Correlation between pores and chemical structures for lignin pyrolysis char

Understanding the mechanism of char structure evolution during lignin pyrolysis can provide a reference for developing and optimizing lignin utilization. In this paper, lignin was pyrolyzed at 700 and 750 ℃ for different residence times to prepare chars with different chemical and pore structures. The pore structures are related to the degree of char crystallinity. The N₂ adsorption specific surface area of the char (S_N2) increases from 3.2 to 187.6 m²/g with L_a increasing from 2.02 to 2.87 Å. The CO₂ adsorption specific surface area (S_CO2) of the 0.45–0.68 nm pores increases from 2.17 to 82.63 m²/g with d₀₀₂ decreasing from 3.93 to 3.76 Å, while the total S_CO2 increases linearly with the S_CO2 of the 0.45–0.68 nm pores (S_0.45-0.68). In order to account for the influence of microcrystal size, the correlation equation previously proposed for the S_N2 in coal pyrolysis and coal char gasification was modified and the new equation is S_N2=N∙L_a-c∙A_Other/A_Total∙exp-n∙A_G/A_Other, where c is the minimum L_a that can form the N₂ adsorption. The modified equation is applicable to both lignin and coal char, and the R² for the coal char has been improved from 0.68 to 0.86. The specific surface area of micropores increases with the increasing aromatic C–H and C*-O-C(−H) structure content. A correlation of the S_0.45-0.68 as a function of d₀₀₂, aromatic C–H and C*-O-C(−H) content was proposed, S_0.45-0.68=4.04×10¹³∙C_Car-H+2.06×10¹³∙C_{C^∗-O-C(-H)}∙exp-6.66∙d₀₀₂.