Abstract: A system is provided for maintaining the concentration of sulfur oxides in a flue duct (60) at a predetermined value. The sulfur oxides being maintained at a concentration to substantially optimize the particulate removal performance of an electrostatic precipitator (70). The sulfur rate control system (100) injects sulfur trioxide into the flue gas in proportion to a deficiency in the content of sulfur oxides in the flue gas, with respect to the proportion to particulates being carried by the flue gas. The sulfur trioxide is formed in a catalytic converter (54) which is supplied with sulfur dioxide and oxygen from a reaction chamber (40). The formation of sulfur dioxide is controlled by metering the source of sulfur responsive to a signal provided from a ratio controller (64).