Study On The Mechanism And Prevention Of Slagging And Ash Deposition In Circulating Fluidized Bed Boiler Burning High-alkali-chlorine Coal

The proved reserves of Shaerhu（SEH）coalfield in Xinjiang,China is about 89.2 billion tons.SEH coal has the advantages of low mining cost,low sulfur,low phosphorus,low ash and high calorific value.However,the contents of sodium（Na）and chlorine（Cl）in this coal are very high.It is a typical high alkali-chlorine coal,which leads to strong slagging and ash deposition characteristics in the process of combustion and utilization and thus greatly limits its application.In order to explore the solution,the ash melting and ash migration experiments were carried in laboratory enviroment,and then the ash samples were taken for analysis along the process of an actual circulating fluidized bed boiler aiming at pure combustion of high alkali-chlorine coal.A sampling device was developed to conduct real-time sampling during the boiler operation,and then the migration behavior of SHE coal ash,the mechanism of slagging and ash deposition during the circulating fluidized bed combustion process were investigated,and the prevention and control methods were proposed on the basis.The main conclusions are as follows:1)SEH coal contains high sodium and chlorine.It is easy to be ignited and burn out,and the ash formation rate decreases with the increase of temperature;Cl in coal is rapidly released at 550～815℃,and completely released at 815℃.Na is released in two stages.It is first rapidly released with in the form of Na Cl at 500℃～700℃,and then in the form of elemental Na or Na₂O at900～1000℃.Na Cl in the SEH coal ash decreases with the temperature increases from 700℃to 1150℃.It disappears after900℃.The calcium aluminum compounds continue to increase,and the ash minerals are mainly composed of calce（Ca₂Al₂Si O₇）at 1150℃.2)After one month of pure burning of SEH coal,the buried pipes of the furnace severely slagged and heavily ashing occured in the tail convective heating area.The slag sample on the buried pipe can be divided into four layers from inside to outside,showing a typical layered structure.The layer close to the heat-exchanger was Na Cl,which enhanced the adhesion of other particles.The middle layer and the outer layer were mainly composed of Ca₂Al₂Si O₇.These substances were co-melted with other substances at a low temperature,and then reducing the ash melting point,which further intensified the formation of slagging on the bonding layer.The ash on the convective heating area had a loose porous structure and was wedge-shaped on both sides.Viscous minerals such as Na Cl,Ca SO₄ and Mg O were selectively deposited on the convective heating surface through thermophoresis and eddy current impact.3)The effects of flue gas temperature and wall temperature on ash deposition were studied.The real-time sampling experiment showed that the ash deposition thickness and rate increased with the increase in the wall temperature of the sampling pipe when the flue gas temperature was 800℃.The variation of ash deposition rate with temperature is:ash deposition rate at 600℃>that at 500℃≥that at 400℃.After 12 hours of pure combustion,the deposition thickness on the windward side and leeward side reached the maximum value of 9.2 mm and 6.4 mm respectively at the wall temperature of 600℃.The ash deposition on the windward side became thinner and denser,and that on the leeward side continued to grow.The ash deposition on the windward side was more serious than that on the leeward side at high temperature flue gas.The ash on both sides can be cleaned easily at the flue gas temperature below 450℃.4)After blending with high-silicon,high-aluminum and high-silicon-aluminum base bed materials,all ash fusion points decreased and then increased,with the lowest deformation temperature of 1159°C.The Na precipitation was inhibited by the addition of the three basic bed materials.The highest Na fixation efficiency of high aluminum base was 37.5%.The Na fixation efficiency of high aluminum base and high silicon aluminum base was high in the range of 550～950°C,and decreased with the increase of temperature,while the high silicon base was high in the range of more than 950℃.Si/Al ratio（Si O₂/Al₂O₃）≤1.8,Si/Al（Si O₂+Al₂O₃）content≥75%can be used as bed material selection method.5)With the furnace outlet temperature lower than 520℃,the hard cohesive ash deposition can be effectively avoided on the tail heating surface when applying three process furnace and medium temperature separation.When the ash deposition on the convective heating area reached dynamic equilibrium,the heat transfer coefficient was only 32%of the designed value.The pneumatic shock wave soot blowing device had a certain effect on the removal of viscous high alkali and high chlorine coal ash.The removal efficiency of Na and Cl reached the highest values when washed twice with water at the radius of coal particle less than 1mm,the water to coal mass ration of 50:1,60 min stirring time,which were 78.6%and 90.4%respectively.The coal is easier to be ignited and burn out after water washing.The effect of different solvents on the removal of Na and Cl in coal was further studied.The results showed the removal efficiency of citric acid>that of ammonium acetate>that of sulfuric acid>that of pure water.