| Summary: | 碩士 === 國立中央大學 === 環境工程研究所 === 105 === This study characterizes the formation and emission of PCDD/Fs and dl-PCBs in two woodchips boilers during different operating periods. Two woodchips boilers (referred to as Plant A and B) investigated in this study are located in northern Taiwan. The sampling program of Plant A was conducted during different operating stages, including start-up, normal operation and shut-down periods. The results indicate that PCDD/F and dl-PCB concentrations (588.2 ng/Nm3) at APCDs inlet during the start-up period are much higher than those measured during normal operation and shut-down periods due to unstable combustion conditions. Concentrations of PCDD/F and dl-PCB (41.05 ng/Nm3) measured at APCDs inlet during the shut-down period were even lower than that measured during the normal operating period. It is due to the lack of carbon and chlorine sources since woodchip was not fed into the boiler during the shut-down period. The Plant B was investigated under three operating conditions, including full feed loading with injection of activated carbon, full feed loading without injection of activated carbon and half feed loading with injection of activated carbon. PCDD/Fs and dl-PCBs concentrations (113.6 ng/Nm3) measured at APCDs inlet during the half feed loading are lower than those measured during the full feed loading (139.4 ng/Nm3) due to the reduction of input woodchips. The removal efficiences of gas-phase PCDD/Fs and dl-PCBs achieved with ACI+BF are 95.60% and 92.05%, respectively, which are higher compared to the case without injection of activated carbon (85.74% and 79.76%, respectively). It demonstrates that gas-phase PCDD/Fs and dl-PCBs emissions can be effectively reduced by activated carbon injection. Durig full feed loading with the injection of activated carbon, PCDD/F and dl-PCB concentrations at APCDs inlet of the Plant A are significantly higher than those measured in the Plant B, due to the lower combustion temperature (500-850 oC) compared with Plant B (850-925 oC). Combustion condition remarkably affects PCDD/F and dl-PCB formation within the woodchip combustion process. The TEQ concentration of PCDD/Fs measured at the stack of the Plant A during different operating stages are significantly higher than the emission limit, while those of Plant B are lower than the regulation limit (0.5 ng I-TEQ/Nm3). For both Plants A and B, solid-phase PCDD/Fs dominate at APCDs inlet during different operating conditions because combustion of woodchips generates high concentration particulate matter. Different operating stages of woodchips combustion do not affect distributions of PCDD/F congeners significantly. Major PCDD/F congeners at APCDs inlet of Plants A and B during different periods include 1,2,3,4,6,7,8-HpCDF, 1,2,3,4,6,7,8-HpCDD and OCDD. The composition of feeding materials and different operating conditions affect the distributions of dl-PCB congeners. Concentrations of dioxin (PCDD/Fs+dl-PCBs) in the BF ash of Plants A and B during full feed loading with injection of activated carbon are measured 979.2 and 406.2 ng/g. Removal efficiencies of PCDD/Fs and dl-PCBs of the Plant A are 95.56% and 88.63%, respectively, while those of the Plant B are 99.29% and 94.94%. PCDD/Fs and PCBs emission factors of Plant A are 17.24 and 1.120 μg WHO-TEQ/ton, respectively. For the Plant B, the emission factors of PCDD/Fs and PCBs are 1.520 and 0.1400 μg WHO-TEQ/ton, respectively.
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