US3884620A

US3884620A - Process and apparatus for continuously heating fine-grained coal

Info

Publication number: US3884620A
Application number: US405233A
Authority: US
Inventors: Roland Rammler
Original assignee: Metallgesellschaft AG
Current assignee: GEA Group AG
Priority date: 1972-11-17
Filing date: 1973-10-10
Publication date: 1975-05-20
Anticipated expiration: 1992-05-20
Also published as: DE2256385B2; IT1001767B; DE2256385A1; CA995617A; AU6104773A; AU475650B2; GB1437245A; FR2207592A5; JPS4981274A

Abstract

Fine-grained coal is continuously heated to a controlled, uniform end temperature by direct contact with hot gases by conveying the coal to be heated to a first elevated heat exchange cyclone and subsequently to one or more heat exchange cyclones positioned on a lower level or on successively lower levels, the temperature of the coal being raised in the cyclones. Thereafter the coal is heated to the end temperature in a fluidized bed heater and the exhaust gas from the fluidized bed heater is supplied as a hot conveying gas to the heat exchange cyclones positioned above the fluidized bed heater. Apparatus for carrying out the process of the invention includes a plurality of heat exchange cyclones for preheating the coal positioned on different levels, a fluidized bed heater positioned below the heat exchange cyclones and means to transfer exhaust gas from the fluidized bed heater to the heat exchange cyclones. A dust collector is also included for collecting fine dust from the exhaust gas leaving the upper most heat exchange cyclone.

Description

United States Patent 1191 DRYER COAL SUPPLY BIN 1 f 5 Ecomeusnon CHAMBER 'Rammler May 20, 1975 PROCESS AND APPARATUS FOR CONTINUOUSLY HEATING FlNE-GRAINED Primary Xa inerCharleS J. Myhre COAL Assistant Examiner-William C. Anderson Attorney, Agent, or Firm-Burgess, Dinklage & [75] Inventor: Roland Rammler, Komgstem, Spruhg Germany [73] Assignee: Metallgesellschaft Aktiengesellschaft, [57] ABSTRACT Frankfurt Germany Fine-grained coal is continuously heated to a con- [22] Fil d; O 10,1973 trolled, uniform end temperature by direct contact with hot gases by conveying the coal to be heated to a [21] Appl' 405233 first elevated heat exchange cyclone and subsequently to one or more heat exchange cyclones positioned on [30] F i A li ti P i it D a lower level or on successively lower levels, the tem- Nov 17 972 Germany I 2256385 perature of the coal being raised in the cyclones. Thereafter the coal is heated to the end temperature [5 0 5 CI u in a fluidized bed heater and the exhaust gas from the 5 201/31 fluidized bed heater is supplied as a hot conveying gas 51 Int Cl. F27b 15/00- F26b 3/08 the heat exchange Cyclones POsitioned the 58 Field of Search 34/10, 57 R, 57 A; 432/15, fluldlzed bed heater- 432/58; 201/31 Apparatus for carrying out the process of the invention includes a plurality of heat exchange [56] References Cited cyclones for preheating the coal positioned on UNITED STATES PATENTS different levels, a fluidized bed heater positioned below the heat exchange cyclones and means to gz gj Ga transfer exhaust gas from the fluidized bed heater to 3 790 334 2/1974 Cohen ::::::::::...::::III:I: 432/58 the heat exchange cyclones- A dust collector is also included for collecting fine dust from the exhaust gas FOREIGN PATENTS OR APPLICATIONS leaving the upper most heat exchange cyclone. 1.160323 8/1964 Germany 432/15 1,150,361 6/1963 Germany 432/58 10 Claims, 1 Drawlng Flgure COLLECTOR CYCLONE HEATER COMBUSTION CHAMBER PROCESS AND APPARATUS FOR CONTINUOUSLY HEATING FINE-GRAINED COAL BACKGROUND This invention relates to a process and the corresponding apparatus for continuously heating finegrained coal particularly caking coal, to an exactly controlled, uniform end temperature by hot gases directly the material to be heated.

It is known to heat coal in fluidized beds. If the coal is heated to be dried, the exhaust gas is at a low temperature, which is desirable for heat economy. This proposal has already been used in practice. As the coal is to be heated to higher temperatures, the thermal efficiency of a fluidized bed decreases and the equipment becomes more expensive. Improved efficiencies can again be obtained only at treating temperatures above 500 600 C. because a combustin can then be carried out in most cases in the fluidized-bed heater. At these temperatures, coal is subjected to a considerable degasiflcations, which is often a desired result of the heating.

For this reason it has been proposed to improve the thermal economy of the heating of coal to temperatures slightly below the point where the degasification begins, i. e., to temperatures up to about 300 C., in that fluidizedbed plants are used which comprise two or more stages and in which the exhaust gas from a hotter fluidized bed is used as a heating and fluidizing fluid in the next colder fluidized bed and the coal is passed through the various stages in the direction in which the temperature increases. Whereas such plants result in a better utilization of heat, they still involve a high structural expenditure, particularly when very fine material is to be treated because lower fluidization velocities are required in this case. Another disadvantage of these plants resides in that the pressure loss is multiplied in dependence on the number of stages.

In other processes, coal is heated in a pneumatic elevator tube, in which heat is supplied to the coal as it is pneumatically conveyed. That concept has proved highly satisfactory for a treatment at relatively low temperatures and has been embodied in numerous forms of suspended-particle dryers for coal and many other materials. It has also been proposed to degasify coal by a blow-coking process, which involves an internal combustion in the presence of the coal.

In that process, difficulties arise also during a treatment at medium temperatures above about 250 C. For the same reason as in the fluidizedbed process, it has been proposed to use the pneumatic elevator tube in two stages and to move the coal and the conveying gas in opposite directions in each stage. The disadvantages arising in this process are the same as in multi-stage fluidized-bed plants. Another substantial disadvantage resides in that the end temperature of the coal is subjected to much larger fluctuations. For this reason, to avoid trouble in pneumatic elevator tube plants used to heat caking coal, the temperature must be kept a larger margin below the softening temperature of the coal than in fluidized-bed plants if an undesirable caking of the coal particles with each other and the wall to be avoided.

German Pat. No. 1,l60,823 describes a process of degasifying non-caking coal, in which the coal is predried and in a suspended-particle dryer is heated in a plurality of series-connected cyclones. The coal and the heat-carrying conveying gas are moved in mutually opposite directions in each stage. This process has also the disadvantage that the treating temperature is subjected to substantial fluctuations and the resulting fin ished product is not uniformly degasified. A process in which the disadvantage is alleviated has been described in DOS 1,671,320. The alteration compared to the prior process resides mainly in that the preceding suspendedparticle dryer is replaced by a separate system of cyclones, which will be referred to hereinafter as heat exchange cyclones. This process has also the disadvantage that the end temperatures of the treatment fluctuate because the very short treatment times which are typical of the process and are inherently desirable, restrict the amount of coal which is contained in the system so that undesires small changes in operating conditions, e. g., in the feed rate, result immediately in a change of the end temperature.

A new development has taken place in recent years in the conventional coking process carried out in the byproduct coke oven and resides in that the coking coal is dried and heated before it is charged into the chambers. The effect of this measure increases the capacity of the batteries, increases the range of cokable coal and improves the quality of the coal, as the coal is preheated to higher temperatures so as to approach in practice the temperature at which the coal begins to soften, which is between 300 and 350 C. In the hot briquetting or hot pelletizing processes of producing shaped coke bodies it may also be desirable or necessary to preheat the caking component before it is charged. Various aspects govern the selection of the temperature to which the coal is preheated, and a preheating to the highest possible temperature is often required.

SUMMARY first heat exchange cyclone and flows from the same to one or more heat exchange cyclones which are disposed on a lower level or on successively lower levels, the temperature of the material being raised in said cyclones, the material is then heated to the end temperature in a fluidized-bed heater, and the exhaust gas from the fluidized-bed heater is supplied as a hot conveying gas into the heat exchange cyclones. The residence times of the material being heated in the fluidized bed amount to a few minutes (e.g. 2 or 3 minutes) to about half an hour (30 minutes) and in most cases to between 5 minutes and 20 minutes.

DESCRIPTION OF THE DRAWING The present invention will be more fully understood from the following description taken in conjunction with the accompanying drawing which is a diagrammatic flow diagram of preferred apparatus for carrying out the process of the invention.

DESCRIPTION In the process according to the invention, the advantages of the heat exchange cyclones are utilized for the initial heating. Because these heat exchangers are operated in a counterflow mode in stages, the exhaust gas therefrom is at low temperatures so that the heat is utilized to a high degree. Besides, the material is rapidly heated in these heat exchangers so that the residence times are short and a small structural expenditure is sufficient.

In the final heating stage, the advantages residing in the extremely rapid temperature equalization taking place within a fluidized-bed heater are utilized according to the invention. This ensures a uniform heating of the material being treated without an occurrence of local temperature differences. At the same time,- higher gas inlet temperatures than in other processes are permissible even if a heat-sensitive feed such as caking coal is to be heated. Because the residence time of the material to be heated amounts to between a few minutes and about half an hour, it is ensured that the coarser particle size fractions which may not have been fully heated through in the heat exchange cyclones are heated to the same temperature as the finer fractions.

Besides, the relatively long residence times in the fluidized bed have the result that the fluidized-bed heater contains a considerable amount of material to be heated so that the overall system has a certain desired thermal inertia as far its response to inevitable fluctuations of the operating conditions is concerned. For this reason, the uniformity as regards locus and particle size function is accompanied by a stability of the heating as regards time.

The heating system according to the invention comprising a fluidized-bed heater as a final stage affords at the same time the advantage that the fluidized-bed heater has such a high storage capacity that there is no need in many cases for an intermediate bin for receiving the heated material because the same can be directly supplied from the fluidized-bed heater to the consuming station, e. g., the mixing station of a plant for making shaped coke bodies.

Fine coal is usually wet. Other materials too are free of moisture only in exceptional cases. For this reason it will be desirable in most cases to provide a dryer which precedes the heating system according to the invention unless drying can be accomplished in a desirable manner in the uppermost heat exchange cyclone. This will be particularly useful if a feed which is free of tar or oil is to be heated to temperature above 500 600 C.

To reduce the costs of the plant, a suspendedparticle dryer is preferable to other kinds of dryers. In the heating system according to the inventionconsisting of a plurality of cyclones and a succeeding fluidized-bed heater, fine-grained material usually flows from top to bottom and for this reason must be charged on an elevated level. The suspended-particle dryer may have a dryer tube which is longer than could be required for drying and combines drying and conveying functions so that it can be combined with the heating system according to the invention in a very simple manner and without an interposition of mechanical conveyors.

The exhaust gases from the fluidized-bed heater are approximately at the end temperature of the treatment in the fluidized bed in a development of the invention these exhaust gases are used as a conveying gas and heat carrier in the heat exchange cyclones. If the feed has a low resistance to abrasion, the gases may entrain a relatively large amount of dust from the fluidized bed. In that case it will be desirable to provide a cyclone which is associated with the fluidized-bed heater and collects dust from the exhaust gases leaving the fluidized-bed heater before they flow through the more elevated heat exchange cyclones. Depending on requirements, the solids collected in this dust-collecting cyclone may be supplied to the fluidized-bed heateror may be admixed to the fine-grained product withdrawn from the fluidized-bed heater, or may be separately withdrawn.

ln fluidized-bed technology, a thin-phase fluidized bed has been developed in recent years, which is fluidized with gas flowing at a relatively high velocity so that solids are entrained thereby at a relatively high rate. To ensure a sufficiently long residence time of the solids in the region heated to the bed temperature, such heaters are succeeded by a cyclone and the solids collected in said cyclone are recycled into the fluidized-bed heater. This technology requires smaller investment costs and may be used also in the present case if this is permissible in view of the properties of the material to be heated. This arrangement results in the cyclic arrangement which has been described in the preceding paragraph and which comprises a fluidized-bed heater and a cyclone for collecting solids which are recycled into the fluidized-bed heater.

An example of the invention will now be explained more fully with reference to the simplified diagrammatic showing in the drawing. This example demonstrates the heating of moist, fine-grained coking coal to 320 C. so as to produce a preheated caking component for the continuous production of shaped coke bodies.

The fine-grained caking coal feed has a moisture content of 9 percent and is at the ambient temperature as it is fed from the supply bin 1 into the suspendedparticle dryer 2. In the present example, the dryer 2 consists of a circulating dryer provided with a sifting hood 3 and a coarse particle-recirculating conduit 3a, which incorprates a hammer mill 4. The dryer 2 is fed with hot combustion gas from a combustion chamber 5, which is fed with air L and gaseous fuel G. The combustion chamber is operated under a slightly superatmospheric pressure and the ratio of fuel gas to air in said chamber is exactly controlled so as to minimize the contents of free oxygen in the combustion gas. Cooled gas from the conduit 18 is recycled to adjust the gas at the outlet of the combustion chamber 5 to a temperature of570 C. Rising in the tube of the dryer 2, the gas is cooled to 140 C.; at the same time, the entrained coal is heated to approximately C. and dried to a residual water content of 1 percent. The coal is separated in the coal-collecting cyclone 6 from the vaporleaving the dryer and is then immediately supplied to further heating means described below.

Because the heat transfer in the suspended-particle dryer 2 is accompanied by a pneumatic conveyance, the coalcollecting cyclone 6 is elevated and there is no need for a further conveyor to move the coal to the feeder for the uppermost heat exchange cyclone 9.

The drying gases from which the coal has been removed are sucked by the blower 7 and are forced into the electrostatic precipitator 8, in which more dust is collected.

The dry coal is conducted from the collecting cyclone 6 through the discharge conduit 6a thereof into the conduit 10a, in which the coal is entrained by hot conveying gases flowing to the heat exchange cyclone 9. Through the discharge conduit 9a of the latter cyclone, the coal enters the conduit 10b, in which it is entrained by gases which are hotter than those in the conduit 10a and flow into the second heat exchange cyclone 10, in which the coal is heated further.

The coal is finally supplied through the conduit 11a to the fluidized-bed heater 11, in which it is heated to its end temperature of 320 C. during a residence time of 12 minutes. At this temperature the coal is with drawn as a finished product through the conduit 20 and is directly supplied to a consuming station. The fluidized-bed heater has such a high storage capacity that there is no need for an intermediate bin.

The fluidized-bed heater 11 is heated with combustion gases from the combustion chamber 12, which like the combustion chamber 5 is operated with a gas-air ratio which is exactly controlled to ensure that the combustion gases contain only traces of free oxygen. This is important because otherwise the caking capacity of the coal could be substantially reduced by oxidation.

Gas at 140 C. is recirculated to adjust the temperature of the gases entering the fluidized-bed heater 11 to 460 C. The recirculated gas is branched by the 'conduits 18 from the exhaust gas stream of the system be hind the electrostatic precipitator and is pressurized by the blower 13 to the required pressure. The remaining exhaust gas is discharged into the open via line 19.

Having flown through the coal bed in the fluidizedbed heater 11, the fluidizing gas leaves the heater at about 320 C. and flows through insulated tubes 17 into the more elevated cyclones. The gas flows first through the collecting cyclone 14, in which entrained dust is collected. Depending on the requirements, this dust is recycled through the conduit 15 into the fluidized-bed heater or is conducted through conduit 16 and admixed to the coarser particles which have been withdrawn from the fluidized bed heater.

When dust has been collected in the cyclone 14 from the gas withdrawn from the fluidized-bed heater, the gas is supplied by the conduits 10b and 10a, respectively, to the heat exchange cyclones l0 and 9 in succession. In said cyclones, the gas is cooled to 140 C. as it transfers heat to the oppositely moving coal which has been supplied through conduits 9a and 6a. At a temperature of 140 C. the gas together with the vapor from the dryer enters the electrostatic precipitator 8, in which dust is collected. The dust collectes in the precipitators is added in the present example to the finished product through the conduit 21.

The example which has been described hereinbefore may be modified in various ways. For instance, the dryer 2 will be omitted if the feed has only a very low moisture content. Whereas the significance of the production of a virtually oxygen-free fluidizing and conveying gas has been emphasized incconnection with the previous example, this requirement need not be fulfilled, e. g., when it is desired to heat fine coal for a subsequent dry distillation or coking. In this case the combustion chamber 12 preceeding the fluidized-bed haater 11 may be omitted and the heater may be operated with cold or preheated air whereas the heat required for the process is produced directly by a partial combustion of coke and volatile constituents in the fluidized-bed heater. A fluidized-bed heater in which an internal combustion is carried out may also be used to heat an incombustible feed. This technology is known per se and is described in German Pat. No. 1,758,244. It calls for a supply of fuel and air through mixing nozzles provided in the bottom of the fluidized-bed heater to produce combustion gases near the nozzles, and for the use of these combustion gases as a fluidizing gas. When coal is heated to temperatures in the degasification range, the electrostatic precipitator shown on the drawing must be replaced by a condenser.

What is claimed is:

1. Process for continuously heating fine-grained coal to a controlled, uniform end temperature by direct contact with hot gases which comprises conveying the coal to be heated to elevated cyclonic zone and subsequently to one or more cyclonic heat exchange zones disposed on a lower level or on successively lower levels, the temperature of the coal being raised in said heat exchange zones, thereafter heating the coal to said end temperature in a fluidized-bed heating zone, the exhaust gas from the fluidized-bed heating zone being supplied to a separate cyclonic collecting zone where dust is collected from the exhaust gas, said gas being then passed as a hot conveying gas through said one or more heat exchange zones, said dust collected in the collecting zone being withdrawn or recycled to the fluidized bed zone.

2. Process of claim 1 wherein a suspended-particle drying zone is used to convey the coal to the first heat exchange zone.

3. Process of claim 1 wherein the residence time of the coal in the fluidized-bed heating zone is between a few minutes and about half an hour.

4. Process of claim 1 wherein said hot conveying gas is conducted in flow paths opposite to the direction in which the coal being heated flows under gravity.

5. Process of claim 1 wherein fine dust is collected from the vapor leaving the dryer and from exhaust gas leaving the first heat exchange zone which is combined with said vapor.

6. Process of claim 1 wherein moist coal to be heated to an end temperature above 500C. is dried in the first heat exchange zone.

7. Process of claim 1 wherein the fluidizing gas required for the fluidized-bed zone is produced in a'preceding combustion zone wherein an almost stoichiometric fuel gas to air ratio is maintained and the fluidizing gas is temperature-conditioned by an addition of cooled and recycled exhaust gas.

8. Process of claim 1 wherein an internal combustion is carried out in said fluidized-bed zone to obtain an end temperature above 500C.

9. Apparatus for heating fine-grained coal to a controlled, uniform end temperature by direct contact with hot gases which comprises means to convey coal to be heated to elevated cyclone means, a plurality of cyclonic heat exchange means for preheating the coal po sitioned on different levels below said elevated cyclone means, fluidized bed means as a final heating stage, means to supply said bed with fluidizing gas, means for transferring exhaust gas from the fluidized-bed means into the heat exchange means including cyclonic collecting means between the fluidized bed means and the lowermost cyclonic heat exchange means for collecting dust from said exhaust gas and withdrawing or recycling the collected dust to the fluidized bed means, and dust collecting means for collecting fine dust from the exhaust gas leaving the uppermost heat exchange means.

10. Apparatus of claim 9 wherein said fluidized-bed means includes means for carrying out an internal com- UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 5, 8 I-,62O v Dated May 975 lnventofls) Roland Rammler It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 8, after"directly" should read contacting Column 1, line 18, "oombustin" should read combustion 0 Signed and Scaled this twenty-fifth D 3) of November 19 75 [SEAL] Arrest:

RUTH C. MASON I C. MARSHALL DANN Altexting Officer (mnmixsiuner nj'Patents and Trademarks

Claims

6. Process of claim 1 wherein moist coal to be heated to an end temperature above 500*C. is dried in the first heat exchange zone.

7. Process of claim 1 wherein the fluidizing gas required for the fluidized-bed zone is produced in a preceding combustion zone wherein an almost stoichiometric fuel gas to air ratio is maintained and the fluidizing gas is temperature-conditioned by an addition of cooled and recycled exhaust gas.

8. Process of claim 1 wherein an internal combustion is carried out in said fluidized-bed zone to obtain an end temperature above 500*C.

9. Apparatus for heating fine-grained coal to a controlled, uniform end temperature by direct contact with hot gases which comprises means to convey coal to be heated to elevated cyclone means, a plurality of cyclonic heat exchange means for preheating the coal positioned on different levels below said elevated cyclone means, fluidized bed means as a final heating stage, means to supply said bed with fluidizing gas, means for transferring exhaust gas from the fluidized-bed means into the heat exchange means including cyclonic collecting means between the fluidized bed means and the lowermost cyclonic heat exchange means for collecting dust from said exhaust gas and withdrawing or recycling the collected dust to the fluidized bed means, and dust collecting means for collecting fine dust from the exhaust gas leaving the uppermost heat exchange means.

10. Apparatus of claim 9 wherein said fluidized-bed means includes means for carrying out an internal combustion.