Study On Migration And Release Characteristics Of Sodium And Ash Deposition During Zhundong Coal Combustion Under Oxy-fuel FB Condition

The Zhundong coalfields have been the largest intact coalfields across PR China.The coal reserves are predicted to reach 390 billion tons,and the accumulated proven reserves are currently213.6 billion tons.The typical features of these coalfields are characterized by a low content of mineral matters,a low content of total sulfur,and high reactivity.Therefore,Zhundong Coal is very suitable for power generation and coal chemical industry.Except for those attractive features,Zhundong coal also labeled as typical sodium-rich coals,because sodium species contents in the coal are relatively high.Due to the strong volatility of sodium species during thermal utilization of Zhundong coal can bring about a series of serious problems,such as slagging,fouling,and corrosion,which has been a barrier for further thermal utilization of Zhundong coal.Oxy-fuel combustion refers to the separation of N₂ in the air through an air separation device,and pure O₂ is used as the oxidant for fossil fuel combustion.In order to reduce the furnace temperature,part of the flue gas(mainly containing CO₂ and H₂O)at the tail of the boiler is circulated to achieve O₂/CO₂ combustion environment that can increase the CO₂ concentration in the tail flue gas to more than 80%.This technology can not only achieve the high concentration of CO₂ in the flue gas to achieve the purpose of CO₂ capture,but also greatly reduce the emission of NO_X and SO_X,thereby achieving the integrated removal of pollutants during the coal combustion process.At present,the research on the fouling and slagging problem of Zhundong coal mainly focuses on the pulverized coal furnace under the air combustion condition,and the relevant research under the Oxy-fuel fluidized bed condition is insufficient.Aiming at the problem of fouling on the convective heating surface when burning Zhundong coal in a fluidized bed with Oxy-fuel atmosphere,this paper has carried out corresponding research.The main work and results are as follows:(1)Sodium species in Zhundong coal mainly exist in four chemical forms,which was classified by the sequential chemical extraction method.According to the sequential chemical extraction method,sodium species in coals were divided into Water-soluble Na,NH4Ac-soluble Na,HCl-soluble Na and Insolule Na.The content of Water-soluble sodium was 68%,which had a low melting point and was easily released into the gas phase during combustion,causing ash accumulation and slagging.The differences in the migration and release characteristics of Na during combustion of Zhundong coal under O₂/CO₂,O₂/H₂O and air atmosphere were compared.The results showed that the residual rate of Na in the ash was the highest under O₂/CO₂ atmosphere.Further analysis found that the difference in the percentage of Insoluble Na in the ash was the most obvious under the three atmospheres,and the O₂/CO₂ atmosphere promoted the conversion of Water-soluble Na to insoluble Na and increased the Na content in the ash.When burning Zhundong coal under O₂/H₂O atmosphere,H₂O not only had a good hole expansion effect,but also participated in the gasification reaction at the beginning of combustion,promoting the migration of Na inside the coal particles,and reducing the residence time in the ash.It reduced the probability of reacting with other minerals in coal,promoting the release of Water-soluble Na into the gas phase.(2)The influence of temperature,steam concentration,and particle size on the tube furnace bench during the combustion process of Zhundong coal was studied.The influence of temperature on the migration and release of Na under the three atmospheres was basically the same.With the increase of temperature,the release of Na in coal was promoted,and the residual rate of ash was significantly reduced.The main influence of temperature was the percentage of Water-soluble Na and Insoluble Na.The increase of steam concentration effectively promoted the development of particle pore structure and the release of Na in ash.The relatively large particle size effectively inhibited the release of Na,and the percentage of Water-soluble Na and Insoluble Na in the ash increased significantly.Temperature,steam concentration,and particle size had no obvious effect on the mineral composition of the ash,but affected the relative content of each component.(3)The influence of additives on the migration and release of Na during the combustion of Zhundong coal was systematically studied.The comparison of the Na residual rate and the existence form in the ash obtained by doping different additives showed that the comprehensive effect of kaolinite was the best.Doping kaolinite promoted the reaction of the soluble Na with Al₂O₃·2Si O₂·2H₂O to form Na Al Si₃O₈ and other Na-containing aluminosilicates effectively,increasing the Insoluble Na percentage during the combustion process.The adsorption of Na by kaolinite is divided into two parts:physical adsorption and chemical adsorption.The percentage of Water-soluble Na and Insoluble Na in the ash after doping raw kaolinite(R-kao)increased by about13%and 57%,respectively,compared with that without doping,indicating that the chemical adsorption effect of kaolinite was relatively more significant.In order to further improve the adsorption performance of kaolinite,this paper modified it by calcination(C-kao)and intercalation-exfoliation.The result showed that the distance between layers of kaolinite modified by potassium acetate(KAc-kao)increased,and Na-containing substances(such as Na Cl)were more likely to enter the adsorption surface,and the amount of adsorption increased significantly.The physical adsorption and chemical adsorption performance of KAc-kao was about 25%and 65%higher than the R-kao respectively.(4)The CASTEP module based on DFT theory was used to calculate the loss of hydroxyl groups on the layers of kaolinite and the influence of surface water molecule concentration on the adsorption energy of Na Cl.The absence of surface hydroxyl groups reduced the sites of action on Na Cl,and the simultaneous presence of H₂O and Na Cl on the surface caused competitive adsorption.H₂O occupied part of the adsorption sites.When the concentration of H₂O was too high,it may also formed a layer of water molecules on the adsorption surface,hindering the adsorption of Na Cl by kaolinite.Using molecular mechanics based on Grand Canonical Monte Carlo theory,the influences of the temperature and flue gas composition on the adsorption of Na Cl on the surface of R-kao,dehydroxy kaolinite and expanded kaolinite was calculated.Under high temperature conditions,the physical adsorption of sodium chloride by kaolinite was inhibited.When the interlayer distance was D<9.8(?),Na Cl molecules hardly entered to the kaolinite's interlayer,and when the interlayer distance was D>11.5(?),the physical adsorption amount increased significantly.Comparing the physical adsorption performance of Na Cl molecules on the intact surface and the De-hydroxyl surface,the result showed that when the interlayer distance was 13.3(?),the K_H of the intact surface is greater than the<13.3(?),and when the interlayer distance was 11.5(?),the physical adsorption of the hydroxyl surface was stronger.The calculation result aboved showed that the distance between layers played a key role in the physical adsorption performance of kaolinite.(5)The ash deposition characteristics of the convective heating surface during the combustion of Zhundong coal under fluidized bed condition were systematically studied,and the prevention and control effect of doped kaolinite on ash deposition was further explored.The results showed that bed temperature,steam and surface temperature of the heated surface affected the ash deposition rate.High bed temperature,high steam concentration,low temperature of the heating surface effectively promoted the formation of a viscous inner layer,resulting in an increase in the ash deposition rate.The viscous inner layer was mainly formed by low melting point compounds containing Na(such as Na₂SO₄,Na₂CO₃,and Na Cl,etc.)in the form of a gaseous phase that flowed with the flue gas to the heating surface,and adhered to the outer surface of the heating surface through condensation or thermophoretic deposition.This greatly improved the efficiency of catching ash particles on the heated surface and worsened the accumulation of ash on the heated surface.Doped kaolinite can effectively inhibit the ash deposition rate,the proportion of the inner ash mass is significantly reduced,and the Na content in the inner ash is significantly reduced.Exploring the effect of kaolinite after potassium acetate expansion on the deposition of ash showed that KAc-kao further inhibited the formation of inner ash,and the deposition of ash was mainly collisional deposition,which was the same conclusion as the tube furnace experiment and calculation.