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Novel power generation models integrated supercritical water gasification of coal and parallel partial chemical heat recovery
Energy 134 (2017) 933e942
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Novel power generation models integrated supercritical water gasification of coal and parallel partial chemical heat recovery Zhewen Chen a, b, Xiaosong Zhang a, b, *, Sheng Li a, b, Lin Gao a, b a b
Institute of Engineering Thermophysics, CAS, China University of Chinese Academy of Sciences, Beijing, 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2016 Received in revised form 5 June 2017 Accepted 6 June 2017 Available online 17 June 2017
Supercritical water gasification (SCWG) of coal is a promising clean coal technology. Supercritical water can effectively and cleanly convert coal to hydrogen-rich syngas. Three power generation models integrated SCWG of coal are proposed and compared in this article. The gasification products have a large amount of sensible heat. Efficient use of the sensible heat can improve the model performance. Compared to the models with total and without chemical heat recovery, the model with partial chemical heat recovery has the advantages of much less exhausted energy and relative small amount of fuel coal used to heat the water to the supercritical state. The efficiency of the model with partial chemical heat recovery is higher than that of other models, and increases with increasing coal-water slurry concentration (CWSC). The efficiencies of the models with partial chemical heat recovery, without chemical heat recovery, and with total chemical heat recovery are 46.60%, 37.56%, and 42.17% when CWSC is 11.3%, respectively. The thermal efficiency of the PCHR model is higher than most conventional coal-fired power plants and coal-based IGCC projects. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Supercritical water gasification Model integration Process optimization Parallel chemical heat recovery
1. Introduction Coal plays a crucial role in the development of China's economy and society. China's coal production and consumption have continuously increased over the past decade. In 2013, the coal production and consumption account for 75.6% of total energy production and 66.0% of total energy consumption, respectively [1]. However, current coal utilization in China faces problems including low energy efficiency and severe environmental issues. Hu conducted data envelopment analysis for APEC economies, and found that China had the lowest energy efficiency [2]. The heat rate of power supply in 2014 was 318 g coal equivalent per kWh, thus the power efficiency was 38.67% after conversion [3]. In the other aspect, the CO2 emission of China approached 8.2 billion in 2012, which was accounted for approximately 26% of the world's total CO2 emissions. Approximately half of the CO2 emissions came from coal combustion [4]. Besides CO2 emissions, serious haze, 90% of the SO2 emissions, 70% of the dust emissions, and 67% of the NOx emissions were the results of coal combustion [5,6]. So there are great demands of more cleaner and efficient ways of using coal for
* Corresponding author. Institute of Engineering Thermophysics, CAS, China. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.energy.2017.06.027 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
China's sustainable development. The properties such as low viscosity, low dielectric constant and high diffusivity make supercritical water (SCW) (T > 374 C, P > 22.1 MPa) an ideal solvent for biomass or coal to take homogenous reactions [7]. Compared to conventional gasification technologies, supercritical water gasification (SCWG) has advantages such as: (1) lower gasification temperature (usually 500e700 C, and more than 1200 C for conventional gasification technologies); (2) faster reaction rate due to lower heat and mass transfer resistance in SCW; (3) cleaner gasification products: elements such as N, P, S, Hg deposits as inorganic salts in SCW [8]. Thus, there is no need for further cleaning of the gasification products. While in conventional gasification, cleaning units such as dust-extraction unit, SOx separation unit are needed to clean the syngas for further using; (4) higher carbon conversion benefit from the inhibiting effect of SCW and catalyst to preventing tar and char formation [9,12]; (5) hydrogen-rich syngas production. The hydrogen yield accounts for almost 50e70% of total syngas yield [10]. Thus, SCWG may be a promising technology for clean and efficient utilization of coal. Currently, studies on SCWG mainly focus on the influences of gasification temperature, pressure, catalyst, oxidation ratio, reactor types, feed concentration, and residence time on the product distribution, numerical investigation of reactors for visualization to
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Nomenclature T h Q W s, t, l
h
HE PCHR NCHR TCHR
Temperature Enthalpy Heat Work Energy ratio Thermal efficiency Heat Exchanger Model with Partial Chemical Heat Recovery Model without Chemical Heat Recovery Model with Total Chemical Heat Recovery
find out the heat and mass transfer performance in the reactor, thermodynamic and kinetics model developing. Molino et al. [11] conducted continuous gasification of glucose in SCW with a bench scale plant under condition of 250 bar and 550 C, and obtained the product distribution in different glucose concentrations. Zhu et al. [12] studied SCWG of glucose in quartz reactors with temperature of 500 C, glucose concentration of 5 wt%, and reaction time between 10 and 1800s. The influences of reaction time and Ru/ Al2O3 catalyst on the product distribution were investigated. The gas yields increased with increasing reaction time, and the Ru/ Al2O3 catalyst promoted the degradation of intermediates to gaseous products. Giuseppe et al. [13] carried out computational fluid dynamics simulations about a continuous flow reactor to understand the complex fluid dynamics inside the reactor, and found that the top section of the reactor behaved like a mixed reactor and the bottom section behaved like a plug flow reactor. Yan et al. [14] developed a non-stoichiometric thermodynamic model based on minimum free energy to predict the hydrogen production from SCWG of biomass. The prediction results were in good agreement with the experimental data. In SCWG processes, approximately 10% of the SCW takes part in the gasification reactions. Thus, the mixture flowing out of the reactor consists of the syngas produced by the gasification reaction and the unreacted SCW, which has a large amount of sensible heat. The ratio of the sensible heat and enthalpy of the coal lies between 1 and 7, when the CWSC (coal-water slurry concentration) is between 2 and 10%, and the ratio is higher in lower coal-water slurry concentration. When integrating SCWG of coal with power generation cycles, methods of using the sensible heat have a great influence on the generation efficiency. One way to use the heat is heating the feed water of a Rankine cycle to obtain hightemperature and high-pressure steam (510.1 C, and 87.21 bar) for power generation. The other way is recovering the heat to preheat water before being heated to the supercritical state. The methods of using the sensible heat can be separately or jointly implemented. If the sensible heat is only used to heat the feed water of a Rankine cycle for power generation, the model is integrated without chemical heat recovery. If the sensible heat is only recovered to preheat water before being heated to the supercritical state, the model is integrated with total chemical heat recovery. If the sensible heat within high-temperature range is used to heat feed water of a Rankine cycle, and the sensible heat within lowtemperature range is recovered to preheat water before being heated to the supercritical state, the model is integrated with tandem partial chemical heat recovery. If the sensible heat is simultaneously used to heat the feed water of a Rankine cycle and recovered to preheat water before being heated to the supercritical state, the model is integrated with parallel partial chemical heat
recovery. Different implementation ways lead to different performances. Chen et al. proposed and analyzed a power generation model integrated SCWG of coal with tandem partial chemical heat recovery. The maximum thermal efficiency of the model is 42.18% [15]. In this work, models with different chemical heat recovery methods are compared with each other. The energy distributions of different models have great influences on the thermal efficiencies. The energy at the outlet of these models is consisted of three parts: energy being transported into a combined cycle for power generation, energy being converted to a Rankine cycle for power generation, and energy being discharged into the atmosphere. The relative values of these three parts energy greatly affect the model performances. The optimal model structure is confirmed through analyses on the energy distributions of different models with different chemical heat recovery methods. 2. Proposal of the novel models The temperature and pressure of supercritical water gasification reactions are usually 500e700 C, and over 22.1 MPa, respectively. The produced syngas mainly consists of H2, CO2, CH4, CO, and C2H6. The yields of H2, CO2, and CH4 account for more than 50%, approximately 30e40%, and approximately 10e20% of the total gas yield, respectively. The yields of CO and C2H6 are small. The gasification reaction is endothermal. Thus, some heat (approximately 700e1000 C) should be provided for the processing of the gasification reaction. The ratio of the provided heat and the LHV of the gasified-coal is approximately 4e15%. The heat can be provided by coal combustion or solar energy. Some heat is converted to chemical energy stored in the syngas through the gasification reactions, and can be released in a combined cycle (approximately 1300e1700 C) with a higher energy level. Thus, the energy level of the provided heat is upgraded through the gasification reactions. Three power generation models integrated with supercritical water gasification of coal are proposed in this section. The main difference of the models is the utilization methods of the sensible heat of the mixture. In the model with partial chemical heat recovery (PCHR), the sensible heat of the mixture is simultaneously transferred to a Rankine cycle for power generation and used to preheat the feed water of the SCW. While in the model with total chemical heat recovery (TCHR), the sensible heat of the mixture is only used to preheat the feed water of the SCW. And in the model without chemical heat recovery (NCHR), the sensible heat of the mixture is only used to heat the feed water of a Rankine cycle for power generation. 2.1. Detailed model description Coal undergoes complex gasification reactions with supercritical water in a gasifier. The main reactions include the carbon-steam reaction, and water-gas shift reaction. After the gasification reactions, most produced syngas is dissolved in the unreacted SCW. As seen in Fig. 1, the flowchart of PCHR is illustrated. HE is the heat exchanger for heat exchange between the mixture and the feed water of the rankine cycle and the water to be preheated. CC is the combine cycle where the syngas is combusted for power generation. The parameter hgasified-coal is the enthalpy of the gasified coal; h1 is the enthalpy of the SCW; h2 is the enthalpy of the mixture consisted of produced syngas and unreacted SCW; h4 is the enthalpy of the mixture that flows out of HE; h5 is the enthalpy of unreacted SCW after decompression, cooling, and separation processes; h50 is the enthalpy of the mixed water; h6 is the enthalpy of the preheated water; hfuel-coal is the enthalpy of the fuel coal; heg is the enthalpy of
Z. Chen et al. / Energy 134 (2017) 933e942
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Fig. 1. Flow sheet of PCHR.
the exhausted flow gas; hg is the enthalpy of the syngas; Tg is the temperature of the gasifier; QTg-T is the heat transferred to the Rankine cycle; QT-T0 is the heat recovered for preheating of the mixed water. The gasified coal is gasified with the SCW in the gasifier, and the syngas is produced through the gasification reactions. The mixture of produced syngas and unreacted SCW flows out of the gasifier, and enters HE. Because the solubility of the syngas in water approaches zero at atmospheric condition, the temperature and pressure of the mixture should be lowered down to atmospheric values to separate the produced syngas and unreacted SCW. Most sensible heat of the mixture is absorbed by the feed water of the Rankine cycle and the water to be preheated in HE. In the heat exchange process, the temperature of the mixture is lowered down to approximately 30 C. After the heat exchange process, the mixture is decompressed to 20 bar to release more than 98% of the syngas. And then the mixture continues to be decompressed to atmospheric pressure through a pressure relief valve, and is cooled down to approximately 25 C in a condenser for the separation of remaining syngas and unreacted water. In the cooling process, some energy is discharged into the atmosphere. If the temperature of the mixture entering the condenser is high, the amount of exhausted energy is large, which leads to poor model performance. After the separation, all the syngas is converted to the combined cycle for power generation, and the unreacted water is recycled. In supercritical water gasification process, approximately 10% of the total supercritical water undergoes complex gasification reactions with the gasified coal in the gasifier. To maintain a continuous operation of the model with a certain CWSC, some make-up water should be added after the separation of syngas and unreacted SCW. The unreacted water is mixed with the make-up water in a mixer. And the mixed water is pumped to 250 bar. Then the mixed water is preheated to approximately 400 C by the sensible heat of the mixture in a preheater. After preheating process, the mixed water is heated to the supercritical state by the high-temperature flue gas produced by fuel coal combustion in a boiler. The power consumption of pumps and losses of pipelines are neglected in the following calculations. The flowcharts of NCHR and TCHR are illustrated in Figs. 2 and 3. In NCHR, the sensible heat of the mixture is only used to produce high-temperature and high-pressure steam for power generation. In TCHR, the sensible heat of the mixture is only used to preheat the water before being heated to the supercritical state. Other parts of the models are similar to the PCHR.
2.2. Thermodynamic analysis To have a comprehensive comparison between the model with parallel partial chemical heat recovery and the models with total chemical heat recovery, without chemical heat recovery, the energy distributions of the models are discussed. As seen in Fig. 4, the energy input of the models is consisted of two parts: enthalpy of the gasified coal hgasified-coal, and enthalpy of the fuel coal hfuel-coal. A part of the heat provided by the fuel-coal combustion is supplied for the gasification reactions, and the other part is used to heat the preheated water to the supercritical state. The energy output of the models is comprised of three parts: energy transferred into the combined cycle (hg), energy transported to the Rankine cycle (Q1), and exhausted energy (Q0 ). The exhausted energy mainly includes the enthalpy of the unreacted carbon in the gasifier, enthalpy of the exhausted flue gas, and energy discharged into the atmosphere in the condenser. In SCWG process, the enthalpy of the syngas is approximately equal to the sum of the enthalpy of the gasified coal and Q [9]. The generation efficiencies of the combined cycle and the Rankine cycle are hcc and hr, respectively. The ratio of energy transferred into the combine cycle and the total energy input of the model is defined as s:
s¼
hg hgasifiedcoal þ hfuelcoal
(1)
The ratio of energy converted to the Rankine cycle and the total energy input of the model is defined as t:
t¼
Q1 hgasifiedcoal þ hfuelcoal
(2)
The ratio of exhausted energy and the total energy input of the model is defined as l:
l¼
Q
0
hgasifiedcoal þ hfuelcoal
(3)
And parameters s, t, and l satisfy the relation as follow:
sþtþl¼1
(4)
The relative values of the dimensionless parameters s, t, and l have a great influence on the thermal efficiency. In one hand, the energy being transferred into the combined cycle is in the form of chemical energy, and the energy being converted to the Rankine
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HE1
h2
Condenser
hg
QTg-T Gasifier
hgasified-coal
Rankine cycle
Tg
Combined cycle
h5
heg
h1
h5'
Boiler
water
Air
hfuel-coal
Make-up water
Fig. 2. Flow sheet of NCHR.
HE2
h2
Condenser
hg
Gasifier
QT-T’
hgasified-coal
h5
Tg h1
water
Combined cycle
heg h6
Boiler Air
Preheater
h5' Make-up water
hfuel-coal Fig. 3. Flow sheet of TCHR.
Energy Output
Energy Input
hgasified-coal
+ hfuel-coal
hcoal+Q hg
Combined cycle
W1
Q1
Rankine cycle
W2
Exhausted energy
Q
Gasifier Heat for gasification reaction Q
Boiler
+ +
Fig. 4. Energy distribution of power generation models integrated with SCWG of coal and chemical heat recovery.
cycle is the sensible heat of the gasification product. That is to say, more available energy is transferred into the combined cycle than to the Rankine cycle if same amount of energy is transferred to the
combined cycle and the Rankine cycle. In the other hand, the generation efficiency of the combined cycle (approximately 55.1%) is higher than that of the Rankine cycle (approximately 35.68%).
Z. Chen et al. / Energy 134 (2017) 933e942
Thus, if more energy is transferred into the combined cycle for power generation than to the Rankine cycle or is discharged into the atmosphere, higher thermal efficiency can be obtained. The exhausted energy mainly includes the unreacted carbon in the gasifier, enthalpy of the exhausted flue gas, and energy discharged into the atmosphere in the condenser. The energy discharged into the atmosphere in the condenser is decided by the temperature of the mixture of produced syngas and unreacted SCW before entering the condenser (T0 ). T0 is relative to Tg because the pinch points of the heat exchanges are set to approximately 10 C. From a system viewpoint, the ways to improve the performances of the models integrated supercritical water gasification with power generation mainly includes: increase the extent of heat recovery, and decrease the energy being discharged into the atmosphere in the condenser. When more energy is recovered to preheat the feed water of the SCW, the energy being transferred into the Rankine cycle for power generation decreases. In the other hand, the amount of the fuel coal decreases. Thus, the enthalpy of the exhausted flue gas in the boiler decreases. When less energy is discharged into the atmosphere in the condenser, the exhausted energy of the model decreases. Thus, if the extent of heat recovery increases and the energy discharged into the atmosphere in the condenser decreases, i.e., s/t and s/l are larger, higher model efficiencies would be obtained. The thermal efficiency is lower when s/t and s/l are smaller; however, there are situations that s/t is higher, but s/l is lower. In these situations, the change trend of the thermal efficiency is complex. When the amount of recovered heat used for preheating is larger, the amount of fuel coal needed to heat the preheated water to the supercritical state is smaller. As seen from Fig. 4, if the amount of gasified coal keeps constant, the amount of fuel coal decreases, and the sum of Q1 and Q0 decreases. However, the change trends of Q1 and Q0 are not clear. If Q0 dramatically increases, and Q1 decreases, i.e., s/t is higher, but s/l is lower, the thermal efficiency may decreases. The thermal efficiency can be calculated as follow:
h¼
shcc þ thr ¼ shcc þ thr sþtþl
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increasing CWSC, more carbon is not gasified. The proximate and ultimate analyses of the gasified coal and fuel coal are listed in Table 2. Aspen Plus 11.1 is used to simulate the model. The choice of property method is of great importance to the accuracy of the simulation results. Because the gasification temperature and pressure are above the critical temperature and pressure of water (i.e. 374 C and 22.1 MPa, respectively), ideal gas law, the PengRobinson (PR), and the Redlich-Kwong (RK) property methods are not capable of accurately predicting the fluid behavior for their applicability at middle and low pressure. Soave RedlicheKwong property method with modified HuroneVidal mixing rule (SRKMHV2) is chosen as property method because this method is already confirmed as suitable property method for chemical systems at supercritical conditions [17,18]. The other key parameters for the calculation are listed in Table 3. And the parameters of key points are shown in Table 4. The gasification temperature and pressure are 660 C and 25 MPa, respectively. The pinch points of HE2 in different models are set to approximately 10 C. The steam temperature and pressure of the single pressure Rankine cycle are 510.1 C and 87.21 bar, respectively [19]. The combined cycle is enhanced with a Siemens SGT-800 gas turbine, and the work output of the combined cycle is approximately 71.4 MW [20]. Supercritical water gasification of coal can be summarized into three reactions as follows:
C þ H2 O/CO þ H2
DH ¼ 132kJ=mol
CO þ H2 O/CO2 þ H2 CO þ 3H2 /CH4 þ H2 O
(R1)
DH ¼ 41kJ=mol
(R2)
DH ¼ 206kJ=mol
(R3)
R1 and R2 are the main reactions in the gasification process. Thus, the gasification process is partially endothermal, and some heat is needed for the gasification process. According to the energy
(5) Table 2 Ultimate analyses and proximate analyses of the gasified coal and fuel coal.
3. Calculation condition To further compare the models, experimental data from Ref. [16] is used for calculation. The gasification temperature and pressure are 660 C and 25 MPa, respectively. The composition and distribution of gasification products are listed in Table 1. The unit of the gas yield is mol/kg feedstock. The carbon conversion means the mass ratio of the carbon in gaseous product and carbon in the feedstock. As seen from Table 1, with increasing coal-water slurry concentration, the hydrogen yield and carbon conversion decrease, the CH4 yield increases. The yields of CO and C2H6 are small. The yields of hydrogen, CO2 and CH4 account for 50e60%, approximately 30%, and approximately 15% of the total gas yield, respectively. With
Proximate analysis Moisture Ash Volatile Fixed carbon Total Ultimate analysis Carbon Hydrogen Oxygen Nitrogen Sulfur Moisture Ash Total LHV(MJ/kg)
Gasified coal (wt%, ar)
Fuel coal (wt%, ar)
2.79 6.84 33.19 57.18 100
8.84 9.98 49.52 31.66 100
74.29 4.69 9.26 1.00 1.12 2.79 6.84 100 28.34
68.55 3.96 6.85 0.74 1.08 8.84 9.98 100 26.71
Table 1 Experimental results of SCWG of coal in a fluidized bed reactor. Coal-water slurry concentration/%
H2 yield
CO yield
CO2 yield
CH4 yield
C2H6 yield
Carbon conversion/%
2.0 3.8 6.6 8.4 11.3
77.50 77.50 59.30 52.60 52.44
2.33 2.59 3.35 2.10 2.56
43.70 43.29 39.05 32.49 33.23
12.82 12.84 17.49 17.91 19.28
1.48 1.65 0.60 2.71 1.68
100 100 98.9 93.95 94.56
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Table 3 The values of key parameters in the calculation. Parameters
values
Parameters
values
Gasification temperature/ C Gasification pressure/MPa hr/%
660 25 35.68
Pinch points of HE2/ C hcc/% Temperature of exhausted flue gas/ C
10 ± 1 55.1 120
48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Temperature/ C
Pressure/bar
Mass Flow/Kg/s
25 660 30 25 25 25 25 406 25 25 120 161 25 25 120 203 25 25 120
250 250 250 20 1 1 250 250 1 1 1 250 1 1 1 250 1 1 1
1 8.745 8.745 2.0 6.739 1.110 7.849 7.849 6.825 0.591 7.416 8.745 16.804 1.455 18.259 8.745 5.532 0.479 6.011
Thermal efficiency/%
Table 4 Key parameters of flows in the models.
44 40 36 32
PCHR NCHR TCHR
28 24
0
2
4
6
8
10
Coal-water slurry concentration/%
12
Fig. 5. Thermal performances of power generation models integrated with SCWG of coal and chemical heat recovery. Table 5 The influence of CWSC on Q. CWSC/%
Qs/kWh/kg gasified coal
2 3.8 8.4 11.3
0.487 0.428 1.118 1.063
balance of the gasifier, the energy absorbed (Q) in the gasification process is listed in Table 5. When the CWSC is 11.3%, the amount of fuel coal for superheating the steam is 0.591kg/kg gasified coal. Thus, in order to get the superheated steam, approximately 33.76mol CO2 is produced according to the ultimate analysis of the fuel coal. The calorific values of the syngas produced by the gasification reactions under different CWSCs are listed in the Table 6. The calorific values of the syngas are independent to the CWSC. If the syngas is combusted, all the carbon in the syngas will be turned into CO2. The CO2 yields are also listed in Table 6. 4. Results and discussions 4.1. Performances of the models In this section, the energy distributions of different models are adopted to analyze the difference between PCHR, NCHR and TCHR. The model efficiencies of these three models are illustrated in Fig. 5.
In different CWSCs, the efficiency of PCHR is higher than that of NCHR and TCHR. When CWSC ¼ 11.3%, the efficiencies of PCHR, NCHR, and TCHR are 46.60%, 37.56%, and 42.17%, respectively. According to previous analysis, the energy output is consisted of three parts: energy being transported to the combined cycle, energy being converted into the Rankine cycle, and energy being discharged into the atmosphere. The energy being discharged into the atmosphere is largely influenced by the energy released in the condenser. The amount of energy released in the condenser depends on the temperature of the mixture at the inlet of the condenser. If more heat is recovered to preheat the water before being heated to the supercritical state, the amount of the fuel coal is smaller. The amount of gasified coal is set to 1 kg in the analysis of this section. As seen in Fig. 6(a), in NCHR, the temperature of the mixture at the inlet of the cooler is 161 C. The energy released in the cooler is approximately 5 MW, which accounts for approximately 18.5% of the sensible heat of the mixture. As seen in Fig. 6(b), in TCHR, the temperature of the mixture at the inlet of the cooler is approximately 200 C, and the energy released in the cooler is approximately 7 MW, which accounts for approximately 25.9% of the sensible heat of the mixture. However, as seen in Fig. 6(c), in TCHR, the water can be preheated to 408 C. Thus, the amount of fuel coal can be reduced from 1.455 kg/kg gasified coal in NCHR to 0.6563 kg/kg gasified coal in TCHR. In PCHR, the temperature of the mixture at the inlet of the cooler can be lowered down to approximately 30 C, and the water
Table 6 The calorific values of the syngas under different CWSCs. CWSC/wt%
2
3.8
6.6
8.4
11.3
Calorific values of syngas/MJ/kg gasified coal CO2 yields after combusting the syngas/mol/kg gasified coal
31.83 61.81
32.17 62.03
30.20 61.10
31.61 57.93
31.32 58.44
Z. Chen et al. / Energy 134 (2017) 933e942
700
700
Thot
600
500
Tcold
500
Temperature
600
400 300 200
Thot Tcold
400 300 200 100
100 0 -2 0
2
4
6
0 -2 0
8 10 12 14 16 18 20 22 24 26 28
2
4
6
Heat duty/MW
8 10 12 14 16 18 20 22 24 26 28
Heat Duty/MW
(a)
(b)
700 600
Thot Tcold
500
Temperature
Temperature
939
400 300 200 100 0 -2
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28
Heat Duty/MW
(c) Fig. 6. The heat exchange curves of the heat exchangers in different models when CWSC ¼ 11.3%: (a) NCHR (b) TCHR (c) PCHR.
W2 13.6MW
Rankine cycle
QTg-T 38.1MW
Exhausted energy 24.5MW
Gasified 116.6MW coal 64.9MW Fuel coal
Gasifier
HE2
Boiler 138.6 MW
W1 71.0MW
247.0MW 240.6MW
heg 8.2MW
Condenser
Combined cycle
128.8MW 128.8MW
Qe
0.009MW
hur 6.4MW
Recovered heat 73.7MW
Exhausted energy 57.8MW
Fig. 7. The energy flow diagram of PCHR when CWSC ¼ 11.3%.
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W1 76.5MW
125.7MW
55.8MW Fuel coal
Gasifier
Gasified coal
HE2 Condenser
Boiler 167.8MW 147.4 MW
Combined cycle 138.9MW
266.2MW 259.4MW hur 6.8MW
heg 6.9MW
Qe 28.9MW
Recovered heat 91.6MW
Exhausted energy 62.4MW
Fig. 8. The energy flow diagram of TCHR when CWSC ¼ 11.3%.
can be preheated to 386 C. The energy discharged in the cooler is small, and the amount of fuel coal in PCHR is 0.7575 kg/kg gasified coal. PCHR has the advantages of much less exhausted energy in the condenser than NCHR and TCHR, and relative small amount of fuel coal used for heating. The thermal efficiency is largely enhanced by these advantages. 4.2. Energy balances of the models To have a comprehensive and clear understanding on the comparison of the models, the energy flow diagrams of the models are illustrated in Figs. 7e9. The total energy inputs of the models are 181.5 MW. The enthalpies of the gasified coal and fuel coal in the models are different, which is due to the different amount of recovered heat. The amount of fuel coal decreases with increasing amount of recovered heat. As seen in Figs. 7e9, the total work outputs of PCHR, TCHR, and NCHR are 84.6 MW, 76.5 MW, and 68.1 MW, respectively. Thus, the efficiencies of PCHR, TCHR, and
NCHR are 46.60%, 42.17%, and 37.56%, respectively. As seen in Figs. 7 and 8, compared to TCHR, the better performance of PCHR is mainly due to the less exhausted energy in the condenser. In PCHR and TCHR, the temperatures of the mixture before entering the condenser are approximately 30 C and 200 C, respectively. The high temperature of the mixture in TCHR causes large energy losses in the condenser. In PCHR, this part of energy is not discharged into the atmosphere. Instead, it is transferred to the Rankine cycle for power generation, which may be the reason of higher efficiency of PCHR. As seen in Figs. 7 and 9, compared to NCHR, the better performance of PCHR is mainly due to two aspects: more energy is transferred to the combined cycle than is transferred to the Rankine cycle for power generation; and less exhausted energy in the condenser and the boiler. Because the large amount of recovered heat in PCHR, the amount of fuel coal that is needed for heating the preheated water to the supercritical state is much smaller than that in NCHR. Thus, the amount of gasified coal in PCHR is larger than that in NCHR. As seen from Fig. 4, the energy
W2 21.5MW Rankine cycle
QTg-T 60.4MW
W1 46.6MW Exhausted energy 38.9MW
76.5MW
Gasified coal
Gasifier
HE1
Boiler 105.0MW Fuel coal
105.0 MW
162.1MW 157.9MW
Condenser Combined cycle 97.5MW 84.6MW Qe 12.9MW
heg 19.4MW
hur 4.2MW Exhausted energy 38.0MW
Fig. 9. The energy flow diagram of NCHR when CWSC ¼ 11.3%.
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1.1
much larger than that in NCHR. In the other hand, the temperatures of the mixture before entering the condenser are approximately 30 C and 150 C in PCHR and NCHR, respectively. Thus, the energy exhausted in the condenser of NCHR is larger than that of PCHR. Furthermore, the enthalpy of the exhausted flue gas of the boiler in NCHR is larger than that in PCHR because more fuel coal is consumed in NCHR. Thus, the efficiency of PCHR is much higher than that of NCHR. As seen in Fig. 10, the parameters s, t, and l of the models are illustrated. Parameter s of PCHR is close to that of TCHR, whereas l of PCHR is much smaller than that of TCHR. The energy transported into the combined cycle in PCHR is close to that in TCHR. However, the exhausted energy in the cooler of TCHR is much larger than that of PCHR. Instead, the additional exhausted energy is converted to electricity in PCHR. As seen in Fig. 10, t of PCHR is much larger than that of TCHR. Thus, the efficiency of PCHR is much larger than that of TCHR. When compared PCHR with NCHR, parameter s of PCHR is larger than that of NCHR; parameters t and l of PCHR are smaller than that of NCHR. The exhausted energy in PCHR is smaller than that in NCHR, and more energy is transported into the combined cycle than into the Rankine cycle for power generation. Thus, the efficiency of PCHR is much larger than that of NCHR. For PCHR, as seen in Fig. 5, the efficiency increases with increasing coal-water slurry concentration. The influences of CWSC on the parameters s, t, and l of PCHR are shown in Fig. 11. As seen in Fig. 11, with increasing CWSC, parameter s increases, t decreases, and l changes little. More energy is transferred into the combined cycle than in to the Rankine cycle for power generation, which lead to higher efficiency in larger CWSC. The comparison of PCHR and conventional coal-fired supercritical power plants is conducted and listed in Table 7. As can be seen in Table 7, the thermal efficiency of PCHR when CWSC ¼ 11.3% is higher than that of conventional coal-fired power plants. The comparison between supercritical water gasification of coal with IGCC systems is also conducted, as can be seen in Table 8. When the CSWC is 11.3%, the efficiency of the system proposed in this article is 46.60%. As for IGCC systems, the actual performances of major coal-based IGCC projects worldwide are shown in Table 8 [25]. As can be seen in Table 8, the efficiencies of the major coal-based IGCC projects worldwide are lower than that of the system proposed in this article. The thermal efficiencies of IGCC systems are approximately within the range of 35e43% [26].
1.0 0.9
Parameters
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
PCHR
TCHR
NCHR
Fig. 10. The dimensionless parameters of different models.
0.8 0.7
Parameters
0.6 0.5 0.4 0.3 0.2 0.1 0.0
0
2
4
6
8
10
941
12
Coal-water slurry concentration/% Fig. 11. The influence of CWSC on dimensionless parameters in PCHR.
transported into the combined cycle is approximately equal to the sum of enthalpy of the gasified coal and the gasification heat Q. Thus, the energy transferred to the combined cycle in PCHR is
Table 7 Comparison between PCHR and conventional coal-fired supercritical power plants.
Highest temperature/ C Highest pressure/bar Thermal efficiency/%
PCHR (CWSC ¼ 11.3%)
[21]
[22]
[23]
[24]
660 250 46.60
540e600 230e300 41.00
597 220e240 45.00
545 260 43.00%
600 253.4 43.19%
Table 8 Design and actual performances of major coal-based IGCC projects. Net plant efficiency/%
Wabash
Tampa
SEP/Demkolec
ELCOGAS
Gas turbine MW design (achieved) Steam turbine MW design (achieved) Auxiliary power MW design (achieved) Net power MW design (achieved) LHV basis design (achieved) HHV basis design (achieved)
192 (192) 105 (98) 35.4 (36) 261.6 (252) 39.2 (41.2) 37.8 (39.7)
192 (192) 121 (125) 63 (66) 250 (250) 41.2 (38.9) 39.7 (37.5)
155 (155) 128 (128) 31 (31) 252 (252) 43 (43) 41.4 (41.4)
182.3 (185); 200 at ISO 135.4; 135 at ISO 35 at ISO 300 at ISO 42.2 41.5
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5. Conclusion Supercritical water gasification technology can cleanly and efficiently convert coal to hydrogen-rich syngas. The gasification product has a large amount of sensible heat. Reasonable use of the heat can largely improve the thermal efficiency. The methods of using the heat conclude: heating the feed water of a Rankine cycle for power generation; and preheat the water before being heated to the supercritical state. The methods can be implemented jointly or separately. In this work, three power generation models integrated supercritical water gasification of coal are proposed and compared. The thermal efficiencies of PCHR, NCHR, and TCHR are 46.60%, 37.56%, and 42.17% when CWSC is 11.3%, respectively. PCHR has advantages of much less exhausted energy than reference models in the cooler and relative small amount of fuel coal used for heating, which leads to higher thermal efficiency than that of NCHR and TCHR. Thus, PCHR is more preferred than NCHR and TCHR if power generation plants integrated SCWG of coal are put into application. The thermal efficiency of the PCHR model is higher than most conventional coal-fired power plants and coal-based IGCC projects. Acknowledgments This study is supported by the The National Key Research and Development Program of China (No. 2016YFB0600105). References [1] National Bureau of Statistics of China. China energy statistical yearbook. Beijing: China Statistics Press; 2014. [2] Hu JL, Kao CH. Efficient energyesaving targets for APEC economies. Energy Policy 2007;35(1):373e82. [3] China Electricity Council. Current status and outlook of China's electric power industry. 2015 [In Chinese)]. [4] IEA. World energy outlook 2014 [London]. 2014. [5] Li MN, Zhang LL. Haze in China: current and future challenges. Environ Pollut 2014;189(6):85e6. [6] Chen WY, Xu RN. Clean coal technology development in China. Energy Policy 2010;38(5):2123e30. [7] Guan Qingqing, Huang Xiaodian, Liu Jing, Gu Junjie, Miao Rongrong, Chen Qiuling, et al. Supercritical water gasification of phenol using a Ru/CeO2 catalyst. Chem Eng J 2016;283:358e65. [8] Guo L, Jin H. Boiling coal in water: hydrogen production and power generation system with zero net CO2, emission based on coal and supercritical water
gasification. Int J Hydrogen Energy 2013;38(29):12953e67. [9] Kruse A, Dinjus E. Hot compressed water as reaction medium and reactant : 2. Degradation reactions. J Supercrit Fluids 2007;41(3):361e79. [10] Li Y, Guo L, Zhang X, Jin H, Lu Y. Hydrogen production from coal gasification in supercritical water with a continuous flowing system. Int J Hydrogen Energy 2010;35(7):3036e45. [11] Molino A, Migliori M, Macrì D, Valerio V, Villone A, Nanna F, et al. Glucose gasification in superecritical water conditions for both syngas production and green chemicals with a continuous process. Renew Energy 2016;91:451e5. [12] Zhu Chao, Guo Liejin, Jin Hui, et al. Effects of reaction time and catalyst on gasification of glucose in supercritical water: detailed reaction pathway and mechanisms. Int J Hydrogen Energy 2016;41(16):6630e9. [13] Caputo Giuseppe, Rubio Patricia, Scargiali Francesca, Marotta Gaspare, Brucato Alberto. Experimental and fluid dynamic study of continuous supercritical water gasification of glucose. J Supercrit Fluids 2016;107:450e61. [14] Yan Q, Guo L, Lu Y. Thermodynamic analysis of hydrogen production from biomass gasification in supercritical water. Energy Convers Manag 2006;47: 1515e28. [15] Chen Z, Zhang Xiaosong, Gao L, Li S. Thermal analysis of supercritical water gasification of coal for power generation with partial heat recovery. Appl Therm Eng 2017;111:1287e95. [16] Guo L, Jin H, Lu Y. Supercritical water gasification research and development in China. J Supercrit Fluids 2015;96:144e50. [17] Dahl S, Dunalewicz A, Fredenslund A, Rasmussen P. The mhv2 model: prediction of phase equilibria at sub- and supercritical conditions. J Supercrit Fluids 1992;5:42e7. [18] Li J, Vanderbeken I, Ye S, Carrier H, Xans P. Prediction of the solubility and gaseliquid equilibria for gasewater and light hydrocarbonewater systems at high temperatures and pressures with a group contribution equation of state. Fluid Phase Equilibria 1997;131:107e18. [19] Mehraban Karim Maghsoudi, Rohani Vahid, Mehrpanahi Abdollah, Naserabad Sadegh Nikbakht. Using two types of heat recovery steam generator for full repowering a steam power plant and its analysis by exergy method. Indian J Sci Res 2014;1(2):106e19. € rstad D, Lindholm A, Pettersson J, et al. Siemens SGT-800 Industrial Gas [20] Lo Turbine Enhanced to 50MW: Combustor Design Modifications, Validation and Operation Experience[C]//ASME Turbo Expo 2013: Turbine Technical Conference and Exposition. 2013:V01BT04A038. [21] Breeze P. Chapter 3-Coal-fired power plants [M]//power generation technologies. Elsevier Ltd; 2014. [22] Zhang D. Introduction to advanced and ultra-supercritical fossil fuel power plants. Ultra-Supercritical Coal Power Plants 2013:1e20. [23] Julia Hentschel, Henning Zindler, Hartmut Spliethoff. Modelling and transient simulation of a supercritical coal-fired power plant: Dynamic response to extended secondary control power output [J]. Energy, 10.1016/j.energy.2017. 02.165. [24] Sanpasertparnich T, Aroonwilas A. Simulation and optimization of coal-fired power plants. Energy Procedia 2009;1(1):3851e8. [25] Holt NAH, Alpert SB. Integrated gasification combined-cycle power plants. Encycl Phys Sci Technol 2003:897e924. [26] Zhang Jianyun, Zhou Zhe, Ma Linwei, Li Zheng, Ni Weidou. Efficiency of wet feed IGCC (integrated gasification combined cycle) systems with coal-water slurry preheating vaporization technology. Energy 2013;51:137e45.
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Novel power generation models integrated supercritical water gasification of coal and parallel partial chemical heat recovery Zhewen Chen a, b, Xiaosong Zhang a, b, *, Sheng Li a, b, Lin Gao a, b a b
Institute of Engineering Thermophysics, CAS, China University of Chinese Academy of Sciences, Beijing, 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2016 Received in revised form 5 June 2017 Accepted 6 June 2017 Available online 17 June 2017
Supercritical water gasification (SCWG) of coal is a promising clean coal technology. Supercritical water can effectively and cleanly convert coal to hydrogen-rich syngas. Three power generation models integrated SCWG of coal are proposed and compared in this article. The gasification products have a large amount of sensible heat. Efficient use of the sensible heat can improve the model performance. Compared to the models with total and without chemical heat recovery, the model with partial chemical heat recovery has the advantages of much less exhausted energy and relative small amount of fuel coal used to heat the water to the supercritical state. The efficiency of the model with partial chemical heat recovery is higher than that of other models, and increases with increasing coal-water slurry concentration (CWSC). The efficiencies of the models with partial chemical heat recovery, without chemical heat recovery, and with total chemical heat recovery are 46.60%, 37.56%, and 42.17% when CWSC is 11.3%, respectively. The thermal efficiency of the PCHR model is higher than most conventional coal-fired power plants and coal-based IGCC projects. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Supercritical water gasification Model integration Process optimization Parallel chemical heat recovery
1. Introduction Coal plays a crucial role in the development of China's economy and society. China's coal production and consumption have continuously increased over the past decade. In 2013, the coal production and consumption account for 75.6% of total energy production and 66.0% of total energy consumption, respectively [1]. However, current coal utilization in China faces problems including low energy efficiency and severe environmental issues. Hu conducted data envelopment analysis for APEC economies, and found that China had the lowest energy efficiency [2]. The heat rate of power supply in 2014 was 318 g coal equivalent per kWh, thus the power efficiency was 38.67% after conversion [3]. In the other aspect, the CO2 emission of China approached 8.2 billion in 2012, which was accounted for approximately 26% of the world's total CO2 emissions. Approximately half of the CO2 emissions came from coal combustion [4]. Besides CO2 emissions, serious haze, 90% of the SO2 emissions, 70% of the dust emissions, and 67% of the NOx emissions were the results of coal combustion [5,6]. So there are great demands of more cleaner and efficient ways of using coal for
* Corresponding author. Institute of Engineering Thermophysics, CAS, China. E-mail address: [email protected] (X. Zhang). http://dx.doi.org/10.1016/j.energy.2017.06.027 0360-5442/© 2017 Elsevier Ltd. All rights reserved.
China's sustainable development. The properties such as low viscosity, low dielectric constant and high diffusivity make supercritical water (SCW) (T > 374 C, P > 22.1 MPa) an ideal solvent for biomass or coal to take homogenous reactions [7]. Compared to conventional gasification technologies, supercritical water gasification (SCWG) has advantages such as: (1) lower gasification temperature (usually 500e700 C, and more than 1200 C for conventional gasification technologies); (2) faster reaction rate due to lower heat and mass transfer resistance in SCW; (3) cleaner gasification products: elements such as N, P, S, Hg deposits as inorganic salts in SCW [8]. Thus, there is no need for further cleaning of the gasification products. While in conventional gasification, cleaning units such as dust-extraction unit, SOx separation unit are needed to clean the syngas for further using; (4) higher carbon conversion benefit from the inhibiting effect of SCW and catalyst to preventing tar and char formation [9,12]; (5) hydrogen-rich syngas production. The hydrogen yield accounts for almost 50e70% of total syngas yield [10]. Thus, SCWG may be a promising technology for clean and efficient utilization of coal. Currently, studies on SCWG mainly focus on the influences of gasification temperature, pressure, catalyst, oxidation ratio, reactor types, feed concentration, and residence time on the product distribution, numerical investigation of reactors for visualization to
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Nomenclature T h Q W s, t, l
h
HE PCHR NCHR TCHR
Temperature Enthalpy Heat Work Energy ratio Thermal efficiency Heat Exchanger Model with Partial Chemical Heat Recovery Model without Chemical Heat Recovery Model with Total Chemical Heat Recovery
find out the heat and mass transfer performance in the reactor, thermodynamic and kinetics model developing. Molino et al. [11] conducted continuous gasification of glucose in SCW with a bench scale plant under condition of 250 bar and 550 C, and obtained the product distribution in different glucose concentrations. Zhu et al. [12] studied SCWG of glucose in quartz reactors with temperature of 500 C, glucose concentration of 5 wt%, and reaction time between 10 and 1800s. The influences of reaction time and Ru/ Al2O3 catalyst on the product distribution were investigated. The gas yields increased with increasing reaction time, and the Ru/ Al2O3 catalyst promoted the degradation of intermediates to gaseous products. Giuseppe et al. [13] carried out computational fluid dynamics simulations about a continuous flow reactor to understand the complex fluid dynamics inside the reactor, and found that the top section of the reactor behaved like a mixed reactor and the bottom section behaved like a plug flow reactor. Yan et al. [14] developed a non-stoichiometric thermodynamic model based on minimum free energy to predict the hydrogen production from SCWG of biomass. The prediction results were in good agreement with the experimental data. In SCWG processes, approximately 10% of the SCW takes part in the gasification reactions. Thus, the mixture flowing out of the reactor consists of the syngas produced by the gasification reaction and the unreacted SCW, which has a large amount of sensible heat. The ratio of the sensible heat and enthalpy of the coal lies between 1 and 7, when the CWSC (coal-water slurry concentration) is between 2 and 10%, and the ratio is higher in lower coal-water slurry concentration. When integrating SCWG of coal with power generation cycles, methods of using the sensible heat have a great influence on the generation efficiency. One way to use the heat is heating the feed water of a Rankine cycle to obtain hightemperature and high-pressure steam (510.1 C, and 87.21 bar) for power generation. The other way is recovering the heat to preheat water before being heated to the supercritical state. The methods of using the sensible heat can be separately or jointly implemented. If the sensible heat is only used to heat the feed water of a Rankine cycle for power generation, the model is integrated without chemical heat recovery. If the sensible heat is only recovered to preheat water before being heated to the supercritical state, the model is integrated with total chemical heat recovery. If the sensible heat within high-temperature range is used to heat feed water of a Rankine cycle, and the sensible heat within lowtemperature range is recovered to preheat water before being heated to the supercritical state, the model is integrated with tandem partial chemical heat recovery. If the sensible heat is simultaneously used to heat the feed water of a Rankine cycle and recovered to preheat water before being heated to the supercritical state, the model is integrated with parallel partial chemical heat
recovery. Different implementation ways lead to different performances. Chen et al. proposed and analyzed a power generation model integrated SCWG of coal with tandem partial chemical heat recovery. The maximum thermal efficiency of the model is 42.18% [15]. In this work, models with different chemical heat recovery methods are compared with each other. The energy distributions of different models have great influences on the thermal efficiencies. The energy at the outlet of these models is consisted of three parts: energy being transported into a combined cycle for power generation, energy being converted to a Rankine cycle for power generation, and energy being discharged into the atmosphere. The relative values of these three parts energy greatly affect the model performances. The optimal model structure is confirmed through analyses on the energy distributions of different models with different chemical heat recovery methods. 2. Proposal of the novel models The temperature and pressure of supercritical water gasification reactions are usually 500e700 C, and over 22.1 MPa, respectively. The produced syngas mainly consists of H2, CO2, CH4, CO, and C2H6. The yields of H2, CO2, and CH4 account for more than 50%, approximately 30e40%, and approximately 10e20% of the total gas yield, respectively. The yields of CO and C2H6 are small. The gasification reaction is endothermal. Thus, some heat (approximately 700e1000 C) should be provided for the processing of the gasification reaction. The ratio of the provided heat and the LHV of the gasified-coal is approximately 4e15%. The heat can be provided by coal combustion or solar energy. Some heat is converted to chemical energy stored in the syngas through the gasification reactions, and can be released in a combined cycle (approximately 1300e1700 C) with a higher energy level. Thus, the energy level of the provided heat is upgraded through the gasification reactions. Three power generation models integrated with supercritical water gasification of coal are proposed in this section. The main difference of the models is the utilization methods of the sensible heat of the mixture. In the model with partial chemical heat recovery (PCHR), the sensible heat of the mixture is simultaneously transferred to a Rankine cycle for power generation and used to preheat the feed water of the SCW. While in the model with total chemical heat recovery (TCHR), the sensible heat of the mixture is only used to preheat the feed water of the SCW. And in the model without chemical heat recovery (NCHR), the sensible heat of the mixture is only used to heat the feed water of a Rankine cycle for power generation. 2.1. Detailed model description Coal undergoes complex gasification reactions with supercritical water in a gasifier. The main reactions include the carbon-steam reaction, and water-gas shift reaction. After the gasification reactions, most produced syngas is dissolved in the unreacted SCW. As seen in Fig. 1, the flowchart of PCHR is illustrated. HE is the heat exchanger for heat exchange between the mixture and the feed water of the rankine cycle and the water to be preheated. CC is the combine cycle where the syngas is combusted for power generation. The parameter hgasified-coal is the enthalpy of the gasified coal; h1 is the enthalpy of the SCW; h2 is the enthalpy of the mixture consisted of produced syngas and unreacted SCW; h4 is the enthalpy of the mixture that flows out of HE; h5 is the enthalpy of unreacted SCW after decompression, cooling, and separation processes; h50 is the enthalpy of the mixed water; h6 is the enthalpy of the preheated water; hfuel-coal is the enthalpy of the fuel coal; heg is the enthalpy of
Z. Chen et al. / Energy 134 (2017) 933e942
935
Fig. 1. Flow sheet of PCHR.
the exhausted flow gas; hg is the enthalpy of the syngas; Tg is the temperature of the gasifier; QTg-T is the heat transferred to the Rankine cycle; QT-T0 is the heat recovered for preheating of the mixed water. The gasified coal is gasified with the SCW in the gasifier, and the syngas is produced through the gasification reactions. The mixture of produced syngas and unreacted SCW flows out of the gasifier, and enters HE. Because the solubility of the syngas in water approaches zero at atmospheric condition, the temperature and pressure of the mixture should be lowered down to atmospheric values to separate the produced syngas and unreacted SCW. Most sensible heat of the mixture is absorbed by the feed water of the Rankine cycle and the water to be preheated in HE. In the heat exchange process, the temperature of the mixture is lowered down to approximately 30 C. After the heat exchange process, the mixture is decompressed to 20 bar to release more than 98% of the syngas. And then the mixture continues to be decompressed to atmospheric pressure through a pressure relief valve, and is cooled down to approximately 25 C in a condenser for the separation of remaining syngas and unreacted water. In the cooling process, some energy is discharged into the atmosphere. If the temperature of the mixture entering the condenser is high, the amount of exhausted energy is large, which leads to poor model performance. After the separation, all the syngas is converted to the combined cycle for power generation, and the unreacted water is recycled. In supercritical water gasification process, approximately 10% of the total supercritical water undergoes complex gasification reactions with the gasified coal in the gasifier. To maintain a continuous operation of the model with a certain CWSC, some make-up water should be added after the separation of syngas and unreacted SCW. The unreacted water is mixed with the make-up water in a mixer. And the mixed water is pumped to 250 bar. Then the mixed water is preheated to approximately 400 C by the sensible heat of the mixture in a preheater. After preheating process, the mixed water is heated to the supercritical state by the high-temperature flue gas produced by fuel coal combustion in a boiler. The power consumption of pumps and losses of pipelines are neglected in the following calculations. The flowcharts of NCHR and TCHR are illustrated in Figs. 2 and 3. In NCHR, the sensible heat of the mixture is only used to produce high-temperature and high-pressure steam for power generation. In TCHR, the sensible heat of the mixture is only used to preheat the water before being heated to the supercritical state. Other parts of the models are similar to the PCHR.
2.2. Thermodynamic analysis To have a comprehensive comparison between the model with parallel partial chemical heat recovery and the models with total chemical heat recovery, without chemical heat recovery, the energy distributions of the models are discussed. As seen in Fig. 4, the energy input of the models is consisted of two parts: enthalpy of the gasified coal hgasified-coal, and enthalpy of the fuel coal hfuel-coal. A part of the heat provided by the fuel-coal combustion is supplied for the gasification reactions, and the other part is used to heat the preheated water to the supercritical state. The energy output of the models is comprised of three parts: energy transferred into the combined cycle (hg), energy transported to the Rankine cycle (Q1), and exhausted energy (Q0 ). The exhausted energy mainly includes the enthalpy of the unreacted carbon in the gasifier, enthalpy of the exhausted flue gas, and energy discharged into the atmosphere in the condenser. In SCWG process, the enthalpy of the syngas is approximately equal to the sum of the enthalpy of the gasified coal and Q [9]. The generation efficiencies of the combined cycle and the Rankine cycle are hcc and hr, respectively. The ratio of energy transferred into the combine cycle and the total energy input of the model is defined as s:
s¼
hg hgasifiedcoal þ hfuelcoal
(1)
The ratio of energy converted to the Rankine cycle and the total energy input of the model is defined as t:
t¼
Q1 hgasifiedcoal þ hfuelcoal
(2)
The ratio of exhausted energy and the total energy input of the model is defined as l:
l¼
Q
0
hgasifiedcoal þ hfuelcoal
(3)
And parameters s, t, and l satisfy the relation as follow:
sþtþl¼1
(4)
The relative values of the dimensionless parameters s, t, and l have a great influence on the thermal efficiency. In one hand, the energy being transferred into the combined cycle is in the form of chemical energy, and the energy being converted to the Rankine
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Z. Chen et al. / Energy 134 (2017) 933e942
HE1
h2
Condenser
hg
QTg-T Gasifier
hgasified-coal
Rankine cycle
Tg
Combined cycle
h5
heg
h1
h5'
Boiler
water
Air
hfuel-coal
Make-up water
Fig. 2. Flow sheet of NCHR.
HE2
h2
Condenser
hg
Gasifier
QT-T’
hgasified-coal
h5
Tg h1
water
Combined cycle
heg h6
Boiler Air
Preheater
h5' Make-up water
hfuel-coal Fig. 3. Flow sheet of TCHR.
Energy Output
Energy Input
hgasified-coal
+ hfuel-coal
hcoal+Q hg
Combined cycle
W1
Q1
Rankine cycle
W2
Exhausted energy
Q
Gasifier Heat for gasification reaction Q
Boiler
+ +
Fig. 4. Energy distribution of power generation models integrated with SCWG of coal and chemical heat recovery.
cycle is the sensible heat of the gasification product. That is to say, more available energy is transferred into the combined cycle than to the Rankine cycle if same amount of energy is transferred to the
combined cycle and the Rankine cycle. In the other hand, the generation efficiency of the combined cycle (approximately 55.1%) is higher than that of the Rankine cycle (approximately 35.68%).
Z. Chen et al. / Energy 134 (2017) 933e942
Thus, if more energy is transferred into the combined cycle for power generation than to the Rankine cycle or is discharged into the atmosphere, higher thermal efficiency can be obtained. The exhausted energy mainly includes the unreacted carbon in the gasifier, enthalpy of the exhausted flue gas, and energy discharged into the atmosphere in the condenser. The energy discharged into the atmosphere in the condenser is decided by the temperature of the mixture of produced syngas and unreacted SCW before entering the condenser (T0 ). T0 is relative to Tg because the pinch points of the heat exchanges are set to approximately 10 C. From a system viewpoint, the ways to improve the performances of the models integrated supercritical water gasification with power generation mainly includes: increase the extent of heat recovery, and decrease the energy being discharged into the atmosphere in the condenser. When more energy is recovered to preheat the feed water of the SCW, the energy being transferred into the Rankine cycle for power generation decreases. In the other hand, the amount of the fuel coal decreases. Thus, the enthalpy of the exhausted flue gas in the boiler decreases. When less energy is discharged into the atmosphere in the condenser, the exhausted energy of the model decreases. Thus, if the extent of heat recovery increases and the energy discharged into the atmosphere in the condenser decreases, i.e., s/t and s/l are larger, higher model efficiencies would be obtained. The thermal efficiency is lower when s/t and s/l are smaller; however, there are situations that s/t is higher, but s/l is lower. In these situations, the change trend of the thermal efficiency is complex. When the amount of recovered heat used for preheating is larger, the amount of fuel coal needed to heat the preheated water to the supercritical state is smaller. As seen from Fig. 4, if the amount of gasified coal keeps constant, the amount of fuel coal decreases, and the sum of Q1 and Q0 decreases. However, the change trends of Q1 and Q0 are not clear. If Q0 dramatically increases, and Q1 decreases, i.e., s/t is higher, but s/l is lower, the thermal efficiency may decreases. The thermal efficiency can be calculated as follow:
h¼
shcc þ thr ¼ shcc þ thr sþtþl
937
increasing CWSC, more carbon is not gasified. The proximate and ultimate analyses of the gasified coal and fuel coal are listed in Table 2. Aspen Plus 11.1 is used to simulate the model. The choice of property method is of great importance to the accuracy of the simulation results. Because the gasification temperature and pressure are above the critical temperature and pressure of water (i.e. 374 C and 22.1 MPa, respectively), ideal gas law, the PengRobinson (PR), and the Redlich-Kwong (RK) property methods are not capable of accurately predicting the fluid behavior for their applicability at middle and low pressure. Soave RedlicheKwong property method with modified HuroneVidal mixing rule (SRKMHV2) is chosen as property method because this method is already confirmed as suitable property method for chemical systems at supercritical conditions [17,18]. The other key parameters for the calculation are listed in Table 3. And the parameters of key points are shown in Table 4. The gasification temperature and pressure are 660 C and 25 MPa, respectively. The pinch points of HE2 in different models are set to approximately 10 C. The steam temperature and pressure of the single pressure Rankine cycle are 510.1 C and 87.21 bar, respectively [19]. The combined cycle is enhanced with a Siemens SGT-800 gas turbine, and the work output of the combined cycle is approximately 71.4 MW [20]. Supercritical water gasification of coal can be summarized into three reactions as follows:
C þ H2 O/CO þ H2
DH ¼ 132kJ=mol
CO þ H2 O/CO2 þ H2 CO þ 3H2 /CH4 þ H2 O
(R1)
DH ¼ 41kJ=mol
(R2)
DH ¼ 206kJ=mol
(R3)
R1 and R2 are the main reactions in the gasification process. Thus, the gasification process is partially endothermal, and some heat is needed for the gasification process. According to the energy
(5) Table 2 Ultimate analyses and proximate analyses of the gasified coal and fuel coal.
3. Calculation condition To further compare the models, experimental data from Ref. [16] is used for calculation. The gasification temperature and pressure are 660 C and 25 MPa, respectively. The composition and distribution of gasification products are listed in Table 1. The unit of the gas yield is mol/kg feedstock. The carbon conversion means the mass ratio of the carbon in gaseous product and carbon in the feedstock. As seen from Table 1, with increasing coal-water slurry concentration, the hydrogen yield and carbon conversion decrease, the CH4 yield increases. The yields of CO and C2H6 are small. The yields of hydrogen, CO2 and CH4 account for 50e60%, approximately 30%, and approximately 15% of the total gas yield, respectively. With
Proximate analysis Moisture Ash Volatile Fixed carbon Total Ultimate analysis Carbon Hydrogen Oxygen Nitrogen Sulfur Moisture Ash Total LHV(MJ/kg)
Gasified coal (wt%, ar)
Fuel coal (wt%, ar)
2.79 6.84 33.19 57.18 100
8.84 9.98 49.52 31.66 100
74.29 4.69 9.26 1.00 1.12 2.79 6.84 100 28.34
68.55 3.96 6.85 0.74 1.08 8.84 9.98 100 26.71
Table 1 Experimental results of SCWG of coal in a fluidized bed reactor. Coal-water slurry concentration/%
H2 yield
CO yield
CO2 yield
CH4 yield
C2H6 yield
Carbon conversion/%
2.0 3.8 6.6 8.4 11.3
77.50 77.50 59.30 52.60 52.44
2.33 2.59 3.35 2.10 2.56
43.70 43.29 39.05 32.49 33.23
12.82 12.84 17.49 17.91 19.28
1.48 1.65 0.60 2.71 1.68
100 100 98.9 93.95 94.56
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Table 3 The values of key parameters in the calculation. Parameters
values
Parameters
values
Gasification temperature/ C Gasification pressure/MPa hr/%
660 25 35.68
Pinch points of HE2/ C hcc/% Temperature of exhausted flue gas/ C
10 ± 1 55.1 120
48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Temperature/ C
Pressure/bar
Mass Flow/Kg/s
25 660 30 25 25 25 25 406 25 25 120 161 25 25 120 203 25 25 120
250 250 250 20 1 1 250 250 1 1 1 250 1 1 1 250 1 1 1
1 8.745 8.745 2.0 6.739 1.110 7.849 7.849 6.825 0.591 7.416 8.745 16.804 1.455 18.259 8.745 5.532 0.479 6.011
Thermal efficiency/%
Table 4 Key parameters of flows in the models.
44 40 36 32
PCHR NCHR TCHR
28 24
0
2
4
6
8
10
Coal-water slurry concentration/%
12
Fig. 5. Thermal performances of power generation models integrated with SCWG of coal and chemical heat recovery. Table 5 The influence of CWSC on Q. CWSC/%
Qs/kWh/kg gasified coal
2 3.8 8.4 11.3
0.487 0.428 1.118 1.063
balance of the gasifier, the energy absorbed (Q) in the gasification process is listed in Table 5. When the CWSC is 11.3%, the amount of fuel coal for superheating the steam is 0.591kg/kg gasified coal. Thus, in order to get the superheated steam, approximately 33.76mol CO2 is produced according to the ultimate analysis of the fuel coal. The calorific values of the syngas produced by the gasification reactions under different CWSCs are listed in the Table 6. The calorific values of the syngas are independent to the CWSC. If the syngas is combusted, all the carbon in the syngas will be turned into CO2. The CO2 yields are also listed in Table 6. 4. Results and discussions 4.1. Performances of the models In this section, the energy distributions of different models are adopted to analyze the difference between PCHR, NCHR and TCHR. The model efficiencies of these three models are illustrated in Fig. 5.
In different CWSCs, the efficiency of PCHR is higher than that of NCHR and TCHR. When CWSC ¼ 11.3%, the efficiencies of PCHR, NCHR, and TCHR are 46.60%, 37.56%, and 42.17%, respectively. According to previous analysis, the energy output is consisted of three parts: energy being transported to the combined cycle, energy being converted into the Rankine cycle, and energy being discharged into the atmosphere. The energy being discharged into the atmosphere is largely influenced by the energy released in the condenser. The amount of energy released in the condenser depends on the temperature of the mixture at the inlet of the condenser. If more heat is recovered to preheat the water before being heated to the supercritical state, the amount of the fuel coal is smaller. The amount of gasified coal is set to 1 kg in the analysis of this section. As seen in Fig. 6(a), in NCHR, the temperature of the mixture at the inlet of the cooler is 161 C. The energy released in the cooler is approximately 5 MW, which accounts for approximately 18.5% of the sensible heat of the mixture. As seen in Fig. 6(b), in TCHR, the temperature of the mixture at the inlet of the cooler is approximately 200 C, and the energy released in the cooler is approximately 7 MW, which accounts for approximately 25.9% of the sensible heat of the mixture. However, as seen in Fig. 6(c), in TCHR, the water can be preheated to 408 C. Thus, the amount of fuel coal can be reduced from 1.455 kg/kg gasified coal in NCHR to 0.6563 kg/kg gasified coal in TCHR. In PCHR, the temperature of the mixture at the inlet of the cooler can be lowered down to approximately 30 C, and the water
Table 6 The calorific values of the syngas under different CWSCs. CWSC/wt%
2
3.8
6.6
8.4
11.3
Calorific values of syngas/MJ/kg gasified coal CO2 yields after combusting the syngas/mol/kg gasified coal
31.83 61.81
32.17 62.03
30.20 61.10
31.61 57.93
31.32 58.44
Z. Chen et al. / Energy 134 (2017) 933e942
700
700
Thot
600
500
Tcold
500
Temperature
600
400 300 200
Thot Tcold
400 300 200 100
100 0 -2 0
2
4
6
0 -2 0
8 10 12 14 16 18 20 22 24 26 28
2
4
6
Heat duty/MW
8 10 12 14 16 18 20 22 24 26 28
Heat Duty/MW
(a)
(b)
700 600
Thot Tcold
500
Temperature
Temperature
939
400 300 200 100 0 -2
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28
Heat Duty/MW
(c) Fig. 6. The heat exchange curves of the heat exchangers in different models when CWSC ¼ 11.3%: (a) NCHR (b) TCHR (c) PCHR.
W2 13.6MW
Rankine cycle
QTg-T 38.1MW
Exhausted energy 24.5MW
Gasified 116.6MW coal 64.9MW Fuel coal
Gasifier
HE2
Boiler 138.6 MW
W1 71.0MW
247.0MW 240.6MW
heg 8.2MW
Condenser
Combined cycle
128.8MW 128.8MW
Qe
0.009MW
hur 6.4MW
Recovered heat 73.7MW
Exhausted energy 57.8MW
Fig. 7. The energy flow diagram of PCHR when CWSC ¼ 11.3%.
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Z. Chen et al. / Energy 134 (2017) 933e942
W1 76.5MW
125.7MW
55.8MW Fuel coal
Gasifier
Gasified coal
HE2 Condenser
Boiler 167.8MW 147.4 MW
Combined cycle 138.9MW
266.2MW 259.4MW hur 6.8MW
heg 6.9MW
Qe 28.9MW
Recovered heat 91.6MW
Exhausted energy 62.4MW
Fig. 8. The energy flow diagram of TCHR when CWSC ¼ 11.3%.
can be preheated to 386 C. The energy discharged in the cooler is small, and the amount of fuel coal in PCHR is 0.7575 kg/kg gasified coal. PCHR has the advantages of much less exhausted energy in the condenser than NCHR and TCHR, and relative small amount of fuel coal used for heating. The thermal efficiency is largely enhanced by these advantages. 4.2. Energy balances of the models To have a comprehensive and clear understanding on the comparison of the models, the energy flow diagrams of the models are illustrated in Figs. 7e9. The total energy inputs of the models are 181.5 MW. The enthalpies of the gasified coal and fuel coal in the models are different, which is due to the different amount of recovered heat. The amount of fuel coal decreases with increasing amount of recovered heat. As seen in Figs. 7e9, the total work outputs of PCHR, TCHR, and NCHR are 84.6 MW, 76.5 MW, and 68.1 MW, respectively. Thus, the efficiencies of PCHR, TCHR, and
NCHR are 46.60%, 42.17%, and 37.56%, respectively. As seen in Figs. 7 and 8, compared to TCHR, the better performance of PCHR is mainly due to the less exhausted energy in the condenser. In PCHR and TCHR, the temperatures of the mixture before entering the condenser are approximately 30 C and 200 C, respectively. The high temperature of the mixture in TCHR causes large energy losses in the condenser. In PCHR, this part of energy is not discharged into the atmosphere. Instead, it is transferred to the Rankine cycle for power generation, which may be the reason of higher efficiency of PCHR. As seen in Figs. 7 and 9, compared to NCHR, the better performance of PCHR is mainly due to two aspects: more energy is transferred to the combined cycle than is transferred to the Rankine cycle for power generation; and less exhausted energy in the condenser and the boiler. Because the large amount of recovered heat in PCHR, the amount of fuel coal that is needed for heating the preheated water to the supercritical state is much smaller than that in NCHR. Thus, the amount of gasified coal in PCHR is larger than that in NCHR. As seen from Fig. 4, the energy
W2 21.5MW Rankine cycle
QTg-T 60.4MW
W1 46.6MW Exhausted energy 38.9MW
76.5MW
Gasified coal
Gasifier
HE1
Boiler 105.0MW Fuel coal
105.0 MW
162.1MW 157.9MW
Condenser Combined cycle 97.5MW 84.6MW Qe 12.9MW
heg 19.4MW
hur 4.2MW Exhausted energy 38.0MW
Fig. 9. The energy flow diagram of NCHR when CWSC ¼ 11.3%.
Z. Chen et al. / Energy 134 (2017) 933e942
1.1
much larger than that in NCHR. In the other hand, the temperatures of the mixture before entering the condenser are approximately 30 C and 150 C in PCHR and NCHR, respectively. Thus, the energy exhausted in the condenser of NCHR is larger than that of PCHR. Furthermore, the enthalpy of the exhausted flue gas of the boiler in NCHR is larger than that in PCHR because more fuel coal is consumed in NCHR. Thus, the efficiency of PCHR is much higher than that of NCHR. As seen in Fig. 10, the parameters s, t, and l of the models are illustrated. Parameter s of PCHR is close to that of TCHR, whereas l of PCHR is much smaller than that of TCHR. The energy transported into the combined cycle in PCHR is close to that in TCHR. However, the exhausted energy in the cooler of TCHR is much larger than that of PCHR. Instead, the additional exhausted energy is converted to electricity in PCHR. As seen in Fig. 10, t of PCHR is much larger than that of TCHR. Thus, the efficiency of PCHR is much larger than that of TCHR. When compared PCHR with NCHR, parameter s of PCHR is larger than that of NCHR; parameters t and l of PCHR are smaller than that of NCHR. The exhausted energy in PCHR is smaller than that in NCHR, and more energy is transported into the combined cycle than into the Rankine cycle for power generation. Thus, the efficiency of PCHR is much larger than that of NCHR. For PCHR, as seen in Fig. 5, the efficiency increases with increasing coal-water slurry concentration. The influences of CWSC on the parameters s, t, and l of PCHR are shown in Fig. 11. As seen in Fig. 11, with increasing CWSC, parameter s increases, t decreases, and l changes little. More energy is transferred into the combined cycle than in to the Rankine cycle for power generation, which lead to higher efficiency in larger CWSC. The comparison of PCHR and conventional coal-fired supercritical power plants is conducted and listed in Table 7. As can be seen in Table 7, the thermal efficiency of PCHR when CWSC ¼ 11.3% is higher than that of conventional coal-fired power plants. The comparison between supercritical water gasification of coal with IGCC systems is also conducted, as can be seen in Table 8. When the CSWC is 11.3%, the efficiency of the system proposed in this article is 46.60%. As for IGCC systems, the actual performances of major coal-based IGCC projects worldwide are shown in Table 8 [25]. As can be seen in Table 8, the efficiencies of the major coal-based IGCC projects worldwide are lower than that of the system proposed in this article. The thermal efficiencies of IGCC systems are approximately within the range of 35e43% [26].
1.0 0.9
Parameters
0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
PCHR
TCHR
NCHR
Fig. 10. The dimensionless parameters of different models.
0.8 0.7
Parameters
0.6 0.5 0.4 0.3 0.2 0.1 0.0
0
2
4
6
8
10
941
12
Coal-water slurry concentration/% Fig. 11. The influence of CWSC on dimensionless parameters in PCHR.
transported into the combined cycle is approximately equal to the sum of enthalpy of the gasified coal and the gasification heat Q. Thus, the energy transferred to the combined cycle in PCHR is
Table 7 Comparison between PCHR and conventional coal-fired supercritical power plants.
Highest temperature/ C Highest pressure/bar Thermal efficiency/%
PCHR (CWSC ¼ 11.3%)
[21]
[22]
[23]
[24]
660 250 46.60
540e600 230e300 41.00
597 220e240 45.00
545 260 43.00%
600 253.4 43.19%
Table 8 Design and actual performances of major coal-based IGCC projects. Net plant efficiency/%
Wabash
Tampa
SEP/Demkolec
ELCOGAS
Gas turbine MW design (achieved) Steam turbine MW design (achieved) Auxiliary power MW design (achieved) Net power MW design (achieved) LHV basis design (achieved) HHV basis design (achieved)
192 (192) 105 (98) 35.4 (36) 261.6 (252) 39.2 (41.2) 37.8 (39.7)
192 (192) 121 (125) 63 (66) 250 (250) 41.2 (38.9) 39.7 (37.5)
155 (155) 128 (128) 31 (31) 252 (252) 43 (43) 41.4 (41.4)
182.3 (185); 200 at ISO 135.4; 135 at ISO 35 at ISO 300 at ISO 42.2 41.5
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5. Conclusion Supercritical water gasification technology can cleanly and efficiently convert coal to hydrogen-rich syngas. The gasification product has a large amount of sensible heat. Reasonable use of the heat can largely improve the thermal efficiency. The methods of using the heat conclude: heating the feed water of a Rankine cycle for power generation; and preheat the water before being heated to the supercritical state. The methods can be implemented jointly or separately. In this work, three power generation models integrated supercritical water gasification of coal are proposed and compared. The thermal efficiencies of PCHR, NCHR, and TCHR are 46.60%, 37.56%, and 42.17% when CWSC is 11.3%, respectively. PCHR has advantages of much less exhausted energy than reference models in the cooler and relative small amount of fuel coal used for heating, which leads to higher thermal efficiency than that of NCHR and TCHR. Thus, PCHR is more preferred than NCHR and TCHR if power generation plants integrated SCWG of coal are put into application. The thermal efficiency of the PCHR model is higher than most conventional coal-fired power plants and coal-based IGCC projects. Acknowledgments This study is supported by the The National Key Research and Development Program of China (No. 2016YFB0600105). References [1] National Bureau of Statistics of China. China energy statistical yearbook. Beijing: China Statistics Press; 2014. [2] Hu JL, Kao CH. Efficient energyesaving targets for APEC economies. Energy Policy 2007;35(1):373e82. [3] China Electricity Council. Current status and outlook of China's electric power industry. 2015 [In Chinese)]. [4] IEA. World energy outlook 2014 [London]. 2014. [5] Li MN, Zhang LL. Haze in China: current and future challenges. Environ Pollut 2014;189(6):85e6. [6] Chen WY, Xu RN. Clean coal technology development in China. Energy Policy 2010;38(5):2123e30. [7] Guan Qingqing, Huang Xiaodian, Liu Jing, Gu Junjie, Miao Rongrong, Chen Qiuling, et al. Supercritical water gasification of phenol using a Ru/CeO2 catalyst. Chem Eng J 2016;283:358e65. [8] Guo L, Jin H. Boiling coal in water: hydrogen production and power generation system with zero net CO2, emission based on coal and supercritical water
gasification. Int J Hydrogen Energy 2013;38(29):12953e67. [9] Kruse A, Dinjus E. Hot compressed water as reaction medium and reactant : 2. Degradation reactions. J Supercrit Fluids 2007;41(3):361e79. [10] Li Y, Guo L, Zhang X, Jin H, Lu Y. Hydrogen production from coal gasification in supercritical water with a continuous flowing system. Int J Hydrogen Energy 2010;35(7):3036e45. [11] Molino A, Migliori M, Macrì D, Valerio V, Villone A, Nanna F, et al. Glucose gasification in superecritical water conditions for both syngas production and green chemicals with a continuous process. Renew Energy 2016;91:451e5. [12] Zhu Chao, Guo Liejin, Jin Hui, et al. Effects of reaction time and catalyst on gasification of glucose in supercritical water: detailed reaction pathway and mechanisms. Int J Hydrogen Energy 2016;41(16):6630e9. [13] Caputo Giuseppe, Rubio Patricia, Scargiali Francesca, Marotta Gaspare, Brucato Alberto. Experimental and fluid dynamic study of continuous supercritical water gasification of glucose. J Supercrit Fluids 2016;107:450e61. [14] Yan Q, Guo L, Lu Y. Thermodynamic analysis of hydrogen production from biomass gasification in supercritical water. Energy Convers Manag 2006;47: 1515e28. [15] Chen Z, Zhang Xiaosong, Gao L, Li S. Thermal analysis of supercritical water gasification of coal for power generation with partial heat recovery. Appl Therm Eng 2017;111:1287e95. [16] Guo L, Jin H, Lu Y. Supercritical water gasification research and development in China. J Supercrit Fluids 2015;96:144e50. [17] Dahl S, Dunalewicz A, Fredenslund A, Rasmussen P. The mhv2 model: prediction of phase equilibria at sub- and supercritical conditions. J Supercrit Fluids 1992;5:42e7. [18] Li J, Vanderbeken I, Ye S, Carrier H, Xans P. Prediction of the solubility and gaseliquid equilibria for gasewater and light hydrocarbonewater systems at high temperatures and pressures with a group contribution equation of state. Fluid Phase Equilibria 1997;131:107e18. [19] Mehraban Karim Maghsoudi, Rohani Vahid, Mehrpanahi Abdollah, Naserabad Sadegh Nikbakht. Using two types of heat recovery steam generator for full repowering a steam power plant and its analysis by exergy method. Indian J Sci Res 2014;1(2):106e19. € rstad D, Lindholm A, Pettersson J, et al. Siemens SGT-800 Industrial Gas [20] Lo Turbine Enhanced to 50MW: Combustor Design Modifications, Validation and Operation Experience[C]//ASME Turbo Expo 2013: Turbine Technical Conference and Exposition. 2013:V01BT04A038. [21] Breeze P. Chapter 3-Coal-fired power plants [M]//power generation technologies. Elsevier Ltd; 2014. [22] Zhang D. Introduction to advanced and ultra-supercritical fossil fuel power plants. Ultra-Supercritical Coal Power Plants 2013:1e20. [23] Julia Hentschel, Henning Zindler, Hartmut Spliethoff. Modelling and transient simulation of a supercritical coal-fired power plant: Dynamic response to extended secondary control power output [J]. Energy, 10.1016/j.energy.2017. 02.165. [24] Sanpasertparnich T, Aroonwilas A. Simulation and optimization of coal-fired power plants. Energy Procedia 2009;1(1):3851e8. [25] Holt NAH, Alpert SB. Integrated gasification combined-cycle power plants. Encycl Phys Sci Technol 2003:897e924. [26] Zhang Jianyun, Zhou Zhe, Ma Linwei, Li Zheng, Ni Weidou. Efficiency of wet feed IGCC (integrated gasification combined cycle) systems with coal-water slurry preheating vaporization technology. Energy 2013;51:137e45.