Releasing Behavior Of Sodium And Kinetics Of Na₂SO₄ Formation In High-alkali Coals

Zhundong coal reserves is abundant,and it has the advantages of low ash,low sulfur,and low pollution emissions,thus it has good prospects for utilization.However,Zhundong coal generally has a relatively high content of alkali metals.During the combustion process,the alkali metals will be released into the gas phase,and react with the sulfur oxides in the flue gas to form a viscous thin layer,further forming an initial deposit layer,causing serious fouling issues.This phenomenon has affected the economic and safe operation of the boiler.In order to deepen the understanding of the mechanism of Na migration and transformation process and provide a theoretical basis for the clean and efficient use of Zhundong high-alkali coals,this paper takes five typical high-alkali coals as the research object,using experiments and simulation methods to study the characteristics of sodium release and transformation during combustion and the generation characteristics of Na₂SO₄ in flue gas.First,the coal ash of high-alkali coals at different temperatures was prepared and a sequential chemical extraction method was used to investigate the occurrence and transformation characteristics of mineral elements in five kinds of high-alkali coals.The results showed that:the sodium content in high-alkali coals is high,the potassium content is low,most of sodium exists in the form of soluble salts or hydrated ions,most of potassium exists in the form of insoluble aluminosilicate;calcium mainly exists in the form of calcite and gypsum,and magnesium mainly exists in the form of dolomite;Iron mainly exists in the form of pyrite and siderite.The 300℃low-temperature ash retains the original mineral composition of the raw coal to a certain extent,and as the ash-forming temperature increases,it transforms into oxides or more complex crystalline phases.Then,the combustion experiments were carried out on a high-temperature tube furnace to investigate the influence of combustion temperature and ash composition on the release of sodium,as well as the mineral conversion process in the combustion process of high-alkali coal with different ash composition.The results showed that:as the combustion temperature increases,more sodium is released into the flue gas.The silica-alumina minerals in coal ash have the effect of capturing sodium,thereby the sodium release rate is reduced,while calcium-containing minerals will weaken this effect.The value of[（Ca+Mg）/（Al+Si）]/Na in coal ash has a good positive correlation with the release rate of sodium at high temperature,and on this basis,the prediction formula of sodium release rate is further fitted.The sodium release rate of high-alkali coal with high calcium content is high,and a large amount of sodium is released in the gas phase at high temperature,thus remaining calcium minerals in coal ash;the sodium release rate of high-alkali coal with high silicon and aluminum content is low,and sodium remains in the coal ash in the form of aluminosilicate at high temperature.Finally,a gas-phase chemical reaction model of C/H/O/N/S/Cl/Na in high-temperature flue gas was established,and on the basis of verifying the accuracy of the model,the formation characteristics of Na₂SO₄ in the theoretical flue gas of high-alkali coal were further studied.The results showed that:the content of Na₂SO₄ and the sulfation rate at the outlet of the reactor decreased significantly with the increase of flue gas temperature.There are four main paths for the formation of Na₂SO₄,and the process of generating intermediate substances Na OH and Na HSO₃ is the most important path.The generation of Na₂SO₄ is more sensitive to the elementary reactions that generate Na OH,H/O radicals and SO₃ in the system.The increase in the content of Na Cl,SO₂,O₂,and H₂O in the flue gas all have a certain promotion effect on the formation of Na₂SO₄.