GB2025453A

GB2025453A - Recovery of ungasified solid fuel particles from suspension in water

Info

Publication number: GB2025453A
Application number: GB7908220A
Authority: GB
Original assignee: Texaco Development Corp
Current assignee: Texaco Development Corp
Priority date: 1978-07-12
Filing date: 1979-03-08
Publication date: 1980-01-23
Also published as: AU521279B2; DE2916199A1; ZA791139B; FR2430974A1; JPS5513773A; NO791105L; GB2025453B; FR2430974B1; CA1154255A; JPS597754B2; AU4518679A

Abstract

A solid carbonaceous fuel is subjected to partial oxidation to produce a gas comprising carbon monoxide and hydrogen and containing unconverted solid fuel particles which gas is quenched and scrubbed to form a water suspension of such particles. The suspension is passed to a settling zone at elevated temperature and pressure to form an upper clarified water portion and a lower more-concentrated water containing suspension settled solid fuel particles, at least a portion of the lower portion being returned to the partial oxidation zone. The suspension is not cooled on passing to the settling zone and the pressure therein is not less than 5.2 bars. The invention is applicable to gasification of any solid carbonaceous fuel containing ash-forming ingredients, e.g. coal, sub-bituminous coal, lignite, petroleum coke, organic waste.

Description

SPECIFICATION Recovery of ungasified solid fuel particles from suspension in water This invention relates to the gasification of solid fuels by partial oxidation. More particularly, it is concerned with the recovery of ungasified solid fuel from the partial combustion products and the return of the recovered ungasified solid fuel to the gasification zone where with additional fresh solid fuel it is subjected to partial oxidation.

Ordinarily in the gasification of solid fuel such as coal or coke, the fuel is subjected to partial oxidation with air, oxygen-enriched air or substantially pure oxygen in a gasification zone with the production of a product gas containing carbon monoxide and hydrogen and also containing minor amounts of CO2 and CH4 and if the feed contains sulfur, H2S and COS. However, since insufficient oxygen is introduced into the gasification zone for complete combustion of the carbon in the solid fuel, some of the solid fuel will proceed through the gasification zone without being converted to an oxide or carbon. When a hydrocarbon liquid is subjected to partial oxidation the unconverted carbon appears in the product gas as fine particles of soot whereas when a solid fuel is subjected to partial oxidation the unconverted carbon appears in the product as particles of solid fuel.In addition, depending on the type of solid fuel feed, ash also appears in varying amounts in the combustion products. It will be appreciated, of course, that few of the unconverted particles are purely ash or purely carbon.

To cool the hot products of partial oxidation leaving the gas generation zone and to remove particles of ash and unconverted solid fuel entrained therein, the hot gas is contacted with a quench medium such as water in a quench zone whereby the gas is cooled and entrained particles are transferred to the quench medium. Larger, denser particles of ash or slag which are low in carbon tend to settle to the bottom of the quench zone and are removed but the finer less dense particles form a suspension in the quench medium. To control the concentration of solid material in the quench medium, a portion is withdrawn continuously or periodically and is replaced with fresh quench medium. For economic and ecological reasons, it is desirable to reuse the quench water and unconsumed fuel.

As mentioned above, when the feed to the gas generation zone is a hydrocarbon liquid, the unconverted carbon appears as fine particles of soot which are microscopic in size whereas when the feed to the gas generation zone is a solid fuel the unconverted carbon is in the form of discrete particles of solid fuel.

The soot formed in the gasification of the hydrocarbon liquid may be recovered from suspension in the quench water by admixture with a hydrocarbon liquid as disclosed in U.S.

Patent No. 2,992,906 issued July 18, 1961 to F.E. Guptill, Jr. and U.S. Patent No.

3,917,569 issued November 4, 1975 to G.N. Richter, W.L. Slater, E.T. Child and J.C.

Ahlborn.

Unfortunately, the unconverted particles of solid fuel do not have the affinity for liquid hydrocarbon as do the soot particles formed by the partial combustion of a liquid fuel and the separation technique used for soot recovery is unsatisfactory for the recovery of unconverted solid fuel particles from the quench water.

In the gasification of liquid fuels, any carbon in the feed which is not converted to an oxide of carbon appears in the product gas in the form of microscopic size particles of soot.

When the hot product gas containing entrained soot particles is quenched, for example in water, the soot particles are transferred to the quench water which is heated and in turn the product gas is cooled. Ordinarily the soot particles are recovered from the water by contacting the water with a low molecular weight hydrocarbon liquid such as naphtha into which the soot particles migrate leaving a suspension of soot in naphtha and clarified water. The naphtha containing suspended soot particles may then be contacted with the liquid hydrocarbon feed to the gas generator to form a hydrocarbon liquid mixture containing suspended soot particles. The mixture is heated to distill off the naphtha which is recycled to recover additional soot particles from the quench water while the suspension of soot particles in the heavy oil feed to the gasifier is subjected to partial combustion.

The soot formed during the conversion of liquid hydrocarbon is quite different in character from the particles of unconverted fuel which appear in the product gas when the fuel to be gasified is a solid fuel. For example, the soot particles resulting from the gasification of a liquid fuel generally have a surface area in excess of 100 m2/g and more usually in excess of 200 m2/g whereas particles of unconverted fuel resulting from the gasification of a solid fuel generally have a surface area of less than 50 m2/g.

The particles which appear in the product gas when the fuel to be gasified is a solid fuel do not have the affinity for hydrocarbon liquids that the soot particles resulting from the gasification of a liquid fuel have. As a result the procedure used for the recovery of soot particles from the quench water is not effective for the recovery of unconverted solid fuel particles from the quench water. For this reason, ungasified solid fuel has been recovered from the quench water by cooling the same and allowing it to settle in open vats.

Such a procedure involves the use of large and costly heat exchangers to recover the sensible heat in the quench water. It also means that all of the quench water is subjected to steam stripping to remove undesirable noxious gases which must be treated before emission to the atmosphere. Additionally the quench water must then be repressured prior to its return to the quench zone.

The invention provides a process for the gasification of a solid carbonaceous fuel which comprises subjecting said solid fuel to partial oxidation in the presence of H20 at a pressure of at least 6.9 bars (100 psig) to produce a gas comprising CO and H2 and containing particles of unconverted solid fuel, contacting said product gas with water in a quench zone to cool said product gas and to form a suspension of said particles of unconverted solid fuel in said water, passing said suspension without cooling to a settling zone maintained at a pressure not less than about 5.2 bars (75 psig), allowing the suspension to settle into an upper clarified water portion and a lower more concentrated water portion containing suspension settled unconverted solid fuel particles and returning at least a portion of said settled particles to the partial oxidation zone.

An advantage of the invention is that it makes it possible to recover unconverted solid fuel from the quench water, and to dispense with heat exchange equipment ordinarily used during the recovery of unconverted solid fuel from synthesis gas. Another advantage is that it makes it possible to minimize the amount of water which must be treated for the presence of noxious gases contained therein. A further advantage is that it makes it possible to conserve the energy present in the quench water as it leaves the quench zone.

The feed to the process according to the invention comprises any solid carbonaceous fuel containing ash-forming ingredients such as coal, sub-bituminous coal, lignite, petroleum coke, organic waste and the like.

The solid fuel is generally ground to a particle size of less than 6.4 mm (one-quarter inch) and preferably ground so that at least 95% passes through a 14 mesh sieve (U.S.

Standard) having openings of 1.41 mm and is introduced into the gas generation zone where it is subjected to partial oxidation with a gas such as air, oxygen-enriched air or substantially pure oxygen that is, oxygen having a purity of at least about 95%. The finely-divided fuel may be introduced into the partial oxidation or gas generation zone as a slurry in a liquid such as water or oil or as a suspension in a gaseous or vaporous medium. such as steam, carbon dioxide or mixtures thereof.In the gas generation zone, the solid fuel is. subjected to partial oxidation at a temperature between about 871 and 1927"C (1600 and 3500"F), preferably between 982 and 1760"C (1800 and 3200"F). The pressure in the gas generation zone may range between about 6.9 and 206.8 bars (100 and 3000 psig) preferably between about 10.3 and 172.4 bars (150 and 2500 psig). The oxygen may be introduced into the gasification zone at an oxygen to carbon atomic ratio of between about 0.7 to 1.6 preferably between 0.8 to 1.2. When the solid fuel is introduced into the gasification zone as a slurry in water, the slurry should contain less than 50 wt. % water as a water content above that value will affect the thermal efficiency of the reaction.

In a preferred embodiment hot product gases containing entrained particles of unconverted fuel pass downwardly through a lower outlet at the bottom of the gasification chamber and are discharged into the quench chamber under the surface of the water contained therein. Larger particles, composed for the most part of ash free from carbon, for example less than 2.0 wt. %, descend gravitationally into the lower part of the quench chamber where they are removed periodically by means of a lock hopper. The finer particles of unconverted solid fuel remain suspended in the quench water due, at least in part, to the agitation supplied by the discharge of the hot gases under the surface of the water.

Separation of the solid particles from the water is effected by transferring the suspension from the quench chamber to a settling zone where the suspension is permitted to settle substantially free from agitation into a clarified upper portion and a more concentrated lower portion. This settling is conducted at elevated temperatures and pressures. In a preferred embodiment the settling zone is maintained at substantially the same pressure and temperature as the quench zone.However, good results are obtained when the settling is effected at a temperature between about 37.8 and 374.4"C (100 and 706"F) and a pressure between about 3.4 and 241.3 bars (50 and 3500 psig), preferably at a temperature between 93.3 and 353.3"C (200 and 668"F) and a pressure between 6.9 and 172.4 bars (100 and 2500 psig).

The residence time in the settling zone will depend on the size of the particles of unconverted fuel which in part is dependent on how finely the feed fuel has been ground. Ordinarily the residence time should be in excess of three minutes with from five minutes to thirty minutes being preferred.

The concentrated suspension in the lower section of the settling zone, which may contain as much as 50 wt. % solids, is recovered from the bottom of the settling zone. If the solid fuel is introduced into the gas generator as a water slurry, the concentrated suspension may be returned directly to the mixing zone where a slurry of fresh feed and water is prepared. If the feed to the gas generator is in the form of solid fuel suspended in a hydrocarbon liquid advantageously the settled fuel may be subjected to water removal treatment and then mixed with the fresh solid fuel feed.

The clarified water removed from the upper end of the settling zone may be used for making additional fresh feed slurry or may be returned to the quench zone.

By making the separation at an elevated temperature and pressure, the settling time is considerably reduced over the time for settling the suspended solid fuel in open vats as in the prior art. In addition, since the separation is made hot, it is not necessary to cool the suspension using the large and costly heat exchangers of the prior art. Furthermore dissolved gases are kept in the system which cuts down on the amount of low pressure sour gas which must be subjected to treatment.

The following examples are submitted for illustrative purposes only and it should not be considered that the invention is restricted thereto.

EXAMPLE I This Example represents conventional practice where the suspended solids are permitted to settle at ordinary conditions.

The charge to the gasification zone is a slurry of petroleum coke, ground so that 94% passes through a 40 mesh sieve (0.42 mm, (0.0165 inch) opening) in a California Reduced Crude oil, the coke forming 49.93 wt.

% of the slurry. The slurry is fed into a 0.06 cu. m. (2.15 cu. ft.) unpacked gas generator at a rate of 1 92 kgs (424 pounds) per hour with 11 9.2 std. m3 per hour (4752 SCFH*) of oxygen and 109 kgs (240.6 pounds) of steam per hour. This represents an oxygen to carbon atomic ratio of 0.808. Temperature in the gasifier is maintained at 1309.4"C (2389"F) and the pressure at 55.85 bars (810 psig).

Quench water containing suspended solids is withdrawn, cooled by heat exchange and allowed to settle in an open container. After a residence time of 3.1 hours, analysis of the clarified water withdrawn from the top of the settler shows a solids content of 0.005 wt. %.

The solids content of the concentrated suspension removed from the bottom of the settler is 37 wt. %.

This run shows the results obtained by settling in a conventional manner at 37.8"C (100"F) and atmospheric pressure.

"standard cubic feet per hour EXAMPLE II This Example represents one embodiment of the process of our invention.

The gasification step here is similar to that of Example I. The feed to the gasifier in this case is a slurry of petroleum coke ground as in Example I, containing 47.7 wt. % coke in California Reduced Crude. Feed rates are 151.5 std. m3 per hour (5656 SCFH) of oxygen, 194.4 kgs (428.8) pounds of slurry per hour and 201.5 kgs (444.3 pounds) of steam per hour, representing an oxygen to carbon atomic ratio of 0.943. Gasification temperature is 1247.2 C (2277"F) and pressure 55.8 bars (810 psig).

The quench water containing suspended unconverted solid fuel is transferred at 210"C (41 0 F) to a continuous settler where it is maintained at a pressure of 55.2 bars (800 psig). After a residence time of 1 2 minutes, analysis of the clarified water removed from the top of the settler shows a solids content of less than 0.001 wt. % solids. The solids content of the concentrated suspension withdrawn from the bottom of the settler is 40.1 wt. %.

It will be noted that by transferring the suspension without cooling from the quench zone to the settling zone at substantially the pressure of the gasification zone, superior settling is obtained at 1 5 times the rate of settling in Example I.

The clarified water is recycled to the quench zone with a minimum of repressuring, and the suspended solids after drying, are returned to the mixer where fresh slurry feed is prepared.

Although the Examples describe the gasification of coke in an oil slurry, the process of our invention may be used equally well in the gasification of a coal in oil slurry, a coal in water slurry or a coke in water slurry, for the separation of unconverted solid fuel particles from the quench water. Our process may also be used when the solid fuel feed is suspended in a gaseous or vaporous medium.

In addition to being applicable to gasification processes in which suspensions are formed by the direct quench of synthesis gas, our invention is also applicable to processes where the hot synthesis gas is partially cooled by indirect heat exchange and then contacted with quench or scrub water.

Various modifications of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be made as are indicated in the appended claims.

Claims

1. A process for the gasification of a solid carbonaceous fuel which comprises subjecting said solid fuel to partial oxidation in the presence of H20 at a pressure of at least 6.9 bars (100 psig) to produce a gas comprising CO and H2 and containing particles of unconverted solid fuel, contacting said product gas with water in a quench zone to cool said product gas and to form a suspension of said particles of unconverted solid fuel in said water, passing said suspension without cool ing to a settling zone maintained at a pressure not less than about 5.2 bars (75 psig), allow ing the suspension to settle into an upper clarified water portion and a lower more con centrated water portion containing suspension settled unconverted solid fuel particles and returning at least a portion of said settled particlesto-the partial. oxidation zone.

2. A process according to claim 1 in which the temperature of the suspension is between 37.a and 374.4"C (100 and 706do).

3 A process according to claim 2 in which the temperature of the suspension is between 93.3 and 374.4 C (200 and 706"F).

4; A process according to any of claims 1 to 3 in. which the pressure in the settling zone is between 5.2 and 241.3 bars (75 and 3500 psig).

5. A process according to claim 4 in which the pressure is between 6.9 and 172.4 bars (1.00 and 2500 psig).

6. A process according to any of claims 1 to 5 in which the residence time of the unconverted solid fuel particles in the settling zone is at least 3 minutes.

7. A process according to claim 6 in which the residence time is between 5 and 30 minutes.

8. A process according to any of claims 1 to: 7 in which the solid fuel is ground to a particle size so that at least 95 percent passes through a 14 mesh sieve (U.S. Standard) having 1.41 mm openings.

9. A process according to any of claims 1 to 8 in which the solid fuel is introduced into the partial oxidation zone as a slurry in water.

10. A process according to any preceding claim in which the settling zone is maintained at. substantially the same pressure and temperature as the quench zone.

11. A process for the gasification of solid carbonaceous fuel as claimed in claim 1 and substantially as hereinbefore described.

12. A process for the gasification of solid carbonaceous fuel substantially as hereinbefore described in Example II.