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 Cl-2, Cl . and possibly HOCl as chlorine transfer agents. Bromination occurs via free Br atoms. Slow combustion of phenol plus acetone (at ca. 630 degrees 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 10(2) fold increase in the level of o-/p-chlorophenol, Also, chlorobenzenes, PCDDs, PCDFs and C2Cl4 were now noticed, at comparable levels.