PROCESS FOR THE PARTIAL COMBUSTION OF HYDROCARBONACEOUS FUELS TO PRODUCE SUBSTANTIALLY SOOT-FREE GASES
United States Patent 3868331
The soot content of a hydrogen and carbon monoxide-containing gas mixture obtained by the partial combustion of a hydrocarbonaceous fuel is substantially reduced by passing the partially-combusted gases through a soot-conversion zone maintained at an elevated temperature and pressure, and retarding the passage of soot particles through the zone for a sufficient period of time that the soot is substantially converted to carbon monoxide. An apparatus suitable for effecting soot-conversion is also disclosed.
US Patent References:
Preparation of gaseous fuel
Hemminger - July 1952 - 2605178
Method for producing oil gas
Haug - May 1955 - 2709646
Gas generation
Keith - January 1957 - 2779667
Self-sustaining regenerative process
Hasche - July 1958 - 2844452
Method for cracking and subsequent gasifying of hydrocarbons
Hilgers - July 1962 - 3042507
Preparation of gaseous fuel
Hemminger - July 1952 - 2605178
Method for producing oil gas
Haug - May 1955 - 2709646
Gas generation
Keith - January 1957 - 2779667
Self-sustaining regenerative process
Hasche - July 1958 - 2844452
Method for cracking and subsequent gasifying of hydrocarbons
Hilgers - July 1962 - 3042507
Application Number:
05/370996
Publication Date:
02/25/1975
Filing Date:
06/18/1973
View Patent Images:
Export Citation:
Assignee:
Shell Oil Company (New York, NY)
Primary Class:
Other Classes:
48/212, 48/202, 48/197R, 48/95, 252/373, 48/215, 48/206
International Classes:
C01B3/22; C01B3/36; C01B3/00; C01B2/14
Field of Search:
48/99,12R,63,89,105,107,95,197R,211,212,213,215,196R,202,206 252/373
US Patent References:
| 3048481 | Method of forming gas tight seal between vessel wall and refractory lining of a synthesis gas generator | August 1962 | Eastman | |
| 3536455 | PLANT FOR THE PRODUCTION OF METALLURGICAL REDUCING GAS | October 1970 | Bogdandy et al. | |
| 3607154 | REFRACTORY BED FOR GAS MACHINE AND PROCESS FOR PRODUCING OIL GAS | September 1971 | White et al. | |
| 3682605 | August 1972 | Wada | ||
| 3715301 | February 1973 | Tassoney et al. |
Primary Examiner:
Bashore, Leon S.
Assistant Examiner:
Kratz, Peter F.
Attorney, Agent or Firm:
Jecminek A. A.
Claims:
What is claimed is
1. In a process for the generation of a hydrogen and carbon monoxide-containing gas mixture by the partial combustion of a hydrocarbonaceous fuel with an oxygen-containing gas in the presence of steam in a partial combustion zone at a temperature of from about 1,000° to about 1,600°C and a pressure from 1 to 300 atmospheres whereby a crude hydrogen, steam, carbon dioxide and carbon-monoxide containing gas mixture containing contaminating amounts of soot in particulate form is obtained, the improvement which comprises substantially reducing the soot content of the crude gas mixture by passing the gas mixture into a soot-conversion zone which contains at least one bed of ceramic material and is maintained at substantially the same elevated temperature and pressure as that of the partial combustion zone, and retarding the passage of the soot particles through said soot-conversion zone for at least 5 seconds by adherence of said soot particles to said ceramic material whereby the soot particles are converted to carbon monoxide by reaction of the soot particles with combined stoichiometric amounts of steam and carbon dioxide in the crude gas mixture, reducing the soot content of the crude gas mixture to less than l percent by weight.
2. The process of claim 1 wherein the pressure in the partial combustion zone is between 10 and 150 atmospheres.
3. The process of claim 2 wherein the time that the soot particles remain in the soot-conversion zone is between 15 and 50 seconds.
4. The process of claim 1 wherein the bed of ceramic material through which the crude gas mixture is flowed comprises layers of perforated bricks, the perforations of the bricks in each lower layer interconnecting with the perforations of adjoining upper layers to form elongated tubes in the direction of gas flow.
5. The process of claim 1 wherein the bed of ceramic material through which the crude gas mixture is flowed comprises a fluid bed of ceramic particles.
6. The process of claim 4 wherein the temperature of the gases in the partial combustion zone is from 1,300°C to 1,500°C and the soot content of the gases leaving the soot-conversion zone is less than 0.3 percent by weight.
7. The process of claim 5 wherein the average diameter of the ceramic particles is between 10 and 1,000 microns.
8. The process of claim 6 wherein the diameter of the elongated tubes through which the crude gas mixture flows is between 0.5 cm and 2.5 cm and the surface area of the elongated tubes is between 30 square meters and 200 square meters per cubic meter of said bricks.
9. The process of claim 7 wherein the superficial velocity of the gases passing through the fluid bed lies between 0.1 and 1 meter per second.
10. A process of claim 9 wherein the temperature of the gases in the partial combustion zone are from 1,300° to 1,500°C and the soot content of the gases leaving the soot conversion zone is less than 0.3% by weight.
1. In a process for the generation of a hydrogen and carbon monoxide-containing gas mixture by the partial combustion of a hydrocarbonaceous fuel with an oxygen-containing gas in the presence of steam in a partial combustion zone at a temperature of from about 1,000° to about 1,600°C and a pressure from 1 to 300 atmospheres whereby a crude hydrogen, steam, carbon dioxide and carbon-monoxide containing gas mixture containing contaminating amounts of soot in particulate form is obtained, the improvement which comprises substantially reducing the soot content of the crude gas mixture by passing the gas mixture into a soot-conversion zone which contains at least one bed of ceramic material and is maintained at substantially the same elevated temperature and pressure as that of the partial combustion zone, and retarding the passage of the soot particles through said soot-conversion zone for at least 5 seconds by adherence of said soot particles to said ceramic material whereby the soot particles are converted to carbon monoxide by reaction of the soot particles with combined stoichiometric amounts of steam and carbon dioxide in the crude gas mixture, reducing the soot content of the crude gas mixture to less than l percent by weight.
2. The process of claim 1 wherein the pressure in the partial combustion zone is between 10 and 150 atmospheres.
3. The process of claim 2 wherein the time that the soot particles remain in the soot-conversion zone is between 15 and 50 seconds.
4. The process of claim 1 wherein the bed of ceramic material through which the crude gas mixture is flowed comprises layers of perforated bricks, the perforations of the bricks in each lower layer interconnecting with the perforations of adjoining upper layers to form elongated tubes in the direction of gas flow.
5. The process of claim 1 wherein the bed of ceramic material through which the crude gas mixture is flowed comprises a fluid bed of ceramic particles.
6. The process of claim 4 wherein the temperature of the gases in the partial combustion zone is from 1,300°C to 1,500°C and the soot content of the gases leaving the soot-conversion zone is less than 0.3 percent by weight.
7. The process of claim 5 wherein the average diameter of the ceramic particles is between 10 and 1,000 microns.
8. The process of claim 6 wherein the diameter of the elongated tubes through which the crude gas mixture flows is between 0.5 cm and 2.5 cm and the surface area of the elongated tubes is between 30 square meters and 200 square meters per cubic meter of said bricks.
9. The process of claim 7 wherein the superficial velocity of the gases passing through the fluid bed lies between 0.1 and 1 meter per second.
10. A process of claim 9 wherein the temperature of the gases in the partial combustion zone are from 1,300° to 1,500°C and the soot content of the gases leaving the soot conversion zone is less than 0.3% by weight.
Description:
BACKGROUND OF THE INVENTION
The present invention relates to a process and an apparatus for the partial combustion of hydrocarbonaceous fuels to produce gases having a substantially reduced soot content.
The gases produced by processes for the partial combustion of hydrocarbonaceous fuels invariably contain a substantial amount of free carbon (i.e., soot) which is undesirable for the subsequent utilization or processing of the gases and which should therefore be removed. The removal of the soot is normally achieved by scrubbing the gases with water. The resulting dispersion of soot particles in water is then processed in order to recover substantially soot-free water and soot. The soot removal facilities are expensive and the recovered soot generally has a low value. It is therefore an object of the present invention to provide a process for the combustion of hydrocarbonaceous fuels to produce gases having a substantially reduced soot content and thus eliminate the need for expensive soot-removal facilities. It is also an object of the invention to increase the efficiency of the partial combustion process by ensuring that substantially all the carbon content of the hydrocarbonaceous fuels combusted is converted into more valuable carbon monoxide.
The presence of soot in the hot gases produced by the partial combustion of hydrocarbonaceous fuels provides difficulties with regard to the recovery of heat from the gases. It is not generally possible to use heat exchangers of the normal straight flame tube type since considerable deposits of soot are formed on the inside walls of the tubes and thus greatly reduce the heat transfer coefficient. It is possible to use a heat exchanger of the helical tube type, but such heat exchangers are expensive and are unsuitable for high pressure steam generation due to the possibility of tube collapse owing to small unroundness of the tubes. Unroundness of a tube signifies its deviation from perfect radial and axial symmetry. It is therefore another object of the present invention to produce gases having a substantially reduced soot content; the recovery of heat from which can be carried out with normal straight tube heat exchangers producing high pressure steam.
SUMMARY OF THE INVENTION
It has now been found that in a process for the generation of a hydrogen and carbon monoxide-containing gas mixture by the partial combustion of a hydrocarbonaceous fuel with an oxygen-containing gas (e.g., oxygen, air, or oxygen-enriched air) in the presence of steam in a partial combustion zone, that the soot content of the resulting crude gas mixture can be substantially reduced by passing the gas mixture containing soot in particulate form into a soot-conversion zone maintained at substantially the same elevated temperature and pressure as that of the partial combustion zone, and retarding the passage of the soot particles through said soot-conversion zone for at least 5 seconds, e.g., from 5 to 50 seconds. It has been found that if the soot particles are retained in the soot-conversion zone for this length of time, they are substantially converted into carbon monoxide by reaction with steam and/or carbon dioxide which is also present in the crude gas mixture. By practice of the invention it is possible to reduce the soot content of the crude gas mixture from amounts of from 2 to 5 percent by weight or higher down to below 1 percent by weight, and generally below 0.3 percent by weight. This substantial reduction in in soot concentration correspondingly reduces or eliminates the need for downstream soot removal facilities and permits the use of conventional straight tube heat exchangers to abstract heat from the hot crude gas mixture, instead of more expensive helical coil heat exchangers which are commonly employed in this service. In addition, the conversion of the carbon to carbon monoxide in the soot conversion zone increases the calorific value of the gases produced.
DETAILED DESCRIPTION OF THE INVENTION
In processes for the partial combustion of hydrocarbonaceous fuels which have so far been proposed, the average residence time of the soot particles in the hot gases is very small because the gases are either immediately quenched with water or immediately passed to a heat exchanger as soon as they pass out of the reactor. Accordingly the soot particles are not allowed sufficient time for further reaction at high temperature. In the present process, however, the gases passing out of the reactor enter a soot-conversion zone in which the passage of soot particles, but not of gases, is retarded. This has the effect of increasing the residence time during which the soot particles are in contact with the hot reactor gases. The soot particles are thus allowed sufficient time in which they can react with steam and/or carbon dioxide in the gases at high temperature and pressure to form carbon monoxide. The reaction is believed to proceed according to the following equations:
C + H 2 O ➝ CO + H 2
C + CO 2 ➝ 2CO
The relative proportions of steam and carbon dioxide present in the partially combusted gases is not critical. It is only required that the combined steam and carbon dioxide concentrations be stoichiometrically sufficient to effect conversion of the free carbon to carbon monoxide in accordance with the above equations.
The operation of the soot-conversion zone can therefore be seen to comprise firstly retarding the passage of soot particles and secondly to allow sufficient time for conversion of the particles so retarded into carbon monoxide. The advantage of this operation is two-fold. Firstly it enables the production of substantially soot-reduced gases, and secondly it allows substantial conversion of the carbon content of the hydrocarbonaceous fuel into carbon monoxide, thus increasing the calorific value of the gases produced as previously mentioned.
In general, the retardation and conversion of the soot particles in the soot-conversion zone is such that the percentage by weight of soot particles present in the gases leaving said zone is reduced to less than 1 percent. Preferably, however, the percentage by weight of soot particles present in the gases leaving said zone is reduced to less than 0.3 percent.
It is important that the soot particles are sufficiently retarded in the soot-conversion zone that their residence time therein allows substantial conversion to carbon monoxide to take place. Accordingly, the soot particles suitably remain in the soot-conversion zone for at least 5 seconds. It is, however, preferred that the time that the soot particles remain in the soot-conversion zone is between 15 and 50 seconds.
Retardation of the soot particles is suitably effected by a soot-conversion zone consisting of at least one bed of ceramic material or any equivalent material which is resistant to high temperatures. In general, any ceramic material capable of resisting the high temperatures present in the soot-conversion zone, e.g., 1,000°-1,600°C or higher, can be employed. Ceramic materials having the requisite refractory properties are known to those skilled in this art and are described, for example, in "Encyclopedia of Chemical Technology" by Kirk and Othmer, Second Edition, 1968, Vol. 4, pages 762-775. Suitable ceramic materials include various carbides, e.g., the carbides of silicon, boron, zirconium, hafnium, tantalum, vanadium, molybdenum, tungsten and niobium; various aluminum silicates, particularly sillimanite and Korund (a special alumina refractory material containing at least 95 percent alumina); various borides, nitrides and sulfides of high melting point metals, and various oxides such as mullite, zircon and the like. The soot particles adhere to the surface of the ceramic material under the influence of Van der Waals-type forces.
The bed of ceramic material advantageously consists of packed perforated bricks. The bricks are preferably packed on top of one another in layers such that the perforations of the bricks in a lower layer interconnect with the perforations of the bricks in an upper layer. In this way the interconnecting perforations form elongated tubes in the direction of gas flow within the soot-conversion zone. The gases pass through the elongated tubes but the soot particles adhere to the walls thereof and are therefore retarded. The perforated bricks are preferably made from Korund, carbides or sillimanite, although they may be made from any other suitable ceramic material which can resist high gas temperatures. The diameter of the elongated tubes may vary over a wide range. However, it should be sufficiently large such that deposition of soot particles does not restrict the flow of gases unduly and sufficiently small such that a large surface area of ceramic material is afforded for soot adhesion and these factors therefore dictate that the diameter preferably is between 0.5 cm and 2.5 cm. The surface area of the elongated tubes for gas contact may also vary considerably. Preferably the surface area is between 30 square meters and 200 square meters per cubic meter of perforated bricks. The average gas velocity through the elongated tubes is not critical, but in general lies between 1 meter per second and 10 meters per second.
The ash particles in the hot gases, as in the case of the soot particles, are temporarily retarded in the soot-conversion zone. However, unlike the soot particles they do not react further with the hot gases but pass through the said zone unchanged. A very small proportion of the ash particles however is retained within the soot-conversion zone and thus a gradual build-up of ash particles therein occurs. This build-up is accelerated if the ash content of the hydrocarbonaceous fuel is high. This eventually leads to plugging of the perforations of the bricks and inefficient soot conversion. Accordingly, after a shorter or longer period of operation it is necessary to remove the old bricks from the soot-conversion zone and install new ones. The intervals between successive brick renewal periods depend to a large extent on the ash content of the hydrocarbonaceous fuel combusted, but in general are between 1 month and 12 months. In order to ensure continuous operation of the soot-conversion zone a swing reactor system is suitably utilized, whereby while a bed of bricks in one reactor is being replaced, another bed of bricks in another reactor is in operation.
In another embodiment of the present invention, the bed of ceramic material suitably consists of at least one fluid bed of ceramic particles. The particles are made from any suitable ceramic material and in particular from Korund, carbides or sillimanite. The diameter of the particles may vary over a wide range, but as in the case of perforated bricks, consideration of gas flow rate through the bed and surface area for gas contact requires the diameter to be in general between 10 and 1,000 microns. The superficial velocity of the gases passing through the fluid bed preferably lies between 0.1 and 1 meter per second, although operation above and below these figures is quite possible.
During operation a certain proportion of the ash particles in the hot gases is permanently retained in the fluidized bed of ceramic particles. A gradual build-up of ash particles therefore occurs and this build-up occurs more rapidly if the ash content of the hydrocarbonaceous fuels combusted is high. Accordingly, after a given period of operation it is necessary to replace the old ash-containing particles with fresh particles. The intervals between successive bed replacements in general are between 2 months and 20 months. A swing reactor system may suitably be employed. Alternatively continuous addition of fresh particles and withdrawal of ash-containing particles may be effected in order to provide continuous operation of the soot-conversion zone so that intermittent bed replacement is not necessary.
The temperature and pressure at which soot conversion in the soot-conversion zone takes place are substantially the same as the temperature and pressure at which partial combustion occurs. The pressure in the partial combustion zone can be from 1 to 300 atmospheres, but preferably is between 10 and 150 atmospheres. The temperature within the partial combustion zone is, in general, from about 1,000°C to about 1,600°C, but preferably from 1,300°C to 1,500°C. These same ranges of pressure and temperature apply to the soot conversion zone.
A considerable advantage of the present invention lies in the fact that the production of substantially soot-reduced gases greatly simplifies and facilitates the subsequent heat recovery operation therefrom. In cases where the soot content of the gases is negligible, it is possible to recover heat from the gases in a conventional straight tube type heat exchanger since soot deposition on the inside walls of the tubes no longer occurs. Thus the more expensive helical tube heat exchangers are no longer necessary and also heat recovery at higher steam pressures becomes possible, since conventional straight-tube exchangers can operate at higher pressures than helical tube exchangers. Hence, cheaper equipment can be used and more efficient heat recovery is achieved. At a sufficiently low content of particulate matter it is even possible to use water-tube heat exchangers, which permit heat recovery at still higher steam pressures enhancing further the thermal efficiency. In cases where small amounts of soot still remain in the gases passing to the heat exchanger it may nevertheless be desirable to use a helical gastube heat exchanger.
Any suitable hydrocarbonaceous or carbonaceous fuel may be partially combusted according to the present invention including natural gas, heavy oils, coal, coke and shale oil, tar sands oil, etc. In general the fuel will be a liquid hydrocarbon fuel. If a solid fuel is employed, it may be combusted in either a dry pulverized form or in a slurry with liquid.
The invention also relates to an apparatus suitable for carrying out the process described hereinbefore. This apparatus comprises:
(a) a brick-lined partial combustion reactor free of internals and solids with an inlet for fuel, oxygen-containing gas and steam, and an outlet for discharging partially combusted, soot particle-containing gases, and
(b) a soot-conversion vessel connected to the partial combustion reactor containing a bed of ceramic material of sufficient thickness and porosity to retard passage of the soot particles through said soot-conversion vessel for at least 5 seconds whereby the soot particles are substantially converted to carbon monoxide, said soot-conversion vessel having an inlet for the partially combusted, soot-containing gases from the partial combustion reactor, and an outlet through which the soot-reduced gases are discharged from the soot-conversion vessel.
In a preferred embodiment, the outlet of the soot-conversion vessel is connected to a waste heat boiler. The waste heat boiler is suitably of the conventional straight tube type if the gases leaving the soot-conversion vessel contain negligible amounts of soot. If small amounts of soot are still present in the gases then a helical tube type heat exchanger may be employed.
The number of beds of ceramic material contained within the soot-conversion vessel is not of critical importance. Accordingly any number of separate beds, within reason, may be contained within the soot-conversion vessel although one single bed is preferred. The bed(s) of ceramic material must be of sufficient porosity to permit the passage of the gases therethrough while retaining the soot particles for the prescribed length of time to effect their conversion to carbon monoxide, i.e., at least 5 seconds.
According to one embodiment of the invention the soot-conversion vessel contains a bed of packed perforated bricks. The perforated bricks are packed on top of one another in layers such that the perforations in the bricks in a lower layer interconnect with the perforations of the bricks in an upper layer. In this way the interconnecting perforations form elongated tubes in the direction of gas flow within the soot-conversion vessel. The total length of the elongated tubes formed by the packed perforated bricks varies according to, inter alia, the surface area required for soot retardation, the diameter of the perforations and the soot content of the combustion gases entering the soot-conversion zone, but in general is between 0.5 and 5 meters. The surface area of the elongated tubes suitably lies between 30 square meters and 200 square meters per cubic meter of the packed perforated bricks.
The number of layers of perforated bricks within the soot-conversion zone depends on the length of elongated tubes required and the size of the individual bricks but advantageously lies between 2 and 100. The size of the individual bricks is not critical, but for reasons of ease of handling suitably lies between 15 cm long, 5 cm wide and 5 cm deep and 60 cm long, 20 cm wide and 20 cm deep. The number of perforations per brick suitably lies between 5 and 750, and the diameter of the perforations between 0.5 cm and 2.5 cm.
In another embodiment, the soot conversion vessel contains at least one bed of fluidizable particles. The size of the particles is not critical but advantageously the average diameter thereof is between 10 and 1000 microns.
A method in which the process and apparatus according to the present invention are suitably used for the partial combustion of hydrocarbonaceous fuels to produce substantially soot-reduced gases is described below with reference to the accompanying drawings, by way of example and without limitation.
FIG. 1 is a diagrammatic representation of an apparatus for the partial combustion of hydrocarbonaceous fuels to produce substantially soot-reduced gases.
FIG. 2 is a partially cutaway, perspective view of a perforated ceramic brick suitable for use in the soot-conversion vessel.
With regard to FIG. 1, hydrocarbonaceous fuel is introduced via line 1 and oxygen is introduced via line 2 into burner part 3 of partial combustion reactor 4. Steam if required may be introduced via both lines 1 and 2. The hot combustion gases pass through a connecting piece 5 which connects the reactor to soot-conversion vessel 6. The latter contains a bed of ceramic material 7 through which the hot combustion gases pass in upflow. The bed of ceramic material can suitably comprise ceramic bricks as shown in FIG. 2, consisting of ceramic material 13 and having perforations 14 through which the soot-containing combustion gases flow, the soot particles adhering to the inside surfaces of the perforations thus retarding their passage and permitting their conversion to carbon monoxide. The gases leaving the top of the soot-conversion vessel enter heat exchanger 8 via line 9 and leave the heat exchanger via line 10. Water enters the heat exchanger via line 11 and steam leaves via line 12.
In a practical embodiment of the invention, a liquid hydrocarbon fuel may be combusted in the presence of oxygen and steam in a reactor and the combustion gases passed at a temperature of 1,400°C and a pressure of 60 atmospheres from the reactor to a soot-conversion vessel containing a bed of packed perforated ceramic bricks. The combustion gases entering the soot-conversion vessel contain 3 percent by weight of soot particles.
The bed is packed with perforated bricks having dimensions of 36 centimeters long, 12 centimeters wide and 10 centimeters deep and the height and diameter of the packed bed are 2 meters and 1.4 meters, respectively. Each brick contains 50 perforations of 1.5 centimeters diameter and the surface area of the tubes formed by the interconnecting perforations is 70 square meters per cubic meter of packed bricks.
If the volume flow rate of the combustion gases flowing through the soot-conversion vessel is 1.45 cubic meters per second and the average gas flow through the tubes is 4.5 meters per second, then the average soot retardation time is 20 seconds and the average thickness of the soot layer formed on the surface of the perforations of the packed bricks is 0.8 millimeters. In this case the gases passing out of the soot conversion vessel contain 0.03 percent by weight of soot particles. Hence a 99 percent conversion of the soot particles present in the gases entering the soot-conversion vessel is achieved.
In a further practical embodiment, a liquid hydrocarbon fuel may be combusted in the presence of oxygen and steam in a reactor and the combustion gases passed at a temperature of 1,400°C and a pressure of 60 atmospheres from the reactor to a soot-conversion vessel containing a fluidized bed of ceramic particles. The combustion gases entering the soot-conversion vessel contain 3 percent by weight of soot particles and 300 ppm of ash particles.
The bed consists of 9.2 cubic meters or 12 tons of Korund particles having an average diameter of 50 to 200 microns. The height and diameter of the fluidized bed are 2 meters and 2.4 meters respectively.
If the superficial velocity of the combustion gases flowing through the fluidized bed is 0.3 meters per second, then the soot retardation time is greater than 20 seconds and the gases passing out of the soot-conversion vessel contain 0.03 percent by weight of soot particles. Hence a 99 percent conversion of the soot particles present in the gases entering the soot conversion is achieved.
Operation of the process at this soot conversion level may be continued for 3 months until the amount of ash which builds up within the fluidized bed is 6.5 tons. At this point replacement of the bed with fresh particles is considered desirable since the bed contains about 30 percent by weight of ash particles.
The present invention relates to a process and an apparatus for the partial combustion of hydrocarbonaceous fuels to produce gases having a substantially reduced soot content.
The gases produced by processes for the partial combustion of hydrocarbonaceous fuels invariably contain a substantial amount of free carbon (i.e., soot) which is undesirable for the subsequent utilization or processing of the gases and which should therefore be removed. The removal of the soot is normally achieved by scrubbing the gases with water. The resulting dispersion of soot particles in water is then processed in order to recover substantially soot-free water and soot. The soot removal facilities are expensive and the recovered soot generally has a low value. It is therefore an object of the present invention to provide a process for the combustion of hydrocarbonaceous fuels to produce gases having a substantially reduced soot content and thus eliminate the need for expensive soot-removal facilities. It is also an object of the invention to increase the efficiency of the partial combustion process by ensuring that substantially all the carbon content of the hydrocarbonaceous fuels combusted is converted into more valuable carbon monoxide.
The presence of soot in the hot gases produced by the partial combustion of hydrocarbonaceous fuels provides difficulties with regard to the recovery of heat from the gases. It is not generally possible to use heat exchangers of the normal straight flame tube type since considerable deposits of soot are formed on the inside walls of the tubes and thus greatly reduce the heat transfer coefficient. It is possible to use a heat exchanger of the helical tube type, but such heat exchangers are expensive and are unsuitable for high pressure steam generation due to the possibility of tube collapse owing to small unroundness of the tubes. Unroundness of a tube signifies its deviation from perfect radial and axial symmetry. It is therefore another object of the present invention to produce gases having a substantially reduced soot content; the recovery of heat from which can be carried out with normal straight tube heat exchangers producing high pressure steam.
SUMMARY OF THE INVENTION
It has now been found that in a process for the generation of a hydrogen and carbon monoxide-containing gas mixture by the partial combustion of a hydrocarbonaceous fuel with an oxygen-containing gas (e.g., oxygen, air, or oxygen-enriched air) in the presence of steam in a partial combustion zone, that the soot content of the resulting crude gas mixture can be substantially reduced by passing the gas mixture containing soot in particulate form into a soot-conversion zone maintained at substantially the same elevated temperature and pressure as that of the partial combustion zone, and retarding the passage of the soot particles through said soot-conversion zone for at least 5 seconds, e.g., from 5 to 50 seconds. It has been found that if the soot particles are retained in the soot-conversion zone for this length of time, they are substantially converted into carbon monoxide by reaction with steam and/or carbon dioxide which is also present in the crude gas mixture. By practice of the invention it is possible to reduce the soot content of the crude gas mixture from amounts of from 2 to 5 percent by weight or higher down to below 1 percent by weight, and generally below 0.3 percent by weight. This substantial reduction in in soot concentration correspondingly reduces or eliminates the need for downstream soot removal facilities and permits the use of conventional straight tube heat exchangers to abstract heat from the hot crude gas mixture, instead of more expensive helical coil heat exchangers which are commonly employed in this service. In addition, the conversion of the carbon to carbon monoxide in the soot conversion zone increases the calorific value of the gases produced.
DETAILED DESCRIPTION OF THE INVENTION
In processes for the partial combustion of hydrocarbonaceous fuels which have so far been proposed, the average residence time of the soot particles in the hot gases is very small because the gases are either immediately quenched with water or immediately passed to a heat exchanger as soon as they pass out of the reactor. Accordingly the soot particles are not allowed sufficient time for further reaction at high temperature. In the present process, however, the gases passing out of the reactor enter a soot-conversion zone in which the passage of soot particles, but not of gases, is retarded. This has the effect of increasing the residence time during which the soot particles are in contact with the hot reactor gases. The soot particles are thus allowed sufficient time in which they can react with steam and/or carbon dioxide in the gases at high temperature and pressure to form carbon monoxide. The reaction is believed to proceed according to the following equations:
C + H 2 O ➝ CO + H 2
C + CO 2 ➝ 2CO
The relative proportions of steam and carbon dioxide present in the partially combusted gases is not critical. It is only required that the combined steam and carbon dioxide concentrations be stoichiometrically sufficient to effect conversion of the free carbon to carbon monoxide in accordance with the above equations.
The operation of the soot-conversion zone can therefore be seen to comprise firstly retarding the passage of soot particles and secondly to allow sufficient time for conversion of the particles so retarded into carbon monoxide. The advantage of this operation is two-fold. Firstly it enables the production of substantially soot-reduced gases, and secondly it allows substantial conversion of the carbon content of the hydrocarbonaceous fuel into carbon monoxide, thus increasing the calorific value of the gases produced as previously mentioned.
In general, the retardation and conversion of the soot particles in the soot-conversion zone is such that the percentage by weight of soot particles present in the gases leaving said zone is reduced to less than 1 percent. Preferably, however, the percentage by weight of soot particles present in the gases leaving said zone is reduced to less than 0.3 percent.
It is important that the soot particles are sufficiently retarded in the soot-conversion zone that their residence time therein allows substantial conversion to carbon monoxide to take place. Accordingly, the soot particles suitably remain in the soot-conversion zone for at least 5 seconds. It is, however, preferred that the time that the soot particles remain in the soot-conversion zone is between 15 and 50 seconds.
Retardation of the soot particles is suitably effected by a soot-conversion zone consisting of at least one bed of ceramic material or any equivalent material which is resistant to high temperatures. In general, any ceramic material capable of resisting the high temperatures present in the soot-conversion zone, e.g., 1,000°-1,600°C or higher, can be employed. Ceramic materials having the requisite refractory properties are known to those skilled in this art and are described, for example, in "Encyclopedia of Chemical Technology" by Kirk and Othmer, Second Edition, 1968, Vol. 4, pages 762-775. Suitable ceramic materials include various carbides, e.g., the carbides of silicon, boron, zirconium, hafnium, tantalum, vanadium, molybdenum, tungsten and niobium; various aluminum silicates, particularly sillimanite and Korund (a special alumina refractory material containing at least 95 percent alumina); various borides, nitrides and sulfides of high melting point metals, and various oxides such as mullite, zircon and the like. The soot particles adhere to the surface of the ceramic material under the influence of Van der Waals-type forces.
The bed of ceramic material advantageously consists of packed perforated bricks. The bricks are preferably packed on top of one another in layers such that the perforations of the bricks in a lower layer interconnect with the perforations of the bricks in an upper layer. In this way the interconnecting perforations form elongated tubes in the direction of gas flow within the soot-conversion zone. The gases pass through the elongated tubes but the soot particles adhere to the walls thereof and are therefore retarded. The perforated bricks are preferably made from Korund, carbides or sillimanite, although they may be made from any other suitable ceramic material which can resist high gas temperatures. The diameter of the elongated tubes may vary over a wide range. However, it should be sufficiently large such that deposition of soot particles does not restrict the flow of gases unduly and sufficiently small such that a large surface area of ceramic material is afforded for soot adhesion and these factors therefore dictate that the diameter preferably is between 0.5 cm and 2.5 cm. The surface area of the elongated tubes for gas contact may also vary considerably. Preferably the surface area is between 30 square meters and 200 square meters per cubic meter of perforated bricks. The average gas velocity through the elongated tubes is not critical, but in general lies between 1 meter per second and 10 meters per second.
The ash particles in the hot gases, as in the case of the soot particles, are temporarily retarded in the soot-conversion zone. However, unlike the soot particles they do not react further with the hot gases but pass through the said zone unchanged. A very small proportion of the ash particles however is retained within the soot-conversion zone and thus a gradual build-up of ash particles therein occurs. This build-up is accelerated if the ash content of the hydrocarbonaceous fuel is high. This eventually leads to plugging of the perforations of the bricks and inefficient soot conversion. Accordingly, after a shorter or longer period of operation it is necessary to remove the old bricks from the soot-conversion zone and install new ones. The intervals between successive brick renewal periods depend to a large extent on the ash content of the hydrocarbonaceous fuel combusted, but in general are between 1 month and 12 months. In order to ensure continuous operation of the soot-conversion zone a swing reactor system is suitably utilized, whereby while a bed of bricks in one reactor is being replaced, another bed of bricks in another reactor is in operation.
In another embodiment of the present invention, the bed of ceramic material suitably consists of at least one fluid bed of ceramic particles. The particles are made from any suitable ceramic material and in particular from Korund, carbides or sillimanite. The diameter of the particles may vary over a wide range, but as in the case of perforated bricks, consideration of gas flow rate through the bed and surface area for gas contact requires the diameter to be in general between 10 and 1,000 microns. The superficial velocity of the gases passing through the fluid bed preferably lies between 0.1 and 1 meter per second, although operation above and below these figures is quite possible.
During operation a certain proportion of the ash particles in the hot gases is permanently retained in the fluidized bed of ceramic particles. A gradual build-up of ash particles therefore occurs and this build-up occurs more rapidly if the ash content of the hydrocarbonaceous fuels combusted is high. Accordingly, after a given period of operation it is necessary to replace the old ash-containing particles with fresh particles. The intervals between successive bed replacements in general are between 2 months and 20 months. A swing reactor system may suitably be employed. Alternatively continuous addition of fresh particles and withdrawal of ash-containing particles may be effected in order to provide continuous operation of the soot-conversion zone so that intermittent bed replacement is not necessary.
The temperature and pressure at which soot conversion in the soot-conversion zone takes place are substantially the same as the temperature and pressure at which partial combustion occurs. The pressure in the partial combustion zone can be from 1 to 300 atmospheres, but preferably is between 10 and 150 atmospheres. The temperature within the partial combustion zone is, in general, from about 1,000°C to about 1,600°C, but preferably from 1,300°C to 1,500°C. These same ranges of pressure and temperature apply to the soot conversion zone.
A considerable advantage of the present invention lies in the fact that the production of substantially soot-reduced gases greatly simplifies and facilitates the subsequent heat recovery operation therefrom. In cases where the soot content of the gases is negligible, it is possible to recover heat from the gases in a conventional straight tube type heat exchanger since soot deposition on the inside walls of the tubes no longer occurs. Thus the more expensive helical tube heat exchangers are no longer necessary and also heat recovery at higher steam pressures becomes possible, since conventional straight-tube exchangers can operate at higher pressures than helical tube exchangers. Hence, cheaper equipment can be used and more efficient heat recovery is achieved. At a sufficiently low content of particulate matter it is even possible to use water-tube heat exchangers, which permit heat recovery at still higher steam pressures enhancing further the thermal efficiency. In cases where small amounts of soot still remain in the gases passing to the heat exchanger it may nevertheless be desirable to use a helical gastube heat exchanger.
Any suitable hydrocarbonaceous or carbonaceous fuel may be partially combusted according to the present invention including natural gas, heavy oils, coal, coke and shale oil, tar sands oil, etc. In general the fuel will be a liquid hydrocarbon fuel. If a solid fuel is employed, it may be combusted in either a dry pulverized form or in a slurry with liquid.
The invention also relates to an apparatus suitable for carrying out the process described hereinbefore. This apparatus comprises:
(a) a brick-lined partial combustion reactor free of internals and solids with an inlet for fuel, oxygen-containing gas and steam, and an outlet for discharging partially combusted, soot particle-containing gases, and
(b) a soot-conversion vessel connected to the partial combustion reactor containing a bed of ceramic material of sufficient thickness and porosity to retard passage of the soot particles through said soot-conversion vessel for at least 5 seconds whereby the soot particles are substantially converted to carbon monoxide, said soot-conversion vessel having an inlet for the partially combusted, soot-containing gases from the partial combustion reactor, and an outlet through which the soot-reduced gases are discharged from the soot-conversion vessel.
In a preferred embodiment, the outlet of the soot-conversion vessel is connected to a waste heat boiler. The waste heat boiler is suitably of the conventional straight tube type if the gases leaving the soot-conversion vessel contain negligible amounts of soot. If small amounts of soot are still present in the gases then a helical tube type heat exchanger may be employed.
The number of beds of ceramic material contained within the soot-conversion vessel is not of critical importance. Accordingly any number of separate beds, within reason, may be contained within the soot-conversion vessel although one single bed is preferred. The bed(s) of ceramic material must be of sufficient porosity to permit the passage of the gases therethrough while retaining the soot particles for the prescribed length of time to effect their conversion to carbon monoxide, i.e., at least 5 seconds.
According to one embodiment of the invention the soot-conversion vessel contains a bed of packed perforated bricks. The perforated bricks are packed on top of one another in layers such that the perforations in the bricks in a lower layer interconnect with the perforations of the bricks in an upper layer. In this way the interconnecting perforations form elongated tubes in the direction of gas flow within the soot-conversion vessel. The total length of the elongated tubes formed by the packed perforated bricks varies according to, inter alia, the surface area required for soot retardation, the diameter of the perforations and the soot content of the combustion gases entering the soot-conversion zone, but in general is between 0.5 and 5 meters. The surface area of the elongated tubes suitably lies between 30 square meters and 200 square meters per cubic meter of the packed perforated bricks.
The number of layers of perforated bricks within the soot-conversion zone depends on the length of elongated tubes required and the size of the individual bricks but advantageously lies between 2 and 100. The size of the individual bricks is not critical, but for reasons of ease of handling suitably lies between 15 cm long, 5 cm wide and 5 cm deep and 60 cm long, 20 cm wide and 20 cm deep. The number of perforations per brick suitably lies between 5 and 750, and the diameter of the perforations between 0.5 cm and 2.5 cm.
In another embodiment, the soot conversion vessel contains at least one bed of fluidizable particles. The size of the particles is not critical but advantageously the average diameter thereof is between 10 and 1000 microns.
A method in which the process and apparatus according to the present invention are suitably used for the partial combustion of hydrocarbonaceous fuels to produce substantially soot-reduced gases is described below with reference to the accompanying drawings, by way of example and without limitation.
FIG. 1 is a diagrammatic representation of an apparatus for the partial combustion of hydrocarbonaceous fuels to produce substantially soot-reduced gases.
FIG. 2 is a partially cutaway, perspective view of a perforated ceramic brick suitable for use in the soot-conversion vessel.
With regard to FIG. 1, hydrocarbonaceous fuel is introduced via line 1 and oxygen is introduced via line 2 into burner part 3 of partial combustion reactor 4. Steam if required may be introduced via both lines 1 and 2. The hot combustion gases pass through a connecting piece 5 which connects the reactor to soot-conversion vessel 6. The latter contains a bed of ceramic material 7 through which the hot combustion gases pass in upflow. The bed of ceramic material can suitably comprise ceramic bricks as shown in FIG. 2, consisting of ceramic material 13 and having perforations 14 through which the soot-containing combustion gases flow, the soot particles adhering to the inside surfaces of the perforations thus retarding their passage and permitting their conversion to carbon monoxide. The gases leaving the top of the soot-conversion vessel enter heat exchanger 8 via line 9 and leave the heat exchanger via line 10. Water enters the heat exchanger via line 11 and steam leaves via line 12.
In a practical embodiment of the invention, a liquid hydrocarbon fuel may be combusted in the presence of oxygen and steam in a reactor and the combustion gases passed at a temperature of 1,400°C and a pressure of 60 atmospheres from the reactor to a soot-conversion vessel containing a bed of packed perforated ceramic bricks. The combustion gases entering the soot-conversion vessel contain 3 percent by weight of soot particles.
The bed is packed with perforated bricks having dimensions of 36 centimeters long, 12 centimeters wide and 10 centimeters deep and the height and diameter of the packed bed are 2 meters and 1.4 meters, respectively. Each brick contains 50 perforations of 1.5 centimeters diameter and the surface area of the tubes formed by the interconnecting perforations is 70 square meters per cubic meter of packed bricks.
If the volume flow rate of the combustion gases flowing through the soot-conversion vessel is 1.45 cubic meters per second and the average gas flow through the tubes is 4.5 meters per second, then the average soot retardation time is 20 seconds and the average thickness of the soot layer formed on the surface of the perforations of the packed bricks is 0.8 millimeters. In this case the gases passing out of the soot conversion vessel contain 0.03 percent by weight of soot particles. Hence a 99 percent conversion of the soot particles present in the gases entering the soot-conversion vessel is achieved.
In a further practical embodiment, a liquid hydrocarbon fuel may be combusted in the presence of oxygen and steam in a reactor and the combustion gases passed at a temperature of 1,400°C and a pressure of 60 atmospheres from the reactor to a soot-conversion vessel containing a fluidized bed of ceramic particles. The combustion gases entering the soot-conversion vessel contain 3 percent by weight of soot particles and 300 ppm of ash particles.
The bed consists of 9.2 cubic meters or 12 tons of Korund particles having an average diameter of 50 to 200 microns. The height and diameter of the fluidized bed are 2 meters and 2.4 meters respectively.
If the superficial velocity of the combustion gases flowing through the fluidized bed is 0.3 meters per second, then the soot retardation time is greater than 20 seconds and the gases passing out of the soot-conversion vessel contain 0.03 percent by weight of soot particles. Hence a 99 percent conversion of the soot particles present in the gases entering the soot conversion is achieved.
Operation of the process at this soot conversion level may be continued for 3 months until the amount of ash which builds up within the fluidized bed is 6.5 tons. At this point replacement of the bed with fresh particles is considered desirable since the bed contains about 30 percent by weight of ash particles.