TEXT BOOK - CHAPTER-11 - Non-Conventional Energy Sources - by G D RAI.pdf

Non-Conventional Energy Sources - chapter -11.

1. STE

= (11)

Hydrogen Energy

11.1. Introduction

Hydrogen as an energy carrier can play an important role as an alternative
to conventional fuels, provided its technical problems of production, storage and
transportation can be resolved satisfactorily and the cost could be brought down to
acceptable limits. One of the most attractive features of hydrogen as an energy
carrier is that it can be produced from water which is abundantly available in
nature. Hydrogen has the highest energy content per unit of mass of any chemical
fuel and can be substituted for hydrocarbons in a broad range of applications,
often with the inereased combustion efficiency. Its burning process is non-polluting
and it can be used in the fuel cells to produce both electricity and useful heat. Use
ofhydrogen as an energy source involves five basic issues: (1) Production, (2) Storage
and transportation, (8) Utilization, (4) Safety and management, (5) Economy. These
are discussed in the following articles.

‘The hydrogen can be used as a fuel directly, or it might be used as a raw
material to produce methanol, ammonia, or hydrocarbons by using either carbon
dioxide or nitrogen from the atmosphere.

‘The combination of hydrogen with oxygen (e.g, from air) results in the
liberation of energy, with water as the sole material product; thus,

H+ 40,110 + energy LD

‘The reaction can be carried out and the energy made available in several
different ways, so that hydrogen is a versatile fuel material. Hydrogen is chemically
very reactive and hence it is not found in its free state on the earth. However,
combined chemically with other elements, it is present in water, fossil hydrocarbons,
biological materials such as cellulose, starch etc., minerals such as biocarbonate
rocks. Energy must be supplied to those compounds to break the chemical bonds to
release hydrogen. Unlike fossil (coal, natural gas and petroleum) and nuclear fuels
and solar radiation, which are primary energy sources, hydrogen is a secondary
fuel that is produced by utilizing energy from a primary source. Much of this energy
can be recovered by recombination of the hydrogen with oxygen in the ways
indicated above.

‘The current and projected needs of hydrogen are growing rapidly,
applications include nitrogeneous fertilizers, coal liquefaction basic chemicals etc.

2. Hydrogen Energy EC
almost three times that of hydrocarbon fuels. So when used in air erafts, it reduces
the take-off load considerably.
‘The possible areas of use for hydrogen in the near future are as follows:
(@ production of electrolytic hydrogen, for full load exploitation of nuclear
power stations;
(i direct addition of hydrogen to the existing natural gas distribution net-
work;
(iii) use of hydrogen in the processing of heavy oil;
(iv) use of hydrogen for the manufacture of synthetic liquid or gaseous fuels;
(v) direct use of hydrogen as a motor vehicle fuel in urban transport,
particularly where air pollution problems are already critical;
(vi) reduction of iron oxides by means of hydrogen in the steel industry;
(vii) direct use of hydrogen as an aircraft fuel in air transport.
While hydrogen can be produced from fossil hydrocarbons it is desirable to
use processes which recover hydrogen by a closed cycle water splitting without the
use of much hydrocarbons, Such an approach has many advantages because
hydrocarbons are not a renewable source and their supply is becoming increasingly
unreliable. The cost of crude oil is escalating day by day and in the near future
alternative technologies will be competitive.
Several cycles have been suggested for closed cycle water splitting, The basic
Westing houso Electro-chemical thermal sulfur cycle is one such promising cycle
which has received some attention in production of hydrogen.
‘The simplest practical way to obtain hydrogen from water is means of
electrical energy. Hence electrical energy can be conveniently stored and
transmitted by way of hydrogen in some situations. Electricity is generated from
distributed primary sources, such as geothermal energy, wind energy and some
other forms of solar energy, often at a distance from a load centre or an electric
utility grid. Conversion into hydrogen fuel would then be a practical means of
energy transmission.
One of the potential advantages of hydrogen as a secondary fuel is that it
can be transmitted and distributed by pipe line in much the same way as natural
gas. Alternatively, the hydrogen gas could be converted into liquid form by cooling
to a very low temperature and transported in insulated tanks by high way rail-
road. On a smaller scale, hydrogen gas could be compressed into transportable
cylinders. Another possibility is to form a solid compound of a metal with hydrogen
from which the gas can be recovered, when required, by heating, Thus, hydrogen
can serve as a means of carrying energy from the place where a primary source is
available to a distant load centre where the energy is used.
‘An important aspect of hydrogen utilization is that it is accompanied by
little or no atmospheric pollution. In many cases, such when hydrogen is burne
pure oxygen, in a flameless (catalytic) burner in air, or in a fuel cell with air or
oxygen, water is the sole product. However, if hydrogen is burned normally in air
or in a spark-ignition engine or a gas turbine, nitrogen oxides may be produced by
a o nd ci leo AES Nel

3. | AA

Properties of Hydrogen

Hydrogen at ordinary temperature and pressure isa light gas with a density
only 1/14th that of air and 1/9th that of natural gas under the same conditions. By
cooling to the extremely low temperature of - 253°C at atmospheric pressure, the
gas is condensed to a liquid with a specific gravity of 0.07, roughly 1/10th that of
gasoline.

‘The standard heating value of hydrogen gas is 12.1 MJ/eu m compared with
an average of 38.3 MJ/eu m for natural gas. The heating value of liquid hydrogen
is 120 MJ/kg or 8400 MJ/cum; the corresponding value for gasoline (or
approximately for jet fuel) is 44 MJ/kg or 32,000 M/cum. Hence, for producing a
specific amount of energy, liquid hydrogen is superior to gasoline (or jet fuel) on a
weight basis but inferior on a volume basis.

‘The flame speed of hydrogen burning in air is much greater than for natural
gas, and the energy required to initiate combustion (i.., the ignition energy) is
less. One consequence of the low ignition energy is that flameless combustion on a
catalytic (finely divided metal) surface is possible with hydrogen at much lower
temperatures than flame burning.

Mixture of hydrogen and air are combustible over an exceptionally wide
range of compositions; thus, the flammability limits at ordinary temperatures
extend from 4 to 74% by volume of hydrogen in air. (Detonation can occur between
18 and 59 per cent). This wide range has an important bearing on the use of
hydrogen fuel in internal combustion engines. The engine will operate, although
not necessarily with the same efficiency, from very rich (excess fuel) to very lean
(excess air) mixtures. The adjustment of air to fuel ratio is thus much less critical
than in a gasoline engine.

11.2. Hydrogen Production

11.2.1. Introduction

Commercial production of hydrogen today is carried out by the steam
reformation or partial oxidation of hydrocarbons (natural gas, naphthas, or crude
oil) depending on supplies and economics. Clearly, with gas and oil in diminishing
supply, these sources for hydrogen will become less attractive as a basis for an
energy delivery system.

Hydrogen made from coal is of questionable importance as a fuel gas. On
one hand, it is relatively inexpensive (compared with the sources we discuss next),
but on the other hand, it would not be as conveniently transported or utilizec
transmission and combustion systems as would be a synthetic conventional fuel
such as methane or liquid hydrocarbons (which are probably as easy to make from
coal as is pure hydrogen). Hydrogen made from coal can probably only be justified
as a fuel for special applications where the unique characteristics of hydrogen can
be put to advantage, such as its weight or its non-polluting characteristics.

Small quantities of hydrogen are commercially produced today by the
electrolysis of water usually in situations where electric power is cheap, or where
reliable unattended operations is required. Electrolysis is the only presently
available technology by which hydrogen can be made from non-fossil energy

4. Hydrogen Energy 467

cycle that would utilise heat to achieve the chemical splitting of water to its elements
without the need for intermediate electricity generation, and without the need to
use the extremity high temperature of 2500°C or more, (which would be required
to dissociate water directly). This so called thermochemical hydrogen production.
Concept is an important building block of a hydrogen energy system because it
offers the potential for non-fossil hydrogen production at a higher efficiency and
lower cost than electricity generation by electrolysis. At present, research is at the
stage where these potentials have not been realized in practico, but progress is
sufficiently encouraging to continue.

‘The methods of producing hydrogen may be classified according to the
immediate source of addition of energy to decompose, thus electrical energy (in
electrolysis), heat energy (in thermo-chemical methods), fossil fuels, and solar energy.

11.2.2. Electrolysis or the Electrolytic Production of Hydrogen

‘The process of splitting mes
water into hydrogen and =3

oxygen by means of a direct

electric current is known as

electrolysis; this is the simplest Anode Cathode
method of hydrogen pro- (9 o
duction. In principle, an elec- Electrolyte
trolysis cell consists of two solution.

electrodes, commonly flat
metal or carbon plates,
immersed in an aqueous
conducting solution called the electrolyte. A source of direct current voltage is
connected to the electrodes so that an electric current flows through the electrolyte
from the positive electrode (or anode) to the negative electrode (or cathode). As a
result, the water in the electrolyte solution is decomposed into hydrogen gas (H,)
which is released at the cathode, and oxygen gas (0); released at the anode.
Although only the water is split, an electrolyte (e.g., KOH solution) is required
because water itself is a very poor conductor of electricity.

Ideally, a voltage of 1.23 volts should be sufficient for the electrolysis of
water at normal temperature and pressure. For various reasons, especially the
slowness of the electrode processes that lead to the liberation of hydrogen and
oxygen gases, higher voltages are required to decompose water. The decomposition
voltage increases with the current density (Le, the current per unit (area of
electrode). Since the rate of hydrogen production is proportional to the current
strength, a high operating current density is necessary for economic reasons. Hence,
in practices the decomposition voltage (per cell) is usually around 2 volts.

‘Theoretically, 2.8 kW-hr of electrical energy should produce one cum. of
hydrogen gas. Because of the higher than ideal decomposition voltage, however
the actual electrical energy requirement is generally from 3.9 to 4.6 KW-hr per
eu.m.). This means that the efficiency of electrolysis (Le, the proportion of the
energy supplied that is used in electrolysis) is roughly 60 to 70%.

D ui

Fig. 11.1. Simple electrolytic cell.

EAS: SAE AN 2 DOIN nn nn

5. 468 Non-Conventional Sources of Energy

commercially, For practical water electrolysis, the electrodes are generally ofnickel-
plated steel, The effective electrode surface area (and hence the rate of the electrode
process) is increased by depositing porous nickel on a wire gauge, or a highly
corrugated steel base. Research is being directed at the development of improved
electrodes that will give better electrolysis efficiency at a reasonable cost.

Diaphragms prevent electronic contact between adjacent electrodes and
passage of dissolved gas or gas bubble; from one electrode compartment to another
(leading to a decrease in current efficiency and possible to explosions), without
themselves offering an appreciable resistance to the passage of current within the
electrolyte. Dissolved gas crossover is serious only in pressure operations; to prevent
the passage of gas bubbles, the diaphragm must consist of small pores whose
capillary pressure is greater than the maximum differential pressure applied across
the cell.

Asbestos is the most common material for cell diaphragms. At atmospheric
pressure, woven asbestos cloth is used, sometimes with fine nickel wire to support
the structure, Pressure electrolyzers usually have a mat made of woven or felted
asbestos fibres that produces a fine pore structure, giving a higher resistance to
the generation of gases. This mat is sometimes supported by the electrodes.

Three major factors determine the usefulness of an electrochemi cal ell for
hydrogen production. One is the energy efficiency, related to the cells operating
voltage: another is the capital cost of the plant, related to the rate of hydrogen
production from a cell of a given size. These two factors are closely inter related.
‘The third factor is the life time ofthe cell and its maintenance requirements, which
involve the materials used in its construction and the operating conditions selected,

A number of advantages can be gained from operating an electrolyzer at
higher pressures, including (a) reduction in specific power consumption, (b) delivery
of gas at pressure, thus reducing or eliminating the cost of gas compressors; and
(0 reduction in the size of electrolysis cells. It can be shown theoretically that the
reversible cell voltage increases with pressure. However, as decreased gas
volume and higher operating pressures result in a reduced over potentially there
usually a small overall reduction in the cell voltage. This real gain in efficiency
offset by increase in the cost of pressure vessels or stronger components. Operating
voltages as stated above can be lowered for a given current by using electrodes
that carry precious metal catalysts or incorporate sophisticated metallurgical
structures, both of which are expensive but increase efficiency.

Several large electrolytic hydrogen plants, consuming over 100 MW, have
been operated successfully, while thousands of smaller units are in use for special
applications.

‘Two types of electrode arrangements are used by industry for the electrolysis,
of water. On this basis they are classified as:

@ Tank type electrolyzer, and
Gi) Filter press or bipolar electrolyzer.
A A A i

6. Hydrogen Energy 469

Hydrogen

Porous
diaphragm 0
SASHA HS HSH
Electrode. ININ!N! ¡Ni

MN SiS: iSiS

t
+ [ [bv

Call battery voltage = No. of ells x 2 V
Fig. 11.2. Schematic diagram of tank type (unipolar) electrolyzer.

by porous diaphragms (e.g., asbestos) impermeable to gas but permeable to the
cell’s electrolyte, that prevent the passage of gas free from one electrode
compartment to another. The whole assembly is hung from a series of gas collectors.
All the anodes in the tank are connected to the same positive terminal ofthe direct
current voltage source, and all the cathodes are connected to the same negative
terminal (See Fig. 11.2).

By connecting the electrodes in parallel in this manner the voltage required
for a tank of several electrode pairs, regardless of the number is little more than
for a single pair (or cell), that is, about 2 volts. As a rule, a number of tanks are
connected in series: the operating voltage is then roughly 2n volts, where n is the
number of tanks so connected.

‘The major advantages of tank type electrolyzers are two-fold relatively few
parts are required and those needed are relatively inexpensive; and individual
cells may be isolated for repair or replacement simply by short circuiting the two
adjacent cells with a temporary busbar connection. Some disadvantages of the
tank electrolyzers are:

(a) Inability to handle high current densities because of cheaper component

parts and

(6) Inability to operate at high temperatures because of heat losses from

the large surface areas of connected cells.
(i) Filter-Press Electrolyzer (Bipolar Electrolyzer).

‘The alternative and more widely used water electrolysis system is called the
filter-press electrolyzer because of its superficial resemblance to a filter press.
Except at the ends of the cell, the electrodes are bipolar that is, one face of each
plate electrode is an anode and the other face is the cathode. (Refer Fig. 11.3). As
in the tank system, porous diaphragms between adjacent electrodes prevent mixing
of the hydrogen and oxygen gases.

7. 470 Non-Conventional Sources of Energy

a battery. Because the cells can be made relatively thin, a large gas output is
achieved from a relatively small volumes.

Cell battery voltage =
Le No. of pairs of electrodes x 2 V ——e|

yt ay!

Fig. 11.3. Filler press (Bipolar) cell construction.

It is usually desirable to circulate electrolyte through the cell, thereby
separating the gas and the electrolyte, and in many designs, this is accomplished
in a separating drum mounted on top of the electrolyzer. The electrolyte, free of
gas, is recirculated through the cells, and the circulation is maintained by gas lift
of the generated oxygen and hydrogen.

‘The filter press electrolyzer is generally preferred, because it occupies less
space and can be operated at a higher current density than the tank type. The
economies are thus more favourable, since a larger hydrogen production is possible
ina plant size.

Although filter press electrolyzers can operate at higher current densities
and appear to occupy relatively less space than tank types, they require a much
closer tolerance in construetion and are more difficult to maintain. Breakdowns in
filter press electrolyzers are rare, but when they occur, rejuvenation is difficult
and repair may take considerable time. If an individual asbestos diaphragm is
damaged, the entire battery must be dismantled. The greater capital costs of bipolar
electrolyzers are offset because these electrolyzers can operate at higher current
densities (producing more hydrogen per unit area of electrode) with virtually the
same operating voltages as the tank type unit.

Electrolyzer Modifications. In addition to the search for improved electrodes,
efforts are being made to inercase electrolyzer efficiencies. For example, operation
at temperatures and pressures above normal is reported to decrease the
decomposition voltage. In a commercial German design, for which a high efficiency
is claimed the pressure in the cell is maintained at some 50 atm. (5 MPa) and the
electrolyte temperature is 90°C.

General Electric Company of Lynn. Massachusetts (U.S.A.), has been
developing a water electrolysis system based on solid polymer electrolyte (SPE)
A o oo

8. Hydrogen Energy 471

when a thin sheet of this material is saturated with water, it is an excellent ionie
conductor, providing low electrical resistance. Used in an electrolysis cel, it is the
only electrolyte required; there are no liquid acids or alkaline in the system.
Hydrated hydrogen ions (H* . xH,0) move through the sheet of the electrolyte.

A cell consists of a thin sheet of SPE with a finely divided precious metal
(eg, platinum) coated on each side to catalyze the electrode processes. Metal plates
pressed against the surfaces of the SPE provide electrical contact with the coatings
and serves as electrodes for connection with voltage source.

‘The electrolysis of water is interest as an economic industrial scale method
of producing hydrogen in cases where (i) fossil fuels such as petroleum, natural
gas and coal (from which hydrogen can also be obtained) are scarce or expensive,
or both, and (i) electrical energy from sources other than fossil fuels is available in
sufficient quantity and relatively cheaply.

Nuclear power stations have to be operated with a load which is constant in
time; hence, during the seasons or times of day when there is a decreased demand
for electricity the energy produced is particularly cheap and could be used to produce
hydrogen.

So far, water electrolysis technology has been developed to the point where
extremely robust and reliable electrolysers are available which operate without
excess pressure or under a pressure of 30 bar. However, most electrolysers operate
with relatively high cell voltages (between 1.8 and 2.2 V). Furthermore these
electrolysers are operated with relatively modest current densities of 0.15 and
0.30 Alem?

High coll voltages imply high energy costs, and low current densities imply
that the electrolysers give small yields per unit time, leading to relatively high
capital costs. For these reasons hydrogen produced electrolytically is at present
twice as expensive as hydrogen produced from fossil fuels.

11.2.8. Thermo-chemical Methods

‘The overall efficieney for the conversion of primary energy from fossil and
nuclear fuels into hydrogen by electrolysis is dependent, in the first place on the
net efficiency of generating electricity. This efficiency may be upto about 38% for
modern fossil fuel plants and 32% for nuclear installations. Assuming that
electrolyzer efficiency can be increased to 80%, the overall efficiency for hydrogen
production would be only 25 to 30%.

A higher conversion efficiency might be possible if the heat produced by the
primary fuel could be used directly to decompose water, without the intermediary
ofelectrical energy. Such direct decomposition into hydrogen and oxygen is possible,
but it requires a temperature of atleast 2500°C. Because of the temperature
limitations and conversion process equipment, direct single step water
decomposition can not be achieved. However a sequential chemical reaction series
can be devised in which hydrogen and oxygen are produced, water is eonsumed
and all other chemical intermediates are recycled. The operation is called a
thermochemical eycle, itis so called because energy is supplied as heat at one or
more of the chemical stages. In the reaction series, water is taken up at one stage,
and hydrogen and oxygen are produced separately in different stages. The net

9. 472 Non-Conventional Sources of Energy

For practical reasons, primarily the availability structural and containment
materials, the maximum temperature in a thermochemical cycle is considered to
be about 950°C. Heat energy should then be convertible into hydrogen energy with
an efficiency around 50%; this is a marked improvement over what is possible by
electrolysis. Unless the upper temperature of the thermochemical cycle is above
700°C, the officieney is little better than for electrolysis.

At present, no commercial process for the thermal splitting of water to
hydrogen and oxygen is in operation. Several workers have proposed many
multistep reaction sequence that thermally decompose water at lower overall
temperatures. An example of such a reaction sequence is as follows:

2CrCl, + 2HC1—> 20rCl, +H, (825°C) 112)
2C1Cl, — 20rCl, + Cl, (875°0) 113)
10 +01 mars Lo, 6500) „ua

As can be seen, in this reaction sequences, only water is split, all other
materials are completely recycled.

‘The maximum temperatures available from newly designed high
temperature gas cooled reactors (HTGRs) are about 800- 900°C. Some what higher
temperatures may be available in the future from the conceptual, ultra-high
temperature gas cooled reactors.

A very large number of thermochemical cycles have been proposed, based
on chemical and thermodynamic considerations. Several of the more promising
cycles are being investigated in many countries. Although the various stages in a
cycle may be thermodynamically possible, there is no assurance, without
experimental studies, that they will take place at a useful rate. Also several cycles
involve highly corrosive substances, such as sulphur dioxide and trioxide, chlorine,
and hydrogen bromide, containment problems may then be dominant on the whole,
the thermochemical production of hydrogen is regarded as a long term project.

‘Some of the possible thermochemical cycles are given in Table 11.1.

‘Table 11.1. Possible Cycle for Hydrogen Production

5 Chemical Reaction Temperature
No. di
1 cao cor, msc
00+ ee — 30 +0 aso
1100, — 200,10, seso
à ano > con, so
cio — cas Lo, 1290
a FCI + AMO FeO, OHG, sue
1 3 !
790,10, — Aro, sue
Bree oHC — aPech + Io
2 2

10. Hydrogen Energy

4 Fe,0, +2H,0 +350, —> 3FeSO,+2H, nc

— rose 250,8 *

so, — 290,280, 280,| mse
an :

Brunei, — ro h

200,1 160, — ro,» 150, avc

250, — 80,0, use

5 tous — ake, we

“cue — sou abu ue

600, — Seach, sore

Cue MgOm, —> Mes 10, soc

Mh, 240 — Myr, He sre

11.2.4, Some Thermochemical eyclic processes

Numerous candidate eycles have been suggested during past few years. Here,
we shall consider only three cyclic processes which are being worked on at the
present time, and are at an advanced stage of development, or are of special
importance. These cyclic processes should be mentioned particularly, for which
demonstration models are already available, constructed mainly of glass or quartz
and giving a continuous production of about 100 I hydrogen per hour. These are
the Westinghouse sulphur cycle, the Ispra Mark 13 bromine—sulphur cycle and
the General Atomic Co. iodine—sulphur process of these, the first two are hybrid
processes.

(1) Westinghouse Electrochemical Thermal Sulphur Cycle

This is a two-step process, where in hydrogen and sulphur acid are produced
electrolytically by the reaction of sulphurous acid and water. SO, is recovered by
reducing SO, obtained from sulphuric acid at high temperature, oxides of sulphur
serve as recycling intermediates within the process. The use of sulfur compounds
results in several process advantages.

‘The process in the most general form consists of two chemical reactions:
one for producing oxygen and the other for producing hydrogen. The production of
oxygen occurs via thermal reduetion of sulphur trioxide obtained from sulphuric
acid.

1450, "o 10,50, as

‘The equilibrium for reaction (11.5) lies to the right at temperatures above
1000°K. Catalysts are available for accelerating the rate of sulphur trioxide
reduction to sulphur dioxide and oxygen. The process is completed by using sulphur
dioxide from thermal reduction step to depolarise the anode of a water electrolyzer.
‘The overall reaction occurring electro chemically is:
2H,0 + 50, H, + H,SO, (1.6)
Reaction (11.6) comprises of two individual reactions.
Cathode 2H* + 2e°—> Hy 017)
‘Anode H.SO. + HO 2H*H 80. + 2 (18)

11. 474 Non-Conventional Sources of Energy

much smaller quantities than those necessary for conventional electrolysis are
needed. The theoretical voltage to decompose water is 1.23 volts with many
commercial electrolyzers requiring over 2 volts. The power requirements (0.17 volts
at unit activity of reagents and products) are seen to be less than 15% of those
required in conventional electrolysis. This aspect changes dramatically the heat
and work requirements for water decomposition and leads to high thermal
efficiencies.
Basie Process Description

‘The process is shown in Fig. 11.4. Hydrogen is generated electrolytically
in an electrolysis cell which anodically oxidises sulphurous acid to sulphurie acid

Heat

acia
mo Sulphor trioxide _L vapouriser
pa reduction reactor
fera
Surge
A
HSO acia Steam
1.80, condenser
Generation Z
CES expander

Fig. 11.4. Westinghouse sulphur cycle flowsheet schematic.

while simultaneously generating hydrogen at cathode. Sulphuric acid formed in
the electrolyzer is sent to a tank from where it is fed to two vaporizers in series.
‘The first of these is a recuperative heat exchanger heated by the effluent from the
high temperature sulphur trioxide reduction reactor. The second is heated by helium
from an ultra high temperature reactor. The sulphur trioxide steam mixture from
the second vaporizer flows to the helium heated reduction reactor where sulphur
dioxide and oxygen are formed. These gases are subsequently cooled against the
incoming acid and unreacted sulphur trioxide is recovornd on sulphuric acid in
knockout system. Steam is first condensed, following with the SO,/O, mixture is
‘compressed and sulphur dioxide recovered.

Bulk sulphur dioxide removal is accomplished by condensation against
cooling water. Final removal is achieved by condensation against low temperature
oxygen. This refrigeration and some auxiliary power produetion is generated by
expansion of the oxygen stream prior to its venting.

Hence in this process sulphur dioxide in sulphuric acid solution is oxidized
electrochemically to sulphuric acid, hydrogen being formed at cathode. The
sulphuric acid is highly concentrated, decomposed at a high temperature, and the

12. Hydrogen Energy 475

However, although at first sight the process appears to be simple, there are
serious difficulties in carrying it out in practice. The electrochemical stage functions
best in dilute sulphurie acid an optimum of 30 wt% has recently been cited whereas

highly concentrated acid is most favourable for the high temperature stage. If,
however, higher concentrations are used in the electrolytic cell both the resistance
of the electrolyte and the anode potential increase considerably. Also, from a
concentration of about 70-75% onwards there is reduction of the acid at the cathode
to sulphur and hydrogen sulphide. All this makes it necessary to optimize the acid
concentration for the two stages and to use considerable amounts of heat in
producing highly concentrated acid.

‘There is a further problem connected with the electrolytic cell. In order to
avoid losses of potential and of hydrogen at the cathode the cathode compartment
must be kept free from sulphur dioxide. This can be achieved either by a higher
static pressure of the electrolyte on the cathode side or by a specially constructed
cell containing two cation exchange membranes, between which there flows an
auxiliary electrolyte. The electrodes are made of porous graphite, graphite gauge
or graphite felt. Both the anodic and the cathodic reactions must be catalysed.

(2) Ispra Mark 13 cycle

359-470 K

2HBr 8-42 K_, H, + Br electrolysi 1110)
Br, + SO, + 2H,0 |, 2HBr + HS, (aq) (11.12)

1,80, = 1,0+80, +40, thermolysis (11.18)

In the first, stage concentrated hydro bromine acid is electrolysed at about
373 K, giving hydrogen and bromine. The latter is separated by distillation, and in
the second stage reacts with sulphur dioxide giving an aqueous solution of hydrogen
bromide and sulphuric acid. One thus obtains gaseous hydrogen bromide and fairly
concentrated sulphuric acid (about 75-80%), which can then be decomposed at high
temperatures, either directly or after further concentration. It is well known that
hydrogen bromide cannot be appreciably decomposed thermally in a single stage
below 1300 K. As in the case of water, direct decomposition is possible only with
the aid of electrical energy. If one compares the standard cell potentials for the two
electrolyses, the small difference is rather discouraging (1.23 — 1.07 = 0.16 V).
However, for hydrobromic acid it is possible to lower the reversible cell voltage
markedly (by several 100 mV), notably by increasing the concentration of hydrogen
bromide. Industrial realization of the electrolysis of hydrobromic acid has been
well investigated.

Graphite is used as the electrode material. The over voltage for the discharge
of hydrogen can be kept low by the addition of small amounts of noble metal salts.
No catalyst is needed at the anode. The over voltage for the discharge of bromine is
very small (in contrast to that for chlorine), and can be neglected in practice.

A me Da CE

13. 476 Non-Conventional Sources of Energy

At the present stage it is difficult to make a comparison with the
Westinghouse process. The somewhat lower cell voltage in the Westinghouse
sulphur process is counterbalanced by the advantage of the higher concentration
of sulphuric acid obtained in the Mark 13 process. It should also be mentioned that
the incorporation of an additional element—bromine in the Mark 13 process may
also be advantageous, since it offers more possibilities in the industrial operation
of the process. For example, the eyele can be modified so that no free bromine is
formed. Purification of the oxygen from sulphur dioxide or separation of the mixture
of oxygen and sulphur dioxide can be achieved by the Bunsen reaction. Electrolysis
can be carried out with either one or two cell eycles. A further reduction of the cell
voltage can be obtained by carrying it out under pressure. Without departing from
the conventional pressure range, the use of a precipitation technique for the
sulphuric acid makes it possible to obtain at 373 K a cell voltage of 0.7 V at about
5 km?

‘These considerations show that further work is necessary to determine which
is the optimum process.

(3) Iodine-Sulphur Cycle

Among the purely thermochemical cyclie processes, those belonging to the
iodine-sulphur family are of most interest at the present time. The following three
stage process has been developed by the General Atomic Co. in particular:

309-800 K

Zu MK pa +1, 1119
1, +80, +2H,0 H,S0, + 2H1 (11.15)
11,50, IE 1,0 +80, + Ou (11.16)

‘The main difficulty in this process lies in the fact that, if side reactions are
to be avoided, the two acids can be obtained only in dilute solution in the second
stage, and are difficult to separate even on a laboratory scale. Separation by
distillation is impossible without decomposition. The General Atomic Co. has
developed a continuous process in which an excess of iodine is used to effect
separation into approximately 56% sulphuric acid and hydriodie acid rich in iodine.
After phosphorie acid has been added to the latter, treatment at a high temperature
yield's liquid iodine and gaseous hydrogen iodide. The further working up of the
solutions consumes a considerable amount of heat. Nevertheless, an overall
efficiency of 47% is attained.

‘The difficulties associated with this process have led in recent years to
numerous investigations and suggestions for modifying the process. For instance,
scientists have attempted to carryout the second stage electrochemically. On short
circuiting the cell the following electrode reactions occur:

Cathode: 1, + 2H* + 22 — 2HI (1117)

Anode: SO, + 2H,O—> H,SO, + 2H* + 20" (11.18)

O O AS tee ae A

14. Hydrogen Energy 477

11.2.5. Fossil Fuel Methods

Mostly a gaseous mixture of carbon monoxide and hydrogen is formed in
the first stage, in the production of hydrogen by using a fossil fuel (ie., natural
gas, petroleum product, or coal). Such a mixture can be made by any method used
for an intermediate heat value fuel gas, synthesis gas or water gas. The procedures
in common use are steam reforming of methane or other hydrocarbon gas or light
liquid hydrocarbon and partial oxidation of a heavier hydrocarbon in the presence
of steam at a high temperature. In all these cases part of the hydrogen produced
originates in the hydrocarbon.

‘To remove the carbon monoxide, the mixture is submitted to the water gas
shift reaction with steam. The carbon monoxide is thereby converted into carbon
dioxide with the formation of additional hydrogen.

CO + H,0 = CO, + H, + 1440 kJ/kg. -L1L19)

‘The carbon dioxide is an acid gas that can be absorbed in an alkaline medium,
If the small amounts of carbon monoxide and dioxide remaining are undesirable,
they can be converted into methane which can be separated as a liquid by cooling
to a moderately low temperature.

Several processes proposed for converting coal into gaseous and liquid
hydrocarbon fuels require a hydrogen rich gas. The hydrogen would then be made
by reacting coal or Char obtained in the early stages of coal treatment, with steam
and a limited amount of oxygen. The heat generated when the carbon in the coal
(or char) reacts with oxygen produces the high temperature required for the carbon
steam reaction. The product is a mixture of hydrogen and carbon monoxide. The
carbon monoxide may be removed in the manner deseribed above.

Air can be used instead of oxygen to supply the heat for the carbon steam
reaction, but about half (by volume) of the product gas is inert nitrogen from the
air. Although this procedure is more economical, the high proportion of nitrogen
which can not be removed easily, is often a drawback. However, the iron steam
process, described below is designed to use air, steam, and coal char to make
hydrogen essentially free from nitrogen and also from carbon monoxide without,
the need for water gas shifts.

‘This method, which depends on the reaction of steam with iron at a
temperature of about 815°C at a pressure of 70 atm (7 MPa). The products are
fairly pure hydrogen gas and a solid iron oxide; thus

Fe+HO — FeO + H, (11.20)
Iron + Steam Tronoxide + Hydrogen

‘The iron is reco-

vered from the oxide in a

separate vessel and [ote 1

returned for further

reaction with steam. The Reducing +

conversion (reduction) of gas FR
Tron ir &

iron oxide to iron is Ion, Iron en

achieved by means of a
pas rer Hydrogen

15. 478 Non-Conventional Sources of Energy

chat process (refer Fig. 11.5). Carbon monoxide and nitrogen are absent from the
product, as the iron-steam reaction occurs in separate vessel.
Coal gasification for the production of Hydrogen

Introduction. In the gasification of coal, there complete conversion of the
organic part of the coal into gas, so that ash alone remains. This is done by reacting
the coal with a gasifying agent, e.g, steam above 700°C. The following account is
concerned solely with coal gasification, the fundamental physical and chemical
principles are explained first, end this is then followed by a discussion of the state
of the art and also new developments.

Fundamental physical and chemical principles

‘The basic reactions are set out in Table 11.2. The main reaction is the
heterogeneous water gas reaction (1), in which earbon is converted with steam to
hydrogen and carbon monoxide. The reaction products can then react either with
the gasifying agent or with each other in two further reactions the homogeneous
water gas shift reaction, (2) and the methanation reaction, (3) with formation of
carbon dioxide and methane. Processes (2) and (3) are exothermic, but the
endothermic main reaction (1) is dominant, so that overall the gross reaction
consisting of the sum of the three reactions listed under “steam gasification” is
heat consuming, or endothermic.

Hydrogasification (4), ie, the direct reaction of hydrogen with the carbon of
the coal, is of great importance for the production of methane as substitute natural
gas from coal. Lastely, the reaction with oxygen (5) and (6) also play a part in
gasification techniques, on the one hand serving as heat producing processes in
association with endothermic steam gasification but on the otherhand also resulting
as in partial combustion (6) in CO. Reaction (2) is of great importance for the
manufacture of pure hydrogen, in the subsequent gas conversion,

Table 11.2. Basic reaction of coal gasification

Sion guien | BART na
WT moe Mo | ns
@| coton mic, | ‘us
| cova, = chemo | “160
Hydrogasification
w | ca on Las
us
© | co, uso
| c+10,— co Lise

‘The steam gasification process and its potential applications

Fig. 11.6 gives a schematic review of the steam gasification process and its
potential applications. Coal and steam are fed into a gas generator, which can be
one of the three different types (two autothermic and one allothermic) depending

16. Hydrogen Energy 479

end product is therefore a lean gas with a calorifie value of the order of 4000-5000
kJ/m® (NTP), which can be used in industrial firing for heating purposes provided
that no lengthy transport distances are involved. Examples are smelting or metal
working plants, glassworks, brickworks, limeworks and the like, which at one time
operated gas generators in Germany itself. This technique is used now a days in
countries where natural gas is not available or can not be distributed because
there are no gas networks, There are numerous plants in operation in developing
countries, e.g., India. The use of a lean gas in power stations provides the
opportunity of first removing, in an intermediate purification stage, all the noxious
substances particularly sulphur, that were originally contained in the coal, and
thus producing environmentally clean electricity from coal in addition, converting
coal into gas opens up the possibility of using gas turbines, which greatly improves
the efficiency. Work is on this technology is therefore in progress in R & D projects
in a number of industrialized countries.

Autothermic gasification using pure oxygen gives an undiluted gas consisting
essentially of hydrogen, carbonmonoxide, methane and carbondioxide. After
purification and conversion, one area of application is to use this to obtain synthesis
‘gas for the manufacture of ammonia, methanol and liquid hydrocarbons. It is mainly
for this purpose that coal gasification plants are being operated today.

Steam gasification

‘Air }+{ Autothermic ] [2 }{ Autothermic | [ Allothermie }—{ Heat

> Li
Fe
1

Lean gas Purification
Industrial] [Power [ |
firing | | stations Y conversion Methanation
1 | Synthesis] Reducing
Heating Electricity SNG
town
Chemical Ore ®
industry reduction
T | Gas networks
NH methanol Pig 1
liquid hydrocarbons iron Heating

Fig. 11.6. Steam gasification process and its applications.
AR Min Mn ae nd te

nn ce

17. 480 Non-Conventional Sources of Energy

this route does not really make any sense either technically or economically. Lastly
it is also possible to convert the CO and H, produced in an autothermic process
into methane in a catalytic reaction in accordance with equation (3), and to
distribute this as substitute natural gas or town gas via the gas networks.

Raw gases with lower CO, contents but of otherwise similar quality are
also produced by allothermic gas generators, which differ in that coal, coal products
or gasification products are burned out-side the generator and the heat is then
introduced into the generator using appropriate methods of heat transfer. This
obviates the need to produce pure oxygen. However, these processes have not yet
won acceptance on a large industrial scale because of their complexity. They will
regain importance in the fairly near future by virtue of the fact that it is intended
to utilize heat from nuclear high temperature reactors for gasification.

Coal gasification plants

‘The coal gasification plants used in industry throughout the world today
have gas generators of the types illustrated in Fig. 11.7. They are based exclusively
on autothermie process in which coal is gasified with steam and oxygen.

ES i
N
ia

18. Hydrogen Energy 481

In the Lurgi gas generator, gasification is carried out in a fixed bed in which
the coal and gasifying agent are introduced in counter current flow. This results in
the formation in the generator of a temperature profile and separate zones, in
which with increasing temperature, the coal is dried and devolatilized and then
gasified and combusted. The process takes place under pressures of between 25
and 35 bar. The coal is therefore fed into the generator via. a lockhopper system
and the ash is removed via a similar system. The generator is a water-cooled
pressurized vessel 3-4 m in diameter.

The fluidized bed process, called the Winkler process after its inventor, was
developed for coal gasification on a large industrial scale as long ago as the 1920s.
Here, coal is gasified in a fluidized bed with steam and oxygen in a cylindrical gas
generator that tapers to a conical-shape at the bottom. The coal is fed in near the
centre of the bed via a screw conveyor. The gasifying agent is introduced firstly at
the bottom and secondly also above the fluidized bed. This ensures that entrained
coal particles being carried out with the gas can also be gasified, resulting in a
high carbon burn-up and because of the high temperatures that build up above the
fluidized bed considerable reduction of the by products. Uptill now, the Winkler
process has been operated solely at atmospheric pressure.

In the Koppers-Totzek entrained-flow reactors pulverized coal is gasified
with oxygen and steam in an entrained flow of coal dust. Because of the short
residence times, the introduction of relatively large quantities of oxygen produces
very high temperatures and high conversion rates are therefore achieved. The ash
runs down the walls as liquid slag and is removed from beneath the generator.
Directly above the generator there is a waste heat boiler for heat recovery.

‘Table 11.3 makes it possible to compare these three currently used coal
‘gasification processes. The most important technical data on the generators have
already been given in the text, As regards the feed coal for the process, it should be
noted that the Lurgi and Winkler processes are suitable at best for moderately
caking coals, whereas caking properties are immaterial with the Koppers-Totzek
process. Also the fixed-bed process does not tolerate fine grained material: only
lump coal can be used, and fines have to be diverted to some other use such as a
power station for the generation of electricity and steam for coal gasification.
However, itis only fair to record that future developments are aimed at broadening
the feed coal tolerance. As regards the gas composition, it should be pointed out
that in the fixedbed-process there is markedly higher methane content in the raw
gas, which is attributable both to the separate zones that form inside the gasifier
and to the elevated pressure. Consequently, the Lurgiprocess is particularly well
suited to the production of a high-heating value gas and was in fact originally
developed for the production of town gas. The raw gas from the Winkler and
Koppers-Totzek processes, in contrast, exhibits a very low methane content and is
therefore very suitable for conversion into pure hydrogen. The table also lists a
number of criteria relevant to the choice of process according to the rank and cost.
of the coal and the target product.

A major field of application of coal gasification is in the manufacture of
ammonia for fertilizer production. For this purpose in various developing countries
such plants are based on hard coal, and the process most widely used is the Koppers-

19. | AA

11.2.6. Solar Energy Methods
‘The following two approaches are under consideration i.
(@ Bio photolysis, and (ii) Photo electrolysis.

Table 11.3. Comparative data on
coal gasification processes in Industrial use

Gas generator Lurgi Koppers-Totzek
Type Fixed-bed Entrained flow
‘Temp. (°C) 700-1000 | Approx.1000 | >1300,
Pressure (bar) 35 1 a
Gas production | 35-50000 | 17-2000. 20000 (2 nozzles)
me) 50000 (4 nozzles)
Coal Rank Lignite and hard | Lignite and hard | All coals
coal upto modera- | coal upto modera-
tely caking tely caking
Grain size (mm)| 6-40 0-8 <o1
Raw gas Vol. & H, 37-39 35-46 a
co 20-23 30-40 58
co, 27-30 18-25 10
CH, 10-12 12 01
Other criteria + Good heat + Notarandoil | + No by products
utilization
+ Little discharge | + High carbon | + High steam
content decomposition:
+ High carbon | + High discharge | + High coal and
content oxygen con.
+ Tarand oil | + Low degree of | sumption
+ High steam gasification | = High discharge
consumption

Biophotolysis. This method utilizes living systems (or material derived
from such systems) to split water into its constituents hydrogen and oxygen. In
normal photosynthesis in green plants the green pigment chlorophyll takes up
energy from sunlight and in a complex series of reactions breaks up water molecules
into oxygen gas, hydrogen ions (i.e., hydrogen with a positive electric charge), and
electrons (ie, particles with a negative charge). The oxygen is evolved from the
green plant, but the hydrogen ions and electrons are removed by interaction with
carbon dioxide (from the air) to produce simple sugars.

Certain single cell green algae are able to make the enzyme (ie, biological
catalyst) hydrogenase. In these algae, the second stage of photosynthesis can be
circumvented by eliminating carbon dioxide. The hydrogen ions and electrons then
ln la lib deudas id eidvaicden to Sewn Irma Minin acia ak

20. Hydrogen Energy [> U

Blue-green algae differ from green algae in several respects. In particular,
in addition to normal photosynthesis cells in which reaction with carbon dioxide
occurs, they contain some larger cells (heterocysts) where hydrogen can be formed.
In the presence of nitrogen (e.g., from the atmosphere), however, the nitrogen
combines with the hydrogen ions and electrons to produce ammonia. By preventing
access of nitrogen (eg, in an inert argon atmosphere), blue-green algae decompose
water in sun light to yield hydrogen and oxygen.

Instead of using living algae to obtain hydrogen from water, a more
convenient approach is to utilize biological materials obtained from plants or
bacteria. One advantage is the ability to vary the conditions to optimize hydrogen
production.

Chloroplasts, the small bodies containing the chlorophyll in green plants,
retain their photosynthetic activity when extracted from the plant, Hydrogen and
oxygen can then be obtained from water by exposing chloroplasts to sunlight
together with the enzyme hydrogenase and ferredoxin, an electron carrier, also of
biological origin. It is possible, although less efficient to replace the ferredoxin by
asynthetic electron carrier and the hydrogenase by an inorganic (platinum) catalyst.
An ultimate objective of research on the decomposition of water by sunlight is the
efficient simulation of biological processes without using biological materials.

Photoelectrolysis. In ordinary electrolysis, water is decomposed into
hydrogen and oxygen by passing an electric current, from an outside source, between
two electrodes in an electrolyte solution. In photoelectrolysis, a current is generated
by exposing on or both electrodes to sunlight. Hydrogen and oxygen gases are
liberated at the respective electrodes by the decomposition of water, just as an
ordinary electrolysis. Atleast one of the electrodes in photoclectrolysis is usually a
semiconductor; a catalyst may be included to facilitate the electrode process. In
the cells studied so far, the efficiency for the conversion of solar energy into hydrogen
oxygen energy has been very low. Research is being directed at increasing this
efficiency by selection of electrode materials, electrolyte solutions, and electrode
catalysts.

Electrolysis is a more attractive way of producing hydrogen with solar
radiation since it ean be operated intermittently and therefore needs no storage.

109 659

‘Turbo

clectrolyser

3

Electricity

‘Condenser

21. 484 Non-Conventional Sources of Energy

‘The solar electricity needed for electrolysis can be produced either
photoelectrically or thermochemically. Both technologies are available today. Solar
electrolytic hydrogen production is therefore a question of the cost of component
and of development. Solar thermal power plant (Tower concept Fig. 11.8) is an
example of a solar electricity generation station. When the break through in high-
temperature electrolysis comes it could lead to an interesting application for
thermomechanical solar electricity generation, only part of the coolant heated in
the receiver is released into the turbo generator; the remainder is used to supply
the heat to electrolyser. Saving can be made with such hybrid electrolysis systems.

11

Hydrogen Storage

In an energy system there is a need to be able to store energy somewhere
between the production point and the utilization point. The need for storage is due
to the almost inevitable mismatch between the optimum production rate of energy
and the fluctuations in demand for energy by the users.

Inthe electric energy system storage, presents considerable difficulty because
electricity itself is not readily storable. In contrast to the electrical energy system,
both the gas energy and the oil energy systems are endowed with the capability
store very large quantities of energy. In the oil industry, liquid fuels are stored in
above and below ground tanks at the refinery, at the distribution centres and even
at the consumers premises. In the gas industry, storage is provided to a small
extent in the pipeline themselves and to a large extent in the underground systems
which can be either depleted gas or oil fields or so called aquifer systems: rock
formations resembling oil fields but containing water.

‘The location of energy storage systems is very important. Ifa large storage
system can be installed very close to customer, the load factor on the transmission
systems is automatically raised, and therefore the transmission cost becomes lower.

One of the advantages often claimed for a hydrogen energy systems is that
hydrogen is storable. However, it must be realized that storage of hydrogen is not
an easy problem compared with storage of liquid fuels such as gasoline or oil. It is
only when it is compared with electricity that storage of energy as hydrogen seems
relatively easy. It is when hydrogen is considered as a replacement fuel
applications currently met by natural gas and oil, that bulk energy storage becomes
very important.

‘There are five principle methods that have been considered for hydrogen
storage, these are:

(1) Compressed gas storage.

(2) Liquid storage (cryogenic storage in vacuum insulated or superinsulated
storage tank).

(3) Line pack system (allowing the pressure in the transmission or
distribution system to vary).

(4) Underground storage (in depleted oil and gas fields or in aquifer systems).

(5) Storage as metal hydrides.

O Lans Le ch ae BE encom: AO PAU ton oa hen:

22. Hydrogen Energy ES
storage on a large scale and on a small scale. The former applies particularly to
stationary applications, and the latter also to mobile ones.

1. Compressed Gas Storage. Hydrogen is conveniently stored for many ap-
plications in high pressure cylinders. This method of storage is rather expensive
and very bulky because very large quantities of steel are needed to contain quite
small amounts of hydrogen. In the conventional industrial hydrogen system, com
pressed gas is used to supply relatively small amounts of hydrogen, but when
hydrogen is considered as a fuel, it is soon realized that tank storage of hydrogen
is not really a practical proposition.

2. Liquid Storage. On a small scale or moderate scale, hydrogen is frequently
stored under high pressure in strong steel cylinders, as stated above; this type of
storage would be too costly fora large scale applications. A more practical approach
is to store the hydrogen as liquid at a low temperature, (ie., eryogenic storage).
Forexample, the liquid hydrogen fuel used as rocket propellant in the space program
is stored in large tanks. Very large facilities for hydrogen liquifaction have been
designed and built, and large storage tanks have also been constructed. One major
difference exists between handling liquid natural gas and liquid hydrogen the
storage temperature. Liquid hydrogen boils at - 253°C and therefore must be
‘maintained at or below this temperature in storage unless pressure build up can
be tolerated. Itis commonly regarded as necessary to use vacuum, insulated storage
vessels, where liquid natural gas can be maintained in the liquid state at a
considerable higher temperature by using superinsulation, but without the need
for vacuum jackets. The principal need for vacuum jacketed containers is that the
liquid hydrogen is below the temperature at which air condenses on the surface,
and thus any air in the contact with the cold walls of the hydrogen container.
‘There is also a flammability danger from the fact that liquefied atmospheric gases
(rich in oxygen) would concentrate in the vicinity of the hydrogen tank. Another
problem concerning storage of liquid hydrogen is the considerable amount of energy
required to convert hydrogen gas into the liquid phase. Not only must the latent
heat of the phase change be removed from the system, but some additional
precooling is required to bring the gaseous hydrogen down below the Joule-
‘Thompson temperature above which liquid hydrogen heats up on expansion. Thus,
aliquid hydrogen plant normally requires some kind of primary refrigeration, such
as a liquid nitrogen plant, to precool hydrogen. The net result is that about 25 to
30% of the heating value of hydrogen is required to liquefy hydrogen.

(3) Line Packing. The use of line pack storage in the natural gas industry
provides a relatively small capacity storage system, but one with a very fast re.
sponse time that can take care of minute by minute or hour by hour variations in
demand. A hydrogen transmission and distribution system running on hydrogen
would have a similar capability although the capacity would be reduced by a factor
of about 3 because of the reduced heating value of hydrogen, compared with natu-
ral gas.

(4) Underground Storage. The cheapest way to store large amounts of
hydrogen for subsequent distribution would probably be in underground facilities
similar to those used for natural gas; these facilities would include depleted oil
and gas reservoirs and aquifers. More expensive alternatives would be caverns

23. 486 Non-Conventional Sources of Energy

(5) Metal Hydrides (Storage in chemically bound form). Considerable interest
has been shown recently in the possibility of storage of hydrogen in the form of a
metal hydride. A number of metals and alloys form solid compounds, called metal
hydrides, by direct reaction with hydrogen gas. When the hydride is heated, the
hydrogen is released and the original metal (or alloy) is recovered for further use.
‘Thus, metal hydrides provide a possible means for hydrogen storage. An important
property of metal hydrides is that the pressure of the gas released by heating a
particular hydride depends mainly on the temperature and not the composition.

‘Ata fixed temperature, the gas pressure remains essentially constant until the
hydrogen content almost exhausted

Several studies are being made to find a metal hydride that would satisfy
the requirements for hydrogen storage. These requirements include the following:

@ the metal (or alloy) should be fairly inexpensive,

(Gi) the hydride should contain a large amount of hydrogen per unit volume
and per unit mass,

(ii) the hydride should be formed without difficulty by reaction of the metal
with hydrogen gas, and it should be stable at room temperature, and

(iv) the gas should be released at a significant pressure from the hydride at
a moderately high temperature (preferably below 100°C).

‘Three of the more promising hydrides are those of lanthanum nickel (LaNi,),
iron titanium (FeTi), and magnesium nickel) (Mg,Ni) alloys. The maximum
hydrogen contents are represented approximately by the formulae (LaNi,)H,,
(FeT)H, and (Mg,NiH,) respectively. These hydrides contain somewhat more
hydrogen than an equal volume, but much less than an equal weight, of liquid.
‘Thus, in theory (LaNi,) Hg contains 1.35% by weight, (Fe Ti)H, contains 1.9%, and
(Mg, NH, contains 3.6% of hydrogen. It appears that hydrides would be of interest
for stationary storage of hydrogen, when the small volume is advantageous. They
might be less useful, however, for mobile storage on a light vehicle, where their
weight would be a drawback ie, the weight of the hydride is more relative to its
hydrogen content.

Hence as cited above it has been shown that certain metals and their alloys
can reversibly store hydrogen chemically or electrochemically by forming metal
hydrides. The formation of various exothermic hydrides depending on pressure
and temperature and their stoichiometry can be determined by means of
concentration/pressure isotherms. If gaseous hydrogen is to react with an alloy,
the molecules first have to dissociate into hydrogen atoms which can then be
absorbed by the metal. For physical reasons, hydride formation always takes place
at constant pressure.

‘The dissociation of the molecular hydrogen into atomic hydrogen and the
bonding in the metal phases means that

+ the gas pressure ofthe hydrogen above the hydride is one to three orders

of magnitude below the values for gas storage in high-pressure cylinders,

+ the hydrogen density in the hydride is greater than that of hydrogen in

liquid form, and

24. Hydrogen Energy 487

In practice, energy densities of from 500 Wh/kg (already achieved) to a
maximum of 1000 Wh/kg can be attained,

If this is compared with the energy density of gasoline (including tank), ie.
some 10,000 Wh/kg and that of a lead battery, i.e., 25—30 Wh/kg (account being
taken of the efficiency of electric propulsion, which is 2-3 times higher than that of
the combustion engine), as a rough approximation it can be said that for the same
energy content a hydride storage unit (with cladding) will always be 10-20 times
heavier than a gasoline tank but 5-10 times heavier than a conventional lead battery
with the same range. Thus, reversible chemical storage of hydrogen in metals gives
technically acceptable figures in terms of weight, volume, pressure safety and energy
consumption.

‘The chemical reaction equation for exothermic formation of the hydride from
hydrogen and metal is:

= free
H, + Me 2% hydride + heat
discharge Pd

‘This means that during charging up with hydrogen heat is always produced
at the same time, and in order to withdraw the hydrogen from the hydride it is
always necessary to add heat,

For the technical application of hydrides the temperature at which their
Aissociaton pressures attain values above 1 bar is of particular interest. Of the
many that exist, tis group of hydrides are called low temperature hydrides (LTH),
because oven at temperatures below freezing point their dissociation pressure is
above 1 bar; all the other hydrides, for which the pressure limit of bar is above
the boiling point of water, are known as high temperature hydrides (HTH)

Ifthe reaction equation for hydride formation is viewed not as a hydrogen
reaction but as a thermal reaction, the basic equation forthe storage of heat in
hydrides is:

Q +MeH, — Me +H,
Heat is stored and hydrogen is given off by the dissociation of the hydrides.
‚The back reaction is accordingly:
H, + Me— Mell, + Q
Now the hydrogen is absorbed and (the formerly stored) heat released. This
means that metal hydrides fulfil a dual role. They store both fuel (hydrogen) and
heat.

Low temperature hydrides (LTH) have a heat storage density of upto 0.3
Mi/kg(TiFeH,, CaNi,H,) or 1.5 x 10? MJ/m in a temperature range from ~ 20°C
to 100°C at a hydrogen pressure of ~ 10 bar.

High temperature hydrides (HTH) have a heat density of upto 3 MJ/kg
(MgH,, Mg,NiH,) or 6 x 10° MJ/m® in a temperature range from 150°C to 550°C
(iH, : 8 Mi/kg, 20 x 10° MJ/m?, T = 500°C) and at a hydrogen pressure of ~ 10
bar.

25. 488 Non-Conventional Sources of Energy

In combustion processes (e.g., H, engines or H, domestic heating etc.) waste
heat is produced and delivered to the hydride to release hydrogen, it is therefore
stored in the metal and not given off into the atmosphere simultaneous with
combustion. To ensure continuous release of hydrogen, the waste heat from the
combustion process has, of course, to be greater than the energy needed for the
release of the hydrogen from the hydride,

If the quantity of heat available, together with all the transmission losses,
is the same as the energy needed for the dissociation of the hydride, the combustion
process produces no waste heat to the outside, since it is then a closed system of
hydrogen combustion/waste heat storage. Only when recharging the metal with
hydrogen (and only then) does the hydride formation lead to the release of the
previously stored heat of combustion. The waste heat energy can be made available
for practical use, e.g., for space heating, in various temperature ranges (by varying
the pressure, in recharging).

‘This interaction of fuel and heat storage can be used to particular advantage
in connection with combustion engines.

11.4. Hydrogen Transportation

Pipe lines. At present, the long distant pipelining of hydrogen is an
operation that is carried out by only a few specialized companies in different parts
of the world. It is of interest to compare the design requirements of a pipeline for
hydrogen with those of a pipeline for natural gas. Heating value of hydrogen in
only 12.1 MJ/cum, as compared to about 38.3 MJ/cum for natural gas. This implies
that to deliver the same quantity of energy, three times the volumes of hydrogen
must be transmitted. On closer inspection, however, one finds that the capacity of
pipeline depends upon the square root of the density of the gas, and because the
density of hydrogen is about one ninth (1/9) that of natural gas, there is a
compensating factor ofthe one third that results in the given pipe having essentially
the same energy carrying capacity for natural gas as for hydrogen. This is true at
atmospheric pressure, As the pressure increases to typical pipeline operating
pressures of 50 kg/cm? (5 MPa) or so, the compressibility factor for hydrogen is
somewhat different that for natural gas, and this results ina slightly unfavourable
carrying capacity for hydrogen. At 50 kg/cm? (5 MPa), the ratio of heating values
for a given compressed volume of hydrogen and natural gas has changed from 3: 1
to 3.83 : 1. Long distances gas transmission lines of lengths greater than about 90
km must be supplied with pipeline compressors at fairly regular intervals. Hydrogen
compressors must handle a considerably greater volume of the gas, somewhere
between three to four times the number of cu m for the same energy capacity.
Secondly, the horse power required to drive a hydrogen compressor is considerably
greater than that needed to drive a natural gas compressor for the same gas energy
throughout. Thirdly, the design of rotory compressors commonly used for natural
gas lines appears to be inadequate for hydrogen operation.

Itis possible to estimate the cost of transmitting hydrogen by pipeline from
a knowledge of the required pipeline diameter, compressor capacity and horse
power, and energy throughout required. In the case of design of a pipeline of
hplepia sspmcaños the bot of Tank end te chive Wins engines ic! concer

26. Hydrogen Energy KC

mechanical strength on exposure to hydrogen; the phenomenon called hydrogen
embrittlement, is specially significant for steel in hydrogen under pressure.
Operating experience with common pipeline steels at pressures up to about
35 kg/em® (8.5 MPa) has shown no problems of consequence. Normal pipeline steels
are satisfactory for pressures up to the typical operating values of 50 atm (5 MPa).
However the behaviour at higher pressure is uncertain, and more experimental
work needs to be carried out to get some definite data.

Liquid Hydrogen Transportation. Hydrogen in bulk can be transported
and distributed as the liquid. Double walled, insulated tanks of liquid hydrogen
with capacities of 7000 gal (26.5 cu m) or more are carried by road vehicles and
upto 84,000 gal (129 cu m) by rail road cars. Distribution of liquid hydrogen by
pipelines, jackted with liquid nitrogen, has been proposed. The costs would be
substantially greater than for gas pipelines, but it might be justifiable for certain
fuel applications where the liquid is required.

Metal Hydride Transportation. Hydrogen can also be transported as a
solid metal hydride. The main drawback, as stated earlier, is the weight of the
hydride relalive to its hydrogen content.

11.5. Utilization of Hydrogen Gas

Hydrogen gas can be utilised:

@ For residential uses

i) For industrial uses

(ii) For as an alternative transport fuel
(iv) For as an alternative fuel for aircraft

(©) For electric power generation (utilities)

() Residential uses. Electricity for lighting and for operating domestic
appliances (e 4, refrigerators) could be generated by means of fuel cells, with hy-
drogen gas at one electrode and air at the other.

Hydrogen can be used in domestic cooking. The burners of domestic
appliances (eg, stoves) would have to be modified if hydrogen were to replace
natural gas. In the first place, since the heating value per unit volume of hydrogen
gas is less than that of natural gas, a larger volume would have to reach the burners
to achieve the same heating effect. Furthermore, the greater flame speed when
hydrogen burns in air would facilitate backward propagation of the flame (e.g,
backfiring) in a natural gas burner. These problems ean be overcome by changing
the design of the burner, including the hole size and the air supply system.

Direct combustion product of hydrogen in air, is water only. Nitrogen oxides
may also be formed at the flame temperature but their amounts should be quite
small. The stove may not require venting to the outside atmosphere unless the
humidity level from the water vapour were objectionable.

Hydrogen would be useful in radiant space heaters, because of the possibility
of flameless combustion on a catalytic surface. Such devices would operate
spontaneously when the gas was turned on and no pilot light or other ignition
system would be required. Because of the low combustion temperature, nitrogen
pdt ap dis rior pater camer rior IR

27. o | AA

replace natural gas in these operations. Hydrogen gas could also be used with
advantage, instead of coal or coal derived gases, to reduce oxido ores (eg. iron ore)
to the metal (iron).

(iii) Road vehicles. The use of hydrogen fuel in internal combus- tion engines
for automobiles, buses, trucks, and farm machinery has attracted interest as a
means of conserving petroleum products and of reducing atmospheric pollution.
Because of the fuel is a gas, the conventional carburetor of a spark-ignition engine,
in which liquid gasoline is vapourized in air, must be modified for use with hydrogen.
‘The methods of using hydrogen as a fuel in CI engines are as follows:

@ A mixture of fuel gas and air, with an approximately constant fuel to air
ratio, is introduced into the eylinder intake manifold. The engine power (i.e, vehicle
speed) is controlled by varying the quantity of mixture entering the eylinder by
means of a throttle value. (The same procedures were used in the otto and ol.lier
early LC. engines with coal gas as the fuel). Stable operation, especially at higher
speeds, may require addition of water vapour to the fuel air mixture; this ean be
achieved by returning part of the exhaust gas to the manifold

(i) The hydrogen gas under pressure is injected through a valve directly
into the engine cylinder, and the air is admitted through another intake valve.
Since the hydrogen and air are supplied separately, an explosive mixture does not
occur except in the cylinder. This scheme is considered to be safer than the provi-
ous one, in which such a mixture is formed in the manifold. The engine power
output is controlled by varying the pressure of hydrogen gas from about 14 atm at
low power to 70 atm at high power. The hydrogen is required to be stored as a
compressed gas.

ii) The hydrogen gas at normal or moderate pressure is drawn through a
throttle value into the engine cylinder during the intake stroke. At the same time,
unthrottled air is drawn in through the intake port. Here, also, there is no explo.
sive mixture,except in the eylinders. The engine power is varied by adjusting the
hydrogen inlet throttle. Since the air supply is unthrottled, there is a change in
the proportion of fuel in the eylinder and consequently a change in the power de-
veloped. This scheme of power variation is possible because of the wide composi-
tion range over which hydrogen air mixture can be ignited.

Another modification arises from the high speed of the hydrogen flame in
air; this require that the ignition time be retarded (e, less spark advance) compared
with a gasoline engine.

Advantages claimed for hydrogen fuel engines are:

(1) They can higher efficiencies ie, utilize a higher proportion ofthe energy
in the fuel than gasoline engine.

Gi) The amount earbon monoxide and hydrocarbons in the exhaust would
be very small since they would originate only from the cylinder lubricating oil

However the nitrogen oxide levels due to high combustion temperature may
be high, it may be reduced by reducing the combustion temperature by injecting
‘water vapour into cylinder from the exhaust as described earlier.

Ma alar war to: willine kenn ana valriclé Baal da the o

28. Hydrogen Energy 491

(iv) Air Craft Applications. The earliest application of liquid hydrogen
fuel is expected to be in a jet air craft; this possibility was demonstrated in a
subsonic air craft in 1957. The main advantage is the much lower overall weight of
the fuel and storage tank than for ordinary jet fuel. The volume of liquid hydrogen
would be greater than for regular fuel, but this could be accommodated on a large
aircraft. The cold liquid hydrogen could be used directly or in directly to cool the
engine and the air frame surfaces of a high speed aircraft. If hypersonic aircraft
(i.e,, speed more than five times the local speed of sound) is ever developed, liquid
hydrogen may be the only practical fuel. Even in the subsonic range liquid hydrogen
makes it possible to reduce the starting weight of high capacity air craft by half,
compared with hydrocarbon fuelled aireraft of the same range and pay load. Because
of the smaller total weight it is possible to achieve shorter take off runs, steeper
climbing paths (less nuisance from noise) and/or smaller engine thrust (less noise
production).

It may also be possible to decrease the size and weight of the engines.

Hydrogen’ favourable diffusion properties and high thermal conductivity
lead to better mixing than with liquid hydrocarbons, even with shorter combustion
chambers. The wide range of ignition for hydrogen air mixtures (5% to 75% by
volume) makes the engine more readily controllable, especially under partial loads,
and reduces the emission of noxious substances. Since it is possible to use engines
of shorter construction, thus reducing the time for which the fuel is at a high
temperature, the emissions of nitrogen oxides (in any case small with hydrogen)
can be still further reduced. The problems, in addition to that of the economic
production of Hydrogen, lie in the provision of the necessary infrastructure at
international air ports and the bridging of the transitional period, which will
necessarily be a long one. A further problem concerns the placing of the fuel tanks,
which are more bulky than those for kerosene. It is not possible to site them in the
wings, since the surface/volume ratio of eryogenic containers must be kept as low as
possible in order to keep down evaporation losses. Foam insulation can be used, in
accordance with current technical practice, but the optimum construction requires
further investigation with present techniques double walled tanks with an
intervening vacuum for insulation would be too heavy. External tanks located on
the wings would increase the aerodynamic drag considerably. Placing this tanks
in the extended fuselage represents a reasonable compromise between space
utilization, aerodynamic drag and safety. The insertion of suitable bulk heads would
provide adequate safety, even in the case of accidents.

‘The heat required to vaporize and heat up the hydrogen for the engines can
be obtained through certain sections of the outer skin of the wings and fuselage. In
this way the boundary layer is cooled so as to produce laminar flow, resulting in a
lowering of the aerodynamic drag and hence of the fuel consumption (laminar flow
control), this could not be achieved to the same extent in any other way.

(0) Electric Power Generation. It is unlikely that hydrogen would serve
as a major fuel for electric power generation by a utility. However, its substitution
for natural gas in peak shaving turbines is possible, Hydrogen could also be used
as a means for storing and distributing electrical energy.

29. | AA

reducing manufacturing costs when producing a large number of units. However,
in the present state of fuel cell technology such applications are feasible only for
pilot plants. A number of developments are still necessary, especially in the fields
of electrodes and catalysts, to make fuel cell systems applicable on a large scale.

It is also of importance that the conversion efficiency of fuel cells is
independent of the load factor over a wide range, so that a high efficiency can be
obtained even with partial loads. Furthermore, with fuel cells which at high
temperatures high-grade waste heat can be used for thermal energy production.

On the basis of experience of electricity generation with small hydrogen-
oxygen assemblies, research and development work is now in progress (especially
in U.S.A.), with the object of developing fuel-cell power stations for the centralized
and local generation of electricity. The present position with regard to various fuel
cells can be described as follows.

Low and medium temperature fuel cells with operating temperature
between 80 and 250°C have been equally for developed in three directions:

() alkaline fuel cells with aqueous caustic potash as electrolyte;

(ii) acid fuel cells with concentrated phosphoric acid solutions as electro-

Iyte;
iii) solid electrolyte fuel cells with acidie ion exchange groups in a polymer
matrix.

Alkaline fuel cells can be used advantageously for producing electricity from
pure hydrogen and oxygen, since high efficiencies can be obtained here with cheaper
catalysts. Fuel cells with acid electrolytes can also be operated with fossil derived
impure hydrogen or hydrocarbons, but as yet require the use of catalysts of platinum
or other noble metals.

Along side the use of fuel-cells operating at low or medium temperatures
and pressures, the development of optimum configuration of large fuel cell modules
therefore depends on the choice of suitable inexpensive materials for electrodes of
sufficient long term stability and enhanced procedures for manufacturing the
individual components and modules.

High temperature fuel cells operate in the temperature range from 600 to
1000*C. Because of their high operating temperature they have the advantage
that no special catalysts are needed to activate the reactions, so that crude gas or
CO/H, mixtures can be used directly for producing electricity, and high grade waste
heat is available for use.

High temperature fuel cells with solid electrolytes based on conduction by
oxide ions represent the reverse of the high temperature electrolysis of steam, and
involve the same problems as those described in that connection. In particular, it
will be necessary to solve the fundamental problems connected with constructional
materials arising from the high operating temperatures (around 1000°C) before
development to an industrial seale is possible.

A more advanced stage of development has been reached with high-
temperature cells with molten carbonate electrolytes, operating at about 600°C.
Laboratory cells have been undergoing tests for sometime now. Further development

30. Hydrogen Energy 493
11

Hydrogen as an Alternative Fuel for Motor Vehicles

‘The consequences of using hydrogen in motor vehicles are best seen by
comparing it with other chemical energy vectors. Table 11.4 shows the required
tank volume and weight of fuel for typical non-conventional fuels compared with
iso-octane, which is typical of the various hydrocarbons that constitute gasoline.
Comparison of isooctane with methane (liquid natural gas), methanol and hydrogen
shows that, although liquid hydrogen requires a larger tank volume, it requires
the lowest weight of fuel on account ofits high minimum calorific value. Except for
the tank volume liquid methane corresponds most closely with gasoline, while
methanol requires an appreciably larger tank volume and also (on account of its
lower calorific value) a considerable heavier weight, of fuel (Table 11.5).

‘Table 11.4. Comparison of Various Alternative
Fuels and Storage

Storage | Density of | Energy density Operating

capacity | Hydrogen temperature
ut. % all [EVE | RIWATE | (at 2bar)

‘Magnesium hydride 7 101 233 336 350€

‘Magnesium nickel

hydride 316 sr 105 2.69 250°C

Iron titanium hydride| 1.75 9 058 318 -10%0

Liquid hydrogen 100 m 333 236 | -20c

Methanol 56 142

Liquid methane 138 58 1610

Ammonia 54 421

Hydrazine 46 4.65,

Tso-octane 127 876

(tis taken here as

ropresentati

gasoline, whi

mixture of low boiling

hydrocarbons)

‘Table 11.5. Weight Required For the Storage of Fuels

Hydrogen | Hydride | Storage Total
container, | weight
ete.
Gg) Ga) Ge Gg)
‘Magnesium hydride 85 121 193
‘Magnesium nickel hydride 85 269 430
Iron titanium hydride 85 485 715
iquid hydrogen 85 50 585
Equivalent quantity of.

31. | AA

In the use of hydrogen as an alternative fuel, the storage of the fuel on
board the vehicle constitutes an important problem. For motor vehicles hydrogen
could in principle be stored as compressed gas, chemically as a metal hydride, or
as a liquid. The storage of compressed gas would be difficult on grounds of safety
because of the high pressures required. Furthermore, even at pressures around
200 bar the quantity of hydrogen stored amounts to only about 1% of the weight of
the container, while the volume of the container is only 20 times greater than that
ofan equivalent gasoline tank. In metal hydrides hydrogen atoms are incorporated
in the crystal lattice of metals or alloys (inter metallic compounds), and each lattice
atom can absorb up to two hydrogen atoms, or in individual cases between two and
three. When using alloys like iron titanium the storage material can contain only
upto 1.75 wt% of hydrogen, since in iron titanium hydride (Fe Ti H 1.95) the iron
and titanium constitute about 98.25 w1% ofthe substance. The amounts of hydrogen
stored are more favourable when the lattice consists of lighter atoms, for example
magnesium.

However, other problems then arise, since the formation ofthe hydride and
its decomposition to give hydrogen occur rapidly enough to produce a sufficiently
high hydrogen pressure only at fairly high temperatures (250-350°C).

‘This temperature can be lowered by suitable additions to the alloy,
particularly of nickel. However, this causes the heat of formation of the hydride,
which must be removed when the storage unit is being charged and supplied when
the hydrogen is being liberated, to assume very high values (about 30% of the
minimum calorific value of hydrogen see Table 11.4). This can cause problems in
running the vehicle, since on account of the high operating temperature of these
hydrides the heat available from the exhaust gases is insufficient to liberate from
the hydride the hydrogen required by the engine. Since hydride vehicles have
already been investigated intensively, a considerable amount of information is
how available about the performance and potential of mobile hydride storage units.

For a given amount of energy stored in the vehicle, hydride storage requires
about the same volume as liquid hydrogen, but 10-20 times the weight. Hydride
vehicles must therefore be regarded as alternatives to electric vehicles, and would
be used mainly for urban transport within a moderate range. However, itis precisely
in urban transport, that carrying the additional weight of the storage system is a
disadvantage; for example, it may around to about 300 kg for a maximum range of
150 km. The consequent increases in fuel consumption in urban transport makes
the. advantages that hydride storage offers over storage as liquid hydrogen, such
as the possibility of loss-free storage at ambient temperatures and the absence of
energy consumption for the liquefaction of hydrogen, seen less attractive. Thus,
although the consumption of primary energy for liquefaction may be regarded as a
particular disadvantage, it does not represent a limitation in comparison with
hydride storage, since on the one hand liquid and gasoline are comparable as regards
weight and range even when the storage tank is taken into account and on the
otherhand a considerable proportion of the energy used in liquefaction can be
regained as mechanical energy when the cold hydrogen is warmed up. It is also of
importance for engine operation that a pump for liquid hydrogen can be used to
obtain the extra pressure needed for internal mixing in the spark ignition engine

32. Hydrogen Energy KC

vehicle engine, or the injection pressure of the hydrogen into the engine would
have to be reduced so much that the fuel could no longer be supplied at the most
favourable moment.

A problem could arise with evaporation of hydrogen gas from the storage
tank of the vehicle, especially when it is parked in an enclosed space such as a
garage. However, according to the most recent observations the rate of evaporation
from the latest vacuum insulated storage containers ean be kept down to a very
low level by taking suitable precautions. Laboratory storage containers are now
available for liquid hydrogen with eapacities of 100-200 litres which can be kept
for up to one year before all the liquid has evaporated. Storage tanks for motor
vehicles giving storage periods of weeks or months are technically quite feasible
today at reasonable cost. The small amount of hydrogen with evaporates can be
used to generate electricity in a fuel cel, thus making it possible to dispense with
a rotary electric generator. In this way the car battery can be charged efficiently
and with little consumption of fuel when the car is stationary and during town
driving, and the emission of hydrogen to the surroundings, which is particularly
dangerous in enclosed spaces, is avoided completely.

11.7. Safety and Management

ince hydrogen is a highly flammable gas, it must be handled with ca
special equipment designed for safety. The extensive use of H, on the grounds is
dangerous. Natural gas and gasoline are also hazardous materials, yet they are in
common daily use. The danger of fire or explosion can be minimized by taking
proper precautions with hydrogen. Larger quantities of the gas and liquid have
has been carried by pipeline, road and rail for use in industry and in space vehicles
in many respects, hydrogen gas is no more, and possible less, hazardous than
natural gas. The lower flammability limit of hydrogen is 4% (by volume) in air,
whereas that of methane in natural gas is 5%. However, hydrogen will escape
from a leak of fixed size about three times as fast as natural gas hence, in a closed
space, the flammability will be reached soon with hydrogen. On the other hand,
the energy contained in the space at the lower flammability limit is about one
fourth that for natural gas. Because of its low density, hydrogen disperses more
readily in an open (or ventilated) space, and it would therefore take a much longer
time for the flammability limit to be reached.

‘The chief danger from hydrogen gas is associated with the very low ignition
energy, which is less than a tenth than of natural gas. Consequently, a spark that
is too weak to ignite a flammable air natural gas mixture may ignite an air hydrogen
mixture. In handling hydrogen, special care is to be taken to avoid flames and to
prevent spark formation

Liquid hydrogen presents another safety problem. The low temperature of
the liquid may cause air in the vicinity to liquefy; since oxygen is more readily
liquefied than nitrogen, an oxygen rich liquid may form. This would greatly increase
the flammability danger. Such a situation could arise if a liquid hydrogen tank or
pipeline were not properly insulated so that the exterior temperature fell low enough
toliquefy oxygen from the air. The special double walled insulation as stated earlier
cathe Br

33. | AA

Hydrogen is used on a large scale today in industry, especially the chemical
industry, where the safe handling of hydrogen is an established technique, Nor
are any essential problems to be expected in this area of use in the future. However,
in considering the possible future public use of hydrogen as an energy vector, for
example as an alternative fuel, it has to be borne in mind that at present hydrogen
(and especially liquid hydrogen) is generally considered as extremely dangerouo
substance to handle, although completely realistic assessments can be made on
the basis of extensive experience in space travel. This is because such experience
is not sufficiently widely known, and because the public has been accustomed for
nearly a hundred years to dealing with gasoline and kerosene, although these too
are by no means free from danger. Natural gas is also generally accepted today as
an energy vector, although in the 1940s it gave rise to a number of extremely
serious accidents with liquid gas tankers and large scale storage in the U.S.A. In
the case of hydrogen this ‘familiarization effect has not yet developed, although in
the past hydrogen has been used in considerable quantities in town gas, which
contains between 40% and 70% of hydrogen.

Although safety problems with hydrogen are somewhat different, in the present
state of knowledge no greater potential dangers are to be envisaged than in dealing
with natural gas, while the unequalled environmental compatibility of hydrogen
must be counted an additional advantage.

‘The hazards—both real and supposed—presented by hydrogen must be
evaluated before it can be used as a fuel and energy vector in any particular system.
‘The public’s questions about safety with have to be answered if hydrogen is to be
introduced for use as a transport fuel, for heating in the home and in industry, for
new industrial processes such as generating electricity at periods of peak demand
or, to a greater extent than present, for industrial processes such as ammonia,
fertilizer and methanol production and coal gasification or liquefaction. This
enlarged hydrogen market will be accompanied by a corresponding growth in the
production of hydrogen in new techniques for its production, in storage capacity
and in transportation across or beneath public territory. All these elements of a
‘hydrogen energy system’ are involved in any discussion of safety.

It must be stressed that hydrogen has been used as a fuel and as a chemical
for more than two centuries. Hydrogen is used mainly as an intermediate product
in the chemical and petroleum industries, the three largest consumers being the
production of ammonia and methanol and the refining of crude oil.

Almost all the hydrogen in the petrochemical industry is produced and
consumed in one place. Only a few small pipeline systems carrying hydrogen
between several plants are in use. The largest hydrogen pipeline system is in
Germany and has a total length of approximately 210 km and an annual throughout
of approximately 3 x 10°m?. Total world production of hydrogen in 1973, was 2.1 x
10"! m}, which is equivalent to 1.8 107 tonnes. These figures show that considerable
experience exists in the handling of hydrogen.

Cas

's of files and preventive measures:

Leaks from seals at flanges and valves are responsible for 65% of the fires.
Fittings that resulted in the most leaks that in turn led to fires were located in the

34. Hydrogen Energy 497

‘The analysis of causes indicates the characteristic tendency of hydrogen to
penetrate normally “air-tight” seals. The volume that leaks through a seal is
determined either by the density of the gas (molecular weight) or by its viscosity,
for a given pressure difference and leakage area. Like methane, hydrogen has a
lower viscosity and molecular weight than air; hence a fitting or component that is
normally air tight is by no mean a necessarily “gas tight” to hydrogen or methane.

When leaking hydrogen has a temperature near or above room temperature
it rises upwards rapidly owing to the difference in density. The density of bydrogen
is only 1/8 that of methane; hydrogen therefore has a corresponding faster rate of
rising. Hydrogen's relatively high diffusion coefficient also promotes dispersion
even under still atmospheric conditions.

In the event of an escape of hydrogen in an enclosed space, its tendency to
rise imposes a limit on the degree to which itis diluted by the air. As recommended
for this reason hydrogen should not be handled in enclosed spaces but always in
the open air waste gas systems and furnaces (heat treatment furnaces and reformer
furnaces) are more often affected by faults than any other components, Compressors
and heat exchangers are likewise frequently affected. The relatively high accident
rate for compressors, furnaces, heat exchangers and reformers is understandable
when it is remembered that these usually contain hydrogen at temperatures close
to its spontaneous ignition point (585°C).

‘The relatively high accident rate for systems in which the operating
personnel arc'not specifically familiar with the hazards presented by hydrogen
should also be mentioned. This in especially noticeable in, for example, nuclear
power stations, chlorine plants and semi conductor factories, where hydrogen is
only an unwanted side product that has to be handled. Leaks from flanges and
failure of waste gas collection systems and in the case of chlorine, the gas collection
system are the fittings and components mostly affected by the faults. In view of
the possible use of water electrolysis as a method of producing hydrogen in the
future, it is important to point out that electrolysis has a high accident rate, most
of the accidents today occurring in chlorine/alkali plants. It is therefore particularly
important to ensure that the hydrogen and the oxygen are kept separate in the
electrolysis cell and in the gas collection system.

‘Traditional fittings and components like pressurized tanks, distillation
columns and heat exchangers as such appears to be affected to a negligible extent.
by hydrogen faults,

Preventive measures. In discussing preventive measures it is necessary
to distinguish between measures intended to prevent accidents inside the system
and outside the system. Three conditions must be fulfilled simultaneously for a
fault inside a system: for example, hydrogen as fuel, oxygen/chlorine as oxidising
agent and, lastely a source of ignition. The elimination of sources of ignition may
be an adequate preventive measure for avoiding hydrogen explosions because of
the extremely small amount of energy required to ignite hydrogen. Consequently,
to avoid explosions in hydrogen systems, itis absolutely essential to prevent oxygen
from entering the system. This can be accomplished either by making the pressure
in the system higher than that of the oxidizing atmosphere or by using an inert
‘gus wlan Cin aba. brad be aise! eut that calls OR volecas of ey dian da:

35. 498 Non-Conventional Sources of Energy

‘The inside of the system can be rendered inert not only by using nitrogen
but also by using carbondioxide, halons or extinguishing powders such as
ammonium phosphate or potassium chloride.

‘The latter can be used for temporarily rendering mixtures of hydrogen and
air inert in order to prevent explosions progressing through the system in this
case they are therefore actually explosion suppressors.

Another way of preventing explosions from propagating through the system
is to install flame traps. Here too, reliable flame traps for hydrogen are difficult to
design because hydrogen requires a very narrow gap to prevent a flame from
passing.

Whilst a high internal pressure and an inert internal atmosphere efficiently
and reliably prevent the formation of explosions in pure hydrogen systems, other
measures are needed in systems containing hydrogen and other products to prevent
the dangerous consequences of explosions.

‘The measures are essentially as follows

1. systems that withstand the force of an explosion,

2. systems that withstant the force of pressure surges,

3. explosion-relief systems,

4. explosion-suppression systems and, for pipeline system alone,

5. flame traps, flame suppressors, explosion relief devices and rapid closing
devices.

Measures for preventing hydrogen explosions outside the system are greatly
limited in effectiveness because an oxidizing agent, air, is always present and the
presence of a source of ignition always has to be assumed even though it may have
previously been demonstrated that numerous leaks, and therefore the presence of
hydrogen in the free atmosphere, had occurred without an explosion and that the
hydrogen had diffused away harmlessly. Consequently a fundamental preventive
measure in the case of hydrogen plants is that they be built in the open air. Further,
it is very important to detect any hydrogen leaks as early as possible. Because of
hydrogen’s low density, standard analysers like gas detectors can be used to only a
limited extent and can be made effective only in open air plant. The other preventive
measures are largely secondary measures giving protection solely against further
effects resulting from fires and explosions. Examples of such secondary measures
are automatic or manual shut down of the process, sprinklers, flooding, water
spraying, extinguishing systems, halon systems, and explosion relief devices for
entire buildings

All these systems have proved their effectiveness in the past. Automatic
sprinklers, flooding systems and water spraying for cooling have a success ratio of
80%. This result seems to confirm the advantages of the qualitative recommendation
made in published guidelines on explosion prevention. Powder extinguishing
systems have a successive ratio of 86% in extinguishing hydrogen fires. Shutting
down the entire hydrogen plant is the most natural method of preventing fire and
has a success ratio of 81%. Explosion relief devices have a success ratio of 90% and
have contributed greatly to preventing damage to apparatus and buildings.

36. Hydrogen Energy KC

(@ Production of hydrogen by photo electrolysis of water using solar energy.

(Gi) Production of hydrogen by blue green algae and by certain bacterial
species.

(ii) Storage of hydrogen through metal hydrides/non metal hydrides (rice
husk and misch metal).

(iv) Problems relating to utilization of hydrogen as a fuel, that is, develop-
ment of suitable engines and burners ete.

(v) Liquid hydrogen production, storage and utilisation ete.

Satisfactory progress has been reported on the various funded projects
advancing the state of art one step further.

In comparison with other important secondary energy vectors, such as
electricity, natural gas, synthesis gas, or remote heating, hydrogen satisfies a
number of necessary conditions for a potential future secondary energy vector.
‘These include availability, manufacture without harm to the environment, good
storage properties and very ready conversion into other forms of secondary energy.

A coupling of hydrogen and electricity in a future energy economy is
technically feasible. Hydrogen can thus successively replace petroleum based
products, including natural gas, and also synthesis gas.

Hydrogen can also be mixed with natural gas or synthesis gas, or can be
used in pure state: it ean therefore play an important part in ensuring gas supplies
‘when fossil energy vectors become scarce.

Hydrogen can play a specially significant role when coupled with
regenerative primary energy sources such as the sun and wind: since it can be
stored, it ean serve here to even out a fluctuating supply of energy and to transport
energy to areas of high demand.

Hydrogen is also a candidate among the various alternative transport fuels,
such as synthetic hydrocarbons, methanol and liquid natural gas. Because of its
favourable environmental characteristics it can be used both in air transport and
in surface transport.

In view of the requisite order of magnitude of a future hydrogen market,
however, enormous capital investment will be required for production, distribution
and storage, and this factor represents the essential obstacle to a rapid introduction
of hydrogen.

QUESTIONS

1. What are the different methods for hydrogen production? Explain in brief.
2. What is an electrolysis? Describe the more popular method of hydrogen production.
3. Describe thermo chemical method, for hydrogen production,
4. What is the Westinghouse electro chemical Thermal Sulphur eyele? Deseribe it.
5. Write short notes on:

(a) Solar energy method of H, production.

(6) Fossil fuel method of H, production.

(©) Metal hydrides

(d) Safety precautions in hydrogen utilization.