Peat and bark extractives and their behaviour in drying processes: Dissertation

The organic compounds released in the thermal drying of peat and bark, and their behaviour in downstream sections, were studied to determine their impact on the formation of deposits in processes and on the environment. The releases were studied using laboratory-scale fluidized-bed drying experiments carried out on peat, pine bark and birch bark at different temperatures. In addition, peat, particle and deposit samples were taken from the drying system of a peat power plant, and bark, condensate and deposit samples from a pressurized steam dryer at a pulp mill. The hydrophilic compounds released were analysed using different chromatographic methods. All the samples were analysed for lipophilic extractives using capillary gas chromatography and mass spectrometry. The lipid composition of Finnish peat and peat extracts was investigated at the beginning of the study. Both hydrophilic and lipophilic compounds were released in the steam and flue gas drying of peat and bark, presumably as a result of thermal degradation or steam distillation. 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 was raised from 190 to 350 °C. Likewise, the amounts for pine bark and birch bark increased from 0.1 to 11 wt% and from 0.3 to 17 wt%, respectively, as the temperature was raised from 150 to 350 °C. The main hydrophilic compounds released from peat and barks were short-chain carboxylic acids, methanol, aliphatic aldehydes, anhydroglucoses and furanoic compounds. The hydrophilic compounds may cause organic load on the recipient or emissions to the air. Their amounts can be decreased by using lower drying temperatures. The major lipophilic compound groups released from peat consisted of fatty acids, those from pine bark of fatty acids and resin acids, and those from birch bark of triterpenoid alcohols. Their amounts depended on their content in the raw material. The percentage released from the free fatty acids of the peat increased from 10 to 70 wt%, as the temperature was raised from 190 to 350°C. The peat lipids, and particularly the fatty acids, were enriched in the particle samples from the gas flows and in the deposit samples from the drying mill and the flue gas blower of the peat power plant. The fatty and resin acids were also found to be enriched in the deposit samples from the heat exchanger tubes and the recycling fan of the bark steam dryer when compared with the bark feed. The released tacky lipids, 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.