Effects of FGR and Changeable Combustion Parameters and Coal/Char Properties on the Formation of Ultrafine PMs during Pulverized Coal Char Combustion under Various O₂/N₂ and O₂/CO₂ Atmospheres

ABSTRACT

Aiming at the wide adoption of flue gas recirculation (FGR) in coal power plants for NOx removal and CO₂ capture, and the imperfect removal of particulate matters (PMs), especially ultrafine PMs which constitute most of the particles discharged into atmosphere, consequently causing atmospheric haze and diseases, the effects of FGR and combustion parameters, and coal/char properties on PMs formation were therefore studied under different O₂/N₂ and O₂/CO₂ atmospheres using a self-developed kinetic model. Modeling results showed that the inclusion vaporization rate and ratio had a dominant effect on PMs number density and size respectively. At the same level of oxygen content, smaller ultrafine PMs were generated under O₂/CO₂ than those generated under O₂/N₂. Increasing oxygen content facilitated the formation of larger and fewer PMs. The PMs size in 27O₂/73N₂ shows the biggest, followed by that in 36O₂/64CO₂, 27O₂/73CO₂, 21O₂/79N₂, and 21O₂/79CO₂ in turn. Compared to without FGR, FGR decreased the size of ultrafine PMs and increased the number, thus hindering PMs removal. With increasing FGR ratio, which had the most significant effect, the ultrafine PMs size decreased due to dilution effect. With increasing dust removal efficiency, the ultrafine PMs size decreased initially and then increased, peaking close to 100% of dust removal efficiency. The effect of recirculated particle size was negligible. Sensitivity analyses revealed that furnace gas temperature, inclusion size, and char density were the most influential factors on inclusion vaporization and PMs formation. The effects of CO/CO₂ ratio, char load, coal ash content, and inclusion content were positive, while the effects of char density and ash content, inclusion size, and moisture were negative. High furnace temperature increased vaporization, but significantly decreased the PMs size. The number density of ultrafine PMs followed mass conservation. These results provide meaningful guidelines for practical applications.

KEYWORDS:

• Particulate matter
• flue gas recirculation
• char combustion
• oxy-fuel
• kinetics

Acknowledgments

The present work was supported by the National Natural Science Foundation of China under Grant 51776161, the National Key Research and Development Program of China under Grant 2016YFC0801904, and Youth talent promotion plan of the Xi’an Association for science and technology. Special thanks to Christopher R. Shaddix for his assistance with the code at Sandia National Laboratories.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China [51776161];National Key Research and Development Program of China [2016YFC0801904];Youth talent promotion plan of the Xi’an Association for science and technology.