AU2008207358A1 - Method and arrangement for controlling the combined drying and pulverizing process of a fan mill for pulverizing coal - Google Patents

Description

P001 Section 29 Regulation 3.2(2)

AUSTRALIA

00 00 0 0 0 Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: Invention Title: Method and arrangement for controlling the combined drying and pulverizing process of a fan mill for pulverizing coal The following statement is a full description of this invention, including the best method of performing it known to us: P11ABAU/1207 Description 00 SMethod and arrangement for controlling the combined drying and t1o pulverizing process of a fan mill for pulverizing coal SThe invention relates to a method and an arrangement for controlling the combined drying and pulverizing process of a fan mill for pulverizing coal.

o00 2 Fan mills for pulverizing coals or beater wheel mills are used in power stations in which Ssolid fossil fuel, in particular raw brown coal is dried and pulverized into coal dust and 00oo 0the coal dust is then fed by means of coal dust ducts to burners and is burnt in the c combustion chamber or furnace of a steam generator. Hot combustion exhaust gas is used for drying and conveying the pulverized material, said combustion exhaust gas being removed from the upper region of the combustion chamber and extracted through a flue gas resuction duct from the fan mill. The raw fuel or raw brown coal to be pulverized and dried is usually provided in coal bunkers and supplied by means of bunker extraction devices and/or coal feeders to the flue gas resuction duct and via this to the fan mill for pulverizing coal. In order to be able to vary or control the coal throughput through the mill or the mills, the rotational speed or conveying speed of the bunker extraction devices or coal feeders is adjustable or controllable.

Mill control in power stations or control of the combined drying and pulverizing process in power station coal mills is generally primarily dependent on the power specification of the block or power station control system. As a result of the specifications of the block control system, the total fuel mass flow to the mills is increased or reduced until the block power is reached. The total fuel mass flow is divided among the operating mills.

The level of the fuel throughput of the individual mills is dependent on their mill drying or ventilation performance. The mills are controlled on the basis of the coal throughput.

Other crucial mill parameters such as the mill rotational speed are controlled by means of pre-control curves depending on the coal throughput represented in the form of feeder rotational speed, bunker discharge size or quantity or similar). In order to V:\PA\APB_269_a. dcc 00 O achieve a certain flexibility in the combined drying and pulverizing process, various pre- Scontrol curves of Ihe mill parameters to be controlled are configured with maximum ;and minimum values or maximum and minimum curves.

The mill controls so far have been developed from the viewpoint of the mill drying or ventilation, i.e. the mill rotational speed and the temperature monitoring after the mill 00 C(the temperature cit the coal dust outlet of the mill). With increasing operating time of the mills, a significant coarsening of the fuel or coal dust produced in the mill comes 00 about as a consequence of the wear of the mill pulverizing tools such as pre-beater heads, beater plates and corner armour and in particular in mills with pre-beaters and without classifiers. For the firing, a coarsening of the fuel dust means that more coarse particles having a higher water content are fed to the burners. The ignition conditions as well as the combustion conditions of the introduced coarsened coal dust thereby deteriorate. Furthermore, the coarsening of the fuel dust leads to deterioration of the transport conditions inside the mill spiral and in the downstream coal dust ducts. In addition, in classifier mills i.e. in fan mills having a classifier disposed at the coal dust outlet, the internal circulation of solids is increased since a larger fraction of coarser pulverized material is fed back to the mill. Worn corner armour increases the gas/solid circulation in the spiral equally in pre-beater mills and in mills with classifiers and leads to a deterioration in the mill and firing conditions.

Document DE 198 53 925 C2 has disclosed a method for controlling the combined drying and pulverizing process of a fan mill for pulverizing coal and a device for carrying out the method in which the combined drying and pulverizing process of the mill is controlled in all operating cases, including with lower quantities of temperaturecontrol gas in such a manner that no impermissible excess temperatures occur in the mill and at its periphery, i.e. in the mill suction duct and the mill outlet duct or mill classifier.

00 O It is now the object of the invention to provide a method for controlling the combined Sdrying and pulverizing process of a fan mill for pulverizing coal, in particular the combined drying and pulverizing process of raw brown coal in a fan mill for pulverizing coal which avoids the aforesaid disadvantages. In particular, it is the object of the invention to provide a method for controlling the combined drying and pulverizing 00 process of a fan mill for pulverizing coal in which the fineness of the fuel or coal dust C produced is main'fained at substantially the same level of fineness over a fairly long period. It is furthermore the object of the invention to propose an arrangement for 00 controlling the combined drying and pulverizing process of a fan mill for pulverizing coal.

The aforesaid object is achieved with regard to the method by the features of claim 1 and with regard to the arrangement by the features of claim 7.

Advantageous errbodiments of the invention can be deduced from the dependent claims.

The solution according to the invention provides a method and an arrangement for controlling the combined drying and pulverizing process of a fan mill for pulverizing coal having the following advantages: Fuel dust fineness and moisture are the same over a fairly long time interval, Conveying stability inside the coal dust ducts is higher for fine fuel dust, Recirculation of solids is lower in classifier mills, Gas/solid circulation in mill spirals is lower, Ignition conditions at the burner are the same for a longer time.

An advantageous embodiment of the invention provides that the wear state Voou.a is determined by measurement on at least one of the pulverizing tools pre-beater heads, beater plates and corner armour and the measured value is included as a correction value in the control of the mill rotational speed pre-control curve VSKmill speod The measurement car be made by means of at least one measuring sensor. By direct 00 measurement of the wear state on the pulverizing tool by means of a measuring sensor, the actual change at the pulverizing tool can be incorporated in the control of the combined drying and pulverizing process. The measurement can be made using r commercially usual equipment, for example by optical or other measuring devices or sensors.

00 M In an advantageous embodiment of the invention, the wear state on at least one of rthe pulverizing tools pre-beater heads, beater plates and corner armour is fixed by 00 empirical determination over the running time of the pulverizing tool and a correction Svalue derived therefrom is included in the control of the mill rotational speed pre-control curve VSKm;i spe.d In this case, firstly, the measuring sensor or sensors can be eliminated and secondly, the many years of experience of wear states on the pulverizing tools over the running time of a fan mill can be incorporated.

In an advantageous embodiment of the invention, the parameter temperature before coal is controlled by means of a pre-control curve VSK,,mp before cool determined as a function of the coal throughput, wherein the pre-control curve VSKtemp before col has a minimum and a maximum curve. This measure has the result that material and process-technology temperature limits can be adhered to in the mill.

An advantageous embodiment of the invention provides that if the allocated maximum or minimum curve of one of the controlled parameters is exceeded or fallen below, at least one other of the controlled parameters is varied for the same coal throughput within its maximum and minimum curves until the overshooting parameter comes to lie within its allocated maximum and minimum curves. This has the result that the combined drying and pulverizing process of a fan mill for pulverizing coal can be operated fully automatically, continuously varying within the limiting values or curves of specific pre-control curves.

A further advantageous embodiment of the invention provides that in the event that by varying at least one of the other controlled parameters, the overshooting parameter does not come to lie within its allocated maximum and minimum curves, the coal 00 0 throughput is varied until the overshooting parameter comes to lie within its allocated Smaximum and minimum curves. This has the result that the combined drying and tb3 pulverizing process of a fan mill for pulverizing coal can be operated fully automatically within the limiting values or curves of specific pre-control curves, wherein however a correction of the coal throughput must be made by lowering or raising said throughput.

00 0Exemplary embodiments of the invention are explained in detail hereinafter with

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00 reference to the drawings and the description.

In the figures: Fig. 1 shows schematically a fan mill for pulverizing coal with classifier in crosssection, Fig. 2 as Fig. 1 but in longitudinal section, Fig. 3 shows scnematically a fan mill for pulverizing coal with pre-beater and without classifier in cross-section, Fig. 4 as Fig. 3 but in longitudinal section, Fig. 5 shows pre-control curves of the controlled mill parameters, Fig. 6 shows schematically the wear state at the pulverizing tools, pre-beater head or beater plate or corner armour.

Figure 1 and 2 show schematically a fan mill for pulverizing coal with classifier 3 in which the method according to the invention is used. In such a fan mill 1, raw fuel, in particular raw brown coal from a coal bunker not shown, is fed through an feeding device 16, for example, a coal feeder, a bunker extraction device or another feeding 00 0 member, and thrcugh a coal inlet 8 into the flue gas resuction duct 7 from which the raw fuel passes through the mill inlet 9 into the mill 1. The raw fuel is dried in the fan mill or beater wheel mill 1 and is pulverised or pulverized by means of the pulverizing tools 13, 14. For drying, combustion exhaust gas or flue gas is extracted by the fan mill 1 from the upper region of the combustion chamber, not shown, of a power plant steam 00oo generator fired with fuel from the mill 1, wherein the combustion exhaust gas is fed Sthrough respectively one flue gas resuction duct 7 to respectively one mill 1.

Furthermore, primary air is added to the combustion exhaust gas, together forming the 00 drying and conveying gas 17 for the raw fuel to be dried and pulverized or for the 0pulverized coal dust after the mill drying. The pulverized coal dust leaves the mill 1 at the outlet 10 as a coal dust/conveying gas mixture 18 and is passed via the classifier 3, in which coarser grain is separated and can be fed back to the mill 1 again, to the coal dust duct 11 through which the coal dust is passed to the burners, not shown, in the combustion chamber. The mill housing 4 of the mill 1 comprises a mill spiral 5 inside whereof the beater wheel 6 configured with beater plates 13 and responsible for pulverising the coal is disposed. The feeding device 16 can be controlled on the conveying quantity side so that the coal throughput is adjusted according to the specification of the block or power plant power. Since a coal-fired block or coal-fired power station comprises a plurality of mills 1, the coal throughput to be implemented in accordance with the predefined block power is divided between the operating mills 1 and their feeding device 16.

In contrast to Figures 1 and 2, Figures 3 and 4 comprise a fan mill 1 without classifier 3 but with upstream pre-beaters 2. Classifying of the pulverized material is therefore omitted but this is already pre-comminuted by pre-beaters 2 before the actual pulverizing inside the mill spiral 5. For this purpose the pre-beaters 2 are configured as pre-beater heads 12 which serve as pulverizing tools.

00 0 In the method according to the invention for controlling the combined drying and pulverizing process of a fan mill for pulverizing coal 1, in particular the combined tbO ;Z drying and pulverizing process of raw brown coal, at least the parameters "mill I. rotational speed", "temperature after mill" and "quantity of primary air" are controlled, wherein the mill rotational speed designates the rotational speed of the mill beater 00 wheel 6, the temperature after mill designates the temperature of the mixture of pulverized material/conveying gas stream 18 at its outlet 10 from the mill 1 and the quantity of primary air designates the quantity of air supplied to the flue gas sucked 00 back by the fan mill 1 from the combustion chamber, not shown, before the inlet 9 into 0 the mill 1. In each case, these parameters are controlled by means of a pre-control curve VSKmill speed VSKIemp fter mill VSKquantity of primary air wherein the pre-control curves VSKm,,i speed VSKemp after mill VSKquantity of primary air are each determined or specified depending on the coal throughput, see Figure 5. The respective pre-control curves VSKil speed, VSKemp after mill i VSKquontity of primary air have minimum and maximum values or minimum and maximum curves which allow a certain flexibility of the respective parameter within the minimum and maximum values or minimum and maximum curves. Furthermore, according to the invention the mill rotational speed pre-control curve VSKmi speed is corrected depending on the wear state of the pulverizing tool or the pulverizing tools 12, 13, 14 of the fan mill 1.

The wear state Vatjal of the pulverizing tool or the pulverizing tools 12, 13, 14 can be determined or measured in situ by means of one or a plurality of measuring instruments, for example, a measuring sensor 15 on the pulverizing tool 12, 13, 14 according to Figure 6 and can be fed as a measured value of the wear state actual value to the controlling system. However, it can also be determined by empirical determination over the operating time of the pulverizing tool 12, 13, 14 and a characteristic or correction value dependent on this can be fed, so to speak as a measured value, to the controlling system. If only one or optionally two measuring sensors 15 are used to determine the wear state Vaua,, of the pulverizing tool or the pulverizing tools 12, 13, 14, it is obvious to place the one or the two measuring sensors on the critical pulverizing tool(s) 12, 13, 14. By means of empirical experience it is possible to determine the wear state of the pulverizing tool or the pulverizing tools 00 0 12, 13, 14, of the pulverizing tools not monitored by means of the measuring sensor In the event that the wear state is determined by means of the measuring ;Z sensor 15 on all the pulverizing tools, i.e. on the pre-beater heads 12, on the beater plates 13 and on the corner armour 14 attention is focused on the most critical pulverizing tool 12, 13, 14 and its wear state Vodo,,,, with regard to the combined drying 00 and pulverizing process control and the mill maintenance. This should mean that the oo pulverizing tool 12, 13, 14 having the most serious or the most advanced wear state Vo.u C 1 is used for controlling the combined drying and pulverizing process and the mill 00 maintenance. When the wear state Vodo u is measured by the measuring sensor or sensors 15, the wear of the pulverizing tools 12, 13, 14 can either be measured or determined in min or in wear fractions (percentage) of the respective wearing pulverizing tool 12, 13, 14. For example, 10 mm of wear or 20% of the wear fraction is determined on the pulverizing tool 12, 13, 14. This means that in the said example, a total of 50 mm of wear material would be available before the worn pulverizing tools need to be replaced by new pulverizing tools 12, 13, 14.

Pulverizing tools liable to wear are substantially to be considered as pre-beater heads 12 on the pre-beater(s) 2, beater plates 13 on the beater wheel 6 and corner armour 14 on the mill housing 4 or at the narrowest point of the mill spiral 5 of the fan mill 1.

Thus, should wear have appeared after a certain operating time of the fan mill 1, for example 1000 hours, on at least one of the pulverizing tools 12, 13, 14, according to the invention, the pre-control curve VSKmil sped of the mill rotational speed will be corrected by the measured value fed to the controlling system. The correction is made in such a manner that the mill rotational speed is raised or increased depending on the wear state V 0 w so that despite the wear which has occurred on the pulverizing tools 12, 13, 14 the fineness of the pulverized coal dust remains substantially constant and no coarsening of the pulverized material occurs. The magnitude of the gradual and quantitative raising or increase in the mill rotational speed depending on the wear at the pulverizing tools 12, 13, 14 to achieve a substantially constant fineness of the pulverized material during the operating phase 00 O with increasing wear at the pulverizing tools 12, 13, 14 can be determined by experiments or specified by empirical values.

SIn addition to the aforesaid parameters, the parameter "temperature before coal" can further be determined to control the combined drying and pulverizing process. This 00 parameter comprises the temperature of the combustion or flue gas before the coal inlet

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c8, i.e. on the flue gas side upstream of the coal inlet 8, in the flue gas resuction duct 7 Sor the control of this temperature. As has already been stated above, the flue gas fed to 00 the mill 1 is extracted from the upper portion of the combustion chamber, not shown, O and is extracted through the flue gas resuction duct 7 from the mill 1. For control of the temperature of the flue gas before the coal inlet 8, a pre-control curve VSKemp before coal is also specified with minimum and maximum curves or minimum and maximum values, the pre-control curve VSKemp before coal again being dependent on the coal throughput, see Figure The respective pre-control curves VSKmill speed, VSKemp after mill, VSKquantity of primary air, VSKtemp before cool and their minimum and maximum curves as a function of the coal throughput which can be seen from Figure 5 are based on process and control-technology calculations and operating experience. The pre-control curves VSKmil speed, VSKemp after mill, VSKqanity of primary air, VSKtemp before coal and their minimum and maximum curves are necessarily dependent on fuel or coal i.e. dependent inter alia on their water and ash content. The minimum and maximum curves of the respective pre-control curves can differ according to the type and mode of action of the system components and depending on the fuel.

The minimum ano maximum curves can, for example, run parallel to the pre-control curve and thus determine a minimum and a maximum curve. See on this matter for example the minimum and maximum curve of the pre-control curve VSKtemp after mill, over the coal throughput range of 0 to 100%. The minimum and maximum curves can however also determine a constant value over a part of or over the entire coal throughput range of 0 to 100%. As an example of this, see the minimum and maximum curve of the pre-control curve VSKqntit of primary air over the coal throughput range of 0 to 100%. Furthermore, the pre-control curve can define a range which is delimited by a 00 Sminimum and a maximum curve such as can be seen, for example, for the pre-control curve VSKtemp before col As a result of the nminimum and maximum values or the minimum and maximum curves of the pre-control curves according to Figure 5, continuously varying operation of the 00oo mill 1 or the combined drying and pulverizing process is possible within the parameter

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limits predefined by these values. If one of the control values or one of the controlled parameters leaves the defined range for the current coal throughput, for example, coal o00 throughput for 80% steam generator or power plant power, then one or more of the Sother controlled parameters will be varied until the deviating or overshooting control quantity lies in the defined range again, i.e. within the minimum or maximum curve. If this is not successful, the coal throughput of the mill 1 is changed, i.e. increased or reduced until all the control quantities or controlled parameters again lie in a defined controlling range, i.e. within the minimum or maximum curves. In this case, the steam generator or power plant power will be accordingly increased or reduced depending on the change in coal throughput.

By taking into account the wear states Vou of the pre-beater heads 12 and/or the beater plates 13 and/or corner armour 14 in the mill control, the fineness of the coal dust produced can be kept at substantially the same level over a fairly longer period.

The conveying conditions inside the mill and inside the coal dust ducts 11 leading to the coal dust burners not shown and also the conditions for combustion of the coal dust inside the combustion chamber remain unchanged for a fairly long time. The changeover cycles for the wear parts 12, 13, 14 can be specifically optimised from the pulverisation technology point of view.

-11- 00 0 Reference list: 1 Fan mill t 2 Pre-beater 3 Classifier 00 4 Mill housing 5 Mill spiral 6 Beater wheel 00 7 Flue gas resuction duct 8 Coal inlet into resuction duct 9 Mill inlet Coal dust outlet 11 Coal dust duct 12 Pre-beater head 13 Beater plate 14 Corner armour Measuring sensor 16 Feeding device 17 Drying and conveying gas 18 Mixture of coal dust/conveying gas

Claims (8)

1. A method for controlling the combined drying and pulverizing process of a fan mill for pulverizing coal, in particular the combined drying and pulverizing process of raw brown coal, wherein at least the parameters mill rotational speed, temperature downstream of the mill and quantity of primary air are controlled, comprising a pre-control curve (VSKmilsped) for controlling the mill rotational speed, 00 a pre-control curve (VSKTemp afteo mill) for controlling the temperature after the mill, 0a pre-control curve (VSKprimaryquantityofair) for controlling the quantity of primary air, wherein the pre-control curves (VSKmillspeed, VSKTemp fte mill, VSKpri,,m ,,quntty ofir) are each determined as a function of the coal throughput, and the mill rotational speed pre-control curve (VSKil speed) is corrected as a function of the wear state of at least one pulverizing tool of the fan mill and wherein the respective pre-control curves have minimum and maximum curves.

2. The method according to claim 1, characterised in that the wear state (Vodual) is determined at least on one of the pulverizing tools pre-beater heads, beater plates and corner armour by measurement and the measured value is incorporated as a correction factor in the control of the mill rotational speed pre-control curve (VSKmill speed)

3. The method according to claim 1, characterised in that the wear state (Vactul) is specified at least on one of the pulverizing tools pre-beater heads, beater plates and corner armour by empirical determination over the running time of the pulverizing tool and a correction factor derived therefrom is incorporated in the control of the mill rotational speed pre-control curve (VSKmilspeed).

4. The method according to claim 1, characterised in that the parameter temperature before coal is controlled by means of pre-control curve (VSKemp bfore col) determined as a function of the coal throughput and the pre-control curve (VSKmp before coal) has a minimum and maximum curve. -13- 00 O N

5. The method according to claim 1, characterised in that when the assigned maximum or minimum curve of one of the controlled parameters is exceeded or fallen below, at least one other of the controlled parameters with constant coal throughput is varied within its maximum and minimum curves until the overshooting parameter comes to lie within its assigned maximum and minimum curves. 00oo

6. The method according to claim 5, characterised in that in the event that when by 0 varying at least one other of the controlled parameters, the overshooting Sparameters does not come to lie within its assigned maximum and minimum curves. the coal throughput is varied until the overshooting parameter comes to lie within its assigned maximum and minimum curves.

7. An arrangement for controlling the combined drying and pulverizing process of a fan mill for pulverizing coal operated with solid fossil fuels, wherein the fan mill for pulverizing coal is configured with wearing pulverizing tools (12, 13, 14) when viewed over the operating time of the fan mill and at least the parameters mill rotational speed, temperature after mill and primary quantity of air can be controlled wherein the mill rotational speed can be controlled by a pre-control curve (VSKmill speed the temperature after mill can be controlled by a pre-control curve (VSKTempofermill the primary quantity of air can be controlled by a pre-control curve (VSKprimory quantity oi air wherein the pre-control curves (VSKmispeed VSKTomp fter mill, VSKprimarquantity of air) can each be determined depending on the coal throughput, and wherein the mill rotational speed pre-control curve (VSK;mil speed) can be corrected depending on ihe wear state (Vo.ual) of at least one of the pulverizing tools (12, 13, 14) of the fan mill and wherein the respective pre-control curves (VSKmillspeed, VSKTempaftermill, VSKprimoryquantityof oir) have minimum and maximum curves. -14- 00 0

8. The arrangement according to claim 7, characterised in that the wear state (VoC"ui) at c least at one of the pulverizing tools pre-beater heads beater plates (13) and Scorner armour (14) can be determined by means of at least one measuring sensor and the determined value can be supplied as a control value to the mill rotational speed pre-control curve (VSKm,,speod) 00 00 00 0o