EP1363097B1 - Pneumatic centrifugal dryer - Google Patents

Abstract

A pneumatic transport rotor, recirculates a mixture of materials to be dried with vapors. In the course of recirculation, it causes the flow of falling material to be accelerated under centrifugal force, against the walls of the dryer. The heated walls act as a heat exchanger. An Independent claim is included for the corresponding dryer.

Description

The invention relates to a method and a device for drying pneumatically conveyable goods, in particular of wood fibers and similar vegetable substances, in a spin dryer with a heated wall.

In the industrial drying of wood chips, the high temperatures, which are usually used for reasons of economy, cause emission problems due to thermal decomposition. Wood fibers are industrially very energy-intensive dried in Stromtrockneren. For the drying of wood pulps increasingly find use Heißdampfrockner, in which the heat of vaporization is entered by superheated Brüdendampf in the dry material. The entry of the heat of vaporization in the form of radiant energy, for example after FR-A-998 617 without back mixing of dry material, is very gentle on the product but not very energy efficient and therefore unsuitable for mass production.

Rohrbündel-Rotationsbockner were previously regarded as the dryer with the least amount of pollutants in the drying of wood pulp. This is because these dryers are gently heated indirectly with steam (or thermal oil). Of great disadvantage, however, is the steam boiler required, unless steam is available anyway. In normal tube bundles, however, wood fibers get caught. For this purpose, a special, complex Heizregistertrockner the company VETTER MASCHINENBAU GMBH KASSEL is suitable.

The EP 0 457 203 B1 claims a drum dryer with closed steam-air circuit, which is particularly suitable for the drying of substances with a strong odor, wherein the vapors are condensed and the non-condensable vapor fractions are burned. For this dryer, the use of the heat of condensation of the vapors is claimed. The patent also describes the heating of the non-condensable vapor fractions by means of flue gas coolers prior to their entry into the combustion chamber.

The EP 0 459 603 B1 claims method and system for drying wood chips, wood fibers or similar bulk materials in a Brüdenkreislauf in which the drier dryer leaving flue gas-free vapors are heated indirectly in a heat exchanger of the hot fuel gases and fed back to the dryer, with a partial flow diverted from this circuit and the thermal decomposition of the pollutants is conducted to the combustion chamber. A vapor condensation does not take place. The same procedure describes the EP 0 508 546 A1 ,

For the drying of substances in which a hard-to-clean vapor condensate is obtained, claimed the EP 0 714 006 B1 an improvement of the process of EP 0 459 603 B1 by heating the Brüdenteilstromes reaching for combustion with the hot, the combustion chamber directly leaving the flue gas, so that the following countercurrent heat exchanger for the extraction of the dryer heat at the hottest point is no longer so much thermally stressed. The residual heat of the flue gases is then still used to heat the combustion air and optionally still for a predrying.

As is known, because of their low density, gases are poor heat carriers and evaporative condensation circuits transfer the heat best. The EP 0 851 194 A3 claims that the hot flue gas of a combustion chamber first heat is removed by a steam generator, before being heated in a second heat exchanger, the vapor cycle. The vapor excess is burned. Preferably, as a dryer obviously a tube bundle rotary dryer with housing or with rotatable drum is used, wherein a current dryer can be connected upstream. With this arrangement, lower exhaust gas temperatures should be achievable. It is not clear from the illustration whether this arrangement has economic advantages over the classic tube bundle rotary dryer with only one heat carrier circuit.

After DE 100 56 459 C1 can fibrous substances with low bulk density and high internal friction, such as wood fibers basically not in a dryer after the EP 0 714 006 B1 or the DE 196 54 043 C2 be dried. Therefore, a method is claimed for a dryer, which is obviously a flow dryer, with heated in the circuit between the dryer and the furnace heat exchanger vapors and with vapor condensation, as already in the EP 0 365 851 B1 was claimed for a drum dryer, wherein the non-condensed vapor fractions are passed over a heat exchanger in the furnace, as in the EP 0 714 006 B1 for the disposal of vapors without prior condensation is claimed. In subclaim 9, the use of a current dryer for the method is then claimed.

In 1999/2000, the DEUTSCHE BUNDESSTIFTUNG UMWELT supported the development of the superheated steam fiber dryer by SCHENKMANN & PIEL, LEVEREKUSEN. This is a 100 m long power dryer operating at a pressure of 0.5 to 1.5 bar with only slight overheating of circulating vapors (<180 ° C) to avoid gaseous emissions. For 1 kg of water evaporation, about 50 kg (!) Of vapor must be circulated in this dryer.

Most drying processes are unfavorable in terms of control engineering. The DE 196 06 472 C1 describes how in a continuous dryer already at start the target residual moisture can be achieved quickly by a complex control. The DE 196 09 530 A2 describes a two-stage drying of biomass, wherein the hot biomass of the first stage is set in a downstream flow dryer by controlled evaporative cooling with conditioned supply air to the exact processing moisture. The EP 1 128 145 A2 describes the control of a power dryer by recording the climate data within the dryer with pressure, temperature and humidity sensors whose signals are evaluated by a controller that regulates the temperature of the hot gas stream.

A problem with multi-stage dryers, for example the widespread combination current dryer for predrying and drum dryer for main drying, is the amount of the guided through the entire plant steam, its separation after the predrying the DE 44 27 709 A1 describes.

With indirectly heated Umgas operated continuous dryers are quite expensive. There are new attempts to reduce the effort for low-emission operation of wood chip dryers. This is done after the WO 01/67016 A1 proposed to return the entire excess of the Umgasstromes a flue gas directly heated Umgastrockners in the furnace and to use the resulting excess of thermally purified flue gases for the likewise direct heating of a pre-drying also rubbed with circulating air, wherein in the pre-drying due to much lower temperatures due to higher humidity less decomposition products are formed.

When drying mechanically insensitive goods, or when working in a process with comminution anyway, very good material and heat transfer are achieved with Schleuderwellentrocknem. Due to the very good separation of the dry material is almost the entire specific outer surface of the dry material at the same time high turbulence for the mass and heat exchange available. An interesting spin-dryers claims the EP 0 862 718 B1 , This pneumatic centrifugal dryer with stationary type, indirectly heated by Umgas, combines in one container an axially standing centrifugal shaft with a classifying flow dryer. Here granules of defined grain size are produced from a suspension. For the drying of wood fibers and similar botanicals, however, this pneumatic centrifugal dryer with Umgasbetrieb seems less suitable.

To EP-A-0 536 650 Dry material is thrown by a whipper dish in such a way against an overheated baffle that a thin liquid film is formed, which flows along the wall. This is a contact dryer, which would be suitable for wood fibers and similar herbal substances only to evaporate the mechanically separable adhesive water, but not for the drying technology relevant bound in the fibers water.

Object of the present invention is a method and apparatus for the indirect drying of pneumatically conveyable goods, especially wood fibers and similar herbal substances, with very good separation of the dry material and use of the wall of the drying room as a heat exchanger.

The solution of this problem is specified in claim 1 or 5.

In FIG. 1 the essential elements of the invention are shown: consisting of induced draft 3, suction pipe 4 and downpipe 5 Schlendertrockner is in an oven 6. The Good 1 is given up wet and the spin dryer leaves Brüdendampf and dried material. 2

The pneumatic centrifugal dryer according to the invention combines the very good heat transfer of the spin dryer with the very simple control behavior of the loop reactor, which corresponds approximately to that of the ideal stirred tank. As a current dryer, it is particularly suitable for drying difficult to handle goods such as wood fibers and similar botanicals. In contrast to the currently used solutions indirectly heated current dryer but eliminates the costly entry of the heat of vaporization via a Brüdenkreislauf with various apparatus and required for low overheating of the vapors extremely high gas circulation or required for lower circulation high Brüdenüberhitzung with product decomposition. According to claim 6, the spin dryer is directly in an oven. The burnt flue gas is cooled by the recirculation abruptly at oven temperature of eg 450 ° C and overheated on the reactor wall vapors of eg 150 ° C is immediately withdrawn by the drying process heat again. For 1 kg of water evaporation, for example 10 kg vapors are circulated. The independent dwell times of the dry material and the vapors are usually 10 to 300 seconds.

The dry material / vapor mixture is circulated with a transport impeller, as it is known from the induced draft of the pneumatic chip conveyor. This is a very robust open radial fan with closed back, where nothing gets stuck. When passing through the transport wheel and when spinning against the reactor wall, the dry material is very well isolated, as in any spin dryer. The particles of dry matter rubbing in the downpipe by centrifugal force on the reactor wall cause a thin PRANDEL boundary layer and thus an excellent heat transfer from the wall to the circulated vapors.

In internal circulation, the existing guide tube is designed as a suction tube, the induced draft is located directly on the suction tube and hurls the dry material free blowing against acting as a heat exchanger reactor wall. To increase the heat exchanger surface, it is advantageous to work with several downcomers, which are fed through a distributor head absolutely uniform.

In principle, the centrifugal dryer according to the invention can also be operated by means of an induced draft at the bottom, wherein the transport impeller can be a floor mixer at the same time. In this case, an external circulation of the dry material / vapor mixture takes place. This design, in which the entire machine technology is installed at ground level, can be very advantageous for smaller dryers.

In the interior of the pneumatic centrifugal dryer according to the invention, the heat is transported from the dryer wall to the dry material without great overheating of the vapors. Due to the thermal inertia of the dryer wall bie disturbance of the charge of dry material can be done very quickly reaching the entire drying room emergency cooling with injected water. Thus, the special fire risk of the drying process of plant substances can be practically excluded. If it should nevertheless come to the fire, this is simply suffocated by Abschiebem of all openings. If an impermissible overpressure is built up, it will be removed via pressure relief dampers.

The pneumatic spin dryer can be controlled very well according to the internal temperature of the loop reactor. The residual moisture correlates very well with this temperature for a given product. To start the dryer is preheated to the operating temperature and then slowly wet material is fed in such an amount that the internal temperature is constantly in the desired value range. The material task is steadily increased until the furnace wall is fully heated and the full firing capacity is available for drying. To shut down the firing is set and continue to dry with decreasing load until it can be switched off safely. The dryer can also be quickly emptied and cooled by evaporation of injected water.

For cogeneration of the pneumatic centrifugal dryer can be heated by means of exhaust gas of a gas turbine or with exhaust steam of a steam engine, a back pressure turbine or a withdrawal condensing turbine. For steam heating, the downpipes can also be designed as double-jacketed pipes. For vapor of an organic liquid, the pneumatic Centrifugal dryers can be used as a capacitor of an ORC power plant.

Particularly suitable is the spin dryer according to the invention for the decentralized processing of renewable raw materials. For this purpose, the plants preferably have a firing capacity 0.3 to 3 MW. High efficiency is achieved by burning biomass. Thus, e.g. in the production of wood fibers by means of twin-screw extruder from LEHMANN MASCHINENBAU GMBH JOCKETA from wet wood chips from Ganzbaumemte the fine fraction, which contains a high proportion of bark and needles or leaves, separated by air separation and burned directly in a Vorofenfeuerung. The air classifier can be adjusted to the respective current fuel demand. In this case, there is only one common line for raw material and fuel.

In the pneumatic centrifugal dryer according to the invention, the dry material is separated very well as in each spin dryer, so that almost the entire specific surface of the dry material for the mass and heat exchange with the vapors is available and due to the high turbulence very good material and Heat transfer can be achieved. Until the break point moisture is the rate-determining step of the heat input into the dryer. For drying up to the break point moisture content, therefore, the residence time behavior of the simple stirred tank is sufficient. For further drying, as is required, for example, for chipboard bonding, two or more dryers can also be connected as a cascade in series in order to drive higher temperatures in the dryer only for the diffusion-controlled final drying below the breakpoint moisture. Also, in the main drying two or more spin dryers can work in parallel and be connected in series with a common spin dryer for final drying.

The dry matter discharged from the spin dryer can be removed pneumatically with air. The sensible heat of the dry material is still used for subsequent drying by evaporative cooling. Even better is the efficiency when the transport air is previously heated with waste heat. The used transport air is advantageously used as combustion air in the furnace of the dryer. In the case of flammable dry material, such as vegetable materials, no further dedusting is required after the product separator. Although the water vapor from the after-drying must be heated to the exhaust gas temperature with the flue gases, this only requires a fraction of the heat of vaporization saved.

The pneumatic spin dryer is preferably also fed pneumatically. If transport air is used for this purpose, the water vapor partial pressure in the dryer drops, which is advantageous for drying, but the dew point temperature of the vapors also drops, which is disadvantageous for the use of waste heat. Brüdendampf should be overheated as transport gas to avoid dew point violations. By such a vapor overheating with an exhaust gas cooler, the efficiency is improved. However, the pneumatic spin dryer can be used e.g. be charged with a screw conveyor.

Brüdendampf with high dew point temperature is known to be very suitable for the use of waste heat. Often, however, the heat requirement at the place of occurrence is missing. Intended for drying renewable Raw materials are often wet. In this case, it may be advantageous to steam the raw materials with broth steam tz in order to save heating in the dryer. The running off steaming water is usually very good biological clean. Such a damper also causes a dedusting of the vapors.

However, the vapor vapor may be e.g. be condensed in an air cooler, the non-condensed fraction is fed directly or indirectly to the furnace and the heated air can be used for pre- or post-drying. For example, chopped green fodder can be predried in a pneumatic spin dryer and then dried on a belt dryer. The green fodder is aerated much better after pre-drying and the heating energy is used twice here. Wood chippings are pre-dried by this warm air for the production of fuel pellets in silos with soil aeration, crushed in a hammer mill, dried in a pneumatic spin dryer and then pelletized.

Example 1: Plant and method for processing wood chips to wood fibers with drying in one embodiment of the dryer according to the invention.

The most important dimensions are approximately: Eight downpipes 508 x 8 mm with 12,750 mm drop height. The suction tube 406.4 x 12 mm, the distribution head of square Holprofil 150 x 10 mm and hemispherical collector 2.600 mm diameter, 1300 mm height and 10 mm wall thickness. This loop reactor is made of material St 37-2. The furnace wall has an inner diameter of 2,400 mm and an outer diameter of 3,000 mm and is stuffed with mineral insulating wool to about 100 kg / m ³ . The inside of the furnace wall consists of a 1 mm thick metal jacket made of material 1.4713, which is provided with cross beads for stiffening and length compensation. The combustion chamber for a wood injection firing and all thermally highly loaded kiln installations are made of material 1.4762. The entire heat exchanger surface is about 180 m ² . Depending on the material and wall temperature, the maximum evaporation capacity for water is approx. 1,200 to 2,400 kg / h. Thereafter, the firing capacity is designed depending on the specific circumstances to 0.99 to 2.7 MW, in principle, the fuel is possible. The specific power consumption of the induced draft is 0.01 to 0.06 kWh / kg water evaporation. The loop reactor with furnace wall is the largest single part for assembly and, with dimensions of 3,000 x 3,000 x 16,000 mm, does not present any problems for road transport.

The wet woodchips are discharged from the bunker by means of a sliding floor for wind protection, where the fine fraction is discharged according to the respective heat requirement of the dryer by means of transport air and blown into a Vorofenfeuerung.

The wood chips separated from the fines pass through a conveyor belt into the damper, where the coarse-grained lumps are well permeated by broth steam. The condensed vapors and the water discharged from the wood chips are subjected to biological purification. Depending on the other heat demand in a condenser, waste heat with an 80 ° C flow temperature is recovered from the vapor dedusted in the damper.

The steamed hot chips are fed to a twin-screw extruder from LEHMANN MASCHINENBAU GMBH JOCKETA for defibration and arrive from there with a screw conveyor in the suction tube of the dryer.

In the spin dryer, the wood fiber-vapor mixture at the lowest point of the collecting device is sucked into the suction pipe, wherein above the collecting device in the suction pipe and the fresh material is supplied. Above the suction tube is a transport wheel, as it is known from the chip conveyor, mounted with a vertical axis in the distributor head hanging. The distributor head corresponds to the housing of a radial blower with the difference that it has several arranged around a symmetry axis of rotation outlets, here eight pieces. The Transportlaufrad hurls the aspirated wood fiber-vapor mixture through the outlets of the header with tangential entry into the downpipes, where the wood fibers against the wall hurled down a spiral path fall down and get along with the circulated vapors in the collection device, where they are sucked back ,

At the bottom of the collection device, a soil agitator ensures that no wood pulp settles and that wood fibers are constantly being transported into the outlet, from where they pass with the vapors to the cyclone separator.

The Einblasfeuerung is designed as a pre-furnace, so that in the fine fraction of the wet wood chips necessary for a clean combustion chamber temperature is reached. The actual furnace chamber of the spin dryer stands on the hemispherical collector of the loop reactor and envelops the entire remaining part of the loop reactor, except for the drive of the transport wheel. The approximately 400 ° C hot exhaust gases of the furnace are cooled in the air preheater to about 200 ° C Schomstein temperature.

Example 2: Process for producing fuel pellets from wet sawdust.

An adjustable feed belt feeds the pneumatic belt dryers with sawdust via a rotary feeder in your pneumatic Baypass. In a cyclone separator, the dried sawdust are separated from the vapors and fed via a rotary feeder directly to a pellet press of the company MÜNCH, RATINGEN. The optimum degree of drying of the sawdust is judged by the appearance of the pellets and the speed of the feed belt is increased or decreased accordingly. The short-time superheated steam eliminates the otherwise necessary conditioning and the intermediate buffer

Example 3: Process for self-sufficient sewage sludge drying with direct combustion and optional phosphate recovery.

The organic dry matter (oTS) of municipal sewage sludge has about the calorific value as dry wood. With filter presses or centrifuges mechanically dewatered sewage sludge with at least 20% oTS can therefore be burned autothermally. According to the prior art, the sewage sludge is dried and the dry material can be burned for the heating of the dryer. Since the sewage sludge passes through a so-called glue phase during drying, it must be adjusted to over 60% DM for drying in convection dryers by back-mixing with dry material. Due to the internal product return, the pneumatic centrifugal dryer according to the invention is ideally suited for this purpose. In the collection device, a kneader is installed, which causes the mixing to crumbly consistency.

The dusty dry sewage sludge is roughly separated from the broth steam in a cyclone and with preheated air into an eddy current burner of the GIV MBH, HALL burned very clean The dusty vapors are fed directly to the secondary combustion chamber and thus thermally cleaned. The steam generator is heated with the completely burnt flue gases. The steam engine of the generator is operated with 16 bar back pressure and the dryer acts as a capacitor.

Mono incineration allows the ash from sewage sludge without heavy metal pollution to be used directly as biocide-free phosphate fertilizer, otherwise it requires further treatment for phosphate recovery.

Claims (10)

1. Procedure for drying of goods in an indirectly heated centrifugal dryer, where the goods are catapulted in downdraft by centrifugal forces against the reactor wall functioning as heat exchanger. This procedure is distinguished by the fact that a goods-exhaust-vapour-mixture is pneumatically deliverer via a transport impeller generating this centrifugal force and circulated within the heat exchanger.
2. Procedure according to claim 1 with pneumatic goods supply to the centrifugal dryer, where exhaust vapour is used as transport medium, which can be overheated as needed, especially in an exhaust gas cooler.
3. Procedure according to claim 1 with exhaust vapour condensate heat discharge, preferably with disposal of not-condensable exhaust vapour components via the firing equipment of the centrifugal dryer.
4. Procedure according to claim 1 with downstream pneumatic suction system for waste heat utilization by evaporative cooling, where the transport air used is preferably feed to the firing equipment of the centrifugal dryer.
5. Fixture to perform the procedure according to claim 1, where the centrifugal dryer or a cascade of centrifugal dryers with pneumatic circulation by one or several transport impeller(s) features a reactor wall heated using a heat carrier medium is the area of the downdraft/ downdrafts.
6. Fixture according to claim 5, where the centrifugal dryer is directly installed in a furnace, which can be heated using waste gases of firing equipment or using hot exhausts.
7. Fixture according to claim 5, where the centrifugal dryer can be indirectly heated using steam in the area of the downdraft/downdrafts.
8. Fixture according to claim 5, where the centrifugal dryer mainly consists of a stranding cylindrical vessel and a suction draught with suction pipe without connected exhaust line, where the suction draught is arranged in such a way that goods-exhaust-vapour-mixture can be drawn from the bottom of the vessel using the suction pipe, and goods can be catapulted against the upper part of the wall of the cylindrical vessel using the suction draught's transport impeller without connected exhaust line.
9. Fixture according to claim 5, where the centrifugal dryer consists of several down pipes, which are connected via tangential inflow opening with the distributor head, which is connected via a transport pipe to the collection fixture arranged underneath the down pipes.
10. Fixture according to claim 8 or 9 with one or several bottom agitator aids for deposit prevention at the bottom of the centrifugal dryer and possibly in the lower part of the suction pipe as well and possibly with agitator aid blading for goods delivery to the suction pipe and/or the outflow.

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