Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND ARRANGEMENT FOR THE CONTINUOUS PRODUCTION OF
LIGNOCELLULOSE-CONTAINING BOARDS
The present invention relates to a method of producing continuously ligno-
cellulosic boards in accordance with the preamble of claim 1, and to an
arrange-
ment for carrying out the method.
Methods of producing boards from lignocellulose-based raw materials are
well known to the art and have found wide use in practice. The manufacture of
such boards includes the following main method steps: disintegration of the
raw
1 o material to fibres andlor particles of appropriate size, drying the
particles and/or fi-
bres to a determined moisture ratio and gluing the material either prior to or
sub-
sequent to said drying process, shaping the glued material to form a mat,
which
may comprise several layers, and optionally cold pre-pressing the mat, preheat-
ing said mat, water-spraying mat surfaces etc., and heat pressing the mat in a
dis-
continuous press or in a continuous press while subjecting the material
simultane-
ously to pressure and heat so as to obtain a finished board.
A well-known problem with present day manufacturing technology, irre-
spective of whether it involves discontinuous presses or continuous presses,
is
that gases are generated in the press during the compression process, which
2 o takes place at high temperatures. These gases consist of water vapour
(steam),
different volatile substances dissolved from wood and glue, so-called Volatile
Or-
ganic Compounds (VOC), and gaseous phenol from wood and glue, etc. It has
been found that long-time exposure to these substances results in irritation,
and
that they are also harmful to personal health when present in sufficiently
high con-
centrations. Consequently, the authorities in the majority of countries in
which
boards are manufactured in accordance with the aforesaid methods have elabo-
rated a set of rules and regulations that state the emission concentrations
that are
permitted in work places and the permitted concentrations permitted in
emissions
to atmosphere.
3 0 Since present day press technology involves the use of homogenous
heating plates or steel bands, only a minor part of the gases generated in
press
will leave the boards through their edges in the compression process. However,
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the major part of these gases will leave the board as it exits from the press.
The
influence of these gases on the working environment can be limited to some ex-
tent with the aid of protective casings and covers, although air at room
tempera-
ture is normally used as transport air because of the large size of the
presses.
Consequently, this air volume will normally exceed the requirement of
combustion
air in the standard heating plant of the factory. This has necessitated the
installa-
tion of complicated and expensive equipment in connection with the majority of
plants in which lignocellufosic sheets and boards are produced. For instance,
the
plants will normally include so-called RTO (Regenerated Thermal Oxidizer)
units
or scrubber systems for purifying press gases.
The object of the present invention is to provide a method and an arrange-
ment for producing lignocellulosic boards without VOC-emissions or formalde-
hyde-emissions to the workshop areas concerned and to the ambient environ-
ment, and also obviating the need to install expensive purification equipment.
This
object is achieved with a method and an arrangement according to the invention
that have the characteristic features set forth in respective claims.
The invention will now be described in more detail with reference to the
accompanying drawing, which is a schematic longitudinal section view of an ar-
rangement in accordance with the invention.
2 o The plant illustrated in the drawing is based on the plants disclosed in
SE
502 272 and SE 504 638, which describe two continuous steam-press processes.
A fibrous mat 1 previously formed in the manufacturing process is compressed
in
a continuous steam-injection press 2 to form a board or sheet 3, which is then
passed through an after-conditioning unit 4. As the fibre mat 1 passes into
the nip
2 5 between two steam-injection rolls 5, steam is delivered and injected into
the mat
through wires 6. The temperature rises very quickly to above 100°C; a
typical
temperature is above 120°C. The mat is herewith formed into a solid
board 3. The
pressure falls as the board leaves the nip between the steam-injection rolls
5, and
the temperature therewith drops very quickly to about 100°C. This takes
place by
3 o virtue of the extremely rapid vaporisation of part of the enclosed
moisture. VOC-
emissions and formaldehyde-emissions accompany the departing steam.
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Because this process takes ptace between two gas-permeable wires 6,
the steam and the gases departing with the steam are able to leave the board a-
cross the whole of its width. Steam and other emissions are captured before
being
able to escape into the workshop area or to ambient atmosphere, by a suction
unit
8 provided to this end inside the press. Air heated to a temperature in excess
of
100°C is transported to this suction unit. The hot air is used together
with leakage
air from the surroundings as a vehicle gas for the steam and said other
emissions.
The hot air, leakage air, steam and emissions are transported to a heating
plant 9
in the factory, for combustion. A hot air delivery unit 11 is connected to a
curing
1 o zone 10 in the press 2, and the hot air supplied is then passed to the
suction unit
8.
The temperature is maintained at a high level partly to prevent the emis-
sions and the steam from condensing out to the suction system and partly to
uti-
lize the fact that the moisture carrying capacity of the air, calculated per
kilogram
of air, increases with increasing temperatures. This enables the total air
volumes
and gas volumes to be maintained at levels which do not exceed the volumes of
combustion air that are required by the standard plant system to generate the
heat
and process steam necessary for the production of such board material. Conse-
quently, no other equipment need be installed to prevent emissions to the sur-
2 o roundings.
Subsequent to the board having been produced in the continuous steam
injection press 2, the board is passed into the after-conditioning unit 4 (see
SE
504 638) where a pre-determined volume of air heated to a pre-determined tem-
perature and having a pre-determined moisture content is sucked through the
board so as to obtain a desired board moisture content and temperature. The
air
leaving the after-conditioning unit will also contain emissions of VOC and
formal-
dehyde, although in smaller quantities; measurements taken in a pilot plant
have
shown that the major part of the emissions occur in the continuous steam-
injection
press. For this purpose, a suction unit 12 is arranged in the after-
conditioning unit
4. Air is sucked in at 13 and heated by a heater 14 and is supplied with steam
through the conduit 15.
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The air leaving the after-conditioning unit is transported to the hot air sup-
ply unit 11 of the steam-injection press 2 and its curing zone 10, by means of
a
suction fan 16. As it passes to the supply unit 11, the air is given
additional energy
through the medium of a heat exchanger 17. If the air from the after-
conditioning
unit 4 is in excess, the excess can be mixed with the flow from the press 2 in
a
closed hood 18 and passed to the heating plant 9. If there is a deficiency of
air to
the curing zone 10, the suction fan 16 draws-in extra air through the closed
hood
18. The air leaving the after-conditioning unit 4 is thus used as hot input
air for the
internal suction unit 8 of the continuous steam-injection press. Measurements
have shown that these volumes are sufficient to fulfil the requisite transport
vol-
umes needed for the continuous steam-injection press.
Subsequent to having passed through the after-conditioning unit 4, the
board 3 may optionally also be passed through a surface-densifying press in ac-
cordance with SE 502 272 (not shown in the drawing). This latter press also in-
cludes a special suction unit that functions to capture in said press those
emis-
sions that are transported to the combustion plant of the factory with the aid
of hot
air, for the production of heat and steam.
