Energy quality factor and exergy destruction processes analysis for a proposed polygeneration system coproducing semicoke, coal gas, tar and power

Energy Conversion and Management 149 (2017) 52–60

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Energy quality factor and exergy destruction processes analysis for a proposed polygeneration system coproducing semicoke, coal gas, tar and power Xuye Jing ⇑, Zhiping Zhu, Pengfei Dong, Guangjun Meng, Kun Wang, Qinggang Lyu Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Road, Beijing 100190, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 3 May 2017 Received in revised form 4 July 2017 Accepted 5 July 2017

Keywords: Exergy destruction Energy quality factor Energy grade Calculation Polygeneration

a b s t r a c t A new polygeneration system was established by combining coal pyrolysis with semicoke combustion, semicoke gasification and a steam Rankine cycle, and calculation approaches were developed to analyze the system. The energy and exergy efficiencies of the system were 68.3% and 76.8%, respectively. The standard energy quality factor ah values of the 17 components and the energy quality factor a of each system stream were obtained based on the benchmark of the selected environmental reference state. The features of the energy grade of the components were elucidated according to their energy quality factors. And the exergy destruction processes in system blocks were exhibited and analyzed in combination with the concept of the energy quality factor. In the DRYER and PYRO, the majority of exergy destructions were used to pay the thermodynamic penalty of the increased energy grade of the coal. In the BUR and GASIF, the energy grade difference between the semicoke and output gas was the main cause of exergy destruction. Especially in the BUR, the energy grade difference was enormous and the exergy destruction was large. In HEX2, the average energy grade difference between QED and QEA was the main cause of the exergy destruction. Finally, from the perspective of the energy quality factor, some potential improvements were analyzed to reduce the exergy destruction in the blocks. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction China is rich in coal, but not oil and gas, therefore, coal will continue to be the major energy source for a long time, making thermal conversion technologies that can convert coal into syngas, liquid fuel, coke and other value-added chemicals attractive [1]. Coal pyrolysis has attracted the attention of many researchers [2–4]. In general, the heat carriers for coal pyrolysis can be categorized as a gas or solid. The representative techniques of solid heat carrier pyrolysis include Lurgi-Rufrgas, Toscoal and DG [5,6], while those of gas heat carrier pyrolysis include LFC, Lurgi-Spuelgas and COED [7,8]. Gas heat carriers may dilute the volatiles, but the heat transfer efficiency and uniformity of gas carriers are higher than those of solid carriers [9]. After extracting the valuable chemicals, e.g., fuel oils, from the coal through pyrolysis, the semicoke can be used for gasification, combustion or other applications; therefore coal pyrolysis technology can be coupled with other technologies to form polygeneration systems. Zhang et al. proposed a dual-bed pyrolysis gasification ⇑ Corresponding author. E-mail address: [email protected] (X. Jing). http://dx.doi.org/10.1016/j.enconman.2017.07.014 0196-8904/Ó 2017 Elsevier Ltd. All rights reserved.

process that combines a coal pyrolyzer and a pneumatic char gasifier to produce pyrolysis oil and gasification gas [10]. In their study, the gasification agent used in the gasifier could be air or air and steam, and the heat carrier for coal pyrolysis was a mixture of char and quartz sand. Yi et al. proposed a system for lignite pyrolysis through solid heat carrier coupled gasification [11]. In their work, the gasification process was relatively independent of the pyrolysis process, and the optional gasification technology was not limited. The heat carrier was quartz sand, and the energy consumed by the system was supplied by char and pyrolysis gas combustion. Guo et al. presented a polygeneration system by integrating a circulating fluidized bed and an atmospheric pressure pyrolyzer [12,13]. In their system, the pyrolyzed volatiles were utilized for the cogeneration of methanol, oil, and electricity, while the char residues were fired in boilers. Dai et al. introduced a process for the integration of pyrolysis and entrained-flow gasification to utilize high moisture low rank coal [14]. In this work, a new polygeneration system for coproducing semicoke, coal gas, tar and power is introduced in Section 2. The energy grade of a material was initially proposed by Rant in 1961 [15]. After estimating the exergies of many homogeneous organic fuels, Rank determined the exergy ratios of gas fuel and

X. Jing et al. / Energy Conversion and Management 149 (2017) 52–60

53

Nomenclature A BUR CON CW DRYER FLA GCHC GASIF H HEX I M m p P PYRO Q REB s S T

ash burner condenser cooling water drying reactor flash tank gas circulating heat carrier gasification reactor specific enthalpy, kJ kg
1 heat exchanger exergy destruction and loss, kJ h
1 mixer mass flow rate, kg h
1 pressure, bar pump pyrolysis reactor heat duty, kJ h
1 reboiler specific entropy, kJ K
1 kg
1 splitter temperature, K

liquid fuel as 0.95 and 0.975, respectively. Szargut and Styrylska [16] attempted to correct Rant’s formulas by considering the chemical composition of fuels, and the mass ratios of H/C, O/C, N/C and S/C were used to describe the chemical compositions. Stepanov investigated several methods for estimating the energies and exergies of fuels and compared the results [17]. In 2009, Zheng and Hou [18] developed the concept of the energy quality factor, i.e., the ratio of exergy to enthalpy, to evaluate the energy grade of a substance. And they calculated the standard exergy data for 81 elements based on the environmental reference state proposed by Kameyama et al. [19]. Furthermore, in 2017, as the benchmark for calculating the standard exergy value of a substance, the environmental reference state was improved by Zheng et al., and the standard enthalpy and standard exergy data for 81 elements were recalculated [20]. Exergy is considered as the energy that can be converted into work. The energy grade of mechanical work is 1; the energy grade of heat flux is measured by the Carnot coefficient, and the concept of energy quality factor provides a reasonable index of a substance’s energy grade. The basis of exergy analysis is that energy has an inherent quality. Hebecker and Bittrich developed a similar concept for the exergetic evaluation of material fluxes; they used the concept to analyze technology in a brewery [21]. By analyzing the energy grade, i.e., the exergy rate, changes between the heat flux and a type of pseudo-work, Zheng and Jing established a system analysis method to reveal the heat conversion mechanism of heat conversion cycles without material conversion, e.g., heat pumps [22]. Then, Jing and Zheng used the proposed energy grade analysis method to elucidate the energy efficiency boosting mechanism of a power/cooling cogeneration cycle [23]. From the perspective of the energy quality factor, Chen et al. investigated the energy performance of a low-rank coal pyrolysis system and suggested several potential improvements [24]. Unfortunately, the values of the energy quality factor calculated in Chen’s work may not be accurate because they cited outdated basic data published by Zheng and Hou in 2009 [18]. Based on the second law of thermodynamics, exergy analysis has been extensively conducted to evaluate the performance of coal-based systems [25–28], and the conventional expressed tools

TUR W x V

turbine work duty, kJ h
1 mass fraction valve

Subscripts ar as received d dry basis gr gross I the first law of thermodynamics II the second law of thermodynamics h standard state i number of the material stream or energy stream Greek letters g efficiency a energy quality factor e specific exergy, kJ kg
1

used for exergy analysis include pie charts, Sankey diagrams and Grassmann diagrams. Analogous to the Carnot coefficient, which is used to measure the quality of heat under different temperatures, the energy quality factor provides a reasonable index of a substance’s energy grade. Therefore, exergy analysis of coalbased systems can be performed in combination with the concept of the energy quality factor. In this study, a polygeneration system based on coal pyrolysis was proposed and simulated. Based on the selected benchmark of the environmental reference state, the enthalpy and exergy values of the streams were calculated using the introduced calculation approach. Then, the standard energy quality factors of the involved components, the actual energy quality factor of the system streams, and the exergy destruction of the system blocks were calculated. Based on the calculation results, the exergy destruction processes were analyzed in combination with the concept of the energy quality factor. 2. Description of the proposed polygeneration system Atmospheric semicoke gasification gas has not only a high temperature but also a high caloric value, and the gas is suitable as a heat carrier for coal pyrolysis to avoid dilution of the pyrolysis gas. Atmospheric semicoke gasification was coupled with coal pyrolysis in this work. The coal drying process also requires energy; however, the quality of the gas heat carrier for coal pyrolysis is high, and it is wasteful to use it as the heat carrier for coal drying. On the other hand, the steam Rankine cycle is usually driven by the high-temperature flue gas of coal combustion, and the middle-temperature waste exhaust could be recovered to dry the input coal before pyrolysis. Therefore, semicoke combustion and a steam Rankine cycle were added to the coal pyrolysis, and a new polygeneration system was established by combining coal pyrolysis with semicoke gasification, semicoke combustion, and a steam Rankine cycle to co-produce semicoke, coal gas, tar and power. The configuration of the polygeneration system is shown in Fig. 1. The raw materials of the system are wet coal, water vapor, oxygen and air, and the products are coal gas, tar, semicoke and

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X. Jing et al. / Energy Conversion and Management 149 (2017) 52–60

Fig. 1. Schematic description of the proposed polygeneration cycle. It describes the configuration of the proposed polygeneration system.

power. The polygeneration system consists of several major components: drying reactor (DRYER), pyrolysis reactor (PYRO), gasification reactor (GASIF), burner (BUR), heat exchanger (HEX), mixer (M), splitter (S), condenser (CON), turbine (TUR), and flash tank (FLA). Wet coal with 20 wt% initial moisture (1) is heated by the hot flue gas (11) in the DRYER to reduce the moisture in the coal to 5 wt% (5). The dry coal (5) is sent to the PYRO and is decomposed into pyrolysis gas, tar, semicoal and pyrolysis water vapor. The energy required for the pyrolysis reaction is supplied by stream 9, i.e., the higher temperature gas circulating heat carrier (GCHC), which releases its heat into the PYRO and becomes a lower temperature GCHC. The gas phase (7) leaving the PYRO is composed of lower temperature GCHC, pyrolysis gas, tar and pyrolysis water vapor, while the solid phase (8) leaving the PYRO is semicoke. The semicoke (8) is cooled through heat recovery unit HEX1 and then separated into three streams. One stream (18) goes to the GASIF, another (12) enters the BUR, and the last (38) is the product output. In the GASIF, the high temperature gasification reaction of semicoke (18) with water vapor (2) and pure oxygen (3) occurs. The high temperature gasification gas (21) leaving the GASIF mixes with GCHC 14 in a specific proportion to produce GCHC 9, whose temperature can be adjusted by altering the proportion. The rest of stream GCHC 16 joins stream 7 to form stream 20, which is cooled and separated in FLA2. The liquid phase leaving FLA2 is the product tar (36), and the gas phase (28) is further cooled and separated in FLA3. After separation, the liquid water (30) and a portion of the coal gas (34) are output from the system, and the rest of the coal gas (32) acts as the GCHC. Then, stream GCHC 32 recovers the heat released by the ash (31) and semicoke (8) in HEX3 and HEX1. The combustion reaction of semicoke (12) with air (4) occurs in the BUR. The temperature of the hot flue gas (10) from the BUR decreases to an appropriate temperature (11) via HEX2, and the released heat is used to drive a steam Rankine cycle to output power. Then, the appropriate temperature flue gas (11) enters the DRYER to heat the wet coal (1). Stream 6 from the DRYER is a mixture of flue gas and water vapor that is cooled and separated in FLA1. The separated flue gas (33) and liquid water (19) are outputs of the system.

3. Calculation approaches and results 3.1. System calculation methods The polygeneration system was simulated using Aspen Plus. In the simulation process, the block RSTOIC reactor was employed to simulate the drying unit by inputting the reaction coal (wet) ? coal (dry) + 0.055H2O and embedding a calculation block to realize the drying process [11,24,29]. The reaction process of coal pyrolysis is complex, and the pyrolysis products are closely related to the coal type, pyrolysis conditions, reactor types, etc. In this study, the amount of each pyrolysis product was determined based on the results of the pyrolysis experiment [10]. The coal pyrolysis product yields (on a dry basis) for tar, gas, char and pyrolysis water were 11.74 wt%, 8.10 wt%, 72.93 wt% and 7.23 wt%, respectively. The proximate and ultimate analyses results of the coal and semicoke are listed in Table 1, and the volume composition of the pyrolysis gas is listed in Table 2 [10]. The pyrolysis unit was simulated by the RYIELD model in which the products and yields of the pyrolysis reaction were expressed [11,29]. To calculate the semicoke atmospheric combustion/gasification reactions, the total Gibbs free energy minimization principle, which has been widely used in process modelling of coal combustion/gasification systems, was employed. When the combustion/ gasification reaction temperature and pressure are fixed, the Table 1 Proximate and ultimate analyses of the coal and semicoke. Item Proximate analysis, wt%

Element analysis, wt%

Qar,gr, kJ kg
1

FCd VMd Ad Mar Cd Hd Nd Sd Od –

Coal

Semicoke

59.98 35.37 4.65 20.00 79.06 4.44 1.20 0.21 10.43 26740

75.58 17.95 6.47 1.00 83.84 2.19 1.01 0.21 6.28 33330

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X. Jing et al. / Energy Conversion and Management 149 (2017) 52–60

When the temperature and pressure are at the environmental state, i.e., T0 = 298.15 K and ph = 1 bar, Eq. (1) can be written as:

Table 2 Volume composition of pyrolysis gas (%). H2

CH4

CO

CO2

CnHm

26.38

41.71

15.5

7.22

9.19

ah ðT 0 ; ph ; xÞ ¼ eh ðT 0 ; ph ; xÞ=Hh ðT 0 ; ph ; xÞ

ð2Þ

where a is the standard energy quality factor, and e and H represent the standard exergy and enthalpy of the substance at the environmental state, respectively. The substances in this study can be divided into two categories: conventional and non-conventional. Substances whose molecular formulas have been determined (e.g., H2O, CO2, CH4 and SiO2) are conventional substances, while the substances whose molecular formulas cannot be determined (e.g., coal, coal tar and coal ash) are considered non-conventional substances. The absolute values of the enthalpy and exergy obtained in Aspen are negative, and they cannot be directly used for the energy quality calculation. The selected energy quality reference state system in this study was created by defining 1 bar and 298.15 K as the pressure and temperature of the environmental reference state and choosing air with an appropriate composition derived from the US Standard Atmosphere 1976 as the reference substance for atmospheric elements, pure water as the reference substance for hydrogen, and the pure compounds in the geosphere as the reference substances for other elements [20]. In addition, the simulated values of the enthalpy and exergy must be converted into values based on the benchmark of the selected environmental reference state. h

Gibbs free energy of a reaction reaches zero, and the compositions of the output stream can be calculated using the constraint condition of element conservation. Therefore, the combustion and gasification processes were simulated using the RGIBBS model [11,29]. As an unconventional component, before entering the RGIBBS model, the semicoke was decomposed into its constituent reactants in the YIELD model using an embedding calculation block. The gasification gas was used as the heat carrier for the coal pyrolysis, and the gasification pressure was consistent with that required for pyrolysis. Therefore, the gasification pressure was 1 bar. For the semicoke atmospheric gasification, the feedstock ratios and reaction temperature were obtained from the literature [30]. The input mass ratios of O2/semicoke and H2O/semicoke were 0.92 and 0.46, respectively, and the atmospheric gasification temperature was 1323.15 K. For the feedstock ratios of semicoke combustion, the excess air factor was 1.1 and the combustion temperature was assumed as 1523.15 K. For the steam Rankine cycle, the temperature and pressure of the turbine inlet stream were 811 K and 99 bar, respectively [31]; the pressure of the turbine outlet stream was 0.1 bar, which was determined by the temperature of the heat sink.

h

h

3.3.1. Calculation of the enthalpy for a conventional substance The denominator H(T, p, x) in Eq. (1) can be calculated using Eq. (3).

3.2. General assumptions The cogeneration system calculation was developed under the following assumptions. (1) The cycle runs in a steady-state, the changes in the potential energy and the kinetic energy of the components are negligible, and the pressure drops within the cycle, e.g., each pipe line, can be neglected. (2) For convenience in calculating the energy quality factor, the ash and CnHm are represented as SiO2 [32] and C2H4, respectively, air is composed of O2 (21%) and N2 (79%), and the tar is represented by a mixture of C7H8, C6H6O, C10H8, C12H10, C14H10 and C16H10 [11,24]. (3) The process of coal drying is treated as a chemical reaction [11,24,29]. (4) The minimum temperature differences in the DRYER, PYRO, HEX1, HEX2 and HEX3 are 100 K, 100 K, 100 K, 50 K and 20 K, respectively. (5) The ash is assumed as to be inert, and it is not involved in the pyrolysis, combustion, and gasification reactions. (6) The isentropic and mechanical efficiencies of the TUR are 0.88 and 0.98, respectively. (7) The mass flow rate of the feed coal is 3600.00 kg h
1.

HðT;p;xÞ ¼

X

xi Hhi ðT 0 ;ph Þþ

8R 9 Rp T X < T C p dT þ ph ½V
Tð@V=@TÞp dp = 0 xi :
RT 2 ½@ lnð^f =f h Þ=@T ; i i p;x ð3Þ Hhi ðT 0 ; ph Þ

On the right side of Eq. (3), the first term represents the standard enthalpy of pure species i, and the thermodynamic h parameters C , V, ^f and f contained in the second term are the p

i

i

heat capacity, volume, fugacity and standard fugacity of species i, respectively. The entire second term can be calculated using the Peng-Robinson equation of state in Aspen Plus. The molecular formula of pure species i could be described as AaBb, and the first term Hhi ðT 0 ; ph Þ in Eq. (3) can be calculated using Eq. (4):

HhAaBb ðT 0 ; ph Þ ¼ aHhA þ bHhB þ Df HhAaBb

ð4Þ

where the standard enthalpies of elements A and B based on the benchmark of the selected environmental reference state, HhA and HhB , can be found in the literature [20], and the standard enthalpy values of the used elements are listed in Table 3. The standard enthalpy of formation of AaBb, Df HhAaBb , can be obtained from a handbook [33]. The enthalpies of the conventional substances in this study at any state can be calculated using Eqs. (3) and (4). 3.3.2. Calculation of the exergy for a conventional substance

3.3. Calculation of the energy quality factor

The numerator e(T, p, x) in Eq. (1) can be calculated using Eq. (5). The energy grade of a substance can be measured according to its energy quality factor. The energy quality factor a is defined as the ratio of the exergy e to the enthalpy H of a substance [20]:

ah ðT; p; xÞ ¼ eðT; p; xÞ=HðT; p; xÞ

ð1Þ

where T, p and x represent the temperature, pressure and chemical composition of the multispecies fluid, respectively. The value of a is between 0 and 1. For a material, a larger a value indicates a higher energy quality.

eðT; p; xÞ ¼ þ

X

X

(

xi

xi ehi ðT 0 ; ph Þ

½Hi ðT; pÞ
Hhi ðT 0 ; ph Þ
T 0 ½Si ðT; pÞ
Shi ðT 0 ; ph Þ h h þRT 0 lnð^f =f Þ þ RTð1
T 0 =TÞ½@ lnð^f =f Þ=@ ln T i

i

i

i

) p;x

ð5Þ On the right side of

e

Eq. (5), the first term hi ðT 0 ; ph Þ represents pure species i, and Si and Shi in the second

the standard exergy of term are the entropy and standard entropy of species i, respec-

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X. Jing et al. / Energy Conversion and Management 149 (2017) 52–60

Table 3 Standard enthalpy and exergy of the involved elements. Elements
1

Standard enthalpy, kJ mol Standard exergy, kJ mol
1

C

H

O

N

S

Si

412.25 410.51

137.08 117.60

32.51 1.94

28.87 0.31

610.71 601.10

856.21 856.21

tively. The second term in Eq. (5) can be calculated using the PR equation of state in Aspen Plus, while the first term ehi ðT 0 ; ph Þ in Eq. (2) can be calculated using Eq. (6):

ehAaBb ðT 0 ; ph Þ ¼ aehA þ behB þ Df GhAaBb

ð6Þ

where the standard exergies of elements A and B, ehA and ehB , can be obtained from the literature [20], and the standard exergy values of the used elements are also listed in Table 3. The standard Gibbs energy of formation of AaBb, Df GhAaBb , can be found in a handbook [33]. The exergies of the conventional substances at any state in this study can be calculated using Eqs. (5) and (6). Additionally, as mentioned in Section 3.2, the non-conventional substances of coal tar and ash are represented as a mixture [11,24] and SiO2 [32], respectively. Therefore, the coal tar and ash can be treated as conventional substances, and their enthalpies and exergies can be calculated using the equations in Sections 3.3.1 and 3.3.2. 3.3.3. Calculation of enthalpy and exergy for coal and semicoke As non-conventional substances, coal and semicoke do not have determined molecular formulas, and Eqs. (4) and (6) are not suitable for calculating their standard enthalpies and exergies. In this study, the standard enthalpies for coal and semicoke were calculated using Eq. (7) [17,24], while their standard exergies were calculated using Eq. (8) [16,29]:

HhCoal ðT 0 ; ph Þ ¼ ð1 þ kÞQ ar;net

ð7Þ

where Qar,net is the net heat value of coal or semicoal. The coefficient k varies with the coal quality. In this study, the k values for wet coal, dry coal and semicoke are 0.2, 0.18 and 0.15, respectively [17,24]:    H O N S eh ðT 0 ; ph Þ ¼ 1:0401 þ 0:1728 þ 0:0432 þ 0:2169 1
2:0628 C C C C  Q ar;net

ð8Þ where C, H, O, N and S represent the carbon, hydrogen, oxygen, nitrogen and sulfur contents of the coal or semicoal, respectively, in wt% (dry basis). The enthalpy of coal or semicoke in any state can be calculated by Eq. (9), which is simplified from Eq. (3). h

h

Z

T

HðT; pÞ ¼ H ðT 0 ; p Þ þ

ð9Þ

C p dT T0 h

h

On the right side of Eq. (9), the term H ðT 0 ; p Þ is calculated using Eq. (7), and the heat capacity Cp is obtained using Aspen Plus. The exergy of coal or semicoke in any state can be calculated using Eq. (10), which is simplified from Eq. (4):

eðT; pÞ ¼ eh ðT 0 ; ph Þ þ

Z

Z

T T0

where the term

T

C p dT
T 0 T0

Cp p
Rln h p T

 ð10Þ

eh ðT 0 ; ph Þ is calculated using Eq. (8).

3.4. Energy and exergy efficiency Based on the first and second laws of thermodynamics, the energy and exergy efficiencies are used to evaluate the thermodynamic performance of the proposed polygeneration system. In the

polygeneration system, the beneficial outputs are oil, coal gas, semicoke and electricity, and the inputs are coal, water vapor, oxygen and air. Therefore, the overall energy and exergy efficiencies of the system are defined as

gI ¼

mCoalgas HCoalgas þ mTar HTar þ mcoke Hcoke þ W mCoal HCoal þ mVapor HVapor þ mo2 Ho2 þ mAir HAir

ð11Þ

gII ¼

mCoalgas eCoalgas þ mTar eTar þ mcoke ecoke þ W mCoal eCoal þ mVapor eVapor þ mo2 eo2 þ mAir eAir

ð12Þ

where gⅠ and gⅡ are the overall energy and exergy efficiencies of the new polygeneration system, respectively; and W is the work output. 3.5. Calculation results Based on the above conditions, the polygeneration system was calculated. Especially, based on the benchmark of the selected environmental reference state, the standard enthalpies, standard exergies and standard energy quality factors for all components involved in this study were calculated, and the data are listed in Table 4. Furthermore, the actual enthalpy, exergy and energy quality factor of each stream of the polygeneration system were calculated. The energy and exergy efficiencies of the system were 68.3% and 76.8%, respectively. 4. Exergy destruction processes analysis using the concept of energy quality factor As a reasonable index of a substance’s energy grade, the energy quality factor reflects the density of the exergy in the energy it contains. When input substances change from one state to another via an open system, the material-conversion process is inevitably accompanied by exergy destruction. Different substances have different energy grades, and the differences in the energy quality factors of related substances in the material-conversion process can be used to analyze the exergy destruction processes. Before analyzing the exergy destruction processes using the energy quality factor, an energy grade assessment of the related substances is necessary. 4.1. Energy grade assessment of the involved components The calculated standard thermodynamic data of the 17 components listed in Table 4 are illustrated in Fig. 2. In the diagram, the two horizontal axes represent the standard enthalpy Hh and standard exergy eh, while the vertical axis represents the standard energy quality factor ah. As illustrated in Fig. 2, tar has the highest ah value of 0.975, indicating that tar has the highest energy grade among the components under the standard environmental reference state. For non-conventional components, the ah values of semicoke, dry coal, coal and ash decrease sequentially. The standard energy quality factors of the other components can also be found in Fig. 2. Based on a proper proportion, through a series of thermodynamic processes, e.g., mixing, heating, compressing and expanding, the components listed in Fig. 2 can form any stream of the fixed thermodynamic state. For instance, according to the mass ratio of 64.17 wt% N2, 25.40 wt% CO2, 0.05 wt% O2, 10.34 wt% H2O and

57

X. Jing et al. / Energy Conversion and Management 149 (2017) 52–60 Table 4 Standard thermodynamic properties of the involved components. Component

DfHh, kJ mol
1 [29]

DfGh, kJ mol
1 [29]

Hh, kJ kg
1

eh, kJ kg
1

ah

CO CO2 CH4 C2H4(CnHm) H2 H2O (l) H2O (g) H2S O2 N2 NO2 SO2 Tar SiO2(Ash) Coal Dry coal Semicoal


110.5
393.5
74.6 52.4 0
285.5
241.8
20.6 0 0 34.2
296.8


137.2
394.4
50.5 68.4 0
237.1
228.6
33.4 0 0 52.3
300.1


910.7





856.29




11933.6 1903.9 55234.5 54731.5 135991.1 1158.5 3599.3 25362.8 2032.1 2061.1 2783.9 5914.7 37589.2 175.6 30530.8 35417.7 37852.0

9826.9 454.3 51770.2 52218.4 116661.7 0 474.4 23561.8 121.1 21.9 1227.9 4759.1 36684.9 0 26938.1 31779.5 34576.0

0.824 0.239 0.937 0.954 0.858 0 0.132 0.929 0.060 0.011 0.441 0.805 0.975 0 0.882 0.897 0.913

Fig. 2. An ah
Hh
eh diagram of the involved components. It describes the calculated standard thermodynamic data of the involved 17 components. The two horizontal axes represent standard enthalpy Hh and standard exergy eh, respectively; while the vertical axis represents the standard energy quality factor ah (2-column fitting image).

0.04 wt% SO2 at 298.15 K and 1 bar, these components are mixed and heated to 423.15 K, and they become stream 6. The actual energy quality factor a of the streams is affected by the stream composition. A larger proportion of the compositions with high ah values indicates a higher a value of the stream. For example, the most abundant component in the coal gas (34) is CO, while that in the flue gas (33) is N2. The ah value of CO is much higher than that of N2, and there is an enormous difference between the a value of coal gas (34) (0.857) and that of flue gas (33) (0.072). 4.2. Analyzing the exergy destruction processes using the actual energy quality factor According to the obtained thermodynamic data of the system streams, the exergy destruction in different blocks and the exergy loss of the non-product outputs were calculated and are listed in Table 5. The diagram shown in Fig. 3 can be used to describe and analyze the exergy destruction processes in combination with the concept of the energy quality factor. The ordinate represents the energy quality factor of the stream or heat flux, while the abscissa represents the unit processes. The numbers next to the

energy quality factor lines represent the exergy (the value above) and the enthalpy (the value below). In the DRYER, the temperature of the hot flue gas (11) is decreased, and heat is released to dry the wet coal (5). The exergy destruction processes in the DRYER are illustrated in Fig. 3. A comparison of the input and output data in Fig. 3 shows that the total enthalpy input is equal to the total output, while the total exergy

Table 5 Exergy destruction and loss distributions. Block

Exergy loss, kW

Block/Stream

Exergy loss, kW

DRYER PYRO BUR GASIF HEX1 HEX2 HEX3 PUMP CON TUR

351.2 679.1 2762.4 1236.5 43.8 352.7 19.8 0.6 55.0 59.6

M1 M2 M3 FLA1 FLA2 FLA3 Sreeam33 Stream35 Stream37


33.7 38.9 0.3 110.6 161.6 145.6 251.4 0.4 0.3


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X. Jing et al. / Energy Conversion and Management 149 (2017) 52–60

Fig. 3. Exergy destruction processes in the DRYER. It describes the processes of exergy destruction in the DRYER in combination with the concept of energy quality factor.

destruction is 351.3 kW. The detailed internal processes are as follows. With the temperature decrease of flow 11, its energy quality factor drops from 0.161 (11) to 0.068 (110 ). The enthalpy difference between flows 11 and 110 is the released heat QDry, and the exergy difference between flows 11 and 110 is the exergy stored in heat energy QDry. Therefore, as shown in Fig. 3, the average energy quality factor of the released heat QDry is 0.585. The released heat QDry is used to dry the wet coal (1). The unbound water and most of the bound water in flow 1 are released as steam, and flow 1 becomes dry coal (5). During the drying process, the energy quality factors of flows 1, 5 and the released steam are 0.882, 0.892 and 0.156, respectively, and the energy quality factor of flow 1 is increased to that of flow 5. The cost of increasing the energy grade for coal is the exergy destruction during the processes of water evaporation and separation from coal. In the PYRO, the temperature of GCHC (9) is decreased, and QPyro is released to decompose flow 5. The processes of exergy destruction in the PYRO are described in Fig. 4. A comparison of Figs. 3 and 4 shows that the exergy destruction processes in the PYRO and DRYER are similar. In the PYRO, the energy grade of flow 5 is further increased to that of semicoke (8), and the irreversibility of the process of decomposing flow 5 into flow 8 and pyrolysis oilgas is the main cause of the exergy destruction. As described in Fig. 4, the total exergy destruction in PYRO is 679.1 kW. In addition, the average energy quality factor of QPyro is higher than that of QDry, indicating that the energy grade of the driving heat used for pyrolysis is higher than that used for drying, which

Fig. 4. Exergy destruction processes in the PYRO. It describes the processes of exergy destruction in the PYRO in combination with the concept of energy quality factor.

indicates that the energy quality of the heat source for pyrolysis is higher than that for drying. The processes of exergy destruction and loss in the BUR and GASIF are described in Figs. 5 and 6, respectively. The exergy destruction and loss in the BUR and GASIF are 2762.4 kW and 1236.5 kW, respectively, and the exergy destruction and loss in the BUR is much higher than that in the other blocks. Among the reactants in the BUR and GASIF, semicoke (12, 18) has the highest energy quality factor of 0.913, and the exergy of the reactants is mainly concentrated in the semicoke (12, 18). For the products in the BUR, the main components in the hot flue gas (10) are N2, CO2 and H2O, which have low energy quality factors; therefore, although the temperature of combustion gas flow 10 is high, the energy quality factor of flow 10 is only 0.3. The enormous difference between the energy quality factors of flows 12 and 10 is the main cause of the high exergy destruction in the BUR. For the products in the GASIF, the most abundant component in the gasification gas (21) is CO, and the energy quality factor of flow 21 is 0.783. In contrast, the difference in the energy quality factors of flows 18 and 21 is far less than that of flows 12 and 10, and the exergy destruction in the GASIF is not as high as that in the BUR. This finding also shows that in general, the thermodynamic irreversibility of coal combustion is larger than that of coal gasification. As representative HEXs and FLAs, the processes of exergy destruction in HEX2 and FLA3 are described in Figs. 7 and 8, respectively. In Fig. 7, the temperature of the hot flue gas (10) is decreased, and Q is released to heat the water (24). The Q value is 1136.9 kW. For the energy donor, the average energy grade of the released heat QED is 0.745 (i.e., (1443
595.9)/1136.9); however, for the energy acceptor, the average energy grade of the accepted heat QEA is 0.345 (i.e., (498
3.9)/1136.9). The larger energy grade difference between the QED and QEA is the main reason for the relatively higher exergy destruction in HEX2. Similar to the other blocks, the exergy destruction processes can also be analyzed using the energy quality factor. From the perspective of the energy quality factor, some potential improvements can be made to reduce the exergy destruction in the blocks. In the DRYER, the wet coal is heated to form dry coal and steam. The energy quality of dry coal is much higher than that of the steam, and reducing the moisture in the wet coal could reduce the exergy destruction in the DRYER. In the PYRO, tar has the highest energy grade among the products, and CH4 has the highest energy grade in the pyrolysis gas. Therefore, measures that increase the yields of tar and the CH4-rich pyrolysis gas could reduce the exergy destruction in the PYRO, e.g., adopting a more

Fig. 5. Exergy destruction processes in the BUR. It describes the processes of exergy destruction in the BUR in combination with the concept of energy quality factor.

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of the drying temperature can reduce the combustion fraction of char. In the GASIF, where H2 has the highest energy grade among the main gasification products, measures that increase the yield of H2 could be taken to reduce the exergy destruction. For the heat exchange process in HEX2, measures that reduce the energy grade difference between QED and QEA could be taken, e.g., using variable temperature evaporation of supercritical water rather than constant temperature evaporation. 5. Conclusion

Fig. 6. Exergy destruction processes in the GASIF. It describes the processes of exergy destruction in the GASIF in combination with the concept of energy quality factor.

Fig. 7. Exergy destruction processes in HEX2. It describes the processes of exergy destruction in HEX2 in combination with the concept of energy quality factor.

A new polygeneration system was established by combining coal pyrolysis with semicoke combustion, semicoke gasification and a steam Rankine cycle to co-produce semicoke, coal gas, tar and power. Calculation approaches were developed to analyze the system. The energy and exergy efficiencies of the system were 68.3% and 76.8%, respectively. The standard energy quality factor ah values of the 17 components and the energy quality factor a of each system stream were obtained based on the benchmark of the selected environmental reference state. The exergy destruction and loss values in the system blocks were calculated, and the main exergy destruction occurred in the DRYER, PYRO, BUR, GASIF and HEX2. The energy grade features of the components were elucidated, and the causes of exergy destruction were explained from the perspective of the energy quality factor. In the DRYER and PYRO, the energy quality factors of the wet coal, dry coal and semicoke were 0.882, 0.892 and 0.913, respectively, and the main exergy destruction in the two blocks was used to pay the thermodynamic penalty of the increased energy grade of the coal. In the BUR and GASIF, the energy quality factors of the main reactant, semicoke and the main products combustion gas and gasification gas are 0.913, 0.3 and 0.783. The energy grade difference between the reactant semicoke and output gas was the main reason of exergy destruction in these two blocks. Especially in the BUR, the energy grade difference between semicoke and combustion gas was enormous and the exergy destruction was large. In HEX2, the average energy grades of QED and QEA were 0.745 and 0.345, respectively, and the energy grade difference was the main cause of the exergy destruction. Finally, from the perspective of the energy quality factor, some potential improvements were analyzed to reduce the exergy destruction in the blocks. Acknowledgments The support provided by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA07010300) for the completion of the present work is gratefully acknowledged. Appendix A. Supplementary material

Fig. 8. Exergy destruction processes in FLA3. It describes the processes of exergy destruction in FLA3 in combination with the concept of energy quality factor.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.enconman.2017. 07.014. References

optimum pyrolysis temperature to yield more tar and more CH4 in the pyrolysis gas. In the BUR, because of the enormous irreversible energy grade difference between the main reactants and products, it is difficult to fundamentally reduce the exergy destruction for the combustion process; therefore, the proportion of semicoke combustion should be appropriately reduced. For example, on the premise of ensuring the drying efficiency, a proper reduction

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