Study On The Deposition And Stripping Characteristics Of Ammonium Bisulfate Ash Scale In Rotary Air Preheater

Coal-fired power plant boilers commonly use SCR denitrification technology to control NOx emissions,but it also leads to serious blockage problems in the downstream air preheater ammonium bisulfate(ABS),which increases fan power consumption and raises exhaust flue gas temperature,reducing boiler operating economy and safety.In the strict environmental requirements of ultra-low NOx and ultra-ultra-low emissions,SCR denitrification ammonia escape can not be completely avoided,from the prevention point of view can only mitigate the air preheater ABS deposition,in order to further solve the air preheater ABS blockage from the governance point of view,based on the existing steam sootblower,hot air recirculation,single column flue to raise the temperature operation,warmers to enhance the cold air temperature and other measures have been carried out on a trial basis,but ABS to ash scale The lack of basic research related to the degree of slabbing,the weakening law of heating precipitated ABS on the strength of the scale and the law of heating precipitated ABS from high-temperature airflow in narrow passages makes the reduction effect of these heat treatment-assisted ABS scale removal techniques uneven.To this end,this paper investigates the deposition and stripping characteristics of ammonium hydrogen sulfate scale in rotary air preheaters through model simulation and experimental analysis,aiming to provide theoretical guidance for heat treatment-assisted removal of ABS scale in air preheaters.Firstly,the heat transfer model between flue gas-secondary air-primary air-heat transfer element during the rotation of air preheater was established,and the finite difference method was applied for iterative calculation.Simulations were carried out for the rotary air preheater of Huaneng Chaohu Power Plant 600 MW coal-fired unit to obtain the temperature distribution of the working mass and the temperature distribution of the element,and to find out the variation law of the ABS scale deposition position on the element surface at the cold end: 1)the ABS deposition position varies periodically with the rotation angle,and the deposition position at the time when the rotor leaves the flue gas bins is closest to the lower edge of the cold end,and the element temperature is at the highest on the circumference;2)the ABS deposition position at high unit load is closer to the bottom of the cold end than at low load;3)the ABS deposition position is further close to the bottom of the cold end after the heater heating raises the temperature of the primary and secondary cold air.In addition,based on the DCS data such as boiler operating parameters and the differential pressure of the flue gas inlet and outlet of the air preheater,the corresponding relationship between the ABS scale deposition thickness on the wall of the air preheater heat exchanger element and the operating time was fitted and analyzed,and the ABS scale deposition thickness during the steam sootblower blowing once per shift(about 8 h)was initially obtained.Under the normal operation of the steam sootblower,the ABS scale deposited every 8h,can be blown away by the steam jet ABS floating ash about 93.31%-95.34%,the residual formation of slab ABS scale accounted for about 4.66%-6.69%,the accumulation of the latter is the main cause of serious blockage in the long-term operation of the air preheater.Secondly,the pressed flakes were prepared by premixing ABS powder with fly ash in the laboratory,and the weight loss experiments were carried out by using thermogravimetric balance,and the experiments of ABS liquefaction causing ash scale slabbing were carried out in the temperature range of 147-220°C,and the experiments of ABS gasification precipitation causing ash scale loosening were carried out in the temperature range of220-300°C.The mechanical strength,ABS content,particle agglomeration morphology and chemical composition of the pressed samples were examined and analyzed to quantify the action law of ABS on the ash scale for long time treatment at different temperatures.The experimental results showed that 1)the compressive strength of the pressed flake samples increased with the increase of ABS content from 0% to 2%,3.5% and 7%,and the compressive strength increased from 147.48 k Pa to 288.33 k Pa in blank samples,which was about 95.50%,and the effect of different heating temperatures on the compressive strength was relatively small in the interval of 147-220°C;2)the effect of different heating temperatures on the ABS precipitated during high temperature heating at 220-300°C,and the ABS content in the pressed samples decreased from 7% to about 0.18%-0.25%,with a maximum decrease of 97.42%,and the compressive strength also decreased to about 155 k Pa,which was close to the compressive strength of the blank reference sample;3)In the process of ABS causing ash scale slabbing to improve the compressive strength,the chemical reaction between ABS and ash scale to generate new substances The influence of the chemical reaction between ABS and gray scale to generate new substances accounts for about 10%-12%,while the influence of physical adhesion of ABS melting to promote particle agglomeration accounts for about 88%-89%,which lays a theoretical foundation for the heat treatment method to assist in solving the problem of ABS deposited gray scale.In addition,a thin layer of ABS deposits with a thickness of 1.0 mm was covered on the surface of the heat exchanger element,and a high-temperature parallel jet transient heating experiment was conducted using a hot air gun,and about 26.18%-33.27% of ABS was vaporized and precipitated during a relatively short heating time of 0.5-2s,corresponding to a surface layer thickness of about 0.25-0.30 mm that reached the vaporization temperature.Then,based on the principle of energy conservation,an unsteady heat transfer model of high-temperature jet in a narrow channel was established for the location and thickness of ABS scale deposited on the surface of the cold-end heat exchanger element of the rotary air preheater,and the influence of high-temperature jet temperature,velocity and heating time on the temperature of different thicknesses of scale along the surface of the element and the temperature of the jet workpiece was studied and analyzed.The correlation between heating time,high-temperature jet temperature and velocity was found with the objective of making all ABS in the surface layer of deposited scale between 200-500 mm from the bottom surface of the cold end element vaporize and precipitate.For the conventional high-temperature jet gas with a temperature of 300℃ and a flow rate of 30m/s,70%-82% of ABS can be precipitated from the deposited layer of 0.3-0.6mm in a minimum of 1s,thus reducing the mechanical strength of the deposited slab scale and creating conditions for the blowing and cleaning of the steam sootblower.Finally,on the premise of strictly controlling the SCR denitrification ammonia escape less than 3μL/L to reduce the ABS scale deposition in the air preheater,based on the study of the ABS deposition location characteristics of rotary air preheater,the ABS slab scale thickness deposition rate,the high temperature heat treatment above 220℃ to reduce the mechanical strength of ABS scale and the narrow channel high temperature airflow transient heating precipitation ABS,the secondary air sub-bin near the The high-temperature jet injector is arranged at the cold air inlet position of the secondary air sub-bin near the flue gas sub-bin(where the ABS scale deposition position is the most downward and the temperature of the component wall is the highest on the circumference),and the high-temperature jet and the secondary cold air are heated in the same direction to precipitate ABS from the ABS scale deposited on the surface of the component,so as to create conditions for the existing steam soot blower to remove the ABS slab scale efficiently.diameter,rotation speed),high-temperature jet parameters(temperature,speed),high-temperature jet blowing residence time,high-temperature jet blowing interval(depending on the growth rate of ABS scale deposition thickness)in one high-temperature jet blowing-assisted steam sootblower to remove ABS blocked slab scale heat treatment strategy.