Another option: Carbon Capture & Storage

What does it entail?

Carbon Capture & Storage (CCS) is defined by the Intergovernmental Panel on Climate Change's (IPCC) Special Report on Carbon Dioxide Capture and Storage as a “process consisting of the separation of CO₂ from industrial and energy-related sources, transport to a storage location and long-term isolation from the atmosphere”. Energy policy experts John Gibbins & Hannah Chalmers offer insights into the three types of carbon separation: post-combustion, pre-combustion and oxyfuel combustion.

Post-combustion capture aims to remove CO₂ just before emission into the atmosphere. An aqueous amine solvent is used to remove the CO₂ from the waste gas at a temperature of approximately 50ᵒC. This solvent can then be re-created for later use by heating it to approximately 120ᵒC before further cooling and recycling. The captured CO₂ is dehydrated and compressed before being transferred to a safe geological storage facility.

Pre-combustion capture involves reforming fossil-fuels with sub-stoichiometric volumes of oxygen at high pressures of approximately 30-70 atmospheres creating a synthesis gas which consists of CO and H₂. Steam is then added before the temperature is reduced to allow for the conversion of CO to CO₂. The CO₂ can then be captured, leaving a Hydrogen-rich gas, as through the use of a physical solvent the CO₂ can be dissolved at higher pressure before being released as the pressure in the system is reduced. Following this, the gas dehydrated and compressed for transporting to a storage site. This method does not require heat to separate and capture the CO₂, and therefore has an advantage over the post-combustion method as it requires less energy for the process to function.

Yukun Hu offers important information on the final process, oxyfuel separation. This method begins by adapting the combustion chamber so that it is filled with almost-pure oxygen using an air separation unit. The fossil fuel is then burnt within the oxygen and a mixture of recycled flue gases which are used as a replacement for Nitrogen (which the carbon would usually burn with in the atmosphere). This produces CO₂ and water vapour which is cleaned and separated during the dehydration and compression processes. Key advantages to this process compared with the other carbon capture methods include the lowest level of CO₂ emissions for all three processes, lower fuel consumptions as well as having the highest combustion efficiency.

Diagram illustrating the processes that take place during the different types of carbon captures.

Transportation:

Unlike the capture methods, there is a fairly unanimous opinion on the best ways to transport captured CO₂. As explained in the IPCC's report, if the gas is only being transported relatively short distances, ships or large vehicles are preferred. However if there is a large transporting distance to the storage site, e.g. thousands of kilometres, then pipelines would be the favoured option. This method of carbon transport is currently in use in nations such as the USA, who have pipelines transporting 40Mt CO₂ over 2,500km every year.

Storage:

Cross-section of a carbon capture storage site, including a
 water-filled reservoir (e.g. Sandstone), cap-rock (e.g. Basalt)
and Oil and Gas fields.
http://www.glossary.oilfield.slb.com/Terms/c/cap_rock.aspx

Favourable geological sites for carbon storage include both onshore and offshore oil and gas fields, Basalt formations, unamenable coal seams and deep saline aquifers. A key requirement for a storage site is depth. To create an optimum density (500kg/m³) for storage, a depth of 1km is needed. Furthermore, a cap-rock (an impenetrable, hard rock which prevents the escape of materials such as oil and gases from escaping to the surface)  would also be required to ensure that the CO₂ can actually be stored underground. 

Although the scientific methods and reasoning behind CCS appear to be reasonable on a practicality level, there remains many further advantageous and problematic aspects to this option will be discussed in the coming blog posts.