Bed – The fluidized bed dryer can be subdivided into multiple sections or beds. The chips flow sequentially from the inlet into the first bed and onward through additional beds to the outlet. The floor of each bed will be slanted toward the outlet end of the the dryer. The length of each bed will depend on how the chips can be moved from bed to bed.

“Blue-Haze” – This refers to the appearance of the dryer gas outlet stack and the corresponding emissions. The appearance of “blue-haze” is an indication that some chips have been overheated and have begun to oxidize. It can usually be prevented by always maintaining the chips below about 150F.

Dryer Gas – The dryer gas is any gas that can carry away the moisture in the wood as water vapor. Air is a good dryer gas if the dryer design can tolerate the oxygen in the air which can cause fires or explosions. In rotary dryers the exhaust gas, which is low in oxygen is recycled to the dryer inlet and mixed with the flue gas from the gas preheat burner.

Drying Curve – This gives the relationship of chip moisture-content versus time during the drying process. It is used during the mathematical modeling process to determine how long the chips must be retained in each bed to attain a specific moisture-content.  The drying curve for wood chips can vary considerably with chip size and wood type, but the process to establish the curve is relatively easy.  Chips with measured moisture content are dried in a simple fluidized bed for a measured amount of time and then checked again for moisture content.  Several runs are necessary to get points at different times.  The air rate must be sufficient to remove the moisture as quickly as it is released from the chips. Equations to represent the drying curves are easily created using the Solver function in MS Excel so they can then be used in dryer model. The temperature effect on drying can be projected by several methods.

Flame Arrester – Normally a device place in a gas duct to prevent a flame in the gas from propagating past it. It normally works by absorbing the heat from the flame so that the gas is cooled below the flame temperature. It is typically limited to a short burst of flame before it heats up.

Fluidized Bed – A fluidized bed is formed when a quantity of a solid particulate substance (usually present in a holding vessel) is placed under appropriate conditions to cause the solid/fluid mixture to behave as a fluid. This is usually achieved by the introduction of pressurized fluid through the particulate medium. This results in the medium then having many properties and characteristics of normal fluids; such as the ability to free-flow under gravity, or to be pumped using fluid type technologies.

Moisture Content - The amount of water in the chips can be expressed as mass-water/mass-wet chips (known as wet-basis), or mass-water/mas-dry chips (known as dry-basis). All calculations in our work is done on a dry-basis.

Pine-Oil – This is the “turpentine” and other volatile organic compounds that occur in most evergreen trees. This includes turpenes and some high value compounds used in cosmetics, soaps and other uses.  Unfortunately, most of the oils are in the “turpentine” range and are not very valuable.

Pneumatic Conveyance – A method of moving solids by blowing them through tubes with gas. Variations on this theme can be used to move chips through a dryer. The patent dispute with NAU is for a riser at the end of a bed that can throw or “pneumatically convey” the chips several feet upward to the top of the next bed. There are other ways to provide pneumatic conveyance.

Pulse-Fluidization – This refers to injecting the fluidizing gas in “pulses” rather than as a steady flow.   When done with a rapidly increasing rate at the start of the pulse it prevents “rat-holing” by never giving the chips a chance to organize. While it works well, it is difficult to implement in a commercial system, and the uneven flow through the unit produces problems with several of the upstream and downstream processing equipment.

Relative Humidity (RH) – This is the % of maximum humidity (at water saturation) of a gas. The upper limit of water removal with a given amount of dryer gas is to saturate it to 100% RH.  At the beginning of the dryer, liquid water is available on the chip surface, or readily available from the pores, and removal is fast, but the rate drops off considerably as the wood dries and diffusion of water vapor from within the chips becomes limiting. This pattern allows very hot dryer gas (up to 500F) to be used at the front end of the dryer without overheating the chips, but dictates much lower temperatures (200F) at the back end of the dryer to prevent “blue haze”.

Riser – In fluid bed terminology this is a segment of a bed where solids are thrown upward by the dryer gas. It is usually a small nearly vertical area between two plates with a higher velocity than in the bed.

Rat-Holes –  “Rat-holes” are stable passages that form in a bed of particles when attempting fluidization.  These are paths of least resistance that pass the gas through the bed without fluidization.  They prevent good gas particle contact. They occur most commonly when trying to fluidize irregular shaped particles (like wood chips). Once established they are difficult to disrupt, They do look like “rat-holes”. One way to prevent “rat-holes is to inject the dryer gas into the bed with a high velocity shearing action, rather than the even distribution through the floor common in most fluidized bed designs.

Temperature Profile - This refers to the temperature from bed to bed.  It can refer to the wood-chip temperatures or the dryer gas-inlet temperatures. For wood chips the desired temperature profile is to always keep the chips below about 150F to prevent “blue haze”, and usually to have the outlet chip temperature below 120F in preparation for further processing. The dryer gas temperature profile can begin at 500F is the chips are wet (they cool immediately on contact with chips due to water evaporation).  However, the dryer gas temperatures must be much lower at the back end of the dryer (under 200F) to prevent overheating the chips.