Unlike the ash, the flue gas continues its path throughout the plant. The flue gas flows through the plant due to the lower pressure produced at the end of the plant by the induced draft fan. The heat generated by the flue gas is utilized in a heat exchanger called the economizer. The economizer preheats the boiler feedwater and decreses the flue gas temperature. During combustion, pollutants such as NOX,CO,Acid gases,SOx,Particulates,PCDD's,PCDF's, and metals are generated and carried out of the burner by the flue gas. Treatment of these pollutants is necessary in order to comply with the permit levels set by the regulatory agencies. The first pollution control device used to collect pollutants is the cyclone. A cyclone is a circular tube which causes the heavier particles in the flue gas to drop out. The remaining pollutants are treated by the dry gas scrubber. The dry gas scrubber uses a lime injection system to treat acidic pollutants in the flue gas. Larger lime particulates are formed through the use of this system. A baghouse is then used to catch these remaining pollutants. The cleaned flue gas is then sent to the stack. Within the stack are continuous emissions monitoring devices (CEM) to ensure that the pollutant levels are within the permit limit.