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Modelling and energy analysis of an integrated coal gasification and pyrolysis system for synthetic natural gas
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 43, Issue 7, Jul 2015 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2015, 43(7), 779-789
RESEARCH PAPER
Modelling and energy analysis of an integrated coal gasification and pyrolysis system for synthetic natural gas LI Chao1,2, DAI Zheng-hua1,2,*, YANG Ji3, YU Guang-suo1,2, WANG Fu-chen1,2 1
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and
Technology, Shanghai 200237, China; 2
Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, China;
3
School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China
Abstract:
The entrained-flow coal gasification is an important and attractive technology in large scale coal-based synthetic natural
gas (SNG) process. In this study, an integrated entrained-flow coal gasification and pyrolysis system is proposed and investigated. The gasifier is divided into two stages in this system: one is mainly for the gasification of char and the other is mainly for the coal pyrolysis. The integrated system is studied by using a process simulation model and compared with a coal gasification system with the radiant and convective coolers. The effects of the operating conditions on the gasification performance are studied and the optimized operating conditions are obtained. It is found that the optimized steam coal ratio of the integrated system is about 250-300 kg(steam)/t (dry coal) while the gasification temperature is 1400°C. The cold gas efficiency of 88.18% of the integrated system is higher than that of 84.14% of the gasification and radiant+conductive cooler system. The coal and O2 consumptions are relative low in the integrated system. The performance of the integrated system is highly related to the yield of tar and CH4 in the pyrolysis stage. The overall energy conversion efficiency of the integrated system (92.26%) is slightly lower than that of the gasification and radiant+conductive cooler system (93.39%). However, the exergy efficiency is enhanced by more than 2.2% in the integrated system. The integration of the gasification and pyrolysis can effectively recover and promote energy grade of the sensitive heat in the hot syngas. Keywords: coal gasification; pyrolysis; integrated system; process modelling; energy analysis
With the significant increasing of natural gas consumption in China since 2009[1], synthetic natural gas (SNG) becomes an attractive technology to meet this huge demand. Coal is considered as the most economic and reliable feedstock in the SNG process because it is still the major part of primary energy supply in China. Therefore, coal gasification is an efficient way to produce the syngas for SNG process and to reduce the pollutant emission during coal utilization. As shown in Figure 1, the typical coal based SNG process includes air separation unit (ASU), gasification and heat recovery, purification, water gas shift (WGS), sulphur/CO2 removal and methanation unit. There are many gasification technologies can be used in coal based SNG projects[2], in which one of the most reliable and promising technologies is entrained-flow coal gasification. The most attractive features of the entrained-flow gasification are the relative flexibility of feedstock and the
scaled up capacity of a single gasifier. A large quantity of heat is converted to the sensitive heat and stored in the hot gas due to the high gasification temperature. The heat recovery methods used in entrained-flow gasification process can be generally classified into two types: the physical heat recovery and the chemical heat recovery. The physical heat recovery methods are all aimed to recover the sensitive heat of hot syngas by directly heat exchange and to generate steam, including quench or radiant + convective syngas cooling processes.
Fig. 1
Flow sheet of typical coal based SNG process
Received: 31-Mar-2015; Revised: 02-Jul-2015. Foundation item: Supported by the National Key State Basic Research Development Program of China (973 Program, 2010CB227000) and the National High Technology Research and Development Program of China (863 Program, 2008AA052310). *Corresponding author: DAI Zheng-hua, Tel: +86-21-64250784, Fax: +86-21-64251312, E-mail: [email protected]. Copyright 2015, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
In the chemical heat recovery method, the sensitive heat of the syngas is to provide coal pyrolysis or gasification reactions to generate syngas or pyrolysis gases. Some of the technologies employed the chemical heat recovery methods have been successfully commercialized or applied in pilot plant, such as the E-Gas coal water slurry (CWS) gasification technology[3], the TPRI gasification technology[4] and the MHI air-blown pulverized coal gasification technology[5]. In these processes, the gasifier is divided into two stages. A part of Table 1
Features of the entrained flow gasification technologies with chemical heat recovery process
Technology
Vendor
Feedstock
Flow direction
Oxidant
E-gas
ConocoPhillips
slurry
up
oxygen
refractory
TPRI
TPRI
dry
up
oxygen
membrane
1.1
Reactor wall
MHI
MHI
dry
up
air
membrane
E-STR
ConocoPhillips
slurry
up
oxygen
refractory
Two stage OMB
ECUST
dry/slurry
down
oxygen
refractory/ membrane
In this study, a chemical heat recovery system combined the coal gasification and pyrolysis together is proposed and studied systematically. In this system, the first stage of the gasifier, named gasification stage, is mainly for char combustion and gasification, while the second stage (pyrolysis stage) is mainly used for coal pyrolysis and heat recovery. The gasification results of the integrated system are studied and compared with a pulverized coal gasification system with radiant and convective cooler. The effects of operating conditions on gasification performance are investigated as well to determine the most appropriate operating parameters. The energy and exergy analyses are also performed to evaluate the energy efficiency of the integrated system in the aspects of both quantity and quality of the energy output.
1
CWS or pulverized coal is injected into the second stage of gasifier. Another integrated system coupled the entrained-flow coal gasification and the fixed bed coal pyrolysis is also proposed by the East China University of Science and Technology (ECUST)[6]. The ConocoPhillips E-STR[7] and researchers in Japan[8] have also developed the processes with introducing all of the feedstock to the second stage of gasifier. The features of these technologies are summarized and listed in Table 1.
Experimental Process description
Fig. 2
The concept of the integrated system is to enhance the energy efficiently by decoupling the endothermic reactions and exothermic reactions and to utilize this part of CH4 generated in coal pyrolysis in a SNG process. To evaluate the performance of the integrated coal gasification and pyrolysis system (G-P), the pulverized coal gasification system with radiant cooler and convective coolers (G-C) is also studied in this article. The flowsheets of these two systems are shown in Figure 2. As shown in Figure 2(a), the two stage entrained-flow gasifier is the key of the G-P system. All of the pulverized coal is transported by N2 or CO2 and injected into the pyrolysis stage of the gasifier. (In order to considering the demand of pyrolysis stage temperature control, part of pulverized coal can also be injected into the gasification stage in real operation.) In this study, the N2 is used as the transport gas. With directly mixing with the hot gas from the gasification stage of the gasifier, the pulverized coal particles pyrolyze rapidly.
Flowsheet of the G-P system (a) and the G-C system (b)
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
The original design conception of this process is to reduce the tar yield of coal pyrolysis through the high temperature of the pyrolysis stage and the steam introduced in gasification stage. However, a small amount of tar will be generated during pyrolysis in real operation. The existence of tar in the syngas usually involves engineering problems, such as gas purification, tar recovery and waste water treatment. Therefore, the tar yield is an important factor which should be highly concerned. The tar yield will be discussed in Section 1.3. The rawgas from the pyrolysis stage of the gasifier is cooled by a heat exchanger firstly to produce high pressure steam. Following it, a separator is used to remove the unreacted char and ash particles from the syngas. The syngas out of the separator is sent to the downstream process for further purification. A small part of the syngas is recycled to the nozzle of the pyrolysis stage for the dispersion of the pulverized coal. The unreacted char and ash particles are transported by N2/CO2 and recycled to the gasification stage of the gasifier by a lock hopper system. The recycled char particles are injected into the gasification stage of the gasifier with steam and oxygen and converted. Most of the slag is deposited on the wall and discharged from the bottom of the gasifier. A portion of the fly ash is entrained to the pyrolysis stage by the hot gas. The G-C system (b) is illustrated in Figure 2(b), the coal, transport N2, O2 and steam are all injected into the gasifier through the coaxial burner. The rawgas and slag from the
Fig. 3
1.3 1.3.1
gasification chamber is cooled in a radiant cooler, and the slag is separated and discharged. A convective cooler is used to recover the residual heat in the low temperature rawgas. Finally, the fly ash entrained in the rawgas is removed in a separator. 1.2
Model implementation
In this study, the simulation is performed by using ASPEN PLUS software. The modelling flowsheets of the G-P system and G-C system are shown in Figure 3(a) and Figure 3(b), respectively. In the G-P system, the units GASIFIER-1 and SEP-1 are combined to model the gasification stage of the gasifier, and the pyrolysis stage is divided into two units of DECOMP and the GASIFIER-2. In the G-C system, the DECOMP and the GASIFIER are used to model the gasifier as well. The yields in the DECOMP unit are determined based on the mass balance of the elements and the yields of the specific species in pyrolysis products[9]. As the RGibbs model based on the Gibbs energy minimization principle has been widely used in process modelling of coal gasification system[10, 11], the RGibbs model is employed to calculate char combustion/gasification and homogeneous reactions in this study. The PR-BM model is employed to calculate the physical properties used in the simulation.
Modelling flowsheets of the G-P system (a) and G-C system (b)
Operating conditions and assumptions Operating conditions
The calculation of this study is based on a commercial scale gasifier with the capacity of ~2200 t(dry coal)·d–1. The operating conditions are listed in Table 2. The coal analysis results are given in Table 3. The outlet temperatures of the separator and the syngas cooler are 350°C in the G-P system. In the G-C system, the outlet temperatures of the radiant cooler and convection cooler (separator) are 800°C and 350°C,
respectively. 1.3.2
Main assumptions
To simplify the calculation and estimate the model parameters, the following main assumptions are employed in this study: The products of the coal pyrolysis process is assumed including CH4, CO, CO2, H2, H2S, NH3, H2O, tar (including light tar and heavy tar) and char. The light tar and the heavy tar are presented as C11H10 and
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
C21H42, respectively[12]. The yield of light tar, heavy tar and CH4 is assumed according to the references, which will be discussed later. The heat loss via the wall of the gasifier is assumed as 0.5% in both the G-P and G-C systems. 1.3.3
Table 2
Tar yield in the G-P system
Tar is considered as a main product of coal pyrolysis. In this study, the temperature of the gasifier pyrolysis stage of G-P system is in the range of 800-1400°C. The tar yield in the syngas is inevitable and its effects on gasification performance cannot be ignored, which will be discussed later. The rapid pyrolysis of coal has been studied by many researchers[13–16]. Table 3
Operating conditions
Variables
Value
Unit
Gasification pressure
4.0
MPa
Coal mass flow
90926
kg·h-1
Coal temperature
80
°C
Transport N2 (Coal)
352
kmol·h–1
Transport N2 (Char)
206
kmol·h–1
N2 temperature
80
°C
Oxygen concentration
99.6
%
Oxygen temperature
25
°C
Steam temperature
350
°C
Gasification temperature
1400
°C
Analysis results of coal
Proximate analysis wd/%
Ultimate analysis wd/%
QHHV,d
FC
V
A
C
H
O
N
S
/(kJ·kg–1)
58.37
32.66
8.97
74.29
4.20
11.44
0.87
0.23
29337
Freihaut et al[14] studied the bituminous coal devolatilization using an entrained-flow reactor. The tar yield reported was about 0.2% of coal (daf basis) under the condition of 1000°C, heating rate >2000°C/s, residence time <1 s and particle size from 20 to 30 μm. Hayashi et al[15, 16] investigated the rapid pyrolysis of brown coal in a drop-tube reactor and a Curie-point reactor. Their results showed that the tar yield was about 1.07 mol-C·(100 mol-C)–1 in coal. The steam reforming of tar was also found significant with the catalytic effect of the metal element in coal ash. It was also reported that the elevated temperature could promote the tar cracking. According to the reports, the tar yield in the G-P system is assumed to be 0.5% of coal (dry basis) in this study and the mass fraction of heavy tar is 50%. In addition, the effects of tar yield on the gasification performance are also investigated.
CH4 yield was also reported[19,20]. Gao et al[21] also reported the promotion effects of CO on the CH4 yield in the pressured rapid pyrolysis of lignite. In this article, the CH4 yield in the G-P system is assumed to be 6.5% (on carbon basis) during coal pyrolysis. To evaluate the effects of CH4 yield, the gasification results with different CH4 yields are compared as well. In order to obtain the more precise results of the tar and CH4 yields of coal pyrolysis in this condition, some experiments are ongoing.
1.3.4
The gasification performances of the G-C system and the G-P system are compared in Table 4. The cold gas efficiency, ηCGE, is used to evaluate the gasification performances of these two systems, which is defined as: QHHV,syngas ηCGE = ×100% (1) QHHV,in
Methane yield in the G-P system
The mole fraction of CH4 in the syngas has great effects on the gasification results due to its high heat value. Chen et al[17] analyzed the gas composition of the coal rapid devolatilization in a radiant coal flow reactor. The temperature was 1300 °C with a heating rate of higher than 10000°C/s. The particle size of Pittsburgh No.8 bituminous was ranged from 75 to 106 μm. The CH4 yield reported in their study was 4.08% of the coal (daf basis) at the pressure of 0.1 MPa. Matsuoka et al[18] conducted a high-pressure coal pyrolysis experiment in a drop tube furnace. The yield of CH4 is 6.38% (on a carbon basis) at 850°C and 1 MPa, while it increases to 6.95% at 800°C and 3MPa. This indicated that the pressure may enhance the CH4 yield. In addition, the promoting effect of H2 atmosphere on
2
Results and discussion
2.1 Comparison between the G-C system and the G-P system
As shown in Table 4, the cold gas efficiency of the G-P system is from 87.19% to 89.14%. Although the CO and H2 fractions of the G-P system are lower than that of the G-C system, the relatively high CH4 fraction of the G-P system promotes the gasification performance significantly. Compared to the G-C system, the G-P system has higher cold gas efficiency and lower consumptions. In addition, with increasing tar yield and keeping the yield of CH4 constant, the cold gas efficiency excluding the heat values of tar decreases
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
greatly. This indicates that the effects of the tar yield on the gasification performance and operation of the G-P system are Table 4 Item
negative, especially in the case without tar removal and recovery unit.
Gasification performances of the G-C and G-P systems
Unit
G-P system
G-C system
Tar yield 0%
Tar yield 0.5%
Tar yield 1.0%
Syngas compositions (dry basis) H2
%
30.59
28.81
28.59
28.35
CO
%
61.22
54.15
54.03
53.92
CO2
%
3.47
5.67
5.91
6.15
CH4
%
0.03
4.30
4.34
4.38
N2
%
4.51
6.87
6.93
6.99
Tar content
g·m–3
0
0
2.23
4.58
1400
1400
1400
1400
Performance & consumptions Gasification temperature
[a]
°C
Outlet temperature
°C
1400
833
843
854
Coal consumption[a]
kg·km–3(CO+H2)
513.5
485.1
490.4(486.3)
496.0(487.6)
Oxygen consumption[a]
m3·km–3(CO+H2)
283.5
220.4
222.8(221.0)
225.3(221.5)
Steam consumption[a]
kg·km–3(CO+H2)
115.7
147.0
148.6(147.4)
150.3(147.7)
ηCGE[a]
%
84.14
89.14
88.18(88.91)
87.19(88.68)
: in the calculations of the coal/oxygen/steam consumptions and cold gas efficiency, each mole of CH4 is calculated as 3.12 mole CO or H2 based on the high heat
values of the specific species. Bracketed value is obtained including the heat value of tar, where the tar is calculated as equivalent CO or H2 based on their high heat values
2.2 Effects of the gasification temperature and steam addition In the G-C system, the gasification temperature is the outlet temperature of the gasifier. In the integrated two stage gasifier, the gasification temperature is defined as the outlet temperature of the gasification stage and denoted by tG. With the specified operating conditions (as shown in Table 2), tar yield (0.5%, w% of dry coal) and CH4 (6.5%, on carbon basis) yield, the effects of the gasification temperature on the cold gas efficiency and the coal/oxygen consumptions of the G-P and G-C systems are shown and compared in Figure 4 and Figure 5. The tar is excluded in the calculations of the consumptions and cold gas efficiency in this section (the same as below). It can be found that while the gasification temperature changes from 1300 to 1550°C, the cold gas efficiency of G-C system drops from 85.70% to 81.75% and the consumptions of coal and oxygen increase by 4.8% and 14.0%, respectively. However, with the same variation of the gasification temperature, the cold gas efficiency of the G-P system decreases by 2.79%, and the coal and oxygen consumptions increase by 3.2% and 11.0%. The gasification temperature has fewer effects on the performance of the G-P system. This may be explained as that some of the sensitive heat of the hot syngas from the gasification stage is recovered in the pyrolysis stage. After the pyrolysis process, the amount of the char recycled to the gasification stage of the integrated
system is less than the coal fed to the pulverized coal gasifier. In other word, the fuel combusted in the gasification stage is “upgraded” after pyrolysis process in the G-P system. Thus, less oxygen is needed in G-P system to obtain the same gasification temperature.
Fig. 4
Effects of gasification temperature on cold gas efficiency
Another key operating parameter of the G-P system is the steam coal ratio. Figure 6 shows the effects of the steam coal ratio and the temperature of the gasification stage (tG) on the gas compositions of the syngas and the cold gas efficiency. It can be obtained that tG has less effects on the gasification results while the steam coal ratio is larger than 250 kg(steam)/t(dry coal). If the ratio is less than 220
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
kg(steam)/t(dry coal), higher tG will lead to higher CO and coal gas efficiency, and lower H2 and CH4 fractions. With increasing steam coal ratio, the mole fraction of H2 in syngas increases monotonically, and the CO mole fraction is enhanced firstly and then decreases when the steam coal ratio is higher than 220 kg(steam)/t(dry coal). These results may be explained from the aspects of the carbon conversion of char particles and the chemical equilibrium of the water gas shift reaction. Figure 7 illustrates the effects of the tG and the steam coal ratio on the carbon conversion (xC) of the gasification stage. The xC is defined as: C1 xC =1(2) C0 where C1 means the mass flow of the residual char in outlet stream of the gasification stage, and C0 is the total carbon mass flow in input coal. Compared with Figure 6(d), it can be obtained from Figure 7 that the residual char ratio dominates the cold gas efficiency of the integrated system. It reduces sharply with the increasing of steam coal ratio when it is less than 250 kg(steam)/t(dry coal). This can also be used to explain the other results of Figure 6. At low steam coal ratio, the gasification reactions of the char in the gasification stage
Fig. 6
dominate the gas composition. Once the carbon conversion of the gasifier first stage has approached 0.99, the increasing of steam coal ratio slightly decrease the cold gas efficiency of this system and the gas compositions will be determined by the chemical equilibrium of the water gas shift reaction.
Fig. 5
Effects of gasification temperature on coal and oxygen consumptions O2 consumption: ■ (G-C system), □ (G-P system); coal consumption: ●(G-C system), ○(G-P system)
Effects of the steam coal ratio and the gasification temperature on gas compositions and cold gas efficiency
The effects of gasification temperature and steam coal ratio on gasification performance and the optimization of the operating conditions can be summarized in Figure 8. The
optimization of the operating conditions depends on the ash fusion temperature (flow temperature, FT), which decides the gasification temperature (generally about 100°C above ash FT)
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
and the carbon conversion. To obtain the high carbon conversion, a relatively fast gasification rate and sufficient gasification agent (steam) are necessary. Taking the limitations of the gasification temperature and the carbon conversion into account, the optimized operating conditions could be confirmed. The steam coal ratio should be set about 250–300 kg(steam)/t(dry coal) when the gasification temperature is 1400°C which is related to the ash FT.
increase. However, due to the relatively low temperature of this stage (<900°C) and the low concentration of H2O+CO2 (<12%), the promotion of the degree of char gasification is negligible. This means a small part of syngas heat value has converted into sensitive heat.
Fig. 9
Effects of the CH4 yield on the cold gas efficiency and consumptions
Fig. 7
Effects of the steam coal ratio and gasification temperature
2.4
Energy analysis
on carbon conversion of the gasification stage
Fig. 8
2.3
Optimization of the operating conditions
Effects of CH4 yield in syngas
In this section, the energy and exergy analyses are performed. The energy conversion efficiency and exergy efficiency are defined and employed to evaluate the energy efficiencies of the G-P and G-C systems, respectively. The following equation gives the definition of the energy conversion efficiency, indicating the degree of the total input energy converted to the useful output energy. Eout ηECE = ×100% (3) Ein The energy in a material stream is calculated as the sum of physical and chemical exergies, and the kinetic exergy and other types of exergies are neglected. (4) Ex=Exch + Exph The following equations are adopted to calculate the physical and chemical exergies: ph Exph = ∑i xi exi (5) Exch =
In order to take the CH4 yield of the pyrolysis stage into account, the effects of CH4 yield on the cold gas efficiency and coal/oxygen consumptions are studied and shown in Figure 9. With increasing CH4 yield from 0 to 13.1% (on carbon basis), the oxygen consumptions decreases by 12.7%. This indicates that the CH4 yield may greatly affect the gasification performance of the G-P system, especially the oxygen consumption. In addition, the promotion of CH4 yield slightly reduces the coal gas efficiency. The reason may be explained as following: with the increasing of CH4 yield, the heat value of syngas decreases slightly and the temperature of the pyrolysis stage
xi (exch i + i
ln
∑
)
(6)
For each component, the physical exergy can be obtained from the following equations. ph (7) exi = hi -hi,0 -Ti,0 si -si,0 The chemical exergy of char is calculated as 410260 kJ/kmol[22]. The chemical exergy of coal is estimated by the following equation[23]. exch coal = 1423.079 ×Cd + 21.97×Nd +25088.31×Hd + 4445.29×Od+8984.93×Sd-298.15×0.71768×Ad+QHHV,d (8) where the Cd, Nd, Hd, Od, Sd and Ad represent the mass fraction of hydrogen, carbon, oxygen and nitrogen in the ultimate analysis of coal (dry basis).
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
heat of the gasification stage with very high energy efficiency. The ηECE of the gasification stage and the pyrolysis stage are 96.83% and 99.69%, respectively. In the G-C system, the ηECE of the gasifier is 98.05%, which is higher than the gasification stage of the G-P system due to the higher CO and H2 yields and the lower steam consumption.
The exergy efficiency of the system is calculated as: Exout ×100% (9) ηEx = Exin The overall energy flowsheets of the G-P and G-C systems are illustrated in Figure 10. In the G-P system, it is found that the cold gas efficiency of the gasification stage is 86.2%, and which of the pyrolysis stage is 99.3%. This result means that the coal is converted in the pyrolysis stage using the sensitive
Fig. 10 Table 5
Energy flowsheets of the G-P system (a) and the G-C system (b) Energy and exergy balances of the G-C system and the G-P system G-C system
Items
G-P system
Mass flow
Energy
Exergy
Mass flow
Energy
Exergy
/(kg·h–1)
E/(kJ·h–1)
/(kJ·h–1)
/(kg·h–1)
E/ (kJ·h–1)
/(kJ·h–1)
90926
2.62×109
2.71×109
90926
2.62×109
2.71×109
8.72×10
6
57832
0.00
7.17×106
2.35×10
5
Input Coal Oxygen
70287
0.00 5
15632
8.96×10
5
3.73×105
26977
8.27×107
1.98×107
1.33×10
8
3.68×107
1.02×10
8
1.02×108
6
8.28×106
Transport N2
9864
5.65×10
Steam
20063
6.15×107
1.47×107
1.95×10
8
5.42×10
7
1.24×10
8
1.24×10
8
6
8.28×10
6
-
8.28×10
1.99×107
-
1.99×107
1.99×107
-
7
1.44×107
9
2.92×109
Boiler feed water ASU
192319 -
Coal grinding
-
8.28×10
Coal transport
-
1.99×107
Char transport Total
-
0.00
0.00 9
383459
3.03×10
181886
2.31×109
130629 -
1.44×10
9
321997
2.98×10
2.05×109
181659
2.41×109
2.14×109
7
2.46×107
3.56×10
8
9.09×107
2.78×10
9
2.26×109
2.94×10
Output Syngas Tar
0
HP Steam Total
374205
ηECE[a] /% ηEx [a]
[a]
192319
/%
0
0
5.24×10
8
2.83×10
9
436
1.34×10
8
2.18×10
9
130629 312724
1.91×10
93.39
92.62 (93.29)
74.15
76.37 (77.40)
: the bracketed value is calculated including the exergy of tar
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
The detailed energy and exergy balances of the G-P system and G-C system are listed in Table 5, from which the ηECE and ηEx can be derived. The ηECE of the G-P system is 92.26%, which is lower than that of the G-C system (93.39%) without taking the tar into account. The higher transport energy consumption and lower steam production may be the main reasons for this result. However, the exergy efficiency ηEx of the G-P system (76.37%) is higher than that of the G-C system (74.15%), showing that the energy quality of the syngas in the G-P system is much higher than that in the G-C system. Table 6 Section
The exergy balance of each section of the G-P and G-C system is listed in Table 6. The result shows the largest exergy loss of either the G-C system or G-P system is in the gasification section. The total exergy loss of the gasification stage and the pyrolysis stage in the G-P system is much lower than the sum of exergy losses of the gasification section and the radiant cooler section in the G-C system. The results show that the design of combination of the gasification stage and the pyrolysis stage can significantly reduce the exergy loss during the coal conversion process.
Exergy balance of each section of the G-C system and the G-P system Exin /(kJ·h–1)
Exout /(kJ·h–1)
Exloss /(kJ·h–1)
ηEx /%
2.74×109
2.28×109
4.62×108
83.11
9
9
9.87×107
95.72
9
7
5.12×10
97.62
2.57×108 (2.57×108)
86.63(86.63)
2.14×108 (1.89×108)
95.21(95.77)
G-C system Gasification Radiant cooler
2.31×10
Convective cooler
2.21×10
9
2.10×10
2.15×10
G-P system Gasification stage[a] [a]
Pyrolysis stage
[a]
Syngas cooler [a]
3
1.92×109 (1.92×109) 9
9
9
9
4.46×10 (4.47×10 ) 4.29×10 (4.31×10 )
1.66×109 (1.66×109) 9
9
9
9
4.25×10 (4.28×10 ) 4.21×10 (4.24×10 )
7
7
7.53×10 (7.53×10 )
98.24(98.25)
: the bracketed value is calculated including the exergy of tar
Conclusions
In this article, an integrated coal gasification and pyrolysis system is studied. The process modelling of the G-P system is performed and the results are compared with the G-C system. The effects of the operating conditions on the gasification performance are also investigated and the optimized operating conditions are proposed. The energy analysis based on the overall energy balance and the exergy analysis is also performed. Following conclusions can be obtained: The cold gas efficiency of the G-P system is 88.18%, which is higher than that of the G-C system. The coal and O2 consumptions are reduced in the integrated system. The tar and CH4 yield have significant effects on the gasification performance of the integrated system. The optimized steam coal ratio of this integrated system is about 250~300 kg(steam)/t (dry coal) at the gasification temperature of 1400°C. Compared with the G-C system, the gasification temperature has less effect on the G-P system. The overall energy conversion efficiency of the G-P system is slightly lower than that of the G-C system. However, the exergy efficiency is enhanced by more than 2.2% in the G-P system. The design of combination of the gasification stage and the pyrolysis stage promotes the grade of the energy recovered from the hot syngas. The higher transport energy consumption and lower steam production may be the main reasons which affect the energy efficiency of the G-P system. The control of tar yield, tar
recovery and treatment of gas purification waste water should be investigated in further study. Nomenclature C1 = the residual char in outlet stream of the gasification stage, kg/h C0 = the total carbon mass flow in input coal, kg/h exiph = physical exergy of component i, kJ/kmol exich = chemical exergy of component i, kJ/kmol exch coal = chemical exergy of coal, kJ/kg Ein/Eout = energy of a stream, energy entering/exiting a unit, kJ/h Exin/Eout = exergy of a stream, exergy entering/exiting a unit, kJ/h QHHV = high heat value of a stream, kJ/h QHHV,d = high heat value of coal (dry basis), kJ/kg R = universal gas constant, 8.3145 kJ/(kmol·K) tG = the outlet temperature of gasification stage of the integrated system, gasification temperature, °C xC = the carbon conversion of the gasification stage, 1
Greek letters ηCGE = cold gas efficiency, % ηECE = energy conversion efficiency, % ηEx = exergy efficiency, %
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
Abbreviations
2008, 87(10/11): 2304–2313. [11] Dai Z H, Gong X, Wang F C, Yu G S, Tan K R, Yu Z H.
ASU = air separation unit
Thermodynamic analysis of entrained-flow pulverized coal
G-P = gasification and pyrolysis
gasification by Gibbs free energy minimization. J Fuel Chem Technol, 2005, 33(2): 129-133.
CWS = coal-water slurry
[12] Stefano J M. Evaluation and modification of Aspen fixed-bed
ECUST = East China University of Science and Technology MHI = Mitsubishi Heavy Industries
gasifier 1 models for inclusion in an integrated gasification
OMB = Opposed Multi-Burner
combined-cycle
G-C = gasification and cooler
Morgantown Energy Technology Center, 1985.
power
plant
simulation.
West
Virginia:
[13] Liu T F, Fang Y T, Wang Y. Rapid pyrolysis of coal at high
SNG = synthetic natural gas
temperature. J Fuel Chem Technol, 2009, 37(1): 20–25.
TPRI = Xi’an Thermal Power Research Institute Co. Ltd
[14] Freihaut J D, Proscia W M, Seery D J. Chemical characteristics
WGS = water gas shift
of tars produced in a novel low-severity, entrained-flow reactor.
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RESEARCH PAPER
Modelling and energy analysis of an integrated coal gasification and pyrolysis system for synthetic natural gas LI Chao1,2, DAI Zheng-hua1,2,*, YANG Ji3, YU Guang-suo1,2, WANG Fu-chen1,2 1
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and
Technology, Shanghai 200237, China; 2
Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, China;
3
School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai 200237, China
Abstract:
The entrained-flow coal gasification is an important and attractive technology in large scale coal-based synthetic natural
gas (SNG) process. In this study, an integrated entrained-flow coal gasification and pyrolysis system is proposed and investigated. The gasifier is divided into two stages in this system: one is mainly for the gasification of char and the other is mainly for the coal pyrolysis. The integrated system is studied by using a process simulation model and compared with a coal gasification system with the radiant and convective coolers. The effects of the operating conditions on the gasification performance are studied and the optimized operating conditions are obtained. It is found that the optimized steam coal ratio of the integrated system is about 250-300 kg(steam)/t (dry coal) while the gasification temperature is 1400°C. The cold gas efficiency of 88.18% of the integrated system is higher than that of 84.14% of the gasification and radiant+conductive cooler system. The coal and O2 consumptions are relative low in the integrated system. The performance of the integrated system is highly related to the yield of tar and CH4 in the pyrolysis stage. The overall energy conversion efficiency of the integrated system (92.26%) is slightly lower than that of the gasification and radiant+conductive cooler system (93.39%). However, the exergy efficiency is enhanced by more than 2.2% in the integrated system. The integration of the gasification and pyrolysis can effectively recover and promote energy grade of the sensitive heat in the hot syngas. Keywords: coal gasification; pyrolysis; integrated system; process modelling; energy analysis
With the significant increasing of natural gas consumption in China since 2009[1], synthetic natural gas (SNG) becomes an attractive technology to meet this huge demand. Coal is considered as the most economic and reliable feedstock in the SNG process because it is still the major part of primary energy supply in China. Therefore, coal gasification is an efficient way to produce the syngas for SNG process and to reduce the pollutant emission during coal utilization. As shown in Figure 1, the typical coal based SNG process includes air separation unit (ASU), gasification and heat recovery, purification, water gas shift (WGS), sulphur/CO2 removal and methanation unit. There are many gasification technologies can be used in coal based SNG projects[2], in which one of the most reliable and promising technologies is entrained-flow coal gasification. The most attractive features of the entrained-flow gasification are the relative flexibility of feedstock and the
scaled up capacity of a single gasifier. A large quantity of heat is converted to the sensitive heat and stored in the hot gas due to the high gasification temperature. The heat recovery methods used in entrained-flow gasification process can be generally classified into two types: the physical heat recovery and the chemical heat recovery. The physical heat recovery methods are all aimed to recover the sensitive heat of hot syngas by directly heat exchange and to generate steam, including quench or radiant + convective syngas cooling processes.
Fig. 1
Flow sheet of typical coal based SNG process
Received: 31-Mar-2015; Revised: 02-Jul-2015. Foundation item: Supported by the National Key State Basic Research Development Program of China (973 Program, 2010CB227000) and the National High Technology Research and Development Program of China (863 Program, 2008AA052310). *Corresponding author: DAI Zheng-hua, Tel: +86-21-64250784, Fax: +86-21-64251312, E-mail: [email protected]. Copyright 2015, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
In the chemical heat recovery method, the sensitive heat of the syngas is to provide coal pyrolysis or gasification reactions to generate syngas or pyrolysis gases. Some of the technologies employed the chemical heat recovery methods have been successfully commercialized or applied in pilot plant, such as the E-Gas coal water slurry (CWS) gasification technology[3], the TPRI gasification technology[4] and the MHI air-blown pulverized coal gasification technology[5]. In these processes, the gasifier is divided into two stages. A part of Table 1
Features of the entrained flow gasification technologies with chemical heat recovery process
Technology
Vendor
Feedstock
Flow direction
Oxidant
E-gas
ConocoPhillips
slurry
up
oxygen
refractory
TPRI
TPRI
dry
up
oxygen
membrane
1.1
Reactor wall
MHI
MHI
dry
up
air
membrane
E-STR
ConocoPhillips
slurry
up
oxygen
refractory
Two stage OMB
ECUST
dry/slurry
down
oxygen
refractory/ membrane
In this study, a chemical heat recovery system combined the coal gasification and pyrolysis together is proposed and studied systematically. In this system, the first stage of the gasifier, named gasification stage, is mainly for char combustion and gasification, while the second stage (pyrolysis stage) is mainly used for coal pyrolysis and heat recovery. The gasification results of the integrated system are studied and compared with a pulverized coal gasification system with radiant and convective cooler. The effects of operating conditions on gasification performance are investigated as well to determine the most appropriate operating parameters. The energy and exergy analyses are also performed to evaluate the energy efficiency of the integrated system in the aspects of both quantity and quality of the energy output.
1
CWS or pulverized coal is injected into the second stage of gasifier. Another integrated system coupled the entrained-flow coal gasification and the fixed bed coal pyrolysis is also proposed by the East China University of Science and Technology (ECUST)[6]. The ConocoPhillips E-STR[7] and researchers in Japan[8] have also developed the processes with introducing all of the feedstock to the second stage of gasifier. The features of these technologies are summarized and listed in Table 1.
Experimental Process description
Fig. 2
The concept of the integrated system is to enhance the energy efficiently by decoupling the endothermic reactions and exothermic reactions and to utilize this part of CH4 generated in coal pyrolysis in a SNG process. To evaluate the performance of the integrated coal gasification and pyrolysis system (G-P), the pulverized coal gasification system with radiant cooler and convective coolers (G-C) is also studied in this article. The flowsheets of these two systems are shown in Figure 2. As shown in Figure 2(a), the two stage entrained-flow gasifier is the key of the G-P system. All of the pulverized coal is transported by N2 or CO2 and injected into the pyrolysis stage of the gasifier. (In order to considering the demand of pyrolysis stage temperature control, part of pulverized coal can also be injected into the gasification stage in real operation.) In this study, the N2 is used as the transport gas. With directly mixing with the hot gas from the gasification stage of the gasifier, the pulverized coal particles pyrolyze rapidly.
Flowsheet of the G-P system (a) and the G-C system (b)
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
The original design conception of this process is to reduce the tar yield of coal pyrolysis through the high temperature of the pyrolysis stage and the steam introduced in gasification stage. However, a small amount of tar will be generated during pyrolysis in real operation. The existence of tar in the syngas usually involves engineering problems, such as gas purification, tar recovery and waste water treatment. Therefore, the tar yield is an important factor which should be highly concerned. The tar yield will be discussed in Section 1.3. The rawgas from the pyrolysis stage of the gasifier is cooled by a heat exchanger firstly to produce high pressure steam. Following it, a separator is used to remove the unreacted char and ash particles from the syngas. The syngas out of the separator is sent to the downstream process for further purification. A small part of the syngas is recycled to the nozzle of the pyrolysis stage for the dispersion of the pulverized coal. The unreacted char and ash particles are transported by N2/CO2 and recycled to the gasification stage of the gasifier by a lock hopper system. The recycled char particles are injected into the gasification stage of the gasifier with steam and oxygen and converted. Most of the slag is deposited on the wall and discharged from the bottom of the gasifier. A portion of the fly ash is entrained to the pyrolysis stage by the hot gas. The G-C system (b) is illustrated in Figure 2(b), the coal, transport N2, O2 and steam are all injected into the gasifier through the coaxial burner. The rawgas and slag from the
Fig. 3
1.3 1.3.1
gasification chamber is cooled in a radiant cooler, and the slag is separated and discharged. A convective cooler is used to recover the residual heat in the low temperature rawgas. Finally, the fly ash entrained in the rawgas is removed in a separator. 1.2
Model implementation
In this study, the simulation is performed by using ASPEN PLUS software. The modelling flowsheets of the G-P system and G-C system are shown in Figure 3(a) and Figure 3(b), respectively. In the G-P system, the units GASIFIER-1 and SEP-1 are combined to model the gasification stage of the gasifier, and the pyrolysis stage is divided into two units of DECOMP and the GASIFIER-2. In the G-C system, the DECOMP and the GASIFIER are used to model the gasifier as well. The yields in the DECOMP unit are determined based on the mass balance of the elements and the yields of the specific species in pyrolysis products[9]. As the RGibbs model based on the Gibbs energy minimization principle has been widely used in process modelling of coal gasification system[10, 11], the RGibbs model is employed to calculate char combustion/gasification and homogeneous reactions in this study. The PR-BM model is employed to calculate the physical properties used in the simulation.
Modelling flowsheets of the G-P system (a) and G-C system (b)
Operating conditions and assumptions Operating conditions
The calculation of this study is based on a commercial scale gasifier with the capacity of ~2200 t(dry coal)·d–1. The operating conditions are listed in Table 2. The coal analysis results are given in Table 3. The outlet temperatures of the separator and the syngas cooler are 350°C in the G-P system. In the G-C system, the outlet temperatures of the radiant cooler and convection cooler (separator) are 800°C and 350°C,
respectively. 1.3.2
Main assumptions
To simplify the calculation and estimate the model parameters, the following main assumptions are employed in this study: The products of the coal pyrolysis process is assumed including CH4, CO, CO2, H2, H2S, NH3, H2O, tar (including light tar and heavy tar) and char. The light tar and the heavy tar are presented as C11H10 and
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
C21H42, respectively[12]. The yield of light tar, heavy tar and CH4 is assumed according to the references, which will be discussed later. The heat loss via the wall of the gasifier is assumed as 0.5% in both the G-P and G-C systems. 1.3.3
Table 2
Tar yield in the G-P system
Tar is considered as a main product of coal pyrolysis. In this study, the temperature of the gasifier pyrolysis stage of G-P system is in the range of 800-1400°C. The tar yield in the syngas is inevitable and its effects on gasification performance cannot be ignored, which will be discussed later. The rapid pyrolysis of coal has been studied by many researchers[13–16]. Table 3
Operating conditions
Variables
Value
Unit
Gasification pressure
4.0
MPa
Coal mass flow
90926
kg·h-1
Coal temperature
80
°C
Transport N2 (Coal)
352
kmol·h–1
Transport N2 (Char)
206
kmol·h–1
N2 temperature
80
°C
Oxygen concentration
99.6
%
Oxygen temperature
25
°C
Steam temperature
350
°C
Gasification temperature
1400
°C
Analysis results of coal
Proximate analysis wd/%
Ultimate analysis wd/%
QHHV,d
FC
V
A
C
H
O
N
S
/(kJ·kg–1)
58.37
32.66
8.97
74.29
4.20
11.44
0.87
0.23
29337
Freihaut et al[14] studied the bituminous coal devolatilization using an entrained-flow reactor. The tar yield reported was about 0.2% of coal (daf basis) under the condition of 1000°C, heating rate >2000°C/s, residence time <1 s and particle size from 20 to 30 μm. Hayashi et al[15, 16] investigated the rapid pyrolysis of brown coal in a drop-tube reactor and a Curie-point reactor. Their results showed that the tar yield was about 1.07 mol-C·(100 mol-C)–1 in coal. The steam reforming of tar was also found significant with the catalytic effect of the metal element in coal ash. It was also reported that the elevated temperature could promote the tar cracking. According to the reports, the tar yield in the G-P system is assumed to be 0.5% of coal (dry basis) in this study and the mass fraction of heavy tar is 50%. In addition, the effects of tar yield on the gasification performance are also investigated.
CH4 yield was also reported[19,20]. Gao et al[21] also reported the promotion effects of CO on the CH4 yield in the pressured rapid pyrolysis of lignite. In this article, the CH4 yield in the G-P system is assumed to be 6.5% (on carbon basis) during coal pyrolysis. To evaluate the effects of CH4 yield, the gasification results with different CH4 yields are compared as well. In order to obtain the more precise results of the tar and CH4 yields of coal pyrolysis in this condition, some experiments are ongoing.
1.3.4
The gasification performances of the G-C system and the G-P system are compared in Table 4. The cold gas efficiency, ηCGE, is used to evaluate the gasification performances of these two systems, which is defined as: QHHV,syngas ηCGE = ×100% (1) QHHV,in
Methane yield in the G-P system
The mole fraction of CH4 in the syngas has great effects on the gasification results due to its high heat value. Chen et al[17] analyzed the gas composition of the coal rapid devolatilization in a radiant coal flow reactor. The temperature was 1300 °C with a heating rate of higher than 10000°C/s. The particle size of Pittsburgh No.8 bituminous was ranged from 75 to 106 μm. The CH4 yield reported in their study was 4.08% of the coal (daf basis) at the pressure of 0.1 MPa. Matsuoka et al[18] conducted a high-pressure coal pyrolysis experiment in a drop tube furnace. The yield of CH4 is 6.38% (on a carbon basis) at 850°C and 1 MPa, while it increases to 6.95% at 800°C and 3MPa. This indicated that the pressure may enhance the CH4 yield. In addition, the promoting effect of H2 atmosphere on
2
Results and discussion
2.1 Comparison between the G-C system and the G-P system
As shown in Table 4, the cold gas efficiency of the G-P system is from 87.19% to 89.14%. Although the CO and H2 fractions of the G-P system are lower than that of the G-C system, the relatively high CH4 fraction of the G-P system promotes the gasification performance significantly. Compared to the G-C system, the G-P system has higher cold gas efficiency and lower consumptions. In addition, with increasing tar yield and keeping the yield of CH4 constant, the cold gas efficiency excluding the heat values of tar decreases
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
greatly. This indicates that the effects of the tar yield on the gasification performance and operation of the G-P system are Table 4 Item
negative, especially in the case without tar removal and recovery unit.
Gasification performances of the G-C and G-P systems
Unit
G-P system
G-C system
Tar yield 0%
Tar yield 0.5%
Tar yield 1.0%
Syngas compositions (dry basis) H2
%
30.59
28.81
28.59
28.35
CO
%
61.22
54.15
54.03
53.92
CO2
%
3.47
5.67
5.91
6.15
CH4
%
0.03
4.30
4.34
4.38
N2
%
4.51
6.87
6.93
6.99
Tar content
g·m–3
0
0
2.23
4.58
1400
1400
1400
1400
Performance & consumptions Gasification temperature
[a]
°C
Outlet temperature
°C
1400
833
843
854
Coal consumption[a]
kg·km–3(CO+H2)
513.5
485.1
490.4(486.3)
496.0(487.6)
Oxygen consumption[a]
m3·km–3(CO+H2)
283.5
220.4
222.8(221.0)
225.3(221.5)
Steam consumption[a]
kg·km–3(CO+H2)
115.7
147.0
148.6(147.4)
150.3(147.7)
ηCGE[a]
%
84.14
89.14
88.18(88.91)
87.19(88.68)
: in the calculations of the coal/oxygen/steam consumptions and cold gas efficiency, each mole of CH4 is calculated as 3.12 mole CO or H2 based on the high heat
values of the specific species. Bracketed value is obtained including the heat value of tar, where the tar is calculated as equivalent CO or H2 based on their high heat values
2.2 Effects of the gasification temperature and steam addition In the G-C system, the gasification temperature is the outlet temperature of the gasifier. In the integrated two stage gasifier, the gasification temperature is defined as the outlet temperature of the gasification stage and denoted by tG. With the specified operating conditions (as shown in Table 2), tar yield (0.5%, w% of dry coal) and CH4 (6.5%, on carbon basis) yield, the effects of the gasification temperature on the cold gas efficiency and the coal/oxygen consumptions of the G-P and G-C systems are shown and compared in Figure 4 and Figure 5. The tar is excluded in the calculations of the consumptions and cold gas efficiency in this section (the same as below). It can be found that while the gasification temperature changes from 1300 to 1550°C, the cold gas efficiency of G-C system drops from 85.70% to 81.75% and the consumptions of coal and oxygen increase by 4.8% and 14.0%, respectively. However, with the same variation of the gasification temperature, the cold gas efficiency of the G-P system decreases by 2.79%, and the coal and oxygen consumptions increase by 3.2% and 11.0%. The gasification temperature has fewer effects on the performance of the G-P system. This may be explained as that some of the sensitive heat of the hot syngas from the gasification stage is recovered in the pyrolysis stage. After the pyrolysis process, the amount of the char recycled to the gasification stage of the integrated
system is less than the coal fed to the pulverized coal gasifier. In other word, the fuel combusted in the gasification stage is “upgraded” after pyrolysis process in the G-P system. Thus, less oxygen is needed in G-P system to obtain the same gasification temperature.
Fig. 4
Effects of gasification temperature on cold gas efficiency
Another key operating parameter of the G-P system is the steam coal ratio. Figure 6 shows the effects of the steam coal ratio and the temperature of the gasification stage (tG) on the gas compositions of the syngas and the cold gas efficiency. It can be obtained that tG has less effects on the gasification results while the steam coal ratio is larger than 250 kg(steam)/t(dry coal). If the ratio is less than 220
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
kg(steam)/t(dry coal), higher tG will lead to higher CO and coal gas efficiency, and lower H2 and CH4 fractions. With increasing steam coal ratio, the mole fraction of H2 in syngas increases monotonically, and the CO mole fraction is enhanced firstly and then decreases when the steam coal ratio is higher than 220 kg(steam)/t(dry coal). These results may be explained from the aspects of the carbon conversion of char particles and the chemical equilibrium of the water gas shift reaction. Figure 7 illustrates the effects of the tG and the steam coal ratio on the carbon conversion (xC) of the gasification stage. The xC is defined as: C1 xC =1(2) C0 where C1 means the mass flow of the residual char in outlet stream of the gasification stage, and C0 is the total carbon mass flow in input coal. Compared with Figure 6(d), it can be obtained from Figure 7 that the residual char ratio dominates the cold gas efficiency of the integrated system. It reduces sharply with the increasing of steam coal ratio when it is less than 250 kg(steam)/t(dry coal). This can also be used to explain the other results of Figure 6. At low steam coal ratio, the gasification reactions of the char in the gasification stage
Fig. 6
dominate the gas composition. Once the carbon conversion of the gasifier first stage has approached 0.99, the increasing of steam coal ratio slightly decrease the cold gas efficiency of this system and the gas compositions will be determined by the chemical equilibrium of the water gas shift reaction.
Fig. 5
Effects of gasification temperature on coal and oxygen consumptions O2 consumption: ■ (G-C system), □ (G-P system); coal consumption: ●(G-C system), ○(G-P system)
Effects of the steam coal ratio and the gasification temperature on gas compositions and cold gas efficiency
The effects of gasification temperature and steam coal ratio on gasification performance and the optimization of the operating conditions can be summarized in Figure 8. The
optimization of the operating conditions depends on the ash fusion temperature (flow temperature, FT), which decides the gasification temperature (generally about 100°C above ash FT)
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
and the carbon conversion. To obtain the high carbon conversion, a relatively fast gasification rate and sufficient gasification agent (steam) are necessary. Taking the limitations of the gasification temperature and the carbon conversion into account, the optimized operating conditions could be confirmed. The steam coal ratio should be set about 250–300 kg(steam)/t(dry coal) when the gasification temperature is 1400°C which is related to the ash FT.
increase. However, due to the relatively low temperature of this stage (<900°C) and the low concentration of H2O+CO2 (<12%), the promotion of the degree of char gasification is negligible. This means a small part of syngas heat value has converted into sensitive heat.
Fig. 9
Effects of the CH4 yield on the cold gas efficiency and consumptions
Fig. 7
Effects of the steam coal ratio and gasification temperature
2.4
Energy analysis
on carbon conversion of the gasification stage
Fig. 8
2.3
Optimization of the operating conditions
Effects of CH4 yield in syngas
In this section, the energy and exergy analyses are performed. The energy conversion efficiency and exergy efficiency are defined and employed to evaluate the energy efficiencies of the G-P and G-C systems, respectively. The following equation gives the definition of the energy conversion efficiency, indicating the degree of the total input energy converted to the useful output energy. Eout ηECE = ×100% (3) Ein The energy in a material stream is calculated as the sum of physical and chemical exergies, and the kinetic exergy and other types of exergies are neglected. (4) Ex=Exch + Exph The following equations are adopted to calculate the physical and chemical exergies: ph Exph = ∑i xi exi (5) Exch =
In order to take the CH4 yield of the pyrolysis stage into account, the effects of CH4 yield on the cold gas efficiency and coal/oxygen consumptions are studied and shown in Figure 9. With increasing CH4 yield from 0 to 13.1% (on carbon basis), the oxygen consumptions decreases by 12.7%. This indicates that the CH4 yield may greatly affect the gasification performance of the G-P system, especially the oxygen consumption. In addition, the promotion of CH4 yield slightly reduces the coal gas efficiency. The reason may be explained as following: with the increasing of CH4 yield, the heat value of syngas decreases slightly and the temperature of the pyrolysis stage
xi (exch i + i
ln
∑
)
(6)
For each component, the physical exergy can be obtained from the following equations. ph (7) exi = hi -hi,0 -Ti,0 si -si,0 The chemical exergy of char is calculated as 410260 kJ/kmol[22]. The chemical exergy of coal is estimated by the following equation[23]. exch coal = 1423.079 ×Cd + 21.97×Nd +25088.31×Hd + 4445.29×Od+8984.93×Sd-298.15×0.71768×Ad+QHHV,d (8) where the Cd, Nd, Hd, Od, Sd and Ad represent the mass fraction of hydrogen, carbon, oxygen and nitrogen in the ultimate analysis of coal (dry basis).
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
heat of the gasification stage with very high energy efficiency. The ηECE of the gasification stage and the pyrolysis stage are 96.83% and 99.69%, respectively. In the G-C system, the ηECE of the gasifier is 98.05%, which is higher than the gasification stage of the G-P system due to the higher CO and H2 yields and the lower steam consumption.
The exergy efficiency of the system is calculated as: Exout ×100% (9) ηEx = Exin The overall energy flowsheets of the G-P and G-C systems are illustrated in Figure 10. In the G-P system, it is found that the cold gas efficiency of the gasification stage is 86.2%, and which of the pyrolysis stage is 99.3%. This result means that the coal is converted in the pyrolysis stage using the sensitive
Fig. 10 Table 5
Energy flowsheets of the G-P system (a) and the G-C system (b) Energy and exergy balances of the G-C system and the G-P system G-C system
Items
G-P system
Mass flow
Energy
Exergy
Mass flow
Energy
Exergy
/(kg·h–1)
E/(kJ·h–1)
/(kJ·h–1)
/(kg·h–1)
E/ (kJ·h–1)
/(kJ·h–1)
90926
2.62×109
2.71×109
90926
2.62×109
2.71×109
8.72×10
6
57832
0.00
7.17×106
2.35×10
5
Input Coal Oxygen
70287
0.00 5
15632
8.96×10
5
3.73×105
26977
8.27×107
1.98×107
1.33×10
8
3.68×107
1.02×10
8
1.02×108
6
8.28×106
Transport N2
9864
5.65×10
Steam
20063
6.15×107
1.47×107
1.95×10
8
5.42×10
7
1.24×10
8
1.24×10
8
6
8.28×10
6
-
8.28×10
1.99×107
-
1.99×107
1.99×107
-
7
1.44×107
9
2.92×109
Boiler feed water ASU
192319 -
Coal grinding
-
8.28×10
Coal transport
-
1.99×107
Char transport Total
-
0.00
0.00 9
383459
3.03×10
181886
2.31×109
130629 -
1.44×10
9
321997
2.98×10
2.05×109
181659
2.41×109
2.14×109
7
2.46×107
3.56×10
8
9.09×107
2.78×10
9
2.26×109
2.94×10
Output Syngas Tar
0
HP Steam Total
374205
ηECE[a] /% ηEx [a]
[a]
192319
/%
0
0
5.24×10
8
2.83×10
9
436
1.34×10
8
2.18×10
9
130629 312724
1.91×10
93.39
92.62 (93.29)
74.15
76.37 (77.40)
: the bracketed value is calculated including the exergy of tar
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
The detailed energy and exergy balances of the G-P system and G-C system are listed in Table 5, from which the ηECE and ηEx can be derived. The ηECE of the G-P system is 92.26%, which is lower than that of the G-C system (93.39%) without taking the tar into account. The higher transport energy consumption and lower steam production may be the main reasons for this result. However, the exergy efficiency ηEx of the G-P system (76.37%) is higher than that of the G-C system (74.15%), showing that the energy quality of the syngas in the G-P system is much higher than that in the G-C system. Table 6 Section
The exergy balance of each section of the G-P and G-C system is listed in Table 6. The result shows the largest exergy loss of either the G-C system or G-P system is in the gasification section. The total exergy loss of the gasification stage and the pyrolysis stage in the G-P system is much lower than the sum of exergy losses of the gasification section and the radiant cooler section in the G-C system. The results show that the design of combination of the gasification stage and the pyrolysis stage can significantly reduce the exergy loss during the coal conversion process.
Exergy balance of each section of the G-C system and the G-P system Exin /(kJ·h–1)
Exout /(kJ·h–1)
Exloss /(kJ·h–1)
ηEx /%
2.74×109
2.28×109
4.62×108
83.11
9
9
9.87×107
95.72
9
7
5.12×10
97.62
2.57×108 (2.57×108)
86.63(86.63)
2.14×108 (1.89×108)
95.21(95.77)
G-C system Gasification Radiant cooler
2.31×10
Convective cooler
2.21×10
9
2.10×10
2.15×10
G-P system Gasification stage[a] [a]
Pyrolysis stage
[a]
Syngas cooler [a]
3
1.92×109 (1.92×109) 9
9
9
9
4.46×10 (4.47×10 ) 4.29×10 (4.31×10 )
1.66×109 (1.66×109) 9
9
9
9
4.25×10 (4.28×10 ) 4.21×10 (4.24×10 )
7
7
7.53×10 (7.53×10 )
98.24(98.25)
: the bracketed value is calculated including the exergy of tar
Conclusions
In this article, an integrated coal gasification and pyrolysis system is studied. The process modelling of the G-P system is performed and the results are compared with the G-C system. The effects of the operating conditions on the gasification performance are also investigated and the optimized operating conditions are proposed. The energy analysis based on the overall energy balance and the exergy analysis is also performed. Following conclusions can be obtained: The cold gas efficiency of the G-P system is 88.18%, which is higher than that of the G-C system. The coal and O2 consumptions are reduced in the integrated system. The tar and CH4 yield have significant effects on the gasification performance of the integrated system. The optimized steam coal ratio of this integrated system is about 250~300 kg(steam)/t (dry coal) at the gasification temperature of 1400°C. Compared with the G-C system, the gasification temperature has less effect on the G-P system. The overall energy conversion efficiency of the G-P system is slightly lower than that of the G-C system. However, the exergy efficiency is enhanced by more than 2.2% in the G-P system. The design of combination of the gasification stage and the pyrolysis stage promotes the grade of the energy recovered from the hot syngas. The higher transport energy consumption and lower steam production may be the main reasons which affect the energy efficiency of the G-P system. The control of tar yield, tar
recovery and treatment of gas purification waste water should be investigated in further study. Nomenclature C1 = the residual char in outlet stream of the gasification stage, kg/h C0 = the total carbon mass flow in input coal, kg/h exiph = physical exergy of component i, kJ/kmol exich = chemical exergy of component i, kJ/kmol exch coal = chemical exergy of coal, kJ/kg Ein/Eout = energy of a stream, energy entering/exiting a unit, kJ/h Exin/Eout = exergy of a stream, exergy entering/exiting a unit, kJ/h QHHV = high heat value of a stream, kJ/h QHHV,d = high heat value of coal (dry basis), kJ/kg R = universal gas constant, 8.3145 kJ/(kmol·K) tG = the outlet temperature of gasification stage of the integrated system, gasification temperature, °C xC = the carbon conversion of the gasification stage, 1
Greek letters ηCGE = cold gas efficiency, % ηECE = energy conversion efficiency, % ηEx = exergy efficiency, %
LI Chao et al / Journal of Fuel Chemistry and Technology, 2015, 43(7): 779-789
Abbreviations
2008, 87(10/11): 2304–2313. [11] Dai Z H, Gong X, Wang F C, Yu G S, Tan K R, Yu Z H.
ASU = air separation unit
Thermodynamic analysis of entrained-flow pulverized coal
G-P = gasification and pyrolysis
gasification by Gibbs free energy minimization. J Fuel Chem Technol, 2005, 33(2): 129-133.
CWS = coal-water slurry
[12] Stefano J M. Evaluation and modification of Aspen fixed-bed
ECUST = East China University of Science and Technology MHI = Mitsubishi Heavy Industries
gasifier 1 models for inclusion in an integrated gasification
OMB = Opposed Multi-Burner
combined-cycle
G-C = gasification and cooler
Morgantown Energy Technology Center, 1985.
power
plant
simulation.
West
Virginia:
[13] Liu T F, Fang Y T, Wang Y. Rapid pyrolysis of coal at high
SNG = synthetic natural gas
temperature. J Fuel Chem Technol, 2009, 37(1): 20–25.
TPRI = Xi’an Thermal Power Research Institute Co. Ltd
[14] Freihaut J D, Proscia W M, Seery D J. Chemical characteristics
WGS = water gas shift
of tars produced in a novel low-severity, entrained-flow reactor.
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