Study on the mechanism of the characteristics of biochar changed by tobacco stalk macromolecules volatile-char interaction

Abstract

Tar and char, as products of the pyrolysis process, do not separate during the reaction in the gasifier. Char particles are continually surrounded by volatiles, and the interaction between volatile-char in the bed layer causes changes in the physicochemical structure of the char. Based on this, a dual-stage connected system was used in a self-constructed solid fuel thermal conversion experimental setup, with demineralized tobacco stalks as the sample. The interlayer interaction between volatile-char at different pyrolysis temperatures were studied, and the effects of these interactions on char were comprehensively analyzed in terms of char gasification reactivity and macromolecular volatile composition. Thermogravimetric analysis results indicate that the deposition of volatiles on the char surface reduces the char's gasification reactivity, leading to an increase in the activation energy required for the gasification reaction. The composition of the tar obtained from the pyrolysis of demineralized tobacco stalks was determined, and it was found to primarily consist of monoaromatic compounds, oxygenated compounds, nitrogenated compounds, aliphatic hydrocarbons, and polyaromatic hydrocarbons. As the pyrolysis temperature increases, the proportion of monoaromatic compounds rises, while the content of oxygenated compounds decreases. Polyaromatic hydrocarbons begin to appear at 600 ℃ and at this temperature, tar molecules further crack or undergo secondary reactions. The interaction has altered the pore structure of the char, leading to a reduction in its active specific surface area from 62.27 m²/g to 33.91 m²/g, while the total pore volume and micro-pore volume have decreased by 33.84 % and 44.26 %, respectively. These interactions involve the transfer of electron pairs from the more stable chemical bonds in the char to the structure of the tar, leading to bond cleavage, the formation of free radicals, and subsequent reactions with the functional groups on the char surface. During this process, the degree of graphitization of the char increases, and the content of oxygen-containing functional groups decreases from 24.19 % to 22.01 %, 21.37 %, and 19.05 %. Oxygen-containing functional groups serve as active sites for the char gasification reaction. A strong linear relationship exists among the char's active specific surface area, the reaction activation energy, and the content of oxygen-containing functional groups.