Thermal Stripping of Sulfur and Chlorine from Cement Hot Meal: An Experimental Feasibility Study

One of the issues connected with the use of alternative fuels in cement production is an increased input of sulfur and chlorine into the pyro process, giving rise to increased deposit formation and corrosion problems. This work explores the feasibility of developing a new thermal stripping process for simultaneously removing S and Cl from the cement hot meal, thereby decreasing the total amount of both S and Cl in the cement plant. The concept is based on the idea that a fraction of hot meal, coming from the last-stage preheater cyclone at a temperature of 850–900 °C, is brought into contact with a reducing gas that will strip the S and Cl by converting CaSO4(s) to SO2(g) and by evaporation of KCl. Industrial hot meal samples, C4 and C5 meals obtained from the fourth and fifth preheater cyclones of a full-scale cement plant, respectively, were subjected to release experiments using a horizontal tube reactor. The effects of temperature and process gas composition were evaluated. Off-gas SO2 analysis was used to quantify the S release. For selected experiments, the elemental release of S, Cl, and K was also quantified based on bulk chemical analysis of the solid residue. It was found that a mildly reducing atmosphere of ∼1% CH4 provided significant S release, and the main release took place in the temperature interval from 800 to 900 °C. At 900 °C and 1% CH4, substantial release of S (∼40%), Cl (∼30%), and K (∼13%) was obtained, and the S was largely released as SO2. At higher temperatures, there was an additional mechanism for S release, emphasizing that only by using a quantification method based on the total S content can the full S release be determined. Partial retention of S even at high temperatures (1100 °C) implied formation of thermally stable phases. Thermodynamic equilibrium simulations suggested that sulfur distributes between the condensed phases CaSO4, K2SO4, Ca2K2(SO4)3, and CaS, and the gas phase SO2, but the system is very sensitive to changes in process conditions and hot meal compositions. For a hot meal with a composition like the present C5 meal, an optimized stripping system with a gas-to-solid ratio (kg/kg) of around 0.25, T = 900 °C, and 0.3% CH4, would yield up to ∼50% S release (owing to decomposition of CaSO4 and CaS) and approximately 40% Cl release (owing to evaporation of KCl(g) and (KCl)2(g)). To “unlock” the maximum achievable release potential for a given hot meal, full decomposition of CaSO4 and CaS must be attained. With increasing S content, calcium becomes an important “sink” for sulfur, and CaS is stabilized by excess S. The latter suggests that for practical applications, a way to ensure robust release conditions with full decomposition of CaSO4 and CaS for a broader range of hot meal compositions could be to apply an alternating reducing/oxidizing atmosphere (facilitating reoxidation of CaS).