Formation of phenols from the low-temperature fast pyrolysis of radiata pine ( Pinus radiata) : Part II. Interaction of molecular oxygen and substrate water

Abstract

In Part I, the influence of molecular oxygen on phenolic compound formation from the low-temperature fast pyrolysis of Radiata pine ( Pinus radiata) was reported. This work is elaborated upon and includes the interaction, or dual effect, of molecular oxygen and substrate water on phenolic compound formation. Experimentation was performed on a bench scale fluidised bed system and gas chromatography/mass spectroscopy was used for product analysis. Oven-dried and ‘as received’ (9.7% moisture content) Radiata pine (150–250 μm) was processed at 290–295 °C using air, nitrogen and oxygen enriched nitrogen as the fluidising gases. The inclusion of both molecular oxygen, in the form of air, and substrate moisture in the reaction system resulted in an increase in the yield of monomeric phenols from 0.07 to 5.03% (m/m). However, the inclusion of either molecular oxygen or substrate moisture resulted in a phenolic compound yield of only 0.60 and 0.27% (m/m), respectively. It was proposed that the molecular oxygen promoted the homolytic, free radical induced, aryl ether cleavage through facilitation of free radical formation as well as through hydroperoxide formation on the aliphatic carbon associated with aryl ether linkages. This explanation was consistent with the significant increase in carbonyl content of the phenolic product when molecular oxygen was included in the reaction atmosphere. However, such carbonyl groups are quite reactive and may participate in cross-linking reactions at elevated temperatures. The positive influence of water on the yield of monomeric phenolic compounds was believed to be indirect. That is, under fast pyrolysis conditions, in which the residence time of both substrate and volatile decomposition products is short, the evaporation of water would have suppressed substrate temperature elevation, thereby suppressing the rate of cross-linking reactions, resulting in the increased preservation of carbonyl containing low molecular weight products.