This smell is made up of volatile organic compounds (VOCs) and the concentration of these emanating from a factory is limited in many parts of the world. Dust is also created when wood particles or fibres are dried, especially if wood and bark dust are used as part of the dryer fuel. Dust emissions are also regulated so panel manufacturers must put in place systems to minimise all air pollution.
All mills use a combination of cyclones and filter screens to minimise dust emissions, but cyclones are not very efficient at removing very fine particles and are ineffective for VOC emissions. However, installing a Wet Electrostatic Precipitator (WESP) can reduce gas and particulate emissions by over 95%.
The first cleaning step in a WESP involves the quenching of the air stream by spraying in water to cool the air and thus condense the vaporised compounds. Some, soluble in water, are collected by the ‘rain’ in the WESP.
The second step is to pass the air through a battery of electrostatic filters. These consist of a discharge tube (often hexagonal in cross-section) and a high voltage electrode in the centre of each tube. The electrode ionises the dust, giving it a negative charge, and thus it is attracted to the positively charged tube walls.
Water sprays are used to continuously wash the discharge tube surfaces so dust should not accumulate in a well-designed and maintained WESP. The dust/VOC/water mix is collected at the WESP base.
Other, more complex WESPs can have a second stage electro-static screen, ozone (O3) removal system (small quantities of ozone are generated by the electrostatic filter), plume minimisation and integrated water treatment.
The photograph shows that the characteristic white plume normally seen from dryer stacks is much reduced and this alone can reduce complaints from nearby residents.
So, WESPs efficiently reduce air pollution, but they generate some polluted water which must be treated. Modern WESPs are designed to clean and recirculate the water to minimize fresh water use. Recirculated water goes through sedimentation and decanting stages before reuse. Some factories also collect rain water to use in their WESPs.
The sludge that is separated is often burnt, which is not energy-efficient but it does provide a practical solution to its disposal. Others use an activated sludge treatment whereby aerobic bacteria break down the organic matter.
Pollution control is neither a simple nor a cheap aspect of panel manufacture; it is, however, an essential one.
