Research On Nucleation And Crystallization Characteristics Of Gypsum From Wet Flue Gas Desulfurization Process

Wet flue gas desulfurization process based on limestone-gypsum method is the mainstream desulfurization technology in large-scale thermal power units at present,but the content of fine particulate matter in the flue gas is increased and "gypsum rain" pollution exists in this process.The results show that gypsum is the main component of fine particulate matter,and the increase of small and medium-sized gypsum particles will lead to the increase of fine particulate matter emissions in desulfurization tower.Therefore,the nucleation and growth characteristics of gypsum which affect the particle size distribution were studied by experimental method,and a preliminary particle size control strategy was proposed.Firstly,in order to fully understand the properties of desulfurization gypsum,the composition,morphology and physical properties of desulfurization gypsum growth system of different power plants were tested.Furthermore,by means of equilibrium method,the solubility of gypsum in water system of 30～60℃ and Cl-system with concentration of 5000-20000 mg/L at 50℃ were determined,using conductivity meter and chemical analysis.Therefore,the basic data for studying the nucleation and growth characteristics of gypsum were expanded.The results show that the main component of desulfurization gypsum is calcium sulfate dihydrate,which is usually in short columnar or rhomboid shape with particle size of 50-100μm.The content of Cl-in the slurry of absorber is the highest,which can reach more than 12000 mg/L.The solubility of gypsum in pure water system increases first and then decreases with temperature,reaching the maximum value at about 40℃.In the sodium chloride aqueous solution,the solubility of gypsum increases with the increase of Cl-ion concentration in the range of 0～20000 mg/L at 50℃,and the solubility reaches more than twice that of the pure water system.Secondly,the nucleation characteristics of desulfurization gypsum were investigated with the classical nucleation theory.The induction time of gypsum nucleation at different supersaturation ratios was obtained by measuring the changes of the electrical conductivity of the system,and then the liquid-solid interfacial tension was deduced,and the boundary point of homogeneous and heterogeneous nucleation was defined.For the pure water system,the primary homogeneous nuclei are dominant in the region of high supersaturation ratio above 1.5,while the non-homogeneous nuclei are dominant in the region of low supersaturation ratio.Sodium chloride aqueous solution system,although the influence of supersaturated ratio on induction time and interfacial tension is similar to pure water system,since the solubility of gypsum under chloride ion system increases,the supersaturated ratio calcium sulfate fells with the enrichment of chloride ions in the slurry,which will reduce the "outbreak" nucleation caused by high degree of supersaturation.The higher the initial supersaturation ratio during nucleation,the higher the proportion of needle-like and plate shape crystals,the more unfavorable of dehydration gypsum products.Furthermore,the growth characteristics of single crystal gypsum were discussed by lab made single crystal growth experiment,and the particle size distribution characteristics of power plant taken gypsum under different operating parameters were discussed by seed experiment.The gypsum size distribution was measured by laser particle size analyzer.The results show that the growth rate of gypsum increases with the increase of supersaturation ratio S.When S=1.0～1.5,the growth rate is proportional to the 1.4 power of supersaturation,which means is not only controlled by volume diffusion,it may be controlled by both volume diffusion and surface chemical reaction.When the supersaturation ratio is from 1.2 to 1.4,with the increase of supersaturation ratio,the proportion of large particles increases,while that of fine particles first decreases and then increases.When the rotating speed was 50 rpm,150 rpm and 250 rpm,the proportion of large particles first increased and then decreased with the increasing of rotating speed,while that of fine particles first decreased and then increased.The presence of chloride ions increases the proportion of large particle size gypsum and decreases the proportion of fine particles.