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Hydrogen Energy Engineering Applications and Products
Chapter 5
Hydrogen Energy Engineering Applications and Products Hirohisa Uchida1, Makoto R. Harada2 1
Professor, School of Engineering, Tokai University/President & CEO, KSP Inc., Japan; Research Adviser, National Institute of Advanced Industrial Science and Technology (AIST), Research Institute for Chemical Process Technology 2
INTRODUCTION This chapter describes the engineering application of hydrogen energy. Applications of hydrogen energy technologies fall into four categories: hydrogen production, storage, transportation, and stationary use. Each field has undergone independent development. Hydrogen production has a long history. The hydrogen production from fossil fuels exceeds a 100-year history. From the 1900s, research and development for mass production, advanced industrial hydrogen production, and progress has been continued to the present. This technology is currently making a great contribution to hydrogen energy technology and has future prospects of additional progress, including hydrogen production from biomass. Hydrogen storage is an indispensable technology for fuel cell mobile applications. Various techniques for storing hydrogen, such as hydrogen storage alloys, high-pressure hydrogen gas storage, liquid hydrogen storage, and the like, have been developed and are currently being used at hydrogen refueling stations. These technologies have enriched the prospects for fuel cell mobile applications, including current fuel cell vehicles. Hydrogen station deployment is actively being carried out in East Asia, Japan, South Korea, and China, as well as in the United States and Europe. Transportation of hydrogen has a very important significance for the supply of feedstock. Currently, research on how to transport hydrogen is under way. Currently, two major projects are progressing in Japan. The first one is a development plan to transport liquid hydrogen by ship, and the other one is hydrogen production using organic hydrides. Stationary fuel cell applications have initially unveiled hydrogen utilization by the society. But at present, mobile/transportation fuel cells, typified by fuel cell vehicles, forklift, buses, and cars, are taking the lead. Also, due to the Science and Engineering of Hydrogen-Based Energy Technologies. https://doi.org/10.1016/B978-0-12-814251-6.00009-5 201 Copyright © 2019 Elsevier Inc. All rights reserved.
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massive production of hydrogen, hydrogen combustion in turbines for power generation will soon start, inducing an expansion of hydrogen energy applications. Concerning vehicles, Toyota first commercialized Mirai in 2014, then Honda began selling Clarity in 2016. Subsequently, Toyota began selling buses in March 2018. In 2019 fuel cell trucks are planned to be sold, and in the near future demand for transportation fuel cells will grow steadily. Sales of forklifts have also started in 2016. As described above, development of hydrogen utilization technology is progressing continuously with the goal of being significantly implemented by 2030, and this trend will continue. Even from a global view, many countries are working on applied technologies of hydrogen energy, fuel economy, testing cogeneration power generation with stationary fuel cells for several world metropolitan cities. Technology development and deployment road maps focus on long-term schedules, up to 2050. Application technology of hydrogen energy has dramatically improved in the past decade, and it is speculated that this trend will continue in the future. In this chapter, the main features concerning applied technology of hydrogen energy are described.
Chapter 5.1
Hydrogen Production Technology From Fossil Energy INTRODUCTION Hydrogen (H2) production from fossil fuels has a long history and has been industrialized since the beginning of the 20th century. Various reaction types or plants have been proposed, but here we will summarize the technically established methods. Hydrogen is presently artificially manufactured in large scale. For this reason, various technological developments have been carried out so far. It has mainly been produced from the electrolysis of water and from fossil fuels [1e3], as will be described here. Hydrogen as an energy carrier is usually found in the state of a compound bonded to other elements. Therefore, for hydrogen to become an energy source, the hydrogen compound must be efficiently decomposed with the use of some external energy. Hydrogen is a well-known and versatile energy carrier that has been produced in large scale, with global amounts exceeding 55 Mt, mainly from
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fossil fuels [4]. Hydrogen production methods such as steam reforming, partial oxidation and auto-thermal reforming, ATR, are used, however, the steam reforming of natural gas is mostly employed, making it less expensive than other procedures [5]. Hydrogen atoms of natural gas are separated into hydrogen molecules in the course of the reaction, and at the same time, carbon dioxide is produced as a by-product. For this reason, the development of CO2 -free hydrogen by renewable energy has been advanced [6]. Here we will explain the hydrogen production method from fossil fuel by explaining the hydrogen production reaction, the reaction process, the catalyst, and so on.
CHARACTERISTICS OF HYDROGEN PRODUCTION PROCESSES First, the various hydrogen production methods will be explained, including steam reforming using steam reforming reaction (partial reforming), partial oxidation using oxygen as an oxidizing agent, and autothermal utilization of reaction heat generated when hydrogen is produced. A combined reforming method combines a steam reforming method and an ATR method [7]. As shown in Fig. 5.1, energy, including hydrogen, can be manufactured industrially from various fossil fuels through various hydrogen production methods. These processes are already established industrially. In this chapter, the technology of production of hydrogen from fossil fuels is discussed, mainly the steam reforming reaction, which is a hydrogen production process also used for fuel cell systems.
FIGURE 5.1 Relationship between types of fossil fuels and various hydrogen production processes.
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Hydrogen Production Reaction Catalytic Steam Reforming The steam reforming process has been carried out for more than 100 years and is the most historically old and industrially constructed process. Therefore, since this hydrogen production method has been described in many places so far, we will first describe the reaction, and then explain items deeply related to hydrogen production such as catalyst and poisoning. It is thought that the steam reforming reaction proceeded simultaneously with the following two reactions. CH4 þ H2O / CO þ 3H2 206 kJ/mol
(5.1)
(5.2) CO þ H2O / CO2 þ H2 þ 41 kJ/mol Eq. (5.2) is called a shift reaction, which is a CO concentration reduction reaction that generates CO2 from CO. ALthough CO2 has become a global warming problem, there is also a reaction called carbon dioxide reforming reaction: CH4 þ CO2 / 2CO þ 2H2 248 kJ/mol
(5.3)
This reaction is used in research to convert H2 and CO2 into useful compounds, which has been drawing attention in recent years. Since this reaction is an endothermic reaction larger than the steam reforming reaction and since carbon deposition is liable to occur in the catalyst layer, a catalyst capable of suppressing carbon deposition currently has to be used. Fig. 5.2 shows a
FIGURE 5.2 Radiant wall-type reformer. Based on Miyasugi et al., J. Jpn. Petrol. Int. 25 (1982) 260.
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schematic diagram of this reactor, but it is an energy-consuming reactor that heats a catalyst bed reactor with several burners from the outside.
Partial Oxidation In this method, oxygen (O2) is used as an oxidizing agent, and hydrogen is produced by utilizing a partial oxidation reaction of a fossil fuel as a feedstock. The amount of O2 to be introduced is an amount corresponding to 30%e50% of the theoretical oxygen amount required for complete combustion. When a flame occurs, CO2 is introduced and the flame is controlled. Most types of fuels can be used. This method is characteristic of a noncatalytic reaction. The partial oxidation reaction formula is m n Cm Hn þ O2 /mCO þ H2 (5.4) 2 2 In case the fuel is methane, it becomes 1 CH4 þ O2 / CO þ 2H2 þ 35.7 kJ=mol 2
(5.5)
Fossil fuel and O2, which is the oxidizer, as well as steam, are injected from the burner to perform partial oxidation reaction. In the partial oxidation reaction, slight carbon deposition occurs. Normally, about 3 wt% of carbon is precipitated with respect to the fuel. Carbon in this product gas is removed as a carbon slurry by water quenching or water washing. Fig. 5.3 shows a
O2
Feedstock
Steam
Syn Gas
Cooler Condenser
FIGURE 5.3 Partial oxidation hydrogen production secondary reformer. Based on Dry et al., Industrials chemicals via C1 processes, ACS Symp. Ser. 328 (1987) 18.
206 Science and Engineering of Hydrogen-Based Energy Technologies
schematic diagram of this reactor. Although it sometimes fills ceramic balls with very high heat resistance, it is fundamentally a very simple structure [7].
Autothermal Reforming ATR is a hydrogen production method combining a partial oxidation method and a steam reforming method. At the inlet of the catalyst layer, O2 (which is an oxidizing agent), fuel, and steam are introduced; part of the fuel is burned; and the steam reforming reaction is carried out by utilizing the reaction heat. Therefore, the partial oxidation reaction mainly occurs on the upstream side of the reactor and the steam reforming reaction mainly occurs at the downstream side of the reactor. The overall reaction is called ATR reaction. In the ATR method, the following reaction occurs. Partial oxidation reaction: CH4 þ 3/2O2 / CO þ 2H2O þ 519 kJ/mol (5.6) Steam reforming reaction: CH4 þ H2O / CO þ 3H2 206 kJ/mol Shift reaction: CO þ H2O / CO2 þ H2 þ 41 kJ/mol
(5.7) (5.8)
O2 is used as the oxidizing agent as in the partial oxidation reactor described earlier, as the temperature at the inlet of the reactor reaches 1000 C or higher. For this reason, a metallic reactor cannot be used. The inner wall of the reactor of the partial oxidation reaction and ATR reaction is coated with refractory brick or refractory cement and a reactor with high heat resistance is used. Fig. 5.4 shows a schematic diagram of this reactor.
Combined Reforming This process is used in the synthesis of ammonia and methanol. Part of fossil fuel as a primary reformer is a method of decomposing fuel using the steam reforming reaction method and then reforming it by the ATR method as a secondary reforming. Fig. 5.5 shows a schematic diagram of this reactor, but basically it is a combination of the steam reforming and ATR reactors. In the secondary reforming, when synthesizing ammonia, air is used as an oxidant, and the gas composition at the ATR reactor outlet is adjusted such that the molar ratio of H2 and N2 required for ammonia synthesis is 1:3. Since the oxidizing agent of the ATR method is air, O2 from the air is utilized, and an expensive oxygen separating apparatus is unnecessary. For this reason, a gas for ammonia synthesis can be produced economically. On the other hand, when methanol synthesis is carried out, O2 is used as an oxidizing agent. The gas composition at the outlet of the ATR reactor has a molar ratio of H2 and CO required for methanol synthesis of 2:1, and it is possible to obtain a gas easily meeting methanol synthesis reaction conditions. This chemical reaction formula is a partial oxidation reaction formula. CH4 þ 1/2O2 / CO þ 2H2 þ 35.7 kJ/mol
(5.9)
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FIGURE 5.4 Autothermal hydrogen production secondary reformer. Based on B. Glo¨ckler, A. Gritsch, A. Morillo, G. Kolios, G. Eigenberger, Autothermal reactor concepts for endothermic fixed-bed reactions, Chem. Eng. Res. Des. 82 (2) (February 2004) 148e159.
Features of the four hydrogen production methods are summarized in Table 5.1. Table 5.2 also summarizes the industrial conditions for producing each product including hydrogen [8].
Industrial Hydrogen Production Process The history of hydrogen production catalysts used for the steam reforming method, partial oxidation method, and others is very old, and even in the experiment note of Michael Faraday, hydrogen, CO, CO2, and the like are generated when steam is brought into contact with heated coal. In these notes, there is a description that white smoke is adsorbed on a metal such as Ni, and the phenomenon of a predictable catalyst is noted. In order to produce hydrogen from fossil fuel, it is essential that the fossil fuel gas component and water vapor are adsorbed to the catalyst; the fossil fuel, which is a hydrocarbon, decomposes and reacts comprehensively with the oxygen molecules in the water vapor. Also, considering chemical reactions, production of CO is essential. Therefore, in the hydrogen production process,
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FIGURE 5.5 Combined reforming reactor. Based on H. Ebrahimi, A. Behroozsaranda, A. Zamaniyan, Arrangement of primary and secondary reformers for synthesis gas production. Chem. Eng. Res. Des. 88 (10) (October 2010) 1342e1350.
a shift reactor is always arranged downstream of the reforming process. Fig. 5.6 shows the industrial hydrogen production process by the steam reforming process. Temperature, pressure, and gas concentration shown in the process are typical examples of the plant actually being operated. Other hydrogen production methods such as the partial oxidation method only replace the steam reforming reactor with other reactors, and there is no change in the process [9]. The hydrogen production process is divided into four steps: a desulfurization step for removing sulfur compounds, a reforming step for producing hydrogen, a shift step for reducing the concentration of carbon monoxide, and a purification step for removing impurities. These four steps will be described.
Desulfurization Step H2S is contained in natural gas, and sulfide typified by methyl mercaptan is contained in a gas state in other hydrocarbon fuels. This sulfur compound adsorbs to the reforming catalyst on the downstream side and the catalyst for low temperature carbon monoxide shifts to lower the activity. In order to
Partial Oxidation System
Autothermal Reforming System
Natural gas, LPG, naphtha up to C6 saturated hydrocarbons. Unsaturated hydrocarbons are difficult to use.
Hydrocarbons, heavy oil, Town gas
Natural gas, LPG, naphtha
Natural gas, LPG, naphtha, saturated fossil fuel, and fossil fuel with few unsaturated hydrocarbons
Reactor type
External heat exchangeetype fixed bed reactor
Adiabatic fixed bed reactor
Adiabatic fixed bed reactor
(Type- I) Two-stage external heat exchangeretype reactor or (type-II) isothermal steam reforming reactor and autothermal reformingetype reactor
Main chemical reaction
Catalytic steam reforming reaction CH4 þ H2O / CO ＋ H2 206 kJ/mol shift reaction CO þ H2O / H2 þ CO2 þ 41 kJ/mol
Catalytic partial oxidation reaction CH4 ＋ 1/2O2 / CO þ H2 þ 38 kJ/mol
Partial oxidation reaction and catalytic steam reforming reaction
Catalytic steam reforming reaction
Oxygen plant
Not required
Required
Required
Not required for type-I, but required for type-II
Process
Steam Reforming System
Available fossil fuel
Two-Stop Reforming System
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Continued
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TABLE 5.1 Hydrogen Production Methods from Fossil Fuels
Process
Steam Reforming System
Partial Oxidation System
Autothermal Reforming System
Catalyst
Ni/Al2O3, Ru/Al2O3
Ni/Al2O3, Rh/Al2O3
Ni/Al2O3, Ni/MgAl2O4, Rh/Al2O3,
Ni/Al2O3
Operating conditions
Maximum bed temperature 750e950 C
Maximum bed temperature 900e1150 C
Maximum bed temperature 900e1150 C
First steam reformer bed temperature 500 C (type-I)
Temperature and pressure
Reactor pressure 1.5e3 MPa
Reactor pressure 3e7 MPa
Reactor pressure 2e6 MPa
Second steam reformer bed temperature 750e950 C Reactor pressure 1.5e3 MPa (type-II) Second reactor max. bed temperature 900e1150 C Reactor pressure 2e6 MPa
Hydrogen/CO ratio
2.8e4.8 (mol/mol)
1.7e2.0 (mol/mol)
1.8e3.8 (mol/mol)
2.2e4.4 (mol/mol)
Hydrogen production scale per series
800e80,000 Nm3/h
7000e70,000 Nm3/h
10000e7,50,000 Nm3/h
50,000e3,00,000 Nm3/h
Two-Stop Reforming System
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TABLE 5.1 Hydrogen Production Methods from Fossil Fuelsdcont’d
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TABLE 5.2 Examples of Industrial Conditions for the Production of Pure Hydrogen and of Hydrogen for the Production of Other Chemical Components H2 for Ammonia Production
H2 for Methanol Productiona
H2 for Syngas Productionb
Condition
H2
Feedstock
Natural gas
Natural gas
Natural gas
Natural gas
Pressure
40
34
16.5
27
H2O/C (mol/atom)
6.5
3.5
2.5
0.6
CO2/C (mol/atom)
0
0
0.3
0.1
Space velocity (volCH4/volcat)
700
1170
1425
e
Maximum temperature of Reference
850
795
950
1050
H2/CO (product)
4.4
4.5
2.1
2
a
W.D. Deckwer, et al., Kinetics studies of Fischer-Tropsch synthesis on suspended iron/potassium catalyst-rate inhibition by carbon dioxide and water, Ind. Eng. Chem. Process Des. Dev. 25(3) (1986) 643e649. b I. Dybkjaer, T.S. Christensen, Syngas for large scale conversion of natural gas to liquid fuels, Stud Surf. Sci. Catal. 136 (2001) 435.
avoid sulfur poisoning, desulfurization is carried out. The sulfur compound is converted to H2S by the hydrodesulfurization catalyst and absorbed by ZnO. Through this catalytic reaction, it is possible to reduce sulfur compounds to about 20e50 ppb.
Reforming Reaction Process Since it has already been explained in the previous section, the reaction is different between the primary reformer and the secondary reformer, although omitted here. Particularly in the primary reformer when the molecular weight of the hydrocarbon is increased, carbon deposition tends to occur easily. Shift Reaction Process The composition of the reformed gas at the reforming process outlet is 72% hydrogen, 3% methane, 13% Carbon monoxide, and 12% Carbon dioxide. Since Carbon monoxide is toxic and we wish to reduce the concentration and increase H2 concentration, the shift reaction shown in Eq. (5.2) is generated. The high temperature Carbon monoxide transformer operates at 300 Ce350 C
212 Science and Engineering of Hydrogen-Based Energy Technologies Air
CO Conversion
Steam
Primary Reformer (Ni/Al2O3) Secondary Reformer (Ni/Al2O3)
600°C Furnace
Desulfuriration MoCo-S ZnO
300350°C Fe3O4 WGS Catalyst
800°C
Nat. gas
CH4 =9-14%
200250°C
CuO WGS Catalyst
9001100°C Steam CH4 =0.5-1.5% CO =10-13%
CO =2-3%
CO =0.2-0.5% CO2 CO2 removable
CO2 0.1% Methanation Ni/Al2O3
H2 99%
Drain
Drain
FIGURE 5.6 Hydrogen production process diagram, operating conditions: S/C ¼ P(H2O)/ P(CH4) ¼ 2.5e4.0, T(exit) ¼ 900e1100 C, P (exit) ¼ 2e3 MPa. From C.H. Bartholomew, R.J. Farrauto, Hydrogen production and synthesis gas reactions, in: Fundamentals of Industrial Catalytic Processes, second ed., John & Wiley Sons, Inc., 2006, pp. 339e371 (chapter 6).
and reduces the outlet Carbon monoxide concentration to about 2%e3%. Then, we operate the low temperature Carbon monoxide transformer packed with Cu-based catalyst at about 200 C and reduce the outlet Carbon monoxide concentration from 0.2% to 0.4%. The Carbon monoxide shift reaction is advantageous at lower temperatures, but the reaction rate is slower.
Purification Step Conventionally, this was a system using decarboxylation with an amine type absorbent and a methanation reaction with a Ni/geAl2O3 catalyst, but recently it was cooled down to room temperature and passed through a pressure swing adsorption device, whereby a purity of 99.9% of hydrogen is obtained. The results are shown in Table 5.3, of representative catalysts used for desulfurization, reforming, CO shift, and methanation reaction.
THERMODYNAMICS The steam reforming reaction, which is a hydrogen production reaction, and the cooccurring shift reaction are chemical reactions dominated by equilibrium. The equilibrium constant of each reaction is shown in Table 5.4 at
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TABLE 5.3 Industrial Hydrogen Production Reaction and Typical Catalyst Process
Reaction
Industrial Typical Catalyst
Hydrode sulfurization
ReH þ 2H2 / H2S þ HeReH
CoMo/Al2O3
Adsorption
ZnO þ H2S / ZnS þ H2O
ZnO
Primary reforming
HC þ H2O / H2 þ CO þ CO2 þ CH4
Ni/MgO
Secondary reforming
2CH4 þ 3H2O / 7H2 þ CO þ CO2
Ni/MgAL2O4, Ni/CaAl2O4
High-temperature CO shift
CO þ H2O / H2 þ CO2
Fe3O4/Cr2O3
Low-temperature CO shift
CO þ H2O / H2 þ CO2
Cu/ZnO/Al2O3
Methanation
CO þ 3H2 / CH4 þ H2O
Ni/Al2O3
TABLE 5.4 Equilibrium Constants
KRa
K Sb
0
0.066 l029
4.555 105
100
0269 l019
3.587 103
200
0.487 l013
2299 102
300
0.682 109
3.973 10
400
0.618 106
1.192 10
500
1.021 104
4.999
600
0.873 103
3.53
700
1
1.305 10
1.541
800
1.759
1.059
900
1.533 10
7.692 101
1000
0.955 102
5.923 101
a
C
Steam reaction equilibrium constant. Shift reaction equilibrium constant.
b
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temperatures up to 1,000 C. In the steam reforming reaction, the equilibrium constant rises as the temperature rises. This is because the steam reforming reaction is an endothermic reaction [10]. The equilibrium constant is very small up to about 400 C, indicating that the reaction hardly proceeds in terms of equilibrium. The inlet temperature of the industrial reformer is set to 450 C or higher, but this equilibrium constant is determined. The fact that the equilibrium constant is small means that the target amount of hydrogen is small, so that it is industrially meaningless to react at 450 C or lower. Since the equilibrium constant of the steam reforming reaction exceeds 1 at 700 C or higher, it is understood that the objective hydrogen production can be performed smoothly. Therefore, the maximum temperature of the catalyst layer in any reformer is 700 C or more. In addition, since the steam reforming reactor is reacted at a high pressure of about 2e3.5 MPa, it is disadvantageous for hydrogen production in equilibrium, so the maximum temperature of the catalyst layer is set as high as 800 C e1000 C. On the other hand, the shift reaction decreases as the temperature rises. Therefore, the shift reaction proceeds at a low temperature, and the reverse shift reaction proceeds at a high temperature of 800 C or higher. Therefore, in the reformer outlet gas, CO and CO2 are detected. CO is processed by a shift reactor installed on the downstream side as shown in Fig. 5.7. The hydrogen production reaction is an equilibrium reaction according to thermodynamics.
(A)
(B)
100
(C)
90
Conversion (%)
80 70 60 50 40 30 20
600 650 700 750 800 850 900
Temperature (°C)
600
650 700
750
800
Temperature (°C)
850
900
600 650 700 750 800 850 900
Temperature (°C)
FIGURE 5.7 Simulation results of conversion of steam reforming reaction of methane. Operating conditions (A) 0.1 MPa, (B) 0.5 MPa, (C) 1 MPa, S/C ¼ 2, , S/C ¼ 3, > S/C ¼ 4, S/C ¼ steam to carbon ratio.
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The higher the pressure and the higher the steam to carbon ration, S/C, the higher the conversion. Using this property, the stationary fuel cell system operates at a low pressure, improves the methane conversion rate, and improves the operation efficiency of the plant.
INDUSTRIAL CATALYST DESIGN An overview of industrial hydrogen production catalyst is outlined. Hydrogen production catalysts are used industrially for the production of methanol and ammonia. Technologies are also established. Especially with old steam reforming catalysts, in addition to Ni, precious metals such as Rh and Ru are sometimes used and are commercialized. Industrially, since the Ni-type catalyst is used most frequently, the components and compositions of representative Ni-type catalysts used industrially are shown in Table 5.5. Although some alkaline earth metals, which are basic oxides, have been introduced as additives (as promoters), there was a catalyst that had previously been added with alkali metals. However, at present, no alkali metal is used as a conversion agent since the alkali metal gradually leaves the catalyst and causes corrosion of the metal piping on the downstream side of the reactor, which causes a failure of the plant. On the other hand, alumina is mostly used as a catalyst carrier, but there are catalysts in which MgAl2O4 or MgO having a spinel crystal structure is used. Hydrogen production is made at 800 C or more, in some cases 1000 C or more, so high temperature heat resistance is required [11]. It is also required to have resistance against carbon deposition described later or resistance to sulfur poisoning. After past decades of development, the present industrial catalyst has been established. Therefore, it seems that the current catalyst will continue to be used in the future.
DEACTIVATION Carbon Formation Carbon deposition is considered to be an inevitable phenomenon in hydrogen production reactions. Carbon deposition occurs in the steam reforming reaction depending on operating conditions. Precipitated carbon is classified into three types. Table 5.6 describes these carbons [12]. The most typical carbon deposition is called whisker carbon and occurs on the surface of the Ni catalyst. Carbon deposition amount increases due to diffusion of C in the fuel. In addition, there are carbon deposits in which carbon is generated as a core of the feedstock radical called encapsulating polymers. This phenomenon is seen as the activity gradually decreases. It is a phenomenon observed at a low temperature of 500 C or less. The third carbon deposition is called pyrolytic carbon, which is generated by thermal decomposition of hydrocarbons as a
Catalyst Composition wt% Feedstock
NiO
A12O3
MgO
MgAl2O4
CaO
SiO2
CeO2
Natural gas
12
88
Natural gas
20
80
Natural gas
20
Natural gas
20
Natural gas
20
Natural gas
30
20
LPG
30
30
40
LPG
35
30
35
Naphtha
80
20
Naphtha
25
25
10
Naphtha
40
30
30
Light hydrocarbon (C8)
60
40
Light hydrocarbon (C10)
75
25
80 40
40 80 20
30
15
25
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TABLE 5.5 A Representative Industrial Ni Catalyst of Steam Reforming
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TABLE 5.6 Different Routes for Three Types of Carbon Formation in Hydrogen Production Reactions [15] Whisker Carbon
Encapsulating Polymers
Pyrolytic Carbon
Formation
Diffusion of C through Ni-crystal: nucleation and whisker growth with Ni crystal at top
Slow polymerization of hydrocarbon radicals on Ni-surface into encapsulating film
Thermal cracking of hydrocarbon: deposition of C-precursors on catalysts
Temperature
>300 C
<500 C
>600 C
Phenomenon
Deactivation breakdown of catalyst and increasing DP
Gradual deactivation
Encapsulation of catalyst particle: deactivation and increasing DP
Critical parameters
Low steam/carbon Low activity Aromatic feed in feedstock Abrupt temperature change
Low temperature Low steam/carbon Low H2/carbon in feedstock Aromatic reed in feedstock
High temperature High void fraction Low steam/carbon high-pressure deactivation
feedstock. For this reason, this phenomenon occurs in a temperature range of 600 C or more. Usually carbon deposition in hydrogen production utilizing steam reforming reaction generates whisker carbon. It depends on contamination of unsaturated hydrocarbons in the feedstocks. Fig. 5.8 is a graph showing the time course of the carbon precipitation amount when using a Ni catalyst as the feedstock of unsaturated hydrocarbon. It is understood that carbon precipitates in a very short time. Fig. 5.9 is the scanning electron microscope (SEM) image of the surface of the Ni catalyst. (A) is a new catalyst. It is evident that the Ni particles are highly dispersed on the alumina carrier. In this way, the Ni particles are initially in a highly dispersed state on the surface of the carrier. Normally, since the Ni catalyst is used at a high temperature of 450 C or higher, sintering occurs and the particle diameter becomes coarse. That is, the Ni particles move on the surface of the carrier. Therefore, the activity decreases as compared with the initial stage. Fig. 5.9(B) is a SEM image of this sintered surface. Ni particles present a liquid-like behavior at high temperatures, 450 C or higher. Ni particles will coarsen by sintering because their
218 Science and Engineering of Hydrogen-Based Energy Technologies 20
mg/g-cat
15 10 5 0 0
1
2
EEthylen
3 h Ben zen
n-Heptan n
Cyc lohexane
4
5
6
n-Hexane Trimethylbutane
FIGURE 5.8 Carbon formation from different feedstock. Thermogravimetric studies. 0.7 g Ni/ MgO/Al2O3 catalyst. S/C ¼ 2, 0.1 MPa, 500 C. Based on J.R. Rostrup-Nielsen, New aspects of syngas production and use, Catal. Today. 63 (2e4) (2000)159.
FIGURE 5.9 SEM of the surface of the Ni catalyst. (A) New catalyst surface. (B) Sintered surface of the catalyst. Ni particles become large according to reaction heat. (C) Surface of the carbon deposit. It is usually caused by a decrease in activity. Plant steam reforming catalyst. S/C ¼ 2.5, P ¼ 0.1 MPa, Ni/Al2O3 catalyst. Feedstock gas is propane.
surface energy is decreased. For this reason, there is a phenomenon in which the activity decreases. When the activity declines, carbon precipitates on the surface of the catalyst as shown in Fig. 5.9(C). The carbon deposition shown in Fig. 5.9(C) is a typical whisker carbon, and filamentary nanotubes are observed. Also, Ni particles acting as a catalyst exist at the tip of the filament. Carbon deposition
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is remarkable, especially when subjected to sulfur poisoning described later. Usually, in a reformer of a fuel cell that is a hydrogen energy device, carbon deposition often occurs on the inlet side of the catalyst layer, and most of it is whisker carbon. The cause is considered to be due to low S/C (molar ratio of steam to carbon) at the start of the reformer, and when carbon is generated, it cannot be removed and thus the reformer is replaced [13].
Poisoning Sulfur is a severe poison for steam reforming catalyst. Sulfur compounds are strongly chemisorbed on the active metal surface so that this activity is for deactivation [14]. Sulfur is the element that takes the most activity in steam reforming catalyst. Sulfur compounds adsorb to the active sites on the active metal surface and inhibit the reaction to block adsorption of reactants. That is, it is the greatest cause of deteriorating the catalytic function. Sulfur poisoning is an inevitable reason to decrease steam reforming catalyst activity since sulfur compounds are contained in small amounts in fuels [15]. For this reason, usually sulfur compounds are treated with sulfur in a desulfurizer located upstream of the plant. However, even if high-performance desulfurization is carried out, H2S passes through 5e30 vol. ppb sulfur poisoning, in which sulfur poisoning is adsorbed on the reforming catalyst packed on the downstream side of the desulfurizer and decreases the activity of the reforming catalyst in addition to causing precipitation of carbon. Generally, as the concentration of H2S increases, the sulfur poisoning of the reforming catalyst becomes more intense. H2S decomposes on the Ni surface and chemical sulfur chemisorbs. H2S þ Ni / S e Ni þ H2
(5.10)
This reaction is usually carried out at 300 C, but since the equilibrium constant at this temperature is Kp ¼ 5.9 106 the reaction proceeds sufficiently. The concentration of sulfur in the fuel at the inlet of the reformer is about 10e50 vol ppm. Fig. 5.10 shows the result of simulating the situation of sulfur poison of the loaded catalyst in the steam reforming reactor. The horizontal axis represents the length of the catalyst layer, and the vertical axis represents the coverage. This coverage is a ratio of how much active sites of Ni surface are adsorbed with sulfur. The operating conditions are as follows: inlet temperature 500 C, outlet temperature 800 C, pressure 3.4 MPa, S/C ¼ 3.3. Even at any concentration of sulfur compounds, the curve is convex downward from the reactor inlet side toward the outlet side. This indicates that when the sulfur compound enters the catalyst bed, sulfur is adsorbed to nickel so that the concentration of the sulfur compound is reduced. As described
Sulfur Coverage of Catalyst Surface [-]
220 Science and Engineering of Hydrogen-Based Energy Technologies 1.0 0.9 0.8 1.00 ppm 0.7 0.50 ppm 0.6
0.20 ppm 0.10 ppm 0.01 ppm 0.005 ppm 0.002 ppm 0.001 ppm
0.5 0.4 0.3 0.0
0.2
0.4
0.6
0.8
1.0
Catalyst Bed Length [-] FIGURE 5.10
Sulfur coverage of catalyst surface in tubular reformer [15].
earlier, sulfur poisoning is strongly poisoned at the inlet of the catalyst layer, and the influence of poisoning toward the outlet tends to weaken. This specific example will be shown in many papers. Here, it is pointed out from the result of the actually operated reformer that the activity decreases as the sulfur poisoning amount increases [16].
CONCLUSION Hydrogen production reaction and process were reviewed. The hydrogen production reaction is a reaction controlled by chemical equilibrium, and the process utilizing chemical equilibrium is systemized. Although research in this field has a long history, many themes remain to be studied, which will continue in the future.
Hydrogen Energy Engineering Applications and Products Hirohisa Uchida1, Makoto R. Harada2 1
Professor, School of Engineering, Tokai University/President & CEO, KSP Inc., Japan; Research Adviser, National Institute of Advanced Industrial Science and Technology (AIST), Research Institute for Chemical Process Technology 2
INTRODUCTION This chapter describes the engineering application of hydrogen energy. Applications of hydrogen energy technologies fall into four categories: hydrogen production, storage, transportation, and stationary use. Each field has undergone independent development. Hydrogen production has a long history. The hydrogen production from fossil fuels exceeds a 100-year history. From the 1900s, research and development for mass production, advanced industrial hydrogen production, and progress has been continued to the present. This technology is currently making a great contribution to hydrogen energy technology and has future prospects of additional progress, including hydrogen production from biomass. Hydrogen storage is an indispensable technology for fuel cell mobile applications. Various techniques for storing hydrogen, such as hydrogen storage alloys, high-pressure hydrogen gas storage, liquid hydrogen storage, and the like, have been developed and are currently being used at hydrogen refueling stations. These technologies have enriched the prospects for fuel cell mobile applications, including current fuel cell vehicles. Hydrogen station deployment is actively being carried out in East Asia, Japan, South Korea, and China, as well as in the United States and Europe. Transportation of hydrogen has a very important significance for the supply of feedstock. Currently, research on how to transport hydrogen is under way. Currently, two major projects are progressing in Japan. The first one is a development plan to transport liquid hydrogen by ship, and the other one is hydrogen production using organic hydrides. Stationary fuel cell applications have initially unveiled hydrogen utilization by the society. But at present, mobile/transportation fuel cells, typified by fuel cell vehicles, forklift, buses, and cars, are taking the lead. Also, due to the Science and Engineering of Hydrogen-Based Energy Technologies. https://doi.org/10.1016/B978-0-12-814251-6.00009-5 201 Copyright © 2019 Elsevier Inc. All rights reserved.
202 Science and Engineering of Hydrogen-Based Energy Technologies
massive production of hydrogen, hydrogen combustion in turbines for power generation will soon start, inducing an expansion of hydrogen energy applications. Concerning vehicles, Toyota first commercialized Mirai in 2014, then Honda began selling Clarity in 2016. Subsequently, Toyota began selling buses in March 2018. In 2019 fuel cell trucks are planned to be sold, and in the near future demand for transportation fuel cells will grow steadily. Sales of forklifts have also started in 2016. As described above, development of hydrogen utilization technology is progressing continuously with the goal of being significantly implemented by 2030, and this trend will continue. Even from a global view, many countries are working on applied technologies of hydrogen energy, fuel economy, testing cogeneration power generation with stationary fuel cells for several world metropolitan cities. Technology development and deployment road maps focus on long-term schedules, up to 2050. Application technology of hydrogen energy has dramatically improved in the past decade, and it is speculated that this trend will continue in the future. In this chapter, the main features concerning applied technology of hydrogen energy are described.
Chapter 5.1
Hydrogen Production Technology From Fossil Energy INTRODUCTION Hydrogen (H2) production from fossil fuels has a long history and has been industrialized since the beginning of the 20th century. Various reaction types or plants have been proposed, but here we will summarize the technically established methods. Hydrogen is presently artificially manufactured in large scale. For this reason, various technological developments have been carried out so far. It has mainly been produced from the electrolysis of water and from fossil fuels [1e3], as will be described here. Hydrogen as an energy carrier is usually found in the state of a compound bonded to other elements. Therefore, for hydrogen to become an energy source, the hydrogen compound must be efficiently decomposed with the use of some external energy. Hydrogen is a well-known and versatile energy carrier that has been produced in large scale, with global amounts exceeding 55 Mt, mainly from
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fossil fuels [4]. Hydrogen production methods such as steam reforming, partial oxidation and auto-thermal reforming, ATR, are used, however, the steam reforming of natural gas is mostly employed, making it less expensive than other procedures [5]. Hydrogen atoms of natural gas are separated into hydrogen molecules in the course of the reaction, and at the same time, carbon dioxide is produced as a by-product. For this reason, the development of CO2 -free hydrogen by renewable energy has been advanced [6]. Here we will explain the hydrogen production method from fossil fuel by explaining the hydrogen production reaction, the reaction process, the catalyst, and so on.
CHARACTERISTICS OF HYDROGEN PRODUCTION PROCESSES First, the various hydrogen production methods will be explained, including steam reforming using steam reforming reaction (partial reforming), partial oxidation using oxygen as an oxidizing agent, and autothermal utilization of reaction heat generated when hydrogen is produced. A combined reforming method combines a steam reforming method and an ATR method [7]. As shown in Fig. 5.1, energy, including hydrogen, can be manufactured industrially from various fossil fuels through various hydrogen production methods. These processes are already established industrially. In this chapter, the technology of production of hydrogen from fossil fuels is discussed, mainly the steam reforming reaction, which is a hydrogen production process also used for fuel cell systems.
FIGURE 5.1 Relationship between types of fossil fuels and various hydrogen production processes.
204 Science and Engineering of Hydrogen-Based Energy Technologies
Hydrogen Production Reaction Catalytic Steam Reforming The steam reforming process has been carried out for more than 100 years and is the most historically old and industrially constructed process. Therefore, since this hydrogen production method has been described in many places so far, we will first describe the reaction, and then explain items deeply related to hydrogen production such as catalyst and poisoning. It is thought that the steam reforming reaction proceeded simultaneously with the following two reactions. CH4 þ H2O / CO þ 3H2 206 kJ/mol
(5.1)
(5.2) CO þ H2O / CO2 þ H2 þ 41 kJ/mol Eq. (5.2) is called a shift reaction, which is a CO concentration reduction reaction that generates CO2 from CO. ALthough CO2 has become a global warming problem, there is also a reaction called carbon dioxide reforming reaction: CH4 þ CO2 / 2CO þ 2H2 248 kJ/mol
(5.3)
This reaction is used in research to convert H2 and CO2 into useful compounds, which has been drawing attention in recent years. Since this reaction is an endothermic reaction larger than the steam reforming reaction and since carbon deposition is liable to occur in the catalyst layer, a catalyst capable of suppressing carbon deposition currently has to be used. Fig. 5.2 shows a
FIGURE 5.2 Radiant wall-type reformer. Based on Miyasugi et al., J. Jpn. Petrol. Int. 25 (1982) 260.
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schematic diagram of this reactor, but it is an energy-consuming reactor that heats a catalyst bed reactor with several burners from the outside.
Partial Oxidation In this method, oxygen (O2) is used as an oxidizing agent, and hydrogen is produced by utilizing a partial oxidation reaction of a fossil fuel as a feedstock. The amount of O2 to be introduced is an amount corresponding to 30%e50% of the theoretical oxygen amount required for complete combustion. When a flame occurs, CO2 is introduced and the flame is controlled. Most types of fuels can be used. This method is characteristic of a noncatalytic reaction. The partial oxidation reaction formula is m n Cm Hn þ O2 /mCO þ H2 (5.4) 2 2 In case the fuel is methane, it becomes 1 CH4 þ O2 / CO þ 2H2 þ 35.7 kJ=mol 2
(5.5)
Fossil fuel and O2, which is the oxidizer, as well as steam, are injected from the burner to perform partial oxidation reaction. In the partial oxidation reaction, slight carbon deposition occurs. Normally, about 3 wt% of carbon is precipitated with respect to the fuel. Carbon in this product gas is removed as a carbon slurry by water quenching or water washing. Fig. 5.3 shows a
O2
Feedstock
Steam
Syn Gas
Cooler Condenser
FIGURE 5.3 Partial oxidation hydrogen production secondary reformer. Based on Dry et al., Industrials chemicals via C1 processes, ACS Symp. Ser. 328 (1987) 18.
206 Science and Engineering of Hydrogen-Based Energy Technologies
schematic diagram of this reactor. Although it sometimes fills ceramic balls with very high heat resistance, it is fundamentally a very simple structure [7].
Autothermal Reforming ATR is a hydrogen production method combining a partial oxidation method and a steam reforming method. At the inlet of the catalyst layer, O2 (which is an oxidizing agent), fuel, and steam are introduced; part of the fuel is burned; and the steam reforming reaction is carried out by utilizing the reaction heat. Therefore, the partial oxidation reaction mainly occurs on the upstream side of the reactor and the steam reforming reaction mainly occurs at the downstream side of the reactor. The overall reaction is called ATR reaction. In the ATR method, the following reaction occurs. Partial oxidation reaction: CH4 þ 3/2O2 / CO þ 2H2O þ 519 kJ/mol (5.6) Steam reforming reaction: CH4 þ H2O / CO þ 3H2 206 kJ/mol Shift reaction: CO þ H2O / CO2 þ H2 þ 41 kJ/mol
(5.7) (5.8)
O2 is used as the oxidizing agent as in the partial oxidation reactor described earlier, as the temperature at the inlet of the reactor reaches 1000 C or higher. For this reason, a metallic reactor cannot be used. The inner wall of the reactor of the partial oxidation reaction and ATR reaction is coated with refractory brick or refractory cement and a reactor with high heat resistance is used. Fig. 5.4 shows a schematic diagram of this reactor.
Combined Reforming This process is used in the synthesis of ammonia and methanol. Part of fossil fuel as a primary reformer is a method of decomposing fuel using the steam reforming reaction method and then reforming it by the ATR method as a secondary reforming. Fig. 5.5 shows a schematic diagram of this reactor, but basically it is a combination of the steam reforming and ATR reactors. In the secondary reforming, when synthesizing ammonia, air is used as an oxidant, and the gas composition at the ATR reactor outlet is adjusted such that the molar ratio of H2 and N2 required for ammonia synthesis is 1:3. Since the oxidizing agent of the ATR method is air, O2 from the air is utilized, and an expensive oxygen separating apparatus is unnecessary. For this reason, a gas for ammonia synthesis can be produced economically. On the other hand, when methanol synthesis is carried out, O2 is used as an oxidizing agent. The gas composition at the outlet of the ATR reactor has a molar ratio of H2 and CO required for methanol synthesis of 2:1, and it is possible to obtain a gas easily meeting methanol synthesis reaction conditions. This chemical reaction formula is a partial oxidation reaction formula. CH4 þ 1/2O2 / CO þ 2H2 þ 35.7 kJ/mol
(5.9)
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FIGURE 5.4 Autothermal hydrogen production secondary reformer. Based on B. Glo¨ckler, A. Gritsch, A. Morillo, G. Kolios, G. Eigenberger, Autothermal reactor concepts for endothermic fixed-bed reactions, Chem. Eng. Res. Des. 82 (2) (February 2004) 148e159.
Features of the four hydrogen production methods are summarized in Table 5.1. Table 5.2 also summarizes the industrial conditions for producing each product including hydrogen [8].
Industrial Hydrogen Production Process The history of hydrogen production catalysts used for the steam reforming method, partial oxidation method, and others is very old, and even in the experiment note of Michael Faraday, hydrogen, CO, CO2, and the like are generated when steam is brought into contact with heated coal. In these notes, there is a description that white smoke is adsorbed on a metal such as Ni, and the phenomenon of a predictable catalyst is noted. In order to produce hydrogen from fossil fuel, it is essential that the fossil fuel gas component and water vapor are adsorbed to the catalyst; the fossil fuel, which is a hydrocarbon, decomposes and reacts comprehensively with the oxygen molecules in the water vapor. Also, considering chemical reactions, production of CO is essential. Therefore, in the hydrogen production process,
208 Science and Engineering of Hydrogen-Based Energy Technologies
FIGURE 5.5 Combined reforming reactor. Based on H. Ebrahimi, A. Behroozsaranda, A. Zamaniyan, Arrangement of primary and secondary reformers for synthesis gas production. Chem. Eng. Res. Des. 88 (10) (October 2010) 1342e1350.
a shift reactor is always arranged downstream of the reforming process. Fig. 5.6 shows the industrial hydrogen production process by the steam reforming process. Temperature, pressure, and gas concentration shown in the process are typical examples of the plant actually being operated. Other hydrogen production methods such as the partial oxidation method only replace the steam reforming reactor with other reactors, and there is no change in the process [9]. The hydrogen production process is divided into four steps: a desulfurization step for removing sulfur compounds, a reforming step for producing hydrogen, a shift step for reducing the concentration of carbon monoxide, and a purification step for removing impurities. These four steps will be described.
Desulfurization Step H2S is contained in natural gas, and sulfide typified by methyl mercaptan is contained in a gas state in other hydrocarbon fuels. This sulfur compound adsorbs to the reforming catalyst on the downstream side and the catalyst for low temperature carbon monoxide shifts to lower the activity. In order to
Partial Oxidation System
Autothermal Reforming System
Natural gas, LPG, naphtha up to C6 saturated hydrocarbons. Unsaturated hydrocarbons are difficult to use.
Hydrocarbons, heavy oil, Town gas
Natural gas, LPG, naphtha
Natural gas, LPG, naphtha, saturated fossil fuel, and fossil fuel with few unsaturated hydrocarbons
Reactor type
External heat exchangeetype fixed bed reactor
Adiabatic fixed bed reactor
Adiabatic fixed bed reactor
(Type- I) Two-stage external heat exchangeretype reactor or (type-II) isothermal steam reforming reactor and autothermal reformingetype reactor
Main chemical reaction
Catalytic steam reforming reaction CH4 þ H2O / CO ＋ H2 206 kJ/mol shift reaction CO þ H2O / H2 þ CO2 þ 41 kJ/mol
Catalytic partial oxidation reaction CH4 ＋ 1/2O2 / CO þ H2 þ 38 kJ/mol
Partial oxidation reaction and catalytic steam reforming reaction
Catalytic steam reforming reaction
Oxygen plant
Not required
Required
Required
Not required for type-I, but required for type-II
Process
Steam Reforming System
Available fossil fuel
Two-Stop Reforming System
209
Continued
Hydrogen Energy Engineering Applications and Products Chapter j 5
TABLE 5.1 Hydrogen Production Methods from Fossil Fuels
Process
Steam Reforming System
Partial Oxidation System
Autothermal Reforming System
Catalyst
Ni/Al2O3, Ru/Al2O3
Ni/Al2O3, Rh/Al2O3
Ni/Al2O3, Ni/MgAl2O4, Rh/Al2O3,
Ni/Al2O3
Operating conditions
Maximum bed temperature 750e950 C
Maximum bed temperature 900e1150 C
Maximum bed temperature 900e1150 C
First steam reformer bed temperature 500 C (type-I)
Temperature and pressure
Reactor pressure 1.5e3 MPa
Reactor pressure 3e7 MPa
Reactor pressure 2e6 MPa
Second steam reformer bed temperature 750e950 C Reactor pressure 1.5e3 MPa (type-II) Second reactor max. bed temperature 900e1150 C Reactor pressure 2e6 MPa
Hydrogen/CO ratio
2.8e4.8 (mol/mol)
1.7e2.0 (mol/mol)
1.8e3.8 (mol/mol)
2.2e4.4 (mol/mol)
Hydrogen production scale per series
800e80,000 Nm3/h
7000e70,000 Nm3/h
10000e7,50,000 Nm3/h
50,000e3,00,000 Nm3/h
Two-Stop Reforming System
210 Science and Engineering of Hydrogen-Based Energy Technologies
TABLE 5.1 Hydrogen Production Methods from Fossil Fuelsdcont’d
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TABLE 5.2 Examples of Industrial Conditions for the Production of Pure Hydrogen and of Hydrogen for the Production of Other Chemical Components H2 for Ammonia Production
H2 for Methanol Productiona
H2 for Syngas Productionb
Condition
H2
Feedstock
Natural gas
Natural gas
Natural gas
Natural gas
Pressure
40
34
16.5
27
H2O/C (mol/atom)
6.5
3.5
2.5
0.6
CO2/C (mol/atom)
0
0
0.3
0.1
Space velocity (volCH4/volcat)
700
1170
1425
e
Maximum temperature of Reference
850
795
950
1050
H2/CO (product)
4.4
4.5
2.1
2
a
W.D. Deckwer, et al., Kinetics studies of Fischer-Tropsch synthesis on suspended iron/potassium catalyst-rate inhibition by carbon dioxide and water, Ind. Eng. Chem. Process Des. Dev. 25(3) (1986) 643e649. b I. Dybkjaer, T.S. Christensen, Syngas for large scale conversion of natural gas to liquid fuels, Stud Surf. Sci. Catal. 136 (2001) 435.
avoid sulfur poisoning, desulfurization is carried out. The sulfur compound is converted to H2S by the hydrodesulfurization catalyst and absorbed by ZnO. Through this catalytic reaction, it is possible to reduce sulfur compounds to about 20e50 ppb.
Reforming Reaction Process Since it has already been explained in the previous section, the reaction is different between the primary reformer and the secondary reformer, although omitted here. Particularly in the primary reformer when the molecular weight of the hydrocarbon is increased, carbon deposition tends to occur easily. Shift Reaction Process The composition of the reformed gas at the reforming process outlet is 72% hydrogen, 3% methane, 13% Carbon monoxide, and 12% Carbon dioxide. Since Carbon monoxide is toxic and we wish to reduce the concentration and increase H2 concentration, the shift reaction shown in Eq. (5.2) is generated. The high temperature Carbon monoxide transformer operates at 300 Ce350 C
212 Science and Engineering of Hydrogen-Based Energy Technologies Air
CO Conversion
Steam
Primary Reformer (Ni/Al2O3) Secondary Reformer (Ni/Al2O3)
600°C Furnace
Desulfuriration MoCo-S ZnO
300350°C Fe3O4 WGS Catalyst
800°C
Nat. gas
CH4 =9-14%
200250°C
CuO WGS Catalyst
9001100°C Steam CH4 =0.5-1.5% CO =10-13%
CO =2-3%
CO =0.2-0.5% CO2 CO2 removable
CO2 0.1% Methanation Ni/Al2O3
H2 99%
Drain
Drain
FIGURE 5.6 Hydrogen production process diagram, operating conditions: S/C ¼ P(H2O)/ P(CH4) ¼ 2.5e4.0, T(exit) ¼ 900e1100 C, P (exit) ¼ 2e3 MPa. From C.H. Bartholomew, R.J. Farrauto, Hydrogen production and synthesis gas reactions, in: Fundamentals of Industrial Catalytic Processes, second ed., John & Wiley Sons, Inc., 2006, pp. 339e371 (chapter 6).
and reduces the outlet Carbon monoxide concentration to about 2%e3%. Then, we operate the low temperature Carbon monoxide transformer packed with Cu-based catalyst at about 200 C and reduce the outlet Carbon monoxide concentration from 0.2% to 0.4%. The Carbon monoxide shift reaction is advantageous at lower temperatures, but the reaction rate is slower.
Purification Step Conventionally, this was a system using decarboxylation with an amine type absorbent and a methanation reaction with a Ni/geAl2O3 catalyst, but recently it was cooled down to room temperature and passed through a pressure swing adsorption device, whereby a purity of 99.9% of hydrogen is obtained. The results are shown in Table 5.3, of representative catalysts used for desulfurization, reforming, CO shift, and methanation reaction.
THERMODYNAMICS The steam reforming reaction, which is a hydrogen production reaction, and the cooccurring shift reaction are chemical reactions dominated by equilibrium. The equilibrium constant of each reaction is shown in Table 5.4 at
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TABLE 5.3 Industrial Hydrogen Production Reaction and Typical Catalyst Process
Reaction
Industrial Typical Catalyst
Hydrode sulfurization
ReH þ 2H2 / H2S þ HeReH
CoMo/Al2O3
Adsorption
ZnO þ H2S / ZnS þ H2O
ZnO
Primary reforming
HC þ H2O / H2 þ CO þ CO2 þ CH4
Ni/MgO
Secondary reforming
2CH4 þ 3H2O / 7H2 þ CO þ CO2
Ni/MgAL2O4, Ni/CaAl2O4
High-temperature CO shift
CO þ H2O / H2 þ CO2
Fe3O4/Cr2O3
Low-temperature CO shift
CO þ H2O / H2 þ CO2
Cu/ZnO/Al2O3
Methanation
CO þ 3H2 / CH4 þ H2O
Ni/Al2O3
TABLE 5.4 Equilibrium Constants
KRa
K Sb
0
0.066 l029
4.555 105
100
0269 l019
3.587 103
200
0.487 l013
2299 102
300
0.682 109
3.973 10
400
0.618 106
1.192 10
500
1.021 104
4.999
600
0.873 103
3.53
700
1
1.305 10
1.541
800
1.759
1.059
900
1.533 10
7.692 101
1000
0.955 102
5.923 101
a
C
Steam reaction equilibrium constant. Shift reaction equilibrium constant.
b
214 Science and Engineering of Hydrogen-Based Energy Technologies
temperatures up to 1,000 C. In the steam reforming reaction, the equilibrium constant rises as the temperature rises. This is because the steam reforming reaction is an endothermic reaction [10]. The equilibrium constant is very small up to about 400 C, indicating that the reaction hardly proceeds in terms of equilibrium. The inlet temperature of the industrial reformer is set to 450 C or higher, but this equilibrium constant is determined. The fact that the equilibrium constant is small means that the target amount of hydrogen is small, so that it is industrially meaningless to react at 450 C or lower. Since the equilibrium constant of the steam reforming reaction exceeds 1 at 700 C or higher, it is understood that the objective hydrogen production can be performed smoothly. Therefore, the maximum temperature of the catalyst layer in any reformer is 700 C or more. In addition, since the steam reforming reactor is reacted at a high pressure of about 2e3.5 MPa, it is disadvantageous for hydrogen production in equilibrium, so the maximum temperature of the catalyst layer is set as high as 800 C e1000 C. On the other hand, the shift reaction decreases as the temperature rises. Therefore, the shift reaction proceeds at a low temperature, and the reverse shift reaction proceeds at a high temperature of 800 C or higher. Therefore, in the reformer outlet gas, CO and CO2 are detected. CO is processed by a shift reactor installed on the downstream side as shown in Fig. 5.7. The hydrogen production reaction is an equilibrium reaction according to thermodynamics.
(A)
(B)
100
(C)
90
Conversion (%)
80 70 60 50 40 30 20
600 650 700 750 800 850 900
Temperature (°C)
600
650 700
750
800
Temperature (°C)
850
900
600 650 700 750 800 850 900
Temperature (°C)
FIGURE 5.7 Simulation results of conversion of steam reforming reaction of methane. Operating conditions (A) 0.1 MPa, (B) 0.5 MPa, (C) 1 MPa, S/C ¼ 2, , S/C ¼ 3, > S/C ¼ 4, S/C ¼ steam to carbon ratio.
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The higher the pressure and the higher the steam to carbon ration, S/C, the higher the conversion. Using this property, the stationary fuel cell system operates at a low pressure, improves the methane conversion rate, and improves the operation efficiency of the plant.
INDUSTRIAL CATALYST DESIGN An overview of industrial hydrogen production catalyst is outlined. Hydrogen production catalysts are used industrially for the production of methanol and ammonia. Technologies are also established. Especially with old steam reforming catalysts, in addition to Ni, precious metals such as Rh and Ru are sometimes used and are commercialized. Industrially, since the Ni-type catalyst is used most frequently, the components and compositions of representative Ni-type catalysts used industrially are shown in Table 5.5. Although some alkaline earth metals, which are basic oxides, have been introduced as additives (as promoters), there was a catalyst that had previously been added with alkali metals. However, at present, no alkali metal is used as a conversion agent since the alkali metal gradually leaves the catalyst and causes corrosion of the metal piping on the downstream side of the reactor, which causes a failure of the plant. On the other hand, alumina is mostly used as a catalyst carrier, but there are catalysts in which MgAl2O4 or MgO having a spinel crystal structure is used. Hydrogen production is made at 800 C or more, in some cases 1000 C or more, so high temperature heat resistance is required [11]. It is also required to have resistance against carbon deposition described later or resistance to sulfur poisoning. After past decades of development, the present industrial catalyst has been established. Therefore, it seems that the current catalyst will continue to be used in the future.
DEACTIVATION Carbon Formation Carbon deposition is considered to be an inevitable phenomenon in hydrogen production reactions. Carbon deposition occurs in the steam reforming reaction depending on operating conditions. Precipitated carbon is classified into three types. Table 5.6 describes these carbons [12]. The most typical carbon deposition is called whisker carbon and occurs on the surface of the Ni catalyst. Carbon deposition amount increases due to diffusion of C in the fuel. In addition, there are carbon deposits in which carbon is generated as a core of the feedstock radical called encapsulating polymers. This phenomenon is seen as the activity gradually decreases. It is a phenomenon observed at a low temperature of 500 C or less. The third carbon deposition is called pyrolytic carbon, which is generated by thermal decomposition of hydrocarbons as a
Catalyst Composition wt% Feedstock
NiO
A12O3
MgO
MgAl2O4
CaO
SiO2
CeO2
Natural gas
12
88
Natural gas
20
80
Natural gas
20
Natural gas
20
Natural gas
20
Natural gas
30
20
LPG
30
30
40
LPG
35
30
35
Naphtha
80
20
Naphtha
25
25
10
Naphtha
40
30
30
Light hydrocarbon (C8)
60
40
Light hydrocarbon (C10)
75
25
80 40
40 80 20
30
15
25
216 Science and Engineering of Hydrogen-Based Energy Technologies
TABLE 5.5 A Representative Industrial Ni Catalyst of Steam Reforming
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TABLE 5.6 Different Routes for Three Types of Carbon Formation in Hydrogen Production Reactions [15] Whisker Carbon
Encapsulating Polymers
Pyrolytic Carbon
Formation
Diffusion of C through Ni-crystal: nucleation and whisker growth with Ni crystal at top
Slow polymerization of hydrocarbon radicals on Ni-surface into encapsulating film
Thermal cracking of hydrocarbon: deposition of C-precursors on catalysts
Temperature
>300 C
<500 C
>600 C
Phenomenon
Deactivation breakdown of catalyst and increasing DP
Gradual deactivation
Encapsulation of catalyst particle: deactivation and increasing DP
Critical parameters
Low steam/carbon Low activity Aromatic feed in feedstock Abrupt temperature change
Low temperature Low steam/carbon Low H2/carbon in feedstock Aromatic reed in feedstock
High temperature High void fraction Low steam/carbon high-pressure deactivation
feedstock. For this reason, this phenomenon occurs in a temperature range of 600 C or more. Usually carbon deposition in hydrogen production utilizing steam reforming reaction generates whisker carbon. It depends on contamination of unsaturated hydrocarbons in the feedstocks. Fig. 5.8 is a graph showing the time course of the carbon precipitation amount when using a Ni catalyst as the feedstock of unsaturated hydrocarbon. It is understood that carbon precipitates in a very short time. Fig. 5.9 is the scanning electron microscope (SEM) image of the surface of the Ni catalyst. (A) is a new catalyst. It is evident that the Ni particles are highly dispersed on the alumina carrier. In this way, the Ni particles are initially in a highly dispersed state on the surface of the carrier. Normally, since the Ni catalyst is used at a high temperature of 450 C or higher, sintering occurs and the particle diameter becomes coarse. That is, the Ni particles move on the surface of the carrier. Therefore, the activity decreases as compared with the initial stage. Fig. 5.9(B) is a SEM image of this sintered surface. Ni particles present a liquid-like behavior at high temperatures, 450 C or higher. Ni particles will coarsen by sintering because their
218 Science and Engineering of Hydrogen-Based Energy Technologies 20
mg/g-cat
15 10 5 0 0
1
2
EEthylen
3 h Ben zen
n-Heptan n
Cyc lohexane
4
5
6
n-Hexane Trimethylbutane
FIGURE 5.8 Carbon formation from different feedstock. Thermogravimetric studies. 0.7 g Ni/ MgO/Al2O3 catalyst. S/C ¼ 2, 0.1 MPa, 500 C. Based on J.R. Rostrup-Nielsen, New aspects of syngas production and use, Catal. Today. 63 (2e4) (2000)159.
FIGURE 5.9 SEM of the surface of the Ni catalyst. (A) New catalyst surface. (B) Sintered surface of the catalyst. Ni particles become large according to reaction heat. (C) Surface of the carbon deposit. It is usually caused by a decrease in activity. Plant steam reforming catalyst. S/C ¼ 2.5, P ¼ 0.1 MPa, Ni/Al2O3 catalyst. Feedstock gas is propane.
surface energy is decreased. For this reason, there is a phenomenon in which the activity decreases. When the activity declines, carbon precipitates on the surface of the catalyst as shown in Fig. 5.9(C). The carbon deposition shown in Fig. 5.9(C) is a typical whisker carbon, and filamentary nanotubes are observed. Also, Ni particles acting as a catalyst exist at the tip of the filament. Carbon deposition
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is remarkable, especially when subjected to sulfur poisoning described later. Usually, in a reformer of a fuel cell that is a hydrogen energy device, carbon deposition often occurs on the inlet side of the catalyst layer, and most of it is whisker carbon. The cause is considered to be due to low S/C (molar ratio of steam to carbon) at the start of the reformer, and when carbon is generated, it cannot be removed and thus the reformer is replaced [13].
Poisoning Sulfur is a severe poison for steam reforming catalyst. Sulfur compounds are strongly chemisorbed on the active metal surface so that this activity is for deactivation [14]. Sulfur is the element that takes the most activity in steam reforming catalyst. Sulfur compounds adsorb to the active sites on the active metal surface and inhibit the reaction to block adsorption of reactants. That is, it is the greatest cause of deteriorating the catalytic function. Sulfur poisoning is an inevitable reason to decrease steam reforming catalyst activity since sulfur compounds are contained in small amounts in fuels [15]. For this reason, usually sulfur compounds are treated with sulfur in a desulfurizer located upstream of the plant. However, even if high-performance desulfurization is carried out, H2S passes through 5e30 vol. ppb sulfur poisoning, in which sulfur poisoning is adsorbed on the reforming catalyst packed on the downstream side of the desulfurizer and decreases the activity of the reforming catalyst in addition to causing precipitation of carbon. Generally, as the concentration of H2S increases, the sulfur poisoning of the reforming catalyst becomes more intense. H2S decomposes on the Ni surface and chemical sulfur chemisorbs. H2S þ Ni / S e Ni þ H2
(5.10)
This reaction is usually carried out at 300 C, but since the equilibrium constant at this temperature is Kp ¼ 5.9 106 the reaction proceeds sufficiently. The concentration of sulfur in the fuel at the inlet of the reformer is about 10e50 vol ppm. Fig. 5.10 shows the result of simulating the situation of sulfur poison of the loaded catalyst in the steam reforming reactor. The horizontal axis represents the length of the catalyst layer, and the vertical axis represents the coverage. This coverage is a ratio of how much active sites of Ni surface are adsorbed with sulfur. The operating conditions are as follows: inlet temperature 500 C, outlet temperature 800 C, pressure 3.4 MPa, S/C ¼ 3.3. Even at any concentration of sulfur compounds, the curve is convex downward from the reactor inlet side toward the outlet side. This indicates that when the sulfur compound enters the catalyst bed, sulfur is adsorbed to nickel so that the concentration of the sulfur compound is reduced. As described
Sulfur Coverage of Catalyst Surface [-]
220 Science and Engineering of Hydrogen-Based Energy Technologies 1.0 0.9 0.8 1.00 ppm 0.7 0.50 ppm 0.6
0.20 ppm 0.10 ppm 0.01 ppm 0.005 ppm 0.002 ppm 0.001 ppm
0.5 0.4 0.3 0.0
0.2
0.4
0.6
0.8
1.0
Catalyst Bed Length [-] FIGURE 5.10
Sulfur coverage of catalyst surface in tubular reformer [15].
earlier, sulfur poisoning is strongly poisoned at the inlet of the catalyst layer, and the influence of poisoning toward the outlet tends to weaken. This specific example will be shown in many papers. Here, it is pointed out from the result of the actually operated reformer that the activity decreases as the sulfur poisoning amount increases [16].
CONCLUSION Hydrogen production reaction and process were reviewed. The hydrogen production reaction is a reaction controlled by chemical equilibrium, and the process utilizing chemical equilibrium is systemized. Although research in this field has a long history, many themes remain to be studied, which will continue in the future.