The Influence of Sulfuric Acid Injection on Deposit Formation in a Waste-to-Energy Plant

Abstract

Waste-to-Energy (WtE) technology is the most competitive and mature technology for producing heat and electricity from municipal solid waste (MSW). However, the electrical efficiency of WtE plants is constrained by the formation of corrosive deposits on the superheaters. The present work deals with the sulfur recirculation technology, which was aimed at reducing the corrosion rate of the superheaters and dioxin formation in WtE plants, by recirculating the sulfur from the wet flue gas cleaning back to the boiler in the form of sulfuric acid. To improve the understanding of how sulfuric acid injection influences the ash chemistry and deposition rate, full-scale measurements in a WtE plant equipped with sulfur recirculation were carried out by using an advanced deposit probe system. The sulfuric acid was injected into two different positions, A (flue gas temperature ∼1000 °C) and B (flue gas temperature ∼730 °C) inside the boiler. Compared to the reference operation without sulfuric acid injection, injecting sulfuric acid equivalent to 170−920 mg SO 2 /Nm 3 reduced the Cl/S molar ratio in fly ash from 2.3 to a minimum of 0.6, and in deposits from 0.8 to a minimum of 0.2. The short-term (14−15 h) deposition rate also decreased from 19 g/h/m 2 to a minimum of 7 g/h/m 2 . The reduction in both the Cl/S ratio and the short-term deposition rate was more effective with sulfur injection at position B compared with position A. This was attributed to the increased generation of SO 3 at position B, which facilitated the sulfation of alkali chlorides. This phenomenon was supported by chemical kinetic modeling. A long-term (1 week) deposition test was conducted with a sulfur dosage of ∼200 mg of SO 2 /Nm 3 at position B after 60 h of reference operation. This resulted in a higher deposition rate of 22 g/h/m 2 compared to the short-term deposition rate (7−13 g/h/m 2 ). This suggests that the deposition rate increases over time, indicating the need for further research to understand this phenomenon.