Zhang, M; Buekens, A; Li, X
Biomass combustion originating both from human activities and behaviour and from natural causes, has caused considerable concern as a result of the numerous pollutants emitted into the atmosphere, including polychlorinated dibenzo-p-dioxins, polychlorinated dibenzofurans and polychlorinated biphenyls, or in brief dioxins. The contribution of dioxins emissions from biomass combustion becomes more and more important, especially since evident guided emissions-principally from waste incineration and metallurgy-have been curtailed drastically. Different types of biofuels feature different chemical components, including cellulose, lignin, and proteins, and accompanying organic and inorganic compositions, thus showing different characteristics of dioxins generation. Combustion modes, either flaming or smouldering combustion, also show considerable influence on the amounts of dioxins emitted from the system and they may host distinct processes for forming dioxins. Lean in chlorine and catalytic copper, native biomass materials usually produce low emission factors. However, various contaminants are inevitably mixed into biofuels during combustion and significantly promote the dioxins generation. Emission factor data from a wide range of biomass burning sources are collated in the present review, suggesting that dioxins emissions are substantially influenced by the facilities used, their operating conditions and combustion processes, fuel composition, accidental addition of contaminants, etc. Their roles in biomass combustion and dioxins formation pathways, however, remain difficult to quantify, resulting in emission factor values stretching over several orders of magnitude and complicating the efforts to build a comprehensive global estimation of dioxins emissions from biomass burning.