US3870480A

US3870480A - Process and apparatus for the production of combustible gases

Info

Publication number: US3870480A
Application number: US234764A
Authority: US
Inventors: Gerald Moss
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1971-03-19
Filing date: 1972-03-15
Publication date: 1975-03-11
Anticipated expiration: 1992-03-11
Also published as: GB1384711A

Abstract

Substantially sulphur-free fuel gas is produced from coal by gasification of particles of coal with a steam/oxygen mixture at an upstream end of a bed of fluidized particles containing an alkaline earth metal oxide, sulphur from the coal being fixed in the particles as alkaline earth metal sulphides. The bed particles travel generally downstream towards a regenerator, and at the downstream end, oil is injected into the bed and gasified to compensate for the reduced coal concentration at the downstream end whereby to avoid dilution of the fuel gas. Bed material from the downstream end is contacted with oxygen or air to convert sulphides to oxides, SO2 being liberated and recovered and the oxide-containing regenerated particles are returned to upstream end of the bed.

Description

United States Patent Moss PROCESS AND APPARATUS FOR THE PRODUCTION OF COMBUSTIBLE GASES lnventor: Gerald Moss, Oxford, England Exxon Research and Engineering Company, Linden, NJ.

Filed: Mar. 15, 1972 Appl. No.: 234,764

Assignee:

Foreign Application Priority Data Mar. 19, 1971 Great Britain 7310/71 11.5. C1 48/71, 48/63, 48/98, 48/128, 48/197 R, 48/200, 48/206, 423/244 1111. c1. Cl0j 3/16 Field 61 Search 48/200, 98, 99, 101, 206, 48/63, 64, 197 R, 71, 72, 73, 62 R, 89, 128; 423/242, 244

References Cited UNITED STATES PATENTS 7/1932 Baufre 48/200 7/1951 Schutte 48/197 R 4/1969 Grantham 423/242 12/1969 Squires 201/17 FOREIGN PATENTS OR APPLICATIONS 1,183,937 3/1970 Great Britain Primary Examiner-S. Leon Bashore Assistant Examiner-Peter F. Kratz Attorney, Agent, or Firm-Reuben Miller [57] ABSTRACT Substantially sulphur-free fuel gas is produced from coal by gasification of particles of coal with a steamloxygen mixture at an upstream end of a bed of fluidized particles containing an alkaline earth metal oxide, sulphur from the coal being fixed in the particles as alkaline earth metal sulphides. The bed particles travel generally downstream towards a regenerator, and at the downstream end, oil is injected into the bed and gasified to compensate for the reduced coal concentration at the downstream end whereby to avoid dilution of the fuel gas. Bed material from the downstream end is contacted with oxygen or air to convert sulphides to oxides, S0 being liberated and recovered and the oxide-containing regenerated particles are returned to upstream end of the bed.

21 Claims, 4 Drawing Figures SHEET 1 pg 2 EUY EFHEUHARI 1 5 1/ II VIII/II FIG. I.

PROCESS AND APPARATUS FOR THE PRODUCTION OF COMBUSTIBLE GASES The present invention relates to the production of combustible gases, and more particularly to the production of substantially sulphur-free combustible gases from sulphur-containing solid or semi-solid carbonaceous material such as the various types of coals, lignites, peats and some shales hereinafter termed, for brevity only, coal.

The specifications of U.K. Pat. No. 1,183,937 and U.K. Pat. No. 1,336,563 described methods and means for converting sulphur-containing fuels, such as hydrocarbon oils and coals to (inter alia) substantially sulphur-free combustible gases. In one mode of making such combustible gases according to those specifications, the fuel is injected into a reactor containing a bed of particles comprising alkaline earth metal oxide(s), such as calcium oxide, or a thermal precursor of alkaline earth metal oxide(s), such as limestone or dole mite, the particles being fluidized in an upwardly moving stream of free oxygen containing gas, such as air, wherein the relative proportions of oxygen and fuel are such that the fuel is partially combusted and thereby converted to combustible gases, the sulphur in the fuel reacting with the alkaline earth metal oxide of the particles to form solid compound(s) comprising alkaline earth metal and sulphur (e.g. calcium sulphide) whereby the conbustible gases leaving the reactor are substantially free of sulphur. Particles are transferred continuously or intermittently from the reactor (preferably from the region of the top of the fluidized bed therein) to a regenerator containing a bed of particles which are contacted by, and preferably fluidized by, an upwardly moving stream of an oxidizing gas. The solid compounds comprising sulphur and alkaline earth metal(s) are converted mainly to alkaline earth metal oxide(s) with the release of concentrated streams of sulphur dioxide which can be utilised for the manufacture of sulphur products, such as sulphuric acid or for conversion to elemental sulphur. The particles entering the regenerator from the reactor preferably do so at a bottom zone of the regenerator bed, and particles are returned to the reactor bed (preferably to a bottom zone thereof) from the regenerator bed (preferably from the region of the top of the regenerator bed) to maintain the inventory of alkaline earth metal oxide(s) in the reactor bed. Optionally, a small bleed of particles from the regenerator and a corresponding make-up of fresh particles maintains the sulphur-removing activity of the reactor bed. In the reactor bed, temperatures are preferably maintained in the range 800950C and in the regenerator, from lO-l 150C.

In the regenerator, the various oxidizable materials which may be present as a result of the reactions in the reactor bed are involved in competitive reactions for the available oxygen, and in these competitive reactions, carbon laid down on the particles is oxidized before the alkaline earth metal(s) --sulphur compound(s) are oxidized. Thus, if the latter are to be successfully oxidized so as to regenerate alkaline earth metal oxide(s) and to release sulphur, e.g. as sulphur dioxide, either the carbon content of the particles transferred to the regenerator must be very low or the residence time of the particles in the regenerator must be sufficiently long for all the carbon to be oxidized. The latter course is to be avoided since the carbon thus oxidized represents lost heat value from the combustible gases, the carbon oxides dilute the sulphur dioxide thereby reducing its value, and the sizes of the regenerator bed, the ducting and the gas circulatory fans must be increased to handle the increased volumes of gas passed into, through and out of, the regenerator.

The reactions which occur in the reactor are cracking of the fuel to hydrogen, and hydrocarbons of lower carbon to hydrogen ratio than the original fuel and to carbon, and some of the products of the cracking reaction are oxidized at least in part to produce the combustible gases which leave the reactor. The cracking reactions are endothermic, and it is the heat of reaction of the partial oxidation which maintains the tempera ture in the reactor sufficiently high for the production of combustible gases, or gasification to proceed continuously.

It has now been found that most of the heat produced in the reactor is from oxidation of carbon rather than oxidation of hydrogen: thus, when a hydrocarbon fuel oil is gasified in the reactor with 25 percent of the oxygen, supplied as air, required for stoichiometric combustion of the fuel oil, about 44 percent of the carbon of the fuel is oxidized whilst only about 2.5 percent of the hydrogen is oxidized. Under these conditions, the carbon is oxidized to approximately equal molar quantities of CO and CO and these gases form about 10 vol. percent of the combustible gases.

It has been found that the preferential oxidation of carbon occurs even when the carbon content of the reactor bed is very low. The reactor beds when used for conversion of hydrocarbon oils to combustible gases, as described in UK. Pat. No. specifications 1,183,937 and 1,336,563, typically contain about 0.3 wt. percent of carbon which is predominantly disposed on the surface of the particles of the reactor bed so that it is readily amenable for oxidation. In preferred modes of preparing combustible gases, the oxygen containing gas is passed into the bottom of the reactor bed through a suitable gas distributor. It has been found that substantially all of the oxygen is utilised very shortly after entering the bed, that is to say, the bulk of the combustion of carbon and other combustible materials occurs in the immediate vicinity of the distributor.

The U.K. Pat. specification Nos. 1,183,937 and 1,336,563 mention, en passant, that coal (as hereinbefore defined) may be converted into combustible gases employing the invention described in those specifications. Coal is of relatively high carbon to hydrogen ratio, and generally speaking, of such physical and chemical constitution, that under the conditions prevailing in the reactor, the coal is initially coked on entering the reactor, the amount of oxidation being relatively small. The coke produced is mainly in the form of discrete particles which distribute themselves by transverse diffusion through the bed. The coke represents about wt. percent of the original coal (if ash is ignored) the remaining 30 percent leaving the bed as hydrocarbon gases, hydrogen, and partially and fully oxidized gaseous products. In view of the fact that in the regenerator, carbon is oxidized in preference to alkaline earth metal sulphide(s) as hereinbefore discussed, and in view of the wastefulness of oxidizing carbon in the regenerator and the commensurate difficulty of temperature regulation therein, it is highly desirable to remove substantially all the carbon or coke produced in the reactor before particles from the reactor are taken off to the regenerator. However, since the amount of coke remaining in the bed after coal pyrolysis is high, the amount of oxygen for the coke gasification would need to be about 50 percent of stoichiometric, so that if air were the source of oxygen, a relatively large reactor would be necessary. Moreover, the heat released by oxidation of the coke could be so high as to present problems of temperature control in the reactor. Another problem which has been encountered with coal usage is that the coked coal particles are not substantially different in size to the original coal particles, and other considerations apart, the oxidation of relatively large carbon masses requires a long residence time in the reactor, and hence a further increase in reactor size in addition to increases for other reasons. This is in marked contrast to the relative ease of oxidation of carbon produced by part combustion of flowable hydrocarbons (e.g. oils, residua, tars), since, with these latter materials, the carbon is almost wholly laid down as a thin coating on the hot particles of the reactor bed, and the thin carbon coating is readily oxidizable as hereinbefore mentioned.

In accordance with one aspect, the invention comprises a method of producing a substantially suplhur free combustible gas from sulphur-containing coal (as herein defined) which comprises introducing sulphurcontaining coal into an upstream region of a reactor bed of particles containing at least one alkaline earth metal oxide (or thermal precursor thereof) at a temperature of 800C to l025C, the particles being fluidized by an oxygen-containing gas, the oxygen content of which is between percent and 50 percent of the amount required for stoichiometric combustion of the coal whereby sulphur in the coal combines with the alkaline earth metal in the particles as alkaline earth metal sulphide and the coal is converted to substantially sulphur-free combustible gas, recovering the substantially sulphur-free combustible gas, particles being transferred from a downstream region of the reactor bed to a regenerator bed wherein said transferred particles are contacted at a temperature in the range of about l000C to 1 150C with an oxygen-containing gas whereby alkaline earth metal sulphide(s) is or are converted to alkaline earth metal oxide(s) with the evolution of sulphur dioxide, and returning particles from the regenerator to the upstream region of the reactor bed, a hydrocarbon oil (as herein defined) being injected into the reactor bed in the said downstream region and steam being provided in the oxygencontaining gas entering at least upstream of the region of oil injection.

In accordance with another aspect of the invention, there is provided apparatus for producing substantially sulphur-free combustible gas from sulphur-containing coal, comprising a reactor vessel having an upstream end and a downstream end, a bed of particles containing at least one alkaline earth metal oxide or precursor of such oxide, a gas distributor supporting said bed, means for monitoring the bed temperature at the downstream end of the bed, means for supplying a hydrocarbon oil to the region of said downstream end, means for regulating the rate of oil supply to maintain the bed temperature within the range of about 800 1025C, means for supplying particulate sulphur-containing coal to said bed upstream of the region to which oil is supplied, means for supplying steam to said bed at least to regions where, during operation, devolatized coal will be present, means responsive to the rate of supply of coal to regulate the rate of supply of steam, means for monitoring the temperature or mean temperature of the reactor bed, means for supplying an oxygencontaining gas underneath said distributor for fluidizing the contents of the reactor bed, means for supplying an inert gas underneath said distributor for diluting the oxygen-containing gas in the bed, first valve means for regulating the rate of supply of oxygen-containing gas, second valve means for regulating the rate of supply of said inert gas, said second valve means being responsive to the bed temperature to close the second valve means progressively as the bed temperature falls towards about 800C, and to open the second valve means progressively as the bed temperature increases towards about 1,025C whereby to maintain the bed temperature between about 800C and about l025C, means responsive to the rate of supply of gas(es) to the reactor bed to progressively close the first valve means as the gas supply rate approaches a selected upper limit and to open the first valve means as the gas supply rate approaches a selected lower limit, whereby to maintain particles in the bed in a substantially selected state of fluidization, a regenerator vessel having an upstream end and a downstream end, a gas distributor in said regenerator vessel, means for transferring'particles from an upper region of the downstream end of the particle bed in the reactor vessel to a region adjacent to the distributor of the regenerator vessel at the upstream end thereof, means for transferring particles from an upper region of the downstream end of the regenerator bed to a region adjacent to the distributor at the upstream end of the reactor vessel, means responsive to the temperature of particles in the regenerator to reduce the rate of particle transfer progressively when the temperature in the regenerator approaches about l,l50C and to increase the rate of particle transfer when the temperature approaches about l000C, means for supplying an oxygen-containing gas at a regulatable rate beneath the regenerator distributor for contact with particles above the distributor, means for monitoring at least one constituent of exhaust gases leaving the re generator vessel, said constituent being chosen from oxygen and S0 and means responsive to signals from the monitoring means for reducing the rate of supply of oxygen-containing gas to the regenerator vessel when the oxygen content of the exhaust gases exceeds a maximum selected concentration and/or the S0 content is lower than a minimum selected concentration.

Preferably, the coal is de-ashed by any convenient process (preferred types de-ashing processes are described in UK. Pat. specifications Nos. 1041547 and 1 123597) whereby ash and non-coal materials of a size comparable with the size of the coal particles is removed, and it is preferred that the coal is substantially dry before it is introduced in the reactor. The coal should preferably be in particulate form and the particule size should be of about the same order as the size of the particles containing the alkaline earth metal compound: preferred sizes are from 1% inch to l/32 inch, or predominantly in this range.

During the performance of the invention, the coal particles are introduced into the upstream end of the reactor bed and the temperature is sufficiently elevated to cause volatilizable material to be liberated, leaving behind in the bed coke particles of substantially the same size as the originating coal particles, and which include most of the inherent ash which was not previously removed in the de-ashing process.

Some of the liberated volatiles may be cracked at the bed temperatures, leaving a thin carbon layer on the surface of the bed particles from which the energy for cracking is supplied, and the remaining volatile material escapes from the reactor bed and is recovered with the combustible gases generated in the reactor bed.

Depending on the source and type of coal, the sulphur content on a dry de-ashed basis can vary from as little as 0.5 wt. percent up to 7 wt. percent or higher, but most coals have a sulphur content of about 2-3 wt. percent.

During the pyrolysis of the coal particules, 50 percent to 70 percent of the sulphur is retained in the coke particles, while the carbon formed by cracking of crackable volatiles will contain 85-95 wt. percent of the volatiles sulphur. In addition, non-crackable sulphur-containing volatile compounds tend to react with the alkaline earth metal oxide(s) of the reactor bed particles if there is sufficient residence time in the bed to form alkaline earth metal sulphides, so that the proportion of original coal particle sulphur retained in the reactor bed may be of the order of 90 wt. percent.

Since reactor bed material is transferred to the regenerator at the downstream end of the reactor and returned at the upstream end after regeneration, it will be appreciated that in addition to the vertical components of bed particle circulation, there will be a flow or current of reactor bed material from the upstream to the downstream end of the reactor bed.

After and/or simultaneously with the endothermic coal pyrolysis at the upstream end, the resulting coke particles and carbon coated bed particles are subjected to the action of the oxygen of the oxygen-containing gas (which may be air) supplied in substoichiometric quantities, preferably about 30 percent to 40 percent of the oxygen for complete combustion, and for most coals, about 35 percent stoichiometric oxygen. This provides for the oxidation of some coke carbon substantially wholly to CO with little CO produced and without generating a temperature which is so high in the reactor that sulphur from the oxidized coke cannot be retained and fixed in the reactor bed particles as sulphides. Any carbon deposited on the reactor bed particles from cracked volatiles would be in the form of a thin coat on each particle and such carbon is readily converted to CO, any sulphur being retained as sulphide by the alkaline earth metal. At temperatures exceeding l,025C, sulphide-containing particles which have circulated to the base of the bed pick up oxygen to form sulphates, and these interact with sulphides in the bed to release S0 according to this type of reaction scheme:

3CaSO CaS -*4CaO 480 and at such temperatures, much of the sulphur thus initially released in the bed as S0 will tend to escape out of the bed as elemental sulphur, COS, H 8 and other reduced sulphur products. At raised operating pressures and/or with increased bed depths, this tendency will be reduced.

At temperatures of 1,025C and lower, the release of sulphur is reduced and is relatively insignificant at temperatures of about 1,000C, the preferred operating temperature although the method of the invention is workable down to temperatures of about 800C.

In order to provide that all of the coke particles are oxidized as far as possible without employing an unduly large reactor bed so that a large coke residence time is avoided, and to improve temperature control, steam is injected at least into those regions of the bed containing pyrolyzed coal. The steam reacts efficiently in the fluidized bed at 1,000C or thereabouts according to the endothermic water-gas reaction:

thus improving the calorific value of the gas leaving the bed.

As the coke particles move with the bed particles in the direction of the downstream end of the reactor, they are progressively reduced in size, and eventually are entrained or elutriated out of the reactor bed (together with any ash and attrited bed particles). The region at which the coke concentration is so small that the fluidizing gas virtually passes through the bed without significantly reacting with coke depends on the lengthwise diffusion rate of the reactor bed particles. the length of the reactor bed and the superficial velocity of gas upwards through the bed.

The dimensions of the bed are so chosen that for a bed particle size in, or predominantly in, a selected range, and a coal particle size of similar range, and with coal of known or determinable pyrolysis and combustion characteristics in fluidized beds, the coke burn-out and elutriation region should be just upstream of the downstream end of the reactor bed.

The coke particle burn-out and elutriation must take place as fully as possible to ensure that substantially no competition between carbon and alkaline earth metal sulphide occurs subsequently in the regenerator. A flowable hydrocarbon oil, which may contain sulphur, is injected into the downstream end region of coke burnout and elutriation where it is partially combusted and converted to combustible gas, any sulphur in the oil being fixed in the particles as alkaline earth metal sulphides. The oil-derived combustible gas maintains the quality of the combustible gas leaving the downstream end of the reactor at about the same calorific value per unit volume or the same Wobbe index as the combustible gas produced upstream of the oil injection region. The preferred oils will usually have a high carbon to hydrogen ratio (e.g. cat. cycle oil, residues, fuel oils) and the part combustion will lead to carbon deposition on the surface of the bed particles. The amount of carbon may be from 0.3 wt. percent up to 10 wt. percent of the bed weight in the coke burn-out and elutriation zone, but since the carbon derived from oilcracking is very amenable to attack by oxygen or oxidizing atmospheres, the bed particles at the downstream end of the bed at the off-take of the regenerator will be sufficiently free of carbon for regeneration to proceed without substantial problems of temperature control and competition for oxygen by the carbon and sulphides.

In order to ensure efficient gasification of the oil, it is preferred to inject the oil into the bed. Oils with very low C/l-l ratios may be injected relatively near the top surface of the bed while heavy oils must be injected low down in the bed. As a general rule, however, it is preferred to inject all oils a few inches above the distributor l to 3 inches is a suitable height for injection while about 2 inches is preferred. This will permit all the required gasification and sulphur fixing reactions to occur below the top layer of the bed which particles overflow or splash into the transfer conduit to the regenerator.

Although steam may be provided in the coke gasiflcation zone only, it may be convenient in practice to mix the steam and oxygen-containing gas, e.g. air, beneath the distributor of the reactor bed so that a steamlair mixture passes into all parts of the bed. Having regard to the exothermic heat of reaction of oxygen with carbon and the endothermic heat of reaction of steam with carbon, it will be appreciated that the minimum mol ratio of oxygen to steam should be of the order of 2.0. Moreover, since the amount of steam required will be proportional to the bed carbon content and therefore substantially proportional to the coal input, a simple control system may be adopted to supply steam at a rate substantially proportional to the coal supply rate.

The coal may be scattered over the surface of the reactor bed or it may be injected within the bed, the distributor beneath the coal introduction point or points having fluidization gas holes of such size distribution that the coal particles are vertically distributed in the reactor bed.

ln order to provide for turn down or reduced output capability of the reactor, the reactor bed temperature is monitored at one or more places and a bed temperature signal generated to regulate the supply of oxygen containing gas, sincewhen less combustible gas is required, less coal is introduced, and correspondingly, less steam, so that in the absence of any control, the ratio of oxygen to carbon in the bed will increase causing a temperature rise. The temperature signal causes the oxygen-containing gas supply rate to be decreased with increasing temperature and increased with decreasing temperature.

A potential problem arising from the temperaturecontrol expedient is that the total amount ofgas passing through the reactor bed is decreased with decreasing gas demand, and the decrease may lead to defluidization of the reactor bed while coal is being introduced at a relatively low rate. To avoid this possibility, which would lead to difficulties in re-fluidizing the bed and to removing carbon and tar accretions in the bed, the rate of gas flow to the bed is preferably monitored and when the gas flow reaches a predetermined lower limit, a valve is opened to cause the mixing with the gas passing to the bed of an inert gas: most conveniently, the inert gas may be a flue gas derived from the combustion of the substantially sulphur-free combustion gases leaving the reactor bed.

The gases leaving the reactor bed will contain a certain amount of fine solids elutriated from the bed. The fine solids will usually be mainly ash particles containing coal or coal residues, and may amount to about 6 wt. percent of the total of coal introduced to the reactor. In addition, some of the finer bed particles will be elutriated, but this is not likely to be more than 2 wt. percent of the coal if care is taken to reduce attrition to the lowest possible level.

If the reactor is operated to produce substantially sulphur-free combustible gases for use in turbines or other engines or applications where the gases must be solidsfree, the elutriated solids may be eliminated by the use of conventional hot cyclones and precipitators. Alternatively, the solids may be trapped by the system described in UK. Pat. No. 1,280,374 and the combustibles returned to the reactor.

In the regenerator, the substantially carbon-free particles containing alkaline earth metal sulphide(s) are initially distributed above the distributor and an oxygen-containing gas passed therethrough. The sulphide(s) are oxidized either directly to oxide(s), e.g.,

tained at from 1,000C and l,l50C, since below 1,000C, regeneration proceeds inefficiently and above about 1,150C, the S-fixing properties of the particles tend to be destroyed. Temperature control depends on the rate of contact of oxygen with sulphide containing particles; it is preferred to vary the throughput of particles through the regenerator 'for temperature control rather than to vary the oxygen containing gas throughput. Thus one or more temperature monitors generate a signal representative of the regenerator bed temperature, and the signal is employed to control suitable means for effecting the transfer of material between the reactor and regenerator.

Gases leaving the regenerator bed may contain, in theory, the same concentration of SO as the entering oxygen-containing gas, e.g. about'ZO percent of the latter is air. However, it is preferred to employ at least a slight excess of oxygen-containing gas so that the S0 containing effluent is somewhat lean e.g. 10 vol. percent, so that there is some surety that complete regeneration of the particles is being effected. The effluent gases are preferably monitored for one or both of O and S0 and if the concentration of 0 should rise, or the S0 fall, a signal indicative of this change is relayed to a valve which regulates the rate of supply of oxygencontaining gas to reduce the gas supply rate. Similarly, if the S0 concentration rises or the 0 concentration falls in the effluent gases, the effluent gas monitor generates a signal causing the oxygen-containing gas supply regulating valve to open further.

Any solids entrained with the regenerator effluent gases will be of small size and of low sulphur content, and are preferably separated from the gases and rejected.

In order to ensure that the S-fixing ability of the particles in the reactor is substantially maintained, the amount of sulphur in the combustible gas produced therein, or in flue gas resulting from the combustion thereof is monitored. If the amount of sulphur is excessive, fresh particles containing alkaline earth oxide or carbonate is added to the reactor bed, and a purge or bleed of equivalent weight removed from the regenerator.

The S0 effluent gases are at a high temperature and concentration any may be converted to sulphur acid, or may be reduced to elemental sulphur, for example by the process taught in the specifications of US. Pat. No. 3,810,972.

The alkaline earth metal oxides may be contained in or derived from lime, limestone, dolomite and calcined colmite. Limestone is preferred for its high" attritionresistance. A small proportion of silica in theparticles sometimes tends to retain sodium and vanadium in the particles (if these are present in the coal) while chlorine, which is a common impurity in coal, is retained by the alkaline earth metal oxide.

The invention will now be described by way of a nonlimitative example and with reference to the accompanying drawings in which:

FIG. 1 is a schematic longitudinal sectional diagram illustrating the principle of the invention,

FIG. 2 is a transverse cross-sectional view of a boiler according to the invention,

FIG. 3 is a longitudinal plan view of a reactor and regenerator in the boiler of FIG. 2, looking down from the plane of line III-III, and

FIG. 4 is a cross-sectional elevation of the reactor and regenerator taken on 'IV-IV of FIG. 3.

In FIG. 1, the reactor is indicated by 10, and contains a bed 11 of limestone particles supported on a gas distributor 12. The bed is fluidized by an oxygencontaining gas which may also contain steam and inerts, such as flue gas, which are passed via pipes 13 into the space between the reactor base 14 and the distributor 12. Combustible substantially sulphur free gases are generated in the bed 11 and leave the reactor via ducting 15.

At one end (the downstream end) of the reactor, a transfer pipe 16 transfers sulphide-containing particles from the top of the bed 11 to a regenerator vessel 17 wherein the particles form a bed 18 supported on a gas distributor 19: the transferred particles enter the vessel 17 just above the distributor 19. The bed 18 is fluidixed by oxygen-containing gases supplied through pipe 20, and the sulphides are oxidized to alkaline earth metal oxides, S being liberated. Regenerated particles are transferred from the top of the bed 18 via transfer pipe 21 to just above the distributor 12 at the upstream end of the reactor for further use, while the So -containing effluent gases from vessel 17 passes via a duct 22 to a cyclone separator 23, fine solids being rejected via line 24 and the sO -containing gases recovered for further processing via conduit 25.

Coal, which has been particulated to about the size of the bed particles and freed of ash as far as possible, is introduced onto or into the upstream end of the end 11 from a gas-tight feed system 26. At the downstream end of the bed 11 where the pyrolysed coal has been burned to the size of elutriatable fines, oil is injected in the bed 11 from an injector 27.

Referring now to FIG. 3, it will be seen that the upstream end and the downstream end of the reactor are adjacent to each other, but separated by a dividing wall 28 (which extends higher than the bed 11). The wall 28 also extends into the regenerator 17 so that the regenerator 17 has an upstream end adjacent the downstream end of the reactor 10, and a downstream end adjacent the upstream end of the reactor 10. As depicted, material in the reactor and regenerator flows clockwise, there being transfer tubes 16 from the top of the reactor bed 11 to the bottom of the regenerator bed 18, and transfer tubes 21 from the top of the regenerator bed 18 to the bottom of the reactor bed 11.

Coal particles are thrown onto the bed 11 from gastight screw feeders supplied from coal hoppers 26 towards the upstream end of the'bed 11. The coal pyrolyses and is gasified with a mixture of air, steam and flue gas as hereinafter explained, and elutriatable coke fines remain in the bed 11 as the downstream end is approached. In the region where coke fines tend to be elutriated in significant quantities, oil is injected into the bed from injectors 27, thereby serving to avoid dilution of the combustible gas by the gases passing through the bed 11, since the oil is itself gasifled.

Sulphur from the coal and oil is fixed as calcium sulphide in the bed particles, and at the downstream end of the bed 11, the particles are substantially free of carbon due to the reaction with oxygen and steam. Referring now to FIG. 2, the reactor 11 is shown incorporated into a stream raising furnace and boiler.

Air is induced via trunking 44 by a fan 45 and passed via a gas flow-meter 46 into line 13 from whence it passes through the distributor 12 into the bed 11.

As hereinbefore explained, steam is also passed into the bed 11 at a rate which is substantially proportional to the rate of coal feed. The steam is supplied from line 48, and a coal feed rate monitor 49 of any type controls the steam feed via a signal line 50, a transducer 51 and an actuator 52 to operate the steam valve 53. The quantity of steam will usually be about 10 vol. percent of the quantity of air.

The rate of injection of oil at the downstream end of the bed 1! depends on the local bed temperature as sensed by the monitor 73. When the temperature rises towards 1,025C, a transducer 74 generates a signal which increases the rate of oil injection from injection from injection pump 27, the cracking and gasiflcation of oil being endothermic. Similarly, if the local temperature should fall much below 950C, the rate of oil injection is decreased.

Temperature control elsewhere in the bed 11 depends on the relative amounts of steam and air and coal, and a number of temperature monitors 60 (only one is depicted) are located in the bed 11 and operate via a signal line 61 and a transducer 62 to control an actuator 63 for an inert gas regulating value 64. If the temperature in bed 11 rises towards 1,025C, the valve 64 is opened, and inert gas from the line 65, and which, in this embodiment is conveniently flue gas, is passed via fan 45, meter 46 and line 13 into the bed 11. If the temperature falls in bed 11, the fall is detected by the monitors 60, and the valve 64 is closed.

It is preferredthat the rate of gas flow through the bed 11 be maintained within selected limits to maintain the bed in a fluidized condition, and to prevent excessive elutriation. Variations in gas flow rate are detected by the meter 46, and signals representative of such variations are transmitted via line 47 to a-transducer and actuator of the air valve in air trunking 44. Thus, as the flue gas valve 64 opens, the air valve tends to shut, and the net effect is to maintain a relatively constant temperature and state of fluidization in bed 11. The reactor bed preferably comprises particles having a size predominantly in the range /a inch to l/32 inch in a depth of at least 21 inches and the fluidizing gases have a superficial velocity therethrough of from 2-10 feet per second.

The combustible gases leaving the bed 11 are mixed with secondary air supplied from an inlet line 54, a fan 540, a control valve 55, a manifold 56 and ports 57. Combustion takes place in the furnace section 58, including combustion of elutriated coke from bed 11, and the resulting substantially sulphur free flue gases pass via flue 59 to conventional air preheaters, solids precipitators or separators, and an effluent stack. A

carbon monoxide detector 66 communicates via line 67 with a main secondary air controller 68 which also receives signals from a

smoke detector

69a, 69b via line 70, and if changes in secondary air rate are necessary, a signal is despatched via line 71 to actuator 72 of the secondary air control valve 55.

The water/steam circulation system is conventional, featuring an inlet 75, an economiser 76, a convection section 77, a superheater 78, and an outlet 79.

Line 80 is for gas or oil supplies to overhead burners for heating the particles in bed 11 for start-up, the particles being mildly fluidised to distribute the heat.

FIG. 4 depicts in greater detail the regenerator l7 and means for transferring particles to an'from the reactor 10 and the regenerator 17.

Referring to the arrangements for transferring particles from the regenertor 17 to the reactor 10, it will be seen that the upper end of transfer conduit 21 is formed with a weir 33 just above the level of the top of bed 18 so that particles will splash from bed 18 into the conduit 21 during fluidization. The bottom of conduit 21 is formed into a short horizontal section, the horizontal and downsloping sections forming an elbow. Particles pack in the conduit 21 and thereby prevent any gas exchange between

beds

11 and 18. At predetermined intervals, a pulse of pressurised air is blown into the horizontal section from line 30 thereby pneumatically transporting particles from the horizontal section of conduit 21 into bed 11. The compressed air pulses are regulated by a valve 31, and air is supplied from line 32. The arrangements for particle transfer via conduits 16 in the opposite direction are exactly the same, and air pulses regulated by the valve 31 pass from line 29 into the horizontal bottom section of conduit 16 to transport particles pneumatically into the bottom of the bed 18. The rates of particle transfer in each direction should be equal.

The oxidation reaction in bed 18 is exothermic, and it is important to avoid temperatures above l,l50C this reduces the sulphur-fixing capacity of the particles. Since the amount of heat released depends on the amount of oxidizable sulphides present, the rate of particles transfer, as regulated by valve 31, depends on the temperature in bed 18. The temperature is monitored by probe 35, and a control signal is provided from transducer 36 and relayed via line 37 to valve 31. lf the temperature should fall overmuch in bed 18, the relatively cool particles returned to bed 11 can affect the gasification efficiency: moreover, a fall in temperature is usually indicative of a fall in the sulphides content of bed 18, and hence when the temperature falls below 1,075C, the rate of particle transfer from bed 11 to bed 18 is decreased.

In the regenerator 17, the bed 18 is fluidized by air from line propelled by a fan 43. The So l-containing gases and entrained solids pass via duct 22 to a cyclone,

and low sulphur fines are rejected via pipe 24, the solids-free gases being recovered from line 25. As previously explained, it is preferred to supply air in excess of that required for sulphides oxidation to ensure that the bed 18 is substantially desulphurized. An S0 or 0 monitor 38 (preferably the former) determines whether or not the effluent concentration is within a target range, or outside. If the S0 is too concentrated, a signal from monitor 38 passes via line 39 to transducer 40 to cause actuator 41 to increase the opening of the air control valve 42. If the S0 is too dilute, or

the 0 too concentrated, the air valve 42 is similarly caused to restrict the air flow rate.

It will be appreciated that while the preferred embodiment of the invention has been described in relation to the production of combustible sulphur-free gases at about atmospheric pressure, the invention can be applied to the production of combustible substantially sulphur free gases under pressure. Variations in the form and/or shape of the apparatus of the invention may also be made: for example, the reactor and/or the regenerator may have a number of transverse baffles to increase the horizontal path of the particles therein. Also, it may be convenient in some situations, to situate the regenerator at least partly within the reactor so that the regenerator walls act as baffles to increase the horizontal path of the particles.

I claim:

1. Apparatus for producing substantially sulphur-free combustible gas from sulphur-containing coal, comprising a reactor vessel having an upstream end and a downstream end containing a bed of particles containing at least one alkaline earth metal oxide or precursor of such oxide, a gas distributor supporting said bed, means for monitoring the bed temperature at the downstream end of the bed, means for supplying a hydrocarbon oil to the region of said downstream end, means for regulating the rate of oil supply to maintain the bed temperature at said region to which oil is supplied within the range of about 800 l,025C., means for supplying particulate sulphur-containing coal to said bed upstream of the region to which oil is supplied, means for supplying steam to said bed at least to regions where, during operation, devolatilized coal will be present, means responsive to the rate of supply of coal to regulate the rate of supply of stream, means for monitoring the temperature of the reactor bed, means for supplying an oxygen-containing gas underneath said distributor for fluidizing the contents of the reactor bed, means for supplying an inert gas underneath said distributor for diluting the oxygen-containing gas in the bed, first valve means for regulating the rate of supply of oxygen-containing gas, second valve means for regulating the rate of supply of said inert gas, said second valve means being responsive to the bed temperature to close the second valve means progressively as the bed temperature falls towards about 800C., and to open the second valve means progressively as the bed temperature increases towards about i ,025 C. whereby to maintain the bed temperature between about 800C. and about 1,025C., means responsive to the rate of supply of gas to the reactor bed to progressively close the first valve means as the gas supply rate approaches a selected upper limit and to open the first valve means as the gas supply rate approaches a selected lower limit, whereby to maintain particles in the bed in a substantially selected state of fluidization, a regenerator vessel having an upstream end and a downstream end, a gas distributor in said regenerator vessel, means for transferring particles from a first region of the downstream end of the particle bed in the reactor vessel to a first region of the regenerator vessel at the upstream end thereof, means for transferring particles from a second region of the downstream end of the regenerator vessel to a second region of the upstream end of the reactor vessel, means responsive to the temperature of particles in the regenerator to reduce the rate of particle transfer progressively when the temperature in the regenerator increases toward about l,l50C. and to increase the rate of particle transfer when the temperature decreases toward about l,000C., means for supplying an oxygen-containing gas at a regulatable rate beneath the regenerator distributor for contact with particles above the distributor, means for monitoring at least one constituent of exhaust gases leaving the regenerator vessel, said constituent beingchosen from oxygen and S and means responsive to signals from the monitoring means for reducing the rate of supply of oxygen-containing gas to the regenerator vessel when the oxygen content of the exhaust gases exceeds a maximum selected concentration or the S0 content is lower than a minimum selected concentration.

2. Apparatus according to claim 1 in which the up stream end and downstream end of the reactor are adjacent and separated by a partition wall having a height exceeding the height of the reactor bed, the wall extending into the regenerator vessel and thereby separating the upstream end thereof from the downstream end.

3. A burner apparatus or boiler for producing a substantially sulphur-free flue gas from sulphur-containing coal comprising the combination of apparatus in accordance with claim 2 with means for supplying secondary air for the combustion of substantially sulphur-free combustible gases from said apparatus.

4. Apparatus according to claim 1 in which the first region of the said downstream end of the particle bed in the reactor vessel is an upper region of the said bed, and the said first region of the regenerator vessel is adjacent to the said distributor.

5. Apparatus according to claim 1 in which the said second region of the said downstream end of the regenerator vessel is an upper region thereof and the second region of the reactor vessel is adjacent to the distributor thereof.

6. A method of producing a substantially sulfur-free combustible gas from a solid carbonaceous material selected from the group consisting of coal, lignite, peat and shale and mixtures thereof, which comprises reacting said carbonaceous material in the upstream region of a reactor bed of fluidized particles containing at least one alkaline earth metal oxide at a temperature of between about 800C. and l,025C. with an oxygencontaining gas having an oxygen content of between about percent and 50 percent of the amount required for stoichiometric combustion of said carbonaceous material to convert said carbonaceous material to a substantially sulfur-free combustible gas and combining the sulfur in said carbonaceous material with the alkaline earth metal of said particles to form alkaline earth metal sulfides, transferring said particles from a downstream region of said reactor bed to a regenerator bed wherein said transferred particles are reacted at a temperature in the range of about l000 to 1100C. with an oxygen-containing gas to thereby convert the alkaline earth metal sulfides to alkaline earth metal oxides with the concurrent evolution of sulfur dioxide, returning said particles from said regenerator to the upstream region of said reactor bed, introducting a hydrocarbon oil in the said downstream region of said reactor bed and introducing steam in the oxygen-containing gas entering at least upstream of the region of said oil introduction.

7. A method according to claim 6 in which the temperature of said reactor bed is monitored and the rate of supply of oxygen in the oxygen-containing gas is progressively increased as the temperature decreases towards about 800C. and progressively decreased as the temperature increases towards about 1,025C.

8. A method according to claim 6 in which said reactor bed has a depth of at least 21 inches of said particles having sizes predominantly in the range between about Vs inch to l/32 inch and said particles are maintained in a fluidized state by passing fluidizing gases therethrough at a superficial velocity of from 2 to 10 feet per second.

9. A method according to claim 6 which comprises the steps of monitoring the sulfur content of the combustible gas to determine the sulfur-fixing activity of the reactor bed, bleeding said particles from the said regenerator bed when the sulfur content of said gas approaches a predetermined maximum and supplying a corresponding amount of fresh particles to the top of the said reactor bed so as to maintain the active inventory of said reactor bed.

10. A method according to claim 6 in which steam is mixed with said oxygen-containing gas entering the upstream and downstream regions of the reactor.

11. A method according to claim 6 in which the mol. ratio of oxygen to steam in the oxygencontaining gas entering the reactor bed is at least 2.

12. A method according to claim 6 in which particles are transferred from an upper region of the reactor bed to a lower region of the regenerator bed, and particles are returned to a lower region of the reactor bed from an upper region of the regenerator bed.

13. A method according to claim 6 in which the hydrocarbon oil is injected into a lower region of the downstream region of the reactor bed.

14. A method according to claim 6 in which the rate of steam supply to the reactor bed is directly proportional to the rate of coal supply.

15. A method according to claim 6 in which, when the reactor bed temperature progressively rises and approaches 1,025C., an inert gas is supplied at a progres sively increased rate to the reactor bed in admixture with the oxygen-containing gas.

16. A method according to claim 15 in which the rate of supply of gases to the reactor bed is monitored, and as a maximum rate is approached, the rate of supply of oxygen-containing gas is reduced progressively.

17. A method according to claim 6 in which the rate of injection of oil into the downstream region of the reactor bed is progressively increased as the reactor bed temperature increases and is progressively decreased as the reactor bed temperature decreases.

18. A method according to claim 6 in which the rate of transfer of particles into the regenerator bed is substantially equal to the rate of transfer of particles out of the regenerator bed, and is decreased as the temperature in the regenerator bed decreased towards 1 C and increased as the temperature in the regenerator bed decreases towards l,000C.

19. A method according to lcaim 6 in which the concentration of S0 or 0 in the gases leaving the regenerator bed is monitored, and the rate of supply of oxygencontaining gas to the regenrator decreased progressively when the SO, concentration decreases towards a selected minimum concentration or the O, concentration increases towards a selected maximum concentration, and increased progressively when the SO, concentration increases towards a selected maximum concen- 21. A method according to claim 6 in which the coal is introduced into at least one part of the reactor bed in which is contiguous with the top surface of the reactor bed and which is downflowing relative to adjacent parts of the reactor bed.

Claims

1. Apparatus for producing substantially sulphur-free combustible gas from sulphur-containing coal, comprising a reactor vessel having an upstream end and a downstream end containing a bed of particles containing at least one alkaline earth metal oxide or precursor of such oxide, a gas distributor supporting said bed, means for monitoring the bed temperature at the downstream end of the bed, means for supplying a hydrocarbon oil to the region of said downstream end, means for regulating the rate of oil supply to maintain the bed temperature at said region to which oil is supplied within the range of about 800* -1,025*C., means for supplying particulate sulphur-containing coal to said bed upstream of the region to which oil is supplied, means for supplying steam to said bed at least to regions where, during operation, devolatilized coal will be present, means responsive to the rate of supply of coal to regulate the rate of supply of steam, means for monitoring the temperature of the reactor bed, means for supplying an oxygen-containing gas underneath said distributor for fluidizing the contents of the reactor bed, means for supplying an inert gas underneath said distributor for diluting the oxygen-containing gas in the bed, first valve means for regulating the rate of supply of oxygen-containing gas, second valve means for regulating the rate of supply of said inert gas, said second valve means being responsive to the bed temperature to close the second valve means progressively as the bed temperature falls towards about 800*C., and to open the second valve means progressively as the bed temperature increases towards about 1,025*C. whereby to maintain the bed temperature between about 800*C. and about 1,025*C., means responsive to the rate of supply of gas to the reactor bed to progressively close the first valve means as the gas supply rate approaches a selected upper limit and to open the first valve means as the gas supply rate approaches a selected lower limit, whereby to maintain particles in the bed in a substantially selected state of fluidization, a regenerator vessel having an upstream end and a downstream end, a gas distributor in said regenerator vessel, means for transferring particles from a first region of the downstream end of the particle bed in the reactor vessel to a first region of the regenerator vessel at the upstream end theReof, means for transferring particles from a second region of the downstream end of the regenerator vessel to a second region of the upstream end of the reactor vessel, means responsive to the temperature of particles in the regenerator to reduce the rate of particle transfer progressively when the temperature in the regenerator increases toward about 1,150*C. and to increase the rate of particle transfer when the temperature decreases toward about 1, 000*C., means for supplying an oxygen-containing gas at a regulatable rate beneath the regenerator distributor for contact with particles above the distributor, means for monitoring at least one constituent of exhaust gases leaving the regenerator vessel, said constituent being chosen from oxygen and SO2, and means responsive to signals from the monitoring means for reducing the rate of supply of oxygen-containing gas to the regenerator vessel when the oxygen content of the exhaust gases exceeds a maximum selected concentration or the SO2 content is lower than a minimum selected concentration.

2. Apparatus according to claim 1 in which the upstream end and downstream end of the reactor are adjacent and separated by a partition wall having a height exceeding the height of the reactor bed, the wall extending into the regenerator vessel and thereby separating the upstream end thereof from the downstream end.

6. A METHOD OF PRODUCING A SUBSTANTIALLY SULFUR-FREE COMBUSTIBLE GAS FROM A SOLID CARBONACEOUS MATERIAL SELECTED FROM THE GROUP CONSISTING OF COAL, LIGNITE, PEAT AND SHALE AND MIXTURES THEREOF, WHICH COMPRISES REACTING SAID CARBONACEOUS MATERIAL IN THE UPSTREAM REGION OF A REACTOR BED OF FLUIDIZED PARTICLES CONTAINING AT LEAST ONE ALKALINE EARTH METAL OXIDE AT A TEMPERATURE OF BETWEEN ABOUT 800*C. AND 1,025*C. WITH AN OXYGEN-CONTAINING GAS HAVING AN OXYGEN CONTENT OF BETWEEN ABOUT 20 PERCENT AND 50 PERCENT OF THE AMOUNT REQUIRED FOR STOICHIOMETRIC COMBUSTION OF SAID CARBONACEOUS MATERIAL TO CONVERT SAID CARBONACEOUS MATERIAL TO A SUBSTANTIALLY SULFURFREE COMBUSTIBLE GAS AND COMBINING THE SULFUR IN SAID CARBONACEOUS MATERIAL WITH THE ALKALINE EARTH METAL OF SAID PARTICLES TO FORM ALKALINE EARTH METAL SULFIDES, TRANSFERRING SAID PARTICLES FROM A DOWNSTREAM REGION OF SAID REACTOR BED TO REGENERATOR BED WHEREIN SAID TRANSFERRED PARTICLES ARE REACTED AT A TEMPERATURE IN THE RANGE OF ABOUT 1000* TO 1100*C. WITH AN OXYGEN-CONTAINING GAS TO THEREBY CONVERT THE ALKALINE EARTH METAL SULFIDES TO ALKALINE EARTH METAL OXIDES WITH THE CONCURRENT EVOLUTION OF SULFUR DIOXIDE, RETURNING SAID PARTICLES FROM SAID REGENERATOR TO THE UPSTREAM REGION OF SAID REACTOR BED, INTRODUCING A HYDROCARBON OIL IN THE SAID DOWNSTREAM REGION OF SAID REACTOR BED AND INTRODUCING STEAM IN THE OXYGENCONTAINING GAS ENTERING AT LEAST UPSTREAM OF THE REGION OF SAID OIL INTRODUCTION.

6. A method of producing a substantially sulfur-free combustible gas from a solid carbonaceous material selected from the group consisting of coal, lignite, peat and shale and mixtures thereof, which comprises reacting said carbonaceous material in the upstream region of a reactor bed of fluidized particles containing at least one alkaline earth metal oxide at a temperature of between about 800*C. and 1,025*C. with an oxygen-containing gas having an oxygen content of between about 20 and 50 percent of the amount required for stoichiometric combustion of said carbonaceous material to convert said carbonaceous material to a substantially sulfur-free combustible gas and combining the sulfur in said carbonaceous material with the alkaline earth metal of said particles to form alkaline earth metal sulfides, transferring said particles from a downstream region of said reactor bed to a regenerator bed wherein said transferred particles are reacted at a temperature in the range of about 1000* to 1100*C. with an oxygen-containing gas to thereby convert the alkaline earth metal sulfides to alkaline earth metal oxides with the concurrent evolution of sulfur dioxide, returning said particles from said regenerator to the upstream region of said reactor bed, introducing a hydrocarbon oil in the said downstream region of said reactor bed and introducing steam in the oxygen-containing gas entering at least upstream of the region of said oil introduction.

7. A method according to claim 6 in which the temperature of said reactor bed is monitored and the rate of supply of oxygen in the oxygen-containing gas is progressively increased as the temperature decreases towards about 800*C. and progressively decreased as the temperatUre increases towards about 1,025*C.

8. A method according to claim 6 in which said reactor bed has a depth of at least 21 inches of said particles having sizes predominantly in the range between about 1/8 inch to 1/32 inch and said particles are maintained in a fluidized state by passing fluidizing gases therethrough at a superficial velocity of from 2 to 10 feet per second.

11. A method according to claim 6 in which the mol. ratio of oxygen to steam in the oxygen-containing gas entering the reactor bed is at least 2.

15. A method according to claim 6 in which, when the reactor bed temperature progressively rises and approaches 1,025*C., an inert gas is supplied at a progressively increased rate to the reactor bed in admixture with the oxygen-containing gas.

18. A method according to claim 6 in which the rate of transfer of particles into the regenerator bed is substantially equal to the rate of transfer of particles out of the regenerator bed, and is decreased as the temperature in the regenerator bed decreased towards 1150*C and increased as the temperature in the regenerator bed decreases towards 1,000*C.

19. A method according to claim 6 in which the concentration of SO2 or O2 in the gases leaving the regenerator bed is monitored, and the rate of supply of oxygen-containing gas to the regenrator decreased progressively when the SO2 concentration decreases towards a selected minimum concentration or the O2 concentration increases towards a selected maximum concentration, and increased progressively when the SO2 concentration increases towards a selected maximum concentration or the O2 concentration decreases towards a selected minimum concentration.

20. A method according to claim 6 in which the coal is introduced into at least one part of the reactor bed contiguous with the base of the reactor bed which is upflowing relative to adjacent parts of the reactor bed.