CA1154255A

CA1154255A - Recovery of ungasified solid fuel particles from suspension in water

Info

Publication number: CA1154255A
Application number: CA000331576A
Authority: CA
Inventors: Robert J. Stellaccio; Warren G. Schlinger; William B. Crouch
Original assignee: Texaco Development Corp
Current assignee: Texaco Development Corp
Priority date: 1978-07-12
Filing date: 1979-07-11
Publication date: 1983-09-27
Also published as: DE2916199A1; JPS5513773A; JPS597754B2; AU521279B2; FR2430974B1; AU4518679A; FR2430974A1; ZA791139B; GB2025453B; GB2025453A; NO791105L

Abstract

RECOVERY OF UNGASIFIED SOLID FUEL
PARTICLES FROM SUSPENSION IN WATER
(D)76, 293-F) ABSTRACT
A continuous process for the partial oxidation of solid carbonaceous fuels in the presence of H2O to produce a gas comprising CO and H2 with recovery and recycling of uncombusted solid fuel particles to the partial oxidation zone is disclosed. The uncombusted fuel particles entrained in the product gas are trapped in quench water through which the product gas is bubbled.
The quench water containing said fuel particles is held in a settling zone for three to thirty minutes in the temperature range of 100°F. and 706°F., and the pressure range of 50 p.s.i.g. and 3500 p.s.i.g., resulting in a concentrated solid fuel particle bearing lower section and a clarified upper section wherein at least a portion of said lower section is returned to the partial oxidation zone.

Description

~ 4~5 This invention relates to the gasiication of solid fuels by partial oxidation~ More particularly, it is concerned with the recovery of ungasified solid uel 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 airp oxygen~enriched air or substantially pure oxygen in a gasificatio~ zone with the production of a product ga3 containtn~ carbon monoxide and hydrogen and also con-taining minor amounts of CO2 and CH4 and if the feed con-tains sulfur, H2S and COS. However, since insufficient oxygen is introduced into the gasification zone for complete combustlon of the carbon in the solid fuel, some of the solid fuel will proceed through the gasiication zone without being converted to an oxide of 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 com-bustion products. It will be appreciated, of course, that few o the uncon~erted particles are purely ash or purely carbon.
To cool the hot products of partial oxidation leavin~ the gas generation zone and to remove particles of ash and unconverted solid fuel entralned therein, the hot ~ 5~

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 ~ut the ~ le~ d~nse 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 periodlcally and is replaced with fresh quench medium. For economic and ecological reasons, it is desirable to reuse the quench water and unconsumecl fuel.
As menttoned above, when the ~eed to the gas generatton zone is a hydrocarbon liquidt the unconverted carbon appears as fine particles of soot which are micro-scopic in slze 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 f.rom suspenslon in the quench water by admixture with a hydrocarbon liquid as disclosed in U. S. Patent 2,992,906 issued July 18~ 13~1 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. Chlld 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 liqu~d fuel and the separation tecAnique used for soot recovery ~s unsat~sfactor~ for the recovery of ~converted solld fuel partlcles from the quench water.
I~ the g~s~ication o~ liq~id ~uels ~ any carbon ~ 255 ln 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 entr~ined soot partlcles 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 reco~ered from the water by contac~ing the ~ater ~ith a lo~ molecular weight hydrocar~on liquid such as ~aphtha into which the soot particles ~igrate leaving a suspension of soot in naphtha and clarified water, The naphtha containing suspended soot particles may then be contacted ~ith the liquid hyarocarbon feed to the gas generator to ~orm a hydrocarbon liquid mixture containing suspendad soot particles. The mixture is heated to distill off the naphtha which is recycled to recover additional soot partlcles from the quench water while the suspension of soot particles in the heavy oil eed to the gasifier is subjec~ed to partial combustion, The soot formed during the conversion of liquid 2C hydrocarbon is quite different in character from the particles of uncon~erted fuel which appear in t~e product gas when the fuel to be gasified is a solid fuel. For example~ the soot particles resulting from the gasification of a licIuid fuel generally have a surface area in excess of 100 m2~cr and more usually in excess of 200 m2~c3 whereas par-ticles of unconverted uel resulting fr~m the gasification of a ~olid fuel generally have a surface area of 1ess 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 afinity for hydrocarbon liqulds that the soot parkicles 1 ~Lr4Z55 resulting from the gasification of a liquid fuel have.
As a resul~ the procedure used for the recovery of soo-t particles ~rom the quench water is not effective for the reco~ery 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 involvas the use of large and costly heat exchangers to recover the sensible heat in the quench water. It also means that all o~ 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 repxessured pxior to its return to the quench zone.
It is therefore an object of this invention to recover unconverted solid fuel from the quench water.
Another object of the invention is to dispense with heat exchange equipment ordinarily used during the recovery of unconverted solid fuel from synthesis gas. Still another o~ject is to minimize the amount of water which must be treated for the presence of noxious gases contained therein.
Yet another o~ject is to conserve the energy present in the quench water as it leaves the quench zone.
According to our in~ention there is provided a process for the gasification of a solid carbonaceous fuel which comprises subjecting said solid fuel to partial oxidation in the presence of H2O and at a pressure of at least 100 psig to produce a gas comprising CO and H2 and containing particles of unconverted solid fue!l, quenching 3Q said pxoduct ~s ~e contacting same with wa~er in a quench ~ 25~

zone to cool said product gas and to form a suspension of said particles of unconverted solid fuel in the quench water, passing said suspension to a settling zone maintained at substantially the same temperature as said quench zone and also malntained at a pressure not less than about 75 psig t allowins the suspension to settle in an upper clariied portion and a lower more concentrated portion containing settled unconverted solid fuel particles and re1;urning at least a portion of said settled particles to the partial oxidation zone.
The feed to the process of our in~ention comprises any solid carbonaceous fuel containing ash-forming ingred-ients such as coal t sub-bituminous coal, lignite, petroleum coke, organic waste and the like.
The solid fuel ground to a particle size o less than one-quarter inch and preferably ground so that at least 95% passes through a 14 mesh sieve (U.S. Standard) is introduced into the gas generation zone where it is subiected to partial oxidation with a gas such as air, oxygen-enriched air or substantially pure oxygen that is, oxygen havin~ a purity of at least about 95~. The finely-di~ided fuel may be introduced into the partia:L 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 parttal oxidation at a temperature between about 1600 and 3500F, preferably between 1800 and 3200F'. The pressuxe in the gas generation zone may range between about 100 and 3000 psig pre~erably between about l50 and 2500 psig.

5~;

The oxygen may be introduced into the gasification zone at an oxygen to carbon atomic ratio of between about 0.7 and 1.6 preferably between 0~8 and 1.2. When the solid fuel is introduced Into the ~asi~ication zone as a slurry in water, the slurry should contaln less than 50 wt. %
water as a water content above that ~alue will affect the the~mal e~ficiency of the r~action.
In a preferred embodiment hot product gases containln~ entrained particles of unconverted fuel pass downwaxdly throu~h a lower outlet at the bottom of the gasiflcatlon chamber and are discharged into the quench ch~mber under the surface of the watex contained therein.
Lar~ex particles,composed ~or the most part of ash free rom carbon, ~or example less than 2.0 wt. ~ t descend gravita-tionally 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 ag~tation supplied by the discharge of the ~ot gases under the sux~ace of the water.
Separation o~ the solid particles from the water is effected by transferring the suspension from the quench chamber to a s~ttllng zone where the suspension is permitted to settle substantially free ~m agitation into a clarified upper portlon and a more concentrated lower portion. This settling is conducted at elevated temperatures and pressures~
In a preferred embodiment the settling zone is maintaine~ at substantLally the same pressure and temperature as the quench zone, However~ good results are obtatned when the settllng is efected at a temperature between about 100F~

2~5 and 706F. and a pressure between about 50 psig and 3500 psig, preferably at a temperature between 200 and 668F. and a pressure between 100 and 2500 psig.
The residence time in the settling zone will depend on the size o the particles of unconverted fuel whi~h in par-t 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 frash 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 sQlid 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 ~one.
By making the separation at an elevated temperature and pressure~ the settling time is considerably reduced o~er 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 ~ 25S

subjected to treabment~
The follo~ing examples are submitted for illustra-tive purposes only and it should not ~e considered that the in~ention is restrlcted thereto.
E~MPLE I
This example represents conventional practice where the suspended solids are permitted to settle at ordinary conditions~
The charge to the gaslfication zone is a s:Lurry or petroleum coke, ground so that 94% passes through a 40 mesh sieve (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 2.15 cu. ft. unpacked gas generator a-t a rate of 424 pounds per hour wi~h 4752 SCF~ of oxygen and 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 2389F. and the pressure at 810 ps~
Quench water containing suspended solids is withdrawn~ cooled by heat exchange and allowed to settle in an open countainer. After a residence time of 3.1 hours, analysis o~ the clarified water withdrawn from the top of the settler shows a solids content of 0~005 wt~ %.
The solids content o~ the concentrated suspension removed from the bottom of the settler is 37 wt. %.
Th~s run shows the xesults obta~ned by settling in a conventional manner at 100F. and atmospheric pressure .

*standard cubic ~eet per hour t"~

EXAMPLE II
, This example represents one embodiment o~ the process o~ oux in~Jention.
The gasi~ication step here is similar to that of Example I~ ThP feed to the ga~ifiex in this ca6e is a slurry of petroleum coke ground as in Ex~mple I containing 47.7 wt. % coke in California Reduced Crude. Feed rates are 5656 SC~H af oxygen, 428.8 pounds of slurry per hour and 444.3 pounds of steam per hour, representing an oxygen to carbo~ atomlc ratio of ~.943. Gasification temperature is 2~77F.and pressure 810 psig.
The quench water containing suspended unconverted solid fuel is transfe~red at 410F. to a continuous settler whexe it is maintained at a pressure of 800 psig. After a residence time of 12 minutes, analysis of the clarified water remo~ed ~rom the top of the settler shows a solids content of less than a~ ool wt. ~ solids. The solids content of the concentrated supension withdrawn from the bottom of the settler is 40.1 wt~ %.
It will be noted that by transferring the sus-pensi~n without cooling from the quench zone to the settllng zone at substantially the pressure of the gasific-ation æone, superior settling is obtained at 15 times the rate of settling in Example I.
The clarified-water i5 xecycled to the quench zone ~ith a minimum of repressuring; and the suspended solids ater drying r are returned to the mixer where fresh slurry feed is prepared.
Althaugh the ex~mples describe the gasification

3~ of coke in an oil slurr~, the process of our in~ention may _g_ 42~

be used equally well in the yasification 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. Qur pro~,ess may also be used when the solid fuel feed Is suspendad in a gaseous or vaporous medium~
In addition to being appli.cable 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 modi~ications of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereo, and thereore, only such limitations should be made as are indicated in the appended claims.

_ln-

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the gasification of a solid carbonaceous fuel which comprises subjecting said solid fuel to partial oxidation in the presence of H2O at a pressure in the range of from about 150 p.s.i.g. and about 2500 p.s.i.g. and a temperature in the range of from about 1600°F. and about 3500°F to produce a gas comprising CO and H2 and containing particles of unconverted solid fuel, discharging said product gas into a quench zone under the surface of water contained therein to cool said product gas and to form a first suspension of said particles of unconverted solid fuel in said water, passing said suspension to a settling zone maintained at a pressure range of from about 50 p.s.i.g.
to about 3500 p.s.i.g. and a temperature range of from about 100°F. to about 706°F., maintaining said suspension in said settling zone for a residence time ranging from about 3.0 minutes to about 30 minutes thereby allowing the suspension to settle in said settling zone into an upper clarified water portion and a lower more concentrated second suspension consisting essentially of settled unconverted solid fuel particles in water and returning at least a portion of said lower more concentrated second suspension consisting essentially of settled unconverted solid fuel particles in water to the partial oxidation zone.

2. The process of Claim 1 in which the temperature of the first suspension is between 200°F. and 706°F.

3. The process of Claim 1 in which the pressure in the settling zone is between 100 p.s.i.g. and 3500 p.s.i.g.

4. The process of Claim 3 in which the pressure is between 100 p.s.i.g. and 2500 p.s.i.g.

5. The process of Claim 1 in which the residence time of the unconverted solid fuel particles in the settling zone is more than about 3 minutes.

6. The process of Claim 5 in which the residence time is between about 5 minutes and about 30 minutes.

7. The process of Claim 1 in which the solid fuel is ground to a particle size so that at least 95 percent passes through a 14 mesh sieve.

8. The process of Claim 1 in which the solid fuel feed is introduced into the partial oxidation zone as a slurry in a hydrocarbon liquid.

9. The process of Claim 1 in which the solid fuel is introduced into the partial oxidation zone as a slurry in water.