the furnace and may be passed to other processing devices by opening valve 56 or they may be passed to a scrubber 58 by means of opening valve 60 the scrubber has an exhaust outlet 62 and a water outlet 64 heretofore in order to support combustion an oxidant such as air was added at the bottom of the furnace additional air was added as deemed necessary in various other hearths throughout the furnace particularly in the fixed carbon burning zone generally it was thought necessary to operate with substantial quantities of excess air frequently with as much as 100 excess air above that theoretically required for supporting combustion for purposes of providing adequate oxygen and for cooling however such excess air caused problems as it tended to entrain or carry with it particulate matter into the exhaust gases for example in other installations such as the one disclosed in a copending application filed on the same date as the present application and entitled 34 method and apparatus for incinerating waste material 34 the air supply is controlled so that on most hearths there is a deficiency of oxygen as compared to that theoretically required for complete combustion this system has certain advantages as indicated in said patent application in both of the above systems the carbon and oxygen react as follows c.