The Reaction Characteristics Of Wet Desulfurization With Carbide Slag And Pilot Test Verification

China’s SO2 emissions are still very large,in order to promote the victory of pollution prevention and control battle,it is necessary to continue to promote the desulfurization work.Limestone desulfurizer occupies the dominant position in the existing desulfurization technology,and the huge demand leads to a sharp decline in its reserves and an increase in costs.Moreover,the limestone mining process is also a process that destroys the natural environment,so the calcium carbide slag shows its outstanding advantages.Calcium carbide slag has strong alkalinity and low price,which can replace limestone for flue gas desulfurization.In this paper,calcium carbide slag was used as desulfurizer to study the reaction characteristics of wet desulfurization and pilot test was carried out.(1)The composition analysis of calcium carbide slag shows that more than 80%of the content of calcium carbide slag is Ca(OH)2,and also contains a small amount of CaCO3,SiO2,Al2O3,Fe2O3,Cl-,S2-and organic C.Thermogravimetric analysis shows that the decomposition of calcium carbide slag can be divided into two stages.The first stage is the decomposition of calcium hydroxide at 400℃,and the second stage is the decomposition of a small amount of calcium carbonate at 600℃.(2)After calcium carbide slag desulfurizer and limestone/calcium carbide slag mixed desulfurizer are dissolved in water,the pH of the slurry is above 12.0,which can be kept above 11.0 for a long time after reacting with 2 mol/L HCl.The large particle size of calcium carbide slag will not affect the release of effective calcium in calcium carbide slag.When the acid strength increases from 1.0 to 3.0,the activity of calcium carbide slag reaches up to 80.25%,which is 2.28%and 22.65%higher than that of calcium carbonate and limestone respectively.Two platforms will appear when the mixed desulfurizer reacts with 2 mol/L HCl.The first is the platform with pH from 11.0 to 12.0.At this time,the calcium carbide slag is absorbed and the decisive step is the gas-liquid mass transfer process dissolved by SO2.The second is the platform with pH from 5.0 to 6.0.At this time,limestone reacts.When the calcium carbide slag is completely consumed and the pH of the slurry drops to 5.6,calcium carbonate begins to accelerate the dissolution and participate in the absorption reaction,but its dissolution rate is slow.Therefore,the SO2 dissolution step and the limestone dissolution step jointly constitute the control step of this stage.(3)Keeping a large amount of desulfurization slurry,and containing a certain amount of calcium sulfite and limestone can buffer the change of slurry pH.The pH stability of the slurry can be controlled by adding calcium carbide slag in the range of pH from 4.8 to 5.0 and sodium acetate additive to the slurry.In the process of SO2 absorption,the pH of slurry changed a lot by calcium carbide slag and mixed desulfurizer under a large flow rate of over 1.0 L/min.And with a small flow rate of 0.2～0.3 L/min,the slurry pH can be slightly adjusted.The higher the proportion of calcium carbide slag in the mixed desulfurizer,the stronger the ability to adjust the pH of slurry.(4)In the desulfurization system,the efficiency of limestone desulfurization is less than 95%,and the number and height of the spray layer had negligible influence on the efficiency.In the desulfurization of calcium carbide slag,the effect of pH on the desulfurization efficiency had difference in different liquid-gas ratio ranges:when L/G is less than 12.0 L/Nm3,the pH of the slurry in the tower had a decisive effect on the efficiency;when L/G is from 12.0 to 22.0 L/Nm3,The desulfurization efficiency is jointly determined by the pH of slurry and the liquid-gas ratio;when L/G is greater than 22.0 L/Nm3,the influence of the slurry’s pH on the desulfurization efficiency becomes dull.Calcium carbide slag desulfurizer and mixed desulfurizer have better desulfurization effect than limestone.(5)The pilot test verified that the continuous slurry supply method could control the change of pH below 0.5 within 2 hours.The pH changed from 5.0 to 6.0 under the condition of L/G is 12.0 L/Nm3,the efficiency of limestone is the highest when the pH is 5.6,which is 88.7%.The desulfurization efficiency of calcium carbide slag increased gradually,and reached 96%when the pH is 6.0.The desulfurization slurry of limestone can be completely oxidized at 5.5h.However,the calcium carbide slag desulfurization slurry cannot be completely oxidized at 5.5h,and it still contained 0.31%CaSO3.