Modeling Of Inorganic Matter Behaviour During Char Conversion Stages Of Pulverized Coal Burning In O₂/CO₂ Environment

Oxy-fuel combustion is a promising technology which can be utilized both in existing power plants and new-built power plants to achieve high temperature and high O₂ concentration combustion. Particulate matter formation during coal combustion not only affects safety and operation of the whole system, but also may result in air pollution if not talcked properly. Inorganic matter vaporization during char conversion is dominant for particulate matter formation. Oxy-coal combustion has been observed to have significant impacts on char combustion characteristics, which is believed also to affect inorganic matter vaporization when comparing to the traditional air-coal combustion. So it is significant to properly describe char conversion stage for modeling inorganic matter vaporization and gasification. Therefore, the main objective of present work is describing inorganic matter vaporization and gasification during char conversion stage of pulverized coal burning in O₂/CO₂ environment with modeling approach.A mathematical sub-model was developed to describe a single char particle burning inO₂/CO₂ atmosphere through extending the sub-model of single char combustion in O₂/N₂ atmosphere to include char CO₂ gasification and detailed description of gas properties. The sub-model was based on single-film approach, assuming no homogenous reaction in particle boundary layer. In the model, C-O₂ and C-CO₂ reactions were empirically described with Langmuir-Hinshelwood kinetics and the gas properties were modeled in detail to consider the transports in the boundary layer and the impact of gas dissociations due to the high temperature in oxygen enriched combustion. The model was validated with literature data on the combustion of pulverized coal char burning in O₂/N₂ and O₂/CO₂ atmosphere, indicating that the present model accurately estimates char burning temperature and burnout time for O₂ molar fraction of < 0.2 - 0.3 in both O₂/N₂ and O₂/CO₂ environment. Furthermore, the decrease in the char burning temperatures when changing from O₂/N₂ to O₂/CO₂ combustion is mainly attributed to the lower O₂ diffusivity in O₂/CO₂ mixture than that in O₂/N₂ mixture at low O₂ fractions and the higher thermal conductivity of O₂/CO₂ mixture at higher O₂ fractions. This confirms the significance of the contribution of the gas properties to the char burning behavior and portrays the importance of properly handling the gas properties in modeling char combustion in O₂-enriched combustion. CO₂ gasification is also demonstrated to be important in O₂/CO₂ combustion as well, which contributes to 1/4 - 1/3 reduction of char burning temperature.A mathematical sub-model was developed to describe for a single char particle burning in oxygen-enriched O₂/CO₂ atmosphere considering the evolution of detailed reaction kinetics and structure of char particle. Double-film model was established by extending single-film model but including modeling CO oxidation in particle boundary layer with a flame sheet. It described mainly global CO oxidation kinetics and its impacts on diffusion and thermal conduction within the particle and in the boundary layers. The model was extensively validated with vast experimental data in open literature, covering a wide range of O₂ concentration of both O₂/N₂ and O₂/CO₂ combustion. The results indicate that the model predicts particle temperatures and burnout times and both generally agree well with those measured results of various coal chars, particle sizes and gas conditions. The performance of double-film model is demonstrated to be better than that of single-film model for predicting char particle temperature, burnout time and reaction rate at higher O₂ levels. These indicate the importance of considering CO oxidation and gas dissociations in the boundary layer to properly model the combustion process of pulverized coal char under oxygen-enriched conditions. The double-film model can be applied in CFD as well since it is not complicate.Systematic experiments were conducted to investigate ash formation behaviors during coal particles burning in O₂/CO₂ atmosphere. With carefully selecting 75-90 ?m Zhundong coal and Victorian brown coal, treating coal samples with water washing, combustion experiments in a high temperature drop tube furnace were conducted separately in 35%O₂/65%CO₂ and air environments to quantify particulate matter behaviors through systematically analysis and characterization of ash samples ?submicron and supermicron particulate matter?. The experimental results indicate that the behaviors of inorganic matter were observed to change obviously due to variation of char particle combustion temperature and existence of high concentration CO₂ when combustion atmosphere transferred from O₂/N₂ to O₂/CO₂. Different amounts of PM0.1 and PM0.1-1 formed during 35%O₂/65%CO₂ compared to air combustion are mainly contributed to the difference of inorganic matter vaporization and gasification, but the change of PM1-10 is due to the difference of fragmentation and agglomeration. These results indicate that it is appropriate to validate the developed inorganic matter vaporization model with experimental data of PM1.A mathematical sub-model was developed to describe the vaporization of inorganic matter during char burning stages in both O₂/N₂ and O₂/CO₂ environment. The model development was to modify the classic ash vaporization model of Quann and Sarofim and extend it to cover oxy-coal combustion over a wide range of oxygen concentration, based on using the double-film char burning sub-model with intrinsic kinetics to describe the burning process of a single char particle. The model was extensively validated with the literature data including the partial pressure and evaporation rate and fraction of alkali, alkali earth elements and silicon. The model was proved to be able to predict the vaporization behavior of the inorganic matter during char conversion stages in O₂/N₂ and O₂/CO₂ environments. The model can reflect the effects of coal type, particle size, ambient temperature and oxygen concentration on the vaporization behavior. These results demonstrate the effectiveness of model development and generalization to extend its application for oxy-coal combustion.