Abstract
Microwave-assisted pyrolysis (MAP) of lignocellulosic biomass is a technique that could potentially be used to produce and upgrade renewable energy carriers. However, there is no available information about the formation of dioxins and other organic pollutants in MAP treatment of woody biomass. In this study, MAP experiments were conducted in lab-scale using virgin softwood, bark, and impregnated wood as feedstocks. The non-condensable gas, liquid (fractionated into aqueous and oil phases), and char fractions generated during pyrolysis were collected and analysed for polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and naphthalenes (PCNs). The concentrations of PCDDs, PCDFs and PCNs in the pyrolysis products ranged from 0.52 to 43.7 ng kg(-1). All investigated compound groups were most abundant in the oil fraction, accounting for up to 68% (w/w) of the total concentrations. The highest PCDD, PCDF and PCN concentrations were found from the pyrolysis of bark, which has relatively high contents of chlorine and mineral matter, followed by impregnated wood, which contains organic and metal-based preservatives. The homologue profiles of all three compound groups were dominated by the less chlorinated homologues. The homologue abundance decreased as the degree of chlorination increased. This trend was observed for all three feedstocks.
Highlights
Because it is not clear how these factors might affect the production of persistent organic pollutants, there is a need to investigate the formation of such compounds including dioxins during Microwave-assisted pyrolysis (MAP) of biomass
The origin and treatment history of the telephone pole could not be confirmed, the treatment of utility poles in Sweden was commonly performed by use of creosote oils or metal-based preservatives such as chromated copper arsenate (CCA), or a mixture of both (Sundqvist, 2009)
Measurements of the PCDD, PCDF and PCN concentrations in the feedstock materials (Table 1) resulted in concentrations within the range reported for woody biomass from unpolluted areas (Lavric et al, 2004)
Summary
Low temperature pyrolysis is a process of partial oxidation that primarily favours formation of a broad spectrum of volatile organic compounds (known as tars), some of which are aromatic (Evans and Milne, 1987). These compounds are released quickly due to the rapid heating achieved during microwave pyrolysis which may influence the mechanism of pyrolysis. Because it is not clear how these factors might affect the production of persistent organic pollutants, there is a need to investigate the formation of such compounds including dioxins during MAP of biomass
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