Study On The Transformation Of Sodium During The Combustion And Gasification Of Zhundong Coal

Zhundong coalfield in Xinjiang with the predicted reserves of 390 billion tons is the largest integrated coal basin in China. With great reserves, low mining costs and good reactivity, Zhundong coal can be potentially used to supply electricity and chemical products. However, during the combustion and gasification process, the high alkali (especially sodium) content in Zhundong coal could induce severe problems, such as slagging and erosion, which influences the safety and efficiency of equipment operation, and there is no fundamental solution by now. Therefore, it has significant research and practical value to further study the migration and transformation properties of sodium in Zhundong coal to find a proper way to control the transformation of sodium.In this dissertation, the migration and transformation of sodium in Zhundong coal during combustion and gasification, as well as the influence of additives on the transformation of sodium was investigated by experimental methods. Besides, the cause of adhesion phenomenon during experiments was also dealt with by the help of thermochemical calculation software. This work has significant guidance meanings on the application of Zhundong coal in circulated fluidized bed gasification.The combustion characteristics was investigated by thermogravimetric analysis, and the influence of ashing temperatures on the ash composition and the migration and transformation of sodium during the combustion process was experimentally studied by the muffle furnace. The results show that Zhundong coal has good combustion characteristics with low ignition and burnout temperatures. The ashing temperatures influence not only the total content of sodium but also the existing form of sodium in ashes. With the increase of ashing temperatures, the total sodium content declines and the main form of sodium transforms form NaCl to sodium aluminosilicates such as albite and nepheline. Ashes obtained at 800℃ and higher temperatures show great adhesion phenomenon because the sodium-containing compounds react with other oxide such as SiO2, Al2O3, Fe2O3 and CaO to form low temperature euteetic. 575℃ is the proper ashing temperature for Zhundong coal ash composition analysis, to ensure the burnout of carbon as well as reducing the loss of alkali metal elements.The gasification characteristics of Zhundong coal, the influence of gasification temperatures and reaction time on the residues’composition and the migration and transformation of sodium during the gasification process were investigated by thermogravimetric analysis and a horizontal tube furnace experimental system. The results show that: Zhundong coal has good gasification reactivity with low starting and ending gasification temperatures; The gasification temperatures and reaction time have great impact on the composition of the residues; As the gasification reaction goes on, the total sodium content declines and the residues’composition changes; When the reaction time is settled as 60 min, and the gasification temperatures as 850℃,900℃,950℃and 1000℃, respectively, the total sodium content declines and the main form of sodium transformes form NaCl to sodium aluminosilicates such as albite and nepheline; The adhesion phenomenon occurs in all the residues, and the higher the temperature is, the severe the adhesion phenomenon is. In contrast with the combustion process, in the gasification process the sodium-containing compounds tend to be easier to form low temperature eutectic by reacting with oxide, such as SiO2 and Al2O3, causing adhesion phenomenon.The co-gasification experiments of nano-SiO2, nano-Al2O3 additives and Zhundong Mulei coal were carried on with a horizontal tube furnace experimental system. The influence of additives on the occurrence and migration of sodium during the gasification process was investigated. The results show that: the SiO2 and Al2O3 additives have different influence on the composition of the residues; With SiO2 as additive, the content of the residues increases with the increase of relative content of Na2O; All of the residues which contain different mass ratio of SiO2 show varied degrees of adhesion phenomenon; SiO2 can effectively reduce the release of sodium and has a two-side influence on the residues’adhesion phenomenon; To be specific, a little content of SiO2 can promote adhesion phenomenon, but a mass of SiO2 can inhibit the adhesion phenomenon. With Al2O3 as additive, the content of the residues increases but the relative content of Na2O decreases; Al2O3 inhibits the mineral composition of raw coal from fixing sodium, thus promoting the release of gaseous sodium and preventing the adhesion phenomenon of the residues.Based on Gibbs free energy minimization theory, thermochemical calculation software, FactSage 6.1, was used to simulate the thermodynamic equilibrium state of Zhundong coal in combustion and gasification progress, respectively, according to the experiments conducted in this dissertation. The influence of reaction atmosphere, reaction temperatures, additives content and component on the migration and transformation of sodium was investigated, and the cause of adhesion phenomenon was also studied. The results show that:during the combustion or gasification process, with the increase of reaction temperatures, the sodium content of the residues decreases, and the decreasing rate is related to different coals; During the process, the maior phase of sodium transferred from NaCl(s) to NaCl(g) and NaAlSiO4(s); When the reaction temperature is higher than 700℃, all of the potassium occurs as gas phase and KCl(s) is the major content; The transformation of chlorine is related to the transformation of sodium and potassium; The content of the gaseous sodium released during the gasification process is higher than the one released during the combustion process; Adding SiO2 makes more sodium resident in the gasification residues as the form of aluminosilicate and silicate, while adding Al2O3 would not obviously achieve that; The phase diagram calculation results of 950℃ gasification process show that the residues of Zhundong Mulei coal gasification alone and with SiO2 has slag phase, while the coal with Al2O3 residues does not have slag phase, thus the adding of Al2O3 inhibit the adhesion phenomenon. The calculation results show good consistency with the experimental results.