Thermochemical Conversion of Biomass Char under Carbon Dioxide Flux in a Thermally Supported Environment: Experimental and One-Dimensional Numerical Investigations

Abstract

The current work reports on the experimental and numerical investigations of biomass char conversion in a CO2-dominated environment. Having established 800 °C as the transition temperature for char conversion in terms of a threshold conversion rate, the mass loss rate of biomass char particles of different sizes (8–20 mm diameter) and two densities (220 and 350 kg/m3) are studied under a wide range of temperatures (800–1000 °C), CO2 concentrations (10%–100%), Reynolds numbers (0.01–100), and combinations thereof. The investigation broadly covers the operating conditions of practical systems. It is observed that increasing the temperature (12.5%) or reactant concentration (33%) or Reynolds number (10 times increase) reduces the conversion time scales by up to 50%. Contrary, a 22% increase in particle size and increase in particle density from 220 to 350 kg/m3 increases the conversion time by 25% and 65%, respectively. It is of interest and significance to note that temperature and reactant concentration in combination explicitly control the conversion regime (kinetic limit to diffusion limit) and as such can be used as explicit control parameters. A numerical model is employed to explore the intraparticle dynamics under different conversion regimes. The numerical model enables analysis of intraparticle temperature and reactant distribution, typically challenging through the experimental approach. Extending the analysis, a novel, first of its kind method to identify the conversion regime based on temperature and reactant concentration is hypothesized and validated. The methodology estimates the conversion regime with sufficient accuracy, with the power-law coefficient being within the 10% band.