1. Introduction
1.1. Energy outlook and salient environmental issues
In 2008 the annual world primary energy consumption was estimated at 11,295 million tonnes of oil equivalent (mtoe) [1]. Fossil fuels accounted for 88% of the primary energy consumption, with oil (35% share), coal (29%) and natural gas (24%) as the major fuels, while nuclear energy and hydroelectricity account for 5% and 6% of the total primary energy consumption, respectively [1]. Given the current technological progress, potential reserves, and increased exploitation of newer unconventional reserves (e.g. for natural gas), it is highly probable that fossil fuels will continue to be available at low cost for a considerable period of time; albeit with the variations in the security of supply arising from geo-political developments, from time to time [2] and [3]. Unfortunately, the potential threat of global climate change has increased, and for a major part, this has been attributed to greenhouse gas emissions from fossil fuel usage [4]. The associated climatic change projections could have major consequences for nature as well as human systems [5], which creates uncertainty regarding the sustainability of current fossil fuel use, not only in relation to the finiteness of the resource, but also on the negative effects of CO2 emissions.
Fossil fuels are the largest contributor of greenhouse gases (GHGs) to the biosphere, and in 2006 associated CO2 emissions were 29 Gtonnes [6]. It is estimated that natural processes remove only about 12 Gtonnes, therefore, compatible mitigation strategies are required to neutralise the excess CO2[7]. With the increase in anthropogenic GHG emissions, mainly due to large scale use of fossil fuels for transport, electricity and thermal energy generation, it has become increasingly important to develop abatement techniques and adopt policies to minimise impacts of global warming. The Kyoto Protocol of 1997 called for a 5.2% reduction in GHG emissions worldwide from 1990 values [8]. To meet the agreed target, a selection of a range of effective technologies, including chemical and biological CO2 mitigation possibilities, has been a focus of research.
The overall implication is therefore a need for enhancement of global strategies for energy security and mitigation of CO2-energy related emissions, for which the salient strategies include, inter alia, the need for: increased energy efficiency (i.e. decreasing energy use per unit of product, process or service); increased use of clean fossil energy (i.e. use of fossil fuels coupled with CO2 separation from flue gases and injection into underground reservoir for gradual release), and; increased use of renewable energy (i.e. development of CO2-neutral energy resources). Given the necessary CO2 emission targets, and the potential of each of the outlined strategies to the timely reduction of CO2 emissions to ‘safe levels’, it has been argued that the three outlined strategies will have to be employed in order to tackle the progression of climatic change [9].