Process for removing contaminants from hot waste gas streams

In a process for removing sulfur dioxide, sulfur trioxide and particulate contaminants from a hot waste gas stream in which the waste gas stream is passed through one or more absorption zones for removal of sulfur dioxide and other contaminants, the absorption zones utilizing as the absorbent a recirculated aqueous salt solution of ammonium sulfite-bisulfite, the improvement comprising

• quenching with water said waste gas stream to approximately its wet bulb temperature,
• passing said waste gas stream through a first cooling zone to said gas and condense the moisture contained therein, the condensed moisture acting to remove particulate contaminants from the gas,
• passing said waste gas stream through at least one additional cooling zone to further cool said gas to a temperature below about 100.degree.F and condense said moisture, the condensed moisture acting to remove substantially all remaining particulate contaminants from the gas,
• passing said waste gas stream in sequence through at least two separate absorption zones,
• continuously withdrawing heat from the recirculated salt solution in each zone so that the temperature of the water gas stream is successively reduced as it passes through each of the zones,
• the recirculated absorbent in the first zone absorbing most of the sulfur dioxide and having an initial pH of from 6 to 7.2 and a salt concentration of from 2 to 8% by weight,
• the recirculated absorbent in the second zone absorbing the sulfur trioxide and the remaining sulfur dioxide and having an initial pH of from 5.2 to 6.5 and a salt concentration of from 0.3 to 1.0% by weight.