Fly Ash Capture of Mercuric Chloride Vapors from Exhaust Combustion Gas DESPINA KARATZA, † AMEDEO LANCIA,* ,† AND DINO MUSMARRA ‡ Dipartimento di Ingegneria Chimica, Universita ` di Napoli “Federico II”, P.le Tecchio 80, 80125 Napoli, Italy, and Istituto di Ricerche sulla Com bustione, CNR, P.le Tecchio 80, 80125 Napoli, Italy Emissions of mercuric chloride from incineration of Municipal Solid Waste (MSW) constitute a severe environmental problem. Recent studies of the emissions from MSW incinerators indicate that, when fabric filters are used, fly ash may promote Hg emission control. In this investigation, a laboratory-scale apparatus was used to evaluate HgCl 2 adsorption on MSW fly ash. A mixture of N 2 and HgCl 2 was used as gas phase, with HgCl 2 concentration varying between 0.5 and 10 mg/m 3 . Fly ash samples collected, on the fabric filter of an industrial size MSW incinerator, were tested to understand the mechanisms involved in mercury fly ash sorption. The experiments led to the determination of the breakthrough curves for the fixed bed and of the adsorption isotherms at different temperatures in the range of 150-250 °C. The adsorption isotherms, which have a characteristic “Langmuir” shape, were used to evaluate the Langmuir parameters at the temperatures investigated and the heat of adsorption for the phenomenon. Introduction Mercury is a pollutant particularly dangerous for human health because it is a mutagenic and teratogenic agent and furthermore isa strongneurologicaland behavioraltoxicant (1). Mercury compounds are emitted into the atmosphere from various anthropogenic and natural sources and are removed through dryand wet deposition processes. Mercury deposited on soil and in water bodies may bioconcentrate in vegetation and fish (1, 2); consumption of produce and fish may lead to adverse health effects on humans and on predator animals (3). The main sources of anthropogenic mercury emissions to the atmosphere are exhaust gases from coal combustion and municipal solid waste (MSW) incineration. Mercury is a trace element both in coal(0.1-0.3 m g Hg/ kg) an d in MSW (0.5-3mgHg/kg)(4). Duringcombustion,mostofmercury (up to 98%) is volatilized and is emitted in the gaseous phase either in the metallic or in the oxidized form (5). Experimental measurements(6, 7)and thermodynamicanalysis(8)indicate that in flue gas from MSW incinerators mercury is mainly found as HgCl2, probably due to the relatively high concen- tration ofHClin the gas. On the contrary,elementalmercury is the prevailing form in emissions from coal combustion processesasconsequence ofthe relativelyhigh concentration of SO2 (6, 9). Mercury adsorption from flue gas is the most common approach to meet air quality standards, due to the low cost inertization and discharge of the solid waste. With the aim of increasing the efficiency of the dry mercury removal processes,the attention ofmanyresearchers was focused on the interactions between mercury vapors and adsorbing materials both from a kinetic and a thermodynamic point of view. Adsorption ofoxidized mercurywasstudied bySchager (7),who found that oxidized mercuryismore easilyadsorbed than elementalmercury. Lancia et al.(8),adsorbingmercuric chloride on calcium hydroxide, found that the lower the temperature,the higher the adsorption capacity. Karatza et al. (10-12), studying the adsorption of HgCl2 on activated carbon and on the same carbon impregnated with Na 2S, found that impregnation of carbon strongly enhances the adsorption capacity and that the adsorption process is exothermic for both sorbent. Karatza et al. (13) used as sorbent Sorbalit , a mixture ofimpregnated activated carbon and Ca(OH)2, expressly designed to capture both mercury vapors and acid gases. They showed that the adsorption capacity depends on temperature in an unusual fashion: it decreases for 70° < T < 150 °C and then increases for 150° < T < 250 °C. Physical adsorption occurs for T < 150 °C, while for T > 150 °C the results fall in a “transition zone” between physical adsorption and chemical adsorption. Tseng et al. (14) studied the mercury removal in a full-scale incineration plants. Theyfound highlyvariable Hgcollection efficiencies (ranging from 0 to 95%) and indicated that the key parameter affecting Hg removal is the presence, down- stream of the acid gas control unit, of a fabric filter for particulate collection. Thisresult wasinterpreted bymaking the hypothesis that the solid particulate collected (a mixture of fly ash and calcium compounds) can adsorb HgCl2 and that fabric filters allow enough contact time between the gas phase and the solid cake deposited on the filter tissue. Furthermore,theyalso report that the operatingtemperature of the filter has to be kept as low as possible, being the adsorption phenomenon strongly sensitive to temperature. However it appears that there is a lack of quantitative information about HgCl2 capture by fly ash and about the mechanisms through which such capture occurs. The main objective ofthe present workhas been the study of the adsorption capacity shown by fly ash collected on the fabric filter operating in a fullscale MSWincinerator located in Northern Italy. Different ash samples were compared each other with regard the HgCl2 adsorption capacity. The adsorption isotherms and the breakthrough curves for the materials tested were determined varying the temperature in the range of 150-250 °C and the HgCl2 concentration in the range of 0.5-10 mg/m 3 . Eventually, the Langmuir parameters for the adsorption isotherms were determined, and a model based on the assumption ofkinetic control was used to evaluate the kinetic parameters of the process. Experimental Section Experiments were performed using the laboratory scale apparatusshown in Figure 1. In thisapparatusthe gasstream at the required temperature and HgCl2 concentration was produced,and the HgCl2 capture on a fixed bed ofadsorbent material was performed. The gas stream was obtained by sublimatingreagent grade solid HgCl2 contained in a stainless steel (AISI 316) cylindrical saturator into a stream of pure nitrogen. The saturator was kept at a fixed temperature by *Corresponding author phone: [39](81)768-2243; fax: [39](81)- 593-6936; e-mail: lancia@unina.it. † Universita ` di Napoli “Federico II”. ‡ Istituto di Ricerche sulla Combustione. Environ. Sci. Technol. 1998, 32, 3999-4004 10.1021/es971074m CCC: $15.00  1998 American Chemical Society VOL. 32, NO. 24, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3999 Published on Web 10/31/1998