For DF Series, Most of the case, we use Multi cyclone and venturi wet scrubber only,without employment of expensive DeSOx tower. Specially if you use Indonesian coal of below 1% sulfur content, SO2 level is lower than the government set value. You have two(2) Options to remove SO2 acording to the content. 1st OPTION is to inject CaCO3 into the Furnace. 2nd OPTION is NaOH solution to be injected to the venturi water scrubber for reaction as indicated in this paper. In anyway, FBC boiler does not require expensive DeSOx tower at the exhaust gas area. This is a remarkable advantage of FBC system.