Zn and Cl have been found in deposits in municipal solid waste (MSW) boilers and industrial boilers. This leads to the general belief that ZnCl 2 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 ZnCl 2 , ZnSO 4 , and ZnO as well as mixtures of ZnCl 2 and NaCl/ KCl was investigated by means of thermogravimetric analysis/differential scanning calorimetry (TGA/DSC). The reactions of the Zn compounds with SO 2 /SO 3 and HCl were also investigated. The results obtained show that ZnCl 2 melts at 320 °C. Above 400 °C, ZnCl 2 vaporizes and is partly oxidized to ZnO. ZnSO 4 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 ZnCl 2 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, ZnCl 2 reacts with NaCl/KCl to form 2NaCl•ZnCl 2 and 2KCl•ZnCl 2 , respectively. These compounds melt at temperatures lower than NaCl and KCl but higher than ZnCl 2 . 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.

Sulfur recirculation is a new technology for reducing boiler corrosion and dioxin formation. It was demonstrated in full-scale tests at a Waste to Energy plant in Göteborg (Sweden) during nearly two months of operation. Sulfur was recirculated as sulfuric acid from the flue gas cleaning back to the boiler, thus creating a sulfur loop. The new technology was evaluated by extensive measurement campaigns during operation under normal conditions (reference case) and operation with sulfur recirculation. The chlorine content of both fly ash and boiler ash decreased and the sulfur content increased during the sulfur recirculation tests. The deposit growth and the particle concentration decreased with sulfur recirculation and the dioxin concentration (I-TEQ) of the flue gas was reduced by approximately 25%. Sulfuric acid dew point measurements showed that the sulfuric acid dosage did not lead to elevated SO3 concentrations, which may otherwise induce low temperature corrosion. In the sulfur recirculation corrosion probe exposures, the corrosion rate decreased for all tested materials (16Mo3, Sanicro 28 and Inconel 625) and material temperatures (450 °C and 525 °C) compared to the reference exposure. The corrosion rates were reduced by 60-90%. Sulfur recirculation prevented the formation of transition metal chlorides at the metal/oxide interface, formation of chromate and reduced the presence of zinc in the corrosion products. Furthermore, measured corrosion rates at 525 °C with sulfur recirculation in operation were similar or lower compared to those measured at 450 °C material temperature in reference conditions, which corresponds to normal operation at normal steam temperatures. This implies that sulfur recirculation allows for higher steam data and electricity production without increasing corrosion.

This study aimed to determine the content of fossil carbon in waste combusted in Sweden by using four different methods at seven geographically spread combustion plants. In total, the measurement campaign included 42 solid samples, 21 flue gas samples, 3 sorting analyses and 2 investigations using the balance method. The fossil carbon content in the solid samples and in the flue gas samples was determined using (14)C-analysis. From the analyses it was concluded that about a third of the carbon in mixed Swedish waste (municipal solid waste and industrial waste collected at Swedish industry sites) is fossil. The two other methods (the balance method and calculations from sorting analyses), based on assumptions and calculations, gave similar results in the plants in which they were used. Furthermore, the results indicate that the difference between samples containing as much as 80% industrial waste and samples consisting of solely municipal solid waste was not as large as expected. Besides investigating the fossil content of the waste, the project was also established to investigate the usability of various methods. However, it is difficult to directly compare the different methods used in this project because besides the estimation of emitted fossil carbon the methods provide other information, which is valuable to the plant owner. Therefore, the choice of method can also be controlled by factors other than direct determination of the fossil fuel emissions when considering implementation in the combustion plants.

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This study focuses on how the temperature of the bed in a waste-fired fluidized-bed boiler affects the chemical composition of ash and deposits. The aim was to reduce the concentration of corrosive elements in the convection pass, which can lead to both less frequent soot blowing intervals and extended superheater lifetimes. Complementary laboratory-scale tests were carried out in a single-pellet reactor to study online alkali and Zn emissions during temperature changes, using an ICP-MS instrument. The full-scale study was based on full-scale experiments at a plant consisting of two 20 MWth fluidized-bed boilers firing a mixture of municipal solid waste and industrial waste. The boilers are normally operated at bed temperatures of about 870 °C. This normal operation was compared in this work to a test case in which the bed temperature was reduced below 720 °C by altering the air staging and flue gas recycling. The experimental work included collecting samples of fuel, ash, and particles under the two different sets of operating conditions. Furthermore, deposits on temperature-controlled probes were sampled upstream of the superheaters. By reducing the bed temperature, the sand consumption of the plant could be reduced by roughly 25%. The measurements showed that the amount of submicrometer particles decreased and the fouling rate on deposit probes was reduced by about 20%. The measured concentration of HCl in the flue gas increased as the bed temperature was reduced. This might be a consequence of the reduced formation of alkali chlorides. In addition, results from the laboratory-scale tests indicated a trend of reduced alkali emissions from the fluidized bed with reduced temperature, and thermodynamic equilibrium calculations confirmed the trends.

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