Research On Characteristics And Treatment Of Ammonium Salt Fouling On The Low-temperature Heating Surface At The Tail Of Power Plant Boilers And Fouling In A Slag-tapping Boiler

Coal-fired power generation is the main form of power generation in China,and the efficient,safe and stable operation of coal-fired boilers is essential to ensure power supply and energy security in China.However,the low-temperature heating surface of the boiler often has a serious ash deposition problem,causing blockage and corrosion,which endangers the safe operation of the boiler.Fuel pretreatment,blending and injection of additives are difficult to deal with the ash that has formed.Even with soot blowing,there are still many ash deposits that cannot be completely removed.In addition,China is rich in high-alkali coal resources,but the characteristics of easy ash deposition and slagging severely limit the large-scale application of high-alkali coal.The slag-tapping boiler has the characteristics of high combustion temperature,high combustion intensity and high slag capture rate,and its own liquid slag can capture alkali metals such as Na and K.It is considered to be an effective solution to the problems of furnace slagging and ash fouling on the heat transfer surfaces when burning high-alkali coal.Based on the above background,this paper investigates the characteristics and treatment of ammonium salt fouling on the low-temperature heating surface at the tail of power plant boilers and fouling in a slag-tapping boiler:Firstly,the heating decomposition effect of different reheating temperatures on the formed ABS ash deposition was investigated by using the self-made air-cooled automatic heating deposition probe by the method of depositing ash first and then heating.The test results showed that when the reheat temperature exceeded 365℃,the deposition on the probe was significantly reduced,and the metal surface could be seen,and the metal surface area enlarged when the reheat temperature was increased.The agglomeration of ash particles in the deposition was improved after reheating.The relative heat density through the probe was improved after increasing the reheat temperature.The NH₄HSO₄ and（NH₄）₂SO₄ in the deposition would decompose during the reheating process.The total amount of NH₄HSO₄ and（NH₄）₂SO₄ in the deposition did not exceed 1%at reheat temperatures of 365°C and 385°C.Secondly,the mechanism of heating decomposition of the ABS ash deposit was studied by heating the ash deposit for different times.During the heating process,the ABS adhering to the surface of the probe head was decomposed first,and then the ABS in the ash deposit layer was decomposed,and the ash deposit became loose and gradually crumbled and peeled off.The metal surface of the probe head was visible after heating for 30 min.This was attributed to the effect of heating on the agglomerated particles.The relative heat density through the probe surface gradually improved with increasing heating time.The total content of NH₄HSO₄ and（NH₄）₂SO₄ in the deposition gradually decreased during heating,and their weight loss stabilized when the heating time was 15 min.Thirdly,the ash deposition characteristics on the low-temperature heating surfaces under three NH₃ co-firing ratios（pure coal,10%NH₃ and 20%NH₃）were investigated in a 200 k W one-dimensional furnace.The test results showed that the deposition growth could be divided into a fast growth stage,a slow growth stage and a stable stage.After the test,the ash thickness was 1.17,1.55 and 1.22 mm for the three conditions,respectively.The ash thickness for the 10%NH₃ condition was the largest,resulting from the combined effect of fly ash amount and NH₄HSO₄ content.The XRD and TGA results showed the presence of NH₄HSO₄ in the ash deposit,and its content increased with the NH₃ proportion,which was 0.16%,0.29%and 0.71%for the three conditions,respectively.Fourthly,the NH₄Cl ash deposition characteristics and the effects of additives（Na OH,Mg（OH）₂ and Ca（OH）₂）on NH₄Cl ash deposition were investigated in a vertical furnace test bed.The test results showed that NH₄Cl was poor at adsorbing ash particles,and NH₄Cl tended to deposit itself on the probe to form the NH₄Cl layer.After adding the additive,the floating ash on the lower surface of the probe appeared to increase.The addition of Na OH resulted in the formation of Na Cl on the upper surface of the probe,leading to a rough ash deposition pattern and a decrease in the relative heat density.Ca（OH）₂ improved the heat transfer on the upper surface of the probe,Na OH improved the heat transfer on the lower surface of the probe,and Mg（OH）₂ had a poor effect.Fifthly,the ash deposition characteristics in a 300 MW slag-tapping boiler were studied by normal coal blended with Na₂CO₃ to simulate high-alkali coal combustion and by complete combustion of high-alkali coal.The boiler operated stably,and the variations of major parameters were within reasonable limits.The main components in liquid slag,ash deposition and fly ash were Si O₂,Al₂O₃,Ca O and Fe₂O₃,with high contents of Si,Fe and K and low contents of Al,Ca and Na in liquid slag,and the opposite in deposit and fly ash.Doping with Na₂CO₃ increased the Na content in all samples,especially in deposition and fly ash,and the increase was greater at low-load conditions.The liquid slag had a rich microporous structure,and the Na content inside the pores was higher.Along the flue gas flow direction,more fine ash particles appeared in the ash deposition and fly ash.The addition of Na₂CO₃ and the change of boiler load had no effect on the mineral composition of the liquid slag,ash deposit and fly ash.Finally,the effect of fly ash reburning on the ash deposition characteristics when burning high-alkali coal in slag-tapping boilers was investigated.The test results showed that with or without fly ash reburning,the main components of the liquid slag and ash samples were Si O₂,Al₂O₃,Fe₂O₃ and Ca O.The liquid slag had a high content of Si and Fe and a low content of Na,with the opposite trend in the ash.Fly ash reburning could increase the Si and Al contents and decrease the Fe and Ca contents in the liquid slag and ash deposits.After fly ash reburning,the Na content in the liquid slag increased by about 10%and decreased in the deposition.The condensation of Na on the ash particles’surface led to a roug surface.The EDX results showed that Na Cl caused the aggregation of the ash particles.The Na in the flue gas was mainly in the form of NaCl.