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Thermal analysis of supercritical water gasification of coal for power generation with partial heat recovery
Accepted Manuscript Research Paper Thermal analysis of supercritical water gasification of coal for power generation with partial heat recovery Zhewen Chen, XiaosongZhang, Lin Gao, Sheng Li PII: DOI: Reference:
S1359-4311(16)32554-6 http://dx.doi.org/10.1016/j.applthermaleng.2016.10.110 ATE 9312
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
23 July 2016 26 September 2016 19 October 2016
Please cite this article as: Z. Chen, XiaosongZhang, L. Gao, S. Li, Thermal analysis of supercritical water gasification of coal for power generation with partial heat recovery, Applied Thermal Engineering (2016), doi: http://dx.doi.org/ 10.1016/j.applthermaleng.2016.10.110
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Thermal analysis of supercritical water gasification of coal for power generation with partial heat recovery Zhewen Chena, XiaosongZhangb, Lin Gaoc, Sheng Lid a
Institute of Engineering Thermophysics, CAS, University of Chinese Academy of Sciences, Beijing, 100190, [email protected]
b
Institute of Engineering Thermophysics, CAS, Beijing 100190, [email protected]
c
Institute of Engineering Thermophysics, CAS, Beijing, 100190, China. [email protected]
d
Institute of Engineering Thermophysics, CAS, Beijing, 100190, China. [email protected]
Abstract:
The methods of utilizing coal for power generation include direct combustion in air integrated with a steam cycle and gasification with O2/air integrated with a combined cycle. Special equipment is needed for the removal of NOx, SOx, CO2, and PM2.5, and the cost is high. The technology needed to perform supercritical water gasification (SCWG) can efficiently convert coal to a clean gaseous product. A novel model integrating SCWG with power cycles and heat recovery for power generation is proposed. The gasification product has a large amount of sensible and latent heat. The heat in the high temperature range is used to produce high-temperature and -pressure steam for power generation, whereas the heat in the low temperature range is recovered to preheat water before being heated to the supercritical state. The temperature that separates the high temperature range and low temperature range is the characteristic parameter T. The influences of T, which represents the extent of heat recovery, and the coal–water slurry concentration (CWSC) on the model performance are studied. The efficiency curves have a maximum value when T equals approximately 250°C in all CWSCs. The model efficiency increases with increasing CWSC at a certain T. The model efficiency can reach 42.18% when CWSC=11.3% and T=250°C. Keywords:
Combined cycle, Power generation, Supercritical water gasification, Thermal analysis, novel model.
1. Introduction Coal has been used as fuel for power generation since the late 19 thcentury [1]. Currently, coal is the single most important fuel for power generation, and accounts for approximately 40% of the electricity generated worldwide [2–3]. The coal utilization pathway involves direct combustion in air to generate the high-temperature and -pressure steam that are required to drive the Rankine cycle in most power plants. As in R1, direct combustion of coal cannot eliminate the emission of NO x, SOx, CO2, heavy metal, and PM2.5 [4]. Therefore, pollutant emission reduction systems are complicated, and their cost is high. A low efficiency of power generation (commonly 30–40%) in the direct combustion pathway is another important issue for
efficient coal use [5–6]. Coal + Air→ CO2 + NOx + SOx+ N2 + PM2.5 + H2O(g)
(R1)
Coal gasification is promising because coal is gasified to syngas before utilization, and the syngas can be used in a combined cycle to generate more electricity [7–8]. The coal gasifier mostly operates in gas environments. Thus, special equipment is required for the purification of syngas, which often contains CO2, H2S, HCN, and COS, which incurs high cost. A great demand for a cleaner and more efficient method of using coal is required. Supercritical water (SCW; T>374°C and P> 22.1MPa) has a single phase with no surface tension and no liquid/gas phase boundary. SCW has a lower dielectric constant and fewer and weaker hydrogen bonds for obtaining complete miscibility with many organic compounds and gases than ambient liquid water. SCW has a sufficient density to attain an appreciable dissolving power, has diffusivity that is higher than that of liquid, and has lower viscosity to enhance mass transport. SCW provides a homogeneous and rapid reaction environment for coal gasification. N, P, S, As, Hg, and other elements in coal are deposited in supercritical
water as inorganic salts to avoid the formation of pollutants, such as NO x, and SOx. CO2 can be easily separated from H2, leading to the strong solubility when the pressure and temperature are in the critical region. Therefore, CO2 capture and sequestration is easy [6, 9–11]. Compared with conventional gasification, supercritical water gasification (SCWG) of coal has advantages, including a higher hydrogen yield, N and S deposits as salts, easy CO2 emission reduction, good coal adaptability, and easy energy recovery [6]. Supercritical water gasification has been proposed for decades. Many experiments have been performed to investigate the influences of concentration of coal slurry, gasification temperature, types of catalysts, residence time, and oxidant equivalent ratio on the product distribution. Ge systematically investigated the influences of the residence time, temperature, oxidation equivalent ratio and feed concentration on the gasification characteristics of coal in supercritical water with quartz batch reactors [12]. In another work [13], Ge mainly studied the influences of various alkaline catalysts, feed concentrations, catalyst loadings, and reaction temperatures on the gasification performance of Yimin lignite in supercritical water with a micro batch reactor. Jin et al. [14] proposed a novel supercritical water gasification kinetics model for high-moisture lignite to predict the product distribution. Jin et al. [15] investigated the gasification and sodium distribution characteristics of Zhundong coal in supercritical water with autoclaves and fluid bed reactors. Rashidi [16] conducted experiments on supercritical water gasification
of
sugarcane
bagasse
in
the
presence
of
unpromoted
and
copper-promoted
carbon-nanotube-supported nickel nanocatalysts, and found that 20 wt. % nickel over CNTs had the best performance. Wang et at. [17] examined the influences of temperature, water density, time, reactant concentration, catalyst amount, and catalytic efficiency on the gasification of alkali lignite in supercritical water. Sheikhdavoodi et al. [18] studied the influences of the catalyst, and reaction temperature on the yield and composition, gas heating value, carbon gasification efficiency, and hydrogen gasification efficiency of lignocellulose in supercritical water in a batch reactor at a constant pressure of 25MPa. The highest amount
of hydrogen was achieved at 800°C with the presence of KOH. Su et al. [19] studied the reaction kinetics of Zhundong coal in supercritical water with a quartz reactor. The influences of the reaction temperature and coal concentration on the gasification performance have also been investigated. In addition, numerical studies on a fluidized bed reactor [20], tubular reactor [21], and continuous down-flow reactor [22] have been conducted to explain the complex fluid dynamics and optimize the reactor configuration to obtain a high gasification performance. After gasification, the mixture of unreacted SCW and syngas has a large amount of sensible and latent heat. The methods of using the heat include the following: producing high-temperature and high-pressure steam to drive a Rankine cycle (with efficiency of approximately 35.68%) for power generation, and preheating water before being heated to the supercritical state. However, no works in the literature have studied the influence of heat recovery on the generation efficiency of a model integrating SCWG and power cycles from both mathematical and physical viewpoints according to our knowledge. In this article, a novel model integrating SCWG with power cycles and partial heat recovery for power generation is proposed. The sensible and latent heat of the gasification reaction product is used to drive a Rankine cycle or is recycled for SCW preheating. In section 2, the model with partial heat recovery will be discussed in detail.
2. Introduction of the model with partial heat recovery Supercritical water gasification of coal usually takes place at 500–700°C, and over 22.1MPa. The gasification products mainly consist of H2, CO2, CH4, CO, and C2H6. The hydrogen yield accounts for more than 50% of the total gas yield. The CO2 yield usually accounts for 30–40% of the total gas yield, and CH4 accounts for approximately 10%-20% of the total gas yield. SCWG of coal can be summarized into three reactions as follows:
C H 2O CO H 2 H 132kJ / mol
(R2)
CO H 2O CO2 H 2 H 41kJ / mol
(R3)
CO 3H 2 CH 4 H 2O H 206kJ / mol
(R4)
R2 and R3 are the main reactions in the gasification process. Thus, the gasification process is partially endothermal, and some heat is needed for the gasification process. The ratio of the heat required and the LHV of coal is 30–40%, whereas the coal–water slurry concentration is 2–10%. Some physical heat is transferred to the chemical energy stored in the syngas through the chemical reactions. Heat can be provided by the recovered sensible and latent heat of the mixture of unreacted SCW and syngas.
2.1 Integration method Because the gasification temperature and pressure are very high, and the amount of unreacted SCW, which accounts for approximately 90% of the total SCW, is large, the mixture of unreacted SCW and produced syngas has a large amount of sensible and latent heat. The amount of sensible and latent heat is 1.1–6.9 times larger than that of the enthalpy of coal, whereas the coal–water slurry concentration is 2–10%. The ratio is higher at lower coal–water slurry concentrations. The cold gas and hot gas efficiencies of the SCGP-1 process with a Shell gasifier are approximately 75–83% and 93–97% [24], respectively. Thus, the sensible heat of the syngas accounts for approximately 16% of the total enthalpy of coal. The sensible and latent heat of the mixture in SCWG is approximately 7-43 times larger than the sensible heat of the syngas in IGCC when the same mass and type of gasified coal are used. Reasonable use of the heat can effectively improve the system performance. In IGCC, the produced syngas in a gasifier should be cooled to a suitable temperature for syngas cleaning. The energy released by the syngas in the cooling process is usually used to heat the feedwater of a Rankine cycle to saturated steam with a pressure of approximately 110 bar and to generate power in the steam turbine. In the model proposed in this article, the produced syngas should be cooled to the appropriate
temperature to separate unreacted water and syngas. There are two primary ways to use the sensible and latent heat, as follows: The first way to use the sensible and latent heat is to generate power through a Rankine cycle, such as IGCC. The mixture of unreacted SCW and syngas flows out of the gasifier with high temperature. The sensible heat and latent heat of the mixture can be transferred to the feedwater of a Rankine cycle to produce high-temperature and high-pressure steam and to drive a steam turbine for power generation. The second way to use the sensible and latent heat of the mixture is to preheat water before being heated to the supercritical state. Before water enters the gasifier, it should be heated to the supercritical state. The heat may only come from coal combustion in a boiler, or both the sensible and latent heat of the mixture and coal combustion. If the sensible and latent heat of the mixture are used to preheat water, and the heat from coal combustion is used to further heat water to the supercritical state, the amount of combusted coal could be reduced. The first way to use the sensible and latent heat of the mixture is to generate power with an efficiency of approximately 35.68% when the temperature and pressure of the steam are 510.1°C and 87.21bar, respectively. The heat can be effectively transferred to the feedwater of the Rankine cycle in the first way because the temperature of the mixture can be lowered to approximately 30°C in the heat exchange process with the feedwater. In the second way, some heat is converted to chemical energy in the syngas through the gasification reaction, and released in the combine cycle with a high efficiency. However, if the pinch point of the pre-heater is set to 10°C, the temperature of the mixture can only be lowered to approximately 200°C. Thus, a large amount of sensible and latent heat cannot be effectively used. These two ways of using the sensible and latent heat of the mixture, which can be used separately or conjointly, achieve different levels of performance in different conditions. Thus, further analysis is necessary to find the best way to use the sensible and latent heat of the mixture in practical application.
2.2 Specific description of the model The ways to use the sensible and latent heat of the mixture mentioned above can be used separately or conjointly. In this section, a model with partial heat recovery is proposed. The sensible and latent heat in the high temperature range is used to generate power, and the heat in the low temperature range is recovered for pre-heating. The characteristic temperature T, which separates the high temperature range and low temperature range, affects the model performance. If T is the gasification temperature, the second way to use the sensible and latent heat of the mixture is implemented. If T is sufficiently low, the first way to use the sensible and latent heat of the mixture is carried out. The influences of T and the coal–water slurry concentration on the model performance are considered in this article.
h3
HE1
h2
T
HE2
h4
Condenser
hg
T’
QTg-T Gasifier
hgasified-coal
Tg
Rankine cycle
h1
heg
Supercritical water
Boiler
QT-T’
h6
hfuel-coal
Preheater
h5
Combined cycle
h5' Make-up water
Air
Fig. 1.Power generation model integrating SCWG of coal with partial heat recovery As shown in Fig. 1, the model integrating SCWG of coal and power generation with partial heat recovery is presented. HE1 and HE2 are the heat exchangers for high-temperature and low-temperature heat exchange, respectively.. CC is the combined cycle. hgasified-coal represents the enthalpy of gasified coal. h1 is the enthalpy of SCW; h2 is the enthalpy of the mixture of unreacted SCW and syngas; h3 is enthalpy of the mixture that flows out of HE1; hg is the enthalpy of the syngas that flows out of HE2; h4 is the enthalpy of the unreacted water that flows out of HE2; h5 is the enthalpy of unreacted water after being cooled in the
condenser; h5’ is the enthalpy of water after being mixed in the mixer; and h6 is the enthalpy of water after being pre-heated. huel-coal is the enthalpy of the fuel coal. heg is the enthalpy of the exhausted flue gas. Tg is the temperature of the gasifier. T is the characteristic temperature. T’ is the temperature of water and syngas that flow out of HE2. The coal undergoes complex reactions with SCW in the gasifier. After gasification, the mixture of unreacted SCW and produced syngas flows out of the gasifier, and enters HE1 to release Q Tg-T for power generation in a Rankine cycle. The temperature of the mixture decreases from T g to T. The mixture then enters HE2 to release QT-T’ for preheating SCW, and the temperature of the mixture reaches T’. The unreacted SCW and syngas are simultaneously separated in the cooling process. The unreacted SCW is recycled, and the syngas is transferred to a combined cycle for power generation. Because a portion of the SCW is consumed in the gasification process, the mass of recycled liquid water is smaller than that of SCW entering the gasifier. Thus, make-up water is needed. The mixed water is pre-heated by QT-T’ and further heated to the supercritical state by Q in HE3. Q is provided by the combustion of the fuel coal. The power consumption of pump and pipe losses are neglected when calculating the model efficiency.
2.3 Thermodynamic analysis of the model with partial heat recovery In the analysis, the pinch points of HE2 are set to approximately 10°C. The efficiencies of the Rankine cycle and combined cycle are ηr and ηcc, respectively. The energy balance of the gasifier is expressed as follows：
hgasified coal h1 h2 hur
(1)
Where hur is the enthalpy of unreacted carbon in the gasifier. The energy balances of HE1, HE2, the pre-heater, and the boiler can be separately expressed as follows:
h2 h3 QTG T
(2)
h3 hg h4 QT T '
(3)
h6 QT T ' h5'
(4)
h6 h fuel coal h1 heg
(5)
The model efficiency can be written as:
=
QTG Tr hgcc hgasified coal h fuel coal
QTG T
QTG Tr hgcc QTG T hg h4 h5' hur heg
r
hg
cc hgasified coal h fuel coal hgasified coal h fuel coal cc r QTG T hg h h h h 4 5' ur eg hgasified coal h fuel coal hgasified coal h fuel coal hgasified coal h fuel coal
(6)
ηr and ηcc are the efficiencies of the Rankine cycle and combined cycle, respectively. In =
QTG T hgasified coal h fuel coal
, σ represents the ratio of energy transferred into the Rankine cycle for power
generation and the total energy input of the model; In =
hg hgasified coal hfuel coal
, ζ denotes the ratio of the energy transferred into the combined cycle for power
generation and the total energy input of the model; In =
h4 h5' hur heg hgasified coal hfuel coal
, δ denotes the ratio of exhausted energy and the enthalpy of the total energy
input of the model. A larger σ means that more energy is transferred into the Rankine cycle, rather than being transported to the combined cycle, which leads to a lower thermal efficiency. A larger ζ means that more energy is transferred into the combined cycle for power generation, which leads to a higher efficiency. A larger δ means that more energy is exhausted into the atmosphere, which leads to a lower efficiency. From Fig. 1, we find that when QTg-T= 0, the model becomes a model with total heat recovery. When QT-T’ = 0, the model becomes a model without heat recovery.
3. Model parameters and calculation conditions The influences of the characteristic temperature T and coal–water slurry concentration on the efficiency of the model are discussed in this article under a certain gasification temperature, gasification pressure, ηcc and ηr. Table 2. Ultimate analyses and proximate analyses of gasified coal and fuel coal. Ultimate analysis, wt%
Proximate analysis, wt%
gasified coal
fuel coal
gasified coal
fuel coal
Car
74.29
68.55
Mar
2.79
8.84
Har
4.69
3.96
Aar
6.84
9.98
Oar
9.26
6.85
Var
33.19
49.52
Nar
1
0.74
FCar
57.18
31.66
Sar
1.12
1.08
LHV, MJ/kg
28.34
26.71
Experiments on the supercritical water gasification of gasified coal in a fluidized bed system are carried out at the State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF). The proximate and ultimate analyses of the gasified coal are listed in Table 2. C2H6
100
CO2
CH4
CO
H2 CE YH2
100
90
80
70 60
CE or YH2
gas fraction/%
80
50 40 30 20
60 40 20
10 0
A
B
C
D
E
0
A
working conditions
B
C
D
working conditions
(a)
(b)
Fig. 2. Experimental results under different working conditions: (a) Gas fractions (b) CE or YH2 Note：1) CE: carbon conversion,
E
2) YH2: hydrogen yield,
The experimental results are taken from [24] and rearranged in Figs. 2(a) and 2(b), whereas the working conditions of the experiments are listed in Table 3. The gasification temperatures and pressures of different working conditions are approximately 660°C and 25MPa, respectively. The experimental results can be considered to be the function of the coal–water slurry concentration. Table 3.Working conditions of different experiments Working condition
A
B
C
D
E
CWSC, wt%
2.0
3.8
6.6
8.4
11.3
From Fig. 2(a), we can see that with an increasing coal–water slurry concentration, the gas fraction of H2 decreases, and that of CH4 increases. A higher coal–water slurry concentration facilitates reaction R4 to produce more methane and reduce the gas fraction of hydrogen. The yields of hydrogen, CO2 and CH4 account for 50–60%, approximately 30%, and approximately 15% of the total gas yield, respectively. The hydrogen yield and carbon gasification efficiency decrease with increasing coal–water slurry concentration, as can be seen in Fig. 2(b). Coal is not fully gasified under the high coal–water slurry concentration condition. The enthalpy of the unreacted carbon will be calculated based on the energy balance of the gasifier. The calorific values of the syngas under different CWSCs are listed in Table 4. The calorific values of the syngas are independent to the CWSC. Table 4. 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
31.83
32.17
30.20
31.61
31.32
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 22.1MPa, respectively), ideal gas law, the Peng-Robinson (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 Redlich–Kwong property method with modified Huron–Vidal mixing rule (SRKMHV2) is chosen as property method because this method is already confirmed as suitable property method for chemical systems at supercritical conditions [27, 28]. Other key parameters of the model are illustrated in Table 5. Table. 5. The values of key parameters in the calculation Parameters
values
Parameters
values
Gasification temperature/°C
660
Pinch points of HE2 /°C
10±1
Gasification pressure/MPa
25
ηcc /%
55.1
Flow rate of gasified coal/kg/s
3.9
Temperature of exhausted flue gas/°C
120
ηr /%
35.68
The Rankine cycle is a single pressure steam cycle, whose steam temperature and pressure are 510.1°C and 87.21bar, respectively [26]. The combined cycle is a type of combined cycle with an enhanced Siemens SGT-800 gas turbine, whose work output is approximately 71.4MW [27].
4. Results and discussions The characteristic temperature T affects the distribution of the sensible and latent heat of the mixture in the Rankine cycle and heat recovery to obtain different model efficiencies. Thus, the final structure of the model, which may be partial heat recovery, total heat recovery or no heat recovery, is affected by T.
46 44 42
Efficiency/%
40 38 36 34 CWSC:
32 30 28 26
0
100
2% 3.8% 6.6% 8.4% 11.3%
200
300
400
T/℃
500
600
700
Fig. 3.The influences of T and CWSC on the efficiency of the model with partial heat recovery
The influences of T and CWSC on the model efficiency is illustrated in Fig. 3. The following conclusions can be drawn from Fig. 3. (1) At temperatures below 250°C, the model efficiencies slowly increase with increasing T for all CWSCs. At temperatures between 250°C and approximately 300°C, the model efficiencies slowly decrease with increasing T. After 300°C, the model efficiencies drastically decrease. After the drastic reductions, the efficiencies of the model with all CWSCs increase with increasing T except for CWSC=2%. CWSC=11.3% is taken as an example to analyze the influence of temperature T on the efficiency of the model with partial heat recovery. Between 100 and 250 °C, the model efficiency increases with increasing T. Between 250°C and approximately 300°C, the model efficiency slowly decreases with increasing T. However, the model efficiency sharply decreases in the range of 300–350°C. After that, the model efficiency increases with increasing T within the range of 350–660°C. Viewed from the system level, the energy output of the system consists of three parts: energy transferred into the Rankine cycle for power generation, energy transferred into the combined cycle for power generation, and energy discharged into the atmosphere. The relative values of the three parts of energy determine the thermal efficiency of the system. When the ratio of energy transferred into the combined cycle and energy transported to the Rankine cycle or discharged into the atmosphere is larger, the efficiency of the system will be higher. With increasing T, the work output of the Rankine cycle decreases, whereas the work output of the combined cycle changes slightly. However, the variation tendency of the exhausted energy is complex with increasing T. The total exhausted energy consists of three parts: exhausted energy of the mixture in the condenser Qe, enthalpy of the exhausted flue gas in the boiler heg, and the enthalpy of the unreacted carbon in the gasifier hur. The influences of T on Qe, heg, hur, and the total exhausted energy are illustrated in Fig. 4. The total
exhausted energy of the model sharply increases with increasing T in the range of 300–350°C. When the amount of gasified coal keeps constant, with increasing T, hur keeps constant, heg slightly decreases. Thus, the drastic increase of total exhausted energy in the temperature range of 300–350°C is due to the drastic increase of Qe.
Exhausted energy/kW
12000
Qe heg
10000
Total exhausted energy hur
8000 6000 4000 2000 0
0
100
200
300
400
500
600
700
T/℃ Fig. 4. The influences of T on the exhausted energy The temperature of the water entering the condenser directly affects the amount of energy exhausted into the atmosphere in the condenser. With increasing T, the change trend of T’ is complex. This will be solved as shown in Fig. 5.
700 Tcold Extension cord of Thot
500
Temperature/℃
d
Thot
600
400
b
300 200
q(T)
c B
C
D
R
e p(T')
100 0
a
A
0
2 4
6 8 10 12 14 16 18 20 22
Heat Duty/MW
Fig. 5. The influences of T on the temperature of water entering the condenser The heat exchange curves of HE2 under the condition of total heat recovery are illustrated in Fig. 5.
Thot represents the temperature change curve of the mixture of unreacted SCW and syngas, and Tcold represents the temperature change curve of the water being preheated. The extension cord of T hot is added to obtain comprehensive analysis. Points a, d, and A, D represent the endpoints of Thot and Tcold, respectively. The mixture has a much higher heat capacity between point b and point c than with other temperatures. The water being preheated has a much higher heat capacity between point B and point C than with other temperatures. In an actual heat exchange process, point p and point q represent T’ and T, respectively. Point A and point R represent the temperatures of the water before being preheated and after being preheated, respectively. For example, for a model with total heat recovery, p, q, and R are e, d, and D, respectively. Because the pinch point of HE2 is set to 10°C, the difference in q and R is 10°C when q is below point c. When q is beyond c, the difference in q and R is no longer necessarily 10°C because the pinch point of the temperature curves has met its minimum value at point c. Tx represents the temperature of point x, where x can be any point on the curves. Points p, q, and R are moving points. When x is below c, and q moves from a to x, R moves from A to the point with a temperature of (Tx -10), and p moves to the point that must satisfy the energy balance of the
250
200
T'/℃
150
100
50
0
0
100
200
300
400
500
600
700
T/℃
Fig.6. The influences of T on the temperature of water entering the condenser heat exchange. If q is between a and b, the heat capacity of the mixture is small and changes little. The heat capacity of water is also small and changes little. Thus, with increasing T in the range of T a and Tb, T’
increases slowly. However, when q is beyond point b, the heat capacity of the mixture dramatically increases, and the heat capacity of the water is still small. Thus, if T has an increase of 10°C in the range of Tb and Tc, T’ should have a dramatic increase to satisfy the energy balance of the heat exchange. When q moves to c, p moves to e. When q is beyond c, because the temperature curves have met their minimum value, p will stay at point e, i.e., T’ will remain constant. From the previous analysis, with increasing T, the change curve of T’ can be depicted as shown in Fig. 6; T’ first increases slowly, and then increases dramatically. T’ remains constant after the dramatic increase. The energy discharged into the atmosphere has a similar change trend to T’. As seen in Fig. 7, the influences of T on the dimensionless parameters are illustrated. Taking CWSC=11.3 as an example, as seen in Fig. 7(e), within the temperature range of 100–250°C, σ decreases, ζ and δ increase slowly, and the decrease rate of σ is larger than the increase rate of ζ and δ, which leads to an increase of the model efficiency with increasing T. Between 250 and 300°C, the increase rate of δ grows larger to cause a slight decrease in the model efficiency. However, between 300 and 350°C, the increase rate of δ is much larger than the decrease rate of σ, and ζ still slowly increases, which leads to drastic decreases in efficiency. Between 350 and 660°C, δ increases slowly, and σ still decreases, so the efficiency increases with increasing T.
1.0
1.0
σ ζ δ
0.6
0.4
0.2
0.0
σ ζ δ
0.8
Parameters
Parameters
0.8
0.6
0.4
0.2
0
100
200
300
T/℃
(a)
400
500
600
700
0.0
0
100
200
300
400
T/℃
(b)
500
600
700
1.0
1.0
σ ζ δ
0.6
0.4
0.2
0.0
σ ζ δ
0.8
Parameters
Parameters
0.8
0.6
0.4
0.2
0
100
200
300
400
500
600
0.0
700
0
100
200
300
400
500
600
700
T/℃
T/℃
(c)
(d)
1.0
σ ζ δ
Parameters
0.8
0.6
0.4
0.2
0.0
0
100
200
300
400
500
600
700
T/℃
(e) Fig. 7. The influences of T on dimensionless parameters: (a) CWSC=2%; (b) CWSC=3.8%; (c) CWSC=6.6%; (d) CWSC=8.4%; (e) CWSC=11.3%. The influence of T on the model efficiency when CWSC=11.3% is discussed in detail in the previous section. And the influences of T on the model efficiencies when CWSCs are 2%, 3.8%, 6.6% and 8.4% are discussed as follows: (a) CWSC=2%: Between 100 and 250°C, the model efficiency increases with increasing T. Between 250°C and approximately 300°C, the model efficiency slowly decreases with increasing T. However, Between 300 and 350°C, the model efficiency sharply decreases. After that, the model efficiency decreases with increasing T within the range of 350–660°C. As can be seen in Fig. 7(a), when T is between 100°C and 250°C, σ decreases, ζ increases, and δ increases slowly. More energy is transferred into the combined cycle for power generation, while the amount of exhausted energy slowly increases. Thus, the model efficiency increases with increasing T in this range. When T is between 250°C and 300°C, the model efficiency has a small
decrease with increasing T because the exhausted energy increases slightly faster. However, within the range of 300–350°C, the model efficiency has a drastic decrease with increasing T, which is due to the dramatically increase of exhausted energy. After the sharp decrease, the model efficiency still decreases with increasing T in the range of 350–660°C, because the exhausted energy remains fast increase in this temperature range. (b) CWSCs=3.8%, 6.6%, and 8.4%: the influences of T on the model efficiencies when CWSCs are 3.8%, 6.6%, and 8.4% are similar to that when CWSC is 11.3%. Within the temperature range of 100–250°C, σ decreases, ζ and δ increase slowly, and the decrease rate of σ is larger than the increase rate of ζ and δ, which leads to an increase of the model efficiency with increasing T. Between 250 and 300°C, the increase rate of δ grows larger to cause a slight decrease of the model efficiency. However, between 300 and 350°C, the increase rate of δ is much larger than the decrease rate of σ, and ζ still slowly increases, which leads to drastic decreases in efficiency with increasing T. Between 350 and 660°C, δ increases slowly, ζ increases, and σ still decreases, so the efficiency increases with increasing T. (2) According to the change curves of the model efficiencies in different CWSCs, there are maximum model efficiencies when T is approximately 250°C in all CWSCs. Because the pinch point of HE2 is set to 10°C, T’ increases with increasing T, i.e., the amount of discharged energy into the atmosphere increases when the extent of heat recovery increases. Although heat recovery does benefit to the model efficiency, this benefit is weakened by the exhausted energy into the atmosphere. In low CWSCs, this offset effect is much larger because the amount of exhaust water is larger in low CWSCs. Thus, in low CWSCs, the model with total heat recovery has no advantage over the model with partial heat recovery.
(3) The efficiency of the model with total heat recovery increases with increasing CWSC. The performance of the model with total heat recovery depends on the competition of useful energy for power generation and useless energy exhausted into the atmosphere. If more energy is transferred into the combined cycle than is discharged into the atmosphere, the model achieves higher efficiency. Because δ
denotes the ratio of energy discharged into the atmosphere and the enthalpy of the total energy input of the model, and ζ denotes the ratio of the energy being transferred into the combined cycle for power generation and the total energy input of the model, as seen in Fig. 7, the ratio of ζ and δ increases with increasing CWSC, which means that in higher CWSC, more energy is transferred into the combined cycle for power generation than is discharged into the atmosphere to obtain higher model efficiency. From a mathematic viewpoint, (6) can be rearranged as follows:
=
hg cc hgasified coal h fuel coal
hg cc hg h4 h5' heg hur
cc
1
h4 h5' hur heg hg
cc 1
(7)
δ/ζ represents the ratio of energy discharged into the atmosphere and the enthalpy of the syngas, and δ/ζ decreases with increasing CWSC. Thus, the efficiency of the model with total heat recovery increases with increasing CWSC. (4) Model selection: according to previous analysis, in terms of model efficiency, the optimal model structure is partial heat recovery with a characteristic temperature T of approximately 250°C, as seen in Fig. 8.
HE1
h2
h3 T=250°C
HE2
h4
Condenser
hg
T’
QTg-T Gasifier
hgasified-coal
Tg
Rankine cycle
h1
heg
Supercritical water
Boiler
QT-T’
h6
Preheater
hfuel-coal
h5
Combined cycle
h5' Make-up water
Air
Fig. 8.Power generation model integrating supercritical water gasification of coal with partial heat recovery when T = 250°C. However, if the economy of the model is considered, in CWSCs when the difference in model efficiencies between models with partial heat recovery and total heat recovery is small, models with total
heat recovery are more attractive because the Rankine cycle unit can be removed. Taking CWSC=11.3% for example, the efficiencies of the model with T=250°C and the model with total heat recovery are 42.18% and 42.17%, respectively, which can be seen in Fig. 3. In this situation, the model with total heat recovery may be more practical. The energy flow diagram of the model with partial chemical heat recovery when CWSC=11.3% and T=250°C is illustrated in Fig. 9. The total energy input of the model is 225.5MW, which is consisted of two parts: 113.3MW of the gasified coal, and 112.2MW of the fuel coal. The energy transferred into the Rankine cycle and the combined cycle are 73.1MW and 125.3MW, respectively, which account for approximately 32.42% and 55.57% of the total energy input, respectively. The exhausted energy is 27.1MW, which accounts for approximately 12.02% of the total energy input. The exhausted energy is consisted of three parts: 15.5MW of the exhausted flue gas, 6.2MW of the unreacted carbon, and 5.4MW of the energy released in the condenser. The thermal efficiency of the model is 42.18% ((26.1+69)/ (112.2+113.3)).
W2 26.1MW
Rankine cycle QTg-T 73.1MW
Gasified 113.3MW coal
112.2MW Fuel coal
W1 69.0MW Exhausted energy 47.0MW
Gasifier
HE1 HE2
Boiler 142.3 MW
240.1MW 233.9MW
160.8MW
Condenser Combined cycle 130.7MW 125.3MW Qe
heg 15.5MW
5.4MW
hur 6.2MW Exhausted energy 56.3MW
Recovered heat 30.1MW
Fig. 9. The energy flow diagram of the model when CWSC=11.3% and T=250°C The characteristic temperature T mainly affects the distribution of the sensible and latent heat of the mixture. When T is high, more energy is recycled to preheat the SCW. More energy is used to heat the
feedwater of the Rankine cycle when T is low. Moreover, CWSC mainly affects the ratio of energy discharged into the atmosphere and energy used to generate power, mainly because the amount of water is different in various CWSCs. The ratio is high when CWSC is low. Thus, the model performs differently with various T and CWSCs, and the model structure is decided according to the model performance. The best performance of the model is achieved with partial heat recovery when T is approximately 250°C in all CWSCs. The model efficiency increases with increasing CWSC.
5. Conclusions SCWG is a promising way to cleanly and efficiently use coal. After gasification, the sensible and latent heat of the products is higher that of the produced syngas in IGCC. Efficient utilization of the sensible and latent heat is important for improving the model efficiency. A novel model integrating SCWG of coal with partial heat recovery and power cycles for power generation is proposed in this article. The sensible and latent heat in the high temperature range is used to produce high-temperature and -pressure steam for power generation, whereas heat in the low temperature range is recovered to preheat water before being heated to the supercritical state. The temperature that separates the high temperature range and low temperature range is the characteristic parameter T. The influences of the characteristic temperature T and CWSC on the performance of the model are investigated from both mathematical and physical viewpoints, and the parameters are transformed into dimensionless forms that have definite physical meaning. According to the performance behavior, the model structure selection strategy is decided: partial heat recovery with T = 250°C in all CWSCs. The model efficiency increases with increasing CWSC. The highest efficiency of the model discussed in this article is 42.18% when CWSC=11.3% and the characteristic temperature is 250°C. If economy is also considered, in CWSCs when the difference in model efficiencies between models with partial heat recovery and total heat recovery
is small, models with total heat recovery are more practical.
Acknowledgments This study is supported by the The National Key Research and Development Program of China (No. 2016YFB0600105).
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Highlights
A novel model integrated SCWG and power generation with partial chemical heat recovery is proposed.
Energetic analysis through energy flow diagram.
Maximum thermal efficiency of 42.18% is obtained.
Optimal model structure is determined through parametric analysis.
S1359-4311(16)32554-6 http://dx.doi.org/10.1016/j.applthermaleng.2016.10.110 ATE 9312
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
23 July 2016 26 September 2016 19 October 2016
Please cite this article as: Z. Chen, XiaosongZhang, L. Gao, S. Li, Thermal analysis of supercritical water gasification of coal for power generation with partial heat recovery, Applied Thermal Engineering (2016), doi: http://dx.doi.org/ 10.1016/j.applthermaleng.2016.10.110
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Thermal analysis of supercritical water gasification of coal for power generation with partial heat recovery Zhewen Chena, XiaosongZhangb, Lin Gaoc, Sheng Lid a
Institute of Engineering Thermophysics, CAS, University of Chinese Academy of Sciences, Beijing, 100190, [email protected]
b
Institute of Engineering Thermophysics, CAS, Beijing 100190, [email protected]
c
Institute of Engineering Thermophysics, CAS, Beijing, 100190, China. [email protected]
d
Institute of Engineering Thermophysics, CAS, Beijing, 100190, China. [email protected]
Abstract:
The methods of utilizing coal for power generation include direct combustion in air integrated with a steam cycle and gasification with O2/air integrated with a combined cycle. Special equipment is needed for the removal of NOx, SOx, CO2, and PM2.5, and the cost is high. The technology needed to perform supercritical water gasification (SCWG) can efficiently convert coal to a clean gaseous product. A novel model integrating SCWG with power cycles and heat recovery for power generation is proposed. The gasification product has a large amount of sensible and latent heat. The heat in the high temperature range is used to produce high-temperature and -pressure steam for power generation, whereas the heat in the low temperature range is recovered to preheat water before being heated to the supercritical state. The temperature that separates the high temperature range and low temperature range is the characteristic parameter T. The influences of T, which represents the extent of heat recovery, and the coal–water slurry concentration (CWSC) on the model performance are studied. The efficiency curves have a maximum value when T equals approximately 250°C in all CWSCs. The model efficiency increases with increasing CWSC at a certain T. The model efficiency can reach 42.18% when CWSC=11.3% and T=250°C. Keywords:
Combined cycle, Power generation, Supercritical water gasification, Thermal analysis, novel model.
1. Introduction Coal has been used as fuel for power generation since the late 19 thcentury [1]. Currently, coal is the single most important fuel for power generation, and accounts for approximately 40% of the electricity generated worldwide [2–3]. The coal utilization pathway involves direct combustion in air to generate the high-temperature and -pressure steam that are required to drive the Rankine cycle in most power plants. As in R1, direct combustion of coal cannot eliminate the emission of NO x, SOx, CO2, heavy metal, and PM2.5 [4]. Therefore, pollutant emission reduction systems are complicated, and their cost is high. A low efficiency of power generation (commonly 30–40%) in the direct combustion pathway is another important issue for
efficient coal use [5–6]. Coal + Air→ CO2 + NOx + SOx+ N2 + PM2.5 + H2O(g)
(R1)
Coal gasification is promising because coal is gasified to syngas before utilization, and the syngas can be used in a combined cycle to generate more electricity [7–8]. The coal gasifier mostly operates in gas environments. Thus, special equipment is required for the purification of syngas, which often contains CO2, H2S, HCN, and COS, which incurs high cost. A great demand for a cleaner and more efficient method of using coal is required. Supercritical water (SCW; T>374°C and P> 22.1MPa) has a single phase with no surface tension and no liquid/gas phase boundary. SCW has a lower dielectric constant and fewer and weaker hydrogen bonds for obtaining complete miscibility with many organic compounds and gases than ambient liquid water. SCW has a sufficient density to attain an appreciable dissolving power, has diffusivity that is higher than that of liquid, and has lower viscosity to enhance mass transport. SCW provides a homogeneous and rapid reaction environment for coal gasification. N, P, S, As, Hg, and other elements in coal are deposited in supercritical
water as inorganic salts to avoid the formation of pollutants, such as NO x, and SOx. CO2 can be easily separated from H2, leading to the strong solubility when the pressure and temperature are in the critical region. Therefore, CO2 capture and sequestration is easy [6, 9–11]. Compared with conventional gasification, supercritical water gasification (SCWG) of coal has advantages, including a higher hydrogen yield, N and S deposits as salts, easy CO2 emission reduction, good coal adaptability, and easy energy recovery [6]. Supercritical water gasification has been proposed for decades. Many experiments have been performed to investigate the influences of concentration of coal slurry, gasification temperature, types of catalysts, residence time, and oxidant equivalent ratio on the product distribution. Ge systematically investigated the influences of the residence time, temperature, oxidation equivalent ratio and feed concentration on the gasification characteristics of coal in supercritical water with quartz batch reactors [12]. In another work [13], Ge mainly studied the influences of various alkaline catalysts, feed concentrations, catalyst loadings, and reaction temperatures on the gasification performance of Yimin lignite in supercritical water with a micro batch reactor. Jin et al. [14] proposed a novel supercritical water gasification kinetics model for high-moisture lignite to predict the product distribution. Jin et al. [15] investigated the gasification and sodium distribution characteristics of Zhundong coal in supercritical water with autoclaves and fluid bed reactors. Rashidi [16] conducted experiments on supercritical water gasification
of
sugarcane
bagasse
in
the
presence
of
unpromoted
and
copper-promoted
carbon-nanotube-supported nickel nanocatalysts, and found that 20 wt. % nickel over CNTs had the best performance. Wang et at. [17] examined the influences of temperature, water density, time, reactant concentration, catalyst amount, and catalytic efficiency on the gasification of alkali lignite in supercritical water. Sheikhdavoodi et al. [18] studied the influences of the catalyst, and reaction temperature on the yield and composition, gas heating value, carbon gasification efficiency, and hydrogen gasification efficiency of lignocellulose in supercritical water in a batch reactor at a constant pressure of 25MPa. The highest amount
of hydrogen was achieved at 800°C with the presence of KOH. Su et al. [19] studied the reaction kinetics of Zhundong coal in supercritical water with a quartz reactor. The influences of the reaction temperature and coal concentration on the gasification performance have also been investigated. In addition, numerical studies on a fluidized bed reactor [20], tubular reactor [21], and continuous down-flow reactor [22] have been conducted to explain the complex fluid dynamics and optimize the reactor configuration to obtain a high gasification performance. After gasification, the mixture of unreacted SCW and syngas has a large amount of sensible and latent heat. The methods of using the heat include the following: producing high-temperature and high-pressure steam to drive a Rankine cycle (with efficiency of approximately 35.68%) for power generation, and preheating water before being heated to the supercritical state. However, no works in the literature have studied the influence of heat recovery on the generation efficiency of a model integrating SCWG and power cycles from both mathematical and physical viewpoints according to our knowledge. In this article, a novel model integrating SCWG with power cycles and partial heat recovery for power generation is proposed. The sensible and latent heat of the gasification reaction product is used to drive a Rankine cycle or is recycled for SCW preheating. In section 2, the model with partial heat recovery will be discussed in detail.
2. Introduction of the model with partial heat recovery Supercritical water gasification of coal usually takes place at 500–700°C, and over 22.1MPa. The gasification products mainly consist of H2, CO2, CH4, CO, and C2H6. The hydrogen yield accounts for more than 50% of the total gas yield. The CO2 yield usually accounts for 30–40% of the total gas yield, and CH4 accounts for approximately 10%-20% of the total gas yield. SCWG of coal can be summarized into three reactions as follows:
C H 2O CO H 2 H 132kJ / mol
(R2)
CO H 2O CO2 H 2 H 41kJ / mol
(R3)
CO 3H 2 CH 4 H 2O H 206kJ / mol
(R4)
R2 and R3 are the main reactions in the gasification process. Thus, the gasification process is partially endothermal, and some heat is needed for the gasification process. The ratio of the heat required and the LHV of coal is 30–40%, whereas the coal–water slurry concentration is 2–10%. Some physical heat is transferred to the chemical energy stored in the syngas through the chemical reactions. Heat can be provided by the recovered sensible and latent heat of the mixture of unreacted SCW and syngas.
2.1 Integration method Because the gasification temperature and pressure are very high, and the amount of unreacted SCW, which accounts for approximately 90% of the total SCW, is large, the mixture of unreacted SCW and produced syngas has a large amount of sensible and latent heat. The amount of sensible and latent heat is 1.1–6.9 times larger than that of the enthalpy of coal, whereas the coal–water slurry concentration is 2–10%. The ratio is higher at lower coal–water slurry concentrations. The cold gas and hot gas efficiencies of the SCGP-1 process with a Shell gasifier are approximately 75–83% and 93–97% [24], respectively. Thus, the sensible heat of the syngas accounts for approximately 16% of the total enthalpy of coal. The sensible and latent heat of the mixture in SCWG is approximately 7-43 times larger than the sensible heat of the syngas in IGCC when the same mass and type of gasified coal are used. Reasonable use of the heat can effectively improve the system performance. In IGCC, the produced syngas in a gasifier should be cooled to a suitable temperature for syngas cleaning. The energy released by the syngas in the cooling process is usually used to heat the feedwater of a Rankine cycle to saturated steam with a pressure of approximately 110 bar and to generate power in the steam turbine. In the model proposed in this article, the produced syngas should be cooled to the appropriate
temperature to separate unreacted water and syngas. There are two primary ways to use the sensible and latent heat, as follows: The first way to use the sensible and latent heat is to generate power through a Rankine cycle, such as IGCC. The mixture of unreacted SCW and syngas flows out of the gasifier with high temperature. The sensible heat and latent heat of the mixture can be transferred to the feedwater of a Rankine cycle to produce high-temperature and high-pressure steam and to drive a steam turbine for power generation. The second way to use the sensible and latent heat of the mixture is to preheat water before being heated to the supercritical state. Before water enters the gasifier, it should be heated to the supercritical state. The heat may only come from coal combustion in a boiler, or both the sensible and latent heat of the mixture and coal combustion. If the sensible and latent heat of the mixture are used to preheat water, and the heat from coal combustion is used to further heat water to the supercritical state, the amount of combusted coal could be reduced. The first way to use the sensible and latent heat of the mixture is to generate power with an efficiency of approximately 35.68% when the temperature and pressure of the steam are 510.1°C and 87.21bar, respectively. The heat can be effectively transferred to the feedwater of the Rankine cycle in the first way because the temperature of the mixture can be lowered to approximately 30°C in the heat exchange process with the feedwater. In the second way, some heat is converted to chemical energy in the syngas through the gasification reaction, and released in the combine cycle with a high efficiency. However, if the pinch point of the pre-heater is set to 10°C, the temperature of the mixture can only be lowered to approximately 200°C. Thus, a large amount of sensible and latent heat cannot be effectively used. These two ways of using the sensible and latent heat of the mixture, which can be used separately or conjointly, achieve different levels of performance in different conditions. Thus, further analysis is necessary to find the best way to use the sensible and latent heat of the mixture in practical application.
2.2 Specific description of the model The ways to use the sensible and latent heat of the mixture mentioned above can be used separately or conjointly. In this section, a model with partial heat recovery is proposed. The sensible and latent heat in the high temperature range is used to generate power, and the heat in the low temperature range is recovered for pre-heating. The characteristic temperature T, which separates the high temperature range and low temperature range, affects the model performance. If T is the gasification temperature, the second way to use the sensible and latent heat of the mixture is implemented. If T is sufficiently low, the first way to use the sensible and latent heat of the mixture is carried out. The influences of T and the coal–water slurry concentration on the model performance are considered in this article.
h3
HE1
h2
T
HE2
h4
Condenser
hg
T’
QTg-T Gasifier
hgasified-coal
Tg
Rankine cycle
h1
heg
Supercritical water
Boiler
QT-T’
h6
hfuel-coal
Preheater
h5
Combined cycle
h5' Make-up water
Air
Fig. 1.Power generation model integrating SCWG of coal with partial heat recovery As shown in Fig. 1, the model integrating SCWG of coal and power generation with partial heat recovery is presented. HE1 and HE2 are the heat exchangers for high-temperature and low-temperature heat exchange, respectively.. CC is the combined cycle. hgasified-coal represents the enthalpy of gasified coal. h1 is the enthalpy of SCW; h2 is the enthalpy of the mixture of unreacted SCW and syngas; h3 is enthalpy of the mixture that flows out of HE1; hg is the enthalpy of the syngas that flows out of HE2; h4 is the enthalpy of the unreacted water that flows out of HE2; h5 is the enthalpy of unreacted water after being cooled in the
condenser; h5’ is the enthalpy of water after being mixed in the mixer; and h6 is the enthalpy of water after being pre-heated. huel-coal is the enthalpy of the fuel coal. heg is the enthalpy of the exhausted flue gas. Tg is the temperature of the gasifier. T is the characteristic temperature. T’ is the temperature of water and syngas that flow out of HE2. The coal undergoes complex reactions with SCW in the gasifier. After gasification, the mixture of unreacted SCW and produced syngas flows out of the gasifier, and enters HE1 to release Q Tg-T for power generation in a Rankine cycle. The temperature of the mixture decreases from T g to T. The mixture then enters HE2 to release QT-T’ for preheating SCW, and the temperature of the mixture reaches T’. The unreacted SCW and syngas are simultaneously separated in the cooling process. The unreacted SCW is recycled, and the syngas is transferred to a combined cycle for power generation. Because a portion of the SCW is consumed in the gasification process, the mass of recycled liquid water is smaller than that of SCW entering the gasifier. Thus, make-up water is needed. The mixed water is pre-heated by QT-T’ and further heated to the supercritical state by Q in HE3. Q is provided by the combustion of the fuel coal. The power consumption of pump and pipe losses are neglected when calculating the model efficiency.
2.3 Thermodynamic analysis of the model with partial heat recovery In the analysis, the pinch points of HE2 are set to approximately 10°C. The efficiencies of the Rankine cycle and combined cycle are ηr and ηcc, respectively. The energy balance of the gasifier is expressed as follows：
hgasified coal h1 h2 hur
(1)
Where hur is the enthalpy of unreacted carbon in the gasifier. The energy balances of HE1, HE2, the pre-heater, and the boiler can be separately expressed as follows:
h2 h3 QTG T
(2)
h3 hg h4 QT T '
(3)
h6 QT T ' h5'
(4)
h6 h fuel coal h1 heg
(5)
The model efficiency can be written as:
=
QTG Tr hgcc hgasified coal h fuel coal
QTG T
QTG Tr hgcc QTG T hg h4 h5' hur heg
r
hg
cc hgasified coal h fuel coal hgasified coal h fuel coal cc r QTG T hg h h h h 4 5' ur eg hgasified coal h fuel coal hgasified coal h fuel coal hgasified coal h fuel coal
(6)
ηr and ηcc are the efficiencies of the Rankine cycle and combined cycle, respectively. In =
QTG T hgasified coal h fuel coal
, σ represents the ratio of energy transferred into the Rankine cycle for power
generation and the total energy input of the model; In =
hg hgasified coal hfuel coal
, ζ denotes the ratio of the energy transferred into the combined cycle for power
generation and the total energy input of the model; In =
h4 h5' hur heg hgasified coal hfuel coal
, δ denotes the ratio of exhausted energy and the enthalpy of the total energy
input of the model. A larger σ means that more energy is transferred into the Rankine cycle, rather than being transported to the combined cycle, which leads to a lower thermal efficiency. A larger ζ means that more energy is transferred into the combined cycle for power generation, which leads to a higher efficiency. A larger δ means that more energy is exhausted into the atmosphere, which leads to a lower efficiency. From Fig. 1, we find that when QTg-T= 0, the model becomes a model with total heat recovery. When QT-T’ = 0, the model becomes a model without heat recovery.
3. Model parameters and calculation conditions The influences of the characteristic temperature T and coal–water slurry concentration on the efficiency of the model are discussed in this article under a certain gasification temperature, gasification pressure, ηcc and ηr. Table 2. Ultimate analyses and proximate analyses of gasified coal and fuel coal. Ultimate analysis, wt%
Proximate analysis, wt%
gasified coal
fuel coal
gasified coal
fuel coal
Car
74.29
68.55
Mar
2.79
8.84
Har
4.69
3.96
Aar
6.84
9.98
Oar
9.26
6.85
Var
33.19
49.52
Nar
1
0.74
FCar
57.18
31.66
Sar
1.12
1.08
LHV, MJ/kg
28.34
26.71
Experiments on the supercritical water gasification of gasified coal in a fluidized bed system are carried out at the State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF). The proximate and ultimate analyses of the gasified coal are listed in Table 2. C2H6
100
CO2
CH4
CO
H2 CE YH2
100
90
80
70 60
CE or YH2
gas fraction/%
80
50 40 30 20
60 40 20
10 0
A
B
C
D
E
0
A
working conditions
B
C
D
working conditions
(a)
(b)
Fig. 2. Experimental results under different working conditions: (a) Gas fractions (b) CE or YH2 Note：1) CE: carbon conversion,
E
2) YH2: hydrogen yield,
The experimental results are taken from [24] and rearranged in Figs. 2(a) and 2(b), whereas the working conditions of the experiments are listed in Table 3. The gasification temperatures and pressures of different working conditions are approximately 660°C and 25MPa, respectively. The experimental results can be considered to be the function of the coal–water slurry concentration. Table 3.Working conditions of different experiments Working condition
A
B
C
D
E
CWSC, wt%
2.0
3.8
6.6
8.4
11.3
From Fig. 2(a), we can see that with an increasing coal–water slurry concentration, the gas fraction of H2 decreases, and that of CH4 increases. A higher coal–water slurry concentration facilitates reaction R4 to produce more methane and reduce the gas fraction of hydrogen. The yields of hydrogen, CO2 and CH4 account for 50–60%, approximately 30%, and approximately 15% of the total gas yield, respectively. The hydrogen yield and carbon gasification efficiency decrease with increasing coal–water slurry concentration, as can be seen in Fig. 2(b). Coal is not fully gasified under the high coal–water slurry concentration condition. The enthalpy of the unreacted carbon will be calculated based on the energy balance of the gasifier. The calorific values of the syngas under different CWSCs are listed in Table 4. The calorific values of the syngas are independent to the CWSC. Table 4. 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
31.83
32.17
30.20
31.61
31.32
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 22.1MPa, respectively), ideal gas law, the Peng-Robinson (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 Redlich–Kwong property method with modified Huron–Vidal mixing rule (SRKMHV2) is chosen as property method because this method is already confirmed as suitable property method for chemical systems at supercritical conditions [27, 28]. Other key parameters of the model are illustrated in Table 5. Table. 5. The values of key parameters in the calculation Parameters
values
Parameters
values
Gasification temperature/°C
660
Pinch points of HE2 /°C
10±1
Gasification pressure/MPa
25
ηcc /%
55.1
Flow rate of gasified coal/kg/s
3.9
Temperature of exhausted flue gas/°C
120
ηr /%
35.68
The Rankine cycle is a single pressure steam cycle, whose steam temperature and pressure are 510.1°C and 87.21bar, respectively [26]. The combined cycle is a type of combined cycle with an enhanced Siemens SGT-800 gas turbine, whose work output is approximately 71.4MW [27].
4. Results and discussions The characteristic temperature T affects the distribution of the sensible and latent heat of the mixture in the Rankine cycle and heat recovery to obtain different model efficiencies. Thus, the final structure of the model, which may be partial heat recovery, total heat recovery or no heat recovery, is affected by T.
46 44 42
Efficiency/%
40 38 36 34 CWSC:
32 30 28 26
0
100
2% 3.8% 6.6% 8.4% 11.3%
200
300
400
T/℃
500
600
700
Fig. 3.The influences of T and CWSC on the efficiency of the model with partial heat recovery
The influences of T and CWSC on the model efficiency is illustrated in Fig. 3. The following conclusions can be drawn from Fig. 3. (1) At temperatures below 250°C, the model efficiencies slowly increase with increasing T for all CWSCs. At temperatures between 250°C and approximately 300°C, the model efficiencies slowly decrease with increasing T. After 300°C, the model efficiencies drastically decrease. After the drastic reductions, the efficiencies of the model with all CWSCs increase with increasing T except for CWSC=2%. CWSC=11.3% is taken as an example to analyze the influence of temperature T on the efficiency of the model with partial heat recovery. Between 100 and 250 °C, the model efficiency increases with increasing T. Between 250°C and approximately 300°C, the model efficiency slowly decreases with increasing T. However, the model efficiency sharply decreases in the range of 300–350°C. After that, the model efficiency increases with increasing T within the range of 350–660°C. Viewed from the system level, the energy output of the system consists of three parts: energy transferred into the Rankine cycle for power generation, energy transferred into the combined cycle for power generation, and energy discharged into the atmosphere. The relative values of the three parts of energy determine the thermal efficiency of the system. When the ratio of energy transferred into the combined cycle and energy transported to the Rankine cycle or discharged into the atmosphere is larger, the efficiency of the system will be higher. With increasing T, the work output of the Rankine cycle decreases, whereas the work output of the combined cycle changes slightly. However, the variation tendency of the exhausted energy is complex with increasing T. The total exhausted energy consists of three parts: exhausted energy of the mixture in the condenser Qe, enthalpy of the exhausted flue gas in the boiler heg, and the enthalpy of the unreacted carbon in the gasifier hur. The influences of T on Qe, heg, hur, and the total exhausted energy are illustrated in Fig. 4. The total
exhausted energy of the model sharply increases with increasing T in the range of 300–350°C. When the amount of gasified coal keeps constant, with increasing T, hur keeps constant, heg slightly decreases. Thus, the drastic increase of total exhausted energy in the temperature range of 300–350°C is due to the drastic increase of Qe.
Exhausted energy/kW
12000
Qe heg
10000
Total exhausted energy hur
8000 6000 4000 2000 0
0
100
200
300
400
500
600
700
T/℃ Fig. 4. The influences of T on the exhausted energy The temperature of the water entering the condenser directly affects the amount of energy exhausted into the atmosphere in the condenser. With increasing T, the change trend of T’ is complex. This will be solved as shown in Fig. 5.
700 Tcold Extension cord of Thot
500
Temperature/℃
d
Thot
600
400
b
300 200
q(T)
c B
C
D
R
e p(T')
100 0
a
A
0
2 4
6 8 10 12 14 16 18 20 22
Heat Duty/MW
Fig. 5. The influences of T on the temperature of water entering the condenser The heat exchange curves of HE2 under the condition of total heat recovery are illustrated in Fig. 5.
Thot represents the temperature change curve of the mixture of unreacted SCW and syngas, and Tcold represents the temperature change curve of the water being preheated. The extension cord of T hot is added to obtain comprehensive analysis. Points a, d, and A, D represent the endpoints of Thot and Tcold, respectively. The mixture has a much higher heat capacity between point b and point c than with other temperatures. The water being preheated has a much higher heat capacity between point B and point C than with other temperatures. In an actual heat exchange process, point p and point q represent T’ and T, respectively. Point A and point R represent the temperatures of the water before being preheated and after being preheated, respectively. For example, for a model with total heat recovery, p, q, and R are e, d, and D, respectively. Because the pinch point of HE2 is set to 10°C, the difference in q and R is 10°C when q is below point c. When q is beyond c, the difference in q and R is no longer necessarily 10°C because the pinch point of the temperature curves has met its minimum value at point c. Tx represents the temperature of point x, where x can be any point on the curves. Points p, q, and R are moving points. When x is below c, and q moves from a to x, R moves from A to the point with a temperature of (Tx -10), and p moves to the point that must satisfy the energy balance of the
250
200
T'/℃
150
100
50
0
0
100
200
300
400
500
600
700
T/℃
Fig.6. The influences of T on the temperature of water entering the condenser heat exchange. If q is between a and b, the heat capacity of the mixture is small and changes little. The heat capacity of water is also small and changes little. Thus, with increasing T in the range of T a and Tb, T’
increases slowly. However, when q is beyond point b, the heat capacity of the mixture dramatically increases, and the heat capacity of the water is still small. Thus, if T has an increase of 10°C in the range of Tb and Tc, T’ should have a dramatic increase to satisfy the energy balance of the heat exchange. When q moves to c, p moves to e. When q is beyond c, because the temperature curves have met their minimum value, p will stay at point e, i.e., T’ will remain constant. From the previous analysis, with increasing T, the change curve of T’ can be depicted as shown in Fig. 6; T’ first increases slowly, and then increases dramatically. T’ remains constant after the dramatic increase. The energy discharged into the atmosphere has a similar change trend to T’. As seen in Fig. 7, the influences of T on the dimensionless parameters are illustrated. Taking CWSC=11.3 as an example, as seen in Fig. 7(e), within the temperature range of 100–250°C, σ decreases, ζ and δ increase slowly, and the decrease rate of σ is larger than the increase rate of ζ and δ, which leads to an increase of the model efficiency with increasing T. Between 250 and 300°C, the increase rate of δ grows larger to cause a slight decrease in the model efficiency. However, between 300 and 350°C, the increase rate of δ is much larger than the decrease rate of σ, and ζ still slowly increases, which leads to drastic decreases in efficiency. Between 350 and 660°C, δ increases slowly, and σ still decreases, so the efficiency increases with increasing T.
1.0
1.0
σ ζ δ
0.6
0.4
0.2
0.0
σ ζ δ
0.8
Parameters
Parameters
0.8
0.6
0.4
0.2
0
100
200
300
T/℃
(a)
400
500
600
700
0.0
0
100
200
300
400
T/℃
(b)
500
600
700
1.0
1.0
σ ζ δ
0.6
0.4
0.2
0.0
σ ζ δ
0.8
Parameters
Parameters
0.8
0.6
0.4
0.2
0
100
200
300
400
500
600
0.0
700
0
100
200
300
400
500
600
700
T/℃
T/℃
(c)
(d)
1.0
σ ζ δ
Parameters
0.8
0.6
0.4
0.2
0.0
0
100
200
300
400
500
600
700
T/℃
(e) Fig. 7. The influences of T on dimensionless parameters: (a) CWSC=2%; (b) CWSC=3.8%; (c) CWSC=6.6%; (d) CWSC=8.4%; (e) CWSC=11.3%. The influence of T on the model efficiency when CWSC=11.3% is discussed in detail in the previous section. And the influences of T on the model efficiencies when CWSCs are 2%, 3.8%, 6.6% and 8.4% are discussed as follows: (a) CWSC=2%: Between 100 and 250°C, the model efficiency increases with increasing T. Between 250°C and approximately 300°C, the model efficiency slowly decreases with increasing T. However, Between 300 and 350°C, the model efficiency sharply decreases. After that, the model efficiency decreases with increasing T within the range of 350–660°C. As can be seen in Fig. 7(a), when T is between 100°C and 250°C, σ decreases, ζ increases, and δ increases slowly. More energy is transferred into the combined cycle for power generation, while the amount of exhausted energy slowly increases. Thus, the model efficiency increases with increasing T in this range. When T is between 250°C and 300°C, the model efficiency has a small
decrease with increasing T because the exhausted energy increases slightly faster. However, within the range of 300–350°C, the model efficiency has a drastic decrease with increasing T, which is due to the dramatically increase of exhausted energy. After the sharp decrease, the model efficiency still decreases with increasing T in the range of 350–660°C, because the exhausted energy remains fast increase in this temperature range. (b) CWSCs=3.8%, 6.6%, and 8.4%: the influences of T on the model efficiencies when CWSCs are 3.8%, 6.6%, and 8.4% are similar to that when CWSC is 11.3%. Within the temperature range of 100–250°C, σ decreases, ζ and δ increase slowly, and the decrease rate of σ is larger than the increase rate of ζ and δ, which leads to an increase of the model efficiency with increasing T. Between 250 and 300°C, the increase rate of δ grows larger to cause a slight decrease of the model efficiency. However, between 300 and 350°C, the increase rate of δ is much larger than the decrease rate of σ, and ζ still slowly increases, which leads to drastic decreases in efficiency with increasing T. Between 350 and 660°C, δ increases slowly, ζ increases, and σ still decreases, so the efficiency increases with increasing T. (2) According to the change curves of the model efficiencies in different CWSCs, there are maximum model efficiencies when T is approximately 250°C in all CWSCs. Because the pinch point of HE2 is set to 10°C, T’ increases with increasing T, i.e., the amount of discharged energy into the atmosphere increases when the extent of heat recovery increases. Although heat recovery does benefit to the model efficiency, this benefit is weakened by the exhausted energy into the atmosphere. In low CWSCs, this offset effect is much larger because the amount of exhaust water is larger in low CWSCs. Thus, in low CWSCs, the model with total heat recovery has no advantage over the model with partial heat recovery.
(3) The efficiency of the model with total heat recovery increases with increasing CWSC. The performance of the model with total heat recovery depends on the competition of useful energy for power generation and useless energy exhausted into the atmosphere. If more energy is transferred into the combined cycle than is discharged into the atmosphere, the model achieves higher efficiency. Because δ
denotes the ratio of energy discharged into the atmosphere and the enthalpy of the total energy input of the model, and ζ denotes the ratio of the energy being transferred into the combined cycle for power generation and the total energy input of the model, as seen in Fig. 7, the ratio of ζ and δ increases with increasing CWSC, which means that in higher CWSC, more energy is transferred into the combined cycle for power generation than is discharged into the atmosphere to obtain higher model efficiency. From a mathematic viewpoint, (6) can be rearranged as follows:
=
hg cc hgasified coal h fuel coal
hg cc hg h4 h5' heg hur
cc
1
h4 h5' hur heg hg
cc 1
(7)
δ/ζ represents the ratio of energy discharged into the atmosphere and the enthalpy of the syngas, and δ/ζ decreases with increasing CWSC. Thus, the efficiency of the model with total heat recovery increases with increasing CWSC. (4) Model selection: according to previous analysis, in terms of model efficiency, the optimal model structure is partial heat recovery with a characteristic temperature T of approximately 250°C, as seen in Fig. 8.
HE1
h2
h3 T=250°C
HE2
h4
Condenser
hg
T’
QTg-T Gasifier
hgasified-coal
Tg
Rankine cycle
h1
heg
Supercritical water
Boiler
QT-T’
h6
Preheater
hfuel-coal
h5
Combined cycle
h5' Make-up water
Air
Fig. 8.Power generation model integrating supercritical water gasification of coal with partial heat recovery when T = 250°C. However, if the economy of the model is considered, in CWSCs when the difference in model efficiencies between models with partial heat recovery and total heat recovery is small, models with total
heat recovery are more attractive because the Rankine cycle unit can be removed. Taking CWSC=11.3% for example, the efficiencies of the model with T=250°C and the model with total heat recovery are 42.18% and 42.17%, respectively, which can be seen in Fig. 3. In this situation, the model with total heat recovery may be more practical. The energy flow diagram of the model with partial chemical heat recovery when CWSC=11.3% and T=250°C is illustrated in Fig. 9. The total energy input of the model is 225.5MW, which is consisted of two parts: 113.3MW of the gasified coal, and 112.2MW of the fuel coal. The energy transferred into the Rankine cycle and the combined cycle are 73.1MW and 125.3MW, respectively, which account for approximately 32.42% and 55.57% of the total energy input, respectively. The exhausted energy is 27.1MW, which accounts for approximately 12.02% of the total energy input. The exhausted energy is consisted of three parts: 15.5MW of the exhausted flue gas, 6.2MW of the unreacted carbon, and 5.4MW of the energy released in the condenser. The thermal efficiency of the model is 42.18% ((26.1+69)/ (112.2+113.3)).
W2 26.1MW
Rankine cycle QTg-T 73.1MW
Gasified 113.3MW coal
112.2MW Fuel coal
W1 69.0MW Exhausted energy 47.0MW
Gasifier
HE1 HE2
Boiler 142.3 MW
240.1MW 233.9MW
160.8MW
Condenser Combined cycle 130.7MW 125.3MW Qe
heg 15.5MW
5.4MW
hur 6.2MW Exhausted energy 56.3MW
Recovered heat 30.1MW
Fig. 9. The energy flow diagram of the model when CWSC=11.3% and T=250°C The characteristic temperature T mainly affects the distribution of the sensible and latent heat of the mixture. When T is high, more energy is recycled to preheat the SCW. More energy is used to heat the
feedwater of the Rankine cycle when T is low. Moreover, CWSC mainly affects the ratio of energy discharged into the atmosphere and energy used to generate power, mainly because the amount of water is different in various CWSCs. The ratio is high when CWSC is low. Thus, the model performs differently with various T and CWSCs, and the model structure is decided according to the model performance. The best performance of the model is achieved with partial heat recovery when T is approximately 250°C in all CWSCs. The model efficiency increases with increasing CWSC.
5. Conclusions SCWG is a promising way to cleanly and efficiently use coal. After gasification, the sensible and latent heat of the products is higher that of the produced syngas in IGCC. Efficient utilization of the sensible and latent heat is important for improving the model efficiency. A novel model integrating SCWG of coal with partial heat recovery and power cycles for power generation is proposed in this article. The sensible and latent heat in the high temperature range is used to produce high-temperature and -pressure steam for power generation, whereas heat in the low temperature range is recovered to preheat water before being heated to the supercritical state. The temperature that separates the high temperature range and low temperature range is the characteristic parameter T. The influences of the characteristic temperature T and CWSC on the performance of the model are investigated from both mathematical and physical viewpoints, and the parameters are transformed into dimensionless forms that have definite physical meaning. According to the performance behavior, the model structure selection strategy is decided: partial heat recovery with T = 250°C in all CWSCs. The model efficiency increases with increasing CWSC. The highest efficiency of the model discussed in this article is 42.18% when CWSC=11.3% and the characteristic temperature is 250°C. If economy is also considered, in CWSCs when the difference in model efficiencies between models with partial heat recovery and total heat recovery
is small, models with total heat recovery are more practical.
Acknowledgments This study is supported by the The National Key Research and Development Program of China (No. 2016YFB0600105).
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Highlights
A novel model integrated SCWG and power generation with partial chemical heat recovery is proposed.
Energetic analysis through energy flow diagram.
Maximum thermal efficiency of 42.18% is obtained.
Optimal model structure is determined through parametric analysis.