hydrogen production and technology: today, tomorrow and beyond

Int. J. Hydrogen Energy Vol. 9. No, 8, pp. 649-668, 1984. Printed in Great Britain.

0360-3199/84 $3.00 + 0.00 Pergamon Press Ltd.

© 1984 International Association for Hydrogen Energy.

HYDROGEN PRODUCTION AND TECHNOLOGY: TODAY, TOMORROW AND BEYOND

W . B A L T H A S A R

Kinetics Technology International Group, Zoetermeer. The Netherlands

(Received for publication 7 June 1983)

Abstract--Hydrogen is produced on a large scale by a wide variety of processes starting with feedstocks like natural gas, crude oil products to coal as well as water-using processes like steam reforming, partial oxidation, coal gasification, metal-water processes and electrolysis. Hydrogen is also recovered from various gas streams especially in refineries.

Depending on the basic energy scenarios to be used, steam reforming natural gas will remain the major hydrogen source from today till tomorrow, i.e. the turn of the century. Coal gasification will significantly increase in its share for hydrogen production. This will be achieved via newly developed coal gasification processes.

The development of thermochemical hydrogen production technology as well as biological hydrogen production technologies will progress, but their widespread application remains to be seen in the next century.

1. I N T R O D U C T I O N

Hydrogen is one of the prime e lements of the universe. We all live on earth from hydrogen energy produced

by the sun. Water, which covers more than 70% of the earth's surface has the molecular formula of H20 . Our major building block of living matter is hydrogen in the

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Fig. 1. Hydrogen production.

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650 W. BALTHASAR

molecular structures of hydrocarbons, proteins, amino acids, and the genetic information in DNA, RNA and the like.

The first discovery of hydrogen is attributed to Par- acelsus at around 1500. In 1700, Lemory showed that a mixture of hydrogen and air explodes in the presence of a flame and, in 1781, Cavendish proved that hydrogen burns in air to form nothing but water. Hydrogen at this time was produced by metal reactions with acids, but later in the early industrial days of the 19th and 20th century electrolysis and coal-water-gas reactions were the main sources of hydrogen. The steam/iron processes using "producers" were used to prepare hydrogen, which was free from carbon monoxide. Only when natural gas became available in the 1940s did steam reforming of natural gas become the major source of hydrogen, and will remain so during the rest of this century [1].

Today hydrogen is considered a utility chemical. It is produced in large quantities as outlined in Fig. 1 [2]. Other contributions are focusing more on this subject [3-5]. Most of the needs for hydrogen are supplied by captive production in which hydrogen manufacture is integrated into the overall processes of refinery operation, ammonia and methanol production. Modern hydrogen production facilities are considered utility operations by producers, who desire to produce totally different products, such as in Fig. 2.

Today hydrogen is mainly produced via the following technologies:

(i) steam reforming of natural gas, (ii) partial oxidation of heavy oils,

(iii) coal gasification, (iv) electrolysis of water.

Hydrogen is also generated in certain refinery systems. In Fig. 3 a rough distribution is given on the various production amounts [6].

Generally the raw material available determines the process applied. Electrolysis of water is only applied when smaller amounts and high purity or distribution considerations render the high costs economically acceptable [7]. There are few pipeline supply systems in the world; the largest is Germany's Ruhr system operated by Chemische Werke Hiils AG [8]. Due to the fact that a hydrogen energy system with its distribution system is not inplace, a lot of by-product hydrogen (i.e. from chlorine/caustic electrolysis) is either blown to atmosphere or recently at least fired in boilers [9].

Hydrogen production will remain based on today's feedstocks and technologies, though improved pro- gressively, until the turn of the century [10]. A shift from natural gas to coal may, by then, be evident worldwide, with some countries and companies standing out in any one process area. This is already evident

REFINING 47%

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Fig. 2. Hydrogen uses.

HYDROGEN PRODUCTION AND TECHNOLOGY 651

Fig. 3. Hydrogen sources.

today, where we see coal going to the power industry in most countries if they use coal at all; and natural gas to industrial, chemical uses and the private public sector. Coal is only used for gasification in those countries, where availability and political reasons versus an absence of gas, oil and money dictate so. Thus coal is used in South Africa and India.

Germany will have demonstrated by tomorrow a variety of new coal gasification technologies including the use of nuclear reactors as heat source. Some countries (France) will use electrolysis of water as storage means of electricity produced in off-peak hours by nuclear or hydro power plants.

It is also important to point out the development of hydrogen purification technologies, which represent unit operations of today's hydrogen production processes having natural gas and coal as energy sources. There are new sulphur resistant shift catalysts available. New gas treatment processes, stretford, selexol, sepasolv, sulfinol-M, complement the long existing processes like rectisol, sulfinol-1, benfield, amine washes, etc. To the cold box separation, Pressure Swing Absorption (PSA) has been added as a proven gas separation process [11]. Membrane gas separation processes like Prism have proven their hydrogen recovery capability from special waste gas streams containing hydrogen [12].

Beyond this time frame technologies using regenerative energy sources will become available for hydrogen production [13]. These technologies will all involve water splitting by one energy source or the other. Fusion energy, the last one to become available, could be used for direct thermal decomposition of steam. Whereas thermochemical and hybrid (thermochemical and elec-

trolysis) processes will be technologically proven, temperature -solar, -geothermal, -wind, temperature gradient, biological-solar, hydrogen production processes will still be in developmental stages, whereby some of them will have to prove their environmental accepta- bility, i.e. a lot of solar panels occupying major areas or wind generators doing the same with the additional creation of noise and landscape problems.

2. PRODUCTION METHODS

2.1. Today

2.1.1. Steam reforming. For several decades, the process of high temperature steam reforming (HTSR) has been the most efficient, economical and practical tech- nique available for conversion of light hydrocarbons to hydrogen and hydrogen/carbon-monoxide mixtures for the manufacture of ammonia, ethanol, oxo-alcohols, and a wide range of petrochemicals and chemicals. Raw materials range from natural gas, methane, and methane containing refinery gases through various combinations of light hydrocarbons including ethane, pro- pane, butane, pentane, light naphtha and heavy naphtha (490 K end point). The reforming reaction typically is carried out with steam-to-carbon ratios in the range of 2.5-5.0 at process temperatures from 970 to l l 0 0 K and pressures up to 3.5 MPa.

With natural gas, or naphtha as feedstocks, the reforming process follows a series of reversible reactions. At the same time the water-gas shift equilibrium is established. All these reactions proceed almost to

652 W. BALTHASAR

equilibrium and the composition of the gas is defined by the dependence of these equilibria on temperature, pressure and steam-to-carbon ratio. In reforming it is found that the CO shift reaction is quickly established at all points in the catalyst. The methane/steam reaction proceeds almost to equilibrium at the exit of the reformer; this is necessary to ensure a minimum concentration of methane in the product gas.

If the feedstock contains sulphur-bearing impurities, the reforming step is preceded by a desulphurization step to avoid poisoning the reformer catalyst, which is usually based on nickel and includes promotors of various other metals. Lately though, various catalysts have become available, which are sulphur resistant up to a certain degree [14, 15].

After the reformer, the process gas mixture of carbon monoxide and hydrogen passes through a heat recovery step and is fed into a shift reaction step, where the carbon monoxide reacts with steam catalytically to form carbon dioxide and hydrogen. In the classical set up the carbon dioxide is scrubbed from the system, and any residual carbon monoxide removed from the hydrogen

stream by passage through a catalytic methanator (Fig. 4).

The process variations possible after the high temperature- and/or low temperature-shift convertor will be discussed later in Section 2.1.6. Hydrogen purity after the above unit operations is about 98%, quite satisfactory for its principal application, the production of ammonia, but not satisfactory for those applications requiring high purity. A principal flow sheet is depicted in Fig. 4 for conventional and PSA-purification with 99.999% purity.

2.1.2. Partial oxidation. The partial oxidation of hydrocarbons is a major commercial process route to hydrogen production [1, 16]. The process requires ton- nage oxygen and proceeds at moderately high pressure with or without a catalyst according to the feedstock and process selected. The partial oxidation reaction is basically the result of a series of flame reactions proceeding when hydrocarbons are burned with only 30- 40% of the stoichiometric amount of oxygen required. The hydrocarbon feedstock may range from natural gas to sulphur containing crude oil, heavy fuel oil or resi-

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Fig. 5. Typical flow scheme for hydrogen from fuel oil by non-catalytic partial oxidation.

duums. The raw gas generation is followed by gas purification consisting of the following steps:

(i) removal of sulphur compounds, (ii) carbon monoxide conversion, followed by (iii) the final gas treatment step consisting of carbon

dioxide removal and methanation of residual carbon oxides.

The two commercially important non-catalytic processes are the Texaco process [17-19] and the Shell process [20-23]. A typical flow sheets is depicted in Fig. 5.

Due to residence time limitations the partial oxidation reactions only approach equilibrium leaving some residual methane and carbon in the raw product gas. The sulphur contained in the feedstock is converted mainly into H2S with some 4-6% into COS. The free carbon produced is removed from the reaction products by water scrubbing and is recycled.

After sulphur removal the gas is treated in the same way as the product gas of the steam reforming process, i.e. carbon monoxide conversion and final purification either by carbon dioxide removal and methanation of residual carbon oxides.

Whereas about 50% of the produced hydrogen in steam reforming is generated from water this figure goes up to 70% in partial oxidation of heavy oils and will finally go to above 80% in coal gasification. Pure

oxygen must be used in the partial oxidation process, because of the difficulty of separating nitrogen from hydrogen to produce a pure product.

The cost of the oxygen plant and the additional costs of the desulphurization steps make such a plant extremely capital intensive.

2.1.3. Refinery. The production of hydrogen as a by-product from the catalytic dehydrogenation of naphtha has for a long time been a major source of hydrogen providing about 80% of the hydrogen demand in refinery processing [7]. The catalytic naphtha reforming process converts straight run and cracked naphthas into high octane gasoline containing substantial quantities of aromatics. The vaporized feed is passed over a platinum based catalyst at operating pressures from 1.4 to 3.5 MPa and temperatures in the range of 720-820 K. Purity of the by-product hydrogen can be in the range of 85-97%, depending on operating severity and recovery system.

2.1.4. Coal gasification. The production of hydrogen from coal or coke proceeds via coal gasification [1, 24--28]. In coal gasification a hydrogen-rich gas is produced, which is then separated by various techniques into hydrogen and the other components. Hydrogen is either recovered from coke oven gases or produced through direct coal gasification by any one of the many autothermic (ATR) or allothermic processes [28]. Hydrogen from coal is today mainly produced via

654 W. BALTHASAR

synthesis gas from coal gasification and, in this specific instance, for ammonia production. In this application the atmospheric Koppers-Totzek gasification had its major success. However, since synthesis gas and hydrogen are normally needed under increased pressure, a trend is observed to operate coal gasifiers under increased pressure. By this operation, compression of the synthesis gas to the necessary synthesis pressure for reaction is reduced and, in some cases like the methanol low pressure synthesis, a further compression of the synthesis gas is completely avoided.

Today, for the production of synthesis gas and hydrogen from coal, three processes are proven on an industrial scale:

(i) the Lurgi pressure gasification process, (ii) the Koppers-Totzek atmospheric gasification

process, (iii) the fluidized bed Winkler gasification process.

During the last 20 years coal gasification plants for the production of synthesis gas have basically been designed, constructed and operated according to the processes from Krupp-Koppers GmbH and Lurgi, both West German. New process developments will be discussed later in Section 2.2.2.

The reaction mechanisms of coal gasification resemble very much those of the partial oxidation of heavy oils. However, due to the low hydrogen content of the coal, much more hydrogen of the final product is supplied from water than from the hydrogen bound in coal.

For hydrogen production the raw synthesis gas is purified from sour gases by one of the available gas treating processes, subjected to a high temperature shift

and, if economically advisable, a low temperature shift in order to remove carbon monoxide and to increase the hydrogen content of the gas, and is further purified in a pressure swing absorption system or another CO2 removal system followed by methanation.

The coal gasification processes are complicated by the necessity to handle a relatively unreactive fuel as a solid and to remove large amounts of ash. The solids-handling problem has a severe impact on costs and prevents much of the technology and equipment developed for petroleum from being used in the conversion of coal. Again a coal gasification unit for producing hydrogen needs pure oxygen for the energy generation in the partial oxidation of the feed coal.

A basic flow sheet is depicted in Fig. 6. 2.1.5. Metal water processes. Historically, the reaction

of reactive metals and reactive metal hydrides with water or acid is the standard way of producing pure hydrogen in small quantities. These reactions involve sodium metal with water to form hydrogen or zinc metal with hydrochloric acid or calcium hydride with water [281.

All these methods are quite outdated and expensive, however, and the only similar method to achieve any larger scale operation is the steam iron process [29, 30]. This involves the reaction of metallic iron or ferrous oxide with steam at elevated temparatures to produce hydrogen. Although a coal-based process, hydrogen by the steam iron process is derived by the decomposition of steam by reaction with iron oxide, rather than synthesis gas generated from coal. Coal is gasified to provide a producer gas for the regeneration of iron oxide.

Condensation of the steam from the oxidizer effluent

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leaves a gas containing 96% hydrogen, 1.6% carbon oxides and 2.4% nitrogen. No carbon monoxide shift or acid-gas scrubbing are needed. A clean-up methanation step reduces carbon oxides to 0.2%.

2.1.6. Electrolysis of water. Although water electrolysis is one of the oldest technologies for producing hydrogen the contribution of electrolysis to hydrogen production is still minor compared to fossil-based processes [1]. Nevertheless, several large electrolysis plants with over 100 MW electrical consumption are in suc- cessful operation, and smaller ones number in the thousands [31, 32].

Water electrolysis is based on the passage of direct current through water that has been made electrically conducting by addition of excess hydrogen or hydroxyl ions, oxygen being liberated at the anode and hydrogen being liberated at the cathode. Alternate electrodes are surrounded by diaphragms that prevent passage of gas into the other electrode compartments.

The state of the art offers very reliable electrolyzers, which, however, have an energy efficiency (based on the hydrogen enthalpy) of not more than 80%, capable of limited current densities of no more than 2 kAm -2, thus requiring relatively large electrode area for a given hydrogen production [32]. These circumstances lead to relatively high production costs of electrolytically generated hydrogen, which is a factor of 3--4 higher than the production cost of hydrogen from fuel or natural gas [29, 34] .The reason for this relatively poor performance is mainly due to the strong kinetic hindrance of the cathodic hydrogen, and the anodic oxygen evolution which demands the application of at least 350 mV anodic and 150 mV cathodic overvoltages in order to obtain current densities of 2-3 kAm -2.

Furthermore, the intrinsic electrical resistance of actual cell configurations is too high, due to the use of relatively thick diaphragms and to gas accumulation in the intra-electrodic gap. This gives rise to excessive resistive heating, or undesired energy losses respectively.

2.1.7. Separation techniques. As has been seen by the above descriptions, hydrogen is normally produced/contained in gas mixtures, from which it has to be separated. These separation techniques/processes are a necessary unit operation of hydrogen production units, but recently have also received widespread attention as hydrogen recovery processes from hydrogen containing gases.

The purpose of gas treating is to remove one or more of several components from a crude gas mixture either because they form a nuisance or because they are val- uable recoverable materials. The variety of gases that, because of these reasons, require treatment is so exten- sive that an almost innumerable number of ways and combinations and sequences thereof have been devised to achieve the imposed purification resp. recovery requirements. The fields of application in these methods largely overlap so that, in most cases, it is impossible to indicate for a specific gas the best treatment method. The choice of the treatment method depends on par-

ameters such as gas flow rate, pressure, temperature, absolute and relative concentrations of impurities/ recoverables, treated gas specifications and also desti- nation of removed/recovered components.

Basically, gas treatment processes can be separated via two basic principles, physical and chemical treatment. Furthermore, separation of the various gas components can be achieved by chemical reactions, chemical or physical wash systems involving solvents, low temperature separation, molecular sieve separation or membrane separations.

2.1.7.1. Physical wash systems. Absorption of sour components of gas mixtures to solvents has been applied successfully for a long time. A process gas mixture is treated in a counter current wash column, whereby one or more components are absorbed to the solvent. After loading the solvent is regenerated in a separate vessel, whereby the gas component with the exception of oxidative gas treatment methods is released in its orig- inal chemical state. Physical and chemical absorption have different loading characteristics and therefore have different typical applications relating to the impurities and the to-be-removed gases. At high partial pressures the physically acting solvent can absorb more than the chemically acting solvent. This means high process pressures and high concentrations of the gas component to be separated favour a physical absorption and vice versa.

A physically loaded solvent releases, during partial pressure reduction, much more gas than at chemical absorption. Physical absorption solvents, therefore, can be better used for pressure swing regeneration than chemical solvents, which have practically always to be regenerated by stripping. Chemical absorption techniques normally use less stages than the physical absorption methods. With physical absorption, the solvent flow is directly proportional to the feed gas flow and reverse proportional to the process gas pressure and independent from concentration of the component to be removed. At chemical absorption the solvent flow is proportional to the to-be-removed gas component flow and therefore strongly dependent on the concentration of this component, whereas it is practically independent from the process pressure.

The oldest absorption process is water wash, but this has been replaced by many different organic solvents. The Rectisol process using methanol at low temperatures is one of the most common processes [62], however, in recent times the Fluor-solvent process using propylene carbonate or the Purisol process using n- methylpyrrolidone (NMP), the Selexol process using a mixture of polyethylene glycoldimethylethers (DMPEG) or the Sepasolv MPE process using a special mixture of oligoethyleneglycolmethylisopropylethers have been developed and applied.

The Sulfinol process combines a physical absorbent tetrahydrothiophenedioxide (Sulfolan) and di-isopro- pylamine and therefore combines features of both chemical and physical wash systems. It allows higher loadings than the pure chemical wash systems, i.e. mon-

656 W. BALTHASAR

oethanolamine (MEA) system, especially at higher partial pressures, where the amine does not benefit but the physical solvent does. For this reason it is frequently utilized in typical "amine applications". Also solvent degradation is less, as is corrosion.

The most used chemical absorption wash systems are amine washes. In the beginning only mono-, di-, and triethanolamine have been used. MEA can be loaded at low partial pressures of the sour gas components and permits high purities in the treated gas. Therefore the regeneration steam consumption is very high. MEA is sensitive to COS contents. Methyldiethanolamine or the ADIP process using di-isopropanolamine as active component are similar amine wash systems as are the Alkazid processes using n,n-dimethylaminoacetic acid or n-methylalanine as reactive components for the removal of sour gases.

The hot carbonate systems utilize the capacity of potassium carbonate to participate in equilibrium reactions with acid gases. The hot carbonate processes are offered today by various licensors differing in kind and amount of additives. The Benfield process, and HiPure process, the Catacarb process and Vetrocoke processes have become most well known.

Oxidative wash processes, which oxidize the sulphur component in the gas to elemental sulphur have become known under the name of Stretford as well as Takahax and Locat. In all these processes an oxidation is carried out using air oxygen as oxidant via various intermediate catalyst chemicals.

2.1.7.2. Low temperature separation. All gas components have varying boiling points at low temperatures and can therefore be separated similar to the air separation processes via low temperature. This low temperature separation is carried out as partial condensation of carbon monoxide and methane at temperatures between 170 and 100 K or as liquid methane wash or liquid nitrogen wash systems, whereby carbon monoxide and methane, as well as argon and traces of oxygen, will be removed.

2.1.7.3. Adsorption. Gas separation with solids using adsorption techniques have been practised and further developed.

Active carbon, apart from its capacity of removing hydrocarbons and a wide variety of solvents from air streams, is useful for the removal of acid components from gases, and for the elimination of sulphur compounds from gaseous streams.

Metal oxides can be used to remove hydrogen disulphide from hydrogen containing gases.

Molecular sieves are used to remove a variety of components, mainly polar ones such as water, carbon dioxide, hydrogen sulphide, sulphur dioxide, ammonia, carbon oxosulphide and mercaptans. Whereas other solid gas purification systems function predominantly by virtue of chemical phenomena, molecular sieves remove impurities due to their polarity. Also, they primarily adsorb small molecules while excluding large ones, so that separations can be made based on differ- ences in molecular size. Molecular sieves are extensively

used in gas purification and consist of zeolites or carbon, but their main applications are gas dehydration and removal of impurities such as methane, water, carbon dioxide, nitrogen and carbon monoxide from hydrogen. Regeneration of molecular sieves makes use of their reduced loading capacity at either higher temperatures or at lower pressures.

The main application of molecular sieves in hydrogen production is executed by purification of hydrogen via the pressure swing absorption (PSA) process [11].

Impurities are adsorbed at the higher pressure at which hydrogen-rich gas is processed. The bed is regenerated by pressure reduction. Although the PSA system of hydrogen recovery and purification is an alternative to the cryogenic hydrogen recovery, it has different characteristics and the systems are not completely inter- changeable. The PSA system produces hydrogen of extremely high purity. Purity levels of 99.999% hydrogen are normally achieved and contaminant removal to levels of less than l p p m has been commercially achieved for impurities such as carbon monoxide, nitrogen and methane. This capability is not inherent in the cryogenic hydrogen recovery where purity levels are normally 90-98% hydrogen. Although hydrogen recovery factors are dependent upon process conditions and design criteria, hydrogen recovery from a cold separation system is typically 95% or better compared with approx. 80% in a 4-bed PSA system. In the polybed PSA [63], using normally 10 beds and applied at high capacity systems, recoveries in the range of 85-90% are achievable. Although capable of producing hydrogen of very high purity, the PSA-system is not limited to doing so. If desired, hydrogen of lower purity can be produced while obtaining some improvement in hydrogen recovery. The two available processes have been developed by Union Carbide Corp. and Bergbaufor- schung GmbH, whereby the UCC process has found widespread and large scale application.

2.1.7.4. Membrane separation. Another method to separate gases has been developed by Du Pont and Monsanto [12]. Gas separation via membrane technology works on the principle that different components in a gaseous mix have diffferent rates of permeation. The driving force for the separation is differential pressure--the operation can take place over a wide range of temperatures and pressures and is restricted only by the membrane's physical limits. Such membrane materials as polysulphone, polystyrene, Teflon and various rubbers have different separation characteristics, which qualify them for specific jobs.

In the capillary or hollow-fibre design, the separator modules resemble shell- and tube-heat exchangers. Anywhere from 10000 to 100000 capillaries, each less than 1 ~m dia., are bound into a tube sheet surrounded by a metal shell. Feed gases are introduced into either the shell or the tube side of the module. If the perme- ability rates of the gaseous components are close, or if higher product purities are desired separator modules can he arranged in series, and feed streams can be recycled.


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It is apparent, that today's hydrogen production processes will be further developed for more economical hydrogen production and wider application of the basic principles. This development will be less dramatic in steam reforming, where most of the new catalysts will broaden the spectrum of permissible fuels by allowing sulphur contamination or heavier liquid fuels like fuel oils and development of other unit operations and process schemes around the actual steam reforming cataly- sis reactor. This incremental development overall may be still more dramatic than the expected development in coal gasification, where more than a few dozens of

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processes are contemplated around the world. This coal gasification development stands in sharp contrast to practically no further development in the field of partial oxidation. Electrolysis of water is a major section of industry and government funded research.

2.2.1. Development of steam reforming. It has been pointed out earlier that equilibrium is closely reached with today's catalysts, which at the same time have an GJ/t acceptable lifetime and carbon deposition resistance. N H3' Steam reforming catalysts are capable of using light hydrocarbon gaseous or liquid fuels with low sulphur 60 content. Recently catalysts have become available, which are sulphur tolerant and thus permit the feed of sulphur containing hydrocarbon distillates [15]. These 50 catalysts show different activities and temperature behaviour and thus give rise to new reformer designs, z 40 i.e. a combination of a regenerative tube high temperature steam reformer with an autothermal reformer, = where oxygen is fed to the HTSR effluent. A basic flow sheet is depicted in Fig. 9. ~ z 30

A regenerative reformer tube is a tube where heat transfer takes place between the feed and the reformer ~ 2o effluent. This leads to increased heat transfer of the z total fired heat to the catalyst, thus reducing the overall ,~ fired duty. The same goal is partially achieved by the ~ ~0 combination of the HTSR and ATR reformers.

One of the major goals of hydrogen production via steam reforming is to lower the energy requirement per ton of hydrogen produced. A drastic development has taken place in this area during the past years as is

depicted in Fig. 10. This overall development is achieved in a series of process and construction material developments during this time including the other unit operations of steam reforming, i.e. these developments

THEORETICAL MINIMUM FEED

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Fig. 10. Energy conservation for ammonia production.


are accordingly applicable to partial oxidation and coal gasification, if these unit operations are applied there. A recent summary has appeared on this subject [35]. These developments include mechanical and process innovations in the areas of burners, insulation, reformer tubes, convection systems, catalysts, process design, conceptual design, and fuel alternatives. In the area of energy conservation the concepts of combusion air pre- heat, improved energy cycles, e.g. use of gas turbines, high efficiency steam turbines and compressors, the use of low level heat recovery systems like Rankine cycles, vapour recompression, etc. have led to a recovery of heat at levels considered highly uneconomical in the past. All these technologies are either already proven today or will be tomorrow.

2.2.2. Development of coal gasification processes. Most of the hydrogen users today and tomorrow need hydrogen at elevated pressures. Since hydrogen compression is highly energy consuming, as is synthesis gas compression, it is important to gasify coal at elevated pressures. This has always been done with the Lurgi coal gasifier [37], however, the gas composition and the by-products do not render the Lurgi gasifier an ideal system for the production of synthesis gas or hydrogen. New technology in coal gasification is based on high- pressure gasification to promote the direct formation of methane. For hydrogen production this is undesirable and the new technology in this area is thus concentrated on high-pressure and/or high-temperature coal gasification. The most prominent new processes in this area are the Texaco coal gasification process [36], the Shell coal gasification process [38], the pressurized Koppers-Totzek process, the Ot to-Rummel process [39], and the pressurized Winkler coal gasification process [40]. All these processes have reached demonstration scale and are thus in the final stage of development.

Synthesis gas from the Ruhrchemie demonstration plant of the Texaco coal gasifier has been used in the grid of that plant, i.e. oxo-alcohol production [41]. Just recently the TVA coal-to-ammonia demonstration plant based on a Texaco coal gasifier has also come on stream. Shell has operated a coal gasification prototype plant in Harburg, Germany. The main difference between these two processes is to be seen in the feed system, where in the Texaco process a coal water slurry is fed to the gasifier and in the Shell process dry powdered coal. Other reviews have been published on coal gasification, whereby main development activities seem to be centered in the U.S.A. , Great Britain and Germany.

From these data it seems conceivable, that fully developed commercial coal gasification processes of the 2nd and/or 3rd generation will be available before the turn of the century. One can also anticipate that a significant portion of hydrogen produced at that time will be derived from coal.

Coal has a dual role in all normal coal gasification processes: coal represents the raw material for carbon monoxide and carbon dioxide formation as well as an energy source for the provision of the reaction heat and for heating of the coal and the steam. In all autothermic

coal gasification processes, which are the ones thusfar described, 30-40% of the feed coal are thus just oxidized in order to produce heat. By use of heat from a high temperature nuclear reactor the energy consumption of a coal gasification process can be supplied, in order to gasify all coal to usable products [43]. Nuclear heat must be lower in cost than heat derived from the combustion of coal for this method to be practical, and, since this is the case in some parts of Europe, interest in nuclear-assisted coal gasification centers there. The decisive technical problem is that of transferring the nuclear heat to the gasifier without risk of contamination of the primary reactor coolant. Various projects have been run in Germany with a high temperature nuclear reactor coupled to simulated water-gas-coal gasification and hydrogenation [43].

With the current slow-down in nuclear power application, including the development of high temperature nuclear reactors, it can be assumed that the development in integrating the nuclear heat source with the coal gasifier will take some time and will not be proven much before the turn of the century. Nevertheless, some pilot units are operated with electrical or by other means heated helium cycles, thereby simulating nuclear heat input [42].

2.2.3. Development of water electrolysis. Research and development in water electrolysis has been stimu- lated and supported by various government and industry organisations [31, 44]. The last years have seen sizable research projects for water electrolysis, which have moved into demonstration stage on commercial size installations [44, 45]. The work on electrolysis, in the flame of the European Commission programmes, has been mostly concentrated on alkaline cells, for which less severe material problems are anticipated and for which a more developed technology already existed. Two main lines of development have emerged up until now:

(i) a low temperature approach (350°390 K = 800 120°C) in which particularly active catalysts are used;

(ii) a medium temperature approach (4100470K, 140-200°C), in which less active catalysts or no catalysts at all are employed, taking advantage of the lowering of overvoltages at higher temperatures;

(iii) diaphragms and materials specific to each alternative have been found;

(iv) a third approach dealing with electrolysis at high temperature (1070-1270K, 80001000°C) in a solid electrolyte, still in its very early stage of development, has also been investigated to some extent.

More efficient and compact cell structures have been proposed for both concepts of low-temperature electrolysis and medium-temperature electrolysis. The total single cell thickness anticipated is between 5 and 10 ram, effectively limiting the ohmic losses, even at high current density, which is around 10 kA m -2, and giving very high

660 W. BALTHASAR

power densities at the expense of more critical control or flow distribution among single cells. The behaviour of bubbles, and their influence on cell losses has been studied extensively as well as their effect on cell optim- ization and design. Long-term testing of these developments is in progress in a few 5-10 kW laboratory prototypes whose cell configuration is similar to that anticipated for full-scale cells of the same type and operating at corresponding high power densities.

As an outgrowth of fuel-cell work a new electrolyzer concept has been developed, in which the only electrolyte is a fluorocarbon-based material capable of conducting hydrogen ions when wet. The electrolyte is attached mechanically to current collectors on either side, and a voltage is applied across these collectors. Considerable development has gone into the construction of the solid-polymer electrolyte (SPE) electrolyzers, the most important of which is the development of the polymer itself [33]. The electrolyte was developed by Du Pont for the chloralkali industry and as such has not yet been optimized for electrolyzer operation. The efficiency goal for this development requires a low cell operating voltage and the cost goal requires a high operating current density as welt as low manufacturing cost [34]. The key advantage of the SPE cells is the superior performance which makes the high current density possible at a low cell voltage. The design current density for most of the applications of the SPE electrolyzer technology is around 100 kA m -2. Moulded carbon current collectors are a major result of the technology development programme. The function of the current collector is to:

(i) provide the flow field for the water and oxygen on the anode (oxygen) side and for the water and hydrogen on the cathode (hydrogen) side,

(ii) separate the oxygen and hydrogen sides, (iii) provide for the conduction of electricity from

one cell to the next.

The collector is moulded from a mixture of carbon and phenolic resin which incorporates an in situ formed titanium foil shield on the anode side to prevent corrosion. High temperature operation (up to 420 K) offers advantages in operating efficiency and counts for about one half of the improvement needed to meet the goal performance.

The plant programme calls for operational evaluation of 50, 200 and 500 kW systems leading to installation of a 5 MW demonstration system during 1983. It is obvious from this programme, that this electrolyzer technology will become commercially available tomorrow.

A typical water electrolysis plant will consist of a water purification plant, the electrolyzer section, gas separation, power transformer, regulator and rectifiers as well as compressor systems. The cost of the electrolyzer section is about 60% of the total investment.

2.2.4. Thermochemical production of hydrogen. The desire to free the production of hydrogen from Carnot efficiency limitations has led to interest in chemical

methods to extract hydrogen from water by heat alone. These methods have come to be called "closed thermochemical cycles" by which is meant to imply a system of linked regenerative chemical reactions which accept only water and heat as feedstocks and produce hydrogen and oxygen (an "open thermochemical cycle" is meant to be one in which feedstocks such as fossil fuels could be accepted and products such as carbon dioxide could be rejected). There are also hybrid thermochemical cycles where in one of the reaction steps not only heat but also electrical power as an energy source is used.

Direct thermal degradation of water requires temperatures above 2500 K. Since there is no technical solution to this problem for tomorrow, one has reverted to the possibility of splitting water by various reaction sequences using chemicals. These chemicals are run in cycle. It is possible to produce hydrogen in a multi- reaction process from water with a higher thermal efficiency as for water electrolysis.

For a hydrogen production in large scale a high temperature nuclear reactor seems to be the most sensible primary energy source. This has led to a maximum process temperature of 1000-1200 K in thermochemical cycles. There are many requirements to be cited for thermochemical cycles to become technically and commercially interesting [46].

The most important of these requirements is the thermochemical efficiency which has been defined as the quotient between the upper heating value of hydrogen and the total fed energy [47]. A high efficiency and technical realisability requires few reaction steps and acceptable fast kinetic reactions. Thermodynamic considerations dictate that below a temperature of 1200 K at least three steps are necessary in order to split water thermochemically. Only hybrid processes can be executed in two reaction steps. The individual reaction products should be produced in high yields in order to keep the purification effort low. Homogeneous gas reactions also require a high total yield in order to prevent losses during separation. Also, side reactions should not occur. Possible by-products should not influence the next reaction step.

All in all, more than 200 thermochemical cycles have been proposed, more than 1000 have been calculated via computer application, however, only a few, ca 30, have been worked out in more detail and only 3 have led to demonstration plants. These are the Westinghouse-sulphur cycle, the Mark 13 or Bromine-sulphur cycle of GRC-ISPRA and the Iodine-sulphur process of General Atomic company.

The first two are hybrid processes. The basic reaction schemes are depicted in Fig. 11.

It is not yet possible to state whether these selected processes are representing the optimum choices. This will also depend very much on the heat source to be used, i.e. whether the high temperature nuclear reactor will reliably supply these or even higher temperatures or whether other heat sources, i.e. solar heat, will be used, which in turn have a totally different characteristic in temperature level, reliability of temperature availa-


G E N E R A L A T O M I C - IOD INE S U L P H U R CYCLE

2 HJ 393 800 K @

J~ + S O 2 + 2H~O 2 3 7 0 K >

H , S O t l 1 1 5 0 K •

H2 + J2

2HJ ~ H2SO 4

H20 * SO 2 * }02

WESTINGHOUSE ~ S U L P H U R CYCLE

SO 2 + 2H20 298 373 K

H2SO 4 925 - 1125 I~

H2SO 4 + H 2 E L E C T R O L Y S I S

H20 + SO 2 ~ }O~ T H E R M O L Y S I S

MARK 13 ISPRA

2 HEIr 353 473 K H 2 * B r 2 E L E C T R O L Y S I S

EIr 2 + SO 2 + 2H20 293 - 373 K 2HBr + H2SO 4 aq

H 2 S O 4 925 - 1125 K H 2 0 + S O 2 + }02 T H E R M O L Y S l S )

Fig. 11. Thermochemical process schemes.

bility, no temperature during night time, etc. The further investigation into other alternative thermochemical cycles is also hindered by missing or unprecise thermodynamic, as well as kinetic, data.

Some of the goals though have been achieved, i.e. the demonstration that such chemical reactions can be operated in the required temperature range without formation of by-products. The processes have been shown to work with a total efficiency of above 40% and have, in this respect, a significant advantage over normal water electrolysis. Some combinations which have to be determined experimentally more closely indicate an efficiency of 50--55 %. However, also the improvements in water electrolysis have to be taken into account when these two competing technologies are compared. Since there have been no technical, commercial installations for thermochemical cycles there is great insecurity in cost estimation of thermochemical hydrogen production. This insecurity is also tied to the slow down of the programs of the high temperature nuclear reactor, which is already needed for demonstration scale projects, but needed even more as dedicated heat source for a commercial size unit of around 1 GW. From these statements it becomes clear that reliable thermochemical cycles in commercial scale for hydrogen production will not be available tomorrow and may be even grouped into the technologies of beyond.

2.3. Beyond

2.3.1. Development of today's and tomorrow's processes. A prediction on the further development of hydrogen production technologies beyond the turn of the century is very much dependent on the scenario and the authors' own imaginations. There are various scen-

arios which can be used to predict these developments. The accuracy depends very much on the basis of the forecast used. However, coal has certainly become a much slower significant participant in synthesis gas production than anticipated ten years ago. This same statement is true for the participation of nuclear energy in the total energy supply.

Oil and, even more so, gas have retained their positions, which they have conquered in the 50's and 60's. Gas, even now, still seems to be progressing, though its known reserves are more limited than those for oil.

If one accepts that coal, in an intermediate scenario, resumes an intermediate position, i.e. coal will supply the energy needs at a larger share, passed on to it from oil and gas, before nuclear energy and even later fusion energy will progress to their significant energy supply positions; if one also takes into account the uneven distribution of oil and gas resources in the world and the far more even distribution of coal and the possible even distributions of nuclear fuels by origin and breeding, one can assume that certain countries will move to a greater independence in energy supply, i.e. coal tomorrow and nuclear beyond. Since this article deals with production methods, and other papers will deal with future hydrogen energy and energy options in total, this scenario will be adopted for the time being.

There are many more coal gasification processes in a developmental stage, than have been outlined so far [48]. Most of these processes are geared towards the production of synthetic natural gas, however, it is obvious from the earlier demonstrated chemistry, that the step from coal to SNG and in between synthesis gas or hydrogen as one constituent of synthesis gas can be achieved without major complications. Therefore, it is impossible to predict which of those many processes at the end will become the superior coal to hydrogen process, but it may be anticipated that the current leading candidate, the Texaco process, is, due to the associated lower theoretical heat efficiency, not the long term final candidate for hydrogen production.

Also, if nuclear energy becomes a major energy source, processes which use the heat input from high temperature nuclear reactors may be superior. These processes have not yet been demonstrated on a larger scale and will not be for another 15 years. In this scenario one may not forget the special situations of individual countries, which already in past history have led to the development and application of competing processes just for know-how, availability, political and commercial reasons at the specific location of the country and even more so the plant. This has already, today, led to the use of coal in South Africa as well as to the coal upgrading process development in Great Britain and Germany. France, in contrast, has pushed ahead with a major nuclear power programme which, in turn, could lead to hydrogen production with off-peak power before the turn of the century.

Any one political development could also change the current slow development and demonstration programmes if, due to this political change, economic rea-

662 W. B A L T H A S A R

sons do not have to be considered that overwhelming anymore. It has to be kept in mind that any one of the major hydrogen production units, as they are contemplated in a hydrogen energy system or even a chemical system, reach investment scales and operating cost scales which will drive any private business within a short period out of business, if a competing feedstock changes price only slightly for reasons which cannot be influenced by the operator of the coal gasification unit.

It is clear from the afore-mentioned, that the development of coal gasification for the production of hydrogen will be kept on a multi-process path far beyond the turn of the century and that significant technological developments are expected still in this area.

Steam reforming and electrolysis of water do not seem to have a major development in this far beyond time frame.

2.3.2. Development of regenerative energy processes. Hydrogen, in principle, can also be produced by various other energy or heat inputs, which have thusfar not been mentioned. The other main alternative is solar energy as heat or photon energy to be used directly or

via biological systems. Other energy sources mainly of the renewable type are formed by geothermal energy, wind energy, water temperature gradient energy.

Solar energy is a clean, non-depletable energy source and has attracted much attention as an alternative to fossil and nuclear energy for future use in a high-grade energy market. The major problem with solar energy is the cost of collecting it as well as the ecological pollution associated with this collection. The surface of the earth receives about 550--1400 W m -2 of radiation, which can be collected without concentrators at less than 520 K. Another feature of solar energy is its diurnal nature which, from an economics standpoint, means that capital tied up in solar collectors is utilized less than 50% of the time, in practice not more than 25%. This effectively quadruples the cost of solar energy over continuous energy sources of comparable capital outlay.

Solar heat can be concentrated to form high grade energy which can be used to produce hydrogen in processes similar to nuclear or combustion heat applications, i.e. steam-cycle/electrolysis or thermochemical cycles [49]. These technologies have been discussed in earlier sections and depend primarily on heat at a suf-

0, 1 - -

0.01 0

H20 I

H

O

' ' 1 ' ' q I

2000 4000

TEMPERATURE, °K

Fig. 12. Water splitting temp. dependence at 132 N m -2

6000

01

001 L 01

H

0

HYDROGEN PRODUCTION AND TECHNOLOGY

I I

H20

1 lO

PRESSURE kN/m 2

Fig. 13. Water splitting pressure dependence at 2800 K.

OH

100

663

ficient temperature level to drive the process efficiently. In addition, collection to very high temperatures may allow direct thermal splitting of water at low pressure. Figures 12 and 13 show the approximate effect of temperature and pressure of the equilibrium radical concentrations of water at high temperature and low pressure. Material problems with the solar heat systems are severe and volumetric efficiencies are low because of the low pressure.

However, a solar heat system can also take the route via conventional power generation and electrolysis of water. Examples of solar power generation exist in a few demonstration units, one of which is Eurelius completed in December 1980 in Italy [50]. It has a power generation capacity of 1 MW with solar collector surfaces in total of 6200 m 2 and a steam generator on a tower of 55 m elevation. These figures give indications of the surface which would be needed in order to get to sensible plant sizes for hydrogen energy systems or hydrogen-use systems in the chemical field.

Solar radiation at wavelengths below 495 nm contains sufficient energy to decompose water directly. Water, however, does not absorb light appreciably at wavelengths above 185 nm and, as a result, direct photolysis does not occur appreciably at ground level. An energy transfer mechanism is required to allow the photon energy to decompose water directly.

Several methods of energy transfer have been proposed which allow the direct use of radiative energy to

produce hydrogen without resorting to a Carnot-limited procedure [51]. These include:

(i) photovoltaic-electrolysis methods, (ii) heterogeneous photo-assisted oxidation-reduc-

tion reactions, (iii) homogeneous photo-assisted oxidation-reduc-

tion reactions, (iv) biological reactions.

Photovoltaic energy conversion is considered a prom- ising technology because it produces electricity directly from sunlight without intermediate gas, steam or mechanical cycles [52]. It therefore appears efficient and benign. Moreover, photovoltaic electricity systems are modular-arrays of identical modules and can be assembled to meet almost any power need, Photovoltaic energy conversion begins with photons, the increments of energy that are carried in all wavelengths of light. Photovoltaic conversion is completed when current flows in a circuit connected to the opposite sides of a solar cell. The essence of a cel l--whether treated silicon or molecular films of selected materials--is its two regions, one electrically positive and one electrically negative. Solar photons dislodge electrons from their chemical bonds in their cell materials. The positive and negative regions encourage electron flow, and contacts on the cell surface channel this flow into a circuit. A single such cell some 7.6 cm in diameter and 300 ~tm thick may represent 0,5 V and generate about 1 W.

664 W. BALTHASAR

Ceils are connected in series to build up the voltage or in parallel to build up the current. A module combines many cells into a package about 1.2 m 2.

Modules and panels (made up of several modules) are the customary design units for assembling photovoltaic arrays that will meet specific power or energy requirements. Photovoltaic systems today show a low efficiency. Today's best single-crystal cells yield efficiencies of 16-17%. Modules of typical mass produced cells seldom exceed 10%, in the average, not more than 8%. There is a major R & D activity established, in order to reduce cost of photovoltaic cell production and increase efficiency at the same time [53]. Nevertheless, for hydrogen production one has to keep in mind that there will be another energy loss during electrolysis of water.

Heterogeneous photo-assisted redox reactions generally refer to reactions at semi-conductor surfaces, and the production of hydrogen peroxide from zinc oxide, cadmium sulphide and cadmium selenide surfaces upon irradiation is well known. Also, irradiation of n-type titanium dioxide semiconductors liberated oxygen and, at a platinum electrode through an electrical connection, liberated hydrogen.

Homogeneous photo-assisted redox reactions refer to direct irradiation of solutions which contain chemicals capable of absorbing solar radiation and transferring it to the water for photolysis. Not much progress has been achieved lately in this area.

Another energy source, which could be used in combination with electrolysis of water would be geothermal energy. Over thousands of years, volcanos, lava flows, hot springs and geysers have been seen as picturesque and inspiring manifestations of the vast heat store that lies beneath the earth crust. Limited electricity generation using natural steam was first successfully demonstrated at Lardarello, Italy, in 1904. It was not until fairly recently, however, that people considered cap- turing this natural heat from the earth--this geothermal energy--for use in space and process heating and in electricity generation on a large scale [53]. There are three basic types of geothermal energy: hydrothermal, geopressured, and petrothermal. In a hydrothermal system, water becomes heated or is vaporized into steam by contact with hot rock. In geopressured systems, water heated in a similar way occupies an underground reservoir deep within deposits of sand and shale. This hot water is sealed off from the surface by impermeable shale layers, and is subjected to pressure from the overlying rock formation. In addition, the pressurized water is saturated with natural gas thought to have been produced by the decomposition of organic matter. In petrothermal systems, magma lying relatively close to the earth surface heats overlying rocks to the high temperature. Water or some other fluid could someday be injected into such a geologic formation and pumped out again, extracting the thermal energy.

All these energy sources can be used to generate electricity which, in turn, can be used to electrolyse water for hydrogen production. Due to the nature of

this energy generation, it is a steady and reliable source of a renewable energy type. One major problem associated with the energy recovery from this source is the relatively low experience available.

However, there is a strong growth in the energy supply industry existing today and the incentives to grow further are strong since, in the United States of America alone, 24 GW of hydrothermal resources have already been identified. It has to be expected though, that the real use and significant contribution to the overall energy supply lies in the time frame beyond the year 2000.

Wind energy is another renewable energy source which can generate electricity which, in turn, can be used to produce hydrogen by water electrolysis. Cur- rently, the most advanced technology being developed by major industrial sources for harnessing wind power is the large, horizontal-axis wind turbine with propeller-type rotor blade. With horizontal machines, the axis of rotation is parallel to the ground. The development has now reached a third generation machine design stage and it is anticipated that it will take some more time to develop this technology fully.

The concept of generating electricity with ocean-thermal energy conversion (OTEC) systems takes advantage of the approx. 22K temperature difference between the surface and the depth of tropical, sub- tropical and equatorial ocean waters. Two basic designs have been investigated: closed cycle and open cycle [55]. In the closed cycle, warm surface water is drawn into the system to heat a working fluid, such as ammonia. The liquid ammonia vaporizes, when its temperature reaches about 289 K and expands to turn a turbine generator; after it has passed through a turbine, it is condensed back to liquid form by cold water drawn from the ocean depths through a huge vertical pipe. In the open cycle system instead, surface water is flashed to steam in a partial vacuum.

The steam drives the turbine generator and is then condensed by cold water from the depth. These systems have to be operated by today's standards under extreme low temperature difference and therefore reach immense proportions. A 100 MW OTEC plant would have a water flow comparable to that of Boulder Dam. Cold water pipes measuring 900 m long and 30 m across must be designed, fabricated and installed in an ocean environment. It can be easily seen that, with these challenges, this energy, if at all, is one for the 21st century.

Solar ponds work on a principle similar to that of OTEC, but here the temperature difference that makes electricity generation possible exists between a layer of highly concentrated salt water and a layer of fresh water that floats on its surface. The sun's heat passes through the fresh water layer, which does not warm up significantly, and is trapped in the salty layer, which can reach about 366 K. Heat exchangers, pumps and turbine generators are used in much the same way as in an OTEC system. The technology has been pioneered in Israel. Although solar ponds may have appeal in special


applications, where appropriate conditions already exist, such as a supply of brackish water and a sunny climate, their requirements for land area and the availability of water and salt make them unlikely to con- tribute in large measure to national energy supplies.

2.3.3. Biological production o f hydrogen. There are, in principle, various ways of producing hydrogen via biological production processes. Hydrogen is produced by some bacteria or algae, directly through their hydro- genesis or nitrogenesis enzyme system [49, 56-58]. Fol- lowing anaerobic digestion or fermentation, biofuels can be produced in gaseous form which, in turn, can be used for power generation and in turn, water electrolysis, for hydrogen production or directly steam reformed, or partially oxidized to synthesis gas and the hydrogen recovered from this by the standard process procedures already outlined.

Finally, the biomass itself could be burned and power generated for water electrolysis to produce hydrogen.

A few unicellar algae and photosynthetic bacteria are able to use light as an energy source in the production of small amounts of gaseous hydrogen [57]. All these organisms use some form of chlorophyll an~/or caro- tenoid molecules as light-trapping agents. Oxygen tends to reduce the production of hydrogen and, under anaerobic metabolism, a simultaneous reabsorption of hydrogen gas occurs. Mutants have been found, where the reabsorption of hydrogen is suppressed. There are also other methods to increase hydrogen photoproduction capability (Fig. 14). Hydrogenases do not them- selves split water; even a hydrogenase system does not

usually begin operating on water, but usually on an organic, intermediate from the food chain. Therefore a total assessment of this hydrogen production possibility cannot be done, since only the basic research has yet been started. Also, these systems produce carbon dioxide.

The other processes for using biomass, oiz. conversion to combustible liquids, gases, and other solids, can be split into two categories: thermochemical and biochemical. The thermochemical processes---pyrolysis, gasification and straight burning--involve heating the biomass. Pyrolysis, heating in the absence of air, is generally a lower-temperature process that forms gases, liquids and solids; gasification is a higher-temperature operation that takes place in the presence of an oxidant (e.g. air) and principally produces gaseous fuels.

The biochemical processes are all fermentations, i.e. microbial transformations of organic feed materials that take place with or without oxygen and are producing alcohols, organic chemicals, such as methane, ethanol, acetic acid, acetone and hydrogen in minimal amounts, as well as carbon dioxide. Before biomass can be processed thermochemically or biochemically, it must be prepared, whether reduced to small chips, dried, com- pressed, or treated with enzymes or acids. In addition to plant-derived biofuels, manure, sewage and munici- pal solid waste could become sources of energy in the thermochemical processes.

2.3.4. Fusion as an energy source. Nuclear fusion involves joining, or fusing, the nuclei of light chemical elements through the application of extreme tempera-

I

INCREASE IN CARBOHYDRATE CONTEN]'

ACCUMULATION OF ELECTRON DONOR SUBSTANCES

I

NITROGEN LIMITED CULTURE

DECREASE IN PROTEIN CONTENT

¢ HIGHER CELLUAR RATIO CARBON / NITROGEN

ENCHANCEMENT OF N 2 ASE (AND H2ASE)SYNTHESIS

I

DECREASE IN PHOTO SYSTEM II ACTIVITY (DEREASE IN CHLOROPHYLL AND PHYCOCYANIN CONTENT)

DECREASE IN 02 L PHOTOPRODUCTIO N ACTIVITY

REDUCTION OF 02 INHIBITION OF N2ASE (AND H2ASE}

ENCHANCEMENT OF HYDROGEN PHOTOPRODUCTION CAPABILITY

Fig. 14. Biological hydrogen production enhancement.

666 W. BALTHASAR

tures and pressures. These conditions are necessary to speed up the nuclei enough to overcome the electrostatic repulsion that normally keeps them apart. When this happens and the nuclei fuse, nuclear particles are freed, and the extremely rapid motion of these particles represents a release of energy that can be recovered for practical use in the form of heat.

When a nucleus of deuterium (one proton and one neutron) is brought together with a nucleus of tritium (one proton and two neutrons) under extreme temperature and pressure, the nuclei fuse, reaching a highly energetic state. However, because this state is highly unstable, the nuclear material then breaks into a new configuration--a helium nucleus (two protons and two neutrons) and a free neutron, which carries most of the energy of the reaction. When billions upon billions of nuclei are involved, as will be the case in a fusion reactor vessel, the energy release is substantial.

The sun, which is a huge fusion reactor, accomplishes the reaction with relative ease. The sun can hold the plasma (the mixture of positively and negatively charged particles of nuclei and electrons) by gravity to densities well in excess of those necessary for the fusion reaction. On earth, a plasma density cannot be achieved that

would permit a fusion reaction at the relatively low temperature of the sun (15 million K); on earth, the temperature must be higher--about 100 million K.

Heating the plasma can be accomplished by a number of means, including electric currents, radio frequency waves and neutral-beam heaters. Containment of the plasma is a much more formidable problem, however, because the plasma would immediately be cooled to sub-fusion temperature by contact with any sort of physical container. There are two containment methods presently being pursued, differentiating fusion research into two areas: magnetic confinement fusion and inertial confinement fusion [59, 60].

The fuel that will probably be used in early fusion devices is a mixture of two isotopes of hydrogen-- deuterium and tritium [61]. Deuterium is easily extracted from ordinary seawater; tritium occurs only rarely in nature, but can be bred from lithium by using well-understood nuclear processes.

Tritium breeding is accomplished in the diffusion blanket, which surrounds the reaction chamber. The blanket also serves the essential function in a fusion power plant of converting the nuclear energy released from the fusion reaction into thermal energy, which can

N E A R T E R M I I I I N T E R M E D I A T E T E R M I F A R T E R M

I I I SUPPORTING RESEARCH AND MATERIALS AND SAFETY

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I I I I I

SOLAR CONVERSION AND WATER SPLITTING r- -

1 ' ' r-- ~ NUCLEAR ENERGY, ELECTROLYSIS AND THERMOCHEMICAL PROCESSES

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HYDROGEN[

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AIRCRAFT AND ENGINES F--

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Fig. 15. Summary of hydrogen production.

HYDROGEN PRODUCTION AND TECHNOLOGY

then be used to generate electricity or split water directly. The blanket can also be used to breed fuels for conventional fission power plants from relatively plentiful non-fissile material in what is known as a fusion-fission hybrid reactor. Whether the fusion blanket is used to generate power or to produce fuel, its development is seen as a complex engineering challenge that is of prime importance to the future application of fusion technology.

Scientists and engineers are fairly confident that fusion energy break-even (as much energy produced as is required to set up the conditions for energy release) can be achieved within five years in fusion devices now being constructed. Thus, the scientific aspects of the fusion reaction could be considered advanced enough, that the feasibility of fusion is demonstrated. The pres- ent transition from fusion science to fusion engineering therefore means that basic decisions have to be made. Trade-offs involving issues of technology, versatility, performance and cost will affect these critical decisions, which, in turn, will determine the type and scale of fusion devices to be used in early commercial fusion plants. The engineering choices made in the near term will profoundly influence the course of fusion energy development for the next 20 years.

These basic engineering decisions will then also determine, whether direct thermal splitting of water to hydrogen and oxygen will be performed in the blanket system of the fusion reactor or whether the detour via thermochemical or even electrolytical water splitting will be taken. Nevertheless, it is anticipated, that this technology will become commercially available during the next century and will then represent a vast energy source as is the sun today.

3. SUMMARY

Hydrogen is produced on a large scale by a wide variety of processes starting with feedstocks like natural gas, crude oil products to coal as well as water using processes like steam reforming, partial oxidation, coal gasification, metal-water processes and electrolysis. Hydrogen is also recovered from various gas streams especially in refineries (Fig. 15).

Depending on the basic energy scenarios to be used, steam reforming natural gas will remain the major hydrogen source from today till tomorrow, i.e. the turn of the century. Coal gasification will significantly increase its share of hydrogen production. This will be achieved via newly developed coal gasification processes.

The development of thermochemical hydrogen production technology as well as biological hydrogen production technologies will progress, but their widespread application remains to be seen in the next century.

Since hydrogen production is a large energy consuming process the development of energy sources will influence significantly the application of the various available production technologies in the next century.

667

The author is a strong believer in technical solutions of production problems also covering safety and environment. Fission and fusion then represent almost renewable energy sources. It is anticipated, that fission and fusion energy will become the main energy sources for hydrogen production in the next century. Solar, geothermal, wind and water temperature gradient will be applied only on a small scale.

REFERENCES

1. J. H. Kelley and E. A. Laumann, Hydrogen Tomorrow. Jet Propulsion Laboratory (1975).

2. J. H. Kelley, W. J. D. Esther and W. van Deelen, CEP 58 (1982).

3. W. van Deelen, Assessment of potential future hydrogen markets for the production of hydrogen from water. IEA (September 1980).

4. A. K. Kashani, Assessment of potential future markets for the production of hydrogen from water, U.S.A. JPL 80- (18 September 1980).

5. J. H. Kelley, Hydrogen market assessment. Private communication (September 1981).

6. W. Balthasar and J. W. M. van Rijnsoever, Hydrogen as an energy vector. Commission of the European Com- munities 664 (1980).

7. H. G. Corneil and F. J. Heinzelmann, Hydrogen: Pro- duction and Marketing. ACS Symposium Series 116, 67 (1980).

8. CWH pipeline (1981). 9. H. Gruber, Dow Chemical GmbH. Personal commu-

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