Emission of nonchlorinated and chlorinated aromatics in the flue gas of incineration plants during and after transient disturbances of combustion conditions: delayed emission effects.

The profiles of different products of incomplete combustion (PIC) in the flue gas of a 1 MW pilot combustion facility were investigated under normal steady-state and disturbed combustion conditions. The behavior of emission profiles after disturbed combustion conditions was investigated in order to obtain a better understanding of emission memory effects. Highly time-resolved, quantitative on-line measurements of several aromatic species down to low ppbv or higher pptv concentrations were performed by a mobile resonance-enhanced multiphoton ionization time-of-flight mass spectrometer. Conventional analytical methods (gas chromatography-mass spectrometry and high-performance liquid chromatography) were also applied for measurement of polycyclic aromatic hydrocarbons (PAH) and polychlorinated dibenzo-p-dioxins and -furans (PCDD/F). The sampling point was located in the high-temperature region of the plant at the outlet of the post-combustion chamber at temperatures between 650 and 880 degrees C, prior to any emission reduction devices. The investigation pointed out that after a short phase of disturbed combustion conditions, e.g., due to process changes, transient puffs, or malfunctions, the composition of combustion byproducts in the flue gas can be changed drastically for a very long time ("memory emission" effect). It is suggested that carbonaceous layers, deposited on the inner walls in the high-temperature zone of the plant, might be responsible for the observed memory emission of some PAH species. Drastic changes in the profiles of the PCDD/F homologues were also observed during memory emission conditions. The PAH memory most likely is due to pyrolytic degradation of the carbonaceous layers, while the altered PCDD/F homologue pattern may be mediated by the high catalytic activity of the freshly formed deposit layers. Finally, it should be emphasized that a rich pattern of aromatic species, including PCDD/F, was found in a temperature regime well above the typical temperature window (approximately 300 degrees C) for de novo PCDD/F formation.