The modelling and validation of one dimensional storage drying of woodchips for fuel
Since wood fuel when used sustainably provides an energy source that does not contribute to greenhouse warming, its use in future is likely to increase. Contractors prefer to chip trees and residues green to minimise power required and machinery wear. For non-specialised combustion systems the green woodchips should then be dried to increase their calorific value and to allow buffer storage without significant dry matter loss or the development of hazardous fungal spores. As the drying of wood chips has never been rigorously defined, a study was undertaken to determine the drying characteristics of green woodchips. A drying bin was built to simulate storage drying in one dimension. Data from three drying runs using spruce and birch whole tree provided data to validate a one-dimensional heat and mass transfer drying model converted from an existing grain drying routine. The specific energy consumption varied from 2.0 MJ/kg of moisture removed using unheated air during summer drying to 5.0 MH/kg during winter when dried with air warmed by 28°C. Dry matter loss in birch during drying and a period of two months cool storage was approximately 2%. Mean evaporation rate ranged from 1.9 kg/h in winter to 2.9 kg/h in summer. This gave a drying rate of 1% and 2.2% per day, inwinter and summer respectively. Hardwoodbirch released its moisture more readily than softwood Sitkaspruce. Airflow rates used ranged from 0.26-0.63 m3/s pertonne of dry matter. The dust levels on the woodchips ranged from 3.0 mg/m3 ofair before drying up to 55 mg/m3 of air after drying. The concentrations of colony forming units on woodchips ranged from2.0 X 104 cfu/m3 of air before drying up to 8.0 X 105cfu/m3 of air after drying.