The NO Release Characteristics And Chemical Kinetics Simulation Of Typical Coals In Oxy-Fuel Combustion

Coal is the main source of energy in china, in current and coming decades, however,nitrogen oxides and carbon dioxide produced during the process of coal combustion alsocause serious pollution to the environment for human survival. CO₂is the source of thegreenhouse effect, while nitrogen oxides lead to acid deposition. Thus, solving the pollutionproblems caused by coal-fired power generation and the development of clean coaltechnology have become imperative. Oxy-fuel combustion technology now has become apromising way for to capture CO₂from the flue gas; at the same time, it can also effectivelyreduce pollutants. Therefore, to carry out research in this area has important scientific andpractical significance.This thesis provided an overview of the present researehes on oxy-fuel coalcombustion, and discussed status and the lack of the NOx control technology in details.This thesis investigated on nitrogen transformation during oxy-fuel condition, and thenfocused on the generation and control of the nitrogen-containing precursor compounds toreveal the regularity and mechanism of the conversion of volatile nitrogen to NO by meansof a model compound of pyridine. Due to the complexity of the char nitrogen, the impact ofof the evolution of char structure to the release of nitrogen-containing product andreactivities were studied, and finally established a model for the NO formation in oxyfuelcombustion..The key findings of dissertation are as follows:Firstly, the release properties of gases produced during the pyrolysis and combustionof pyridine were studied and the influences on the conversion of pyridine to NO and N₂were studied. The results showed that the major pyrolysis gases for pyridine including CH₄,C₂H₂and HCN increased with increasing temperature, while NH3decreased. HCN contentin the product gases was the highest. The conversion of pyridine to NO increased withtemperature. However, it was consistently lower in reducing atmosphere than that inoxidizing atmosphere and the discrepancy between them was enlarged at highertemperatures. O₂/CO₂atmosphere had an optimal temperature for NO reduction of900℃.The data obtained in O₂/Ar presented higher values than the corresponding results in O₂/CO₂atmosphere. Moreover, the gap between them developed with excess oxygencoefficient increased. The Terasa09mechanism model had a better capability of predictingthe combustion behaviors of pyridine than Sarma08mechanism. HNO、NH and NCOradicals played critical roles in the final conversion of the pyridine nitrogen.The influences of several factors on NO formation and reduction by char in O₂/CO₂atmosphere were studied in a bench scale fixed bed reactor. The results showed that the NOreduction by char was greatly affected by several factors. SX lignite char had the bestreduction effect while NCP anthracite char was the worst. High temperature conditionswere conducive to NO heterogeneous reduction with char. The increase of initialconcentration of NO resulted in the increasing reaction intensity, the peak value of NO andCO of coal appeared ahead of time and peak width decreased, while the CO peak for chardecreased. Proper amounts of CO₂and CO had positive effects on NO reduction. NOreleased from char were similar in both atmospheres while the NO concentration wassignificantly reduced in O₂/CO₂atmosphere for coal. The NO release with O₂concentrationof10.5%was greatly enhanced in O₂/CO₂atmosphere. The increase of O₂concentrationwas inclined to promote the conversion of char nitrogen to NO.Coal char and biomass char were prepared in N₂and CO₂using a small scale fluidizedbed reactor. The release properties of gas products were analyzed on-line. The structuraldiscrepancies of char resulted from the different fluidizing gases were examined byultimate analysis, Raman, FTIR, SEM and N₂isothermal adsorption/desorption method.Then the reactivities of the chars were measured by TGA-FTIR technology in air andoxy-fuel conditions with O₂concentration of21%. The derived activation energy fordifferent samples were correlated with the Raman structural parameters. The results showedthat the major nitrogenous gases produced during pyrolysis and gasification of coal andbiomass were HCN and NO. CO₂atmosphere could postpone the release of HCN from coaland enhance the release of NO from biomass. The weight losses for all samples under CO₂were larger than that under N₂. It was found that additional new O-containing functionalstructures would be introduced into the char structure when gasified in CO₂. The effect ofCO₂reacting with coal could enhance dehydrogenation of hydroaromatics and the growthof aromatic rings, which resulted in the formation of more disordered char structures when gasified in CO₂. The CO₂atmosphere reduced the content of hydroxyl groups and olefinicC=C bonds, and increased the content of ν（C O） vibrations in primary C OH andsecondary C OH and promoted the conversion of C O bonds to ethers structure for coalchar; as for biomass, the CO₂promoted the rupture of hydrogen-bonds and increased theamount of methyl or methylene substituents. The CO₂atmosphere led to the occurrence ofsoftening, melting for biomass char. Many wide blow holes were formed on the surface andthe particles appeared almost transparent, resulting from more intense release of volatiles.The coal char prepared in CO₂showed little fluidity and had a relatively solid structure witha low porosity. The reactivity was higher for the CO₂char than N₂char in both combustionatmospheres, while the combustion atmospheres rarely affected the char reactivity. CO₂played a more important role on the devolatilization processes than combustion in mediumtemperatures. The activation energy had good correlations with Raman parameters.Finally, the oxy-fuel combustion of pulverized coal was studied in a drop tube furnace.The release property of NO was analyzed on-line. A chemical kinetics model wasdeveloped to investigate the influences of excess oxygen coefficient, oxygen concentrationand combustion temperature on NO conversion. Then the heterogeneous reduction of NOby char and the improvement of the model were discussed in detail. The results showed thatthe NO conversion in O₂/N₂and O₂/CO₂atmosphere increased from0.39and0.20to0.48and0.27, respectively, with the highest decline up to20%in O₂/CO₂atmosphere. Excessoxygen coefficient in the oxidizing atmosphere had a more obvious impact on the NOconversion in O₂/N₂. With the increase of O₂concentration or temperature from900℃to1200℃, the conversion of NO increased monotonically, until reaching a peak value of0.24at1200℃. The improved chemical kinetics model had capability of predicting theconversion NO at different excess oxygen coefficients and oxygen concentrations while themodeling results exhibited a certain extent of deviation with the variation of temperatures.In the absence of the heterogeneous reaction between NO and char, the modeloverestimated the NO conversion for14%in oxidizing atmosphere and only5%in thereducing atmosphere, indicating that heterogeneous reactions were important for the finalformation of NO, particularly under oxidizing conditions.