| Nitrogen oxides (NOX), which are one of the major air pollutants, are primary controlled atmospheric pollutants in emission reduction targets of the China 12th Five-Year Plan. And it has posed a serious threat to human health and environment security. Because of its global transfers and biological toxicity, gaseous mercury (Hg0) has aroused a lot of public attention as the significant air pollutants. With the development of modern industry, the proportion of coal energy in the internal primary energy consumption is higher than 75%. The energy consumption in China is currently dominated by coal, which determined the characteristic of Chinese air pollution is the coal-fired flue gas pollution. Coal-fired flue gas, which including a large amount of nitrogen oxides (NOx), gaseous mercury (Hg0) and dust, can cause serious environmental pollution problems. In Chinese most coal fired power plants, the exhaust gas with high concentration of SO2 can be removed by wet process firstly, then carried on the de-nitrification and de-mercury removal processing. The SCR technology which was used most widely, has a mature processing, a high efficiency and a wide applications, has the problems of high reaction temperature. Take into account the circumstances that there are selective catalytic reduction (SCR) devices in Chinese most coal-fired power, we proposed the simultaneous removal method of NOX and Hg0 from coal fired flue gas at low temperature which inspired by the low-temperature SCR of NO and selective catalytic oxidation of Hg0. The new SCR method would to achieve the low-temperature catalytic removal of Hg0 on the basis of the original high performance of de-NOx. According to the public literature and a scientific experiment scheme, we have investigated the effects of catalyst preparation conditions (e.g the kinds of supports, the kinds of active components, the loading of active component and calcination temperature) and the other reaction conditions on the catalytic activity, which to try a high-efficiency and stability catalysts of simultaneous removal method of NOx and Hg0 at low temperature. Meanwhile, we have discussed the reaction mechanism by the method of BET, XRD, NH3-TPD, CO chemisorption and in suit-DRIFTS.The simulated flue gas flow rate was fixed at 500 cm3·min-1, which equivalent to a gas hourly space velocity (GHSV) of 30000 h-1 and including 613.5mg/m3(500ppm) NO,25-35μg·m-3 Hg,417.2 mg/m3(600ppm) NH3 in this experiment. Firstly, the selective of catalysts supports and the effect of active components, loading amount and calcination temperature for the catalysts synthesis process were discuss, and ensured the best synthesis technics. Through the comparison of six different supports, the microwave coconut shell activated carbon (MCSAC) was selected as the best catalyst support in this study. The catalysts experiment results indicated that the NO and Hg removal efficiencies over the MCSAC modified by Cu(Cu/MCSAC) catalyst were higher than modified AC by other metal components. It showed 15w.t% is the optimal loading of Cu, which has a 75.7% de-NOx efficiency,64.9% de-Hg0 and greater than 60% N2 selectivity at 200-225℃. Calcination temperature has a complicated effect on removal efficiency:too high or too low calcination temperature both were harmful to catalytic activity. Thus the optimum calcination temperature has been determined at 400℃ by a series experiments..After we have chosen the 15%Cu/MCSAC catalyst (which was modified by CuCl2 calcinated at 400℃)as the best catalyst, we carried out the secondary load test of the 15%Cu/MCSAC catalyst in the following step to inprove the removal performance. By studying the effect of the metal species, loading and Fe precursor on the removal efficiency, we draw the following conclusions:15%Cu-5%Fe/MCSAC catalyst modified by Fe(NO3)3 has the best removal efficiency, which can reach the de-NOx efficiency was 86% and the de-mercury efficiency was 61%, the N2 selectivity greater than 62% at 225℃.Then, the effects of some technological parameters, such as NH3/NO ratio, O2 concentration, GHSV, H2O and SO2 and space velocity on the low-temperature simultaneous NOx and Hg0 removal activity of Cu-Fe/MCSAC catalyst were investigated. The results demonstrated that 600ppm(638mg/m3) is the best NH3 concentration, which the best NH3/NO ratio is 1.2; the optimum O2 concentration is 3%; with the increase of GHSV from 20000h-1 to 70000h-1, de-NOx efficiency and de-Hg0 efficiency have decreased gradually:de-NOx efficiencies were 90%,74%, de-Hg0 efficiency were 66%,44% and the N2O concentrations were 70ppm,89ppm. So the optimum GHSV has been determined as 30000h-1 by a series comparison.When the H2O and SO2 were passed, the de-NOx efficiency was reduced from 88% to 54%, de-Hg0 efficiency was reduced from 60% to 24% and N2 selectivity was reduced from 62% to 40%. The introduction of H2O and SO2 both inhibited the reaction of catalytic removal, which may be because that the presence of H2O will increase the adsorption of SO2, and adsorbed SO2 would be oxidized to SO3, which further reacted to form sulfate accumulated on the catalyst surface, which leads to the decrease of catalytic active sites on catalyst surface and the reduction of removal efficiency.Finally, synthesizing the in suit-DRIFTS characterization results and activity test analysis, we believe that the catalytic reaction over Cu-Fe/MCSAC catalyst in the low-temperature simultaneous removal of NOx and Hg0 reaction mainly through the Langmuir-Hinshelwood mechanism and mercury oxidation mechanism, which the adsorption of NH3, the adsorption Hg0 and the adsorption-oxidation reaction of NO were occurred firstly; and then adsorbed ammonia react with nitrite or nitrate to generate N2, Hg0 react with O2 to generate HgO, led to the removal of NOx and Hg0. |
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