Method for binding resin and tar substances in wet-cleaning and wet precipitation systems

A method and apparatus are provided for binding emulsified resin and tar substances in circulating water of a wet-cleaning and wet precipitation system for waste gas produced in the wood industry. Wood dust is metered into the circulating water and receives the emulsified substances adsorptively. The wood dust is removed from the water circulation path and sedimented.

BACKGROUND OF THE INVENTION 
The invention relates to a method of binding emulsified resin and tar 
substances in circulating water of a wet-cleaning and wet precipitation 
system for waste gas produced in the wood industry. 
Austrian Patent AT-PS E 87227 describes a wet precipitation method for 
exhaust air cleaning of a timber desiccation system in which the exhaust 
air is wetted with circulating water, then cooled to the dew point of 
50-70.degree. C., whereupon the condensing aerosols are ionized by means 
of high voltage and finally precipitated in a wet precipitator. In this 
case, circulating water is used for all cleaning steps, and the method is 
performed without waste water, because the water balance is negative due 
to the evaporation cooling. 
If the raw, unprocessed wood (e.g. pine) contains a large amount of resin 
and is desiccated at high temperatures (600-800.degree. C.), a quantity of 
steam-volatile resin, tar and fatty acids is stripped out in addition to 
the cellular water (wood moisture). These substances are still present as 
gases in the desiccation waste air at 120.degree. C. In the following 
cooling to the dew point of 50-70.degree., these substances condense as 
aerosols ("blue haze") and can therefore first be precipitated as a 
tar-like coating on the precipitation surfaces of the wet precipitator and 
subsequently washed from there with circulating water. 
A property of the tar-like, resinous substances, however, is that they form 
stable emulsions with the water and therefore cannot be precipitated, or 
only poorly precipitated, with the installed centrifuges for removal of 
solids. Hence, these substance build up in the circulating water and, 
consequently, severe contamination and clogging occur. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a method and a system with 
which the tar substances described above can be adsorptively bound and 
precipitated out. 
The above and other objects are accomplished in accordance with the 
invention by the provision of a method of binding emulsified resin and tar 
substances in circulating water of a wet-cleaning and wet precipitation 
system for waste gas produced in the wood industry, comprising: metering 
into the circulating water a wood dust which receives the emulsified 
substances adsorptively; removing the wood dust from the circulating 
water; and sedimenting the removed wood dust. 
The method of the invention thus has the following features: wood dust for 
receiving the emulsified substances adsorptively is metered into the 
circulating water, and this wood dust is removed from the circulating 
water and sedimented. 
With the aid of the invention, the formation of coatings in a wet-cleaning 
and wet precipitation system can be effectively prevented. In a preferred 
embodiment of the invention, a quantity of wood dust amounting to 100% to 
800% of the original waste gas dust proportion is added to the waste gas. 
Advantageously, the wood dust is metered into the waste gas supplied to 
the wet-cleaning and wet precipitation system. In accordance with a 
further feature of the invention, sediment which is formed can either be 
recirculated in a desiccator or burnt in a furnace with other wood waste 
products. 
Wood dust produced in large quantities in the particle board industry has 
proven to be the optimum substance for performing the method of the 
invention. This dust is produced during surface smoothing in grinding 
machines. Wood sanding dust possesses good properties for the purposes of 
the invention because it has a good affinity for resin and tar, and 
because it darkens. The sanding dust also has a corresponding fineness and 
therefore a large surface area. Moreover, the loaded, wetted sanding dust 
sediments well. 
In accordance with a further aspect of the invention, the residual product 
can be disposed of, in the form of drained sediment, either by burning, or 
composting. It has proven particularly advantageous to use a substance 
which corresponds to the natural raw material wood. 
In a comparison of methods and systems currently in use, it has been seen 
that some systems operate problem-free without coating buildup, while 
others tend toward excessive baking-on. In an analysis of operating 
conditions, a link has been found between the type of wood and the 
system-stipulated dust content present in the waste gas following cyclone 
separations. Systems that have a high dust content and a low resin content 
are always free from coating buildup. With the addition of wood dust 
according to the invention, it has been determined that no worsening of 
the dust content of the clean gas occurs, and the quality of the 
circulating water is significantly improved. Therefore, the metering of 
wood dust into the circulating water permits universal application of the 
wet precipitation method in the timber industry. 
Within the scope of the method according to the invention, the sanding dust 
can be supplied into the gas line, into the washer, directly into the 
circulating water or into the circulating water line of the sludge removal 
device. A system according to the invention for performing the method 
includes a washing container to which the waste gas to be cleaned is 
supplied via a gas line at the lower end, and which has a washing device 
and a wet precipitation arrangement downstream thereof in the direction of 
gas flow. A wood dust supply device associated with the container has a 
supply and a metering device, by means of which a metered quantity of wood 
dust can be introduced into the circulating water. A sludge removal device 
is connected to the lower end of the container. 
The wood dust supply device preferably discharges into the wetting device 
of the waste gas supply line via an injection device. In accordance with 
another feature of the invention, the sludge removal device has a sludge 
pump and a decantering centrifuge downstream thereof, which is connected 
to the container by way of a water line. 
The invention is described in detail below by way of an embodiment of a 
wet-cleaning and wet precipitation system and in conjunction with the 
accompanying drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a waste gas line 1 for supplying waste 
gas or exhaust air of a wood desiccation system (not shown) to a lower end 
of a vertical container 3 of a wet-cleaning and wet precipitation system. 
A pre-wetting device 2 is disposed in the waste gas line. The supplied 
waste gas or exhaust air is introduced to container 3 through an inlet 3'. 
The gas flows upwardly in container 3 through a washing device 4 and is 
cooled in the process to the dew point of 60-70.degree. C. In a wet 
precipitation means which, as shown in the figure, may include a 
honeycomb-shaped device 5 disposed downstream in the direction of gas 
flow, to which an electrical high voltage is applied by a high-voltage 
unit 6, condensing aerosols in the gas are ionized and precipitated out in 
a wet precipitator 7 or de-mister disposed downstream of honeycomb-shaped 
wet precipitation, means 5. The gas exits the upper end of the column 3 as 
clean gas through an outlet 3". 
The residue formed in container 3 is removed as sludge by a sludge pump 7 
and a decantering centrifuge 8 connected to container 3 by a water line 
8'. The residue is then deposited as sediment S. In a circulation path, 
the washing water is conveyed by a circulation pump 9 up to the washer and 
for rinsing the honeycomb, on the one hand and, on the other hand, to the 
pre-wetting device 2 by a pre-wetting pump 10. 
A wood dust supply device 11 is provided in a region of waste gas line 1 
and includes a wood dust silo 12 which has a SCREW-CONVEYOR 12' and a 
ventilator 13 which sprays the wood dust, preferably wood sanding dust, 
into pre-wetting device 1. 
The wood dust blown into the pre-wetted waste gas is introduced into 
container 3 with the waste gas, and enters the washing water circulation 
path in this way. The metering in of the wood dust can be controlled by 
the silo discharge SCREW-CONVEYOR 12' to correspond to operating 
requirements. A quantity of wood dust amounting to 100% to 800% of the 
original waste gas dust proportion is added to the waste gas. 
The described embodiment can be modified within the scope of the general 
concept underlying the invention. Thus, the wood dust can be added 
directly into the container, the washing water circulation path or the 
circulating water line of the sludge removal device instead of into the 
crude gas.