Study On The Transformation Mechanism Of Nitrogen Oxides In Dilute Phase In Circulating Fluidized Bed Furnace

For a long time,as an important fossil energy,coal has played an important role in promoting the operation and development of the whole world economy,the coal-based energy structure will be difficult to change in the future for a long time.However,the process of coal combustion produces a large number of pollutants,such as smoke sulfur dioxide nitrogen oxides,so the emission standards of coal smoke pollutants are increasingly strict.As an efficient low NO_x combustion technology,low oxygen combustion combined with post-combustion technology in circulating fluidized bed furnace ensures low NO_x emission while ensuring combustion efficiency.The application of this technology makes the dense phase area and dilute phase area in the circulating fluidized bed furnace present completely different combustion atmosphere.Therefore,as an important combustion area in the furnace,the dilute phase area has a profound impact on NO_x transformation in the furnace,so it is necessary to study the transformation of NO_x in the circulating fluidized bed systematically.In order to explore the influence of various factors on the process of NO_x reduction in the dilute phase region,this paper systematically studied the relevant influencing factors and initial reaction mechanism of NO_x reduction in the dilute phase region by using the method of TG-MS&ECSA,the high-temperature vertical tubular furnace experiment and the basic physicochemical test and analysis.In this study,three types of recycled ash from Bengbu,Xingang and Shenmu circulating fluidized bed boiler were selected and raw coal samples were used to produce high-temperature pyrolysis semi-coke on the horizontal tubular furnace test bed.The chemical composition,physical structure and surface morphology of raw coal and semi-coke were analyzed and compared by means of elemental analysis and nitrogen adsorption desorption test and SEM.By using high temperature two-section vertical tubular furnace system,exploring the relationship between the reduction of NO by combustible residual carbon of different properties and the temperature of coal type CO in reducing and oxidizing atmosphere.In addition,the reaction mechanism of graphite reduction decomposition of CaSO₄ was deeply explored from the perspective of escaping gas and solid products by combining TG-MS&ECSA with the solid experiment of horizontal tube furnace,which laid a foundation for the study on the influence of sulfur products on NO reduction process.Finally,through the vertical tubular furnace experiment,the reaction temperature and reaction mole ratio were changed to preliminarily explore the influence of sulfur fixation products such as CaSO₄ on the NO_x conversion process in the reducing atmosphere,which provided a preliminary theoretical basis for the realization of combined desulfurization and denitrification by combining low-oxygen combustion with supplementary combustion technology.The research results of physical and chemical characteristics show that the micro surface of raw coal is mostly flat with sharp edges and corners.After pyrolysis,the semi-coke surface obtained shows a state of dense coking,with a decrease in volatiles and an increase in combustible carbon content per unit mass.The specific surface area and pore volume of semi-coke both increase,the average pore size decreases,the micro-pore structure is more than that of raw coal,the pore structure is more developed and the reactivity is higher.The recycled ash contains more combustible carbon than ash and other impurities,and the surface of the material in the microscopic form has undergone wear and tear.Compared with raw coal and semi-coke,the reactivity has been greatly reduced.The experimental results of vertical tubular furnace show that the main reducing agent in the NO conversion process is carbon,and the reduction effect is positively correlated with the carbon content,and the higher temperature,the more obvious effect.Under the reducing atmosphere,the reduction effect of semi-coke is greater than that of raw coal and that of circulating ash below 850.At 900,for the Shenmu sample with higher carbon content,the difference in the final reduction conversion rate between raw coal and semi-coke is not big,but the reaction speed of semi-coke will be faster.In addition,the reduction process is a reversible endothermic reaction,and the higher the temperature,the faster the reaction rate.In the actual production,O₂ and CO will exist in the dilute phase region,and the reaction atmosphere in the vertical tubular road will be changed.It can be found that the addition of O₂ will consume the combustible residual carbon as a reducing agent at high temperature.The consumption degree of semi-coke is greater than that of raw coal than that of circulating ash,and the influence of O₂ is the greatest.As the reaction of carbon thermal reduction of NO is also strengthened with the increase of temperature,there is NO overall difference in the conversion rate of NO in oxidizing atmosphere with the change of temperature.CO gas will not undergo reduction reaction with NO gas in the presence of its own,however,the addition of CO in the reducing atmosphere will have a partial inhibition on the process of C thermal reduction of NO,and the inhibition effect is not related to the solid reactant gray semi-coke and raw coal.Under the oxidizing atmosphere,CO will have a synergistic effect on the reaction of NO_x reduction by carbon heat.This synergistic effect will promote the conversion of NO_x to N₂ and increase the conversion rate of NO under the original oxidizing atmosphere.Calcium sulfate?CaSO₄?is the main component of sulfur fixation products produced by calcium desulfurization spraying in the furnace.TG-MS&ECSA and horizontal tube furnace experiments show that the self-decomposition reaction will occur at 1100?,and the carbon will reduce the initial decomposition temperature of CaSO₄.The reaction occurs at about 900?,mainly generating CaO,CO₂,SO₂.When the temperature exceeds 1100?,the graphite may react with CaSO₄ to form a small amount of CaS.Below 850?,the addition of CaSO₄ has little impact on the process of carbon-thermal reduction of NO.Above 850?,CaSO₄ has an inhibitory effect on the process of carbon-thermal reduction of NO.As the temperature increases,the inhibitory effect becomes more obvious.At 850?,with the gradual increase of the molar ratio of CaSO₄,the reduction effect of the cyclic gray semi-coke and the raw coal per mole ratio was not significantly different,but the overall change showed a slow downward trend.As can be seen from the weight loss curve of the thermogravimetric experiment,excessive CaSO₄ will promote the consumption of residual carbon and have a negative impact on the NO_x reduction process.The results provide a preliminary theoretical basis for the reduction of combustible carbon by changing the combustion atmosphere in the furnace,and for the realization of combined desulphurization and denitration by the combination of low oxygen combustion and supplementary combustion.