The Formation of Polyaromatic Hydrocarbons and Dioxins During Pyrolysis: A Review of the Literature with Descriptions of Biomass Composition, Fast Pyrolysis Technologies and Thermochemical Reactions

This review on the formation of polyaromatic hydrocarbons (PAH) and dioxins during biomass fast pyrolysis was funded by the Washington State Department of Ecology. It includes a worldwide literature review of what is known and not known about this timely area. Areas needing further research and development work are identified and, as such, serve as a base for further work. For example, the need for lignin research is identified on page 31. While it was not possible to find any references to the presence of leachable polyaromatic hydrocarbons or dioxins in chars produced from the fast pyrolysis of woody biomass, small amounts of PAHs have been reported in bio-oils produced from this process. In general, fast pyrolysis oils typically tend to contain under 10 parts per million (ppm) of PAHs, which is lower than the values obtained in slow pyrolysis oils (exceeding 100 ppm). These small amounts of PAHs seem to be generated from a very poorly known mechanism involving polycondensation reactions of all biomass organic components, which occurs in certain char structures. The mechanism leading to the formation of these unstable structures in chars has to be further investigated. Fast pyrolysis bio-oil is less damaging to air and water quality than slow pyrolysis derived tars. In comparison to traditional petroleum derived fuels, bio-oil biodegrades faster, and is considerably less toxic. Fast pyrolysis oils do not need special precautions in terms of explosive concerns or toxic or ecotoxic emissions but these oils are corrosive and irritating to skin due to the presence of carboxylic acids. PAH production levels are significantly reduced at lower temperatures (below 700°C, page 19). In addition, dioxin production is significantly reduced when chlorine and metals are missing (page 13). It was considered convenient to add several sections to this review devoted to describing biomass composition, fast pyrolysis technologies and thermo-chemical reactions in order to create a self-contained document that could offer a more complete overview of the complex phenomena associated with the formation of these undesirable compounds. The review starts with a brief introduction describing some basic elements of biomass composition and existing pyrolysis technologies, and goes on to focus on known pathways for the formation of polyaromatic hydrocarbons (PAHs) and dioxins, their toxicity, and ways to control their production during pyrolysis. The possible relationships between the composition of the biomass, the reaction conditions and the presence of PAHs and dioxins in bio-oils and chars are also discussed.