The Influence Of Kaolinite Structural Derivation On The Collaborative Control Of Sodium And Arsenic During Low-Rank Coal Combustion

The release and transformation behavior of alkali metals and trace elements in low-rank coal at high temperatures have a significant influence.Superabundant sodium will lead to the fouling and slagging of the heat exchange surface,and the release of arsenic will easily lead to the poisoning of selective catalytic reduction catalysts and cause environmental pollution.This study investigated the collaborative control of sodium and arsenic by kaolinite in furnace.The key scientific issues involved are:identifying the physical and chemical properties that affect the adsorption capacity of kaolinite for sodium and arsenic,and analyzing the correlation between structural derivation and adsorption capacity of kaolinite in the wide temperature range;the collaborative control mechanism of kaolinite on gaseous sodium and arsenic;the influence of kaolinite on the transformation of sodium and arsenic in the heat transfer zone along the coal combustion process and verify the mechanism of collaborative control.Given the above problems,this paper has carried out a series of studies,and the main results are as follows.The characteristic structure of kaolinite was changed directionally by various treatment methods,and the Al coordination was determined to be the key factor affecting the adsorption capacity of kaolinite at high temperatures.With the chemical composition,specific surface area,and pore structure basically unchanged,the“intercalation-exfoliation”method and“thermal pretreatment”method were used to regulate the Al coordination of kaolinite,reduce the amount of Al-OH on the surface of the layered stacking structure of kaolinite,increase the unsaturated tetra-coordination Al(^[IV]Al),and improve the degree of amorphism.At the temperature of 1273 K,the adsorption efficiency of treated kaolinite for Na Cl increased by44%.With the addition of treated kaolinite,the fixed amount of Na in coal ash increased.A coupled adsorption platform was self-designed to quantitatively describe the correlation between the derivation of the Al coordination structure of kaolinite in the wide temperature range and its ability to adsorb gaseous sodium and arsenic separately.At 523 K to 723 K,the sodium and arsenic adsorption efficiency of kaolinite showed a slowly increasing trend.When the temperature further increased to 1273 K,kaolinite contained more^[IV]Al with strong adsorption capacity,and^[IV]Al+Na/As→^[V]（Al+Na/As）was the dominant reaction.When the reaction temperature was more than 1273 K,the adsorption capacity decreases rapidly.By comparing the changes of Al coordination between“intercalation-exfoliation”kaolinite and raw kaolinite before and after the adsorption of sodium and arsenic compounds,and combining with the quantum chemical calculation results,it was shown that arsenic atoms are easy to bind to the central position of Al ring in the Al-O layer of kaolinite,while sodium atoms are easier to be fixed to the bridge position between aluminum atoms.The staged adsorption experiments were designed to confirm the oxidation of arsenic and related mechanism.Compared with the adsorption of arsenic alone,the adsorption capacity of kaolinite with Na Cl or Na OH vapor increased by 1.9-3.4 times,respectively.Na vapor can affect the valence state and binding form of As in the adsorption products.More than 60%of As existed in the form of low toxicity As（V）,and in the form of high stability Al.The results of thermodynamic calculation and absorption experiments show that the oxidation rate of As（III）in the homogeneous reaction was less than 10%.The oxidation of As occurred in the heterogeneous reaction.The characterization results showed that in the co-adsorption products,As was closer to the Al-O layer,while Na was distributed in the outer layer of the stable structure.Based on the optimized quantitative calculation model,it was determined that Na and As were immobilized in the form of Na-O-As（V）-O-Al in the co-adsorption products,and the new structure was verified by acid site test.Based on the 3 MW_th coal pilot-scale test,it was further verified that the introduction of kaolinite can achieve the collaborative control of sodium and arsenic in the low-temperature reheat area and the tail flue area.The sodium content of molten ash at high temperature decreased along the path.In the low-temperature reheat zone and the tail flue,kaolinite can fix sodium and arsenic,and convert 80%sodium into silicate aluminate and 50%arsenic into Al binding form.Sodium content in ash increased by 21%-27%,and arsenic content increased by 47%-100%.The element concentration distribution in single ash particle indicated that the Si/Al ratio of ash is close to that of kaolinite,which had a strong Na binding ability.Kaolinite also promoted the enrichment of arsenic on fine ash particles(especially PM_1-2.5).The experimental results of adding kaolin in coal-fired power plants further support the collaborative control effect.The species of sodium and arsenic in coal ash and the enrichment phenomenon in fine particle size ash were consistent with the pilot test results.