The organic compounds released in the thermal drying of biomass, and their behaviour in downstream sections, were studied to determine their effect on the formation of deposits in processes and on the environment. The releases were studied by using laboratory-scale fluidizedbed drying experiments carried out on peat, pine bark, and birch bark at different temperatures. In addition, bark, condensate, and deposit samples were taken from a pressurized steam dryer at a pulp mill. Aging experiments were carried out for deposit samples in a microautoclave. All the samples were analysed for lipophilic extractives by using capillary-gas chromatography and mass spectrometry. The hydrophilic compounds released were analysed by using various chromatographic methods.
Different organic hydrophilic and lipophilic compounds were released in drying. The amount of identified compounds released from peat increased from 1 to 10 wt% of feed, calculated on a dry ash-free basis, as the temperature rose from 190 to 350°C. The amounts for pine bark and birch bark at 150–350°C were 0·1–11 wt% and 0·3–17 wt%, respectively.
The main hydrophilic compounds released from raw materials were carboxylic acids, methanol, aldehydes, anhydroglucoses, and furanoic compounds. The hydrophilic compounds may cause an organic load on the recipient or emissions to the air. The major lipophilic-compound groups released from peat consisted of fatty acids, those from pine bark of fatty and resin acids, and those from birch bark of triterpenoid alcohols. The fatty and resin acids were found to be enriched in the deposit samples from the heat-exchanger tubes and the recycling fan of the bark dryer, when compared with the bark feed. The tacky lipids released, which consisted mainly of fatty and resin acids, were assumed to be potential precursors to deposits found on the surfaces of peat and bark dryers.