Effect of Alkali Metal Compounds on Limestone Calcination in Desulfurization Process: Molecular Dynamics Simulations

Abstract

In this study, the change in the desulfurization characteristics of limestone induced by adding some alkali metal compounds and chlorides to limestone was investigated by thermobalance experiments. In addition, molecular dynamics MD simulations were performed to elucidate the effect of alkali-metal chloride additions on the CaO calcination for the desulfurization process. The experimental results obtained show that the addition of NaCl into the limestone is the most effective in improving the desulfurization efficiency. The X-ray diffraction analyses of the calcined seashell, limestone and NaCl-doped limestone found that the calcined seashell, as well as the calcined NaCl-doped limestone, have a similar structure as the CaO crystal. These two materials also show a high desulfurization efficiency. The MD simulations show that NaCl doping into the CaO contributes to the crystallization of CaO, while KCl and LiCl dopings have a small contribution. Therefore, the NaCl doping has a predominant effect on the CaO crystallization leading to a high desulfurization efficiency. Key-words : Desulfurization, Calcination, Limestone, Alkali metal compounds, Molecular dynamics simulation, Sodium chloride, Calcium oxide, Seashell 1. Introduction Recently, acid rain has been recognized as one of the most serious global environmental problems. Sulfur oxides SO x are one of the main sources of acid rain. In a bubbling and circulating fluidized bed coal combustors, limestone is directly supplied into the combustors as a desulfurizer in order to decrease the emissions of SO x from coal. Those combustors are operated at temperatures ranging from 1073 to 1173 K. The limestone is calcined and sulfurized optimally under this temperature condition. In practical boilers however, limestone, which mainly consists of CaCO 3 , is fed excessively to improve the contact efficiency between the CaO produced and the SO 2 emitted. When accomplishing more than 90! of the desulfurization efficiency, or burning coal with a sulfur content of more than 1 mass!, it is necessary to add limestone more than four times as much as Ca to S mole ratio. Otherwise, the particle size of the limestone should be fine. 1 ,2 This is because pores inside the CaO particles ought to be plugged as a result of CaSO 4 formation. 3 As the main component of the limestone is CaCO 3 , which is similar to that of seashells, it is possible to use wasted seashells as a desulfurizer. In this case, the CO 2 produced during the calcination of the seashells is fixed again to the seashells and recycled naturally. Naruse et al., 4 reported that the desulfurization efficiency of the seashells was over twice as high as that of the limestone. This suggests that the seashells could be used as an effective desulfurizer. To explain the mechanisms and reasons for the high desulfurization efficiency of the seashells, the chemical compositions and the mean pore size of the seashell and the limestone were analyzed. As a result, the seashell was found to