Since the start of the industrial revolution, the concentration of atmospheric CO₂ has risen from 270 parts per million to 380 parts per million, with the expectation that it will continue to rise for the remainder of this century, leading to global warming and climate change.

The projections of the International Energy Agency (IEA) and the Intergovernmental Panel on Climate Change (IPCC) indicate that the use of fossil fuels will increase rather than decrease this century, with much of the increase in the developing world. Atmospheric CO₂ concentrations could double by the end of the 21st century, to more than 700ppm, unless action is taken.

There is a need to consider both mitigation and adaptation responses. We must encourage greater energy efficiency, switching to low carbon-intensity fuels (including nuclear in many countries) and using more renewable energy. But it appears unlikely that these actions will produce the deep cuts to CO₂ emissions that will be necessary if we are to stabilise atmospheric CO₂ concentrations at a level of say 550ppm by the end of this century.

Options such as radically decreasing world economic growth or denying developing countries the opportunity to have ready access to reliable electricity derived from fossil fuels are unrealistic and in some cases morally indefensible, unless developed countries are prepared to apply those same actions to their economies, which for the most part they are not.

Growing economies such as China and India are likely to meet much of their growing energy needs from their extensive coal deposits. Australia is likely to continue to rely on its abundant coal and natural gas deposits to meet its base load electricity generation requirements. Therefore unless the nuclear option is pursued with greater enthusiasm than is currently the case in most countries, we have to assume growing use of fossil fuels.

The most promising technology for significantly decreasing emissions from large-scale fossil fuel-based stationary sources of CO₂ (coal-fired power stations, cement plants, gas processing facilities, etc) involves separation and capture of the CO₂, compression and then storage of it in geological formations where it cannot leak back into the atmosphere.

CO₂ capture

Capture of CO₂ from natural gas is a low-cost and widely used technology at the present day (Figure 1). There are early opportunities for low-cost CO₂ capture in urea, steel and cement plants, all of which produce high CO₂ emissions. But post-combustion capture of CO2 from flue gases emitted from conventional coal-fired power stations (Figure 1), while technically feasible, is costly and results in additional power requirements.

Methods currently used for CO₂ separation include the use of physical and chemical solvents, particularly monoethanolamine (MEA); various types of membranes; adsorption onto zeolites and other solids and cryogenic separation. However, extensive research is under way and is likely to bring down post-combustion costs very significantly in the future from the $40 to $50 per tonne of CO₂ avoided that currently applies.

For example, burning coal in an oxygen-rich atmosphere produces a CO₂-rich emission stream, thereby cutting CO₂ capture costs. There are significant costs associated with oxygen separation. Nonetheless oxyfuels systems are a promising area of research and development which could significantly decrease capture costs.

Similarly, pre-combustion capture of CO₂ (Figure 1) involving coal- or gas-fired power generation systems such as Integrated Gasification Combined Cycle (IGCC) systems which produce concentrations of CO2 in the emissions, potentially cut capture costs very considerably, but the initial capital costs appear to be high and more development work is required before there will be widespread application of IGCC to electricity generation or hydrogen production.

It is important to remember that 20 to 25 per cent of global emissions are related to transport and obviously if we are to stabilise emissions, then transport-related emissions must be decreased. It is not possible to capture CO₂ from non-stationary sources such as motor vehicles, but it is possible to use electric cars (with the electricity generated from stationary sources) or hydrogen cars (with the hydrogen centrally generated from fossil fuels) accompanied by geosequestration at the centralised energy production facility. This could provide near-zero emission (and non-polluting) vehicular transport in the future.

There are technical and cost impediments to large scale take-up of hydrogen or electric cars, but again vigorous R&D programs aimed at bringing down costs are under way and already technology roadmaps are under development that will help to take us towards the hydrogen economy.

CO₂ Storage

Once the CO₂ is captured from a major stationary source of emissions, it is compressed, usually to a dense supercritical fluid, transported (mainly by pipeline) to a suitable location and then injected into suitable deep rock formations (sediments) at depths of 800 to 1000 metres or greater. The CO₂ will then remain geologically stored in the rocks (geosequestered) for many years – essentially for geological time.

In some instances the CO₂ will remain in a supercritical state; some of it will go into solution in ground water, and over time some will react with components in the bedrock to form mineral carbonates.

Globally there appears to be massive CO₂ storage potential in sedimentary basins. The main storage capacity appears to be in deep saline geological reservoirs, but depleted oil and gas fields and deep coals (accompanied by CO₂ enhanced coal-bed methane) also offer promise (Figure 2). Geosequestration uses safe, proven technology that the oil and gas industry has used for the past 50 years. CO₂ has been injected into deep geological formations since the late 1970s, particularly in North America as part of enhanced oil recovery and acid gas injection projects. There is strong evidence from existing natural gas storage projects and from natural petroleum and CO₂ systems, that stored CO₂ is most unlikely to leak to the surface, provided the geological storage site is chosen carefully.

In other words, CO₂ can be safely stored in the subsurface for thousands of years and longer. Storage costs appear to be on average around $10 or less a tonne of CO₂ avoided depending on the distance between the CO₂ source and the storage site.

In the longer term the aim must be to develop ‘zero emission hubs’ that will bring together CO2-emitting industries so that they can collectively address capture and storage issues in a cost-effective manner that is economically and environmentally sustainable.

There is a need to more fully characterise potential CO₂ storage sites which will require a detailed geological investigation. We must also improve our capacity to monitor and verify the effectiveness of storage in order to convince the public that geosequestration is a sustainable mitigation option.

Additionally, it is essential to have an appropriate regulatory regime and clarity on long-term liability issues in order to ensure public confidence in the technology. But none of these challenges are seen as representing major ‘show-stoppers’ and progressively more developed and developing countries are looking to greenhouse gas technologies, particularly geosequestration, to play a major part in achieving deep cuts in greenhouse gas emissions.