Carbon Capture and Storage
Introduction
Increasing emissions of greenhouse gas (GHG) by fossil fuels have been recognized as the major contributor to global warming and significant changes in climate. Carbon dioxide (CO2) is one of the primary anthropogenic GHG, and its global emissions have reached a historic high of 32.5 Gt in 2017. Studies by the International Energy Agency (IEA) indicate that the mean concentration of CO2 in the earth’s atmosphere has nearly reached 404 ppm and caused almost 1 °C increase compared with pre-industrial levels. China has overtaken the United States as the world's biggest producer of CO2 since 2006 with 5.95 billion tons of CO2 from fuel combustion. In the last decade, China's energy use has doubled, amounting to 3.13 billion tons oil equivalent in 2017. Large stationary industries contribute the most CO2 emissions as they are the major energy consumers of the nation. To control CO2 emissions, the Chinese government has launched The Thirteenth Five-Year Plan forward to increasing the proportion of non-fossil energy in primary energy consumption to about 15% of the total in 2020, and a peaking target of the national CO2 emissions by 2030 has been also proposed. This intense demand for CO2 mitigation by China and other industrialized countries opens challenges and opportunities as well for the development of the approaches aiming at CO2 emissions reduction.
Working Principle
Carbon capture and storage?(CCS) or?carbon capture and sequestration?is the process of capturing?carbon dioxide?(CO2) before it enters the atmosphere, transporting it, and storing it for centuries or millennia.
Usually, the?CO2?is captured from large?point sources, such as a?chemical plant or biomass power plant, and then stored in an underground?geological formation. The aim is to prevent the release of?CO2?from?heavy industry?with the intent of?mitigating the effects of climate change.?Although?CO2?has been injected into geological formations for several decades for various purposes, including?enhanced oil recovery, the long-term storage of?CO2?is a relatively new concept.?Carbon capture and utilization?(CCU) and CCS are sometimes discussed collectively as carbon capture, utilization, and sequestration (CCUS). This is because CCS is a relatively expensive process yielding a product with an intrinsic low value (i.e.?CO2). Hence, carbon capture makes economically more sense when being combined with a utilization process where the cheap?CO2?can be used to produce high-value chemicals to offset the high costs of capture operations.
The general concept structure of CCS supply chain is presented in Fig. 1. It has three primary components:
Carbon Dioxide Capture
CO2?can be captured from large sources, such as power plants, natural gas processing facilities and some industrial processes. Capture from the open atmosphere is also possible. Where fossil fuels are burnt at power plants, there are three techniques to remove or ‘scrub’ CO2:
Post-combustion
In this process, CO2?is removed after burning fossil fuel. CO2?is captured (‘scrubbed’) from the exhaust (or ‘flue’) gases. This is the method that would be applied to most conventional power plants as it can be retrofitted. The technology is well understood and is currently used in other industrial applications.
Pre-combustion
This technique traps CO2?before burning fossil fuel. First, the fossil fuel is partially burned in a ‘gasifier’ to form synthetic gas. CO2?can be captured from this relatively pure exhaust stream. The process also produces hydrogen, which can be separated and used as fuel. Pre-combustion is used in the production of fertiliser, chemical gas fuel and power production. It is a cheaper option than post-combustion but cannot be retrofitted to older power plants.
Oxyfuel combustion
In oxyfuel combustion, fossil fuel is burned in oxygen instead of air. The resulting flue gas consists of mainly CO2?and water vapour. The water condenses through cooling and the result is almost pure CO2?that can be transported and stored.
Electricity plant processes based on oxyfuel combustion are sometimes referred to as ‘zero emission’ as nearly all the CO2?is captured. It is possible that some CO2?will dissolve in the condensed water, so the water may have to be further treated. Whilst it may be the most effective method of the three, the initial oxygen burning process is energy intensive.
Oxyfuel combustion plant at Schwarze Pumpe, Germany. Lignite and hard coal are combusted in a mixture of oxygen and re-circulated CO2, which also contains water vapour. The flue gas is treated and sulphur oxide particles and other contaminants are removed. Finally, the water is condensed and the concentrated CO2 compressed into liquid.
Hydrogen production
Another way of reducing atmospheric emissions of CO2?in some areas, such as heavy goods vehicles, where battery-based electric power is unsuitable, and trains, and domestic heating and gas cookers, is to convert to hydrogen.
Whilst hydrogen produced by a process called electrolysis of water using renewable power has very low CO2?emissions, it is currently expensive. A cheaper and more mature option to enable the conversion to hydrogen to be started is to produce hydrogen from natural gas. This process is called methane reformation. However, the CO2?produced in the reforming process must also be captured and stored.
Direct air capture
It is possible to capture CO2?directly from the open atmosphere, but this is still being researched.
The estimated energy needed for air capture is only slightly more than for capture from large emission sources. The costs may be higher as well, but may be feasible for dealing with emissions from diffuse sources.
Carbon Dioxide Transport
After capture, CO2?must be transported to suitable storage sites. Pumping it though pipelines is the cheapest form of transport and is a well known and reliable technology.
There are 5800?km of CO2?pipelines in the United States transporting CO2?to oil production fields, where the CO2?is injected to help produce more oil. This process is called?enhanced oil recovery?or EOR.
However, the increased number and carrying capacity of pipelines needed for a large-scale CCS industry will require further studies of pipeline safety, particularly in heavily populated areas or areas of high earthquake activity. Ships and road tankers can also be used to transport CO2?for small scale applications.
Carbon Dioxide Storage
CO2?can be stored in two main ways:
领英推荐
Deep ocean storage will increase ocean acidification, a problem that also stems from the excess of CO2?already in the atmosphere and oceans.
Geological formations are currently considered the most promising storage sites. Areas such as the North Sea and the US Gulf Coast are believed to contain a large amount of geological storage space.
The?Intergovernmental Panel on Climate Change?(IPCC) says that for well-selected, well-designed and well-managed geological storage sites, CO2?could be trapped for millions of years, retaining over 99 per cent of the injected CO2?over 1000 years.
Deep geological formations
Storage in deep geological formations is also known as ‘geo-sequestration’. In this technique, CO2?is converted into a high pressure, liquid-like form known as ‘supercritical CO2’. Supercritical CO2?behaves like a runny liquid, and is injected directly into sedimentary rocks. The rocks may be in old oil fields, gas fields, or in saline formations — rocks with porous spaces filled with salty water. Unmineable coal seams and some volcanic rocks are also suggested storage sites.
Various physical structures prevent CO2?from escaping to the surface. These include impermeable ‘caprocks’ and geochemical trapping mechanisms.
Enhanced oil recovery (EOR)
This process is already understood and has been carried out for many years. In the United States, approximately 30–50?million tonnes of CO2?are injected annually into declining oil fields, to increase oil production. This option is attractive for CO2?storage because costs of injection may be partly offset by the sale of additional oil that is recovered. However, the subsequent burning of the additional oil recovered through EOR will offset much or all of the reduction in CO2?emissions.
‘Unmineable’ coal
‘Unmineable’ coal, which is coal that is too deep or difficult to mine, can be used to store CO2.?The coal absorbs the CO2, provided the coal is permeable enough to allow CO2?to penetrate. During this process, the coal releases previously absorbed methane (CH4), which can then be recovered and used. This is called enhanced coal-bed methane or ECBM.
The sale of the CH4?can offset some of the cost of the CO2?storage. However, as in EOR, burning the resultant CH4?would produce CO2?which would reduce some of the benefit of storing the original CO2.
Saline aquifers
Some deep rock formations contain highly concentrated brine (salty water). This is present in the rock pores and acts like a huge sponge. These are known as ‘saline aquifers’. Their main advantage for CCS is their large storage potential and their abundance. For example, large saline aquifers underlie much of the North Sea, mainland Europe and the Gulf Coast of Texas in the USA.
The main disadvantage of saline aquifers is that relatively little is known about them compared to oil fields. However, current research shows that several trapping mechanisms immobilise the CO2?underground, reducing the risk of leakage. Unlike storage in oil fields or coal beds, there is no useful byproduct to offset the cost of storage.
Mineral storage
In mineral storage, captured CO2?is reacted with naturally occurring iron (Fe), magnesium (Mg) and calcium (Ca) minerals. This is called ‘mineral carbonation’ and occurs naturally in the weathering of rock over time. Such minerals are very abundant and very stable. As a result, the re-release of CO2?into the atmosphere does not happen. However, these carbonation reactions are very slow under normal conditions and to speed it up would need energy to increase temperature and pressure to ideal levels. The?IPCC?estimates that a power plant equipped with CCS using mineral storage will need 60–180 per cent more energy than a power plant without CCS.
What are the costs and risks of CCS??
CO2?storage regulations require that storage operations be rigorously monitored for a number of reasons, including:
Regulatory frameworks governing geological CO2?storage are being developed worldwide. In Europe, an EC Directive says that the issues of leakage and potential long-term stewardship of storage sites must be addressed if the potential for CO2?capture and storage to provide substantial reductions in atmospheric CO2?emissions is to be realised.
Current progress
There have been several CCS pilot projects throughout the world in operation such as Val Verde Natural Gas Plant in the USA, Sleipner project in Norway and Weyburn-Midale project in Canada, and several large commercial-scale CCS projects for power plant are in various stages of planning or construction in the USA, Canada, Australia and China.
How much will CCS cost?
Capturing and compressing CO2?requires a lot of energy and increases the fuel needs of a coal-fired electricity plant by 25–40 per cent. These and other costs are estimated to increase the cost of electricity from a new power plant with CCS by 21–91 per cent. These estimates apply to purpose-built plants near a storage location. However, applying the technology to pre-existing plants or plants far from a storage location will be more expensive. As a result, CCS makes electricity power stations more expensive to build and electricity more expensive to buy. However, the costs of the impacts of climate change will be far higher.
The economic feasibility of CCS on a global scale largely depends on the value and price that governments and people put on environmental and ecosystem viability. If the penalty price for emitting CO2?is high then there is a financial incentive to adopt CCS and it will become economical quickly. If the penalty price remains low, CCS will be slow to develop because there is no incentive. When CCS technology is better developed, its costs may lower. Some people suggest that money spent on CCS will divert investments away from other solutions to climate change.
References
https://www.iea.org/reports/ccus-in-clean-energy-transitions/a-new-era-for-ccus#growing-ccus-momentum
https://www.lse.ac.uk/granthaminstitute/explainers/what-is-carbon-capture-and-storage-and-what-role-can-it-play-in-tackling-climate-change/
https://www.bgs.ac.uk/discovering-geology/climate-change/carbon-capture-and-storage/#deepstorage