GB2073386A - Wall structure for gasifier - Google Patents

Abstract

A wall structure for a gasification region of an entrained flow gasifier apparatus provides, at a specified gas temperature, velocity and residence time, a stabilized flowing slag layer as the primary insulation for the region. The wall structure is cooled and may have an inner surface such as metal 11 that will not react with the slag and that is provided with grooves 15 for receiving prior to use a suitable refractory material or cement. Alternatively, the inner surface may be flame sprayed to provide, for example, multiple layers grading from a cermet to ceramic. In use, slag adheres to the refractory material and builds up to cover and insulate the entire inner wall surface where desired. By choice of wall structure, one can select areas to be slagged and areas in which slagging is to be avoided. <IMAGE>

Description

SPECIFICATION Gasifier This invention relates to wall structure defining at least a portion of the walls of a gasification region of a gasifier apparatus for converting carbonaceous matter to synthesis gas or fuel gas.

The two stage entrained flow gasifier apparatus developed at Bituminous Coal Research, Inc., Pittsburgh, Pennsylvania, U.S.A. in the 1 960's has great potential for further development. The preferred embodiment of the present invention is illustrated and described as an improvement in the wall structure of such a two stage entrained flow gasifier apparatus.

In the two stage type, pulverized coal is introduced into a second or gasification stage to produce a process gas and a process char.

This process char is separated from the process gas and recycled and reacted with oxygen and steam in a first or combustion stage to produce hot combustion gas. As used herein "combustion gas" includes predominantly carbon dioxide and water vapor with lesser amounts of hydrogen and carbon monoxide.

The hot.combustion gas from the combustion stage is introduced into the aforementioned second stage and contacts the pulverized coal introduced into this stage. Here the coal is heated and reacted in contact with the combustion gas and steam to produce synthesis gas, some methane, and process char. This gasification reaction in accordance with the prior art is carried out typically at low gas flow velocities of the order of 2-1 2 feet per second, pressures of about 60 atmospheres and temperatures of about 1 200o K. Residence time in the second stage is substantially in excess of one hundred milli-seconds.

The pressure and temperature of the combustion gas produced in the first stage are such that in the second or gasification stage, the classic heterogeneous carbon/steam and carbon/carbon dioxide reactions take place to produce CO and H2.

Upon issuing from the second stage, the exiting gases and entrained char may be passed into a quenching zone to cool the gas and char to an acceptable downstream process temperature. Thereafter, the process stream is separated into its gaseous and char components.

The two stage entrained flow gasifier process has the ability to produce a tar-free, low sulfur content char product in addition to a gaseous product.

In gasifiers of the type here concerned, the degree and rapidity of initial mixing is related to the heating rate of individual coal particles, and one desires to attain a heating rate for individual coal particles which is only limited by the natural physical limit of heat conduction/convection from a hot gas to a coal particle which it surrounds. This limit is about 7 = 1 msec. for a d = 201l diameter particle in a hot gasifier environment and scales as ?-d2.

In situations of poor initial mixing of the stream of hot gas, the system becomes "mixing limited", dT/dt for individual particles is reduced, and hence the ultimate yield of volatiles is reduced by virture of "low activation energy" reactions having a significant role, i.e., by the time the coal particle temperature gets up to where the high temperature reactions can occur, it-is too late for them to make a significant contribution.

The second kind of mixing has to do with mixing the evolving volatiles with the background H20 and CO2 to allow "stabilizing" reactions before "cracking" reactions between hydrocarbon volatile species occur to form soot and hydrogen.

The benefits of a total gasification process are measured in terms of overall capital costs and operating costs and it is virtually impossible to assess the effect of an individual unit process on the cost of synthesis gas (SNG) by judging it by itself. A total process has been shown to have attractive capital cost and operating cost characteristics. One reason is that such a process makes good use of oxygen in a physical way, and thus uses minimal oxygen consistant with obtaining a product stream of syn-gas plus some methane, but free of oils and tars which require subsequent or separate reforming if syn-gas or SNG is the ultimate goal. Lower oxygen means less of the original coal carbon appears as CO2 (more as CO) and, hence, not only is the oxygen plant reduced, but so is the substantial acid gas (CO2) removal equipment.The reactor or gasification stage utilizes H20 rather than H2 as in hydro-gasification processes and thus does not pay the price of hydrogen production for this purpose. Finally because of the utilization of high velocities and short residence times in the gasification stage, there is a benefit of a small low cost gasifier itself.

In single stage entrained flow gasifiers, coal is injected into a hot environment together with oxygen and stream. As coal particles devolatilize, the hydrocarbon volatiles that come off undergo homogeneous gas phase reactions in the background mixturc they react via the fastest reaction, which means they react with the oxygen. The oxygen is thus used up and makes the heat which is required, but one is left with the residual char to be reacted heterogeneously with the steam-a slow and inefficient reaction compared to reacting char with oxygen. A better approach is to use the valuable oxygen to combust and burn out the char, which is more difficult to react, and use the steam (and the CO2) to stabilize the more reactive volatiles.

This is the approach and the advantage of the two stage entrained flow gasifier compared to a single stage device. A single stage gasifier typically has internal region(s) of very high temperature, where oxidation of the coal has occurred, but gasification has not proceeded to a significant degree. In these regions, the coal slag is typically molten on the gasifier walls.

The pyrolitic devolatilization concept of a two stage entrained flow gasification process provides compatible conditions to achieve high thermal efficiency when producing clean fuel gases. In the first or combustion stage, residual char preferably is combusted with oxygen in a near stoichiometric mix with carbon to form primary carbon dioxide and steam. The principal objective in operating at near total combustion is not for the production of hydrogen and carbon monoxide as in partial oxidization, but rather to provide a source of heat for pyrolysis which occurs in the second stage.

In the second or gasification stage, fresh coal is contacted with the product gases from the combustion stage at a temperature preferably in the order of about 3500"F. Aided by rapid and efficient mixing at high temperatures, pyrolytic devolatilization can result in the conversion of up to or greater than 60 percent of the coalcarbon into gas phase constituents. The use of steam in both combustion and pyrolysis provides a stabilizing background atmosphere designed to prevent solid carbon formation. The subsequent interaction of the chemical species generated by rapid devolatilization yields a clean fuel gas free of ethane and heavier hydrocarbons at an equilibrium temperature in the range of 1 600 F.

As may now be seen, it is desirable to operate a gasifier with some regions or components exposed to very high temperature flow. This can result in development of a molten slag coating on the internal wall surface. A two stage gasifier may optimally be operated with wall slagging in the combustion stage, the coal injection region, and part of the pyrolyzer.

Prior art technology in the gasifier field utilizes a wall structure which typically consists of an outer pressure shell with an inner lining of a refractory in the form of bricks, cast, or rammed material. This serves as the primary insulation structure. When the gasifier temperature is high enough for slag fusion to occur as noted above, the refractory inner wall structure typically serves as a bonding surface on which a slag layer develops.

Prior art methods of wall structure such as noted above which are generally satisfactory for installations having large, exposed surface areas, low velocity flows and/or low temperature operation have the primary disadvantage of being incompatible with long duration high temperature operation. Typically, the refractory forming part of the wall is fluxed out by the flowing slag in a non-uniform manner, resulting in a partially insulated wall. This can result in increased heat losses and changes in the gasifier flow field associated with the changed wall structure geometry. A further disadvantage is the thermal stress limit of such a wall, requiring slow system heatup or cooldown to avoid loss.

According to the invention, there is provided wall structure defining at least a portion of the walls of a gasification region of a gasifier apparatus for converting carbonaceous matter to synthesis gas or fuel gas, the gasifier apparatus being of the type in which a stream of hot products of combustion at a temperature exceeding 2300"F. and at a velocity of at least 100 feet/second is introduced with a separate stream of finely divided carbonaceous fuel into the gasification region, the hot products of combustion including slag having mineral constituents in part, at least some of which are not solid, and having a residence time of from 1 millisecond to 100 milliseconds in the portion of the gasification region in which the temperature exceeds 2300"F., wherein the wall structure is provided with an inner member which carries means to which the slag in the hot products of combustion will adhere, and with means for maintaining the inner member and the means carried thereby at a temperature less than the fusion temperature of the slag.

Such a wall structure is specifically designed to give a stabilized flowing slag layer as the primary insulation for single and multistage entrained flow gasifiers. It may comprise a cooled metallic, carbide or nitride wall structure, with which is incorporated refractory oxide elements and is designed so that at no point does the temperature of metal or refractory approach or exceed the slag fusion temperature. The refractory chosen may be, for example, predominantly a single oxide such as Al2O3, MgO, or ZrO2; or other refractory compounds such as, for example, an aluminosilicate or zirconium silicate. One method of fabricating the wall structure is to machine normal to the direction of gas flow, grooves on the hot face of the cooled wall, and to cast into those preferably square or retangular grooves a refractory cement, e.g., phosphate bonded zirconia. Another method may comprise applying a flame spray to the wall surface typically comprising multiple layers grading from cermet to ceramic. Wall structural design, cooling, and ceramic type application and dimensions are chosen to obtain metal and ceramic temperature within certain bounds. It is preferable to operate the metal or other structural material at as low a temperature as possible, while avoiding a temperature below the dew point of the working fluid.

The ceramic component of the wall structure is preferably designed thermally to operate with the peak ceramic temperature less than the fusion temperature of slag/ceramic com positions to avoid ceramic loss by solution in the flowing slag and washout. It is also dedirable to operate with at least a portion of the ceramic, for example that near the center of grooves or the like, hot enough and with low enough thermal diffusion capability in the absence of a slag coating to effect bonding of slag deposited on it as droplets, vapor or liquid flow. This is desirable in order to facilitate establishment of a bonded slag coating on an initially uncoated wall. The present invention has the primary advantage of providing a stable, predictable slagged wall structure compatible with high temperature entrained flow gasifier operation.It has the further advantage, by choice of wall structure, of selecting those wall areas which are to be slagged, and those which are not. In particular, while the walls in the high temperature regions may be designed to be slagged with all of the advantages resulting therefrom, the downstream walls can be designed to avoid slagging, thus avoiding fouling in low temperature regions. It has the further advantage of being compatible with both single and multistage entrained flow gasifiers. It has the still further advantage of providing a controllable, erosion resistant structure for use in high temperature, high velocity locations of a gasifier.

A potential disadvantage of the present invention is the large amount of heat that would be lost through the cooled wall structure in an installation having a large area of exposed wall surface. On the other hand, it is ideally suited to use in a high velocity entrained flow gasifier because only a relatively small gasifier size and correspondingly limited wall area is required for a high to very high throughput.

Thus, due to the amount of heat that of necessity must be lost through gasifier walls in accordance with the present invention, the present invention may be advantageously used only in certain types of gasifiers. These types are entrained flow gasifiers that operate with temperature locally in excess of about twenty-three hundred degrees Fahrenheit with a gas flow velocity of at least about one hundred feet per second, and wherein the gas in such portion of the gasifier in which the temperature is in excess of twenty-three hundred degrees Fahrenheit has a residence time of about one millisecond to one hundred milliseconds.

In order that the invention may be more fully understood, it will now be described in conjunction with the accompanying drawing, in which: Figure 1 is a fragmentary view in section of a wall structure in accordance with the invention; and Figure 2 is an enlarged fragmentary side elevation of the wall structure of Fig. 1 having a deposited layer of slag on the inner surface thereof.

Since the present invention is directed to, and concerned only with, the prdvision of improved wall structures for gasifiers, no detailed discussion of the construction and operation of suitable gasifiers is deemed necessary and is omitted for purposes of clarity and convenience.

Referring now to Fig. 1, there is shown a portion of the wall structure bounding a gasification stage or region of a gasifier, comprising an outer metal pressure shell 10 and an inner metal wall portion 11 spaced from the outer metal pressure shell to provide therebetween a coolant flow region 1 2. Coolant, such as water, is continuously supplied to and exhausted from the coolant flow region 1 2 in a conventional manner by coolant manifolds 1 3 to remove heat from the inner wall 11. Provided in the exposed surface of the inner metal wall 11 are a series of grooves 14 at least substantially normal to the direction of gas flow, which for this example is from left to right as indicated by the arrow in Fig. 1.

Disposed within each groove is a refractory material 1 5 which may be conveniently a refractory cement such as, for example, phosphate bonded zirconia.

The protective slag layer 16, which forms during operation as more fully discussed hereinafter, while shown in Fig. 2, is not shown in Fig. 1 which is representative of a gasifier wall structure in accordance with the invention prior to its use in actual practice.

The wall structure is arranged, and adapted to the intended gasifier operating conditions, to maintain the inner metal wall 11 and the refractory material 1 5 carried by it at a temperature less than the slag fusion temperature of the coal or other carbonaceous material being used. The grooves 14 may, for example, have a width about twice the local equilibrium slag layer thickness, a depth about equal to or greater than the width, and a spacing of metal or other suitable wall material about equal to the groove width. The local slag layer steady state equilibrium thickness is controlled by gasifier conditions, slag properties, and wall thermal design, and may typically be in the range of from one to ten millimeters.The provision of the refractory on the inner wall is essential to initiate slag formation which eventually results in the covering of the inner wall 11.

The depth of the grooves need be only sufficient to retain the refractory material and of sufficient width that slag will adhere to the refractory material and build up as more fully discussed hereinafter. The spacing of the grooves one from another should not be in excess of that which will permit bridging of the slag from one groove to the next.

Upon startup of a gasifier incorporating the wall structure shown in Fig. 1, slag initially begins to attach or become bonded to the refractory filled grooves 1 4. Development of local bumps or the like typically results in small scale streamer runoff across the exposed metal between the grooves (due to high shear loading from the gas flow or gravitational force). This streamer runoff is then followed by broader curtain-like structures until the exposed metal is bridged locally and a continuous flowing slag coating develops.

Typically, final development of a steady state continuous flowing slag coating occurs as a result of shear flow in the gas flow direction initiated near the upstream end of the wall structure, or in a region of high slag deposition rate on the wall. Fig. 2 shows, on an enlarged scale for convenience of illustration, the grooved wall structure with a continuous insulating slag layer 1 6 formed on the exposed inner surface of the inner metal wall 11.

In high temperature operation, the slag surface may be expected to equilibrate at from about 2500 to 3000"F., controlled by shear and body forces on the slag. The slag viscosity is strongly dependent on temperature.

Peak metal wall temperatures for such slag surface temperatures may be maintained in the range of from about 300 to 500"F. to inter alia, provide low chemical corrosion rates and preserve integrity of the wall structure.

Broadly, the qualitative growth behavior and steady state conditions of the slag layer on the grooved wall surface may be expected to be substantially independent of both wall surface and gas velocity up to supersonic velocities. Accordingly, embodiments of the invention may use inner walls of various metals and with gas flows up to at least supersonic.

During use, boundary layer slag transport and shear flow are the primary causes of slag deposition on the wall. Re-entrainment of slag in the gas flow, together with droplet removal and surface vaporization, are the primary causes of slag removal. Local steady state, constant thickness slag flow is generally the result of a balance between local deposition and local shear flow of the molten slag.

If desired, the provision of grooves may be omitted and the inner surface of the metal wall 11, may in accordance with the invention, be provided with a smooth and continuous flame sprayed coating over its entire surface to provide a base for attachment of the slag. Such a coating may be provided by building up, by flame spraying, successive layers of a graded conventional cermet or layers grading from a conventional cermet to a conventional ceramic. Such a coating may comprise, for example, up to four or more layers each having a thickness of from about five to thirty or more micrometers. The coating material may be provided in conventiorial manner as a powder feed to a conventional oxyacetylene spray device.

An advantage of a flame sprayed inner wall in accordance with the invention is that, as compared to the grooved wall construction, the flame sprayed wall exhibits more uniform local slag development with stronger bonding of the slag to the wall surface. Further, flame sprayed walls as described above tend to retain their slag layer longer everywhere during startup and shutdown transients.

At some particular downstream location, the temperature will have decreased to a point that the slag will solidify. Upstream of such a location and/or at any location where the formation of slag on the inner wall surface is undesirable, such may be accomplished by providing only a cooled but smooth metal, nitride, carbide or other surface not wetted by slag. This may be an exposed smooth surface of the inner metal wall 11. Further, at such a location, to further decrease the possibility of slag attachment, the inner wall surface may be made slightly divergent, thereby reducing the tendency of slag to contact the wall at this point, and to prevent impact of re-entrained slag on the downstream structure, thus minimizing erosion.

The various features and advantages of the invention are thought to be clear from the foregoing description. Various other features and advantages not specifically enumerated will undoubtedly occur to those versed in the art, as likewise will variations and modifications of the preferred embodiment illustrated, all of which may be achieved without departing from the scope of the invention.

Claims (9)

1. Wall structure defining at least a portion of the walls of a gasification region of a gasifier apparatus for converting a carbonaceous matter to synthesis gas or fuel gas, the gasifier apparatus being of the type in which a stream of hot products of combustion at a temperature exceeding 2300"F. and at a velocity of at least 100 feet/second is introduced with a separate stream of finely divided carbonaceous fuel into the gasification region, the hot products of combustidn including slag having mineral constituents in part, at least some of which are not solid, and having a residence time of from 1 millisecond to 100 milliseconds in the portion of a gasification region in which the temperature exceeds 2300"F., wherein the wall structure is provided with an inner member which carries means to which the slag in the hot products of combustion will adhere, and with means for maintaining the inner member and the means carried thereby at a temperature less than the fusion temperature of the slag.

2. Wall structure according to claim 1, wherein the inner member is metal and the temperature maintaining means is a coolant arrangement for removing heat from said inner member.

3. Wall structure according to claim 1 or 2, wherein the means carried by the inner member is a material which maintains its integrity during operation of gasifier apparatus.

4. Wall structure according to any of claims 1 to 3, wherein the means carried by the inner member is a refractory material.

5. Wall structure according to claim 4, wherein the refractory material is disposed in grooves in the inner member.

6. Wall structure according to claim 5, wherein the grooves are spaced one from another and are disposed at least substantially normal to the direction of gaseous flow in the gasification region.

7. Wall structure according to any of claims 1 to 4, wherein the means carried by the inner member is a coating which maintains its integrity during operation of a gasifier apparatus.

8. Wall structure according to any of claims 1 to 4, wherein the means carried by the inner member is a coating made of a refractory material.

9. Wall structure according to claim 2, wherein the means carried by the metal inner member is carried thereby only where the wall structure defines said portion of the walls of the gasification region and is omitted from the metal inner member where the wall structure defines a further portion or portions of said walls.

1 0. Wall structure defining at least a portion of the walls of a gasification region of a gasifier apparatus for converting carbonaceous matter to synthesis gas or fuel gas, constructed and arranged substantially as herein described with reference to the accompanying drawing.