Study On Species Transformation And Emission Characteristic For Mercury During Coal Combustion In Power Station With Sea Water Flue Gas De-Sulphurization

Mercury has been identified as a persistent pollutant with characteristics of high toxicity, volatility, bioaccumulation and mutagenicity to human health and environment. Since the coal contains mercury, the coal-fired power plants have become the main source for anthropogenic discharge of mercury to the atmosphere. The emission features and the control of mercury exhausted from coal-fired power plants are currently an active topic and highlight interest of research.This research chose the mercury emitted from coal combustion as the objective, investigated the exhaust process of flue gas from the large-scale industrial boiler equipped with flue gas cleaning devices (FCDs) in the power plant near sea, and focused on the mercury species transformation and emission characteristic during coal-firing. Based on the study results, the mercury control measures are also discussed. The detail information is as follows:(1) The mercury contents in bituminous coals were analyzed. It is observed that the mercury contents had a positive correlation with the sulphur contents in coal. A small-scale tube-furnace was assemblied for simulating mercury species transformation in flue gas. The result showed that the release percentage of mercury from coal was higher than 95% under 600℃combustion temperature. The content of Hg²⁺ in the flue gas increased with the increase of combustion temperature and air flow rate, while Hg°decreased with the increase of the combustion temperature and air flow rate.(2) Ontario-Hydro (OH) method had been applied to determine the mercury concentrations and species in the flue gas emitted from a 300 MW coal-fired boiler. Mercury concentrations in raw coal, bottom ash and fly ash of the boiler, seawater at the inlet and outlet of SO₂ absorption reactor and the drainage of aeration sink were also analyzed. The results indicated the concentrations of mercury in flue gas ranged from 12.63μg/Nm³ to 15.71μg/Nm³, the mercury content in fly ash was close to that in coal, while in bottom ash was only about 1%-10% of that in fly ash. The mercury concentration in the seawater of aeration sink drainage was 5-6 times higher than that in fresh seawater. The calculation from materiel equilibrium showed that the mercury produced by a unit of electric-power generation was 62.40μg/kW·h, and the field experimental data revealed that the amounts of mercury exhausted to atmosphere, remained in fly ash and bottom ash, discharged into seawater for flue gas de-sulphurization (FGD) were 13.78, 3.53, 33μg/kW·h, respectively.(3) The data of OH experiment, the analysis of remnant carbon contents and sizes distribution of particulate in fly ash, and the concentration variety of mercury in flue gas supervised by an on-line continuous emission monitor (CEMs) were employed to further study the effects of FCDs on mercury emission from coal-fired boiler. It is found that Selective Catalytic Reduction (SCR) De-NOx device had strong impact on the mercury species transformation, and the conversion rate of Hg⁰ to Hg²⁺ by the catalyst was as high as 83.4%. As the reducing reagent in SCR technology, NH₃ had no discernable effect on the concentration and species of mercury in the flue gas. After passing through the electrostatic precipitator (ESP), particulate mercury in the flue gas dropped from 5.66% of the total amount at the inlet of the ESP to zero at the outlet, and the removal efficiency of particulate mercury by the ESP was almost 100%. The amounts of mercury in the fly ash in the hoppers of the ESP had a negative correlation with the particulate sizes, on the other hand, showed a positive correlation with the remnant carbon contents in the fly ash. With seawater flue gas de-sulphurization (SW-FGD), the removal efficiency of mercury was as high as 73.6%. The amount of mercury that could have been exhausted into the atmosphere if without the SW-FGD, however, was discharged into the seawater. The total removal of mercury by FCDs combined with SCR+ESP+SW-FGD reached 74.1%. The study showed that FCDs in coal-fired power plant play an important role on mercury emission, which is feasible to be applied for cutting down the amounts of mercury discharged into the atmosphere.(4) The method of gold amalgamation was employed to determine the concentration of total gaseous mercury (TGM) in the air above the aeration sink for the SW-FGD, and mercury concentration in the seawater of the sink was also analyzed for evaluating the effect of aeration technology on the amount of mercury released. The results showed that the average concentration of TGM was about 10.01 ng/m³, which was 20 times higher than that of local background. The concentrations of TGM above the aeration sink presented a positive correlation with mercury concentrations in seawater of the FGD and aeration intensity. The experiment also found that the TGM concentrations decreased as the vertical height near the aeration sink increased, and the TGM concentrations above the aeration sink in daytime, especially during noon time, were higher than those at night, which could be explained by the photo-reduction of mercury.(5) In order to evaluate the discharge of mercury from sea water of the FGD into the sea areas nearby, a three dimensional hydrodynamic numerical model with 6-node triangular element and two order shape function were developed. The model simulated and predicted the concentration distribution of mercury from seawater of the FGD into the sea. The field study was also carried out for verifying the model.