Effect Of Alkaline Metal Sulfates On SO₃ Formation In Coal-fired Flue Gas

SO3 in coal-fired flue gas is very harmful,and some countries have issued relevant standards.More attention will be paid to the governance of SO3 in the future.However,the current research on SO3 is mainly focused on gas-phase formation and catalytic formation by metal oxide,and there are few studies on SO3 formation by fly ash.In addition,due to the strong ability to capture sulfur(such as Zhundong coal and Shenhua coal),the coal with high alkali content will contain more sulfates.Therefore,it is of practical significance to study the effects of sulfate in fly ash and fly ash on the SO3 formation.In order to confirm the effect of sulfate in coal ash etc factors on SO3 formation,SO3 formation experiments were carried out by adding four different sulfates to fly ash.The production and formation ratio of SO3 increased with the increase of temperature.The effect of temperature on the catalytic SO3 formation of coal ash is not consistent with that of others.It may be due to the differences of ash composition used.For different coal types,the catalytic capacity of Minqiao ash is stronger than that of Shikutai.It is because the Fe2O3 content of Minqiao ash is obviously higher than that of Shikutai ash.The effect of sulfate on SO3 formation shows before and after adding sulfate into the ash,the trend of SO3 formation with temperature remained unchanged and increased with increasing temperature.However,the addition of different sulfate had different effects on the formation of SO3.According to the influence of sulfate on SO3 formation,sulfate can be divided into three types:Na2SO4 and K2SO4;MgSO4;CaSO4.After adding Na2SO4 and K2SO4,the SO3 formation increased significantly,and the increase of SO3 formation increased with the increase of temperature.After adding CaSO4,the SO3 formation decreased slightly at lower temperatures.With the increase of temperature,the SO3 formation increased gradually,approaching that of original ash at 900 C and slightly higher than that of coal ash at 1000 C.After adding MgSO4,the effect of MgSO4 is similar to that of CaSO4 at lower temperature,but different at higher temperature.At 900 0C,the SO3 formation increased significantly,much higher than that of original ash,and even more than that of adding TNa2S04 at 1000 0C.Before and after adding sulfates into ash,SO3 formation gradually increased and formation ratio gradually decreased with the increase of SO2 concentration.When the oxygen content increased from 2%to 5%,the formation of SO3 increased significantly and the oxygen content increased from 5%to 8%.The change of oxygen concentration had little effect on SO3 formation and the formation ratio tended to be stable.The effect of sulfates on ash fusion temperature(AFT)shows AFT decreased obviously after adding sulfate.The reasons for the promoted SO3 formation after adding sulfate to ash are as follows,when Na2SO4 and K2SO4 are added,the increase of SO3 is mainly due to the release during the process of forming aluminosilicate;when MgSO4 is added,a part of promoted SO3 is due to its own decomposition at high temperature,and the other part is due to the formation of aluminosilicate;when CaSO4 is added,the SO3 formation is not affected at low temperature,and a small amount of decomposition result SO3 increasing slightly at high temperature(1000 C).It is found that sulfate in fly ash can promote SO3 formation in flue gas through research.