Hydrogen Manufacture by Electrolysis, Thermal Decomposition and Unusual Techniques    This book deals with sources and processes for the production of hydrogen. Hydrogen is often produced from natural gas and by petroleum refining. However, it is extremely important to understand and develop other means for obtaining this essential energy carrier. Hydrogen, like electricity, is not a naturally occurring energy form, but must be manufactured from basic energy resources. Again, like electricity, any basic energy resource could be used to produce hydrogen. Hydrogen Manufacture by Electrolysis, Thermal Decomposition and Unusual Techniques presents a survey of existing and future technologies for hydrogen pro­duction with an eye to their usefulness for a "hydrogen economy." Published technological studies were the basis for this review. The available material has been organized, abstracted and excerpted so as to provide YOU with a comprehensive overview of investigations and analysis of different hydrogen production practices that have been accomplished.

 

The most likely long-term energy economy will utilize mixed hydrogen/electric means to carry energy to end-use.  Hydrogen and electricity are the obvious technological complements because neither electricity nor hydrogen is the best choice for all applications.

Today, hydrogen is made mainly from fossil fuels.  Natural gas is the primary fossil fuel source, but some is made from petro­leum fractions. Petroleum refining accounts for most of this production. However, coal is the cheapest, nearest-term, large-scale source of hydrogen because coal and water are the basic chemical feed stocks needed to produce coal-derived hydrogen by well understood processes.

Nuclear power is the leading intermediate-term source of energy for hydrogen production, either by proven electrolytic means or by proposed, but unproven, thermochemical means. Both technologies require substantial improvement to compete economically with hydrogen derived from coal. Nuclear energy is also a likely long-term source of energy for hydrogen production.

Solar energy is a long-term candidate for production of hydrogen as well, either through electrolytic or thermochemical means; ultimately, solar-produced hydro­gen is likely to become competitive with hydrogen derived from nuclear energy.

Most of the data in this book are based on federally funded studies. Because the information in this book is taken from many sources, it is possible that certain portions of this book may disagree or conflict with other parts of the book. This is especially true of monetary values and opinions of future potential. These different points of view, were included however, in order to make the book more valuable to the reader. Cost figures provided are those given in the cited report, the date of which is always given in the bibliography. When the dates of the cost figures are given, they are included.

The first chapter of Hydrogen Manufacture by Electrolysis, Thermal Decomposition and Unusual Techniques presents a full survey of a wide range of hydrogen technologies. The second chapter encompasses those methods which produce hydrogen from fossil fuels. Water electrolysis, as it might be used in a hydrogen economy, is covered in the third chapter, while the fourth chapter surveys proposed thermochemical means for splitting water to produce hydrogen. In the fifth chapter, more unusual sources and processes which may someday be useful in the conversion to a hydro­gen economy are presented. The sixth chapter provides an overview of the eco­nomics of a hydrogen economy and the final chapter discusses various aspects of hydrogen safety. A complete bibliography of the published source material on which this book is based is also included.

The table of contents is organized in such a way as to serve as a subject index and provides easy access to the information contained in this book. Each chapter is followed by a list of references giving further details on these timely topics and the bibliography at the end of the volume lists highly important government re­ports.

 

 

 

 

 

 

 

INTRODUCTION

HYDROGEN PRODUCTION FOR THE HYDROGEN ECONOMY

Hydrogen Production from Fossil Fuels

 

 

FIGURE 1.1:  TYPICAL HIGH TEMPERATURE, ATMOSPHERIC PRESSURE COAL GASIFICATION PROCESS

 

 

 

 

 

 

 

 

 

 

 

 

Hydrogen from Water by Electrolysis

Closed-Cycle Thermochemical Decomposition of Water

Mixed Thermal/Electrolytic (Hybrid)

Hydrogen from Thermonuclear Fusion

Comparison of Hydrogen Production Processes

References
 

HYDROGEN PRODUCTION FROM FOSSIL FUELS

Production of Hydrogen by Coal Gasification

Gasification Reactions for Production of Hydrogen

Gas Producers

 

 

 

 

 

Gas Producers

In its simplest form, a gas producer consists of a vertical brick-lined vessel on top of which is a charging hopper and on the bottom of which is a grate with the air and steam blast distributor plates. A diagrammatic sketch of a gas pro­ducer is shown in Figure 2.1 (12).

The fuel bed in the gasifier travels vertically from top to bottom, being consumed in the process of coal gasification. One can divide the bed into a succession of horizontal zones: the ash zone, the combustion (or oxidation) zone, the reduc­tion zone, and the distillation zone.

(1)     The ash zone extends from the grate upward toward a carbon combustion zone. Air and steam enter as uniformly as pos­sible over the whole section and move upward through the bed. The air and steam cooled ash is removed by continu­ously rotating or intermittently moving the grate. Any clinker formed earlier is crushed. The ash is usually dropped into water-sealed troughs. For proper operation, it is most important that the blast distribution is uniform, that no gas channeling occurs, that no significant quantities of carbon re­main and that the ash does not fuse.

(2)     The combustion zone occupies a rather thin (about six inches thick) region in which oxygen reacts with carbon in the fuel. The temperature of this zone may reach about 3000°F. In the bottom of the zone the carbon oxidation product is pri­marily carbon dioxide. Near the top of this zone carbon mon­oxide is formed. The temperature must be closely controlled to prevent the formation of large clinker lumps, by the proper proportioning of steam and air.

(3)     The reduction zone extends for a distance from one to five feet, in which the temperature drops from about 2200° to about 1500°F. Here no significant oxygen remains, most of the car­bon dioxide is converted to carbon monoxide and a signifi­cant concentration of hydrogen is first noticed. Near the top of the zone no significant gasification of fixed carbon occurs.

(4)     The distillation zone is a zone in which the hot gases from below preheat the coal and cause the volatilization and the cracking of the more volatile coal constituents. The character of the products of the distillation zone depends most strongly on the type of coal fed into the gasifier.

 

 

 

 

 

 

 

 

Lurgi Gasifiers

Koppers-Totzek Gasification Process

Winkler Gasification Process

Other Coal Gasification Processes

Comparison of Various Processes of Coal Gasification

Conclusions

Commercial Technology for Hydrogen Production

Catalytic Steam Reforming of Natural Gas

Partial Oxidation of Hydrocarbons

250 x 10^9 Btu of Hydrogen per Day from Montana Subbituminous Coal

By the Koppers-Totzek Process

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

By the U-GAS Process

By the Steam-Iron Process
Use of High Temperature Nuclear Heat from an HTGR for Hydrogen Production

Hydrogen from Natural Gas

Hydrogen from Coal

By Direct Gasification of Coal with Steam
By Lurgi Gasification of Oklahoma Coal
Selection of Fossil-Based Hydrogen Production (Coal Gasification) System
Hydrogen Production from Coal Liquefaction Residues
Texaco Pilot Plant Process
References
 

HYDROGEN PRODUCTION BY ELECTROLYSIS

An Overview of the Technology

Principles of Electrolysis

Energy Requirements for Electrolysis

Effect of Pressure on the Decomposition Voltage

Basic Designs of Electrolyzer Cells

 

Basic Designs of Electrolyzer Cells The oldest form of industrial electrolysis of water uses the tank electrolyzer in which a series of electrodes, anodes and cathodes alternately, are suspended verti­cally and parallel to one another in a tank partially filled with electrolyte. Alter­nate electrodes, usually cathodes, are surrounded by diaphragms that prevent the passage of gas from one electrode compartment to another. The diaphragm is im­permeable to gas, but permeable to the cell's electrolyte. The whole assembly is hung from a series of gas collectors. A single tank-type cell usually contains a number of electrodes, and all electrodes of the same polarity are connected in parallel, electrically, as pictured in Figure 3.2. This arrangement allows an individual tank to operate across a 1.9 to 2.5 volt dc supply. In general, the cost of electrical conductors increases as the current load increases, but the cost of ac-dc rectification equipment per units of output de­creases as the output voltage increases. This is one important consideration in the design of tank-type electrolyzers. There are two major advantages to tank-type electrolyzers. (1)     Relatively few parts are required to build a tank-type electrolyzer, and those parts that are needed are relatively inexpensive. Be­cause of this feature, tank-type electrolyzers tend to optimize at a lower thermal efficiency than do more sophisticated electro­lyzer structures. Therefore, tank-type electrolyzers are usually selected when electric-power costs are at their lowest. (2)     Individual cells may be isolated for repair or replacement simply by short-circuiting the two adjacent cells with a bus bar. This feature allows maintenance to be carried out with a minimum of downtime for the entire plant.

 

Electrolyzer-System Designs

Survey of Types of Industrial Electrolyzers

 

In the static water feed system, the water to be electrolyzed is supplied to the cell electrolyte as a vapor. Each cell is divided into three main compartments: a water-feed compartment, a hydrogen-gas compartment, and an oxygen-gas compartment. Compartment separation and liquid-vapor phase separation are achieved by the capillary action provided by liquid-filled asbestos sheets. Cat­alyzed porous-nickel plaques support the cell matrix, forming a composite elec­trolysis site. Plastic screens similarly support the water feed matrix. The cell configuration is given in schematic form in Figure 3.7, and Figure 3.8 shows the principle of cell operation. The latter represents a thermally insulated box enclosing two bowls of electrolyte. When power is applied to the electrodes, water in the cell electrolyte is consumed. As a result, the concentration of the cell electrolyte increases, causing its vapor pressure to drop below that of the feed-compartment electrolyte. This difference in vapor pressure is the driving force that causes the water vapor to diffuse across the hydrogen cavity to the cell matrix.

 

Comparative Evaluation of Various Electrolyzers

Ultimate Potential

Research and Development Requirements–Engineering Problems

Electrolyzer-Feedwater Quality Standards

Survey of Industrial Electrolyzers

Electrolyzer Corporation Ltd

Teledyne Isotopes

Lurgi Apparate-Technik GmbH

Bamag Verfahrenstechnik GmbH

Norsk Hydro Verksteder A/S

Life Systems, Inc.

General Electric

Comparison of Electrolysis Cells

Operation and Design of Large-Scale Plants

General Construction of an Electrolytic Cell

Conventional and Modified Water Electrolysis Systems for Large-Scale Production

 

 

 

 

Figure 3.17 shows the schematic construction of a few cell types. Table 3.3 lists some of the technical data of typical, conventional water electrolysis methods.

1  Cell container 2   Electrolyte 3   Cathode 4   Anode 5   Hydrogen outlet 6   Oxygen outlet

7   Gas jars 8   Diaphragm 9   Output cathode 10 Output anode 11 Bipolar electrode 12 Isolators

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE 3.3: SOME CHARACTERISTIC FIGURES FOR CONVENTIONAL WATER ELECTROLYSIS SYSTEMS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Dedicated Thermal Electric Plants

Component Characteristics

Resource Impact

Use of Heat from High-Temperature Nuclear Sources

Comparative Data on Electrolyzers

Environmental Constraints

Water Electrolysis Plants

Electrolyzer Selection

Nuclear Electrolytic Hydrogen Production Facility

Methods for Approaching Ideal Efficiencies for Hydrogen Production
Comparison of Cell Potential-Current Density Relations in Acid and Alkaline Water Electrolysis CelIs
Effect of Temperature on the Performance of Acid and Alkaline Water Electrolysis Cells
Effect of Temperature on Hydrogen and Oxygen Overpotential at Nickel Electrodes in Concentrated Potassium Hydroxide Electrolyte
Evaluation of Separator Materials to Replace Asbestos in Alkaline Solution

Conclusions

Solid Electrolyte and Elevated Temperature Water Electrolysis

General Electric Solid Polymer Electrolyte Cells

Solid Oxide Electrolyte Cells
Alkaline Water Electrolysis Cells Operating at Temperatures Between 120° a nd 150°C

 

 

 

 

Electrocatalysis of Hydrogen and Oxygen Evolution Reactions in Alkaline Solu­tion at 80°C: A systematic study of the electrocatalytic activities of a number of metals for the hydrogen and oxygen evolution reactions from 30% KOH at 80°C have been carried out (83). There is a periodic variation of hydrogen over-potential with atomic number (Figure 3.33). The overpotentials (at a current density of 2 mA cm-2) have minimum values for Ni, Pd and Pt which have d2s2, d10s0 and d9s1 electronic configurations respectively. High overpotentials are ob­served for Zn, Cd and Hg, all with d10s2 electronic configurations. The periodic trend for hydrogen overpotential shows a high correlation with the work func­tion of the elements.

 

 

 

 

 

 

 

 

 

 

 

 

Current Status of Advanced Water Electrolysis Cells

Photoelectrochemical Effects

References

HYDROGEN PRODUCTION BY THERMOCHEMICAL PROCESSES

Water Electrolysis vs Thermochemical Production

Electrolytic Production—Off-Peak Power

Electrolytic Production—Dedicated Nuclear Plant

Thermochemical Production of Hydrogen

Hydrogen Production Co^q Computations

Sensitivity Analysis

 

 

The data in Figure 4.2 assume an efficiency of one-third for the electric produc­tion efficiency and treat the efficiency of producing process heat, f, as variable, i.e., 0.5, 0.7 and 0.9. In a process heat HTGR, f can be related to the installa­tion of intermediate heat exchanger between the reactor and the hydrogen pro­duction plant.

Based on the best information available, the ratio of capital costs for the corre­sponding nuclear facilities, i.e., process heat vs electric production will probably be in the range of 0.2 to 0.3. If the expected value is 0.3, the ratio of the re­spective production efficiencies must be 0.3 for f = 0.7. Thus, if thermochemical production of hydrogen can achieve an efficiency of 30%, it will be competitive if its capital cost is equal to the capital cost of an electrolysis plant with a pro­jected efficiency of 90%.

In actual fact, the efficiency of electricity production may also improve to levels beyond one-third. This tends to make water electrolysis more competitive. For example, for a capital cost ratio of 0.3, the hydrogen production efficiency ratio changes from 0.33 to 0.36 for f = 0.7, when the electric production efficiency

is increased from one-third to one-half. Thus, it can be concluded that thermo­chemical production plants must strive to achieve practical efficiencies greater than 30% based on the expectations of current R&D on water electrolysis systems.

Estimates for processes under study suggest that theoretical efficiencies are in

the range of 45 to 50% (6)(7)(8). Electricity generation plants are able to achieve approximately two-thirds of the theoretical Carnot efficiency. Thus, on the same basis, these processes must achieve two-thirds of the theoretical efficiency in a practical system at a capital cost equal to, but no greater than, the capital cost of a competing electrolytic production plant to be competitive. A compari­son of electrolytic and thermochemical hydrogen production costs at various process efficiencies is shown in Figure 4.3.

 

 

 

Sensitivity of Capital Costs to Efficiency Variations

It is also possible by means of a parametric analysis to determine the allowed capital cost differential of the two types of plants as a function of the efficiency of electrolytic and thermochemical production processes. Again, it is possible to make such comparisons without specifying the absolute capital costs involved in each type of production plant. The results shown previously in Figure 4.2 were arrived at on the assumption that the capital and O&M costs of both types of plants are equal. In this case the ratio of the capital cost of a nuclear process heat plant ($/kWt) to the cost of a nuclear electric plant ($/kWe) vs the ratio of thermochemical to electrolytic production efficiency for various reactor heat utilization efficiencies was examined and cost ratio varied from 0.1 to 0.6. By choosing a fixed cost ratio of one-third and assuming a reactor heat utilization efficiency of 90% for the thermochemical process heat plant, the allowed capital cost differential between an electrolytic and thermochemical production plant, Ce-Ct, are calculated for various efficiencies of the two different production schemes. In this case, the O&M costs of both plants are assumed to be equal.

The results of this analysis are given in Figure 4.4. Ce and Ct are the respective cost of the electrolytic and thermochemical production plants in $/kWt of hydro­gen output. In Figure 4.4, Ce-Ct is plotted vs the thermochemical production efficiency for various efficiency values of the electrolytic plant. Positive values of Ce-Ct correspond to the condition where the thermochemical plant capital cost must be greater than the electrolytic plant capital cost to be competitive. When Ce-Ct is negative, thermochemical plant capital cost can be greater than the corresponding cost of the electrolytic plant.

It is evident from Figure 4.4 that if the electrolytic plant production efficiency achieves 90%, the thermochemical efficiency must be at least 30% to be compet­itive. If the thermochemical plant achieves 40% efficiency, the capital cost can be higher than for the electrolytic plant by $75/kWt hydrogen output, i.e., the value of Ce-Ct. Using Figure 4.4 it is possible to specify various electrolytic and thermochemical efficiencies and determine the allowed capital cost differential, Ce-Ct, for the two different production schemes. A more explicit comparison of electrolytic and thermochemical hydrogen production costs, as a function of the process equipment cost is shown in Figure 4.5. These results are valid for the basic cost assumptions noted in this figure.

 

Off-Peak Power Costs

Break-Even Efficiency Ratio

Sensitivity of Capital Costs to Efficiency Variations

Conclusions

Survey of Thermochemical Methods

Basic Thermodynamic Considerations

Efficiency Calculations for Thermochemical Cycles

Evaluating Cycles

Heat Source Specifications and Availability

Status of the Technology

Halide Processes

Reverse Deacon Processes

Metal Processes

Ultimate Potential
Selecting A Water Decomposition Process Using Heat From High Temperature Nuclear Sources

Study of the ISPRA Mark-10 Process

Assumptions and Calculation Bases

Subsystem Functional Descriptions

Side Reactions with Potential Process Significance

Materials Availability

Environmental Constraints and Plant Effluents

Discussion of Results Assessment of Economics of Hydrogen Production from Lawrence

Livermore Laboratories ZnSe Cycle

Process Description

Key Equipment Requirements

Reference Plant
Design of an Integrated Nuclear Hydrogen Production Plant Using the Sulfur Cycle

The Westinghouse Sulfur Cycle Water Decomposition Process
Energy Sources for the Westinghouse Sulfur Cycle Water Decomposition System

References
 

INVESTIGATIONS OF UNUSUAL METHODS OF HYDROGEN PRODUCTION

Biophotolysis of Water

Survey of Photosynthetic Processes
Solar Energy Conversion of Water to Hydrogen Using a Ruthenium Complex

 

 

 

 

 

 

 

 

This insoluble ruthenium complex having the metal atom at the center of three bipyridyl residues and with long octadecyl tails attached to the ring has the unusual property of separating the reactive components of water to form molecular hydrogen and oxygen.

 

 

 

 

 

 

 

 

 

 

 

Radiolysis of Water by Dissolved Fission Products

A Survey of Other Processes

Hydrogen Production from Waste Materials

Hydrogen Production by Radiation

Direct Thermal Decomposition of Water to Produce Hydrogen

Steam-Iron System for the HYGAS Process

 

 

 

FIGURE 5.5:  IGT'S HYGAS PROCESS WITH HYDROGEN GENERATION BY THE STREAM-IRON PROCESS

 

Figure 5.5 is a flow diagram of the steam-iron process integrated with the HYGAS process. The steam-iron system comprises three principal vessels: (1) the pro­ducer, in which residual HYGAS char is converted to reducing gases; (2) the reducer, in which iron oxide is reduced with producer gas; and (3) the oxidizer, in which the iron is reoxidized and the hydrogen is produced.

In integrated operation, spent char from the HYGAS process is fed directly to a producer vessel in which it reacts with air and steam to generate a gas capable of reducing iron oxide to iron. (In actual operation, the producer zone is con­tained within the hydrogasifier pressure shell.)

 

 

 

 

 

 

 

 

 

 

 

References

ECONOMICS OF HYDROGEN PRODUCTION

Development of Cost Estimates

Hydrogen Production Costs

Hydrogen from Coal

Hydrogen from Electrolysis of Water

 

 

 

 

 

FIGURE 6.2:  COST OF HYDROGEN FROM COAL AS A FUNCTION OF COAL COST

TABLE 6.4:  COST ESTIMATES FOR PRODUCING HYDROGEN FROM COAL

 

 

 

Hydrogen from Thermochemical Processes

Costs of Other Fuels Compared to Hydrogen Systems

References

HYDROGEN SAFETY

Complexity of the Safety Question

Properties of Hydrogen

Hydrogen Behavior

 

FIGURE 7.1:  MAXIMUM VERTICAL CROSS SECTIONS OF FLAMES

Note that in 2 seconds the flames are nearly all dispersed upwards. Extreme care must be exercised, however, in drawing inferences from this figure about the behavior of a large spill. (A railroad tank car holds about 28,000 gallons of liquid hydrogen. This is more than 1000 times larger than the spill shown in Figure 7.1.) If liquid hydrogen were to leak from a small orifice, the effects would be less dramatic than in a large rupture spill but they would still be im­pressive, especially since a stream leaking from a small orifice often ignites be­cause of static electric discharges (6).

The description above illustrates that blanket statements about hydrogen safety are inadequate because the final result of the spill depends upon the nature of the rupture, the degree of confinement, the nature of the actual nearby objects, and whether combustion is initiated. With the present state of knowledge about hydrogen and its accidents, discussions of its safety usually assume the form of "on the one hand this, but on the other hand that." Some of these kinds of discussions follow the figure.

 

 

Experience in the Space Program and the Hindenburg

Special Hazards

Summary of Physical and Chemical Aspects of Safety

Perceptions of Safety Hazards—A Key to Policy

Voluntary vs Involuntary Exposure to Hazards

References

BIBLIOGRAPHY

 

 

 

 

Hydrogen Manufacture by Electrolysis, Thermal Decomposition and Unusual Techniques    This book deals with sources and processes for the production of hydrogen. Hydrogen is often produced from natural gas and by petroleum refining. However, it is extremely important to understand and develop other means for obtaining this essential energy carrier. Hydrogen, like electricity, is not a naturally occurring energy form, but must be manufactured from basic energy resources. Again, like electricity, any basic energy resource could be used to produce hydrogen. Hydrogen Manufacture by Electrolysis, Thermal Decomposition and Unusual Techniques presents a survey of existing and future technologies for hydrogen pro­duction with an eye to their usefulness for a "hydrogen economy." Published technological studies were the basis for this review. The available material has been organized, abstracted and excerpted so as to provide YOU with a comprehensive overview of investigations and analysis of different hydrogen production practices that have been accomplished.