| The clean and efficient utilization of Zhundong coal is of great significance for ensuring China’s energy security and achieving the goal of“dual carbon”strategy.The carbon conversion and mineral migration characteristics of coal combustion are essential for guiding the numerical modeling of pulverized coal combustion,optimization of new burner design,and so on.The laser spectroscopy diagnostic technology can obtain the in-situ combustion field information online and plays an important role in coal combustion research.Since there is not enough understanding of the physicochemical mechanisms of the coupled multi-process combustion of Zhundong coal in the high-temperature transient laminar flow field,this dissertation investigates the ignition and combustion characteristics at the early stage of coal combustion,the char structure evolution characteristics at the middle and late stages of combustion,and the mineral migration characteristics during the combustion process,based on the laser spectroscopic diagnostic techniques.This aim is to reveal the kinetic characteristics of carbon conversion and mineral migration in the combustion of Zhundong coal and provide the theoretical foundation and data support for new combustion technologies.Firstly,a laser spectroscopy diagnostic platform was established to study Zhundong characteristics at the early stage of coal combustion based on a multi-premixed Mc Kenna flat flame burner since the current research status that the transient reaction characteristics in high-temperature laminar combustion fields are still unclear.Diagnostic data,such as free-radical concentration,particle velocity,and particle temperature in the high-temperature combustion field,were obtained using OH-PLIF,CH*chemiluminescence,and a particle pyrometer.The ignition and combustion characteristics of the volatiles and char particles are quantitatively characterized using the multi-source spectral fusion analysis.The study revealed that homogeneous oxidation dominates the volatile reaction of Zhundong coal upstream,and the heterogeneous ignition of char particles occurs upstream and then controls the heterogeneous reaction downstream.A first-order kinetic model was established to describe the combustion behavior of combustible materials of pulverized coal.The model fully couples the combustion process of volatiles and char particles,which is verified by the data of char samples during combustion.This model yielded the expressions of the Arrhenius activation energies of volatiles and char particles,such as 173.1 and 278.0 kJ/mol in O2/N2 atmosphere and 217.0 and 291.1 kJ/mol in O2/CO2 atmosphere,respectively.With the help of the fast Fourier transform,the gas-phase flame fluctuation characteristics of the oxy-fuel combustion of pulverized coal were investigated,and a tendency for the maximum frequency of flame fluctuations was found to increase and then stabilize with the increasing temperature in the range of 1200-1800 K at 30%O2.However,the maximum fluctuation frequency increased from 244.8 Hz to 268.7 Hz in the range of 20–40%O2 at 1800 K,and there is no limit to the fluctuation frequency.Secondly,the structural evolution characteristics of char particles after pyrolysis and during combustion were investigated to address the problem that the kinetic model established at the early stage of coal combustion cannot accurately predict the char combustion behavior in the middle and late stages of combustion.Therefore,the pyrolysis chars of Zhundong coal and Jincheng coal were used as study subjects,while demineralized coal samples were used as control groups.For the first time,the aromatization degree of char particles was analyzed using Raman spectroscopy and laser-induced breakdown spectroscopy（LIBS）techniques to obtain the effect of pyrolysis structural evolution on LIBS molecular emission.It was found that Raman drift coefficientαdecreases,and C2 and CN molecular emissions diminish when the char structure is aromatized.Based on the correlation between Raman and LIBS,the evolution characteristics of the char structure during Zhundong-coal and its demineralized-coal combustion were investigated using LIBS at 1800 K and 20%O2in the O2/N2 atmosphere.It is found that the correlation between the coefficientαand the CN molecular rotational temperature was a linear fitting with R2 of 0.77 and an exponential fitting with R2 of 0.95.The correlation indicates that the evolution of char structure may affect the CN molecular emission,and char particles’aromatization was found to decrease the char oxidation reaction rate.Finally,regarding the serious problems of coking,slagging,and corrosion caused by mineral migration,as well as the significant alteration of flame dynamics,which greatly limits the prediction accuracy of numerical models and restricts its industrial application scale,research was carried out on the migration and release characteristics of mineral elements during the combustion process of Zhundong coal,based on the study of ignition characteristics during the early stages of combustion and the burning characteristics of coke in the middle and later stages.A movable multi-point LIBS measurement system was established to analyze the kinetic properties of laser-induced plasma,such as electron temperature,electron density,and self-absorption,which laid the foundation for the quantitative study of the migration and release of mineral elements.The chemical reaction mechanism of the release and recombination was investigated for three representative mineral elements:Na,Ca,and Fe.The Na matter released in the devolatilization stage was recombined to the char surface under high-temperature conditions,which resulted in a Na concentration trend of increasing and then decreasing during the char combustion.The Ca release trend is similar to Na during char combustion,while the Fe is almost uninvolved.Furthermore,a first-order kinetic model was developed for the Na and Ca migrations in the char combustion stage.This model yielded the Arrhenius activation energies as 87.2 and 150.8 kJ/mol for Na and Ca migrations in O2/N2 atmosphere,respectively.The results presented in the dissertation will be of importance,both scientifically and practically,to improve the understanding of the numerical simulation,guide the design and optimization of new burners,and promote the engineering application of the clean,efficient,and low-carbon Zhundong coal combustion technology. |
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