**1. Introduction**

Excessive carbon dioxide emissions have caused serious environmental problems, especially for global warming [1]. Synthetic gas (H2 + CO) via gasification is the most important intermediate product in the highly efficient technologies [2,3]. Steam and carbon dioxide (CO2) are two regular gasification agents and can usually control the overall conversation process [4]. Although it has been widely used as a gasification agent, CO2 gasification is important as it is the slowest among gasification reactions and considered as the rate determining step, as well as the key to making a balance between air or oxygen and steam to generate optimum heat for driving endothermic gasification reactions [5,6]. The process of coal CO2 gasification can be divided into pyrolysis and char gasification reaction [7], and the char gasification formed in situ from pyrolysis process is the rate-determining step [8]. Therefore, the kinetics of char gasification are vital in the design and operation of the gasifier. The gasification of tri-high coal chars often proceeds under the effect of the mineral content in particular, by significantly affecting the gasification rate, which will complicate the kinetics [9,10]. There are several combined chemical and physical steps involved in the conversion rate of coal char [11–14], such as the external mass transfer, inter-particle diffusion and surface reactions. All these steps are associated with changes in the pore structure and the chemical composition of the coal char. However, in some conditions, with the effects of high-ash contents on the chemical composition and porous structure during coal

gasification, the gasification characteristics will vary significantly. In southwest of China, tri-high coal, as the most representative type of coal, is characterized by high-ash content, high-sulfur content and high-ash fusion, which will limit its use in gasification process. However, it is uncertain whether the existing environment of ash is really a factor determining the rate of gasification [15]. In our previous work [3], the derivations and variations of char structures throughout the char gasification process were studied, indicating that both of the porous structure and carbon crystallites can affect the char CO2 gasification kinetics [16].

The most reasonable way to quantify the influence of mineral content on the char gasification kinetics and its relationship with the initial coal structure is to remove the mineral content from the coal (named ash-free coal), and then investigate the gasification characteristics and structures of the ash-free coal [17–19]. There are two methods of char preparation to investigate the influence of mineral content on the char gasification: (1) removal of mineral content by pickling of char derived from pyrolysis, and (2) pyrolysis of coal, of which mineral content has been removed by pickling or other methods [20–22]. However, coal pyrolysis is the initial stage of coal gasification, and closely related to coal composition and structure, which will significantly affect the char gasification characteristics. The mineral content has a great influence on the formation of coal char, such as the cracking of organic matter in coal, volatilization of low-molecular-weight pyrolysis products, polycondensation of cracking residues, decomposition and combination of volatile products during emission, and further decomposition and repolycondensation of the polycondensation products during the pyrolysis process. The composition and structure of coal are directly related to the coal gasification kinetics [9,10,23]. Thus, the method (2) was chosen to produce ash-free coals (AFCs) in this research.

In this paper, two tri-high coals were selected to remove mineral contents by pickling method. The CO2 gasification characteristics of raw coals and ash-free coals were investigated by using a thermo-gravimetric analyzer. Meanwhile, the structure of the raw coals (RCs), ash-free coals (AFCs) and their chars were characterized by SEM, BET, XRD, Raman and FTIR spectroscopy. The specific method can be obtained in our previous work [3].

### **2. Materials and Methods**

### *2.1. Materials*

Two tri-high coal samples were collected for the experiments from Guizhou Province in southwest of China. The AFC samples were produced by acid pickling. The nitric acid solution was added to the RC at a solution to coal ratio of 20:1 by weight, and the slurry was stirred for 24 h to ensure the coal wetting. After filtration, the filtered coal was mixed with hydrofluoric acid, and then stirred for 24 h and again filtered. Mixing with deionized water, stirring and filtering were repeated until the PH = 7 to ensure complete removal of coal ash. All the agents used above were analytical reagents and the concentration was not diluted. For comparison, the RCs with similar particle size to the AFCs (75–106 μm) were used as well. The char was prepared at 950 ◦C under a nitrogen atmosphere in a horizontal tubular furnace. During the char preparation process, approximately 20 g coal was placed in a corundum crucible, and then heated at 20 ◦C/min to the designed temperature under a nitrogen atmosphere. Finally, the sample was held at 950 ◦C for 30 min. The proximate and ultimate analyses of the AFCs and the corresponding RCs are summarized in Table 1.

On a dry basis, RC-I and AFC-I samples had 9.42 wt.% and 13.67 wt.% of volatile, respectively, while RC-II and AFC-II samples had 13.79 wt.% and 52.34 wt.% of volatile, respectively. The ash contents of RC-I, AFC-I, RC-II and AFC-II were 21.45 wt.%, 0.19 wt.%, 25.26 wt.% and 0.16 wt.%, respectively. The increased Fixed carbon of AFC-I was consistent with the result of Rubiera et al. [24], while that of AFC-II decreased. This probably due to the nitric acid showing poor effect on the mineral content, hydrofluoric acid dominated the acid pickling process. In addition, nitric acid played a role of oxidizing agent led to the increase in O content after pickling.


**Table 1.** Proximate and ultimate analysis of raw coals and ash-free coals.

db, dry basis; and daf, dry and ash-free; a, oxygen content by difference.
