||SO2 emissions from coal combustion have several harmful effects on the environment, including their contribution to acid rain. A variety of approaches have been developed to control SO2 pollution. Direct dry sorbent (limestone) injection is a relatively simple and low-cost process. Fluidized beds have been one of the most popular furnaces for the application of direct sorbent injection. In addition, oxy fuel combustion is a promising, practical method to reduce greenhouse gas emissions. Thus, studies on the mechanisms related to the production of SO2 emissions under oxy fuel conditions are important. The sulfation mechanisms (direct or indirect) of limestone depend on whether the limestone is calcined. Direct sulfation takes place in an uncalcined state while an indirect sulfation happens with calcined limestone. Usually, in fluidized bed combustion conditions, direct sulfation occurs in oxy fuel combustion due to CO2 inhibition, and indirect sulfation occurs in air combustion. A bench-scale bubbling fluidized bed (BFB) and a 330 KW pilot-scale circulating fluidized bed (CFB) were used to investigate SO 2 behavior in air and oxy coal combustion. SO2 release with and without limestone sorbent were investigated in N2/O2 and CO2/O2 environments for the following conditions: temperature range (765-902°C;), O2 concentration range (10-30%), and a wide range of Ca/S ratios. The bench-scale experiments without recycled flue gas (RFG) and the equilibrium calculations of NASA Chemical Equilibrium with Applications (CEA) show no effect of combustion diluent (N 2, CO2) on SO2 emissions. Limestone addition shows greater SO2 capture efficiency in air firing than that in oxy firing. In addition, sulfation behavior of limestone in N2/O 2/SO2 and CO2/O2/SO2 atmospheres was studied, exploring the mechanisms of indirect and direct sulfation in a wide range of temperatures (765-874°C;). A significant temperature effect on sulfation behavior of limestone was seen during direct sulfation reactions. However, limited effect was seen for an indirect sulfation reaction. A scanning electron microscope (SEM) and energy dispersive x-ray spectrometer (EDS) were used to exam the microstructure of sulfated limestone and their sulfur distribution. A mathematical and computational framework for a single particle model was developed to understand the differences between indirect and direct sulfation and single coal particle combustion processes. The model framework presented here, however, provides a broader flexibility that could be used to address a range of particle sizes. The shrinking core particle model and fine single particle model are two specific applications of my general single particle model.