PRIOR ART Wood raw material such as bio-fuel, chips or fibres are dried before being used for energy production or for manufacturing wood based boards, such as particle boards or oriented strand boards. During the drying process, ter- penes and other volatile organic compounds are aforced out. Such organic compounds are often referred to as VOC-short for Volatile Organic Com- pounds-and they will be thus denoted in the following. VOC evaporate along with moisture for instance, into the gases which during the drying process flow through the drying device and the wood material, and leave the drying device along with said moisture containing waste gases from the pro- cess. Normally the temperature of the waste or process gases vented during the drying process is between 80°C-150°C, depending on the residual moisture contents of the wood material. Depending on the drying conditions, and to some extent on the composition of the wood raw material, essentially all VOC evaporate during the initial stages of the drying process while, un- der other conditions or with other compositions, essentially all VOC evapo- rate during the final stages of the drying process. Since it is desired to re- duce the discharged amount of VOC, the VOC present in the process gases are destroyed, normally using either catalytic or thermal oxidation. In order to secure an efficient destruction of the VOC present in the process gases, the exhaust gases are heated to a temperature of approximately 350°C for catalytic oxidation, and 850°C for thermal oxidation. A required gas tem- perature for achieving sufficient oxidation is determined by the composition of the waste gases, which varies according to the composition, type and quality of the wood raw material. According to prior art, the waste gases are heated to the elevated temperature by means of a complicated and costly system, comprising heat exchangers of regenerating or recuperative type, where the heat of the waste gases, from which the VOC have been elimi- nated, is used for heating the gases still containing VOC, before they are de- livered to the reactor. It is often, however, necessary to supply additional heat to the waste gases upstreams or in the reactor, in order to achieve the desired reduction of the VOC contents. According to prior art, the waste gases are vented from the drying device in only one place. This means that the whole amount of process gas leaving the drying device has to be pre- heated prior to oxidation in the reactor. This, together with the fact that the process gases often are heated to a higher temperature than necessary in order to secure an efficient reduction of the VOC discharge from the drying device, leads to increasing demands on, and size of, the heat exchange sys- tem arranged at the reactor, as well as an increased required energy for the destruction of VOC, and for the drying process as a whole.

AIM OF THE INVENTION The primary aim of the invention is to provide a method for drying wood raw material, comprising a destruction of VOC evaporated during the drying with a more efficient use of energy, and lower investment needs. The inventive method is cost efficient, with low maintenance requirements and a low en- ergy needs, which secures the oxidation of essentially all of the VOC which evaporate into the drying gases prior to venting the waste gases into the at- mosphere.

Another aim of the invention is to provide a method for drying wood raw material which, irrespective of variations regarding the type, composition and quality of the wood raw material, secures the oxidisation of all of the VOC evaporated into the drying gases before venting the drying gases into the atmosphere.

Further advantages of the present invention are apparent from the following description of the invention, and from the described preferred embodiments.

THE INVENTION In order to achieve said results, the present invention suggests a method for drying wood raw material according to the preamble of claim 1, where said wood raw material is dried in at least two sub-steps, said drying being con- trolled so that essentially all VOC are removed during one sub-step, and so that said VOC from the wood raw material is concentrated in the process gas which is removed during said sub-step, said gas being separated from the remaining process gases. The gas containing VOC is heated before it is transferred to a reactor where it is oxidised catalytically, at a gas tempera- ture which exceeds 180°C. A suitable gas temperature for the oxidisation of the VOC is from 200°C to 500°C. Preferably the VOC is oxidised within a temperature interval from 200°C to 350°C. The increase in gas temperature to a level which is necessary for catalytic oxidisation in the reactor is achieved by mixing the drying gases, with hot gases upstreams from the re- actor, preferably combustion flue gases. By dividing the drying process into two or more sub-steps, while at the same time controlling the drying process so that the VOC is removed from the wood during one of the sub-steps, which leads to the concentration of essentially all of the VOC in the gases which are removed from said sub-step, the amount of gas to be treated is reduced significantly. At the same time, the VOC concentration in the amount of process gas to be treated is increased. Preferably the process gases, which no longer contain VOC, are returned to the drying process as drying gas. Process gas which has been treated in the catalytic oxidation re- actor is supplied to the drying process as drying gas, downstreams from the reactor, in a different sub-step in the drying process.

According to a preferred embodiment of the invention, the wood raw mate- rial is dried in an essentially continuous process, comprising at least two sub-steps. Sub-flows of process gas are removed from the different sub- steps and the drying process is controlled so that essentially all VOC evapo- rate from the wood raw material during one sub-step, thereby concentrating essentially all the VOC to the sub-flow from said sub-step. The gas tem- perature in said sub-flow is increased before the VOC are removed by cata- lytic oxidisation at an elevated temperature in a reactor.

As was described above, the process gas containing VOC is heated before oxidisation, preferably by mixing it with hot gases upstreams from the re- actor. According to a preferred embodiment of the invention, hot drying gases are used for increasing the temperature to a required level before oxi- disation of VOC in the reactor. After oxidisation in the reactor, the process gas, now relieved from its VOC contents, is transferred to a further process step. Normally, the hot drying gases are generated by combustion of a suit- able fuel, such as oil, gas or wood or other suitable bio-fuel. It may be suit- able to use waste material from the on-site production facility in this fuel. As an alternative or complement to the combustion gases, it is of course possi- ble to use other heat sources, such as gas heaters based on electrical resis- tance elements, heat exchangers for extracting excess heat from the drying process or, even better, from another process with waste gases with a high heat content.

It has, during application of the inventive process, turned out that in order to achieve an efficient decrease of the VOC contents in the process gas, the gas temperature should exceed 180°C. It is advantageous to heat the proc- ess gas, before the oxidation, to a gas temperature in the interval between 200° and 500°C. Preferably a gas temperature in the interval between 200°C and 350°C is chosen, in order to secure the destruction of essentially all the VOC, without unnecessarily influencing the energy efficiency in a negative way.

DRAWINGS The invention will in the following be explained in closer detail, and be ex- emplified with preferred embodiments, with reference to the appended drawings.

Fig. 1 illustrates, schematically, a two step method for drying wood raw material according to a preferred embodiment of the invention.

Fig. 2 illustrates, schematically, a two step method for drying wood raw material according to an alternative embodiment of the invention.

Fig. 3 illustrates, schematically, evaporated VOC over time, in the embodi- ments shown in fig. 1 and fig. 2.

DETAILED DESCRIPTION OF THE INVENTION According to the preferred embodiment of the invention, shown in fig. 1, the wood raw material is dried by means of a two step method. Wet wood raw material 10 is supplied to the process in step 1, I, in which it is processed, whereupon the wood material 10 is transferred to step 2, II, in which it is processed further, before it leaves the process as dried wood raw material 19. In fig. 1, this two step method is shown as a continuous or semi- continuous process where the wood material 10 passes through two drying devices or drying chambers 11,12, e. g. drying ovens or autoclaves-the flow of the wood raw material 10 is denoted by filled bold face arrows. Drying gas 20, in the form of hot gases, is generated in some kind of gas generator 13 as described above, e. g. by combustion of a fuel such as wood waste mate- rial 14, in the presence of combustion air 15. Part of said hot drying gas 21 is provided to the drying oven 11 where the wood material 10 is processed during step 1, I, by allowing the drying gas 21 to pass through or over the wood material 10, making VOC and water evaporate from the wood material.

The conditions inside oven 11 are controlled such, that VOC evaporated from the wood material are concentrated to step 1, I. Process gas 22 is vented from step 1, I, and mixed with hot drying gas 20 so that the tem- perature of the process gas is increased. The heated process gas 23 is intro- duced into a reactor 16 in which the VOC are oxidised. Then the process gas 24 coming from the reactor 16, which has been relieved from its VOC con- tents, is introduced into an oven 12 provided for step 2, II, of drying process.

Process gas 25 is removed from step 2, II, and is preferably transferred to a conventional device for purifying exhaust gases, e. g. dust separation, not shown in the figures. In fig. 1 a parallel flow is shown during step 1, I, and cross-flow is shown during step 2, II, only for the purpose of illustrating the process in a clear schematic way. Thus, the type of relation between the re- spective flows of the wood raw material and the drying gas through the dry- ing facilities, essentially counter flow, cross-flow or parallel flow, may be chosen freely according to what is deemed suitable.

As an alternative to the continuous or semi-continuous process in which the wood raw material is processed in two drying devices 11,12, the two step method according to this embodiment may be performed in batches, said wood raw material being introduced into a drying oven or an autoclave, processed with drying gas in a first step 1. Then the step 1 process gas is mixed with more drying gas and heated to a temperature suitable for cata- lytic oxidation. The VOC contained in the process gas is now oxidised before the process gas, now relieved from its VOC contents, is returned to the dry- ing oven or autoclave, as drying gas for step 2.

According to the alternative inventive two step method for drying wood raw material which is shown in fig. 2, wet wood raw material is, in the above de- scribed way introduced into an oven or an autoclave 11, in which the wood material is processed in step 1,1. The wood material is transferred to a new autoclave 12, where it is processed further in step 2, II, before it leaves the process as dried wood raw material 19. Also according to this alternative embodiment, hot gases are used as drying gas 20, a part of said hot drying gas 32 being supplied to the drying process during step 2, in which it passes through or over the wood raw material, the conditions in the drying device 12 during step 2, II, being so controlled, that essentially all VOC is evapo- rated from the wood material during step 2. Process gas 33 is removed from the drying facility 12 for step 2, II, and is mixed with hot drying gas 31 in order to increase the temperature if the process gas. The heated process gas 34 is, after being heated, introduced into a reactor 16 in which the VOC are oxidised, after which the process gas 35 relieved from its VOC contents, downstreams of the reactor 16 is supplied as drying gas into a drying oven or autoclave 11 provided for step 1 of the drying process, where it passes through or over the wood raw material. Process gas 36 is removed from step 1, I, and is advantageously transferred to a conventional device for purifica- tion of process gases from a drying facility, not shown. In fig. 2, cross-flow is shown in step 1, I, and counter-flow is shown in step 2, II, solely for the purpose of achieving a clear schematic illustration of the process. Thus, the type of relation between the flow of the wood raw material and the drying gases through the drying facility, essentially counter-flow, cross-flow or par- allel flow, may also in this case be chosen freely according to what is deemed appropriate. In the application of said alternative drying process for drying batches of wood raw material, two batch processes should be run in parallel, so that process gas removed from step 2, II, in the one process can, after VOC oxidation, be provided to step 1, I, of the other process, in order to maintain an efficient use of energy.

The evaporation of VOC from the wood raw material over time, in carrying out the above described two step methods according to the present invention is shown in fig. 3. The solid curve A shows that in carrying out the process according to fig. 1, such conditions are upheld during step 1, I, that the evaporation of VOC from the wood raw material is essentially completed during said step 1, I, and that only a negligable amount of residual VOC evaporates during step 2, II. As can be seen from the dotted curve B, such conditions are upheld during step 1, I, that essentially no, or only negligable amounts of VOC evaporate, while such conditions are upheld during step 2, II, that VOC evaporation from the wood raw material essentially takes place during step 2, II. In order to achieve the desired VOC evaporation during the process, the process is controlled regarding process variables such as tem- perature, pressure, dew point, velocity of the drying gas flow, flow distribu- tion in the oven, et c. The choice between a process with VOC evaporation according to curve A and a process with VOC evaporation according to curve B is made based on conditions such as the structure of the drying facility, type and composition of the wood raw material, and demands on the dried wood material in a subsequent process or use.