Modelling of Energy Demand in a Sawmill M.C. McCurdy J. Li and S. Pang Department of Chemical and Process Engineering University of Canterbury, Private Bag 4800, Christchurch 8020, NEW ZEALAND E-mail: jingge.li@canterbury.ac.nz Abstract An empirical model was developed to predict the electrical energy used by a sawmill comprising of debarking, chipping, sawing and grading operations. The input parameters for the model were log volume, surface to volume ratio of timber products and conversion rate to timber and chips. The model can predict the energy requirements of the sawmill with an error of less than 5% for normal production levels. 1. INTRODUCTION The wood processing industry in New Zealand consumes approximately 9% of the country’s total primary energy supply. About half of this is from renewable biomass energy usually in the form of heat generated from waste and by-products. The remainder comes from geothermal, fossil fuels and purchased electricity. Only a small amount of electricity is supplied from biomass cogeneration. Woody Biomass Integrated Gasification Combined Cycle (BIGCC) technology offers the potential for the wood processing industries to provide all of their thermal and electrical energy needs from a single energy plant utilising the self generated wood residues (Rutherford et al., 2006). The objective of the work presented in this paper was to develop a model to quantify the energy demand in solid timber processing which will be integrated into a BIGCC system model for assessing the technology feasibilities. Modelling of thermal energy demand in a sawmill is also underway. 1.1. The Sawmilling Process The main operations in a sawmill are shown in Figure 1. The first operation is the logyard where the green logs from the forest are stored prior to processing. The logs are usually delivered by truck or train and unloaded using a front-end loader. The logyard also normally contains a log loading system that sorts and orientates the logs before they enter the debarker. When logs are delivered to the sawmill they still retain some of the bark that was on the tree. This bark is removed before the log enters the sawmill as bark contamination can severely degrade chip quality as byproduct. The bark itself is also a valuable byproduct that is in demand from garden centres, which also require low wood content in the bark. The debarker is a once through process. After debarking, the logs are sawn into green timber, with trims to chips and sawdust in the sawmill. The purpose of the log trimming operation is to remove the curvature on the log to aid the cutting of boards and to remove curved edges on the boards. The trimming operation also effectively removes the taper from the log as well. Wood chips are also a valuable byproduct that can be sold for pulp or MDF production. Figure 1. Simplified flow diagram of sawmill process In most sawmills, the timber sawing is done either using bandsaws or circular saws. In the sawing process a fraction of the log volume is converted into sawdust. Usually there is a headrig that does the initial sawing of logs into cants and another gang-saw that reduces the cants into boards. In this case the log makes multiple passes through the headrig with a cant removed on each pass. There are also sawmills that have multiple saws so that the log only makes one pass through each saw. There are also edging saws that remove any remaining curvature from the boards.