasifier with a coal slurry as adopted by Texaco is advantageous from the viewpoints of operational reliability and availability. Savings in capital costs have also been reported when compared with solids feeding as in Shell and Prenflow, for example. However, there is an efficiency penalty in feeding a gasifier with a slurry of coal with water, which can simply be related to the latent heat of evaporation of the water content of the slurry. With all these systems there is a requirement for either or both fine grinding/crushing to a relatively fine size and partial drying of the as received coal. A second object of the present invention is therefore to define a novel method which overcomes the aforementioned problems with current technology relating to coal feed preparation and its feeding into a pressurized gasifier. The solution is to take as received coal and transport it directly into the gasifier without the need for pressure lock hoppers and the like and also to eliminate the need for crushing/grinding and partial drying external to the gasifier system itself.

Accordingly, the present invention provides a liquid metal transport gasifier system comprising:- (i) a pressurised gasifier having a gasification region at a first end region of the gasifier and a heat recovery region at a second end region of the gasifier, the gasifier housing a horizontal bath of liquid metal which is forcibly circulated from a first end of the bath towards its second end, (ii) a liquid metal fuel gas quencher, (iii) a fuel gas filter, (iv) a liquid metal fuel gas reheater, (v) means for introducing carbonaceous material to be gasified onto the surface of the molten metal at the first end of the bath, (vi) a liquid metal flowpath loop extending from an outlet in the bath down through the fuel gas reheater and fuel gas quencher and back into the bath towards its first end, (vii) a fuel gas flowpath extending from an outlet intermediate the gasification and heat recovery regions of the gasifier up through the fuel gas quencher whereby, in use to quench the fuel gas by a countercurrent of liquid metal, through the filter and up through the fuel gas reheater whereby to reheat the fuel gas by a counter current of liquid metal, and (viii) a flowpath for a mixture of steam and air extending from an inlet at the second end of the gasifier, through heat recovery tubes located in the heat recovery region of the gasifier and into top blowing lances located in the first end region of the gasifier, whereby in use, fuel gas is generated by combustion of carbonaceous material transported on the circulating molten metal through the gasification region of the gasifier, thereby producing a slag from which heat is extracted as it is transported on the molten metal through the heat recovery region of the gasifier prior to its removal from the gasifier as a glassy/solid slag.

Consider now the transportation of the coal through the gasifier whilst it undergoes the various stages of gasification. With the entrained flow and fluidised bed approaches this is not a problem provided the feed system delivers coal that is properly sized for the particular system. With moving bed gasifiers such as BGL there is extensive charge preparation involving briquetting/agglomeration followed then by the use of a mechanical stirring device within the bed to ensure smooth passage of solids down through the gasifier.

The new method now being proposed transports both fine and coarse solids as a layer floating on a liquid metal carrier medium in a simple uncomplicated fashion through a horizontal gasifier directly incorporating an in-line slag removal system, which is effectively an adaptation of the well established Float Glass Process, used commercially throughout the world for flat glass manufacture.

The Float Glass Process relies on a static bath of molten tin to support via an interaction of gravitational and surface tension forces a ribbon of molten glass on a liquid tin bath and as it progresses along the horizontal bath cools and solidifies to produce a perfectly flat sheet of plate glass of uniform thickness without surface imperfections. Since LAB Pilkington first published his account of this new technology in the Proceedings of the Royal Society of London (1969-1970) the process has revolutionised glass manufacture worldwide. In this context it is also relevant to note that a paper published in America in 1981 by Serth et al. (Energy Communications, 7 (2), 167) described a modified float glass process for the recovery of thermal energy from industrial slags.

Returning now to gasification of carbonaceous materials, the new process uses a shallow bath of molten tin or other suitable liquid metal such as lead or lead-bismuth eutectic to transport coal or other carbonaceous material, here-in-after referred to collectively as simply coal, through a zone of top blowing jets or transverse slots delivering a preheated mixture of air and steam to the floating coal layer, gasifying the carbon and generating enough heat for transforming mineral matter ultimately into a sheet of liquid slag. Unlike the float glass process the liquid metal bath is not static but rather is force circulated in an external closed loop comprised of heat recovery and hot gas cleanup functions.

Coal is admitted to the pressurised gasifier bath via a vertical column of liquid metal of sufficient height to balance the high pressure or a so-called barometric leg having its upper free surface terminating in a vessel with its liquid surface at substantially atmospheric pressure. Solids to be gasified are added to the top surface and drawn down and dispersed into the melt by a specially designed mechanical agitator adapted for the draw down of solids floating on a liquid. The solid material is entrained in the molten metal after having its moisture driven off possibly as it floats along a flash dryer which is effectively the return channel containing the circulating liquid metal at close to atmospheric pressure after release of system pressure by means of a barometric leg of molten metal. This low temperature in-line treatment dries the coal without loss of volatile matter so that it enters the gasifier in a preheated dry state.

Upon liquid metal and entrained coal entering the gasifier via the barometric leg, a simple phase disengagement arrangement overflows coal and a minor amount of associated liquid metal onto the surface of the moving metal bath proper. The bulk of the molten metal at around 240°C to 350°C after phase disengagement is returned to the external closed loop circulation as feed liquid for further entrainment of solids. The principal liquid metal circulation is derived from a liquid metal quench. This system cools the raw fuel gas from say 1550°C to around 250°C and simultaneously physically removes a very large proportion of the solid particulates by inertial deposition on the droplets and rivulets characteristic of nonwetting flow in packed beds irrigated with liquid metals. The principal liquid metal circulation through the gasifier itself is derived from the high temperature end of the liquid metal quench system and enters the gasifier at temperatures typically between 1150 and 1450°C for an air-blown system and closer to 1000°C for an oxygen blown system. In turn the floating solid layer, semi carbonized coke/char, perhaps a raft of sintered carbonaceous material and finally a slag ribbon are all transported along the length of the gasifier top blown zone in association with the flow of liquid metal from feed to discharge end of the horizontal bath.

The flowing liquid metal is withdrawn at an intermediate point towards the remote end from the feed but leaving enough room for what is effectively a static bath of molten metal forming an in-line float glass chamber. It is here that an adaptation of float glass technology is used to lift the solidified slag ribbon off the liquid metal bath. Heat has to be removed from the metal bath to cool the liquid slag so that it solidifies and, for example, this can be achieved advantageously by radiative heat transfer to a serpentine arrangement of air/steam preheater tubes or other suitable configuration located in the gas space above the slag layer to either top-up the preheat conducted elsewhere or to provide most of the preheat dictated by heat balance considerations.

It is important to note that the liquid metal flowing through the gasifier is not involved in thermal energy transfer or heat recovery in the gasification process itself but is there merely to support and transport the solid materials, as rafts and slag through the array of top blowing lances to effect gasification.

The design of the top blowing lances is critically important to the success of the whole system. They must induce the correct amount of entrainment by momentum transfer between the jet and its surrounding gaseous environment so that after reaction in the gas phase the oxygen potential of the gas on impingement is below that required to oxidize the liquid metal bath but still sufficiently high to ensure rapid gasification of carbon. Clearly, residual free oxygen must be avoided and the oxidising components of the gas effecting gasification must be comprised principally of carbon dioxide and water at levels as high as possible without causing oxidation of the liquid metal bath.

The liquid metal itself is protected from oxidation so long as carbonised material remains floating as a coherent layer on the liquid metal/gas interface, thereby precluding access of the gaseous oxidants other than in reduced form. Accordingly, not all the feed is gasified in a single pass or alternatively, a protective carbon-containing hearth layer is deposited on the liquid metal surface in advance of the material to be gasified. The bulk of this carbon- containing material is recycled after physical separation from the solid slag eventually issuing from the gasifier.

Notwithstanding the precautions taken above to avoid oxidation of the liquid metal bath, some dissolution of atomic oxygen into the bath is bound to occur at the very high temperatures involved. Where the liquid metal is tin, volatile stannous oxide SnO gas will be present in the fuel gas leaving the gasifier. The extent to which this occurs can be reduced by introducing a non-oxidising purge gas to screen the metal surface once a molten slag layer is formed. However, as the gases pass through the liquid metal quench, SnO (g) becomes unstable as the temperature falls and elemental tin is redeposited preferentially on the copious shower of molten tin irrigating the moving packed bed rather than forming a homogeneously nucleated aerosol and is thus removed down to negligibly low levels by the time the fuel gas is filtered at around 250°C.

The dimensions of the bath required to achieve gasification of a given material at a required rate and establishment of a stable layer of liquid slag floating along on the surface of the molten metal can be readily evaluated by those skilled in the art, on the assumption that the top-blowing reactions are principally controlled by gaseous diffusion. Once gasification is effectively completed, the bath temperature will return to around 1450°C provided the air/steam is preheated to an appropriate temperature as determined by heat balance considerations. This preheating of the air/steam mixture is undertaken before admission to the top-blow manifold and is effected by radiant heat exchange between the liquid slag and metal surfaces using ceramic materials in combination with heat resisting alloy tubes in the cooler regions. The liquid metal bath is cooled to around 800°C and the slag solidifies under these conditions and is removed as in the commercial float glass process. Molten metal leaves the bath typically at about 800°C in order to irrigate a packed bed countercurrent to the upward flow of fuel gas, which is reheated from 250°C to 750°C or perhaps hotter by direct contact with the liquid metal irrigant. The upper temperature depends on the chemical composition of the fuel gas and thus its propensity to promote the formation of volatile stannous oxide.

The fuel gas is now in a condition for advanced power generation duties, provided the sulphur content of the solid fuel material being gasified is relatively low. With most coals, petroleum cokes and similar materials, however, steps will need to be taken to desulphurise the fuel gas at perhaps 250°C or 750°C, whichever is the more appropriate using existing fuel gas desulphurisation technology or alternatively, conventional flue gas desulphurisation can be utilised to meet increasingly stringent emissions regulations.

However, the preferred choice in the present case is to utilise the liquid metal transport medium as the means for desulphurisation. For example, molten tin containing dissolved sulphur at low thermodynamic activity will dissolve gaseous tin sulphide as well as reacting with both hydrogen sulphide H2S and carbonyl sulphide COS. In all cases, elemental sulphur dissolves in the molten tin with dissolution continuing so long as the sulphur activity in the melt is less than the sulphur activity in the gas phase. In other words, desulphurisation can progress under low melt sulphur activity conditions even at very high temperatures. At 1450°C, for example, the principal gaseous sulphur component in contact with molten tin is SnS gas rather than H2S or COS and provided the thermodynamics permit, sulphur is transferred from the fuel gas at 1450°C by what is effectively a gas absorption process. The key issue is how to maintain a low enough sulphur activity in the molten tin to permit sulphur absorption to occur. This forms the substance of the third object of the present invention.

Liquid metals have the ability to remove sulphur without cooling from gases, in particular a fuel gas obtained by gasification of coal or petroleum residues, to extremely low levels.

For example, the gas produced by injecting coal in the powdered form into a molten iron bath is typically of the order of 100 to 600ppmv of H2S + COS but as pointed out in US Patent 4,852, 995 this level of desulphurisation is insufficient for certain uses of the gas. A higher level of desulphurisation can be obtained by the methods described in US Patent No 3,690, 808 for removing sulphur compounds by reacting combustion gases with molten copper or as described in US Patent No 3,954, 938 for treating reducing gases without cooling with a melt of molten lead and metal sulphide.

An account has also been published in the literature by workers in Germany at the University of Karlsruhe (K. Hedden, B. R. Rao and F. Reitz, Proc. 1986 Int. Gas Research Conf. , Toronto, 1124, (1986) ) alluding to both tin and lead as suitable wash liquids but indicating their clear preference for tin, except in those cases where the manufactured gas has higher ratios of steam/hydrogen and carbon dioxide/carbon monoxide because of the possibility of oxidising tin. The Karlsruhe workers refer to separation of formed solid sulphide from the liquid metal using density differences. The sulphide is then converted to oxide and elemental sulphur followed by reduction of the oxide back to the metal, which is recycled back to the gas/liquid scrubber along with the bulk of the liquid metal. Clearly, the bulk of the molten tin or molten lead in these cases is saturated with the metal sulphide when it is recycled back to the desulphurisation reactor. This simple fact places a severe limitation on the level of sulphur removal which can be obtained by such treatment.

For heat resisting alloys after long term exposure, it is known that even a chromia layer fails in an oxidising-sulphidising environment. Furthermore, the phenomenon of internal sulphidisation can occur by one or more of three mechanisms, general internal sulphidisation, preferential attack along grain boundaries and a mixture of these mechanisms.

In the context of hot gas cleanup (HGCU), the best possible way to alleviate sulphidisation of heat resisting alloys containing chromium is not to rely on kinetic constraints but rather to employ advanced desulphurisation technology, which for the temperature levels where internal sulphidisation is potentially a problem, reduces sulphur to such low levels that chromium sulphide is thermodynamically unstable. This is not attainable by the liquid metal processes already referred to but this breakthrough can be secured by the present invention.

From what has already been stated it should be clear that sulphur removal in advanced power generation needs to be improved by at least an order of magnitude above current practice in HGCU. To achieve this using liquid metals demands very high dilution of sulphur in the liquid metal. Attention must no longer be focused on solid chemical compounds such as SnS or PbS as being involved in the separation of sulphur from the liquid metal in the course of its regeneration to restore its absorptive capacity. The emphasis must be changed to physical solution of sulphur in the liquid metal and the reversibility that this implies. In contra-distinction to the teachings of US Patent No 4, 308, 037 the end result of contacting a fuel gas with liquid metal is not the formation of a product which is non-volatile such as stated categorically in Column 4, lines 19-20. In fact at the higher temperatures envisaged in the present invention, gaseous tin sulphide is more stable in fuel gas than is either hydrogen sulphide or carbonyl sulphide, but the stability is reversed over the range of temperatures involved in the higher temperature liquid metal quench of the present invention.

The reversibility of physical absorption permits both absorption and desorption of sulphur depending entirely on the concentration levels in both the liquid metal and the associated gas phase. It is this ability to remove in the vicinity of 99.99 percent of sulphur in the liquid metal absorbent with relative ease that opens up the exciting scenarios already referred to that are directly responsible for potential attainment of higher cycle thermal efficiency than other competing coal-based processes for advanced power generation.

It is recognised that the goal of sulphur removal to a level that is low enough to preclude chromium sulphide formation is unlikely to be achieved directly in the fuel gas itself so a new approach is needed. The key to the new method is to desulphurise the liquid metal used to cool the raw fuel gas to an exceedingly low level and then re-contact it countercurrently with hydrogen, helium or other suitable gas or gas-mixture with exceptionally good heat transfer characteristics in a closed loop circulation containing a network of conventional heat exchangers for fuel gas reheating, steam generation, steam reheating and gasifier air/steam preheating, for example. It is this network of heat exchangers that will be protected against internal sulphidisation and with only a small addition of oxygen, steam or carbon dioxide to the hydrogen or other carrier gas in the closed loop, stable chromia scales can be assured without the deleterious effects of sulphur.

The associated high level desulphurisation of the liquid metal prior to its countercurrent contacting in the hydrogen (fuel gas) circuit, for example, can be achieved in a number of ways which will be familiar to those skilled in the art. In the particular case of coal gasification in the slagging ash mode, the temperature of the liquid metal (probably tin) will be in the region of 1450°C or even higher so that desorption of volatile SnS or PbS into an appropriate strip gas at reduced pressure is seen to be a preferred method, although at lower temperatures addition of metallic zinc to precipitate out a zinc sulphide could be advantageous.

US Patent No 4,852, 995 and US Patent No 5,538, 703 have identified zinc vapour or its precursors as effective agents for desulphurising coal-derived and similar fuel gases.

However, there are a number of inherent disadvantages which have collectively contributed to the lack of commercial use of metallic zinc for desulphurisation purposes. The product of reaction between zinc and volatile sulphur compounds is zinc sulphide and invariably some zinc sulphide will nucleate and grow on associated solid particulate matter in advance of filtration, which results in a heavily contaminated zinc sulphide product and depending on the type of gasifier, carbonaceous char or soot is also contaminated with zinc sulphide. The subsequent treatment of these process intermediates introduces problems that are best avoided if at all possible.

Also with metallic zinc addition to a raw fuel gas at high temperature, there is always the attendant risk of aerosol formation as the gas is cooled for cleanup prior to its admission to the combustor of a gas turbine. Such aerosols can penetrate conventional filters and potentially there is a danger that carryover of zinc to the turbine will exceed specification limits designed to ameliorate deposition of zinc compounds on the turbine blades and elsewhere within the turbine. In addition, there is the possibility that zinc oxide could pass through the turbine and be emitted to the atmosphere in quantities exceeding increasingly stringent environmental regulations.

Notwithstanding the disadvantages referred to above, there still exists a strong incentive for the development of an effective metallic zinc-based desulphurisation process. In advanced power generation, for example, the electric power company could install straightforward technology in which bought in zinc metal is transformed to a zinc sulphide product, which is environmentally benign and stored readily prior to its return to a zinc smelter. The transaction could involve the payment of a treatment charge as already established worldwide within the non-ferrous metallurgical industry in return for supply of refined zinc ready for re-use. This reduces to an absolute minimum the chemical processing to be undertaken by the power company and contrasts markedly with the complex technology more at home in a petroleum refinery that currently dominates gasification power projects.

A fourth object of the present invention is to introduce improved methods for using metallic zinc which do not suffer from the disadvantages already discussed whilst securing the positive attributes. It calls for total isolation of zinc from the fuel gas itself.

The sulphur compounds in the fuel gas are caused to react at a gas/liquid metal interface to permit elemental sulphur to be absorbed countercurrently into a stream of liquid metal initially unsaturated with respect to sulphur. An appropriate gas/liquid contactor is used for this purpose such as the moving packed bed type described in the Report, Coal R127 entitled"Zinc-based Clean Technology for Desulphurisation in Advanced Power Generation"published in 1997 as part of the Department of Trade and Industry's Coal Research Development Programme. This provides a means for simultaneously absorbing sulphur and transferring heat from the hot raw fuel gas into the liquid metal coolant and transporting the solid packing elements down through the packed bed and then by entrainment in the liquid metal elevating them to a cleaning station to dislodge accreted solid particulates scrubbed from the gas by the fine droplets and rivulets characteristic of non-wetting liquid metal irrigation of a packed bed. Since"air-lift"action is used to transport the liquid metal and its associated individual solid packing elements there are no mechanical moving parts so the method is ideally suited to operation at high temperatures in aggressive environments. The liquid metal stream containing dissolved sulphur discharges from the liquid metal quench normally somewhere between 1000 and 1450°C depending on the particular flowsheet adopted and flows into the front end of the liquid metal transport gasifier ready to accept the solids to be gasified and float them along under the top blow jets of air/steam in to effect gasification. After passing through the gasification zone the molten metal is then subjected to a reduced pressure or vacuum, -preferably in the presence of a strip gas, which may be either inert or chemically reactive, so that sulphur or a volatile sulphide is desorbed from solution in the liquid metal and the so-treated liquid metal then continues its progress in the gasification/HGCU circuit. In this case the strip gas containing the sulphur or sulphur compounds is later contacted with metallic zinc to form a zinc sulphide product in one of the various ways outlined in the Report, Coal R127, except that in the present invention the zinc is added non-intrusively, quite independent of the principal fuel gas circuit and therefore totally avoiding any possible carryover of zinc or its compounds to the gas turbine.

In the gasification scheme previously described, the temperature of the liquid tin as it progresses along the transport gasifier returns to a maximum temperature, 1450°C in the example quoted. If the tin flow at this juncture is exposed to a vacuum strip or contacted countercurrently with hydrogen at reduced pressure, sulphur will be desorbed from the melt into the gas phase and the melt will be desulphurised to a very high level. Accordingly, when the cooled fuel gas is reheated by direct contact with the desulphurised metal at around 800°C, the sulphur content of the fuel gas will be exceedingly low and could be rightly classified as superclean.

The fifth object of the present invention is to provide a means for the containment of submicron aerosols without sacrificing cycle efficiency in advanced power generation.

In relation to coal combustion in direct-fired gas turbine cycles concern has been expressed in the literature about the small fraction of the coal ash, increasing with increasing temperature, which is vaporized and forms a submicron aerosol upon condensation as the gas cools down and deposits on turbine blades. Thus, for example, Pressurized Bed Combustion (PFBC) is currently a focus for further development because it is recognized that the use of advanced turbine designs will require improved hot gas cleanup and a topping cycle to increase turbine inlet temperatures. According to the developers of advanced PFBC the lower temperatures employed in fluidised bed combustors alleviate concerns about submicron aerosol formation and makes PFBC the preferred choice over pulverized coal combustion for direct fired gas turbine application. The fundamental reasoning here has widespread implications elsewhere in advanced power generation and if substantiated this could well preclude the use of hot gas cleanup (HGCP) for cycles employing slagging gasifiers in general. This would be of lesser importance for oxygen blown system, but clearly HGCP is required for air blown cycles to avoid a serious reduction in cycle efficiency.

If, however, the fuel gas can be cooled down to say around 240°C without condensation of its water vapour content, filtration with conventional fabric filters is feasible at this temperature for removing residual particulates remaining after the relatively efficient liquid metal scrub and quench in the hot gas cleanup circuit. As a further precaution, if so required, the fuel gas could be passed through a high efficiency particulate filter system as used, for example, in radioactive waste vitrification processes and the like. Therefore, it can be concluded that the submicron aerosol problem can be dealt with using existing technology at low temperature, so the real issue to be confronted is the development of an effective means for reheating the fuel gas from around 240°C back to higher temperatures.

If cycle efficiency is not to be sacrificed, this fuel gas reheating has to be accomplished without electric power or additional fuel consumption.

A fuel gas reheat of 900°C or even hotter cannot be achieved at the upper temperature level using conventional metallic materials. Therefore a ceramic heat exchanger is needed or the teachings of US Patent No 4,308, 037 could be adapted for this service. A preferred means to achieve the highest temperature level is to use a liquid metal irrigated packed bed continuously supplied with ultra-low sulfur liquid metal so the fuel gas is not contaminated with sulfur compounds or other volatile pollutants. In the particular case of Integrated Gasification Combined Cycle power generation, what must be avoided, however, is simple heat exchange between the hot liquid metal and the relatively cold fuel gas as illustrated in US Patent 4,308, 037. This would de-grade the high level heat available making it unavailable for other heat input requirements at high temperature levels which are mandatory if the full potential of the advanced circuit is to be realized and high cycle efficiency secured. The principal requirements for attaining these important objectives have been identified. In summary, it is desirable to incorporate in the flowsheet some or all of the following components: 1. A liquid metal transport gasifier or other appropriate slagging gasifier 2. A high temperature liquid metal quench of the raw fuel gas incorporating liquid metal- based desulphurisation with liquid metal exiting desulphurisation at a very high temperature, typically around 1450°C 3. Desulphurisation of the liquid metal to produce a high temperature stream of ultra-low sulfur content liquid metal 4. High intensity heat transfer between the dispersed liquid metal and the fuel gas to effect in a countercurrent fashion, a second stage of reheating up to a temperature level of 900°C or possibly even higher for strongly reducing fuel gases without constraints relating to volatile stannous oxide formation after the initial reheat described below.

5. Dispersed countercurrent contact of the hot liquid metal with hydrogen in a closed loop recirculating configuration consisting of a network of conventional metallic heat exchangers arranged in parallel to effect supercritical steam superheating, high pressure steam reheating, air/steam preheating for the gasifier and the first stage of fuel gas reheating. Next in series would be intermediate pressure steam generation and then a hydrogen boost compressor to return the hydrogen back to the liquid metal countercurrent contactor to complete the closed loop circulation.

Special steps have to be taken to ensure that the metallic heat exchangers referred to above are not likely to be corroded by contaminants in the gas phase or fouled by deposition of entrained solid particulates rendering heat transfer inefficient and requiring difficult and expensive maintenance and repair. Thus although US Patent No. 4,308, 037 has identified one means for transferring heat between two gas streams using a liquid metal and it is desirable to utilise certain aspects of such teachings, the alternative and improved system now proposed ensures that the cycle efficiency and operability are maximised.

A sixth object of the present invention is to provide means for the removal of mercury and trace elements from coal-derived or other manufactured fuel gases using liquid metal scrubbing and vacuum evaporative distillation at high temperature.

US Patent No 4, 308, 037 discloses a method for removing pollutants from combustion/gasification-derived fuel gases by scrubbing the raw fuel gas with a liquid metal in which the pollutant gaseous species are at least to some extent soluble in the liquid metal. Clearly, volatile elements such as mercury, arsenic, cadmium and lead should all respond to such treatment. One difficulty in making use of this procedure in practice stems from the necessity that continued re-use of the liquid metal by recycling as a scrubbing agents depends critically on the ability to regenerate the liquid metal back to its original absorptive capacity.

The present invention provides the means for either reduced pressure gaseous stripping or vacuum evaporative distillation for the simultaneous recovery of elemental mercury and regeneration of mercury-lean liquid metal for recycling without expending a large amount of fuel or electricity in bringing the liquid metal up to an appropriately high temperature so that equipment size and capital cost are kept to a minimum. It is also vitally important that the mercury is recovered in the elemental state without excessive cross-contamination or the generation of solid/liquid residues which if produced would clearly be regarded as hazardous and would entail considerable costs in disposal.

To achieve the worthwhile objectives just referred to requires gas absorption at relatively low temperature of mercury and other trace metal contaminants such as arsenic, cadmium and lead into a refined liquid metal feed of a closed loop liquid metal circuit, operating over a range of liquid metal temperatures differing by many hundreds of degrees. Mercury is absorbed in a low temperature scrubber along with sulphur, lead, arsenic and cadmium.

At the higher temperature end of the closed liquid metal circuit, the increased temperature of the mercury contaminated liquid metal imparts sufficient volatility of mercury for it to be capable of high level separation from the relatively non-volatile metal stream, for example, by vacuum evaporative distillation using a strip gas which is either inert or chemically reactive depending on the particular circumstances.

For coal and petroleum coke gasification in the slagging mode, the higher temperature gas/liquid contacting, liquid metal Scrub A, is undertaken in the region of 1500°C. Besides mercury and sulphur this would also desorb arsenic, lead, cadmium and other pollutants.

In the lower temperature liquid metal Scrub B the fuel gas is cooled from around 1500°C to 250°C-140°C depending on whether a pure liquid metal or eutectic mixture or alloy is being used as the solvent. Whatever the choice, the same liquid metal must be used for the separate Scrubs A and B. In one preferred embodiment using liquid tin as the scrubbing agent, Scrub A is fed with molten tin at about 1450°C containing dissolved sulphur, mercury and the other"air toxic"elements already mentioned and Scrub B is fed with highly refined or"superclean"tin at 240°C, for example.

By the means just described the fuel gas is cooled in Scrub B down to around 250°C for tin as the scrubbing liquid but the thermal efficiency need not suffer adversely by this procedure provided of course that the fuel gas is efficiently reheated before admission to the gas turbine combustor without using extra fuel or consuming excessive electrical power in the process. The means for securing this vitally important attribute have been discussed previously in this document. The key requirement is the ability to transfer heat back to the fuel gas at high intensity. This can be achieved using a combination comprised of conventional shell and tube heat exchangers with hydrogen as the thermal carrier gas, heated by direct dispersed contact with the hot liquid metal at high temperature and by direct contact of the partially reheated fuel gas with ultra-low sulfur content dispersed liquid metal at the higher temperature end. Alternatively, the fuel gas is simply reheated only by direct contact with high temperature ultra-low sulphur content tin. By these means a fuel gas reheat to 900°C or even higher can be achieved without concerns about materials of construction or re-contamination of the fuel gas.

Particular examples of the invention will now be given as applied to the gasification of carbonaceous solid materials in order to produce fuel gas for electricity generation using gas turbines, either standing alone or in combination with steam turbines, in what is known as Integrated Gasification Combined Cycle (IGCC), with reference to Fig. l, Fig. 2 and Fig.

3.

Fig. 1 is a schematic general arrangement in elevation of the basic plant for liquid metal transport gasification.

Fig. 2 is a schematic general arrangement in elevation of a liquid metal transport gasification plant for IGCC with low sulphur and low mercury-containing feedstocks, where efficiency gains are sought by employing ultra-supercritical steam conditions.

Fig. 3 is a schematic general arrangement of a liquid metal transport gasification plant employing a steam injected gas turbine (STIG), requiring only sub-critical steam but demanding ultra-high level elimination of sulphur, mercury and other"air-toxic" emissions.

Referring now to Fig. 1, at 1 as received crushed coal, petroleum coke, biomass etc. , are added to the top surface of a launder returning molten tin from a liquid metal direct contact fuel gas reheater 2 via a pump 3. Moisture in the feed material is flashed off before the solids are entrained into a barometric leg 4 of liquid tin using a specially designed mechanical agitator 5 capable of drawing down floating solids. A simple phase separation device 6 overflows 7 the charge solids onto the moving surface 8 of a liquid tin bath. The solids float downstream under an array of top blowing lances 9, supplied with a mixture of steam and air, preheated by radiation from the molten slag/molten tin surface using a serpentine or other appropriate arrangement of ceramic or heat resisting alloy tubes 10, depending on the temperature level of the preheat dictated by the overall heat balance and the entry temperature of the steam/air mixture at 11. Raw fuel gas leaves the gasifier via a refractory lined flue 14 and flows into the lower region of a liquid metal irrigated moving packed bed 12 or multi-functional tower. This tower performs a number of important duties, including liquid metal quenching of the hot raw fuel gas, cleaning the gas of solid particulates via inertial scrubbing involving small droplets and rivulets of molten tin associated with non-wetting flow of a liquid metal through a packed bed of relatively larger solids. Typically, the fuel gas is cooled from around 1500°C to 250°C during the countercurrent contact operation. The cool gas is filtered at 250°C or perhaps a lower temperature using a conventional filter 13, before passing into the fuel gas reheater 2, which involves countercurrent direct contacting of fuel gas and liquid tin to give preheated fuel gas 15 ready for transmission to the combustors of a gas turbine, provided the feed material is very low in sulphur and contains negligible mercury of other"air-toxic" elements. The mineral matter or ash associated with the feed initially forms a liquid ribbon of molten slag, analogous to float glass manufacture, which then solidifies into a plate of glassy slag 28, which is extracted from the pressurised gasifier vessel 16 using a adaptation of the established glass making technology.

Referring now to Fig. 2, the liquid metal transport gasifier is identical to that already described in Fig. 1, so only the new plant items will be detailed for the special case of a readily available charge material containing virtually no sulphur or mercury or other"air toxics". Maybe energy taxes related to COs emissions per kWh of electricity generated demand that attention be focused on high cycle efficiency with increased capital costs of lesser importance. IGCC with supercritical steam conditions is then the preferred option.

However, the heat recovery steam generator (HRSG) associated with conventional plant does not normally provide high enough temperatures to make ultra-supercritical steam conditions a realistic option. With a liquid metal transport gasifier, there is considerable scope for altering the basic flowsheet to accommodate this requirement. For example, Fig.

2 shows the final fuel gas reheater 2 topping-up the fuel gas temperature after an initial reheat as part of a Heat Exchanger Network (HEN) 17. HEN is a major constituent of a closed loop hydrogen circuit employing direct contact heat exchange at high intensity between the molten tin exiting the second stage fuel gas reheater and a recirculated hydrogen carrier gas. This approach eliminates concerns about the corrosive nature of molten tin towards metallic materials required for effective heat transfer and facilitates higher steam temperatures than normally considered for IGCC and thus leads to higher combined cycle efficiencies. The principal new plant items involved in addition to HEN are a packed tower or direct heat exchange column 18, a hydrogen turbo-booster 19 and probably an intermediate pressure (IP) steam, generator 20 to supply the steam requirements for the gasifier. The individual heat exchangers of generally conventional design constituting HEN 17 may include, for example, a first stage fuel gas reheater, an air/steam preheater, a supercritical steam heater to enhance the temperature of supercritical steam initially generated in the HRSG and a heater to provide a significant contribution to the total steam reheat thermal load.

It is also worth noting that contrary to what is shown in Fig. 2, it may be desirable to operate the hydrogen heat exchange circuit with very hot tin, immediately it leaves the gasifier. This would eliminate concerns about the formation of volatile SnO, if the fuel gas is not highly reducing and would mean that the direct contact of fuel gas with tin is avoided at the higher temperature levels. However, the practicality of this approach would probably be contingent upon the availability of reliable ceramic heat exchanger technology.

Referring now to Fig. 3, except for the necessity to maintain a low sulphur atmosphere, if metallic tubing is used for air/steam preheating within the gasifier, by compartmentalising the gas in this region and adding a purge of low sulphur fuel gas, the liquid metal transport gasifier is identical with that already described in Fig. 1 so only the new plant items will be detailed. These are a direct liquid metal irrigated contactor or reduced pressure desorber 21, purged with recycled hydrogen gas, a liquid metal scrub or reduced pressure quench tower 22, in which zinc-based desulphurisation of the purged gas is conducted along the lines proposed in the Report Coal 127, which involves adding molten zinc or tin-zinc eutectic to an independent recirculating loop of liquid tin to balance the stochiometric requirement as zinc is desorbed from the liquid metal and reacts with the sulphur gases carried in the purge gas to form zinc sulphide (ZnS), deposited preferentially on a moving bed of ceramic balls, which are circulated by entrainment in the irrigating liquid tin. ZnS deposits are removed mechanically and or by thermal shock to provide a ZnS product for export to a zinc smelter, which returns zinc ingots (or molten zinc if nearby) based on the payment of a treatment charge. This practice is well established in the primary zinc industry. The molten tin leaving the multi-functional tower 22 is cooled in a direct contact tin cooler 23 using a closed loop hydrogen circuit, employing a gas/gas heat exchanger 24 and a hydrogen turbo-booster 25. The main hydrogen strip gas circuit is serviced by a vacuum or reduced pressure booster set 26 operating over the pressure range of 20mm Hg or thereabouts to approximately 1 atm. Compression of the purged gas plus cooling results in deposition of elemental mercury in the condenser 27. Mercury removal in this fashion is a major positive attribute and the cost savings thus secured must be taken properly into account.

A flowpath 30 of sulphur-containing molten tin extends from the downstream end of the gasification zone 16a to the desorber 21 and a flowpath 32 returns ultra-low sulphur tin back into the gasifier a short distance downstream of the sulphur- containing tin flowpath 30. A broad crested weir (not shown) is provided between the two tin flowpaths 30,32. Although there is a thin layer of tin over the weir, the tin bath 8 is effectively divided into two: a sulphur containing bath upstream of the weir in the gasifiction zone 16a and an ultra low sulphur bath downstream of the weir. The floating liquid slag sheet is pushed forward across the weir by the ram effect of the advancing raft of solid charge material floating on the melt and itself moving forward by virtue of the drag force on the solid charge raft exerted by the liquid metal advancing. The net force forward results from the ram force exerted by the upstream solid raft over a substantial length of the melt bath minus the smaller drag force between the two liquid layers (i. e. the liquid slag sheet and very thin layer of the liquid metal bath over the weir over which only a small amount of tin is actually returning back into the gasification zone bath in order to prevent sulphur-containing tin advancing into the ultra-low sulphur (ULS) bath. This means that a minor amount of tin has to be withdrawn at the feed end to balance not only the flow over the weir but also tin which is entrained with the solid feed after phase separation following entry via the barometric leg arrangement.

The other concern is that gaseous sulphur compounds (e. g. H2S/tin sulphide gas) will contaminate the atmosphere above the ULS bath and therefore sulphidise the metallic radiant tube air/steam preheater so in addition to the weir referred to above, some baffling will be required in the gas space above the melt to prevent the two gas atmospheres intermixing and possibly a small amount of desulphurised gas will be needed to purge the interface area between the raw gas atmosphere on one side and the ULS gas atmosphere on the other.

Finally, there is the issue of what happens to the"air toxics"desorbed from the main circuit circulation of molten tin in the packed desorption tower at very high temperature and reduced pressure. Arsenic, lead, cadmium and other trace elements will build up in concentration in the secondary molten tin loop servicing the towers 22 and 23. These concentrations will require monitoring and the tin will need periodic replenishment with refined tin, again a treatment charge with a secondary smelting company is envisaged and the possibility of a contractual arrangement with a satellite smelter company across the fence from the power generator may be a viable option. The philosophy here is that it is very much better to contain"air toxics"under controlled conditions rather than dispersing them at high dilution into the surrounding ecosystem.

For gasification and hot gas cleanup, clearly many different scenarios exist and each will need detailed evaluation to find the most cost-effective solution. The three embodiments depicted in Fig. 1, Fig. 2 and Fig. 3 are just three examples. Other challenges may exist to which the methodology proposed in this document could provide an effective solution.

The invention of the liquid metal transport gasifier could be of paramount significance in the future.