Influence of wet flue gas desulfurization on the pollutants monitoring in FCC flue gas

Fluid catalytic cracking (FCC) unit emits a large amount of flue gas, which is a major concern of environmental protection supervision. Wet flue gas desulfurization (WFGD) technologies have been widely used to control the emissions of SO 2 in refineries. In this study, stack tests for pollutants emission of a typical FCC unit were conducted. The emission characteristics of the FCC unit indicated that WFGD would cause a large amount of water vapor in the flue gas, which indirectly leads to large quantities of salt pollutants entrained in the flue gas including ammonium sulfite ((NH 4 ) 2 SO 3 ) and ammonium sulfate ((NH 4 ) 2 SO 4 ). A strong correlation among the concentrations of SO 2 , NH 3 , and H 2 O was observed, and factor analysis shows that these concentrations are dominated by a common factor. It was also found that a mass quantity of NH 4 + and SO 3 2− existed in the condensate water of the flue gas. The TG-MS analysis shows that (NH 4 ) 2 SO 3 could be decomposed at 94.1 °C, and NH 3 , SO 2 , and H 2 O are released as reaction products in the form of gas. Therefore, a part of the NH 3 and SO 2 obtained by Fourier transform infrared spectroscopy (FTIR) monitoring may be derived from the decomposition of (NH 4 ) 2 SO 3 in the flue gas due to the high temperature during the sampling process, which was also confirmed in a lab experiment. The hot and wet sampling process will lead to overestimation of NH 3 and SO 2 emissions rather than FTIR method itself when monitoring the high-humidity FCC flue gas. Thus, the concentration of H 2 O in the flue gas and the type of sampling process need to be taken into consideration during the monitoring process.