Thermal and catalysed halogenation in combustion reactions

Organic vapors in hot air with HCl can form organic chlorine derivatives, parallel to their oxidative degradation, even without catalysis (fly-ash). Upon slow combustion in an inert quartz reactor with incomplete conversion, phenol - a key precursor for PCDDs - indeed gave o-/p-chlorinated phenols, together with a variety of other PICs. With HBr plus HCl at ca. 900K, the (net) production rate of brominated phenol was ca 10 x that of Cl-phenol. These uncatalysed processes can be explained via free-radical mechanisms, with Cl2, Cl• and possibly HOCl as chlorine transfer agents. Bromination occurs via free Br atoms. Slow combustion of phenol plus acetone (at ca. 630°C) also led to methylchloride. In real (waste) combustion, metal salts catalyse (oxy)chlorination, and presumably oxidation also. In a model experiment, a piece of quartz tubelet ‘aged’ with inorganics emanated from real municipal solid waste was placed in a clean quartz reactor - with a volume 70 x that of the tubelet; at 900K this led to a 102 fold increase in the level of o-/p-chlorophenol. Also, chlorobenzenes, PCDDs, PCDFs and C2Cl4 were now noticed, at comparable levels.