Study On The Migration-Transformation Mechanisms And Emission Control Of Arsenic/Selenium From Coal Combustion

The arsenic/selenium emitted from coal combustion has high harmfulness,mobility and bioaccumulation,which needs to be effectively controlled.The migration-transformation behaviors of arsenic/selenium during coal combustion are complex,and arsenic/selenium is usually released in the form of vapors and fine particles that are difficult to remove.The atmospheric arsenic/selenium emission of coal-fired power plants is of large quantities,and the arsenic/selenium in fly ash is easily leached out because of the poor stability.Based on these situations,from the perspective of analysis on the migration-transformation mechanisms and development of emission control methods,this dissertation clarified the migration and distribution behaviors of arsenic/selenium in coal-fired power plants,and revealed the gas-solid conversion processes of arsenic/selenium during coal combustion,and established the strengthening removal methods of arsenic/selenium in the flue gas based on in-furnace adsorption and tail flue gas temperature regulation,and developed the stabilization methods of arsenic/selenium in fly ash.The detailed research works and main conclusions are as follows:Firstly,the migration and emission characteristics of arsenic/selenium in ultra-low emission coal-fired power plants were clarified by field sampling,and the distribution characteristics of arsenic/selenium in particulate matters were cleared out.The results showed that the total removal efficiency of arsenic and selenium by air pollution control devices reached 99.8%and 97.4%,respectively.The emission factors of arsenic and selenium were 0.052 g/10¹²J and 0.320 g/10¹²J,respectively.The increase of operation load resulted in the decrease in selenium removal efficiency of WFGD.Arsenic and selenium were distributed in PM₁ with high concentration and large mass ratio.Arsenic in particulate matters was mainly in the form of calcium-bound arsenic generated by chemical reaction.Selenium in particulate matters was mainly in the form of iron-bound selenium generated by chemical reaction,and there also existed SeO₂（s）generated by physical processes.Secondly,the temperature range for the formation of particulate-bound arsenic/selenium was determined by 3 MW pilot-scale coal combustion experiments.The key components affecting the formation of particulate-bound arsenic/selenium and gas-solid conversion mechanism were revealed by coupling adsorption experiments and quantitative calculations.The results showed that the particulate-bound arsenic was mainly formed at medium-high temperatures（650～1350 ^oC）,while the formation of particulate-bound selenium mainly occurred at medium-low temperatures（<650 ^oC）.Calcium sulfate and iron oxides played an important role in the formation of particulate-bound arsenic and particulate-bound selenium,respectively.The arsenic/selenium vapors were mainly deposited on particulate matters by chemical reactions and then underwent gas-solid conversion process.Then,the adsorption experiment and field test were conducted to illustrate the removal mechanism of in-furnace adsorption and tail flue gas temperature regulation on the fine particulate-bound arsenic/selenium and gaseous selenium in the flue gas.The results showed that limestone owned excellent arsenic/selenium adsorption performance at 900 ^oC.The calcium injection in the furnace promoted the enrichment of arsenic/selenium in coarse particles and inhibited the formation of fine particulate-bound arsenic/selenium.The temperature decrease of flue gas induced the condensation of H₂SO₄ vapors on the surfaces of particulate matters and promoted the agglomeration and growth of fine particulate matters.At the same time,it reduced the specific resistance of particulate matters and increased the residence time of particulate matters in ESP.Furthermore,it improved the physical adsorption capacity of particulate matters for gaseous selenium.Finally,the leaching characteristics of arsenic/selenium in fly ash were investigated based on the leaching experiments,and the stabilization methods of different fly ashes were developed.The results showed that the calcium components except for calcium sulfate in fly ash were dissolved and ionized,which made the leaching solution appear alkaline.The leaching of arsenic/selenium underwent dissolution,re-adsorption and equilibrium,and calcium sulfate in fly ash was the key component for the re-adsorption process.Ash blending could regulate the alkalinity and the sulfur content of fly ash to promote re-adsorption process,and addition of 3 wt.%elemental iron could fix arsenic/selenium ions on its surface through adsorption and reduction processes,and the addition of 15 wt.%magnesium-aluminum double-layer metal oxide could fix selenium ions in its lattice,thereby achieving the stabilization of arsenic/selenium in fly ash.