METHOD OF DRYING LIGNOCELLULOSE MATERIAL

Abstract: A method of drying lignocellulosic fibrous material in two steps in connection with the manufacture of fiberboard. The material is transported with the help of a first drying gas through a first drying line (11) to a first cyclone (13) where the first drying gas is conducted away from the material. Thereafter the material is transported with the help to a second drying gas through a second drying line (17) of a second cyclone (19) where the second drying gas is conducted away while the material is advanced to a subsequent step. The second drying gas, which is conducted away from the second cyclone (19), is moved to the first drying step and used as a portion of the first drying gas. A portion of the first drying gas, which is conducted away from the first cyclone (13), is reclycled and used as drying gas at the first drying step.

Description: Method of drying lignocellulose material This invention relates to a method of drying Hgnocellulosic fibrous material in connection with the manufacture of fiber- board, for example MDF (Medium Density Fiberboard). The fibrous material consists of mechanically defibered size-coated fibers. For achieving an optimum drying result, the drying is carried out in two steps with the help of drying gas. At the first step the fibrous material is transported with a first drying gas through a first drying line to a first cyclone where the drying gas is separated from the material. Thereafter the material is transported with a second drying gas through a second drying line to a second cyclone where the drying gas is separated from the material. From the second drying step the material is moved to subsequent steps in the process for the manufacture of fiberboard. At two-step drying in this way, both steps can be carried out with drying gas, which consists of air, which directly or indirectly is heated with flue gas or in some other way. At dryers of this kind great environmental problems arise due to the emission of fiber dust, formaldehyde and organic volatile compounds (VOC = Volatile Organic Compounds). One way of reducing these problems is to recycle the separated second drying gas from the second cyclone to the first step. In order to bring about additional cleaning of the gas emitted from the dryer it is, however, necessary to use separate cleaning equipment, for example gas scrubbers, RTO (Regenerative Thermal Oxidizer) or WESP (Wet Electrostatic Precipitator) . This implies, however, that the costs for the drying plant increase considerably . This invention is directed to additional improvements of this kind of drying. Its characterizing features are defined in the attached claims . This invention is described in greater detail in the following, with reference to the accompanying drawing, which schematically shows a plant according to the invention. At the plant shown in the Figure the fibrous material is supplied through a line 10 in a first drying line 11. With the help of a first fan 12 a first drying gas is blown into the drying line 11 and transports the fibrous material at simultaneous drying to a first cyclone 13. There the drying gas is separated through an outlet 14 while the material via a sluice means 15 and a line 16 is advanced to a second drying line 17. With the help of a second drying gas and a second fan 18 the material is transported at simultaneous additional drying to a second cyclone 19. The second drying gas contains fresh air, which is supplied through a line 20 and heated directly or indirectly in a heating means 21. The heating can be made with flue gas, gas from gas burner or in some other way by direct supply or heat exchange. In the second cyclone 19 the drying gas is separated through a discharge line 22, and the fibrous material is taken out via a sluice means 23 and line 24 to be transported further to a subsequent step in the process for fiberboard manufacture. The separated second drying gas is moved through the discharge line 22 to the first drying step, where it is introduced into the first drying line 11 and thereby constitutes a part of the first drying gas. An additional closure of the drying system is obtained in that a portion of the drying gas separated from the first cyclone 13 through the outlet 14 is recycled via a line 25 to the first drying step where it is introduced into the first drying line 11. The first drying gas, thus, contains a mixture of the drying gas separated from the second drying step and a portion of the drying gas separated from the first drying step. In a heating means 26 the first drying gas is heated directly or indirectly. The heat source used there is flue gas, gas from gas burner or other, and the heating is carried out directly or in a heat exchanger. Fresh air can be introduced also into the first drying step through a line 27. In the drying plant described above the temperature of the first drying gas at the inlet to the first step should be 130-200°C, and at the outlet from the first cyclone 45-90°C Corresponding inlet and outlet temperatures of the second drying gas should be 80-150°C and, respectively, 30-80°C. With an ingoing moisture content of the fibrous material of 50-150% the outgoing moisture content can be reduced to 3-15%. By recycling the drying gas from the second drying step to the first drying step the heat content in this drying gas can be recovered in the system. The separated second drying gas is also relatively dry and, thus, does not cause condensation problems in the system. This recycling of drying gas implies that the total emission of gas can be reduced and at the same time the heat economy can be improved. Moreover, by recycling a portion of the separated first drying gas to the first drying step, the heat economy can be improved still more. This recycling further implies that the wet air flow out of the dryer can be reduced substantially. With the return only of the second drying gas flow the total wet air flow out of the dryer can be reduced by 20-25%. If in addition the first drying gas flow partially is recycled, the total wet air flow out of the dryer can be reduced to 40-50% of the wet air flow without drying gas recycling, i.e. 50-60% is recycled. The reduced wet air flow out of the dryer implies that the costs for the separate cleaning equipment, which can be necessary to install, are considerably lower. The method according to the invention implies that the entire required fresh air amount can be supplied to the drying system at the second drying step through the line 20. The re-circulated flow of the first drying gas is controlled so that condensation does not occur in the system The size of the portion of the first drying gas flow which is recycled depends, thus, on other parameters, such as ingoing fiber moisture, gas temperature, etc. Compared with conventional plants, the method according to the invention implies, that outgoing fiber moisture can be controlled very effectively, the energy consumption can be reduced, the capacity of the plant can be increased, the emissions detrimental to the environment can be reduced, and the costs for separate cleaning equipment can be decreased . The invention, of course, is not restricted to the embodiment described above, but can be varied within the scope of the claims .

Claims: Claims 1. A method of drying Hgnocellulosic fibrous material in two steps in connection with the manufacture of fiber- board, where the material with the help of a first drying gas is transported through a first drying line (11) to a first cyclone (13), where the first drying gas is conducted away from the material, whereafter the material with the help of a second drying gas is transported through a second drying line (17) to a second cyclone (19), where the second drying gas is conducted away while the material is advanced to a subsequent step, c h a r a c t e r i z e d i n that the second drying gas, which is conducted away from the second cyclone (19), is moved to the first drying step and used as a portion of the first drying gas, and that a portion of the first drying gas, which is conducted away from the first cyclone (13), is recycled and used as drying gas in the first drying step . 2. A method as defined in claim 1, c h a r a c t e r i z e d i n that the re-circulated flow of the first drying gas is controlled so that condensation in the system is avoided . 3. A method as defined in claim 1 or 2, c h a r a c t e r i z e d i n that the entire fresh air amount required for the drying system is supplied to the second drying step. 4. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that 50-60% of the separated first drying gas is recycled to the first drying step.

Inventors: ERIKSSON LENNART V, LENNARTSSON STIG, ERIKSSON, LENNART, V., LENNARTSSON, STIG
Applicants: ERIKSSON LENNART V, LENNARTSSON STIG, VALMET FIBERTECH AB, ERIKSSON, LENNART, V., LENNARTSSON, STIG
Application year: 2000
Main classification: B27N1
IPC codes: B27N1, B27N3