US2782109A

US2782109A - Method of gasifying solid fuel

Info

Publication number: US2782109A
Application number: US492917A
Authority: US
Inventors: Roberts Irving
Original assignee: Individual
Current assignee: Individual
Priority date: 1951-08-29
Filing date: 1955-03-08
Publication date: 1957-02-19
Anticipated expiration: 1974-02-19

Description

Feb. 19, 1957 v I I. ROBERTS 2,782,109

METHOD OF GASIFYING SOLID FUEL Original Filed Aug. 29, I951 Top of coalbed /y/uq 736527:

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United States Patent METHOD OF GASIFYING SOLID FUEL Irving Roberts, Meadowbrook, Pa.

Original application August 29, 1951, Serial No. 244,172. Divided and this application March 8, 1955, Serial No. 492,917

3 Claims. (Cl. 48-206) This invention relates to a method of gasifying solid fuels, especially caking bituminous coal. The application is a division of my copending patent application, Serial No. 244,172, filed August 29, 1951 now abandoned for Apparatus for Gasifying Solid Fuel.

In a conventional updraft gas producer, a continuous blast of steam and air is introduced into the bottom of a bed of burning fuel to produce a product gas consisting of .a mixture of carbon monoxide and hydrogen with some carbon dioxide and unreacted steam. The nitrogen in the air blast appears in the product gas as an inert diluent. When coke or anthracite coal is used as the fuel, the bed is maintained five or six feet deep to obtain heat transfer between the product gas and the incoming fuel so that a considerable amount of heat will be recovered from the product gas before it leaves the producer. On the other hand, if the fuel is a caking bituminous coal, it will begin to fuse somewhere between about 750 and 950 F. and will form large masses of coke. These large lumps are slow to react with the gases in the bed, and the rising product gas tends to by-pass the lumps and form channels or blowholes through which partially reacted gas flows and mixes with the product, thereby reducing its quality. Consequently, when such a fuel is used, the depth of the bed is reduced to about two feet so that the temperature of the surface of the bed can be raised well above the fusion temperature of the coal. A rake or similar mechanical device is'arranged to pass over the surface of the bed to prevent the formation of any large masses of coke and to push coal into any blowholes which may appear. Satisfactory gas quality and output are obtained in this manner, but the temperature of the product gas leaving the producer is raised to about 1350-1550 F- This represents a substantial loss in thermal efficiency of the producer. Such a loss is especially important in a synthesis process, such as the synthesis of gasoline from solid fueL'Where in order to avoid the presence of nitrogen in the synthesis gas relatively pure oxygen is used in place of air in the blast. Since the oxygen, which must be produced in an air distillation plant, is quite expensive, it is highly desirable to reduce the quantity consumed in the gas producer as much as possible. As the heat losses from the gas producer require the generation of more heat in it by the addition of more oxygen, substantial reductions in oxygen requirements may be made by improvements in thermal efiiciency of the producer. Thermal efiiciency, however, can not be improved as long as the product gas leaves the producer at the high temperature it has in the past when caking bituminous coal has been used as the fuel. been used in the production of a synthesis gas. Instead, the more expensive coke or anthracite coal has been used in the making of gas for synthetic methanol and synthetic ammonia. Such fuel is too expensive to use in the manufacture of synthetic gasoline, and, as pointed The result has been that such coal has not ice out above, the thermal efiiciency of a gas producer using bituminous coal is too low.

It is among the objects of this invention to provide a method of gasifying solid fuel, which has high thermal efiiciency and which eliminates caking difiiculties when caking bituminous coal is used.

in accordance with this invention, fuel is fed to the upper ends of a pair of updraft gas producer's to maintain deep beds therein. Steam and oxygen-containing gas are piped to the upper ends of the producers, while oxygen-containing gas also is piped to the bottoms of the producers to support combustion of the fuel in them. Connected to the top of each producer is an outlet pipe for product gas, while a conduit connects the bottoms of the producers. Valves are provided for closing the outlet pipes alternately, and further valves are provided for directing the steam and oxygen-containing gas into the top of the producer having the closed outlet pipe at that time and for directing oxygen-containing gas into the bottom of the other producer. The product gas flowing upward alternately in the two producers transfers a great amount of its heat to the deep beds of fuel which are cooled periodically by the steam and oxygen-com taining gas that flows down through them alternately. When the fuel is caking bituminous coal, the flow of oxygen-containing gas down through the deep bed destroys the caking properties of the coal before it reaches the zone in which it otherwise would cake.

The invention is illustrated diagrammatically in the accompanying drawings, in which Fig. 1 represents a vertical section through two producers with their connecting conduits; and Fig. 2 illustrates a modification of one of the producers.

Referring to Fig. l of the drawings, each producer A and B may be a conventional type in which a bricklined cylinder 1 has a tapered lower end that opens into a pan 2 filled with water to form a seal. The pan may be rotated to remove ash from the producer. Of course, other ways of removing the ash may be used. Connected to the top central portion of each cylinder is a conduit 3, through which fuel can be delivered to it. For convenience the two conduit are shown connected to a single supply conduit 4. The cylinders are much taller than those used heretofore so that total depth of the bed of fuel and ash in each can be from 15 to 25 feet. Also connected to the tops of the cylinders are outlet pipes 6 for product gas. The pipes are connected to a main 7 and are provided with valves 8 and 9 for closing them alternately. Also connected to the tops of the two producers are inlet pipes 11 which may be connected to a common supply pipe 12 for delivering steam to the producers. The inlet pipes are provided with valves 13 and 14 which are closed alternately.

Extending up through the ash pans and into the lower ends of the gas producer cylinders are the opposite ends of a cross-over conduit 16 that may have vertical end portions connected by a horizontal intermediate portion. Mounted a short distance above each end of this conduit is a hood 17 to prevent coal or ash from entering the conduit. Extending up through each, vertical end portion of the conduit and projecting above the adjoining hood is a blast pipe 18 which has a hood 19 located a short distance above it. Below the cross-over conduit the blast pipes are provided with

valves

21 and 22 and maybe connected by a common supply pipe 23 to a Source of oxygen-containing gas under pressure. This gas may be air or a gas containing a larger proportion of oxygen, even nearly pure oxygen. The steamzqs'upply pipe 12 and the lower gas supply pipe 23 are connected by a throttle valve 24 so that some of the oxygen-containing gas can be fed into the steam pipe to mix with the steam. v

To start this apparatus in operation where a calting bituminous coal is to be gasified, the two producers first are filled with coke to a depth of from to feet. Valves 8, 14, 21 and Marc closed and valves 9, 13 and 22 are open, sov that thesteamat a temperature of about 250 F., will enter the top of producer A and flow down- Ward through the coke bed therein and then pass through the cross-over conduit 16 into the bottom of producer B. At the same time oxygen-containing gas, nearly pure oxygen for example, will flow into the bottom of producer B through its blast pipe and producer B, after ignition, will start to operate with the gases flowing upward through the coke bed and out through valve 9 into the product main. The product gases flowing upward through the tall fuel bed exchange heat with it, heating the coke while they are cooled by it. After a suitable interval of time, valves 9, 13 and 22 are closed and

valves

8, 14 and 21 are opened. The steam now flowing downward through the coke inproducer B ispreheated by it, thereby reducing the temperature of the coke, before entering the bottom of producer A by way of the crossover conduit. Aftcr'p'roducer A'has been started in operation, the entering preheated s'tearn makes it possible to reduce the oxygen flow through the blast'pipe below the original rate. The product gases now flow upward through the coke bed in producer A, giving oif heat to the coke and leaving through valve 8. Switching of the valves is repeated at intervals, whereby each unit is operated alternately as a steam preheater and as a gas producer. After a number of such cycles the tall fuel bed above the gasifica-tion zone in each cylinder tends to take on an approximately linear temperature distribution that decreases from about 2000 F. at the top of the gasification zone to about 350 F. at the top of the bed. When this occurs it is time to start feeding the bituminous coal into the tops of the two producers.

The coal can be fed continuously or intermittently. If intermittent feed is used, it should be frequent enough to allow only a slight variation in the height of the beds so as to assure efficient cooling of the product gas. After the coal feed has been started, throttle valve 24 is opened and adjusted so as to allow a portion (say 10 to 50%) of the total oxygen used to mix with the steam flowing throughpipe 12. By mixing a portion of the oxygen with the steam entering the tops of the producers, the coal is exposed to an oxygen-rich atmosphere during every half cycle.v This downwardly flowing atmosphere produces slow oxidation of the fusible constituents of the coal so that the coal loses its caking properties before it sinks far enough down in the bed to reach the temperature range of about 750-950 R, which is a common range in which fusion begins. The rate of oxidation is a function of the type of coal, the size of the coal particles, the oxygen content of the surrounding gas and the temperature.

The proportion of oxygen mixed with the steam entering the top of the bed should be as large as possible for most effective destruction of the caking properties of the coal. On the other hand, the oxygen content of the steam must be limited to that amount which will just avoid setting up of a self-sustaining gasification reaction at the bottom of the producer which is in preheating service.

As the gasification proceeds, consuming fuel from the bottom of the bed, each coal particle gradually moves downward and is exposed to regions of successively higher temperatures. By using a tall bed of coal, the coal is exposed to oxygen-rich gas for a long time composed of many preheating half-cycles. As an example, with a gasification rate of 25 pounds of coal per hour per square foot of cross section of bed, the coal moves downward at an average rate of about 0.25 foot per hour, moving 0.5 foot per hour during the gasitieation half-cycleand" being almost stationary during the preheating and oxygen exposure half-cycle. If the height of coal above the temperature zone of 750-950 F. is six feet, an exposure time of 12 hours to oxygen-rich gas is allowed before the fusion temperature is reached. Under these conditions the coal does not cake, and each coal particle remains a separate entity which is gradually transformed into coke during its passage down through the producer without formation of large lumps or .blowholes.

The height of the coal bed should be adjusted for the type and size of coal being used to allow sufiicient oxygen exposure time to avoid caking difficulties at the normal fusion temperature. Alternatively, the coal may be crushed to a size that will give satisfactory operation with the bed height desired to be used. However, with some coals, if the bed height is small, the particle size required may be so small as to require reduction of the allowable flow rates in order to keep the product gas from carrying away the coal particles.

The use of a deep bed of coal also results in unusually high thermal etficiency. The regenerative heat transfer effect of the two coal beds allows preheating of the coal, the steam and a portion of the oxygen feed by-heat recovered entirely from the product gas. For example, it is calculated that with steam entering at 250 F. and with coal and oxygen entering at F. the product gas will leave the producer at a temperature between 400 and 450 F. The switching time is adjusted to produce a rise and fall of temperature in the coal beds consistent with good heat transfer. Thus, with a fuel bed fifteen feet high above the gasification zone, and a gasification rate of 25 pounds of coal per hour per square foot of bed cross section, the switching time per half-cycle may be five minutes This allows a temperature swing of about 16 If the switching time is increased, the temperature swing also will increase. An increased temperature swing will give poorer heat transfer but will produce less wear on the switching valves. In some cases, switching times as long as 30 minutes per half'cycle may be economical. V

For certain grades of bituminous coal which are fed to the producers in relatively large lumps, it may not be possible to obtainsufiicient destruction of their cal-:ing properties with a bed of reasonable height. In such cases, each of the two producers can be provided with jet pipes to introduce a portion of the steam-oxygen mixture into the bed in a region just above the fusion zone. Such an arrangement is shown in Fig. 2, where a series of laterally spaced pipes 26 extend from a header 27 through the side wall of a producer cylinder 1 and across it at a level just above the normal fusion zone. The undersides of the pipes are provided with small holes from which a mixture of steam and oxygen issues in jets that penetrate into the fused coal while it is still in molten condition. The jets break up the material, preventing the formation of large lumps which would impede the operation of the'producer, and provide a high oxygen content locally: for destruction of the fusible constituents of the coal. The total amount of oxygen used is about the same as with the apparatus shown in Fig. 1, which is the maximum that can be used without sustaining gasificationat the bottom of the producer that is in preheating service.

This apparatus has the advantages of very good thermal eificiencyeven when the solid fuel is not a caking fuel, because the steam and 'the down-flowing oxygencontainiug gas are preheated by the fuel beds which removed heat from the product gas. Thus, the product gas is cooled by the fuelbeds to a lower temperature than heretofore, even if the beds are no deeper than in the past. Also, if the fuel is non-caking, the thermal efficiency is improved even though no oxygen-containing gas is delivered :to the top of the fuel beds, because the beds will be cooled'inaterially by'the steam passing down through them. V l I According to the provisions of the patent statutes, I

have explained the principle of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. The continuous method of efiiciently gasifying solid fuel comprising directing steam down through a hot first bed of said fuel to preheat the steam, conducting the preheated steam from the bottom of said bed to the bottom of a second bed of said fuel while directing oxygen-containing gas up through the second bed to sustain combustion therein in order to generate product gas, conducting the product gas away from the top of the second bed until the temperature of the top of that bed has been raised a predetermined amount by heat from the product gas, then stopping the withdrawal of product gas from the top and the delivery of steam and oxygen-containing gas to the bottom of the second bed, directing steam down through the hot second bed to preheat the steam, conducting the preheated steam from the bottom of the second bed to the bottom of the first bed while directing oxygen-containing gas up through the first bed to sustain combustion therein in order to generate product gas, conducting the product gas away from the top of the first bed until the temperature of the top of that bed has been raised a predetermined amount by heat from the product gas, and continuing to reverse the cycle periodically, whereby steamis preheated in each bed alternately by heat absorbed by the bed from the product gas that has just left it.

2. The continuous method of efficiently gasifying caking bituminous coal, comprising directing steam and oxygen-containing gas down through a previously heated first deep bed of said coal to preheat the steam and to reduce the tendency of the coal to cake, limiting the amount of said oxygen to less than will sustain a gasification reaction at the bottom of said bed, conducting the preheated steam from the bottom of said bed to the bottom of a second bed of said coal while directing oxygencontaining gas up through the second bed to sustain combustion therein in order to generate product gas, conducting the product gas away from the top of the second bed until the temperature of the top of that bed has been raised a predetermined amount by heat from the product gas, then stopping withdrawal of product gas from the top and the delivery of steam and oxygencontaining gas to the bottom of the second bed, directing steam and oxygen-containing gas down through the second heated bed to preheat the steam and to reduce the tendency of the coal therein to cake, limiting the amount of said last-mentioned oxygen in the same way as the first-mentioned oxygen delivered to the first bed conducting the preheated steam from the bottom of the second bed to the bottom of the first bed while directing oxygen-containing gas up through the first bed to sustain combustion therein in order to generate product gas, conducting the product gas away from the top of the first bed until the temperature of the top of that bed has been raised a predetermined amount by heat from the product gas, continuing to reverse the cycle periodically, whereby steam is preheated in each bed alternately by heat absorbed by the bed from the product gas that has just left it, and adding coal to the top of the beds to maintain their depth.

3. The method of gasifying caking bituminous coal as defined in claim 2, in which some of said steam and oxygen-containing gas is conducted before heating directly to a predetermined zone of the bed in which the steam is being preheated, said zone being the zone in which the coal is in molten condition.

References Cited in the file of this patent UNITED STATES PATENTS 1,160,908 Koster Nov. 16, 1915 1,867,102 Russell July 12, 1932 2,131,696 Brondegee et a1. Sept. 27, 1938 2,592,591 Odell Apr. 15, 1952 FOREIGN PATENTS 344,742 Great Britain Mar. 12, 1931

Claims

1. THE CONTINUOUS METHOD OF EFFICIENTLY GASIFYING SOLID FUEL COMPRISING DIRECTING STEAM DOWN THROUGH A HOT FIRST BED OF SAID FUEL TO PREHEAT THE STREAM, CONDUCTING THE PREHEATED STEAM FROM THE BOTTOM OF SAID BED TO THE BOTTOM OF A SECOND BED OF SAID FUEL WHILE DIRECTING OXYGEN-CONTAINING GAS UP THROUGH THE SECOND BED TO SUSTAIN COMBUSTION THEREIN IN ORDER TO GENERATE PRODUCT GAS, CONDUCTING THE PRODUCT GAS AWAY FROM THE TOP OF THE SECOND BED UNTIL THE TEMPERATURE OF THE TOP OF THAT BED HAS BEEN RAISED A PREDETERMINED AMOUNT BY HEAT FROM THE PRODUCT GAS, THEN STOPPING THE WITHDRAWAL OF PRODUCT GAS FROM THE TOP AND THE DELIVERY OF STREAM AND OXYGEN-CONTAINING GAS TO THE BOTTOM OF THE SECOND BED, DIRECTING STEAM DOWN THROUGH THE HOT SECOND BED TO PREHEAT THE STEAM, CONDUCTING THE PREHEATED STREAM FROM THE BOTTOM OF THE SECOND BED TO THE BOTTOM OF THE FIRST BED WHILE DIRECTING OXYGEN-CONTAINING GAS UP THROUGH THE FIRST BED TO SUSTAIN COMBUSTION THEREIN IN ORDER TO GENERATE PRODUCT GAS, CONDUCTING THE PRODUCT GAS AWAY FROM THE TOP OF THE FIRST BED UNTIL THE TEMPERATURE OF THE TOP OF THAT BED HAS BEEN RAISED A PREDETERMINED AMOUNT BY HEAT FROM THE PRODUCT GAS, AND CONTINUING TO REVERSE THE CYCLE PERIODICALLY, WHEREBY STEAM IS PREHEATED IN EACH BED ALTERNATELY BY HEAT ABSORBED BY THE BED FROM THE PRODUCT GAS THAT HAS JUST LEFT IT.