Effect of H2-pressure on the structures of bio-oils from the mild hydropyrolysis of biomass

Abstract

Low intensity hydropyrolysis of biomass has been investigated as a likely process route for upgrading agricultural and municipal waste, aiming to produce less highly oxygenated bio-oils with more stable storage characteristics than atmospheric pressure pyrolysis oils. A high-pressure wire-mesh pyrolysis reactor, recently re-designed to allow continuous sweeping of volatile products away from the reaction zone, was used during these experiments. This reactor configuration allows the determination of bio-oil yields and the recovery of condensible products for analytical characterisation in the relative absence of extra-particle secondary reactions. Samples of Swedish pine-wood and pine-wood lignin have been pyrolysed at temperatures between 300–700°C, at hydrogen pressures up to 70 bar. Product bio-oils have been characterised by size exclusion chromatography (SEC), UV-fluorescence spectroscopy (UV-F) and gas chromatography-mass spectrometry (GC-MS). Bio-oil yields from both pine-wood and lignin decreased by relatively small amounts over the pressure range, but significant changes in liquid product structures were observed: product bio-oils with progressively smaller molecular masses and lower polarity have been obtained with increasing pressure. Nevertheless, product bio-oils were characterised by a preponderance of oxygenates, including syringol and guaiacol derivatives and carboxylic acids, rather than alcohols, ketones, terpenoid and polynuclear aromatic hydrocarbons. Considerable differences have been observed between hydropyrolysis yields from pinewood and lignin: bio-oil and total volatile yields from the parent wood were greater than those observed for the lignin sample by about 30%. Comparison by GC-MS and UV-F suggests that the lignin derived bio-oils contain considerably more aromatic and less polar structures than the wood derived oils. At 70 bars, traces of di- and tri-bromodibenzodioxins have been detected in pinewood bio-oils from experiments at 400 and 700°C, apparently liberated from the wood during hydropyrolysis; it is presumed that the precursor fungicide molecules had previously been incorporated into the wood structure.