The most important air pollution problem of the pulp and paper industry is emissions of odoriferous sulphur compounds and particulates from kraft pul mills. Without any ery boiler, the kraft mills would emit 10 ? 5 kg of hydrogen sulphide (HzS) and 0.1 to 1 k each of methyl mercaptan disulphide (CH,),SZ per tonne of pulp produced. About 80 to 90% of these emissions originate from the (conventional) recovery furnace, with 290% of the total odoriferous compounds being HzS from this particular source. Outside of certain types of power boilers, the recovery boiler is also responsible for most of the mills particulates emissions. The latter, if not properly controlled, can range between 2-10 kgk, mostly consisting of NaZS04 with a particulate size range of <1 to 10 pm.For these reasons, in the early seventies, we gave the highest priority within PPRIC to finding a suitable low cost technological solution to the problems of recovery furnace flue gas emission control.In 1969, when our R&D in this field started, the industry had three principal options to reduce odoriferous sulphur emissions from recovery boilers.