Written by Kasparas Spokas and Ryan Edwards
The world faces an urgent need to drastically reduce climate-warming CO2 emissions. At the same time, however, reliance on the fossil fuels that produce CO2 emissions appears inevitable for the foreseeable future. One existing technology enables fossil fuel use without emissions: Carbon Capture and Sequestration (CCS). Instead of allowing CO2 emissions to freely enter the atmosphere, CCS captures emissions at the source and disposes of them at a long-term storage site. CCS is what makes “clean coal” – the only low-carbon technology promoted in President Donald Trump’s new Energy Plan – possible. The debate around the role of CCS in our energy future often includes questions such as: why do we need CCS? Can’t we simply replace fossil fuels with renewables? Where can we store CO2? Is storage safe? Is the technology affordable and available?
The global climate-energy problem
The Paris Agreement called the globe to action: limit global warming to 2°C above pre-industrial temperatures. To reach this goal, CO2 and other greenhouse gas emissions need to be reduced by at least 50% in the next 40 years and reach zero later this century (see Figure 1). This is a challenging task, especially since global emissions continue to increase, and existing operating fossil fuel wells and mines contain more than enough carbon to exceed the emissions budget set by the 2°C target.
Fossil fuels are abundant, cheap, and flexible. They currently fuel around 80% of the global energy supply and create 65% of greenhouse gas emissions. While renewable energy production from wind and solar has grown rapidly in recent years, these sources still account for less than 2.1% of global energy supply. Wind and solar also face challenges in replacing fossil fuels, such as cost and intermittency, and cannot replace all fossil fuel-dependent processes. The other major low-carbon energy sources, nuclear and hydropower, face physical, economic, and political constraints that make major expansion unlikely. Thus, we find ourselves in a dilemma: fossil fuels will likely remain integral to our energy supply for the foreseeable future.
CO2 storage and its role in the energy transition
CCS captures CO2 emissions from industrial sources (e.g. electric power plants) and transports them, usually by pipeline, to long-term storage sites. The ideal places for CO2 sequestration are porous rock formations more than half a mile below the surface. (Target rocks are filled with water, but don’t worry, it’s saltwater, not freshwater!) Chosen formations are overlain, or “capped,” by impermeable caprocks that do not allow fluid to flow through them. The caprocks effectively trap buoyant CO2 in the target rocks (see Figure 2).
Scientists estimate that suitable rock formations have the potential to store more than 1,600 billion tonnes of CO2. This amounts to 70 years of storage for current global emissions from capturable sources (which are 50% of all emissions). Large-scale CCS could serve as a “bridge,” buying time for carbon-free energy technologies to develop to the stage where they are economically and technically ready to replace fossil fuels. CCS could even help us increase the amount of intermittent renewable energy by providing a flexible and secure “back-up” with low emissions. Bioenergy combined with CCS (BECCS) can also deliver “negative emissions” that may be needed to stabilize the climate. Furthermore, industrial processes such as steel, cement, and fertilizer production have significant CO2 emissions and few options besides CCS to reduce them.
In short, CCS is a crucial tool for mitigating the worst effects of global warming while minimizing disruption to our existing energy infrastructure and buying time for renewables to improve. Most proposed global pathways to achieve our targets include large-scale CCS, and the United States’ recently released 2050 decarbonization strategy includes CCS as a key component.
While our summary makes CCS seem like an obvious technology to implement, important questions about safety, affordability, and availability remain.
Is CCS Safe?
For CCS to contribute substantially to global emissions reduction, huge amounts of emissions must be stored underground for hundreds to thousands of years. That’s a long time, which means the storage must be very secure. Some worry that CO2 might leak upward through caprock formations and infiltrate aquifers or escape to the atmosphere.
But evidence shows that CO2 can be safely and securely stored underground. For example, the Sleipner project has injected almost 1 million tonnes of CO2 per year under the North Sea for the past 20 years. (For scale, that’s roughly a quarter of the emissions from a large coal power plant.) The oil industry injects even larger amounts of CO2 – approximately 20 million tonnes per year – into various geological formations in the United States without issue in enhanced oil recovery operations to increase oil production. Indeed, the oil and gas deposits we currently exploit demonstrate how buoyant fluids (like CO2) can be securely stored in the subsurface for a very long time.
Still, there are risks and uncertainties. Trial CO2 injections operate at much lower rates than will be needed to meet our climate targets. Higher injection rates require pressure management to prevent the caprock from fracturing and, consequently, the CO2 from leaking. The CO2 injection wells and any nearby oil and gas wells also present possible leakage pathways from the subsurface to the atmosphere (although studies suggest this is likely to be negligible). Leading practices in design and maintenance can minimize well leakage risks.
Subsurface CO2 storage has risks, but experience suggests the risks can be mitigated. So, if CCS has such promise for addressing our climate-energy problem, why has it not been widely implemented?
The current state of CCS
CCS development has lagged, and deployment remains far from the scale required to meet our climate targets. Only a handful of projects have been built over the past decade. Why? High costs and a lack of economic incentives.
Adding CCS to coal and gas-fired electricity generation plants has large costs (approximately doubling the upfront cost of a new plant using current technology). Greenhouse gases are free (or cheap) to emit in most of the world, which means emitters have no reason to make large investments to capture and store their emissions. In order to incentivize industry to invest in CCS, we would need to implement a strong carbon price, which is politically unpopular in many countries. (There are exceptions – Norway’s carbon tax incentivized the Sleipner project.) In the United States, the main existing economic incentive for capturing CO2 is for enhanced oil recovery operations. However, the demand for CO2 from these operations is relatively small, geographically localized, and fluctuates with the oil price.
Inconsistent and insufficient government policies have thwarted significant development of CCS (the prime example being the UK government’s last-minute cancellation of CCS funding). Another challenge will be ownership and liability of injected CO2. Storage must be guaranteed for long timeframes. Government regulations clarifying liability, long-term responsibility for stored CO2, and monitoring and verification measures will be required to satisfy investors.
The future of CCS
The ambitious target of the Paris Agreement will require huge cuts in CO2 emissions in the coming decades. The targets are achievable, but probably not without CCS. Thus, incentives must increase, and costs must decrease, for CCS to be employed on a large scale.
As with most new technologies, CCS costs will decrease as more projects are built. For example, the Petra Nova coal plant retrofit near Houston, a commercial CCS project for enhanced oil recovery that was recently completed on time and on budget, is promising for future success. New technologies also have great potential: a pilot natural gas electricity generation technology promises to capture CO2 emissions at no additional cost. A technology that could capture CO2 from power plant emissions while also generating additional electricity is also in the works.
Despite its current troubles, CCS is an important part of solving our energy and climate problem. The recent United States election has created much uncertainty about future climate policy, but CCS is one technology that could gain support from the new administration. In July 2016, a bipartisan group of senators introduced a bill to support CCS development. If passed, this bill would satisfy Republican goals to support the future of fossil fuel industries while helping the United States achieve its climate goals. Strong and stable supporting policies must be enacted by Congress – and governments around the world – to help CCS play its key role in the climate fight.
Kasparas Spokas is a Ph.D. candidate in the Civil & Environmental Engineering Department at Princeton University studying carbon storage and enhanced oil recovery environments. More broadly, he is interested in studying the challenges of developing low-carbon energy systems from a techno-economic perspective. Follow him on Twitter @KSpokas
Ryan Edwards is a 5th year PhD candidate in Princeton’s Department of Civil & Environmental Engineering. His research focuses on questions related to geological carbon storage, hydraulic fracturing, and shale gas. He is interested in finding technical and policy solutions to the energy-climate problem. Follow him on Twitter @ryanwjedwards.