Method at manufacture of board

Abstract: A method for eliminating contaminating emissions at the manufacture of board from lignocellulosic material. The board manufacture comprises disintegration of the material, glueing, drying in two stages and forming to a mat and hot pressing the same to a ready board. Exhaust air separated from the drying and pressing is used in a heat energy plant. The exhaust air (16) from the pressing is heated together with fresh air (15) and used as drying air in the two drying stages (3,9). The exhaust air (20) from the second drying stage (9) is introduced as drying air to the first drying stage (3). One part of the exhaust air (18) from the first drying stage is reheated and re-used as drying air (19) in the first drying stage (3) and another part is used as combustion air (23,24,25) in the furnace (14) of the heat energy plant.

Description: This invention relates to the manufacture of board from lignocellulosic material according to the dry method, for example particle board and fiberboard. The raw material can be wood, straw, bagasse etc. The invention, more precisely, relates to a heat generation system, which is integrated with the board manufacture, and where contaminating emissions can be eliminated. The manufacture of board of the aforesaid kind is carried out by disintegration of the material, glueing, drying and forming to a mat, which is hot pressed to a ready board. These steps are known from the method disclosed in US-A-5482666. At the manufacture, gaseous organic impurities, such as formaldehyde and other volatile hydrocarbons (VOC, volatile organic compounds), are generated and emitted. The impurities are emitted primarily from the drying, but to a certain extent also from the hot pressing. These substances contribute to the greenhouse effect, and there is an increasingly stronger demand for reducing the emissions. In addition, also particular impurities in the form of wood dust are emitted from the drying, and fly ash is emitted from the heat energy plant. At the manufacture of board relatively much heat energy is consumed. The most heat consuming steps at the manufacture of MDF (medium density fiberboard) are the defibering process, the drying of the fiber material and the hot pressing of the fiber mat. In the defibering process steam is used as heating medium. The fiber dryer and press can be heated with steam, hot oil or hot water. The fiber dryer can also be heated directly with hot flue gases from a burner in the dryer or from a common heat energy plant. The exhaust air from the drying contains also fly ash from the flue gases. At the use of continuous press technology the main part of the organic substances emitted at the pressing operation can be sucked off with a limited air volume. The exhaust air can be used as combustion air in the heat energy plant where the organic impurities are transformed to water vapour and carbon dioxide. Exhaust air from a dryer, however, is much larger in quantity than the air volum from a press exhaust by suction. This fact implies that the exhaust air from the dryer cannot simply be used as combustion air in the same way as the exhaust air from the press. At stringent environmental requirements it can, therefore, be necessary to apply expensive and complicated cleaning technology, for example in the form of gas scrubber, wet electrostatic precipatator (WESP) or regenerative thermal oxidation (RTO), depending on the requirements by the authorities concerned. These technologies are expensive in investment as well as operation costs. The present invention offers a solution of the aforesaid problems. According to the invention, it is thus possible to minimize the total exhaust air volume from the hot press and fiber dryer to a level where this contaminated air can be used to 100% as combustion air in the heat energy plant. The volatile organic impurities and the wood matter in the dryer exhaust air can thereby be combusted in the energy plant furnace and form carbon dioxide, water vapour and a small quantity of ash. The ash can be separated in an electorstatic precipitator, before the flue gases are emitted to the atmosphere. The characterizing features of the invention are apparent from the attached claims. The invention is described in greater detail in the following, with reference to the accompanying Figures illustrating an embodiment of the invention. Fig. 1 shows schematically a plant for the manufacture of fiberboard according to the dry metnod; Fig. 2 shows a flow diagram for air and flue gases in a plant for the manufacture of board according to the invention. The plant shown in Fig. 1 comprises a defibration apparatus 1 for defibering the fiber material. The fiber material is directed from there through a blow line 2 to a first drying stage 3, for example in the form of a flash tube-type dryer. The drying of the fiber material takes place while it is simultaneously transported by hot drying air, which is supplied through a line 4. After the drying stage 3 the drying air is separated in a first cyclone 5, while the fiber material is moved through a line 6 to a mixer 7 for the admixture of hot setting glue, for example urea resin or phenol resin. The glue is supplied through an inlet 8. Alternatively, the glue can be admixed in the blow line before the drying stages. The glued material is introduced from the mixer 7 to a second drying stage 9, which can be a flash tube-type dryer. The drying takes place while the material is transported by hot drying air, which is supplied trhough a line 10. The material and drying air are transferred from the drying stage 9 to a second cyclone 11, where the gas is separated, and the material is led to a subsequent forming station 12, where the fiber material is formed to a mat, which thereafter is moved to a hot press 13 to be pressed to fiberboard. Fig. 2 shows schematically the flow of drying air and, respectively, exhaust air to and from the drying steps 3,9 via the two cyclones 5,11 and the flue gas flow from a combustion furnace 14 in the heat energy plant. The air required for the drying is supplied in the form of fresh air 15 from the atmosphere, and press air 16 is supplied from the exhaust of the hot press 13. These two air flows are combined and passed through a first heat exchanger 17 for heating. The main part of this heated air is directed via a line 10 to the second drying stage 9, while the remaining part is led via a line 19 to the first drying stage 3. To the first drying stage 3 are also moved via a line 20 the exhaust air from the second drying stage separated in the second cyclone 11 and via a line 21 recycled preheated exhaust air from the first drying stage. The preheating of this recycled exhaust air takes place in a second heat exchanger 22. The exhaust air separated from the first drying stage 3 is discharged through a line 18. Part of this exhaust air is directed after the preheating in the heat exchanger 22 via a line 23 as primary air to the furnace 14. Another part of the exhaust air from the first drying stage 3 is moved directly from the cyclone 5 to the furnace 14 as secondary air via lines 24,25. The combustion furnace 14 is formed with an inlet 26 for fuel, which can be biofuel in the form of bark, wood waste etc., which is generated at the manufacture of the board. Sanding dust suitably is supplied through a separate inlet 27 via a dust-burner 29. It is also possible to use oil or gas as additional fuel. Air required for the combustion is supplied, as stated above, through the lines 23,24 and 25. The flue gases are discharged through a line 29, which passes through the two heat exchangers 17,22. Via a branch line 30 from the line 29 the flue gases pass through a heat exchanger 31 for other heating purposes, for example steam generation to the defibering apparatus 1 and heating of thermal oil for the hot press 13. The heat supply to the drying stages, thus, takes place by fresh air and press air, which is heated by flue gas in heat exchangers. Owing to the fact that flue gases always have a higher moisture content than fresh air and even the press air is relatively dry, the introduction of moisture into the drying process is reduced, whereby the exhaust air amount can be reduced. Due to the fact that the drying of the fiber material is carried out in a two-stage fiber dryer with recycling of the dry exhaust air from the second drying stage to the inlet in the first stage the total exhaust air volume can be reduced by about 35%. The primary air as well as the secondary air to the furnace consist of only exhaust air from the drying. The combustion temperature in the furnace is chosen so that the main part of the volatile, organic substances is combusted, normally at about 850°C. The generated flue gases are cooled in connection with the heat exchange in the heat exchangers 17,22,31 in order thereby to cover the heat demand of the plant. The flue gases finally are emitted to the atmosphere after dust cleaning in an electrostatic precipitator 32. The flue gases according to the present system hold a temperature of about 300°C, which can be compared with 75°C in a system without air cleaning, where the flue gases are used for direct heating. This implies that the thermal efficiency is reduced to about 70% from about 90%. The difference is what the air cleaning costs in extra heat demand. A thermal efficiency of 70%, however, is comparable to what is achieved in a conventional system with indirectly heated fiber dryer. With the system according to the invention the entire heat demand can be covered with different types of biofuels. As a comparison can be mentioned that the aforesaid RTO-technique requires high-quality fuels such as gas or crude oil with low sulphur content. Another advantage is that, if the evaporation capacity in the dryer is reduced, the recycling degree in the fist drying stage increases. This implies that the thermal efficiency for the entire plant can be held high even at partial load. The table below shows expected emissions to the atmosphere for some different alternative heat energy plants. 1) Conventional plant with flue gas heated one-stage dryer; 2) Plant with flue gas heated two-stage dryer with exhaust air recycling; 3) Same as 2) with gas scrubber; 4) Same as 2) with wet electrostatic precipitator (WESP); 5) Same as 2) with regenerative thermal oxidation (RTO); 6) Plant according to the invention. Gas flow Particles Formaldehyde Other VOC Nm 3 /hr mg/Nm 3 kg/day mg/Nm 3 kg/day mg/Nm 3 kg/day 1 346 000 50 380 15 115 100 760 2 220 00 45 220 23 110 150 720 3 220 000 15 75 7 35 100 480 4 220 000 10 50 10 50 50 240 5 220 000 10 50 2 10 5 25 6 110 000 50 120 5 12 20 50 The numeral information is based on a fiber capacity of 21 ton per hour with dry unresinated fiber corresponding to a production capacity of 650 m 3 15 mm MDF per day at a running time of 22 hrs/day. Regarding the reduction of organic gases in the exhaust air, according to the invention, thus, substantially the same cleaning degree is achieved as with a regenerative thermal oxidation at considerably lower investment and operation costs. The invention, of course, is not restricted to the embodiment shown, but can be varied with the scope of the invention as defined in the claims.

Claims: A method for the manufacture of board from lignocellulosic material, comprising disintegration of the material, glueing, drying in two stages and forming to a mat and hot pressing the same to a ready board, where exhaust air from drying and pressing is used in a heat energy plant, characterized in that the exhaust air (16) from the pressing together with fresh air (15) is heated and used as drying air in the two drying stages (3,9), that the exhaust air (20) from the second drying stage (9) is introduced as drying air to the first drying stage (3), and that the exhaust air (18) from the first drying stage is both reheated and re-used as drying air (19) in the first drying stage (3) and used as combustion air (23,24,25) in the furnace (14) of the heat energy plant. A method as defined in claim 1, characterized in that the flue gases from the furnace (14) of the heat energy plant are used for heating the drying air for the two drying stages (3,9) by heat exchange. A method as defined in claim 1 or 2, characterized in that a part (23) of the re-heated exhaust air from the first drying stage (3) is used as primary air in the furnace (14). A method as defined in any one of the preceding claims, characterized in that the part (24,25) of the exhaust air (18) separated from the first stage (3) is used directly as secondary air in the furnace (14).

Inventors: MIKAELSSON STEFAN, SAEFSTROEM CHRISTER, MIKAELSSON, STEFAN, SAEFSTROEM, CHRISTER, Säfström, Christer 
Applicants: VALMET FIBERTECH AB
Application year: 1998
Main classification: B27N1
IPC codes: B27N1