Haps generation and removal

HAPs emissions from veneer dryers are lowest in the final zone where the temperature is the highest. This runs contrary to expectations, since high temperatures usually increase emissions. Air drawn in through the cooling section and is probably diluting the emissions from the last zone. An analysis of pinene emissions suggests that there is partial mixing of the air between adjoining zones. The overall HAPs emissions are an order-of-magnitude higher than the proposed production-based limit. The gap is unlikely to be closed through small changes in operating conditions. HAPs emissions from an OSB mill that dries at a high inlet temperature of 1,200°F averaged 3 lbs/ton. While the HAPs dropped with increasing moisture, the HAPs load was 1.5 lbs/ton even at 8% MC, which is still well above the production-based limit. Similar measurements made at a mill that dried at lower inlet temperatures showed wide variations in inlet temperature that could increase HAPs generation. Fines generate a disproportionate quantity of HAPs through overdrying, and several mill trials were conducted in order to determine whether the quantity of fines produced increased with the age of the flaker knife. This was true at one mill but not at others, possibly because of the effect of additional variables such as log diameter.