Wood Chip Drying in Fixed Beds: Drying Kinetics and Economics of Drying at a Municipal Combined Heat and Power Plant Site

Abstract

Drying of wood chips at a power plant site increases the lower heating value and decreases composting and dry matter losses in long-term storage. In this study, experimentally measured drying curves for wood chips are produced to study the drying kinetics of the chips. Drying curves have been used to assess whether or not wood chip drying in a batch-type fixed bed dryer at a municipal Combined Heat and Power (CHP) plant site is profitable when using external excess heat or solar energy as a heat source in drying, and what could be the reasonable drying parameters for that. The curves have been determined by changing bed heights (100, 300, 500 mm), temperature of inlet air (30, 50, 70, 90°C) and air velocities (0.3 m/s, 0.5 m/s, 0.7 m/s) in the tests. Air has been used as a drying gas. The results show that drying time decreases considerably when the temperature of the drying air increases from 50°C to 70°C. The influence of drying air temperature on the drying time is no longer so remarkable as the temperature increases from 70°C to 90°C. This indicates that the inlet air temperature should be at least 70°C. The results also indicate that the air velocity should be at least 0.5 m/s in order to achieve reasonable drying times. The economics of drying have been evaluated in the study by calculating the payback period for the dryer. According to the results, drying could be profitable if investment costs are appropriate (not much higher than 1,500 €/m2) and the price of the wood chips is sufficiently high (higher than ∼15–20 €/MWh). It is, however, important to consider that drying at a municipal CHP plant site is always case-dependent.