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Carbon capture and storage (Rev #5, changes)

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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 from ambient air, either by geoengineering, or biological methods such as biochar.

We need a listing of different methods of CCS. 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!

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!

Enhanced weathering of rocks

In Without the hot air, David MacKay writes:

Is there a sneaky way to avoid the significant energy cost of the chemical approach to carbon-sucking? Here is an interesting idea: pulverize rocks that are capable of absorbing CO2, and leave them in the open air. This idea can be pitched as the acceleration of a natural geological process. Let me explain.

Two flows of carbon that I omitted from figure 31.3 are the flow of carbon from rocks into oceans, associated with the natural weathering of rocks, and the natural precipitation of carbon into marine sediments, which eventually turn back into rocks. These flows are relatively small, involving about 0.2GtC per year (0.7GtCO2 per year). So they are dwarfed by current human carbon emissions, which are about 40 times bigger. But the suggestion of enhanced-weathering advocates is that we could fix climate change by speeding up the rate at which rocks are broken down and absorb CO2. The appropriate rocks to break down include olivines or magnesium silicate minerals, which are widespread. The idea would be to find mines in places surrounded by many square kilometres of land on which crushed rocks could be spread, or perhaps to spread the crushed rocks directly on the oceans. Either way, the rocks would absorb CO2 and turn into carbonates and the resulting carbonates would end up being washed into the oceans. To pulverize the rocks into appropriately small grains for the reaction with CO2 to take place requires only 0.04 kWh per kg of sucked CO2. Hang on, isn’t that smaller than the 0.20 kWh per kg required by the laws of physics? Yes, but nothing is wrong: the rocks themselves are the sources of the missing energy. Silicates have higher energy than carbonates, so the rocks pay the energy cost of sucking the CO2 from thin air.

I like the small energy cost of this scheme but the difficult question is, who would like to volunteer to cover their country with pulverized rock?

For more details, read about Olivine weathering and Enhanced weathering of other minerals in the page on Carbon negative energy.

Methane hydrate reactions

Another idea is to pump liquid CO2 into methane hydrate formations. It is claimed 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, see:

hydrate

hydrate loci, pressure vs. temperature

hydrates

References

For starters, try:

There is undoubtedly a vast literature, so this paper is the tip of the iceberg:

  • D’Alessandro et al, Carbon dioxide capture: prospects for new materials, Angew. Chemie Int. Ed. 49 (2010), 2–27.
To quote:

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.

Here’s a quote from Without the hot air

Enhanced weathering of rocks

Is there a sneaky way to avoid the significant energy cost of the chemical approach to carbon-sucking? Here is an interesting idea: pulverize rocks that are capable of absorbing CO2, and leave them in the open air. This idea can be pitched as the acceleration of a natural geological process. Let me explain.

Two flows of carbon that I omitted from figure 31.3 are the flow of carbon from rocks into oceans, associated with the natural weathering of rocks, and the natural precipitation of carbon into marine sediments, which eventually turn back into rocks. These flows are relatively small, involving about 0.2GtC per year (0.7GtCO2 per year). So they are dwarfed by current human carbon emissions, which are about 40 times bigger. But the suggestion of enhanced-weathering advocates is that we could fix climate change by speeding up the rate at which rocks are broken down and absorb CO2. The appropriate rocks to break down include olivines or magnesium silicate minerals, which are widespread. The idea would be to find mines in places surrounded by many square kilometres of land on which crushed rocks could be spread, or perhaps to spread the crushed rocks directly on the oceans. Either way, the rocks would absorb CO2 and turn into carbonates and the resulting carbonates would end up being washed into the oceans. To pulverized the rocks into appropriately small grains for the reaction with CO2 to take place requires only 0.04 kWh per kg of sucked CO2. Hang on, isn’t that smaller than the 0.20 kWh per kg required by the laws of physics? Yes, but nothing is wrong: the rocks themselves are the sources of the missing energy. Silicates have higher energy than carbonates, so the rocks pay the energy cost of sucking the CO2 from thin air.

I like the small energy cost of this scheme but the difficult question is, who would like to volunteer to cover their country with pulverized rock?

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