CHEMICAL ENGINEERING TRANSACTIONS
VOL. 81, 2020
A publication of
The Italian Association
of Chemical Engineering
Online at www.cetjournal.it
Guest Editors: Petar S. Varbanov, Qiuwang Wang, Min Zeng, Panos Seferlis, Ting Ma, Jiří J. Klemeš
Copyright © 2020, AIDIC Servizi S.r.l.
ISBN 978-88-95608-79-2; ISSN 2283-9216
Energy and Carbon Emission Optimisation
of Coal to Syngas Process
Yitong Gao, Guilian Liu*
School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi Province, China
guilianliui@mail.xjtu.edu.cn
Coal gasification to syngas is a common technique for coal utilization and consumes large amount of energy.
The design and operation of gasifier, washing column and heat exchangers affect the energy consumption and
carbon emission of the whole system. In this work, a coal to syngas process is simulated by Aspen Plus software.
Based on the simulation results, the integration among the coal gasification, shift process and rectisol process
are analysed; the heat exchanger network (HEN) is optimised to minimize the energy consumption and the
carbon dioxide emission. The utility consumption is reduced by 17.4 %, and the emission of CO2 is reduced by
5.1 %.
1. Introduction
Syngas is an important intermediate product in the application of coal and produced through coal gasification.
Its main components are carbon monoxide and hydrogen, and can be used to produce methanol, butanoctanol,
ethylene glycol, ammonia, hydrogen, etc. Compared with the direct utilization of coal, coal gasification to syngas
is an efficient and more environmentally friendly way to utilize coal, and can improve its utilization value.
In the process of coal to syngas, large amount of energy is consumed. Taking Shanghai Coking Co., Ltd. as an
example, the energy consumption of coal-based syngas is 4.216×107 kJ/t syngas (Zhang, 2011). In the
production process, the main feed stream, coal-water slurry (cold stream) need to be heated, while other process
streams (hot streams) require to be cooled. In order to reduce the energy consumption as much as possible, it
is necessary to optimise the HEN composed by these hot and cold streams. Along with this, the minimum carbon
dioxide emissions can be achieved.
For coal gasification, Wen and Chaung (1979) developed a mathematical model to simulate the Texaco gasifier
using coal-water slurries as feed material. Zhang et al. (2014) proposed an Aspen Plus based coal gasification
model, in which the gasifier was simulated by two blocks. For the separation of sour gas in the rectisol process,
two configurations of single-stage rectisol wash and two-stage configurations are analysed and simulated in
Aspen Plus by Sun et al. (2013). Both of them can fulfil the separation requirement, while have different power
and other energy demands. Linnhoff and Flower (1978) proposed a two-stage approach to solve the problem of
HENs. In the first stage, the preliminary network with the maximum heat recovery is generated. In the second
stage, the most satisfactory network is obtained based on the evolution starting from the preliminary network.
Tan et al. (2015) evaluated the utilization of energy and carbon emission of a typical Texaco coal gasification
process. Pan et al. (2018) built a complex mixed integer nonlinear programming (MINLP) to study the heat
transfer intensification in HEN retrofitting; details of exchanger geometry, stream bypassing and splitting, limited
minimum temperature difference (LMTD) and its correction, temperature-variation of properties and pressure
drops are considered. Jiang et al. (2018) introduced the performance simulation into HEN retrofit model to
reassess the performance of reused heat exchange units. Jiang et al. (2020) proposed a multiple objective
optimisation model with energy, economic, environmental and engineering quantity indexes considered in the
HEN retrofit.
Although many literatures have studied the HEN optimisation of coal gasification process, the three sub-
processes are evaluated and optimised independently. There is no report on the evaluation and optimisation
with these sub-processes taken as a whole. This work aims to study the integration of three sub-processes of a
DOI: 10.3303/CET2081002
Paper Received: 26/03/2020; Revised: 22 April 2020; Accepted: 24/04/2020
Please cite this article as: Gao Y., Liu G., 2020, Energy and Carbon Emission Optimisation of Coal to Syngas Process, Chemical Engineering
Transactions, 81, 7-12 DOI:10.3303/CET2081002
7
coal to syngas process of China and minimize its energy consumption. Aspen Plus will be used to simulate this
process. Then, the minimum energy consumption will be identified according to the simulation results and Pinch
Technique. At last, the HENs of three sub-processes are integrated to achieve the minimum energy
consumption, as well as the minimum carbon dioxide emission.
2. Simulation of coal to syngas process
Coal to syngas process includes three sub-processes, coal gasification, water-gas shift process and rectisol
process. Gasification process has three sections, pretreatment, gasification and grey water treatment. In this
process, coal or coal char and gasification agent (air or oxygen) are partially oxidized at high temperature, to
convert them into crude gas. Crude gas mainly contains CO, H2 and H2O, a small amount of H2S and COS. It
is sent to the Water-Gas Shift Reactor (WGSR) of the water-gas shift unit, where CO reacts with H2 to generate
CO2. The shifted gas is processed in the rectisol unit to remove impurities, such as CO2 and H2S, and obtain
the refined syngas. In the process, 5.21×108 kg/y coal is processed and 1.47×107 Nm3/y syngas is produced.
The main parameters are shown in Table 1. The main flowsheet of this process is shown in Figure 1.
Table 1:Main parameters of coal to syngas process
Parameter Gasifier First shift reactor Second shift reactor Third shift reactor
CO2 and H2S absorber
(stage number is 60)
T (ºC) 1,300 450 280 250 -30 4.5
P (MPa) 6.5 6.15 6.04 5.97 5.45 5.56
R102R101
COAL
WATER
S101
RAWGAS
ASH
E201
R201
E202
R202
E203
R203
E204
S201
SHIFTGAS
WATER
E301
T301-1
REFINED SYNGAS
E302E303E304
RICH METHANOL
F301F302
T302
NITROGEN
WASTE GAS WASTE GAS WATSE GAS
P301
E305
METHANOL
METHNOL
R101-Coal decomposition reactor Pump-P301
R102-Gasifier
R201-First shift reactor Separators-S101 S201
R202-Second shift reactor
R203-Third shift reactor Flash Column-F301 F302
T302-Methanol regeneration column
CO2 and H2S absorber- T301-1 T301-2 T301-3 T301-4
Heat Exchangers- E201 E202 E203 E204 E301 E302 E303 E304 E305
OXYGEN
T301-2T301-3T301-4
Figure 1: Flowsheet of the coal to syngas process
Based on this flowsheet, the simulation model is built by Aspen Plus 10. In this model, the gasifier is simulated
by two reactors, one is RStoic (R101), the other is RGibbs (R102). Rsoic is a stoichiometric coefficient reactor
with known reactants and products, and is used to simulate the coal decomposition reaction. The coal is
assumed to decompose into elementary substance (C, S, H2, N2, O2 and Cl2) and ASH in this reactor. These
intermediates are sent to the RGibbs reactor, in which the product composition is calculated based on the
minimum Gibbs free energy of the reaction equilibrium. In the RGibbs reactor, the main reactions are shown by
Eqs(1)-(9).
2C+O2→2CO (1)
2CO+O2→2CO2 (2)
C+H2O→CO+H2 (3)
C+CO2→2CO (4)
8
CO+H2O→CO2+H2 (5)
C+2H2→CH4 (6)
C+O2→CO2 (7)
2CO+O2→2CO2 (8)
2H2+O2→2H2O (9)
After ash is separated from the gasifier product, the raw gas is obtained and successively sent to the three shift
reactors to convert part of CO into CO2 and H2 . The aim is to increase the content of H2 in syngas. The main
reaction is shown by Eq(10).
CO+H2O→CO2+H2 (10)
Since the reaction is a strong exothermic reaction, a lot of heat is produced and can be used to generate steam.
The shift gas is sent to the rectisol section, where low-temperature methanol is used as the absorbent to remove
the impurities such as CO2 and H2S and obtain the refined syngas. Four Rad-Frac blocks are used to simulate
the absorber, and are represented by the desulfurization part (T301-1) and decarbonization section (T301-2,
T301-3, T301-4).
3. Optimisation of the HEN
3.1 Analysis of HEN
The HEN of current process is shown in Figure 3. The energy is recovered in each sub-process, and there is
no heat exchange among three sub-processes. The hot utility consumption (HUC), cold utility consumption
(CUC) and CO2 emission of each sub-process are shown in Table 2. The total HUC is 188,419.6 kW, and the
CUC is 149,300.1 kW.
Table 2:Utility consumption and CO2 emission data
Name HUC
(kW)
CUC
(kW)
CO2 emission
(kg/h)
Min. HUC
(kW)
Saving
potential (%)
Min. CUC
(kW)
Saving
potential (%)
Gasification 161,848.3 29,972.8 38,381.3 128,466.4 20.62 0 0
Shift 0 89,524.7 0 0 0 89,524.7 0
Rectisol 26,571.3 29,802.6 148,649.1 8,011.3 69.85 11,231.4 62.3
Total 188,419.6 149,300.1 187,030.4 136,477.7 27.57 100,756.1 32.5
Figure 2: Comparison of Grand Composite Curve before and after optimisation
Based on the simulation data, the Grand Composite Curves (GCC) of three sub-processes are plotted by Aspen
Energy Analyzer, as shown in Figure 2. It can be identified that, the minimum HUC of the gasification sub-
process is 128,466.4 kW. The minimum CUC for of the shift sub-process is 91,605.8 kW. The minimum HUC
and CUC of the rectisol sub-process is 8,011.3 kW and 11,231.4 kW. The detailed data and energy saving
potential are shown by Table 2.
-100.0
0.0
100.0
200.0
300.0
400.0
500.0
0 50 100 150 200
T
e
m
p
e
ra
tu
re
(℃
)
Enthalpy (MW)
gasification
shift
rectisol
gasification opt
shift opt
rectisol opt
9
It can be seen from Figure 2, there is a large amount of heating demand in the gasification section and cooling
demand in the shift section, and the temperature of the former is less than that of the latter. The hot stream of
the shift section can be used to provide energy to the cold stream of the gasification section, and the redundant
energy can be used to generate steam.
3.2 Optimisation of the HEN
In order to reduce energy consumption, the HEN is optimised based on the Pinch method. The optimal HEN is
shown in Figure 4, and the HUC, CUC, GUC and CO2 emission of each sub-process is shown in Table 3. In
Figure 2, the Grand Composite Curve of the HEN after the optimisation is compared with that before the
optimisation.
H1 55.9 45.0
H2 1390.0
H3 178.3
H4 68.9
H5 107.6
H6 55.5
H7 242.8
H8 199.8
H9 163.8
H10 126.5
H11 31.0
H12 69.0
H13 253.0
H14 449.0
H15 286.0
H17 180.0
H18 75.0
H19 242.1
H20 40.0
H27 -4.6
H28 -6.7
H29 -0.9
1386
168.0
45.0
105.0
45.0
242.3
178.3
126.5
68.9
25.0
59.1
252.1
258.0
248.0
75.0
40.0
239.0
-12.7
-50.0
-50.0
-30.0
C1 25.2
C2 25.3
C3 40.0
C4 25.0
C5 105.6
C6 239.0
C7 -14.9
C8 -48.4
C9 -30.2
50.0
40.0
49.4
30.0
152.4
305.0
0.0
0.0
25.0
620.1 kW
E-2039
E-20533
350.3 kW
E-3009
10,923.4 kW
E-3017
12,019.7 kWE-4001
1,991.0 kW
E-4012
252.4 kW E-22020
1,826.8 kW
E-30032
4,904.7 kW
E-30051
1,864.9 kWE-30151
365.8 kW E-30210
69.9 kW E-40081
2,156.2 kW
E-20380
3,901.3 kW E-2101
8,239.5 kW
E-2102
6,158.4 kW
E-2103
9,508.1 kW
C
E-2107
4,640.4 kW
C
E-2108
1,659.5 kW
E-2109
22,518.3 kW
H16 251.0 180.0
C
E-2105
7,626.6 kW
C
E-2106
4,589.0 kW
E-2112
24,610.4 kW
E-2104
4,762.1 kW
E-2110
3,071.0 kW
E-2111
381.0 kW
E-2201
4,380.3 kW
E-2203
1,351.8 kW
E-2204
15,736.3 kW
E-2205
8,334.2 kW
E-20101
1,840.9 kW
E-2018
518.4 kW
E-R20531
159,489.0 kW
E-2202
6,584.4 kW
E-2206
18,248.7 kW
E-2207
1,738.2 kW
GasificationShiftRectisol
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
Figure 3: HEN of the current process
10
Through the comparison, it can be identified that both the HUC of gasification section and shift section are
reduced. Although the CUC of the Rectisol section increases, it’s heating utility consumption decreases to zero.
In the gasification section, there is a large amount of energy demand. However, the corresponding cold
stream can only be heated by a furnace and cannot match with other hot streams, as it corresponds the
gasification reaction (a strong endothermic reaction) and it is difficult to recover the reaction heat in the furnace.
In the optimal HEN, hot streams H4, H11 and H12 are matched with cold streams C7, C8 and C9. Most of the
hot streams (H14, H15, H16, H19) in the shift section are used to produce steam (298.1 °C, 0.5 MPa). The total
amount of the generated stream is 8.0 × 108 kg/y.
H1 55.9 45.0
H2 1390.0
H3 178.3
H4 68.9
H5 107.6
H6 55.5
H7 243.0
H8 199.8
H9 163.8
H10 126.5
H11 30.8
H12 69.0
H13 253.0
H14 449.0
H15 286.0
H17 180.0
H18 75.0
H19 242.1
H20 40.0
H27 -4.6
H28 -6.7
H29 -0.9
1386
168.0
45.0
105.0
45.0
242.3
178.3
126.5
68.9
25.0
59.1
252.1
258.0
248.0
75.0
40.0
239.0
-12.7
-50.0
-50.0
-30.0
C1 25.2
C3 25.3
C4 25.0
C5 105.5
C6 239.0
C7 -14.9
C8 -48.4
C9 -30.2
50.0
35.3
30.0
151.2
305.0
0.0
0.0
25.0
H16 251.0 180.0
E-6
1,841.0 kW
E-2039
620.1 kW
E-20533
350.3
E-R20531
159,489.0 kW
10,807.4 kW
E-3009
E-4012
247.0 kW
E-3017
500.6 kW
E-22020
2,628.8 kW
E-30032
4,973.9 kW
E-30051
E-30151
332.9 kW
E-30210
67.7 kW
E-40081
340.9kW
E-20380
3,901.3 kW
E-2101
8,239.5 kW
E-2103
15,666.5 kWE-2104
4,762.1 kW
E-2109
28,836.9 kWE-5
C10 -21.4 25.0
2,154.8 kW
E-2110
1,311.2 kW
E-2111
387.5 kW
E-2112
36,826.0 kW
E-2
3,744.0 kW
E-2201
636.3 kW
E-2202
6,500.7 kW
E-2203
E-2204
9,775.2 kW
E-2205
4,659.4 kW
5,961.1 kW
E-4
1,738.2 kW
3,674.7 kW
E-1
E-3
4,708.6 kW
S Heat exchanger for Generating steam
C
C
C
C
C
C
1,869.6 kW
C
C
C
S
S
S
S
C
C
C
C
C
C
1,351.8 kW
E-2206
H
Gasification
Shift
RectisolNew Heat exchangers
Figure 4: Optimised HEN
11
Table 3:Utility consumption and CO2 emission data after optimisation
Section HUC (kW) CUC (kW) GUC (kW) CO2 emission (kg/h)
Gasification 159,489.0 15,415.1 174,904.1 37,821.8
Shift 0 87,790.3 87,790.3 0
Rectisol 0 16,422.8 16,422.8 139,625.1
Total 159,489.0 119,628.1 279,117.1 177,473.8
Under the condition of no extra heat exchangers is added, the integration of HEN is carried out among sections.
The GUC is reduced by 17.4 %, and the CO2 emission is reduced by 5.1 %, as shown in Table 4. It can be seen
that establishing the heat transfer among different sections is an effective way to reduce the energy consumption.
Table 4: Reduction of utility consumption and CO2 emission
Name HUC (%) CUC (%) GUC (%) CO2 emission (%)
Gasification 1.5 48.6 8.8 1.5
Shift 0 1.9 1.9 0
Rectisol 100 44.9 70.9 6.1
Total 15.4 15.8 17.4 5.1
4. Conclusions
In this paper, the coal to syngas process is simulated and optimised. With the HEN optimised and the heat-
exchange among three sections carried out, the utility consumption is reduced by 17.4 % and the emission of
CO2 is reduced by 5.1 %. In this work, only the optimisation of HEN is considered. If the optimisation of shift
reactors is considered together with the HEN integration, the energy consumption of the system can be reduced
further. This will be studied in the future work.
Acknowledgements
Financial support provided by the National key research and development program of China (2017YFB0602603)
are gratefully acknowledged.
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