Article pubs.acs.org/EF
Thermal Stability of Zinc Compounds Frida Jones,*,†,‡ Honghi Tran,§ Daniel Lindberg,† Liming Zhao,§ and Mikko Hupa† †
Åbo Akademi Process Chemistry Centre, Biskopsgatan 8, FIN-20500 Åbo, Finland Energy Technology, SP Technical Research Institute of Sweden, Box 857, SE-501 15 Borås, Sweden § Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada ‡
ABSTRACT: Zn and Cl have been found in deposits in municipal solid waste (MSW) boilers and industrial boilers. This leads to the general belief that ZnCl2 may play a role in corrosion of heat-transfer tubes, owing to its low melting temperature and high corrosivity. In this study, the thermal stability of the compounds ZnCl2, ZnSO4, and ZnO as well as mixtures of ZnCl2 and NaCl/ KCl was investigated by means of thermogravimetric analysis/differential scanning calorimetry (TGA/DSC). The reactions of the Zn compounds with SO2/SO3 and HCl were also investigated. The results obtained show that ZnCl2 melts at 320 °C. Above 400 °C, ZnCl2 vaporizes and is partly oxidized to ZnO. ZnSO4 is stable up to 680 °C, at which it decomposes and, subsequently, forms ZnO above 900 °C. ZnO is stable at a much higher temperatures but can be chlorinated to ZnCl2 in the presence of HCl at temperatures around 300 °C. In the presence of a large amount of NaCl/KCl, which is typically the case in actual boilers, ZnCl2 reacts with NaCl/KCl to form 2NaCl·ZnCl2 and 2KCl·ZnCl2, respectively. These compounds melt at temperatures lower than NaCl and KCl but higher than ZnCl2. The findings show the significance of understanding the thermal stability of various zinc compounds, how they interact with one another, and whether they react with alkali chlorides and other compounds in the deposits to assess the role of Zn in deposit formation and corrosion in boilers burning Zn-containing fuels.
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INTRODUCTION Previous studies indicate that Zn from the fuel can accumulate in the bed material, in the ash,1 participate in the formation of deposits in boilers,2,3 and also increase the deposit formation rate.4 Ljungdahl and Zintl1 showed both experimentally and with thermodynamic equilibrium calculations that Zn present in the bed material of a fluidized bed can cause severe agglomeration of the bed if the stoichiometric conditions are changed after a long period of oxidizing to reducing. In addition, they suggested that metallic Zn that enters a boiler stays in the bed as a metallic smelt, is volatilized to ZnO during oxidizing conditions, and condensates on the furnace wall tubes as metallic Zn in reducing conditions. Lind et al.5 found that most zinc was removed with the bottom ash and only 0.1−0.3% was in the gas phase at 830 °C when studying combustion of forest residues. If present in deposits, it is possible that the compound ZnCl2 contributes to the corrosion of boiler heat-exchanger surfaces as it melts at low temperatures, creating sticky melts.6 Åmand et al.2 found small fractions of Zn in their deposit samples during an experimental campaign in a 12 MWth circulating fluidizedbed (CFB) boiler. They also identified Zn compounds in collected aerosol particles. Furthermore, they could conclude that, in one experiment where they had injected ZnO particles with a size of 20−25 μm, they had an increase of Zn in particles
