Study On The Coupling Characteristics Of Nitrogen Element Transformation And Combustion Process Of Pulverized Coal Combustion In O₂/CO₂

Today,China’s power production is dominated by thermal power generation.Aiming the carbon peaking and carbon neutrality goals,how to achieve carbon reduction in coal-fired power plants is an important task to achieve carbon neutrality in the energy field currently.At present,there are three technical methods to reduce carbon dioxide emissions for coal-fired power plants,i.e.post-combustion decarbonization,pre-combustion decarbonization,and oxygen enriched combustion technology.Among these three technologies,oxygen enriched combustion technology features easy realizability and low economic cost,which is a relatively potential carbon dioxide emission reduction technology at present.Meanwhile,this technology can effectively reduce the emission of NOx,such as in the oxygen enriched combustion,there is no participation of N2 and no large amount of thermal NOx will form.For Shenmu bituminous coal,the present paper conducts oxygen enriched combustion of single pulverized coal particle and pulverized coal to investigate the combustion characteristics and the change rule of N-element of coal,and the coupling relationship between the two factors.Firstly,the paper conducted single coal particle combustion experiments on a horizontal tube furnace.The focus of the experiment is to investigate the oxygen enriched combustion of Shenmu coal with two different particle sizes,while varying the oxygen concentrations ranging from 21%to 80%.The experiments studied the influence of oxygen concentration and coal particle size on the ignition and combustion characteristics of coal.The results have shown that for single coal particle measuring 400-500 μm,homogeneous ignition occurs when the oxygen concentration ranges from 21%to 40%.Additionally,the ignition mode is joint heterohomogeneous ignition when the oxygen concentration is within the range of 40%to 50%.As the oxygen concentration increases to 60%-80%,the ignition mode shifts heterogeneous ignition.When the particle size of coal particle decreases to 100-200 μm,the critical oxygen concentration of ignition mode transition is founded to be 50%.However,the combustion state of coal would change.When the concentration of O2 drops below 50%,the single coal particle only undergoes volatile combustion,and the coke cannot be ignited at 1000℃.After obtaining the relationship between characteristics of coal ignition mode and flame migration with the particle size and oxygen concentration,the study conducted oxygen enriched combustion experiments on pulverized coal using a drop tube furnace.The experiments focused on oxygen concentrations ranging from 21%to 80%,and conducted online monitoring of coal combustion with the size range of 0 to 70 μm.The experiments studied the influence of oxygen concentration on combustion characteristics and the change rule of N-element.The results have shown that with the increase of oxygen concentration at 1000℃ furnace wall temperature,the ignition mode of pulverized coal in the oxygen enriched conditions changes from homogeneous ignition to heterogeneous ignition with 60%oxygen concentration as the boundary.As oxygen concentration continuously increases,the emission of NO exhibits an initial rise followed by a subsequent decline.The moderate increase of oxygen concentration is conducive to reducing the release of NO.The study found that the lowest emissions of NO are achieved at 30%O2 concentration,while the peak emission occurs at 50%O2 concentration.When the oxygen concentration exceeds 60%,the NO emissions decrease.In the paper,it has been found that the combustion characteristics such as ignition mode of coal,ignition time of volatile and coke have complex relationship with the change rule of N-element and reducing gases such as CO.Within the oxygen concentration range of 30%to 50%,homogeneous ignition occurs in pulverized coal.As oxygen concentration increases,the ignition time interval between volatile and coke shortens.During the early stage of combustion,surface oxidation and gasification reactions involving CO2 can generate a large amount of CO.Due to the presence of oxygen,this CO is consumed,inhibiting the reduction reaction of NO and leading to increasing of NO emissions.At the same excess air coefficient,when the oxygen concentration ranges from 60%to 80%,heterogeneous ignition occurs in pulverized coal.The ignition time of the coke greatly advances,and the coke enters a violent reaction state during the initial stage of combustion.At this time,the combustion of coal coke is dominated by surface oxidation reaction,owing to the high oxygen concentration.Consequently,a large amount of CO and free active points on the coke surface that have been formed accelerate the heterogeneous reduction reaction of coke to NO,and reduce the production amount of NO.