Abstract

Climate change and fossil fuel depletion are the main reasons leading to hydrogen
technology. There are many processes for hydrogen production from both conventional
and alternative energy resources such as natural gas, coal, nuclear, biomass, solar and
wind. In this work, a comparative overview of the major hydrogen production methods
is carried out. The process descriptions along with the technical and economic aspects of
14 different production methods are discussed. An overall comparison is carried out,
and the results regarding both the conventional and renewable methods are presented.
The thermochemical pyrolysis and gasification are economically viable approaches
providing the highest potential to become competitive on a large scale in the near future
while conventional methods retain their dominant role in H2 production with costs in
the range of 1.34–2.27 $/kg. Biological methods appear to be a promising pathway but
further research studies are needed to improve their production rates, while the low
conversion efficiencies in combination with the high investment costs are the key
restrictions for water-splitting technologies to compete with conventional methods.
However, further development of these technologies along with significant innovations
concerning H2 storage, transportation and utilization, implies the decrease of the
national dependence on fossil fuel imports and green hydrogen will dominate over the
traditional energy resources.

Introduction

The introduction of greenhouse gases (GHG) into the atmosphere due to the continuous
burning of fossil fuels, pose a serious threat to the global environment and consequent
climate change [1]. In addition, the growing energy demand has imposed the increase of
conventional fuel prices which are declining, exposing national economies which are
dependent on their import. For the long-term treatment to climate change along with
the reduction of the dependence on oil imports, future energy sources must meet the
requirements of being carbon-free and renewable [2], [3], [4], [5].

The expansion of the amount of renewable sources in the supply system is restricted by
their intermittent and unpredictable nature. The increase in the contribution of
renewable energy sources (RES), with simultaneous adaptation of production to
demand, would not be feasible without the use of energy storage systems [6], [7], [8].
The major challenge for a storage device is to maintain the energy stored as long as
needed and, when required, to be able to supply it as soon as possible. For this purpose,
several studies in their effort to provide a clean and reliable alternative to traditional
fossil fuels, which enjoy this particular feature, were led to hydrogen technology.

Unlike fossil fuels, hydrogen is not readily available in nature. It can be however
produced from any primary energy source and to be then used as the fuel either for
direct combustion in an internal combustion engine or in a fuel cell, only producing
water as a byproduct [9], [10], [11], [12]. As the only carbon-free and possessing the
highest energy content compared to any known fuel (Table 1), hydrogen is globally
accepted as an environmentally benign secondary form of renewable energy, alternative
to fossil fuels [13], [14], [15]. A further advantage is that, supported by appropriate
storage technologies, hydrogen can be utilized for domestic consumption as it can be
safely transported through conventional means [16], [17], [18], [19], and in order to be
fed to stationary fuel cells, it can be stored as compressed gas, cryogenic liquid or solid
hydride [20], [21], [22]. Currently the annual production of hydrogen is about 0.1 GT
which is mainly consumed on-site, in refining and treating metals [23], [24]. A small
fraction is already used to fuel driving cars while in the near future applications
including power generation and heating in residential and industrial sectors are
expected [23], [25], [26].
The major problem in utilization of hydrogen gas as a fuel is its unavailability in nature
and the need for inexpensive production methods [27]. A wide variety of processes are
available for H2 production which according to the raw materials used could be divided
into two major categories namely, conventional and renewable technologies. The first
category processes fossil fuels and includes the methods of hydrocarbon reforming and
pyrolysis. In hydrocarbon reforming process, the participating chemical techniques are
steam reforming, partial oxidation and autothermal steam reforming.

The second category accommodates the methods which produce hydrogen from
renewable resources, either from biomass or water. Utilizing biomass as a feedstock,
these methods can be subdivided into two general subcategories namely,
thermochemical and biological processes. Thermochemical technology mainly involves
pyrolysis, gasification, combustion and liquefaction, whereas the major biological
processes are direct and indirect bio-photolysis, dark fermentation, photo-fermentation
and sequential dark and photo-fermentation. The second class of renewable
technologies regards the methods, which can produce H2 through water-splitting
processes such as electrolysis, thermolysis and photo-electrolysis, utilizing water as the
only material input. The various pathways for hydrogen production are shown in Fig. 1.

Based on the extensive literature review, there has not yet been a comprehensive
discussion, assessment and comparison of the operating principles along with the cost
components relating to both H2 production, storage, transportation and utilization. In
this work a comparative overview of the major hydrogen production methods is carried
out. The operating principles together with the technical features of the systems that
comprise each technology are analyzed. Also, the raw materials used and the energy
requirements relating to each method are reviewed. Finally, the associated production
costs are provided and a qualitative comparison between the various production
processes is undertaken, in order to evaluate the feasibility of such systems and their
future contribution in the development of sustainable hydrogen economy.

In Section 2 the methods which produce hydrogen from fossil fuels are presented and
discussed in detail, whereas Section 3 deals with renewable technologies. In Section 4,
an overall comparison of the technical and economic aspects relating to each method is
carried out and the issues concerning hydrogen storage, transportation and utilization
are mentioned in Section 5. The conclusions are summarized in Section 6.

Research paper on hydrogen production !