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Abstract
Coal blending is a popular approach to improve combustion efficiency in a blast furnace for ironmaking as well as pf boiler for power generation. The synergistic effect of combustion behaviour of coal blends is often uncertain. This study aims to clarify the factors that may affect the synergistic phenomena with emphasis on modification of char characteristics during treatment under different gas environments. Coal samples were subjected to various thermal conditions using a DTF, double-layered FBR, TGA and the resulting char specimens were characterized using a XRD, NMR, SEM, BET, etc. Char oxidation rate with O2 and gasification rates with CO2 were measured using a TGA and FBR respectively.
The study showed that total weight loss and maximum decomposition rate of blends exceeded compared to the expected values, and confirmed the synergistic effect of coal blending on thermal decomposition rate and the nature as well as magnitude of pyrolysis products. The study confirmed that molecular structure of char is modified during pyrolysis due to interaction with the pyrolysis gases evolved from blend components, and impacts the char reactivity in accordance with known association with molecular structure.
During combustion at high temperatures in a DTF, coal blends did not show notable synergistic effect under both O2/N2 and O2/CO2 conditions. The reaction rate of pyrolyzed char samples of coal blends was lower compared to their weighted constituent coal chars. The observed trend was more apparent in case of larger difference of their volatile matters, and attributed to changes to their molecular structure, particularly the number of total attachments per cluster.
The study revealed that N2 substitution by CO2 increases coal conversion, and the increase was attributed to the influence of CO2-char gasification. The extent of coal conversion varied with coal type and influenced by modification of char physical structure during devolatilization. The NMR molecular data further confirmed that aromatic fraction of chars made with CO2 condition was higher and the number of aliphatic side chains was less compared to N2 condition. The study has implications for enhancing the scientific basis of coal blending practice for combustion phenomena.