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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
An exploration of current and possible future hydrogen storage technologies, written from an industrial perspective. The book describes the fundamentals, taking into consideration environmental, economic and safety aspects, as well as presenting infrastructure requirements, with a special focus on hydrogen applications in production, transportation, military, stationary and mobile storage. A comparison of the different storage technologies is also included, ranging from storage of pure hydrogen in different states, via chemical storage right up to new materials already under development. Throughout, emphasis is placed on those technologies with the potential for commercialization.
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Seitenzahl: 383
Veröffentlichungsjahr: 2012
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Contents
Chapter 1: Introduction
1.1 History/Background
1.2 Tanks and Storage
Chapter 2: Hydrogen – Fundamentals
2.1 Hydrogen Phase Diagram
2.2 Hydrogen in Comparison with Other Fuels
2.3 Hydrogen Production
2.4 Hydrogen Storage Safety Aspects
Chapter 3: Hydrogen Application: Infrastructural Requirements
3.1 Transportation
3.2 Filling Stations
3.3 Distribution
3.4 Military
3.5 Portables
3.6 Infrastructure Requirements
Chapter 4: Storage of Pure Hydrogen in Different States
4.1 Purification of Hydrogen
4.2 Compressed Hydrogen
4.3 Liquid/Slush Hydrogen
4.4 Metal Hydrides
Chapter 5: Chemical Storage
5.1 Introduction
5.2 Materials and Properties
5.3 Hydrogen Storage in Hydrocarbons
5.4 Hydrocarbons as Hydrogen Carrier
5.5 Application: Automotive
5.6 Ammonia
5.7 Borohydrides
Chapter 6: Hydrogen Storage Options: Comparison
6.1 Economic Considerations/Costs
6.2 Safety Aspects
6.3 Environmental Considerations: Waste, Hazardous Materials
6.4 Dimension Considerations
6.5 Sociological Considerations
6.6 Comparison with Other Energy Storage System
Chapter 7: Novel Materials
7.1 Silicon and Hydropolysilane (HPS)
7.2 Carbon-Based Materials – General
7.3 Microspheres
Index
Related Titles
The Editors
Dr.-Ing. Agata Godula-Jopek
EADS Innovation Works
Dept. IW-EP
81663 München
Germany
Walter Jehle
ASTRIUM GmbH
Dept.: TO 51
88090 Immenstaad
Germany
Prof. Jörg Wellnitz
Privat-Institut für Technik
und Design e.V.
Marie-Curie-Str. 6
85055 Ingolstadt
Germany
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Library of Congress Card No.: applied for
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A catalogue record for this book is available from the British Library.
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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2012 Wiley-VCH Verlag & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
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Print ISBN: 978-3-527-32683-9
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Preface
Energy is an essential component of life. After coal and oil, hydrogen will play a very important role as energy carrier. Hydrogen has the advantage that there are several options for producing it, including use of renewable energy sources. Hydrogen, as energy carrier, can also be used for energy storage and can be easily converted to electric energy and heat. Hydrogen could also be used for mobility as its use promises low impact to the environment. This technology is environment friendly.
During the past years, the necessity to review alternative energy sources has become increasingly important. Environmental issues related to global warming, climate change, and the need to reduce CO2 emissions stimulated the interest in looking into new technologies that may overcome the effect. There are numerous applications in which hydrogen can be used directly, as in road transport or space vehicles or in stationary applications.
The key to a new pathway in mobility will be definitely laid in the mid- and long-term use of hydrogen for general transportation. Hydrogen is the lightest and most energy-efficient element in the world, which is available as an almost unlimited source on our planet. From the perspective of the authors and the hydrogen community, storage of hydrogen plays the key role for the future transportation solutions for passenger cars, trucks, and general transportation. The solution and the cost-effective feasibility of hydrogen storage in reliable tank systems will be the gateway to CO2-free emission driving. This book should provide experts’ view to the scientific community to understand the issue of hydrogen storage and to show case solutions for storage. In addition, this book describes some of the significant developments that have emerged in this field. The reader should be able to understand the complexity of storage systems and to design own solutions for transportation vehicles of his choice. We are very happy to have this extraordinary book in our hands today, because we think this is the main contribution to the next-generation transportation.
June 2012
Agata Godula-Jopek
Walter Jehle
Jörg Wellnitz
Chapter 1
Introduction
1.1 History/Background
Storage of hydrogen is still one of the key issues of the usage of hydrogen itself for vehicle transportation. Main activities on these fields were recorded in the early times of space flight, whereas launching-systems/liquid propelled systems were driven by hydrogen and oxygen fuel.
The development of lightweight tank systems played – from the beginning – a very important role [1], as well as the adjusted issues of pipes, hoses, and dressings.
In Figure 1.1 a typical setup of a pressurized hydrogen or oxygen tank is given, designed with a filament winding method from the early 1950s and 1960s of the last century.
Figure 1.1 H2 and O2 tanks manufactured with filament winding technique in Lockheed–Martin laboratories.
From these issues the storage of hydrogen can be mainly divided into two major technology approaches:
Pressure storage, CH
2
, using high pressurized H
2
in special tank systems in order to store an amount of n-kg of mass for the use in vehicles.
Liquid storage, LH
2
, where the gaseous agent is liquefied below 50 K with moderate pressure (less than 10 bar) and held in a thermo-insulated tank setup.
In this chapter the emphasis will lie on the background of pressurized storage of hydrogen with a special focus on ground transportation, automotive, and tracking.
In Figure 1.2 the basic storage capacities of LH2 and CH2 are given with respect to cost and manufacturability margins.
Figure 1.2 CH2 and LH2 storage outlines with respect to design, cost, and weight margins.
The early developments of hydrogen tanks are closely linked with space flight programs such as Mercury, Gemini, Delta, and the Apollo program, only to mention NASA projects.
The demands of space flights as a part of early application in transportation are very high, on the other hand cost and manufacturing issues played a minor role in this field of application. Due to technical boundary conditions, mainly cylinder- or elliptical-cylinder-tanks were designed which were the best fit for the fuselage of launchers. This layout was mainly driven by static determinations of the current limited ultimate load-cases where the burst and rupture strength of the structure were taken into account for layout. In addition to that, dressing systems were mainly driving the weight-penalty of the structure, by demanding special in- and out-design-features as an interface to composite materials.
Figure 1.3 explains the problem field of interfaces and connectors between dressing-systems and the homogeneous and monolithic tank structure.
Figure 1.3 Problem field of interconnection between dressings and the monolithic composite.
The usage of tank systems for ground transportation or vehicles had already been introduced in the early 19th century within the use of hydrogen carriages. Figure 1.4 shows the earliest hydrogen application ever recorded on a carriage system by De Rivaz from 1808.
Figure 1.4 The earliest application of hydrogen in a ground transportation vehicle – carriage – the earliest predecessor of cars.
The development of tank systems for automotive application in the second half of the last century was mainly driven by BMW automotive tank systems, which were using internal combustion engines (ICEs) as the main propulsion system (Figure 1.5). This project, among other parallel developments such as Ford, Man, Mazda, see [2], was one of the main drivers of hydrogen storage, as well as this, BMW has decided to use the storage system LH2 from the very beginning in order to carry more kilograms of hydrogen and therefore to extend the range.
Figure 1.5 Tank system with the use of CH2 for PSA automotive application using a cylindrical tank rack system in a conventional passenger car.
The recent projects founded by the European government – mainly StorHy (2004–2008) – were focused on the cost critical and production critical issues of the storage of hydrogen and have also lined out major criteria for a practical use of hydrogen in passenger car vehicles.
In [3], a brief overlook at the results of the StorHy project is given, the main aspects of the developments of this project are also used by the authors as an input for future strategies of hydrogen storage.
1.2 Tanks and Storage
The storage of a gaseous agent in tank systems under pressure or high pressure can be linked to the conventional task of layout of pressurized vessels in engineering mechanics.
Mainly the gas, as the fuel for the propulsion system, will be put under high pressure and stored in mostly rotationally symmetric tank systems. The pressure especially for hydrogen will vary between 100 and 750 bar, in particular driven by the demand of the vehicle. This pressure range allows between 0.5 and 3 kg of H2 to be carried. Figure 1.6 shows a conventional tank system by the manufacturer Dynetek with the daily operational pressure of 350 bar and the burst pressure of about 1000 bar.
Figure 1.6 Conventional CH2 pressurized tank system for automotive usage at 350 bar.
This tank is designed with an aluminum liner on the inside to avoid critical H2 permeation and a filament winded carbon fiber hull is used to cope with the high circumferential stresses.
With a storage mass of about 1 kg of hydrogen a typical range of about 100 km can be reached using an internal combustion engine (ICE) and about 150 km on conventional fuel cell applications (F/C). This would automatically demand a typical required mass of at least 3 kg hydrogen for a passenger car, in order to establish practical useful ranges for the customer.
Pressurized gas in tank systems will lead to high circumferential normal stresses, which can be calculated using the pressure-vessel theory. Pressure vessels based on a rotationally symmetric topology can be calculated with the so-called half membrane theory, which will include the membrane stress state and a set of transfer forces for the static equilibrium balance. The normal forces and the shear stresses can be calculated as a function of the metric of the tank system with the conventional orthotropic shell theory [4].
In Figure 1.7 the stresses are shown as a function of the cutting reactional forces of the shell under internal pressure load.
Figure 1.7 Cutting reactional forces of a shell element under internal pressure.
With the help of the statical determination by balancing all forces and moments, the main stresses of the vessel under internal pressure load can be calculated. The investigation of this phenomenon shows that all designed tank systems following this strategy would be inwardly statically determined, whereas the determined stresses are only a function of metric and wall thickness.
If the loads of the shell mid place are described by the load vector
that is relating to their unit of area, the membrane equilibrium state of the shell element follows – under the condition of neglected shear forces Qα = 0 – to
The partial derivative of the covariant basis vectors
is here reflected in the covariant derivative . The term
results in the consideration of the curvature in the equilibrium of forces in normal direction. Further deduction leads to the balance of forces in component notation
Showing the main circumferential stresses as a function of pR/t the layout of the vessel system can mainly be focused on this simple formula. Following the metric of cylinder/paraboloid shells versus spherical shells the critical stresses in spheres are only half of the main axial stresses in the other shells.
Because of the character of spherical shells storage, the advantage of this geometry is not only in stresses but also in having a high mass of hydrogen in a very limited amount of space which means the smallest room. This was the reason why spherical shells were mainly used for space-flight applications as well as for submarine and sea-vessel usages.
The complexity of the dressing system and the necessity of putting in and out hoses and wirings lead to so-called edge-design, which can cause the introduction of sharp bending cutting reactional bending moment gradients. Figure 1.8 shows a typical peak-stress environment on a conventional tank system filled with a liquid source.
Figure 1.8 Typical bending moment peak-gradient on the transition phase between the cylinder and bottom wall.
With the calculation of the main-stress-state of the tank system, the strength of the structure can be derived mainly by using conventional failure criteria, as shown by Von Mises and Tresca. Using these criteria would lead to the insight that – for the fatigue loading of tanks with storage pressure of more than 200 bar – a conventional steel design is no longer feasible. The yield strength of modern steel alloys, even for multiphase steels lies in a region of 700–1100 MPa.
Comparing the strength allowable with the calculated main stresses, low or negative margins of safety for homogeneous monolithic tanks using isotropic materials is shown. In order to gain the required strength of the to-be-designed tank structure monolithic materials such as steel or aluminum alloys on a sheet metal base are no longer sufficient. High stresses due to internal pressure loading up to 750 bar require the usage of new fiber material such as carbon or polyamide fibers. This requirement leads to the introduction of high tensile carbon fibers, which can provide strength values up to 2000–3000 MPa. In addition to that, low rupture strain characteristics of the fibers will lead to a reliable layout of the new tank generation. The combination of carbon fiber with resin material such as epoxy or PUR will lead to a new generation of carbon fiber reinforced plastics (CFRP).
In Table 1.1 the basic material properties of new fiber generations are given. It is clearly visible that only carbon or PA fibers can cope with the demand of high circumferential daily fatigue stresses of the hydrogen tank.
Table 1.1 Tensile strength values and Young’s moduli of modern fiber generations.
CFRP with thermoset resin systems would have high permeation rates for hydrogen molecules. These values lay more than 103 times higher than conventional metal alloys such as aluminum or steel. For this reason an inner- or outer-lining system has to be introduced in order to minimize the leakage rate. Such lining systems are mainly used for ground transportation or car usages and will be produced by 3D-rollforming. The aluminum vessel can be used as a mold for filament winding processes or tape-layup CFRP production techniques. Following that the carbon fiber reinforcement will strengthen the aluminum liner to provide high rupture strength in the matter of a circumferential wrapped filament. The permeation rate of aluminum sheet metal is very low, so aluminum is a favorite material for any containment issues for hydrogen.
The key issue for the implementation of pressure tanks for compressed hydrogen is strongly linked with the application of carbon fibers. This is mainly driven by the requirements of the strength of the structure and the subsequent prevention of burst cases. Following this philosophy a 750 bar tank has to have a design using high tenacity (HT) carbon fibers, which will be an issue of cost and availability. The carbon fiber market worldwide is saturated, carbon fiber raw material costs will exceed 20–25 Euro/kg, worldwide availability of carbon fibers is around 50,000 tons per annum. These boundary conditions will restrict the must-production usage of CFRP for hydrogen tanks for conventional cars.
The solution of this problem is the key to a new generation of hydrogen tank systems, whereas new promising materials for high strength fibers or filaments are currently being investigated. The most promising approach is the usage of basalt-fibers and/or super-light-weight PA-fibers such as Innegra. Table 1.1 indicates new material families for the usage of filament winding processes for pressure tanks. In the European project StorHy the pathway to a new generation of pressure tanks was given, also considering its current limitations to costs and availability as well as for all aspects of safety, see [5].
The usage of conventional 200 bar tanks, which may carry 1 kg of hydrogen, can be very cost efficient when used for small cars or racing projects, such as HyRacer, Formula H, and so on (Figures 1.9 and 1.10). These applications offer a wide range of easy-to-use hydrogen tank systems with ICE and conventional mechanical pressure-regulators and -reducers.
Figure 1.9 Tank assembly of the Formula H racing car, developed by the University of Applied Sciences Ingolstadt and RMIT Melbourne, see [6].
Figure 1.10 Formula H racing car at a glance.
Chapter 2
Hydrogen – Fundamentals
A lot has been written and said about hydrogen because of its independent discovery by Antoine Lavoisier and Henry Cavendish in the eighteenth century. In 1766, Henry Cavendish discovered a gas he called “inflammable air.” Nearly a century later, in 1839, Swiss professor Christian Friedrich Schönbein [1] and London lawyer William Robert Grove described the first idea of fuel cell effect, generating electricity and heat by electrochemical conversion of hydrogen and oxygen from air [2].
Introducing hydrogen into world still dependent on fossil fuels is not easy and certainly will face many challenges. However, it has to be underlined that more recently there is a clear trend to become less dependent on oil and fossils toward reduced carbon consumption and increased use of hydrogen, to develop the use of alternative fuels, and to use renewable energies. There is also a big issue related to climate changes and pollution. It has been found that carbon-rich fuel is responsible for the global warming due to greenhouse gas emission to the atmosphere. As stated by Jeremy Rifkin [3],
There are rare moments in history when a generation of human beings are given a new gift with which to rearrange their relationship to one another and the world around them. This is such a moment. . . . Hydrogen is a promissory note for humanity’s future on Earth. Whether that promise is squandered in failed ventures and lost opportunities or used wisely on behalf of our species and our fellow creatures is up to us.
Hydrogen being the simplest and the most abundantly available element in the universe (0.9 wt%) is present everywhere. Being an energy carrier, hydrogen (a means of storing and transporting energy) is not an energy source itself, but it can only be produced from other sources of energy, such as fossil fuels (natural gas, coal, and petroleum), renewable sources (biomass, wind, solar, and geothermal), or nuclear power (using the energy stored in fissile uranium) by means of several different energy-conversion processes.
Hydrogen is nontoxic, colorless, odorless, and tasteless gas, causing no problems when inhaled with ambient air. It is environment friendly and nonpollutant; releasing hydrogen has no effect on atmosphere (no greenhouse gas effect) or water (under normal atmospheric conditions, hydrogen is a gas with a very low solubility in water of 0.01911 dm3 dm−3 at 25 °C, 0.1 MPa). Hydrogen is highly combustible, therefore a proper ventilation and sensing must be assured when hydrogen diffuses into nonflammable concentrations. During combustion, only water vapor is produced.
Thermodynamic data of hydrogen at 298.15 K and 0.1 MPa are presented in Table 2.1 [4].
Table 2.1 Basic thermodynamic data of hydrogen.
Selected physical properties of hydrogen based on Van Nostrand [5] are as follow (Table 2.2).
Table 2.2 Selected physical properties of hydrogen, reprinted by permission.
Parameter
Value
Unit
Molecular weight
2.016
mol
Melting point
13.96
K
Boiling point (at 1 atm)
14.0
K
Density solid at 4.2 K
0.089
g
−3
Density liquid at 20.4 K
0.071
g cm
−3
Gas density (at 0 °C and 1 atm)
0.0899
g L
−1
Gas thermal conductivity (at 25 °C)
0.00044
cal cm s
−1
cm
−2
°C
−1
Gas viscosity (at 25 °C and 1 atm)
0.0089
Centipoise
Gross heat of combustion (at 25 °C and 1 atm)
265.0339
kJ g
−1
mol
−1
Net heat of combustion (at 25 °C and 1 atm)
241.9292
kJ g
−1
mol
−1
Autoignition temperature
858
K
Flammability limit in oxygen
4–94
%
Flammability limit in air
4–74
%
The energy content of hydrogen is 33.3 kWh kg−1, which corresponds to 120 MJ kg−1 (lower heating value, LHV) and 39.4 kWh kg−1 corresponding to 142 MJ kg−1 (upper heating value, HHV). The difference between the HHV and LHV is the molar enthalpy of vaporization of water, which is 44.01 kJ mol−1. HHV is achieved when the water steam is produced as a result of hydrogen combustion, whereas LHV is achieved when the product water (i.e., steam) is condensed back to liquid.
2.1 Hydrogen Phase Diagram
Hydrogen phase diagram is presented in Figure 2.1. The phase diagram of any substance shows the areas of the pressure and temperature where the different phases of this substance are thermodynamically stable. The borders between these areas (phase equilibrium lines) determine the pressure and temperature conditions, p and T, where two phases are in equilibrium. The critical point is defining the critical conditions for the physical state of the system separating the states with different properties (liquid–gas), which cannot distinguish between the two states (liquid and gas). For example, for pure gaseous substances, critical point means the critical temperature (and corresponding critical pressure – maximal pressure over the liquid) above which gas cannot condense, independent of its size.
Figure 2.1 Simplified phase diagram for hydrogen [7], reprinted by permission.
From the diagram, it can be seen that the molecule of hydrogen can be found in different states/forms depending on the temperature and pressure conditions. Critical parameters of hydrogen are: Tc=33.25 K (−239.9 °C), pc=1.28 MPa, and Vc=64.99 cm3 mol−1 [6].
At low temperatures, hydrogen exists as a solid with density of 0.089 g cm−3 at 4.2 K.
Liquid hydrogen with density of 0.071 g cm−3 at 20.4 K exists between the solid line and the line from the triple point and the critical point at 33.25 K. Hydrogen is a gas at higher temperatures with a density of 0.0899 g L−1 at 0 °C and 1 atm. At ambient temperature of 25 °C (297.15 K), hydrogen gas can be described by the following van der Waals equation:
(2.1)
where:
a (dipole interaction or repulsion constant) and b (volume occupied by hydrogen molecules) are called van der Waals coefficients. For hydrogen, the coefficients a and b are 0.2476 atm L2 mol−2 (a) and 2.661×10−2 L mol−1 (b), respectively [6],
R is the gas constant (8.134 JK−1 mol−1),
n is the number of moles,
p is the gas pressure,
V is the volume of the gas.
In order to store hydrogen gas, the big volume of gas has to be reduced significantly (under ambient conditions, 1 kg of hydrogen has a volume of 11 m3). Either a certain work has to be performed to compress hydrogen or the temperature has to be decreased below the critical temperature of 33.25 K, or the repulsion must be reduced by the interaction of hydrogen with another material [7]. Hydrogen can be stored as a gas or a liquid. In addition, other methods of storing hydrogen in compounded form such as in metal hydrides, chemical hydrides, carbon materials, glass microspheres are also possible.
Following parameters for storing methods have been given [7] (Table 2.3).
Table 2.3 Hydrogen storage methods, parameters, [7] reprinted by permission.
2.2 Hydrogen in Comparison with Other Fuels
Table 2.4 shows the properties of hydrogen compared with other fuels.
Table 2.4 Mass and volume energy density of hydrogen in comparison with other fuels.
From Table 2.4, it is clear that hydrogen yields much higher energy per unit weight than any other fuel. Hydrogen has a high energy-to-weight ratio (around three times more than gasoline, diesel, or kerosene) but is less flammable than these fuels. The required concentration for hydrogen to combust in air is four times higher than gasoline; hydrogen concentrations in air below 4%, and above 75%, will not burn. In comparison, gasoline concentrations of only 1% are flammable in air (flammability limits of petrol are 1–7.6 vol.%). Flammability range is highest for hydrogen, but as long as it stays in the area of proper ventilation, it is difficult to reach the limit. In addition, hydrogen has a relatively high ignition temperature of 858 K compared to an ignition temperature as low as 501 K for gasoline. Once ignited, even the flame temperature for hydrogen is lower than that for gasoline −2318 K (2470 K for gasoline) [8]. Due to low density, hydrogen does not cumulate near ground but dissipates in the air, unlike gasoline and diesel fuel. Hydrogen and methanol with regard to safety, economics, and emissions aspects have been evaluated by Adamson and Pearsons [9]. Comparative risks analysis in case of accident, self-assessed by the authors in enclosed and ventilated areas showed that both hydrogen and methanol are safer than petrol; there is no winner between the first two fuels, but in certain situations hydrogen may be at higher risk than methanol. Hake et al. [10] compared different fuels and fuel storage systems of exemplary passenger car including safety characteristics of gasoline, diesel, methanol, methane, and hydrogen and concluded that “there are no technical safety or health aspects that generally exclude the introduction of hydrogen as a fuel.” The risk of hydrogen with the infrastructure exists and is different when compared to diesel or gasoline but not higher. In addition, it has to be noted that hydrogen is environmental friendly, producing water vapor as the only waste, compared to other fuels like gasoline. High energy-to-weight ratio and clean by-products are main factors for the automobile industry that make hydrogen a future fuel source. Assuming proper storage and infrastructure, hydrogen can be used in many applications as an alternative fuel or even the fuel of the future.
2.3 Hydrogen Production
There are several ways of producing hydrogen. Most of the technologies are well technically and commercially developed, but some of them are competing to a certain extent with existing energy technologies. Presently, most of the industrial hydrogen is produced from fossil fuels (natural gas, oil, and coal), mainly by steam reforming of natural gas as the leading process on a large scale, partial oxidation of hydrocarbons, and coal gasification, contributing significantly to CO2 gas emission to the atmosphere as a result. It is reported that each kilogram of hydrogen produced by the steam reforming emitted 13.7 kg of equivalent CO2 [11]. A special attention is paid to the renewable production options that include water electrolysis using renewable power (e.g., wind, solar, hydroelectric, and geothermal), biomass gasification, photoelectrochemical and biological processes, and high-temperature thermochemical cycles. The use of biomass for producing hydrogen instead of fossil fuels contribute to the reduction of the atmospheric emissions, but biomass-related processes are used on a minor scale hydrogen production and still in the demonstration phase. Solar hydrogen generated by water electrolysis with solar cells, direct photocatalytic water, photobiological water splitting, or solar thermal processes represents a highly desirable, clean, and abundant source of hydrogen. Water electrolysis is said to be rather efficient, above 70%, but is expensive and estimated at more than $20/GJ assuming a cost of about $0.05 kWh−1 [12]. Water electrolysis is suitable in combination with photovoltaics (PVs) and wind energy and is in direct relation with the availability of electricity. In general, hydrogen production methods (without photovoltaics) can be connected with the nuclear reactor, providing heat and electricity for the process. It is estimated that one of the most promising thermochemical cycles for large-scale hydrogen production is the iodine–sulfur (I–S) cycle (General Atomic) with thermal to hydrogen efficiency of 52% and UT-3 cycle developed at the University of Tokyo with efficiency of around 50%. General pathways of hydrogen production by different processes and from different primary energy sources are presented in Figure 2.2.
Figure 2.2 General pathways of hydrogen production.
The end users of produced hydrogen can be classified in various sectors as presented in Figure 2.3.
Figure 2.3 Hydrogen by means of different sources and its potential application in stationary, transportation, and portable sectors.
In fact a big part of hydrogen produced is used in industry for various syntheses and polygeneration processes and turbines or in households.
Hydrogen can be used in transportation sector to power vehicles by using conventional engines (Diesel, Otto), gas turbines in vehicles. One of the most important advantages is that hydrogen engines emit less pollutants to atmosphere compared to gasoline engines. Combustion product is mainly water vapor and some nitrogen oxides. Certain interest has been risen by aeronautic sector by using liquid hydrogen for direct combustion in gas turbines. Considerable attention has been paid for using hydrogen as a fuel for fuel cells, which are of high and still increasing interest of stationary, portable, and transport groups. Fuel cells are recently one of the most attractive and promising hydrogen utilization technologies. It must be noted that a special attention must be given toward issues related to hydrogen gas transmission, storage and distribution systems, as well as end-user infrastructure. Integrity, durability, safety aspects, standards and norms, and the public acceptance come along with hydrogen application in wide understanding.
The application of hydrogen for different sectors is described above in Section 1.2.
2.3.1 Reforming Processes in Combination with Fossil Fuels (Coal, Natural Gas, and Mineral Oil)
Fossil fuels, coal, oil, and natural gas, are a nonrenewable source of energy. Although combustion of fossil fuel produces significant amount of greenhouse and toxic gases, such as CO2, SO2, NOx, and other pollutants, contributing to the global warming and acid rain, they are still a significant source of hydrogen and electricity production by means of different processes. However, it has to be mentioned that an effort is being made to look for clean and renewable alternatives.
2.3.1.1 Steam Reforming of Natural Gas
Steam reforming of natural gas is currently the cheapest, well technically, and commercially established way to produce hydrogen, mainly used in petrochemical and chemical industries. The cost of hydrogen production from steam reforming strongly depends on the costs and availability of the natural gas feedstock. Steam reforming is highly endothermic (heat is absorbed) gas phase conversion process and requires high reaction temperatures, typically above 600 K (823 °C) in the presence of Fe- or Ni-based catalysts conventionally supported on AL2O3 and MgAl2O4 and pressure of about 3 MPa. As a result, a syngas containing CO and hydrogen is generated. The efficiency of steam reforming is around 65–70%.
General steam-reforming reaction can be described by the following equation:
(2.2)
In addition to carbon monoxide and hydrogen, a certain amount of unreacted steam can be found in the reformate gas as well as some unreacted fuel and carbon dioxide. CO2 is formed by the following water gas shift reaction (WGSR):
(2.3)
WGSR increases the hydrogen concentration of reformate; methane is usually formed in large amount. It occurs in two stages, as high-temperature shift (HTS) at around 350 °C and low-temperature shift (LTS) at around 200 °C. Ideally, WGS reactions should reduce the CO level down to less than 5000 ppm. The reaction is moderately exothermic with low level of CO at low temperatures but with favorable kinetics at higher temperatures. In industrial applications, catalysts based on Fe–Cr oxide are used for HTS and Cu–ZnO–Al2O3 for LTS to achieve a good performance under steady-state conditions [13]. Hydrogen produced by steam-reforming process may require additional purification like desulfurization and CO2 removal. Currently, most of the hydrogen plants use pressure swing adsorption, producing very pure hydrogen (99.9%). In modern hydrogen plants, reforming temperatures can operate above 900 °C and steam-to-carbon (S/C) ratio below 2.5, even below 2.0. Under advanced steam-reforming conditions (high temperature of reforming condition combined with low steam ratio), it is possible to operate hydrogen plants with efficiency less than 3 Gcal/1000 Nm3 of hydrogen. The theoretical efficiency of the hydrogen production from methane by steam reforming is 10.8 GJ/1000 Nm3 H2 when starting from water vapor and 11.8 GJ/1000 Nm3 H2 starting from liquid water [14]. Haldor Tops⊘e developed two types of convection reformers to improve the thermal efficiency of steam reforming, process gas heated reformer (Haldor Tops⊘e exchange reformer, HTER) and the convective flue gas heated reformer (Haldor Tops⊘e convective reformer, HTCR). Using HTCR technology, the reforming efficiencies increased to 80% with capacities up to around 25 000 Nm3 h−1 [15].
Steam reforming of light hydrocarbons is also a well-established industrial process. One of the benefits of using methanol is that the reforming reaction can be carried out at lower temperature – the endothermic steam reforming can be carried out at around 300 °C over a Co–Zn catalyst [16], whereas about 800 °C is required for hydrocarbons. Theoretically steam reforming of methanol can produce 75% hydrogen concentration at 100% CO2 selectivity; in practice, it is greater than 70% with various catalysts [13]. Higher range of temperature (850–1500 °C) is required for the steam reforming of ethanol because of its C─C bond [17].
2.3.1.2 Partial Oxidation and Autothermal Reforming of Hydrocarbons
Partial oxidation reaction is much faster than steam reforming. It is exothermic reaction with oxygen at moderately high pressure with or without catalyst depending on the feedstock and selected process. Noncatalytic Texaco process (TGP) operates at temperatures in the range of 1200–1500 °C and pressure above 3 MPa. Catalytic partial oxidation uses lower temperatures around 1000 °C, but production of pure hydrogen is less efficient and more costly than steam reforming. Catalysts include supported nickel (NiO–MgO), nickel-modified hexa-aluminates, platinum group metals Pt, Rh, Pd/alumina, on ceria-containing supports or on titania [13].
Partial oxidation is the conversion of fuels under oxygen-deficient conditions according to the following formula [18]:
(2.4)
A definite advantage of partial oxidation is that only fuel and feed air are needed for the reaction; there are no evaporation processes. Typical by-product of the reaction is methane and coke formation. The disadvantage of partial oxidation process is catalyst deactivation due to coke deposition and carbon monoxide. Coke formation may be formed due to the following reaction:
(2.5)
It is to be noted that the amount of CO is higher when compared with steam-reforming reaction (clean-up step may be needed in case of connecting with fuel cells). Kolb [18] reports two reaction mechanisms for partial oxidation: (i) reaction begins with catalytic combustion followed by reaction of lower rate (steam reforming, CO2 reforming, and water–gas shift) and (ii) direct POX at very short residence time.
Autothermal reforming (ATR) or oxidative steam reforming combines the endothermic steam-reforming process with the exothermic partial oxidation reaction. It is a combination of both processes in which the energy generated by the partial oxidation reaction provides the energy for the steam-reforming reaction. These systems can be very productive, fast starting, and have been demonstrated with methanol, gasoline, and natural gas. Autothermal reforming has several advantages including improved heat integration, faster start-up, and lower operating temperatures. Catalyst choice for autothermal reforming depends on the fuel and operating temperature. For example, for methanol, Cu-based catalysts similar to commercial methanol synthesis catalysts and LTS catalysts are used. For higher hydrocarbons, catalysts containing Pt, Rh, Ru, and Ni supported or deposited on oxides have been reported; highest selectivity in reforming of iso-octane was obtained on Pt formulation (Pt/Ce0.8Sm0.15Gd0.05O2) [13].
The autothermal reaction can be described as follows [18]:
(2.6)
Air addition should be in limited amounts to prevent coke formation on the catalyst. Usually, an optimum atomic O/C (oxygen to carbon) ratio exists for each fuel under thermally neutral conditions to achieve optimum efficiency. The maximum efficiency available from autothermal process for various hydrocarbons was given by Hagh [19] at fixed S/C ratio and pressure. Simulation on autothermal reforming on low and high molecular weight hydrocarbons (LHCM and HHCM, respectively) showed 80% efficiency for LHCM at 700 °C at S/C 3.5 and O/C ratio 0.28% and 80% efficiency for HHCM at the same temperature, S/C 3.5, O/C 0.5 [20]. önsan [17] reported that higher S/C ratios and reactor inlet temperature favor hydrogen production. Less fuel has to be burned for the reaction and as a consequence lower O/C ratio can be used. Optimal S/C ratios have been reported for different fuels: 4 for methane, 1.5 for methanol, 2.0 for ethanol, and 1.3 for surrogate gasoline. Increase of S/C ratio favors hydrogen production [17]. CO formation is depressed at higher S/C ratios especially at higher temperatures. Higher operational temperatures enhance CO formation and reduce hydrogen production.
2.3.1.3 HyPr-RING Method to Produce Hydrogen from Hydrocarbons
Innovative hydrogen production method, HyPr-RING (hydrogen production by reaction-integrated novel gasification), using hydrocarbons and water was proposed by Lin et al. [21]. General idea of the concept is the integration of water carbon reaction, water–gas shift reaction, and CO2 absorption reaction in a single low-temperature reactor to produce clean and highly concentrated hydrogen, as shown in Figure 2.4.
Figure 2.4 General concept of the HyPr-RING process [21], reprinted by permission.
Hydrocarbons are mixed with water in the reactor operating at temperature 650–700 °C and high pressure (10–100 MPa). Following reactions occur in the reactor:
(2.7)
(2.8)
(2.9)
(2.10)
The experimental results show that all hydrocarbons can produce high concentrations of hydrogen, up to 80% of the product gas. Besides hydrogen, methane was also present in the gas as well as trace amounts of CO and CO2 [21].
The advantages of the HyPr-RING process is that it can be applied to hydrocarbon sources such as coal, heavy oil, biomass, plastic, and organic waste (Figure 2.5). In a single reactor, these materials can produce sulfur-free gas containing up to 80% of hydrogen and 20% of methane. Hydrogen produced is of high quality and can be fed directly to the fuel cell [22].
Figure 2.5 Gas production by HyPr-RING process from various hydrocarbons [22].
2.3.1.4 Plasma-Assisted Production of Hydrogen from Hydrocarbons
Thermal and nonequilibrium plasma are under consideration as a way for hydrogen production from hydrocarbons. The advantages of thermal plasma chemical methods are very high specific productivity of the apparatus, low investment, and operational costs. The disadvantage is high-energy consumption. The gas and electron temperatures and the energy content are very high; the gas temperature is between 10 000 and 100 000 K. Thermal plasma is an equilibrium plasma because temperatures of the gas and electrons are nearly equal.
Nonthermal plasmas are known as a very high energy density media, able to accelerate the reactions at low temperatures, nonequilibrium properties, and low-power requirements. In addition, if active species generated by the nonthermal plasma can promote many cycles of chemical transformation, high productivity of plasma can be combined with low-energy consumption of conventional catalysts (plasma catalysis). Nonthermal plasma reforming shows interesting results of efficiency, conversion rate, and hydrogen production (Figure 2.6). As an alternative for hydrocarbons reforming for the development of fuel cells in vehicles, nonequilibrium plasma technique has been implemented over the past two decades. Various plasma types have been used: plasmatron, gliding arc, dielectric barrier discharge, corona, microwave, and pulsed discharge [23]. Hydrocarbon conversion in gliding arc showed following results: 3 kWh Nm−3 (cost 2.5 eV mol−1) of syngas for steam reforming, 0.11 kWh Nm−3 (cost 0.09 eV mol−1) of syngas for partial oxidation in oxygen, 0.3 kWh Nm−3 (cost 0.25 eV mol−1) of syngas for steam–oxygen conversion, and 0.98 kWh Nm−3 (cost 0.82 eV mol−1) of syngas for air–steam conversion in the presence of nickel catalyst [24]. In combination with the pulse microwave discharge by small addition of microwave energy (up to 10% of thermal energy input), a significant increase of conversion degree was observed, resulting in reduction of hydrogen energy costs. In this process, hydrocarbons were first preheated up to 427–727 °C in a conventional heat exchanger followed by nonequilibrium pulse microwave discharge. In case of methane decomposition, conversion degree increased from 7% to 18% (H2 plasma energy cost 0.1 eV mol−1), ethane decomposition from 19% to 26% (H2 plasma energy cost 0.25 eV mol−1), ethanol decomposition from 23% to 62% (H2 plasma energy cost 0.1 eV mol−1), methane–steam decomposition from 10% to 16% (H2 plasma energy cost 0.35 eV mol−1), and ethanol–steam decomposition conversion rate increased from 41% to 58% (H2 plasma energy cost ∼0.1 eV mol−1) [24]. Reforming of hydrocarbons (methane) and alcohols (methanol and ethanol) in mixtures with CO2 or H2O has been performed in dielectric barrier discharge [25]. No higher hydrocarbons were formed during the reforming of the fuels; the majority of the products were hydrogen and carbon monoxide. For methanol and ethanol, 100% conversion was obtained for relatively high flow of the reactants. Calculated energy consumed in the discharge showed that 1 mol of hydrogen can be produced by energy approximately 4 kWh for methane, 0.6 kWh for methanol, and 0.3 kWh for ethanol [25].
Figure 2.6 Conversion rates of different nonequilibrium plasma-assisted technologies [23], by kind permission of MINES ParisTech – CEP, Plasma Group.
Novel sliding discharge reactor for hydrogen generation from hydrocarbons has been introduced by GREMI, University of Orleans, France [26]. This reactor was used for steam reforming of methane and propane at atmospheric pressure. The main products of the plasma process were hydrogen (50%), carbon monoxide (up to 30%), and nonconsumed methane or propane. For the same experimental conditions, methane conversion rate was higher than those of propane.
Major works concerning nonequilibrium plasma-assisted reforming have been presented and compared, among others at the Plasma Science and Fusion Center at MIT, USA; Drexel Plasma Institute, USA; Siemens AG, Germany; Kurchatov Institute, Russia; ECP, France; Waseda University, Japan, and so on. A comparison between nonequilibrium plasma reforming reactors (efficiency, conversion rate) for hydrogen production from various hydrocarbons showed that arc discharge-based technologies meet in the best way performances due to their relative simplicity of the set up, high-energy densities, and their ability to create a large reactive volume [23]. The GAT reactor reached the top value with 79% (Figure 2.7). The conversion rates and energetic efficiencies of different nonequilibrium plasma-assisted technologies are presented below.
Figure 2.7 Energy efficiencies of different nonequilibrium plasma-assisted technologies [23], by kind permission of MINES ParisTech – CEP, Plasma Group.
2.3.1.5 Coal Gasification
Systems based on gasification can utilize coal, petroleum coke, biomass, municipal, and hazardous wastes. In principle, the process is similar to partial oxidation of heavy oils and has three main steps, (i) conversion of coal feedstock in the presence of oxidant (typically oxygen or air and steam) to syngas at high temperatures of 1000–1500 °C in gasification reactor, (ii) catalytic shift reaction, (iii) and purification of the produced hydrogen, mainly residual carbon and ash. Depending on the gasification technology, certain amount of water, carbon dioxide, and methane can be present in the syngas including traces components, for example hydrogen cyanide (HCN), hydrogen chloride gas (HCl), hydrogen sulfide (H2S), and carbonyl sulfide (COS). Generated syngas can be used directly to produce electricity or be further processed to pure hydrogen for hydrocracking of petroleum or ammonia production. A lot of research has been performed on integrating the gasifier with a combined cycle gas and steam turbine (IGCC – integrated gasification combined cycle) and a fuel cell (IGFC – integrated gasification fuel cell). IGFC systems tested in the United States, Japan, and Europe with gasifiers by Texaco, Eagle, and Lurgi showed energy conversion at efficiency of 47.5% (HHV), higher than the efficiency of conventional coal gasifiers [27]. Most of the carbon components are removed before combustion when the gas turbine is used before conversion of the feedstock in air or oxygen/steam step. The largest worldwide IGCC Power Plant with 318 MWel is build in Puertollano, Spain by Elcogas. Worldwide, there are 117 operating plants, 385 gasifiers with a total production capacity of around 45 000 MWth [28]. Major gasification technologies are developed by Texaco, E-Gas, Shell, Kellogg, British Gas/Lurgi, KRW, and PRENFLO [27]. In gasification process, other fuels like biomass may also be used (see Section 2.3).
Figure 2.8 presents schematic process based on gasification. In this process, carbon-based feedstock is converted in the reactor (gasifier) to synthesis gas, being a mixture of hydrogen and CO. This process takes place in the presence of steam and oxygen at high temperatures and moderate pressure.
Figure 2.8 Gasification-based energy-conversion options.
Stiegel and Ramezan [29] estimate that the availability of the gasifier must be greater than 97% for hydrogen production from coal. Department of Energy (DOE) Cost Targets in 2017 for coal gasification are estimated to be less than $1.10 for central production scale (with and without carbon capture and storage) [30].
2.3.2 Water-Splitting Processes (Hydrogen from Water)
Hydrogen may be produced from water by water electrolysis in low-temperature process and from steam at elevated temperatures. Another way to generate hydrogen from water is the direct water-splitting process at high temperatures.
