Calcium Bisulfite Oxidation in the Flue Gas Desulfurization Process Catalyzed by Iron and Manganese Ions D. Karatza, † M. Prisciandaro, ‡ A. Lancia,* ,† and D. Musmarra § Dipartimento di Ingegneria Chimica, Universita ` di Napoli “Federico II”, Piazzale Tecchio 80, 80125 Napoli, Italy, Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita ` di L’Aquila, Monteluco di Roio, 67040 L’Aquila, Italy, and Dipartimento di Ingegneria Civile, Seconda Universita ` degli Studi di Napoli, Real Casa dell’Annunziata, Via Roma 29, 81031 Aversa (CE), Italy Among different flue gas desulfurization processes for control of sulfur dioxide emissions from combustion of fossil fuels, wet limestone scrubbing is the most widely used. Forced oxidation in the scrubber loop substantially improves the dewatering properties of the sludge, leading to the formation of gypsum (CaSO 4 ‚2H 2 O). In view of this, the present paper reports the experimental study of calcium bisulfite oxidation in the presence of catalysts (ferrous and manganese ions) both separately and simultaneously added in the reaction vessel. A laboratory-scale apparatus was used; the experiments were performed at a fixed oxygen partial pressure (21.3 kPa) and at a temperature of 45 °C. In particular, the effect of the simultaneous addition of both catalysts has been studied. The analysis of the experimental results, carried out by using the theory of mass transfer with chemical reaction, indicated that the slow reaction regime has been explored and the transition from the kinetic to the diffusional subregime identified. Experimental results, as compared with those obtained in the presence of the single catalytic species (Mn 2+ alone and Fe 2+ alone) allowed one to observe the synergistic effect that the two catalysts added simultaneously have on the oxidation reaction. Introduction Wet limestone scrubbing is the most common flue gas desulfurization (FGD) process (or limestone/gypsum process) for control of sulfur dioxide emissions from combustion of fossil fuels because it gives excellent results for SO 2 removal; moreover, it is simpler with respect to other processes, such as the dual alkali process, which requires a double step not needed in the limestone process, thus resulting in the process being noncompetitive. Calcium bisulfite oxidation is consid- ered to be an important issue in limestone/gypsum processes because forced oxidation in the scrubber loop improves the dewatering properties of the sludge, lead- ing to the formation of gypsum (CaSO 4 ‚2H 2 O), a byprod- uct with a low adverse impact on the environment in comparison with the solid mixture of CaSO 3 ‚ 1 / 2 H 2 O and CaSO 4 usually produced. 1 The calcium bisulfite oxidation reaction has been extensively studied, both in the absence 2,3 and in the presence 4,5 of catalysts; a number of researchers devoted their studies to comprehension of the oxidation reaction path, 4,6 in particular when the occurrence of a chemical reaction is accompanied by the diffusion of oxygen in the liquid phase, which causes complex interactions that are rather difficult to enlighten. 7 This circumstance is encountered when the oxidation reaction takes place in the so-called heterogeneous conditions, achieved by contacting a sulfurous solution with an oxygen-contain- ing gas phase, that is, the operating condition in FGD processes. On the other hand, the study of sulfite oxidation in homogeneous conditions (by contacting a sulfite solution with an oxygen-saturated solution) led to relatively consistent results: in a previous work, Lancia et al. 1 showed how the following equation appears to be the most appropriate to describe the kinetics of the oxidation reaction for a pH range of 7.5-9 in homogeneous conditions: where r is the reaction rate expressed as moles of SO 4 2- produced per unit time and volume, k is the kinetic constant, c M is the catalyst concentration, and c S(IV) is the total sulfite concentration. A partial agreement, between results reported in the literature, exists only on the value of the kinetic constant at 25 °C, which ranges between 2 × 10 6 and 36 × 10 6 m 3 /mol, while the activation energy ranges from 50 to 150 kJ/mol. Equa- tion 1 can be interpreted by assuming that the reaction takes place via a free-radical mechanism, with a chain initiated by the catalyst autoxidation or by the action of UV light. 8,9 On the other hand, in both homogeneous and heterogeneous oxidation, the reaction is highly sensitive to operative conditions, such as the liquid- phase composition (sulfite concentration, dissolved oxy- gen, and pH) and the presence, even in traces, of catalysts (Co 2+ , Cu 2+ , Mn 2+ , and Fe 2+ ) and inhibitors (alcohols, phenols, and hydroquinone). The previous experimental study 1 of the bisulfite oxidation reaction in heterogeneous conditions has given the kinetic equation for calcium bisulfite oxidation that follows, which is of zero order in oxygen and three halves in HSO 3 - ions: * To whom correspondence should be addressed. Tel.: +39 81 7682243. Fax: +39 81 2391800. E-mail: lancia@unina.it. † Universita ` di Napoli “Federico II”. ‡ Universita ` di L’Aquila. § Seconda Universita ` degli Studi di Napoli. r ) kc M 1/2 c S(IV) 3/2 (1) r ) k u c HSO 3 - 3/2 (2) 4876 Ind. Eng. Chem. Res. 2004, 43, 4876-4882 10.1021/ie030836l CCC: $27.50 © 2004 American Chemical Society Published on Web 07/02/2004