Study on gasification reaction and energy conversion characteristics of the entrained flow coal gasification based on chemical kinetics simulation

Abstract

Coal gasification that converts coal into syngas is a promising technology for efficient and clean utilization of coal. Current simulations of coal gasification are mostly based on equilibrium reactions, which cannot reflect the residence time and carbon conversion rate. In this paper, the reliable chemical kinetics simulation of the coal gasification is carried out considering the solid residence time and the effects of the main parameters on the gasification performance are analyzed. The results show that the higher the O₂/coal ratio, the higher the carbon conversion rate until it reaches 100 %. However, an excessively high O₂/coal ratio will reduce the cold gas efficiency. Therefore, the ideal cold gas efficiency is further proposed, revealing the energy saving potential of the system by reusing the ungasified coal. Based on this, the energy conversion characteristics of the combined cycle system were analyzed, showing that the effective conversion of coal to synthetic gas is the key to improving efficiency.

Nomenclature

Abbreviation
IGCC	integrated gasification combined cycle
Variables
xc	carbon conversion rate
η	cold gas efficiency
η*	the ideal cold gas efficiency
V	the volume of gas, m3/h
G	the mass flow of coal, kg/h
VC	the carbon element ratio
Vvolatile	the volatile yield
Vv,ar	the volatile ratio of dry ash-free basis coal
rc	the unreacted core radius
Rp	coal particle radius
x	the coal conversion rate after the completion of pyrolysis
f	the coal conversion rate at the completion of pyrolysis
P	the partial pressure, atm
R	the reaction rate, g/cm2·s
vs	the solids entrainment speed, m/s
vt	the particle final settling velocity, m/s
vs,i	the solids inlet velocity, m/s
vg	the gas flow rate, m/s
ρs	the solid phase density, kg/m3
ρg	the gas phase density, kg/m3
μ	the gas phase viscosity, Pa·s
dp	the solid particle diameter, m.
h	the height of the gasifier, m
e	the sum of common square errors
yS i	the simulation values of the component
yr i	the reference values of the component
Parameters
Qgas,LHV	the low heating value of gas, kJ/m3
Qcoal,LHV	the low heating value of coal, kJ/kg
kdiff	the gas film diffusion coefficient, g/cm2·atm·s
ks	the gas film diffusion coefficient, g/cm2·atm·s
kdash	the ash layer diffusion coefficient, g/cm2·atm·s
ε	the ash layer porosity
n	a coefficient between 2 and 3

1. Introduction

Coal remains the largest source of electricity in the world today, accounting for 36 % of the global total [1]. In 2022, coal demand hit a record high of 8415 Mt, an increase of 4%of which the demand for coal for power generation to 5687 Mt [2]. Due to the coal-dominated energy structure, it is not feasible to adjust the energy structure in the short term to meet the challenges of climate change. How to efficiently and cleanly utilize the current energy resources is of great significance. One of the most promising technologies at present is coal gasification technology, and the integrated gasification combined cycle (IGCC) system has been developed based on it [3].

Coal gasification technology is a non-combustion process in which coal reacts with a gasification agent (e.g. O₂, H₂O, air, or CO₂) at high temperature into combustible gases such as CO, H₂ and CH₄, as well as certain CO₂, H₂O, N₂ and other hydrocarbon compounds. The products can be used to produce heat, electricity or cogeneration, etc. Gasification technology can be divided into three types according to the gasification process: fixed bed (moving bed) gasification, fluidized bed gasification and entrained bed gasification [[4], [5], [6]]. The entrained bed gasification process has the advantages of high gasification intensity, high carbon conversion rate x_c, adaptability to many types of coal, and large production volume. Coal gasification research can be carried out by means of experiments and simulations. The simulation can obtain the variation of gasification process parameters, which can provide theoretical guidance, reduce the workload and guide the direction of the experiment. The currently accepted mathematical models that can describe the coal gasification process mainly include the equilibrium model and the kinetic model.

The equilibrium model [[7], [8], [9], [10], [11]] is based on the assumption that all parameters of the gasification process are in equilibrium, and predicts the composition and temperature of the syngas through the balance of mass and energy. Although the methodology has been well researched and proven effective over the years, it is difficult for the gasifier to achieve complete equilibrium in actual operation.

The kinetic model considers mass transfer, energy transfer, momentum transfer and its kinetic behavior in the gasification process, which can reflect the real gasification process to a certain extent. This model has always been the focus of gasification simulation research. For this reason, a variety of mathematical models have been established for this model, mainly including the zero-dimensional model, one-dimensional model, cell model and multi-dimensional model.

The zero-dimensional model considers that the components in the gasifier are mixed sufficiently. Assuming that all chemical reactions in the gasification process reach equilibrium, it is solved in combination with chemical reaction kinetics, mass conservation, energy conservation or the principle of minimum Gibbs free energy. Similar to the equilibrium model, the zero-dimensional model [[12], [13], [14], [15]] can only obtain the final solution, but cannot describe the actual operation process. The one-dimensional model [14] establishes a conservative differential equation in the flow direction of the material in the gasifier, which can reflect the parameter distribution in the flow direction. It is suitable for gasification technology similar to the plug flow model. The cell model [12,13,15,16] is also a series zero-dimensional model, which divides the gasifier into multiple cells. Considering the mass and energy conservation and minimum Gibbs free energy of each cell, all cells are integrated for the solution. This model has been studied a lot, and it can easily obtain the distribution of parameters such as component concentration, temperature, and carbon conversion rate x_c in the direction of material flow.

Compared with the models mentioned above, the multi-dimensional model takes more comprehensive considerations. Based on the principles of computational fluid dynamics, computational heat transfer and computational chemical kinetics, the discrete differential equations can be solved by utilizing numerical methods. The multi-dimensional model [14,17] can deeply reveal the reaction process in the gasifier and the distribution of parameters in the reaction gasifier. However, due to the problems of multi-factor coupling, poor modeling replicability and long calculation time, this model has not yet been applied to guide industrial design.

Consequently, most studies [[18], [19], [20], [21], [22], [23], [24]] focus on kinetic parameters during gasification based on experiments. Aprianti et al. [18] selected four iso-conversional models for determining the activation energy through thermogravimetric analysis and determined that Starink is the most appropriate method for determining the activation energy coal gasification. Meng et al. [20] carried out the gasification experiments of small coal char particle and large coal char particle under different gasification temperatures, and calculated the effective factors and kinetic parameters in the conversion process to investigate the diffusion effect and the evolution of kinetic parameters in the CO₂ gasification process of coal char. Mandapati and Ghodke [19] evaluated the applicability of the commonly used pyrolysis kinetic model of Indian lignite through experiments under different conditions. If the temperature is known, the proposed kinetic parameters can be used to determine the yield of coke and other gaseous substances at any time.

Most current coal gasification chemical kinetics models are developed based on the devolatilization model. The semi-coke and volatile yields of the process are based on the empirical matrix equation proposed by D. Merrick [25]. Some studies simplify the equation without considering the formation of tar, and some studies further discuss the coefficients of the equation [17]. However, different studies have shown that the semi-coke yield and composition of coal produced at different temperatures are different, which can only be obtained by experiment or assumption, and the equation is still largely inseparable from the actual experimental results.

The novelty of this paper is that a simplified but reliable kinetic model of entrained flow gasification is established, which reduces the dependence on experimental results and can reflect the effects of residence time and carbon conversion rate. In the proposed kinetic model, coal is directly cracked into elemental substances, and part of the solid-phase C elemental substance and the gas phase are separated for kinetic simulation. And the carbon conversion rate x_c, the main synthesis gas composition and temperature were compared with the operation value, experiment value and the literature value, and the simulation results were in reasonable agreement. The model was used to simulate and analyze the effects of gasifier size, pressure, O₂/coal ratio and H₂O/coal ratio on synthesis gas composition, temperature, residence time, carbon conversion rate and cold gas efficiency. The entrained-flow gasifier model established in this paper can provide some theoretical data and guidance for the design and operation of entrained flow coal gasifier. On this basis, the energy conversion characteristics of the integrated gasification combined cycle system were further analyzed by estimating the efficiency.

2. Process and evaluation index

2.1. Entrained flow coal gasification

Entrained flow gasifier has the advantages of large capacity, high adaptability to coal types, strong variable load capacity, high cold gas efficiency η and carbon conversion rate x_c. It is one of the most widely used and experienced gasification processes at present. In the gasifier, coal is first pyrolyzed into semi-coke, gas (the main components are regarded as CO₂, CO, H₂, CH₄, H₂O, H₂S and N₂) and tar (regarded as C₆H₆). The reaction is shown as Eq. (1):

$$\text{coal}\to \text{semi}-\text{coke}+\text{CO}+{\mathrm{H}}_2+{\mathrm{H}}_2\mathrm{O}+{\text{CO}}_2+{\text{CH}}_4+{\mathrm{H}}_2\mathrm{S}+{\mathrm{N}}_2+\text{tar}\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$$ (1)

The semi-coke after pyrolysis is further gasified with gas. The main reactions [4,5,26] in the gasification process are shown as Eqs. (2), (3), (4), (5), (6), (7), (8), (9), (10), (11), (12):

$$\mathrm{C}+\frac{1}{2}{\mathrm{O}}_2\begin{array}{c}\underset{‗}{}\end{array}\text{CO}\Delta \mathrm{H}=-111\text{MJ}/\text{koml}$$ (2)

$$\mathrm{C}+{\mathrm{O}}_2\begin{array}{c}\underset{‗}{}\end{array}{\text{CO}}_2\Delta \mathrm{H}=-393.5\text{MJ}/\text{koml}$$ (3)

$$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\begin{array}{c}\underset{‗}{}\end{array}\text{CO}+{\mathrm{H}}_2\Delta \mathrm{H}=+131\text{MJ}/\text{koml}$$ (4)

$$\mathrm{C}+{\text{CO}}_2\begin{array}{c}\underset{‗}{}\end{array}2\text{CO}\Delta \mathrm{H}=+172\text{MJ}/\text{kmol}$$ (5)

$$\mathrm{C}+2{\mathrm{H}}_2\begin{array}{c}\underset{‗}{}\end{array}{\text{CH}}_4\Delta \mathrm{H}=-75\text{MJ}/\text{kmol}$$ (6)

$${\mathrm{H}}_2+\frac{1}{2}{\mathrm{O}}_2\begin{array}{c}\underset{‗}{}\end{array}{\mathrm{H}}_2\mathrm{O}\Delta \mathrm{H}=-242\text{MJ}/\text{kmol}$$ (7)

$$\text{CO}+\frac{1}{2}{\mathrm{O}}_2\begin{array}{c}\underset{‗}{}\end{array}{\text{CO}}_2\Delta \mathrm{H}=-283\text{MJ}/\text{kmol}$$ (8)

$${\text{CH}}_4+2{\mathrm{O}}_2\begin{array}{c}\underset{‗}{}\end{array}{\text{CO}}_2+2{\mathrm{H}}_2\mathrm{O}\Delta \mathrm{H}=-890\text{MJ}/\text{kmol}$$ (9)

$$\text{CO}+{\mathrm{H}}_2\mathrm{O}\begin{array}{c}\underset{‗}{}\end{array}{\text{CO}}_2+{\mathrm{H}}_2\Delta \mathrm{H}=-41\text{MJ}/\text{kmol}$$ (10)

$${\text{CH}}_4+{\mathrm{H}}_2\mathrm{O}\begin{array}{c}\underset{‗}{}\end{array}\text{CO}+3{\mathrm{H}}_2\Delta \mathrm{H}=+206\text{MJ}/\text{kmol}$$ (11)

Reactions (2) and (3) are combined as follows:

$$\mathrm{C}+\frac{1}{\phi }{\mathrm{O}}_2\begin{array}{c}\underset{‗}{}\end{array}2\left(1-\frac{1}{\phi }\right)\text{CO}+\left(\frac{2}{\phi }-1\right){\text{CO}}_2$$ (12)

whereφ represents the combined components of CO and CO₂.

2.2. Evaluation index

The performance of gasification is mainly reflected by carbon conversion rate x_c and cold gas efficiency η. The higher the carbon conversion rate x_c and cold gas efficiency η, the more chemical energy of coal is converted into chemical energy of syngas, which is more conducive to improving the efficiency of the subsequent utilization process. The definition and calculation of the two indicators are shown as follows.

• (1)Carbon conversion rate x_c,

Carbon conversion rate x_c refers to the percentage of carbon contained in coal converted into carbon content in syngas in the gasifier. To ensure the coal gasification efficiency, the carbon conversion rate x_c should be as high as possible. The carbon conversion rate x_c is shown as Eq. (13).

$$x_{\mathrm{c}}=\left(1-\frac{\text{remaining}\text{carbon}\text{mass}}{\text{total}\text{carbon}\text{mass}}\right)\times 100\%$$ (13)

• (2)Cold gas efficiency η,

The cold gas efficiency η is shown as Eq. (14).

$$\eta =\frac{\text{chemical}\text{energy}\text{of}\text{syngas}}{\text{chemical}\text{energy}\text{of}\text{coal}}=\frac{V\cdot Q_{\text{gas},\text{LHV}}}{G\cdot Q_{\text{gas},\text{LHV}}}$$ (14)

where Q_gas,LHV is the low heating value of gas, kJ/m³. Q_coal,LHV is the low heating value of coal, kJ/kg. V is the volume of gas, m³/h. G is the mass flow of coal, kg/h.

Carbon may not be completely converted in the gasification process. If the remaining coal has not been gasified is reused, higher energy utilization efficiency can be achieved, and the ideal cold gas efficiency η* is proposed. The ideal cold gas efficiency η* is shown as Eq. (15).

$${\eta }^{*}=\frac{\text{chemical}\text{energy}\text{of}\text{syngas}}{\text{chemical}\text{energy}\text{of}\text{coal}-\text{chemical}\text{energy}\text{of}\text{ungasified}\text{coal}}=\frac{V\cdot Q_{\text{gas},\text{LHV}}}{\left(G-G_{\text{re}}\right)\cdot Q_{\text{gas},\text{LHV}}}$$ (15)

3. Modeling process

3.1. Modeling purpose and assumptions

Although the equilibrium model established based on minimum Gibbs free energy can predict the composition of syngas, it is based on the premise of complete conversion of carbon. It is unknown how long and under what operating conditions the carbon conversion rate x_c can reach 100 %. The focus of this model is not to predict the actual gasification reaction process from the starting to ending, but to predict the influence of reaction condition changing on the thermodynamic and kinetic characteristics of gasification reaction for a gasifier that is in a quasi-stable state and in a varying work condition operation. This means the chemical kinetics model is suitable to relatively accurately predict the gasification property (such as the main syngas composition, the residence time and the carbon conversion rate) as the gasification reaction is nearly completed, and to predict the influence of the reaction conditions changing on the gasification property after the coal gasification process enters the gasification stage.

Reasonable assumptions are as follows.

• (1)The starting process of coal gasification (such as coal coking and volatile partial combustion) is simply assumed as a specified state, and then, the chemical kinetics mechanism of the main gas-solid reactions is focused.
• (2)The influence of flow and heat transfer imbalance is not considered, only homogeneous and heterogeneous chemical kinetics are considered, and heat loss is not considered;
• (3)The components in the gasifier are fully mixed. For a relatively slow gas-solid reaction, the gas-phase combustion process can be completed instantly, and the heat released by combustion provides energy for the gas-solid reaction.

3.2. Chemical kinetics mechanism description for the simplified model

The current coal gasification chemical kinetics models are basically based on the devolatilization model, and the yield of the process is based on the empirical matrix equation proposed by D.Merrick [25], as shown as Eq. (16):

$$\left[\begin{array}{cccccccccc}0.98& 0.75& 0.8& 0.4286& 0.2727& 0.85& 0& 0& 0& 0\\ 0.02& 0.25& 0.2& 0& 0& 0.082& 1& 0.1111& 0.1765& 0.0588\\ 0.02& 0& 0& 0.5714& 0.7273& 0.049& 0& 0.8889& 0& 0\\ 0.1& 0& 0& 0& 0& 0.009& 0& 0& 0.8235& 0\\ 0.006& 0& 0& 0& 0& 0.01& 0& 0& 0& 0.9412\\ 1& 0& 0& 0& 0& 0& 0& 0& 0& 0\\ 0& 1& 0& 0& 0& 0& 0& 0& 0& 0\\ 0& 0& 1& 0& 0& 0& 0& 0& 0& 0\\ 0& 0& 0& 1& 0& 0& 0& 0& 0& 0\\ 0& 0& 0& 0& 1& 0& 0& 0& 0& 0\end{array}\right]\left[\begin{array}{c}\text{CHAR}\\ {\text{CH}}_4\\ {\mathrm{C}}_2{\mathrm{H}}_6\\ \text{CO}\\ {\text{CO}}_2\\ \text{TAR}\\ {\mathrm{H}}_2\\ {\mathrm{H}}_2\mathrm{O}\\ {\text{NH}}_3\\ {\mathrm{H}}_2\mathrm{S}\end{array}\right]=\left[\begin{array}{c}\mathrm{C}\\ \mathrm{H}\\ \mathrm{O}\\ \mathrm{N}\\ \mathrm{S}\\ 1-\mathrm{V}\\ 1.31\mathrm{H}\\ 0.22\mathrm{H}\\ 0.32\mathrm{O}\\ 0.15\mathrm{O}\end{array}\right]$$ (16)

In this matrix, the first equation is carbon balance, so the first row corresponds to the C content of semi-coke, CH₄, C₂H₆, etc. Rows 2 to 5 represent the balance of H, O, N, and S. Therefore, the first five entries in each column include the final analysis for species such as semi-coke, CH₄, C₂H₆, etc. Row 6 represents the overall mass balance. Rows 7 to 10 represent the correlations of CH₄, C₂H₆, CO and CO₂ production, respectively. The first column in this matrix equation and the coefficients in rows 7 to 10 on the right side of the equation all are obtained based on experimental values. The equation is still largely inseparable from the actual experimental results.

To avoid the limitation of experimental data, in this model, semi-coke (Char) is regarded as C, and the remaining pyrolysis and volatile combustion products are CH₄, CO, CO₂, H₂, H₂O, N₂, H₂S and TAR (C₆H₆). Firstly, the coal is decomposed into C, H, O, N, S and ash (treat the Cl element as part of the ash), and part of the C element is separated. Since the combustion process is very rapid and almost completely reacted, the remaining element reacts with O₂ to simulate the yield of each component when the volatile combustion is completed. Then they react with the separated solid C based on chemical kinetics. Since a large amount of heat is released from the combustion reaction, the temperature is relatively high when the combustion is completed, which provides heat for the subsequent gasification. The ratio of the separated solid C is calculated by Eqs. (17), (18):

$$V_{\mathrm{C}}=1-V_{\text{volatile}}$$ (17)

$$V_{\text{volatile}}=V_{\mathrm{v},\text{ar}}-0.36{V_{\mathrm{v},\text{ar}}}^2\left[25\right]$$ (18)

where V_C is the carbon element ratio. V_volatile is the volatile yield. V_v,ar is the volatile ratio of dry ash-free basis coal.

It can be seen from 2.1 that gas-solid reaction and gas-phase reaction mainly occur in coal gasification process. The unreacted shrinking reaction core model is used to describe the gas-solid reaction, taking into account the effects of the ash layer diffusion, the gas film diffusion, and the chemical reaction. It is shown as Eq.(19), (20), (21):

$$R=\frac{1}{\frac{1}{k_{\text{diff}}}+\frac{1}{k_{\mathrm{s}}{Y}^2}+\frac{1}{k_{\text{dash}}}\left(\frac{1}{Y}-1\right)}\left(P_{\mathrm{i}}-P_{\mathrm{i}}^{*}\right)$$ (19)

in which,

$$k_{\text{dash}}=k_{\text{diff}}·{\epsilon }^{\mathrm{n}}$$ (20)

$$Y=\frac{r_{\mathrm{c}}}{r_{\mathrm{p}}}={\left(\frac{1-x}{1-f}\right)}^{\frac{1}{3}}$$ (21)

where k_diff is the gas film diffusion coefficient, g/cm²·atm·s. k_s is the gas film diffusion coefficient, g/cm²·atm·s. k_dash is the ash layer diffusion coefficient, g/cm²·atm·s. ε is the ash layer porosity and n is a coefficient between 2 and 3. In this model, ε and n are set as 0.75 and 2.5 respectively [27]. r_c and r_p the unreacted core radius and coal particle radius. x is the coal conversion rate at any time after the completion of pyrolysis. f is the coal conversion rate at the completion of pyrolysis. (P_i- P* i) is the partial pressure of element i, atm. R is the reaction rate, g/cm²·s.

The kinetic parameters of the main reactions in the coal gasification process are as Table 1, Table 2. 

$$v_{\mathrm{s}}=v_{\mathrm{s},\mathrm{i}}{e}^{-\text{bt}}+\left(v_{\mathrm{g}}+v_{\mathrm{t}}\right)\left(1-e^{-\text{bt}}\right)$$ (22)

in which,

$$b=\frac{18\mu }{{\rho }_{\mathrm{s}}{d}_{\mathrm{p}}^2}$$ (23)

$$v_{\mathrm{t}}=\frac{\left({\rho }_{\mathrm{s}}-{\rho }_{\mathrm{g}}\right)d_{\mathrm{p}}^{2}g}{18\mu }$$ (24)

where v_s is the solids entrainment speed, m/s. v_t is the particle final settling velocity, m/s. v_s,i is the solids inlet velocity, m/s. v_g is the gas flow rate, m/s. ρ_s and ρ_g are the solid phase and gas phase density, kg/m³. μ is the gas phase viscosity, Pa·s. d_p is the solid particle diameter, m.

$$h={\int }_0^t{v}_{\mathrm{s}}dt=\frac{v_{\mathrm{s},\mathrm{i}}}{b}\left(1-e^{-\text{bt}}\right)+\left(v_{\mathrm{g}}+v_{\mathrm{t}}\right)\left(t-\frac{1-e^{-\text{bt}}}{b}\right)$$ (25)

where h is the height of the gasifier, m.

Table 1.

Kinetic parameters of gas-solid reaction.

$k_{\text{diff}}$	$k_{\mathrm{s}}$	$P_{\mathrm{i}}-{P_{\mathrm{i}}}^{*}$	$K_{\text{eq}}$	Reaction
$\frac{10\times {10}^{-4}{\left(\frac{T}{2000}\right)}^{0.75}}{P_{\mathrm{t}}{d}_{\mathrm{p}}}$	$247e^{-\frac{21060}{T}}$	$P_{{\mathrm{H}}_2\mathrm{O}}-\frac{P_{{\mathrm{H}}_2}{P}_{\text{CO}}}{K_{\text{eq}}}$	$K_{\text{eq}}=e^{17.644-\frac{30260}{1.8T}}$	(4)
$\frac{7.45\times {10}^{-4}{\left(\frac{T}{2000}\right)}^{0.75}}{P_{\mathrm{t}}{d}_{\mathrm{p}}}$	$247e^{-\frac{21060}{T}}$	$P_{{\text{CO}}_2}$	–	(5)
$\frac{1.33\times {10}^{-3}{\left(\frac{T}{2000}\right)}^{0.75}}{P_{\mathrm{t}}{d}_{\mathrm{p}}}$	$0.12e^{-\frac{17921}{T}}$	$P_{{\mathrm{H}}_2}-\sqrt{\frac{P_{{\text{CH}}_4}}{K_{\text{eq}}}}$	$K_{\text{eq}}=\frac{0.175}{34173}{e}^{\frac{18400}{1.8T}}$	(6)
$\frac{0.292\phi \left(\frac{4.26}{T}\right){\left(\frac{T}{1800}\right)}^{1.75}}{P_{\mathrm{t}}{d}_{\mathrm{p}}}$	$8710e^{-\frac{17967}{T}}$	$P_{{\mathrm{O}}_2}$	–	(12)

Table 2.

Kinetic parameters of gas phase reaction.

$R$	Reaction
$8.83\times {10}^5{e}^{-\frac{9.976\times {10}^4}{8.315T}}{C}_{{\mathrm{H}}_2}{C}_{{\mathrm{O}}_2}$	(7)
$30.9e^{-\frac{9.976\times {10}^4}{8.315T}}{C}_{\text{CO}}{C}_{{\mathrm{O}}_2}$	(8)
$3.552\times {10}^{11}{e}^{-\frac{9.304\times {10}^5}{8.315T}}{C}_{{\text{CH}}_4}{C}_{{\mathrm{O}}_2}$	(9)
$0.554\times {10}^5\left(\frac{P_{\text{CO}}}{P_{\mathrm{t}}}-\frac{1}{P_{\mathrm{t}}}·\frac{P_{{\text{CO}}_2}{P}_{{\mathrm{H}}_2}}{e^{-3.6893+\frac{7234}{1.8T}}{P}_{{\mathrm{H}}_2\mathrm{O}}}\right)·e^{-\frac{27760}{1.987T}}·P_{\mathrm{t}}^{0.5-\frac{P_{\mathrm{t}}}{250}}·e^{-8.91+\frac{5553}{T}}$	(10)
$312e^{-\frac{30000}{1.987T}}\left(C_{{\text{CH}}_4}-\frac{C_{\text{CO}}·C_{{\mathrm{H}}_2}^3}{e^{33.1371-\frac{25014.0499}{T}}·C_{{\mathrm{H}}_2\mathrm{O}}}\right)$	(11)

The solids residence time can be calculated as Eq.(22), (23), (24), (25) [16].

3.3. Implementation of the simplified chemical kinetics model by Aspen Plus

Based on the ASPEN PLUS software, the coal gasification process under the proposed chemical kinetics model is simulated as shown in Fig. 1. Coal is firstly cracked directly into elements in the RStoic reactor. After the coal is cracked into elements, the semi-coke and volatile yields are calculated. The semi-coke is regarded as C, part of C (calculated by Eq. (17)) is separated in the SEP as the C stream and enters the RPlug reactor. The other components are separated out as the MIX steam and enter the RGibbs reactor, then enter the RPlug reactor and react with C element based on chemical kinetics to produce syngas.

Fig. 1.

Kinetic model of entrained flow gasifier with C separation.

In the established ASPEN PLUS model, coal is considered an unconventional component that can not react directly. The RStoic reactor is used to convert unconventional components to conventional components. This process is accomplished by programming the conservation of elements into the calculator module. The RGbiss reactor is used to calculate the yield of each component at the end of volatile combustion. The RPlug reactor is used to calculate the syngas based on chemical kinetics. The kinetic parameters from the previous section are compiled in FORTRAN and linked into the RPlug reactor to produce an object module file to calculate the gasification product components affected by chemical kinetics. Besides, the kinetic model does not consider the heat transfer in the gasifier, and the heat loss is neglected. Fig. 2 shows the framework of the entrained flow gasifier kinetic model.

Fig. 2.

Framework of the entrained flow gasifier kinetic model.

4. Case study

4.1. Basic data

The coal types and inlet conditions selected in this simulation are shown in Table 3 (Cl is regarded as ash composition in simulation) . For coal with unknown heating value, the Boie relationship is used in Aspen Plus to calculate, and the result is the dry basis high heating value. The Boie relation is shown in Eq. (26):

$$Q=\left(151.2w_{\mathrm{C}}+499.77w_{\mathrm{H}}+45.0w_{\mathrm{S}}-47.7w_{\mathrm{O}}+27.0w_{\mathrm{N}}\right)\times {10}^2-189.0$$ (26)

where w_C, w_H, w_S, w_O and w_N are the numerical coefficients of the elements of C, H, S, O and N, respectively [28].

Table 3.

Coal used in the model.

Coal	Gasification pressure	Coal feed	Gasification agent/Coal			Ultimate analysis							Number
	MPa	kg/h	m(O2)/m(coal)	m(H2O)/m(coal)	m(N2)/m(coal)	C	H	N	Cl	S	O	A
Illinois No.6①	2.431	275.98	0.866	0.241	–	74.05	6.25	0.71	0.37	1.77	1.32	15.53	a
Illinois No.6②	2.431	275.98	0.768	0.314	–	73.04	5.82	0.73	0.48	1.37	1.7	16.83	b
Illinois No.6③	4.083	41670	0.86	0.5	0.017	69.55	5.34	1.25	0	3.86	10	10	c
	2.413	41670	0.8	0.08	0.13								d
Shangwan coal	1	8333.33	0.78	0.32	0.36	74.83	4.07	0.86	0	0.65	11.84	7.75	e

4.2. Model validation

The inlet parameters of the equilibrium model and the chemical kinetics model are consistent. The equilibrium model and the chemical kinetics model are simulated respectively and compared with the experimental values and literature values. The final comparison results of gas components are shown in Table 4, Table 5, Table 6. Table 4 lists the simulation results of Shangwan coal. Table 5 lists the simulation results of Illinois No.6③ under different operating conditions. Table 6 lists the simulation results of Illinois No.6 ① and Illinois No.6 ②. Among them, the references in Table 4 only cover the range of industrial operation values. The difference between “dry” and “converted” is whether N₂ is considered. The reference in Table 5 do not give the carbon conversion rate. The reference in Table 6 has the most complete data.

Table 4.

The simulation results and the main syngas of industrial operation values of the Shangwan coal model.

Syngas	Industrial operation e [17]	Zhu [17]	Zhu (dry)	Zhu (converted)	Equilibrium model	Equilibrium model (dry)	Equilibrium model (converted)	Chemical kinetics model	Chemical kinetics model (dry)	Chemical kinetics model (converted)
Temperature/K		1609.00			1517.78			1519.57
CO	54–56	47.19	50.15	57.69	48.06	52.15	59.48	48.23	52.47	59.78
H2	37–29	27.93	29.68	34.14	27.53	29.87	34.07	27.23	29.63	33.98
CO2	7–8	6.68	7.10	8.17	5.21	5.65	6.45	5.03	5.48	6.23
H2O		5.91			7.84			8.09
H2S		–			0.17	0.19		0.17	0.19
N2		12.29	13.06		11.19	12.14		11.20	12.18
CH4		–			0.00	0.00		0.04	0.05

Table 5.

Comparison of model simulation results and experimental values for Illinois No,6③ under different operating conditions.

Syngas	Experimental c [10]	Calculated by wu [29]	Equilibrium model	Chemical Kinetics Model	Experimental d [10]	Calculated by wu [29]	Equilibrium model	Chemical Kinetics Model
Temperature/°C	E		1281.85	1265.06	E		1485.86	1490.98
CO	41.00	40.90	42.38	42.59	61.50	61.20	60.58	60.63
H2	29.80	29.90	29.57	28.77	30.60	31.01	29.73	29.47
CO2	10.20	9.70	9.05	8.55	1.60	1.20	1.08	1.03
CH4	0.30	0.06	0.02	0.09	0.00	0.02	0.01	0.09
N2	0.80	0.75	0.39	0.93	4.70	5.20	5.42	5.42
H2S	1.10	0.98	1.07	1.07	1.20	1.20	1.28	1.28
H2O	17.10	17.60	17.50	17.99	–		1.90	1.03
Error with experimental value		0.59	3.68	7.17		0.67	2.41	2.89
Error with equilibrium model				1.47				0.84

E is the experimental temperature. It is not given in the reference, and expressed like this.

Table 6.

Carbon conversion rate x_c of Illinois No. 6 ① and Illinois No. 6 ② model simulation results.

Syngas	Temperature /K	CO/vol%	H2v ol%	CO2 /vol%	CH4 /vol%	N2 /vol%	H2S /vol%	Error	$x_{\mathrm{c}}$
		(g/s)	(g/s)	(g/s)	(g/s)	(g/s)	(g/s)
Experimental a	E	57.57	39.13	3.95	0.12	0.12	0.06		98.64 %
		(123.77)	(6.01)	(9.985)	(0.15)	(0.53)	(0.133)
Wen and chaung [16]	1421.9	56.6	39.84	2.92	0.16	0.208	0.27	2.56	98.88 %
		(123.94)	(6.23)	(10.04)	(0.2)	(0.454)	(0.726)
Govind and shah [12]	–	55.46	39.99	3.95	0.109	0.219	0.272	5.25	98.10 %
		(120.8)	(6.22)	(13.51)	(0.135)	(0.476)	(0.72)
Equilibrium model	1416.48	57.55	38.16	2.99	0.49	0.25	0.55	2.26	100 %
		(124.57)	(5.94)	(10.16)	(0.62)	(0.54)	(1.44)
Chemical kinetics model	1428.17	57.4	38.58	3.07	0.19	0.25	0.55	1.37	97.96 %
		(122.29)	(5.92)	(10.60)	(0.22)	(0.54)	(1.44)
Experimental b	E	53.06	41	5.15	0.46	0.07	0.21		90.66 %
		(112.52)	(6.211)	(17.2)	(0.56)	(0.1513)	(0.59)
Wen and chaung [16]	1421.9	54.11	41.39	3.89	0.193	0.204	0.212	2.93	93.29 %
		(119.78)	(6.54)	(13.54)	(0.242)	(0.451)	(0.57)
Govind and shah [12]	–	51.794	41.688	5.95	0.15	0.219	0.214	2.83	88.26 %
		(102.03)	(6.26)	(19.65)	(0.18)	(0.461)	(0.547)
Equilibrium model	1301.06	54.97	37.89	4.28	2.16	0.27	0.44	17.06	100 %
		(121.29)	(6.02)	(14.82)	(2.73)	(0.59)	(1.18)
Chemical kinetics model	1337.31	53.42	41.04	4.8	0.18	0.28	0.22	0.38	90.92 %
		(114.69)	(6.31)	(16.35)	(0.22)	(0.59)	(1.18)

a,b: Illinois No.6① and Illinois No.6②.

The error is analyzed by the sum of square errors [10,[29], [30], [31]], and the definition formula is as shown as Eq. (27):

$$e=\sum _{i=1}^n{\left(y_{\mathrm{i}}^{\mathrm{s}}-y_{\mathrm{i}}^{\mathrm{r}}\right)}^2$$ (27)

in which, e is the sum of common square errors, yS i, yr i are the simulation and reference values of the component, respectively. n is the number of syngas components.

It can be seen from the table that the simulation values of syngas composition and temperature show a reasonable agreement compared with experimental data and verifies the reliability of the model and the calculating method. Considering the carbon conversion rate x_c, the accuracy of the kinetic model is higher than that of the equilibrium model for the prediction of syngas composition.

Among the main components of syngas, CO, H₂ and CH₄, the prediction accuracy of CO and H₂ is higher, while the accuracy of CH₄ is slightly lower. Since the volume of CH₄ in the actual syngas is very small, the error of the simulation results has little effect on the calorific value of the syngas, which can be regarded as a reliable simulation of the main components of the syngas. The prediction results of other components in syngas, H₂S and N₂, are slightly poor. This is because of the simplification of the simulation process to improve improve the calculation speed, N in gasification coal is regarded as all converted to N₂ and S is regarded as all converted to H₂S. In the actual operation process, S and N will be converted to more than one product of H₂S and N₂, and the gas-solid reaction kinetics of S is not considered in the model. As a result, there is a slightly higher error between the predicted value and the experimental value. In Table 5 can be seen, the error between the simulated value and the experimental value is higher than the error between the simulated value of the equilibrium model and the simulated value of the kinetic model. This may be since the heating value of coal is calculated, which may be inconsistent with the heating value of the original experiment, so even the equilibrium model has some errors. Moreover, the carbon conversion rate x_c of c, d and e is also unknown, which may also affect the final component. In the current study, it is considered that the minimization of Gibbs free energy is reliable for predicting gas composition, so the basic agreement between the chemical kinetics model and the equilibrium model can be regarded as reliable.

Table 6 shows the simulation results for cases a and b. The height of the gasification stage of the gasifier is set as 3.1 m, the diameter is 1.5 m, and the coal particle size is set as 350 μm refer to Ref. [32]. In addition to syngas composition, carbon conversion rate x_c and residence time are further compared. And the error of the comparison results is reasonable. The model is considered reliable. And it can be seen from the results of this table that when the carbon conversion rate x_c is not high enough, the error between the predicted results of the equilibrium model and the actual operating value is larger.

Fig. 3, Fig. 4 show the main syngas components and the changes in the carbon conversion rate x_c during the gasification process. It can be seen from the figure that the simulation results of the carbon conversion rate x_c and the changing trend of the main syngas components in the whole process are consistent with the results in the literature [12,16]. The fit is slightly lower in the front part of the figures. This is because the gasifier of this model is established after the coal is cracked into semi-coke and volatiles, and the volatiles are rapidly combusted. Since the coal cracking and volatile combustion process always occurs rapidly, this assumption is reliable.

Fig. 3.

(a) Syngas composition calculated by Wen et al. [16]

Fig. 3-(b) Syngas composition calculated by Govind et al. [12]

Fig. 3-(c) Syngas composition calculated by the model proposed in this paper.

Fig. 4.

(a) Temperature and carbon conversion rate x_c calculated by Wen et al. [16]

Fig. 4-(b) Carbon conversion rate x_c calculated by Govind et al. [12]

Fig. 4-(c) Carbon conversion rate x_c calculated by the model proposed in this paper.

It can be seen from the above results that the chemical kinetics model can reasonably reflect the entrained-flow gasification reaction process. The final synthesis gas composition prediction is accurate, and the model is reliable.

5. Impact on gasification reaction characteristics

It can be seen from the theory and the results from the model that under the same gasification conditions, the carbon conversion rate x_c of the gasifier is related to the residence time of coal particles in the gasifier. The longer the residence time, the more sufficient the reaction and the higher the carbon conversion rate x_c (until equilibrium). However, there are many influencing parameters of the gasification reaction. A factor enhances the gasification reaction, but at the same time may shorten the residence time. Thus it has different effects on the carbon conversion rate x_c and the cold gas efficiency η. Based on these reasons, this paper further analyzes the influence of different influencing factors on the gasification process. In order to study the influence of gasification parameters on entrained flow gasification, this paper takes the coal type ‘a’ model in Table 6 as the basic model to carry out a series of analysis on the gasification process. Based on the above model, the effects of gasification parameters such as gasifier size, pressure, residence time, O₂/coal ratio and H₂O/coal ratio on the gasification process and syngas composition are discussed in detail, and the mechanism and law of these effects are analyzed.

5.1. Effect of gasifier size

Taking the gasification model in 4.2 as the benchmark, keeping the same inlet parameters of the gasifier, the volume of the gasifier is expanded to 3.25 times in equal proportion at intervals of 0.25. The results are shown in Fig. 5.

Fig. 5.

Variation of gasification characteristics with gasifier volume.

It can be seen from Fig. 5(a–d) that with the increase of gasifier volume, the carbon conversion rate x_c increases from 97.96 % to 99.94 %, residence time increases from 8.50 s to 30.74 s, cold gas efficiency η increases from 81.89 % to 83.92 %, ideal cold gas efficiency η* increases from 83.59 % to 83.98 %, and the syngas temperature decreases from 1439.20 K to 1402.61 K, respectively. It has little effect on the volume of each main gas in the syngas. Since the inlet parameters of coal and gasifying agent remain unchanged and the gasifier becomes larger, the residence time increases. The carbon conversion rate x_c and cold gas efficiency η are improved with the increase of the volume of gasifier. It is because the residence time is longer due to the larger gasifier volume, the reaction is more complete, and there is less coal remaining.

It can be seen from Fig. 5-(d) that the main syngas composition change little in several cases. As the reaction progresses, the carbon conversion rate x_c increases and the composition of carbon-containing compounds further decreases. Although there is little difference in composition, it has a great impact on the heating value of syngas, thereby affecting cold gas efficiency η. And the volume fraction of CO and H₂ increases slightly, while the volume fraction of CO₂ and H₂O decreases slightly, which is consistent with the trend of temperature.

In order to clarify the radial and vertical effects of gasifier, the influence of gasifier diameter and height on gasification results is further analyzed. Taking the gasification model in 4.2 as the benchmark, keeping the same inlet parameters of the gasifier, the size of the gasifier is respectively (1) keeping the height of the gasifier unchanged, and the diameter of the gasifier shall be enlarged to 3.25 times in equal proportion with an interval of 0.25; (2) keeping the diameter of the gasifier unchanged, and the height of the gasifier shall be enlarged to 3.25 times in equal proportion with an interval of 0.25. The results obtained are shown in Fig. 6 (a-d)-7 (a-d).

Fig. 6.

Variation of gasification characteristics with gasifier diameter.

It can be seen from the figures that the influence trend of gasifier volume, diameter and height on gasification characteristics is the same. With the increase of gasifier size (diameter/height), the carbon conversion rate x_c increases from 97.96 % to 98.11 % and 99.90 %, residence time increases from 8.50 s to 9.05 s and 28.91 s, cold gas efficiency η increases from 81.89 % to 82.05 % and 83.89 %, ideal cold gas efficiency η* increases from 83.59 % to 83.62 % and 83.97 %, and the syngas temperature decreases from 1439.20 K to 1436.24 K and 1402.97 K, respectively.

Although the carbon conversion rate x_c, cold gas efficiency η and residence time all increase with the increase of diameter or height, it can be seen that the height of the gasifier has a great influence on the characteristics of syngas, while the diameter has little influence on it. This is because the height of the gasifier can directly affect the residence time as shown in Eq. (25). As for the diameter, while keeping the height of the gasifier unchanged and changing the diameter, the gas phase density ρ_g changes, which affects the changes of v_t, v_g and v_s. So the residence time change, but not significantly. Although the larger the size of the gasifier, the higher the carbon conversion rate x_c and cold gas efficiency η, the larger the size means the higher the capital cost and operation cost of the gasifier, which may not bring higher economic benefits. As can be seen from Fig. 5, Fig. 6, Fig. 7, if the ungasified coal can be reused, the gasifier can achieve a higher ideal cold gas efficiency η* when the size is small, which can improve the economy.

Fig. 7.

Variation of gasification characteristics with gasifier height.

5.2. Effect of O₂/coal ratio

Differences in reactant concentrations lead to differences in reaction rates, which affect residence time. Therefore, the effect of the ratio of coal and gasification agent on the syngas is further analyzed, and the results of O₂/coal ratio are shown in Fig. 8(a–d). Keeping the inlet coal amount unchanged, changing the oxygen amount, when the O₂/coal ratio increases from 0.75 to 1.05 at the intervals of 0.05, the carbon conversion rate x_c increases from 82.91 % to 100 %, where x_c already reaches 100 % when the O₂/coal ratio reaches 0.9. The cold gas efficiency η first increases from 71.80 % to 82.34 % until the O₂/coal ratio reaches 0.9 and then decreases from 82.34 % to 74.98 %. And the ideal cold gas efficiency η* first increases from 82.78 % to 83.32 % until the O₂/coal ratio reaches 0.85 and then decreases from 83.32 % to 74.98 %.

Fig. 8.

Variation of gasification characteristics with O₂/coal ratio.

The difference from the previous analysis results is that the previous carbon conversion rate x_c and the residence time t have the same trend of change. In this case, the carbon conversion rate x_c is increasing until equilibrium. The residence time t increases first and then decreases, although the overall decline was not significant. It can be seen that the residence time is not positively correlated with the oxygen-to-coal ratio. Fig. 8 - (d) shows the main syngas components at different O₂/coal ratios. It can be seen from the figure that the composition of syngas does not change monotonically with the change of O₂/coal ratio. The proportion of CO first increases and then decreases slightly with the increase of O₂/coal ratio. The proportion of H₂ decreases with the increase of O₂/coal ratio while the proportion of CO₂ remain relatively steady. With the increase of O₂/coal ratio, the proportion of H₂O first decreases slightly and then increases slightly, and the proportion of H₂O first decreases slightly and then increases with the increase of O₂/coal ratio. This is due to the increase of O₂/coal ratio, that is, the increase of oxygen flow, so that the combustion reaction is more intense and the temperature in the gasifier increases, which also means that more CO and H₂ react with O₂ to produce CO₂ and H₂O. However, at the same time, the increase of temperature will cause the equilibrium point of water gas reaction to shift to the left, which will lead to the increase of CO and H₂ and the decrease of the proportion of CO₂ and H₂O. There is a competitive relationship between the two. Fig. 8 - (c) shows the change results of syngas temperature with O₂/coal ratio. It can be seen from the figure that syngas temperature increases from 1439.20 K to 2025.96 K with the increase of O₂/coal ratio, which is also consistent with the trend of carbon conversion rate x_c.

The analysis shows that it is too high O₂/coal ratio will reduce the cold gas efficiency η. And the resulting high temperature greatly improves the requirements for materials. Considering the carbon conversion rate x_c and cold gas efficiency η, the optimal O₂/coal ratio is about 0.9 for this gasification situation. When the O₂/coal ratio is 0.85, although the carbon conversion rate x_c is lower, the ideal cold gas efficiency η* is the highest. If the coal that has not been gasified can be reused, it may be possible to achieve higher energy utilization efficiency.

5.3. Effect of H2O/coal ratio

Keeping the inlet coal amount and O₂/coal ratio unchanged, changing the H₂O/coal ratio, and analyzing the effect of the H₂O/coal ratio on the gasification characteristics, the results are shown in Fig. 9(a–d). It can be seen from the figure that the carbon conversion rate x_c, the cold gas efficiency η and the residence time t all increase first and then decrease with the increase of the H₂O/coal ratio, while the ideal cold gas efficiency η* continues to decrease. The carbon conversion rate x_c increases from 97.67 % to 98.15 % with the increase of H₂O/coal ratio from 0.2 to 0.35, and then decreases from 98.15 % to 97.56 % with the increase of H₂O/coal ratio from 0.35 to 0.6. The cold gas efficiency η increases from 81.69 % to 81.95 % with the increase of H₂O/coal ratio from 0.2 to 0.3, and then decreases from 81.95 % to 80.65 % with the increase of H₂O/coal ratio from 0.33 to 0.6. Meanwhile, the ideal cold gas efficiency η* decreases from 83.63 % to 82.66 %.

Fig. 9.

Variation of gasification characteristics with H₂O/coal ratio.

The increase in H₂O increases the water-gas reaction and reduces the temperature of the system, thereby shifting the equilibrium point of the reaction to the right, producing more CO₂ and H₂. The higher the concentration of reactants, the more complete the reaction and the higher the carbon conversion rate x_c. When the H₂O/coal ratio exceeds 0.35, it can be seen that the carbon conversion rate x_c begins to decrease slightly. At this time, the temperature is the main influence factor, resulting in a decrease in the carbon conversion rate x_c. As for the cold gas efficiency η, it is also affected by the amount of H₂O. And the amount of inlet H₂O also affects the syngas volume, resulting in changes in the cold gas efficiency η. It can be seen from the figure that the carbon conversion rate x_c reaches its peak at around 0.35, and the cold gas efficiency η reaches its peak at around 0.3. Therefore, the steam coal ratio is better between 0.3 and 0.35.

Fig. 9-(c) shows that the syngas temperature decreases as the H₂O/coal ratio increases, which is slightly different from the relationship between carbon conversion rate x_c and temperature described earlier. This is because the temperature is also affected by the amount of H₂O at this time. Fig. 9-(d) shows the results of the main syngas composition as a function of the H₂O/coal ratio. It can be seen from the figure that the proportion of H₂ basically does not change, the proportion of CO decreases with the increase of the H₂O/coal ratio, and the proportion of H₂O and CO₂ increases with the increase of the H₂O/coal ratio. In addition to the reasons mentioned above, the increasing inlet H₂O also has an impact on the proportion of the main syngas.

It is obvious that when other inlet parameters remain unchanged, and the H₂O/coal ratio increases, the increase in the amount of H₂O and the decrease in temperature are the main influencing factors of the gasification process. The increase of H₂O to a certain extent increases the carbon conversion rate x_c and reduces the temperature of the synthesis gas. It can be seen from the above analysis that the increase of CO₂ will also increase the carbon conversion rate x_c. It is possible to consider replacing H₂O with other gasification agents such as CO₂, or directly selecting coal with higher water content to reduce part of the water vapor gasification agent.

5.4. Effect of gasification pressure

The effect of pressure on gasification was further analyzed, and the results are shown in Fig. 10(a–d). It can be seen from the figure that with the increase of pressure from 20 atm to 40 atm, the carbon conversion rate x_c increases from 97.55 % to 98.84 %, the residence time increases from 8.32 s to 8.95 s, the cold gas efficiency η increases from 81.47 % to 82.75 %, the ideal cold gas efficiency η* increases from 83.52 % to 83.71 %, respectively. The increasing range of residence time and ideal cold gas efficiency η* is small. Generally, as the pressure increases, the gas volume fraction increases and the reaction rate increases. Under the same gasifier size and inlet parameters, the faster the reaction rate, the more complete the reaction proceeds, which is consistent with the trends of residence time, carbon conversion x_c and cold gas efficiency η in the figure. The temperature decreases from 1446.99 K to 1424.12 K with the increase of pressure. It shows that with the increase of gasification pressure, the endothermic reaction in the gasification reaction is enhanced. It can be seen that the pressure has little effect on the main syngas temperature.

Fig. 10.

Variation of gasification characteristics with gasification pressure.

6. Impact on energy conversion characteristics

Fig. 11 shows the IGCC system considering carbon conversion rate. When the carbon conversion rate is less than 100 %, the crude syngas produced by coal gasification is purified and then enters the combined cycle for power generation, while the remaining carbon directly enters the thermodynamic cycle for power generation.

Fig. 11.

The IGCC system considering carbon conversion rate.

Without considering heat loss and carbon sensible heat, the energy of coal gasification products can be divided into three parts: syngas chemical energy, syngas sensible heat physical energy, and carbon chemical energy. Based on the results of chemical kinetics simulation, the energy conversion characteristics of coal gasification technology for power generation can be obtained. The calculation method for thermal conversion of gasification products in this analysis process is shown in Fig. 12 and Table 7.

Fig. 12.

Utilization of gasification products.

Table 7.

Calculation method for efficiency.

Items	Syngas (Chemical energy combined cycle)	Syngas (Sensible steam cycle)	Carbon(Chemical energy steam cycle)
Value	57.8 %	40 %	40 %

Fig. 13 is the change of the energy proportion of syngas with residence time, and Fig. 14 is the change of the combined cycle efficiency with time. As can be seen from Fig. 13, the proportion of chemical energy in syngas increases sharply and then tends to plateau as the gasification process progresses. The proportion of sensible heat and carbon energy first decreases sharply and then tends to plateau. As can be seen from Fig. 14, the combined cycle efficiency considering the reuse of carbon significantly improves when the conversion rate of C is low. And its estimated combined cycle efficiency is consistent with the trend of syngas chemical energy, indicating that the effective conversion of coal to synthetic gas is the key to improving energy conversion efficiency.

Fig. 13.

The conversion of energy of syngas products with time.

Fig. 14.

Estimated combined cycle efficiency.

7. Conclusion

In this paper, the simplified chemical kinetics model of the entrained flow gasification reflecting the effects of residence time and carbon conversion rate is established. The reliability of the model is verified by comparison with the carbon conversion rate x_c and the main syngas composition from other data. On this basis, the impacts of critical parameters (e.g. gasifier size, O₂/coal ratio, H₂O/coal ratio and gasification pressure) on the performance of entrained flow gasification process (e.g. synthesis gas composition, temperature, residence time, carbon conversion rate x_c, cold gas efficiency η and ideal cold gas efficiency η*) were simulated and detailly analyzed for gasifier design and operation. The conclusions are as follows.

• (1)Considering that part of the coal has not been gasified, the concept of ideal cold gas efficiency η* is proposed. It is found that the value of η* can be high despite insufficient the carbon conversion rate x_c and the cold gas efficiency η. That is, the energy utilization efficiency can be improved if the ungasified coal can be suitably reused.
• (2)With the increase of gasifier size (volume, diameter, and height), the carbon conversion rate x_c, cold gas efficiency η and ideal cold gas efficiency η* all increase, while the syngas temperature decreases. The gasifier size has little effect on the volume fraction of each main gas in the syngas. It is found that the height of the gasifier is the main factor affecting the gasification characteristics of the coal while the diameter of the gasifier has little effect.
• (3)O₂/coal ratio has the greatest influence on carbon conversion rate x_c and cold gas efficiency η. The carbon conversion rate x_c increases with the increase of O₂/coal ratio, and the cold gas efficiency η and η* first increases and then decreases. When the O₂/coal ratio is 0.85, although the carbon conversion rate x_c is lower, the ideal cold gas efficiency η* is the highest. The energy utilization efficiency can be improved if the ungasified coal can be reused, such as separating the ungasified coal into thermodynamic cycle for power generation.
• (4)When the H₂O/coal ratio increases, the temperature decreases and the water vapor concentration increases. The carbon conversion rate x_c and the cold gas efficiency η first increases and then decreases, but the effect of H₂O/coal ratio is much smaller than that of O₂/coal ratio. The results of the main syngas composition show that the increase of CO₂ will also increase the carbon conversion rate x_c. That is, by replacing H₂O with other gasification agents such as CO₂, or directly using the coal with higher water content to reduce part of the water vapor gasification agent, higher x_c can be obtained.
• (5)Based on the chemical kinetics model, the energy conversion characteristics of the combined cycle system were analyzed. The estimated IGCC efficiency is consistent with the trend of the proportion of chemical energy in syngas over time, showing that the effective conversion of coal to synthetic gas is the key to improving efficiency.

In all, the entrained-flow gasifier model established in this paper can provide some theoretical data and suggestions for gasifier design and operation.

CRediT authorship contribution statement

Lingjie Feng: Writing – review & editing, Writing – original draft, Validation, Software, Methodology, Conceptualization. Guoqiang Zhang: Writing – review & editing, Supervision, Methodology, Conceptualization. Rongrong Zhai: Writing – review & editing, Supervision, Methodology, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.