2932 Chapter 24 



At the discharge end of the dryer, a drop-out box is provided in which air 

 velocity diminishes so that the flakes pass a fire dump and drop through an 

 airlock to a conveyor to screens. The fines are carried beyond the drop-out box to 

 a cyclone which collects them for use as dry fuel. 



Recirculation. — Recirculation of exhaust gases from the dryer collector to 

 the dryer inlet can reduce fuel required and reduce particulates in exhaust gases. 

 The amount of recirculation varies inversely with the water evaporation require- 

 ment of the system. For example, if a dryer is operating at about 80 percent of its 

 rated capacity, then only 20 to 30 percent of the gases could be recirculated. But 

 if the dryer was using only 30 percent of its capacity, up to 50 to 60 percent could 

 be recirculated. 



Recirculation can increase fire hazard by burning of particles in the recirculat- 

 ed gases, particularly in the event of a plugged collector. Although commonly 

 used in other industries, recirculation is being used in only a few board plants to 

 date. Dryer manufacturers make special design provisions for recirculation, 

 including furnace breaching for burning the particles while returning the gases to 

 the dryer inlet, and extra elbows in ductwork to extinguish sparks; they should be 

 consulted if recirculation of gases is being considered. 



Pollution control. — Emission of particles with the gases from the primary 

 dryer collector generally exceeds pollution standards. Dust collection systems 

 following the dryer collector have difficulties because the steam content in the 

 gases condenses in cold weather. Secondary, high efficiency multiple cyclone 

 collectors are not always adequate. Impact or impingement type collectors will 

 generally not work with fines because of their low mass and inertia. Bag 

 collectors are particularly susceptible to any condensation, even with high- 

 temperature resistant bags, and any spark or fire in the system will destroy the 

 bags and frames, requiring costly replacement and down time. 



Raddin (1975) noted that the Aerodyne Dust Collector (fig. 24-15) has proven 

 effective. This unit has no moving parts, is constructed of steel, can be preheated 

 to prevent condensation of steam, and is not easily damaged by fire. Its efficien- 

 cy with wood dust and fiber is very high and particulate matter in the outlet gases 

 to atmosphere will be about 0.01 to 0.02 grains per standard cubic foot, which is 

 adequate to meet most emission standards. It is used as a secondary collector 

 following the primary dryer collector which removes most of the conveyed 

 material. 



Emissions of particulates from dryer collectors have been measured in the 

 range of 0.07 to 0.60 grains per standard cubic foot (dry without the dilutive 

 effect of the steam). Standards call for 0. 1 to 0.2 grains per standard cubic foot. 

 Of the total emission, the proportion of hydrocarbons can be as high as 40 

 percent, particularly with conifers. The hydrocarbons are evident as a haze in the 

 gases, and are measured by gas opacity. 



The temperature at the dryer inlet will affect the generation of haze — the 

 lower the better. Consequently, multiple stage drying at lower temperatures 

 could reduce haze to acceptable levels. Recirculation of gases reduces haze also 

 because the hydrocarbons burn when reintroduced to the furnace. Water scrub- 

 bing reduces hydrocarbons by agglomeration and cooHng, but not substantially. 



