Effect Of H₂O And Its Resulting Groups On Nitrogen Evolution During Oxy-steam Combustion Of Coal

Carbon capture is necessary to solve the issue about high CO2 emissions from coalfired power plant and to meet the climate target of ’carbon peaking and carbon neutrality’goals.Among them,oxy-steam combustion is as one of the most potential carbon capture technologies.Compared to conventional air combustion,H2O is used instead of N2 to mix with O2,thus solid fuels are burned in the mixed atmosphere of O2 with H2O.This improves the CO2 net efficiency,and also reduces the formation of thermal NOx by moderating the temperature in boiler.As so far,most of studies focused on the NOx release under conventional combustion.However,there is little research addressing the evolution mechanism of coal nitrogen during oxy-steam combustion,and there is a lack of clear knowledge about the effect of H2O on nitrogen evolution.In this paper,the distribution of nitrogen-containing functionalities on particle surface and the release of nitrogen-containing species in gas-phase during coal devolatilization and char combustion were investigated by means of experiments.According to the experimental results,using molecular dynamics simulation and quantum chemistry calculation,the evolution paths of nitrogen were traced and the elementary reaction of nitrogen chemistry were analyzed.Based on the thermodynamics data,the combustion kinetics models based on nitrogen chemistry were established via transition state theory,in order to find the dominant route in nitrogen evolution and meets the target of semiquantitative prediction and analysis for the process of micro-mechanism to macrophenomenon under oxy-steam combustion.Main contents are expressed as follows:First,the distribution of nitrogen-containing functionality and the release of nitrogencontaining precursor were characterized by the isothermal fixed-bed experiments combined with XPS and FTIR tests.The release of NO and NO2 during char combustion was monitored.Results showed that during coal devolatilization,the high concentration of H2O inhibits the migration of the nitrogen located inside aromatic rings to edge,but promotes the conversion of N-6 to N-5 with lower thermal stability.Furthermore,the release of nitrogen-containing precursor HCN and NH3 from nitrogen-containing functionality are also inhibited under oxy-steam combustion.During char combustion,the NO and NO2 emissions under oxy-steam combustion are higher than that under air combustion.The higher the temperature and H2O concentration is,the more significant the positive effect of H2O becomes.According to FTIR analysis,considerable OH groups derived by H2O are the main cause of the difference between oxy-steam and air combustion.Second,the systems of coal devolatilization and char combustion were established by means of ReaxFF MD simulation,in order to trace the evolution path of nitrogen at various state,find the source of HCN and NH3 during devolatilization,illustrate the oxidation channel of char-N during char combustion,as well as quantify the difference in the content of OH group between oxy-steam and air combustion.Results showed that most of HCN and NH3 are derived from the decomposition of N-6 and N-5 during coal devolatilization.During char combustion,the HCN is formed by the interaction of char-N with oxygencontaining functionality,and then it is oxidized to NOx in gas phase,together with volatileN.Furthermore,the contents of H2O and OH groups in oxy-steam combustion are much higher than that in air combustion.In particular,the number of OH radicals in oxy-steam combustion is about 10 times higher than that in air combustion during char combustion.After that,according to the nitrogen conversion path from ReaxFF MD simulation,the nitrogen chemistry during coal devolatilization and char combustion was illustrated and the elementary reactions and thermodynamics parameters were obtained by quantum calculation based on density functional theory,in order to know the effect of OH group on nitrogen chemistry.Results showed that the hydroxyl group on carbonaceous surface formed by OH adsorption strengthens the interaction of nitrogen with aromatic ring,leading to a lower reaction rate and thus inhibiting the migration of N-Q to N-6 and the release of HCN and NH3.At the stage of char combustion,OH has little influence on the decomposition of HCN from char-N.However,the homogeneous oxidation paths of HCN and NH3 vary in the presence of OH radical,promoting the conversion of HCN to NHi.The fact makes the NHi become important in the NO formation under oxy-steam combustion.The reaction path is described as follow:HCN→NH2→NH→N→HNO→NO.For the NO reduction,the presence of OH decreases the energy barriers of the NO+CO heterogeneous reduction on both char and char-N surfaces,but has little influence on the NO heterogeneous reduction without CO on char-N surface.Finally,based on the elementary reaction and thermodynamics parameters from quantum calculations,the combustion kinetics models of nitrogen conversion during coal devolatilization and char combustion were built by means of transition state theory,in order to explore the competition and cooperation among elementary reactions of nitrogen chemistry,find the dominant elementary reaction and key intermediate,as well as to reveal the effect of H2O and its resulting groups on nitrogen conversion.Results showed that NO release is largely determined by the homogeneous oxidation of HCN/NH3 during coal combustion.The presence of OH inhibits the release of HCN and NH3,but it accelerates HCN and NH3 oxidation to NO.By comparison,the positive effect of OH on NO formation is more significant,leading to higher NOx emissions under oxy-steam combustion than air combustion.In this case,the elementary path NH3+OH→NH2+O2→NO dominates the nitrogen evolution under oxy-steam combustion.