GB2031012A

GB2031012A - Improved Operation of Slagging Gasifiers

Info

Publication number: GB2031012A
Application number: GB7917759A
Authority: GB
Original assignee: Continental Oil Co
Current assignee: ConocoPhillips Co
Priority date: 1978-10-10
Filing date: 1979-05-22
Publication date: 1980-04-16
Also published as: DE2921339A1

Abstract

In a process for gasifying caking coal in a slagging gasifier, the process is started up or returned to normal operation after operating upsets by charging a non-caking carbonaceous feedstock (e.g. coke) to said gasifier vessel until said gasifier vessel is operating smoothly and thereafter omitting said non-caking carbonaceous feedstock and charging said caking coal to said gasifier. <IMAGE>

Description

SPECIFICATION Improved Operation of Slagging Gasifiers This invention resulted from work pursuant to a contract with the United States Department of Energy.

This invention relates to slagging gasifiers.

This invention further relates to an improvement in the operation of slagging gasifiers wherein caking coals are charged as a feedstock to the slagging gasifier.

In view of the recent shortage of fuel materials such as natural gas and the like, considerable effort has been devoted to the development of apparatus and processes whereby solid or semisolid carbonaceous fuel such as coal, lignite, peat, coal extract and the like can be converted into gaseous or liquid hydrocarbon fuels. One such approach is the use of a slagging gasifier. Some representative slagging gasifiers are shown in "Chemistry of Coal Utilization" Suppl. Vol., H. H.

Lowrey, Ed., 1963, John Wiley 8 Sons, Inc., pp.

973-989. A report describing the operation of a slagging fixed bed gasification vessel is Bureau of Mines Report Investigation, Rl-7408 entitled "Slagging Fixed Bed Gasification of North Dakota Lignite at Pressures to 400 psig" by Gronhovd, Harak, Fegley and Severson, 1970. Both these references are hereby incorporated in their entirety by reference. In the use of such gasifiers, a problem arises when caking coal is used as a feedstock in that the caking coal tends to cake and coke in the upper portion of the gasifier, especially during gasifier start-ups and during gasifier upsets, thereby preventing the free downward movement of the solid feedstocks and inhibiting the intimate mixture of such solids with the upwardly moving gases produced in the combustion zone.Since many coals have caking tendencies, it would be highly desirable if a startup method could be developed whereby such coals could be charged to such slagging gasifiers to produce gaseous products for use as a fuel, a feedstock to a methanation plant, a feedstock to a chemical plant, or the like so that such problems can be minimized or eliminated.

It has now been found that an effective startup method for charging caking coal as a feedstock to a slagging gasifier comprises charging a noncaking carbonaceous feedstock to the gasifier vessel until the gasifier is operating smoothly and thereafter omitting the non-caking feedstock and charging said caking coal.

The Figure is a schematic diagram of a slagging gasifier of a type with which the improvement of the present invention is effective.

A slagging gasifier 10 is shown in the Figure and comprises a gasifier vessel 1 2 having positioned therein a refractory lining 14. Feed, which may be any suitable solid carbonaceous fuel such as a caking coal, is introduced into vessel 12 via a lock hopper 1 6 which comprises a hopper 18 having inlet means 20 and 22, a discharge means 24, and a pair of valves 26 and 28. A pressure adjusting means 30 is provided for varying the pressure in hopper 18. A feed distributor 32 positioned in vessel 12 is driven by a gear 34 which meshes with a second gear 36 which is operatively associated with a shaft 38 which extends outside vessel 12 to an external power source 40. Stirrers 42 are provided to agitate the carbonized coal, coke, char or the like solids in vessel 12, break up lumps, and the like.

An oxidant inlet, shown as a pair of tuyeres 46, is provided to inject an oxidant such as air, oxygen, or the like in mixture with steam, CO2, mixtures thereof, or the like, into a combustion zone 48 inside vessel 12. vessel 12 is also provided with a gas outlet 50 through which product gases are recovered and at least one slag tap 52 through which molten slag is removed. While only one slag tap is shown, clearly a plu-rality of such taps could be used. A slag pool may form and overflow through slag tap 52, although as shown, slag tap 52 is near the bottom of vessel 12. In other words, in some embodiments, no slag pool forms, but in others, a slag pool is allowed to form with the slag draining from vessel 12 through slag tap 52 in either event. The slag removed from vessel 12 via slag tap 52 is recovered in a slag receiver 54 which contains water.Receiver 54 is provided with a water inlet 56 and a water outlet 58. A water level 60 is maintained by control of water inlet 56 and water outlet 58 at a level suitable for cooling the slag as it flows from slag tap 52 into receiver 54. A lock hopper 62 comprising a hopper 64, a pair of valves 66 and 68, and a discharge line 70 is used to recover the slag from receiver 54. The product gases recovered through line 50 are passed to a quench vessel 72 where they are countercurrently contacted with a water stream entering vessel 72 through a line 74 and leaving vessel 72 through a line 76. The product gases are recovered through a line 78 and passed to further treatment or use as a fuel.

In the use of the gasifier shown in the Figure, feed comprising coal of various types such as lignite, sub-bituminous, anthracite, bituminous, etc. is charged to vessel 12 by positioning the coal in one of the inlets 20 or 22 to lock hopper 1 6 and lock hoppering the coal into gasifier 1 2.

As the coal passes into gasifier 12, it is substantially uniformly distributed by feed distributor 32 which comprises a rotating plate having openings 44 positioned therein. The coal is normally of a relatively small particle size and typically is from about 0.25 inches in diameter up to about 1.75 inches in diameter. Such coal particles are relatively uniformly distributed across the diameter of vessel 12 by feed distributor 32. As the coal passes downwardly through vessel 12, it is subjected to increasingly high temperatures which result in carbonizing the coal to coke, char or the like which is ultimately converted into gaseous products in combustion zone 48.Caking coals have been observed to form substantially solid cakes in the area beneath stirrers 42 in some instances during start-ups and during gasifier upsets leading to extreme complications and difficulties in the continuous operation of such slagging gasifiers. In many instances, stirrers 42 break up such coal agglomerates, but in some instances it has been observed that the coal has not completely agglomerated by the time it passes beneath stirrers 42 and that agglomerates formed beneath stirrers 42 tend to plug vessel 12. Obviously such plugging results in difficulties in continued operation of gasifier 10.

By the method of the present invention, it has been found that such caking coals are readily charged to gasifier 10 during the gasifier start-up.

When the gasifier charging a non-caking carbonaceous feedstock is operating smoothly on the non-caking feed then the caking coal is substituted as a feed to the gasifier. Desirably, the caking coal charged to the gasifier vessel is of a particle size range such that the largest particle diameter is no more than about 2.5 times the smallest particle diameter. Such a particle size range refers to the normal consistency of the caking coal charged to the gasifier vessel and is not to be construed to include trace quantities of extremely small particles nor occasional large lumps which may enter vessel 12. Desirably, the particles are from about 0.25 to about 1.75 inches in diameter.

Desirably, the non-caking darbonaceous feedstock which is preferably coke, semi-coke, anthracite or the like, charged to gasifier vessel 12, is of a particle size range such that the largest particle diameter is no more than 2.5 times the smallest particle diameter. This size range also refers to the average consist of the material charged to vessel 12 and does not include minor quantities of various small particles which may be present in the feedstock or occasional lumps which may be present. Desirably, the particles are more than about 0.25 inches in diameter and less than about 1.75 inches in diameter.

It has been found that maximum gas permeability and desirable gas flow upwardly through gasifier 10 is accomplished when the coal and coke particle size is within relatively narrow limits as set forth above. As is known to those skilled in the art, particles less than about 0.25 inches in diameter have a tendency to be carried over with the product gases in such gasifier vessels. Accordingly, the size ranges set forth are required in order that suitable gas flow upward through the gasifier is accomplished and so that the coal feedstock is not carried out of the gasifier vessel with the product gases. Desirably the pressure drop per unit height in the gasifier is equal to or less than the density of the feedstock, i.e. the pressure drop per foot shall not exceed the weight of a column of feedstock one foot in height.

Since the ash component of the coal is recovered from such vessels as molten slag, it is necessary that the rate of steam to oxygen fed to the system be adjusted to achieve the proper temperature. In addition, it is desirable that a flux be used to modify and control the slag properties such as viscosity, melting point, and the like.

Accordingly, a flux material which desirably contains calcium compounds such as lime, limestone and the like, is added to either or both the caking coal and the non-caking feedstock.

This flux material can be added by any suitable method but is desirably admixed with the feedstock prior to charging it to vessel 12.

Suitable flux materials are lime, limestone and the like, but particularly desirable results have been accomplished wherein blast furnace slag having unreacted quantities of lime and limestone contained therein were used. The slag has a relatively low and relatively well-defined melting point so that it is liquid at most reaction temperatures encountered in the lower portion of vessel 12 and serves to control and modify the properties of the molten slag. The calcium compounds in their pure form, have relatively high melting points, and in some instances, may not be molten even in combustion zone 48, and as a result, will react more slowly with the molten slag from the coal to produce molten slag of the desired consistency.Accordingly, the blast furnace slag is preferred although such variations and modifications in the slag used are well-known to those skilled in the art and need not be discussed in great detail.

When gasifier upsets occur because of improper control, feedstock variations, etc. similar problems to those encountered during start-up, may occur, i.e. the caking coal tends to cake and coke in the upper portion of the gasifier, thus inhibiting uniform reaction and uniform gas flow.

In such instances, erratic temperatures in various portions of the gasifier may be encountered, undesirable variations in the product gas composition and the like may be observed. Such upset conditions may be restored to smooth operation by charging a non-caking carbonaceous feedstock into the gasifier as feed until smooth operation is restored. Normally at least one, and preferably three, complete vessel charges of noncaking feedstock should be used to ensure a return to smooth operation. Such a non-caking feedstock may conveniently be stored in one of inlet means 20 or 22 for use during gasifier upsets and the like with the other feed inlet being used for normal caking coal charging operations.

Upon restoring smooth operation, the non-caking feedstock may be omitted with the charge being changed to all caking coal.

Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments described are illustrative rather than limiting in nature, and that many variations and modifications are possible within the scope of the present invention. Many such modifications and variations may appear obvious and desirable to those skilled in the art, based upon a review of the foregoing description of preferred embodiments.

Claims

1. A process for gasifying caking coal in a slagging gasifier vessel, said process comprising: charging said caking coal into an upper portion of said gasifier vessel; injecting an oxidant stream into a lower portion of said gasifier vessel to convert said coal into gaseous products and molten slag; recovering said gaseous products from said upper portion of said gasifier vessel and recovering said molten slag from said lower portion of said gasifier vessel; the start-up method for said process comprising the steps of charging a non-caking carbonaceous feedstock to said gasifier vessel until said gasifier vessel is operating smoothly and thereafter omitting said non-caking carbonaceous feedstock and charging said caking coal to said gasifier.

2. The process of claim 1 wherein said noncaking carbonaceous feedstock is coke.

3. The process of claim 1 wherein said caking coal charged to said gasifier vessel is of a particle size range such that maximum gas permeability in the solid feedstock passing downwardly through said gasifier vessel is achieved.

4. The process of claim 1 wherein said caking coal particles are greater than about 0.25 inches in diameter and less than about 1.75 inches in diameter.

5. The process of claim 1 wherein said noncaking carbonaceous feedstock charged to said gasifier vessel is of a particle size range such that maximum gas permeability in the solid feedstock passing downwardly through said gasifier vessel is achieved.

6. The process of claim 1 wherein said noncaking carbonaceous feedstock particles are greater than about 0.25 inches and less than about 1.75 inches in diameter.

7. A process for gasifying caking coal in a slagging gasifier vessel, said process comprising: charging said caking coal into an upper portion of said gasifier vessel; injecting an oxidant stream into a lower portion of said gasifier vessel to convert said coal into gaseous products and molten slag: recovering said gaseous products from said upper portion of said gasifier vessel and recovering said molten slag from said lower portion of said gasifier vessel; the method for returning said gasifier vessel to normal operation after operating upsets, comprising the steps of charging a non-caking carbonaceous feedstock to said gasifier vessel until said gasifier vessel is operating smoothly and thereafter omitting said non-caking carbonaceous feedstock and charging said caking coal to said gasifier.

8. The process of claim 7 wherein said noncaking carbonaceous feedstock is coke.

9. The process of claim 7 wherein said caking coal charged to said gasifier vessel is of a particle size range such that maximum gas permeability in the solid feedstock passing downwardly through said gasifier vessel is achieved.

10. The process of claim 7 wherein said caking coal particles are greater than about 0.25 inches in diameter and less than about 1.75 inches in diameter.

11. The process of claim 7 wherein said noncaking carbonaceous feedstock charged to said gasifier vessel is of a particle size range such that maximum gas permeability in the solid feedstock passing downwardly through said gasifier vessel is achieved.

12. The process of claim 7 wherein said noncaking carbonaceous feedstock particles are greater than about 0.25 inches and less than about 1.75 inches in diameter.

13. The process of claim 1, substantially as described herein with reference to the accompanying drawing.

14. The process of claim 7 substantially as described herein with reference to the accompanying drawing.