Investigation of polycyclic aromatic hydrocarbons in fly ash from fluidized bed combustion systems

A laboratory scale fluidized bed reactor and a bench scale 0.1 MWth fluidiz ed bed combustor were used to study the effect of operating conditions on t he formation of Polycyclic Aromatic Hydrocarbons (PAHs) in fly ash from flu idized bed combustion systems. A high volatile bituminous coal was chosen t o investigate PAH emissions during the entire pyrolysis to oxygen-rich comb ustion process. During the experiments, the fluidized bed reactor was opera ted at temperatures between 700 degrees C and 900 degrees C, while the exce ss air ratio was varied from 0 to 1.3. An extraction and GC/MS analysis of PAHs was used in this study. Approximately 40 different PAHs were identifie d during the tests, of which only a few are specified by the U.S. EPA. The experimental results indicate the majority of the PAHs in the solid phase ( bed and fly ash) are derived from the breakdown reactions during the proces ses of combustion and/or pyrolysis in a Fluidized Bed Combustion (FBC) syst em, although FBC systems have an efficient solid-gas mixing process and rel atively long residence time. The total amount of PAHs in the fly ash was mu ch higher than that in the raw coal and in the gas phase. Three and four-ri ng aromatic compounds were the major PAHs from pyrolysis conditions, while naphthalene (two-rings) is the dominant compound in bed ash collected from oxygen-rich combustion conditions. Only naphthalene was detected in the bed ash in the FBC system. High-speed secondary air (air staging) injected int o the freeboard of the FBC system is an effective method for minimizing PAH emissions, along with the other benefits including minimizing NOx and SOx emissions.