1.

1 Introduction Owing to an increasing world population and demands for higher standards of
living and better air quality, the future energy demand is expected to increase signifi cantly. To
meet this demand poses great challenges. Currently, most of the world energy requirement for
transportation and heating (which is two-third of the primary energy demand) is derived from
petroleum or natural gas. These two fuels are generally favored due to the ease of transport of
liquid or gaseous forms. Unfortunately, the combustion of hydrocarbon fuels for transportation
and heating contributes over half of all greenhouse gas emissions and a large fraction of air
pollutant emissions. Hence, today’s world is facing an urgency in developing alternative fuels.
Among various alternatives, hydrogen fuel offers the highest potential benefi ts in terms of
diversifi ed supply and reduced emissions of pollutants and greenhouse gases. For the past 40
years, environmentalists and several industrial organizations have promoted hydrogen fuel as the
solution to the problems of air pollution and global warming. The key criteria for an ideal fuel are
inexhaustibility, cleanliness, convenience, and independence from foreign control. Hydrogen
possesses all these properties, and is being evaluated and promoted worldwide as an
environmentally benign replacement for gasoline, heating oil, natural gas, and other fuels in both
transportation and nontransportation applications. A number of reports are now available on
several aspects of hydrogen [1–25]. Similar to electricity, hydrogen is a high-quality energy carrier,
which can be used with a high effi ciency and zero or near-zero emissions at the point of use. It has
been technically demonstrated that hydrogen can be used for transportation, heating, and power
generation, and could replace current fuels in all their present uses [2–6]. Hydrogen can be
produced using a variety of starting materials, derived from both renewable and nonrenewable
sources, through many different process routes. At present, two basic process technologies— (1)
reformation of natural gas and (2) electrolysis of water—are widely used. In the advent of
hydrogen economy, the principal focus of hydrogen technology has shifted to the safe and
affordable utilization of hydrogen as an alternative fuel based on seamless integration of
generation, distribution, and storage technologies. Inaccuracies, inconsistencies, and
contradictions abound in the seemingly persuasive arguments targeting the general public and
politicians regarding the merits of the hydrogen case. These inaccuracies tend to create the global
perception that hydrogen will become an active source for our energy needs, replacing today’s
relatively less-effi cient machines with clean fuel cells, which will effi ciently power cars, trucks,
homes, and businesses, ending global warming and air pollution. The key assertions of the
initiative for hydrogen production and utilization are based on the premise that the fuel cell is a
proven technology and hydrogen is in abundant supply on Earth [10–12], but unfortunately, most
of the hydrogen CRC_4575x_CH001.indd 4 5/29/2008 10:11:28 AM Fundamentals and Use of
Hydrogen as a Fuel 5 TABLE 1.1 United States and World Hydrogen Consumptions by End-Use
Category Captive Users United States World Total U.S. Share of Billion m World Total (%) 3 Share
(%) Billion m3 Share (%) Ammonia producers 33.7 38 273.7 61 12 Oil refi ners 32.9 37 105.4 23 31
Methanol producers 8.5 10 40.5 9 21 Other 3.4 4 13.6 3 25 Merchant users 10.8 12 16.1 4 67 Total
89.3 100 449.3 100 20 Source: Adapted from SRI Consulting Inc., Chemical Economics Handbook
2001, Menlo Park, CA, July 2001; Wee, J.H., Renewable Sustainable Energy Rev., 11, 1720–1738,
2007. on Earth is in the fully oxidized form as H2O, which has no fuel value, and there are no
natural sources of desirable molecular hydrogen (H2). At present, hydrogen production is a large
and growing industry. Globally, some 50 million t of hydrogen, equivalent to about 170 million t of
petroleum, were produced in 2004. And the production is increasing by about 10% every year. As
of 2005, the economic value of all hydrogen produced worldwide was about $135 billion per year
[3]. The current global hydrogen production is 48% from natural gas, 30% from petroleum, 18%
from coal, and 4% from electrolysis [4]. Major end users of the hydrogen are listed in Table 1.1.
Hydrogen is primarily consumed in two nonfuel uses: (1) about 60% to produce NH3 by the Haber
process for subsequent use in fertilizer manufacturing [14] and (2) about 40% in refi nery,
chemicals, and petrochemical sectors. If nonconvenentional resources, such as wind, solar, or
nuclear power for hydrogen production were available, the use of hydrogen for hydrocarbon
synfuel production could expand by 5- to 10-fold [4]. It is estimated that 37.7 million t per year of
hydrogen would be suffi cient to convert enough domestic coal to liquid fuels to end U.S.
dependence on foreign oil imports, and less than half this fi gure to end dependence on Middle
East oil. Figure 1.1 shows various application areas of hydrogen energy, out of which the use of
hydrogen energy for vehicular application is of current focus [26]. 1.2 Physical Properties
Hydrogen atom is the lightest element, with its most common isotope consisting of only one
proton and one electron. Hydrogen atoms readily form H2 molecules, which are smaller in size
when compared to most other molecules. The molecular form, simply referred to as hydrogen is
colorless, odorless, and tasteless and is about 14 times lighter than air, and diffuses faster than
any other gas. On cooling, hydrogen condenses to liquid at −253°C and to solid at −259°C. The
physical properties of hydrogen are summarized in Table 1.2. Ordinary hydrogen has a density of
0.09 kg/m3 . Hence, it is the lightest substance known with a buoyancy in air of 1.2 kg/m3 . Solid
metallic hydrogen has a greater electrical conductivity than any other solid elements. Also, the
gaseous hydrogen has one of the highest heat capacity (14.4 kJ/kg K). CRC_4575x_CH001.indd 5
5/29/2008 10:11:29 AM 6 Hydrogen Fuel: Production, Transport, and Storage TABLE 1.2 Properties
of Hydrogen Property Value Molecular weight 2.01594 Density of gas at 0°C and 1 atm. 0.08987
kg/m3 Density of solid at −259°C 858 kg/m3 Density of liquid at −253°C 708 kg/m3 Melting
temperature −259°C Boiling temperature at 1 atm. −253°C Critical temperature −240°C Critical
pressure 12.8 atm. Critical density 31.2 kg/m3 Heat of fusion at −259°C 58 kJ/kg Heat of
vaporization at −253°C 447 kJ/kg Thermal conductivity at 25°C 0.019 kJ/(ms°C ) Viscosity at 25°C
0.00892 centipoise Heat capacity (Cp) of gas at 25°C 14.3 kJ/(kg°C) Heat capacity (Cp) of liquid at
−256°C 8.1 kJ/(kg°C) Heat capacity (Cp) of solid at −259.8°C 2.63 kJ/(kg°C) Source: Adapted from
Kirk-Othmer Encyclopedia of Chemical Technology. Fundamentals and Use of Hydrogen as a Fuel.
3rd ed., Vol. 4, Wiley, New York, 1992, 631p. FIGURE 1.1 Application areas for hydrogen energy.
(Reproduced with permission from Elsevier; Midilli, A., Dincer, I., and Rosen, M.A., Renewable
Sustainable Energy Rev., 9(3), 255–271, 2005.) Fuel cells Gas turbines Hydrogen plants
Applications for power generation Hydrogen energy Vehicle applications Fuel cells Internal
combustion engines Combustion Efficiency improvement Defense industry Transport Domestic
applications Industrial applications Navigation applications Space applications Heating Cooking Air
conditioning Pumping Power generation Ship engines Defense Communication Transportation
Tourism Pollution control Energy storage Gas turbines Jet engines Defense industry Rockets
Antimissile Space industry Energy storage Ammonia synthesis Fertilizer production Petroleum
refineries Metallurgical applications Energy storage Flammable mixtures Electronic industry Glass
and fiber production Nuclear reactors Power generation systems CRC_4575x_CH001.indd 6
5/29/2008 10:11:29 AM Fundamentals and Use of Hydrogen as a Fuel 7 The hydrogen atom (H)
consists of a nucleus of unit positive charge and a single electron. It has an atomic number of 1
and an atomic weight of 1.00797. This element is a major constituent of water and all organic
matters, and is widely distributed not only on the earth but also throughout the Universe. There
are three isotopes of hydrogen: (1) protium—mass 1, makes up 99.98% of the natural element; (2)
deuterium—mass 2, makes up about 0.02%; and (3) tritium—mass 3, occurs in extremely small
amounts in nature, but may be produced artifi cially by various nuclear reactions. The ionization
potential of hydrogen atom is 13.54 V [7]. Hydrogen is a mixture of ortho- and para-hydrogen in
equilibrium, distinguished by the relative rotation of the nuclear spin of the individual atoms in the
molecule. Molecules with spins in the same direction (parallel) are termed ortho-hydrogen and
those in the opposite direction as para-hydrogen. These two molecular forms have slightly
different physical properties but have equivalent chemical properties. At an ambient temperature,
the normal hydrogen contains 75% ortho-hydrogen and 25% para-hydrogen. The ortho-to-para
conversion is associated with the release of heat. For example, at 20 K, a heat of 703 kJ/kg is
released for ortho-to-para conversion. The conversion is slow but occurs at a fi nite rate (taking
several days to complete) and continues even in the solid state. Catalysts can be used to
accelerate the conversion for the production of liquid hydrogen, which is more than 95% para-
hydrogen. The vapor pressure of liquid normal hydrogen is given by P (Pa) = 10 [−_______ 44.9569
T (K) +6.79177+0.0205377 (K)] Hydrogen has a low solubility in solvents; for example, at ambient
conditions, only 0.018 and 0.078 mL of gaseous H2 dissolves into each milliliter of water and
ethanol, respectively. However, the solubility is much more pronounced in metals. Palladium is
particularly notable in this respect, which dissolves about 1000 times its volume of the gas. The
adsorption of hydrogen in steel may cause “hydrogen embrittlement,” which sometimes leads to
the failure of chemical processing equipment [4].