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Energy and exergy analyses of hydrogen production by coal gasification
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Energy and exergy analyses of hydrogen production by coal gasification S.S. Seyitoglu a,*, I. Dincer b, A. Kilicarslan a a
Faculty of Engineering, Department of Mechanical Engineering, Hitit University, Cevre Yolu Bulvarı No: 8, 19030, Corum, Turkey b Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada
article info
abstract
Article history:
In this study, we examine an integrated coal based gasification system developed for
Received 25 February 2016
hydrogen production and power generation. The proposed plant consists of air separation
Received in revised form
unit, gasification unit, gas cooling and cleaning unit, pressure swing absorption (PSA) for
24 July 2016
hydrogen production, high temperature electrolyzer for hydrogen production, Brayton
Accepted 26 August 2016
cycle, steam Rankine cycle and organic Rankine cycle (ORC) system for power generation.
Available online xxx
We investigate this system through energy and exergy analyses for hydrogen production. Six coal types, such as Beypazari, Tuncbilek, Can, Yatagan, Elbistan and Soma are
Keywords:
considered in this study and energy and exergy efficiencies of these coals are compared to
Hydrogen production
each other for assessment and evaluation. The Aspen Plus and Engineering Equation Solver
Coal gasification
(EES) software packages are used for system simulation and system analyses. The results
Energy
show that the overall energy and exergy efficiencies of the entire system become 41% and
Exergy
36.5%, respectively.
Efficiency
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Energy is one of the most important issues in many applications by all means in the world. Nowadays, two of the most common raw materials for energy production are coal and petroleum. The rapid increase in the energy demand and decrease in the fossil fuel resources have forced people to look for alternative energy production methods in recent years. One of those several alternative energy sources is hydrogen energy. The use of gasification is a common method to produce hydrogen from coal.
Furthermore, the world's energy need depending on developing technology and population is increasing day by day. Since the coal reserves are running out as that of many of the other fossil fuel resources, the available resources of coal should be used efficiently. Also, these resources have to be used more efficiently in terms of problems such as global warming and air pollutions [1,2]. Coal is the most abundant and one of the oldest fossil fuel resources in the world. According to World Coal Association, there is 892 million ton coal in the world and these reserves will end after 110 years [3]. On the whole, coal is used for
* Corresponding author. E-mail addresses: [email protected] (S.S. Seyitoglu), [email protected] (I. Dincer), [email protected] (A. Kilicarslan). http://dx.doi.org/10.1016/j.ijhydene.2016.08.228 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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burning for production of energy and need of heating all over the world. Gasification is a thermochemistry transformation process generating gas from coal [4,5]. In other words, solid fuel is converted to gas fuel. The aim of the gasification is to decrease harmful emission occurring when the coal is burned and to increase the fuel's density. The gasification of coal is a wellknown technology for a long time [4]. The syngas occurring by coal gasification is directly in a boiler or applied to various processes with hydrogen production. Also, the syngas that comes out from the reactor can be used in chemical industries and in energy generation. Hydrogen production by coal gasification is an expensive method as it is compared to other fossil fuel production methods [5]. On the other hand, mining and transportation costs of coal are rather low and coal is abundant. Nowadays, 18% of hydrogen production in the world is acquired from coal [6]. For this reason, instead of using burnt coal, using gasified coal is more efficient way in terms of energy and harmful emission. Conversion of lower quality coal types into syngas rather than directly burning them is crucial for environmental sustainability. There are many methods for high purity hydrogen production. One of them is electrolysis of water. This method uses electricity to separate water into hydrogen and oxygen. Electrolysis is a well-known method and this is considered as the simplest method for hydrogen production. The disadvantage of the method for producing hydrogen by electrolysis system is that it uses too much electricity. For this reason, this method is quite expensive. Therefore, this system should be developed. The High Temperature Electrolysis (HTE) system, which is in a developing stage nowadays, is more efficient than other electrolysis systems. As a result, that is a promising hydrogen production method [7]. The HTE uses thermal energy and electricity to separate water. Also, this system uses high temperature (800e1000 C) to improve the performance of electrolysis [7]. The steam enters the cathode side. After that, the steam separate into the hydrogen gas and oxygen ions then the oxygen gas is formed by discharging the oxygen ions that are flowed through the ceramic material. There are many studies related to High Temperature Electrolysis [7e12]. In the HTE method, the energy decreases to resolve water, however the system's efficiency increases. The HTE is used to produce more hydrogen in this system. The energy decreases for the electrolysis of water due to the usage of high temperature steam in the developed system. If systems are operated integrally, the overall system efficiency and output rate are increased. Recycling of high temperature exhaust gas in different systems increases energy production. Thus, emission of valuable gases to the atmosphere is avoided. By the help of these integrated systems, lots of productions are made such as hydrogen production, heating and electric production. There are many studies about coal gasification, coal gasification based hydrogen production systems and coal gasification based multigeneration systems. Prabu and Jayanti [13] investigated underground coal gasification as a hydrogen generator and used solid oxide fuel cell to generate electrical power. They carried out thermodynamic analysis of a combined system. As a result, they have showed the system efficiency yields to 32%. Khalid et al. [14] analyzed a biomass-solar integrated cycle for
multigeneration system. They found that energy and exergy efficiencies are 66.5% and 39.7%, respectively. Ozcan and Dincer [15] investigated energy and exergy analysis of a novel trigeneration system for hydrogen production, heating and power. Eventually, energy and exergy efficiencies of their system were found to be 56.7% and 45.05%, respectively. Herdem et al. [16] studied energy and exergy analysis of a coal gasification based combined system for hydrogen production. They used water gas shift reactor and alkaline electrolyzer in order to produce hydrogen. They demonstrated energy and exergy analysis of their combined system 58% and 55%, respectively. Bicer and Dincer [17] conducted a thermodynamic analysis of the novel integrated system included underground coal gasification, solid oxide fuel cell, integrated gasification combined cycle and electrolyzer for hydrogen production and power. As a result, their results revealed that energy and exergy efficiencies are 29.2% and 26%, respectively. Gnanapragasam et al. [18] studied energy and exergy analysis comparison of integrated coal gasification combined cycle power generation system with coal gasification based hydrogen production system. They showed that coal gasification based hydrogen production system was better than coal gasification based hydrogen production system in terms of energy and exergy efficiencies. El-Emam et al. [19] investigated a coal gasification based cogeneration system including solid oxide fuel cell. They found total energy and exergy efficiencies 38.1% and 27%, respectively. Martelli et al. [20] performed coal gasification based integrated gasification combined cycle contains both with and without carbon dioxide capture and storage. As a result, they showed CO2 capture added to the system reduces the plant efficiency. Xu et al. [21] investigated coal gasification based cogeneration system with CO2 capture system for hydrogen production and electricity. As a result, the exergy efficiency was found to be 54.3%. Ghosh and De [22] examined a coal gasification and solid oxide fuel cell (SOFC) in terms of exergy analysis. They calculated exergy destruction of each component in system. Duan et al. [23] analyzed a new integrated gasification combined cycle together a molten carbonate fuel cell with CO2 capture. They offered three IGCC systems which are conventional IGCC system without CO2 capture, IGCC system with pre-combustion capture and IGCC system with oxyfuel combustion capture. Adams and Barton [24] conducted a new process to produce power with high efficiency. They used SOFC and the gasification process in their system. They found that SOFC based system efficiency was 44.8% while the integrated gasification combined cycle system efficiency was 38.2%. Seyitoglu et al. [25] analyzed an integrated gasification combine cycle based trigeneration system involving hydrogen production, power and various FischereTropsch synthesis products. As a result, they found the energy and exergy efficiencies of the overall system 53% and 46%, respectively. Ozcan and Dincer [26] investigated thermodynamic analyses of an internal reforming solid oxide fuel cell system using three different practical gasifier products. They showed energy and exergy efficiencies of the SOFC system 42.2% and 36.5%, respectively. In this study, a thermodynamic analysis through energy and exergy approaches is performed to investigate system performances for different coal types. The purpose of this paper is to evaluate the low ranking coals for gasification purposes. Also, according to the properties of coals, the system is examined for hydrogen production. In this study, some
Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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specific Turkish lignites are used. Although the coal reserve of Turkey is high, the heating value of those are low. The data were gathered from the coal reserves of six different regions and the simulation tests were carried out based on these data.
System description During the gasification method used in this study, slurry is obtained by initial grinding and powdering of the coal and then mixed with the water. Then the gasification was applied to the slurry with pure oxygen. The resultant gas was then used as a fertilizer formation and energy production due to its similarity to natural gas. The system consists of several components such as an air separation unit (ASU), a coal gasifier, a low and a high temperature water gas shift (WGS) reactor, H2S and CO2 removal units, a pressure swing absorption (PSA) unit, a combustor, high temperature electrolyzer, organic Rankine cycle and steam Rankine cycle including power unit. Fig. 1 shows an illustrative diagram belonging to the multi-generation energy system presented in this study. In the gasification system suggested in this study, GEE entrained flow gasifier is used. Coal slurry having a ratio of 60e70% coal dust was sent to the gasifier along with high purity of oxygen. Nitrogen and oxygen are decomposed from the air inside the ASU. The operation temperature and pressure of the gasifier used in the system is quite high (1250e1600 C and 20e70 bar). Additionally, the gasifier requires high purity of oxygen for the operation [4]. Two types of syngas cooling unit are used as the gasification system such as radiant and convective cooling. Afterwards, syngas is sent to hydrogen production unit. At the hydrogen production unit, the syngas enters the high temperature WGS (HTWGS) and low temperature WGS (LTWGS) reactor respectively. The WGS reactors used in the system converts CO to CO2 with chemical reactions for more hydrogen production. The respective chemical reactions are written as CO þ H2 O4CO2 þ H2
(1)
COS þ H2 O4CO2 þ H2 S
(2)
3
The HTWGS reactor operates at the temperatures of 300e450 C while operating temperature of LTWGS reactor is 120e300 C [16]. After those chemical reactions, the syngas is subjected to the acid gas removal process where H2S and CO2 are removed from it with two stages. After the acid gas removal unit, the high purity of hydrogen is recovered from syngas at the PSA unit. This unit operates at the pressures of 20e30 bars [4]. The purge gas enters to the combustor after the hydrogen recovery. After the PSA unit, the purge syngas is sent to the combustor to produce power. Steam Rankine cycle, organic Rankine cycle and system turbine are used for the power production unit of the designed system. An open feed water heater (OFWH) and reheat are used to improve the efficiency of SRC. The hot gas coming out of the SRC as waste is sent to ORC for the energy generation. R245fa is preferred in ORC. Thus, the waste gas emission of the system occurred at a very low temperature and in a useless form.
System simulation The system simulation was performed with Aspen Plus software package. In this system, the steam cycle was simulated using STEAMNBS method which is International Association for the Properties of steam formulation. The physical properties of the system were predicted using the PengeRobinson EOS with BostoneMathias function (PR-BM) [24]. The system studied is shown in Fig. 2. Organic Rankine cycle, steam Rankine cycle and HTE were simulated with EES (engineering equation solver) software package. R245fa was used as the working fluid in the ORC. The low ranking coals were used in this study. The properties of coals considered in the system are shown in Table 1. Also, the assumptions made for the system analysis and the values with the constants are tabulated in Table 2.
Thermodynamic analysis Thermodynamic analysis of a coal gasification based hydrogen production system is examined for power and hydrogen generation. Some assumptions are made for the energy and exergy analyses of the system. The assumptions made for analysis and assessment are listed below: The changes in kinetic and potential energy are negligible along the system. The integrated system components are analyzed in a steady state and homogeneous regime. The dead state temperature and pressure are 25 C and 1 bar, respectively. The turbines, compressors and pumps are modelled as adiabatic. The heat transfers in the pipe lines are neglected in this system. Pressure drops throughout piping are neglected.
Chemical reactions
Fig. 1 e Main Schematic diagram of the system.
There are some reactions occurring inside the gasifier. The gasification reactor is an equilibrium reactor. Also as a
Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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Fig. 2 e The combined coal gasification system. chemical equilibrium model Gibbs free energy minimization was adopted. The main chemical equilibrium reactions occurring inside the gasifier can be written [3,4] Combustion reactions;
Cþ
(8)
CO þ H2 O4CO2 þ H2
The CO shift reaction;
1 O2 /CO 2
C þ 2H2 4CH4
The methanation reaction;
(9)
(3) The steam methane reforming reaction;
1 CO þ O2 /CO2 2
(4)
1 H2 þ O2 /H2 O 2
(5)
(10)
CH4 þ H2 O4CO þ 3H2
Energy and exergy analyses
The Boudouard reaction; The water gas reaction;
C þ CO2 42CO
(6)
C þ H2 O4CO þ H2
(7)
The first law of thermodynamic, in other words, energy analysis is the main method to examine the systems. The system energy quantity is measured in energy analysis. This method does not show the realistic results to evaluate system's efficiency. The required energy to evaluate the systems really is usable energy. Usable energy is determined by second
Table 1 e Properties of coals. Coal type
Proximate analysis As received
Beypazarı Tuncbilek Can Yatagan Elbistan Soma
Elemental analysis
Dry
Refs.
Dry
Moisture
Ash
FC
VL
Ash
C (%)
H (%)
N (%)
S (%)
O (%)
Ash
16.4 14.06 11.6 12.6 12.6 15.1
28.8 16.07 15.8 13.6 15.45 13.66
30.62 47.11 41.72 44.06 42.7 45.33
34.92 34.18 40.4 42.34 41.85 41.01
34.5 18.71 17.87 13.6 15.45 13.66
44.93 62.24 54.78 56.42 58.25 61.82
3.28 4.42 4.19 4.41 3.89 4.49
1.05 2.24 1.4 1.3 1.52 1.55
3.21 0.43 5.09 4.75 4.4 3.19
13.03 10.87 16.67 19.53 16.49 15.28
34.5 18.7 17.9 13.6 15.5 13.7
[27] [28] [29] [30] [30] [30]
Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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Table 2 e The input parameters of the system. Input parameters in the system
Refs.
Water-slurry, % Input coal, kg/s Gasifier temperature, C Gasifier pressure, bar Radiant cooler temperature, C Convective cooler temperature, C O2 purity, %vol Pressure of O2 and N2 delivered by ASU, bar Ratio of O2 inlet pressure to gasifier, to the gasifier pressure NH3 and HCl percentage of removal CO2 final delivery pressure, bar CO2 compressor isentropic efficiency, % Syngas turbine Isentropic efficiency, % HTE water conversion efficiency HTE operation temperature, C Hydrogen content in the hydrogen product, %
60 30 1370 42.4 815 260 99 10 1.2 100 153 85 90 80 900 100
[31] [33] [34] [34]
excoal ch ¼
[32]
m_ in
X
m_ out ¼ Dm_ sys
[16,24]
[16] [16]
m_ in hin
X
(11)
_ sys m_ out hout ¼ DEn
(12)
where hin and hout are the specific enthalpies at the inlet and outlet of the system, respectively. Entropy equation was analyzed with following equation [35]: X
m_ in sin
X
m_ out sout þ S_gen þ
X Q_ T
! ¼ DS_sys
(13)
cr
where sin, sout and S_gen are the specific entropies at the inlet, outlet and generation of the system, respectively. Balance equation for exergy was analyzed with following equation [35]: X
m_ in exin
X
_ Des Ex _ Heat ¼ DEx _ sys m_ out exout Ex
(14)
where exin and exout are the specific exergy at the inlet or outlet _ heat _ Des and Ex of the system components, respectively. Also, Ex are destructed exergy rate and exergy rate due to heat transfer, respectively. The specific exergy is divided into two parts. They can be defined as follows: ex ¼ exph þ exch
(15)
where exph and exch are the physical exergy and chemical exergy, respectively. The physical and chemical exergies can be defined as follows; exph ¼ ðhin hout Þ T0 ðsin sout Þ exch ¼
X
xi ex0ch þ RT0
X
xi lnðxi Þ
(18)
where CCV is net calorific value, w is the moisture content, hfg is the latent heat of water at T0, and s is the mass fraction of sulfur in the fuel. b can be defined in terms of the dry organic substances contained in the coal for an O/C mass ratio less than 0.667 as follows: b ¼ 0:1882
[8] [8]
where m_ in and m_ out are inlet and outlet mass flow rate of the system. Balance equation for energy was analyzed with following equation [35]: X
CCV0 þ w*hfg *b þ 9417s 0
law of thermodynamic by taking the environmental conditions into consideration. In the light of the above information, the performance of all system was carried out using mass, energy and exergy balances. Firstly, balance equation for mass was analyzed with the following equation [35]: X
The chemical exergy equation is chemical exergy for gas components of a chemical gas mixture. The chemical exergy values of each stream are estimated from the standard chemical exergies of the substances. The specific chemical exergy values of gases used in this study are summarized in Table 3. The chemical exergy of the coal is written as [16,19]
(16)
h o n þ 0:061 þ 0:0404 þ 1:0437 c c c
(19)
where c, h, o and n represent the mass fractions of carbon, hydrogen, oxygen and nitrogen, respectively. The energy and exergy efficiencies of all system can be defined as the ratio of total useful output to the system input. General definitions for energy and exergy efficiencies can be written as hen ¼
_ out En _ in En
(20)
hex ¼
_ out Ex _ in Ex
(21)
_ and Ex _ stand for energy and exergy per where subscripts En unit time, respectively. In this study, the total efficiency of the system is defined as the ratio of the total of the lower heating value of the hydrogen produced in the system to the lower heating value of the input coal. The exergy efficiency can be defined as the ratio of the total of the exergy of the produced hydrogen to the exergy of the input coal. The equations of the overall energy and exergy efficiencies of the system can be written as hEn;system ¼
_ H2 LHVH2 m _ coal LHVcoal m
(22)
hEx;system ¼
_ H2 exH2 m _ coal excoal m
(23)
Table 3 e Standard chemical exergy values of different components [36]. Substance
Standard chemical exergy (kJ/kmol)
CO CO2 H2 H2O (liquid) H2O (gas) N2 H2S HCL O2
275,430 20,140 238,490 3120 11,710 720 804,770 85,950 3970
(17)
Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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_ H2 is the total mass flow rate of hydrogen in the syswhere m _ coal is mass flow rate of the coal fed to the gasifier. tem. Also, m
Results and discussion In this study, the amount of oxygen provided to the gasifier was proportional to the carbon content of the coal type. In the designed system, the producible amount of hydrogen was dependent on the coal types. In addition, depending on the production rates, energy and exergy efficiencies of the system were investigated. Firstly, the synthesis gas obtained by the gasification of the coal, it was sent to the gas cooling unit and then sent to the WGS reactors for hydrogen production. Hydrogen content of the syngas is enriched in HTWGS and LTWGS reactors. Enriched syngas is sent to the acid gas removal unit in order to separate H2S and CO2 from syngas. Thus, harmful gases in the syngas are removed. Then, the syngas is sent to the PSA unit and H2 is produced. The purge gas is sent to the combustor for the energy production. In this study, variation of H2 and CO ratio in syngas with respect to the gasifier temperature, gasifier pressure and slurry ratio is investigated. Finally, along with their energy and exergy efficiencies, the maximum hydrogen yields of various types of coal were compared. Exergy efficiencies of the system elements were calculated by using the highest hydrogen yielding coal type. According to the analyses, the highest amount of electricity was consumed in the system components such as air compressor, air separation unit and MAC. In system, there was a CO2 removal and storage unit, too. This section of the system was not modelled elaborately. Based on the collected values from the literature, the electric consumption of the capture and storage unit was determined. H2eCO ratios of the syngas based on the slurry ratio are given in Fig. 3. There, Tuncbilek coal was analyzed. Gasifier pressure and temperature are assumed to be 42.4 bar and 1371 C, respectively. It is observed that as the water ratio in the slurry increased, H2 and CO ratio decreased. This ratio affected the rate of hydrogen production and consequently affected energy and exergy efficiencies. In Fig. 4, the correlation between the pressure of the gasifier and the H2eCO ratio in the syngas is illustrated. In this analysis, Tuncbilek lignite was preferred. It is assumed that the gasifier temperature was 1371 C and kept constant during the analysis. As it can be inferred from Fig. 4 that, as the gasifier pressure increases, H2 and CO rates slightly decrease. The results shown in Fig. 5 are obtained by varying the gasifier temperature from 1300 C to 1500 C. Gasifier pressure is kept constant at 42.4 bar. Tunc¸bilek lignite was preferred. As it can be inferred from Fig. 5, as the gasifier temperature increases, H2 rate decreases while CO rate increases. In Fig. 6, breakdown of the generated and consumed electric energy among the system elements are given. As it can be seen, MAC and ASU are the largest electric consumers in the system. Electricity is generated by system turbine and steam Rankine cycle. In addition, an SMR was also added to the system in order to increase electric outcome by recovering waste energy to generate electric energy.
Fig. 3 e The effect of slurry ratio on the syngas mole fraction.
Fig. 4 e The effect of gasifier pressure on the syngas mole fraction.
Fig. 5 e The effect of gasifier temperature on the syngas mole fraction.
Fig. 7 depicts the exergy efficiencies of the system elements. ASU and combustor were the least efficient elements in the system. The ASU's low exergy efficiency was due to its working principle while that of the combustor was due to the chemical reactions that occur during the combustion.
Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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Fig. 6 e Performance of the system component for Tuncbilek coal.
Fig. 7 e The exergy efficiencies of the system components.
Fig. 8 e Hydrogen production of the coal gasification system. Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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Fig. 9 e Energy and Exergy efficiencies of the system for considered coals. The amount of hydrogen that could be produced by the system is given in Fig. 8. The unit of the figure is kg/h. Hydrogen was of high purity and produced by PSA unit and high temperature electrolyzer. According to the Fig. 8, Tuncbilek coal was able to provide the highest hydrogen yield among all types of coal. The overall energy and exergy efficiencies of the system with respect to the generated outputs and coal amounts loaded are given in Fig. 9. The Tuncbilek coal, which is followed by Soma coal, provided the highest energy and exergy efficiency.
Nomenclature _ En _ Ex _ m P Q_ T _ W
Energy rate, kW Exergy rate, kW Mass flow rate, kg s1 Pressure, Bar Heat transfer rate, kW Temperature, K Work rate, kW
Greek letters h Efficiency
Conclusions This study is presented to provide a thermodynamic analysis, through energy and exergy approaches, of coal gasification in an integrated system fashion. This system achieves the production of hydrogen with PSA unit and High Temperature Electrolysis. The present system requires no electricity input to the system. The system uniquely includes CO2 capture and removal which makes this system unique and environmental friendly. The sub-systems MAC and ASU spend the all electricity. The steam Rankine cycle and organic Rankine cycle provide more electricity by using exhaust gases so it affects system's efficiency. Thermodynamic performance of the low ranking coals is conducted for the system and its components. The hydrogen produced by the system is achieved at the highest level. According to the hydrogen production of this system, Tuncbilek coal shows the best performance as their values from PSA and HTE appears to 8640 kg/h and 324 kg/h, respectively. Also, Soma coal has the second best performance as it is compared to the other types of coals used in the gasification system. When the performance of the system using Tuncbilek as a coal is compared to the other types of coals used in the system, the highest overall system energy and exergy efficiencies appear to be 41% and 36.5%, respectively.
Subscripts ch Chemical des Destruction h Enthalpy HX Heat exchanger in Inlet out Outlet Ref Reference Ph Physical Sys System 0 Ambient or reference condition Acronyms FC Fixed carbon HTE High Temperature Electrolyzer LHV Lower heating value HT High Temperature LT Low temperature VL Volatile Matter
references
[1] World Energy Outlook. Website: http://www. worldenergyoutlook.org. [Accessed: 10.01.2016].
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[2] Ghosh S, De S. Thermodynamic performance study of an integrated gasification fuel cell combined cycle-an energy analysis. J Power Energy 2003;217(2):137e47. [3] World Coal Association. Website: http://www.worldcoal.org/ coal. [Accessed 15.02.2016]. [4] Higman C, Burgt MVD. Gasification. 2nd ed. Elsevier Inc; 2008. [5] Acar C, Dincer I. Comparative assessment of hydrogen production methods from renewable and non-renewable sources. Int J Hydrogen Energy 2014;39:1e12. [6] Miller BG. Coal energy systems. Elsevier Inc; 2005. [7] Balta MT, Dincer I, Hepbasli A. Thermodynamic assessment of geothermal energy use in hydrogen production. Int J Hydrogen Energy 2009;34:2925e39. [8] Mansilla C, Sigurvinsson J, Bentemps A, Marechal A, Werkoff F. Heat management for hydrogen production by high temperature steam electrolysis. Energy 2007;32:423e30. [9] Sigurvinsson J, Mansilla C, Lovera P, Werkoff F. Can high temperature steam electrolysis function with geothermal heat. Int J Hydrogen Energy 2007;32:1174e82. [10] Fujiwara S, Kasai S, Yamauchi H, Yamada K, Makino S, Matsunaga K, et al. Hydrogen production by high temperature electrolysis with nuclear reactor. Prog Nucl Energy 2008;50:422e6. [11] Mingyi L, Bo Y, Jingming X, Jing C. Thermodynamic analysis of the efficiency of high-temperature steam electrolysis system for hydrogen production. J Power Sources 2008;177(2):493e9. [12] Herring JS, O'Brien JE, Stoots CM, Hawkes GL, Hartvigsen JJ, Shahnam M. Progress in high-temperature electrolysis for hydrogen production using planar SOFC technology. Int J Hydrogen Energy 2007;32(4):440e50. [13] Prabu V, Jayanti S. Integration of underground coal gasification with a solid oxide fuel cell system for clean coal utilization. Int J Hydrogen Energy 2012;37(2):1677e88. [14] Khalid F, Dincer I, Rosen MA. Energy and exergy analyses of a solar-biomass integrated cycle for multigeneration. Sol Energy 2015;112:290e9. [15] Ozcan H, Dincer I. Thermodynamic analysis of a combined chemical looping based trigeneration system. Energy Convers Manag 2014;85:477e87. [16] Herdem MS, Farhad S, Dincer I, Hamdullahpur F. Thermodynamic modelling and assessment of a coal gasification and alkaline water electrolysis system for hydrogen production. Int J Hydrogen Energy 2014;39(7):3061e71. [17] Bicer Y, Dincer I. Energy and exergy analyses of an integrated underground coal gasification with SOFC fuel cell system for multigeneration including hydrogen production. Int J Hydrogen Energy 2015;40(39):13323e37. [18] Gnanapragasam NV, Reddy BV, Rosen MA. Hydrogen production from coal gasification for effective downstream CO2 capture. Int J Hydrogen Energy 2010;35(10):4933e43. [19] El-Emam RS, Dincer I, Naterer GF. Energy and exergy analyses of an integrated SOFC and coal gasification system. Int J Hydrogen Energy 2012;37(2):1689e97.
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[20] Martelli E, Kreutz T, Consonni S. Comparison of coal IGCC with and without CO2 capture and storage:shell gasification with standard vs. partial water quench. Energy Proc 2009;1(1):607e14. [21] Xu T, Zang G, Chen H, Dou B, Tan C. Co-production of hydrogen and electricity based on coal partial gasification with CO2 capture. Int J Hydrogen Energy 2012;37:11805e14. [22] Ghosh S, De S. Exergy analysis of a cogeneration plant using coal gasification and solid oxide fuel cell. Int J Energy Res 2006;30:647e58. [23] Duan L, Sun S, Yue L, Qu W, Yang Y. Study on a new IGCC (integrated gasification combined cycle) system with CO2 capture by integrating MCFC (molten carbonate fuel cell). Energy 2015;87:490e503. [24] Adams TA, Barton PI. High efficiency power production from coal with carbon capture. AIChE J 2010;50(12):3120e36. [25] Seyitoglu SS, Dincer I, Kilicarslan A. Assessment of an IGCC based trigeneration system for power, hydrogen and synthesis fuel production. Int J Hydrogen Energy 2016;41:8168e75. [26] Ozcan H, Dincer I. Performance evaluation of an SOFC based trigeneration system using various gaseous fuels from biomass gasification. Int J Hydrogen Energy 2015;40(24):7798e807. [27] Karaca H, Ceylan K, Olcay A. Catalytic dissolution of two Turkish lignites in tetralin under nitrogen atmosphere: effects of the extraction parameters on the conversion. Fuel 2001;80:559e64. [28] Yagmur E, Simsek EH, Aktas Z, Togrul T. Effect of demineralization process on the liquefaction of Turkish coals in tetralin with microwave energy: determination of particle size distribution and surface area. Fuel 2005;84:2316e23. [29] Bilgen S, Kaygusuz K, Sarı A. Second law analysis of various types of coal and woody biomass in Turkey. Energy Sources 2004;26:1083e94. [30] Demirbas A. Relationships proximate analysis results and higher heating values of lignites. Energy Sources Part A 2008;30:1876e83. [31] General Directorate of mineral research and exploration. Website: www.mta.gov.tr. [Accessed: 09.04.2015]. [32] U.S Department of energy, reportMajor environmental aspects of gasification-based power generation technologies final report, www.netl.gov. [Accessed: 23.11.2015]. [33] Salkuyeh YK, Adams TA. Combining coal gasification, natural gas reforming, and external carbonless heat for efficient production of gasoline and diesel with CO2 capture and sequestration. Energy Convers Manag 2013;74:492e504. [34] Frey HC, Akunuri N. Development and application of optimal design capability for coal gasification systems: performance, emissions, and cost of Texaco gasifier e based systems using Aspen. Technical report. Carnegie Mellon University; 2001. [35] Dincer I, Rosen MA. Exergy, energy, environment and sustainable development. Oxford: Elsevier; 2007. [36] Kotas TJ. The exergy method of thermal plant analysis. Florida: Krieger; 1995.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Energy and exergy analyses of hydrogen production by coal gasification S.S. Seyitoglu a,*, I. Dincer b, A. Kilicarslan a a
Faculty of Engineering, Department of Mechanical Engineering, Hitit University, Cevre Yolu Bulvarı No: 8, 19030, Corum, Turkey b Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario, L1H 7K4, Canada
article info
abstract
Article history:
In this study, we examine an integrated coal based gasification system developed for
Received 25 February 2016
hydrogen production and power generation. The proposed plant consists of air separation
Received in revised form
unit, gasification unit, gas cooling and cleaning unit, pressure swing absorption (PSA) for
24 July 2016
hydrogen production, high temperature electrolyzer for hydrogen production, Brayton
Accepted 26 August 2016
cycle, steam Rankine cycle and organic Rankine cycle (ORC) system for power generation.
Available online xxx
We investigate this system through energy and exergy analyses for hydrogen production. Six coal types, such as Beypazari, Tuncbilek, Can, Yatagan, Elbistan and Soma are
Keywords:
considered in this study and energy and exergy efficiencies of these coals are compared to
Hydrogen production
each other for assessment and evaluation. The Aspen Plus and Engineering Equation Solver
Coal gasification
(EES) software packages are used for system simulation and system analyses. The results
Energy
show that the overall energy and exergy efficiencies of the entire system become 41% and
Exergy
36.5%, respectively.
Efficiency
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Energy is one of the most important issues in many applications by all means in the world. Nowadays, two of the most common raw materials for energy production are coal and petroleum. The rapid increase in the energy demand and decrease in the fossil fuel resources have forced people to look for alternative energy production methods in recent years. One of those several alternative energy sources is hydrogen energy. The use of gasification is a common method to produce hydrogen from coal.
Furthermore, the world's energy need depending on developing technology and population is increasing day by day. Since the coal reserves are running out as that of many of the other fossil fuel resources, the available resources of coal should be used efficiently. Also, these resources have to be used more efficiently in terms of problems such as global warming and air pollutions [1,2]. Coal is the most abundant and one of the oldest fossil fuel resources in the world. According to World Coal Association, there is 892 million ton coal in the world and these reserves will end after 110 years [3]. On the whole, coal is used for
* Corresponding author. E-mail addresses: [email protected] (S.S. Seyitoglu), [email protected] (I. Dincer), [email protected] (A. Kilicarslan). http://dx.doi.org/10.1016/j.ijhydene.2016.08.228 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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burning for production of energy and need of heating all over the world. Gasification is a thermochemistry transformation process generating gas from coal [4,5]. In other words, solid fuel is converted to gas fuel. The aim of the gasification is to decrease harmful emission occurring when the coal is burned and to increase the fuel's density. The gasification of coal is a wellknown technology for a long time [4]. The syngas occurring by coal gasification is directly in a boiler or applied to various processes with hydrogen production. Also, the syngas that comes out from the reactor can be used in chemical industries and in energy generation. Hydrogen production by coal gasification is an expensive method as it is compared to other fossil fuel production methods [5]. On the other hand, mining and transportation costs of coal are rather low and coal is abundant. Nowadays, 18% of hydrogen production in the world is acquired from coal [6]. For this reason, instead of using burnt coal, using gasified coal is more efficient way in terms of energy and harmful emission. Conversion of lower quality coal types into syngas rather than directly burning them is crucial for environmental sustainability. There are many methods for high purity hydrogen production. One of them is electrolysis of water. This method uses electricity to separate water into hydrogen and oxygen. Electrolysis is a well-known method and this is considered as the simplest method for hydrogen production. The disadvantage of the method for producing hydrogen by electrolysis system is that it uses too much electricity. For this reason, this method is quite expensive. Therefore, this system should be developed. The High Temperature Electrolysis (HTE) system, which is in a developing stage nowadays, is more efficient than other electrolysis systems. As a result, that is a promising hydrogen production method [7]. The HTE uses thermal energy and electricity to separate water. Also, this system uses high temperature (800e1000 C) to improve the performance of electrolysis [7]. The steam enters the cathode side. After that, the steam separate into the hydrogen gas and oxygen ions then the oxygen gas is formed by discharging the oxygen ions that are flowed through the ceramic material. There are many studies related to High Temperature Electrolysis [7e12]. In the HTE method, the energy decreases to resolve water, however the system's efficiency increases. The HTE is used to produce more hydrogen in this system. The energy decreases for the electrolysis of water due to the usage of high temperature steam in the developed system. If systems are operated integrally, the overall system efficiency and output rate are increased. Recycling of high temperature exhaust gas in different systems increases energy production. Thus, emission of valuable gases to the atmosphere is avoided. By the help of these integrated systems, lots of productions are made such as hydrogen production, heating and electric production. There are many studies about coal gasification, coal gasification based hydrogen production systems and coal gasification based multigeneration systems. Prabu and Jayanti [13] investigated underground coal gasification as a hydrogen generator and used solid oxide fuel cell to generate electrical power. They carried out thermodynamic analysis of a combined system. As a result, they have showed the system efficiency yields to 32%. Khalid et al. [14] analyzed a biomass-solar integrated cycle for
multigeneration system. They found that energy and exergy efficiencies are 66.5% and 39.7%, respectively. Ozcan and Dincer [15] investigated energy and exergy analysis of a novel trigeneration system for hydrogen production, heating and power. Eventually, energy and exergy efficiencies of their system were found to be 56.7% and 45.05%, respectively. Herdem et al. [16] studied energy and exergy analysis of a coal gasification based combined system for hydrogen production. They used water gas shift reactor and alkaline electrolyzer in order to produce hydrogen. They demonstrated energy and exergy analysis of their combined system 58% and 55%, respectively. Bicer and Dincer [17] conducted a thermodynamic analysis of the novel integrated system included underground coal gasification, solid oxide fuel cell, integrated gasification combined cycle and electrolyzer for hydrogen production and power. As a result, their results revealed that energy and exergy efficiencies are 29.2% and 26%, respectively. Gnanapragasam et al. [18] studied energy and exergy analysis comparison of integrated coal gasification combined cycle power generation system with coal gasification based hydrogen production system. They showed that coal gasification based hydrogen production system was better than coal gasification based hydrogen production system in terms of energy and exergy efficiencies. El-Emam et al. [19] investigated a coal gasification based cogeneration system including solid oxide fuel cell. They found total energy and exergy efficiencies 38.1% and 27%, respectively. Martelli et al. [20] performed coal gasification based integrated gasification combined cycle contains both with and without carbon dioxide capture and storage. As a result, they showed CO2 capture added to the system reduces the plant efficiency. Xu et al. [21] investigated coal gasification based cogeneration system with CO2 capture system for hydrogen production and electricity. As a result, the exergy efficiency was found to be 54.3%. Ghosh and De [22] examined a coal gasification and solid oxide fuel cell (SOFC) in terms of exergy analysis. They calculated exergy destruction of each component in system. Duan et al. [23] analyzed a new integrated gasification combined cycle together a molten carbonate fuel cell with CO2 capture. They offered three IGCC systems which are conventional IGCC system without CO2 capture, IGCC system with pre-combustion capture and IGCC system with oxyfuel combustion capture. Adams and Barton [24] conducted a new process to produce power with high efficiency. They used SOFC and the gasification process in their system. They found that SOFC based system efficiency was 44.8% while the integrated gasification combined cycle system efficiency was 38.2%. Seyitoglu et al. [25] analyzed an integrated gasification combine cycle based trigeneration system involving hydrogen production, power and various FischereTropsch synthesis products. As a result, they found the energy and exergy efficiencies of the overall system 53% and 46%, respectively. Ozcan and Dincer [26] investigated thermodynamic analyses of an internal reforming solid oxide fuel cell system using three different practical gasifier products. They showed energy and exergy efficiencies of the SOFC system 42.2% and 36.5%, respectively. In this study, a thermodynamic analysis through energy and exergy approaches is performed to investigate system performances for different coal types. The purpose of this paper is to evaluate the low ranking coals for gasification purposes. Also, according to the properties of coals, the system is examined for hydrogen production. In this study, some
Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
specific Turkish lignites are used. Although the coal reserve of Turkey is high, the heating value of those are low. The data were gathered from the coal reserves of six different regions and the simulation tests were carried out based on these data.
System description During the gasification method used in this study, slurry is obtained by initial grinding and powdering of the coal and then mixed with the water. Then the gasification was applied to the slurry with pure oxygen. The resultant gas was then used as a fertilizer formation and energy production due to its similarity to natural gas. The system consists of several components such as an air separation unit (ASU), a coal gasifier, a low and a high temperature water gas shift (WGS) reactor, H2S and CO2 removal units, a pressure swing absorption (PSA) unit, a combustor, high temperature electrolyzer, organic Rankine cycle and steam Rankine cycle including power unit. Fig. 1 shows an illustrative diagram belonging to the multi-generation energy system presented in this study. In the gasification system suggested in this study, GEE entrained flow gasifier is used. Coal slurry having a ratio of 60e70% coal dust was sent to the gasifier along with high purity of oxygen. Nitrogen and oxygen are decomposed from the air inside the ASU. The operation temperature and pressure of the gasifier used in the system is quite high (1250e1600 C and 20e70 bar). Additionally, the gasifier requires high purity of oxygen for the operation [4]. Two types of syngas cooling unit are used as the gasification system such as radiant and convective cooling. Afterwards, syngas is sent to hydrogen production unit. At the hydrogen production unit, the syngas enters the high temperature WGS (HTWGS) and low temperature WGS (LTWGS) reactor respectively. The WGS reactors used in the system converts CO to CO2 with chemical reactions for more hydrogen production. The respective chemical reactions are written as CO þ H2 O4CO2 þ H2
(1)
COS þ H2 O4CO2 þ H2 S
(2)
3
The HTWGS reactor operates at the temperatures of 300e450 C while operating temperature of LTWGS reactor is 120e300 C [16]. After those chemical reactions, the syngas is subjected to the acid gas removal process where H2S and CO2 are removed from it with two stages. After the acid gas removal unit, the high purity of hydrogen is recovered from syngas at the PSA unit. This unit operates at the pressures of 20e30 bars [4]. The purge gas enters to the combustor after the hydrogen recovery. After the PSA unit, the purge syngas is sent to the combustor to produce power. Steam Rankine cycle, organic Rankine cycle and system turbine are used for the power production unit of the designed system. An open feed water heater (OFWH) and reheat are used to improve the efficiency of SRC. The hot gas coming out of the SRC as waste is sent to ORC for the energy generation. R245fa is preferred in ORC. Thus, the waste gas emission of the system occurred at a very low temperature and in a useless form.
System simulation The system simulation was performed with Aspen Plus software package. In this system, the steam cycle was simulated using STEAMNBS method which is International Association for the Properties of steam formulation. The physical properties of the system were predicted using the PengeRobinson EOS with BostoneMathias function (PR-BM) [24]. The system studied is shown in Fig. 2. Organic Rankine cycle, steam Rankine cycle and HTE were simulated with EES (engineering equation solver) software package. R245fa was used as the working fluid in the ORC. The low ranking coals were used in this study. The properties of coals considered in the system are shown in Table 1. Also, the assumptions made for the system analysis and the values with the constants are tabulated in Table 2.
Thermodynamic analysis Thermodynamic analysis of a coal gasification based hydrogen production system is examined for power and hydrogen generation. Some assumptions are made for the energy and exergy analyses of the system. The assumptions made for analysis and assessment are listed below: The changes in kinetic and potential energy are negligible along the system. The integrated system components are analyzed in a steady state and homogeneous regime. The dead state temperature and pressure are 25 C and 1 bar, respectively. The turbines, compressors and pumps are modelled as adiabatic. The heat transfers in the pipe lines are neglected in this system. Pressure drops throughout piping are neglected.
Chemical reactions
Fig. 1 e Main Schematic diagram of the system.
There are some reactions occurring inside the gasifier. The gasification reactor is an equilibrium reactor. Also as a
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Fig. 2 e The combined coal gasification system. chemical equilibrium model Gibbs free energy minimization was adopted. The main chemical equilibrium reactions occurring inside the gasifier can be written [3,4] Combustion reactions;
Cþ
(8)
CO þ H2 O4CO2 þ H2
The CO shift reaction;
1 O2 /CO 2
C þ 2H2 4CH4
The methanation reaction;
(9)
(3) The steam methane reforming reaction;
1 CO þ O2 /CO2 2
(4)
1 H2 þ O2 /H2 O 2
(5)
(10)
CH4 þ H2 O4CO þ 3H2
Energy and exergy analyses
The Boudouard reaction; The water gas reaction;
C þ CO2 42CO
(6)
C þ H2 O4CO þ H2
(7)
The first law of thermodynamic, in other words, energy analysis is the main method to examine the systems. The system energy quantity is measured in energy analysis. This method does not show the realistic results to evaluate system's efficiency. The required energy to evaluate the systems really is usable energy. Usable energy is determined by second
Table 1 e Properties of coals. Coal type
Proximate analysis As received
Beypazarı Tuncbilek Can Yatagan Elbistan Soma
Elemental analysis
Dry
Refs.
Dry
Moisture
Ash
FC
VL
Ash
C (%)
H (%)
N (%)
S (%)
O (%)
Ash
16.4 14.06 11.6 12.6 12.6 15.1
28.8 16.07 15.8 13.6 15.45 13.66
30.62 47.11 41.72 44.06 42.7 45.33
34.92 34.18 40.4 42.34 41.85 41.01
34.5 18.71 17.87 13.6 15.45 13.66
44.93 62.24 54.78 56.42 58.25 61.82
3.28 4.42 4.19 4.41 3.89 4.49
1.05 2.24 1.4 1.3 1.52 1.55
3.21 0.43 5.09 4.75 4.4 3.19
13.03 10.87 16.67 19.53 16.49 15.28
34.5 18.7 17.9 13.6 15.5 13.7
[27] [28] [29] [30] [30] [30]
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Table 2 e The input parameters of the system. Input parameters in the system
Refs.
Water-slurry, % Input coal, kg/s Gasifier temperature, C Gasifier pressure, bar Radiant cooler temperature, C Convective cooler temperature, C O2 purity, %vol Pressure of O2 and N2 delivered by ASU, bar Ratio of O2 inlet pressure to gasifier, to the gasifier pressure NH3 and HCl percentage of removal CO2 final delivery pressure, bar CO2 compressor isentropic efficiency, % Syngas turbine Isentropic efficiency, % HTE water conversion efficiency HTE operation temperature, C Hydrogen content in the hydrogen product, %
60 30 1370 42.4 815 260 99 10 1.2 100 153 85 90 80 900 100
[31] [33] [34] [34]
excoal ch ¼
[32]
m_ in
X
m_ out ¼ Dm_ sys
[16,24]
[16] [16]
m_ in hin
X
(11)
_ sys m_ out hout ¼ DEn
(12)
where hin and hout are the specific enthalpies at the inlet and outlet of the system, respectively. Entropy equation was analyzed with following equation [35]: X
m_ in sin
X
m_ out sout þ S_gen þ
X Q_ T
! ¼ DS_sys
(13)
cr
where sin, sout and S_gen are the specific entropies at the inlet, outlet and generation of the system, respectively. Balance equation for exergy was analyzed with following equation [35]: X
m_ in exin
X
_ Des Ex _ Heat ¼ DEx _ sys m_ out exout Ex
(14)
where exin and exout are the specific exergy at the inlet or outlet _ heat _ Des and Ex of the system components, respectively. Also, Ex are destructed exergy rate and exergy rate due to heat transfer, respectively. The specific exergy is divided into two parts. They can be defined as follows: ex ¼ exph þ exch
(15)
where exph and exch are the physical exergy and chemical exergy, respectively. The physical and chemical exergies can be defined as follows; exph ¼ ðhin hout Þ T0 ðsin sout Þ exch ¼
X
xi ex0ch þ RT0
X
xi lnðxi Þ
(18)
where CCV is net calorific value, w is the moisture content, hfg is the latent heat of water at T0, and s is the mass fraction of sulfur in the fuel. b can be defined in terms of the dry organic substances contained in the coal for an O/C mass ratio less than 0.667 as follows: b ¼ 0:1882
[8] [8]
where m_ in and m_ out are inlet and outlet mass flow rate of the system. Balance equation for energy was analyzed with following equation [35]: X
CCV0 þ w*hfg *b þ 9417s 0
law of thermodynamic by taking the environmental conditions into consideration. In the light of the above information, the performance of all system was carried out using mass, energy and exergy balances. Firstly, balance equation for mass was analyzed with the following equation [35]: X
The chemical exergy equation is chemical exergy for gas components of a chemical gas mixture. The chemical exergy values of each stream are estimated from the standard chemical exergies of the substances. The specific chemical exergy values of gases used in this study are summarized in Table 3. The chemical exergy of the coal is written as [16,19]
(16)
h o n þ 0:061 þ 0:0404 þ 1:0437 c c c
(19)
where c, h, o and n represent the mass fractions of carbon, hydrogen, oxygen and nitrogen, respectively. The energy and exergy efficiencies of all system can be defined as the ratio of total useful output to the system input. General definitions for energy and exergy efficiencies can be written as hen ¼
_ out En _ in En
(20)
hex ¼
_ out Ex _ in Ex
(21)
_ and Ex _ stand for energy and exergy per where subscripts En unit time, respectively. In this study, the total efficiency of the system is defined as the ratio of the total of the lower heating value of the hydrogen produced in the system to the lower heating value of the input coal. The exergy efficiency can be defined as the ratio of the total of the exergy of the produced hydrogen to the exergy of the input coal. The equations of the overall energy and exergy efficiencies of the system can be written as hEn;system ¼
_ H2 LHVH2 m _ coal LHVcoal m
(22)
hEx;system ¼
_ H2 exH2 m _ coal excoal m
(23)
Table 3 e Standard chemical exergy values of different components [36]. Substance
Standard chemical exergy (kJ/kmol)
CO CO2 H2 H2O (liquid) H2O (gas) N2 H2S HCL O2
275,430 20,140 238,490 3120 11,710 720 804,770 85,950 3970
(17)
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_ H2 is the total mass flow rate of hydrogen in the syswhere m _ coal is mass flow rate of the coal fed to the gasifier. tem. Also, m
Results and discussion In this study, the amount of oxygen provided to the gasifier was proportional to the carbon content of the coal type. In the designed system, the producible amount of hydrogen was dependent on the coal types. In addition, depending on the production rates, energy and exergy efficiencies of the system were investigated. Firstly, the synthesis gas obtained by the gasification of the coal, it was sent to the gas cooling unit and then sent to the WGS reactors for hydrogen production. Hydrogen content of the syngas is enriched in HTWGS and LTWGS reactors. Enriched syngas is sent to the acid gas removal unit in order to separate H2S and CO2 from syngas. Thus, harmful gases in the syngas are removed. Then, the syngas is sent to the PSA unit and H2 is produced. The purge gas is sent to the combustor for the energy production. In this study, variation of H2 and CO ratio in syngas with respect to the gasifier temperature, gasifier pressure and slurry ratio is investigated. Finally, along with their energy and exergy efficiencies, the maximum hydrogen yields of various types of coal were compared. Exergy efficiencies of the system elements were calculated by using the highest hydrogen yielding coal type. According to the analyses, the highest amount of electricity was consumed in the system components such as air compressor, air separation unit and MAC. In system, there was a CO2 removal and storage unit, too. This section of the system was not modelled elaborately. Based on the collected values from the literature, the electric consumption of the capture and storage unit was determined. H2eCO ratios of the syngas based on the slurry ratio are given in Fig. 3. There, Tuncbilek coal was analyzed. Gasifier pressure and temperature are assumed to be 42.4 bar and 1371 C, respectively. It is observed that as the water ratio in the slurry increased, H2 and CO ratio decreased. This ratio affected the rate of hydrogen production and consequently affected energy and exergy efficiencies. In Fig. 4, the correlation between the pressure of the gasifier and the H2eCO ratio in the syngas is illustrated. In this analysis, Tuncbilek lignite was preferred. It is assumed that the gasifier temperature was 1371 C and kept constant during the analysis. As it can be inferred from Fig. 4 that, as the gasifier pressure increases, H2 and CO rates slightly decrease. The results shown in Fig. 5 are obtained by varying the gasifier temperature from 1300 C to 1500 C. Gasifier pressure is kept constant at 42.4 bar. Tunc¸bilek lignite was preferred. As it can be inferred from Fig. 5, as the gasifier temperature increases, H2 rate decreases while CO rate increases. In Fig. 6, breakdown of the generated and consumed electric energy among the system elements are given. As it can be seen, MAC and ASU are the largest electric consumers in the system. Electricity is generated by system turbine and steam Rankine cycle. In addition, an SMR was also added to the system in order to increase electric outcome by recovering waste energy to generate electric energy.
Fig. 3 e The effect of slurry ratio on the syngas mole fraction.
Fig. 4 e The effect of gasifier pressure on the syngas mole fraction.
Fig. 5 e The effect of gasifier temperature on the syngas mole fraction.
Fig. 7 depicts the exergy efficiencies of the system elements. ASU and combustor were the least efficient elements in the system. The ASU's low exergy efficiency was due to its working principle while that of the combustor was due to the chemical reactions that occur during the combustion.
Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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Fig. 6 e Performance of the system component for Tuncbilek coal.
Fig. 7 e The exergy efficiencies of the system components.
Fig. 8 e Hydrogen production of the coal gasification system. Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228
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Fig. 9 e Energy and Exergy efficiencies of the system for considered coals. The amount of hydrogen that could be produced by the system is given in Fig. 8. The unit of the figure is kg/h. Hydrogen was of high purity and produced by PSA unit and high temperature electrolyzer. According to the Fig. 8, Tuncbilek coal was able to provide the highest hydrogen yield among all types of coal. The overall energy and exergy efficiencies of the system with respect to the generated outputs and coal amounts loaded are given in Fig. 9. The Tuncbilek coal, which is followed by Soma coal, provided the highest energy and exergy efficiency.
Nomenclature _ En _ Ex _ m P Q_ T _ W
Energy rate, kW Exergy rate, kW Mass flow rate, kg s1 Pressure, Bar Heat transfer rate, kW Temperature, K Work rate, kW
Greek letters h Efficiency
Conclusions This study is presented to provide a thermodynamic analysis, through energy and exergy approaches, of coal gasification in an integrated system fashion. This system achieves the production of hydrogen with PSA unit and High Temperature Electrolysis. The present system requires no electricity input to the system. The system uniquely includes CO2 capture and removal which makes this system unique and environmental friendly. The sub-systems MAC and ASU spend the all electricity. The steam Rankine cycle and organic Rankine cycle provide more electricity by using exhaust gases so it affects system's efficiency. Thermodynamic performance of the low ranking coals is conducted for the system and its components. The hydrogen produced by the system is achieved at the highest level. According to the hydrogen production of this system, Tuncbilek coal shows the best performance as their values from PSA and HTE appears to 8640 kg/h and 324 kg/h, respectively. Also, Soma coal has the second best performance as it is compared to the other types of coals used in the gasification system. When the performance of the system using Tuncbilek as a coal is compared to the other types of coals used in the system, the highest overall system energy and exergy efficiencies appear to be 41% and 36.5%, respectively.
Subscripts ch Chemical des Destruction h Enthalpy HX Heat exchanger in Inlet out Outlet Ref Reference Ph Physical Sys System 0 Ambient or reference condition Acronyms FC Fixed carbon HTE High Temperature Electrolyzer LHV Lower heating value HT High Temperature LT Low temperature VL Volatile Matter
references
[1] World Energy Outlook. Website: http://www. worldenergyoutlook.org. [Accessed: 10.01.2016].
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e9
[2] Ghosh S, De S. Thermodynamic performance study of an integrated gasification fuel cell combined cycle-an energy analysis. J Power Energy 2003;217(2):137e47. [3] World Coal Association. Website: http://www.worldcoal.org/ coal. [Accessed 15.02.2016]. [4] Higman C, Burgt MVD. Gasification. 2nd ed. Elsevier Inc; 2008. [5] Acar C, Dincer I. Comparative assessment of hydrogen production methods from renewable and non-renewable sources. Int J Hydrogen Energy 2014;39:1e12. [6] Miller BG. Coal energy systems. Elsevier Inc; 2005. [7] Balta MT, Dincer I, Hepbasli A. Thermodynamic assessment of geothermal energy use in hydrogen production. Int J Hydrogen Energy 2009;34:2925e39. [8] Mansilla C, Sigurvinsson J, Bentemps A, Marechal A, Werkoff F. Heat management for hydrogen production by high temperature steam electrolysis. Energy 2007;32:423e30. [9] Sigurvinsson J, Mansilla C, Lovera P, Werkoff F. Can high temperature steam electrolysis function with geothermal heat. Int J Hydrogen Energy 2007;32:1174e82. [10] Fujiwara S, Kasai S, Yamauchi H, Yamada K, Makino S, Matsunaga K, et al. Hydrogen production by high temperature electrolysis with nuclear reactor. Prog Nucl Energy 2008;50:422e6. [11] Mingyi L, Bo Y, Jingming X, Jing C. Thermodynamic analysis of the efficiency of high-temperature steam electrolysis system for hydrogen production. J Power Sources 2008;177(2):493e9. [12] Herring JS, O'Brien JE, Stoots CM, Hawkes GL, Hartvigsen JJ, Shahnam M. Progress in high-temperature electrolysis for hydrogen production using planar SOFC technology. Int J Hydrogen Energy 2007;32(4):440e50. [13] Prabu V, Jayanti S. Integration of underground coal gasification with a solid oxide fuel cell system for clean coal utilization. Int J Hydrogen Energy 2012;37(2):1677e88. [14] Khalid F, Dincer I, Rosen MA. Energy and exergy analyses of a solar-biomass integrated cycle for multigeneration. Sol Energy 2015;112:290e9. [15] Ozcan H, Dincer I. Thermodynamic analysis of a combined chemical looping based trigeneration system. Energy Convers Manag 2014;85:477e87. [16] Herdem MS, Farhad S, Dincer I, Hamdullahpur F. Thermodynamic modelling and assessment of a coal gasification and alkaline water electrolysis system for hydrogen production. Int J Hydrogen Energy 2014;39(7):3061e71. [17] Bicer Y, Dincer I. Energy and exergy analyses of an integrated underground coal gasification with SOFC fuel cell system for multigeneration including hydrogen production. Int J Hydrogen Energy 2015;40(39):13323e37. [18] Gnanapragasam NV, Reddy BV, Rosen MA. Hydrogen production from coal gasification for effective downstream CO2 capture. Int J Hydrogen Energy 2010;35(10):4933e43. [19] El-Emam RS, Dincer I, Naterer GF. Energy and exergy analyses of an integrated SOFC and coal gasification system. Int J Hydrogen Energy 2012;37(2):1689e97.
9
[20] Martelli E, Kreutz T, Consonni S. Comparison of coal IGCC with and without CO2 capture and storage:shell gasification with standard vs. partial water quench. Energy Proc 2009;1(1):607e14. [21] Xu T, Zang G, Chen H, Dou B, Tan C. Co-production of hydrogen and electricity based on coal partial gasification with CO2 capture. Int J Hydrogen Energy 2012;37:11805e14. [22] Ghosh S, De S. Exergy analysis of a cogeneration plant using coal gasification and solid oxide fuel cell. Int J Energy Res 2006;30:647e58. [23] Duan L, Sun S, Yue L, Qu W, Yang Y. Study on a new IGCC (integrated gasification combined cycle) system with CO2 capture by integrating MCFC (molten carbonate fuel cell). Energy 2015;87:490e503. [24] Adams TA, Barton PI. High efficiency power production from coal with carbon capture. AIChE J 2010;50(12):3120e36. [25] Seyitoglu SS, Dincer I, Kilicarslan A. Assessment of an IGCC based trigeneration system for power, hydrogen and synthesis fuel production. Int J Hydrogen Energy 2016;41:8168e75. [26] Ozcan H, Dincer I. Performance evaluation of an SOFC based trigeneration system using various gaseous fuels from biomass gasification. Int J Hydrogen Energy 2015;40(24):7798e807. [27] Karaca H, Ceylan K, Olcay A. Catalytic dissolution of two Turkish lignites in tetralin under nitrogen atmosphere: effects of the extraction parameters on the conversion. Fuel 2001;80:559e64. [28] Yagmur E, Simsek EH, Aktas Z, Togrul T. Effect of demineralization process on the liquefaction of Turkish coals in tetralin with microwave energy: determination of particle size distribution and surface area. Fuel 2005;84:2316e23. [29] Bilgen S, Kaygusuz K, Sarı A. Second law analysis of various types of coal and woody biomass in Turkey. Energy Sources 2004;26:1083e94. [30] Demirbas A. Relationships proximate analysis results and higher heating values of lignites. Energy Sources Part A 2008;30:1876e83. [31] General Directorate of mineral research and exploration. Website: www.mta.gov.tr. [Accessed: 09.04.2015]. [32] U.S Department of energy, reportMajor environmental aspects of gasification-based power generation technologies final report, www.netl.gov. [Accessed: 23.11.2015]. [33] Salkuyeh YK, Adams TA. Combining coal gasification, natural gas reforming, and external carbonless heat for efficient production of gasoline and diesel with CO2 capture and sequestration. Energy Convers Manag 2013;74:492e504. [34] Frey HC, Akunuri N. Development and application of optimal design capability for coal gasification systems: performance, emissions, and cost of Texaco gasifier e based systems using Aspen. Technical report. Carnegie Mellon University; 2001. [35] Dincer I, Rosen MA. Exergy, energy, environment and sustainable development. Oxford: Elsevier; 2007. [36] Kotas TJ. The exergy method of thermal plant analysis. Florida: Krieger; 1995.
Please cite this article in press as: Seyitoglu SS, et al., Energy and exergy analyses of hydrogen production by coal gasification, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.228