Effects of physical structure of high heating-rate chars on combustion characteristics

The combustion performance of high heating-rate chars is observably affected by the evolution of its physical structure during devolatilization. An effort has been devoted to building understandable relationships between the heating rates of different rank coals, char structure and subsequently detailed combustion characteristics. In this work, the physical structure parameters (internal surface area, porosity, and swelling ratio) of bituminous coals and lignites are qualitatively summarized as a function of heating rates ( > 10(4) K/s). Meanwhile, the physical structure effects on char combustion characteristics are investigated separately by using the self-developed kinetics model, where the transient char properties, ash distribution (ash film, ash dilution, and ash vaporization) and char combustion performances are taken into account. The relative importance of the three structure parameters on char burnout is also performed. The internal surface area of high heating-rate chars exerts positive effects on combustion with higher burning temperature and shorter burnout time, whereas the swelling ratio and porosity exert complex effects. The combustion rate initially increases then goes down with char conversion, owing to the integrated effects of formation and destruction of internal surface area. The importance of physical structure parameters on char combustion follows the order: internal surface area > swelling ratio > porosity. These results provide in-depth insight into the effects of physical structure during char combustion and suggest higher heating rates are favorable in industrial boiler applications.