Process-based control of HAPs emissions from drying wood flakes.
Industrial wood flake drying generates methanol, formaldehyde, and other hazardous air pollutants (HAPs). A simple theoretical model shows that particles smaller than 400 microm will begin to thermally degrade and release disproportionately large quantities of HAPs. This is confirmed in full-scale practice where particles smaller than 500 microm show visible signs of charring. Laboratory measurement of the activation energy for the breakdown of wood tissue into methanol and formaldehyde led to a value of about 17 kcal/mol. The apparent activation energy measured in the field was higher. This result was obtained under nonisothermal conditions and is biased high by the fines fraction of the furnish, which is exposed to elevated temperatures. It is proposed that a combination of screening out the fines fraction smaller than 500 microm and reducing the dryer inlet temperature will substantially reduce emissions, possibly to the point where control devices can be downsized or eliminated. Our findings allow these HAPs reductions to be semiquantitatively estimated.