| Summary: | 碩士 === 國立中央大學 === 環境工程研究所 === 94 === On this study, stack samplings are conducted in three municipal solid waste incinerators (MSWI) with different air pollution controlling devices (APCDs) in Taiwan are conducted. To evaluate the removal efficiencies of heavy metals from the stationary sources, and the emission factors of heavy metals. Besides, this study also investigates Hg emission from a zinc recovery process (Zinc-D) which does not apply activated carbon injection (ACI) to control the emission of heavy metals in March, 2005. After applying ACI in November, 2005, stack samplings are conducted again at the end of the year. In order to understand how SCR affects the removal efficiency of Hg, this study selects three coal-burning plants equipped with SCR for NOX removal to conduct stack sampling.
The results indicate the emission factors of mercury at MSWI-A, MSWI-B and MSWI-C are quite different (72.5, 257.1 and 12.1 mg /ton, respectiviely). If we compared the results with other countries, MSWI-A and MSWI-C are relatively low, and the emission factor of mercury at MSW-B lies in between the range of other countries. The difference is caused by injection of activated carbon in domestic MSWIs to control dioxin emission. MSW-B is equipped with ACI, but it is not operated to avoid the increase of dioxin emission. As for the zinc recovery process with (DSC+CY+FF), the emission factor of mercury is calculated as 622.9 mg/ton waste, mercury emission from zinc recovery process is obviously higher than that of MSWI-A and MSWI-C, and it is also higher than MSW-B which does not apply ACI as APCD. When the zinc recovery process applies ACI, results indicate the emission factor of mercury is much lower than before (46.2 mg/ton). Although the feeding materials are different, we at least make sure factories which do not apply activated carbon injection technology might result in significant mercury emissions. The emission standard of Hg for large-scale MSWI in Taiwan is 0.3 mg/Nm3, all of the MSWIs investigated in this study are lower than the limit.
SCR is primarily applied to cut down NOX emissions from the power plants, however, SCR can transform elemental mercury into oxidized mercury. Results obtained in this study indicate that the removal efficiency of heavy metals (not including mercury) could reach 95%. As for mercury, the overall removal efficiency could also reach 90%, despite the removal efficiency of elemental mercury is relatively low. Based on this study (Plant-E, Plant-F and Plant-G), the SCR of all power plants can transform elemental mercury into oxidized mercury, but we only observe a little change of transformation from Plant-G, because the flue gas only passes through one layer of SCR reactor. When elemental mercury is transformed into oxidized mercury, it is easier to remove from gas streams through the APCD downstream such as wet scrubbers, and the overall mercury removal efficiency in power plant can be enhanced. Plant-H is equipped with EP only, so the percentage of vapor-phase mercury of the stack reaches 98%. We suspect the removal efficiency of mercury is not good at this plant.
Previous studies indicate that HCl concentration and temperature can affect mercury speciation. Our research changes the HCl concentration to investigate the percentage of mercury speciation. Besides, our study also simulates the mercury speciation of typical MSWI and power plant.
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