Combustion Characteristics And Pollutant Formation Of Pulverized Coal In Pressurized Oxy-fuel Atmosphere

In recent years, to capture and storage CO2generated from large scale stationary power plants, oxy-fuel combustion (O2/CO2combustion) has caused wide attention of scholars both at home and abroad. However, both air separation unit (ASU) and the compression unit (CPU) for oxy-fuel combustion are operated at evaluated pressure, whereas the combustion is nonpressurized, thus serious energy loss occurs due to the change of operating pressure in different subsystems. Pressurized oxy-fuel combustion (POFC) is a novel and efficient combustion technology that derived from basic oxy-fuel combustion. In the POFC cycle, the ASU, combustion and heat transfer and CPU are all operated under pressure. By operating at elevated pressure, the thermal efficiency of the boiler increased and the power consumption of CO2condensation reduced which in turn make up for the power consumption of pressurization. In addition, more latent heat from flue gas can be recovered owing to higher condensation temperature of water vapor under pressure, and hence a larger fraction of thermal energy of the flue gases can be recovered to generate steam and the plant efficiency is improved. There are uncertainties regarding the characteristics of combustion and pollutant emission in pressurized oxy-fuel atmosphere due to the markedly different reaction conditions between POFC and traditional combustion. Therefore, both experimental and theoretical analysis are developed to investigate the burning process of pulverized coal in pressurized oxy-fuel atmosphere, main contents and results of the study include as follows:(1) The associative characteristics between pyrolysis behavior and ignition mechanism of pulverized coal were studied with a thermal analysis system at atmospheric pressure, and then the effect of pyrolysis characteristic on NO formation were discussed in a fixed-bed reactor. The pyrolysis behavior of pulverized coal at low and moderate temperatures has significant effects on the ignition mechanism in oxy-fuel atmosphere. The ignition mechanism varies with the pyrolysis characteristic of pulverized coal during oxy-fuel combustion. The combustion performance can be improved by increasing oxygen concentration, but the transformation of ignition mechanism has little influence on the combustion characteristic of pulverized coal. Pyrolysis behavior, oxygen concentration and temperature all have an influence on the NO formation laws. During burning process, there are obvious differences on both of the release procedures of NO and the NO conversion rates of coals with different pyrolysis characteristics. The impact of oxygen concentration and temperature on the NO conversion rate depends primarily on the volatile-N oxidation and reduction reactions competing with each other. (2) The combustion characteristics of pulverized coal in pressurized oxy-fuel atmosphere were investigated with a pressurized thermal analysis system. With the increase of pressure, the heterogeneous ignition first converts to homogeneous ignition, then begins to change to heterogeneous ignition gradually, and finally becomes heterogeneous ignition. When the pulverized coal ignites homogeneously, the burning rate of volatile increases with the rise in pressure and more volatile is generated. For the change of the ignition mechanism at different pressures, the ignition temperature and burnout temperature and the corresponding flammability index and burnout index don’t change linearly with pressure to a maximum value. As a result, the combustibility index increases firstly and then reduces with the rise in pressure. In addition, the increase of oxygen concentration can enhance the intensity of combustion but does not affect the ignition mechanism of pulverized coal.(3) The conversion of fuel-N of POFC was studied with Fourier transform infrared spectroscopy (FTIR), and the morphological features and mineral conversions were investigated with the combination of scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The conversion rates of precursors are different from each other with the rise in pressure due to the different formation principles. Along with the increase of pressure and the yield of volatiles, the conversion rates of NO and NO2rise rapidly. Although the NO2conversion rate drops somewhat, the NOx conversion rate rises linearly with pressure to a maximum value. With the increase of pressure, the porous char particles fracture frequently which lead to a large number of fine ash particles. The combustion temperatures may be different due to the different ignition mechanisms. Because of the higher combustion temperature of heterogeneous ignition at atmospheric pressure, some minerals, such as mullite, generate in the ash, while some other minerals, such as illite, are found in the ash owing to the lower combustion temperature of homogeneous ignition at elevated pressure. As the pressure continues rising, with increase of combustion temperature the homogeneous ignition transforms to the heterogeneous ignition and low temperature minerals converts to high temperature minerals.(4) The emission characteristics of NO were investigated with a test bench of pressurized bubbling fluidized bed. The results indicate that oxygen concentration has a significant effect on NO emission characteristics. With the increase of system pressure, because of the low oxygen concentration the oxygen consumed during combustion process can not get prompt supplement. As a result, the conversion rate of NO gradually decreases due to the strongly reducing atmosphere around particles. However, the oxygen consumed can get prompt supplement with high oxygen concentration. Thus the NO conversion rate increases by degrees due to the strongly oxidation atmosphere. (5) Based on the exact solution of physical properties of actual gas mixtures, a heat transfer model of initial heating stage was set up to study the transient heat transfer around a particle. The gas temperature around the particle increases gradually with the rise in pressure, which leads to remarkable effect of the heating of gas on the particle. Therefore, the particle temperature increases quickly and the pyrolysis of pulverized coal moves up. The increase of environmental temperature can raise the transient gas temperature around the particle and particle temperature, but the change of atmosphere has little effect.