US 2683657 A
Description (OCR text may contain errors)
July 13, 1954 P. W. GARBO GASIFICATION 0F CARBONACEOUS SOLIDS Fiied May 29, 1948 50L /.D P5571705 IN V EN TOR.
PULWGAO A TTORNE Y5 Patented July 13, 1954 UNITED OFFICE GASIFICATION OF CARBONACEOUS SOLIDS Application May 29, 1948, Serial N 0. 30,023
This invention relates to a method and apparatus for gasification of a solid carbonaceous material. In one of its more specific aspects, this invention relates to a process of gasification of a solid carbonaceous material containing volatile constituents wherein distillation of volatile constituents and reaction of the residual solid with a gaseous reactant are carried out.
Solid carbonaceous materials which may be treated by the present process include coal, coke, oil shale, lignite, and the like. The invention is particularly applicable to treatment of coal and similar carbonaceous materials comprising volatile constituents wherein both carbonization and chemical reaction are employed to gasify substantially all of the non-mineral content of the carbonaceous material. The process of this invention is also useful for carbonization of coal, or partial gasification, as well as for complete gasification.
Of the chemical reactions employed in gasifification, reactions in which carbon is chemically combined with oxygen or hydrogen are most commonly used and best adapted to the process of this invention. Illustrative gasifying reactants include oxygen, steam, carbon dioxide, hydrogen or mixtures of these gases.
Many and diverse methods have been practiced and proposed for the gasification of solid carbonaceous materials. technique has been applied more or less successfully to processes for the gasification of various carbonaceous materials. For example, coal in finely-divided form may be treated in a fluidized bed with hot inert gases to effect removal of volatile constituents or with a gas containing reactive oxygen to effect gasification by chemical reaction to any desired extent. Fluid bed gasification is particularly adapted to the treatment of coke or hard coal, such as anthracite, which have little or no tendency to agglomerate under reaction conditions. Coking coal may be treated prior to gasification to prevent agglomeration. Such pretreatment may consist of heating the coal to drive off a portion of the volatile constituents therefrom or partial preoxidation of the coal with an oxygen-containing gas, nitric acid or other oxidizing agent. Agglomeration may be avoided or minimized by admixing the raw coal with sufficient carbon, char, ash or inert refractory material in finely-divided form to substantially prevent agglomeration of the raw coal particles.
The gasification of coal and the like to produce a mixture comprising carbon monoxide and hy- The fluidized solid drogen is often desirable. It is known that carbon monoxide and hydrogen may be converted to various hydrocarbons. The conversion of carbon monoxide and hydrogen to hydrocarbons suitable for use as motor fuels is satisfactorily carried out under elevated temperatures and pressures in the presence of a catalyst, generally one comprising an element of the iron group of the periodic table of the elements.
At the present time the synthesis of hydrocarbons from carbon monoxide and hydrogen is of considerable importance commercially. The conversion of solid carbonaceous materials to high B. t. u. value heating .gas is also of increasing importance. For these reasons, it is most desirable to develop a simple and economical process for the conversion of solid carbonaceous materials to gaseous reactants suitable as feed stock for the hydrocarbon synthesis or to gaseous mixtures of high B. t. u. value for fuel purposes.
A problem in the gasification of carbonaceous materials which has not been solved entirely satisiactorily is that of economically converting fresh coal to gaseous products of value as fuel gas or snythesis gas.
An object of the present invention is to provide an improved process for the gasification of solid carbonaceous material.
Still another object is to provide an improved process for combined carbonization and gasification of solid carbonaceous materials containing volatile constituents.
A further object of the present invention is to provide such a process which is particularly applicable for use in gasification of coal, oil shale, lignite, and the like.
The present invention provides a simple improved method for the gasification of solid carbonaceous materials to produce either a gaseous product suitable as feed for the synthesis of hydrocarbons and oxygenated derivatives or a heating gasof relatively high B. t. u. value. The process may be operated to produce a high grade char as a solid residue or may be operated for substantially complete gasification of the feed material, leaving only ash as the residual solid.
In accordance with this invention, the solid carbonaceous material of a particle size suitable for fluidization, generally less than about 0.1 inch in diameter, is charged to the top of a fluidized bed of said particles in a gasifier. The gasifier is characterized by the unusual feature that despite good fluidization of the particles there are zones in which top-to-bottom mixing of solid particles is inhibited. This is accomplished by providing zones of the gasifier with a number of partitions disposed vertically in the bed of fluidized solid particles. Thus the advantages of fiuidization may be retained while at the same time obtaining advantages peculiar to a non-fluidized moving bed, articularly, countercurrent flow of gases and solids and a temperature gradient along the vertical dimension of the gasifying vessel. The particles of carbonaceous material flow downwardly through the gasifier countercurrent to a stream of fluidizing gas. The fluidizing gas is introduced in the lower portion of the bed of solid particles and may comprise a reactant gas, preferably free oxygen. The efiiuent gas or product gas is withdrawn from a point above the fluidized bed and comprises any products of distillation, as well as the gases resulting from chemical reaction.
As the carbonaceous material flows downwardly through the gasifier, it passes progressively through a solid preheating zone, which may be a distillation or carbonizing zone; a reaction zone; and, preferably, also a gas preheating zone which may also be a carbon cleanup zone. These zones preferably are all included within a single vessel. In the uppermost or solid preheating zone, the coal is brought into contact with hot gaseous products from the reaction zone. Heat from the gaseous products is transferred to the particles, preheating them and drivin off any volatile constituents contained therein. This solid preheating zone is designed to prevent or greatly minimize top-to-bottom mixing of the solids moving downwardly therethrough. A temperature gradient is thus established along the height of this zone with the lowest temperature at the top of the zone and the highest at the bottom. In the reaction zone, the carbonaceous material is reacted with the gasifying reactant in the fluidizing gas stream, for example, oxygen, liberating heat and generating gaseous products comprisin carbon oxides. In the reaction zone, the solids are free to move with the complete turbulence characteristic of fluidization so that a substantially uniform temperature exists throughout the I The residual solid material,
reaction zone. which may be low carbon char or ash, may be withdrawn directly from the reaction zone but preferably is withdrawn after it has been made to pass downwardly through a gas preheating zone wherein it transfers heat to the incoming gaseous stream. A reactant gas may be passed upwardly through this zone to improve carbon cleanup.
Where a gas preheating zone is employed, it is like the solid preheating zone in that the fluidized solids flow downwardly through elongate channels of restricted horizontal cross-section which curtail top-to-bottom mixing of the solids. Under these circumstances, the incoming gases flow countercurrently to the solids, establishing a temperature gradient along the vertical dimension of the channels with the highest temperature at the top thereof and the lowest at the bottom. Where the incoming gases are reactive with the carbon residue leaving the reaction zone, a desirable composition gradient is also established in the channel of the gas preheating zone, that is, the residue has the highest carbon content at the top of the channels and the lowest at the bottom.
This process effects an economical gasification of coal and other solid carbonaceous materials particularly those containing volatile constituents. Since the heat from the solid residue is transferred to the incoming gas stream and the eflluent 4 gases give up heat to the incoming fresh solid feed material, a highly efficient system is provided which utilizes heat in a most economical manner.
The present invention will be described in detail with reference to coal as the carbonaceous material to illustrate the operation of the process of this invention. It will be understood that coal is used as a specific example and that the method of the invention as described is not limited to the use of coal as the feed material. Since the gasification of various materials is known in the art, the application of the present invention to other solid carbonaceous materials will be evident to one skilled in the art from the detailed description of this invention and the illustrative example of its application to treatment of coal.
Fig. 1 of the accompanying drawing is a diagrammatic elevational view in cross-section of apparatus suitable for carrying out the process of the present invention.
Fig. 2 is a horizontal section through the apparatus illustrated in Fig. 1 taken along the horizontal plane 2-2.
With reference to the drawing, the coal is fed into a vessel l0 through a line H at a rate regulated by a control valve [2, suitably a rotary or slide valve for handling solids. The vessel [0 suitably is a pressure vessel, generally similar in construction to conventional reactors wherein a fluidized bed of catalyst or carbonaceous material is utilized. Thus the vesesl is provided with a conical bottom [3 for distribution of the fluidizing gas, and with an enlarged upper section M for separation of solids from the effluent gas above the fluidized bed. The gasifier is, in general, somewhat more elongated than the conventional fluidized bed reactor so that the bed is relatively deeper than the typical fluidized bed, the purpose of which will be brought out more fully hereinafter.
Gas for fluidization and reaction is introduced to the vessel through line !6 and distributed by a suitable perforated distribution ring I 1. Residual solid material from the bottom of the vessel is withdrawn through line l8 at a rate controlled by the control mechanism [8 which may suitably be a screw-type conveyor or a rotary valve. Effluent gases are withdrawn from the vessel at a point above the upper level of the fluidized bed through line 2|. A suitable filter 22 may be provided at the gas outlet to effect removal of fines from the efliuent gas stream.
The gasifier is divided essentially into three zones, namely, a solid preheat zone 23, a reaction zone 24 and a gas preheat zone 25. Zones 23 and 25 are provided with a plurality of spaced partitions 26 which are disposed longitudinally within the reactor to divide these zones into a multiplicity of longitudinally extending passages or channels 21 disposed axially along the reactor. These passages 21 substantially prevent top-tobottom mixing of the solid material in the solid and gas preheat zones of the reactor. The partitions 26 may take various forms to provide longitudinally extending cells or passages of any desired cross-sectional configuration. Fig. 2 illustrates only one example of an arrangement of partitions which is suitable for the purpose. Numerous other arrangements will occur to those skilled in the art which will be functionally equivalent to the arrangement illustrated.
The passages should be so dimensioned that they have an effective size of a pipe having an internal radius falling within the range from about inch to about 2 inches, preferably from inch to about 1 inch. Thus if a passage has an eflective pipe size corresponding to an internal radius of, for example, inch, no solid particle flowing therethrough will be spaced from a wall by distance greater than inch, and the distance between a particle and the wall farthest away will be not greater than 1 inch. With the partitions spaced within the range above indicated, and passing a gaseous medium therethrough at a suitable velocity, readily determined by trial, depending on the density and particle size of the solid material, top-to-bottom mixing of the solid particles in the passages is prevented to an extent sufiicient to maintain a temperature gradient therethrough.
It is advantageous in the practice of this invention that the solid carbonaceous material be in the form of a powder, substantially all of which passes a 40 mesh secreen. The most advantageous particle size for any given system will depend upon the density of the material, the shape of the particles, the density and velocity of the fiuidizing gas, the size of the passageways, etc.; the optimum particle size for any given system is readily determinable by simple preliminary experiments conducted under conditions simulating those of actual operation.
The carbonaceous material treated is maintained in the reactor in a state of dense phase fiuidization. Under these conditions the particles are agitated by the gas stream and individually exhibit random movement. The upper surface of the bed or mass assumes a level, substantially above the normal level of the settled particles, which level is commonly known as a pseudo-liquid level. Under fluidized conditions the upper surface of the bed is disrupted by movement of the gas therethrough and resembles in appearance the surface of a boiling liquid.
In the conventional fluidized bed, there is thorough top-to-bottom mixing of the particles in the bed so the entire bed becomes a homogeneous mixture having a uniform temperature throughout. In the present invention, on the other hand, top-to-bottom mixing is prevented in the heat transfer zones of the gasifier.
During the flow of the gaseous medium through the bed or mass of particles in the gasifier, the
individual particles rise and fall but the general direction of movement of the particles is downward through the gasifier. Thus as the operation of the process progresses, fresh incoming particles form the upper surface of the mass and gradually progress downwardly toward the bottom of the bed. As the particles progress downwardly they are subjected to preheating, chemical reaction, and heat transfer to the incoming fluidizing gas. The residual solid material is removed from the lower portion of the gasifier.
The solids withdrawn from the lower portion of the gasifier may comprise char of any desired carbon content or ash substantially free from carbon. In general, the residual solid obtained from treatment of coal, lignite and the like, is a char of low carbon content.
The operation may be carried out so as to accomplish distillation of volatile constituents from the feed material, e. g., coking of coal, with only sufiicient oxidation to supply heat required for the distillation. In such an operation, with coal as feed, a high quality char may be obtained as the residual solid. This char may find use as a high grade smokeless fuel or it may be used wherever a high carbon char is indicated.
In some instances it is economically desirable to produce low grade char of limited fuel value. In such an instance the char may serve, under favorable conditions, as fuel for power generation or the like, particularly where the char may be used as fuel in the locality of the gasifier. In other circumstances, it is desirable to convert substantially all of the carbon content of the feed material to gases. Under these circumstances, substantially carbon-free ash is desired as the residual solid. The desired degree of conversion is readily controlled by the process of my invention by control of the rate of flow of solids and gases through the gasifier.
An outstanding characteristic of the gasifier described above is the ability to achieve countercurrent flow of the gases and fluidized solids and maintain a temperature gradient in the gas and solid preheating zones. Thus, in the solid preheating zone, the fluidized solid moves downwardly through the channels generally countercurrent to the upflowing gas stream. The partitions substantially eliminate top-to-bottom mixing of the particles. Thus, contrary to the situation existing in a conventional fluidized bed, the particles of solid in the solid preheating zone are at a considerably higher temperature at the bottom of the preheating zone than at the top. The extent of preheating may be determined by design, the highest temperature being close to the reaction temperature. Similarly, the solids are progressively cooled as they pass downwardly through the gas preheating zone below the reaction zone. The gas is progressively heated as it flows upward through the gas preheating zone. In the zone where chemical reaction takes place it is preferable to permit conventional fluidization with top-to-bottom mixing and a uniform temperature therethroughout.
The use of powdered fuel in a state of dense phase fluidization provides exceptionally high surface area of the solid fuel per unit of volume in the heat exchange zones and exceedingly intimate contact between the gas and the solid particles. The agitation of the solid particles promotes high rates of heat transfer. This insures optimum conservation of heat and most efficient transfer of heat between the solid particles and the gas streams.
It is contemplated that in some instances means will be provided for supplying heat from an external source to the fluidized reaction zone of the gasifier. For example, fire-tubes or elec tric arcs or electric resistance heaters may be disposed in reaction zone 24 of the gasifier l0 shown in the drawing.
It is noteworthy that the process of the invention is applicable to coking coals which may be fed to the gasifier without any pretreatment to alter their coking properties. Since the coal particles charged to the gasifier of this invention pass through a preheat zone in which the temperature of the particles is gradually increased and the particles flow countercurrently to gases ascending from the reaction zone, it is possible to alter at least the surfaces of the coal particles so that they exhibit little or no agglomerating tendency during processing by this invention.
Obviously, many modifications and variations of the invention as above set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.
*aeeacm I claim:
1. In a process for the gasification of a solid carbonaceous material, the improvement which comprises passing a mass of solid carbonaceous .top of said zone to the bottom thereof and having a relatively small, uniform cross-sectional area; passing hot gaseous products of reaction upwardly through said zone at a velocity which maintains the particles in said streams in a state of dense phase fiuidi'zation thereby maintaining an ascending temperature gradient in said streams of particles and a descending temperature gradient in said gaseous products of reaction; discharging the resulting preheated streams of particles directly into the top of a common bed of said particles in an unimpeded reaction zone; passing a preheated gaseous reactant upwardly through said bed of particles in said reaction zone at an elevated temperature effecting partial gasification of said particles and forming said hot gaseous products of reaction and a particulate residual solid; withdrawing said residual solid from the bottom of said reaction zone and passing said residual solid downwardly through a second heat exchange zone as a plurality of immediately adjacent separately confined parallel streams, each of said streams ex tending from the top of said second heat exchange zone to the bottom thereof and having a relatively small, uniform cross-sectional area; passing said gaseous reactant at a, temperature below said reaction temperature upwardly through said second heat exchange zone at a velocity which maintains said residual solid in a state of dense phase fluidization thereby maintaining a descending temperature gradient in said streams of residual solid and an ascending temperature gradient in said gaseous reactant; passing the resulting heated gaseous reactant to said reaction zone as said preheated gaseous reactant and withdrawing the resulting cooled streams of residual solid from said'second heat exchange zone.
2. A process as defined in claim 1 wherein the gaseous reactant passing upwardly in said second heat exchange zone comprises a mixture of steam and oxygen.
3. A process as defined in claim 1 wherein said solid carbonaceous material is coal.
References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,898,967 Schneider et al. Feb. 21, 1933 2,359,310 Hemminger Oct. 3, 1944 2,394,814 Snuggs Feb. 12, 1946 2,436,225 Ogorzaly et al Feb. 17, 1948 2,444,990 Hemminger July 13, 1948 2,451,804 Campbell et al Oct, 19, 1948 2,509,745 Riggs May 30, 1950 2,541,186 Anderson Feb. 13, 1951 2,595,365 Odell May 6, 1952 FOREIGN PATENTS Number Country Date 394,747 Great Britain July 6, 1933 586,391 Great Britain Mar. 18, 1947