The effect of copper speciation on the formation of chlorinated aromatics on real municipal solid waste incinerator fly ash

Abstract

A limited amount of information exists regarding the relationship between the chemical form of copper and the formation of chlorinated aromatics in fly ash. To understand the effects of the various forms of copper on the formation of chlorinated aromatics in real fly ash, we determined the chemical forms of copper present in various types of real fly ash using X-ray absorption near edge structure (XANES) and evaluated the relationship between the chemical forms of copper and the formation of chlorinated aromatics. Copper chloride hydroxide (CuCl₂ · 3Cu(OH)₂) and cuprous chloride (CuCl) were the predominant copper species found in real fly ash. Although pure cupric chloride (CuCl₂) is known to be the most active catalyst for the formation of chlorinated aromatics under experimental conditions with synthetic fly ash, CuCl₂ was not found in every real fly ash sample. The amount of copper chloride hydroxide was positively correlated with the formation of chlorinated aromatics in real fly ash and is, consequently, considered to be one of the key species involved in the formation of chlorinated aromatics.

Introduction

Understanding the mechanisms involved in the formation of chlorinated aromatics in fly ash is necessary in order to control the emission of dioxins and other toxic organic compounds; such compounds are secondarily formed in a post-combustion zone which is considered to occur primarily on and in fly ash, as first described by Stieglitz and Vogg (1986).

According to many studies, copper compounds are regarded as influential catalysts; large amounts of polychlorinated dibenzo-p-dioxin/dibenzofuran (PCDD/DF) and related compounds have been found to be generated in experiments involving heat, synthetic fly ash, and copper chloride (Stieglitz et al., 1989, Luijk et al., 1994, Addink and Olie, 1995, Schoonenboom et al., 1995, Addink and Altwicker, 1998). On the other hand, some studies on the relationship between the amount of PCDD/DF generated and the changes in copper compounds in synthetic fly ash under conditions similar to those leading to the deactivation of catalysts have reported that the catalytic activities for the formation of chlorinated aromatics differed among copper compounds (Lippert et al., 1991, Gullett et al., 1992, Raghunathanm and Gullett, 1996). In a similar manner, much research has been performed using synthetic fly ash, but only limited information on the relationship between the chemical forms of copper in real fly ash and the formation of chlorinated aromatics is available from the literature. Although the results of experiments investigating the formation of PCDD/DF using synthetic fly ash have established that copper chloride acts as a catalyst in real fly ash, the presence and potential of copper chloride in real fly ash are yet to be confirmed.

Our purpose for this study was to investigate the effect of the chemical form of copper present in real fly ash on the formation of chlorinated aromatics. Chlorobenzene (CBz) and polychlorinated biphenyls (PCBs) were chosen as representative chlorinated aromatic compounds for this study because they have been found to react similarly to PCDDs and DFs in heating experiments involving fly ash (Stieglitz et al., 1989, Huang and Buekens, 1995), are well known precursors of dioxin (Kato et al., 2000, Heger et al., 2001), and are easily managed and analyzed. We determined the chemical forms of copper present in various types of real fly ash using X-ray absorption near edge structure (XANES) and evaluated the relationship between the formation of chlorinated aromatics and the chemical form of copper.