Abstract: A flue gas conditioning system (FGC) for conditioning the flue gas flowing in a flue from a boiler to an electrostatic precipitator by injecting sulfur trioxide into the flue gas upstream of the electrostatic precipitator. The FGC has a source (52) of granulated sulfur which is transported by a conveyor (54) to a sulfur furnace (60) where it is combusted in sulfur dioxide. The sulfur dioxide flows from the sulfur furnace (60) into a catalytic converter 62 which generates sulfur trioxide therefrom. The sulfur trioxide flows from the catalytic converter 62 into probes 64 which are mounted in the flue duct. Alternatively, the FGC has a source (70) of emulsoid sulfur which is pumped by a sulfur pump (72) to the inlet of an atomizing spray nozzle (200) which atomizes the emulsoid sulfur and sprays it into sulfur furnace (50).