Study on the Hg emission and migration characteristics in coal-fired power plant of China with an ammonia desulfurization process

- The emission and migration characteristics of Hg in a power plant with an ammonia desulfurization process were reported.
- The combustion of high chlorine content coal is beneficial to improve the overall Hg removal efficiency of the power plant.
- The re-emission of Hg during the desulfurization process was not observed due to the presence of forced oxidation.
- Attention should be paid in the ammonium sulfate drying system for lots of Hg escaped from that position.

Migration and emission characteristics of mercury in coal-fired power plant with an ammonia-based wet flue gas desulfurization (Am-based WFGD) process were studied based on the Ontario Hydro Method (OHM). The temperature-programmed desorption (TPD) method was used to identify the mercury species enrichment mechanisms in fly ash and the by-products of desulfurization. The SCR has a 95.41% mercury oxidation efficiency attributed to the high chlorine content in feed coal. SCR + ESP + WFGD combination has 94.15% gaseous mercury removal efficiency. Mercury distribution analysis showed that most of incoming mercury was deposited in solid (76.46%), and only 4.94% gaseous mercury was emitted to atmosphere. The TPD results indicate that HgCl2 and HgO are the dominant mercury species in fly ash while mercury in WFGD products exists in HgCl2 form. The re-emission of mercury during ammonia desulfurization did not occur due to the presence of forced oxidation process, while mercury release from desulfurization product was monitored suggesting the mercury re-emission from the ammonium sulfate separate and drying system could not be ignored. The mercury emission factor of the coal-fired power plant is calculated as 0.319 g/1012 J, less than the mean value of Chinese power plant. With the equipment of Am-based WFGD, the mercury content in ammonium sulfate is 1.126 mg/kg, therefore the environmental influence from ammonium should be taken into account.

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