WO2003018720A2

WO2003018720A2 - Apparatus and prcess for discharging ash from a high pressure gasifier

Info

Publication number: WO2003018720A2
Application number: PCT/IB2002/003483
Authority: WO
Inventors: Paul Smit Baumann; Jacobus Albertus Bernardus Van Niekerk; Johannes Willem Van Zyl
Original assignee: Sasol-Lurgi Technology Company (Pty) Limited
Priority date: 2001-08-28
Filing date: 2002-08-27
Publication date: 2003-03-06
Also published as: WO2003018720A3; AU2002328185A1

Abstract

A method of discharging ash from a gasifier (12) includes discharging hot ash from a gasification chamber (34) of the gasifier (12), through an ash discharge opening in or adjacent a floor of the gasifier (12), into an ash chamber (40). The hot ash is allowed to mix with a carrier liquid in the ash chamber (40) and an ash/carrier liquid mixture is withdrawn from the ash chamber.

Description

GASIFIERS

THIS INVENTION relates to gasifiers. It relates in particular to a method of discharging ash from a gasifier, and to a gasifier installation.

According to a first aspect of the invention, there is provided a method of discharging ash from a gasifier, which method includes discharging hot ash from a gasification chamber of a gasifier, through an ash discharge opening in or adjacent a floor of the gasifier, into an ash chamber; allowing the hot ash to mix with a carrier liquid in the ash chamber; and withdrawing an ash/carrier liquid mixture from the ash chamber.

The ash chamber thus has an ash inlet at a high level and a mixture outlet at a lower level, and the carrier liquid may be present in the ash chamber as a body or volume of the carrier liquid above the mixture outlet. The body or volume of the carrier liquid thus acts to quench the hot ash as well as to accommodate pressure relief across the ash chamber due to its incompressibility, thereby permitting the mixture to be discharged to atmospheric pressure, while the gasifier operates at elevated pressure. The carrier liquid may, in particular, be water. The discharging of the hot ash from the gasifier may be effected in a continuous manner. More particularly, this may be effected by continuously rotating an ash grate located above the ash discharge opening of the gasifier.

The withdrawal of the ash/water mixture from the ash chamber may be effected continuously, intermittently, or semi-continuously, ie cyclically or at regular intervals. Thus, the withdrawal of the ash/water mixture from the ash chamber may be effected by continuously rotating a discharge member, having a discharge cavity, around an axis, with the discharge cavity thus moving from a first position in which it is aligned with the mixture outlet of the ash chamber so that some of the ash/carrier liquid mixture enters it, to a second position in which the ash/carrier liquid mixture is discharged under gravity from the discharge cavity and in which the discharge member closes off the mixture outlet, whereafter the discharge cavity again rotates into register with the mixture outlet, and so on. Thus, the withdrawal of the ash/water mixture from the ash chamber may be effected by means of a rotary valve located below the mixture outlet of the ash chamber, with the discharge member forming part of the rotary valve.

The level of the ash/water mixture in the ash chamber may thus be controlled by regulating the rotational speed of the rotary valve.

The gasifier can, in principle, be any gasifier in which a solid particulate carbonaceous material or feedstock such as coal, waste material, coal/waste material mixtures, or the like, can be burnt. However, it is envisaged that the method of the present invention will have particular application on a coal gasifier. The coal gasifier may then, still more particularly, be a fixed bed coal gasifier.

According to a second aspect of the invention, there is provided a gasifier installation, which includes a gasifier having a floor, a roof spaced from the floor, and a circular cylindrical wall between the floor and the roof, with a solid particulate carbonaceous material inlet being provided in or adjacent the roof while an ash outlet is provided in or adjacent the floor, and providing a gasification chamber; ash discharge means in the gasification chamber, above the ash outlet, for discharging ash from the gasification chamber; a vessel below the ash outlet of the gasifier, the vessel providing an ash chamber and having an ash inlet aligned with the ash outlet of the gasifier as well as an outlet at a lower level than the ash inlet; carrier liquid introduction means for introducing a carrier liquid into the ash chamber; and discharge means at the vessel outlet, for discharging a mixture of ash and carrier liquid from the ash chamber.

The ash discharge means may, in particular, be a rotatable grate.

The carrier liquid may, as hereinbefore set out, be water. The carrier liquid introduction means may include at least one water conduit leading into the ash chamber, and the conduit may be fitted with a control valve for regulating the water supply to the ash chamber. The installation may include level measurement means, such as a nuclear level measurement device, for measuring the level of the ash/water mixture in the ash chamber.

The discharge means at the vessel outlet may comprise a rotatable discharge member having a discharge cavity. Thus, in use, by rotating the discharge member, the discharge cavity will move from a first position in which it is aligned with the mixture outlet of the ash chamber so that some of the ash/carrier liquid mixture enters it, to a second position in which the ash/carrier liquid mixture is discharged under gravity from the discharge cavity and in which the discharge member closes off the mixture outlet, whereafter the discharge cavity again rotates into register with the mixture outlet, and so on. Typically, the rotatable discharge member can be driven by a variable speed electric or hydraulic motor via a chain, gear or belt drive.

The discharge member may form part of a rotary valve mounted to the vessel outlet.

The gasifier may, in particular, be a coal gasifier, and still more particularly may be a fixed bed coal gasifier.

The invention will now be described by way of example with reference to the accompanying drawings.

In the drawings, FIGURE 1 shows, diagrammatically, a part longitudinal or vertical sectional view of a fixed bed coal gasifier installation according to the invention; and

FIGURE 2 shows, diagrammatically, an enlarged part longitudinal or vertical sectional view of part of the installation of Figure 1 .

In the drawings, reference numeral 10 generally indicates a fixed bed coal gasifier installation.

The installation 10 includes a gasifier, generally indicated by reference numeral 1 2. The gasifier 12 has a floor 14, a roof 1 6 spaced from the floor, and a circular cylindrical wall 18 between the wall and the roof. The floor 1 , roof 1 6 and wall 18 are of jacketed or double-wall construction so that they have cavities or spaces 20 through which water can circulate as a cooling medium. A coal inlet, generally indicated by reference numeral 22, is provided in the roof 1 6. The inlet 22 is defined by a cylindrical member 24, to the upper end of which is mounted a coal lock 26. To the lower end of the cylindrical member 24 is attached a static coal distribution device 28.

An ash outlet, generally indicated by reference numeral 30, is provided in the floor 14, while a rotatable ash grate 32 is provided within the gasifier 12 above the ash outlet 30. The grate 32 is mounted immediately above the floor 14, and the floor 14 slopes downwardly inwardly from the wall 18 to the ash outlet 30.

The gasifier 12 provides a coal gasification chamber, generally indicated by reference numeral 34. In use, an ash bed 36 will be present in the coal gasification chamber 34 immediately above the grate 32, with a coal bed 38 being provided above the ash bed 36.

The installation 10 also includes a vessel, generally indicated by reference numeral 40, providing an ash chamber 41 . The vessel 40 includes a cylindrical wall 42. The upper end of the wall 42 is closed off with a domed top 44 provided with a flanged connection 46 which provides an ash inlet 48 in the domed top 44. The flanged connection 46 is connected to a flanged connection 50 around the ash outlet 30 of the gasifier 12.

The vessel 40 also includes a downwardly inwardly tapering floor 52 which has, at its apex, a flanged connection 54 providing an outlet 56.

An upper conical closure member 58 is provided within the chamber 41 , immediately below the inlet 48. The closure 58 provides a top pressure seal and can be actuated by means of an actuating arm (not shown) to enable isolation of the chamber 41 for maintenance. A temperature measurement point 60 is provided in the domed top 44 as is a high pressure steam arrangement 62 for pressurizing the vessel 40 and for purging it.

The vessel 40 also includes a nuclear level measurement device (not shown) for measuring the level of an ash/water mixture in the vessel 40.

Water conduits 64, 66, fitted with control valves 68, 70 respectively, lead from a high pressure water pump (not shown) into the chamber 41 .

The installation 10 also includes a rotary valve, generally indicated by reference numeral 80. The rotary valve 80 has a body 82 provided with an inlet flanged connection 84 as well as an outlet flanged connection 86. The inlet flanged connection 84 is connected to the flanged connection 54 of the vessel 40, while the outlet flanged connection 86 is connected to the flanged connection 88 of an ash chute 90. A rotatable valve component or discharge member 92, having a discharge cavity 94 for receiving a water/ash mixture as hereinafter described, is located within the body 82.

In use, the fixed bed coal gasifier 12 is operated in known fashion so as to have the ash bed 36 and the coal bed 38, with coal being introduced into the gasification chamber 34 from the coal lock 26 as desired. Hot ash is continuously discharged through the ash outlet 30 by means of the rotating ash grate 32.

A reaction zone (not shown) is provided between the ash bed 36 and the coal bed 38. Fresh coal thus enters the reaction or gasification zone from the coal bed 38, is burnt in the reaction zone, and then passes as ash into the ash bed 36 from where it is discarded or discharged through the ash outlet 30. The reaction or gasification zone in turn has three zones (not shown) in which drying, devolatilization and gasification is effected. By means of the water conduits 64, 66 and control valves 68, 70, a body of water 96, having a substantially constant volume, is maintained within the chamber 41 . As the hot ash which is discharged from the gasifier 12 enters the water 96 it is quenched, and a ash/water mixture is formed. This mixture is withdrawn from the chamber by means of the rotary valve 80. The ash/water mixture is thus withdrawn semi-continuously from the chamber 40 as the valve component 92 rotates.

The water 96, apart from acting to cool or quench the hot ash particles 98, will also, in view of its incompressible nature, accommodate pressure relief across the chamber 41 . Thus, the gasifier 12 operates at an elevated pressure of about 30 bar. However, due to the incompressible nature of the body of water 96, the ash/water mixture can be discharged from the relatively high pressure of the gasifier, which will also be the pressure in the upper section of the chamber 41 , to atmospheric pressure in the chute 90, by means of the rotary valve 80.

The fixed bed coal gasifier 12 may, in particular, be that available under the trade name LURGI.

The Applicant is aware of a gasifier installation similar to that of the installation 10. However, instead of the ash chamber 40, an ash lock is mounted to the flanged connection 50 of the gasifier 12. The ash lock has an upper ash inlet and a lower ash outlet. The ash lock has an upper closure or valve member, similar to the closure member 58, for closing off the ash inlet, and also has a similar bottom closure member or valve closing off the outlet of the ash lock. The top and bottom valves are both operated by arm or lever mechanisms. In this known installation, hot ash is removed from the gasifier 1 2 by means of a batch process, as follows: When the ash lock is full of ash, rotation of the ash grate 32 is stopped to suspend flow of ash into the ash lock. The ash lock top valve is then closed to isolate the ash lock from the gasifier 12. The ash lock is then depressurised through a water filled expansion drum, whereafter the ash lock bottom valve (which has been closed up to this point) is opened. Dry ash then flows from the ash lock into the ash chute or sluiceway 90 and is carried away in a water canal. The bottom valve is then closed and the ash lock repressurized to the gasifier operating pressure by high pressure steam, whereafter the top valve is again opened. The grate is then brought back into operation and ash again flows into the ash lock.

In this known installation, ash removal is thus effected in a cyclic batch-like manner. An ash lock loading/unloading cycle as hereinbefore described typically takes about 30 minutes. The Applicant has found that this cyclic operation gives rise to problems as discussed hereinafter.

Firstly, the stop-start operation of the ash grate introduces operational instabilities. The function of the ash grate is two-fold: (i) to remove ash from the gasification chamber of the gasifier, thereby to allow fresh coal to be fed into the gasifier, and (ii) to distribute the gasification agent, ie a mixture of steam and oxygen. With the grate in the stationary mode, a build-up of ash on top of the grate occurs. This in turn forces the reaction zone, ie the interface of the coal bed 38 with the ash bed 36, to move vertically up towards the top of the gasifier 12, resulting in a rise in temperature in the upper parts of the gasifier. If unchecked, such a rise in temperature can lead to a trip or load cutback scenario. With the grate in stationary mode, gasification agent distribution is suspended which could lead to non- homogeneous distribution of isolated reaction zones, giving rise to process instability. Once the grate is put into motion again, the reaction zone or fire bed is lowered, and temperatures in the upper part of the gasifier decline while temperatures in the lower part thereof increase. These temperature fluctuations associated with the ash removal lead to unstable operation and fluctuating raw gas (a product from the gasifier) temperatures. These cyclic temperatures lead to equipment fatigue. The cyclic temperature fluctuations introduce the degenerative mechanism of thermal fatigue in the mechanical equipment of the installation. Special considerations and specifications, ie post weld heat treatment, have to be met for equipment in cyclic operation to ensure safe and predictable operation. The ash lock has, as a result of repeated weld repairs that are required, a limited life.

Operational failures of the pressure seals provided by the top and bottom closures or valves are also experienced. The cone-and-seat configuration used to close off the ash lock inlet and outlet relies on a metal-to-metal seal for pressure containment. Any hard foreign objects, such as discards from mining operations to produce the coal that is used, and that get caught between the seat and the conical valve member, inadvertently damage the sealing surfaces. This leads to leak through situations requiring extensive maintenance and ultimately production losses.

The hot ash that is produced in the gasifier 1 2 is an extremely abrasive medium. The flow of the ash over the ash lock walls cause accelerated metal loss that requires extensive weld repairs to restore the wall thickness to safe limits.

Two independent operating or actuating arms are required to actuate the top and bottom closure members or valves of the ash lock. This is a mechanically intensive configuration. The arms are driven by an extensive hydraulic system that requires synchronous operation. The arms extend through pressure bearing envelopes, and pressure tight seals are accomplished by means of packing arrangements. These operating arms are maintenance intensive and require daily preventative actions to ensure reliability. Substantial supporting equipment is also required in the known installations. These include the expansion vessel hereinbefore described and pressurizing valves, which are both maintenance intensive. To allow depressurising of the ash lock prior to the unloading of the ash to the sluiceway, an expansion vessel is used as hereinbefore described. This vessel is subject to statutory maintenance requirements, and often gives rise to failures associated with blockages and fouling. The related pressurizing and depressurising valves are also prone to a high rate of failure.

It is believed that by means of the ash chamber 40 containing the body of water 96 according to the invention and the rotary valve 80, that functions in a semi-continuous manner as hereinbefore described, these problems can at least be alleviated as follows: semi-continuous ash removal will permit the ash grate to rotate continuously, thus resulting in a more stable reaction zone and ultimately more stable gasifier operation; semi-continuous ash removal will eliminate the cyclic pressure and temperature fluctuations associated with the known installations, and thus extend the life expectancy of the equipment; - semi-continuous ash removal in accordance with the invention eliminates the top and bottom cone-and-seat closures or valves from operation, thus reducing the number of pressure seal breaches; semi-continuous ash removal in accordance with the invention will substantially reduce erosion; - semi-continuous ash removal in accordance with the invention will render the operating arms of the bottom closure, and related vessel internals, obsolete; and semi-continuous ash removal in accordance with the invention will render the expansion vessel, pressurizing and depressurising valves obsolete. Additionally, in the installation 10, ie in the installation according to the invention, as a result of the ash/water mixture level measurement that can be effected in the vessel 40 by means of the nuclear level measurement device, the rate of ash removal can be varied or adjusted to match the rate of ash production. Still further, since the body of water 96 provides the necessary pressure relief, the chamber 40 does not operate by way of pressurizing/depressurising cycles. It is also expected that the installation 10 will be subject to less maintenance as a result of the elimination of supporting equipment such as the expansion vessel. It is also expected that the installation 10 will have improved reliability in view of its mechanical simplification as compared to the known installation; that raw gas quality will be improved as a result of the stable reaction conditions in the gasifier 10; and that an improved mean gas production rate will be brought about by the reduction of load cutback events associated with ash removal system failures.

Further advantages are improved carbon efficiencies of the gasifier 10, no ash dust pollution as is currently experienced when the ash lock is emptied, improved energy efficiencies of the gasifier 10, and a more environmentally friendly gasifier 10.

Claims

1 . A method of discharging ash from a gasifier, which method includes discharging hot ash from a gasification chamber of a gasifier, through an ash discharge opening in or adjacent a floor of the gasifier, into an ash chamber; allowing the hot ash to mix with a carrier liquid in the ash chamber; and withdrawing an ash/carrier liquid mixture from the ash chamber.

2. A method according to Claim 1 , wherein the ash chamber has an ash inlet at a high level and a mixture outlet at a lower level, and with the carrier liquid being present in the ash chamber as a body or volume of the carrier liquid above the mixture outlet, the body or volume of the carrier liquid thus acting to quench the hot ash as well as to accommodate pressure relief across the ash chamber due to its incompressibility, thereby permitting the mixture to be discharged to atmospheric pressure, while the gasifier operates at elevated pressure.

3. A method according to Claim 2, wherein the carrier liquid is water.

4. A method according to Claim 2 or Claim 3, wherein the discharging of the hot ash from the gasifier is effected in a continuous manner.

5. A method according to any one of Claims 2 to 4 inclusive, wherein the withdrawal of the ash/water mixture from the ash chamber is effected semi-continuously.

6. A method according to Claim 5, wherein the withdrawal of the ash/water mixture from the ash chamber is effected by continuously rotating a discharge member, having a discharge cavity, around an axis, with the discharge cavity thus moving from a first position in which it is aligned with the mixture outlet of the ash chamber so that some of the ash/carrier liquid mixture enters it, to a second position in which the ash/carrier liquid mixture is discharged under gravity from the discharge cavity and in which the discharge member closes off the mixture outlet, whereafter the discharge cavity again rotates into register with the mixture outlet.

7. A method according to Claim 6, wherein the withdrawal of the ash/water mixture from the ash chamber is effected by means of a rotary valve located below the mixture outlet of the ash chamber, with the discharge member forming part of the rotary valve.

8. A method according to any one of Claims 1 to 7 inclusive, wherein the gasifier is a fixed bed coal gasifier.

9. A gasifier installation, which includes a gasifier having a floor, a roof spaced from the floor, and a circular cylindrical wall between the floor and the roof, with a solid particulate carbonaceous material inlet being provided in or adjacent the roof while an ash outlet is provided in or adjacent the floor, and providing a gasification chamber; ash discharge means in the gasification chamber, above the ash outlet, for discharging ash from the gasification chamber; a vessel below the ash outlet of the gasifier, the vessel providing an ash chamber and having an ash inlet aligned with the ash outlet of the gasifier as well as an outlet at a lower level than the ash inlet; carrier liquid introduction means for introducing a carrier liquid into the ash chamber; and discharge means at the vessel outlet, for discharging a mixture of ash and carrier liquid from the ash chamber.

10. A gasifier installation according to Claim 9, wherein the ash discharge means is a rotatable grate.

1 1 . A gasifier installation according to Claim 9 or Claim 10, wherein the carrier liquid is water and wherein the carrier liquid introduction means includes at least one water conduit leading into the ash chamber, with the conduit being fitted with a control valve for regulating the water supply to the ash chamber.

12. A gasifier installation according to any one of Claims 9 to 1 1 inclusive, wherein the discharge means at the vessel outlet comprises a rotatable discharge member having a discharge cavity, so that, in use, by rotating the discharge member, the discharge cavity will move from a first position in which it is aligned with the mixture outlet of the ash chamber so that some of the ash/carrier liquid mixture enters it, to a second position in which the ash/carrier liquid mixture is discharged under gravity from the discharge cavity and in which the discharge member closes off the mixture outlet, whereafter the discharge cavity again rotates into register with the mixture outlet.

13. A gasifier installation according to Claim 12, wherein the discharge member forms part of a rotary valve mounted to the vessel outlet.

14. A gasifier installation according to any one of Claims 9 to 13 inclusive, wherein the gasifier is a fixed bed coal gasifier.