High-temperature pyrolysis modeling of a thermally thick biomass particle based on an MD-derived tar cracking model

Tao Chen,Bodil S.A Karlsson,Tian Li,Xiaoke Ku,Henrik Ström,Jonas Sjöblom

doi:10.1016/j.cej.2020.127923

Abstract

• New tar cracking model developed with ReaxFF molecular dynamics method. • Thermally thick modelling based on multistep kinetic scheme and MD tar cracking model. • Particle-resolved pyrolysis simulation of a single biomass particle over 800 °C. • Tar evolution inside and outside porous biomass particle. Biomass pyrolysis in the thermally thick regime is an important thermochemical phenomenon encountered in many different types of reactors. In this paper, a particle-resolved algorithm for thermally thick biomass particle during high-temperature pyrolysis is established by using reactive molecular dynamics (MD) and computational fluid dynamics (CFD) methods. The temperature gradient inside the particle is computed with a heat transfer equation, and a multiphase flow algorithm is used to simulate the advection/diffusion both inside and outside the particle. Besides, to simulate the influence of intraparticle temperature gradient on the primary pyrolysis yields, a multistep kinetic scheme is used. Moreover, a new tar decomposition model is developed by reactive molecular dynamic simulations where every primary tar species in the multistep kinetic scheme cracks under high temperature. The integrated pyrolysis model is evaluated against a pyrolysis experiment of a centimeter-sized beech wood particle at 800–1050 °C. The simulation results show a remarkable improvement in both light gas and tar yields compared with a simplified tar cracking model. Meanwhile, the MD tar cracking model also gives a more reasonable prediction of the species yield history, which avoids the appearance of unrealistically high peak values at the initial stage of pyrolysis. Based on the new results, the different roles of secondary tar cracking inside and outside the particle are studied. Finally, the model is also used to assess the influence of tar residence time and several other factors impacting the pyrolysis.

Highlights

Biomass combustion/gasification in packed or fluidized bed reactors are important technological applications in the efforts to realize a renewable energy system [1,2]
In the pyrolysis of thermally thick biomass particles, light gas and tar yields are highly dependent on the intraparticle heating process
In our previous work [42], we found that the original kinetic constants result in the appearance of an unrealistic second peak in the methane production rate under high-temperature conditions

Summary

Introduction

Biomass combustion/gasification in packed or fluidized bed reactors are important technological applications in the efforts to realize a renewable energy system [1,2]. During incomplete combustion in packed-bed or fluidized-bed reactors, tar generation leads to the for mation of PAHs (C6H6, C7H8, C10H8 etc.), which, under appropriate conditions, will further convert to soot that cause air pollution [5]. Catalytic cracking of tar is a commonly-used method for the gas cleaning during gasification. The organic tar compounds, how ever, can condense on the catalyst surface and form coke, which significantly reduces the catalytic activity of tar cracking in the gas cleaning process of fluidized-bed gasification [7]

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