Combustion pressure may be the least important factor in SOx formation. There are so many other factors influencing SOx formation and reduction. I know of an EPRI report, but they are hard to obtain and ridiculously expensive. You may be able to get a copy on loan from a library operated by a power company who is a member of EPRI, but EPRI may not allow them to loan their reports out.

SOx formation in combustion process is different from NOx formation. NOx formation, especially thermal-NOx, is limited by reaction kinetics. (Fuel-N to NOx is also complex, through HCN ang NH3 precursors.) The actual air-N2 to NOx conversion is at least a few orders of magnitude less than equilibrium conversion.

The conversion of fuel-S to SO2 and SO3 is usually complete, with only trace amount of COS and very little other species formed during typical coal combustion processes, in which excess air is supplied. However, the SO3/SO2 ratio increases with increasing pressure (you asked about the pressure effect), increasing O2 concentration, and decreasing temperature. It shows a much greater influence of chemical equilibrium than NOx formation. Therefore, instead of using a kinetic model with complicated reaction mechanisms, you may consider using an equilibrium-based model for SOx. The tricky part is, however, how to model the effect of minerals in ash (e.g. CaO, MgO) on the final SO2 and SO3 concentration since they can retain some sulfur as sulfates if the temperature is below 1300 deg C.

Dear Kalimuthu Selvam 

One of the easiest and best references about mechanism of SOx formation is ANSYS theory guide. In this reference you could find the mechanism of SOx  formation and its' equations in different type of fuels like liquid fuels, coal and etc.

Good Luck

Amir Javad Ahmadian Hosseini