Elsevier

Combustion and Flame

Volume 213, March 2020, Pages 63-86
Combustion and Flame

CFD simulation of the steam gasification of millimeter-sized char particle using thermally thick treatment

https://doi.org/10.1016/j.combustflame.2019.11.033Get rights and content

Abstract

A detailed char gasification model is developed using a multiphase Eulerian–Lagrangian algorithm and thermally thick treatment. The model is first validated by both gasification and combustion experiments of a millimeter-sized char particle. Temperature and mass loss histories as well as the particle morphology evolution correspond well with the existing results. Then the steam gasification of a 5 mm char particle is simulated and detailed physical and chemical conversion processes inside the particle are explored. During gasification, three distinct layers, i.e., the outer ash layer, the intermediate layer and the core layer, are identified based on the intraparticle porosity distribution. Simulation results show that the highest H2O and CO2 mass fractions locate in the ash layer, while the intermediate and core layers contain the highest H2 and CO mass fractions, respectively. Moreover, effects of several parameters are also explored. It is found that the Stefan flow caused by the mass transfer plays a key role in determining the diffusion and convection behavior during gasification. The strength of the Stefan flow in the intermediate layer appears to be two orders of magnitude smaller than that of the inflow and has an influence on the shifting from a kinetically-controlled mode to a diffusion-controlled mode. In addition, the char consumption rate in the intermediate layer increases with an increase in steam mass fraction, gasification temperature and inflow velocity while it decreases with increasing particle diameter. Meanwhile, the char consumption rate caused by CO2 is much smaller than that due to steam.

Introduction

Char gasification is an important stage during the thermochemical conversion of solid fuels (e.g., coal and biomass). To improve the gasification efficiency, steam or CO2 are usually added into the gasifying agent in industrial applications like fluidized-bed and entrained-flow reactors [1,2]. Taking biomass for example, the amount of fixed carbon usually lies in the range 10–20% after pyrolysis, which makes a large contribution to the synthetic gas [3]. Char normally has a porous structure and consists primarily of carbon, ash and some minor and trace elements like H, O, N and S [4]. Due to the fact that the pores inside a char particle varies significantly by the material type and pyrolysis conditions, char gasification or oxidation is a highly complex process [5], [6], [7], [8], [9]. Besides, in real applications of fluidized-bed reactors, the fuel particle size usually lies in the range of several millimeters to centimeters, making the gasification mechanism of large fuel particles quite different from that of very small particles that used in entrained-flow reactors, whose diameter normally lies in the range of several to hundred microns.

Despite the large and porous characteristics, the thermochemical conversion of char particles is often simulated with zero/one-dimensional models for simplification purpose, among which sharp interface models and finite reaction zone models are frequently used [10]. For the sharp interface models, char reactions are assumed to take place only at the surface of the shrinking core [11], [12], [13]. While for the finite reaction zone models, char consumption happens in a finite zone near the surface of the shrinking core [14]. Higuera [15] studied the gasification and combustion of a coal char particle by using the particle-resolved CFD method to simulate the char consumption process. It was found that the presence of CO2 in the gasifying agent can greatly affected the gasification process. However, particle porosity was not taken into account in his model. Schulze et al. [16] considered the porosity and gas species transport inside the particle. It was shown that the gas production rate for char gasification in an entrained-flow reactor was well predicted. However, their model was only suitable for very small particles since a uniform temperature was assumed within the particle. Nikrityuk and Meyer [5] investigated the gasification of a char particle by considering both porosity evolution and temperature gradient inside the particle. Different oxidation regimes were observed for small and large particles. Nevertheless, their model still needs to be improved because the gas convection and radiation were ignored, which could be very important for high-temperature gasification.

One of the drawbacks of zero/one-dimensional models is that particle morphology change cannot be accurately resolved. Existing studies showed that the change in particle size and morphology could have a significant impact on the particle trajectory and its way out of the fluidized-bed reactor [17,18]. Furthermore, these simplified models are not easy to take Stefan flow, i.e., the diffusion and convection caused by mass transfer inside the particle, into account, which actually plays an important role in the thermochemical conversion of char particles [19,20]. Therefore, high fidelity simulations are needed to explore the detailed evolution of a single char particle during conversion.

Lee et al. [21] used a surface-resolved method to study the transient combustion behavior of a single char particle. It was found that the char oxidation process changed from a fast burning mode to a heterogeneous kinetically-controlled mode when the Reynolds number increased from 1 to 10. However, porosity evolution inside the particle was not taken into account. Kestel et al. [22] also chose a surface-resolved approach to research the steam gasification behavior of a char particle. They claimed that increasing steam mass fraction and Reynolds number greatly changed the CO2 distribution around the particle. But, particle porosity was still not considered. Lin et al. [23] established a discrete model to investigate the external diffusion-controlled gasification of a coal char particle. It was shown that there was a close relationship between the char gasification rate and local porosity, which was different from that predicted by one-dimensional models. However, gas species evolution was not included in their simulations. More recently, Xue et al. [24] modeled the pseudo-steady-state char oxidation process via pore-resolved CFD simulation. They reported that the flame structure around the particle had a close relationship with the particle size and pore structure. Besides, the oxidant diffusion near the reaction front seemed to be highly affected by Stefan flow. Beckmann et al. [25] established a time-dependent two-dimensional char combustion model with two types of governing equations to simulate the internal and external flow fields. Although the Stefan flow was not fully taken into account, they still found that the diffusion of gas species inside the particle had a great effect on the chemical reaction process. Richter et al. [26] investigated the gasification of a porous char particle by using a pore-resolved CFD method, in which the porous particle was represented with a monodisperse particle cluster. Dierich et al. [27] proposed a novel CFD-based method which could track the chemically reacting particle interface and particle porosity. They found that the Stefan flow could change the hydrodynamic boundary layer around the particle and significantly affected the mass transfer.

The works mentioned above reveal that, pore structure evolution and gas species diffusion/convection are highly coupled with the chemical reactions during char gasification. Although the particle-resolved CFD method can take all these effects into account, it is too time-consuming to study the thermochemical conversion of large fuel particles. Meanwhile, the previous works [28,29] indicated that the porous structure inside the char particle might be very complex depending on the raw material type and specific thermochemical processing method. For real cases, the diameter of these pores can range from several nanometers to micrometers [30,31]. Therefore, resolving all the pore structures and their connections by particle-resolved CFD method is still a big challenge.

In this work, we focus on the coupled effect of pore structure evolution, gas species diffusion/convection and chemical reactions of a char particle during gasification by developing a simplified gasification model compared with particle-resolved CFD method. The steam gasification of a millimeter-sized char particle is chosen as the tested case which involves typical homogeneous and heterogeneous chemical reactions. The gasification model is established based on a recently developed pyrolysis model in our group [32]. In Section 2, we briefly introduce the extension of the original pyrolysis model by adding chemical reaction submodels. In Section 3, simulation setup is presented. Then, validation of the integrated model is carried out in Section 4.1. Moreover, the char steam gasification behavior and effect of various parameters (e.g., inflow temperature, inflow velocity, steam mass fraction and particle diameter) are explored in the following subsections. Finally, conclusions are drawn in Section 5.

Section snippets

Mathematical model

The integrated CFD model for char gasification/combustion is established based on a well-validated pyrolysis algorithm developed in our group [32]. The main idea of the algorithm is to discretize the whole particle into a cluster of virtual particles, which is a commonly used strategy to deal with a porous char particle [26]. These virtual particles keep stationary and shrink with the ongoing of chemical reactions. Then an existing Eulerian–Lagrangian multiphase flow algorithm developed for the

Computational framework

The conservation equations listed in Table 1 and the chemical reactions R1-R7 are implemented in the framework of OpenFOAM [36]. The thermophysical properties used in the equations are also determined by the thermophysical model in OpenFOAM. For example, the mass diffusion coefficient for species is calculated by Deff = κ/(ρgcpLe) where Le is the Lewis number and assumed to be unity. A second-order finite volume method is utilized to solve these equations. An implicit Euler scheme is used as

Results and discussion

In this section, to make a comprehensive validation of the established model, the algorithm is first evaluated by both the combustion and gasification experiments of millimeter-sized char particle. Then the steam gasification of a single char particle is simulated to systematically explore the gasification mechanism of fuel particle in thermally thick regime. Discussions about the gasification behavior and the impact of several influential factors including gasification temperature, inflow

Conclusions

A detailed char gasification model is established based on an Eulerian–Lagrangian algorithm and thermally thick treatment. Chemical reactions, structure evolution and flow fields both inside and outside the particle are all resolved. The integrated model is comprehensively validated by both char combustion (complete oxidation) and gasification (partial oxidation) experiments. A good agreement is achieved between the simulation results and the experimental data.

The steam gasification of a 5 mm

Declaration of Competing Interest

None.

Acknowledgments

The present work is financially supported by the National Natural Science Foundation of China (Grant nos. 91634103, 51876191, and 11632016) and the China Postdoctoral Science Foundation (Grant no. 2018M632469). We would also like to thank Henrik Ström at Chalmers University of Technology for the constructive discussions.

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