On-line spectroscopic measurements of wood chips before a continuous digester

A VIS spectrometer was evaluated during 25 days of on-line operation. The spectrometer was installed above a conveyor belt carrying softwood chips to a continuous digester that was charged with 100 tonnes chips/h. We found that VIS spectroscopy can be used on-line both to predict chip properties and to monitor a continuous digester. Prediction models for pine, bark and dry content were made using partial least square (PLS) regression models. The prediction models were based on data from a designed experiment that was performed in the lab before the on-line installation. Spectral information taken from the VIS range can be used for monitoring the digester, as the information correlates well to process variables. For 3 days, an NIR spectrometer, calibrated via a designed experiment to make the same predictions as did the VIS spectrometer, was also installed at the conveyor belt. The VIS and NIR predictions proved to be similar.