Elsevier

Fuel

Volume 285, 1 February 2021, 119067
Fuel

Full Length Article
Modeling and numerical simulations of lignite char gasification with CO2: The effect of gasification parameters on internal transport phenomena

https://doi.org/10.1016/j.fuel.2020.119067Get rights and content

Highlights

  • Char internal structure change is interpreted by an experimental-based function, f(X).

  • The structure of f(X) function is justified as the sum of two Gaussians.

  • The contribution of pressure term in diffusion equation is found negligible.

Abstract

The purpose of this study is to develop an experimental data-based char particle gasification model in order to assess the effects of particle size, gasification temperature and char generation heating rate on global gasification parameters. Also, the effect of initial porosity is observed by performing parametrical numerical simulations. A continuum-based model is used to solve the gasification inside a char particle and within the external boundary layer. The intrinsic rate of CO2 gasification reaction is computed according to Langmuir-Hinshelwood (LH) mechanism. External mass transfer is modeled by Stefan-Maxwell relations, and Cylindrical Pore Interpolation Model (CPIM) is used for intra-particle molecular diffusion. In the model, all the effects due to particle internal structure changes are represented by a global conversion function, f(X) which is computed from local reaction rate values. In this study, f(X) is deduced from experimental results instead of phenomenological models almost impossible to validate. The best reproduction of the experimental gasification results is obtained for the function f(X) postulated as a summation of two Gaussian functions which represent the char particle random pore structures and their dynamics during gasification. Comparative simulation results show that the Gaussian for low conversion interval is shifted to even lower conversion values for higher gasification temperature and higher initial porosity. Thereby, the Gaussian function for low conversion rates (large particle sizes) is interpreted as representative of the diffusion-limited gasification regime in conjunction with the network of macropores and molecular diffusion rates. The modification of the pore structure due to char generation heating rates causes a shift of the second Gaussian towards higher conversion rates. It is therefore postulated that the second Gaussian function corresponds to the boundary layer diffusion-controlled regime related to available outer surface area of the particle.

Introduction

Coal is the largest source of global energy production, accounting for over 40% of global power generation. A significant portion of the CO2 emissions originates from the electricity generation and the commercial heat generation out of coal. Despite tightening limits on CO2 emissions and the expansion of renewable technologies, coal production is still increasing [1], [2]. Low-rank/high-ash coals are particularly responsible for particulate matter emissions. More efficient energy transition technologies with low carbon emissions are necessary. Coal gasification offers a cleaner and more effective conversion of low-rank/high-ash coals. Moreover, synthesis gas which is a product of gasification can directly be used for clean energy production processes. Low-rank coals are more reactive since they have a higher amount of mineral matter and a larger average pore size; which makes them a good candidate for gasification [3]. There is a substantial amount of both experimental and modeling work into the understanding of the reaction chemistry and the effect of structural evolution of coal particles on their gasification and combustion. In gasification studies, experimental work is conducted using equipment such as small size reactor beds, entrained flow type (drop tube) reactors, and thermogravimetric analyzers. Recent research has focused on the gasification and combustion under different temperatures and ambient gas composition [4]. Gasification (with CO2) and the combustion (with O2) rates at different conditions are observed with recent measurement methods [5]. The variation of gasification rate with the pore structure [6], and with the reactor pressure [7] were studied in entrained flow reactors. Simultaneous gasification with steam and CO2 has drawn special attention recently [8], [9]. In the context of steam/CO2 gasification, influence of the coal char generation process [10], [11], the influence of porosity [12], the volatile-char interactions [13], and the pressure [14], [15] are investigated.

The gasification processes are categorized into three regimes by Smith [16] based on the governing physical and chemical mechanisms which determine the gasification rate. In a first regime, the chemistry solely controls gasification reactions which proceed uniformly on the particle inner surfaces. Larger particle sizes or higher reaction rates result in internal pore-diffusion limitations. In the second regime, the rate is controlled by the combined effects of pore diffusion resistance and surface gasification reactions. With further increase of the reaction rates the external boundary layer diffusion control of species concentration gradients sets in [16]. In this third regime, the gasification may be considered to occur at the outer surface of the particle. Here the external boundary layer, namely the Knudsen boundary layer, refer to the outer gas shell in which the molecular diffusion dominates the convective mass transfer. The gasification regimes are determined by the particle properties such as particle size, particle porosity, pore structure, active site density etc., and the gasification atmosphere.

The change in particle properties and the internal pore network are investigated experimentally in many studies for different gasification conditions [10], [17], [18], [19], [20], [21]. Hungwe et al. studied high and low ash particles for CO2 gasification and observed the change in porosity using Scanning Electron Microscopy (SEM) at different conversions [20]. They reported that surface area increases significantly up to 50% conversion for both chars and for high ash particle (nearly 20% ash), they observed the maximum area change between 50 and 75 % conversions. The surface area decreases in high ash particles because of pore coalescence and carbon accumulation within the pores in addition to the formation of macropores having lower surface areas due to the mesoporous structure. Nie et al. reported different pore shapes for various coal samples [17]. They concluded that different pore structures affect the adsorption and transport properties. For high volatile matter-containing coals (nearly 33%), micropores increase while mesopores becomes significantly smaller. Jayaraman et al. reported that the char preparation heating rate affects the pore structure by changing the volatilization rate [10].

Experimental procedures only provide global measurements of the conversion, the change in composition in the gas phase, and observations of changing coal particle structure. On the other hand, numerical modeling can be used for the assessment of gasification and, similarly, the combustion processes within the coal particle and the boundary layer. Numerical models enable understanding the time variation of the internal porous structure and the gas composition inside the particle. Two commonly used approaches for simulation are the continuum and the discrete modeling.

The discrete modeling approach, which relies on the percolation theory, mainly accounts for the randomness of the pore structure and the heterogeneity of the particle composition. Discrete models are suited for the fragmentation analysis to predict the changes in the morphology during coal particle combustion and gasification. In the diffusion limited regime, percolation modeling may show the fragmentation behavior with a variation of the porosity [22] and the local Thiele modulus [23]. The transient increase in burning rate due to the morphology change can be analyzed using unsteady simulation [24]. The discrete models also enable the use of sub-models for the effect of ash mobility, ash diffusivity [25], the influence of swelling mechanism on gasification rates [26] and the ash agglomeration mechanism below ash fusion temperature [27].

The continuum models address the physics of the external flow with the boundary layer, with or without taking into account the intraparticle mass transfer and gasification [28]. One-dimensional models without intraparticle mass transfer resolve the boundary layer with the surface and the gas-phase chemistry at steady [29] or transient conditions [30]. Recent work also provides highly resolved simulations of the flow around burning and gasifying coal particles [31], [32].

The models, which consider the gas–solid reactions in the particle and the external boundary layer, solve spherically symmetric, one-dimensional reaction–diffusion equations. The one-dimensional reaction–diffusion model can be used for both the pore diffusion and the reaction limited regimes. The effect of pore structure evolution on the heterogeneous reaction rate is a main challenge in the continuum modeling approach. This problem can be solved by implementing simple models such as the random capillary model [33], the random pore model [34], or the adaptive random pore model [35], etc. Other sub-modeling approaches for the annealing, the peripheral fragmentation, and the ash agglomeration are alternatives to improve continuum models [35]. Additionally, a chemical looping mechanism could be assessed using a coupled one-dimensional quasi-steady model [36], [37].

With the continuum approach, Dai et al. used experimental conversion rate results to avoid submodels for the physical phenomena at a micro-scale [38]. This approach represents the local (spatially) dependence of the heterogeneous reactions on the variations of the particle inner structure by combining all non-modeled phenomena in a global function derived from experiments.

The present work focuses on CO2 gasification of lignite char particles. The internal structure change function, f(X) is obtained from the experimental conversion vs. reaction rate data reported by Jayaraman and Gökalp [11] and then, it is modelled by the summation of two Gaussians. Starting from an initial guess for f(X) based on the experimental conversion data, optimization is performed by minimizing the differences between the experimental and modeled conversion vs time data. Following a similar approach as in [44], instantaneous quasi-steady gasification rate is calculated from Langmuir-Hinshelwood kinetics. The species conservation, the species diffusion and the momentum conservation equations are solved for both the inner porous field and the outer Knudsen boundary layer. The intraparticle mass transfer is modeled according to the Cylindrical Pore Interpolation Model (CPIM). The external mass transfer is modeled using Stefan-Maxwell relations. The effects on the particle internal structure changes of several parameters are investigated. They are the char generation (pyrolysis) heating rate, the gasification temperature, the particle size, and the initial particle porosity.

Section snippets

Experimental

The material used in this study is a high ash content Turkish lignite from the Thrace region in Turkey. The experimental data are taken from Jayaraman and Gökalp [11]. Ultimate and proximate analyses of the coal are given in Table 1 below.

In the reported work, the raw lignite particles were pre-dried and sieved to two mean diameters, 800 µm and 3 mm. The char particles were produced by pyrolysis in Ar atmosphere at the temperature used for gasification, i.e. 950 °C. During the pyrolysis

Gasification with CO2

Radial variations of CO fluxes, CO mole fractions, and intraparticle pressure with respect to conversion rate are given in this section for the base case; 3 mm char particle size prepared at 100 K/min heating rate and gasified at 900 °C.

It should be noted that in all figures, dimensionless radius equals to 1 means the particle surface. The values below 1 correspond to the inner radial distance from the center of the particle while dimensionless radius values higher than 1 correspond to the

Conclusions

A continuum-based model for char particle gasification was used to investigate the effect of char generation heating rate, gasification temperature, particle size and initial particle porosity. One dimensional spherically symmetric mass and momentum conservation equations for porous particle and external Knudsen layer were solved. The model was constructed on the idea of putting all unknown phenomena, which are related to the morphology change of solid particle, into a function of conversion,

CRediT authorship contribution statement

Mehmet Karaca: Investigation, Methodology, Writing - review & editing. Deniz Kaya: Formal analysis, Investigation, Software, Visualization, Writing - original draft. Ahmet Yozgatligil: Funding acquisition, Supervision. Iskender Gokalp: Funding acquisition, Investigation, Supervision, Validation, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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