Alteration in formation behaviors of chloroaromatic precursors of PCDD/Fs: An experimental study on the effect of extrinsic and intrinsic oxygen on chlorination.

Due to the promotion on Cl radical generation by enhanced oxidation, chlorination of hydrocarbon intermediates becomes a potential formation path for chloroaromatic precursors of PCDD/Fs (polychlorinated dibenzo-p-dioxins and dibenzofurans) in both MSW (municipal solid waste) incineration and gasification-combustion processes, in which intrinsic oxygen might have a significant effect on the competition between oxidation and chlorination. Thus, chlorination of benzene and phenol was experimentally studied on a homogeneous flow reaction system. Effects of temperature and ER (equivalence ratio) were assessed, and comparison was carried out to clarify the alteration in formation behaviors of chloroaromatics by extrinsic and intrinsic oxygen. At 600 °C, chlorobenzenes were already largely formed in benzene chlorination, and the addition of extrinsic oxygen barely affected it. On the contrary, with intrinsic oxygen, phenol tended to decompose to light compounds. With rising temperature, oxidation was promoted and extrinsic oxygen strongly inhibited the formation of chloroaromatics in benzene chlorination at 900 °C and higher temperature. For phenol chlorination, chlorobenzenes were still rarely generated. However, high proportions of octachloronaphthalene and octachlorodibenzofuran were observed, due to the enhancement in polymerization by high temperature. When increasing ER, oxidative decomposition was also promoted in both the chlorination of benzene and phenol. Extra extrinsic oxygen led to a further reduction of chloroaromatics during benzene chlorination, and till ER = 1.0 at 1000 °C, comparable performance to intrinsic oxygen could be achieved in the control of chloroaromatics. Based on these results, formation pathways of the major chloroaromatics from chlorination, oxidation and polymerization were summarized, and the roles of extrinsic and intrinsic oxygen in altering their formation behaviors were revealed.

Experimental study on dioxin formation in an MSW gasification-combustion process: An attempt for the simultaneous control of dioxins and nitrogen oxides.

A gasification-combustion conversion process has been proposed for MSW disposal, and the positive effect of "homogeneous conversion" and "staged combustion" on the stabilization of Cl atoms and the rupture of CCl has been proved previously in fundamental experiments. To verify and evaluate their inhibition effect on dioxin generation, a bench-scale experimental system of the proposed process was established in the present work and an experimental study on the formation of dioxins in the process was carried out. Since both dioxins and nitrogen oxides should be strictly controlled while applied in industry, the simultaneous control of nitrogen oxides was also considered. Results indicated that "homogeneous conversion" has a clear inhibition effect on dioxins in both the syngas from gasification and the flue gas from combustion, which is a very important process in the stabilization of Cl atoms and the control of dioxins. During the "staged combustion" of the syngas, the increase of SR1, the rise of temperature and the extension of residence time all have a clear inhibition effect on dioxins in the flue gas. The extension of residence time seems to be more efficient, and while increasing SR1 and temperature, the regeneration of active chlorine species and the increase of NO need to be concerned. By the combination of "homogeneous conversion" with a temperature of 700 °C and "staged combustion" with a SR1/SR2 ratio of 0.7/0.4, a temperature of 900 °C, a residence time of 241 ms, a satisfactory simultaneous control of both dioxins and nitrogen oxides was obtained in the experiments of the present work.

Inhibition of CCl formation during the combustion of MSW gasification syngas: An experimental study on the synergism and competition between oxidation and chlorination.

For the safe disposal of MSW, a four-step gasification-combustion conversion process is proposed in this work, consisting of material gasification, ash melting, syngas conversion and combustion. Based on the control method of dioxin in gasification process which has been studied previously, experiments of tar chlorination process under oxidative atmospheres were carried out in a homogeneous flow reaction system, using benzene as the tar model compound, to find a way for the inhibition of CCl formation during the syngas combustion process. Results indicated that Cl2 reacts with benzene more easily than O2 under low temperatures, and has a positive effect on both oxidative cracking and polymerization. For chlorination reactions, high temperature enhances the chlorination degree and leads to the formation of perchlorinated hydrocarbons, but also promotes the rupture of the weak CCl bonds. With the rise of temperature, hexachlorobenzene became the major product, the amounts of all chlorinated hydrocarbons decreased rapidly, and the conversion direction depended on the amount of O2. O2 generally promotes the formation of hydrogen chloride, and inhibits the chlorination of hydrocarbons. At a temperature above 900 °C, the total amount of chlorinated hydrocarbons was very low under oxidative atmospheres, even only with a equivalence ratio of 0.2. However, during the oxidation process under low temperatures, CCl can also be formed on cyclopentadienyl and 1,3-butadienyl radicals, whose chlorination products were observed. The synergistic and competitive effects between oxidation and chlorination are concluded and the major benzene conversion pathways are summarized according to the products detected.