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Dust Explosions:An Overview 735

1.5.3.3
Wood, Cellulose, and Peat Dusts

The fire and explosion hazards in the production of chipboard, hardboard, and wood
powder have been recognized for a long time. As new insight and technology have become
available, the methods of preventing and mitigating the hazards have been improved.
Thelning and Laufke (1970) mainly focused on mitigation, in particular by venting and
automatic suppression of explosions, and fire extinction by carbon dioxide and water
vapor. Schmid (1972) gave detailed recommendations for both fire, explosion and envi-
ronmental protection of chip board producing plants. He included prevention of ignition
sources by recommending removal of foreign stone and metal objects before admitting
the raw material to the process, and avoidance of overheating. The specific processes of
chip preparation, pressing and cutting of the board, and the final finishing of the board
surface were considered separately.
Arvidsson, Back, and Ostman (1977) conducted a very comprehensive investigation
of the explosion and fire hazards in the production of chipboard. They summarized their
recommendations for preventive and mitigating measures in a list of 59 specific points,
paying particular attention to removal of foreign objects in the plant feed, transport,
storage, and further size reduction of undried wood chip, drying of the chip, storage of
the dried chip, milling of the dried chip, finishing of the board, and general housekeep-
ing. Special attention was paid to the chip drying process. Continuous control of the
oxygen concentration in the drying gas, maintaining it as low as possible, was strongly
recommended.
The work ofkvidsson et al. (1977) was presumably not known to Drossel (1984), who
suggested a similar list of means to prevent and mitigate dust explosions and fires in chip-
board production. However, Drossel included automatic extinction of potential ignition
sources in the form of small glowing wood or board fragments in gas and dust extrac-
tion ducts as an additional element. This method, which resulted from newer technological
development, has proven particularly useful in the wood industry and was described in
greater detail by Schroder (1984) (see also Section 1.4.4).
Scholl(l975) investigated the flame development following ignition inside mobile
vacuum collectors for wood dust and wood chips. He found that only smoldering or
open fires occurred, but no dust explosions. Furthermore, fire was only initiated if the
ignition source was comparatively energetic, and the dust/chip mixture contained
an appreciable fraction of fine dust (<lo0 pm). May et al. (1981) concentrated on the
prevention of fire and explosion in wood chip dryers. They found that considerable
overall improvement could be achieved by adequate process control and energy econ-
ornization. Pfeiffer, Kuhnen, and Armbruster (1985) investigated particle size distri-
butions of airborne dusts from wood sawing and finishing operations. The particle size
at which 30% of the dust mass was finer varied between 22 pm and 10 pm depending
on operation and wood type. The mass fractions of very fine dust (<7 pn) varied
between 20 and 2%.
Natinral cellulosic dusts that can give rise to explosions are also generated in the cotton
and linen textile industries. A catastrophic linen dust explosion in Harbin, Peoples
Republic of China, is discussed in Chapter 2. Kuczynski (1987) suggested a compre-
hensive scheme for protecting cotton processing plants against dust explosions. Particular