GB2030690A

GB2030690A - Method of Preparing Dried Coal and Steam From Raw Coal

Info

Publication number: GB2030690A
Application number: GB7928433A
Authority: GB
Original assignee: Brennstoffinstitut Freiberg
Current assignee: Brennstoffinstitut Freiberg
Priority date: 1978-08-18
Filing date: 1979-08-15
Publication date: 1980-04-10
Also published as: FR2433570B1; IN152964B; GR67673B; FR2433570A1; AT381501B; YU201679A; BG33384A1; DD143712A3; GB2030690B; ATA453179A; CS260952B1; DE2926524A1; TR21157A; PL217787A1; DE2926524C2; HU184053B; PL127060B1

Abstract

A method for preparing dried coal and steam from raw coal e.g. lignite including generating hot flue gas preferably at 500 to 1000 DEG C, by combusting the coal in a cooled combustion chamber (3), supplying the hot flue gas to a drier (4) without the interposition of any means for separating unburnt residue and fly ash, in which drier (4) raw coal is dried from an initial high water content to a low water content, using the sensible heat of the said flue gas, and wherein the chamber (3) is cooled by a cooling system constructed in the form of a steam generator (6), the heat capacity of the cooled chamber (3) is adapted to correspond with the heat requirement of the said drier (4) and the steam produced in the chamber (3) is used as required. <IMAGE>

Description

SPECIFICATION Method of Preparing Dried Coal and Steam From Raw Coal The present invention relates to a method for preparing dried coal and steam from raw coal, preferably lignite or a salt-containing lignite.

Soft lignite in the conveyed state is characterised by a low calorific value, high water content and low stability of shape. It is therefore generally necessary to convert this soft lignite into service energy carriers with a high serviceability for the final consumer of the energy. A typical example of such refining of lignite is conversion into gas for the public supply or as an intermediate product for chemical synthesis (synthesis gas) in accordance with the following scheme: Precomminution.

Drying.

Possible shaping or further comminution or classification.

Gasification.

Gas purification and recovery of by-products such as tar.

Delivery of the purified gas as town gas or synthesis gas.

For some of these method steps (drying, gasification, gas purification) and for additional method steps for the preparation of intermediates and the processing of by-products there is a need for steam at various pressure ievels. This steam requirement -- insofar as it is not covered by utilization of waste heat - is produced in a coal-fired back-pressure power plant which generally forms a combined economic unit with other refining, supply and subsidiary plants.

Soft lignite which is characterised by a very high content of sodium chloride and other alkali metal salts in the raw coal, so-called salt coal is known to occur. It is usual to designate as "raw lignite containing salt" those coals in which the content of sodium calculated as Na20 is greater than 0.5% by weight based on the anhydrous state. A typical value is, for example 2.5% by weight of Na20 based on the anhydrous state, the sodium being bound in the coal in the form of humates and chlorides. The high content of alkali metal salts in the coal leads to considerable difficulties during burning in steam generators on account of the formation of mineral deposits on the heating surfaces.It is characteristic that these deposits, which are partially attributable to sublimation of alkali metal salts which have been evaporated during the burning, partially to the adhesion of fly ash by means of eutectic partial melting, mainly occur in the region of the ancillary heating surfaces.

Despite intensive development work, it has not yet been possible to find a technically and economically satisfactory solution for being able to use raw salt-containing lignite in coal-dust fired steam generators. The state hitherto reached, despite the use of particular constructions and restrictions with regard to efficiency and travel times, restricts the maximum permissible alkali content of the raw coal in coal-dust fired steam generators to 0.1 to 0.3% by weight of Na20 based on the anhydrous substance. Higher salt contents, for example up to 2.5% by weight of Na20 based on the anhydrous state were hitherto only mastered in grate-fired steam generators for a maximum steam production of up to 60 t/h.

For a lignite refining combination of economically optimum size on the basis of using raw saltcontaining lignite, this state means that for the operation of the necessary back-pressure power plant either coal of another origin which is completely or at least substantially free of salt must be used or the power plant section must be equipped with a plurality of small grate boilers with the disadvantages of a low working productivity and a high investment. Both alternatives are unsatisfactory.

The object of the invention is a method which obviates or minimises one or more of the above mentioned disadvantages and operates satisfactorily from raw coal and especially from lignite or a saltcontaining lignite.

According to the present invention there is provided a method for preparing dried coal and steam from raw coal including generating hot flue gas by combusting the coal in a cooled combustion chamber, supplying the said hot flue gas to a drier without interposition of any means for separating unburnt residue and fly ash in which drier raw coal is dried from an initial high water content to low water content, using the sensible heat of the said flue gas, and wherein the combustion chamber is cooled by a cooling system constructed in the form of a steam generator, the heat capacity of the cooled combustion chamber is adapted to correspond with the heat requirement of the said drier, and the steam produced in the combustion chamber is used in the process or is used as required.The hot flue gas is preferably generated at a temperature of 500-1 0000C, more preferably at 600--10000C.

Desirably the method is conducted in an integrated plant for the production of gas.

The combustion chamber may be provided with at least one heating surface constructed and arranged so that in normal operation the temperature of the flue gas leaving the combustion chamber corresponds-at least substantially to the temperature required for the drying operation. The production of the hot flue gas may be effected in a multi-partitioned combustion chamber, the flame being maintained in a first uncooled part or slightly cooled part and in one or more other parts of the combustion chamber the hot flue gas is cooled at least substantially to the temperature required for the drying operation, in normal operation, by at least one heating surface.Thus in order to ensure favourable ignition and burning conditions in the combustion chamber this may be divided two or more times, the first part of the combustion chamber seen in the direction of flow being not cooled or only cooled a little. This can be achieved, for example by a refractory lining instead of steam-generator heating surfaces or at the side of such heating surfaces adjacent to the flame. in this case, the steam production may be effected entirely or substantially in the following parts of the combustion chamber equipped with at least one heating surface, preferably at least one radiation heating surface.

The heating surfaces and the geometry of the said combustion chamber may be so designed that, under normal conditions, the temperature of the flue gas emerging from the combustion chamber corresponds to the temperature required for the drier. For the correction of different operating conditions, for starting and finishing operation and for the achievement of partial-load states the temperature of the flue gas, after emerging from the combustion chamber and before entering the drier, may be lowered to a required value by admixture of air or recycled waste gas from the drier which is cold in comparison with the flue gas entering the drier or be increased to a desired value by supplying a less severely cooled partial stream branched off from the combustion chamber.Instead of the less severely cooled component stream branched off from the combustion chamber, an additional stream of hot gas produced in a smaller uncooled auxiliary combustion chamber may be used for the temperature correction.

In order to achieve the same purpose it is also possible to exclude the whole hot stream of flue gas from the combustion chamber from contact with a part of the installed heating surface which may be variable in size, by suitable technical possibilities.

Thus in one embodiment of the invention temperature correction of the stream of flue gas after leaving the combustion chamber, is effected by regulating the supply of fire coal and/or of air or of recycled flue gas or by regulating the supply of a separate stream of flue gas at a higher temperature than the temperature of the gas leaving the combustion chamber. Temperature correction of the stream of flue gas may be effected under partial-load operation and during starting and stopping operations.

In another embodiment of the invention a regulation of the ratio of dry coal production is obtained to a limited extent by regulating admixture of a separate stream of flue gas at a higher temperature than that which corresponds to the temperature of the flue gas leaving the combustion chamber to the stream of flue gas before it enters the drier.

In a further embodiment there is provided a separate stream of flue gas at a higher temperature than the temperature of the flue gas leaving the combustion chamber and such separate stream constitutes a partial stream from a hot.,r point of the combustion chamber or from a part of the combustion chamber which is operated at a higher temperature and is supplied to a mixing region, bypassing one or more cooling surfaces.

In another embodiment there is provided a separate stream of flue gas at a higher temperature than the temperature of the flue gas leaving the combustion chamber and such separate stream is produced in an additional combustion which is slightly cooled or is uncooled and is smaller than the first-mentioned combustion chamber.

In a further embodiment there are effected correction of the temperature of the stream of flue gas at the exit from the combustion chamber and, to a limited extent, regulation of the ratio of dry coal production and steam production by excluding one or more heating surfaces of the first-mentioned combustion chamber from contact with the hot flue gas of the combustion chamber.

The travelling round the cooling surfaces or some of the cooling surfaces by the stream of flue gas or a component stream of flue gas or the operation of an additional auxiliary combustion chamber renders possible, apart from the regulation of the heat offered, also an adaptation of drier capacity and steam production within certain limits.

In an embodiment of the invention which is advantageous in many cases, the control of the process, especially the adaption of steam production and dry coal production, is effected by altering the air-fuel ratio, that is to say the air ratio for the operation of the combustion chamber. This can be effected either by regulating the supply of firing coal or by altering the amount of combustion air.

Increasing values of lower the specific steam production per kg of dry coal produced. The oxygen content in the flue gas should not, however, exceed an upper limiting value of 13% for example, for safety reasons.

The carbon dioxide content or the oxygen content of the flue gas on leaving the combustion chamber may be used in known manner as a measure of the air ratio or as a pulse for a regulating system.

The water content of the dry coal can be corrected in known manner by varying the ratio of flue gas to amount of raw coal used in the drier, with the same conditions otherwise.

It is possible to use no raw coal or not only raw coal but some of the dry coal produced in the drier as fuel for the first-mentioned combustion chamber. For instance, the first-mentioned combustion chamber may be heated with raw lignite or salt-containing lignite. The said combustion chamber may be heated with some of the dry coal produced in the drier. There is likewise the possibility of using proportions of dry coal or dry coal dust which are not needed in the further refining process or which cannot be utiiized, for heating the combustion chamber. Such proportions which cannot generally be utilized are, for example, coarse-grained fractions which would be disturbing in the cloud of fly in following processes or fractions in the form of dust which are unwanted in flud bed processes.

Finally the heating of the combustion chamber may be effected wholly or partially by combustible waste products or residues which occur elsewhere in the whole refining process. If the following refining process is a gasification process, this applies, in particular, to the flue dust containing carbon separated out of the raw gas.

Thus, part of the dry coal produced in the drier and used for heating the first-mentioned combustion chamber may consist wholly or partially of grain size fractions separated from the dry coal.

Heating of the first-mentioned combustion chamber may be effected wholly or partially with a combustible residue obtained as a by-product, e.g. a combustible flue dust separated from a gaseous refining product.

The fly ash occurring during the burning of coal in the combustion chamber may be transferred to the drier by the flue gas. In this case, the separation of ash from the flue gases, otherwise usual, may be dispensed with and the ash also received by the coal to be dried. In the case of a following gasification it may be bonded in the ash or slag during the gasification. The known technical devices may be used to remove the part of the ash remaining in the combustion chamber (combustionchamber residue). However, it is also possible to separate at least some of the fly ash entrained by the flue gas out of the flue gas before the drier in known manner.

The method according to the invention is intended primarily for the preparetion of process steam in integrated plants for the refining of raw lignite containing salt, particularly for the production of gas from raw lignite containing salt. Its use is not restricted, however, to the use of salt coals, but it can be used in all cases where, apart from a demand for dried raw lignite, there is also a demand for steam for process, drive or heating purposes.

By way of example a preferred form of the invention will be described with reference to the accompanying diagrammatic drawings, wherein Figure 1 is a scheme showing in outline the successive stages for the production of synthesis gas from raw coal; Figure 2 is a graphical representation of values of the air ratio A (abscissa) against values (ordinate) of steam production per kg of dry lignite or fire-coal requirement per kg of dry lignite; and Figure 3 is a graphical representation of values of the air ratio A (ordinate) against values of oxygen content (abscissa) in flue gas leaving the combustion chamber.

Synthesis gas is obtained from raw salt-containing lignite, in a plant. the raw coal has a water content of 54% by weight and a calorific value of 2500 kcal/kg. Reference is made to Figure 1. The main amount of the raw lignite containing salt supplied is comminuted in a preparation plant to a grain size of up to 6 mm and supplied to a fire-gas drier 4.A further component supply of the raw coal is comminuted in a mill to a dust fineness (less than 1 mm) and used as fuel for a combustion chamber 3 equipped with heating surfaces Steam with values of 81 kp/cm2 (abs.) and 460"C is produced with the heating surfaces installed in the combustion chamber 3, and the flue gas cools to temperatures of 700--8000C. At this temperature, the flue gas enters the fire-gas drier 4 in which the comminuted main amount of the raw coal is dried to a water content of 10% by weight using the heat content of the flue gases. The flue gases and the vapours formed during the drying leave the drier with a temperature of 1 200C and enter the atmosphere after the removal of dust in a filter 5. The amount of dust separated in the filter is admixed with the dry coal from the drier 4.The dry coal is supplied to the generator 6 working on the principle of gasification in a fluid bed ("winkler generator") and gasified with a mixture of oxygen and steam at about 9000 0. After separation from flue dust containing carbon in a separator 7 the raw gas produced passes through the usual plants to a waste-heat utilizer, part of which, not illustrated in Figure 1, serves for the preheating of feed water, cooling and condensation 9 and desulphurization 10 and then goes for further processing and use.

The high-pressure steam produced in the combustion chamber 3 is used as drive energy, as illustrated in Figure 1 , for example for a steam-driven turbo-compressor of oxygen plant 11 and for further usual applications, while the process steam requirements of the generator 6 are covered by the waste-heat steam from the waste-heat utilization plant 8 of the gasification process.

The flue dust containing C occurring in the separator 7 has a calorific value of about 3500 kcal/kg and is added to the fuel for the combustion chamber 3 with a corresponding reduction in the demand for raw coal.

Based on an amount of 100 t/h of dry lignite used for the gasification, the following specific data result, with respect to a flue-gas entry temperature of the flue gases into the drier of 7500C, an air ratio A=1.20, a drier waste-gas temperature of 1 200C and a feed-water temperature on entry into the heating surface of the combustion chamber of 1 1 50C:: Flue-dust return no yes Amounts tlh raw coal 258 230 of which for drying 195 195 for combustion chamber 63 35 Flue-dust return - 20 Dry coal for the gasificiation 100 Steam generation in combustion 104 chamber Amounts m3i.Nlh Oxygen requirement 28,000 Synthesis gas production 138,000 In the condensation operation the amount of steam produced corresponds to a drive power of 25 MW, against which there is a requirement for the oxygen production of about 14 MW. The excess is used for other consumers of drive energy in the plant.

The control of the process, particularly the alteration ot the ratio ot steam production TO ury (;udl production is effected by adopting the combustion-air ratio of the combustion chamber, and under the design conditions of the illustrated process the relationship illustrated in Figure 2 of air ratio A and steam production per kg of dry lignite or fire-coal requirement per kg of dry lignite results. The relationship between the air ratio A and the O2 content, which can easily be measured in known manner, in the flue gas on leaving the combustion chamber is illustrated in Figure 3. In Figure 2 a possible return of flue dust from the gasification process as additional fuel for the combustion chamber is not taken into consideration, but the influence on the steam production is negligible.

Claims

1. A method for preparing dried coal and steam from raw coal including generating hot flue gas by combusting the coal in a cooled combustion chamber, supplying the said hot flue gas to a drier without the interposition of any means for separating unburnt-residue and fly ash, in which drier raw coal is dried from an initial high water ontent to a low water content, using the sensible heat of the said flue gas, and wherein the combustion chamber is cooled by a cooling system constructed in the form of a steam generator, the heat capacity of the cooled combustion chamber is adapted to correspond with the heat requirement of the said drier, and the steam produced in the combustion chamber is used in the process or is used as required.

2. A method according to claim 1, wherein the hot flue gas is generated at a temperature of 500-1 0000C.

3. A method according to claim 2, wherein the hot flue gas is generated at a temperature of 600--10000C.

4. A method according to any preceding claim, wherein the raw coal is lignite or a salt containing lignite.

5. A method according to any preceding claim, which is conducted in an integrated plant for the production of gas.

6. A method according to any preceding claim, wherein the combustion chamber is provided with at least one heating surface constructed and arranged so that in normal operation the temperature of the flue gas leaving he combustion chamber corresponds at least substantially to the temperature required for the drying operation.

7. A method according to any preceding claim, wherein the production of the hot flue gas is effected in a multi-partitioned combustion chamber, the flame being maintained in a first uncooled part or slightly cooled part and in one or more other parts of the combustion chamber the hot flue gas is cooled at least substantially to the temperature required for the drying operation, in normal operation, by at least one heating surface.

8. A method according to claim 7, wherein at least one heating surface is a radiation heating surface.

9. A method according to any preceding claim, wherein temperature correction of the stream of flue gas after leaving the combustion chamber, is effected by regulating the supply of fire coal and/or of air or of recycled flue gas or by regulating the supply of a separate stream of flue gas at a higher temperature than the temperature of the gas leaving the combustion chamber.

10. A method according to claim 9, wherein temperature correction of the stream of flue gas is effected under partial-load operation and during starting and stopping operations.

11. A method according to any preceding claim, wherein a regulation of the ratio of dry coal production is obtained to a limited extent by regulating admixture of a separate stream of flue gas at a higher temperature than that which corresponds to the temperature of the flue gas leaving the combustion chamber to the stream pf flue gas before it enters the drier.

1 2. A method according to any preceding claim, wherein there is provided a separate stream of flue gas at a higher temperature than the temperature of the flue gas leaving the combustion chamber and such separate stream constitutes a partial stream from a hotter point of the combustion chamber or from a part of the combustion chamber which is operated at a higher temperature and is supplied to a mixing region, by-passing one or more cooling surfaces.

1 3. A method according to any one of claims 1 to 11, wherein there is provided a separate stream of flue gas at a higher temperature than the temperature of the flue gas leaving the combustion chamber and such separate stream is produced in an additional combustion which is slightly cooled or is uncooled and is smaller than the first-mentioned combustion chamber.

14. A method according to any preceding claim, wherein there are effected correction of the temperature of the stream of flue gas at the exit from the combustion chamber and, to a limited extent, regulation of the ratio of dry coal production and steam production by excluding one or more heating surfaces of the first-mentioned combustion chamber from contact with the hot flue gas of the combustion chamber.

1 5. A method according to any preceding claim, wherein control of the method is effected by altering the air ratio A as hereinbefore defined for the operation of the combustion chamber.

1 6. A method according to claim 1 5, wherein regulation of the ratio of dry coal production and steam production is effected by altering the air ratio A as hereinbefore defined.

1 7. A method according to any preceding claim, wherein the first-mentioned combustion chamber is heated with raw lignite.

1 8. A method according to claim 17, wherein the lignite is a salt-containing lignite.

1 9. A method according to any one of claims 1 to 16, wherein the first-mentioned combustion chamber is heated with some of the dry coal produced in the drier.

20. A method according to claim 1 9, wherein part of the dry coal produced in the drier and used for heating the first-mentioned combustion chamber consists wholly or partially of grain size fractions separated from the dry coal.

21. A method according to any one of claims 1 to 1 6, wherein heating of the first-mentioned combustion chamber is effected wholly or partially with a combustible residue obtained as a byproduct.

22. A method according to claim 21, wherein the said combustible residue is a combustible flue dust separated from a gaseous refining product.

23. A method according to claim 1 substantially as herein described and exemplified.

24. Dried coal and/or steam which has or have been produced by the method claimed in any preceding claim.