Modeling Pyrolysis of Thermally Thick Biomass Particles in Co2/H2o Atmosphere at High Heating Rates

Abstract

Computational fluid dynamics (CFD) modeling, known for being time-efficient and cost-effective, has been widely employed in fuel thermal-chemical conversion processes. While the isothermal assumption has conventionally been employed for updating particle temperature, its applicability diminishes when dealing with thermally thick particles. As the initial step, pyrolysis plays a fundamental role and critically influences the subsequent conversion processes of solid fuel. A user-defined discrete phase model, integrated with a thermally thick model, is developed to simulate the pyrolysis of thermal-thick biomass particles in high concentrations of CO2 and H2O. Results demonstrate that the developed model can effectively predict the temperature gradient inside the biomass particles over residence time, while the temperature and devolatilization rate of large biomass particles are significantly overestimated when the isothermal model is coupled. As the pyrolysis temperature increases, a notable rise in the cumulative heat flux is observed from both convection and radiation to the particle surface. As a result, the particle heating rate is improved. The char gasification products become non-negligible when the pyrolysis temperature exceeds 1300 K. The char content inside biomass particles is completely consumed in 32 s in the case of 1750 K. Char gasification with H2O becomes more significant as the increase of H2O concentration. Compared to the gasification pathway of char-CO2 reaction, the char-H2O reaction rate is higher at lower temperatures. For large biomass particles, the volatile release and char gasification stages can be regarded as sequential processes.

Keywords: Volatile release, large biomass particle, thermal thick, char gasification