The rotary dryer is one of the most commonly used technologies for drying wood. It is effective to handle both sawdust and chips [18, 20, 25]. The most commonly used rotary dryers in industry are direct contact type, which consists of a hollow, rotational metal cylinder providing space for direct contact between the material to be dried and the drying medium, usually hot air. The heat and mass transfer between these two streams is high with a direct contact and can be further enhanced by installing a series of flights on the inner wall of the cylinder to promote contact of the two streams. In addition, uniform MC distribution of the dried product can be achieved because every piece of the solid material has an equal chance to contact with the hot air.

For the co-current rotary dryer, the wet material and the hot air enter from one end, and the dried material and the humid air exit from the other end. The wet material and the hot air enter the drum from the opposite ends and move inside the dryer in the op­posite directions. A typical drying temperature used in commercial drying of woody biomass is up to 500 °C that ensures high drying rate and energy efficiency of drying. The co-current mode is usually employed for industrial application as this ensures the dried biomass will not overheat and cause self-ignition. The operation conditions have to be well monitored to prevent fire hazard. This requires reliable measurement techniques of dryer temperature and MC in order to develop a prediction model for biomass drying.

5.2.1.1 Packed Moving Bed Dryer

For packed moving bed dryers, the wet biomass is fed from one side to a moving bed that has openings on it, allowing the drying medium (hot gas) to flow through. In order to increase energy efficiency of the drying, the drying medium is recycled by flowing back through the biomass bed in the second half of the dryer. The overall air flow rate is only half of that in the arrangement where the drying air always flows upwards. Because with the drying air reversal in the second half of the dryer, the exhaust gas has low temperature and high humidity, the corresponding equilibrium moisture content (EMC) is relatively high for the bio-originated material. This indicates that if low final MC is required, the exhaust gas temperature must be kept higher than a certain value to achieve the required dryness. On the other hand, higher exhaust air temperature results in greater heat losses [13, 16, 17].

During drying, the MC varies across the bed thickness [26]. In a co-current arrangement, the bottom layers of biomass in the first half of the dryer dry faster than that of top layers. Although the reverse flow of the drying air reduces the uneven moisture distribution, the MC gradient may not be totally eliminated because the recycling drying gas in the second half of the dryer has a lower drying temperature and higher humidity; thus, the drying rate is lower compared to the first half of the dryer.

In the case that the low final MC is required for the dried biomass, a countercurrent arrangement can be used where the hot gas is fed from the second half of the dryer, flowing upwards, and is then reversed to flow through the biomass bed in the first half of the dryer. In this improvement, the drying efficiency is higher and the required final MC can be achieved [13].