E&E Exchange

Carbon capture and storage (CCS) (aka carbon capture and sequestration) refers to the relatively new process used to reduce carbon dioxide (CO₂) emissions into the atmosphere. It involves the retrieval of carbon dioxide gas from the exhaust streams of power plants and other sources, and pumping it into the ground for permanent storage.

Here's a look at where in the ground the CO₂ can be stored.

(Credit: Climate Change Dispatch)

CCS is quite a controversial arena, as it is a costly process and is a direct result of the debate on climate change and the perceived need to cut down on greenhouse gas emissions. But new research may mean another wrench in the mix; earthquakes could cause problems for large-scale CCS implementation.

Capture and Storage Costs

The costs associated with CCS are typically very high. For starters, capturing CO₂ is no easy business. Different point-capture methods include post-combustion (via scrubbing), pre-combustion (typically utilizing gasification), and oxy-fuel combustion (burning in pure oxygen). All of these methods have their difficulties and restrictions: power plants using post-combustion capture (the most well-developed) suffer from significant reductions in overall output efficiencies; the pre-combustion method is limited to IGCC (Integrated Gasification Combined Cycle) plants and those few existing facilities which can be cost effectively modified; plants utilizing oxy-fuel "zero emission" cycles have very steep operating costs that limit their effectiveness and feasibility.

After capture, CO₂ is compressed for storage at very high pressures (100-150 atmospheres) as a supercritical fluid. This fluid is then transported to an adequate storage site and pumped into the ground through an injection pipe. The costs of this sequestration can be high as well. One study on the net-energy and GWP (global warming potential) of a coal-fired power plant revealed a 25% reduction in output capacity (600 MW to 457 MW) when utilizing post-combustion carbon capture and storage.

The Earthquake Factor

Research by Stanford University indicates that large-scale CCS could trigger earthquakes which would release the gas otherwise trapped deep underground. The oil and gas industry already uses similar techniques for extracting fuels and disposing wastewater (particularly in hydraulic fracturing). Some have blamed wastewater injection for recent earthquakes in Arkansas, Ohio, and the border of Colorado and New Mexico.

(Map of seismic activity sites. Red dots indicate seismicity induced by the creation of water reservoirs. Credit: Zoback et al. / PNAS)

On a small scale, the researchers say earthquake risks can be mitigated through proper storage site selection. But when considering the large-scale impact that CCS advocates are shooting for, implementation becomes a lot more problematic. "You have to be far more restrictive," explains one researcher, when choosing a repository that will store CO₂ for hundreds to thousands of years.

This seems to goes against the claims of a study from MIT, which estimate that there is enough space in saline aquifers to store the emissions of current US coal plants for over a century. Saline aquifers, formations of water permeable rocks saturated with salt water, are the most promising type of geological storage site for CO₂ sequestration.

How Critical Is CCS?

The need for CCS is only as urgent as the need for the reduction/elimination of carbon dioxide as a greenhouse gas. From my own perspective, the reduction of more immediately harmful gases (VOCs, NOx, SOx, etc.) seems more important, especially since some of these have a much higher GWP than CO₂.

(Credit: The Economist -->)

But if carbon dioxide is a concern, carbon capture and storage is currently the only process that could foreseeably reduce the manmade production of carbon dioxide on a large-scale, save for eliminating/replacing fossil fuel consumption altogether. However, the expense of the process coupled with this potential earthquake factor doesn't put CCS in a very good light.

Source

Technology Review