Study On Oxidized Mercury Reduction BehaviorsIn Wet FGD System And The Stability Of Mercury In FGD Gypsum

Mercury emission, due to its high toxicity, persistentence and bioaccumulation in the environment, has been attracted increasing concerns recently. The greatest proportion of anthropogenic mercury emissions to the atmosphere comes from China and the mercury emission from coal-fired flue gas accounts for a considerable proportion of total mercury emission of China. Thus, controlling the mercury emission from flue gas becomes an urgent requirment. Simultaneous removal of mercury in wet fuel gas desulfurization (WFGD) system has become an important way to control mercury emission by oxidizing the Hg0 into oxidized mercury and then captured in the WFGD slurry. However, as there are many reducing agent in WFGD slurry (such as sulfite ions), a portion of absorbed oxidized mercury may be deoxidized into Hg0 and re-emite to the atmosphere, thereby lowering the removal efficiency. In addition, more mercury would enter the desulfurization gypsum which may affect its reusage. Thus, the stability and risk of mercury in gypsum needs to be evaluated.As for the mercury re-emission in different Wet FGD system, the effects of different ions or additives on mercury reduction in carbide slag desulfurization process, multi-pollutants simultaneous absorption systemand Mg-based desulfurization process have been investigated in this dissertation. Firstly, the inhibited effects of soluble thiocyanate species and sulfide species brought from carbide slag in WFGD system on mercury reduction were evaluated. The experimental results suggested that the inhibition effects of sulfide species due to the generation of HgS via the reaction between oxidized mercury and sulfide ions, which followed different mechanism under acidic or alkaline condition. Under alkaline condition, sulfide ions would directly combine with Hg2+ to from HgS. However, under acidic condition, the sulfide ions could be oxidized into elemental sulfur by sulfite ions or Hg2+ under low pH value, where part of element mercury generation might not be captured by S0 immediately, hence weakening their inhibition effects on mercury re-emission. Furthermore,it was found that suppression effects of thiocyanate species were mainly attributed to the formation of HgSO3SCN and Hg (SCN)x(x-2)(x=2-4).Seconedly, the reduction behaviors of oxidized mercury in simultaneous removal of SO2, NOx and oxidized mercury system have been studied. The results revealed that accumulation of NO2- in this system would greatly affect the oxidized mercury reduction process. The NO2- could react with SO32- and Hg2+ to generate HgSO3NO2-, which was very unstabe and its decomposition to release Hg0 was the main pathway to mercury reduction. And when the concentration of NO2- was much higher than SO32-, the generated HgSO3NO2- could transform to Hg(NO3)x-(x-2), which was relatively stable therefore slightly suppressing the mercury reduction. In the presence of Cl- ions, HgSO3NO2- could combined with Cl- to form HgSO3NO2Cl2-, which is very stable and interrupt the mercury reduction paths, leading to the great inhibition of mercury reduction.Finally, mercury reductions in Mg-based system by adding oxidation inhibitors were assessed and the results suggested that ascorbic acid, formaldehyde and hydroquinone addition could increased the mercury re-emission. However, sodium thiosulfate addition was found to somewhat inhibit the bivalent mercury reduction due to its strong interation with bivalent mercury. The pH variation showed dramatic effects on elemental mercury re-emission in the solution containing ascorbic acid or hydroquinone and the relative mercury emission amount raised about 4.3 times and twice respectively when pH was adjusted from 6 to 8. This finding also confirmed that the ascorbic acid was the main reducer of mercury reduction under alkaline condition.Moreover, the distributions of mercury in gas-liquid-solid phases and the thermostabilities and leaching toxicity of mercury in gypsum captured were also evaluated by adding four heavy metal capturing agents (including DTCR, TMT18-B, NaHS and TMT15) into the absorbent. The results indicated that the bivalent mercury could be effecicently captured in solid phase and inhibited the mercury re-emission with heavy metal capturing agents addition. It was also found that around 93% of captured mercury in gypsum treated by DTCR could be released after a thermal treatment at 160 ℃. And the minimum mercury release amount of those four kinds of gypsum was TMT15 treated one (about 67% of the captured mercury). Sequential chemical extractions tests showed that the acid soluble mercury share smaller proportion in the gypsum treated by heavy metal capturing agents compared to the base one. However, leaching toxicity tests suggested that since the addtion of heavy metal capturing agents would lead to the increase the mercury content in resulted gypsum, the total leached mercury was greatly higher than that of the base one. These results indicated that it need to be paid much attention to the risk of mercury release during desulfurization gypsum treatment and disposal as heavy metal capture agents were used.