It will speed up the transition to a carbon-free energy system
To stop global warming, we have to commit to ambitious objectives to decarbonise the global economy. In fact, there are no affordable decarbonisation technologies yet. To achieve a decarbonised world, energy efficiency and a completely renewable energy mix must be combined with Carbon Capture and Storage Technologies (CCS), using fossil fuels or biomass (Bioenergy with Carbon Capture and Storage, BECCS). Some projections predict that CCS technologies will contribute to the reduction of 14% of the total CO2 emissions.
There are three consecutive stages in carbon capture and storage: (1) separation of CO2 from industry and other energy generation sources from the rest of gases; (2) transportation of CO2 to a storage place and (3) long-term isolation from the atmosphere (centuries or millennia). The most direct way to reduce CO2 is in the power plants directly, as they are focalized CO2 sources and represent the 42% of the global anthropogenic CO2 emissions.
Fumes generated in the combustion of fossil fuels are made up of 4-14% CO2, depending on the fuel used, the rest is mainly N2. If we would like to directly store the whole flow, the transportation costs would be prohibitive, due to the difficulty to move such a large volume of flow and the technical difficulties to comprise a diluted CO2 flow. That is the reason why we need to use technologies to separate CO2 from the rest of the combustion fumes. This is the most expensive stage of the whole process. There are three possible ways to capture CO2 in power plants:
In this case, CO2 is separated from the combustion gases produced during the combustion of primary fuel with air, as shown in Figure 1. This separation can be carried out using different methods: physical or chemical absorption, membranes, cryogenic methods, adsorption, etc. Nowadays, the most developed processes are the chemical absorption methods using amines. The carbon capture process is carried out in absorption units where downflowing amines are sprinkled on upflowing combustion gases. They absorb a huge part of the CO2 in the process. Then, amines are regenerated in regeneration units using steam stripping, producing a concentrated flow of CO2 to be stored.
Figure 1. Carbon capture after combustion
These processes have a high energy penalty as the absorbent must be regenerated to be reused. Moreover, the CO2 separation has to be carried out in a diluted flow —CO2 concentration 4-14%— and at atmospheric pressure, that increases the operation and investment costs. It is estimated that the total energy loss using these technologies ranges from 8 to 16% in thermal power stations and from 5% to 10% in natural gas combined cycle power plants .
This process consists in the removal of carbon before the combustion —pre-combustion—. Figure 2 shows a general diagram of the pre-combustion carbon capture systems. In this process, the gasification or reforming of fossil fuels produce a flow that is rich in CO and H2. This synthesis gas is taken to a reactor where the water gas shift (WGS) transforms the CO into CO2 producing H2 in the process. Finally, CO2 is separated from the gas flow in a similar process to the one used in the post-combustion carbon capture, obtaining an almost pure H2 flow. The great advantage of this process is the generation of H2 as a product, as it can be used in different ways: in the combustion of gas turbines to generate electrical power, in fuel cells for transportation and/or in internal combustion motors.
Figure 2. Pre-combustion carbon capture
Pre-combustion processes have less energy penalties, the separation of CO2-H2 is carried out at medium pressure (20-40 atm), and high CO2 concentration (~40%), so the energy penalty of the separation process is less than in the previous case. However, it must be pointed out that the gasification process uses O2, that must be separated using cryogenic methods. An energy penalty ranging from 4% to 11% is estimated in natural gas reforming and from 7% to 13% in coal gasification .
Combustion without N2 or oxy-fuel combustion
Combustion systems without nitrogen (oxy-fuel combustion) use oxygen instead of air to combust fuel in order to produce a combustion gas made up just of water vapor and CO2. The process produces a flow with high CO2 concentration that is easily separated from water vapor by condensation. Figure 3 shows a diagram of the process.
Figure 3. Carbon capture using oxy-fuel combustion
Furthermore, combustion using O2 would highly increment the temperature of the combustion chamber; so, CO2 must re-flow to reduce the combustion temperature. The disadvantage of this technology is that it uses pure O2, whose production implies an important energy penalty due to the process to separate O2 and air. A performance loss ranging from 6 to 9% is estimated in thermal power plants and from 5 to 12% in natural gas power plants.
Regarding the financial feasibility, the costs of CCS technologies could be acceptable depending on the fuel price and the costs of CO2 emissions. The fuel price is basic to know the future costs per ton of CO2 avoided. For example, for a supercritical pulverized coal combustion power plant improved with CCS systems, costs may vary from $20 to $90 per ton of CO2 avoided. In the case of a natural gas combined-cycle power plant with CCS systems, costs may vary from $60 to $100 per ton of CO2 avoided.
Regarding oxy-fuel combustion technologies, even though the cost per ton of CO2 avoided is similar to post-combustion ones, it is estimated that the electrical cost is slightly lower for a power plant equipped with CCS technologies (63€/MWh versus 67 €/MWh). So, this technology is potentially the cheapest option of the three. However, as it is the youngest technology, it still requires a great research and financial effort to improve it and reduce the present costs. That is the reason why, researchers are looking for new carbon capture processes to reduce their energy penalty and costs.
Among the combustion systems without nitrogen (oxy-fuel combustion), the Chemical Looping Combustion (CLC) has been proposed as a feasible alternative to energy production with CO2 capture. This capture process is cheaper than any other assessed process (post-, pre- or oxy-fuel combustion). In fact, it is estimated that in the CLC process, the cost per ton of CO2 avoided ranges €10-20, against €30-40 in a combined-cycle power plant with integrated gasification or €30-60 in oxy-fuel combustion processes.
The CLC process is based in the transference of oxygen from the air to the fuel using a metal oxide (MexOy) as an oxygen carrier, so fuel never gets in contact with the air. For that, we use two interconnected fluidized bed reactors with the oxygen carrier continuously flowing between them, as shown in Figure 4 . Such a configuration, that is similar to the one of a circulating fluidized bed boiler (CFB), allows a good solid-gas contact and the suitable circulation of the solid carrier of oxygen between both reactors.
In this process, oxides of different metals can be used (Fe, Cu, Mn, etc.) as oxygen carriers. In case we use the Fe2O3/Fe3O4 system as oxygen carrier and CH4 as fuel, the following process takes place. In the fuel reactor, the metal oxide (Fe2O3) is reduced to a metal (Fe3O4) as it reacts with the fuel. When the fuel oxidizes, it just generates CO2 and water vapor, that are easily separated by condensation. So, the remaining CO2 gas flow is ready to be transported and stored. In the air reactor, the reduced oxygen carrier (Fe3O4) is regenerated as it oxidizes to Fe2O3 after it gets in contact with the oxygen in the air, producing a N2 flow together with O2, in the case too much air has been introduced.
Figure 4. Conceptual diagram of CLC process
The energy generated in the combustion is equivalent to the one obtained in conventional combustion. This system has a low energy penalty as CO2 must not be separated from any other gas (except H2O). This is the main advantage of the CLC system against any other carbon capture system (chemical or physical absorption, adsorption systems, etc.).
Different studies have been carried out with different prototypes that back up the feasibility of this process in CLC plants, both with gaseous fuels (CH4, synthesis gas, etc.) in a power ranging from 10 to 140 kWt, and with solid fuels (coal and biomass), reaching 3MWt in some pilot plants. Also using oxides of different metal as oxygen carriers (Ni, Cu, Fe, Co or Mn).
We have no future projections in Spain to quantify the degree of penetration of CCS in next years. However, it is estimated that to meet a 10% of the carbon mitigation that Spain should reach before 2030 (5700Mt of equivalent CO2), it would be necessary a 1000 MWe thermal power plant with carbon capture from year 2020 to 2030. It also would imply to have a place to store 725Mt of CO2 for the same period.
In the long-term, CCS potential can be much greater, depending on the evolution of fuel and carbon prices and in the implementation of CCS technologies applied to biomass and motor fuels. In this case, if we want to meet with CCS a 20% of the carbon mitigation in Spain until 2050, we would need to have a storage capacity of 5000Mt/CO2 between 2020-50. Besides the magnitude of these figures, we must again highlight the technical and financial feasibility of CCS technologies to mitigate climate change. Carbon capture and storage technologies can be key tools to drastic reduce CO2 emissions in Spain using the present technologies.
Carbon Transportation and Storage
Before the captured CO2 is stored, it has to be compressed and transported to the injection point. The properties of the transported CO2 will vary depending on the process and fuel used. These properties, mainly the impurities present, the carbon pressure and temperature, will have a bearing on the materials of the pipeline. The transportation can be carried out using a pipeline, in this case, carbon must be compressed to a pressure of 100-200bar, so its density makes it behave as a liquid, and/or by ship, in this case, it must be liquefied at -30ºC and between 15-20bars of pressure.
Regarding carbon transportation by land using a pipeline, there is large experience thanks to the technology used to extract oil (EOR). USA has the largest carbon transportation grid. They transport 45Mt/year of CO2 through 6300km of pipelines.
Finally, the last step is the storage of the captured CO2. There are several ways to store it, but all of them must fulfil the following criteria:
- Safety: stability in the storage. No leaks. Storage for centuries or millennia
- Low cost, including transportation
- Minimization of the environmental impact and risks
- Abide the present legislation
Two main options are being studied to store CO2: geological storage and ocean storage . The global carbon storage capacity depends on the chosen location. As shown in Table 1, ocean storage has the greater capacity, followed by saline aquifers and empty gas and oil wells. The main problem of the ocean storage is the environmental impact, as carbonates and bicarbonates formed reduce the pH of the area. This is the reason why this option has been pushed into the background. Research efforts must be increased to minimize the possible effects of this kind of storage.
|Option||Capacity (Gt CO2)|
|Ocean||18000 – 7*107|
|Saline Aquifers||1700 – 3700|
|Empty Gas & Oil Wells||675 – 900|
|Non-Exploitable Carbon Pits||3 – 200|
Table 1. Carbon storage capacity in different locations. Present emissions ~27Gt CO2 a year
Now, the most feasible storage option is in saturated saline aquifers located in a great depth and immobilized at 800-900m. The technology necessary for this kind of storage is well-developed and tested in the process Enhance Oil Recovery (EOR), use in improved oil extraction. This technology has been successfully used since the 70s. Now there are a lot of projects in the world in operation using this technology. One of them is the Sleipner platform, it has been the first commercial project dedicated to carbon storage in a deep saline aquifer (800m deep). The Sleipner platform is located 250km North of the Norwegian coast in the Northern Sea. Natural gas with a high CO2 concentration (around 9%) is extracted from it. Since 1996, 1Mt of CO2 a year is being injected in a saline formation of the platform, that is expected to accommodate near 20Mt of CO2 in its lifespan .
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Iñaki Adánez Rubio
PhD in Chemical Engineering at University of Zaragoza. Researcher at the Combustion and Gasification Group of the Carbochemistry Institute (ICB-CSIC) in Zaragoza, where he contributed to the experimental demonstration of the CLOU process (Chemical Looping with Oxygen Uncoupling). His doctoral thesis received the following mentions: Environmental Prize 2015 of the Region of Aragón —Innovation and Research Field—; Prize to the best doctoral thesis about Technologies to Carbon Capture, Transportation and Storage and Uses of CO2 in the 2nd Edition of Prizes PTECO2 2015; Finalist of the 8th Prize for Young Researches of the Spanish Carbon Group 2017. Now, he prepares his post-PhD Juan de la Cierva in the group of Thermochemical Processes of the University of Zaragoza.