Comparative Study of the Gasification of Coal and Its Macerals and Prediction of the Synergistic Effects Under Typical Entrained-Bed Pulverized Coal Gasification Conditions

This research is focused on the gasification performance of coal and its corresponding macerals as well as on the interactions among macerals under typical gasification conditions by Aspen Plus modelling. The synergistic coefficient was employed to show the degree of interactions, while the performance indicators including specific oxygen consumption (SOC), specific coal consumption (SCC), cold gas efficiency (CGE) and effective syngas (CO+H2) content were used to evaluate the gasification process. Sensitivity analysis showed that the parent coal and its macerals exhibited different gasification behaviours at the same operating conditions, such as the SOC and SCC decreased in the order of Inertinite> Vitrinite>Liptinite, whereas CGE is changed in the order of Liptinite>Vitrinite> Inertinite. The synergistic coefficients of SOC and SCC for the simulated coals were in the range of 0.94 to 0.97, whereas the synergistic coefficient of CGE was from 1.05 to 1.13. Moreover, it was found that the relationships between synergistic coefficients of gasification indicators were correlated well with maceral contents. In addition, the increase of temperature was


Introduction
According to IEA report 2017, coal constitutes approximately 27% of the global energy mix. It is estimated that the quantity of coal for power generation will contribute to 55% of China's energy demand in the next five years [1] and coal will still continue playing a major role in meeting human being's energy demand worldwide [2,3]. Coal gasification, which converts solid fuels into a gaseous product at elevated pressure and temperature, is considered the most effective method to realize the clean, efficient and economical utilization of coal [4][5][6]. Currently, there are three major types of gasifier that have been commercially used worldwide, i.e., fixed-bed gasifier, fluidized-bed gasifier and entrained-bed gasifier. Their unique characteristics are summarized in the literature [7].
Entrained-bed gasifiers have the advantages of dealing with any type of coals with high carbon conversion and high throughput [8,9]. Shell entertained-bed gasifier is one of the representative gasification technologies and takes up a significant portion of installed capacities in the world [7,8].
According to the classification of International Commission for Coal Petrology (ICCP), macerals are divided into three groups and named as vitrinite, liptinite and inertinite [10]. The determination of different groups lies in the distinguishable colour under reflective light and morphology of the macerals. Except for the differences in appearance, maceral groups differ in their chemical composition, which brings distinct technical performances and therefore affects the burnout of the fuel [11,12]. Therefore, an insight into macerals is the most fundamental step to understand the properties of the parent coal and subsequently, the efficiency of maceral-enriched feedstock [13].
In the past few decades, experimental studies on the structural transformation of macerals and the change in chemical reactivity during pyrolysis/gasification have attracted significant attention [13][14][15][16][17][18]. For example, Sun et al. [14] compared the structural variations of the macerals before and after pyrolysis and found that vitrinite led to the yield of more aliphatic C-H and lowered aromaticity than inertinite. It is reported [13,16] that at a short gasification residence time (10 s), the conversion is in the order of liptinite> vitrinite> inertinite, while at a long residence time (200 s), the extent of gasification was found to be inertinite> vitrinite>liptinite. Moreover, Sun et al. [17] conducted CO2 gasification of vitrinite char and inertinite char in a pressurized thermobalance at a temperature up to 950 ºC and reported that the vitrinite char was more reactive than the inertinite char with or without a catalyst. However, more recently, Wang et al. [18] stated that the gasification reactivity of vitrinite was lower than that of inertinite under CO2 gasification atmosphere.
The interaction between macerals during thermal processing is of significance for the basic understanding of the coal chemistry, developing new coal utilization technology and improving thermal efficiency. Sun et al. [19] compared the volatile yield of the pyrolysis of parent coal and its macerals and concluded the existence of synergism among macerals. Chang et al. [20] also studied the interaction during the pyrolysis of inertinite and vitrinite using FTIR, TG and fixed bed reactor and gave a thorough explanation of the interaction mechanisms at molecule levels. Later, the synergistic effect of macerals during hydropyrolysis was also reported by Sun et al. [21], whereas the maximum synergism reached 14.1% at 500 ºC and 3 MPa. Zubkova et al. [22] also explored the interactions of macerals during carbonization and obtained a denser coke than theoretical expectation.
To date, researchers have conducted a significant amount of work on the understanding of the reactivity of macerals during pyrolysis and gasification as well as on the determination of the interactions between macerals during pyrolysis, but few of them have paid attention to the differences of gasification products, cold gas efficiency, syngas content, specific oxygen consumption and specific coal consumption among macerals and parent coal. Besides, the synergistic effects of macerals during gasification have rarely been investigated. Moreover, the influence of process operating parameters on the synergistic effect has not been discussed although Aspen Plus has been widely applied in the study of solid fuel gasification systems [23][24][25][26].
In this study, the comparatively study of the gasification behaviours of the parent coal and its maceral components under actual entrained-bed gasification conditions was carried out by using Aspen Plus. The quantitative evaluation of the interactions between macerals as well as sensitivity analyses were performed. Besides, the relationship between the synergistic coefficient and maceral contents was investigated. In addition, impacts of typical operating parameters on the interactions among macerals were revealed.

Methodology
Shell coal gasification technology is a commercial technology that is capable of dealing with a large range of coals at a high energy conversion efficiency [7,27]. The Aspen Plus diagram of a Shell gasification process is illustrated in Fig.1. Milled coal is dried to 5% moisture content and mixed with N2 in lock-hopper before being fed into the gasifier. The coal is gasified under the conditions of medium pressure using 95 vol% oxygen derived from a stand-alone air separation unit [27]. The commercial operating pressure is around 4.0 MPa, and the gasification temperature is in the range of 1350 to 1550 ºC . The steam to coal mass ratio varies from 0.01 to 0.16 and the feed oxygen to coal mass ratio is in the range of 0.5 to 1.1. The reactions considered in this study are the ones being considered in the literature [28]. The gas product from the gasifier is quenched by recirculated cold syngas to a temperature of around 900 ºC [29]. After quench, the heat of the raw gas is recovered by a syngas cooler generating steam for power generation. The syngas is sent to a candle filter to remove particulate matters.
The gasification process is mainly simulated by using a combination of RYIELD and RGIBBS modules in Aspen Plus. The function of RYIELD model is to convert the unconventional coal into standard components such as H2, N2, O2, S, H2O, Cl2 and ash, and their yield distribution is programmed using FORTRAN codes according to the ultimate analysis of coal [30][31][32]. The RGIBBS is a phase and chemical equilibrium model based on Gibbs free energy minimization and is commonly employed to model coal pyrolysis and gasification in the Shell gasifier [33]. In addition, the PR-BM method is used to calculate the thermodynamic properties of materials stream [34]. In order to understand the gasification behaviours of the parent coal and its corresponding macerals, the existing analytical data of Pingshuo bituminous coal and its macerals were taken as the feedstock for this study. Maceral groups are separated based on their density difference using ZnCl2 liquid [35].
The composition, together with the petrographic analysis of the feed coal and maceral groups, are listed in Table 1. The study of interaction among macerals is based on the petrological features of the Pingshuo Bituminous coal, which is shown in Table 2 [35]. The main process parameters and conditions of the gasification of the coal and its macerals are shown in Table 3.

System evaluation indicators
The evaluation indicators for the gasification of coal and its macerals mainly include specific oxygen consumption, specific coal consumption, syngas lower heating value (LHV), cold gas efficiency and the content of effective syngas (CO+ H2) in the product gas.
The LHV (MJ/Nm 3 ) of the syngas is calculated as [39], Where CO, H2, CH4 is the volume fraction in the production of gas from the gasification.

The higher heating value (HHV) of coal/macerals is obtained by the correlation proposed by
Channiwala et al. [40], The LHV of the coal is predicted using the following equation [41], Where ZC, ZH, ZO, ZN and ZS are the mass concentration of the carbon, hydrogen, oxygen, nitrogen and sulfur in the feedstock, respectively, as shown in Table 1.
The specific oxygen consumption (SOC) is defined as the amount of oxygen consumed per volume of effective syngas production.
The specific coal consumption (SCC) represents the ratio of coal consumption or macerals consumption to the volume of effective syngas generated in the gasification.
SCC=kg coal/(CO+H2) kNm 3 Synergetic coefficient (aij) accounts for the interactions among macerals is determined as following Where i is the number of simulated coals, i=1 to 9; j stands for the gasification products and evaluation parameters, for example, j can be the mole fraction of CO, H2 and the value CGE, etc. x is the numerical value of gasification products and the evaluation indicators calculated from Aspen plus.
The physical meaning of y stands for theoretical values without considering interaction, which is obtained by the addition algorithm taking into account the mass weight fraction of each maceral in the simulated coal as tabulated in Table 2.
Where z is the mass concentration of the k th independent macerals in the i th simulated coal.

Results and Discussion
Based on the data shown in Table 1 and Table 2, together with the simulation conditions indicated in Table 3, the gasification performance of each type of feedstock is determined and compared under the same operating conditions. For comparison, the benchmark operating parameters are as follows: gasification temperature is at 1450 ºC , the OTC and STC values are 0.8 and 0.08, respectively.

Simulation results
To validate the simulation, the comparison of the syngas composition from the gasifier between the simulation results and industrial data described in the reference [36] is shown in Table 4. As shown in Table 4, the simulation values are agreeable well with the industrial data [36], which demonstrates the reliability of this model. The syngas composition and performance indicators for coal and its macerals are summarized in   Table 6 presents the summary of Aspen plus simulation and performance indicators of the simulated coal (as shown in Table 2). The input data of ultimate and proximate analysis to Aspen Plus for the simulated coals are calculated using simple addition algorithm according to the mass percentage of macerals (as shown in Table 1). It can be seen from Table 6 that the mixed simulated coals have better thermodynamic performances concerning SOC, SCC, effective syngas and CGE than those from the parent coal and each maceral group.

Synergistic effects
Synergistic effect indicates that the products and performances arising from the simulated coals are higher or lower than the sum of their individual maceral. When the synergistic coefficient is not equal to 1, it indicates the interactions among macerals showing an influence on the gasification performance. Table 7 shows a summary of the matrix of the synergistic coefficients calculated by Eqs. (7). It can be seen that interactions among macerals during gasification exist. The synergistic coefficients of H2 and CO contents are higher than 1, while those of the other gases such as CO2 and  The relationships between synergistic coefficient and maceral contents for various performance indicators are investigated by using a direct three-order polynomial correlation method based on the data shown in Table 7. Fig.2 correlates the relations of synergistic coefficient with maceral contents for SOC and SCC respectively.

Effect of gasification temperature.
In order to track the different gasification behaviours of parent coal and macerals under different gasification temperatures, the plot of gasification performance indicators versus operation temperature varied from 1350 to 1550 ℃ is displayed in Fig.4. It can be noted from Fig.4 (a) that the gasification temperature has a slightly negative influence on SOC and SCC of the coal and its macerals. However, from Fig. 4

Fig.4 Effect of gasification temperature on the gasification performance parameters: (a) SOC and SCC, (b) CGE and (CO+H2)%
The effect of temperature on the synergistic coefficients of SOC, SCC, effective syngas and CGE with the variation of maceral contents is shown in Fig.5. In order to have a better quantitative comparison of the synergistic coefficients at different temperatures, three fitting curves (denoted as "FC") are presented at the temperatures of 1350, 1450 and 1550 ℃ as shown in Fig.5. It can be observed in Fig.5 (a) and (b) that the synergistic coefficients of SOC and SCC exhibit similar properties. When gasification temperature is below 1450 ℃, the impact of temperature is not obvious, whereas when gasification temperature is higher than 1450 ℃, the synergistic coefficient detrimental value is about 0.005. This suggests that higher gasification temperature is favourable to the maceral interactions and leads to the decrease in oxygen and coal consumptions. It can be seen from Fig. 5(c) that the gasification temperature does not significantly affect the synergistic coefficient of CGE. Fig.   6(d) depicts a slightly fluctuating phenomenon regarding the synergistic coefficient curves at 1350, 1450 and 1550 ℃. Nevertheless, the fluctuation range is limited to 0.05% demonstrating that temperature has little impact on the effective syngas content.

Effect of oxygen to coal (OTC) mass ratio
The effects of oxygen to coal mass ratio on SCC, SOC, CGE and effective syngas content of parent coal and its macerals have been studied and are shown in Fig.6.  Fig.7 (a) and (b) that the synergistic coefficients of SCC and SOC exist a minimum value which is found to be varied from 0.94 to 0.97 when OTC equals to 0.8. However, when OTC changes from 0.5 to 1.1, the synergistic coefficients of SCC and SOC increase initially and decrease afterwards. From Fig. 7(c), it can be clearly seen that the synergistic coefficient of CGE maintains the highest at OTC=0.8 than that at any other OTC values in the whole range of Inertinite variation. Fig.7(d) shows that the synergistic coefficient is enhanced at the OTC of 0.8. However, the coefficient is lower than 1 at OTC>0.8, which indicates that interactions among macerals exist a slightly mutual inhibition effect.
It can be concluded from Fig.7 that OTC is greater than 0.8, the interactions among macerals are no longer in existence or even existing inhibition effect and at the OTC=0.8, the synergistic coefficients of SOC, SCC, CGE and effective syngas achieve maximum efficiencies.  As can be observed from Fig.8(a), both the SOC and SCC of coal and Liptinite are not sensitive to the addition of steam, while both the SOC and SCC regarding Vitrinite and Inertinite decrease.  Due to the complex physical and chemical properties of coal, it is hard to prove the existence of synergistic effect between macerals gasification directly. The present work is to compare the performance indicators from simulated coals and the calculated values based on the weight of the macerals assuming additive properties apply. According to the previous studies [19,21,[42][43][44], the reasons for synergistic effect among macerals might be concluded as below. Liptinite holds the highest H/C followed by Vitrinite and Inertinite, when macerals are blended in gasifier, a large amount of hydrogen donors (H and OH radicals) produced from Liptinite involve in the decomposition of the remained macerals and suppress re-polymerization and crosslinking reactions of free radicals during gasification [43,44]. On the other hand, based on the works of [19,21,42], because Liptinite and Vitrinite occupy more hydrocarbon aliphatic and lower aliphatic, they are prone to produce more metaplast, which acts as the hydrogen donor solvent and stabilize more rupture fragments and free radicals produced by the Inertinite, resulting in enhancement of gasification performances.

Conclusions
This study revealed the gasification performance of a coal and its corresponding macerals and the interactions among macerals based on Aspen Plus process modelling. For the first time, the synergistic coefficient was quantified to show the extent of the interactions among macerals during gasification. Sensitivity analyses were conducted to demonstrate the effects of gasification temperature, oxygen to coal mass ratio and steam to coal mass ratio on the gasification performance of coal and individual macerals and also on the synergistic coefficients. The main conclusions are: