EP0420859B1 - Process and apparatus for low-emission drying of wood chips - Google Patents

Process and apparatus for low-emission drying of wood chips Download PDF

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Publication number
EP0420859B1
EP0420859B1 EP89905579A EP89905579A EP0420859B1 EP 0420859 B1 EP0420859 B1 EP 0420859B1 EP 89905579 A EP89905579 A EP 89905579A EP 89905579 A EP89905579 A EP 89905579A EP 0420859 B1 EP0420859 B1 EP 0420859B1
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EP
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Prior art keywords
drier
combustion chamber
line
waste
stage
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EP89905579A
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German (de)
French (fr)
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EP0420859A1 (en
Inventor
Alfred Schmidt
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M Kaindl OG
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M Kaindl Holzindustrie KG
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B23/00—Heating arrangements
    • F26B23/02—Heating arrangements using combustion heating
    • F26B23/022—Heating arrangements using combustion heating incinerating volatiles in the dryer exhaust gases, the produced hot gases being wholly, partly or not recycled into the drying enclosure
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B3/00—Drying solid materials or objects by processes involving the application of heat
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F26—DRYING
    • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
    • F26B2200/00—Drying processes and machines for solid materials characterised by the specific requirements of the drying good
    • F26B2200/24—Wood particles, e.g. shavings, cuttings, saw dust

Definitions

  • the invention relates to a method and a device for low-emission drying of wood chips, in which the moist wood chips are pre-dried in a first preferably directly heated drying stage and post-dried in a second drying stage. Furthermore, the invention relates to a device for carrying out the method, with a first, preferably directly heated dryer for predrying the moist wood chips and a second dryer connected downstream thereof.
  • Series dryers for drying materials are e.g. known from DE-0S 26 40 508 and 35 34 260. Furthermore, the thermal cleaning of exhaust gases is known in principle from DE-PS 36 16 333. Furthermore, from DE-0S 28 21 689 a method and a device of the type mentioned are known. However, no statements are made about the degree of drying, the type of dryer heating and the treatment of the exhaust gases.
  • Wood chips are known to require the production of chipboard in large quantities, the wood chips first being mixed with a binder, usually a urea-formaldehyde resin, and then pressed into boards at high pressure and high temperature in presses. If there is too much water in the wood chips used, when the freshly pressed plates are removed from the presses, the plates are exploded by the suddenly expanding water vapor inside the plate. It is therefore necessary to pre-dry the wood shavings to a water content of at most 6% by weight (based on absolutely dry wood weight) before pressing.
  • a binder usually a urea-formaldehyde resin
  • the wood contains, in addition to cellulose, hemicellulose and lignin, resinous constituents (terpenes, predominantly ⁇ -pinene). Some of these substances have a relatively low boiling point and are also still volatile in water vapor. It is therefore the case that the exhaust gas from chip dryers also contains such substances in addition to water vapor. Although these substances are not harmful in the narrow sense, they do give the drying off of the dryer a characteristic unpleasant one smell. Although this load can be reduced by using the much resin-rich hardwood instead of the relatively resin-rich softwood, so that lower terpene emissions occur, another problem arises: in addition to the substances mentioned, the dryer also contains wood dust in the exhaust gas small amount included.
  • the dryer exhaust gas can also contain substances that have formed through thermal decomposition of one or more wood constituents (cellulose, lignins, resins, etc.), e.g. various aldehydes and acids, such as formaldehyde, acetaldehyde, acetic acid etc.
  • wood constituents cellulose, lignins, resins, etc.
  • aldehydes and acids such as formaldehyde, acetaldehyde, acetic acid etc.
  • the wood chips should not be dried using the usually customary, directly heated dryers, that is to say by direct drying with hot flue gases, but by means of indirectly heated dryers. Since the temperature of the heating medium (steam, pressurized water, heat transfer oil) can be reduced to a maximum of 200 ° C in indirectly heated dryers, whereas in the case of directly heated dryers, the flue gas inlet temperature into the dryer is usually around 400 to 600 ° C, a significantly reduced thermal decomposition of the wood components can be achieved with indirectly heated dryers, but the problem of terpene emissions remains unsolved, since their emission from the wood chips is largely independent of temperature.
  • the heating medium steam, pressurized water, heat transfer oil
  • the invention has for its object to provide a method for low-emission drying of wood chips of the type described above, in which these disadvantages are avoided and drying of the chips to a low final moisture content can be carried out without substantial emission of harmful substances.
  • the invention solves this problem in a method of the type mentioned in that the wood chips pre-dried in the first dryer stage to a moisture content of about 20 to 50% are finished dried in the second dryer stage downstream of this dryer stage to a moisture content of 1 to 6%, whereby the second dryer stage is indirectly heated so that the organic substances contained in the exhaust gases of both dryer stages are released into the atmosphere by oxidation at elevated temperature by heating the exhaust gases of both dryer stages in at least one combustion chamber, preferably in this combustion chamber as the first and a second, before the exhaust gases are released into the atmosphere , in relation to the exhaust gas flow in the combustion chamber in series, and that with the entire amount or a partial amount of the smoke or smoke leaving the first combustion chamber.
  • Exhaust gases are heated in the first dryer stage.
  • the procedure according to the invention enables a low-emission but economical drying process (device) for wood chips.
  • the thermal afterburning of the dryer vapors - both with direct as well as indirect heating of the dryer - would destroy the pollutants contained therein alone, but would require an economically unjustifiable energy consumption due to the high energy requirement.
  • the indirectly heated dryer stage also results in a gentler drying of the wood chips with simultaneous removal of water vapor from this dryer stage, which is not disturbed by flue gases.
  • waste heat from the post-combustion according to the invention is furthermore tied to the presence of an indirectly heated dryer.
  • the waste heat can be obtained in farm from steam or thermal oil; the use is based on the presence of a customer bound for this heat transfer medium, ie when used for drying an indirect heating of at least part of the drying.
  • the solid, liquid and gaseous argan substances contained in the exhaust gases are practically completely oxidized and an essentially pollutant-free exhaust gas is obtained.
  • the organic substances contained in the exhaust gases are heated to at least 700 ° C., preferably 700 to 1000 ° C.
  • the temperature required for the oxidation of the argan substances can be reduced if, in the context of the invention, the organic substances contained in the exhaust gases in the presence of oxidation catalysts, e.g. Platinum, chromium or copper oxide on ceramic supports are destroyed at elevated temperature, since this temperature can then be reduced to at least 300 ° C., in particular 300 to 550 ° C.
  • the exhaust gas from the first dryer stage can be completely returned to the first combustion chamber within the scope of the invention, the one leaving this combustion chamber Gas is divided into two partial flows, one of which is led to the first dryer stage, whereas the second is led into the second, in particular provided with a catalyst, combustion chamber. Since the gas coming from the first combustion chamber already has a sufficient temperature, no additional fuel is required in the second combustion chamber.
  • the exhaust gas from the first dryer stage in the second combustion chamber downstream of it must be brought to the required temperature with the addition of further fuel, that is to say reheated.
  • a device according to the invention of the type mentioned at the outset for carrying out the method according to the invention is characterized in that the second dryer is indirectly heated, that the two dryers are followed by a first combustion chamber and, if appropriate, a second combustion chamber for heating the exhaust gases of these two dryers, and in that the first dryer is connected via a line to the first combustion chamber for heating it with all or part of the waste or flue gases of this combustion chamber.
  • the process according to the invention can thus be carried out with little expenditure on equipment, the use of differently heated dryers, namely an indirectly heated dryer for the second drying stage, bringing favorable temperature conditions with regard to a favorable thermal efficiency of the drying, since the heat transfer medium is essential in the indirectly heated dryer is less hot than the flue gases used to heat a direct dryer.
  • a particularly advantageous embodiment of the device according to the invention consists in that a line for the exhaust gases leads from the second dryer into the first combustion chamber for heating the first dryer, from which a further line leads into the first combustion chamber or a line into the second combustion chamber for heating the exhaust gases.
  • the exhaust gases from the second dryer therefore pass one after the other into two combustion chambers, resulting in a complete combustion of those contained in them Is harmful.
  • a line can branch off from the line leading to the combustion chamber for heating the exhaust gases and leads back to the combustion chamber for heating the first dryer. There is also a double combustion for the exhaust gases flowing in the latter line.
  • the arrangement can be made within the scope of the invention in such a way that from the line which carries the exhaust gases for heating the first dryer , before the junction of an input line for the moist chips, branches off a line which leads into the second combustion chamber equipped with a catalyst.
  • a waste heat boiler can be connected to the combustion chamber for utilizing the heat of the exhaust gases, which has a waste heat line leading to the second dryer and possibly also to a chip combustion system. Furthermore, a line for the cooled exhaust gases can lead from this boiler to a condenser, to the gas discharge line of which a hot air line heated by the waste heat boiler is connected.
  • the plant shown in Fig. 1 should dry about 60 t of wood chips per hour with an initial moisture content of 100% water to an end moisture content of 2% water.
  • the moist wood chips are fed via an input line 1 with the aforementioned amount of 60 t / h to the drop lock of a first dryer 2, which is designed as a directly heated current dryer and is heated by a combustion chamber 3 via a line 32.
  • the combustion chamber 3 is fed via a line 4 with about 4 t / h of wood dust, such as is incurred as waste in the production of the wood chips, but also in the grinding of the finished wood chip boards.
  • This wood dust is burned together with the exhaust gases from the mentioned first dryer 2 and a second dryer 5, which exhaust gases are supplied via lines 6 and 7, respectively be and have temperatures of about 200 ° C and 100 ° C.
  • the flue gas developed in the combustion chamber 3 with a temperature of approximately 550 ° C. flows into the first dryer 2 and dries the moist wood chips to a moisture of approximately 30% water in the usual way.
  • the exhaust air from the dryer 2 is separated in a conventional material separator 8 from the wood chips carried along, which are fed via a line 9 to the second, indirectly heated dryer 5.
  • An exhaust gas line 10 is connected to the material separator 8, to which line 6, on the one hand, and a line 12 leading to a further combustion chamber 11, on the other hand, are connected.
  • the exhaust gases entering the combustion chamber 11 via the line 12 are heated there to about 750 to 800 ° C. with the aid of approximately 1400 Nm 3 / h of natural gas, which natural gas is supplied to the combustion chamber 11 via a line 13.
  • a waste heat boiler 14 is connected to the combustion chamber 11, in which a heat transfer oil is heated to a temperature of about 200 ° C. in a pipe system 15, which leaves the waste heat boiler 14 via a line 16 at this temperature.
  • This heat transfer oil is used on the one hand via a line 17 connected to line 16 for heating the presses used for pressing the chipboard, on the other hand line 16 leads to indirect heating 18 of the second dryer 5 and from there again to the pipe system 15 of the waste heat boiler 14 via a Line 19 back to which a line 20 coming from the presses is connected.
  • the waste heat boiler 14 there is also a further pipe system 21, in which air supplied via a line 22 is preheated, which on the one hand is fed as a purge air to the second dryer 5 at a temperature of about 120 ° C. and on the other hand at a temperature of about 200 ° C via a line 24 of an exhaust pipe 25 which leads to the chimney.
  • the second dryer 5 is designed as an indirectly heated tube dryer, in which the chips are brought to the desired final moisture content of about 2% water.
  • the hot air supplied via line 23 with an amount of approximately 34000 Nm 3 / h is used, the entire exhaust air flowing via line 7 into the combustion chamber 3.
  • the dry chips leave the dryer 5 in an amount of approximately 30.6 t / h via a discharge line 26.
  • the exhaust gas from a heat exchanger forming a narrow waste heat boiler 14 contains the entire amount of water in the chips and that from the lines 4 and 13 supplied fuels water in the form of steam. A considerable part of the heat content of this exhaust gas can be recovered in a condenser 27 to obtain hot water of approximately 75 ° C., the condenser 27 being connected via a line 28 to the outlet of the waste heat boiler 14. The hot water mentioned leaves the condenser 27 via a line 29, to which a branch line 30 leading to a receiving water can be connected.
  • the gases escaping from the chimney have a dew point of around 20 ° C and, when mixed with the ambient air, do not result in condensation in most weather conditions, i.e. they are not visible. In addition - apart from small amounts of wood ash - they contain practically no pollutants and cause practically no odor nuisance.
  • the amount of exhaust gas discharged through the chimney is about 60,000 Nm 3 / h
  • the amount of cooling water fed to the condenser 27 via a line 31 with a temperature of about 10 ° C. is about 300 m 3 / h.
  • the amount of heat supplied to the presses via line 17 is approximately 12.6 GJ / h.
  • a directly heated dryer 2 and as an indirectly heated dryer 5 constructions known per se can be used, which need not be described in detail.
  • a directly heated dryer has a hot air pipe heated by the combustion chamber 3, which connects the combustion chamber to the actual dryer and into which the wet chips are fed in via an inlet lock.
  • the incoming chips are transported to the pre-dryer section by the flue gases from the furnace.
  • the chips then enter a rotating dryer drum, which consists of tubes which are pushed together and firmly connected to one another and which are designed with lifting blades. From this drum, the chips pass over one Separator for the separation of heavy goods to the material separator 8.
  • a heating register formed by rotating tube bundles is provided as the heater 18, with lifting and transporting blades rolling or transporting the material to be dried through the dryer and often trickling it over the heating register.
  • the preheated fresh air is expediently blown into a central main pipe or laterally into a trough in which the heating register rotates.
  • the predried wood chips supplied via line 9 can also be fed in parallel to several indirectly heated dryers 5.
  • Such an arrangement can be expedient for performance reasons, the dryers 5 connected in parallel being fed uniformly with heat transfer oil from the waste heat boiler 14.
  • An electric filter (not shown) for fly ash which responds to wet gas, can be embedded in front of or behind the combustion chamber 11 in the exhaust gas stream. Such training is useful when there is a high dust content in the exhaust gases.
  • the embodiment according to FIG. 2 is similar to that according to FIG. 1, but warm air is used as an additional energy source for the combustion chambers 3, 11, which is obtained from the waste heat boiler 14 or its pipe system 21 via a line 34, which leads to the combustion chambers 3, 11 branches.
  • This embodiment like that according to FIG. 1, is intended for the case in which the temperature of the gas leaving the first combustion chamber 3 is not sufficient to oxidize the organic substances in the second combustion chamber 2, so that there the exhaust gas from the first dryer 2 in the combustion chamber 11 must be brought to the required temperature by supplying additional fuel (natural gas via line 13).
  • a line 33 branches off from the line 32, via which the exhaust gases of the combustion chamber 3 for heating the first dryer 2 are fed to it, before the opening of the input line 1 Combustion chamber 3 leads directly into the second combustion chamber 11, for example at a temperature of 500 ° C.
  • the exhaust gas from the first dryer stage 2 is completely returned from the material separator 8 via the line 6 into the combustion chamber 3 of the first stage and - as mentioned - the gas leaving this combustion chamber 3 is divided into two partial streams, which via the two lines 32 and .33 the dryer 2 or the combustion chamber 11 are supplied. Since the gas coming from the first combustion chamber 3 already has a sufficient temperature for heating the second combustion chamber 11 for the purpose of oxidizing the organic components, no additional fuel is required in the second combustion chamber 11.
  • the line 13 of the embodiments according to FIGS. 1 and 2 can therefore be omitted in the construction according to FIG. 3, as can the energy supply via line 34 for the combustion chamber 11.
  • a heat carrier oil boiler (not shown) can of course be switched on in the circuit of the heat transfer oil (lines 16, 19).
  • FIG. 1 further developments of the device according to the invention are described with dashed lines.
  • a condenser water line and / or a feed water line 42 is guided over a heat exchanger 34 in the waste heat boiler 14 in order to generate superheated high-pressure steam.
  • This is expanded in a counter-pressure turbine 35 and generates electricity in a generator 36.
  • the relaxed steam (line 38) can then be used for drying or for the heat requirement of presses.
  • the electricity generated (line 37) can cover the energy requirement of a chip processing system downstream of the second dryer stage or operate presses heated or heated at high frequency.
  • a heat exchanger 41 is arranged in the exhaust line of the material separator 8 in order to recover heat or to reduce the air volume that arises.

Abstract

A process for low-emission drying of wood chips comprises a first drier (2), heated directly by a combustion chamber (3). Connected in series to this drier (2) is a second, indirectly heated drier (5), the exhaust air from which is recycled to the combustion chamber (3) through a duct (7). The exhaust gases emitted by the drier (2) enter a second combustion chamber (11), where they are heated to at least 300°C to eliminate the noxious substances. The heat used for this purpose can be recovered by means of a waste heat boiler (14) and used to heat the second drier (5).

Description

Die Erfindung bezieht sich auf ein Verfahren und eine Vorrichtung zur emissionsarmen Trocknung von Holzspänen, bei welchem die feuchten Holzspäne in einer ersten vorzugsweise direkt beheizten Trockenstufe vorgetrocknet und in einer zweiten Trocknerstufe nachgetrocknet werden. Weiters bezieht sich die Erfindung auf eine Vorrichtung zur Durchführung des Verfahrens, mit einem ersten, vorzugsweise direkt beheizten Trockner zur Vortrockung der feuchten Holzpäne und einem diesem nachgeschalteten zweiten Trockner.The invention relates to a method and a device for low-emission drying of wood chips, in which the moist wood chips are pre-dried in a first preferably directly heated drying stage and post-dried in a second drying stage. Furthermore, the invention relates to a device for carrying out the method, with a first, preferably directly heated dryer for predrying the moist wood chips and a second dryer connected downstream thereof.

Hintereinandergeschaltete Trockner zur Trocknung von Materialien sind z.B. aus der DE-0S 26 40 508 und 35 34 260 bekannt. Ferner ist die thermische Reinigung von Abgasen prinzipiell aus der DE-PS 36 16 333 bekannt. Ferner sind aus der DE-0S 28 21 689 ein Verfahren und eine Vorrichtung der eingangs genannten Art bekannt. Aussagen über den Grad der Trocknung, die Art der Trocknerbeheizung und die Behandlung der Abgase werden allerdings nicht getroffen.Series dryers for drying materials are e.g. known from DE-0S 26 40 508 and 35 34 260. Furthermore, the thermal cleaning of exhaust gases is known in principle from DE-PS 36 16 333. Furthermore, from DE-0S 28 21 689 a method and a device of the type mentioned are known. However, no statements are made about the degree of drying, the type of dryer heating and the treatment of the exhaust gases.

Holzpäne werden bekanntlich in großer Menge die Erzeugung von Spanplatten benötigt, wobei die Holzpäne zunächst mit einem Bindemittel, zumeist einem Harnstoff-Formaldehydharz, vermischt und sodann bei hohen Druck und hoher Temperatur in Pressen zu Platten gepreßt werden. Ist in den eingesetzten Holzspänen zu viel Wasser enthalten, so kommt es bei der Entnahme der frisch gepreßten Platten aus den Pressen zu einer explosionsartigen Zerstörung der Platten durch den im Inneren der Platte befindlichen, plötzlich expandierenden Wasserdampf. Es ist daher erforderlich, die Holzspäne vor der Verpressung auf einen Wassergehalt von höchstens 6 Gew.-% (bezogen auf absolut trockenes Holzgewicht) vorzutrocknen. Da aber frische Holzspäne je nach Holzart, Jahreszeit der Schlägerung und Lagerbedingungen 50 bis 150 Gew.-%, im Durchschnitt etwa 100 Gew.-%, bezogen auf das Holztrockengewicht, Feuchtigkeit enthalten, erfordert die Trocknung der Holzspäne von diesem relativ hohen Wassergehalt auf die erforderlichen 1 bis 6 Gew.-% sehr viel Wärme, so daß bei technischen Trocknungsverfahren auf den thermischen Wirkungsgrad besonders zu achten ist.Wood chips are known to require the production of chipboard in large quantities, the wood chips first being mixed with a binder, usually a urea-formaldehyde resin, and then pressed into boards at high pressure and high temperature in presses. If there is too much water in the wood chips used, when the freshly pressed plates are removed from the presses, the plates are exploded by the suddenly expanding water vapor inside the plate. It is therefore necessary to pre-dry the wood shavings to a water content of at most 6% by weight (based on absolutely dry wood weight) before pressing. However, since fresh wood chips contain moisture depending on the type of wood, season of felling and storage conditions, 50 to 150% by weight, on average around 100% by weight, based on the dry wood weight, the drying of the wood chips requires this relatively high water content required 1 to 6% by weight of a great deal of heat, so that special attention must be paid to the thermal efficiency in technical drying processes.

Zudem ergibt sich bei der Trocknung der Holzpäne noch das weitere Problem, daß im Holz neben Cellulose, Hemicellulose und Lignin noch harzartige Bestandteile (Terpene, vorwiegend α-Pinen) enthalten sind. Ein Teil dieser Stoffe hat einen relativ niedrigen Siedepunkt und ist außerden noch wassendampfflüchtig. Es kommt daher dazu, daß im Abgas von Spantrockenanlagen neben dem Wasserdampf auch solche Stoffe enthalten sind. Obwohl diese Stoffe nicht schädlich im engeren Sinn sind, geben sie doch den Abdämpfen der Trockner einen charakteristischen unangenehmen ruch. Zwar kann diese Belastung dadurch gemindert werden, daß statt des relativ harzreichen Nadelholzes das wesentlich harzärmere Laubholz ein gesetzt wird, so daß eine geringere Terpenemission anfällt, doch tritt dabei ein anderes Problem auf: Neben den erwähnten Stoffen ist nämlich im Abgas der Trockner auch Holzstaub in geringer Menge enthalten. Es hat sich gezeigt, daß gewisse Holzstaubarten, vorwiegend jene von Buchen- oder Eichenholz, möglicherweise krebserregend sind, so daß bei Einsatz dieser Holzarten die zulässigen Grenzwerte für den Staubgehalt im Abgas auf sehr niedrige Werte herabgesetzt wurden. Die üblichen Entstaubungsvorrichtungen, z.B. Multizyklone, reichen hier nicht mehr aus, es müssen Spezialfilter, insbesondere Gewebefilter, eingesetzt werden, wobei aber der hohe Wassergehalt im Trocknerabgas zu häufigen Betriebsstörungen führt.In addition, when wood chips are dried, there is another problem that the wood contains, in addition to cellulose, hemicellulose and lignin, resinous constituents (terpenes, predominantly α-pinene). Some of these substances have a relatively low boiling point and are also still volatile in water vapor. It is therefore the case that the exhaust gas from chip dryers also contains such substances in addition to water vapor. Although these substances are not harmful in the narrow sense, they do give the drying off of the dryer a characteristic unpleasant one smell. Although this load can be reduced by using the much resin-rich hardwood instead of the relatively resin-rich softwood, so that lower terpene emissions occur, another problem arises: in addition to the substances mentioned, the dryer also contains wood dust in the exhaust gas small amount included. It has been shown that certain types of wood dust, predominantly those of beech or oak, may be carcinogenic, so that when these types of wood are used, the permissible limit values for the dust content in the exhaust gas have been reduced to very low values. The usual dedusting devices, such as multicyclones, are no longer sufficient here; special filters, in particular fabric filters, must be used, but the high water content in the dryer exhaust gas leads to frequent malfunctions.

Schließlich können im Trocknerabgas auch noch Stoffe enthalten sein, die sich durch thermische Zersetzung eines oder mehrerer Holzinhaltsstoffe (Cellulose, Lignine, Harze usw.) gebildet haben, so z.B. verschiedene Aldehyde und Säuren, wie Formaldehyd, Acetaldehyd, Essigsäure usw. Die Bildung solcher Stoffe tritt insbesondere bei hohen im Trockner verwendeten Trocknungstemperaturen auf, wie sie insbesondere bei direkt beheizten Trocknern zumeist vorliegen. Diese Stoffe sind zwar in der Regel nur in geringen Konzentrationen (etwa 10 bis 30 % von jenen der Terpene) im Abgas enthalten, sind aber doch hygienisch bedenklich und haben teilweise einen unangenehmen, stechenden Geruch, wirken also umweltbelastend.Finally, the dryer exhaust gas can also contain substances that have formed through thermal decomposition of one or more wood constituents (cellulose, lignins, resins, etc.), e.g. various aldehydes and acids, such as formaldehyde, acetaldehyde, acetic acid etc. The formation of such substances occurs in particular at high drying temperatures used in the dryer, as is usually the case with directly heated dryers. Although these substances are usually only present in low concentrations in the exhaust gas (about 10 to 30% of those of the terpenes), they are hygienically questionable and sometimes have an unpleasant, pungent odor, which means they are environmentally harmful.

Die bisherigen Versuche zur Abscheidung der störenden Stoffe aus den Abgasen von Holzspantrocknern blieben weitgehend erfolglos. Eine Waschung der Abgase mit Wasser zwecks Entfernung der organischen Stoffe ist zwar möglich, entfernt jedoch nur einen Teil der Schadstoffe, etwa 70 bis 80 %: verbleiben infolge des hohen Dampfdruckes der betreffenden Substanzen im Abgas. Außerdem muß das Waschwasser einer komplizierten Reinigung unterzogen werden, bevor es in den Vorfluter abgegeben werden kann.The previous attempts to separate the interfering substances from the exhaust gases from wood-chip dryers have largely been unsuccessful. Washing the exhaust gases with water to remove the organic substances is possible, but only removes some of the pollutants, about 70 to 80%: remain in the exhaust gas due to the high vapor pressure of the substances in question. In addition, the wash water has to undergo complicated cleaning before it can be discharged into the receiving water.

Es wurde ferner bereits vorgeschlagen, die Trocknung der Holzspäne nicht mittels der zumeist üblichen, direkt beheizten Trockner, also durch direkte Trocknung mit heißen Rauchgasen durchzuführen, sondern mittels indirekt beheizter Trockner. Da bei indirekt beheizten Trocknern die Temperatur des Heizmittels (Dampf, Druckwasser, Wärmeträgeröl) auf maximal 200°C abgesenkt werden kann, wogegen bei direkt beheizten Trocknern die Rauchgaseintrittstemperatur in den Trockner in der Regel bei 400 bis 600°C liegt, ist mit indirekt beheizten Trocknern eine erhebliche Verringerung der thermischen Zersetzung der Holzbestandteile erzielbar, jedoch bleibt das Problem der Terpenemissionen ungelöst, da deren Abgabe aus den Holzspänen weitgehend temperaturunabhängig ist.It has also already been proposed that the wood chips should not be dried using the usually customary, directly heated dryers, that is to say by direct drying with hot flue gases, but by means of indirectly heated dryers. Since the temperature of the heating medium (steam, pressurized water, heat transfer oil) can be reduced to a maximum of 200 ° C in indirectly heated dryers, whereas in the case of directly heated dryers, the flue gas inlet temperature into the dryer is usually around 400 to 600 ° C, a significantly reduced thermal decomposition of the wood components can be achieved with indirectly heated dryers, but the problem of terpene emissions remains unsolved, since their emission from the wood chips is largely independent of temperature.

Die Erfindung setzt sich zur Aufgabe, ein Verfahren zur emissionsarmen Trocknung von Holzspänen der eingangs geschilderten Art zu schaffen, bei welchem diese Nachteile vermieden sind und eine Trocknung der Späne auf eine geringe Endfeuchte ohne wesentliche Emission von schädlichen Stoffen durchführbar ist. Die Erfindung löst diese Aufgabe bei einem Verfahren der eingangs genannten Art dadurch, daß die in der ersten Trocknerstufe auf einen Feuchtigkeitsgehalt von etwa 20 bis 50 % vorgetrockneten Holzspäne in der dieser Trocknerstufe nachgeschalteten zweiten Trocknerstufe auf einen Feuchtigkeitsgehalt von 1 bis 6 % fertiggetrocknet werden, wobei die zweite Trocknerstufe indirekt beheizt wird, daß die in den Abgasen beider Trocknerstufen enthaltenen organischen Stoffe vor der Abgabe der Abgase in die Atmosphäre durch Oxydation bei erhöhter Temperatur durch Erhitzung der Abgase beider Trocknerstufen in zumindest einer Brennkammer, vorzugsweise in dieser Brennkammer als erste und einer zweiten, in Bezug auf den Abgasstrom in Serie liegenden Brennkammer, zerstört werden und daß mit der gesamten Menge oder einer Teilmenge der die erste Brennkammer verlassenden Rauch-bzw. Abgase die erste Trocknerstufe beheizt wird. Die erfindungsgemäße Vorgansweise ermöglicht eine emissionsarmes aber wirthschaftliches Trocknungsverfahren (Vorrichtung) für Holzspäne. Die thermische Nachverbrennung der Trocknerbrüden - und zwar sowohl bei direkter, als auch bei indirekter Beheizung der Trockner - würde allein zwar die darin enthaltenen Schadstoffe zerstören, würde aber wegen des sher hohen Energiebedarfes einen wirtschaftlich nicht zu rechtfertigenden Energievenbrauch bedingen.The invention has for its object to provide a method for low-emission drying of wood chips of the type described above, in which these disadvantages are avoided and drying of the chips to a low final moisture content can be carried out without substantial emission of harmful substances. The invention solves this problem in a method of the type mentioned in that the wood chips pre-dried in the first dryer stage to a moisture content of about 20 to 50% are finished dried in the second dryer stage downstream of this dryer stage to a moisture content of 1 to 6%, whereby the second dryer stage is indirectly heated so that the organic substances contained in the exhaust gases of both dryer stages are released into the atmosphere by oxidation at elevated temperature by heating the exhaust gases of both dryer stages in at least one combustion chamber, preferably in this combustion chamber as the first and a second, before the exhaust gases are released into the atmosphere , in relation to the exhaust gas flow in the combustion chamber in series, and that with the entire amount or a partial amount of the smoke or smoke leaving the first combustion chamber. Exhaust gases are heated in the first dryer stage. The procedure according to the invention enables a low-emission but economical drying process (device) for wood chips. The thermal afterburning of the dryer vapors - both with direct as well as indirect heating of the dryer - would destroy the pollutants contained therein alone, but would require an economically unjustifiable energy consumption due to the high energy requirement.

Die Ausführung der Trocknung in zwei Stufen verbessert zwar den energetischen Wirkungsgrad der Trocknung, bringt aber keine wesentliche Verringerung der Schadstoffemissionen mit sich. Die Kombination beider Maßnahmen ermöglicht es, die Trocknung der Späne sowohl emissionsarm, als auch mit vertretbarem Energieaufwand auszuführen, da die Abwärme der Nachverbrennung der Trocknerbrüden vollständig für die Trocknung genutzt werden kann.Carrying out the drying in two stages improves the energy efficiency of the drying, but does not result in a significant reduction in pollutant emissions. The combination of both measures enables the chips to be dried with low emissions as well as with a reasonable amount of energy, since the waste heat from the afterburning of the dryer vapors can be fully used for drying.

Die indirekt beheizte Trocknerstufe bewirkt ferner eine schonendere Trocknung der Holzspäne bei gleichzeitiger durch Rauchgase ungestörter Wasserdampfabfuhr aus dieser Trocknerstufe.The indirectly heated dryer stage also results in a gentler drying of the wood chips with simultaneous removal of water vapor from this dryer stage, which is not disturbed by flue gases.

Die Mäglichkeit der erfindungsgemäßen Nutzung der Abwärme der Nachverbrennung ist ferner an das Vorhandensein eines indirekt beheizten Trockners gebunden. Die Abwärme kann in Farm von Dampf oder von Wärmeträgeröl gewonnen werden; die Nutzung ist an das Vorhandensein eines Abnehmers für diesen Wärmeträger gebunden, d.h. bei Nutzung für die Trocknung an eine indirekte Beheizung zumindest eines Teiles der Trocknung. Die in den Abgasen enthaltenen festen, flüssigen und gasförmigen arganischen Stoffe werden praktisch vollständig oxydiert und es wird ein im wesentlichen schadstofffreies Abgas erhalten.The possibility of using the waste heat from the post-combustion according to the invention is furthermore tied to the presence of an indirectly heated dryer. The waste heat can be obtained in farm from steam or thermal oil; the use is based on the presence of a customer bound for this heat transfer medium, ie when used for drying an indirect heating of at least part of the drying. The solid, liquid and gaseous argan substances contained in the exhaust gases are practically completely oxidized and an essentially pollutant-free exhaust gas is obtained.

Im Rahmen der Erfindung werden die in den Abgasen enthaltenen organischen Stoffe auf zumindest 700°C, vorzugsweise 700 bis 1000°C, erhitzt. Es kann jedoch die zur Oxydation der arganischen Stoffe erforderliche Temperatur gesenkt werden, wenn im Rahmen der Erfindung die in den Abgasen enthaltenen organischen Stoffe im Beisein von Oxydationskatalysatoren, z.B. Platin, Chrom- ader Kupferoxyd auf keramischen Trägern bei erhöhter Temperatur zerstört werden, da dann diese Temperatur auf zumindest 300°C, insbesondere 300 bis 550°C, abgesenkt werden kann.In the context of the invention, the organic substances contained in the exhaust gases are heated to at least 700 ° C., preferably 700 to 1000 ° C. However, the temperature required for the oxidation of the argan substances can be reduced if, in the context of the invention, the organic substances contained in the exhaust gases in the presence of oxidation catalysts, e.g. Platinum, chromium or copper oxide on ceramic supports are destroyed at elevated temperature, since this temperature can then be reduced to at least 300 ° C., in particular 300 to 550 ° C.

Reicht die Temperatur des die erste Brennkammer verlassenden Gases, insbesondere bei Verwendung von Katalysatoren zur Oxydation der organischen Stoffe in der zweiten Brennkammer aus, so kann im Rahmen der Erfindung das Abgas der ersten Trocknerstufe vollständig in die erste Brennkammer rückgeführt werden, wobei das diese Brennkammer verlassende Gas in zwei Teilströme geteilt wird, von denen der eine zur ersten Trocknerstufe geführt wird, wogegen der zweite in die zweite, insbesondere mit einem Katalysator versehene Brennkammer geführt wird. Da das aus der ersten Brennkammer kammende Gas bereits eine ausreichende Temperatur aufweist, ist in der zweiten Brennkammer kein zusätzlicher Brennstoff erforderlich.If the temperature of the gas leaving the first combustion chamber is sufficient, in particular when using catalysts for the oxidation of the organic substances in the second combustion chamber, the exhaust gas from the first dryer stage can be completely returned to the first combustion chamber within the scope of the invention, the one leaving this combustion chamber Gas is divided into two partial flows, one of which is led to the first dryer stage, whereas the second is led into the second, in particular provided with a catalyst, combustion chamber. Since the gas coming from the first combustion chamber already has a sufficient temperature, no additional fuel is required in the second combustion chamber.

Reicht hingegen die Temperatur des die erste Brennkammer verlassenden Gases zur Oxydation der organischen Stoffe nicht aus, so muß das Abgas der ersten Trocknerstufe in der ihr nachgeschalteten zweiten Brennkammer unter Zufuhr weiteren Brennstoffes auf die erforderliche Temperatur gebracht, also nachgeheizt werden.If, on the other hand, the temperature of the gas leaving the first combustion chamber is not sufficient to oxidize the organic substances, the exhaust gas from the first dryer stage in the second combustion chamber downstream of it must be brought to the required temperature with the addition of further fuel, that is to say reheated.

Die Verwendung zweier, in Bezug auf den Durchlauf der zu trocknenden Holzspäne in Serie geschalteten Trockenstufen ergibt durch die Verwendung der indirekt beheizten zweiten Trockenstufe die Möglichkeit, das beschriebene Verfahren besonders energiesparend durchzuführen. Hiebei ist es gemäß einer bevorzugten Variante des erfindungsgemäßen Verfahrens möglich, die Abwärme der erhitzten Abgase zumindest zur teilweisen Beheizung der zweiten Trocknerstufe zu verwenden. Hiebei ist es im Rahmen der Erfindung besonders günstig, die Trocknung der Holzspäne in der ersten Trocknungsstufe so weit erfolgen zu lassen, daß die aus der Erhitzung der Abgase verfügbare Abwärme zumindest zur Deckung des Wärmebedarfes der zweiten Trocknungsstufe ausreicht. Besonders günstig ist es im Rahmen der Erfindung, dies so durchzuführen, daß auch der Wärmebedarf einer der zweiten Trocknerstufe nachgeschalteten Späneverarbeitungsanlage, insbesondere der Wärmebedarf von Pressen, gedeckt wird. Dies kann in einfacher Weise durch eine in den gereinigten Abdampf der Trockner eingeschaltete weitere Stufe durchgeführt werden, welche die in diesem Abdampf enthaltene Wärme rückgewinnt. Durch eine solche Ausführung der Holzspantrocknung gelingt es, die Trocknung mit hohem thermischen Wirkungsgrad, dabei aber praktisch emissionsfrei zu führen, denn die Feuchte der Holzspäne, die aus der ersten Trocknerstufe austreten, ist durch entsprechende Regelung der Beheizung dieser Trockenstufe wählbar. Auf diese Weise ist es möglich, die Trocknung an die wechselnde Eingangsfeuchte der frischen Holzspäne und an den wechselnden Wärmebedarf von Plattenpressen anzupassen, dabei aber den Brennstoffverbrauch minimal, d.h. den thermischen Wirkungsgrad der Trocknung optimal zu halten.The use of two drying stages connected in series with respect to the passage of the wood chips to be dried results in the possibility of carrying out the described method in a particularly energy-saving manner by using the indirectly heated second drying stage. Here, according to a preferred variant of the method according to the invention, it is possible to use the waste heat of the heated exhaust gases at least for partial heating of the second dryer stage. Hiebei it is particularly favorable in the context of the invention, the drying of the wood chips in the first Allow drying stage to take place so far that the waste heat available from the heating of the exhaust gases is at least sufficient to cover the heat requirement of the second drying stage. It is particularly favorable within the scope of the invention to do this in such a way that the heat requirement of a chip processing system downstream of the second dryer stage, in particular the heat requirement of presses, is also covered. This can be carried out in a simple manner by means of a further stage which is switched on in the cleaned exhaust steam of the dryer and which recovers the heat contained in this exhaust steam. Such a design of wood chip drying enables drying to be carried out with a high thermal efficiency, but practically emission-free, because the moisture in the wood chips emerging from the first dryer stage can be selected by regulating the heating of this drying stage accordingly. In this way it is possible to adapt the drying to the changing input moisture of the fresh wood chips and to the changing heat requirements of plate presses, while keeping fuel consumption minimal, ie keeping the thermal efficiency of the drying optimal.

Eine erfindungsgemäße Vorrichtung der eingangs genannten Art zur Duchführung des erfindungsgemäßen Verfahrens ist dadurch gekennzeichnet, daß der zweite Trockner indirekt beheizt ist, daß den beiden Trocknern eine erste Brennkammer und gegebenenfalls eine zweite Brennkammer zur Erhitzung der Abgase dieser beiden Trockner nachgeschaltet ist und daß der erste Trockner über eine Leitung an die erste Brennkammer für seine Erwärmung mit der gesamten Menge oder einer Teilmenge der Ab- bzw. Rauchgase dieser Brennkammer angeschlossen ist. Damit läßt sich mit geringen apparativem Aufwand das erfindungsgemäße Verfahren durchführen, wobei die Verwendung unterschiedlich beheizter Trockner, nämlich eines indirekt beheizten Trockners für die zweite Trockenstufe, günstige Temperaturbedingungen im Hinblick auf einen günstigen thermischen Wirkungsgrad der Trocknung bringt, da im indirekt beheizten Trockner das Wärmeträgermedium wesentlich weniger heiß ist als die zur Beheizung eines direkten Trockners eingesetzten Rauchgase.A device according to the invention of the type mentioned at the outset for carrying out the method according to the invention is characterized in that the second dryer is indirectly heated, that the two dryers are followed by a first combustion chamber and, if appropriate, a second combustion chamber for heating the exhaust gases of these two dryers, and in that the first dryer is connected via a line to the first combustion chamber for heating it with all or part of the waste or flue gases of this combustion chamber. The process according to the invention can thus be carried out with little expenditure on equipment, the use of differently heated dryers, namely an indirectly heated dryer for the second drying stage, bringing favorable temperature conditions with regard to a favorable thermal efficiency of the drying, since the heat transfer medium is essential in the indirectly heated dryer is less hot than the flue gases used to heat a direct dryer.

Eine besonders vorteilhafte Ausführungsform der erfindungsgemäßen Vorrichtung besteht darin, daß vom zweiten Trockner eine Leitung für die Abgase in die erste Brennkammer für die Beheizung des ersten Trockners führt, aus dem eine weitere Leitung in die erste Brennkammer bzw. eine Leitung in die zweite Brennkammer zur Erhitzung der Abgase führt. Die Abgase aus den zweiten Trockner gelangen daher hintereinander in zwei Brennkammern, was einer völligen Verbrennung der in ihnen enthaltenen Schadstoffe förderlich ist. Falls gewünscht, kann von der zur Brennkammer für die Erhitzung der Abgase führenden Leitung eine Leitung abzweigen, die zurück zur Brennkammer für die Beheizung des ersten Trockners führt. Für die in letzterer Leitung strömenden Abgase ergibt sich ebenfalls eine zweifache Verbrennung. Ist jedoch die Temperatur des die erste Brennkammer verlassenden Gases für die Oxydation der organischen Stoffe in der zweiten Brennkammer ausreichend hoch, so kann im Rahmen der Erfindung die Anordnung so getroffen sein, daß von der Leitung, welche die Abgase für die Beheizung des ersten Trockners führt, vor der Einmündung einer Eingabeleitung für die feuchten Späne eine Leitung abzweigt, welche in die zweite, mit einem Katalysator ausgerüstete Brennkammer führt.A particularly advantageous embodiment of the device according to the invention consists in that a line for the exhaust gases leads from the second dryer into the first combustion chamber for heating the first dryer, from which a further line leads into the first combustion chamber or a line into the second combustion chamber for heating the exhaust gases. The exhaust gases from the second dryer therefore pass one after the other into two combustion chambers, resulting in a complete combustion of those contained in them Is harmful. If desired, a line can branch off from the line leading to the combustion chamber for heating the exhaust gases and leads back to the combustion chamber for heating the first dryer. There is also a double combustion for the exhaust gases flowing in the latter line. However, if the temperature of the gas leaving the first combustion chamber is sufficiently high for the oxidation of the organic substances in the second combustion chamber, the arrangement can be made within the scope of the invention in such a way that from the line which carries the exhaust gases for heating the first dryer , before the junction of an input line for the moist chips, branches off a line which leads into the second combustion chamber equipped with a catalyst.

Wie bereits erwähnt, ist es besonders vorteilhaft, die Abwärme der aus der Brennkammer zur Erhitzung der Abgase abströmende Wärme nutzbringend zu verwerten. Hiezu kann im Rahmen der Erfindung an die Brennkammer für die Ausnutzung der Hitze der Abgase ein Abhitzekessel angeschlossen sein, der eine zum zweiten Trockner und gegebenenfalls auch zu einer Späneverbrennungsanlage führende Abwärmeleitung hat. Weiters kann von diesem Kessel eine Leitung für die abgekühlten Abgase zu einem Kondensator führen, an dessen Gasabfuhrleitung gegebenenfalls eine vom Abhitzekessel beheizte Warmluftleitung angeschlossen ist.As already mentioned, it is particularly advantageous to utilize the waste heat of the heat flowing out of the combustion chamber to heat the exhaust gases in a useful manner. For this purpose, within the scope of the invention, a waste heat boiler can be connected to the combustion chamber for utilizing the heat of the exhaust gases, which has a waste heat line leading to the second dryer and possibly also to a chip combustion system. Furthermore, a line for the cooled exhaust gases can lead from this boiler to a condenser, to the gas discharge line of which a hot air line heated by the waste heat boiler is connected.

Wenn im Vorstehenden von "Abgasen" der Trocknerstufen gesprochen ist, so sollen darunter auch die dampfförmigen Anteile verstanden werden.If the foregoing speaks of "exhaust gases" from the dryer stages, this should also be understood to mean the vaporous fractions.

In der Zeichnung ist das erfindungsgemäße Verfahren an Hand von schematischen Darstellungen dreier zur Durchführung dieses Verfahrens geeigneten Vorrichtungen näher erläutert, die in den Fig.1,2 und 3 dargestellt sind.In the drawing, the method according to the invention is explained in more detail with the aid of schematic representations of three devices suitable for carrying out this method, which are shown in FIGS. 1, 2 and 3.

Die in Fig.1 dargestellte Anlage soll etwa 60 t Holzspäne pro Stunde mit einer Anfangsfeuchte von 100 % Wasser auf eine Endfeuchte von 2 % Wasser trocknen. Hiezu werden die feuchten Holzspäne über eine Eingabeleitung 1 mit der erwähnten Menge von 60 t/h der Einfallschleuse eines als direkt beheizter Stromtrockner ausgeführten ersten Trockners 2 zugeführt, der von einer Brennkammer 3 über eine Leitung 32 beheizt wird. Die Brennkammer 3 wird über eine Leitung 4 mit etwa 4 t/h Holzstaub beschickt, wie er als Abfall bei der Herstellung der Holzspäne, aber auch beim Schleifen der fertigen Holzspanplatten anfällt. Dieser Holzstaub wird zusammen mit den Abaasen des erwähnten ersten Trockners 2 und eines zweiten Trockners 5 verbrannt, welche Abgase über Leitungen 6 bzw.7 zugeführt werden und Temperaturen von etwa 200°C bzw. 100°C aufweisen. Das in der Brennkammer 3 entwickelte Rauchgas mit einer Temperatur von etwa 550°C strömt in den ersten Trockner 2 und trocknet dort in üblicher Weise die feuchten Holzspäne auf eine Feuchte von etwa 30 % Wasser. Die Abluft des Trockners 2 wird in einem üblichen Materialabscheider 8 von den mitgeführten Holzspänen getrennt, welche über eine Leitung 9 dem zweiten, indirekt beheizten Trockner 5 zugeführt werden. An den Materialabscheider 8 ist eine Abgasleitung 10 angeschlossen, an welche einerseits die Leitung 6, anderseits eine zu einer weiteren Brennkammer 11 führende Leitung 12 angeschlossen sind. Die über die Leitung 12 in die Brennkammer 11 gelangenden Abgase werden dort mit Hilfe von etwa 1400 Nm³/h Erdgas auf etwa 750 bis 800°C erhitzt, welches Erdgas über eine Leitung 13 der Brennkammer 11 zugeführt wird. An die Brennkammer 11 ist ein Abhitzekessel 14 angeschlossen, in welchem in einem Rohrsystem 15 ein Wärmeträgeröl auf eine Temperatur von etwa 200°C erhitzt wird, das den Abhitzekessel 14 über eine Leitung 16 mit dieser Temperatur verläßt. Dieses Wärmeträgeröl wird einerseits über eine an die Leitung 16 angeschlossene Leitung 17 zur Beheizung der für die Verpressung der Holzspanplatten verwendeten Pressen verwendet, anderseits führt die Leitung 16 zur indirekten Heizung 18 des zweiten Trockners 5 und von dort wieder zum Rohrsystem 15 des Abhitzekessels 14 über eine Leitung 19 zurück, an welche eine von den Pressen kommende Leitung 20 angeschlossen ist. Im Abhitzekessel 14 befindet sich ferner ein weiteres Rohrsystem 21, in welchem über eine Leitung 22 zugeführte Luft vorgewärmt wird, die einerseits mit einer Temperatur von etwa 120°C über eine Leitung 23 als Spülluft dem zweiten Trockner 5 zugeführt wird, anderseits mit einer Temperatur von etwa 200°C über eine Leitung 24 einer Abgasleitung 25, die zum Kamin führt.The plant shown in Fig. 1 should dry about 60 t of wood chips per hour with an initial moisture content of 100% water to an end moisture content of 2% water. For this purpose, the moist wood chips are fed via an input line 1 with the aforementioned amount of 60 t / h to the drop lock of a first dryer 2, which is designed as a directly heated current dryer and is heated by a combustion chamber 3 via a line 32. The combustion chamber 3 is fed via a line 4 with about 4 t / h of wood dust, such as is incurred as waste in the production of the wood chips, but also in the grinding of the finished wood chip boards. This wood dust is burned together with the exhaust gases from the mentioned first dryer 2 and a second dryer 5, which exhaust gases are supplied via lines 6 and 7, respectively be and have temperatures of about 200 ° C and 100 ° C. The flue gas developed in the combustion chamber 3 with a temperature of approximately 550 ° C. flows into the first dryer 2 and dries the moist wood chips to a moisture of approximately 30% water in the usual way. The exhaust air from the dryer 2 is separated in a conventional material separator 8 from the wood chips carried along, which are fed via a line 9 to the second, indirectly heated dryer 5. An exhaust gas line 10 is connected to the material separator 8, to which line 6, on the one hand, and a line 12 leading to a further combustion chamber 11, on the other hand, are connected. The exhaust gases entering the combustion chamber 11 via the line 12 are heated there to about 750 to 800 ° C. with the aid of approximately 1400 Nm 3 / h of natural gas, which natural gas is supplied to the combustion chamber 11 via a line 13. A waste heat boiler 14 is connected to the combustion chamber 11, in which a heat transfer oil is heated to a temperature of about 200 ° C. in a pipe system 15, which leaves the waste heat boiler 14 via a line 16 at this temperature. This heat transfer oil is used on the one hand via a line 17 connected to line 16 for heating the presses used for pressing the chipboard, on the other hand line 16 leads to indirect heating 18 of the second dryer 5 and from there again to the pipe system 15 of the waste heat boiler 14 via a Line 19 back to which a line 20 coming from the presses is connected. In the waste heat boiler 14 there is also a further pipe system 21, in which air supplied via a line 22 is preheated, which on the one hand is fed as a purge air to the second dryer 5 at a temperature of about 120 ° C. and on the other hand at a temperature of about 200 ° C via a line 24 of an exhaust pipe 25 which leads to the chimney.

Der zweite Trockner 5 ist als indirekt beheizter Röhrentrockner ausgeführt, in welchem die Späne auf die gewünschte Endfeuchte von etwa 2 % Wasser gebracht werden. Zur Entfernung des verdampften Wassers, dessen Menge etwa 8,4 t/h beträgt, wird die über die Leitung 23 mit einer Menge von etwa 34000 Nm³/h zugeführte Warmluft verwendet, wobei die gesamte Abluft über die Leitung 7 in die Brennkammer 3 strömt. Die trockenen Späne verlassen über eine Abfuhrleitung 26 den Trockner 5 in einer Menge von etwa 30,6 t/h.The second dryer 5 is designed as an indirectly heated tube dryer, in which the chips are brought to the desired final moisture content of about 2% water. To remove the evaporated water, the amount of which is approximately 8.4 t / h, the hot air supplied via line 23 with an amount of approximately 34000 Nm 3 / h is used, the entire exhaust air flowing via line 7 into the combustion chamber 3. The dry chips leave the dryer 5 in an amount of approximately 30.6 t / h via a discharge line 26.

Das Abgas des einen Wärmetauscher bildengen Abhitzekessel 14 enthält die gesamte Wassermenge der Späne und das aus den über die Leitungen 4 und 13 zugeführten Brennstoffen entstandene Wasser in Form von Dampf. Ein erheblicher Teil des Wärmeinhaltes dieses Abgases kann in einem Kondensator 27 unter Gewinnung von Heißwasser von etwa 75°C rückgewonnen werden, wobei der Kondensator 27 über eine Leitung 28 an den Ausgang des Abhitzekessels 14 angeschlossen ist. Das erwähnte Heißwasser verläßt den Kondensator 27 über eine Leitung 29, an welche eine zu einem Vorfluter führende Abzweigleitung 30 angeschlossen sein kann.The exhaust gas from a heat exchanger forming a narrow waste heat boiler 14 contains the entire amount of water in the chips and that from the lines 4 and 13 supplied fuels water in the form of steam. A considerable part of the heat content of this exhaust gas can be recovered in a condenser 27 to obtain hot water of approximately 75 ° C., the condenser 27 being connected via a line 28 to the outlet of the waste heat boiler 14. The hot water mentioned leaves the condenser 27 via a line 29, to which a branch line 30 leading to a receiving water can be connected.

Da durch die erwähnte Wärmerückgewinnung im Kondensator 27 die über die Leitung 28 zugeführten Abgase erheblich abgekühlt werden, muß das über die Abgasleitung 25 aus dem Kondensator 27 abgeführte, gekühlte Abgas vor Austritt in die Atmosphäre wieder etwas aufgewärmt werden, um einen entsprechenden Auftrieb im Kamin zu erzeugen. Dies erfolgt zweckmäßig durch Zumischung einer geringen Menge (maximal 10.000 Nm³/h) an vorgewärmter Luft über die Leitung 24.Since the exhaust gases supplied via line 28 are cooled considerably by the aforementioned heat recovery in the condenser 27, the cooled exhaust gas discharged from the condenser 27 via the exhaust line 25 must be warmed up a little before being released into the atmosphere in order to provide a corresponding buoyancy in the chimney produce. This is expediently carried out by admixing a small amount (maximum 10,000 Nm 3 / h) of preheated air via line 24.

Die am Kamin austretenden Gase weisen einen Taupunkt von etwa 20°C auf und ergeben bei der Vermischung mit der Umgebungsluft bei den meisten Witterungsbedingungen keine Kondensation, d.h. sie sind nicht sichtbar. Außerdem enthalten sie - abgesehen von geringen Mengen an Holzasche - praktisch keine Schadstoffe und verursachen praktisch keine Geruchsbelästigung.The gases escaping from the chimney have a dew point of around 20 ° C and, when mixed with the ambient air, do not result in condensation in most weather conditions, i.e. they are not visible. In addition - apart from small amounts of wood ash - they contain practically no pollutants and cause practically no odor nuisance.

Die durch den Kamin abgeführte Abgasmenge beträgt etwa 60 000 Nm³/h, die dem Kondensator 27 über eine Leitung 31 zugeführte Menge an Kühlwasser mit einer Temperator von etwa 10°C beträgt etwa 300 m³/h. Die über die Leitung 17 den Pressen zugeführte Wärmemenge beträgt etwa 12,6 GJ/h.The amount of exhaust gas discharged through the chimney is about 60,000 Nm 3 / h, the amount of cooling water fed to the condenser 27 via a line 31 with a temperature of about 10 ° C. is about 300 m 3 / h. The amount of heat supplied to the presses via line 17 is approximately 12.6 GJ / h.

Als direkt beheizter Trockner 2 und als indirekt beheizter Trockner 5 können Konstruktionen an sich bekannter Bauart verwendet werden, die nicht im Detail beschrieben zu werden brauchen. Wie bekannt, hat ein direkt beheizter Trockner eine von der Brennkammer 3 beheizte Heißaasrohrleitung, welche die Brennkammer mit dem eigentlichen Trockner verbindet und in welche über eine Einfallschleuse die Aufgabe der nassen Späne erfolgt. Der Transport der einfallenden Späne in die Vortrockenstrecke erfolgt durch die Rauchgase aus der Feuerung. Die Späne gelangen sodann in eine rotierende Trocknertrommel, die aus ineinandergeschobenen und fest miteinander verbundenen Rohren besteht, die mit Hubschaufeln ausgebildet sind. Von dieser Trommel gelangen die Späne über einen Abscheider zur Abtrennung von Schwergut zum Materialabscheider 8.As a directly heated dryer 2 and as an indirectly heated dryer 5, constructions known per se can be used, which need not be described in detail. As is known, a directly heated dryer has a hot air pipe heated by the combustion chamber 3, which connects the combustion chamber to the actual dryer and into which the wet chips are fed in via an inlet lock. The incoming chips are transported to the pre-dryer section by the flue gases from the furnace. The chips then enter a rotating dryer drum, which consists of tubes which are pushed together and firmly connected to one another and which are designed with lifting blades. From this drum, the chips pass over one Separator for the separation of heavy goods to the material separator 8.

Bei einem indirekt beheizten Trockner ist bekanntlich ein von rotierenden Röhrenbündeln gebildetes Heizregister als Heizung 18 vargesehen, wobei Hub- und Transportschaufeln das zu trocknende Material durch den Trockner wälzen bzw. transportieren und oftmalig über die Heizregister rieseln lassen. Die Einblasung vorgewärmter Frischluft erfolgt zweckmäßig in ein zentrales Hauptrohr oder seitlich in eine Mulde, in welcher das Heizregister umläuft.In the case of an indirectly heated dryer, as is known, a heating register formed by rotating tube bundles is provided as the heater 18, with lifting and transporting blades rolling or transporting the material to be dried through the dryer and often trickling it over the heating register. The preheated fresh air is expediently blown into a central main pipe or laterally into a trough in which the heating register rotates.

Anstelle eines einzigen Trockners 5 können die über die Leitung 9 zugeführten vorgetrockneten Holzspäne auch mehreren indirekt beheizten Trocknern 5 parallel zugeführt werden. Eine solche Anordnung kann aus Leistungsgründen zweckmäßig sein, wobei die einander parallel geschalteten Trockner 5 vom Abhitzekessel 14 gleichmässig mit Wärmeträgeröl beschickt werden.Instead of a single dryer 5, the predried wood chips supplied via line 9 can also be fed in parallel to several indirectly heated dryers 5. Such an arrangement can be expedient for performance reasons, the dryers 5 connected in parallel being fed uniformly with heat transfer oil from the waste heat boiler 14.

Vor oder hinter der Brennkammer 11 kann in den Abgasstrom ein nicht dargestelltes Elektrafilter für Flugasche eingebettet sein, das auf Naßgas anspricht. Eine solche Ausbildung ist bei hohem Staubanteil der Abgase zweckmäßig.An electric filter (not shown) for fly ash, which responds to wet gas, can be embedded in front of or behind the combustion chamber 11 in the exhaust gas stream. Such training is useful when there is a high dust content in the exhaust gases.

Das Ausführungsbeispiel nach Fig.2 ähnelt jenem nach Fig.1, jedoch wird als zusätzliche Energiequelle für die Brennkammern 3,11 Warmluft herangezogen, welche vom Abhitzekessel 14 bzw. dessen Rohrsystem 21 über eine Leitung 34 bezogen wird, die sich zu den Brennkammern 3,11 verzweigt. Diese Ausführungsform ist ebenso wie jene nach Fig.1 für den Fall gedacht, daß die Temperatur des die erste Brennkammer 3 verlassenden Gases zur Oxydation der organischen Stoffe in der zweiten Brennkammer 2 nicht ausreicht, so daß dort das Abgas des ersten Trockners 2 in der Brennkammer 11 durch Zufuhr von weiterem Brennstoff (Erdgas über die Leitung 13) auf die erforderliche Temperatur gebracht werden muß.The embodiment according to FIG. 2 is similar to that according to FIG. 1, but warm air is used as an additional energy source for the combustion chambers 3, 11, which is obtained from the waste heat boiler 14 or its pipe system 21 via a line 34, which leads to the combustion chambers 3, 11 branches. This embodiment, like that according to FIG. 1, is intended for the case in which the temperature of the gas leaving the first combustion chamber 3 is not sufficient to oxidize the organic substances in the second combustion chamber 2, so that there the exhaust gas from the first dryer 2 in the combustion chamber 11 must be brought to the required temperature by supplying additional fuel (natural gas via line 13).

Fig.3 zeigt eine Anlage, welche für den Fall gedacht ist, daß die Temperatur des die erste Brennkammer 3 verlassenden Gases zur Oxydation der organischen Stoffe in der zweiten Brennkammer 11 ausreicht. Dem kann insoferne nachgeholfen werden, als die zweite Brennkammer 11 mit Oxydationskatalysatoren ausgerüstet ist, durch welche die zur Oxydation der organischen Stoffe erforderliche Temperatur auf etwa 300 bis 550°C abgesenkt werden kann. Für diesen Fall zweigt von der Leitung 32, über welche die Abgase der Brennkammer 3 für die Beheizung des ersten Trockners 2 diesem zugeführt werden, vor der Einmündung der Eingabeleitung 1 eine Leitung 33 ab, welche die erwähnten Abgase der Brennkammer 3 direkt in die zweite Brennkammer 11 führt, z.B. mit einer Temperatur von 500°C. In diesem Fall wird das Abgas der ersten Trocknerstufe 2 aus dem Materialabscheider 8 über die Leitung 6 vollständig in die Brennkammer 3 der ersten Stufe zurückgeführt und - wie erwähnt - das diese Brennkammer 3 verlassende Gas in zwei Teilströme aufgeteilt, die über die beiden Leitungen 32 bzw.33 dem Trockner 2 bzw. der Brennkammer 11 zugeführt werden. Da das aus der ersten Brennkammer 3 kommende Gas bereits eine ausreichende Temperatur für die Beheizung der zweiten Brennkammer 11 zwecks Oxydation der organischen Bestandteile aufweist, ist in der zweiten Brennkammer 11 kein zusätzlicher Brennstoff erforderlich. Die Leitung 13 der Ausführungsformen nach den Fig.1 und 2 kann daher bei der Konstruktion nach Fig.3 entfallen, ebenso die Energiezufuhr über die Leitung 34 für die Brennkammer 11.3 shows a system which is intended for the case in which the temperature of the gas leaving the first combustion chamber 3 is sufficient for the oxidation of the organic substances in the second combustion chamber 11. This can be helped to the extent that the second combustion chamber 11 is equipped with oxidation catalysts, by means of which the temperature required for the oxidation of the organic substances can be reduced to approximately 300 to 550 ° C. In this case, a line 33 branches off from the line 32, via which the exhaust gases of the combustion chamber 3 for heating the first dryer 2 are fed to it, before the opening of the input line 1 Combustion chamber 3 leads directly into the second combustion chamber 11, for example at a temperature of 500 ° C. In this case, the exhaust gas from the first dryer stage 2 is completely returned from the material separator 8 via the line 6 into the combustion chamber 3 of the first stage and - as mentioned - the gas leaving this combustion chamber 3 is divided into two partial streams, which via the two lines 32 and .33 the dryer 2 or the combustion chamber 11 are supplied. Since the gas coming from the first combustion chamber 3 already has a sufficient temperature for heating the second combustion chamber 11 for the purpose of oxidizing the organic components, no additional fuel is required in the second combustion chamber 11. The line 13 of the embodiments according to FIGS. 1 and 2 can therefore be omitted in the construction according to FIG. 3, as can the energy supply via line 34 for the combustion chamber 11.

Bei allen Ausführungsformen kann selbstverständlich in den Kreislauf des Wärmeträgeröles (Leitungen 16,19) ein nicht dargestellter Wärmeträgerölkessel eingeschaltet sein.In all embodiments, a heat carrier oil boiler (not shown) can of course be switched on in the circuit of the heat transfer oil (lines 16, 19).

In Fig.1 sind mit strichlierten Linien Weiterbildungen der erfindungsgemäßen Vorrichtung beschrieben. Eine Kondensatorwasserleitung und/oder eine Speisewasserleitung 42 ist über einen Wärmetauscher 34 im Abhitzekessel 14 geführt, um überhitzten Hochdruckdampf zu erzeugen. Dieser wird in einer Gegendruckturbine 35 entspannt und erzeugt in einem Generator 36 Strom. Anschließend kann der entspannte Dampf (Leitung 38) für die Trocknung oder für den Wärmebedarf von Pressen genutzt werden. Der erzeugte Strom (Leitung 37) kann den Energiebedarf einer der zweiten Trocknerstufe nachgeschalteten Späneverarbeitungsanlage decken bzw. mit Hochfrequenz beheizte bzw. heizende Pressen betreiben.In FIG. 1, further developments of the device according to the invention are described with dashed lines. A condenser water line and / or a feed water line 42 is guided over a heat exchanger 34 in the waste heat boiler 14 in order to generate superheated high-pressure steam. This is expanded in a counter-pressure turbine 35 and generates electricity in a generator 36. The relaxed steam (line 38) can then be used for drying or for the heat requirement of presses. The electricity generated (line 37) can cover the energy requirement of a chip processing system downstream of the second dryer stage or operate presses heated or heated at high frequency.

Ferner kann es zweckmäßig sein, wenn in der Abgasleitung des Materialabscheiders 8 ein Wärmetauscher 41 angeordnet wird, um Wärme rückzugewinnen bzw. das anfallende Luftvolumen zu verringern.Furthermore, it may be expedient if a heat exchanger 41 is arranged in the exhaust line of the material separator 8 in order to recover heat or to reduce the air volume that arises.

Claims (19)

  1. A method for the low-emission drying of wood chips, in which the damp wood chips are pre-dried in a first, preferably directly heated, drier stage and are dried again in a second drier stage, characterised in that the wood chips pre-dried in the first drier stage to a moisture content of about 20 to 50 % are dried off in the second drier stage, connected downstream of this first drier stage, to a moisture content of 1 to 6 %, the second drier stage being heated indirectly, in that the organic substances contained in the waste gases of the two drier stages are destroyed, before the release of the waste gases into the atmosphere, through oxidation at a raised temperature by the heating of the waste gases of both drier stages in at least one combustion chamber, preferably in this combustion chamber as a first combustion chamber and in a second combustion chamber situated in series with respect to the waste gas flow, and in that the first drier stage is heated with the whole quantity or part quantity of the flue or waste gases leaving the first combustion chamber.
  2. A method according to claim 1, characterised in that the organic substances contained in the waste gases are destroyed in the presence of oxidation catalysts, e.g. platinum, chromium- or copper oxide on ceramic carriers, at a temperature of at least 300°C, more particularly 300 to 550°C.
  3. A method according to claim 1, characterised in that the organic substances contained in the waste gases are heated to at least 700°C, preferably 700 to 1000°C.
  4. A method according to one of claims 1 to 3, characterised in that all the waste gas of the first drier stage is fed back into the first combustion chamber, and in that the gas leaving this combustion chamber is divided into two flows, one of which is passed to the first drier stage, while the second is passed into the second combustion chamber, in particular provided with a catalyst.
  5. A method according to one of claims 1 to 4, characterised in that the waste gas of the first drier stage is reheated in the second combustion chamber connected downstream of the said first drier stage, with further fuel being supplied.
  6. A method according to one of claims 1 to 5, characterised in that the waste heat of the heated waste gases is used at least for the partial heating of the second drier stage.
  7. A method according to one of claims 1 to 6, characterised in that the drying of the wood chips in the first drier stage takes place to the point where the waste heat available from the heating of the waste gases is at least sufficient to cover the heat requirement of the second drier stage.
  8. A method according to one of claims 1 to 7, characterised in that the drying of the wood chips in the first drier stage takes place to the point where the waste heat available from the heating of the waste gases is sufficient to cover the heat requirement of the second drier stage and the heat requirement of a wood chip processing unit, e.g. of presses, connected downstream of the second drier stage.
  9. A method according to one of claims 1 to 8, characterised in that the waste gases of the second drier stage, mixed with additional fuel, are burned in the first combustion chamber, the waste heat of which heats the first drier stage, of which the waste gases are mixed with additional fuel in the second combustion chamber and burned.
  10. A method according to one of claims 1 to 9, characterised in that at least a biogenous fuel, e.g. wood dust, is used to heat the first drier stage, and a fluid or gaseous fossil fuel, particularly natural gas, is used to heat the second drier stage.
  11. A device for implementing the method according to one of claims 1 to 11, comprising a first, preferably directly heated drier for pre-drying the damp wood chips and a second drier connected downstream of this first drier, characterised in that the second drier (5) is indirectly heated, in that a first combustion chamber (3) and, if appropriate, a second combustion chamber (11) is connected downstream of the two driers (2, 5) to heat the waste gases of these two driers (2, 5), and in that the first drier (2) is connected by way of a line (32) to the first combustion chamber (3) for heating the said first drier with the whole quantity or a part quantity of the waste or flue gases of this combustion chamber (3).
  12. A device according to claim 11, characterised in that a line (7) for the waste gases runs from the second drier (5) into the first combustion chamber (3) for heating the first drier (2), from which drier a further line (6) runs into the first combustion chamber (3) or a line (12) runs into the second combustion chamber (11) to heat the waste gases.
  13. A device according to claim 11 or 12, characterised in that branching off the line (12) leading to the second combustion chamber (11) for heating the waste gases is a line (6) which runs back to the combustion chamber (3) for heating the first drier (2) (Fig. 1, 3).
  14. A device according to claim 11 or 12, characterised in that branching off the line (32) which conveys the waste gases for heating the first drier (2), before the junction of a feed line (1) for the damp wood chips, is a line (33) which runs into the second combustion chamber (11), provided with a catalyst, and in that a line (6) is run from the first drier (2) to the first combustion chamber (3).
  15. A device according to one of claims 11 to 14, characterised in that connected to the combustion chamber (11) for recovering heat from the waste gases is a waste heat boiler (14) which has a line (16) for conveying waste heat running to the second drier (5) and, if appropriate, also running to a wood chip processing unit.
  16. A device according to claim 15, characterised in that a line (28) for the cooled waste gases runs from the waste heat boiler (14) to a condenser (27), to whose line (25) for removing the gases there is connected, if appropriate, a line (24) heated by the waste heat boiler (14) for conveying warm air.
  17. A device according to claim 15 or 16, characterised in that the waste heat boiler (14) comprises a heat exchanger (34) for producing superheated high pressure steam from condensate- and/or feed water, this steam being conveyed via a back pressure turbine (35) with generator (36).
  18. A device according to claim 17, characterised in that a heat exchanger (41) is disposed in the waste gas line (7) of the directly heated drier (2).
  19. A device according to one of claims 11 to 18, characterised in that oxidation catalysts, e.g. platinum, chromium- or copper oxide on ceramic carriers or the like, are provided at least in the combustion chamber (3) to lower the temperature necessary for destroying the organic substances contained in the waste gases.
EP89905579A 1988-05-10 1989-05-10 Process and apparatus for low-emission drying of wood chips Expired - Lifetime EP0420859B1 (en)

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AT0122588A AT399044B (en) 1988-05-10 1988-05-10 METHOD AND DEVICE FOR LOW-EMISSION DRYING OF WOODCHIPS
AT1225/88 1988-05-10

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DK268090A (en) 1990-11-08
BR8907425A (en) 1991-04-02
WO1989011072A1 (en) 1989-11-16
AU626872B2 (en) 1992-08-13
ATA122588A (en) 1994-07-15
DK268090D0 (en) 1990-11-08
AU3572989A (en) 1989-11-29
US5263266A (en) 1993-11-23
EP0420859A1 (en) 1991-04-10
AT399044B (en) 1995-03-27

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