METHOD AND APPARATUS FOR UTILIZATION OF WASTE HEAT IN DESICCATION AND HEAT TREATMENT OF WOOD

Abstract: Method for utilization of waste heat in desiccation and/or heat treatment of timber, in which desiccation/heat treatment the timber (0) is treated in a treatment space (6), such as a drying chamber. In the method, steam/combustion gas (7) generated in the combustion of a fuel used for heating is supplied into the treatment space (6) and at least a proportion of the steam/combustion gas is scrubbed/humidified before being passed into the treatment space (6), or steam/gas generated in the treatment space e.g. during heat treatment is circulated back into the treatment space.

Description: METHOD AND APPARATUS FOR UTILIZATION OF WASTE HEAT IN DESICCATION AND HEAT TREATMENT OF WOOD The present invention relates to a method as defined in the preambles of claims 1 and 2 for utilization of waste heat in desiccation and heat treatment of wood. The invention also relates to an apparatus for utilization of waste heat in desiccation and heat treatment of wood. A saw mill produces a significant amount of humid fuel (bark, chips and saw- dust), which is utilized in a chips-fuelled boiler at the saw mill for the production of heat needed in the drying plant. The final temperature of the fuel gases exhausted via the chimney is typically 150°C, which means that the evaporation heat adsorbed in the water is lost with the fuel gases flowing out into the environment. In the present invention, the free energy contained in humid combustion gases can be utilized in preliminary heating, drying and heat treatment of wood, so the treatment of the wood material can be carried out at lower running costs. Fl patent specification 106578 presents a system for the drying of fresh wood wherein waste wood is burned in a combustion gas generator to produce hot combustion gas and smoke, whereupon the hot combustion gas is fed into a wood drying chamber via a combustion gas feed channel. It has been established that microcracking occurring at the beginning of the desiccation of sawn timber is the greatest factor causing cracking of sawn timber. To prevent cracking, some drying plants use a system of feeding steam into the circulation air to saturate the air, thus preventing the timber from getting dried during the initial warming-up stage. Typically, of the extent of cracking prevailing in the final situation in a compartment kiln, about one half could be avoided if the preliminary warming-up could be implemented without the timber getting dried at this stage. However, steam generated by the traditional method with primary energy is expensive to use for drying. Via heat treatment performed after the drying of the timber, it is possible to modify the properties of the timber, e.g. moisture-induced deformations and decay resistance. In the heat treatment, the temperature of the timber is typically increased to a level of about 200°C. The structure of the wood undergoes changes when water and various extractives are further removed from it. Externally, the changes occurring in the wood become visible as a change of color into different shades of brown and as a reduction of the weight of the wood. Heat treatment involves a serious risk of fire. To prevent inflammation, steam is supplied into the heat treatment chamber during the treatment to make it inert. In the present invention, it is possible to utilize the completely mixed humid combustion gases having a low oxygen content produced in the combustion of e.g. moist wood chips and passed via a scrubber or equivalent and, depending on the phase of the wood preparation process, subsequently into a space where the preliminary warming-up, drying or heat treatment of timber is performed. In the desiccation of timber, the timber is first warmed up with saturated combustion gas obtained from a scrubber to the temperature of the dewpoint of the combustion gas, after which, by reducing the relative humidity and increasing the temperature of the combustion gas, the drying of the timber can be accomplished in a controlled manner. The heat treatment can be implemented by using completely mixed, humid combustion gas having a low oxygen content passed via a scrubber, by heating the gas indirectly. This is a reliable way to ensure that no high-oxygen areas involving a risk of fire will appear in the heat treatment space. In a corresponding manner it is possible to utilize e.g. the vapors produced in heat treatment, by passing them via a strainer back into the heat treatment process. The details of the features characteristic of the method and apparatus of the invention are presented in the claims below. By applying the invention, the waste heat produced in the combustion of wood waste or in heat treatment can be effectively utilized in the desiccation of timber. In addition, as compared with traditional methods, the system of the invention is environmentally friendly because it reduces the amount of impurities, e.g. vola- tile organic compounds (VOC) emitted into the environment from the timber treatment process. The invention can also be used to increase the desiccation capacity of an existing plant without a costly boiler investment, which would otherwise be needed for the generation of drying steam. In the following, the invention will be described in detail by the aid of examples with reference to the attached drawing, wherein Fig. 1 presents an apparatus according to the invention, and Fig. 2 presents another apparatus according to the invention. Example 1 Combustion gases 7, typically at a temperature of about 150°C, coming from a boiler fuelled with wood chips are passed into a scrubber 1. In the scrubber, the solids contained in the combustion gases are separated and the water vapor content of the combustion gases is increased. The combustion gas coming from the scrubber is saturated; its temperature is about 70°C, depending on the temperature of the gas being scrubbed, humidity of the fuel being used, hydrogen content of the fuel, auxiliary humidification and humidity of the combustion air. The condensate and sediment produced in the scrubber during combustion of wood chips are alkaline, which is why no separate treatment for neutralizing the condensate is needed. I. Preliminary warminα-up of timber: The combustion gases scrubbed during preliminary warming-up are not preheated, but the gases are passed via route 8-9 into a timber (timber stack 0) treatment space 6 (chamber or the like), hereinafter referred to as process kiln. Since the surface temperature of the timber at the preliminary warming-up stage is lower than the dewpoint temperature of the scrubbed combustion gases, wa- ter is condensed on the surface of the timber, preventing drying and cracking of the surfaces during preliminary warming-up. Some of the condensed water is absorbed into the wood, increasing the moistness of the wood, while most of the water flows down into floor drains. As a result of condensation, heat is effectively transferred from the gas into the wood, so a high rate of increase of wood temperature can be achieved, thus allowing the drying phase to be started sooner. The combustion gases leaving the process kiln 6 are in a saturated condition (at a reduced temperature), so that, by passing the combustion gases 10 from the apparatus into a heat recovery unit, it will be possible to utilize a proportion of the heat to preheat and humidify the combustion air. The condensation temperature of the combustion gases can be raised by heating with electricity 14 and using humidification 2. II. Drying of timber: The drying energy needed at the drying stage is supplied using a separate heat exchanger (not shown) provided in the kiln 6, and the water evaporating from the wood is enough to maintain sufficiently humid conditions. Steam is only needed at the final phase of the process when internal moisture variations within the timber are to be equalized by increasing humidity in the kiln 6. The steam needed is obtained by scrubbing some or all of the combustion gases from the boiler and passing them via the heat exchanger 3 into the proc- ess kiln 6. HI. Heat treatment of timber: At the heat treatment stage, a proportion of the combustion gases from the chips-fuelled boiler is scrubbed and passed via the heat exchanger 3 into the process kiln 6. By means of a hot current 14, the temperature of the combustion gases supplied into the process kiln 6 is raised to the desired timber treatment temperature. To achieve a better control of the temperature, some of the combustion gases may bypass the heat exchanger 3 and the bypass current is mixed with the heated combustion gases before reaching the process kiln 6. The combustion gases leaving the process kiln 6 during the heat treatment are at a high temperature and contain organic compounds released from the wood, which is why the combustion gases are passed back to the scrubber 1 via route 11. An amount of combustion gas corresponding to the amount returned is removed from the system into the heat recovery stage via route 13-12. The heat treatment of timber does not require any large energy currents as heat losses can be prevented by properly Insulating the process kiln 6 and the combustion gas ducts 4. The steam can also be superheated using a battery (not shown) provided in the kiln 6. In the above-described system, it is also possible to use other fuels instead of wood chips. If the combustion gases supplied into the scrubbing stage are dry and at a low temperature, then it is necessary to use additional humidification because otherwise the final temperature in the scrubber will remain low. A need for additional humidification leads to an increasing power consumption, and the additional power is obtained from the hot current 14. This current 14 is received either from a separate auxiliary boiler or it can be obtained as a shunt current of combustion gases from the chips-fuelled boiler. Example 2 In the heat treatment, steam 17 generated in the process kiln 16 and scrubbed can also be fed back into the process kiln 16. The steam coming from the process kiln during heat treatment is typically at a temperature in the range of 180...230°C. The steam is superheated and the partial pressure of water vapor is low. In addition, it contains combustible tars and other VOC's. Before being fed back into the heat treatment process, it has to be oxygenated and scrubbed/humidified. The oxygenation and scrubbing/humidification are performed outside the chamber in separate devices 18, 19. After oxygenation (calcination), the temperature may be 400...800°C, which is decreased by spraying water into the gas, causing the water to evaporate. By adjusting the amount of water, it is possible to adjust the temperature. In preliminary warming-up and drying, it is possible to utilize steam having a temperature of the order of 80...120°C, in heat treatment the steam may be at a temperature of 100...230°C, depending on the phase of the process. The process kiln 6 com- prises piping 20 for feedback. Energy is supplied to device 18 via pipes 21 , and water is supplied to the scrubbing/humidifying device 19 via pipes 22. In addition, the piping 20 is provided with an exit flow channel 23 placed after the scrubbing/humidifying device 19. The heat can be recovered from the exit current by means of a heat exchanger (not shown). It is obvious to the person skilled in the art that different embodiments of the invention are not limited to the example described above, but that they may be varied within the scope of the claims presented below.

Claims: CLAIMS 1. Method for utilization of waste heat in desiccation and/or heat treatment of timber, in which desiccation/heat treatment the timber (0) is treated in a treat- ment space (6), such as a drying chamber, and in which method steam/combustion gas (7) generated in the combustion of a fuel used for heating is supplied into the treatment space (6), characterized in that in said method, at least a proportion of the steam/combustion gas is scrubbed/humidified before being passed into the treatment space (6). 2. Method for utilization of waste heat in desiccation and/or heat treatment of timber, in which desiccation/heat treatment the timber (0) is treated in a treatment space (16), such as a drying chamber, characterized in that steam/gas (17) generated in the treatment space (16) e.g. during heat treatment is circulated back into the treatment space (16). 3. Method as defined in claim 2, characterized in that, in the method, at least a proportion of the steam/combustion gas is scrubbed/humidified before being passed into the treatment space (6, 16). 4. Method as defined in any one claims 1 - 3, characterized in that, before the scrubbing/humidification, the steam/combustion gas is oxygenated. 5. Method as defined in any one claims 1 - 3, characterized in that the scrubbing/humidification is performed by adding liquid, and that the temperature of the steam/combustion gas is controlled by controlling the amount of liquid. 6. Method as defined in claim 1 , characterized in that, in the desiccation process, the timber (0) is first heated by saturated steam/combustion gas obtained from a scrubber or equivalent substantially to the temperature of the dewpoint of the steam/combustion gas, whereupon the relative humidity is reduced and the temperature of the steam/combustion gas is raised. 7. method as defined in claim 1, characterized in that the low-oxygen, mixed, humid steam/combustion gas is heated. 8. Apparatus for utilization of waste heat in desiccation and/or heat treatment of timber, in which desiccation/heat treatment the timber (0) is treated in a treatment space (6, 16), and which comprises elements (9, 20) for conveying steam/combustion gas (7, 17) generated in the treatment space (6, 16) and/or steam/combustion gas (7, 17) generated in the combustion of a fuel used for heating into the treatment space (6, 16), characterized in that the apparatus comprises a device, such as a scrubber (2), by means of which at least a proportion of the steam/combustion gas is scrubbed/humidified before being passed back into the treatment space (6, 16). 9. Apparatus as defined in claim 8, characterized in that it comprises a calcination apparatus (18) by means of which the steam/combustion gas is oxygenated before being scrubbed/humidified. 10. Apparatus as defined in claim 8, characterized in that it comprises a heating apparatus by means of which the low-oxygen, mixed, humid steam/combustion gas is heated. 11. Apparatus as defined in claim 8, characterized in that the heating apparatus is a heat exchanger (3), the heating being effected in an indirect manner. 12. Apparatus as defined in claim 8, characterized in that it comprises bypass elements via which the exhaust gases from the warming-up stage of the timber treatment process and from the low-temperature desiccation stage are caused to bypass the heating circulation. 13. Apparatus as defined in claim 8, characterized in that it comprises ele- ments whereby the gases from high-temperature desiccation and heat treatment of the timber are caused to return to the scrubber (1) or equivalent and to exit from the apparatus after the scrubbing, preferably before the vaporizing system of the scrubber (1 ).

Inventors: HAUKKA PAULI MATIAS, SONNINEN HEIKKI, WALLEN VESA VIKTOR, HAUKKA, PAULI, MATIAS, SONNINEN, HEIKKI, WALLEN, VESA, VIKTOR
Applicants: HAUKKA PAULI MATIAS, SONNINEN HEIKKI, TEKMA KUIVAAMOT OY, WALLEN VESA VIKTOR, HAUKKA, PAULI, MATIAS, SONNINEN, HEIKKI, WALLEN, VESA, VIKTOR
Application year: 2001
Main classification: F26B21
IPC codes: F26B21, F26B23