ENERGY CONTROL ENGINEERING Lake City, Minnesota 55041

A Synopsis of the Process

Whole wet logs are processed through shaving mills. Shavings produced are fed through a rotary drum dryer where their moisture is reduced from 45% to 12% (or lower). Dry shavings are then fed over a screen. Small "dry sawdust"-like particles are screened out and used as fuel to heat the dryer.

A Schematic of the Operation

This writer speaks from experience on the subject of system pollution emissions. Referring to the system schematic above, note there is only a singular exhaust to the atmosphere: that being from the dryer's exhaust cyclone. Here are the factors that contribute to the emissions, or lack thereof, from that exhaust.

• The fuel being burned in the Webb Burner™ is the clean small sawdust-like wood particles from the shavers. The fuel is dry (having just exited the dryer). The fuel is fired in semi-suspension combustion with a generously low volumetric heat release. Combustion is maintained at approximately 1400°F. The combustion controls are automatic. It can be said with proven certainty that this Webb Burner™, with its high combustion temperatures, will produce no visible emissions, no smoke, no carbon monoxide, and no volatile organic compounds. The dry sawdust fuel contains no sulfur, so there will be no SOx emission. Yet combustion temperatures are not so high as to cause onset of NOx emissions (NOx generally requires temperatures neighboring 2000°F or higher). Combustion of this fuel does result in the yield of dead inert ash. I estimate dead inert ash production at a rate of 20 lbs/hr. Virtually all of this ash becomes entrained in the hot flue gases leaving the burner (and being drawn into the dryer). Note that none of the ash or flue gases exit from the burner to the atmosphere. The stack atop the burner is not an exhaust point to the atmosphere: it is an opening for ambient air intake. The dead inert ash will be carried along with the hot flue gases into the dryer where gases and ash come in direct contact with wet shavings being dried. In the dryer, all the 20 pounds of ash agglomerates onto the many thousands of pounds of wet tumbling shavings. Ash is thereby forever removed from the flue gas stream.
  

• Hot flue gases come in direct contact with wet shavings in the dryer. If the gases are too hot, they can "cook out" some of the wood's resins and cause a so-called "blue haze" emission. We ask that operators limit their dryer inlet temperatures to no higher than 850°F to preclude the emission of blue haze. (The entire state of Wisconsin has taken this recommendation and mandated an upper limit on dryer inlet temperatures of 800°F.) This drying system will operate very clean, by design, with upper inlet temperatures neighboring 750°F. Dry shavings are pneumatically carried by the warm dryer exhaust gases to the exhaust cyclone. Here, the warm dry shavings are removed from the bottom and warm exhaust gases exit to the atmosphere. Gases exit at about 200°F. Gases are laden with water vapor. In winter there will be a white condensed water vapor plume visible from the stack.
  

• The cyclone is not perfect in its removal of dried wood particles from the exhaust gas. A small amount of very fine wood particulate will escape as an emission.The amount is so little and so fine that state Pollution Control Agencies have not yet warranted testing for grain loading in exiting identical systems.
  

You can view existing identical systems in operation at many locations both in America and internationally. Some have been operating for 20+ years. Obviously, these systems work very well!

If you are planning a shavings drying operation and need help talking with your Pollution Control authorities, please call Jackson Lumber Harvester at 715-926-3816.

W. B. Webb, Physicist, Registered Professional Engineer