METHOD AND DEVICE FOR DISCHARGING PARTICULATE MATERIAL FROM A PRESSURIZED CONTAINER

Abstract: The present invention relates to a method and a device for discharging particulate material from a container (10) which is under pressure and located at a higher level to a store (16) which is under a lower pressure and located at a lower level via a preferably vertical tube (18) and further via a flow-controlling material feeder (26, 27). The particulate material which is discharged forms a column of particles (22) in the tube (18), which by their flow resistance achieve across this particle column (22) a pressure reduction in the gas which from the higher container (10) flows through the particle column (22). The discharge is stabilized in relation to variations in the gas flow by the particle column (22) traversing at least one pressure stabilizer (20) comprising a vessel (21) which is connected to an environment by means of a throttle means (34), the pressure level of the particle column (22) at the pressure stabilizer (20) being maintained stable in relation to the pressure at the orifice of the discharge tube (18) since pressure reductions in the gas accompanying the particles are limited by extracting gas to the environment via the throttle means (34).

Description: Method and device for discharging particulate material from a pressurized container TECHNICAL FIELD The invention relates to a method and a device for dischar¬ ging particulate material from a container which is under pressure into a container at a lower pressure, usually atmospheric pressure. The invention is primarily intended for discharging residual products from a combustor. BACKGROUND ART Discharging and handling solid residual products such as bed material, ash and slag products from a combustor which operates at pressure levels up to 20 bar or more, for example a PFBC power plant, entail problems which are difficult to master. Lock hopper systems can be used. The material is often greatly erosive, which entails conside- rable stresses on valves in the lock hopper systems and a rapid reduction of their sealing capacity. The service life is short and the maintenance costs high. The problems with discharge of solid material from a pressurized combustor are dealt with in a publication from Argonne National Labora- tory, Argonne Illinois, ANL/CEN/FE-81-3 entitled "Discharge and Handling of solids from Pressurized Fluidized Bed Combustors" by John E. Hanway and W. F. Podolski. European patent specification 0 108 505 describes a pressure-reducing discharge system without valves. One object of the present invention is to provide a simpli¬ fied discharge system with low service needs for particulate material from a pressurized container. The particulate mate¬ rial primarily relates to residual products in the form of ash and dust particles from a pressurized combustor with combustion in a fluidized bed. Another object is to achieve a more uniform discharge than in a lock hopper system. Still other objects are to reduce the number of valves and the gas consumption in discharge systems. Swedish patent application 8803599-3 describes a proposal for a technical solution of the above-mentioned problems in connection with the discharge of partici late material, in the following also referred to by the term particles, and which, in addition, fulfils certain of the above-mentioned objects of the present invention. The Swedish specification shows a device in which the reduction of pressure when dis¬ charging particles from a pressurized container is brought about by discharging the particles via a substantially ver¬ tical tube, wherein an upright column in the form of partic¬ les of discharged particulate material is built up in the tube. By allowing this particle column to become suffici¬ ently high, the particle column gives rise to a pressure reduction in the accompanying gas from the pressure con¬ tainer, as the gas is forced to flow through the upright particle column. If the particle column is sufficient high, a pressure reduction of the gas down to atmospheric pressure can be obtained. Particulate material in the lowermost part of the particle column is discharged by means of a rotary- vane feeder. A disadvantage with pressure reduction in a particle column of the above known kind is that instability may arise. As an example, the discharge of the particulate material must be controlled by some form of control device. This will give the particle column different heights in course of time. When the particle column is shorter, a higher pressure difference arises in the particle column along each section of the particle column, which entails a higher speed of the gas flow and a higher propelling force on the material in the particle column under these circumstances. Also the dis- charged material may influence the stability in the dis¬ charge device. If suddenly a larger percentage of coarser particles or fine particles cemented together into lumps are included in the particle column, gas paths may arise there- in. This causes a more rapid gas flow which in its turn influences the particle flow through the discharge device, whereby the pressure reduction which was to be achieved by the resistance of the particle column to gas flow does not function as intended. SUMMARY OF THE INVENTION The present invention relates to a method and a device for discharging particulate material from a container which is under pressure and located at a higher level to a store which is under a lower pressure and located at a lower level, via a preferably vertical tube and further via a flow-controlling material feeder. The particulate material which is discharged forms in the tube a column of particles which by their flow resistance achieve a pressure reduction over this particle column in the gas which from the higher container flows through the particle column. The discharge is stabilized with respect to variations in the gas flow by the particle column extending through at least one pressure stabilizer comprising a vessel which is connected by a throttle means to an environment, the pressure level of the particle column at the pressure stabilizer being maintained stable in relation to the pressure at the orifice of the discharge tube since increases of the pressure of the gas accompanying the particles are limited by discharging gas to the surroundings via the throttle means. The . great flow resistance in the particle column conside- rably restricts the gas flow. Extraction of gas via the throttle means of the pressure stabilizer takes place con¬ tinuously. The pressure stabilizer comprises a vessel, into which the particle column opens out. A throttle means, for example in the form of a Laval nozzle, is connected to the vessel, and gas is extracted to the surroundings, usually the atmosphere, through this nozzle. In the pressure stabi¬ lizer the flow out of gas is formed such that the particu¬ late material does not move with the extracted gas flow which is determined by and passes out through the throttle means. From the lower part of the vessel of the pressure stabilizer, the discharge tube with its particle column extends further down into a material feeder. If, for example, the container with the ligher pressure from where particulate material is fed out is subjected to a pressure increase, the pressure in the pressure stabilizer vessel will rise. As a further example, the pressure will rise in the pressure stabilizer vessel at a position where the flow resistance is reduced in the particle column up¬ stream of the pressure stabilizer. In case of such changes of the pressure conditions, the discharge of particulate material via the material feeder is stopped at the bottom part of the discharge tube. Gas is now automatically extrac¬ ted via the throttle means from the pressure stabilizer vessel to the surroundings with a larger extracted gas flow than normal until the pressure in the pressure stabilizer returns to normal conditions, whereupon the material feeder can resume the discharge of dust. The pressure increase in the discharge tube caused by the above abnormal conditions must not reach the orifice of the particle column in the material feeder, which is avoided with the extra extraction of gas via the throttle means as described above. Since a pressure difference between the upstream and down¬ stream sides of the material feeder can be avoided in the manner described, the material feeder is not subjected to stresses caused by pressure differences between its inlet and outlet, which allows a simple rotary vane feeder to be used and good reliability to be attained. The discharge of particulate material also functions without a pressure difference across the particle column since the particulate material by its own weight moves downwards in the discharge system, where discharge of particles takes place from a higher to a lower level. When no pressure difference occurs, the discharge system can be emptied and filled with particulate material without any need of the accessory devices described in this invention. Discharge of occasional larger lumps (sinters etc.) func- tions under all operating conditions in the discharge device. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically illustrates a discharge device for dust from a pressurized container according to the inven¬ tion. Figures 2a - 2c show a number of modifications of the embo- diment of the throttle means which controls the gas flow out of the pressure stabilizer. Figure 3 illustrates a modified embodiment of a material feeder in the form of an L-valve. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the invention will be described with reference to the figures. In the embodiment reference is made to the method of discharge and the device in a PFBC power plant, where discharge of particulate material accor¬ ding to the invention can be utilized both for discharge of ashes and residual products from a bed vessel and discharge of separated ashes from dust separators, for example cyclo- nes, which separate dust in waste gases from the plant. In both cases, the discharge is performed from a pressurized container. Figure 1 shows a first container 10 for particulate mate- rial, preferably ashes. In this first container, the pressure Pi prevails, which considerably exceeds the atmos¬ pheric pressure. A second container 14 is a container with the pressure PQ which in a store 16 receives fed-out parti- culate material. The pressure Po is usually the same as the atmospheric pressure but may, of course, have a different value. In the case where the pressure Po is the atmospheric pressure, the material store 16 may consist of only a store 16 for particulate material, where a second container 14 for the store 16 is not required. A discharge tube 18, divided into an upper tube section 18a and a lower tube section 18b, from the first container 10 is vertically oriented but may enjoy a different inclination, for example to reduce the necessary overall height for the discharge device. The upper tube section 18a extends into a pressure stabilizer 20, which comprises a vessel 21. From the vessel 21, gas is extracted which flows through a particle column 22 formed in the tube 18 during discharge. Downstream of the pressure stabilizer 20, the discharge tube 18 continues with the lower tube section 18b, to the outlet of which a rotary vane feeder 26 is arranged. The lower tube section 18b need not have any real extension in the longitudinal direction but may quite simple constitute an inlet opening to the rotary vane feeder 26 or to another type of dust discharge device, for example an L valve 27. An outlet tube 28 from the rotary vane feeder 26 opens out into the second container 14. The rotary vane feeder 26 is driven by a motor M, the speed of which can be controlled such that the capacity of the rotary vane feeder 26 can be varied. The vessel 21 is provided with an outlet 32 for extraction of gas which flows through the particle column 22 in the discharge tube 18 during dis¬ charge. The outlet 32 in its turn is equipped with a throttle means 34 for determining the extracted gas flow. The throttle means 34 can suitably consist of only a simple throttling of the gas flow achieved by means of a Laval nozzle or by means of a simple fixed throttle washer, both variants being illustrated in Figures 2b and 2c, respec¬ tively, at 35. However, other types of throttle means may be used as well. Thus, the throttle means 34 may consist of a pressure-maintaining valve 36. Further, according to Figure 2b, the throttle means 34 can be arranged in the form of a fixed throttle means 35 in parallel with a pressure-main- taining valve 36. A further example is shown in Figure 2c, where a pressure-maintaining valve 36 in series with a simple fixed throttle means 36 is connected in parallel with an additional pressure-maintaining valve 36. During the discharge, a particle column 22 is formed in the tube 18a. The particles in the particulate material 12 fall down into the discharge tube 18 by their own weight and while being influenced by the gas which flows from the first container 10 down into the tube 18. The upper tube section 18a of this tube 18 is inserted a certain distance into the vessel 21 and opens out approximately halfway down into the vessel 21. The orifice 23 of the upper tube section 18a is conically shaped. When particles in the particle column 22 exit through the orifice 23, they are collected in the vessel 21, whereby particles will be stored to a certain level in the vessel 21. Part of the gas flowing through the particle column 22 will be extracted via the outlet 32 and part of the gas will flow on through the lower tube section 18b of the discharge tube 18 down to the rotary vane feeder 26. To prevent particles in the fed-out dust from accompa¬ nying the gas which is extracted via the outlet 32, the orifice 23 is conical, which means that the particles with their greater inertia cannot accompany the extracted gas but are slowed down ahead of the outlet 32 without at the same time creating too much turbulence at the orifice of the dis¬ charge tube 18 inside the vessel 21. The gas speed past the conical orifice 23 and elsewhere in the vessel 21 must not become so high (locally or generally) that the movement in the dust becomes so great as to cause instability. Such instability may, for example, occur if the gas speed in the vessel 21 becomes so high that particulate material is forced by the gas flow to the outlet 32 or that the parti¬ culate material is fluidized upstream of the outlet 32. The area ratio between different areas in the pressure stabili¬ zer 20 determines the stability of the discharge. The area of the cone orifice 23 in relation to the area at the outlet 32 is an example of such an area ratio. A number of other parameters determine this area ratio. In a PFBC plant, for example, the area ratio described is dependent on the grain size of the particulate material. For bed ash, using a rough measure as an example, the ratio of the cone orifice 23 to the outlet 32 may be 100:1, whereas the corresponding area ratio for fine cyclone ash may roughly be described as 10000:1. By choosing the diameter and length of the discharge tube 18 and hence the height of the particle column 22, the desired flow resistance can be attained in the particle column 22 and a tolerable gas flow from the first container 10 be obtained. By simultaneous choice of parameters for gas extraction in the throttle means 34, a tolerable fall of pressure across the dust discharge device (26, 27) can be obtained.

Claims: CLAIMS 1. A method of discharging particulate material (12) from a first container (10) to a store (16) which is under a con- siderably lower pressure and wherein the material (12) is discharged via a preferably vertical discharge tube (18) permanently containing a particle column (22) of the parti¬ culate material, characterized in that the particle column (22) extends through at least one pressure stabilizer (20) where gas which flows through the particle column (22) is extracted to an environment via a throttle means (34) to thus stabilize the discharge and make this independent of pressure variations in the particle column (22) upstream of the pressure stabilizer ( ' 20) . 2. A method of discharging particulate material according to claim 1, characterized in that the material is discharged via an upper tube section (18a) from the first container (10) located at a higher level, a pressure stabilizer (20), a lower tube section (18b) and a material feeder (26, 27) to a store (16) for receiving the discharged material, wherein the material columns in the two tube sections (18a, 18b) are utilized for reducing the pressure and the material feeder (26, 27) is utilized for control of the quantity of dis- charged material. 3. A method of discharging particulate material according to claim 2, characterized in that if a pressure increase occurs in the particle column (22) upstream of the pressure stabilizer (20), the material feeder (26, 27) is stopped until the pressure in the pressure stabilizer (20) returns to the normal level by extraction of gas via the throttle means (34) . 4. A method of discharging particulate material according to claim 2, characterized in that the extracted gas flow is controlled by the design of the throttle means (34) . 5. A discharge device for carrying out the method of dis¬ charging a particulate material from a first container (10) to a store (16) which is under a considerably lower pressu¬ re, characterized in that the device comprises a prefe- rably vertical discharge tube (18) connected to the first container (10) , a pressure stabilizer (20) arranged between the upper tube section (18a) of the discharge tube (18) and the lower tube section (18b) thereof, and a flow-controlling discharge device (26, 27), downstream of the pressure stabi- lizer (20), for the particulate material, wherein the press¬ ure stabilizer (20) extracts gas to an environment via a throttle means (34), said gas flowing through an upright particle column (22) formed in the discharge tube (18) in order thus to achieve a stable pressure value at the mate- rial feeder (26, 27) . 6. A discharge device according to claim 5, characterized in that the pressure stabilizer (20) comprises a vessel (21) for receiving the particulate material from the upper tube section (18a) of the discharge tube (18), an outlet (32) at the upper end of the vessel (21) for gas which is extracted via a throttle means (34) . 7. A discharge device according to claim 5, characterized in that the pressure stabilizer (20) consists of a vessel (21) , into which the upper tube section (18a) of the dis¬ charge tube (18) opens out at a considerable distance from the upper wall of the vessel (21) . 8. A discharge device according to claim 5, characterized in that the throttle means (34) comprises a) a fixed throttle means (35) in the form of a Laval nozzle or a simple fixed throttle washer, or b) a pressure-maintaining valve, or c) an embodiment comprising a throttle means according to a) and a throttle means according to b) in parallel with each other, or d) an embodiment comprising a throttle means according to a) and a throttle means according to b) in series with each other, or e) an embodiment comprising a throttle means according to b) in parallel with a throttle means according to d) . 9. A discharge device according to claim 5, characterized in that the flow-controlling material feeder consists of a rotary vane feeder (26) or of a material-feeding L valve (27) .

Inventors: VIND TYGE, VIND, TYGE
Applicants: ABB CARBON AB, VIND TYGE
Application year: 1993
Main classification: B01J3
IPC codes: B01J3, B01J8, F23C10, F23J1