| Coal is the largest source of non-renewable energy in our country. In recent years,coal consumption for power generation has increased gradually. Heavy metals in coal are released into the environment during combustion in the form of slag, fly ash, gypsum,flue gas, posing a serious threat to the ecosystem and human health. In this work, we focused on studying the transformation and distribution behavior of mercury Hg,arsenic(As), lead(Pb), chromium(Cr) during coal combustion, as well as the control by hybrid particulate collector(HPC) and wet flue gas desulphurization(WFGD), with experimental and computational methods.Sequential chemical extraction method was applied to determine the mode of occurrence of arsenic, lead, chromium in coals and the corresponding combustion products. After the combustion process, the three heavy metals bound to carbonates,sulfides, organic matter in coals are mostly transformed into the residue of slag and fly ash. The exchangeable arsenic and the residual lead in fly ash increase remarkably with the decrease of the particle size. Although coal burning reduces the relative mobility of heavy metals, it increases the relative proportion of 1.76 times attributed to the easily soluble fraction of arsenic. The arsenic in fine particles of fly ash has the largest migration capability and the harmfulness to the surrounding environment among all the combustion products.Taking a 320 MW unit equipped with HPC and WFGD as an example, the content of Hg, As, Pb, Cr in flue gas after WFGD was measured by applying the US Enviroment Protection Agency(EPA) method 29, coal and other combustion products were also sampled. The heavy metals enrichment characteristics and removal efficiency of HPC and WFGD were analyzed. The results show that As, Pb, Cr are obviously enriched in fabric filtration ash, but distributed more in electrostatic precipitation ash. Fabric filtration can effectively capture gaseous mercury in flue gas, the relative enrichment factor of Hg is 1.01. Heavy metals in WFGD system all migrate to gypsum in a large scale. The average removal efficiency of As, Pb, Cr by HPC and WFGD is 85% and 82%respectively. The total Hg removal efficiency of HPC is 50%, while that of WFGD is91%.The collaborative control of HPC and WFGD on heavy metals emission under the load of 75% and 100% were also compared. The results show that the relative enrichment factors of Hg, As, Pb, Cr in electrostatic precipitation ash is higher under lower boiler load, while the concentrations of heavy metals in gypsum decrease significantly. With the rise of load, the gas and particle concentrations of heavy metals at the outlet of WFGD increase by different degrees, especially the gas concentration of As and Cr. In descending order of their concentration in flue gas is Cr > As > Pb > Hg, while there is no difference between them under low load. Under different boiler load, over90% of Hg, As, Pb, Cr can be removed by HPC+WFGD. HPC plays a more important role in heavy metals emission control, especially under low boiler load. |
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