Coal char characteristics variation in the gasification process and its influencing factors

Underground coal gasification is a burgeoning coal exploitation technique that coal is directly converted into gaseous fuel by controlled combustion. In this paper, the gasification experiments of Inner Mongolia lignite, Xinjiang subbituminous coal, and Hancheng medium volatile bitumite were conducted respectively by using the tube furnace coal gasification experiment system. The gasification process was conducted under 3°C/min increment within the range of 600–900°C. The gas composition was analyzed by gas chromatography and the pore structure of the coal char was detected by low-temperature N2 adsorption. The results show that the gasification temperature, gasification agent, and coal type have an important influence on the gasification reaction. With the increase of gasification temperature, the effective component, gas calorific value, and gas production rate increase. When CO2 is used as the gasifying agent, the effective components in the gas are mainly CO. When H2O(g) is used as the gasifying agent, the effective component of gas is H2. The coal gasification performance with low thermal maturity is obvious better than the high rank coal with higher coalification. N2 adsorption–desorption experiments show that the pore is mainly composed by transition pore and the micropores, the specific surface area is chiefly controlled by a pore size of 2–3 nm. With the increase of coalification degree, the adsorption amount, specific surface area, and total pore volume show a decreasing trend. The gasifying agent has a great influence on the pore structure of the coal char. The gasification effect of H2O (g) is significantly better than that of CO2. Analyzing the gasification characteristics and pore changes of different coal rank coals under different gasification agents, we found that Inner Mongolia lignite is more conducive to the transport of gasification agents and gaseous products in coal.


Introduction
The permeability of chars is helpful for transporting the gasifier and product gas in coal. Therefore, it is of great scientific significance to study the gasification of coal and its influence on pore structure.

Coal samples
In this paper, the samples of Inner Mongolia lignite in Erlian basin (NM), Xinjiang subbituminous coal in Junggar basin (XJ) and Hancheng medium volatile bituminous coal in Ordos basin (HC) were used as raw materials to prepare gasified coal char, and CO 2 and H 2 O(g) were used as gasification agent. The effects the of gasification agent on the structure and properties of the three coal chars were investigated. The sampling positions were shown in Figure 1.

Experimental method and procedure
Gasification temperature is one of the important factors that affect the process and effect of coal gasification (Akbarzadeh and Chalaturnyk, 2014;Sharma et al., 2002). In order to investigate the effect of gasification temperature on gasification characteristics, taking lignite from Inner Mongolia, Xinjiang subbituminous coal, and Hancheng medium volatile bitumite as raw materials, CO 2 and H 2 O (g) was gasification agent, a simulated gasification experiment was carried out under 600 C, 750 C, and 900 C.
The pore structure of coal char was measured by Micromeritics ASAP2020-specific surface analyzer. The adsorption-desorption isotherms of three coal char samples under different gasification agents were obtained. The specific surface area of the samples was calculated by the BET (Brunauer-Emmett-Teller) equation (Brunauer et al., 1938). The pore size distribution of the samples was obtained by the BJH (Barrett-Joyner-Halenda) equation (Barrett et al., 2002).
Firstly, the ash, moisture, and volatile matter of three coal samples were analyzed according to GB/T 12-2008. The raw coal was pyrolyzed at 600 C, 750 C, and 900 C, the pyrolyzed coal chars were divided into two groups. Secondly, crushing and screening, then the 3-mm coal chars were filled in the gasifier, then the gasification agent is introduced, and the temperature is raised to the target temperature at the rate of 3 C/min for gasification The gasified products were cooled, washed, and dried, the components and percentage content ( Figure 2) were tested by gas chromatography. Thirdly, the pore size change rule was determined by the liquid nitrogen adsorption test at low temperatures. The elemental analysis was shown in Table 1.
It can be seen from the table that the carbon content of char is over 90% after high temperature gasification (Table 1). With the deepening of the coalification degree, the content of carbon decreases slightly and the content of oxygen increases. The reason is that the  coal with a low degree of coalification is active in chemical properties, the side chains and branched chains of a large number of oxygen-containing functional groups are broken, and the degree of aromatic nucleus condensation is deepened, so that the content of carbon increases greatly.
Analysis of gas composition and pore structure variation The content of coal char gasification gas from the three regions was tested by gas chromatography (GC $ 2014, Shimadzu, Japan). When the temperature of the laboratory gasifier reached the final target temperature (600,750, and 900 C), the gasification gas began to flow into the gas chromatograph, after the airflow is stabled, the volume fraction of H 2 , CO, CH 4 , O 2 , and N 2 was measured. The gasification coal char was crushed and ground, then the gasified solid products were crushed into 60-80 meshes. The Micromeritics ASAP2020 specific surface analyzer was used to determine the specific surface area, pore volume, pore diameter, pore distribution, and isothermal adsorption and desorption analysis (Chen et al., 2017;Fu et al., 2017;Li et al., 2019aLi et al., , 2019bLi et al., , 2019cLi et al., , 2020.

Results and discussion
The influence of temperature on gas-phase products Effects of gasification temperature on gas components. Figure 3 is the variations of effective components of coal gas with gasification temperatures under different gasification agents. It can be seen from Figure 3(a) that when CO 2 is used as gasification agent, the main effective component of the gas is CO, and the contents of H 2 and CH 4 are relatively low, and the gaseous products increase with the increase of gasification temperature, while the content of CO 2 decreases gradually with the increase of temperature, which indicates that the higher the temperature, the more favorable the reduction reaction between carbon and CO 2 . When CO 2 is used as a gasifying agent, the content of H 2 is relatively low, generally between 0.52% and 6.45%. With the increase of temperature, the content of H 2 increases first and then decreases, reaching the maximum value around 750 C. When the gasification agent is H 2 O (g), H 2 plays the dominant role, generally up to 60%. The process of vaporization of water vapor is an endothermic reaction (Figure 3(b)). The higher the temperature, the more favorable the reaction is. Therefore, the H 2 content in the gas continuously increases with the increase of the gasification temperature and the CO content is relatively small. There are two other reasons for the decrease of CO: firstly, H 2 O (g) is decomposed into H 2 and O 2 at high temperatures, and O 2 rapidly oxidizes the generated CO to CO 2 , thus reducing CO; secondly, a layer of gas film will be formed when coal char adsorbs CO 2 . H 2 O (g) is blocked by CO 2 gas film, which hinders the reaction between carbon and H 2 O (g). CH 4 decreases with increasing gasification temperature. The reasons are manifold: firstly, part of CH 4 produced during gasification may be reduced with CO 2 ; secondly, CH 4 will decompose at high temperature. Besides, from a thermodynamic point of view, the reaction of C and H 2 to form CH 4 is exothermic, and the excessive temperature affects the formation of CH 4 ; H 2 O(g) is decomposed into H 2 and O 2 . The increase in H 2 also reduces the relative content of CH 4 in the gaseous product. Figure 4 is the relationship between the gasification temperature and the gas production rate. As the gasification temperature increases, with the temperature increase, the energy of carbon atoms increases, and the carbon bonds easily break in the coal. Besides, the reaction of coal with CO 2 and H 2 O(g) is endothermic. The high temperature also facilitates the gasification reaction and produces more CO and H 2 .
The calorific value of gas is related to its type, content, and calorific value of each component. The calorific value of CH 4 is much greater than the calorific value of H 2 and CO,  that is, the higher the CH 4 content in gas is, the higher the calorific value. When CO 2 is used as a gasification agent, the calorific value of the gas increases with the increase of temperature in Figure 5, which is consistent with the change of gas composition. With H 2 O(g) as a gasification agent, the calorific value of the gas increases with the increase of temperature, which is also consistent with the variation of as composition in Figure 3.

Effects of gasification agent on gas components
The gasification agent has a significant effect on gasification characteristics of coal (Yang and Ding, 2009). When CO 2 is used as a gasifying agent, CO is the main active component of the gas, H 2 and CH 4 are relatively low. When H 2 O (g) is used as a gasifying agent, H 2 is the absolute dominant, followed by CO. Figure 3 shows that the effective component content is higher when H 2 O (g) is gasification; Figure 4 indicates that the gas production rate of coal is higher when H 2 O (g) as gasification agent. All these conclusions indicate that the activity of carbon and the gasification characteristics is higher under the condition of H 2 O (g). One of the reasons is the coal structure and the molecular characteristics of gasification agents. There are a large number of pores with different pore sizes in coal. At the same time, the molecular diameter of gasifier water is smaller than that of CO 2 . H 2 O (g) can enter micropores with pore size above 0.6 nm and react with carbon. However, CO 2 can only enter micropores above 1.5 nm, so H 2 O (g) can penetrate into the finer pores of coal, occupy more active surfaces and react with carbon. Secondly, When H 2 O (g) and CO 2 enter the coal pore, O is first separated from the coal pore, and H 2 O (g) and CO 2 dissociate oxygen play a key role in the reaction. The formation of hydrogen bonds of water molecules is weaker than the formation of double bonds of carbon dioxide molecules (Messenbock et al., 1999). H 2 O (g) is easier to dissociate oxygen than CO 2 . In addition, the reaction of coal to CO 2 requires more heat than water gas. Therefore, the gasification activity under H 2 O (g) condition is better than that under CO 2 conditions. The calorific value of gas is obviously higher. Under the condition of H 2 O (g) in Figure 5, which is consistent with the higher content of CH 4 in gas with H 2 O (g) as gasifier in Figure 3.

Effects of different coal char on gas components
With the increase of coalification degree, the volatile content in the coal decreases and the gasification performance is poor (Table 1). With the increase of coal coalification, the carbon matrix in coal increases, and the size of carbon microcrystals increases. Additionally, the side chain functional groups of coal decrease, and the active number of coal char surface decreases, resulting in the reactivity of coal decreases. With CO 2 as a gasifying agent, the CO content of Xinjiang subbituminous coal and Inner Mongolia lignite gas is higher than that of Hancheng medium volatile bituminous coal; with H 2 O (g) as a gasifying agent, the H 2 content of Xinjiang subbituminous coal and Inner Mongolia lignite gas is also higher than that of Hancheng medium volatile bituminous coal, so the calorific value of the gas is higher than that of Hancheng medium volatile bituminous coal under the same gasifying agent (Figures 3 and Figure 5). From the calorific value of gas, the calorific value of lignite in Inner Mongolia is obviously higher than that of subbituminous coal in Xinjiang and medium volatile bituminous Hancheng when H 2 O (g) is gasification agent and that of lignite in Inner Mongolia is higher than that of subbituminous coal in Xinjiang and that of medium volatile bituminous coal in Hancheng when CO 2 (g) is gasification agent. Also, it can be seen from Figure 4 that the gas production rate of lignite in Inner Mongolia is obviously higher than that of subbituminous coal in Xinjiang and medium volatile bituminous Hancheng, which indicates that the coal type has an important influence on the gasification characteristics of coal.
The process of gasification is the process of breaking various molecular bonds and the process of re-variation of pores. Under the condition of the final gasification temperature, the void change of different coal rank samples after gasification with different gasification agents shows different regularity.
Measurement of pore structure (2-100 nm) with N 2 adsorption-desorption N 2 adsorption-desorption isotherms in different coal char The saturation pressure and tested pressure of N 2 are represented by P and P o . As shown in Figure 6, the isotherms of the three char gasification products are quite different. For the isotherms of Inner Mongolia and Xinjiang coal char, the adsorption curves did not increase significantly at the initial stage. With the pressure increasing, the adsorption capacity increased rapidly. When P/P o was less than 0.5, the adsorption and desorption curves coincided completely, and the adsorption capacity increased slowly, indicating that the main occurrence of this stage was micropore filling. When P/P o is greater than 0.5, the adsorption and desorption curves appear to separate and the adsorption capacity increases sharply. When the relative pressure is close to 1, the curve increases sharply containing an obvious turning point. The results show that N 2 begins to coagulate at higher relative pressure and the adsorption capacity increases suddenly, indicating the existence of larger pores (Yao et al., 2006). For the isotherm of Hancheng coal char, the adsorption curve did not increase obviously at the initial stage. With the pressure increasing, the adsorption capacity increased rapidly. When P/P o was less than 1, the adsorption and desorption curve coincided completely, and the adsorption capacity increased slowly, indicating that the main occurrence of this stage was micropore filling. When the relative pressure is close to 1.0, there is a significant turning point in the sharply increasing curve, which indicates that N 2 begins to coagulate under higher relative pressure, and the adsorption capacity increases suddenly, indicating that there are larger pores. To the different coal rank, the adsorption volume of the coal char is various. When CO 2 is used as a gasification agent (Figure 6(a), (b), and (c)), the adsorption volume of lignite in Inner Mongolia is 64 mL/g, that of subbituminous coal in Xinjiang is 12 mL/g, and that of Hancheng medium volatile bituminous coal is about 8.3 mL/g. When H 2 O(g) is used as a gasification agent (Figure 6(d), (e), and (f)), the adsorption volume of lignite in Inner Mongolia is 86 mL/g, that of subbituminous coal in Xinjiang is between 16 mL/g and that of Hancheng medium volatile bituminous coal is about 14 mL/g. It can be seen from the diagram that under the same gasifier conditions, the adsorption capacity of pyrolysis final temperature products decreases with the increase of coal rank.
In the coal of low-degree coalification, due to the edge of the coal molecular structure layer with cross-linked structure, the coal is characterized by the loose spatial structure, presenting a relatively large inner surface area; with the degree of coalification deepening, functional groups reduce and the structure of coal gradually stabilizes, resulting in the decrease of surface activity of coal char and the reduce of adsorption amount. Figure 6 is the N 2 adsorption-desorption isotherm of char under different gasification agents. It can be seen from the figure that the gasification agent is different and the adsorption volume of the coal char sample is different. For Inner Mongolia coal, when the gasification agents are CO 2 and H 2 O(g), the corresponding adsorption volume is 64 mL/g and 86 mL/g, respectively. For Xinjiang subbituminous coal, when the gasification agents are CO 2 and H 2 O(g), the corresponding adsorption volume is 12 mL/g and 16 mL/g, respectively. For Hancheng medium volatile bituminous coal, when the gasification agents are CO 2 and H 2 O(g), the corresponding adsorption volume is 8.3 mL/g and 14 mL/g, respectively.

N 2 adsorption-desorption isotherms in different gasification agent
The principle of N 2 adsorption-desorption is consistent with the adsorption theory of porous media. Therefore, the variation characteristics of the adsorption and desorption curves can reflect the type of pore shape (Krooss et al., 2002). According to the type of adsorption heat classified by Sing (1985), the N 2 adsorption-desorption isotherms of coal samples are classified into types A and types B. Comparing the adsorption-desorption curves from different gasifying agents, under relatively high pressure, it can be known that when the gasifying agent is H 2 O(g), the desorption curve of NM-H 2 O (Figure 6(d)) and XJ-H 2 O (Figure 6(e)) is a type A desorption curve with obvious hysteresis loop (P/P o > 05, pore diameter >2.76 nm). It can be seen that there is a certain pore size and the required relative pressure is higher than the relative pressure required for desorption evaporation. That is, the curve indicates a connected pore with open ends, e.g., parallel plate holes or cylindrical holes, which is favorable for adsorption, desorption, and diffusion of coalbed methane. When the gasifying agent is CO 2 , the desorption curve is type B, there is no hysteresis loop or only a small hysteresis loop, reflecting that this type of pore has the same relative pressure in terms of adsorption concentration and desorption evaporation rate. This kind of curve mainly renders pores with poor semi-open pore connectivity, such as one-side closed wedge-shaped and cylindrical slit-like pores. This type of pores has the weakest adsorption and accumulation ability, but is beneficial to the adsorption and diffusion of coalbed methane. It is reflected that this type of pore has the same relative pressure in adsorption concentration and desorption evaporation (Sing, 1985).

Pore size distribution in different coal char
The specific surface area and pore volume of coal char under different gasification agents are shown in Table 2. It can be seen from the Table 2 that when CO 2 is used as the gasifying agent, the specific surface area of the Inner Mongolia coal char sample is 38.72 m 2 /g, and the pore volume is 0.103 mL/g, according to the duty cycle of pore volumes at different porosities, the transition pore (10-100 nm, 51.93%) is most developed, followed by micropore (>100 nm, 25.91%), and the proportion of mesopore is small (<10 nm, 22.61%). The specific surface area of the coal char sample in Xinjiang is 5.14 m 2 /g and the pore volume is 0.017 mL/g. According to the duty cycle of pore volumes at different porosities, the transition pore (10-100 nm, 49.64%) is most developed, followed by medium (>100 nm, 30.17%), the proportion of micropore is small (>10 nm, 20.19%). The specific surface area of Hancheng coal char sample is 4.52 m 2 /g and the pore volume is 0.014 mL/g. According to the duty cycle of the pore volumes under different porosities, the mesopore (>10 nm, 48.05%) is most developed, followed by transitional pore (>100 nm, 34.14%), the proportion of micropore is small (10-100 nm, 22.61%). Figure 7 shows the distribution characteristics of pore volume and specific surface area of three coal samples under different pore sizes. The results show that under the condition of a single gasification agent (CO 2 ), the pore volume distribution of the samples shows a multimodal distribution under different pore diameters. The peak value of Inner Mongolia coal sample is at 30-40 nm, which means that the pore volume is mainly contributed by this part of the pore. The distribution of pore specific surface area to the pore diameter size as a single-peak model and the peak value appears within 2-3 nm, which means that the specific surface area is mainly contributed by this part of the pore (Figure 7(a)). The peak value of Xinjiang coal sample at 10-100 nm is higher than the one at 2-10 nm, especially for pores with diameters of 80-90 nm, indicating that the pores of this section have the biggest contributions to the pore volume. The distribution of pore specific surface area to the pore diameter as the single-peak model and the peak value appears in the range of 2-3 nm, which means that the specific surface area is mainly contributed by this part of the pore (Figure 7(b)). The peak value of Hancheng coal sample at >100 nm is higher than the one at 2-10 nm, especially for pores with diameters of about 100 nm, indicating that the pores of this section have the biggest contributions to the pore volume. The distribution of pore specific surface area to the pore diameter is a singlepeak model, and the peak value appears in the range of 2-3 nm, which means that the specific surface area is mainly contributed by this part of the pore (Figure 7(c)). When H 2 O(g) is used as a gasification agent, the three kinds of coal char have the same trend (Figure 7(d), (e), and (f)). It can be seen from Table 2 that the degree of coalification has a great influence on the structure of coal char. With the increase of coalification degree, the specific surface area and total pore volume show a decreasing trend, the micropores and transition pores are significantly reduced. It shows that with the increase of coalification degree, it is conducive to the development of large and medium pores, and the low degree of coalification is conducive to the development of micropores and transition pores. Compared with Xinjiang coal char and Hancheng coal char, Inner Mongolia coal char has a rich pore structure under the same gasification condition and its specific surface area and total pore volume can reach 38.72 m 2 /g and 0.103 cm 3 /g. It is more conducive to the transportation of gasification agents and gasification products.

Pore size distribution in different gasification agent
The specific surface area and pore volume of coal char under different gasification agents were shown in Figure 7. It can be seen from Figure 7(a) and (d) that taking the Inner Mongolia coal char as an example, when CO 2 is a gasifying agent, the specific surface area of the Inner Mongolia coal sample is 38.72 m 2 /g. The pore volume is 0.103 mL/g and the transition pore (10-100 nm, 51.93%) is most developed, followed by micropore (>100 nm, 25.91%), and the proportion of mesopore was small (<10 nm, 22.61%). When H 2 O(g) is a gasifying agent, the specific surface area of the inner coal sample is 75.04 m 2 /g, the pore volume is 0.119 mL/g, and the micropore (<100 nm, 48.88%) is most developed, followed by the transition pore (10-100 nm, 35.45%). the proportion of large and medium holes is small (>10 nm, 15.67%). Figure 7(a) and (d) shows the distribution of pore volume and specific surface area of Inner Mongolia coal sample under different gasification. The results show that when CO 2 is a gasifying agent, the pore volume distribution of Inner Mongolia samples shows a multi-peak distribution under different pore sizes. The peak value of the pores of 80-90 nm is the highest, this shows that the pores diameter of this section contributes the most pore volume. The distribution of pore specific surface area to the pore diameter is a single-peak model, and the peak value appears in the range of 2-3 nm, which means that the specific surface area is mainly contributed by this part of the pore. That is to say, within 2-100 nm, the pore volume mainly comes from the contribution of 80-90 nm pores, and the more pores develop in 2-3 nm, the bigger the specific surface area is. When H 2 O(g) is a gasifying agent, the pore volume distribution of the Inner Mongolia sample shows a multimodal distribution under different pore sizes, and the peak value of the pore diameter within 3-4 nm is the highest, indicating that the pore volume of this section has the biggest contributions to the pore volume. The distribution of pore specific surface area to the pore diameter as the single-peak model and the peak value appears in the range of 1-2 nm, which means the specific surface area is mainly contributed by this part of the pore. That is to say, in the range of 2-100 nm, the pore volume mainly comes from the contribution of 3-4 nm pores, and the more pores develop in 1-2 nm, the larger the specific surface area is. The same trend is also observed for Xinjiang coal char and Hancheng coal char (Figure 7(b), (c), (e), and (f)). The pore structure parameters of residual coke under different gasification agents are given in Table 2. The results show that the specific surface area and total pore volume of H 2 O (g) coal char is higher than that of CO 2 , and the transition pore of CO 2 coal char is more developed, and the micropore of H 2 O (g) coal char is developed.
The analysis shows that the gasifying agent has a great influence on the pore structure of the coal char, the effect of H 2 O(g) is obviously better than that of CO 2 . The reason may be that the O-H bond is more likely to be broken, H 2 O(g) is easier to dissociate oxygen than CO 2 , and the activation energy of H 2 O(g) chemisorption is also lower, the H 2 O(g) can be preferentially adsorbed on the coal char surface, so the vaporization effect of water vapor is better. The molecular diameter of the gasifying agent water molecule is smaller than that of CO 2 . H 2 O(g) can enter the pores with a pore diameter of 0.6 nm or more and gasification reaction with carbon, while CO 2 can only enter the pores of 1.5 nm or more, so H 2 O(g) can penetrate deeper into the pores of coal, occupy more active surface and react.

Conclusions
1. With the increase of gasification temperature, the effective component, gas calorific value, and gas production rate increase. When CO 2 is used as the gasifying agent, the effective components in the gas are mainly CO. When H 2 O(g) is used as the gasifying agent, the effective component of gas is dominant in H 2 . The coal gasification performance with a lower degree of coalification is obviously better than that of coal with higher coalification. 2. The pore volume is composed by transitional pores and followed by the micropores. The specific surface area is composed by a pore diameter within 2-3 nm. With the increase of coalification degree, the adsorption amount, specific surface area, and total pore volume show a decreasing trend. 3. The gasifying agent has a great influence on the pore structure of the coal char. When the gasification agent is H 2 O (g), the specific surface area and total pore volume of coal char are high and the micropore is developed. When the gasification agent is CO 2 and the transition pore is more developed. The gasification effect of H 2 O (g) is significantly better than that of CO 2 . 4. When H 2 O(g) is used as gasification agent and Inner Mongolia lignite as gasification coal, the effective components are high, the calorific value is higher, the gas production rate is faster, and the pores are more developed after gasification. The gasification of low-grade coal by H 2 O(g) has a better gasification effect.

Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The National Natural Science Foundation Project, China (Grant No. U1703126) and the Fundamental Research Funds for the Central Universities, China.