WO2001059381A1

WO2001059381A1 - Flue gas heating of conveyor dryer for wood strands

Info

Publication number: WO2001059381A1
Application number: PCT/SE2001/000005
Authority: WO
Inventors: Stefan Mikaelsson
Original assignee: Valmet Fibertech Ab
Priority date: 2000-02-11
Filing date: 2001-01-04
Publication date: 2001-08-16
Also published as: AU2001227201A1

Abstract

Methods are disclosed for drying lignocellulosic material including providing hot flue gas from a combustion chamber, cooling the hot flue gas, mixing the partially cooled flue gas with cold air, drying the lignocellulosic material with the cooled flue gas to provide an exhaust gas, recirculating part of the exhaust gas stream to the combustion chamber, and recirculating another part of the exhaust gas to cool the hot flue gas.

Description

FLUE GAS HEATING OF CONVEYOR DRYER FOR WOOD STRANDS FIELD OF THE INVENTION

The present invention relates to methods for drying lignocellulosic material. More particularly, the present invention relates to methods for drying lignocellulosic material utilizing a hot flue gas from a combustion chamber as an energy source for such drying process. Still more particularly, the present invention relates to apparatus for drying lignocellulosic material. BACKGROUND OF THE INVENTION

The manufacture of various wood composition fiberboard products, such as in place of natural paneling, sheathing, decking lumber, and the like, has become more and more popular. One such product known as oriented strand board or OSB is an improved fiberboard composite which has proven to be rather successful. This OSB product can be manufactured from flakes created from logs and which are then subjected to forces to break the flakes into strands having a length parallel to wood grain several times the width of the strand. These strands can then be oriented on the board-forming apparatus .

One important aspect of the manufacture of such OSB products comprises the forming of these materials into a mat and the hot pressing of the mat into a finished board. During such manufacture, gaseous organic impurities, such as formaldehyde and other volatile hydrocarbons (known as VOC or volatile organic compounds) are generated and emitted. This generally takes place during the drying step, and, in fact, these substances can be deleterious to the environment. Indeed, in accordance with a process disclosed in U.S. Patent

No. 5,989,465 to Safstrόm et al., a process for manufacturing board products for lignocellulosic material is disclosed which includes drying in a pair of dryer stages in which part of the exhaust air produced in the second drying step is used as a source of drying air for the first drying step, and the exhaust air from the first drying step is used as a source of drying air for the first step and as a source of combustion air in a furnace .

It is also well known for the fiber dryer to be heated directly from hot flue gas from the burner in the dryer or from a common heat energy plant. The exhaust air from the drying step also contains fly ash from the flue gases.

A known system employing hot flue gas as an energy source and also recycling the exhaust stream containing most of the VOC compounds for combustion is set forth in FIG. 1 hereof. Thus, in this figure, the hot flue gas from combustion chamber 1 passes through line 13 and eventually through fan 14 to stack 12. In line 13, however, the hot flue gas is contacted with heater/boiler 3 , in which the heat from the hot flue gas is used to create steam, heated oil, heated water, or other heating medium, which passes through line 15 with pump 16 therein. This heating medium is then used to heat cold air in heat exchanger 4. The cold air enters through line 17 and blower 18, and is then_. Jtieated by indirect contact with the heated medium from line 14 in the heat exchanger 4. The cooler heating medium then returns to heater/boiler 3 through line 20. The now heated air stream from line 17 then passes through line 6 as dryer inlet air. In the dryer 5, which includes a conveyor for the wood strands passing therethrough, the dryer inlet air from line 6 contacts the wood strands for drying same. The exhaust air from the dryer then exits dryer 5 through line 7. A portion of the exhaust air from the dryer can also be recirculated through line 8 to be used as combustion air in the combustion chamber 1 after passing through combustion air fan 2. As is further shown in FIG. 1, the dryer exhaust air in line 8 can be passed into various sections of combustion chamber 1 as desired.

It has been most desirous, however, to devise systems which not only employ the hot flue gas to provide energy for the drying process, but which are more effective and efficient and less costly than these prior art systems. SUMMARY OF THE INVENTION

In accordance with the present invention, this and other objects have now been realized by the invention of a method for drying lignocellulosic material comprising providing hot flue gas from a combustion chamber, cooling the hot flue gas to provide a partially cooled flue gas, mixing the partially cooled flue gas with cold air to provide a cooled flue gas stream, drying the lignocellulosic material with the cooled flue gas stream thereby providing an exhaust gas stream, recirculating at least a portion of the exhaust gas stream to the combustion chamber, and recirculating at least another portion of the exhaust gas stream to the cooling of the hot flue gas. In a preferred embodiment, the lignocellulosic material comprises wood strands. Preferably, the wood strands are used to produce oriented strand board (OSB) .

In accordance with one embodiment of the method of the present invention, the hot flue gas is provided at a temperature of greater than about 800°C. In a preferred embodiment, the cooling of the hot flue gas provides a partially cooled flue gas at a temperature of less than about

400°C.

In accordance with another embodiment of the method of the present invention, the drying of the lignocellulosic material with the cooled flue gas stream provides the exhaust gas stream comprising a plurality of the exhaust gas streams. In a preferred embodiment, the method includes transporting the lignocellulosic material on a conveyor from a first end to a second end, wherein the plurality of exhaust gas streams comprises a first exhaust gas stream proximate to the first end of the conveyor and a second exhaust gas stream proximate to the second end of the conveyor. Preferably, the recirculating of the at least a portion of the exhaust gas stream to the combustion chamber comprises recirculating the first exhaust gas stream and the recirculating of the at least another portion of the exhaust gas stream comprises recirculating the second exhaust gas stream. Most preferably, the second exhaust gas stream comprises a plurality of second exhaust gas streams. In accordance with the apparatus of the present invention, apparatus has been devised for drying lignocellulosic material comprising a combustion chamber providing a hot flue gas, a first mixing chamber for cooling the hot flue gas by contact with a first cooling gas to provide a partially cooled flue gas, a second mixing chamber for cooling the partially cooled flue gas by contact with cold air to provide a cooled flue gas stream, a dryer for drying the lignocellulosic material by contact with the cooled flue gas stream to provide an exhaust gas stream, first recirculation means for recirculating at least a portion of the cooled flue gas stream to the combustion chamber, and second recirculation means for recirculating at least another portion of the cooled flue gas stream as the first cooling gas. In a preferred embodiment, the dryer for drying the lignocellulosic material provides a plurality of the exhaust gas streams. Most preferably, the dryer comprises a conveyor including a first end and a second end, and wherein the plurality of exhaust gas streams comprises a first exhaust gas stream proximate to the first end of the conveyor and a second exhaust gas stream proximate to the second end of the conveyor.

In accordance with a preferred embodiment of the apparatus of the present invention, the first recirculation means recirculates the first exhaust gas stream and the second recirculation means recirculates the second exhaust gas stream.

In accordance with another embodiment of the apparatus of the present invention, the second exhaust gas stream comprises a plurality of second exhaust gas streams. BRIEF DESCRIPTION OF THE DRAWINGS The present invention may be more fully appreciated with reference to the following detailed description, which, in turn, refers to the figures, in which:

FIG. 1 is a schematic representation of a system for drying lignocellulosic material in accordance with the prior art ; FIG. 2 is a schematic representation of a system for drying lignocellulosic material in accordance with the present invention; and

FIG. 3 is a partial schematic representation of a portion of a preferred embodiment of the method for drying lignocellulosic material shown in FIG. 2. DETAILED DESCRIPTION

Referring to FIG. 2, in which like reference numerals refer to like elements of those shown in FIG. 1, the Figure shows schematically the flow of drying air and exhaust air to and from the dryer for wood strands in accordance with this invention. Hot flue gas is provided from combustion chamber 1 through exit line 20 therefrom. This hot flue gas is used to provide a major portion of the medium for direct drying of the lignocellulosic material or wood fiber in the dryer 5. The remainder of the air required for drying purposes is supplied in the form of fresh air 17 from the atmosphere.

Thus, the hot flue gas from the combustion chamber 1 exiting through line 20 passes into the mixing chamber 11. In this energy plant mixing chamber (the combustion chamber 1 is associated with the energy plant for the associated equipment) , hot flue gas at temperatures of greater than about 800 °C is cooled down to about 400°C or less by contact with an air inlet recirculation stream 10, which is discussed in more detail below. Due to its cooling as described herein, the now partially cooled flue gas can then be transported and cleaned in normal mild steel ductwork and the like. In any event, the partially cooled flue gas stream then passes through fan 22 into a second mixing chamber 9. This drying mixing chamber is a chamber in which the partially cooled flue gas from line 20 is then contacted with cool or cold air from line 17 to provide flue gas at a suitable temperature for the dryer 5. The cooled flue gas stream then passes through line 23 and fan 24, and enters dryer 5 through dryer inlet air stream 6. After the drying process in dryer 5, dryer exhaust air exits the dryer 5 through line 7. This dryer exhaust air includes the volatile organic compounds (VOC) , and at least a portion of same can thus be recirculated back to the combustion system to be used as combustion air in combustion chamber 1 in which the VOC would then be combusted. This occurs through recirculation through line 8 with the aid of fan 2. Turning to FIG. 3, a preferred embodiment is shown in which the dryer 5 is divided into three sections, 5a, 5b and 5c. The dryer inlet air in line 6 from the dryer mixing chamber 9, including both the cold air from line 17 and the partially cooled flue gas from line 20, passes into the first portion of the dryer 5a for contact with the lignocellulosic material therein. Dryer exhaust air from chamber portion 5a exits through line 26. This material will include the largest portion of VOC produced in the dryer, and will be the warmest section of the dryer 5. Additional dryer exhaust air will vent the dryer sections 5b and 5c from lines 27 and 28, respectively. This exhaust gas, having less VOC, passes through line 30 into particulate cleaning apparatus 32 (known in the art) for transfer therefrom through line 34. This supply of cleaned exhaust dryer gas can be vented, or used in another portion of the plant, or also returned to the combustion chamber 1 along with the more impure dryer exhaust gas from line 26.

It can therefore be seen that in accordance with this invention, and particularly as compared to the prior art, mixing chambers 9 and 11 for the various air and gaseous streams are provided instead of heat exchangers, therefore demonstrating a substantial cost reduction in that manner alone. Furthermore, no corresponding heating medium, such as hot oil, steam or hot water, and boilers for these products, are required to produce the heating medium itself at a further substantial reduction in cost.

By using the flue gas from the combustion chamber 1 directly being fed to the drying mixing chambers themselves, there are no stack losses as would be the case when producing hot oil, steam or hot water in accordance with the prior art. Furthermore, there is a substantial concomitant increase in energy efficiency with a reduction in overall operating costs. Finally, direct heating without these stack losses provides for a smaller combustion plant in general and a further cost reduction.

As for the combustion chamber 1 itself, this is generally conventional. It includes an inlet for fuel, which can be biofuel in the form of bark, wood waste, or the like, which is generated during manufacture of the board material itself. Sanding dust can also be supplied through a separate inlet into the burner. It is also possible to use oil or gas as additional fuel. As is noted above, however, the air required for combustion is supplied in the embodiment shown in FIG. 2 through line 8 and various lines attached thereto.

The actual combustion temperature in the combustion chamber 1 is selected so that the main portion of the volatile organic substances in the exhaust air is combusted, and this is normally at about 850°C, but generally greater than 800°C.

Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A method for drying lignocellulosic material comprising providing hot flue gas from a combustion chamber, cooling said hot flue gas to provide a partially cooled flue gas, mixing said partially cooled flue gas with cold air to provide a cooled flue gas stream, drying said lignocellulosic material with said cooled flue gas stream thereby providing an exhaust gas stream, recirculating at least a portion of said exhaust gas stream to said combustion chamber, and recirculating at least another portion of said exhaust gas stream to said cooling of said hot flue gas.

2. The method of claim 1 wherein said lignocellulosic material comprises wood strands.

3. The method of claim 1 wherein said hot flue gas is provided at a temperature of greater than about 800°C.

4. The method of claim 3 wherein said cooling of said hot flue gas provides a partially cooled flue gas at a temperature of less than about 400°C.

5. The method of claim 1 wherein said drying of said lignocellulosic material with said cooled flue gas stream provides said exhaust gas stream comprising a plurality of said exhaust gas streams .

6. The method of claim 5 including transporting said lignocellulosic material on a conveyor from a first end to a second end, wherein said plurality of exhaust gas streams comprises a first exhaust gas stream proximate to said first end of said conveyor and a second exhaust gas stream proximate to said second end of said conveyor.

7. The method of claim 6 wherein said recirculating of said at least a portion of said exhaust gas stream to said combustion chamber comprises recirculating said first exhaust gas stream and said recirculating of said at least another portion of said exhaust gas stream comprises recirculating said second exhaust gas stream.

8. The method of claim 7 wherein said second exhaust gas stream comprises a plurality of second exhaust gas streams.

9. Apparatus for drying lignocellulosic material comprising a combustion chamber providing a hot flue gas, a first mixing chamber for cooling said hot flue gas by contact with a first cooling gas to provide a partially cooled flue gas, a second mixing chamber for cooling said partially cooled flue gas by contact with cold air to provide a cooled flue gas stream, a dryer for drying said lignocellulosic material by contact with said cooled flue gas stream to provide an exhaust gas stream, first recirculation means for recirculating at least a portion of said cooled flue gas stream to said combustion chamber, and second recirculation means for recirculating at least another portion of said cooled flue gas stream as said first cooling gas.

10. The apparatus of claim 9 wherein said dryer for drying said lignocellulosic material provides a plurality of said exhaust gas streams.

11. The apparatus of claim 10 wherein said dryer comprises a conveyor including a first end and a second end, and wherein said plurality of exhaust gas streams comprises a first exhaust gas stream proximate to said first end of said conveyor and a second exhaust gas stream proximate to said second end of said conveyor.

12. The apparatus of claim 11 wherein said first recirculation means recirculates said first exhaust gas stream and said second recirculation means recirculates said second exhaust gas stream.

13. The apparatus of claim 10 wherein said second exhaust gas stream comprises a plurality of second exhaust gas streams .