Production of hydrogen from renewable resources and its effectiveness

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journal homepage: www.elsevier.com/locate/he

Review

Production of hydrogen from renewable resources and its effectiveness Olga Bi
ca´kova´*, Pavel Straka Institute of Rock Structure and Mechanics of the AS CR, v.v.i., V Hole
sovi
cka´ch 41, 18209 Prague 8, Czech Republic

article info

abstract

Article history:

At present, hydrogen is used mainly in a chemical industry for production of ammonia and

Received 3 January 2012

methanol. In the near future, hydrogen will become a significant fuel which can solve the

Received in revised form

local problems connected with an air quality. Because the hydrogen is most widespread

11 May 2012

component on the Earth, it can be obtained from a number of sources, both renewable and

Accepted 12 May 2012

non-renewable, moreover, by various processes. Pure hydrogen can be acquired by the

Available online 17 June 2012

energy-demanding electrolysis of water. Global production has so far been dominated by hydrogen production from fossil fuels, with the most significant contemporary technolo-

Keywords:

gies being the reforming of hydrocarbons, pyrolysis and co-pyrolysis. In the near future,

Hydrogen production

biological method can be used.

Biological processes

This work is aimed to an evaluation of possibilities of the hydrogen production from the

Conventional methods

renewable sources by biological methods and comparison of effectiveness with the

Pyrolysis

conventional methods.

Electrolysis

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Hydrogen is presently used predominantly for the production of methanol and ammonia and in the refining industry. Nevertheless, hydrogen production has become a subject of interest for many global companies for its broad application and ecological aspects, and in a number of countries an intensive R&D of the methods of obtaining hydrogen through affordable technologies is being conducted. The annual production of hydrogen is now ca 55 million tons, with its consumption increasing by approximately 6% p.a. Hydrogen can be produced in many ways from a broad spectrum of initial raw materials. Hydrogen is produced predominantly from fossil fuels; roughly 96% of hydrogen is produced by steam reforming of natural gas [1e3]. Perhaps 4% of hydrogen

is produced by the electrolysis of water. Electrolytic and plasma processes demonstrate a high efficiency of hydrogen production but unfortunately consume the most energy [4e8]. It can be predicted that in the future, besides the steam reforming of natural gas and the gasification of coal, hydrogen production will be provided by the gasification of biomass and by enzymatic processes. It is therefore necessary to devote some attention to the biological methods, especially their efficiency. The fundamental question lies in the discovery of alternatives to hydrogen production from fossil fuels with its utilization especially for means of transportation. This problem can be resolved by the utilization of alternative renewable resources and the related methods of production, e.g. the gasification or pyrolysis of biomass or photolytic

* Corresponding author. Tel.: þ420 266009251; fax: þ420 284680105.
a´kova´). E-mail address: [email protected] (O. Bic 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.05.047

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cracking of water using solar energy and microorganisms as well as dark fermentation. It is not possible to consider only the ecological perspective, because e.g. photolytic cracking of water is very environmentally friendly, but its efficiency for industrial use is very low. It is clear that the processes must be taken into account not only in terms of ecology but at the same time also in terms of economics. In the first place, it is possible to consider thermochemical processes. For instance, through two-step co-pyrolysis, a significant amount of hydrogen can be obtained from mixes of waste polymers or biomass with a marginal share of coal [9,10]. Though, very promising processes for hydrogen production are the biological processes of the treatment of biomass. Biomass is one of the most prospective renewable resources, because its energetic usage, which includes also hydrogen production, has multifaceted importance. Currently, the share of biomass in the contemporary global energy supply is around 12%, but in many developing countries its share is 40e50% [11]. An advantage is that biomass and the derived phytofuel contains hardly any sulfur and the emissions of sulfur dioxide are thus negligible. The disadvantage of the energetic utilization of biomass is its as-yet insufficient competitiveness with fossil fuels. The yield of hydrogen that can be produced from biomass is relatively low, 16e18% based on dry biomass weight [12]. In terms of economics is likely that pyrolysis or co-pyrolysis of biomass, namely thanks to the valuable byproducts and low costs, represent the best possibilities. The oxidation of hydrocarbons or biomass for hydrogen production through the cracking of sugars in the vapor phase with catalysts (e.g. 150  C, Pt/Al2O3; 225  C, 22 bar, Ni, Pd, Pt, Ir, Ru and Rh/Al2O3) [13,14] is for the time being in the stage of development. From the outline above, it is clear that the selection of the resources and methods of hydrogen production must be carefully evaluated, because besides the environmental viewpoint also the economic perspective is important, given other than by the availability of the resources mainly by the effectiveness of the process. The aim of this work is to provide an overview of the promising and developing technologies of hydrogen production and determine the most promising methods for acquiring hydrogen in the future.

processes usually work with various types of the anaerobic bacteria or algae. The effect of the microorganisms differs from one another by the type of substrate and the process conditions. The objective of the development of the processes of biohydrogen production is the acquisition of a higher yield of the produced hydrogen through an economically acceptable method. Biological hydrogen production as a byproduct of the metabolism of the microorganisms includes newly developed technologies utilizing various renewable resources, which can be divided into five separate categories: direct biophotolysis, indirect biophotolysis, biological wateregas conversion, photofermentation and dark fermentation. All of these processes are controlled by enzymes producing hydrogen, particularly nitrogenase and hydrogenase, whose properties are shown in Table 1 [15]. The main components of nitrogenase are the molybdenumeiron protein and iron. The creation of hydrogen by nitrogenase can be described by the chemical reaction (Eq. (1)) 4ATP þ 2Hþ þ 2e /H2 þ 4ADP þ 4Pi

where ATP is adenosintriphosphate, ADP is adenosindiphosphate and Pi is inorganic phosphate, respectively. In the majority of photosynthetic microorganisms, hydrogenases exist as acceptor and reversible hydrogenases. The important components of acceptor hydrogenase are NiFe and NiFeS, which consume molecular hydrogen by the reaction (Eq. (2)) [15,16] H2 /2e þ 2Hþ

Interest in research in the area of biohydrogen has increased in the last decades because of the rising amount of waste materials and the need for their minimization. Biological processes are in contrast with electrolysis and thermochemical processes catalyzed by microorganisms in an aqueous environment under atmospheric pressure and at an ambient temperature. These processes can be utilized in localities with a well-accessible source of biomass or another suitable waste material, which leads to a reduction of the energy costs and the costs for the transport of the initial raw material. The major criteria for the selection raw materials are cost, content of carbohydrate, biodegradability and availability. Biological

(2)

Reversible hydrogenases have the ability to create molecular hydrogen as well as to consume it depending on the reaction conditions H2 42e þ 2Hþ

(3)

The initial material for the creation of hydrogen through a photolytic process is water, with a fermentation-process

Table 1 e The properties of nitrogenase and hydrogenase [15]. Property

2. Biological processes of hydrogen production

(1)

Substrates Products Number of proteins Metal components or sulfur Optimal temperature Optimal pH Inhibitors Stimulators

Nitrogenase þ

ATP, H or nitrogen, electrons H2, NHþ 4

Hydrogenase þ

H , hydrogen

2 (MoeFe and Fe)

ATP, Hþ, hydrogen, electrons 1

Mo, Fe

Ni, Fe, S

30  C (A. vinelandii)

55  C (R. rubrum) 70  C (R. capsulatus) 6.5e7.5 (R. sulfidophilus) CO, EDTA, O2, some organic compounds Absence of organic compounds (R. rubrum, R. capsulatus)

7.1e7.3 (A. vinelandii) N2, NHþ 4 , O2, high N:C ratio of H2 production Light

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biomass. All of the processes have advantages and disadvantages, which are described below. On the whole, the biological processes described below are not very developed and a further investigation toward to commercial scale is needed.

2.1.

Direct biophotolysis

The production of hydrogen by direct biophotolysis utilizes the photosynthetic system of microalgae for the transformation of solar energy into the chemical energy needed for the cracking of the molecule of water while yielding hydrogen (Eq. (4)) [17,18] solar energy

2H2 O





! 2H2 þ O2

(4)

The complex sets of reactions which occur within photosythesis include two photosynthetic systems, known as PSI and PSII. The systems use the ability of specialized microorganisms like the algae Chlamydomonas reinhardtii and Anabaena sp. for the generation of oxygen and hydrogen ions and microalgae like green algae or blue-green algae cyanobacteria for hydrogen production [19,20]. The important media are ferredoxin, reduced ferredoxin (Fd) and reverse hydrogenase (Eq. (5)) [16,21,22]. H2 O/PSII/PSI/Fd/Hydrogenase/H2 Y O2

(5)

Indirect biophotolysis

The process of indirect biophotolysis includes four steps: the production of biomass by photosynthesis, the concentration of the biomass, aerobic dark fermentation (with a yield of 4 mol of H2/mol of glucose in the cell of the algae along with 2 mol of acetate) and the conversion of 2 mol of acetate into hydrogen. Indirect biophotolysis utilizes cyanobacteria and takes place following reactions (Eq. (6) and (7)) [27,28]. solar energy

6CO2 þ 6H2 O





! C6 H12 O6 þ 6O2 solar energy

C6 H12 O6 þ 6H2 O





! 12H2 þ 6CO2

Cyanobacteria contain photosynthetic dyes, which can serve oxidation photosyntheses under acceptable nutritional conditions: air, water, mineral salts and light. In this process, hydrogen is produced by both hydrogenase and nitrogenase. The hydrogen produced by cyanobacteria can be fixed by nitrogen or without nitrogen. Examples of organisms with fixed nitrogen are the non-marine bacteria Anabaena sp. or marine cyanobacteria Calothrix sp. and Oscillatoria sp. [24]. Organisms of the type Synechococcus sp., Gloebacter sp. and Anabaena sp. are not fixed by nitrogen [22,29]. These bacteria are suitable for the development of a greater amount of hydrogen in comparison with the other types of cyanobacteria. One of the known cyanobacteria producing hydrogen is Anabaena cylindrica, although in recent years Anabaena variabilis with a higher efficiency of hydrogen production has demonstrated a rate of the order of 0.355 mmol/h per liter. The production of hydrogen occurs also under anaerobic conditions [30]. An important role is again played here by nitrogenase. The production takes place following the equations below (Eq. (8) and (9)). a) with nitrogen: N2 þ 8Hþ þ 8e þ 16ATP/2NH3 þ H2 þ 16ADP þ 16Pi

Hydrogen production will be successful if the oxygen content is maintained below 0.1%, because hydrogenase as has been mentioned in Table 1 is very sensitive to the presence of oxygen. This condition can be fulfilled by using specialized microorganisms, like the green algae C. reinhardtii. The advantage of this technology lies in the primary supply of inexpensive and easily available water. The disadvantage is its low efficiency, ca 5%, which even through advanced research was raised only to 15%. The activity related to hydrogenase was observed also with other similar algae: Scenedesmus obliquus, Chlorococcum littorale, Platymonas subcordiformis and Chlorella fusca of three type strains Chlorella, where were isolated from soil and water in the Algerian Sahara. Chlorella sorokiniana strain Ce, Chlorella salina strain Mt, Chlorella sp. strain Pt6 [19,23e26]. Recently, their mutants have been derived from the microalgae; the mutants bear the presence of oxygen better and also increase the production of hydrogen. Direct biophotopysis is at an early stage of development.

2.2.

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(6) (7)

(8)

b) or without nitrogen: 16ATP þ 8Hþ þ 8e /4H2 þ 16ADP þ 16Pi

(9)

Also simple sugars were used for the production of hydrogen. Markov et al. [31] attained a production of 12.5 ml H2/g per cell in dry state by indirect biophotolysis with the cyanobacteria A. variabilis exposed to solar radiation. Currently, the speed of hydrogen production with Anabaena sp. is relatively low in comparison with dark fermentation or photofermentation. Another study of indirect biophotolysis with the bacteria Cyanobacterium gloeocapsa alpicola [32] has shown that the maintaining medium for the optimal production of hydrogen is a pH value between 6.8 and 8.3. The temperature increases from 30  C to 40  C can double the production of hydrogen.

2.3.

The biological reaction of wateregas conversion

Although the biological reaction of wateregas (CO þ H2) conversion is currently in the stage of laboratory development, suitable microorganisms working in the environment of carbon monoxide have been discovered. For hydrogen production, this process is favorable, because the dominating products carbon dioxide and hydrogen are balanced. Organisms growing at the expense of this process are such photoheterotrophic gram-negative bacteria as Rhodospirillum rubrum and Rubrivivax gelatinosus and such gram-positive bacteria as Carboxydothermus hydrogenoformans [20,30,33e38]. Under anaerobic conditions, the synthesis is activated by carbon monoxide, namely immediately with several proteins including the carbon monoxide dehydrogenase, the FeeS protein and the carbon monoxide hydrogenase. The electrons released from carbon monoxide through its oxidation are converted via the FeeS protein to hydrogenase for hydrogen

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production. The process takes place at low temperatures and pressures, during which the transformation to CO2 and H2 is significantly assisted thermodynamically. The speed of the transformation is high in comparison with other biological processes but requires a source of carbon monoxide and darkness. This requirement is fulfilled by the photoheterotrophic bacteria R. rubrum working in a dark environment. It was discovered that the generation of this bacteria under the conditions of dark fermentation and in the presence of nickel takes less than 5 h in the case of the oxidation of CO to CO2 along with the reduction of the protons to hydrogen [7,36]. Nevertheless, for its growth it needs light. If the partial pressure of carbon monoxide increases above 0.02 MPa, the produced hydrogen is suppressed. An alternative tested bacterium for hydrogen production is the bacteria Citrobacter sp.Y19 [39], whose maximum activity of hydrogen production was 27 mmol/g cells per hour and is three times higher than that provided by R rubrum. Another bacterium capable of assisting wateregas conversion under anaerobic conditions, atmospheric pressure and at an ambient temperature is the non-sulfur purple photosynthetic bacterium R. gelatinosus. It was determined that this bacterium can work up to an overpressure of 0.4 MPa [35]. In comparison with the photosynthetic or aerobic way, the anaerobic biological conversion produces much less energy for metabolic activity. The drop of energy production leads to a slowing of cell growth. The advantage of the biological system of wateregas conversion lies in that it can work in a conventional closed reactor. This reactor is similar to a biological filter that is used for cleaning waste waters and thus does not require any expensive photobioreactor. While the aerobic reactions provide the organisms with enough energy for more intensive cell growth per mole of CO, they will not produce hydrogen under the given conditions. The reactor cannot function stably unless the speed of cell growth is less than the speed of their natural mortality. In every biological system, cells constantly die; therefore the nutrients are recycled in the dying cells. The rate of the cell growth is important for starting the reactor and the reacquisition of the cells from a reversible process. Changes in pH or temperature can lead to the loss of biological activity as a result of cell damage or death. A higher growth rate of the organisms will provide a faster renewal of the reactor. This opportunity is offered by R. gelatinosus with nutrients in the form of acetates, malates, or inexpensive sources of sugar (e.g. corn extract). The process should be managed so that if it is contaminated, the system must be quickly sterilized and again restreaked. The ideal conditions of growth can be favorably predetermined precisely by the bacterium R. gelatinosus. Currently, the reactors are operated on a laboratory scale, which work for the period of a few months with little or even breakeven hydrogen production.

2.4.

The photofermentation method

and can be easily combined with hydrogen fermentation. The optimal temperature is 30e35  C and pH 7.0. One of the groups of microorganisms capable of photofermentation are nonsulfur purple bacteria, which under anaerobic conditions utilize simple organic acids. The electrons released from the organically bound carbon are in the presence of nitrogen used by nitrogenase for the reduction of molecular nitrogen to ammonia. If the process takes place without nitrogen and solar energy, organic acids or biomass are reduced to hydrogen by equation (Eq. (10)) CH3 COOH þ 2H2 O

! 4H2 þ 2CO2

The benefit of bacteria lies in their flexible metabolic potential. They can be used in a wide range of conditions, because they lack the mentioned PSII system, which precludes a reaction with oxygen and also suppresses hydrogen production. Phototrophic bacteria require organic and inorganic sources of electrons for the governance of photosyntheses and permit work with a number of inexpensive compounds like dried seaweed, agars gel, porous glass, polyurethane foam etc. [22,40]. The total biochemical reaction of the photofermentation method can be expresses as follows (Eq. (11)) [3]:

(11)

The disadvantage of this method is the limited applicability of organic acids and nitrogenase enzymes, which are slow. The method requires a relatively significant amount of energy and reverse hydrogen oxidation. By maintaining a suitable ratio of C:N in the nutrients, nitrogenase activity can be raised and at the same time the energy demands may be reduced [41]. Hydrogenase enzymes recycle the produced hydrogen back into nitrogenases and thus support cell growth. In the current research, genetically modified bacteria are used for the suppression of hydrogenase enzymes. In the last few years, experiments have been conducted focusing on hydrogen production from industrial and agricultural wastes. The efficiency of the proposed methods differs greatly as can be seen in Table 2 (the production of hydrogen by the photofermentation of bacteria from biomass and waste water) [15,24,42,43]. The production of hydrogen from a sugar solution was also examined. The solution had been acquired by the hydrolysis

Table 2 e The effectiveness of hydrogen production from photofermentation [15,24,42,43]. Matter

Photofermentation is a process in which organic materials or biomass are converted into hydrogen and carbon dioxide by photosynthetic bacteria under the simultaneous use of solar energy. The process takes place under anaerobic conditions

(10)

Lactic acid Lactates Waste water

Bacterial system

Efficiency

Rhodobacter sphaeroides RV Rhodobacter capsulatus Rhodobacter sphaeroides (immobilized)

80e86% 30% 53%

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of wheat remnants in an acidic solution in an autoclave for a period of 15 min at pH ¼ 3 and a temperature of 90  C. The resulting sugar solution was neutralized and nutrient additions were added in the form of Rhodobacter sphaeriodes RV, NRLL and DSZM for hydrogen production by photofermentation [40e42]. The highest amount of hydrogen created was determined for the bacteria R. sphaeriodes RV 178 ml with the specific rate of hydrogen production being 46 ml of hydrogen per gram of biomass and hour, at a concentration of sugar solution of 5 kg/m3. It was also found that hydrogen production depended on the concentration of the initial solution. With an increasing concentration of the sugar solution from 2.2 to 8.5 kg/m3, also the amount of hydrogen produced increased from 30 to 232 ml. In summary, it can be said that the nitrogenase enzymes used have high energy demands, a low efficiency of the conversion of solar energy and significant requirements on the area of the anaerobic photobioreactor. In its current form, the photofermentation process is therefore not competitive and must be further investigated.

2.5.

Dark fermentation

The fermentation utilizes predominantly anaerobic bacteria, although for the cultivation of a substrate rich in sugars ‘in the dark’ also some algae can be used. Hydrogen is produced at relatively low temperatures, 30e80  C. For conversion by classical thermochemical processes, dry biomass is a suitable material. Biomass with high water content is from an economic perspective non-utilizable this way (it can be used in biotechnological processes catalyzed by microorganisms in an aqueous environment at low temperatures and under low pressures). Very efficient fermentation microorganisms to produce hydrogen include Enterobacter cloacae, Enterobacter aerogenes, Clostridium sp. and Bacillus sp. [44e46]. The biomass used for the fermentation procedures should be easily biologically degradable, inexpensive, accessible in large quantities and with a high carbohydrates content, e.g. in agricultural remains, food waste etc. Simple and biologically easily degradable sugars like glucose, lactose and sucrose can be considered [47]. In the last few years, hydrogen production from delignified wood fibers and a-cellulose through the method catalyzed by singly-used Clostridium thermocellum culture with a molar yield of hydrogen around 2 has also been tested [48]. The products of dark fermentation are hydrogen and carbon dioxide, to a lesser extent there are other gases present, e.g. methane and sulfane, unlike the biophotolysis process which produces only hydrogen. The composition of the gases from the fermentation process however depends on the course of the process and the type of substrate used. The amount of hydrogen produced by dark fermentation depends further on the pH value, the dwell time and partial pressure of the gas. The optimal pH value for the production of hydrogen should be maintained between 5 and 6 [49]. With the rising concentration of hydrogen, also its partial pressure rises, which leads to a shift of the metabolic way and to the creation of smaller substrates (lactates, acetone, ethanol etc.), which reduce the production of hydrogen. It has been determined that the optimal dwell time for the maximum

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production of hydrogen is half a day. The rate of hydrogen production can significantly decrease from 198 to 34 mmol/l/ day if the time dwell is extended to three days. The carbohydrates in the waste water were degraded with the increasing efficiency from 70% to 97% [15]. The fermentation way with a model substrate, glucose, achieves the theoretical biological maximum of 4 mol of hydrogen/per mole of glucose at best a 33% yield (Eq. (12)) [27,50,51]. At the same time, 2 mol of acetates are created, in which another 4 mol of hydrogen are fixed. The fermentation process produces acetic acid, butyric and further organic acids that reduce the hydrogen yield. Their production requires additional treatment of the waste water (simple sugars, starch or cellulose containing wastes, food industry waste, waste sludge), which increases the costs of the process. The economy of the process can be increased by further use of the waste acetate e.g. via photofermentation. C6 H12 O6 þ 2H2 O/2CH3 COOH þ 2CO2 þ 4H2

(12)

Currently, the speeds of hydrogen production with dark fermentation or photofermentation are relatively high in comparison with indirect biophotolysis.

2.6.

Microbial electrolytic cell

Microbial electrolytic cells (MECs), also called bioelectrochemically assisted microbial reactors (BEAMR), utilize electrochemical hydrogenation for the direct transformation of biologically degradable material into hydrogen [27,52]. A microbial electrolytic cell is actually a modified microbial fuel cell (MFC), whose principle lies in the breakdown of the organic mass by microbes in the anaerobic environment of the anode [53]. In this metabolic process, electrons, protons and carbon dioxide are created. The protons pass through the electrolyte toward the cathode through the proton membrane and the electrons stream through the resistance of the circuit to the cathode under the production of current. The protons and electrons combine with the acid on the cathode while yielding water. The efficiency of this technology is low, but on the other hand it makes it possible to provide energetically for the operation of sewage treatment plants for whose operation bacteria that are common in waste water suffice. MECs use similar components as are used in solid fuel cells. Since the membranes used in MFCs increase the ohmic resistance, new membranes of the Nafion type have been successfully developed for MECs, which lowered the voltage used from 1 V (when using a gas diffusion membrane) to 0.5 V and in a construction without a membrane even to 0.4 V. The efficiency (h) is a function of the low heating value of hydrogen divided by the low heating value of the organic material plus the electrical energy provided (Eq. (13)) [7,54] h ¼ Pn  1

nhydrogen DHc;hydrogen  IEap Dt  I2 Rex Dt þ nsubstratum DHc;substratum

(13)

where I ¼ current, Eap ¼ voltage used, Dt ¼ increment of time (s) in “n” reference points measured during the group cycle and Rex ¼ external resistance, which was 10 U. The usage of the equation (Eq. (13)) significantly increased the efficiency from 23% with the application of the gas

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diffusion membrane to 53% for the Nafion membrane and all the way to 76% with the use of a membraneless reactor. Under these conditions, the rate of hydrogen production is 3.12 m3 H2/m3 per reactor day [50,53,54]. In a BEAMR, hydrogen is developed on the cathode under a simultaneous removal of oxygen. A low voltage is added to the circuit and Geobacter, Shewanella sps. or Rhodoferax ferrireducens bacteria are used. The theoretical voltage for hydrogen production in a neutral environment is 0.61 V (the voltage on the cathode opposite the Ag/AgCl electrode) [52]. The anode produced in the oxidation of the organic mass by special microorganisms is thus approximately 0.5 V. The minimal voltage needed is hence 0.11 V. For hydrogen production by the bioelectrolysis of an acetate, this minimal voltage used is greater than 0.25 V because of the ohmic resistance and electrode overvoltage (Eq. (14)e(16)) [52,55]. C6 H12 O6 þ 2H2 O/2CH3 COOH þ 2CO2 þ 4H2

(14)

Anode : CH3 COOH þ 2H2 O/2CO2 þ 8Hþ þ 8e

(15)

Cathode : 8Hþ þ 8e /4H2

(16)

The BEAMR process differs from the MFCs by the loss of hydrogen, which diffuses from the cathode chamber through the membrane. The technology allows with an acceptable energetic efficiency the production of very pure hydrogen (>99.5%) from a wide range of biological waste material [3,52]. This process is advanced developed in laboratory scale, and further investigations toward an industrial scale are needed.

2.7.

The multi-stage integrated method

The multi-stage process of hydrogen production leading to the maximum yield of hydrogen was initially composed of two steps: dark fermentation and photofermentation. In the first step, anaerobic fermentation of the sugars or organic wastes takes place under the creation of intermediate products with low molecular mass, e.g. organic acids. In the second step, these are transformed in a photobioreactor to hydrogen by a photosynthetic bacterium. The general reactions of the process can be described in this way (Eqs. (17) and (18)) [21,51,56,57]. Stage 1: Dark fermentation (selective anaerobes): C6 H12 O6 þ 2H2 O/2CH3 COOH þ 2CO2 þ 4H2

(17)

Stage 2: Photofermentation (photosynthetic bacteria): CH3 COOH þ 2H2 O/2CO2 þ 4H2

(18)

From the equations above it is clear that one of the suitable substrates for dark anaerobic fermentation is glucose. Its dominant metabolic product is acetic acid. Theoretically, it is possible to acquire through this combined approach 12 mol of hydrogen from waste water using of purple non-sulfur photosynthetic bacteria and anaerobic bacteria [58,59]. Also multi-stage procedures comprised of three or four steps have been proposed (Fig. 1) [7]. The initial raw material in the reactor with dark fermentation is biomass, which is subsequently degraded into hydrogen and waste water. Waste water containing organic acids is further treated with the

Visible light Water

H2

Green algae

H2

IR light

Photofermentation

Celullose biomass

Cellular biomass Sugar

H2

Dark fermentation

Small organic molecules Electrical energy

Microbial electrolysis

H2

Fig. 1 e A multi-stage integrated system for biohydrogen production.

photofermentation approach. In the first place, the photofermentation process utilizes the infrared component of light. The next stage utilizes microbial electrolytic cells, which produce hydrogen from some organic acids while not requiring light. The ammonia contained in waste water produced from the first stage however suppresses the second stage. The waste water consequently must be neutralized before the connection of the second stage. When using multi-stage methods, however, problems arise connected both with the actual implementation and control of the process and with its operation and maintenance.

3. Conventional methods of hydrogen production The technologies of the treatment of fuels transform raw materials like gasoline, hydrocarbons, ammonia, methanol or ethanol into gaseous substances rich in hydrogen. Most hydrocarbon fuels contain a certain amount of sulfur, whose removal is a significant task in the planning of the hydrogen economy. The thermochemical processes are important. They are characterized by the fact that during them the temperature exceeds the limits of the stability of the given substance under the given conditions and is in the range of approximately 200  Ce3000  C. In terms of the character of the chemical reaction, the thermochemical processes can be divided into oxidation and reduction processes. In the oxidation processes, the amount of the oxidizer in the reaction zone is higher or equal to stechiometric combustion, whereas in reduction processes the amount of oxidizer is substechiometric or even zero (gasification, pyrolysis). Hydrogen can be produced only by processes of the second group. It is believed that in the near future biomass can become an important source of hydrogen. But the cost of biomass gasification must be reduced by designing new processing routes and especially catalytic systems. Hydrogen can be acquired from hydrocarbon fuels through three basic techniques: steam reforming (SR), partial oxidation (POX) and autothermal reforming (ATR). These methods produce a great deal of carbon monoxide. The subsequent step therefore uses one or more reactors for the wateregas shift (WGS) reaction (see below). All conventional processes of hydrogen production are used in commercial scale.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 1 5 6 3 e1 1 5 7 8

3.1.

Steam reforming

3.2.

Steam reforming is currently one of the most widespread and at the same time least expensive processes of hydrogen production, through which more than 90% of the hydrogen used is produced [60]. Its advantage comes from the high efficiency of its operation and low operational and production costs. The most frequently used raw materials are natural gas and lighter hydrocarbons, but coke oven gas too [61]. The process has two stages. In the first stage, hydrocarbon raw material is fed into steam (500e900  C, 0.3e2.5 MPa) in a tube reactor filled with a catalyst on the basis of nickel oxide (or Ni þ MgO, Pt, Rh) [62,63]. The catalytic process requires a desulfurized initial raw material. During its reaction, syngas (H2 þ CO) is produced along with a lower proportion of CO2 (Eqs. (19) and (20)). The necessary temperature is achieved by the addition of oxygen or air for the combustion of a part of the raw material (heating gas) inside the reactor. The reaction products are led through the boiler for the production of steam and through the condenser, where they are cooled to approximately 360  C. In the second stage, the cooled gas is led into the converters, where carbon monoxide is converted by means of steam into carbon dioxide (Eq. (21)). CH4 þ H2 OðgÞ /CO þ 3H2

endothermic

CH4 þ 2H2 OðgÞ/CO2 þ 4H2 CO þ H2 OðgÞ/CO2 þ H2

endothermic

exothermic

CO2 þ H2 4CO þ H2 O exothermic

(19) (20) (21) (22)

The nascent carbonic gas is removed by a reversible exothermic reaction (Eq. (22)) usually implemented in two stages. In the first, so-called high-temperature stage, the temperature of the products is raised to almost 500  C, which has the result of lowering the balanced yield of CO2 and H2. The products are then cooled to approximately 360  C and are led to the low-temperature converter filled with a highly active copper catalyst (the second stage), where the concentration of CO is lowered to 0.2e0.3 vol.% at low temperatures of 180e230  C [7]. The gaseous products exiting the low-temperature converter are further cooled and led to the absorber with ethanolamines, in which the CO2 is washed. An important factor characterizing steam reforming is the H:C proportion in the initial raw material. The higher is this proportion, the lower the production of carbon dioxide emissions. A membrane reactor can replace both reactors of the conventional SR process in a achieving the overall reaction (Eq. (20)) [64]. Hydrogen is produced on an industrial scale by the steam reforming of methane, during which the heat efficiency of the process is around 70e85% [65]. A number of other raw materials will be possible to process in this way in the near future: solid communal waste, wastes from the food industry, oils, purposefully cultivated or waste agricultural biomass and fuels of a fossil origin, e.g. coals. The disadvantage is the high production of CO2, around 7.05 kg per 1 kg of hydrogen produced [66].

11569

Partial oxidation

Another relatively widespread process of the production of hydrogen and syngases is POX and the catalytic partial oxidation of hydrocarbons. The gasified raw materials can be methane, biogas but primarily heavy oil fractions (vacuum remnants, heating oil), whose further treatment and utilization are difficult [67]. POX is a non-catalytic process, in which the raw material is gasified in the presence of oxygen (Eqs. (23) and (24)) and possibly steam (Eq. (25)) (ATR) at temperatures of 1300e1500  C and pressures of 3e8 MPa. In comparison with steam reforming (H2:CO ¼ 3:1), more CO (H2:CO ¼ 1:1, 2:1) is created. The process is therefore complemented by a steam conversion of the carbon monoxide into hydrogen and carbon dioxide. This reaction contributes to the maintenance of the equilibrium between the individual reaction products. CH4 þ O2 /CO þ 2H2

exothermic

(23)

CH4 þ 2O2 /CO2 þ 2H2 O exothermic

(24)

CH4 þ H2 OðgÞ/CO þ 3H2

(25)

endothermic

The gaseous mixture created through partial oxidation contains CO, CO2, H2O, H2, CH4 and hydrogen sulfide (H2S) and carbon oxysulfide (COS). A part of the gas is burned to provide enough heat for the endothermic processes. The soot created by the decomposition of acetylene as a transitional product is an undesired product. Its amount depends on the proportion of H:C in the initial raw material. There has therefore been, like with SR an endeavor to shift to raw materials with a higher ratio of H:C, e.g. to natural gas. While the operation of the reactor is less expensive in comparison with steam reforming, the subsequent conversion makes this technology more expensive. The theoretical efficiency of this process is similar to that of conventional SR (60e75%), but less water is required [68]. Since the process does not require the use of a catalyst, it is not necessary to remove sulfurous elements from natural gas, which would lower the efficiency of the catalyst. The sulfurous compounds contained in the gasified raw material are converted into hydrogen sulfide (ca 95%) and carbon oxysulfide (ca 5%). However, POX conducted at lower working temperatures of 700e1000  C and pressures in the range of 6e8 MPa works in the presence of catalysts. The typical catalysts for the POX of natural gas are those based on nickel or rhodium. POX is suitable for small-scale conversion, such as in a motor vehicle with fuel cells.

3.3.

Autothermal reforming

As has already been mentioned, if steam is added to the gasified raw material and an oxidant (O2), it is an ATR. Autothermal reforming utilizes mixtures of air or oxygen and steam in suitable proportion according to the further application of syngas, e.g. for FischereTropsch synthesis. Nickelbased catalysts are used to lower the energy demands of the process. The heat from POX reduces the need for an external heat source, which simplifies the given system and shortens the heating rise time. The autothermal reforming of methane

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(Eq. (26)) has a heating efficiency comparable to partial oxidation, i.e. 60e75%, and slightly lower than steam reforming. Gasoline and other high hydrocarbons may be converted to hydrogen on board cars by the autothermal processes, using suitable catalysts [69,70]. CH4 þ H2 O þ O2 /3CO þ 7H2

CH4 + H2O

Steam reformimg 700 – 800 °C Catalyst Ni

COx+H2 H2O (g)

H T S

L T S

Purification or methanation

(26)

It is evident from Table 3 how the selection of the way of transforming the natural gas can influence the composition of the syngas created [71].

3.4.

WGS reactor

WGS – water-gas conversion HTS – high-temperature step LTS – low-temperature step

Fig. 2 e A diagram of steam reforming with the subsequent conversion.

Wateregas shift

Syngas can be further modified according to other requirements, e.g. through the conversion of CO þ H2O to CO2 and H2 or through the addition of gas of another composition to increase the content of a desired gaseous component or through the complete removal of carbon monoxide. Wateregas shift serves for the transformation of water and carbon monoxide to carbon dioxide and hydrogen by means of a reaction with steam at temperatures of 400e500  C in the presence of catalysts Cr2O3, Fe2O3 (Eq. (27)). This leads to a drop in the concentration of CO down to 0.5e1 mol%. CO þ H2 O/CO2 þ H2

exothermic

(27)

The reaction has an exothermic course, so the reaction equilibrium will be shifted to the right and will support the formation of hydrogen and carbon dioxide at lower temperatures. In terms of kinetics, a higher temperature is preferred. For this reason, it is common to use two successive steps, first a high-temperature and then a low-temperature step. The conversion in the high-temperature reactor takes place at temperatures of 350e370  C and is limited by the balanced composition at those temperatures. Here an almost 90% conversion of carbon monoxide occurs. A higher level of the conversion of carbon monoxide to hydrogen by steam is achieved when the gas exiting the high-temperature reactor is cooled to 200e220  C and fed into the low-temperature reactor. The remaining amount of carbon monoxide can be removed in the third step, including catalytic methanation or purification (Fig. 2). The methanation reactor transforms the residual carbon monoxide (Eq. (28)) or carbon dioxide (Eq. (29)) into methane, thus lowering its concentration down to the desired 10 ppm, but at the cost of higher hydrogen demand, which leads to a lower overall yield [34].

CO þ 3H2 /CH4 þ H2 O

(28)

CO2 þ 4H2 /CH4 þ 2H2 O

(29)

On the other hand, at higher temperatures the equilibrium shifts to the left and is limited by the complete transformation of carbon monoxide and water to hydrogen by reaction the below mentioned (Eq. (30)). CO2 þ H2 /CO þ H2 O

(30)

The wateregas shift reaction is the basis of the global industrial production of hydrogen from natural gas by steam reforming. All three processes can take place simultaneously in one reactor and the final composition of the products depends on the type of catalyst used. The most common catalysts for steam reforming are Co/ZnO, ZnO and Rh/Al2O3. The most utilized catalysts for WGS are copper-based catalysts (for a low-temperature reactor), although currently even molybdenum carbide (MoC2) catalysts are used. For a hightemperature reactor, they are catalysts based on FeePd alloys. Other suitable catalysts for WGS are Ru/ZrO2 and Pt/ CeO2 catalysts, or Cu/ZnO and Fe/Cr2O3 catalysts, more suitable in terms of price. The main emphasis is placed on the removal of catalytic poisons, i.e. carbon monoxide and sulfur compounds. Technologies dealing with their removal are quite widespread in the industry. At the same time, hydrogen is purified by also alternative approaches, namely pressureswing adsorption, cryogenic distillation and membrane technologies, which can ensure the necessary purity of hydrogen, 98e99%. The most advantageous gas-purification method is

Table 3 e The composition of the syngases according to transformation method of natural gas (%). Steam reforminga and autothermal reforming by

H2 CO CO2 N2 CH4

Autothermal reforming by steam and

oxygen

air

oxygen

air

66e68 22e24 8e9 0.5e1 0.5

56e57 10e15 7e12 22e23 0.2e0.3

68 20 10 0.2 0.4

53 15 10 22 0.3

a A two-stage process.

Partial oxidation by steam and oxygen

60 35 3 1.5 0.3

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4.

Pyrolysis and co-pyrolysis

Another currently promising method of hydrogen production is pyrolysis, or co-pyrolysis. Raw organic material is heated and degasified at a pressure of 0.1e0.5 MPa to a temperature of 500e900  C [15]. The process takes place in the absence of oxygen as well as air, and therefore the formation of dioxins can be almost ruled out. Carbon oxides (CO, CO2) are formed in a minority, which leads to the exclusion of the use of secondary reactors (WGS, POX etc.). As a result a significant lowering of emissions is achieved. The advantages of this process lie in the adaptability of the fuel and its relative simplicity. The by-product is a pure carbon. The reaction can be generally described by the following equation (Eq. (31)). Organic material þ heat/H2 þ CO þ CH4 þ other products (31) Based on the temperature range, pyrolysis processes are divided into low- (up to 500  C), medium- (500e800  C) and high-temperature (over 800  C). Fast pyrolysis is one of the latest processes for the transformation of organic material into products with a higher energetic content. The products of fast pyrolysis appear in all of the phases, i.e. solid, liquid and gaseous. The application of the co-pyrolysis of a mixture of coal with organic wastes has recently come to the fore of interest in industrially advanced countries, as it should limit and lighten the burden of wastes in waste disposal (waste and pure plastics, rubber, cellulose, paper, textiles and wood) [72e78]. Thermogravimetric analysis (TGA) is one of the most common techniques used to investigate thermal events and kinetics during pyrolysis coal or coal with plastics [79e81]. It found out, that the polymers played the role of hydrogen donor solvent for coal. The potential of high-temperature co-pyrolysis of waste materials (polymers e acrylonitril butadiene styrene, epoxide, mixed municipal waste, rubber, textile, sawdust, straw) with bituminous coal or lignite has been devoted extensive studies [10,82e84]. The experiments were conducted in the laboratory scale on the model pyrolysis unit with a stationary bed and in an inclined rotary kiln. The continuous pyrolysis of an admixture of lignite with 30 wt.% additive (rubber, ABS, cellulose) in a rotary kiln at 900  C is a two-product technology. We obtain pyrolysis gas and a solid product (carbonaceous residue) [10,85]. This temperature led to incomplete decomposition of all hydrocarbons present in the steamegas mixture to hydrogen and pyrolytic carbon. The higher amount of methane however has a positive influence on the high heating value of the gas (ca 17 MJ/m3), through which we obtain energetically wellutilizable gas [43]. In order to obtain gas with high hydrogen content the twostage co-pyrolysis of coal with organic materials was investigated [9,10]. The pyrolysis unit was expanded by a degradation

module of liquid and gaseous products coming from the stationary reactor, which runs at 1200  C. The liquid products from the first stage are quantitatively cracked in the degradation module and transformed into gaseous components. A further step lies in the use of organic materials, e.g. rubber and mixed plastics, using the two-stage co-pyrolysis the gas with high content hydrogen is formed up to 81% vol. (Fig. 3) [9,10,83,84]. An increase in the yields of gaseous products in hydrogen production is reached at high temperatures with high heating rates and longer dwell times at the volatile phase. These parameters can be influenced by a suitable selection of reactor type and regime of temperature transfer. Some inorganic salts like chlorides, carbonates and chromates have a favorable influence on the speed of the pyrolysis reaction. The influence was tested of also other types of catalysts: Ni-based catalysts, zeolites, K2CO3, Na2CO3 and CaCO3 as well as various metal oxides (Al2O3, SiO2, ZrO2, TiO2 and Cr2O3) [15,86,87]. The presence of NieMgeAl catalyst in the gasification bed was significantly increased the production of hydrogen from 0.015 g H2/g waste plastics (without catalyst) to 0.258 g H2/g waste plastics with catalyst during gasification temperature 800  C [88]. The most efficient of the carbonate-based catalysts is Na2CO3. Catalysts of the rare metals ruthenium and rhodium are much more efficient than nickel-based ones and are less sensitive to the formation of carbon. However, they are not commonly used because of their high prices. Pyrolysis and co-pyrolysis are good developed and they could be used in commercial scale.

5. The production of hydrogen in a small scale Besides the classic and most used conventional methods of acquiring hydrogen, research has proceeded also along other paths by which hydrogen can be produced on a small scale. They include for example plasma cracking, where the reactions are the same as with conventional cracking, but the energy and free radicals are provided by plasma formed by electricity or heat [4e6]. When feeding water or steam along

90

70 60

80.6

78.3

77.4

80

vol. % of hydrogen

the pressure-swing adsorption for its high efficiency (>99.99%) and flexibility.

61.2

59 53.6

50 40 30 20 10 0 bituminous coal

15 % rubber one-stage

15 % plastics

two-stage

Fig. 3 e The dependence of the hydrogen content on the type of the additive from one-stage and two-stage copyrolyzes with bituminous coal.

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with fuel, also electrons are formed besides the radicals of H, OH and O, which creates the conditions for both reduction and oxidation reactions. This method may be applied also within the pyrolysis process and in processes facilitating SR, ATR or POX. A significant problem however is lowering the energy consumption. There are two categories of plasma cracking: thermal and non-thermal. In non-thermal plasma, the temperature rises to markedly high values (>5000  C) [4e6]. Since the electrons are immediately excited, wattage of only a few hundred watts is needed. Non-thermal plasma cracking can be divided into gliding arc plasma (GAP), dielectric barrier discharge (DBD), microwave plasma and corona discharge [4e6]. The first three use dynamic discharge for the formation of plasma, whereas corona discharge is created by a static discharge [6]. The efficiency of the plasma technologies differs markedly. The most effective seems to be non-thermal cracking in gliding arc plasma, which reaches an efficiency of as much as 85%. A promising method of obtaining hydrogen is also dielectric discharge with an efficiency of 55%. The problem in the use of this process is predominantly in the energy demand for reaching a plasma state. Plasma cracking is still in the developmental stage. Hydrogen production in a small scale also includes the decomposition of methanol and ammonia. Cracking of ammonia was designed in the first place for use in fuel cells. Hydrogen can be acquired in a small scale by cracking methanol with steam. The reaction takes place at temperatures of 300e400  C, with a pressure of 3 MPa and in the presence of catalysts of ZnO and Cr2O3 (Eq. (32)). CH3 OH þ H2 OðgÞ/CO2 þ 3H2

(32)

The efficiency of the cracking is as high as 90%, whereas the performance can be changed within the range of 20e100%.

6. Alternative resources and some promising methods of hydrogen production for the future

further industrial application. However, in comparison with the foregoing methods, it is a highly energy-demanding technology. The energetic efficiency of the electrolysis of water (chemical energy acquired/electrical energy supplied) in practice reaches 50e70% [65].

6.1.

Alkaline electrolyzer

The most common electrolysis technologies are based on the electrolysis of alkaline solutions. The addition of an electrolyte (salt) increases the conductivity of water, and hydrogen is a by-product e.g. in the production of chlorine or sodium hydroxide. On the other hand, a more effective technology in which more protons are exchanged utilizes proton-exchange membrane (PEM) electrolyzers and the electrolysis units of solid oxide electrolysis cells (SOEC). While the solid oxide electrolyzers are more efficient in terms of electricity, this technology has problems with corrosion, thermal circulation and chromium migration. PEM electrolyzers are more efficient than alkaline ones, are corrosion resistant unlike SOEC, but their price is higher than that of a classic alkaline system. Alkaline systems are hence the most advanced and their market price is the lowest. The efficiency is low 50e60%, depending on the calorific value of hydrogen, and the price of the electricity supplied is the highest of all the systems. Alkaline electrolyzers are composed of electrodes, a microporous separator and an alkaline solution of ca 30 wt.% of potassium hydroxide or sodium hydroxide [51]. The most commonly used cathode material is nickel with a catalytic layer of platinum. The anode is formed of nickel or copper covered by oxides of mangan, wolfram and ruthenium. The electrolyte is not expended in the reaction but must be continuously complemented because of the loss of other systems like the regeneration of hydrogen. In the alkaline electrolyzer, water is fed into the cathode, where it is disassociated into hydrogen and hydroxyl ions (OH), which pass through electrolytic material to the anode, where oxygen is, formed (Eq. (34)e(36)). Cathode :

The cracking of water can be divided into three categories: electrolysis, thermolysis and photoelectrolysis. A promising method for the acquisition of hydrogen in the future could be water electrolysis. Currently, approximately 4% of hydrogen worldwide is produced in this way [1,2]. The electrolysis of water, or its cracking into hydrogen and oxygen, is a wellknown method which began to be used commercially already in 1890. Electrolysis is a process in which a direct current passing through two electrodes in a water solution results in the cracking of the chemical bond of water into hydrogen and oxygen (Eq. (33)). 2H2 O/2H2 þ O2

Anode : Total :

2H2 O þ 2e /H2 þ 2OH 4OH /O2 þ 2H2 O

H2 O/H2 þ 1=2O2

(34) (35) (36)

Hydrogen leaves an alkaline solution and is separated from water in a gaseliquid separation unit outside of the electrolyzer. The typical current density is 100e300 mA/cm2. The efficiency of alkaline electrolyzers reaches ca 50e60% according to the calorific value of hydrogen [7,65]. This process is good developed and used in commercial scale. Contrary, the methods mentioned below are at an early stage of development.

(33)

The electrolysis processes take place at room temperatures. A commonly used electrolyte in water electrolysis is sulfuric acid and the electrodes are of platinum, which does not react with sulfuric acid. The process is ecologically clear, because no greenhouse gases are created and the oxygen produced has

6.2. Proton exchange in a polymer electrolyte membrane electrolyzer A proton-exchange membrane electrolyzer works with the fuel cell with a polymer electrolyte membrane (PEM). The base

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of the typical PEM electrolyzers is the use of black platinum, iridium, ruthenium and rhodium for electrode catalysts and the thin Nafion polymer membranes, which serve as a gas separator [51]. Water in the PEM electrolyzer is fed to the anode, where it is split into a hydrogen cation and oxygen. The hydrogen cations pass through the polymer membrane to the cathode. At the cathode, the hydrogen cations are compatible with the electrons flowing from the outer circuit, which results in the creation of gas hydrogen (Eq. (37)e(39)). The efficiency of a PEM electrolyzer is around 55e70% [51]. 2H2 O/O2 þ 4Hþ þ 4e

Anode : Cathode :

4Hþ þ 4e /2H2

The total is the same as for the alkaline electrolyzer : H2 O/H2 þ 1=2O2

(37) (38)

ð39Þ

The overall efficiency of the electrolytic production of hydrogen can be attributed mainly to the efficiency of the electrical energy production (30e40% for conventional resources). The overall efficiency of electrolysis is thus roughly in the range of 25e35%. An advantage is the simultaneous production of oxygen for its wide application.

6.3.

Solid oxide electrolysis cells

Generally, solid oxide electrolysis cells are essentially the solid oxides of fuel cells which use a solid material as an electrolyte. The electrolyte selectively transfers oxygen anions at increased temperature. SOEC work like the alkaline system, because the oxygen ion passes through the electrolyte while leaving hydrogen in a non-reactive flow of steam. The reactions have been listed in Chapter 6.1. The SOEC work at high temperatures of 500e800  C in comparison with PEM electrolyzers, which work at temperatures of 80e100  C, and alkaline electrolyzers, which work in the temperature range of 100e150  C. The solid oxide electrolyzers can effectively utilize the heat available at these higher temperatures (from various sources, including nuclear energy) to reduce the amount of electrical energy necessary for the production of hydrogen from water. Higher temperatures increase the efficiency of the electrolyzer at decreasing anode and cathode overvoltage, which causes a drop in performance during electrolysis. For instance, an increase of temperature from 100  C to 770  C lowers the combining of thermal and electrical energy of the apparatuses nearly to 35%. The materials used for SOEC are similar to the materials developed for solid oxide fuel cells such as the Y2O3 in ZrO2. The material for the anode is CoeZrO2 or NieZrO2, whereas LaMnO3 doped with strontium is used for the cathode [89]. The efficiency of hightemperature electrolysis depends on the temperature and the thermal source and can reach values of 85e90%.

6.4.

Thermochemical cracking of water

Thermochemical cycles have been developed already since the 1970s and 1980s, when they were to contribute to the search for new sources of the production of alternative fuels

during a petroleum crisis. The thermochemical cracking of water, also labeled as thermolysis, is the decomposition of water into hydrogen and oxygen by means of a series of subsequent chemical reactions, which are initiated by heat or electrical energy if they are hybrid cycles. The total attainable efficiency of the closed cycles of these processes is around 50% [90]. Water breaks down at extremely high temperatures over 2500  C, for which we do not yet have suitable construction materials and thermal resources. Chemical reagents should therefore be designed for lower temperatures. So far, more than 300 various cracking cycles of water have been described. All of the processes markedly lowered the working temperature from 2500  C, at the price of higher pressure. The cycles mentioned below are closed, which means that the chemical substances used are recycled during the process and re-enter the process [90e92]. Only the initial raw material, i.e. water, is complemented, with the resultant products being hydrogen and oxygen. An example of a cycle is presented below (Eq. (40)e(44)). Kapra Mark-10 [6]: 

2H2 O þ SO2 þ I2 þ 4NH3 /2NH4 I þ ðNH4 Þ2 SO4

T ¼ 50 C



2NH4 I/2NH3 þ H2 þ I2

T ¼ 630 C

(40) (41)

ðNH4 Þ2 SO4 þ Na2 SO4 /Na2 S2 O7 þ H2 O þ 2NH3



T ¼ 400 C (42)

Na2 S2 O7 /SO3 þ Na2 SO4 SO3 /SO2 þ 1=2O2



T ¼ 550 C

(43)



T ¼ 870 C

(44)

The UT-3 thermochemical cycle is developed to be connected with nuclear energetic reactors [91,93]. The efficiency of the UT-3 cycle is in the range of 40e50% and includes these reactions (Eq. (45)e(48)). 

CaBr2 þ H2 O/CaO þ 2HBr T ¼ 730 C CaO þ Br2 /CaBr2 þ 1=2O2

(45)



T ¼ 430 C

(46) 

Fe3 O4 þ 8HBr/3FeBr2 þ 4H2 O þ Br2

T ¼ 300 C

3FeBr2 þ 4H2 O/Fe3 O4 þ 6HBr þ H2

T ¼ 540 C



(47) (48)

In terms of kinetics, the first reaction is the slowest of the above-mentioned reactions and limits the speed of the whole cycle. Therefore it is indispensable that all of the reactions take place at a certain rate for the continuity of the cycle, the efficiency of the process is adversely affected by the lower rate of the hydrolysis of calcium bromide. Another thermochemical cycle lies in the passing of zinc oxide through the reactor heated to 1800  C. At this temperature, zinc oxide breaks down and the cooled zinc reacts with water to form hydrogen gas and solid zinc oxide, which is recycled back into the process (Eqs. (49) and (50)) [93]. ZnO/Zn þ 1=2O2



T ¼ 1800 C

Zn þ H2 O/ZnO þ H2



T ¼ 475 C

(49) (50)

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In the middle of the 1970s, sulfureiodine thermochemical cycle was developed in General Atomics (San Diego, USA), which allowed inexpensive and at the same time effective hydrogen production by means of atomic energy [94,95]. The initial raw material is water in a liquid state and high-potential heat. Iodine and sulfur dioxide are in a liquid state as well and are recycled during the process and reused. The resulting raw materials are hydrogen, oxygen and low-potential heat. The efficiency of the whole hydrogen production cycle is 40e52% (ca 50% at 950  C). With rising temperature, the efficiency of the cycle will increase. In comparison with electrolysis, the efficiency is higher, because losses do not occur in the production of electrical energy. The disadvantage of this cycle lies in the requirement of high temperatures and in the high aggressiveness of both of the produced acids. This places high demands on the chemical resistance of the materials used. A variation of the sulfureiodine cycle is the BowmaneWestinghouse cycle, which includes reactions with bromine instead of iodine and the electrolysis of hydrobromic acid rather than the thermal decomposition of hydroiodic acid [94]. The main disadvantage of this cycle lies in the maintenance of a low concentration of sulfuric acid, leading to a higher energetic consumption of concentrated acid and to a separation process. A new thermochemical sulfureammonia cycle has therefore been designed for the decomposition of water into hydrogen and oxygen [93]. The cycle includes three steps (Eq. (51)e(55)). The cycle has the potential of attaining high total efficiency by the use of commonly available and inexpensive chemicals. The source of heat is solar energy and a part of the UV radiation is used for the photocatalytic redox reaction. ðNH4 Þ2 SO3 þ H2 O/ðNH4 Þ2 SO4 þ H2 

T ¼ 80 C photocatalytic step

ð51Þ 

ðNH4 Þ2 SO4 /2NH3 þ H2 SO4

T ¼ 350 C thermochemical step (52) 

H2 SO4 /SO3 þ H2 O T ¼ 400 C thermochemical step SO3 /SO2 þ 1=2O2



T ¼ 850 C thermochemical step

(53) (54)

SO2 þ 2NH3 þ H2 O/ðNH4 Þ2 SO3 

T ¼ 25 C chemical adsorption

ð55Þ

In terms of environmental protection and the economic issues, the FeeCl cycle [90,91] seems to be ideal, because it is a reaction of ideal elements (Fe, Cl, H and O). Ten different cycles have been identified in the wide temperature range of 100e900  C with the efficiency to 49%. At present time, these processes are however not able to compete with the other technologies of hydrogen production in terms of their costs and efficiency [90,94]. The combination of high temperatures, high pressures and corrosion result in a need for new, more resistant materials.

6.5.

Photoelectrolysis

Photoelectrolysis is one of the renewable ways of hydrogen production, exhibiting promising efficiency and costs,

although it is still in the phase of experimental development [53,99,100]. Currently, it is the least expensive and most effective method of hydrogen production from renewable resources. The photoelectrode is a semiconducting device absorbing solar energy and simultaneously creating the necessary voltage for the direct decomposition of the water molecule into oxygen and hydrogen. Photoelectrolysis utilizes a photoelectrochemical (PEC) light collection system for the driving of the electrolysis of water. If the semiconductor photoelectrode is submerged in an aqueous electrolyte exposed to solar radiation, it will generate enough electrical energy to support the generated reactions of hydrogen and oxygen. When generating hydrogen, electrons are released into the electrolyte, whereas the generation of oxygen requires free electrons. The reaction depends on the type of semiconductor material and on the solar intensity, which produces a current density of 10e30 mA/cm2. At these current densities, the voltage necessary for electrolysis is approximately 1.35 V. The photoelectrode is comprised of photovoltaic (semiconductor), catalytic and protective layers, which can be modeled as independent components [96]. Each layer influences the overall efficiency of the photoelectrochemical system. The photovoltaic layer is produced from light absorbing semiconductor materials. Testing has been conducted on various materials, e.g. TiO2, Fe2O3, WO3, n-GaAs, nGaN for a photoanode, and e.g. CIGS/Pt (Cu-In-Ga-diselenid), p-InP/Pt an p-SiC/Pt for a photocathode [51,96]. The light absorption of the semiconductor material is directly proportional to the performance of the photoelectrode. Semiconductors with wide bands provide the necessary potential for cracking water. An increase in light absorption can be supported by modifying the semiconductor material or adding a photosensibilizator like e.g. dyes absorbing the greater part of the solar spectrum. The most promising dyes are the color N3 and the color black [7]. The catalytic layers of the photoelectrochemical cell also influence the performance of the electrolysis and require suitable catalysts for water cracking. The encased layer is another important component of the photoelectrode, which prevents the semiconductor from corroding inside the aqueous electrolyte. This layer must be highly transparent to be able to provide the maximum solar energy so that it could reach the photovoltaic semiconducting layer.

7.

Some economic aspects

At the present time, the most widely used and also the cheapest method of hydrogen production is the steam reforming of methane (natural gas) (SMR). This method includes about half of the world hydrogen production, and hydrogen price is around U.S.$ 7/GJ [97]. A comparable price of hydrogen is provided by partial oxidation of hydrocarbons. However, greenhouse gases generated by thermochemical processes must be captured and stored, thus, an increase the hydrogen price by 25e30% must be considered [28]. The further used thermochemical processes include gasification and pyrolysis of biomass. Price hydrogen thus obtained is about three times greater than the price of

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hydrogen obtained by SMR process [98], therefore, these processes are generally not considered as competitive steam reforming. The price hydrogen from gasification of biomass ranges from U.S.$ 10 to 14/GJ, and pyrolysis roughly between U.S.$ 8.9 to 15.5/GJ. It depends on the equipment, availability, and cost of feedstock [2,99,100]. Electrolysis of water is one of the simplest technologies for producing hydrogen in large quantities without by-products. Electrolytic processes can be classified as highly effective; on the other hand, the input electricity costs are relatively high and play a key role in the price of hydrogen obtained. Biological processes have different costs compared to chemical and electrochemical processes because they are catalyzed by microorganisms in an aqueous environment at room temperature and pressure. They are substantially less energy-consuming than chemical and electrochemical processes, and are suitable for decentralized energy production on a small scale in areas of easy availability of biomass or waste. In this way you can lower the cost of transport and

Table 4 e A synopsis of the technologies of hydrogen production and their effectiveness [7]. Technology Steam reforming Partial oxidation Autothermal reforming Plasma reforming Pyrolysis Co-pyrolysis Photolysis Dark fermentation Photofermentation Microbial electrolysis cells Alkaline electrolyzer PEM electrolyzer Electrolyzing cells of solid oxides Photoelectrochemical water splitting

Raw material

Effectiveness (%)

Hydrocarbons Hydrocarbons Hydrocarbons

70e85a 60e75a 60e75a

Hydrocarbons Coal Coal þ waste material Solar energy þ water Biomass Biomass þ solar energy Biomass þ electric energy Water þ electric energy Water þ electric energy Water þ electric energy þ heat Water þ solar energy

9e85b 50a 80a 0.5c 60e80d 0.1e 78f 50e70g 55e70g 40e60h 12.4i

For instance, SOEC operating from advanced high-temperature nuclear reactors may be able to achieve up to 60% efficiency. If the initial thermal energy is ignored, the efficiency is ca 90%. a Thermal efficiency based on the higher heating values. b It does not include hydrogen purification. c The conversion of solar energy to hydrogen by water splitting; it does not include hydrogen purification. d Theoretical maximum of 4 mol H2 for 1 mol of glucose. e The conversion of solar energy to hydrogen by organic materials; it does not include hydrogen purification. f Total energy efficiency including the applied voltage and energy in the substrate. It does not include hydrogen purification. g The low heating value of the hydrogen produced, which the participation of the electric energy of the electrolyzing cells. h The efficiency of the high-temperature electrolysis is dependent on the working temperature of the electrolysis and the efficiency of the thermal energy source. i The conversion of solar energy to hydrogen by water splitting; it does not include hydrogen purification.

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avoid energy expenditure [21]. In the case of indirect and direct biophotolysis, Hallenbeck and Benemann [16] estimated the price of hydrogen at U.S.$ 10/GJ and 20/GJ, respectively. Economically preferable method is connection of dark fermentation with photofermentation in two-stage process. By 2030 the dominant methods of hydrogen production will be steam reforming of natural gas and catalyzed biomass gasification. In a relatively small extent both coal gasification and electrolysis will be used. The use of solar energy in a given context is questionable but also possible. Probably, role of solar energy will increase by 2050 [2]. From the biological processes seems to be very promising dark fermentation with photofermentation for high efficiency and technological feasibility. It may therefore be a part of the production mix by 2030. Other presented biological methods should be intensively developed.

8.

Conclusion

In the near future, hydrogen will become an important fuel which may be able to resolve local problems connected with air quality. Hydrogen-propelled transport means are being developed and are already used in the automobile industry. Since the combustion of hydrogen does not produce any emissions of carbon oxides but only water, hydrogen is considered as a key fuel of the future. Hydrogen is abundantly present all over space and can be obtained from a number of resources, be they renewable or non-renewable. Numerous processes have a minimal impact on the environment. The development of these technologies should lower the dependence primarily on fossil fuels. Global production has so far been dominated by hydrogen production from fossil fuels, with the most significant contemporary technologies being the reforming of hydrocarbons, pyrolysis and co-pyrolysis. Plasma cracking is still in the developmental stage. The preferred method of hydrogen production on an industrial scale is steam reforming of natural gas for its low operational and production costs. Pyrolysis processes have acceptable investment costs and besides the production of hydrogen also satisfactory yields of oils. Two-stage co-pyrolysis is suitable considering its acquisition of a high amount of hydrogen from mixed charges. It is apparent that the co-pyrolysis of organic materials with coals is a process for hydrogen production capable of competing. With its conditions, electrolysis connected with renewable energy approaches low-emission technologies. However, for its energetic consumption, it is among the expensive technologies. In recent years, significant progress has been made in the development of systems of alternative hydrogen production including the thermochemical cracking of water and photoelectrolysis. While photoelectrolysis is currently the least expensive and most effective method of hydrogen production, thermal processes from non-renewable resources remain a less expensive method of hydrogen production. The methods of the biological treatment of water and biomass into hydrogen have diverse efficiencies. In Table 4, both the technologies of hydrogen production and for comparison the efficiencies of the individual processes are shown synoptically. It arises from the table that the biological

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processes for hydrogen production are acceptable in terms of ecology and also consume less energy as compared to thermochemical and electrochemical processes. The process of direct biophotolysis has very low efficiency and requires large areas and enough light for the production of hydrogen. It is therefore not promising for hydrogen production. Contrary, dark fermentation is a pronouncedly promising process for hydrogen production. It shows an efficiency of 60e80%, which is comparable with the most-commonly used conventional methods. Its advantage is that it does not require large areas or solar energy, which increases its market value. A still more promising process of the biological production of hydrogen seems to be the BEAMR method, which can achieve efficiency of as much as 92% with the initial substrate being acetate.

Acknowledgments This work was supported by a grant from the Czech Science Foundation reg. No. 105/07/1407 and by the Institutional Research Plan No. AVOZ 30460519.

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