FIELD OF THE INVENTION

The invention concerns a system for drying according to the preamble of claim 1. The invention also concerns a method for drying according to the preamble of claim 12.

BACKGROUND OF THE INVENTION

During drying of organic particle-shape material such as sawdust, hydrocarbon compounds are emitted from the material intended for drying in concentrations that are higher the higher the temperature is of the drying gases.

These emissions result in an environmental load, since the emissions of hydrocarbons influence the environment. When the dried material is to be used as fuel in furnaces of different kinds, expelling of hydrocarbons results in a technically negative aspect, since the thermal value of the material is reduced.

In a previously known plant for drying sawdust for subsequent pellet manufacture, a flue gas conduit is connected from a furnace to a drying device, wherein used flue gases/drying gases contain high concentrations of hydrocarbon compounds .

For reducing these concentrations, according to the background art firing of the flue gases can be used and/or different types of per se known flue gas cleaning systems. The known systems for drying organic particle-shaped material have, however, proved to be relatively energy consuming and thereby economically disadvantageous. The exhaust gas control in the known system is further relatively complex, in order to lower the polluting concentrations to an acceptable level. AIM AND MOST IMPORTANT FEATURES OF THE INVENTION

One aim of the present invention is to provide a system and a method as indicated above wherein the problems with the background art are addressed and at least reduced. This is achieved according to the invention in that the gas heating device includes a heat exchanger, for heat exchange between a flow of flue gases generated in the combustion device and a flow of said drying gases driven forward by a driving device in a conduit circuit, and that the conduit circuit exhibits on the one hand an air intake for oxygen containing air, on the other hand a gas discharge for the connection to an intake for air for combustion to the combustion device.

The flue gases from the furnace can through modern firing technology be brought to contain much lower or negligible concentrations of for example hydrocarbon compounds.

By providing the conduit circuit for the drying gases with an air intake for oxygen containing air, and discharging the air for combustion to the combustion device from there, a number of advantages are obtained. Firstly, it is achieved that hydrocarbon compounds absorbed into the drying gases in the drying device are passed over the gas outlet to the combustion device in order to participate in the combustion process therein and thus, add useful energy to the process. Secondly it is achieved that no particular purification device or the like is necessary for eliminating or reducing the hydrocarbon compounds in the flue gases emitted by the system. Thirdly, it is achieved that the great amounts of water, which emanate from the material intended for drying are supplied to the combustion process in the combustion device, which can be a great advantage during subsequent possible flue gas condensation with adherent possible energy recovery. Particularly advantageous is that said gas outlet is positioned downstream of the drying device and upstream of the heat exchanger, wherein in particular it is preferred that said air intake is positioned downstream of said gas outlet and downstream of the heat exchanger.

It is preferred that a flue gas condenser having means for heat recovery is placed in a flue gas conduit downstream of the heat exchanger, and in particular that said means for heat recovery is connected to a hot air generator for the production of a stream of hot drying gases to a second drying device .

This second drying device is preferably a low temperature dryer, wherein relatively low emissions of hydrocarbon compounds are produced. It is preferred that used drying gases emanating from this second drying device are passed on to and united with a conduit for flue gases from the furnace after the heat exchanger as well as the flue gas condenser.

By the conduit circuit for drying gases being provided with a by-pass conduit for leading away a portion of gases flowing through the drying device passed the heat exchanger, it is achieved that the drying gas temperature in the drying device can be controlled and typically be reduced from the relatively the high temperature which prevails directly after the heat exchanger. Hereby it is possible to adapt to and increase the quality of the dried material.

Further, the possibility is obtained of a greater drying gas flow through the drying device and with maintained high contents of terpenes in the dried material. Further, the risk of self-ignition in the drying device is reduced. By arranging a preheating device for preheating air for combustion supplied through the combustion device, it is achieved that problems with the condensation in the conduit for air for combustion is reduced without affecting the energy- balance in the system negatively.

In a system according to the invention there is suitably arranged a control unit for controlling at least one from the group: gas flows, flow of heat carrying medium, fuel flow to the combustion device, flow of material intended for drying.

The control is advantageously based of anyone or more of: sensed oxygen contents in the flue gases after the combustion device, heat requirement in the drying device, the quantity of material to be dried etc.

It is desirable that the flow of air for combustion is controlled such that the oxygen contents in the flue gases is as low as possible, since hereby no unnecessary extra air has to be driven around in the process . Corresponding advantages are obtained through the corresponding method features .

Further features and advantages are apparent from the following detailed description.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described in more detail by way of embodiments and with reference to the annexed drawings, wherein :

Fig. 1 diagrammatically shows a system for drying according to the invention, and

Fig. 2 diagrammatically shows a flow schedule over a method sequence according to the invention.

DESCRIPTION OF EMBODIMENT The invention concerns a system and a method for drying an organic particle-shaped material, wherein the invention in particular is used in respect of drying sawdust, chips or the like material emanating from wood. In particular such materials are dried in order to subsequently be used as fuel in furnaces for renewable fuels, such as furnaces for pellets, furnaces for firing wood chips etc. As concerns the subsequent production of pellets, typically dried particle-shaped material is pressed together into pellets of suitable size for subsequent combustion.

As concerns sawdust, this is a residual product from saw mills and as a rule has a very high water contents because of the cooling of the saw blades during sawing with flows of water. Resulting sawdust hereby absorb water to a high extent and must be subjected to drying to be useable as fuel.

Calculations indicate that a very great part of the raw wood material which is taken out from forest lands and is delivered to the sawmills can not be used as sawed products, but instead, besides being residual material in the form of sawdust, must be disintegrated into chips of different quality, whereof a part is used for making different kinds of pressed board.

Because of the present very great demand for renewable energy sources, using wooden raw material is interesting for heating of individual homes and other buildings, for district heating etc.

A plant based on a system according to the present invention can very well be positioned close to a saw mill for locally being an important component in the chain for providing directly usable renewable fuel and for avoiding long transports of raw material in the form of highly moist sawdust, wood chips etc.

In Fig. 1 is diagrammatically shown, within the rectangle marked with interrupted lines, a system according to the invention, wherein 1 indicates a combustion device for firing organic material such as for example bark, wood chips or the like transported over a transportation device indicated with 10.

2 indicates a combustion gas channel inside which flue gases produced in the combustion device 1 are passed on to a heat exchanger 3 wherein the flue gases are heat exchanged to drying gases, which in turn are circulated in a circuit 4, 4'.

After heating in the heat exchanger, the drying gases are passed on in the form of a hot gas stream through a conduit portion 4' driven through a blower 12 into a drying device 5. The drying device 5 can be of a conventional kind and is suitably a drum dryer, through which the material intended for drying is transported continuously and brought to come into direct contact with the drying gases in the conduit circuit 4, 4' . Material intended for drying is led into the drying device 5 over a first conduit 13 and the dried material is discharged from the drying device 5 over a discharged conduit 14.

In a conduit portion 4 which is positioned in the conduit circuit downstream of the drying device 5, in operation, the drying gases will contain water to an extent that is determined by the providing parameters, but it is preferred that the operation is such at the drying gases directly downstream of the drying device are essentially saturated with moisture .

In this portion 4 exits a gas discharge 7', which joints to a channel 7, which leads to the combustion device 1. In the channel 7 is driven, through the action of a blower 11, the air which is to be used as air for combustion in the combustion device 1. In order to facilitate this, to the conduit circuit 4, 4' is supplied a flow of oxygen gas containing air/gas through a channel 6 to an air intake 6', which in the shown embodiment is positioned in the conduit circuit downstream of the gas discharge 7' and upstream of the heat exchanger 3. Oxygen contained in the air/gas supplied through the air intake 6' will thus be mixed into the drying gases in the conduit portion 4, to pass through the heat exchanger 23, where it is heated, be driven by the blower 12 and participate in the drying process inside the drying device 5 in order to subsequently be taken out through the gas discharge 7' and subsequently end up inside the combustion device 1 as oxygen to the combustion process in the combustion device 1.

Through this supply of oxygen and the discharge of oxygen containing gas through a gas discharge 7', in an equilibrium condition, the same amount of oxygen which is a supplied through the channel 6, will also be supplied to the combustion device 1. Further, from the material intended for drying, as concerns sawdust and the like, emitted hydrocarbon compounds will be taken out from the conduit circuit 4 continuously in order to be supplied to the combustion device 1 and there participate in the combustion and a contribute as useful energy for the system.

Further, from the drying process in the drying device 5 emitted amounts of water from the material to be dried, will be supplied to the combustion device 1. This water is in principle no problem in connection with the combustion inside the combustion device 1 and can together with water formed during the combustion and possibly contained in the fuel to the combustion device, be condensated out in a flue gao condenser 16 with heat recovery in a per se known manner.

Recovered heat from the flue gas condenser 16 can be used in a desired way, but a preferred use is to use the heat in a heating element for drying gases, which are used for drying organic particle-shaped material in a second drying device 17. Thereto means 18 belong for creating an air stream through a channel 23 with a fresh air intake, a heat exchanger 19 for heating drying gases being connected to the flue gas condenser over a heat carrier circuit 38, to the second drying device 17. Material intended for drying is led into the drying device 17 over a first conduit 43 and a dried material is discharged over a discharge conduit 44. From this second drying device 17 a relatively small amount of hydrocarbon compounds will be emitted because of the relatively low drying temperature.

Emitted and used drying gas from the second drying device 17 is led over a discharge channel 21 to a flue gas conduit 22 downstream of the heat exchanger as well as the flue gas condenser 16 for further transport for example to a chimney belonging to the system.

According to an embodiment of the invention, the channel 6 for oxygen gas containing air/gas has a direct intake for oxygen containing fresh air, which is indicated with interrupted line at 9.

According to a preferred embodiment of the invention, however, the channel 6 for oxygen gas containing air/gas has instead of a fresh air intake (at 9) a connection to a channel 20, which has a connection at 20' to the discharge channel 21 from the second drying device 17. A portion of the gases in this discharge, which is preferably saturated with water, is led as oxygen containing gas component to the conduit circuit 4, whereupon the formed mix is heated, as is indicated above, in the heat exchanger 3, in order there from, in a heated state, to form the drying gas for the drying device 5. Finally, gas with oxygen contents according to the above is discharged from the conduit circuit 4 through the channel 7 to the combustion device 1. The advantages with this embodiment are that air which is rich of energy with great water contains after the drying device 17 is supplied to the combustion device over the conduit circuit 4. More energy can thereby be recovered by flue gas condensing in the condenser 16.

A part of the (small) amount of a hydrocarbon compounds, that in spite of this exists in the channel 21 after the second drying device 17 will through this aspect of the invention contribute to the combustion in the combustion device 1.

A control unit 15 is arranged to control the process, whereby preferably all gas flows can be controlled in respect of their speeds as flows, flows of gases and heat carrying medium are controlled, the fuel flow to the combustion device 1 is controlled as well as the flows of material intended for drying through the drying device 5 as well as the second drying device 17. As input data (see interrupted lines in the figure) to the control unit 15 are supplied signals concerning composition of flue gases after the heat exchanger 3 respectively after the end of the discharge channel 21. Further, signals from sensors for temperature in heat exchanger, drying devices and possibly also in gas conduits. As a complement, the channel 7 is suitably provided with a device for preheating the air for combustion (not shown) , i.a. in order to avoid condensation problems in the conduit 7. Further, a by-pass conduit 8 is suitably provided in the conduit circuit for drying gases 4, 4', wherein a portion of the gases flowing through the drying device are led passed the heat exchanger 3 in order i.a. to ensure a lower drying gas temperature through the drying device 5. A by-pass conduit can be arranged such that it joins the channel 7 over the channel 8 to the conduit portion 4' . For controlling the gas flows, different types of controllable throttle valves or the like (not shown) can be provided, for example in the channels 2, 4, 4', 6, 7, and of course blowers, such as the blowers 11, 12 and 18, can be controllable such that the flow through the respective channels can be regulated.

In Fig. 2 there is diagrammatically shown a flow chart concerning a method sequence according to the invention, wherein

Position 30 indicates the start of the sequence.

Position 31 concerns leading material intended for drying into a drying device in order to come into contact with drying gases therein, Position 32 indicates generating flue gases in a combustion device and heat exchanging of the flue gases with a flow of said drying gases,

Position 33 concerns intake of air into the conduit circuit for drying gases and discharge of gas for transfer as air for combustion to the combustion device.

Position 34 indicates condensation of flue gases in connection with heat recovery in a flue gas conduit downstream of the heat exchanger,

Position 35 indicates the end of the sequence.

EXAMPLE 1

As an example can be mentioned that in a first plant according to the invention, wherein a combustion device generates an effect of 10 MW, 20 000 kilograms of moist sawdust is transferred to a drying device per hour and 10 000 kilograms of dried sawdust (10% moist contents) is discharged from the drying device per hour. Thus, 10 000 kilogram water per hour is led to the combustion device 1 over the discharge 7' and the channel 7 in Fig. 1. In a flue gas condenser the said 10 000 kilograms of water per hour is condensed and further 2800 kilograms of water per hour emanating on the one hand from water generated through the combustion itself, on the other hand moisture contained in the fuel. In the condenser, hereby 12.8 MW is recovered and from a second drying device is taken out from an inlet flow of 24 000 kilogram moist sawdust per hour 12 000 kilograms of dried sawdust (10% water) per hour, whereas water absorbed by the drying gases in the second drying device is passed on to a chimney for the system.

Directly downstream of the heat exchanger 3 in the flue gas conduit 22, the proportion of hydrocarbons typically is very close to zero because of use of a good combustion in the combustion device 1.

As fuel to the combustion device 1 is suitably used bark, chips of the like, whereby is supplied 6350 kilograms of such fuel per hour.

The invention can be modified further within the scope of the following claims and thus, different types of combustion devices can be used as well as different types of drying devices. It is thus preferred that there is provided flue gas heat recovery after the heat exchanger and in particular that this is made, as is indicated above, in the form of so called low temperature drying. It is, however, not excluded that the recovered energy is used for other purposes such as for example heating of buildings through district heating, local heating or the like.