Fuels:There are unconventional fuels that may serve as near term major replacements for conventional mineral oil and natural gas. These include fuels from oil shale and bitumen, liquid fuels from coal, methane from methane hydrates, biofuels and the secondary fuel hydrogen. Here, these fuels will be reviewed as to their presumable stocks and life cycle wastes, emissions and inputs of natural resources.

The unconventional fuels are usually characterized by a relatively poor source-to-burner energy efficiency when compared with current conventional mineral oil and gas. Apart from some varieties of hydrogen and biofuel, their life cycles are characterized by relatively large water inputs, emissions, and wastes.

Fuels

The unconventional fuels shale oil, bituminous oil, coal liquids, and methane from methane hydrates are based on natural resources which are practically finite. This does not hold for biofuels, but the sustainable supply thereof is severely limited when large numbers of people also have to be fed. In view of the problems associated with unconventional fuels, there is a case to consider fuel-less replacements for many fuel applications.

1. Introduction

Current energy inputs by humankind into the world economy are mainly fuels. Fossil fuels, nuclear fuels, and biofuels supply ∼97% of the energy input, whereas power sources generating usable energy without the intermediate use of fuels such as hydroelectric systems, solar collectors and cells, and windmills supply about 3%.Fossil fuels cover about 80% of worldwide energy demand, with conventional mineral oil (including heavy acid oil) and natural gas (including coal bed methane) together supplying ∼60% of that demand .

In the near future, there will probably be a large shift in the position of the major fuels powering the world economy. Actual consumption of conventional natural gas and mineral oil exceeds the rate of their geological formation during the last 400 million years by a factor of probably at least 10⁶ .

Major price rises over the 2004–2008 period suggest that supply has difficulty in keeping up with expanding demand for conventional mineral oil and natural gas. It is moreover expected that production of these fossil fuels will start to fall in the not too distant future, which will necessitate replacement.There is concern about the geopolitical implications of most of the remaining conventional oil and gas reserves being in a limited number of countries .

Also, the consumption of fossil fuels in general is under pressure because of its share in climate change, with coal most objected to because, in supplying energy, coal is (ceteris paribus) associated with much higher CO₂ emissions than conventional mineral oil and natural gas .

Biofuels may be used as long as photosynthesis remains operational, which is for a species such as Homo sapiens an indefinite time. However, there are constraints on its supply, following from the demand for food and the requirement to maintain ecosystem services and photosynthetic productivity.

For terrestrial biofuels, the latter for instance requires maintaining the stocks of freshwater, nutrients, and soil organic carbon. This in turn strongly limits the amount of terrestrial biofuels which can be supplied in a sustainable way in the case that also large numbers of people have to be fed

However, sustainability is also a term that currently has many meanings. Here, the term will be used in the original meaning in the modern environmental debate, linked to a steady state economy . Thus sustainable use of fuels (or energy) may be defined as a type of use that can be continued indefinitely, without jeopardizing the supply of natural resources, which does not lead to the accumulation of pollutants which may negatively affect future generations and maintains available ecosystem services.

Unconventional fossil fuels do not belong in this category of sustainable fuels, as their underlying natural resources have a finite character and their use leads to the accumulation of pollutants which may negatively affect future mankind. The finite character of unconventional fossil fuels clearly emerges when they are energetically compared with solar irradiation of the earth. The total presumable stocks of fossil fuels are, energetically speaking, smaller than 1 month of solar irradiation, and the latter will continue with at least the same intensity for ∼5 × 10⁹ yearsThe stock of methane hydrates may be larger than the stock of fossil fuels (Chatti et al.

but still the stock is finite and its large scale use will lead to the accumulation of carbonaceous gases such as CO₂ (a conversion product of methane) in the atmosphere which will be conducive to climate change affecting future generations. Full removal of emitted CO₂ from the atmosphere has been estimated to take 30,000–35,000 years

There would seem to be more scope for a sustainable physical conversion into energy for economic use of solar irradiation and derivatives thereof such as wind energy. the physical conversion of solar irradiation into electricity is relatively efficient, if compared with photosynthesis. Also, such conversion can be partially integrated in the built environment. Both characteristics are beneficial for maintaining ecosystem services.

The focus of sustainability within the framework of physical energy conversion technologies should be on use of materials which allows for indefinite supply thereof and the prevention of the accumulation of pollutants. Life cycle assessments of current conversion systems suggests that this may well be within the realm of what is possible, if such systems and the components thereof are designed for recyclability and if

Fuels