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Several options are available in the short range to limit or reduce greenhouse gas emissions. The efficiency of current processes which consume fossil fuels can be improved to increase the amount of useful energy per unit of CO2 emitted. This can, for example, be achieved by the construction of more efficient electric power plants using improved technologies, by driving cars with higher fuel efficiencies, and by employing appliances that consume less energy. Conservation, perhaps through better insulation of commercial buildings as well as private homes, can also result in tremendous cuts in heating or air-conditioning energy consumption. These measures not only reduce the CO2 emissions and other environmental impact, but they are also economically advantageous as they reduce the amounts of fuel necessary, and therefore the costs. At the same time they can reduce the dependency of energy-importing countries, such as Europe, the United States and Japan, from oil and gas suppliers. Such savings alone, however, can only extend the availability of needed and accessible fossil fuels in the relatively short term.

A switch to fuels that emit less or no CO2 per unit of energy produced will be necessary. Natural gas will continue to play, as long as it is readily available, an important role in emission reduction by partially replacing coal in electricity generation. Non-fossil fuel energy sources will, however, need to play an increasingly important role to provide for our future energy needs. Among them, hydropower is already widely used and well developed for the generation of electricity, but suitable hydropower resources (rivers, waterfalls, etc.) are limited by their nature. Wind, solar and geothermal energy and energy from the combustion of biomass represent an increasing - but still small - fraction of our energy needs. One of the main obstacles to a wider application of these renewable energy sources is their cost, as well as technological limitations. All this makes the use and extension of nuclear fission power, which is a well-established and reliable source of energy that does not emit CO2, inevitable on a massive scale for the future. Of course, nuclear power should be made even safer, and problems of the storage and disposal of radioactive waste must be solved. There is also a need to develop new generations of nuclear reactors, including breeder reactors and eventually controlled fusion.

As long as we use fossil fuels, the CO2 produced during their combustion should be captured from industrial exhausts, particularly flue gases from coal-or gas-fired power plants or cement factories, in order to avoid its emission to the atmosphere. The removal and capture of CO2 from exhausts can be achieved using a variety of techniques, including chemical absorption, adsorption onto solids, and permeation through membranes. Until now, none of these has been applied on the scale of large commercial power-plants, but the system is feasible and has been tested on a limited scale at a few locations. The removed CO2 can then be compressed and, according to present plans, sequestrated underground in geological formations, depleted oil or gas reservoirs, or even at the bottom of the sea [46]. For example, since 1996, Statoil in Norway has been re-injecting CO2 contained in the natural gas produced at its Sleipner platform back into a deep saline aquifer beneath the North Sea [47]. CO2 can also be used in existing oil fields for enhanced oil recovery. But CO2 sequestering provides only a temporary solution, and in the long run the "Methanol Economy" (see Chapters 10 to 14) will allow recovered CO2 to be chemically recycled to methanol used for energy storage, as fuel, or to be converted to synthetic hydrocarbons and their products. This will also provide economic value for the re-use of CO2, thus lowering the cost of CO2 removal.

Among the greenhouse gases other than CO2, methane and nitrous oxide production from agricultural origins (livestock, animal waste, rice culture, nitrogen fertilizer) could be reduced by making adequate changes in farming procedures. Methane eventually also can be separated and recycled. Methane from landfills is already used as a source of energy and for other uses. N2O emissions from industrial productions (e.g., of adipic acid) are being progressively eliminated.

Fluorinated gases are only produced industrially, and their presence in the atmosphere is purely of human origin. Due to their very high global warming potential, these gases should be monitored and controlled rigorously. Emissions can

Figure 7.8 An overview of proposed CO2 sequestration technologies.

be minimized through process changes, improved recovery, recycling or containment, or avoided by the use of alternative compounds and processes.

Two decades ago, with the discovery of a hole in the Earth's ozone layer due to the emission of CFCs, it was clear that we already faced an environmental challenge of global dimension. This problem threatened to have long-term consequences and could only be resolved by a concerted worldwide response. For the first time, international action was taken through the ratification in 1987 of the Montreal Protocol, progressively phasing out the manufacture and use of CFCs, which had been linked to ozone depletion.

Today, scientific evidence for humanity's responsibility for at least part of the observed climate change is becoming clear. However, this problem is infinitely more complex than that of CFCs, which could be relatively easily banned and replaced, necessitating truly major and difficult international action. This was the intention of the Kyoto Protocol, which was ratified by some 140 nations, but not by the United States, Australia, and some others. The Protocol rightly stated the problem, and suggested regulatory limits for greenhouse gas emissions but, besides the trading of carbon quotas, it offered no technical solutions. Today, new technological answers to the problem are essential, and these may be offered by the suggested "Methanol Economy", which is based on the recycling of CO2 to produce new fuels and materials. Moreover, such an approach would provide a renewable, inexhaustible carbon source, whilst mitigating any human-caused global climate change and liberating mankind from its dependence on diminishing fossil fuel reserves.

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