The need for desulfurization is set by air regulations/quality limits set for your area by the appropriate government agencies. You'll need to start there.
What's the Boiler capacity? For small boilers you don't need all that. Generally Chimneys are designed(Thermal) to create small negative draft at the inlet unless it is meant for creating some draft for boiler. Minimum Temp should be to take care of above and any acid condensation.
Since if I go for Flue Gas Wet Desulfurization, the temperature will be lowered. So I need the minimum temperature needed at STACK to avoid acid condensation and corrosion.
Say Boiler capacity of 2 to 6 ton capacity.
Please also tell me approximate quantity of emitted Sulphur Oxides and Sulphur in Furnace oil and Wood in each Kg.