Summary

In the Netherlands 18 million loading pallets are made of wood. A small part of the produced pallets are dried in special kilns to at least 25% moisture content. The air circulates through the stacked pallets by a number of fans. Usually the drying air is heated by direct-fired natural gas heaters to maintain the air temperature at 60-85 °C. Vents in the roof allow dehumidifying. However, existing dry kilns are designed to dry stacked lumber in stead of pallets, which leads to unequal air distribution and variable drying rates. Flow control is also restricted due to the natural draft principle. In co-operation with a Dutch pallet drying company in the Netherlands the contract-research organisation TNO Process Physics developed design specifications to improve existing dry kilns. Firstly, energy-efficiency can be substantially improved by increasing air temperature and air humidity and minimizing air discharge. Maximum energy-efficiency is reached when airless steamdrying is applied. This method, also called High Temperature (HT) drying, uses superheated steam instead of air and operates at atmospheric pressure and at temperatures till 125 °C. Also drying conditions become more uniform when free space between the pallets and the kiln roof is reduced by installing bulkheads or baffles. An efficient drying process requires an average air speed of 3 m/s. The researchers also advise not exposing the pallets to critical drying conditions for a too long period. This prevents excessive deformation and cracking of the pallet. TNO developed also an innovative design for a batch dryer and a continuous dryer (tunnel). Compared with conventional drying procedures, the drying results are the same or even better with distinct savings in energy used, constant product quality and better logistics. Total energy savings amount to 0,36 m³ of natural gas per pallet (17 kg dry weight). The measures are mainly applicable in wood processing companies.