Study On The Treatment Of Low-concentration Sulfur Dioxide Flue Gas By Sludge Biochar

In recent years,industrial emission of sulfur dioxide flue gas has shown the characteristics of low concentration and large gas volume.Although the activated carbon in the traditional adsorption method can effectively treat low-concentration sulfur dioxide flue gas,the cost of desulfurization is relatively high.Therefore,the surplus sludge of the sewage treatment plant containing a large amount of organic matter,abundant nitrogen,phosphorus and metal ions is used as a raw material to prepare a cheap biochar adsorbent through a chemical method.While realizing the utilization of the remaining sludge in the sewage plant,it also processed low-concentration sulfur dioxide flue gas to achieve the purpose of treating waste with waste.The paper studies the influencing factors of biochar preparation:different anaerobic conditions,pyrolysis temperature,heating rate and holding time,taking the sulfur dioxide adsorption capacity and physical properties of sludge biochar as indicators,combined with the analysis of orthogonal experiment results to determine the best preparation process conditions of the sludge biochar.The results show that in this experiment,the best preparation process conditions of sludge biochar are:pyrolysis temperature 650℃,heating rate 2.5℃/min,stagnation time 2h.The penetration time of the prepared sludge biochar is 19.7-21.7min,the penetration adsorption capacity is 8.3723-9.30622 mg/g,the adsorption saturation time is 132.38-145.18 min,the saturated adsorption capacity is 24.6710-25.4561 mg/g.The prepared sludge biochar shows strong adsorption performance.By changing the adsorption conditions of sulfur dioxide adsorption experiments in fixed bed,the effects of initial sulfur dioxide concentration,empty bed gas velocity and bed temperature on the adsorption performance of sludge biochar were studied.The results show that the initial sulfur dioxide concentration of 1500ppm,the empty bed gas velocity of 0.044m/s-0.088m/sand the bed temperature of 20℃are the best fixed bed adsorption conditions.At this time,the penetration time of the fixed bed was 55.7 min,the penetration adsorption capacity was 11.9 mg/g,the adsorption saturation time was 282 min,and the saturation adsorption capacity was 29.7 mg/g.The fixed bed had a higher desulfurization efficiency for sulfur dioxide.The experimental data of sulfur dioxide adsorption by sludge biochar was fitted with adsorption kinetics and adsorption thermodynamics to explore the mechanism of adsorption of sulfur dioxide gas by sludge biochar.The results show that the fitting of pseudo-first-order kinetics is better than that of pseudo-second-order kinetics,indicating that the initial adsorption of sludge biochar to SO₂ is mainly physical adsorption and is controlled by diffusion.Elovich equation fitting shows that the adsorption process is a single molecule chemical adsorption process.The intra-particle diffusion equation indicates that intra-particle diffusion is not the only factor controlling the adsorption rate,and the adsorption process is affected by multiple adsorption mechanisms.In adsorption thermodynamics,the Gibbs free energy is less than zero,and the adsorption proceeds spontaneously;the enthalpy is less than 40 k J/mol,indicating that the adsorption of sulfur dioxide gas by sludge biochar is mainly physical adsorption.Through XPS energy spectrum analysis,it was found that the main elements of sludge biochar are C/O/N/Ca/Si/Al/Mg/P/Fe.Among them:Ca/Si/Al/Mg/Fe,etc.mainly exist in the form of metal silicates（aluminum silicate,calcium silicate,iron silicate,etc.）,as well as Si O₂ or metal oxides（iron oxide and magnesium oxide）.After adsorption of SO₂ by sludge biochar,SO₂ chemically adsorbs to polysaccharides to form organic sulfur species（R-SH）,and also exists to be oxidized to SO₃,and then forms sulfate with Fe,Mg,etc.on the surface of biochar.In addition,SO₂ reacts with a small amount of H₂O on the surface of sludge biochar to generate H₂SO₃.