GB2080824A - Process for producing an upgraded product from brown coal - Google Patents

Abstract

A process for producing from lumpy brown coal of high water content an upgraded product of high energy content characterised in that the raw brown coal having a particle size of more than 3 mm is in a first stage partially dewatered by a discontinuous treatment with saturated steam in one or more autoclaves, and in that subsequently the coal is dried in a second stage with steam having a temperature higher than 300 DEG C. Preferably coal introduced into the second stage is comminuted to a particle size of not more than 10 mm.

Description

SPECIFICATION Process for producing an upgraded product from brown coal This invention relates to a process for producing from lumpy brown coal of high water content an upgraded product of high energy content. The term "brown coal" includes a large number of types of brown coals of differing composition. It is characteristic of brown coals that they contain in addition to volatile constituents a large amount of water in the range 30% to 70% by weight. Due to this high water content the transport of raw brown coal is attended by difficulty. The water content of the brown coal must therefore be reduced to a tolerable amount, and drying of the brown coal is effected in the vicinity of the coal mine.

The final water content of the dried coal is also and mainly dependent on the subsequent use of the dried coal, as is also the particle size. Due to the geographic world-wide distribution of the brown coal deposits and the potential consumption areas for brown coals, transport of large quantities of brown coals over long distances will assume increasing importance.

Further requirements must however be met in providing an upgraded brown coal product which is to be transported. Thus the upgraded product should be in an optimum physical condition for transport, the water content should be as low as possible and the energy content as high as possible. The upgraded product should in addition be completely safe with respect to any tendency for self-ignition, and must not reabsorb water.

Up to the present time the only process available for producing dry coal in lumpy form from lumpy brown coal high in water content is the Fleissner drying process using saturated steam. A coal obtained by drying raw brown coal having a water content of 30% to 70% by weight by such a process has a residual water content of 10% to 30% by weight. The residual water content of the dry coal is determined by the pressure of the saturated steam used for drying, and also by the readsorption tendency of the dried coal product.

The said residual water content of the dry coal is however not acceptable for transporting the coal product over long distances. This applies particularly for the upper limit of the water content which is characteristic for a coal product obtained by drying brown coals of extremely high water content by means of saturated steam. A still further reduction of the water content of the brown coal by means of saturated steam is however not possible due to the required high pressure of the saturated steam, which would result in very high investment costs of the drying plant. The brown coal product obtained frequently has a tendency to self-ignition and is not suitable for being briquetted without binding agent due not only to its high content in residual water but also to its modified surface properties.

Finally the energy content of the brown coal product is only partially made use of despite its enhanced energy content.

It is an object of the invention to avoid the disadvantages inherent with dried brown coals produced by the Fleissner method with saturated steam, and to obtain an upgraded product of higher quality with low cost. The invention provides a process for producing from lumpy brown coal of high water content an upgraded product of high energy content, characterised in that a lumpy raw brown coal having a particle size of greater than 3 mm and preferably less than 50 mm is, in a first stage partially dewatered by a discontinuous treatment with saturated steam in one or more autoclaves (which may be vertically or horizontally disposed) and in a second stage dried by means of steam having a temperature higher than 350"C, preferably higher than 400"C.

By the practice of the invention an upgraded brown coal product can be produced which substantially corresponds to the requirements for an optimally suitable product, thereby fully making use of the advantages of the usual process according to the Fleissner system for drying materials with saturated steam and particularly with respect to a very favourable utilisation of the heat energy during this drying process.

An additional upgrading step can, if required, follow the treatment with hot steam. It is convenient and in fact necessary for dry coal which has to be transported over long distances, for the dry coal to be further processed in an upgrading equipment, for example a hot briquetting plant to be operated with fine-grain brown coal, immediately following downstream of the drying plant.

In carrying out the invention pre-comminuted raw brown coal is passed over a sieve, the fraction having a particle size of more than 3 mm, preferably a particle size within the range of 5 to 50 mm, being dewatered to a residual water content of 10% per weight in batches within horizontally or vertically arranged autoclaves at a pressure of 8 to 45 bar of the saturated steam and the fine-grain raw coal having a particle size of less than 5 mm being used for the production of steam. The dry coal coming from the drying stage operated with saturated steam and having a particle size within the range of O to 40 mm is preferably treated in a disintegrating stage having a double effect.Firstly, the dry coal is brought to a particle size within the range of O to 10 mm, which particle size is of particular advantage for the following treatment with hot steam, and secondly additional evaporation of the water present in the coal by the sensible heat of the dry coal is favourized by increasing the free surface of the coal. The vapors generated during disintegrating the coal act as an inert gas atmosphere.

The process is preferably performed such, that the comminuted coal is supplied to a drying stage operated with hot steam, within which drying stage the partially dried coal is completely dried at elevated pressure with hot steam having a temperature of 400 to 500"C and in which the partially dried coal is further upgraded by partial removal of carboxy groups. The pressure maintained within the drying stage operated with hot steam is preferably at least 5 bar. The treatment of the coal with hot steam can in a simple manner be effected at a pressure of preferably 10 to 1 5 bar within a fluidized bed reactor. By treating the partially dewatered dry coal by means of hot steam having a temperature of at least 350"C, the dry coal is completely dewatered and further upgraded by degradation of carboxy groups.Simultaneously, the bitumen present within the coal becomes activated and the coal is brought to a temperature of at least 300'C required for subsequent hot briquetting. In an advantageous manner, the dry coal having been dewatered in the manner described and having a temperature of 350"C can immediately subsequently be formed to briquettes in a hot briquetting plant, for example by means of extruders or annular roll presses. In case that the partially upgraded product shall, subsequently to the described upgrading process, be immediately further processed, particularly gasified, the hot briquetting stage is conveniently renounced.

The over-all process can, in this case, conveniently be performed such, that the sensible heat of the coal treated with hot steam is without substantial losses made use of in the following gasifying plant. The briquettes obtained on hot briquetting are finally cooled, inertized and shipped.

The partially cooled hot steam coming from the hot steam stage is preferably subjected to a purification step, thereby removing coal particles carried along by the steam and noncondensable constituents (CO2, CO, CH4) generated on decomposition of coal constituents.

Fine dry coal obtained when purifying the hot steam can be recycled into the flow of coal supplied to the hot briquetting stage, whereas CO2 can advantageously be used for inertizing the hot briquettes produced.

The purified and possibly saturated hot steam can, with a pressure of 10 to 1 5 bar, be recycled via a storage and distributing means into the drying stage operated with saturated steam. This storage end distributing means can be combined with the purifying means for the hot steam to one single unit and can conveniently be equipped with a means for separating CO2.

Conveniently, the coal contained within the autoclave can be subjected to the treatment with hot steam in the course of the treatment known per se with saturated steam after a first pressure-release step. In this case, the coal having been pressure-released to a pressure of preferably 10 to 1 5 bar is treated with the purified hot steam at a temperature of at least 300"C and a pressure of preferably 10 to 15 bar.

Furthermore, the purified and possibly saturated hot steam can, after removal of the CO2, also be brought into a saturated condition, the saturated steam being supplied to the usual drying stage operated with saturated steam.

The vapors generated on comminuting the dry coal can be used for cooling the briquettes.

The steam required for the drying stage operated with saturated steam and for the drying stage operated with hot steam can be produced in a central boiler plant using as fuel fine-grained raw coal. Superheating of a portion of the saturated steam can, after a corresponding pressure reduction, be effected within a heater means associated with the boiler plant.

The process described is characterized by an optimum utilisation of the process heat at dispoSal.

In the following, the invention is further illustrated with reference to the drawings.

In the drawings Figure 1 shows a block diagram illustrating the process according to the invention, Figure 2 is diagrammatically illustrating the tendence of the upgrading degree of the product, of the mass yield and of the thermal efficiency dependent on the treating tempera- ture and Figure 3 graphically illustrates the dependence of the mass yield and of the caloric value of the product on the treating temperature.

For brown coal of the Yallourn rnine, the total energy-requirement for producing one metric ton of upgraded product in form of hot briquettes wars 2.33GJ (gigajoule), the amount of heat required for expelling or evaporating one metric ton water was 1.02 GJ.

In contrast thereto, the usual drying processes require 3 to 4GJ for evaporating one metric ton of water. The mass yield obtained by the described upgrading process is, based on raw coal used as starting material, 28.8% and, based on dry substance of the raw coal used as starting material, 81.8%.

The caloric value of the raw coal used as starting material is, based on coal free of ash and free of water, 8.9 MJ/kg (megajoule per kilograms) and the calorific value of the final product (hot briquettes) is 29 MJ/kg in a raw condition and 30.5 MJ/kg on the basis of coal free of water and free of ash. The specific caloric value, which is defined as that mass of fuel which is equivalent to an amount of heat energy of 1 GJ (measured as caloric value is reduced from 11 2 kg to 34 kg by the upgrading process described. By treating the brown coal in the described manner also oxygen present in the coal as a ballast element is partially removed beside the complete removal of the water from the coal.The oxygen content of the coal is reduced from 26.3% per weight to 15.6% per weight, based on coal free of water and free of ash.

The more oxygen is removed from the coal the more is the coal enriched in carbon. Based on coal free of water and free of ash, the carbon content of 78.2% per weight of the final product is for 10.2% per weight higher than the carbon content of the raw coal used as starting material. The mentioned dependences can be derived from the Figures 2 and 3.

In Fig. 1, reference numeral 1, designates a pre-disintegrating stage in which the raw brown coal is disintegrated to a maximum particle size of 50 mm. The disintegrated product is passed over sieves and the particles having a particular size of less than 5 mm ( < 5mm) are used as fuel for a plant for producing saturated steam. The remaining portion of the disintegrated coal is at 3 subjected to a treatment whith saturated steam at a pressure of 30 to 40 bar. The waste water expelled from the coal on drying is discharged via a conduit 4 and the water is allowed to evaporate from the lumpy dry coal which is subsequently further disintegrated in the process stage 5. The further disintegrated product is then at 6 subjected to a treatment with hot steam which is obtained by superheating in the stage 7 the saturated steam coming from the steam producing plant 2.The contaminated hot steam is purified in a stage 8. The purified hot steam is recycled as hot steam to the treating stage 3 operated with saturated steam via a storage unit and/or a distributor 9 or is recycled as saturated steam via a saturating stage 10.

The process stage 6 operated with hot steam is, in a simple manner, designed as a fluidized bed reactor. The brown coal coming from the process stage 6 operated with hot steam and being slightly carbonized at low temperatures represents a fine-grained upgraded product which can be subjected to a hot briquetting stage or to an other posttreatment, for example to a gasification.

In Fig. 1, the hot briquetting stage is designated 11, the material coming from the process stage 6 operated with hot steam with a temperature of approximately 350"C which is necessary for briquetting the coal without binding agent. To the hot briquetting stage 11 there is also supplied the brown coal which has been separated in the purification stage 8 for the hot steam and which has been slightly carbonized at low temperatures. The hot briquettes coming from the hot briquetting stage 11 are cooled in a cooling stage 1 2 within which the briquettes are inertisized. For this purpose, the cooling stage for the briquettes is supplied with CO2 coming from the purification stage for hot steam and with vapors coming from the process stage 5, said vapors and the CO2 forming an inert gas atmosphere. The upgraded products of high energy density obtained by cooling the briquettes can be shipped over long distances.

Claims (24)

1. A process for producing from lumpy brown coal of high water content an upgraded product of high energy content characterised in that the raw brown coal having a particle size of more than 3 mm is in a first stage partially dewatered by a discontinuous treatment with saturated steam in one or more autoclaves, and in that subsequently the coal is dried in a second stage with steam having a temperature higher than 300"C.

2. A process as claimed in Claim 1, wherein in the said second stage the hot steam has a temperature higher than 400"C.

3. A process as claimed in Claim 1 or Claim 2, wherein the raw brown coal has a particle size less than 50 mm.

4. A process as claimed in any of Claims 1 to 3, wherein the coal treated with saturated steam is, prior to the treatment with hot steam in the second stage, comminuted to a particle size of not more than 10 mm.

5. A process as claimed in any of Claims 1 to 4, wherein the said comminution is effected after having pressure-released the coal from the autoclave(s) to atmospheric pressure.

6. A process as claimed in any of Claims 1 to 5, wherein the coal is introduced into the said second stage in which the partially dried coal is completely dried at elevated pressure by means of hot steam having a temperature of at least 300"C. and within which the thustreated coal is further upgraded by partially removing carboxy groups therefrom.

7. A process as claimed in any of Claims 1 to 6, wherein the said second stage steam is operated under a pressure of at least 5 bar.

8. A process as claimed in any of Claims 1 to 7, wherein the treatment in the said second stage is effected in a fluidized bed reactor.

9. A process as claimed in any of Claims 1 to 8, wherein the coal product from the said second stage is supplied to a subsequent and directly-coupled further treating plant.

10. A process as claimed in any of Claims 1 to 9, wherein the said further treating plant is a hot briquetting plant.

11. A process as claimed in Claim 9, wherein in the said hot briquetting plant the pre-heated dried coal is pressed to hot briquettes by means of extrusion presses or by means of annular roll presses.

12. A process as claimed in Claim 10 or Claim 11, wherein the hot briquettes produced are subjected to a cooling and inertizing step.

1 3. A process as claimed in Claim 11, wherein the said cooling is effected by means of the vapours coming from the comminution stage of Claim 4.

14. A process as claimed in any of Claims 1 to 13, characterised in that hot steam coming from the said second stage is subjected to a purifying step including removal of coal particles and of CO2, CO and OH4.

1 5. A process as claimed in Claim 13, wherein the CO2 removed during the purification of the hot steam is used for inertizing the briquettes in the process of Claim 1 3.

16. A process as claimed in Claim 14, wherein the purified hot steam is recycled to the process of Claim 1.

17. A process as claimed in Claim 16, wherein recycling of the purified hot steam is effected after a pressure-release during treatment in the first stage.

1 8. A process as claimed in Claim 1 6 or Claim 17, wherein the purified hot steam is prior to recycling, introduced into a container acting as a storage and distributing unit.

19. A process as claimed in Claim 18, wherein CO2 is separated from the gases within the storage container.

20. A process as claimed in any of Claims 1 6 to 18, wherein the purified hot steam is saturated to the degree present in the said first stage, and is recycled to the said first stage.

21. A process as claimed in any of Claims 10 to 20, wherein dry coal separated in the said second stage is passed to the hot briquetting stage.

22. A process as claimed in Claim 1, substantially as hereinbefore described and illustrated in the accompanying drawing.

23. An upgraded brown coal obtained by a process according to any of Claims 1 to 9.

24. Briquettes obtained by a process according to any of Claims 10 to 22.