Process modelling of two-stage entrained-bed gasification composed of rapid pyrolysis and gasification processes

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Process modelling of two-stage entrained-bed gasification composed of rapid pyrolysis and gasification processes ⁎

⁎

Rui Gao, Bo Huang, Jian Huang, Jianliang Xu, Zhenghua Dai , Fuchen Wang East China University of Science and Technology, Shanghai 200237, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Two-stage Entrained-bed gasification Rapid pyrolysis Tar Framework

A novel two-stage entrained-bed gasification system composed of the rapid pyrolysis of pulverized coal and the gasification of char is proposed to obtain high-quality synthetic gas with low tar content and high methane content. A new framework containing a modified multi-step kinetic pyrolysis model and a non-linear programming (NLP) process is developed to simulate the rapid pyrolysis process of pulverized coal. The method can be used to simulate the volatile composition, which includes only the high-weight hydrocarbons/distillable liquids (benzene, toluene, xylene, naphthalene, pyrene, phenol) based on the proximate and ultimate analysis data of coal. The framework for coal rapid pyrolysis and the simulation of the two-stage entrained-bed gasification are validated by experimental data and semi-industrial data, respectively. This paper analyses the effects of the final pyrolysis temperature, the gas residence time, and the steam/coal ratio on the performance of the two-stage entrained-bed gasification system. The results indicate that by adjusting the gas residence time to 15.5 s at the final pyrolysis temperature of 1000 °C, the tar yield can be reduced to less than 500 mg/Nm3, the molar fraction of methane in the syngas can be approximately 5.84%, the exit temperature can be approximately 943 °C, and the higher heating value (HHV) of syngas and the cold gas efficiency (CGE) will be 11.41 MJ/(Nm3) and 91.23%, respectively.

1. Introduction Due to the strong market interest and the requirement to reduce greenhouse gas emissions, China’s energy industry and chemical industry have taken considerable interest in the clean conversion process of coal, especially for the development of coal-converted natural gas demonstration projects [1] and the removal processes of pollutants from raw fuel gas in the gasification process [2]. The Lurgi Fixed bed gasifier has been widely used in the traditional SNG (synthetic natural gas) production process [3]. However, it is restricted by some defects, such as the low capacity of a single furnace and the ability to use only lump coal with a particle size above 6 mm [4]. In addition, some pollutants such as tar, particulate matter, NH3, H2S, HCl and SO2 are inevitably produced during the gasification process, causing serious problems for downstream applications and the environment [2,5–7]. A two-stage gasifier that separates the fuel pyrolysis process from the char gasification process by physical separation can effectively promote the respective reaction processes [8]. Chen et al. [9–11] produced a two-stage gasification technology with an entrained bed and a fluidized bed that can produce CH4-rich syngas and tar, though tar formation is the main problem that restricts the further development of

⁎

the fluidized bed gasifier [12]. To overcome the adverse effect of tar, some researchers [4,13–15] have developed two-stage gasification technologies involving fluidized bed and fixed bed and found that syngas with a low tar content can be obtained at moderate temperatures by adjusting the system parameters or adding catalysts. The entrained-bed gasifier has been widely applied due to its high gasification intensity and wide adaptability to coal types [16]. According to the two-stage entrained-bed gasification technology of char gasification coupled with pulverized coal rapid pyrolysis, the 7 t/d laboratory-scale device and the 100 t/d pilot-scale plant [17] were established in Japan, mainly for the production of char, coal gas and tar. The experimental results [17] indicate that the content of heavy tar in the produced syngas is 70 g/Nm3 and the content of light oil (mainly homologue of benzene) is 30 g/Nm3. However, due to the limitations of the separation technology and the low temperature of approximately 600–800 °C for the produced syngas, the tar produces a large number of fines and cannot meet quality requirements. Fuel gas with a tar content of between 30 and 500 mg/Nm3 can be directly used in engines [18]. Therefore, the standard for high-quality syngas is that the tar content in the syngas is below 500 mg/Nm3. Therefore, it is essential and necessary to develop a novel

Corresponding authors. E-mail addresses: [email protected] (Z. Dai), [email protected] (F. Wang).

https://doi.org/10.1016/j.fuel.2019.116531 Received 9 March 2019; Received in revised form 20 August 2019; Accepted 28 October 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Rui Gao, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116531

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Nomenclature

APyr , coal Vppyr , j mfcoal wi, coal MWpyr , i Npyr , j Nai, j mfpyr , j ypyr , j R L G rate kdiff ks kdash Pgas, j Y x coal f S

conversion The initial internal reactive area The volatile yields in the proximate analysis [dry and ashfree basis], wt% The fixed carbon yields in the proximate analysis [dry and ash-free basis], wt% Higher heating value, MJ/Nm3 Cold gas efficiency, % Lower heating value, MJ/kmol

S0 VM

Coal elements set Pyrolysis products set The mass flow rate of coal, kg/s Mass fraction of element i in coal, % Molar weight of element i in coal, kg/kmol The molar flow rate of pyrolysis product j, kmol/s Number of atoms of element i in the molecular formula of pyrolysis product j The mass flow rate of pyrolysis product j, kg/kmol Mass fraction of pyrolysis product j, % Relaxation variables of the constraints of the composition ratio in the pyrolysis gas Relaxation variables of the constraints of the composition ratio of tar Relaxation variables of the constraints of the distribution ratio among the pyrolysis gas, tar, and water reaction rate, g·((cm2 of coal area)·s)−1 gas film diffusion constant, g·(cm2·atm·s)−1 surface reaction constant, g·(cm2·atm·s)−1 ash film diffusion constant, g·(cm2·atm·s)−1 The partial pressure of gas-phase pyrolysis product j, atm The thickness of the ash layer The conversion ratio of the original dry coal at any time after the end of pyrolysis The conversion ratio of the original dry coal at the end of pyrolysis The internal reactive area at any time during char

FC HHV CGE LHV

Greek symbols κ, φ γ ε ψ

Weighting factor of R Weighting factor of L Weighting factor of G voidage in the ash layer the structure parameter for a particular char particle

Subscripts i j s k z

Element index Pyrolysis product index Solid display index The equation index of the constraints of the composition ratio of the pyrolysis gas The equation index of the constraints of the composition ratio of tar

analysed in this study. As shown in Fig. 1, the two-stage entrained-bed gasifier mainly consists of two reaction zones: the lower gasification zone of the char and the upper reduction zone of the pulverized coal. In this system, pulverized coal of approximately 100 µm in average particle size is fed into the upper reduction zone and moves upward with the high-temperature syngas from the lower gasification zone. In the

gasification process that can produce low-tar content fuel gas using pulverized coal. In this study, a novel two-stage entrained pyrolysisgasification system including entrained-bed pyrolysis and entrainedbed gasification has been proposed and studied systematically. Different from the Japanese two-stage entrained bed gasification technology [17], all the pulverized coal is transported to the second-stage furnace, and then all the char produced from the second-stage furnace is fed into the first-stage furnace for gasification. The Aspen Plus simulation software was used to simulate a two-stage entrained-bed pyrolysis-gasification system. In addition, a new framework for a coal rapid pyrolysis process with strict element conservation was proposed, and it could determine the composition of volatiles, including only the high-weight hydrocarbons/distillable liquids (benzene, toluene, xylene, naphthalene, pyrene, and phenol) based on the proximate and ultimate data of coal. Moreover, based on the assumption of the tar component in this study, the predictive multi-step kinetic pyrolysis model of coal proposed by Sommariva [19] has been modified. According to the pyrolysis yield obtained through the modified multi-step kinetic pyrolysis model, a nonlinear programming (NLP) optimization model was established. The modified multi-step kinetic pyrolysis model and the two-stage entrained-bed gasification system model were verified according the available pyrolysis data for two coal samples [20,21] and semi-industrial data [17], respectively. Finally, the effects of the final pyrolysis temperature, the gas residence time and the steam/coal ratio on the two-stage entrained-flow system were investigated. By optimizing the system parameters, the tar content in the exit syngas can be reduced to below 500 mg/Nm3.

GAS

COAL GAS+CHAR CYCLONE POROUS FILTER

REDUCTOR

H2

N2 COAL

D2

2. Description of the two-stage entrained-bed gasification system

OXYGEN STEAM

To obtain high-quality methane-rich coal gas, a novel two-stage entrained system consisting of the entrained-flow pyrolysis of pulverized coal and the entrained gasification of char is proposed. Fig. 1 represents a schematic drawing of the two-stage entrained-bed gasifier

N2

GASIFIER

CHAR

H1

D1

Fig. 1. Schematic diagram of the updraft two-stage entrained-bed gasifier. 2

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H2O, C2H4, N2, H2S, NH3, SO, SO2, HCN, COS, C6H6, C7H8, C8H10, C10H8, C14H10, and C6H6O. The last 14 products can only be generated from the pyrolysis process. 4. The unconventional substance ash in coal is inactive and has no effect on any chemical reactions. 5. In the entrained-bed gasification process, the pyrolysis heating rate is very high (> 104 K/s) due to the very small coal particle size (< 150 μm). The complete release time of volatiles during the rapid pyrolysis process of coal is approximately 50 ms.

upper reduction zone, the drying process, rapid pyrolysis process, and gasification process of pulverized coal occur successively. After the drying and pyrolysis periods, the pyrolysis products flow upward with the high-temperature syngas (steam, hydrogen and carbon dioxide) and further gasify to produce small molecular compounds. The products flowing out of the gasifier include unreacted char and the required methane-rich high-quality coal gas. Then, after separation by a cyclone, the unreacted char is transported to the lower gasification zone by nitrogen. After the combustion and gasification of char with a gasification agent, the high-temperature syngas produced from the lower gasification zone is introduced into the upper reduction zone. Numerous studies [22–24] have shown that the syngas temperature in the entrained flow gasification process is high (often greater than 1600 K), and the composition of the exit syngas is close to that of thermodynamic equilibrium. For simplification, the Gibbs equilibrium model can be used to simulate the entrained-flow bed gasification. Subsequently, the high-temperature syngas enters the upper reduction zone and transfers heat with the pulverized coal, and then the reactions of pulverized coal mentioned above in the upper reduction zone begin.

3.1. Coal drying process model In the entrained-bed gasifier, the physical moisture in the pulverized coal can be rapidly released into the gaseous phase due to the high heating rate of pulverized coal. To simplify the simulation process, it is assumed that the drying process of the pulverized coal is instantaneous and that the dried coal does not contain any physical moisture. In our model, the drying process consists of two blocks, the RYield block and the SEP-1 block, respectively. The RYield is used to simulate the drying process of the complete release of moisture bound within the coal. The produced water vapor is then separated from the solid phase using the SEP-1 block.

3. Model development The updraft two-stage entrained-bed gasification system model was established in Aspen Plus software. In addition, the heterogeneous gasification kinetic reactions model is inserted into Aspen Plus simulator using an external FORTRAN 11.0 subroutine. The accuracy of the pulverized coal rapid pyrolysis model is crucial for the successful simulation of the two-stage entrained-bed gasification system. Therefore, a new framework is proposed to simulate the rapid pyrolysis process of pulverized coal using GAMS 24.8. The proximate and ultimate data of the coal used in this study are listed in Table 1. The Aspen Plus simulation model of the updraft two-stage entrained-bed gasifier is depicted in Fig. 2. According to the actual processes, three Aspen plus blocks, including a drying zone, pyrolysis zone, and gasification and combustion zone, are used to simulate the twostage entrained gasification. All of the above units are strictly modelled in Aspen Plus v9.0. The above model explains the main physical and chemical processes occurring in the gasifier, including the drying process of pulverized coal, the rapid pyrolysis process of dry coal, the secondary reactions of volatile components, and the combustion and gasification process of char. Table 2 lists the representative block operations used in the Aspen Plus simulation model. To make a reasonable comparison with the two-stage entrained-bed gasification system in Japan [17], the reaction pressure, the reactor size and the state of each inlet flow are the same as those of the Japanese gasifier. Four different pyrolysis temperatures are selected to investigate the effect of the final pyrolysis temperature on the overall system performance. To simplify the simulation of the two-stage entrained bed gasification system, the basic assumptions considered in this study are as follows:

3.2. Coal rapid pyrolysis framework The pyrolysis process of coal is extremely complicated due to the simultaneous occurrence of a large number of chemical and physical transformations. The RYield model has been widely used to simulate the pyrolysis process of pulverized coal in Aspen Plus software [25,26]. However, conventional pyrolysis products mainly comprising C(s), S(s), H2, N2, Cl2, O2, H2O, and ash cannot fully describe the composition of the pyrolysis products. A multi-step pyrolysis kinetics model [19], which is considered suitable for simulating slow pyrolysis and rapid pyrolysis processes, has been proposed recently. Additionally, this model adapts to a wide variety of coal types and does not require other coal characterization parameters in addition to the elemental composition of carbon, hydrogen and oxygen. The reason behind not considering the evolution of nitrogen and sulphur in coal is that these elements have little effect on the overall pyrolysis process [19,27–31]. Additionally, the distribution of N and S in the gas phase composition were discussed in the next Section 3.2 (vi). Although the multi-step kinetic pyrolysis model has wide adaptability and high accuracy and can define the elemental compositions of char and tar, the given specific molecular formulas of tar, such as C12H11, C14H10O, and C11H10O2, are not real substances, which makes this model not directly applicable to the Aspen simulation software. The composition of tar is very intricate and has not been fully analysed. Benzene, toluene, naphthalene, and their homologs are often used as substitutes for tar in simulations and experiments [3,32]. Based on the predictive multi-step kinetic pyrolysis model [19] and actual experimental results [33], the composition of tar produced by coal rapid pyrolysis is assumed to be C6H6 (benzene), C7H8 (toluene), C8H10 (xylene), C10H8 (naphthalene), C14H10 (pyrene) and C6H6O (phenol). The reasons for choosing the six conventional substances above are that their contents are relatively high in the respective tar representative components and because they can be applied in the Aspen Plus software. According to the above conventional components, the product

1. The two-stage entrained-bed gasifier operates under a steady state, and the pressure loss in the two-stage gasifier is negligible. 2. After pyrolysis, the unreacted char contains only C, H, O and ash, and it is further decomposed into C, H2, O2, and ash to facilitate the treatment of solid reactions in the GASIFIER block. 3. The gaseous products of the two-stage gasification are H2, CO, CH4,

Table 1 Properties of the prototype coal [17]. Proximate analysis, wt% (dried basis)

Coal a

Ultimate analysis, wt% (dried basis)

Fixed carbon

Volatile

Ash

C

H

O

N

S

52.38

42.92

4.7

76.26

5.29

11.5

1.68

0.57

3

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Fig. 2. Aspen Plus simulation model of the updraft two-stage entrained-bed gasifier.

Plus, a nonlinear programming (NLP) model is proposed to fit the composition and yield of the pyrolysis products. The framework of the coal rapid pyrolysis model is depicted in Fig. 3. Based on the modified multi-step kinetic pyrolysis model, the expected pyrolysis yield was quantitatively optimized using a nonlinear optimization method, in which, there was a special focus on correcting the composition and the yield of char and pyrolysis products containing the elements of nitrogen and sulphur. The NLP optimization process is described below.

coefficients of the predictive multi-step kinetic pyrolysis model proposed by Sommariva et al. [19] have been modified, as listed in Table 3. The kinetic parameters of the sub-reactions in the original multi-step kinetic pyrolysis model [19] are unchanged. As a statistical method, the NLP prediction model enables the calculated results optimized by this method to be as similar as possible to the literature and experimental data. Therefore, the NLP prediction methods can be applied to the prediction of the yield of coal pyrolysis products based on pilot or semi-industrial experimental data [3,34–36]. In the modified multi-step kinetic pyrolysis model, the coal elemental composition is corrected and simply normalized to three elements, which are carbon, hydrogen, and oxygen, based on a dry, ash-free basis containing no nitrogen or sulphur. Because the above modified multistep kinetic pyrolysis model does not contain the evolution processes of N and S elements and because the composition of the predicted char contains three elements (C, H, and O) which cannot be applied in Aspen

i. The coal elemental composition is based on a dry, ash-free (daf) assumption. ii. The set of all atoms APyr , coal is defined as

APyr , coal = {C , H , O, N , S } i ∈ [1, 5]

(1)

where i represents the five elements of carbon, hydrogen, oxygen, nitrogen, and sulphur.

Table 2 Description of the representative block operations used in the Aspen Plus model. Block ID

Block type

Description

DRYING

RYield

PYROLYS

RYield

SEPELEM

RStoic with built-in FORTRAN code

GASIF1

RGibbs

GASIF2

RPlug with external FORTRAN code

SEP

SEPARATOR

MIX

Mixer

HEXC

Heater

HEATER

HeatX

(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3) (4) (1) (2) (3) (1) (2) (1) (2) (1) (2) (1) (2)

Reduces the moisture content of coal Temperature = 473 K Pressure = 0.3 Mpa RYield reactor converts non-conventional coal into non-conventional char and other conventional components Temperature = 973.15, 1073.15, 1173.15, 1273.15 K Pressure = 0.3 Mpa RStoic reactor converts non-conventional char into conventional components (C, H2, O2, ASH) Temperature = 973.15, 1073.15, 1173.15, 1273.15 K Pressure = 0.3 Mpa Uses RGibbs reactor to represent the 1st char gasification Calculates the phase equilibrium and chemical equilibrium Pressure = 0.3 Mpa Heat loss = 1.5% of the coal combustion heat Uses RPlug reactor to represent the 2nd char gasification and secondary gas-phase reaction Length = 4 m; Diameter = 0.28 m Pressure = 0.3 Mpa Separates gases from solids by specifying split fractions Pressure = 0.3 Mpa Pressure = 0.3 Mpa Heat loss = 0 Pressure = 0.3 Mpa Heat exchange with DRYING, PYROLYS, and SEPELEM Recovers waste heat from crude syngas Exit syngas temperature = 473.15 K

4

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Table 3 Modified multi-step kinetics model of coal devolatilization. COAL1 (−C12 H11 − ) ; TAR1 (C10 H8) 1

COAL1 → 5CHARH + 0.1CHARC + 0.2H2 + 0.9CH4 + 0.5C2 H4∗

2 3 4

COAL1 → TAR1∗ + CHARH + H2 COAL1 → 5CHARH + 0.25CHARC + 0.5H2 + 0.75CH4 + 0.5C2 H4 COAL1 → TAR1∗ + CHARH + H2

5

TAR1∗ → TAR1

6

TAR1∗ + CHARH → 4.3CHARH + 3CHARC + 1.55H2 + 0.4CH4

7

TAR1∗ + CHARC → 3.3CHARH + 4CHARC + 1.55H2 + 0.4CH4

8

COAL2 (−C14 H10 O − ) ; TAR2 (C14 H10) COAL2 → 3.94CHARH + 2CHARC + 0.265H2 + 0.31CH4∗ + 0.055C2 H4∗ + 0.04BTX ∗ + 0.15CO2∗ + 0.18CO∗ + 0.41H2 O∗ + 0.11COH2∗

9

COAL2 → 1.95CHARH + 5.15CHARC + 0.65H2 + 0.75CH4 + 0.7CO + 0.1H2 O + 0.12BTX ∗ + 0.2COH2∗

10 11 12

COAL2 → 0.9TAR2∗ + 0.45CHARH + 0.275H2 + 0.5CO2 COAL2 → 0.9TAR2 + 0.45CHARH + 0.275H2 + 0.5CO2 TAR2∗ → TAR2

13

TAR2∗ + CHARH → 7CHARH + 1.5CHARC + H2 + 0.5CH4

14

COAL3 (−C12 H12 O5 − ) ; TAR3 (C6 H6 O) COAL3 → 1.84CHARH + 3.03CHARC + 0.46H2 + 0.18CH4∗ + 0.055C2 H4∗ + 0.04BTX ∗ + 0.5CO2∗ + 0.8CO∗ + 2.2H2 O∗ + 0.6COH2∗ + 0.6CO2

15

COAL3 → COAL3∗

16

COAL3∗ → 1.13CHARH + 2.14CHARC + 0.8H2 + 0.32CH4 + 0.37C2 H4 + 0.06BTX ∗ + 0.65CO2 + 2.03CO + 1.2H2 O + 0.47COH2∗

17 18 19

COAL3 → 1.5TAR3∗ + 0.0625H2 + H2 O + 0.875CHARH + 1.25CO2∗ COAL3 → 1.5TAR3 + 0.0625H2 + H2 O + 0.875CHARH + 1.25CO2 TAR3∗ → TAR3

20

TAR3∗ + CHARH → 2.75CHARH + 1. 25CHARC + 0.775H2 + 0. 075C2 H4 + 0.1CH4∗ + COH2*

Coal

First step

Second step

Modified Multi-stepkinetic model

NLP calculator

calculator

Third step

Ryield Pyrolysis reaction

Pyrolysis zone

Fig. 3. The framework of the coal rapid pyrolysis model.

H2S. The conservations of the elements N and S are shown in Eqs. (5) and (7), respectively.

iii. The pyrolysis products Vppyr , j which include char, gas, tar, and pyrolysis water are defined as follows:

Vppyr , j = {Cs, Hs , Os , CO, CO2, H2, CH4 , N2, C2 H4 , H2 S, NH3, HCN , C6 H6, C7 H8, C8 H10, C6 H6 O, C10 H8, C14 H10, H2 O}, j ∈ [1, 19]

wi, coal ⎞ 0.4mfcoal ⎛⎜ ⎟ = MW pyr , i ⎠ ⎝

(2)

where j denotes the pyrolysis product in the rapid pyrolysis reaction. Cs, Hs, and Os represent three elements in char: carbon, hydrogen, and oxygen.

Npyr , NH3 = Npyr , HCN

iv. The element balances during coal pyrolysis are listed as follows:

wi, coal ⎞ 0.8mfcoal ⎛⎜ ⎟ = MW pyr , i ⎠ ⎝

wi, coal ⎞ mfcoal ⎛⎜ ⎟ = MW pyr , i ⎠ ⎝

19

∑ Npyr,j × Nai,j , j=1

i ∈ [1, 5]

3

∑ Npyr,j × MWj j=1

Npyr , j × Nai, j , i = 4, (5) (6)

∑

Npyr , j × Nai, j , i = 5

j = H2 S

(7)

where mf , w , MW , Na, and N represent the mass flow rate, mass fraction, molar weight, number of atoms and molar flow rate, respectively. vii. The constraint relationships between the reference yields obtained from the modified multi-step kinetic pyrolysis model and the expected yields are shown below. The constraints of the composition ratio of the pyrolysis gas are shown in Eq. (8).

(3)

v. Ash-free char only contains C, H, and O, whose element conservation is listed as follows:

mfcoal × wchar =

∑ j = N2

(4)

vi. During the coal conversion process, the N atom in coal is mainly converted into nitrogen (N2), ammonia (NH3), and hydrogen cyanide (HCN) [27–29]. In particular, 40% of N atoms in coal are converted to N2 [27–29], while the remaining atoms are converted to HCN and NH3, and the ratio of HCN to NH3 is close to 1 [27]. Ma et al. [31] indicate that all the S atoms in coal are associated with

mf

y

⎧ pyr , j − pyr , j ≥ −Rk ypyr , j + 1 ⎪ mfpyr , j + 1 j ∈ [4, 9]; k ∈ [1, 5] ⎨ mfpyr , j − ypyr , j ≤ Rk ⎪ mfpyr , j + 1 ypyr , j + 1 ⎩

(8)

The constraints of the composition ratio of tar are shown in Eq. (9). 5

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mf

y

⎧ pyr , j − pyr , j ≥ −Lz ypyr , j + 1 ⎪ mfpyr , j + 1 j ∈ [13, 18]; z ∈ [1, 5] ypyr , j ⎨ mfpyr , j L − ≤ z ⎪ mfpyr , j + 1 ypyr , j + 1 ⎩

11

⎨ ⎪ ⎩

∑11 j = 2 mf pyr , j ∑18 j = 12 mf pyr , j

−

16

⎧ ∑ j = 12 mfpyr , j − ⎪ ∑j = 17 mfpyr , j ⎨ ⎪ ⎩

∑16 j = 12 mf pyr , j ∑j = 17 mf pyr , j

−

ypyr , gas ypyr , tar

≥ − G1

ypyr , gas ypyr , tar

≤ G1

ypyr , tar ypyr , water ypyr , tar ypyr , water

(10)

min ∑

5

(R4)

S + H2 → H2 S

(R5)

1 1

+

1 Y 2k

S

+

1 k dash

1

( Y − 1)

∗ (Pgas, j − Pgas ,j)

(14)

Here, rate is the reaction rate in g·((cm2 of coal area)·s)−1. Pgas,jP*gas,j is the effective reaction partial pressure of the gas-phase pyrolysis product j in atm. kdiff is the gas film diffusion constant in g·(cm2·atm·s)−1. kdash is the ash film diffusion constant in g·(cm2·atm·s)−1 and can be calculated from Eq. (16). ks is the surface reaction rate constant in g·(cm2·atm·s)−1. Y is the parameter reflecting the thickness of the ash layer and can be calculated from Eq. (15).

(12)

Y = [(1 − x coal )/(1 − f )]1/3

(15)

kdash = kdiff (ε 2.5)

(16)

2

∑ ∑ (κRk + φLz + γGr )

k=1 z=1 r=1

(13)

Here, ε is the voidage in the ash layer, and it is assumed to be a constant value of 0.75 or 0.5 [37,38]. The value of ε is specified as 0.75 in this work. xcoal is the conversion ratio of the original dry coal at any time after the end of pyrolysis. f is the conversion ratio of the original dry coal at the end of pyrolysis. In this study, the shrinking-core model is used to calculate the kinetics of reaction and diffusion, while the random-pore model is used to calculate the evolution of the particle-reactive surface. The internal surface reactive area during char conversion can be calculated from Eq. (17).

The objective function indicates that the component prediction of pyrolysis yields can be estimated by minimizing the relaxation variables under the above constraint conditions. The symbols κ, φ, and γ represent the weighting factors, which denote the relative importance of the constraint coefficients R, L and G, respectively. In this work, the values of κ, φ, and γ are specified as 1. GAMS 24.8 is used to optimize the expected yield of the rapid pyrolysis model. The global solver BARON is used to solve this model and locate its global optimal solution to obtain the yield of the expected pyrolysis products. 3.3. Coal gasification and combustion processes model

S = (1 − x ) 1 − ψ ln(1 − x ) S0

In the two-stage gasification system, the exit syngas of the 1st furnace can reach a chemical equilibrium state due to the high reaction temperature [37]. However, the lower overall temperature of the 2nd furnace makes it impossible to reach the chemical equilibrium state. Therefore, the RGibbs block with the chemical equilibrium reaction model can be used to simulate the gasification and combustion processes of the 1st furnace, and the RPlug block with the chemical reaction kinetics model should be used to simulate the gasification process of the 2nd furnace. In the 2nd furnace, five heterogeneous reactions and thirty-six homogeneous reactions have been considered. The five heterogeneous reactions [38] are shown in R1–R5, and the corresponding kinetics parameters can be found everywhere. In this study, C6H6 (benzene), C7H8 (toluene), C8H10 (xylene), C10H8 (naphthalene), C14H10 (pyrene) and C6H6O (phenol) are used as substitutes for tar to describe in detail the reactions of tar in the gasification process, such as oxidation, steam reforming, pyrolysis, and hydrogenation. The kinetics parameters of specific heterogeneous gasification reactions [32,38–53] are shown in the supporting information, and they include the oxidation reaction, the steam reforming reaction, the pyrolysis reaction, the hydrogenation reaction, and the pollutant conversion reaction.

C+

C + 2H2 → CH4

k diff

The Eqs. (8)–(12) are the logistic constraints, where R, L and G represent three relaxation variables. The relaxation variables represent the differences between the expected yields and the reference yields. Then, the expected yields can be used in the Aspen Plus software. viii. The objective function is as shown in Eq. (13). 5

(R3)

rate =

(11)

The model constraints are shown below.

⎧ Rk ≥ 0, Lz ≥ 0, G1,2 ≥ 0, Npyr , j ≥ 0 ⎨ ⎩

C + CO2 → 2CO

For the heterogeneous-phase reaction, the reaction rate is related to the gas-phase diffusion rate, the heterogeneous-phase diffusion rate and the surface reaction rate [37]. The shrinking-core model is first compiled using an external FORTRAN code and then loaded into the Aspen Plus simulator. The heterogeneous reactions kinetics model can be written as Eq. (14).

≥ −G2 ≤ G2

(R2)

(9)

The constraints of the distribution ratio among the pyrolysis gas, tar, and pyrolysis water are shown in Eqs. (10) and (11)

⎧ ∑ j = 2 mfpyr , j − ⎪ ∑18j = 12 mfpyr , j

C + H2 O → CO + H2

1 1 2 O2 → 2 ⎜⎛1 − ⎟⎞ CO + ⎜⎛ − 1⎟⎞ CO2 ϕ ϕ⎠ ⎝ ⎝ϕ ⎠

(17)

Here, S0 denotes the initial internal reactive area in cm2·(cm3)−1. According to the literature [54], the typical order of magnitude value of the initial internal reactive area S0 for common porous solids is 104 cm2/cm3. When the gasification temperature is lower, less than 1100 °C, the heterogeneous reactions of char with H2O, CO2, and H2 in the 2nd furnace are slow chemical reactions, and the overall reaction rate depends on the chemical kinetics [3,55]. Under the above circumstances, the change of the initial internal reactive area has little effect on the results of the overall gasification system. In this study, the value of 12500 cm2·(cm3)−1 is assigned to S0. S represents the internal reactive area at any time during the gasification process. ψ is the structure parameter for a particular char particle. The value of ψ is specified as 12 in this work. In the two-stage entrained-bed gasification system, an RPlug block is used to simulate the second-stage furnace. The modification process of the residence time for the solids in the updraft two-stage entrainedbed gasifier is analogous to the process of Wen and Chaung [38]. The solids residence time is modified by adjusting the inner diameter of the furnace. Because the gasifier in this work is in the upward flow direction, it is assumed that the initial radial velocity of the particles is zero, and the vector direction of the settling velocity of the particles is opposite to the flow direction.

(R1) 6

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4. Model validation

char yield. However, this slight difference does not affect the application of the modified multi-step kinetic pyrolysis model in the whole gasification system. In the entrained-bed gasifier, pulverized coal particles take a very short time to complete the devolatilization process for the rapid heating rate [21,53,56]. The rapid pyrolysis process of coal in the pressurized dropper furnace is similar to the process in the entrained-bed gasifier. Table 4 lists the comparison between previous experimental results [20] and the numerical results under different pyrolysis termination times in a pressurized dropper furnace. The results show that the numerical results under different pyrolysis termination times are in good agreement with the experimental results. Therefore, it can be considered that the devolatilization process of pulverized coal is basically completed within 50 ms when the particle heating rate is very fast, approaching 104K/s.

4.1. The validation of the modified multi-step kinetic pyrolysis model The modified multi-step kinetic pyrolysis model is validated using previous experimental results [20,21]. Fig. 4 compares the pyrolysis yield predicted by the modified multi-step kinetic pyrolysis model with the experimental results for three types of coal [21], which are (a) Illinois No. 6 coal, (b) Zap North Dakota lignite, and (c) Montana Rosebud coal. The numerical results show that the pyrolysis process of pulverized coal is basically completed within 50 ms with the rapid heating rate (> 104 K/s). The model prediction results of Illinois No. 6 coal and Zap North Dakota lignite are in good agreement with the experimental results, except for Montana Rosebud coal. The RSMSE (root mean square error) between the model prediction results and the experimental results is 5.90%, which indicates that the model can be used to predict the distribution of pyrolysis products. As shown in Fig. 4(c), the char and gas yield predictions of Montana Rosebud coal are slightly different from the experimental results, which can be attributed to the fact that this model ignores the diffusion effects of volatiles inside the coal particles. Under the same pyrolysis conditions, the distribution of the pyrolysis composition is also affected by different coal species. Since different coal types have different pore structures, the diffusion coefficient of volatile substances into the pores is also different [56]. During the fast pyrolysis process, the secondary reaction of the metaplastic substance is insufficient due to the effects of diffusion, so that it cannot be completely released, which leads to parts of the metaplastic substance remaining in the char. In the abovementioned pyrolysis model, the release process of H2 from the metaplastic state and the crosslinking reactions of heavy tar components are not considered, which leads to a slight increase in the gas yield and a slight decrease in the

4.2. The validation of the simulation of the two-stage entrained-bed gasification system In this section, the pyrolysis yields obtained from the mathematics framework have been inserted into the two-stage entrained-bed gasification system model coupled with a heterogeneous chemical reaction kinetics subroutine. To our knowledge, there are few numerical or experimental studies on tar yields in entrained-bed gasification. The semiindustrial data obtained from a development scale (7 t/d) two-stage entrained gasifier [17] is used to validate the feasibility of the whole gasification model. Table 5 lists the material parameters and operating conditions of the two-stage entrained gasifier. Additionally, the heat loss of two-stage entrained-bed gasifier is set as 1.5% of the coal combustion heat [57]. The comparisons of the system simulation results and the industrial

100

100

80

80

40

Yield /%

Char This model Char Gas Tar

60

20

40

0 10

20

30

Tar

0

40 50 Illinois No. 6 coal

Time /ms

Gas

20

Gas

0

Char

This model Char Gas Tar

Tar

0

10

20

Time /ms

30

40 50 Zap North Dakota lignite

(b)

(a) 100

80

60

Yield /%

Yield /%

60

Char

This model Char Gas Tar

40

Gas Tar

20

0 0

10

20

30

Time /ms

40 50 Montana Rosebud coal

(c) Fig. 4. The evolutions of char, tar, and gas for (a) Illinois No. 6 coal; (b) Zap North Dakota lignite; (c) Montana Rosebud coal. 7

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Table 4 Mass fraction of the released products and elemental compositions of tar and coal char. Adaro coal (unit: wt.%)

Predictions

Experiments [20]

Pyrolysis time

50 ms

1000 ms

≈1000 ms

H2 H2O CO CO2 CH4 C2H4 C3–C5 Total gas BTX PAH Ox PAH Heavy PAH Total liquid/TAR CHAR

0.558 5.50 14.07 8.47 3.04 2.00

0.65 5.54 14.08 8.48 3.70 2.14

33.63 2.64 2.73 8.19

34.57 2.64 4.19 8.20

0.7 10.5a 9.9 8 3.7 2.9 0.5 36.20 2.6

13.56 52.18

15.02 49.86

TAR

CHAR

Predictions Pyrolysis time C H O

9.5 4.4 16.50 47.30

50 ms 83.12 6.59 10.29

1000 ms 84.66 5.99 9.35

Experiments [20]

Predictions

≈1000 ms 89.5 5.6 5.0b

50 ms 91.97 3.52 4.51

Experiments [20] 1000 ms 92.41 2.96 4.63

≈1000 ms 90.5 2.2 7.3b

(a poor accuracy in the literature; b ultimate analysis of O includes the S content; the conditions for the rapid pyrolysis of Adaro coal are at 1073 K and 1 MPa[18]. “wt.%” means weight fraction %.)

obtain high-quality syngas with a low tar content by adjusting the system parameters, the accuracy of the current model results is relatively acceptable.

Table 5 Material parameters and operating conditions. Parameter

Unit

Two-stage entrained-bed gasifier [17]

Pressure Coal mass flow The average diameter of the particle Coal temperature 1st Oxygen mass flow Oxygen temperature Steam mass flow The mass ratio of N2/solid Steam temperature The ratio of cycled char Length of the 2nd gasifier Diameter of the 2nd gasifier Length of the 1st gasifier Diameter of the 1st gasifier

Mpa kg·h−1 μm °C kg·h−1 °C kg·h−1

0.3 300 100 80 88.8 180 54.3 0.2645 300 0.55 4 0.28 0.6 0.5

°C m m m m

5. Results and discussion 5.1. Effect of the final pyrolysis temperature Unlike the previous two-stage entrained-bed gasifier [17], the objective of this work is to reduce the tar content in syngas as much as possible. In this work, all the pulverized coal is transported into the second-stage furnace, and the total unreacted char produced from the second furnace are recycled into the first-stage furnace to produce syngas. Based on the above pyrolysis framework model, the yields of pyrolysis products and the compositions of char at different final pyrolysis temperatures are shown in Table 7. As shown, with the increase of the final pyrolysis temperature, the yields of gases and liquids increase gradually, while the char yield decreases gradually, which is caused by the gradual decrease of volatiles remaining in the char. Additionally, the total mass of char decreases as the volatiles in the char decrease, so the relative mass fraction of ash in the char increases. It is worth noting that at 900 °C and 1000 °C, the yields of tar are significantly higher than they are under other pyrolysis temperature conditions, which is not

data are listed in Table 6. The results show that the main exit parameters of the system simulation are in relatively good agreement with those of the industrial data. Except for the difference in the coal gas volume of 7.47%, the differences of all other exit parameters are within 3%. It should be stated that since the main purpose of this study is to

Table 6 Comparison results between the system simulation (Cal.) and industrial data (Ind.) Two-stage entrained-bed gasifier

The yield of char Recycled char Coal gas volume Calorific value of gas The yield of BTX The yield of heavy tar Exit temperature

Unit

Cal.

Ind. [17]

kg/(100 kg coal) kg/(100 kg coal) Nm3/(100 kg coal) MJ/Nm3 kg/(100 kg coal) kg/(100 kg coal) K

25.24 29.19 105.17 15.18 3.89 6.11 1859 904

25.5 29.5 97.7 17.87 3.5 7.2 1773.15 ~ 1923.15 883.15 ~ 1083.15

Stage 1 Stage 2

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results show that with the increase of the final pyrolysis temperature, the exit temperature of the gasifier increases gradually, the yields of carbon monoxide and hydrogen decrease gradually, the yields of carbon dioxide, steam and methane increase gradually, and the cold gas efficiency of the system decreases gradually. The simulation results indicate that increasing the pyrolysis final temperature can increase the degree of secondary gas reaction of tar to reduce the content of tar in the syngas.

Table 7 Released products and compositions of char at different final pyrolysis temperatures. Coal a

Pyrolysis temperature

Unit: wt.%

700 °C

800 °C

900 °C

1000 °C

CO CO2 CH4 C2H4 H2 H2O N2 H2S NH3 HCN Total gas BTX Ox PAH Heavy PAH Total liquid/TAR CHAR

6.16 1.93 0.87 0.21 0.16 2.17 0.67 0.62 0.62 0.98 14.39 3.61 4.86 4.19 12.65 72.96

6.96 2.19 1.15 0.27 0.19 2.51 0.67 0.62 0.62 0.98 16.17 3.88 4.86 4.85 13.59 70.25

7.10 2.23 1.92 0.30 0.24 2.51 0.67 0.62 0.62 0.98 17.19 3.88 4.86 8.94 17.68 65.13

7.13 2.24 1.46 0.55 0.26 2.52 0.67 0.62 0.62 0.99 17.06 3.89 4.88 10.52 19.29 63.65

Char composition Fixed carbon Volatile Ash

71.80 21.76 6.44

74.57 18.74 6.69

80.42 12.37 7.21

84.30 8.32 7.38

5.2. Effect of the gas residence time The gas residence time is one of the most important parameters of the gasification system. In this section, the pyrolysis final temperature was set to 1000 °C, and the effect of the gas residence time was investigated by adjusting the size of the 2nd furnace, while the other input parameter remained unchanged. The gas residence time of an entrained-flow gasifier is mostly at the level of seconds. To reasonably investigate the effect of the gas residence time, it was set to between 1 and 20 s in this work.

C + H2 O → CO + H2 C + CO2 → 2CO

consistent with other experimental data [58]. This is because the pyrolysis model proposed by Sommariva et al. [19] did not take into account the secondary gas-phase reaction of tar components at high temperatures, which leads to over-prediction of the yield of tar. In this work, we supplemented the secondary gas-phase reaction of tar in the subsequent system simulation. The two performance parameters of the gasification system, HHV and GCE, are defined as follows [59,60]:

HHV (MJ /Nm3)

CGE (%) =

(18)

LHVgas *mfgas LHVcoal *mfcoal

ΔH = +172.509kJ / mol

(20) (21)

The effect of the gas residence time on the performance of the gasification system is shown in Fig. 5. As shown in Fig. 5(a), the exit temperature decreases with the increase of the gas residence time. When the gas residence time is less than 7.5 s, the molar fractions of CO and CH4 increase slightly with the increase of the gas-phase residence time, while the molar fractions of H2O and CO2 decrease slightly. When the reaction does not reach equilibrium, as the gas residence time increases, the gasification reaction of coal with water and carbon dioxide gradually proceeds towards the positive reaction direction. When the gas residence time is greater than 7.5 s, the molar fraction of each gas remains basically unchanged. As shown in Eqs. (20) and (21), the gasification reactions of carbon with water and carbon dioxide are both endothermic reactions. The decrease of the exit temperature of the syngas with the increase of the gas residence time can further indicate that the gasification reactions of char with water and carbon dioxide proceed towards the positive reaction direction. As shown in Fig. 5(b), the total yield of tar, including BTX and heavy tar (PAH), and the yields of heavy tar and BTX decrease with increasing gas residence time. In accordance with the specification for a tar concentration of between 30 and 500 mg/Nm3, as shown in the zoom domain, the yield of heavy tar is basically below 500 mg/Nm3 when the gas residence time is approximately 5.87 s, at which time the exit temperature is approximately 958 °C. The total tar yield is basically below 500 mg/Nm3 when the gas residence time is approximately 15.5 s, at which time the exit temperature is approximately 943 °C. As shown in Fig. 5(c), according to Eqs. (18) and (19), HHV and CGE both increase gradually with the

(“wt.%” means weight fraction %.).

= 12.75{H2} + 12.63{CO} + 39.84{CH4} + ...;{mole fraction}

ΔH = +131.318kJ / mol

(19)

where HHV is the higher heating value and CGE is the cold gas efficiency. LHVgas and LHVcoal are the lower heating values of the exit gas and coal, respectively. mfgas and mcoal are the mass flow rates of the exit gas and coal, respectively. According to the above pyrolysis data, the final pyrolysis temperature, which represents the temperature of the stream after the mixing of the pulverized coal and the high-temperature syngas from the 1st furnace, was set to 700, 800, 900 and 1000 °C. As shown in Table 8, the Table 8 The results of the Aspen simulation at different final pyrolysis temperatures.

Pyrolysis temperature Unit 1st Oxygen mass flow Exit temperature CO H2 CO2 H2O CH4 N2 Tar yield Gas residence time HHV CGE

−1

kg·h °C molar yield/% molar yield/% molar yield/% molar yield/% molar yield/% molar yield/% mg/(Nm3 syngas) s MJ/Nm3 %

700 °C

800 °C

900 °C

1000 °C

189.09 746.30 49.60 28.97 2.15 2.22 0.73 15.44 32945.56 0.8578 10.25 83.71

193.97 843.81 48.49 26.24 3.40 3.21 2.08 15.67 32593.31 0.7557 10.30 82.15

197.71 943.37 46.55 23.45 5.47 4.41 4.05 15.70 31035.14 0.7314 10.35 81.17

202.60 1011.32 45.80 22.34 6.96 3.46 4.66 15.79 29139.96 0.6573 10.50 80.20

9

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50

30

20 950 10

2.0x104 1000

1.5x104

1.0x104

5

10

15

750

3

250

0 14

16

18

20

Gas residence time/s

0.0

20

0

5

10 Gas residence time/s

Gas residence time/s

15

20

(b)

(a) 13

100

12 90

85 11

HHV (MJ/Nm3)

HHV CGE

95

Eff (%)

500(mg/Nm (syngas))

500

5.0x103

900

0

liquids yield(mg/Nm3(Syngas))

1000

liquids yield(mg/Nm3(Syngas))

H2O Exit temperature

CH4

0

BTX Heavy PAH Total tar

CO

Exit temperature(°C)

40

Mole fraction (%)

2.5x104

1050 H2 CO2

80

75

10 0

5

10 Gas residence time/s

15

20

(c) Fig. 5. Effect of the gas residence time on (a) the species mole fractions and the exit temperature, (b) the yield of liquids, and (c) the CGE and HHV.

by increasing the partial pressure of water vapor in the gasifier. Meanwhile, the forward reaction rate of the water-gas shift (WGS) is greater than the forward reaction rate of the steam shift reaction of methane. Therefore, the molar fraction of CO decreases with the increase of the steam. Fig. 6(b) illustrates the variations of the yield of liquids produced by changing the steam/coal ratio. According to the figures, the yields of BTX and heavy tar (PAH) decrease with the increase of the steam/coal ratio, indicating that the increase of steam is conducive to the cracking of tar, which is similar to the results of Wang [61]. The CGE and HHV variations produced by changing the steam/ coal ratio are plotted in Fig. 6(c). As shown, the two parameters for coal a are decreased upon increasing the steam/coal ratio. Therefore, it can be concluded that increasing the steam/coal ratio is conducive to the elimination of tar, but it reduces the CGE and HHV.

increase of the gas residence time, but their growth rate decreases gradually. When the gas residence time is approximately 15.5 s, HHV and CGE basically reach their maximum values, which are 11.41 MJ/ (Nm3) and 91.23%. An increasing gas residence time is conducive to the secondary reaction of tar components to increase the yield of effective gas (CO, H2, and CH4), thereby improving the HHV and CGE of the gasification system.

5.3. Effect of the steam/coal ratio The inlet steam mass flow plays a decisive role in the overall performance of the gasification system. Adding water or steam to the gasifier can effectively adjust the internal temperature of the furnace and increase the hydrogen content in the syngas [57]. Therefore, it is necessary to investigate the effect of the inlet steam mass flow on the twostage entrained-bed gasifier. To investigate the effect of the steam/coal ratio on the gasification system, the mass ratio of the added steam to the total coal was adjusted within the range of 0.15 to 0.6, while maintaining a gas residence time of 15 s and a final pyrolysis temperature of 1000 °C. The effect of the steam/coal ratio on the performances of the gasification system is shown in Fig. 6. As shown in Fig. 6(a), the exit temperature gradually decreases with the increase of the steam content due to the high heat capacity of steam. With the increase of the steam/ coal ratio, the molar fractions of CO and CH4 decrease gradually, while those of H2 and CO2 both increase gradually. This can be attributed to the increase of the steam/coal ratio enhancing the intensity of the water–gas shift (WGS) reaction and the steam shift reaction of methane

6. Conclusions A novel two-stage entrained bed pyrolysis-gasification system, including entrained-bed pyrolysis and entrained-bed gasification, has been proposed and studied. To obtain accurate rapid pyrolysis yields, a new framework for the coal rapid pyrolysis process with strict element conservation was proposed, and it could determine the composition of volatiles, including the high-weight hydrocarbons/distillable liquids (benzene, toluene, xylene, naphthalene, pyrene, and phenol). A steadystate kinetics system model for the two-stage entrained-bed pyrolysisgasification system has been established using the Aspen Plus software, into which an external FORTRAN subroutine-compiled heterogeneous kinetics model has been loaded. The mathematical framework of the 10

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Fig. 6. Effect of the steam/coal ratio on (a) the exit temperature and species mole fractions, (b) the yield of liquids, and (c) the CGE and HHV.

Key R&D Program of China (Grant 2018YFB0605000 and Grant 2018YFC0808500 and Grant 2017YFB0602600) and the National Natural Science Foundation of China (Grant No. 21776087).

rapid pyrolysis of pulverized coal and the steady-state kinetics system model were verified by experimental data and industrial data, respectively, indicating the feasibility of the proposed models. Based on the steady-state kinetics system model, the effects of three key operating parameters, including the final pyrolysis temperature, gas residence time and steam/coal mass ratio, on the overall performance of the gasification system have been investigated. The simulation results show that increasing the final pyrolysis temperature can increase the degree of secondary gas reaction of tar to reduce the content of tar in the syngas. The gas residence time exerts a strong impact on the elimination of tar. By adjusting the gas residence time to 15.5 s at the final pyrolysis temperature of 1000 °C, the tar yield can be reduced to less than 500 mg/Nm3, the molar fraction of methane in the syngas can be approximately 5.84%, the exit temperature can be approximately 943 °C, and the HHV and CGE will be 11.41 MJ/(Nm3) and 91.23%, respectively. In addition, increasing the steam/coal ratio is conducive to the elimination of tar, but it reduces the CGE and HHV.

Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116531. References [1] Xu J, Yang Y, Li Y-W. Recent development in converting coal to clean fuels in China. Fuel 2015;152:122–30. [2] Karmakar MK, Chandra P, Chatterjee PK. A review on the fuel gas cleaning technologies in gasification process. J Environ Chem Eng 2015;3(2):689–702. [3] He C, Feng X, Chu KH. Process modeling and thermodynamic analysis of Lurgi fixed-bed coal gasifier in an SNG plant. Appl Energy 2013;111:742–57. [4] Zeng X, Wang F, Li H, et al. Pilot verification of a low-tar two-stage coal gasification process with a fluidized bed pyrolyzer and fixed bed gasifier. Appl Energy 2014;115(4):9–16. [5] Wang W, Han H, Zhang J, et al. Progress in treatment technologies of coal gasification wastewater. Chem Ind Eng Prog 2013;32(3):681–6. [6] Munajat NF, Erlich C, Fakhrai R, et al. Influence of water vapour and tar compound on laminar flame speed of gasified biomass gas. Appl Energy 2012;98(5):114–21. [7] Di Gregorio F, Parrillo F, Salzano E, et al. Removal of naphthalene by activated carbons from hot gas. Chem Eng J 2016;291:244–53. [8] Matsuoka K, Hosokai S, Kuramoto K, et al. Enhancement of coal char gasification using a pyrolyzer–gasifier isolated circulating fluidized bed gasification system. Fuel Process Technol 2013;109:43–8. [9] Chen Z, Gao S, Xu G. Simultaneous production of CH4-rich syngas and high-quality tar from lignite by the coupling of noncatalytic/catalytic pyrolysis and gasification

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. Acknowledgement The authors are grateful for the financial support from the National 11

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[36]

[37] [38] [39] [40]

[41] [42] [43] [44] [45] [46] [47]

[48] [49] [50] [51] [52]

[53] [54] [55] [56] [57]

[58]

[59]

[60]

[61]

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