Hydrogen production
Hydrogen production
César A. C. Sequeira*, Diogo M. F. Santos
Chemical and Biological Engineering Department, Instituto Superior Técnico, 1049-001 Lisboa, Portugal
*
ABSTRACT: Possible means of producing hydrogen are discussed. Emphasis is given on the electrolytic hydrogen production from water electrolysis, at large scale, via the use of renewable electricity (solar, wind, tidal, etc.). Its storage, transport and possible end-uses are also considered.
Keywords: Hydrogen production; Electrolyser; Electrolytic hydrogen; Renewable electricity; Fuel cells.
RESUMO: Discutem-se processos de obtenção de hidrogénio. Em particular, considera-se a produção electrolítica de hidrogénio em meio aquoso, à escala industrial, e à custa de energia eléctrica renovável (solar, vento, marés, etc.). Fazem-se ainda algumas previsões acerca do armazenamento, transporte e possíveis aplicações do hidrogénio electrolítico.
Palavras chave: Produção de hidrogénio; Electrolisador; Hidrogénio electrolítico; Energia eléctrica de fontes renováveis; Pilhas de combustível.
1. Introduction
Hydrogen, the most common element on earth, is widely seen as the ultimate form of clean energy [1,2]. The proposition that hydrogen should be a sustainable energy medium has become known as the Hydrogen Economy. This term is thought to have been coined in 1970 by Neil Triner at the General Motors Technical Laboratory in Warren, USA. But the concept of using hydrogen had in fact been suggested much earlier in such diverse publications as Jules Vernes science-fiction novel The Mysterious Island (1874) and J.B.S. Haldanes essay Daedalus, or, Science and the Future (1923) (Fig. 1). It is further notable that Haldane proposed the use of wind power to produce hydrogen via electrolysis of water; the gas would be liquefied and stored in vacuum-jacketed reservoirs that would probably be sunk in the ground. The overall scheme of the Hydrogen Economy is illustrated conceptually in Fig. 2, which outlines the many different possible routes to hydrogen from both conventional and novel primary energy sources, the storage and transportation modes for hydrogen, and its end-uses in fuel cells (Fig 3, Fig. 4), engines, and industrial processes. This is a broad canvas and many authors restricted the use of the term Hydrogen Economy (or Hydrogens Energy) to the production of hydrogen from non-fossil sources, its distribution and storage, and its combustion in a fuel cell to generate electricity.
Fig. 1 - First hydrogen car invented by Francois Isaac de Rivaz in 1807.
Fig. 2 - The Hydrogen Economy: a summary diagram showing possible means of producing, storing and transporting hydrogen, as well as potential end-uses.
Fig. 3 - Use of hydrogen in a modern fuel cell bus.
Fig. 4 - The hydrogen car from Instituto SuperiorTécnico (IST) in the Shell Eco-Marathon competition.
Hydrogen has many potential attractions as a new fuel. It may be derived from non-fossil sources, it burns cleanly to water with no pollutants being emitted, it is suitable for use in a fuel cell to generate electricity directly, and it has a high energy content expressed on a per mass basis (Table 1). Unfortunately, these attractive features are counter-balanced by many practical engineering and economic considerations that explain why hydrogen does not already find extensive use as a fuel.
Table 1. Technical comparison of hydrogen with other fuels.
As discussed in a previous paper [3], hydrogen is produced today from fossil fuels by chemical reforming reactions, and its major uses are in the refining of crude oil and in the manufacture of ammonia. Lesser, non-energy, applications are found in the production of other chemicals, as well as in the food, plastics, metals, electronics, glass, electric power and space industries (Table 2). In contrast, the present use of hydrogen for electricity generation via fuel cells is still negligible. In the short term, however, there will be a significant environmental benefit in converting fossil fuels into hydrogen to serve as a clean fuel for fuel cells or internal-combustion engines. This benefit stems from the relative ease of pollution management at a central production facility compared with dispersed sites. Moreover, emissions of carbon dioxide are, in principle, more easily captured and sequestered at a single plant than when fossil fuels are deployed in the field. A prototype plant for the conversion of coal to hydrogen, with sequestration of carbon dioxide has been announced, in 2003, by the USA government. It is called FutureGen a 10-year research project to build the worlds first coal-fired station to generate electricity with zero rate electricity and hydrogen with zero emissions.
Table 2. Principal non-energy uses of hydrogen
Basically, when heated coal or coke is reacted with steam, the water-gas reaction occurs (Eq. 1).
The gas produced by the water-gas reaction may be upgraded in terms of hydrogen content by the water-gas shift reaction. The gas is reacted with steam over a catalyst that converts carbon monoxide to carbon dioxide and increases the amount of hydrogen (Eq. 2).
The carbon dioxide may then be separated in a form suitable for sequestration (e.g. in geological structures), while the hydrogen is used for power generation in gas and steam turbines, and/or in fuel cells. The equivalent gross electricity output for the USA project (FutureGen) was of 250 MW.
In the present paper, a different future, possibly 20-30 years away, is looked ahead, in which hydrogen is produced by the electrolysis of water on a large scale via the use of renewable electricity (solar, wind, tidal, etc.). It would then be stored in one of several different forms, distributed to where it is needed, and then reconverted to electricity in a fuel cell. The technology for this vision is still in embryonic form, mainly because the economics of such energy production and use are not favourable. Nevertheless, for almost 30 years, much attention has been focused on the considerable challenges that would confront the practical introduction of hydrogen as an energy vector. Many conferences have been held and a specialist journal, the International Journal of Hydrogen Energy, is devoted to the subject.
At first, the interest in hydrogen energy arose from the shortfall in fossil fuels especially oil that was anticipated in the mid-1970s, and from a projected surplus of off-peak electricity from nuclear power stations. At that time, it was envisaged that nuclear power would expand much more rapidly than it has, and that surplus night-time electricity would be available because nuclear plant normally operates on base-load and is not really switched on and off. In parallel, there was a large interest in using hydrogen to store electricity from renewable energy sources. The attractions of using hydrogen as an energy storag (...truncated)
