Examination of PCDD/F Formation in Thermal Systems Using Simple Mathematical Models

A set of empirical models which accounts for the formation of gas phase polychlorinated dibenzo-p-dioxins and furans (PCDD and PCDF), and solid phase PCDD/F by the de novo mechanism is described.In each case, competing formation and destruction reactions are considered to operate.The effect of the time-temperature history on their formation is then examined.At high temperatures, steady-state is reached in fractions of a second, resulting in the observed low product concentrations.Rapid cooling as found in furnaces produces higher PCDD/F nett formation rates than slower cooling over the same temperature range, but with less overall yield.In addition, a cooling process will result in more PCDD/F production than heating at the same rate. Thus the conventionally-regarded temperature “windows” for formation are misleading, as in practical conditions PCDD/F are produced at higher temperatures.Simulations carried out of a pilot scale municipal solid waste (MSW)incinerator, a commercial fluidised bed boiler burning wood as a fuel, and of the laboratory scale thermal “annealing” of particulates taken from iron ore sintering off-gases illustrate the effects.There is sufficient promise in the approach to suggest that better characterisation of particulates will lead to acceptable predictions.