A simple kinetic model of PCDD/F formation by de novo synthesis

A simple fixed-bed reactor model is used to describe experimental data about the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) from fly ash carbon. The heterogeneous oxidation of graphite-like carbon is represented by a reaction which is first order in carbon and one half order in oxygen. The same orders of reaction are assumed to be valid for PCDD/F formation, such that the oxidation of fly ash carbon would have three parallel channels leading to COx, PCDD and PCDF. In addition, PCDD/F degradation by oxygen is considered and appropriate rate constants were fitted to simulate the experimental data. The effects of water vapour and particulate copper on PCDD/F formation are included. As a result, an overall kinetic model of PCDD/F formation is obtained which describes most of the experimental data with reasonable accuracy. The main reaction channel, COx formation, needs to be understood more completely, in order to interpret PCDD/F reaction pathways better. The model can be extended to include other chlorinated aromatics and the gas/particle partitioning of the semivolatiles. It provides a basis for the qualitative and quantitative interpretation of fixed-bed and flow reactor results and it is hoped to be applicable to industrial installations like waste incinerators and metal smelters.