Flue gas conditioning with spiking gas containing sulfur trioxide

In a process for removing fly ash from flue gas at a temperature of from about 140.degree. C to about 160.degree. C by

• a. treating the flue gas with a spiking gas containing sulfur trioxide in a sufficient amount for conditioning the flue gas in a mixing zone, and then
• b. precipitating the fly ash from the flue gas by electrostatic means, the improvement which comprises:
• 1. forming a spiking gas containing from about 0.15 to 1.2 mol percent sulfur trioxide, and
• 2. maintaining the spiking gas at a temperature sufficiently high between a temperature of from about 300.degree. to about 750.degree. C such that on treating the flue gas with the spiking gas, the temperature of any mixture of spiking gas and flue gas from the point of introduction to the point of dilution as determined by the equation ##EQU4## wherein:
• T = Temperature of mixture of spiking gas and flue gas in degrees centigrade,
• N = Mols of spiking gas per mol of flue gas
• T.sup.s = Temperature of the spiking gas in degrees Centigrade,
• T.sup.f = Temperature of the flue gas in degrees Centigrade,
• C.sub.p.sup.s = Heat capacity of the spiking gas in Cal/mol/.degree. C,
• c.sub.p.sup.f = Heat capacity of the flue gas in Cal/mol/.degree. C,
• is not more than 10.degree. C below the dew point temperature of the gas mixture as determined by the equation
• 1T.sub.DP = 0.002276 - 0.00002943 ln p.sub.H.sbsb.2.sub.O - 0.0000858 ln p.sub.H.sbsb.2.sub.SO.sbsb.4 + 0.00000620(ln p.sub.H.sbsb.2.sub.SO.sbsb.4) (ln p.sub.H.sbsb.2.sub.O)
• wherein:
• T.sub.DP = Dew Point .degree. K and, p.sub.H.sbsb.2.sub.O and p.sub.H.sbsb.2.sub.SO.sbsb.4 are partial pressures in mm Hg.