Development of an Automated Heating System for a Prototype Rotary Dryer for Drying of Saw Dust for Pellet Production

Abstract

Saw dust based pellets are one of the most popular biomass based renewable energy sources in the current context where rotary dryers being used to dry the main raw material: saw dust in order to reduce moisture content to match with process requirements. However, in most of the industrial scale pellet producing plants in Sri Lanka, rotary dryers are operated as open-loop systems with minimum controllability. Since saw dust bulk quantities arrive from different sources, there is a significant variation mainly in the moisture content in the raw form of saw dust. When such varying input is given to conventional rotary dryers, there is significant fluctuation of moisture content and temperature of output produced by rotary dryers which are immediately fed in to pelleting machines. This can lead to serious quality controlling problems. For instance it has been reported that in one of the leading industrial scale pelleting plant in Ulapane, Sri Lanka, over 40% of production output has got defected due the above reason, ha this background, this project aimed to develop a closed loop control system applicable for rotary dryers. In order to conveniently test and verify the control strategy to be used, a small scale rotary dryer was developed having an automated heating system controlled by a closed loop feedback system. The system runs by measuring the moisture content and temperature of the input and output sawdust flows, temperature of input air and mass flow rate of saw dust. In order to maintain the output saw dust temperature and moisture content at set values, the flow rate of the hot air supply and its temperature is controlled. A series of experiments were conducted using the apparatus and it was observed that the proposed feedback controlling system is effective in maintaining the properties of the saw dust output flow within acceptable limits, compared to open-loop controlling. The control strategy developed in this project can be applied to automate most of the industrial rotary dryers in order to improve quality performance and energy efficiency.