Mechanism And Effect Of Calcium And Aluminum-based Sulfur-fixation Agent Desulfurization During Coal Combustion

In allusion to the problem which is well known that the desulfurization rate is low and traditional desulfuration production CaSO₄ is easy to decompose during coal briquette combustion at high temperature(>1050℃),the formation of high-temperature desulfurization phase calcium sulfoaluminate (C₄A₃S) was studied and developed systematically. The characteristic of C₄A₃S formation and decomposition reaction was analyzed by X-ray diffraction,SEM visualization,TG-DTA. The experiments of sulfur capturing of briquette were studied.This thesis was support by the project of national nature science foundation titled "Desulfurization on calcium and aluminum-based/expansion mineral during coal combustion and utilization of residues resource" (No.40602008).Based on calculation of calcium sulfoaluminate thermodynamic parameters, C₄A₃S formation reaction process from mineral system CaCO₃-Al₂O₃-CaSO₄·2H₂O was studied by differential thermal analyses technique and dynamic X-ray diffraction. The research results show that C₄A₃S has begun to form between 950-1000℃,the content has reached the maximum at 1240℃.The re$earch results of C₄A₃S formation reaction experiments in different atmosphere show that the productivity of C₄A₃S was enhanced when CO₂ concentration has increased and CO₂ atmosphere can promote the formation of C₄A₃S.O₂ atmosphere can inhibit the decomposition of calcium sulphoaluminate below 1300℃.Thermal analysis was utilized to study the formation reaction kinetic parameters of C₄A₃S.The kinetic model function was calculated. C₄A₃S reaction kinetic process was studied in different atmosphere( the static air atmosphere, the flowing O₂ atmosphere and the flowing Ar atmosphere). The dynamic experimental results show that the activation energy of the C₄A₃S formation reaction is 228 kJ /mol in the static air atmosphere, 68kJ /mol in the flowing O₂ atmosphere, 254 kJ/mol in the flowing Ar atmosphere. O₂ atmosphere can decrease activation energy of the reaction effectively, while inert atmosphere (Ar) increases activation energy in minor degree. The solid state reaction of high-temperature phase C₄A₃S formation could be described by the third order reaction kinetics and could belong to n=3 nucleus formation mechanism,the kinetic curve could be satisfactorily described by the function g(x)=[-ln(1-x)^1/3].The experiments of sulfur capturing of coal briquette which used CaCO₃, CaCO₃-Al₂O₃ additive and red mud-bauxide additive as the desulphurization agent were studied by intelligent sulfur detector. Effect of CaCO₃ additive was discussed. Because of decomposition of CaSO₄, sulfur fixing rate was reduced when temperature increased. When Ca/S ratio=2, combusted at 1050,1200℃,sulfur retention ratio were 52 %, 10.5 % respectively. Sulfur retention ratio of coal briquette which has CaCO₃-Al₂O₃ additive was 26.8 % at 1050℃and 29.7 % at 1200℃.The experimental results show that Al₂O₃ has dual effects on Ca-based desulfurization efficiency,the formation of calcium aluminate at low remperature inhibits desulfurization reaction, but the formation of C₄A₃S at high temperature improves desulfurization efficiency. Sulfur retention ratio of coal briquette of red mud-bauxide additive was 78 % at 1050℃and 20 % at 1200℃.Fe₂O₃ in the red mud generated to catalyst effect to the formation of CaSO₄ at low remperature.Al₂O₃ could promote the formation of C₄A₃S, on the one hand Fe₂O₃,SiO₂ in the red mud improves the decomposition of CaSO₄,on the other hand they could generated high melting point materials to cocoon CaSO₄ and C₄A₃S to improve desulfurization efficiency. Sulfur retention ratio was considerably enhanced in the process by increasing Ca/S molar ratio, the best value of Ca/S molar ratio was between 3-4, CaSO₄ and C₄A₃S were the major desulfurization phase between 1050-1200℃.The pore structure parameters of Ca-based sulfur-fixation agent was analyzed by Nitrogen sorption surface area instrument, the experimental results show that average pore diameter of red mud were 5-20 nm in favor of desulfurization reaction. The dynamic characteristic of sulfur release during coal briquette combustion was studied , sulfur eduction rate was increased as reaction time increased. The sulfur release characteristics of sample which has red mud-bauxide additive has two peaks. The reason is that red mud possessed well pore structure and contain Fe³⁺,Mg²⁺, Na⁺,K⁺,Mn²⁺ and other metal ions which give rise to more lattice defects and make Ca⁺diffusion easily. The desulfuration reaction rate constant and effective diffusion coefficient of product-layer were enhanced, which could promote the formation of C₄A₃S.The phase characterization of desulfurization residues was analyzed by X-ray diffraction,SEM visualization. The research results show that there were CaSO₄ crystal and C₄A₃S crystal in the residues, and CaSO₄ and C₄A₃S were the major desulfurization phase between 1050-1200℃.