Study On Ash Deposition Characteristics Of High Alkali Coal Solo Firing And Co-Firing With NH₃

The large-scale emission of greenhouse gases such as CO₂ was considered to be the main cause of global climate degradation.In recent years,how to convert fossil fuel based energy systems to reduce CO₂ emissions and achieve carbon neutrality has become the hottest topic in the world.During the 13th Five-Year Plan period,under the policy guidance of supply side structural reform and demand side management of the power industry,the proportion of coal in total energy consumption has been reduced year by year,and the rapid growth of thermal power installed capacity has been effectively controlled.However,the resource endowment and economic social development stage of China indicate that it is unrealistic to completely phase out coal-fired power plants in a short period of time.Therefore,how to realize clean,efficient,low carbon coal-fired power unit is a key issue in the process of carbon peak and carbon neutralization in China.According to the coal resources of China,it can be found that developing the technology of Zhundong coal burning alone,and conducting in-depth research on the ammonia-coal co-firing can promote the integration and complementarity of coal-power units and renewable power,which is an important part of the process of coal-power transformation.This article takes the ash deposition characteristics as the main point of penetration to study the effects of two technical methods,namely full combustion of high alkali coal in liquid slagging cyclone furnace and high alkali coal-NH₃ co-firing technology,on ash deposition on different heat exchange surfaces of boilers.Firstly,Hongshaquan coal was burned in a 200 MW horizontal liquid slagging cyclone furnace,and the element migration and ash deposition characteristics of high alkali coal were studied.The results showed that the final stable relative heat flux of probe 1 and probe 2 is 0.585 and 0.850,respectively.Na,K,Ca,Mg,S and other elements would be enriched in the low temperature zone of the boiler,forming strong adhesion sulfate.These sulfates also adhered to the surface of the ash particles,forming flocculent aggregates.The main mineral phases of slag samples were silicates and iron-containing compounds.Iron was enriched in the slag and formed low temperature eutectic with the silica-calcium-magnesium-aluminum system,reducing the ash melting point.Due to the rapid cooling rate of granulated slag in the cooling pool,low Na₂O and Ca O content,and high silicon aluminum ratio,the crystallinity was lower,which was closer to the characteristics of glassy slag.Secondly,the combustion experiment of high-alkali coal was carried out in the 300k W vertical liquid slagging experimental system designed and built independently.Ash deposition probes and CCD cameras were used to monitor the growth process of deposition line.The results showed that the stable deposition thicknesses of probe 1and probe 2 is 0.85 mm and 3.30 mm,respectively.A large amount of fly ash in the liquid slagging furnace left the furnace with the liquid slag.Therefore,compared with the traditional coal-fired furnace,the amount of fly ash in the flue gas of liquid slagging furnace was much smaller.In addition,SiO₂ and Al₂O₃ were enriched in the high temperature region,and the fly ash in the low temperature region could not form viscous ash,thus reducing the possibility of ash deposition on the surface of the heat exchanger.Thirdly,the combustion characteristics and emission characteristics of coal-ammonia co-firing were studied in a 200 k W one-dimensional furnace designed independently.With the increase of the NH₃co-firing ratio,the CO₂concentration showed a downward trend,while the opposite was the gradual increase in H₂O emissions.The total ammonia slip rate of the three co-firing ratio conditions was0.26%,0.29%and 0.67%,respectively.With the increase of ammonia co-firing ratio,the proportion of unburned carbon increased slightly.The NOx emission concentration during pure coal combustion was 283.34 ppm.Then reached the maximum of 347.05 ppm when the ratio of ammonia co-firing was 10%,and then decreased as the NH₃co-firing ratio increase to 20%.Ammonia co-firing did not change the composition of ash particles,but had a certain deterioration of heat transfer of heat exchange surface in high temperature zone of furnace.Fourthly,the influence of ammonia co-firing ratio（0%,5%,10%,20%）and furnace location on the ash deposition characteristics of ammonia-coal co-firing was studied.The results showed that with the increase of ammonia co-firing ratio,the input of pulverized coal decreases,and the quality of deposition on the probe decreases gradually.When the ammonia co-firing ratio increased from 0 to 20%,the final stable relative heat flux through the probe was 0.836,0.821,0.648,0.770,respectively.This was due to the fact the unburned ammonia attached to the surface of ash particles react with SO₃ and cover the surface of ash particles,significantly increasing the adhesion of particles.In addition,ammonia containing sulfides would also condense on the low temperature surface of probe to form submicroscopic particles,promoting the formation of the initial layer on the probe surface.The quality of deposition on the probe increased as the furnace location of the probe decreased.Finally,in order to study the effect of coal-ammonia co-firing on ash deposition on low-temperature heat exchange surfaces and explore the impact mechanism of ammonium bisulfate on ash particle rebound.The changes of restitution coefficient and critical adhesion velocity of ash particles when ammonium bisulfate adheres to coal ash and target surface were mainly studied by using an AOS high-speed camera.In addition,the effects of impact velocity,impact angle,and airflow temperature on the rebound characteristics of ash particles during impact were also studied.The research showed that with the increase of impact velocity,the rebound velocity,rebound angle gradually increased.As the impact angle increased,the normal restitution coefficient gradually decreased first,and then tended to stabilize.The tangential restitution coefficient first decreased and then increased with the increase of the impact angle.The total restitution coefficient decreased as the impact angle increased.The influence of gas temperature on the rebound characteristics of ash particles was not significant.The impact of ammonium bisulfate on the rebound characteristics of ash particles was mainly reflected in the reduction of normal rebound velocity rather than tangential rebound velocity.And the influence of ammonium bisulfate coating was more remarkable than that of ammonium bisulfate blending.The critical adhesion velocities of ash particles under the conditions of original ash,ammonium bisulfate blending,and coating were 0.103 m/s,0.141 m/s,and 0.229 m/s,respectively.