EP0459603B1 - Process and apparatus for continuous drying of wood chips, wood fibres or other bulk materials - Google Patents

Abstract

The process according to the invention, in which the heat from flue gases generated in a combustion chamber is utilised in a drier charged with the material to be dried, is characterised in that the flue gas-free vapours leaving the drier are recirculated to the drier and, upstream of the drier, heated indirectly by the flue gases leaving the combustion chamber to the temperature required for drying the material to be dried, a vapour part-stream being branched off from this circulation and taken to the combustion chamber. <??>The apparatus for carrying out the process provides that, from the cyclone or cyclones (12) used in such processes for precipitating the dry material, a pipe (13) for the vapours leads back to the drier (8), that a heat exchanger (7) for the indirect heating of the vapours by the flue gases leaving the combustion chamber (1) is arranged in the pipe (13), and that a line (6), branching off from the pipe (13) upstream of the heat exchanger (7) in the direction of flow and leading to the combustion chamber (1), is provided. This invention makes it possible, at low cost, to maintain lower pollutant emission limits than those hitherto prescribed, to improve the efficiency and at the same time to reduce the explosion risk. <IMAGE>

Description

The invention relates to a method and a plant for the continuous drying of wood chips, wood fibers or other bulk materials, in which the heat of flue gases generated in a combustion chamber is used in a dryer charged with the material to be dried.

It is customary to introduce the flue gases directly into the dryer.

When wood shavings, wood fibers or other bulk materials are dried, organic pollutants are released which, in addition to the fine dust contained in the flue gases and vapors, are a burden on the environment. The exhaust gases leaving the dryer must therefore be cleaned before being discharged into the atmosphere. This is done by dedusting and, if necessary, by further, subsequent cleaning processes such as wet dedusting, electrostatic filters, cloth filters, gravel bed filters, chip bed filters and thermal afterburning. With such cleaning processes, the emissions can be reduced, but this requires high investment and operating costs.

DE-A-24 61 415 discloses a drying system with a dryer heated directly by flue gases, the flue gases being generated in a combustion chamber upstream of the dryer. The mixture of flue gases and vapors leaving the dryer, which is contaminated with pollutants, is cleaned by means of cyclone dusting and then divided into two partial flows. One part of the stream is returned to the combustion chamber and injected into the hottest zone for the combustion of the pollutants, so that there is intensive mixing with the flue gases, which are then fed back to the dryer. The other partial flow is released into the atmosphere via an exhaust pipe.

This method has the advantage of a reduced amount of exhaust gas, which means that fewer pollutants leave the system, but, with due regard to the legal regulations on emission limitation, it also does not manage without the additional cleaning methods described above.

From DE-A-23 60 467 a method and a device for drying solids contained in liquid with two independent combustion chambers is known. Part of the vapors is fed to the main combustion chamber, from which the combustion gases are led into the atmosphere. The auxiliary combustion chamber, which contains its own burner, is supplied with a portion of the vapors after preheating by the exhaust gases from the first combustion chamber in a heat exchanger, the combustion gases being introduced from the auxiliary combustion chamber into the dryer charged with the material to be dried. Here too, oxygen-containing combustion gases are used for drying, so that there is an increased risk of explosion when drying wood chips or wood fibers.

The object of the invention is to provide a method and a system of the type mentioned, whereby pollutant emission limit values can be maintained with less effort than previously, which are smaller than previously prescribed, and which increases the efficiency and at the same time the Risk of explosion is reduced.

According to the invention, this object is achieved by a method according to the teaching of claim 1 and by a system according to claim 9.

Accordingly, the vapors, which are polluted with pollutants but are free of flue gas, are returned to the dryer in the circuit and used as a drying agent for direct heating of the dryer. The heat content required for this is obtained by indirect heating before entering the dryer. It is heated with smoke gases that are generated in a combustion chamber.

At the same time, this combustion chamber is the 'lock' for a vapor partial flow which has to be removed from the circuit due to the mass balance to be maintained, i.e. pollutants can only leave the circuit via the combustion chamber. There they are burned and in this way provide part of the energy required to heat the circuit.

With this solution, it is possible to keep the emissions of solid, gaseous and / or condensed organic pollutants below the limit values of the statutory regulations without the additional cleaning processes otherwise required. The additional investment costs for the indirect heating of the circuit can be amortized at short notice due to the unprecedented overall cleaning effect. In addition, these costs are almost offset by savings due to comparatively small system parts such as dryers, separators, fans, filter devices and chimneys. It is particularly advantageous that the circuit in which the vapors move through the dryer is very low in oxygen due to the complete exclusion of flue gases, so that the risk of an explosion is greatly reduced. The low oxygen content in turn enables near-stoichiometric combustion, so that the fuel is used to a high degree and a high degree of efficiency can be achieved.

If it is difficult to mix the relatively cool vapor partial flow into the hot combustion chamber, a further vapor partial flow can be generated after the indirect heating of the Circuit branched from this and introduced into the combustion chamber. Since this vapor partial flow is heated, it can be mixed more effectively with the flue gases. In this case, the first vapor partial flow is reduced by the volume of the second vapor partial flow due to the mass balance to be maintained in the circuit.

Other advantageous features of the invention emerge from the further subclaims.

The invention is explained in more detail below using an exemplary embodiment. The accompanying drawing shows a basic circuit diagram of a system for the continuous drying of wood chips, wood fibers or the like.

In this system, a dryer 8, a separator 10, a fan 11, one or more cyclones 12 and a recuperative heat exchanger 7 are connected together in a circuit via pipes. Design measures ensure that the leakage in this circuit is low.

The system also has a combustion chamber 1 with a burner 2, which is connected to the heat exchanger 7 on the flue gas side, but is not part of the circuit. Supply lines for air 3, light oil or gas 4 and wood dust 5 lead to burner 2. The fuels can be burned individually or in any mixture.

The material to be dried is entered between the heat exchanger 7 and dryer 8 via a material lock, not shown, for example a rotary valve, and a pipeline 9 into the circuit. The material can also be fed directly into the entrance area of the dryer 8, which, as is customary for this application, can be a nozzle tube dryer, single-pass, multi-pass drum dryer or the like.

From the pipeline 13 arranged between the cyclone or cyclones 12 and the dryer 8 for returning the vapors to the dryer 8, a pipeline 6 branches off in the flow direction upstream of the heat exchanger 7 to the combustion chamber 1.

The system works as follows:

The flue gases generated in the combustion chamber 1 are fed to the heat exchanger 7. There is an indirect heat exchange between the hot flue gases and the relatively cool vapors which are circulated, as a result of which the vapors heat up to the process temperature required for drying the material to be dried. After this heat exchange process, the hot vapor stream is fed to the dryer 8 for direct heating via the section of the pipeline 13 arranged between the heat exchanger 7 and the dryer 8. During the drying process, further vapors and harmful ingredients are released from the material to be dried, which are removed from the dryer 8 together with the dried material. The coarser fractions of the dried material are separated in the subsequent separator. The fine fractions of the dried material remaining in the vapors are then fed together with the vapors via a blower 11 to one or more cyclones 12. This is where the fine fractions of the dried material are separated. The vapors leaving the cyclone (s) 12 are contaminated with fine dust and the pollutants formed during drying. They are returned to the dryer 8 via the pipeline 13, passing through the heat exchanger 7 and being heated up. This closes the almost inert vapor cycle.

Since new vapors containing pollutants are constantly formed in the circuit during the drying process, the circulating vapor flow must be continuously reduced for reasons of the mass balance to be observed technologically in the circuit. This is done by means of the branch line 6, via which a vapor partial flow passed into the combustion chamber 1 and burned there. In the event that the mixing of the relatively cool vapor partial flow into the hot flue gases of the combustion chamber 1 is not effective, a further branch line 14 can be led into the combustion chamber 1, which branches off from the circuit behind the heat exchanger 7. The heated vapors contained therein are easier to mix with the hot flue gases from the combustion chamber 1. The sum of the two vapor partial streams is always the same because of the mass balance to be maintained in the circuit.

The cooled flue gases leaving the heat exchanger 7 are conveyed by means of a fan 15 into a filter device 16, which is designed as a dry filter in the form of a bag filter or the like, where they are cleaned. This filter device can be omitted if ash-free fuel is used. After cleaning, the flue gases are passed into a heat exchanger 17 and used there to preheat the combustion air. This is fed to the burner 2 via the pipeline 19. After the heat exchanger 17, the flue gases are passed into the atmosphere via a chimney 18.

The invention is not only limited to the drying of wood chips, wood fibers or the like, but also for other bulk materials, e.g. Fish meal, peat, coal, grain and sewage sludge, applicable.

Claims (12)

1. A process for continuous drying of wood chips, or wood fibres, from flue gases generated in a separate combustion chamber by means of heat in a dryer loaded with the material to be dried characterised in that residual vapour from the dryer returns to the dryer in a flue gas free cycle, and immediately before the dryer passes over a heat exchanger through which the combustion chamber waste gases are heated to a temperature required for drying of material to be dried, whereby a subsidiary vapour stream is diverted from that cycle before the heat exchanger and is returned to the combustion chamber.
2. A process according to claim 1, characterised in that a further subsidiary vapour stream is diverted from the cycle following heat exchange.
3. A process according to one of the preceding claims, characterised in that a greater vapour current is flowing in the cycle than in the subsidiary vapour stream.
4. A process according to one of the preceding claims, characterised in that the entire combustion is substantially stoichiometric.
5. A process according to one of the preceding claims, characterised in that the primary combustion material for the generation of the flue gases comprises oil, preferably light oil, gas or combustible powder, either alone or optionally included in a mixture.
6. A process according to one of the preceding claims, characterised in that after the heat exchange the resulting cooled flue gases are purified in a dry filtration process.
7. A process according to one of the preceding claims, characterised in that the residual heat of the flue gases is used for preheating a burner of one of the combustion chambers for generation of flue gas combustion air flow.
8. A process according to one of the preceding claims, characterised in that the vapours flowing in the cycle are heated to drying operation temperature of between 400°C and 600°C.
9. Apparatus for carrying out a method according to one or more of the preceding claims, with a separate combustion chamber for generation of drying heat for a dryer, a feeding device for the material to be dried in the dryer, conveyor means for transfer of dried material and vapour from the dryer and one or more cyclones for separation of the dried material from the vapour, characterised in that a tubing (13) from the cyclones (12) returns the vapour separated from the flue gas to the dryer (8), a heat exchanger (7) is arranged in the tubing (13) for heat exchange between the combustion chamber waste flue gases and the vapour, and a branch (6) extends from the tubing (13) prior to the heat exchanger (7) in the direction of current flow to the combustion chamber (1).
10. Apparatus according to claim 9, characterised in that a ventilator (15) is arranged for removal of waste flue gases from the heat exchanger (7) to a chimney (18) above a filter device (16).
11. Apparatus according to claim 9, characterised in that the filter device (16) is formed as a drying filter in the form of a tube filter or the like.
12. Apparatus according to claim 11, characterised in that a heat exchanger (17) is adjacent to the filter device (16), by means of which the residual heat of the flue gases is used to heat the combustion atmosphere, which arrives in the burner (2) above a tubing (19).

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