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Carbon Capture Vol 29
Sequestration and Storage
By R. E. Hester, R.M. Harrison
The Royal Society of ChemistryCopyright © 2010 The Royal Society of Chemistry
All rights reserved.
Comparative Impacts of Fossil Fuels and Alternative Energy Sources
KLAUS S. LACKNER
Growing concerns over the consequences of climate change may severely limit future access to fossil fuels. A forced choice between energy and environment could precipitate a major economic crisis, an environmental crisis, or both. Averting such a crisis will be difficult, because fossil energy resources are an essential part of the world's energy supply and climate change is mainly driven by the build-up of carbon dioxide in the atmosphere. Carbon dioxide (CO2) is the unavoidable product of fossil fuel consumption. Therefore, the use of fossil fuels collides directly with global environmental concerns. Unfortunately, fossil fuels are difficult to replace, but stabilising the atmospheric concentration of carbon dioxide requires a nearly complete transition to a carbon-neutral economy. This implies either the abandonment of fossil fuels or the introduction of carbon capture and storage, whereby for every ton of carbon extracted from the ground another ton of carbon is put back.
This chapter discusses the scope of the required reduction in carbon dioxide emissions and the options available for achieving such reductions. It puts the continued use of fossil fuels, with carbon capture and storage, in context with other approaches toward achieving a carbon-neutral energy infrastructure or otherwise avoiding serious climate change impacts.
The vast scale of energy infrastructures emerges as the central theme. There are very few energy resources that are large enough to cope with modern global energy demand. Any technology that will be able to satisfy these demands will unavoidably interfere with natural dynamic systems. Just like some of the large natural cycles, human energy systems are operating on a global scale. It is the vast scale of human energy demand that shapes the available options.
2 Climate Change
The idea that greenhouse gases in the atmosphere control climate is not new. While travelling with Napoleon through Egypt, Fourier was the first to recognise that the composition of a planetary atmosphere regulates a planet's surface temperature. Some sixty years later, Tyndall measured the absorption spectrum of CO2 in the infrared region. His laboratory measurements showed that carbon dioxide is a powerful greenhouse gas, which is largely responsible for the habitable temperature range on Earth. In 1898, Arrhenius was the first to quantify the greenhouse effect and estimate the impact of anthropogenic emissions of CO2. While extensive research and numerical studies have added much detail to our understanding, his initial ideas remain unchanged. Computer models and observations corroborate the basic insights developed in the nineteenth century.
Fossil fuels provide 81% of the world's commercial energy supply. Consumption of fossil fuels produces nearly 30 Pg (petagram) of carbon dioxide annually. Until now, nearly all of this carbon dioxide has been released to the atmosphere. In the past, the atmospheric sink was considered large enough to accommodate any additional carbon dioxide, but the carbon dioxide content of the atmosphere has now risen by more than a third since the beginning of the industrial revolution, from 280 parts per million by volume (ppm) to 385 ppm today.
Fossil fuel combustion is the single most important contributor to this change. The total carbon dioxide produced in the combustion of fossil fuel since the beginning of the industrial revolution actually exceeds the observed increase in the atmosphere. At present, the carbon dioxide content of the atmosphere is rising by 2 ppm per year, suggesting that more than a third of the fossil carbon dioxide produced does not stay in the atmosphere.
The rapid increase in the atmospheric concentration of carbon dioxide has raised the spectre of severe climate change, and much effort has gone into understanding the likely scale and the implications of global warming. Today it is generally accepted that doubling of the carbon dioxide in the atmosphere would create serious harm and an often-cited goal for stabilising carbon dioxide in the atmosphere is 450 ppm, which at current rates of increase would be breached in about 30 years.
Carbon dioxide is an important greenhouse gas and the most obvious impact of CO2 release is global warming. However, CO2 is also physiologically active in plants and animals, it is of great importance to ecological systems and it is an acid that critically affects the chemistry of ocean water.
While the focus of the climate scientist is on the impact of CO2 on global warming, an important focus for the engineer developing a sustainable energy infrastructure is to eliminate the environmental impacts that arise from the release of carbon dioxide to the atmosphere. Even more broadly, the energy engineer has to consider the environmental consequences of generating power. In this context, it is the unintentional mobilisation of large quantities of carbon that needs to be eliminated. With a fossil energy infrastructure, the production of large quantities of oxidised carbon is unavoidable; their release into the atmosphere can and must be avoided.
The climate scientist will lump CO2 together with other greenhouse gases; the engineer of a sustainable energy infrastructure must find ways of stopping CO2 emissions. This will eliminate the climate change impact of carbon dioxide, as well as other impacts of excess carbon. The control or elimination of other greenhouse gases may also be necessary for stabilising climate. However, the control of these other greenhouse gases raises rather different issues and may occur outside of the energy sector. Thus, their management should be considered separately.
Unlike other emissions, carbon dioxide is not a problem at the point of emission. Carbon dioxide rarely reaches concentrations that constitute a local hazard. The ambient background level of CO2 is so high that mixing of CO2-rich plumes with the atmosphere reduces excess concentrations to a small fraction of the background already in the vicinity of the source. Carbon dioxide differs from other power-plant emissions like sulfur dioxide (SO2), because it is not the local impact of CO2 emissions, but the impacts arising from the accumulation of CO2 in the environment that need to be controlled. In the past, when the local impact of other sour gases was recognised as a serious hazard, dilution of CO2 still provided an adequate solution. Today, the CO2 emissions from power plants have become so large that their impact on the entire mobile carbon pool can no longer be ignored.
Conceptually it is useful to consider the various carbon pools on earth and separate them into stable pools that are isolated from other pools, and mobile pools that interact rapidly. Carbon is either tied up in permanent and stable carbon pools, like carbonate rocks or coal seams deep underground, or it is part of the mobile carbon pools on the surface of the Earth. The stable pools are much larger than the mobile pools. The mobile carbon pools consist of the atmosphere, the biosphere carbon and the ocean. These three reservoirs are in rapid exchange with each other, but are essentially decoupled from the other carbon pools.
Before the industrial revolution, the atmosphere contributed less than 600 Pg (i.e. 600 x 1015g) to this pool, today it is 800 Pg. The biomass contribution is also around 600 Pg. Soil carbon provides another 1500 Pg. The ocean contains about 39 000 Pg of dissolved inorganic carbon, which is part of the mobile pool, but cannot easily be changed. The ocean carbon pool may be mobile in the sense that any carbon atom can enter or leave, but it is persistent in the sense that it cannot be increased or decreased by large amounts. The amounts that could be added to the ocean by, for example, doubling the partial pressure of CO2 over the ocean are between 1000 and 1400 Pg. Thus, the total flexibility in the mobile surface carbon pool is several thousand petagrams (Pg).
Fossil fuel consumption adds to the mobile carbon pool. Fossil carbon which is taken from stable carbon pools is oxidised and released to mobile pools, particularly to the atmosphere. Past fossil fuel consumption has already added 350 Pg. This is a substantial amount. The exchange between the mobile carbon pool and the naturally sequestered permanent pool is very small, involving a small fraction of a petagram of carbon per year. As a result, human influences completely dominate the change in size of the mobile pool, even if the transfer rates between the various parts (e.g. between the biomass pool and the atmospheric pool) are far larger than the annual human input to the pool. It will take several tens of thousands of years before the total mobile carbon pool will re-establish its equilibrium with the permanent carbon pools.
This deviation from equilibrium matters beyond just climate change. For example, excess carbon leads to the acidification of the ocean. It has been shown that such a modification of the ocean chemistry stunts coral growth. Excess CO2 in the atmosphere also leads to the eutrophication of terrestrial and oceanic ecosystems. While environmental concern over climate change may be the leading reason for managing anthropogenic carbon, climate change is only one concern of many.
At the heart of the problem is the introduction of excess carbon into the mobile carbon pool. Any human infrastructure which ignores the continued build-up of excess carbon in the mobile carbon pool cannot be sustained. Technologies which purport to stop global warming, while allowing the rise in the mobile carbon pool to continue, are at best emergency measures to bridge a gap, but they are guaranteed to fail over time. Albedo engineering, for example by adding sulfates to the stratosphere, can fix one symptom but it does not address the underlying problem.
Practical solutions will need to stop or even reverse the build-up of CO2 in the environment. The build-up of carbon must be stopped not just in the atmosphere, but also in the surface ocean and throughout the entire mobile carbon pool. This means stopping the mobilisation of additional carbon, or compensating for the mobilisation of carbon by demobilising an equal amount.
3 The Urgent Need for Energy
Energy is central to economic growth. Without access to adequate energy supplies, a world population of six to ten billion people would not be possible. Empirically, economic growth and energy consumption are closely linked, even allowing for the fact that the energy efficiency of most industrial and commercial processes can be improved, and indeed is improving. The dependence of a modern society on metals and synthetic materials, on transportation and information processing, makes access to energy paramount. Every sector of the economy requires energy and even the most basic needs of humanity could not be supplied without access to plentiful energy. Energy is necessary in the production of food and in the provision of clean water.
If the environmental constraints on fossil energy resources cannot be overcome, the resulting serious shortfall in energy would very likely precipitate a crisis of unprecedented proportion. Even without the added concerns of climate change, the world's energy systems are in a precarious state. Rapid economic growth is constantly pushing the existing infrastructure to its limits. It would be extremely difficult to provide sufficient energy for rapid world economic growth while at the same time phasing out fossil energy for environmental reasons.
Energy demand, which had been outstripping supplies in the last few years, led to enormous price increases, even though the bottleneck was only a few percent of the total supply. This shows how little flexibility there is in the energy supply sector and how difficult it is to increase the world's energy supply. Even the recent sudden drop in demand only makes the point how inelastic the world's energy supplies are, but this time with the opposite sign. If these relatively small variations in energy supply and demand can have such a dramatic impact, consider what might happen if over a few decades 80% of the entire energy base became off-limits, and if the most cost-effective source of energy could no longer be deployed in the construction of a new energy infrastructure.
While it may be necessary to learn how to manage with much less energy, if the development of a sustainable energy supply fails, the highest priority should be given to developing energy solutions that can provide plentiful energy for everyone in the decades to come. Supporting a growing world population and their demand is paramount to political stability and the eventual stabilisation of the world population. Unfortunately, it is very difficult to achieve this goal while phasing out fossil fuels.
Higher living standards and increased energy consumption are intrinsically related. Fighting poverty worldwide will require a means of raising the world's living standards to levels the developed nations take for granted. This will involve the introduction of a basic energy infrastructure and consumption patterns that are not very different from those found today in developed countries, where these infrastructures have been built over the last hundred years.
It has been suggested that developing nations might stop at a level of about 2kW of primary energy per person. At this level, basic human needs are satisfied and consumption would still be only a fraction of that in Europe or in the United States. However, it seems unlikely that countries would voluntarily give up their potential for growth, particularly as long as there are other countries that enjoy a much higher standard of living.
Even though one can expect significant improvements in efficiency and a generally reduced energy intensity of the world economy, it is unlikely that developing countries could find a way to leap-frog developed nations and arrive at a far less energy-intensive economy that nevertheless delivers a high standard of living. Developing basic transportation infrastructures, a decent housing stock with the attendant need for heating and cooling, the development of basic food supplies and basic manufactured goods will likely require an energy infrastructure of similar size to that built up in the developed countries.
It may well be possible to reduce some energy consumption by applying more advanced technology, but this is likely to remain the exception rather than the rule. For example, the need to build a wire-based telephone network may well be avoided, but most of the infrastructure will be similar in energy intensity to those known from the developed nations. Furthermore, rapidly growing economies tend to be less efficient in their implementations of technology, because there is a large opportunity cost in squeezing out the last bit of efficiency. Indeed it is generally the case that the energy intensity (i.e. the ratio of energy consumption to GDP) is lower in developed countries than in developing nations.
Much of what will be implemented initially in a developing country are low cost, and hence often less efficient versions of technology than those already deployed elsewhere. Over time, both developing and developed countries will adopt similar technologies and the two types of economies will converge. Catching up with today's developed countries would increase world energy demand by a factor of five to ten.
It is difficult to see why rich countries would refrain from raising their living standards. Economic slowdowns are typically fought at great cost and policy makers have every incentive to keep their local economies on a growth trajectory. Economic growth will bring with it additional energy demands, which are difficult to predict. In the developing countries, one can assume that, at least to some approximation, the development is likely to retrace the steps already taken by the developed nations. However, much of future growth in the developed countries will arise around new and innovative technologies that either do not yet exist, or that are still in their infancy. It is not clear what will be the next technological wave and how much energy it will demand. It is, however, worthwhile to point out that the last technological wave, which was focused on computation and information processing, was exceptional in its low energy intensity. Thus, the past trend of a continued reduction in energy intensity may not be maintained over the next few decades. This would put in disarray all predictions of future energy consumption, as it has been taken for granted that the energy intensity of the world economy should drop at a rate of at least 1 percent per year.
Excerpted from Carbon Capture Vol 29 by R. E. Hester, R.M. Harrison. Copyright © 2010 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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