Wood Pellet Cooling

"The RTO alone can require at least $2million/year to operate."

Many energy-transition initiatives around the world are focusing on wood pellets as an alternative fuel source to coal.European regulators have confirmed that burning wood pellets for electric power would be considered a carbon-neutral energy source. Wood pellets are small pieces of compressed wood used as fuel to power homes and businesses. They are used to differing degrees in electric power plants (primarily European and Asian countries).

The pellets are created by first removing moisture from incoming wood fiber and then milling into fine powder and compressed into pellets that are approximately 0.25 to 0.3 inches in diameter and up to 1.6 inches in length. The pellets are heated, softening the lignin in the wood, which acts as a binder. At this point the pellets are typically between 150Fand 200F, making them too hot for storage and logistics. The pellets must be cooled to around 95F for safe storage and transporting.

Two heat exchanger technologies can be used in the cooling process: direct with air or indirect with fluid.

Cooling directly with air, the pellets enter a cooling bin where cold air is blown through the pile from bottom up in a single pass and then exhausted to an RTO (regenerative thermal oxidizer) to ensure clean emissions to the environment. This is a very inefficient means of cooling and a very costly one. The RTO alone can require at least $2million/year to operate.  

Cooling indirectly with fluid, the pellets enter a vertical moving bed heat exchanger where they flow slowly between a series of parallel heat exchanger plates that contain counter-current flowing cooling water. The heat transfer is indirect by means of conduction and working only with sensible heat to maintain the highest degree of efficiency.The operating cost of indirect cooling is two to three times less than that of direct cooling with air and removes the necessity for an RTO.  

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