Study On Combustion Characteristics Of Pulverized Coal In O₂/CO₂ Atmosphere

In O₂/CO₂ combustion technology, CO₂ separation and SO₂ treatment become easier and NO_x emission is reduced. O₂/CO₂ combustion technology is one of the new clean coal combustion technology that can control pollutant emission. The research work carried out on the O₂/CO₂ combustion technology has great significance.In this paper, experimental studies were carried out on the flame formation process, flame structure, flame length, and NO emission characteristics of different sizes of pulverized coal in O₂/CO₂ atmosphere.High speed camera was used to record the coal ignition and flame propagation process for pulverized coals with different particle sizes in O₂/CO₂ atmosphere. Compared with large particle size of pulverized coal, the flame structure of small particle size coal is more stable and brighter. The flame propagation distance increased with the mass ratio of small size of pulverized coal.NO emission characteristics of different particle sizes in O₂/CO₂ atmosphere were studied with the fixed-bed reactor experiment. Compared with the NO emission in the air atmosphere, the NO emission was lower in O₂/CO₂ atmosphere. Compare with the large size pulverized coal, the conversion rate of coal-N to NO is lower when the small size mass mixing ratio is 1:1. The total NO emission increases with the temperature increasing.In order to further understand the influence of particle size on combustion characteristics and pollutant emission, coal combustion in drop-tube furnace was carried out. The NO emissions in furnace centerline line in the O₂/CO₂ atmosphere basically present single maximum when the furnace temperature is 900℃ and the amount of NO emission decreases with the particle size of pulverized coal. The NO emissions in furnace centerline line in the O₂/CO₂ atmosphere basically present “M” type when the furnace temperature is 1100 ℃. The volatile nitrogen is released and then reacts with the ambient oxygen immediately. Therefore, NO emissions reach the first maximum rapidly in the initial devolatilization processes. The heterogeneous burning stage is mostly attributed to the second maximum of NO emissions. The amount of NO emission for the small size pulverized coal shows higher than that of large size of pulverized coal. The amount of NO emission of two-size pulverized coal between that of the two single size of pulverized coal.The structural parameters at 1100℃ were analysised from coal infrared spectra. The infrared spectra were divided into the peak fitting treatment. The resualts show that chemical structure such as C-H and C-O function groups would be released combined with volatile matters emission during pyrolysis or combustion process. The CH₂/CH₃（estimate the length and degree of branching aliphatic side chains） of coal combustion residue decreases with the particle size reducing. The concentration of C=O functional groups have a reduction effect on the NO formation. The coal aromaticity of coal combustion residues increases with the decreasing of coal particle sizes.