Efeito catalítico da zeólita HZSM-5 e do ácido nióbico HY-340 para a pirólise rápida de lignina Kraft torrificada

Abstract

Kraft lignin is an industrial waste of pulp and paper production that can be used as raw material to obtain chemical compounds of high industrial interest through thermochemical conversion, such as catalytic pyrolysis. In order to improve the characteristics of lignin in the pyrolysis process, torrefaction was used as thermal pretreatment of lignin. The present study aimed to evaluate the influence of catalysts (zeolite HZSM-5 and niobium acid HY-340) on the deoxygenation of constituents in the vapor phase of catalytic pyrolysis of industrial Kraft raw and torrified (493, 533 and 573 K) lignin to produce aromatic hydrocarbons and determine the kinetic and thermodynamic parameters of the volatilization stage of these samples. The physical, chemical and thermal characterization was carried out, as well as the triplet kinetics of the raw and torrified lignins. Analytical pyrolysis (723, 823 and 923 K) of raw lignin was performed without and with acid catalysts (HZSM-5 and HY-340) in different catalyst/biomass mass ratios (1:1, 5:1 and 10:1) to identify the pyrolysis products. A 3k factorial design was performed considering the torrified lignins to evaluate the effects of the independent variables (pyrolysis temperature and catalyst/biomass ratio) on the production of aromatic hydrocarbons. Torrified lignin at 573 K was used in fast pyrolysis (823 K) in fluidized bed reactor and produced bio-oil was characterized. The results of the physical, chemical and thermal characterization of lignins demonstrate compatible characteristics for the pyrolysis process, for example, high content of volatile materials and high calorific value. The results of the master plots showed that the second-order reaction model (N2) represents the decomposition kinetics of raw and torrified lignin (493 and 533 K), while the three-dimensional Ginstling–Brounshtein model (D4) represented the kinetics of decomposition the torrified lignin at 573 K. Still according to the master plots, the activation energy (Ea) and the Arrhenius pre-exponential factor (A) for the in-nature lignin were 106.05 kJ mol-1 and 3.64×106 s-1, respectively. For torrified lignin at 493 K the Ea was 130.91 kJ mol-1 and the A was 3.10×108 s-1. For torrified lignin at 533 K the Ea was 137.20 kJ mol-1 and the A was 4.10×108 s-1. For torrified lignin at 573 K the Ea was 176.81 kJ mol-1 and the A was 4.64×108 s-1. The analytical pyrolysis tests showed that the pyrolytic vapors are formed by oxygenated compounds, mainly phenols, with a maximum of 74% at 923 K for the raw lignin. Catalytic pyrolysis provided the deoxygenation of the vapors, reaching a maximum value of 57.84% obtained in the experimental condition of 10:1 at 923 K with HZSM-5, as well as 87.24% and 86.75% with HY-340 in the catalyst/biomass ratio of 10:1 at 823 and 923 K, respectively. The results of factorial designs showed the influence of independent variables on the formation of aromatic hydrocarbons. It was observed that the maximum catalyst/biomass ratio provided the highest percentage of aromatic hydrocarbons. The maximum percentages with HZSM-5 were 41.95, 53.72 and 92.84% and with HY-340 were 29.29, 50.02 and 90.02% for the torrified lignins to 493, 533 and 573 K, respectively. Finally, the bio-oil analysis proved that, for its use as fuel, it is necessary to perform upgrading due to the high acidity and the presence of oxygenated compounds. It was observed that the lignin is a potential source for compounds of industrial interest and that the use of HZSM-5 and HY-340 catalysts provided deoxygenation and increased the aromatic hydrocarbons selectivity.