Nitrogen Conversion In Char-O₂/H₂O System Under High-temperature Entrained Flow Condition

In China,due to the coal-based energy structure,the problem of emission related to power plant is ever increasing serious.It is urgent to find an effective means to control the emission of main greenhouse gas,CO₂,and main pollutant,NO,produced during coal combustion.For CO₂ reduction,the new Oxy-steam combustion（O₂/H₂O combustion）is a promising technology for low-cost,high efficiency CO₂ capture.However,the process of char-O₂/H₂O combustion is so complex that few researchers conducted an in-depth study of its char-nitrogen（char-N）conversion mechanisms.For NO reduction,the NO produced from volatile-nitrogen（volatile-N）has been effectively controlled by various methods,but the NO produced from char-N is less controllable.Therefore,the development of low-CO₂,low-NO combustion technology requires a better understand of the char-N conversion mechanisms.Based on this,this study investigates the char-N conversion characteristics during traditional char-O₂ combustion and new char-O₂/H₂O combustion experimentally,and a corresponding numerical model is established.The reliability of the numerical calculation is verified based on the experimental results,and the effects of various factors on the conversion of char-N to NO are analyzed according to the numerical results,which perfects the conversion mechanisms of char-N to NO during char combustion.Firstly,according to the experimental results of carbon conversion during char O₂ combustion,the intrinsic O₂ combustion reactivity parameters of JJM-char and demineralized-char are obtained by numerical calculations,and the reliability of numerical calculation is verified.For the carbon conversion during char-O₂ combustion,the char-O₂ combustion is in Regime II.With the increase of O₂ concentration or reaction temperature,the char-O₂ combustion is pushed to Regime III.Meanwhile,the intraparticle distribution of normalized O₂ concentration,local carbon conversion,and local porosity become more nonuniform,and the total pore surface area shrinks.For char-N conversion,the enrichment of nitrogen on the surface of partialreacted during char-NO reaction is not observed though XPS analysis,therefore the adsorption of NO on surface active sites is the rate-determining step of C-NO reaction,which leads to the synchronous release of char-N with carbon during char-O₂ combustion.For O₂ combustion of JJM-char,SH-char,and demineralized-char,the fractional conversion of char-N to NO（FCCN）increases with the increase of O₂ concentration for low O₂ oxygen concentration cases,but it decreases with the increase of O₂ concentration for high O₂ oxygen concentration cases.For low O₂ concentration cases,reaction temperature has small influence on the FCCN.However,for high O₂ concentration cases,the FCCN is lower at higher reaction temperature.The numerical results predict the experimental findings well,therefore the main factors affecting char-N conversion are decoupled and analyzed based on the numerical results.It is found that the crucial factors that affecting the FCCN are different for different O₂ concentration cases,which interprets the experimental findings well through the quantitative analysis on each factor.This study also found that the main primary nitrogen-containing product of char-N-O₂ reaction is NO.This study also investigates the char-N conversion characteristics during char-O₂ combustion with the initial atmosphere containing NO experimentally and numerically.The experimental findings are well interpreted by the quantitative analysis on the competition between char-N release and NO reduction based on the numerical results.Secondly,according to the experimental results of carbon conversion during char H₂O gasification,the intrinsic H₂O gasification reactivity parameters of demineralized-char are obtained by numerical calculations,and the reliability of numerical calculation is verified.The inhibition of H2 on C-H₂O reaction is considered using L-H mechanism.It is found that the char-H₂O gasification is almost not controlled by H₂O diffusion and is very close to Regime II,due to the low rate constant of C-H₂O reaction and the high effective diffusion coefficient of H₂O.Finally,the demineralized-char-O₂/H₂O combustion process is investigated experimentally and numerically.For carbon conversion,the experimental results show that the effect of H₂O addition on the carbon conversion varies at different O₂ concentrations or different reaction temperatures.The experimental findings are reproduced well by the numerical calculations based on common active sites assumption for C-O₂ and C-H₂O reactions.According to the numerical results,with the addition of H₂O,the particle temperature is lowered by the endothermic C-H₂O reaction,and H2,one of the products of C-H₂O reaction,occupies the surface active sites through associative adsorption,leading to a decrease of C-O₂ reaction rate.With the addition of H₂O,the final carbon conversion depends on increase of carbon conversion caused by C-H₂O reaction and the decrease of carbon conversion caused by the decrease of C-O₂ reaction rate.For char-N conversion,the experimental results show that H₂O addition increases the FCCN,which becomes more significant with the decrease of O₂ concentration.The experimental findings are reproduced well by the numerical calculations based on common active sites assumption for C-NO,C-O₂,and C-H₂O reactions.According to the numerical results,the effects of additional H₂O on charN release,heterogeneous NO reduction,and char-N conversion pathways are decoupled and analyzed quantitatively,based on which the crucial factors affecting the conversion of char-N to NO are summarized.During char-O₂/H₂O combustion,the homogeneous reduction rates of NO by HCN and NH₃ are extremely low,and thus the C-NO heterogeneous reaction is the main pathways for NO reduction.With increase of H₂O concentration,the associative adsorption of H2 on surface active sites and the decrease of particle temperature decrease the C-NO reaction rate,which is a major reason for the increase of the FCCN.Most of HCN converts to NO outside the particle through HCN → NH₃ → NO pathway.Therefore,with the increase of H₂O,the increase of primary HCN is the other major reason for the increase of the FCCN.With the increase of O₂,the proportion of HCN in primary nitrogen-containing products decreases,which is the main reason for the weaken influence of H₂O on the decrease of FCCN.Moreover,the values of FCCN is nearly the same under the assumptions of primary HCN and NH₃,because both of HCN and NH₃ convert to NO outside the particle.