Catalytic Destruction of Chlorobenzene and Model Dioxin Compounds with Iron Oxide in a Small Size Incinerator

The current study was intended to investigate the catalytic destruction of iron oxide in chlorobenzene and model dioxin compounds combustion. First, chlorobenzene was used to correlate dioxin formation with chloro content of chlorobenzene. Further, the regenerated ferric oxide catalyst was used at different combustion temperatures to evaluate its impact on dioxin formation. Finally, a referenced dioxin material was used to exanimate the capability of ferric oxide catalytic oxidation. The results indicate that PCDD/F concentrations are linearly dependent on chloro content in the feed stream. PCDD/F concentrations are significantly affected by gas temperature in the presence of ferric iron catalyst. Furthermore, the PCDD/F concentrations were significantly reduced, or from 0.78 ng-TEQ/Nm (580 C) to less than 0.1 ng-TEQ/Nm at temperature greater than 750 C at the catalyst loading 100 g/hr. The destruction efficiency of the model dioxin compound is greater than 95% (in terms of TEQ) at catalyst of 300 g/hr. Among the various dioxin congeners, the destruction of the most toxic congener 2,3,7,8-TeCDD is the most effective. * To whom all correspondence should be addressed. E-mail address: [email protected] INTRODUCTION Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are the most concerned air toxics generated from combustion practices. Many studies have been performed concerning dioxin emission control from municipal solid waste incinerators (MSWI), and to a lesser extent, on industrial waste combustion [1]. In Japan, the total dioxin emission was estimated to be 3000 gTEQ/year, of which one third was released from the industrial waste incinerators [2]. In Taiwan, in addition to 19 regional municipal waste incinerators, there are over 4000 small size incinerators used for combusting a variety of industrial wastes [3]. These small size incinerators with numerous types including different furnaces were widespread everywhere in the island. It is therefore complicated and difficult to develop an effective means to control dioxin generation. Two major mechanisms have been proposed for dioxin generation: precursor formation from existing smaller chlorinated molecules and de novo synthesis [4]. Both mechanisms occur simultaneously and/or independently [5]. The alternatives used in combustion facilities to control PCDD/Fs to compile with regulations can be categorized into three approaches: waste management; in-line inhibition; and end of process treatment [6,7]. The widely employed technologies for end of pipe treatment are activated carbon adsorption and catalytic destruction [8]. With these technologies, it is highly possible to meet the stringent emission standards, say below 0.1 ng-TEQ/ Nm in flue gas [7]. Almost all MSWIs in Taiwan use activated carbon adsorption process to ensure satisfactory dioxin removal before flue gas is emitted into ambient environment. However, activated carbon may react with chlorine atom or molecules to form species that can react to form PCDD/Fs [9]. Additionally, the disposal of spent activated carbon contaminated with dioxin becomes problematic as it creates another hazardous wastes to treat [8]. Catalytic materials were initially developed on industrial gas cleaning systems to destroy waste pollutants such as hydrocarbons, aromatics, halogenated derivatives, and NOx [10]. For example, some organic compounds, such as methane, toluene and styrene have been catalytically oxidized to carbon dioxide and water by noble metals (e.g., Pt and Pd), and MnO/Fe2O3 catalysts [11-13]. Recent efforts in developing catalysts have been focused on the PCDD/F removal. As a result, there are several metal 42 Journal of the Chinese Institute of Environmental Engineering, Vol. 14, No. 1 (2004) oxides such as PdO, CuO, V2O5, MoO3, TiO2, and Fe2O3 that have been demonstrated for their capability in suppressing dioxin formation [14]. For example, the removal efficiencies of PCDD/Fs up to > 99% could be achieved with the TiO2/V2O5 -based catalyst [10,15]. Imai et al. [16-19] have successfully applied hematite (α-Fe2O3) to catalyze oxidation of CO to near 100% destruction, and reduction of dioxin more than 80% in different small MSWIs. Recently, Weber et al. [14] used fly ash-containing Fe2O3 to reduce concentration of 1,2,3,4,6,7,8,9-octachlorodibenzo-pdioxin in an industrial waste incinerator. Therefore, adding metal oxides catalyst either in the mixture with waste or in the gas stream to reduce dioxin formation and emission has become a promising means for small size incinerators [20]. Dioxin formation as function of CO, temperature and chloro content are well known factors in MSWI [21]. However, that is little research as their effects under catalytic conditions. Consequently, in the previous study [22], iron oxide, regenerated from the ferrite sludge, was used for successful CO destruction and dioxin suppression in a pilot-scale incinerator. For the present study, the catalytic destruction of iron oxide for chlorobenzene and model dioxin compounds was furthermore discussed. First, chlorobenzene was used to correlate dioxin formation with chloro content of chlorobenzene. Further, the ferric oxide catalyst was used at different combustion temperatures to evaluate its impact on dioxin formation. Finally, a referenced dioxin material was used to exanimate the capability of ferric oxide catalytic oxidation. It is the intention of this work to develop an environmentally acceptable and cost-effective technology to reduce the risk of dioxin emission from small size incinerators. MATERIALS AND METHODS