EP2852799B1 - Method for processing pit-moist raw brown coal - Google Patents

Description

The invention relates to a process for the treatment of gruel wet lignite coal, in particular for thermal utilization in a power plant boiler, the raw lignite is first pre-broken and then comminuted in at least one grinder and fed to a subsequent drying and wherein the drying in a fluidized bed using at least one indirectly heated Fluidized bed dryer is performed, which is operated with steam as the fluidizing medium.

A method for drying hydrous lignite in a fluidized bed is for example from DE 29 01 723 C2 known.

In the fluidized-bed drying process described therein, it is provided to feed the feed coal to the fluidized-bed dryer by means of a screw conveyor

The method known from the aforementioned publication is intended to be suitable for drying lumpy materials whose size is in the range of, for example, 0.3 cm to 10 cm. For this purpose, the material to be dried, for example lignite, is made to vibrate in a denser material such as quartz sand. The method requires that the dried solid material be removed in conjunction with a portion of the particulate fluidizable material and that the solid material and the fluidizable material be separated so that the separated fluidizable material is recycled back to the fluidized bed. The process is complicated and not readily practical.

Another method for drying lignite, in particular for use in a power plant boiler, is for example from DE 196 20 047 A1 known. This process is operated exclusively with lignite as a solid and with steam as the fluidizing medium. The lignite to be supplied to the dryer is used for the purpose of fluidization in the Fluidized bed relatively finely ground, for example, on a mean grain diameter d ₅₀ of about 1 mm.

Depending on the nature of the lignite, such a fluidized-bed drying process can take place in a stationary fluidized bed relatively stable and trouble-free.

As already stated in the DE 29 01 723 C2 This depends essentially on the fluidisability of the solid in the fluidized bed dryer. The fluid mechanical processes within a stationary fluidized bed are extremely complex and only limitedly simulatable. In a pilot plant operated by the applicant, malfunctions of the mixing dynamics have occasionally occurred, depending on the mass flow to be applied. As a result of such disturbances, deposits of crude lignite are preferably formed on the heat exchangers of the fluidized bed dryer. As a result, the heat transfer deteriorates, the performance of the dryer is significantly reduced, in the worst case, the fluidized bed collapses.

The DE 10 2010 003 612 A1 relates to a method and apparatus for drying lignite which comprises admixing the pulverized coal separated from the vapor of the fluidized bed dryer and / or admixing the dried coal taken from the fluidized bed dryer to the coal to be dried before it is fed to the fluidized bed dryer. The method provides a control such that the recycled portion of the separated from the vapor pulverized coal and / or discharged from the fluidized bed dryer coal is adjusted such that the dryer to be supplied to be dried coal or discharged from the fluidized bed dryer, dried coal a has certain, desired water content.

Experiments with fluidized bed dryers at various throughput rates have shown that the fluidizability of crude lignite not only from the Utilization of the fluidized bed dryer, but also depends on the type and composition of the raw lignite.

The invention is therefore based on the object to provide a method for the treatment of pit-wet raw lignite using at least one indirectly heated fluidized bed dryer, which ensures a substantially stable and trouble-free fluidized bed operation even at high throughput, even with very diverse carburizing.

The object is achieved with the features of claim 1. Advantageous embodiments of the invention will become apparent from the dependent claims.

The invention relates to a process for the treatment of gruel wet raw lignite, in particular for thermal utilization in a power plant boiler, wherein the raw lignite is first pre-broken and then comminuted in at least one grinder and fed to a subsequent drying, wherein the drying in a fluidized bed is carried out using at least one indirectly heated fluidized bed dryer, which is operated with steam as Fluidisierungsmedium, wherein the raw lignite is discharged as bulk material having a mean grain diameter d ₅₀ of at most 2 mm in the fluidized bed and wherein from the dried lignite behind the fluidized bed dryer Branch stream is diverted and the raw lignite is mixed before drying.

Ensuring the particle size distribution can be effected, for example, by influencing the rotational speed of one or more mills upstream of the fluidized-bed dryer. As mills find, for example, beater mills application, which achieve a different Mahlfeinheitsgrad depending on the speed. The particle size distribution is checked by sieving, and sampling may be provided daily or in batches. The samples are checked in the laboratory by sieving classification. Alternatively, the particle size distribution can also be measured volumetrically via an online method.

For the stability of the fluidized bed, it is primarily important that the oversize fraction (> 2 mm) does not become too large, which could otherwise impair the stability of the fluidized bed.

The grain size distribution can alternatively be ensured by following the sieving of the dried lignite with a sieve classification, wherein the oversize fraction is screened off and subjected to subsequent grinding.

The invention is based on the finding that pit-lignite raw lignite, depending on their origin, have a different cohesive behavior which has more or less great influence on the fluidisability of the coal.

It is known that crude lignite is a natural product obtained by mining in terms of water content, carbon content and mineral composition different according to origin. Certain properties of the raw lignite must be taken into account when used as given.

The applicant was able to find out, by way of experiment, that the fluidizability of the raw lignite coal is closely related to its flowability and that the flowability of the lignite coal can surprisingly be positively influenced by admixing dry brown coal of the same type of coal.

According to the invention, therefore, the raw lignite coal to be dried is to be added to the same type of coal in the form of dry lignite before the drying process.

The stability of the treatment process according to the invention, in particular the stability of the fluidized bed within the fluidized bed dryer, can surprisingly be improved by circulating a portion of the dried brown coal within the process, so that poorly or non-fluidisable crude lignite can also be added in this way simultaneously relatively high throughput in the fluidized bed dryer can be used and dried easily.

The problem of fluidizability of the fluidized bed also depends on the capacity of the fluidized bed dryer. Disruptions of the fluidized bed can occur especially at a high load of the fluidized bed dryer.

In the method according to the invention, the mixture of crude lignite coal and recycled lignite coal is preferably introduced via a rotary valve into the fluidized-bed dryer under slight overpressure, introduced above the fluidized bed and distributed onto the fluidized bed.

It is particularly preferred to mix the proportion of dry lignite between 10 and 30% by mass, preferably between 10 and 20% by mass, of the raw lignite, based on the total bed.

According to the invention, it is provided that the proportion of recirculated dry lignite is regulated as a function of the amount of crude lignite fed to the fluidized bed. A regulation can be provided in such a way that a specific ratio of dry brown coal to crude lignite is set and the dry lignite is dosed accordingly. Depending on the set plant load, the recycled dry lignite coal quantity can be automatically adjusted at a constant ratio.

For the purposes of the present application, a regulation means an automated regulation for which a corresponding process control technology is provided.

The raw lignite can have a fluctuating water content / moisture content of up to 65 ma%. The water content of the dry lignite coal withdrawn from the fluidized bed dryer is determined in the hygroscopic region at constant system pressure by the fluidized bed temperature or by the course of the desorption isobars. The dry lignite coal withdrawn from the fluidized-bed dryer may have an average particle diameter d _{50 of} from 0.4 mm to 0.8 mm, preferably from 0.1 mm to 0.4 mm, if appropriate also from 0.1 mm to 0.2 mm. The moisture content of the dry lignite may be between 10 and 15% by mass, preferably between about 15 and 18% by mass. The overpressure within the fluidized bed dryer can be up to 10 bar. Also, the moisture content of the dry lignite may be between 8 and 20 ma%, preferably between 10 and 16 ma%.

As already mentioned above, the fluidized-bed dryer is preferably heated indirectly with steam as the heating medium. A portion of the dryer vapor or, alternatively, extraneous steam from a coupled power plant process may be used to fluidize the fluidized bed or raw lignite within the fluidized bed dryer.

The branched dry lignite can be intimately mixed using at least one mixing device with the raw lignite prior to task in the fluidized bed dryer. Such mixing is not required under all circumstances, but significantly improves the adhesion of fine dry coal particles to the particles of wet lignite coal used.

For example, the dry lignite can be cooled behind the fluidized bed dryer and subjected to a post-milling, with a partial stream of dry lignite is diverted behind the post-grinding.

It is of course also possible to divert the returned dry lignite immediately behind the fluidized bed dryer, but cheaper cooling and Nachmahlung, so that, for example, the dry lignite with an average grain size d ₅₀ from 0 mm to 1 mm of the lignite coal is added. The particle size distribution can also be determined volumetrically by sieve classification, so that, if the particle size distribution differs, the subsequent grinding can be carried out accordingly.

The reclaimed dry lignite can be added to the lignite coal between two milling stages or behind a final milling stage. Of course, it is also possible to supply the recycled lignite coal to the lignite before a first refining.

In this case, an intimate mixing is already ensured by a common grinding of raw lignite and dry lignite, so that optionally separate mixing devices are unnecessary. In the sense of a relatively effective or energy-efficient process management, however, it makes sense to circulate as little as possible dry lignite and to keep the route of the retired lignite to be carried too short as possible. However, consideration should also be given to aspects of explosion protection. For the latter reason, it is expedient and useful, the recycled lignite coal behind a Fine grinding the raw lignite this give up and perform, for example by means of static or dynamic mixing devices in the feed to the fluidized bed dryer mixing with the raw lignite. A mixing can be carried out, for example, via a conventional conveyor.

It is expedient if the proportion of dry brown coal mixed with the raw lignite coal is varied as a function of the adhesion properties of the raw lignite coal and / or the load condition of the fluidized-bed dryer. As will be discussed below, it has been found that the rheology and adhesion characteristics of the coalmine are to a large extent dependent on the compressibility of the raw lignite and hence the bulk density of the lignite coal in the compacted state.

Advantageously, the bed of raw lignite in the fluidized bed by means of at least one arranged above the fluidized bed rotary Verteilerschurre, preferably according to a predetermined distribution based on the cross-sectional area of the fluidized bed dryer.

The invention will be explained below with reference to the accompanying drawings.

Figur 1:

eine schematische Darstellung des Verfahrensprinzips des Aufbereitungsverfahrens gemäß der Erfindung, und

Figur 2:

ein Diagramm, in welchem die Schüttgutfestigkeit von Materialien unterschiedlicher Fließfähigkeit in Abhängigkeit der Verfestigungsspannung dargestellt ist.

Show it:

FIG. 1:

a schematic representation of the process principle of the treatment process according to the invention, and

FIG. 2:

a diagram in which the bulk material strength of materials of different flowability as a function of the solidification stress is shown.

It is first referred to in the FIG. 1 illustrated process principle, which is the flow diagram of a fluidized bed drying plant illustrated, which may be connected, for example, to a power plant boiler for burning lignite.

From a lignite opencast mining, for example, pre-shredded lignite coal with an average grain size of 0 mm to 80 mm is fed to a raw lignite bunker 1. From the raw lignite coal bunker 1, the lignite coal is ground for fine grinding in two mills arranged one behind the other to an average grain size (d ₅₀ ) of about 0 mm to 2 mm. The raw lignite is then fed to a fluidized bed dryer 3 with mixing with dry brown coal, as will be described below. The fluidized-bed dryer 3 is indirectly heated with steam, for example via corresponding heat exchanger installations. The steam feed for charging the heat exchanger is in the process sketch ( Fig. 1 ) is provided with the reference numeral 4. The fluidized-bed dryer 3 is operated in a known manner under slight overpressure, wherein the raw lignite is fed via a rotary valve, not shown, and via a arranged in the upper part of the fluidized bed dryer 3 Verteilerschurre on the fluidized bed. The withdrawn from the fluidized bed dryer 3 vapor 5 is fed to dedusting in an electrostatic precipitator 6 various other uses. This can be released, for example, to the atmosphere. This can alternatively be condensed, wherein the low-temperature heat from the vapor condensation can be coupled, for example, in the boiler feedwater preheating of a power plant process. The vapor can be further compressed alternatively and returned to the fluidized bed dryer 3 for the purpose of heating. The energy from the vapor can also be coupled into an ORC process (organic rankine cycle).

In any case, a partial stream 7 of the vapor 5 is fed to the fluidized-bed dryer 3 as a fluidizing medium. The withdrawn from the fluidized bed dryer 3 dry brown coal 8 is first cooled in a condenser 9, then re-ground in a mill 10 and fed to a dry lignite silo 11. Possible points of extraction of dry brown coal from the brown coal stream intended for thermal utilization are E1 to E4 E1 denotes a removal point behind the fluidized bed dryer 3 and before the radiator 9, E2 denotes a removal point behind the radiator 9 and before the mill 10, E3 denotes a removal point behind a mill 10 and before the dry lignite silo 11. E4 finally designates a removal point behind the dry lignite silo 11.

Preferably, the partial stream to be recycled of the dry lignite is taken at E4, since a better meterability of the recirculating partial flow is ensured because of the stocking provided in the dry lignite silo 11.

The Trockenbraunkohlesilo 11 may be provided on the discharge side with a Austragszellenrad which is operable at variable speed. About the speed control of the Austragszellenrads the dry lignite can be dosed so that a ratio between recycled dry lignite and raw lignite can be adjusted depending on the cohesiveness of the lignite coal and / or depending on the load or the load condition of the fluidized bed dryer 3. The dry lignite silo 11 can two one of which is for dry lignite recycle or dry lignite recycle, the other is for the deduction of dry lignite as a recoverable product of the drying process. The use of two separate prints has in particular the regulatory technical advantage that the subsequent transport can also be used to fill the fluidized bed dryer 3 before starting with dry lignite. Before a first task of raw lignite coal when commissioning the fluidized bed dryer 3, it is necessary to first build a fluidized bed with dry lignite 8, since the raw coal is not fluidizable because of their cohesive properties.

Another advantage of such an interconnection is that, in the event of a possible loss of raw coal feed, a dry lignite recycle can compensate the dust discharge from the fluidized bed and the fluidized bed and all control circuits of the fluidized bed dryer can continue to operate normally.

The positions R1 to R4 denote return points for the dry lignite to be returned, the return point R1 being provided immediately behind the raw coal bunker 1, the return point R2 between a first and a second mill, the return point R3 behind a second mill and before the fluidized-bed dryer 3 Return point R4 is provided immediately before the fluidized bed dryer 3.

The Applicant has surprisingly found that the fluidity of various raw coal is directly related to their fluidizability in the fluidized bed. By mixing dry lignite coal into the poorly fluidizable lignite coal, the flowability of the lignite coal to be introduced into the drying process can be significantly improved.

To prove this connection, the Applicant has examined various raw lignite mines from various opencast mines as well as dry lignite obtained from these raw lignites with regard to their flowability. The various coal samples, hereinafter referred to simply as Samples 1 to 5, and the resultant dry lignite, hereinafter referred to as TBK 1 to TBK 5, were each subjected to fluidity tests, whereby flowability was determined as a ratio of a solidification stress to a compressive strength. The flowability results to:
ff _c = σ ₁ to σ _c , where ff _c denotes the flowability, σ ₁ the hardening stress and σ _c the compressive strength. Such a yield strength can be determined both by means of a known uniaxial compression test and by commercially available ring shear devices. Such a ring shear device is, for example, from Dr. Ing. Dietmar Schulze Bulk Solids Measurement (Ring shear ST-XS) commercially available. Various other types of ring shear devices are available.

The yield strength of bulk material can be classified as follows: ff _c <1 not fluent 1 <ff _c <2 very cohesive to not fluent 2 <ff _c <4 cohesive 4 <ff _c <10 easily flowing 10 <ff _c free flowing.

The bulk material capacity σ _c as a function of the solidification stress σ ₁ for areas of different flowability is, for example, in FIG. 2 shown.

The samples examined by the applicant were examined at a temperature of about 19 ° C at a relative humidity of about 30% relative humidity. The result of the measurements for flowability forth below Table 1, where σ _c denotes the bulk strength or compressive strength of the bulk material, after it σ with the voltage was compacted _1, ff _c, the ratio of σ ₁ to σ _c denotes ρ _b in kg / m ³ denotes the bulk material density, φ _e denotes the measure of the internal friction angle of the bulk material during steady-state flow, φ _lin the gradient angle of the straight-line linearized flow location and φ _sf the internal friction angle during steady-state flow. Table 1: sample σ ₁ [Pa] σ _c [Pa] ff _c [-] ρ _b [kg / m ³ ] φ _e [°] φ _lin [°] φ _sf [°] 1 4020 1703 2.4 530 48 37 40 2 4067 2320 1.8 501 53 36 42 2 3890 1946 2.0 504 49 35 40 4 4443 2007 2.2 554 49 37 42 5 4104 2096 2.0 529 50 35 41 TBK from 1 4219 514 8.2 543 41 38 38 TBK from 2 4109 514 8.0 604 44 41 39 TBK from 3 4192 403 10.4 574 40 38 38 TBK from 4 4022 524 7.7 650 40 37 37 TBK from 5 4243 534 7.9 607 42 39 39

For raw lignite samples 1 to 5, it can be seen that the flowability ff _c in Table 1 is 1.8 to 2.4 for the solidification stress considered. Thus, the samples are classified as cohesive to very cohesive without the influence of time consolidation. The most unfavorable flowability is in the sample 2, the most favorable flowability results for the sample 1, based on the bulk material strength σ _c of sample 1, the sample 2 has a greater strength by 1/3.

As far as the dry lignite, samples 6 to 10 are concerned, the fluidity of the lignite is significantly more favorable than that of the lignite.

The raw lignite sample 2 shows the most unfavorable flow properties. The raw lignite sample 2 was then mixed with various proportions of TBK 2 dry lignite (dry lignite from sample 2) in proportions by weight of 5%, 10%, 15% and 20%. Then, the mixture was examined for flowability, the measurement result is shown in Table 2. Table 2: Share TBK σ ₁ [Pa] σ _c [Pa] ff _c [-] ρ _b [kg / m ³ ] φ _e [°] φ _lin [°] φ _sf [°] 0% 4067 2320 1.8 501 53 36 42 5% 3780 2148 1.8 502 51 33 39 10% 3935 1923 2.0 511 48 33 39 15% 3780 1650 2.3 519 47 35 38 20% 3758 1356 2.8 523 43 33 37 100% 4109 514 8.0 604 44 41 39

The bulk density was determined both in the uncompressed loose state and after a solidification of about 4 kPa (values from Table 1). Bulk density in both the unconsolidated and solidified states of the sample for both Samples 1 through 5 and TBK 1 through TBK 5 and various mixtures is reported in Table 3 below. Table 3: sample Bulk density ρ _b [kg / m ³ ] at hardening stress -> 0 kPa about 4 kPa 1 433 530 2 380 501 3 410 504 4 467 554 5 447 529 TBK from 1 453 453 TBK from 2 603 604 TBK from 3 574 574 TBK from 4 640 650 TBK from 5 604 607 2 + 0% TBK 380 501 2 + 5% TBK 380 502 2 + 10% TBK 400 511 2 + 15% TBK 410 519 2 + 20% TBK 429 523 100% TBK 603 604

Sample 2 has a particularly low bulk density. This corresponds to the unfavorable flowability for this sample, as indicated in Table 1. If a bulk material has an unfavorable flowability, then the individual particles are not mobile in the bed, so remain cavities and the bulk density is low. Consequently, the samples are compressible, so that the bulk density depends on the load. The greatest compressibility is also found in sample 2.

The dry lignite also differ significantly in bulk density. However, there is no correlation between the raw lignite and the dry lignite, a low bulk density of Raw lignite does not necessarily mean a low bulk density of dry lignite. The dry lignite are only very small to measurable compressible in the considered range of solidification stress.

As the proportion of dry lignite increases, both the bulk density and the bulk density increase below a solidification stress of approximately 4 kPa. This can be explained by the better flow resistance after admixture of the dry lignite in that a more favorable flowability allows a tighter packing with a lower void content and thus a higher bulk density.

As a result, it can be stated that an admixture of dry brown coal, for example, to sample 2, has a favorable influence on the flowability of the mixture.

reference numeral

1

Rohbraunkohlenbunker

2

mills

3

Fluidized bed dryer

4

steam injection

5

vapors

6

electrostatic precipitator

7

Partial flow of the vapor

8th

Dry lignite

9

cooler

10

Mill

11

Trockenbraunkohlesilo

E1 - E4

Outlets for dry lignite

R1 - R4

Recovery points for dry lignite

Claims (7)

1. Process for the beneficiation of pit-moist raw brown coal, in particular for thermal utilization in a power station boiler, wherein the raw brown coal is firstly precrushed and subsequently comminuted in at least one milling apparatus and passed to subsequent drying, where the drying is carried out in a fluidized bed using at least one indirectly heated fluidized-bed dryer which is operated using steam as fluidization medium, where the raw brown coal is introduced as bulk material having an average particle diameter d₅₀ of not more than 2 mm into the fluidized bed and a substream is branched off from the dried brown coal downstream of the fluidized-bed dryer and mixed into the raw brown coal before drying, where the proportion of recirculated dry brown coal is regulated as a function of the amount of raw brown coal introduced into the fluidized bed and/or is varied as a function of the bulk density of the raw brown coal in the compacted state.
2. Process according to Claim 1, characterized in that a proportion of recirculated dry brown coal in the range from 10% by mass to 30% by mass, preferably from 10% by mass to 20% by mass, based on the total bed, is mixed into the raw brown coal.
3. Process according to either Claim 1 or 2, characterized in that the dry brown coal branched off is intimately mixed with the raw brown coal using at least one mixing apparatus before production into the fluidized-bed dryer.
4. Process according to any of Claims 1 to 3, characterized in that the dry brown coal is cooled and subjected to after-milling downstream of the fluidized-bed dryer and in that the substream of the dry brown coal to be recirculated is branched off downstream of the after-milling.
5. Process according to any of Claims 1 to 4, characterized in that the dry brown coal is mixed into the raw brown coal between two milling stages or downstream of a last milling stage.
6. Process according to any of Claims 1 to 5, characterized in that mixing of raw brown coal and dry brown coal is carried out using at least one static mixing apparatus in a bulk material feed line to the fluidized-bed dryer.
7. Process according to any of Claims 1 to 6, characterized in that the raw brown coal is poured into the fluidized bed by means of at least one rotating distributor chute arranged above the fluidized bed, preferably according to a prescribed distribution based on the cross-sectional area of the fluidized-bed dryer.

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