Flash pyrolysis of heavy metal contaminated hardwoods from phytoremediation: Characterisation of biomass, pyrolysis oil and char/ash fraction

Flash pyrolysis of heavy metal contaminated hardwoods originating from phytoremediation is studied. Different kinds of hardwoods, i.e. Salix fragilis (crack willow), Salix jorunn (“Jorunn” willow) and Populus grimminge (Grimminge poplar) are compared in a preliminary phase. Salix fragilis scores the best on both remediation capabilities and pyrolysis characteristics. Therefore, this cultivar is chosen for in-depth research. S. fragilis stems, leaves and stems mixed with leaves are pyrolysed. The effect of the pyrolysis temperature (623, 723 and 823 K) and the use of a hot-gas filter on the yield and properties of pyrolysis oil and char/ash residue are studied. Pyrolysis of stems at 723 K yields the most pyrolysis oil; while highest char production is obtained when pyrolysing leaves at 623 K. Pyrolysis of mixed stems and leaves at 723 K yields more gas than the pyrolysis of stems or leaves at 723 K. The use of a hot-gas filter does not influence the composition of the pyrolysis oil significantly. In all the experiments performed, the pyrolysis oil main constituents are phenolics. Pyrolysis of stems at 623 and 723 K yields a pyrolysis oil which does not contain considerable amounts of heavy metals.