Investigation of mechanism and kinetics of non-isothermal low temperature pyrolysis of perhydrous bituminous coal by in-situ FTIR

Abstract

Perhydrous coal has advantages as a fuel source and raw material for hydrocarbon production over other coal with similar rank. Good understanding of thermal characterization of perhydrous coal can provide important information for control of the pollutants release and enhancement of the conversion rate of coal during its utilization. A perhydrous bituminous coal from Dingji Coal Mine, China was selected for the investigation of the non-isothermal pyrolysis process below 500 °C in N₂ flow by in-situ transmission FIIR. Evolutions of the main functional groups in coal, including aliphatic groups, aromatic CC and CH, carbonyl, CO and hydroxyl could be divided into several stages. The thermal characterization of each stage was discussed. The aliphatic groups and aromatic CC content decreased slightly with the volatilization of small molecules below 400 °C, while the amount of aromatic CH increased with the loss of substituent groups. The conversion of CO to CO contributes to the slight increase in the CO amount below 370 °C, and the decrease of hydroxyl was attributed to loss of water and the condensation and elimination of hydroxyl groups. The amount of all functional groups except for the aromatic CH declined rapidly with the cracking of CC bonds after 400 °C. The kinetic characterization of pyrolysis stages for three functional groups, including aliphatic groups, CO and hydroxyl, was studied with modified integral method. Activation energy of pyrolysis stages of the three functional groups increased with rising temperature. Kinetics for the different stages of functional groups were fitted with different kinetic models. The initial loss of aliphatic groups and CO was controlled by diffusion. Then, the reaction of the three functional groups fitted on the first-order or third-order reaction models. The results obtained by in-situ FTIR revealed more detail information about thermal characterization and kinetic characterization and can be expected to lead to good understanding of pyrolysis of a perhydrous coal.

Introduction

Perhydrous coal is a type of coal that is enriched with hydrogen (4.6–5.6, wt.%) or has a higher atomic ratio of hydrogen and carbon (H/C) compared to other types of coal of similar rank [1]. The hydrogen enrichment gives rise to the modifications in the macromolecular structure of the coal, thus affecting the properties and behavior of the coal [2], [3], such as high calorific value [4], higher volatile matter content and enhancement of oil/tar potential [2]. Perhydrous coal was used as a good fuel source and applied in the field of hydrocarbon generation [5].

However, the complexity of coal composition and structure limits its utilization, and consumption of with large quantities of coal releases a high amount of pollutants such as SO₂, CO₂ and organic matter [6]. Thermal decomposition may be the initial step during the main processes of coal utilization including combustion and gasification [7]. Good understanding the mechanism of coal pyrolysis can help controlling the release of pollutants and enhance the conversion rate of coal, beneficial for clean and effective utilization of coal. Additionally, the decomposition reaction of can provide basic information concerning the structure of perhydrous coal molecules [8]. Furthermore, there is a similarity between the artificial pyrolysis and natural coalification, so that pyrolysis was often employed to study coalification process [9], [10].

In the past decades, many studies have addressed the pyrolysis of coal. General pyrolysis reactions and processes were established [11], [12], [13] and can be summarized as follows. At low temperature, some weak non-covalent bonds such as hydrogen bond are cracked and reduced. Compounds bonded by these weak bonds are slightly vaporized and released from solid coal. With rising temperature, bridge bonds begin to be cleaved, resulting in the formation of free radical groups and subsequent production of gas, tar and char. Finally, the solid residue undergoes further condensation reactions with the release of secondary gases (mainly CO and H₂). Nevertheless, studies focusing on the detailed information about reactions and processes during pyrolysis have been limited, especially in the case of dynamic processes. Thermo gravity analysis (TGA) [14] was applied in investigation of the mechanism and kinetic characterization of coal pyrolysis. However, the results based on TGA are macroscopic and do not provide the information on the correlation between the coal structure and reactions. The coal pyrolysis model of Functional Group-Depolymerization Vaporization Crosslinking (FG-DVC) firstly proposed by Solomon established the correlation between coal structure and reactions and characterized the reaction of functional groups as the main processes of coal pyrolysis [12]. Nevertheless, this model is based on the evolution of the gas product by TGA-Fourier Transform Infrared Spectroscopy (TGA-FTIR) [15] and TGA-Mass Spectrum (TGA-MS) [16]. However, data obtained using these two characterization methods can not directly reflect the changes in solid coal during heating process, while in-situ transmission FTIR can determine the direct changes in the solid coal. Some reports on in-situ FTIR measurement have been published [17], [18], [19], [20]. Unlike ex-situ FTIR, the in-situ FTIR can reflect the online evolution of functional groups, and using ex-situ FTIR make the presence of secondary reactions inevitable during the coal processing [21], [22], [23], [24]. In addition, Qi et al. reported smoother and more precise curve of evolution of functional groups obtained by in-situ FTIR than that obtained by ex-situ FTIR during coal oxidation [25].

In coal structure, aliphatic groups (CH₃, CH₂, CH), aromatic groups (aromatic CC and CH), and oxygen-containing groups (hydroxyl, carbonyl, carboxyl and ether) are the most abundant functional groups that appear in the infrared spectrum and are strongly correlated with CO₂, CO and tar [26]. Understanding of the evolution of these functional groups can help optimize thermal treatment of coal processes such as gasification and production of coke and tar.

For kinetic characterization of coal pyrolysis, several methods derived from TGA have been used for isothermal and non-isothermal conditions. These include the Ozawa [27] method and the integral method [28], [29]. However, with the exception of the work by Li et al., reports on the kinetic properties based on quantitative assessment of the functional groups are rare [30]. Li et al. investigated the decomposition kinetics of hydrogen bonds by the in-situ diffuse reflectance FT-IR (DRIFT) based on the loss of absorption of different types of hydrogen bonds.

In our previous works, the structural and thermal characterization of perhydrous coal was investigated by TGA-MS [16], [31], while in the present study in-situ FTIR was used to investigate the pyrolysis process of a perhydrous bituminous coal under non-isothermal heating condition. The spectra obtained at certain temperatures were deconvolved into peaks assigned to the corresponding functional groups. The main objectives of this study are (a) to investigate the pyrolysis mechanism of perhydrous bituminous coal based on the direct loss of functional groups in solid coal; (b) to establish a kinetic method for evolution of functional groups; (c) to determine kinetic characterization of evolution of functional groups during pyrolysis. The result of this study is expected to provide a useful basis for the optimization of coal treatment.