Numerical modeling and validation of hydrothermal liquefaction of a lignin particle for biocrude production

Abstract

Lignin liquefaction process under catalyst-free conditions in a temperature range from 573 K to 647 K is investigated with this mathematical model. Based on the theoretical understanding of the physical and chemical processes of the liquefaction process in subcritical temperatures, a comprehensive mathematical model for the decomposition of lignin by hydrolysis reaction pathway is developed on the results of a series of batch experiments. The model consists of four main sections. They are liquefaction of lignin particle, oily film, and inorganic (ash) layer formation behavior during the liquefaction, kinetic model to model further liquefaction process of initial products, and the layer model for the intraparticle processes. Hydrolysis of the lignin particle is modeled using the shrinking core concept. The formation of oily film and an inorganic layer around the lignin particle and their behavior is modeled considering water transport through layers, diffusion of products, and dissolution of products in water. Moreover, the layer model is used to obtain surface and center point temperatures of the particle using mass transfer. .The kinetic model consists of ten components and 21 reactions. . Variations of aromatic hydrocarbons and phenolic compounds are given significance. In the experimental study highest biocrude yield of 0.28 w/w0 is obtained at an operating temperature of 573 K. Aromatic hydrocarbons are reduced from 0.23 w/w0 to 0.145 w/w0 with the increase of operating temperature from 573 K to 623 K. For an increase of operating temperature from 573 K to 623 K, phenol shows an increase from 2.5 × 10−4 w/w0 to 3 × 10−3 w/w0. At 573 K and with a particle of radius 0.08 mm, oily film and ash layer show a maximum thickness of 2 × 10−12 m and 7.5 × 10−3 m, respectively. Both oily film and ash layer show a faster formation and faster dissolution in water with increasing operating temperature. Finally, the model's liquefaction results are analyzed and validated with the experimental data and the literature data, where it shows a reasonable agreement.