Kinetic Rate Laws of Cd, Pb, and Zn Vaporization during Municipal Solid Waste Incineration QUENTIN FALCOZ, † DANIEL GAUTHIER,* ,† ST ´ EPHANE ABANADES, † GILLES FLAMANT, † AND FABRICE PATISSON ‡ Laboratoire Proc ´ ed ´ es Mat ´ eriaux et Energie Solaire (CNRS-PROMES), 7 Rue du Four Solaire, Odeillo, 66120 Font-RomeusFrance, and Laboratoire de Sciences et G ´ enie des Mat ´ eriaux et de M ´ etallurgie (LSG2M), Ecole des Mines de Nancy, Parc de Saurupt, 54000 Nancy Received November 3, 2008. Revised manuscript received January 5, 2009. Accepted January 23, 2009. The kinetic rate laws of heavy metal (HM) vaporization from municipal solid waste during its incineration were studied. Realistic artiﬁcial waste (RAW) samples spiked with Pb, Zn, and Cd were injected into a ﬂuidized bed reactor. Metal vaporization was tracked by continuous measure of the above metals in exhaust gases. An inverse model of the reactor was used to calculate the metal vaporization rates from the concentration vs time proﬁles in the outlet gas. For each metal, experiments were carried out at several temperatures in order to determine the kinetic parameters and to obtain speciﬁc rate laws as functions of temperature. Temperature has a strong inﬂuence on the HM vaporization dynamics, especially on the vaporization kinetics proﬁle. This phenomenon was attributed to internal diffusion control of the HM release. Two types of kinetic rate laws were established based on temperature: a fourth- or ﬁfth-order polynomial rate law ( r( x) ) k 0 e -E A /RT p( x)) for temperatures lower than 740 °C and a ﬁrst-order polynomial ( r( x) ) k 0 e -E A / RT( q-q f )) for temperatures higher than 740 °C. 1. Introduction Incineration as a waste treatment alternative with volume reduction, stabilization, sanitation, and energy generation beneﬁts is playing an increasingly important role in municipal solid waste (MSW) management. However, heavy metals contained in MSW are concentrated in the incineration byproducts, such as bottom ash, boiler ash, ﬁlter ash, and air pollution control (APC) residues (1, 2). Many factors inﬂuence the heavy metal (HM) partitioning during incineration (3-6): their physicochemical properties, which inﬂuence their evaporation or reaction kinetics; the physicochemical conditions inﬂuencing the incineration, such as temperature, chlorine content in the waste, moisture content, etc., and the parameters inﬂuencing combustion kinetics, such as temperature, retention time, or mixing conditions. High-temperature thermal treatment does not destroy metals. A fraction of the toxic metal compounds vaporizes and then condenses to form particulates during ﬂue gas cooling or deposits on available surfaces (7). The submicrometer metal particulates and gaseous metals may pass through the APC devices, allowing some vaporized metals, which are extremely hazardous for both human health and the environment, to be emitted. It is thus essential to understand the release mechanism of metals during high-temperature waste treatment in order to better understand their behavior and to more effectively control their emissions. Most research has dealt with model wastes to investigate the factors inﬂuencing HM fate (transfer and partitioning) during incineration (8-11) or with thermodynamic calculations to discuss and predict their chemical speciation and fate (12-15). Oxygen and chlorine contents have been identiﬁed as the main parameters inﬂuencing the ﬁnal HM partitioning. However, calculations based on equilibrium thermody- namics cannot predict the temporal evolution of the system and thus that of the HM. This evolution can be investigated only by kinetic studies. Rate laws were determined by Ho et al. (16) to describe the metal behavior during thermal treatment of soil. They carried out experiments in order to identify kinetic parameters in their model. Abanades et al. (9) studied the kinetics of HM vaporization from model wastes in a ﬂuidized bed. Both organic and mineral model wastes were used to study the inﬂuence of operating conditions on the extent of HM release in fumes. Liu et al. (17) determined the rate laws of toxic metal release during thermal treatment of model waste. A ﬁrst-order rate was determined for a mineral matrix, and a second-order rate was determined for realistic model waste. The objective of this study was to identify the kinetic rate law for metal release from realistic artiﬁcial waste. The behaviors of three metals of most concern (Cd, Pb, and Zn) were studied. The inverse method, developed and validated previously (18), was used to determine vaporization rates at the particle level based on experimental concentration proﬁles in the outlet gas of a ﬂuidized bed reactor. Such a reactor was chosen for better control of both temperature and mass transfer. The concentra- tion proﬁles were obtained by online analysis according to a measurement method involving customized ICP-OES spec- trometry (inductively coupled plasma- optical emission spec- trometry), which was described in detail by Falcoz (19). For calibration, the different standard gases are created by nebuliz- ing and vaporizing liquid solutions of known metal concentra- tions. Experiments were carried out at several temperatures in order to determine the kinetic parameters and to obtain speciﬁc rate laws as functions of temperature. The main improvements with respect to our previous results (17) are the following: (i) the accuracy of the experimental curves is signiﬁcantly improved because the new online analysis system provides measurement resolution of one point per second, thus permitting us to take into account the short-term variations of the vaporization rate, and (ii) the measurements are now quantitative due to the latest developed calibration protocol. 2. Experimental Setup and Procedure 2.1. Fluidized Bed Reactor. The experimental setup scheme is shown in Figure 1. The high temperature reactor is a ﬂuidized bed made of AISI 316 L stainless steel, 4.5 × 10 -3 m thick. It is a 0.105 m i.d. and 0.4 m high cylinder topped by a 0.2 m disengaging height. The reactor is insulated and electrically heated by two half-cylinder radiative shells. The bed is composed of sand with mean particle diameter of 0.7 × 10 -3 m (bed mass: 1.6 kg; initial bed height: 15 cm), into which a given mass of reactive metal-spiked particles is injected when the reactor is at thermal steady state. The particles are directly * Corresponding author phone: +33 468 307 757; fax: +33 468 302 940; e-mail: Daniel.Gauthier@promes.cnrs.fr. † Laboratoire Proce ´de ´s Mate ´riaux et Energie Solaire. ‡ Laboratoire de Sciences et Ge ´nie des Mate ´riaux et de Me ´tallurgie. Environ. Sci. Technol. 2009, 43, 2184–2189 2184 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 6, 2009 10.1021/es803102x CCC: $40.75  2009 American Chemical Society Published on Web 02/18/2009