ReaxFF-based molecular dynamics simulation of the initial pyrolysis mechanism of lignite

Highlights

• The initial pyrolysis mechanism of lignite was studied by ReaxFF-MD simulations.

• The evolution trend of pyrolysis products was analyzed and the simulation results are consistent with the previous experimental results.

• Unlike bituminous coal and subbituminous coal, light tar is the main component of pyrolysis tar.

• The pyrolysis mechanism of lignite was further analyzed through the migration behavior of the the main elements.

Abstract

In this paper, a series of ReaxFF molecular dynamics (ReaxFF-MD) simulations were employed to explore the characteristics of pyrolysis products, transformation behavior of major elements, and thermal decomposition mechanism of lignite. The results suggest that the pyrolysis of lignite mainly undergoes decomposition of macromolecular structure and breakage of bridge bonds at lower temperatures. At relatively high temperatures, further cracking of tar fragments and condensation of aromatic structures occur. The relationship between the main pyrolysis gases and the structural characteristics of lignite has been studied. The results show that the formation of H₂O, CO₂, and C₂H₄ is associated with the typical structure of lignite i.e., hydroxyl groups, carboxyl groups, and methylene carbon, respectively. Unlike bituminous coal and subbituminous coal, light tar is the main component of pyrolysis tar, which is due to large amounts of monocyclic and bicyclic structures in lignite. The pyrolysis mechanism of lignite was further analyzed through element migration behavior, and the simulation results are consistent with the above reaction mechanism. This work provides in-depth insight into the initial reaction mechanism of lignite pyrolysis and may be useful for the industrialization of lignite clean utilization.

Introduction

Coal, which is the principal traditional source of energy, occupies a dominant position in world energy consumption, especially in China [1]. With the amount consumption of high rank coal, lignite has attracted considerable interest due to its vast reserves, low selling price, high chemical reactivity, and low pollution-forming impurities [2,3]. However, self-defection (e.g., low calorific value, high moisture content, and a high tendency of spontaneous combustion) makes it unfit for direct utilization [4]. Upgrading lignite to higher quality products by means of pyrolysis is considered to be a promising way to achieve economically efficient utilization of lignite [5,6]. Solid residues (coke or char) with high calorific value can be utilized for direct combustion or preparation of slurry fuels [7]. Pyrolysis gases contain large amounts of combustible gases, such as H₂, CO, and hydrocarbon gas [8]. Tars produced from lignite pyrolysis are suitable for liquid fuels and valuable chemicals [9].

Numerous efforts have been devoted to study the pyrolysis behavior of lignite. Liu et al. [10] investigated the interrelation between lignite structural characteristics and product distributions. They concluded that the formation of pyrolysis products is related to the breakage of chemical bonds and the decomposition of functional groups. Xu et al. [11] reported pyrolysate distributions and kinetics characteristics of Zhaotong lignite. They found the optimal reaction conditions for generation of tar and pyrolysis gas. Meng et al. [12] studied the morphology and chemical structure of low-temperature pyrolysis chars. They found that temperature has an apparent influence on product yields, surface morphology, and the evolution of chemical functional groups of chars. He et al. [13] analyzed the thermal degradation of Shengli lignite using TG–GC–MS. The results indicated that benzene series are the major gaseous products, and small molecular structures are the main components of aliphatic hydrocarbons. According to experimental studies, some progress has been made in understanding the characteristics of pyrolysis products and the chemical reaction mechanism of lignite pyrolysis. However, it is quite difficult to understand the complicated thermal decomposition reactions in depth through experimental methods alone because of the heterogeneity of lignite and the complexity of the pyrolysis process. Furthermore, vast free radicals are generated in an exceedingly short time, and these are difficult to detect in laboratories [14]. Undoubtedly, computational approaches would provide a promising platform for further researching the lignite pyrolysis mechanism.

Quantum chemistry (QC) modeling is applicable for studying chemical reactions with high accuracy. However, QC-based methods are computationally expensive and intensive, thus they have rarely been applied in models with more than 100 atoms [15]. Classical molecular dynamics (MD) has the ability to model molecular systems that contain more than 10,000 atoms, but it is not suitable for exploring the cleavage and production of bonds [16]. Fortunately, the Reactive Force Field (ReaxFF), introduced by van Duin et al., can address the chemical interactions of atoms and molecules [17]. ReaxFF-MD combines the advantages of MD and ReaxFF, which can be used to study the chemical reactions of large and complicated molecular systems. Additionally, ReaxFF-MD has been proven to maintain basically the calculation precision of QC but with much reduced computational costs [18,19].

In the last decade, ReaxFF-MD has been successfully used to simulate the combustion and pyrolysis characteristics of complex compounds, such as coal [20], biomass [21], oil shale kerogen [22] and char [23], with the goal of trying to understand the reaction process in depth. Zhan et al. [24] investigated the reaction mechanism of subbituminous coal via ReaxFF-MD. The calculation results indicated that the pyrolysis process is initiated by breaking unstable CC and CO bonds, which is followed by intramolecular hydrogen transformation. Castro-Marcano et al. [25] adopted ReaxFF-MD to study the combustion processes of coal char. They found that aromatic structures are more liable to oxidize and combust under fuel lean and higher temperature conditions. Zheng et al. [26] employed ReaxFF-MD to analyze the overall reaction stages for Liulin bituminous coal. The simulation results showed that the pyrolysis processes are roughly divided into four stages according to the breakage behavior of chemical bonds. These four stages are the activation stage, primary pyrolysis stage, secondary pyrolysis stage, and recombination dominated stage. Chen et al. [27] studied the pyrolysis and combustion behavior of biomass using ReaxFF-MD. The simulation results indicated that environment and temperature play a significant influence on the dissociation of chemical bonds and product distributions. Extensive studies of the chemical reaction mechanism using ReaxFF-MD simulations have been reported. Nevertheless, coal type is a critical factor that greatly affects the pyrolysis process. Compared with the structural features of bituminous coal and subbituminous coal, the molecular structure of lignite contains more oxygen-containing functional groups, aliphatic side chains, and single aromatic ring structures [28]. Published studies on the pyrolysis behavior of lignite are rare [29]. Further research is needed to better understand the complex pyrolysis process, especially the relationship between pyrolysis products and lignite structure.

The main purpose of this work is to explore the pyrolysis behavior of lignite using ReaxFF-MD simulations. The paper is organized as follows: First, the characteristics of pyrolysis products, especially tar and gas, are analyzed in detail. Second, the transformation behavior of carbon, hydrogen, and oxygen during the pyrolysis process are revealed. Finally, two typical temperatures (2000 K and 3000 K) are selected for an in-depth investigation of the pyrolysis reaction mechanism. The elucidation of these problems may provide more constructive information of high-efficiency clean technology for converting lignite.