The Azimuth Project
Carbon capture and storage (Rev #8)

Contents

The idea

Carbon capture and storage, also known as Carbon capture and sequestration or CCS, describes a variety of ways to reduce the carbon dioxide emitted by burning fossil fuels. Most commonly it refers to methods based on capturing carbon dioxide from large point sources such as fossil fuel power plants and storing it so that it does not enter the atmosphere. It also refers to ways of removing CO2 directly from ambient air, for example by biological methods such as biochar.

In the following paper:

  • D’Alessandro et al, Carbon dioxide capture: prospects for new materials, Angew. Chemie Int. Ed. 49 (2010), 2–27.
the authors summarize two of the key issues as follows:

Two points must first be made with respect to capture materials and potential capture technologies, given the sheer magnitude of global CO2 emissions. First, any chemical employed to capture CO2 will rapidly exhaust its global supplies if it is used in a once-through manner; and second, any chemical produced from CO2 as a reactant will rapidly saturate global markets for that chemical. These considerations underscore the necessity that capture materials must be regenerable. In this case the energy input for regeneration is one of the key factors in determining the efficiency and cost.

List of methods

We need a listing of different methods of CCS. What follows is a start. Then: how they work, how well they can be scaled up, and some scientific and engineering challenges that need to be addressed for each one!

Direct capture from the air

In capturing CO2 from point sources, the dominant cost is supposedly the cost of capture rather than storage. However, David Keith has argued that direct capture from the air could be viable:

Enhanced weathering

One of the main long-term mechanisms that removes carbon dioxide from the ocean and atmosphere is the natural weathering of rocks. We can vastly accelerate this process by digging up suitable rocks, crushing them into powder and spreading them around. The rock dust then ‘weathers’ by reacting with carbon dioxide. This method of carbon capture and storage is called enhanced weathering. In principle it can also serve as a source of carbon negative energy.

For more details, see Enhanced weathering.

Coal bed methane extraction

Disused and uneconomic coal mines continue to emit methane. They can also be used to sequester carbon dioxide. Coal has a stronger affinity for CO2 than methane. So, putting CO2 in mine shafts displaces the methane, which can be captured and used as it rises to the surface. This idea is called coal bed methane extraction. It may constitute a source of carbon negative energy.

See:

Methane hydrate reactions

Another idea, due perhaps to ‘Uncle Al’ of internet fame, is to pump liquid CO2 into methane hydrate formations. He claims that the recovered natural gas plus sequestered CO2 has net zero carbon footprint. The reactions involved are

CH4 + nnH2O(s) \to CH4(g) + nnH2O(l) + 54.44 kJ/mol gas

CO2 + nnH2O(s) \to CO2(g) + nnH2O(l) + 63.6 kJ/mol gas

For more on the chemistry of methane hydrates (also known as methane clathrates), see

References

For starters, try: