AU2014202422A1 - System for drying particulate materials - Google Patents

Abstract

Abstract The specification discloses a system for drying a feed material in a particulate form such as brown coal, the system including: (a) a dryer through which the particulate material is passed 5 and including: (i) a fluidised bed section having at least one heat exchanger supplied with heating fluid at a temperature above 100'C, and (ii) a non-fluidised section downstream of the fluidised bed section to which the particulate material from the fluidised bed section passes, the non-fluidised section having at least one associated heat exchanger to further heat or to maintain heat within the particulate material passing therethrough, whereby the particulate feed material is o partially dried within the fluidised bed section and is subsequently further dried within the non fluidised section, (b) a feeding system for feeding the feed material in particulate form to the dryer at a controlled feed rate, and (c) a discharge system through which the particulate material emerging from the non-fluidised section is passed from the dryer. The heat exchangers of the fluidised bed section and of the non-fluidised section are supplied with heated gas at a 5 temperature of greater than 100'C, the heated gas preferably being superheated or saturated steam. A particle loosening system is in the non-fluidised section to promote continual flow of particulate material therethrough, the particle loosening system including gas distributors located beneath the non-fluidised section whereby intermittent supply of pressurised gas (e.g. superheated or saturated steam or nitrogen or inert gas) therethrough loosens particulate material 0 in the overlying non-fluidised section. The particulate brown coal passing from the fluidised bed section to the non-fluidised section has a water content of about 9% to 18% by weight at or near its critical water content (e.g. no more than 20% above and no less than 20% below the critical water content), and when passing from the non fluidised section has a water content of about 3% to 7% by weight. < 00 H o~ LUj - 00 CV'H Lf.) (0 0 0 Co

Description

SYSTEM FOR DRYING PARTICULATE MATERIALS Field of the Invention The present invention relates to the drying of materials in a particulate form, particularly for drying mineral materials such as brown coal or lignite so as to reduce the water content before 5 further processing or use of the material. Background of the Invention The chemical industry in general, and particularly the processing of some mined materials that have a high water content, there is frequently a need for materials to be dehydrated or to have a substantially reduced water content before further processing or utilisation. For most materials 0 with high water content, the drying process usually can be divided into two periods: above a critical water content for the material is a constant drying rate period; and below the critical water content for the material is a falling drying rate period. A particular example is brown coal or lignite which, when first mined, can have a water content as high as 60%-70% by weight and its critical water content is about 12% - 15% by weight. Such brown coal needs to be dried so as 5 to have a water content of about, or less than, 7% by weight and for some applications 3-5% by weight. In the past it has been proposed to use flue gas from the combustion of brown coal, e.g. in electricity generating plants, or to use steam as the heat source for the drying of the lignite. However the use of the both flue gas and steam have disadvantages. Flue gas creates substantial risk of explosion or combustion and because flue gas has a high volatility and it is difficult to 20 control the temperature of such gas when used for the drying. The use of steam can have advantages of the higher calorific value and the process can be made controllable and therefore reliable, however higher investment is needed, e.g. for a high pressure steam boiler and the consumption of considerable fuel or use of other energy sources to generate the steam. In patent specification AU 42969/78 there is described a process for drying brown coal in 25 particulate form by passing superheated steam upwardly through a body of the coal so as to create a fluidised bed. The use of steam as the fluidising medium enables the water evaporating from the particles in the fluidised bed to mix with the fluidising steam and this can be recovered for reuse or used to perform useful work. 1 In patent specification AU- 17824/83 there is another proposed system for drying brown coal by creating a fluidised bed of coal particles. In this system superheated steam is passed through heat exchange tubes past which the coal particles to be dried flow thereby being heated by the heat exchange tubes whereby water is evaporated from the particles. 5 In patent specification WO 2010/057509 there is another proposed heated fluidised bed dryer having superheated steam passing through heat exchange tubes throughout the zone where the fluidised coal particles are located. The heated and dried coal particles collect at the bottom beneath the fluidising inlets enabling recovery and subsequent further processing or use. The above references to and descriptions of prior proposals or products are not intended to be, o and are not to be construed as, statements or admissions of common general knowledge in the art in Australia or elsewhere. In the prior proposed coal drying system outlined above using heating and drying of the coal particles in a fluidised bed, the efficiency of drying in the constant drying rate period is good for coal particles with high water content which is above the critical water content in the fluidised 5 bed. The effectiveness of the drying however reduces as the particles are progressively dried so that drying efficiency in the falling drying rate period falls progressively as the water content of the coal particles falls below its critical water content. In the falling drying rate period, below critical water content, the drying rate is controlled mainly by the material water transport (internal conditions), but not the fluidised state of the coal particles. The desired drying effect 0 can be nevertheless achieved by maintaining the speed and volume and temperature of flow of the fluidising gas through the coal particles but at relatively large energy cost for the final few degrees or percentages of water removal. Objects of the Present Invention An object of the present invention is to provide a system for drying particulate materials which 25 can be relatively more efficient and/or effective than prior proposed drying systems. It is a preferred object to provide a system for drying particulate materials which can use less energy to achieve a pre-determined level of water reduction in the particulate materials. A further preferred object is to provide a process for drying particulate materials in a reliable, safe, and relatively efficient manner. 2 Summary of the Invention According to the present invention there is provided a system for drying a feed material in a particulate form, the system including: (a) a dryer through which the particulate material is passed and in which the particulate 5 material is dried, the dryer including: (i) a fluidised bed section to which the particulate material is supplied, the fluidised bed section including at least one heat exchanger which effects the heating of the particulate material, the heat exchanger being supplied with heating fluid at a temperature above 100'C, and 0 (ii) a non-fluidised section downstream of the fluidised bed section to which the particulate material from the fluidised bed section passes, the non-fluidised section having at least one associated heat exchanger arranged and operative to further heat or to maintain heat within the particulate material passing therethrough, whereby the particulate feed material is partially dried within the 5 fluidised bed section and is subsequently further dried within the non-fluidised section, (b) a feeding system for feeding the feed material in particulate form to the dryer at a controlled feed rate, and (c) a discharge system through which the particulate material emerging from the non 20 fluidised section is passed from the dryer. In a preferred construction the non-fluidised section is located lower than, and preferably directly beneath, the fluidised bed section so as to receive the particulate material from the fluidised bed section by gravity. Most desirably, the dryer comprises a chamber in which both the fluidised bed section and the non-fluidised section are located. 25 The heat exchanger of the fluidised bed section, or the heat exchanger of the non-fluidised section, or both, are preferably supplied with heated gas at a temperature of greater than 100'C, and most preferably at about 140'C - 200'C, the heated gas preferably being superheated or saturated steam. The steam is preferably saturated steam. 3 The dryer preferably further includes a particle loosening system for loosening particulate material in the non-fluidised section to promote continual flow of particulate material therethrough, the particle loosening system including an arrangement of gas distributors located within the dryer beneath the non-fluidised section whereby intermittent supply of pressurised gas 5 through the gas distributors loosens particulate material in the overlying non-fluidised section. Preferably the pressurised gas introduced by the gas distributors is superheated or saturated steam or nitrogen or inert gas. In a preferred embodiment, the fluidised bed section includes a fluidising gas distributor which is supplied with pressurised fluidising gas at a pressure and flow rate controlled to achieve the o desired drying effect for a particular material flow rate through the fluidised bed section in which the particulate material is dried to nearly its critical water content in the constant drying rate period, the fluidising gas distributor being supplied with fluidising steam which is composed of at least a proportion of the water removed from the particulate feed material and discharging from the dryer in vapour or gas phase and which is returned to the fluidising gas distributor. 5 The dryer preferably includes a fluid discharge outlet optimally above the fluidised bed section through which fluidising gas, water in vapour or steam phase removed from the particulate material, and dust or fines are discharged, and the system further includes a dust removing system operative to remove dust or fines by at least one of the processes of cyclone separation, filtration, electrostatic dust separation, and wet separation. In this embodiment, a proportion of 0 the heated gas discharge of the dust removing system including water in vapour or steam phase is pressurised, further heated (if necessary), and returned for supply to the fluidised bed section as fluidising gas. In a system being used with particulate brown coal with a water content of 30% to 70% by weight, the particulate brown coal passing from the fluidised bed section to the non fluidised 25 section may have a water content of about 9% to 18% by weight, at or near its critical water content, and the particulate brown coal passing from the non fluidised section may have a water content of about 3% to 7% by weight. For example, for a critical water content in the range of 12% to 15% by weight, the brown coal can be up to about 20% below or 20% above the critical water content, i.e. 9% to 18%. 30 To substantially obviate the possibility of combustion, the particulate from coal is preferably processed throughout the dryer in an atmosphere consisting of at least 99.9% water vapour. 4 Brief Description of the Drawing Possible and preferred features of the present invention will now be described with particular reference to the accompanying drawing. However it is to be understood that the features illustrated in and described with reference to the drawing are not to be construed as limiting on 5 the scope of the invention. In the drawings: Fig. 1 illustrates schematically one possible embodiment of a drying system for particulate materials according to the present invention. Detailed Description of Embodiments Referring to the drawing, this shows a drying system particularly for particulate brown coal. 0 Brown coal particles having a relatively high water content are introduced into a dryer 4 in which the coal particles are dried before being discharged from the dryer 4 having a substantially reduced water content. The brown coal to be dried is crushed and screened to render the coal in the particulate form having the appropriate particle sizes (e.g. below about 6mm in average diameter) and the coal is fed to the silo 1. The water content of the particulate coal in the silo 1 5 may be for example 30 - 50% by weight but some brown coals have higher water content, e.g. 60% - 70% by weight, and the system of the invention is useable with such coals also. The coal is fed from the silo 1 by means of the feeding system which includes a discharge valve 2 and screw conveyor 3 for supplying the particulate coal to the dryer 4 at a predetermined and controllable rate. 20 The dryer 4 shown as having a single chamber 41. In the chamber 41 there is a fluidised bed section 42 located above a non-fluidised section 56 whereby coal particles can pass by gravity flow through the upper fluidised bed section 42 and thence to the underlying non-fluidised section 56. The fluidised bed section 42 includes heat exchanger 43a, 43b, 43c. The heat exchangers 43a, 25 43b, 43c are each comprised by arrangements or arrays of heat exchange elements 44 such as tubes or panels through which a heated fluid is passed so that the coal particles travelling therethrough are heated by contact with and by passing in close proximity to the elements 44. In the illustrated embodiment, a suitable heat source 45 is provided for heating the fluid supplied to the heat exchangers 43a, 43b and 43c. The heated fluid is supplied at greater than 100'C so as 5 to efficiently and effectively evaporate water from coal particles. Preferably the heated fluid is superheated or saturated steam supplied, for example, by a steam boiler delivering the saturated steam at a gauge pressure of 0.6 - 1.OMPa(G). As shown the saturated steam can be supplied from the source 45 through a common supply line 46 and then through respective supply 5 manifolds 47a, 47b and 47c to the heat exchangers 43a, 43b and 43c. Parameters, e.g. pressure and temperature and flow rate, of each heated fluid passing through the branches downstream of supply line 46 can be adjusted if the process requires by regulating particular valves or other mechanisms before entering respective supply manifolds 47a, 47b and 47c. The steam after passing through the heat exchange elements 44 of the heat exchangers 43a, 43b and 43c is o collected, combined and passed through a return line 48, e.g. for reuse such as by return to the heating source 45. The coal particles in the fluidised bed section 42 are maintained in the fluidised state by continuous supply of a fluidising gas supplied under pressure through distributor 62 located beneath the heat exchangers 43a, 43b and 43c at the bottom of the fluidised bed section 42. The 5 distributor can be of conventional construction and operation and can be supplied with pressurised fluidising gas from the compressor 6. The rate of flow of the fluidising gas through the distributor 62 and passing up through the heat exchangers 43a, 43b and 43c can be controlled so as to maintain the required residence time of the coal particles in the fluidised bed section 42 until the water content thereof has fallen to a predetermined target level at or near its critical 0 water content, e.g. about 9% to 18% water by weight. The fluidising gas supplied by the compressor 6 through the distributor 62 preferably comprises steam, preferably superheated steam at, for example, 1 10 C-150'C. Preferably the fluidising gas is 100% steam, i.e. there is no proportion of air or other gas. It will be understood by persons skilled in this technology that various parameters of the process 25 occurring in the fluidised bed section 42 determine the water content of the particles emerging from that section and falling by gravity to the non-fluidised section 56 located towards the bottom of the chamber 41. Such parameters include the coal feed rate, the temperature and water content of the coal fed into the dryer 4, the average particle size of the particulate coal being fed, the configuration of the heat exchanges 43a, 43b and 43c, the temperature and calorific value of 30 the heated fluid passing through the heat exchange elements 44, the dimensions of the apparatus, the temperature and composition and flow rate of the fluidising gas. Appropriate control systems can be provided to continually monitor at least some of these variable parameters, particularly to monitor the water content of the emerging coal so as to thereby adjust parameters such as flow 6 rates and temperatures to thereby maintain the desired target water content of the emerging coal particles. Non-fluidised section 36 in the drawing is shown with a heat exchanger 57 which can be generally similar in construction and configuration to the heat exchangers 43a, 43b and 43c, 5 although other configurations may be usable. Heated fluid is supplied to the heat exchanger 57 from supply 58 which can be, for example, operated to supply superheated or saturated steam at a gauge pressure of 0.4 - 1.3MPa(G) and at a temperature of 140'C - 200'C. The superheated or saturated steam, preferably saturated steam, is supplied through supply line 59 and the steam passing from the heat exchanger 57 can be combined with the steam from the heat exchangers 0 43a, 43b and 43c and passed to return line 48. The coal particles settling by gravity from the upper fluidised bed section 42 and entering the non-fluidised section 56 can settle in the non-fluidised section to form a relatively loose bed or body of coal particles of reduced water content below its critical water content, but not yet reduced to a desired target moisture content for the system which may, for example, be about 3% 5 to 7% by weight. The partially dried particulate coal bed or body in the non-fluidised section 56 is maintained heated by the contact with and close proximity to the elements of the heat exchanger 57 so that the particles continue to evaporate water therefrom which will pass upwardly from the section 56 through the fluidised bed section 42 to the top of the vessel 41. The continued removal of water as steam from the coal particles in the non-fluidised section 56 0 can be more energy efficient because the energy expended in fluidising the particles is not required. The coal particles dried to the final target water content of say 3-5% by weight settle or pass by gravity from the bottom of the non-fluidised section 56 to the discharge system 7 shown schematically as having collecting funnels 71 or the like and discharge valves 72 for removing 25 dried coal particles at a metered rate. The dried coal particles having a water content of say 3 5% by weight is suitable for subsequent combustion e.g. in power generation facilities. The illustrated system also includes a particle loosening system 65 which is located at the bottom of the non-fluidised section 56. Because the coal particles in the non-fluidised section 56 form a bed or body which is not fluidised, they can tend to aggregate or compress or clog and thereby 30 stop settling and moving downwardly continually through the heat exchanger 57. The particle loosening system 65 therefore is preferably provided to disrupt any aggregation or clogging 7 preferably by intermittently disturbing the coal particles in the non-fluidised section 56. In the particular illustrated embodiment, the particle loosening system 65 comprises a gas distributor 66 which may be similar or analogous to the fluidising gas distributor 62. The distributor 66 is supplied with pressurised gas from supply 67. The supply 67 is preferably a superheated or 5 saturated steam or nitrogen or inert gas supply and preferably the supply of pressurised gas to the distributor 66 is carried out in an intermittent or pulsed manner. The intermittent or pulsed direction of pressurised gas by the distributor 56 upwardly into the non-fluidised section 56 is insufficient to fluidise the coal particles therein but periodically disturbs the bed or body of particles to inhibit aggregation of particles and clogging of the flow. o In an alternative possibility (not illustrated), the particles loosening system 65 may comprise a vibrating arrangement which for example may be intermittently operated to shake or vibrate the non-fluidised section 56 or elements located in that section to thereby inhibit formation or to break up any aggregation of coal particles in the bed or body within the section 56. The top of the dryer 4 has an outlet 50 through which gases and dust or fines flow out of the 5 chamber 41. The gases will consist of or predominantly comprise steam, including steam from the fluidising gas distributor 62 passing up through the fluidised bed section 42, steam from the distributor 66 of the particle loosening system 65, steam from water evaporated from the particles in the fluidised bed section 42, and steam from the coal particles in the body settled in and passing through the non-fluidised section 56. The dust and fines will be entrained in the gas o flow. This flow is supplied to a dust removing system 5 which is operative to separate the dust and fines using, for example, any one or more known types of dust removing apparatus, including cyclone separators (such as the first and second stages 53, 54 illustrated schematically in the drawing), filtration apparatus (which can be incorporated in the cyclone separators as known in the art), electrostatic dust separation such as that schematically illustrated as the final 25 stage 55 in the drawing, and also, if desired, wet separation apparatus. Preferably the dust and fines separation occurs in the dust removing system 5 while maintaining the flow insulated against substantial heat loss so that the gas in discharge 51 consists of or predominantly comprises superheated steam e.g. at 1 10'C - 150'C. This can be recycled at least partially to the compressor 6 for use as the fluidising gas for the fluidised bed section 42 without reheating 30 being necessary. Because of the evaporated water from the coal particles, there will be more steam emerging from the dust removing system 5 than is need for the fluidising gas so a proportion can be divided and passed through line 81 and, for example, discharged through air draft fan 8 to atmosphere. 8 It will be seen that the particular possible embodiment of the system according to the present invention described in relation to the drawing can combine the features and advantages of particle drying in a constant drying rate period in the fluidised bed section in which an efficient drying of particles having a high water content above its critical water content occurs, followed 5 by a non-fluidised drying section or stage in which the particles enter with an already reduced water content at or near its critical water content, pass through that non-fluidised section at relatively slow speed enabling continued drying to a substantially lower water content of the particles in a falling drying rate period without expenditure of the energy required for fluidisation. This system can therefore enable effective and efficient reduction of the water 0 content of particles, particularly high water content particles of brown coal, so as to yield a dried particulate product for further processing or utilisation (such as combustion of the dried brown coal particles). The drying process can be carried out in a safe manner particularly if the introduced gases for fluidising in the fluidised bed section and for particle loosening in the non fluidised section consists entirely of superheated or saturated steam since this will mean that the 5 drying is conducted in a substantially oxygen-free environment, preferably with a water vapour content of over 99.9%, thereby substantially reducing if not eliminating the risk of combustion and explosion of the particulate coal particularly the coal dust and fines. Also it can be seen that the illustrated system, by providing the preferred particle loosening system associated with a non-fluidised section can enable the required relatively long residence 0 time of coal particles in the final drying stage without expending substantial energy on fluidisation whilst avoiding or reducing the risks of clogging or blocking of particulate flow by agglomeration or bridging of particles. The preferred use of intermittent or pulsed superheated or saturated steam or nitrogen or inert gas to loosen but not fluidise the particles in the non fluidised section also helps to convey away from that section the water being evaporated from 25 the particles in that final drying stage. It is to be understood that various alterations, modifications and/or additions may be made to the features of the possible and preferred embodiment(s) of the invention as herein described without departing from the spirit and scope of the invention. 30 9

Claims (12)

1. A system for drying a feed material in a particulate form, the system including: (a) a dryer through which the particulate material is passed and in which the particulate material is dried, the dryer including: 5 (i) a fluidised bed section to which the particulate material is supplied, the fluidised bed section including at least one heat exchanger which effects the heating of the particulate material, the heat exchanger being supplied with heating fluid at a temperature above 100'C, and (ii) a non-fluidised section downstream of the fluidised bed section to which the 0 particulate material from the fluidised bed section passes, the non-fluidised section having at least one associated heat exchanger arranged and operative to further heat or to maintain heat within the particulate material passing therethrough, whereby the particulate feed material is partially dried within the fluidised bed section and is subsequently further dried within the non-fluidised 5 section, (b) a feeding system for feeding the feed material in particulate form to the dryer at a controlled feed rate, and (c) a discharge system through which the particulate material emerging from the non fluidised section is passed from the dryer. 20

2. A system as claimed in claim 1 wherein the non-fluidised section is located lower than the fluidised bed section so as to receive the particulate material from the fluidised bed section by gravity.

3. A system as claimed in claim 1 or 2 wherein the dryer comprises a chamber in which both the fluidised bed section and the non-fluidised section are located. 25

4. A system as claimed in any one of the preceding claims wherein the heat exchanger of the fluidised bed section or the heat exchanger of the non-fluidised section or both are supplied with heated gas at a temperature of greater than 100C, the heated gas preferably being superheated or saturated steam. 10

5. A system as claimed in any one of the preceding claims and further including a particle loosening system for loosening particulate material in the non-fluidised section to promote continual flow of particulate material therethrough, the particle loosening system including an arrangement of gas distributors located within the dryer beneath the non-fluidised section 5 whereby intermittent supply of pressurised gas through the gas distributors loosens particulate material in the overlying non-fluidised section.

6. A system as claimed in claim 5 wherein the pressurised gas introduced by the gas distributors is superheated or saturated steam or nitrogen or inert gas.

7. A system as claimed in any one of the preceding claims wherein the fluidised bed section o includes a fluidising gas distributor which is supplied with pressurised fluidising gas at a pressure and flow rate controlled to achieve the desired drying effect for a particular material flow rate through the fluidised bed section in which the particulate material is dried to nearly its critical water content in the constant drying rate period, the fluidising gas distributor being supplied with fluidising steam which is composed of at least a proportion of the water removed 5 from the particulate feed material and discharging from the dryer in vapour or gas phase and which is returned to the fluidising gas distributor.

8. A system as claimed in any one of the preceding claims wherein the dryer includes a fluid discharge outlet through which fluidising gas, water in vapour or steam phase removed from the particulate material, and dust or fines are discharged, the system further including a dust 0 removing system operative to remove dust or fines by at least one of the processes of cyclone separation, filtration, electrostatic dust separation, and wet separation.

9. A system as claimed in claim 8 wherein a proportion of the heated gas discharge of the dust removing system including water in vapour or steam phase is pressurised and returned for supply to the fluidised bed section as fluidising gas. 25

10. A system as claimed in any one of the preceding claims wherein the particulate material is brown coal with a water content of 30% to 70% by weight, wherein the particulate brown coal passing from the fluidised bed section to the non fluidised section has a water content of about 9% to 18% by weight at or near its critical water content, and wherein the particulate brown coal passing from the non fluidised section has a water content of about 3% to 7% by weight. 11

11. A system as claimed in claim 10 wherein the particulate brown coal passing from the fluidised section to the non-fluidised section has a water content no more than 20% above and no less than 20% below the critical water content.

12. A system as claimed in claim 10 or 11 wherein the particulate brown coal is processed 5 throughout the dryer in an atmosphere consisting of at least 99.9% water vapour. 12