Experimental And Mechanism Research Of Removal Of Sulfur Dioxide From Coal-fired Flue Gas In Fulvic Acid Solution

The effective control of sulfur dioxide emission from coal-fired flue gas is significant for the containment of acid rain and haze pollution.The flue gas desulfurization with recovery of sulfur resource can effectively control the emission of sulfur dioxide and conform to the national circulation economic development,so it is future desulfurization development direction.As an agricultural country,China annually produces a lot of agricultural and forestry wastes such as straw.The abandonment and open burning of straw wastes can not only waste the valuable biomass resources but also cause environmental pollution and even fires,therefore the way out of straw residues needs to be solved.Fulvic acid（FA）can be extracted from biomass residues such as straw with microbial fermentation.FA has acid-base buffering capacity,because of two significant acidic oxygen-containing functional groups（-COOH and-OH）.Considering the recovery of sulfur resource and the utilization of biomass residues such as straw,the paper puts forward two kinds of novel wet flue gas desulfurization（WFGD）processes（regenerative FA WFGD and aqueous ammonia-FA WFGD）.For two different kinds of WFGD,the paper aims to investigate the SO₂ absorption characters and the mechanisms of SO₂ absorption,etc.（1）SO₂ absorption mechanism of regenerative FA WFGD.First,the experiment of SO₂absorption by FA solution is conducted in a lab-scale bubbling reactor.The experimental results show the FA solution(FA=0.04 g mL^-1,pH 5.5)could excellently absorb SO₂ with the maximum absorption efficiency of 97.5%and the durative high-efficiency absorption time,which confirms the desulfurization performance of FA.Then,chemical mechanism of SO₂ absorption in FA solution is discovered by FTIR,NEXAFS and XPS,etc.Trace amounts of carboxylate on FA have proton-binding sites,so carboxylate can bind H⁺from the dissociation of SO?aq?to form free carboxylic groups by protonation,the mass-transfer resistance of SO₂ in the liquid phase reduces,and the amount of SO₂ into the liquid phase increases.The absorbed SO₂ can be desorbed from the SO₂-loaded FA solution by heating.Meanwhile,the spent FA absorbing solution can be regenerated.By eight successive absorption-desorption cycles,it is found that the eight absorption of FA absorption solution still has high absorption efficiency of above 95%and the DTHE（time of above 95%）of above 10 min,which confirms that FA has a good desulfurization regeneration performance.Moreover,the thermal stability and regenerability of FA has been evaluated by using TG-DTG and XRD,respectively,and the results demonstrate that FA has good thermal stability and regenerability.（2）Characters of SO₂ absorption in FA absorption solution.First,the mass-transfer and the reaction process of SO₂ in FA absorption solution are analyzed on the basis of two-film theory.Then,the influences of nine operating parameters on SO₂ absorption are investigated by batch absorption experiments,and the results show that the FA concentration and initial pH value significantly influence SO₂ absorption efficiency,and the influence of initial pH value is greater than FA concentration.FA concentration,SO₂ inlet concentration,initial pH value and temperature can significantly affect the DTHE,and the influence of SO₂ inlet concentration on DTHE is biggest.The influence of initial pH value on amount of SO₂absorption is biggest.The range of pH suitable for FA desulfurization is 2.8-5.5,in which absorption efficiency can be above 90%.The optimum concentration of FA solution is 0.08 g mL^-1.Lower CO₂ and SO₂ inlet concentration and temperature are conducive to DTHE.NO₂can accelerate the mass-transfer of SO₂ in FA solution.（3）Mechanism and character of SO₂ absorption in aqueous ammonia-FA solution.In view of the phenomenon of ammonia escape in ammonia-based WFGD,the paper prevents ammonia escape by using FA.The experimental results indicate that free volatile aqueous ammonia is effectively captured by-COOH and–OH on FA,and the capacity of ammonia escape inhibiting by FA is approximately 5ppm g?FA?.The mechanism of ammonia escape inhibiting by FA is demonstrated by FTIR:FA<reacts with free ammonia,and is transformed into stable FA<.And,the amount of free ammonia in the liquid phase is lowered,which realizes the inhibiting of ammonia escape.For the absorption of SO₂ in aqueous ammonia-FA solution,FA<is the critical absorbent.The paper researchs the character of SO₂ absorption in ammonia-FA solution using a bubbling reactor,and analyzes the effect of absorption parameters on the DTHE and absorption efficiency,achieves the optimal operating parameters for desulfurization.The results show that the desulfurization capacity of aqueous ammonia is enhanced by the synergic action of FA,and initial pH value can significantly affect SO₂ absorption efficiency and DTHE.FA concentration can significantly affect DTHE,and the reason is that the number of FA<depends on FA mass.Ammonium sulfate,O₂ and SO₂ inlet concentration have slight effect on DTHE and absorption efficiency.CO₂ significantly affects DTHE and absorption capacity.High fly ash can increase desulfurization capability of FA-aqueous ammonia solution.Lower gas flow rate is beneficial to SO₂ absorption in FA-aqueous ammonia solution,and room temperature is optimal for the absorption.To get high absorption efficiency（η99.0%）,the pH value of absorption liquid should be above 4.1.When pH 8.7⁴.1,COO groups of FA<are the crucial ingredient of absorbing SO₂.The result of FTIR also confirms SO₂ absorption mechanism in FA-aqueous ammonia.（4）Kinetic analysis of SO₂ absorption in aqueous ammonia-FA solution.According to two-film theory,the mass transfer of SO₂ absorption in aqueous ammonia-FA solution is discussed,and the model of SO₂ overall gas-side volumetric mass transfer coefficient is obtained.Moreover,the relative deviation of model is provided.The results show that there are three significant negative correlations（T,C and C）in K a,and the contribution of Q in K a is biggest,and the contribution of C is smallest.The results of chemical reaction kinetics indicate that the reaction in slow reaction zone is about zero-order reaction,and the reaction in fast reaction zone is about first-order kinetics,and the reaction apparent activation energy is 14.028kJ mol for SO₂ absorption in aqueous ammonia-FA solution.（5）Oxidation kinetic of ammonium sulfite of FA-aqueous ammonia desulfurization process.The influences of FA mass concentration,ammonium sulfite concentration,ammonium sulfate concentration,air flow,initial pH value and temperature on oxidation rate of ammonium sulfite are investigated by bubbling oxidation experiments.Results show that the decrease of FA mass concentration（or initial pH value,or ammonium sulfate concentration）and the increase of initial ammonium sulfite concentration（or air flow,or temperature）can increase the oxidation rate of ammonium sulfite,when the initial concentration of ammonium sulfite is 0.04⁰.5mol L.According to the initial rate method,the reaction order of FA is-0.9,the reaction order of initial pH value is-3.3,and the oxidation reaction apparent activation energy E is 37.65 kJ mol.