Influence of cooling rate on structure and combustion reactivity of char during char preparation process

Abstract

Different chars with various cooling rates were obtained in the novel concentrating photothermal reactor to investigate the influence of cooling rate on the structure and combustion reactivity of char. The chars were prepared at 900°C with holding time of 10s, 30s and 120s, respectively. Subsequently, the cold (ex-situ) chars were prepared from the hot (in-situ) chars by controlling different cooling rates of 180°C/s (fast cooling rate), 30°C/s (sub-fast cooling rate), 6.8°C/s (moderate cooling rate) and 1.3°C/s (slow cooling rate). The ex-situ chars were then employed for the combustion in 30%O2/70%CO2 atmosphere at 900°C for 180s. For comparison, the in-situ char was conducted by instantaneously switching the reaction atmosphere. For the chars with holding time of 10s, the mass fraction of in-situ char was higher than the char yield of ex-situ char, and the char yield was decreased with the decrease of cooling rate. The char yield with slow cooling rate was decreased by 11.16% than that with fast cooling rate. The maximum and average mass loss rate of char in O2/CO2 was decreased as the cooling rate decreased. Reduced influence of cooling rate on the aromatic ring systems of char with holding time of 30s and its reactivity was displayed, while almost no difference of those with holding time of 120s was illustrated. The reactivity was closely related to the chemical structure of the char, but no positive correlation was found between the combustion reactivity and the specific surface area of char. The residues proportion of volatile components was the decisive factor of char reactivity.