Abstract
Laboratory wastes are discharged from experimental, testing, or analysis processes, and contain various toxic chemical compounds with a high heating-value and a high chlorine content (＞ 9%). Elevated PCDD/F (polychlorinated dibenzo-pdioxin and furan) emissions during start-up stages are caused by combustion of waste with high chlorine contents, incomplete combustion, and so called ”memory effects”. Even though the duration of cold start-up is short compared with the hours of continuous steady operation in a waste incinerator, its negative effects with regard to PCDD/F emissions on both human health and the environment cannot be neglected. A full-scale laboratory-waste incinerator which is operated for 10 days in each run and has 15 to 20 runs annually was investigated in this study. Eleven PCDD/F samples of stack flue gas were collected during the cold start-up periods (for 60.5 hrs). The gas temperature of the primary combustion chamber was above 850°C, and was maintained at between 850 and 900°C by injecting diesel fuel without waste feed. For first 1.5-7.5 hours, the PCDD/F concentration in the stack flue gas was as high as 0.656-1.15 ng I-TEQ/Nm^3. Afterward, during hours 10.5-35.5 and 54.5-60.5, this reduced to 0.159-0.459 and 0.218-0.254 ng I-TEQ/Nm^3, respectively. Based on principal component analysis (PCA) and the L/H ratio, the results revealed a higher L/H ratio (1.23) before hour 32, indicating that less chlorinated PCDD/F homologues (tetra and penta) dominated, while after hour 32 more chlorinated PCDD/F homologues (hexa, hepta and Octa) had a higher mass concentration and the L/H ratio fell to 0.42. These results indicate that the PCDD/F emissions during cold start-up were caused by memory effects and thermal desorption. Therefore, in order to reduce the PCDD/F emissions from the stack flue gas of waste incinerators, is highly recommended that a higher amount of activated carbon injection is used in front of the bag filters.
Highlights
Laboratory wastes containing various hazardous chemical reagents and reactants are often discharged from experimental, testing, or analysis processes
These results indicate that the PCDD/F emissions during cold start-up were caused by memory effects and thermal desorption
PCDD/F Congeners Profiles in Stack Flue Gas According to the operation guide, most waste incinerators start feeding and increasing feed rate of waste
Summary
Laboratory wastes containing various hazardous chemical reagents and reactants are often discharged from experimental, testing, or analysis processes. The PCDD/Fs are often found to from the emissions of thermal treatment processes and have been drawing extensive concern because of their significant toxicity and adverse effects on human health. Those PCDD/Fs with chlorines on the 2, 3, 7, and 8 positions are especially toxic (Rappe et al, 1987; McKay, 2002). Several researchers studied the emission of waste incinerators during start-up stage and reported that higher concentration of PCDD/F emissions were observed during the cold start-up stage (Löthgren and van Bavel, 2005; Neuer-Etscheidt et al, 2006; Wang et al, 2007b). After an incinerator has been shut and cooled down to room temperature for several days for maintenance, a cold startup is needed, which include the following procedures: (1) burning with the fuel oil at the maximum feed rate until the gas temperature of combustion chamber exceeds 850°C. (2) start and increase the waste feed rate until the design load is reached (Wang et al, 2007b)
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