Since fossil fuels started to be used as energy sources at the end of the 17th century, the concentration of greenhouse gas emissions in the atmosphere has increased and increased. Since the 70s, anthropogenic emissions of these gases —mainly CO2, CH4, N2O, HFCs and PFCs— have increased over 80%, reaching almost 50Gt of CO2 equivalent in 2017 globally. Just in Spain, over 340 million tons of CO2 were emitted in 2017, 4% more than last year and 18% more than the levels of 30 years ago. CO2 represent 2/3 of the total greenhouse gas emissions and almost 70% come from fossil fuels. Consequently, CO2 concentration in the atmosphere has reached an historical maximum of 413ppm in 2019 (see Figure 1). Effective actions in the energy generation and industrial sector are essential to face the problem known as climate change. At the 21st International Conference on Climate Change (COP21), that took place in Paris in 2015, an historical universal agreement was reached to limit to 2ºC the increase of temperature during the 21st century and to give a boost to mechanisms that can help us reach a more ambitious aim: reduce global warming to just 1.5ºC during next decades.
Figure 1. Evolution of CO2 concentration in the atmosphere (1960 – today). Measure at Mauna Loa Observatory, Hawaii. (Source: Scripps)
Several measures have to be implemented drastic reduce CO2 emissions in the short and medium term, such as: improve the energy efficiency of energy conversion processes, use and distribution, promote the development of renewable energies, use fuels with less carbon content such as natural gas and develop carbon capture technologies. Carbon capture is expected to contribute to a reduction of 15% of the total emissions —100Gt approximately— by 2050. Any carbon capture technology aims to generate a concentrated flow of this gas suitable to be compressed, transported and permanently stored or, as an alternative, to be reused mainly in the synthesis of chemical products and synthetic fuels. These technologies can be applied to large static carbon emission sources such as thermal plants, cement factories, refineries, steelworks, etc. They cannot be applied to other carbon emission sources such as residential areas or transportation as they are small or mobile.
Depending on the moment when carbon is extracted, there are mainly three types of carbon capture technologies: post-combustion, oxy-fuel combustion and pre-combustion (See Figure 2). Post-combustion technologies separate the carbon present in a combustion gas, a normal situation in air combustion processes at thermal plants, cement factories and steelworks. The challenge is to perform the operation in large low-pressure gas flows with CO2 dilute in nitrogen. The chemical absorption with amines is the alternative most use in this technology. In Oxy-fuel combustion, combustion is carried out in the presence of pure oxygen, instead of air. So, the product is directly a flow rich in CO2, making the separation before its storage or reuse easier. This process usually uses cryogenic methods, entailing a high energy penalization and it needs installations adapted for the combustion of pure O2. In pre-combustion technologies, fuel is transformed in a mixture of H2 and CO2. The separation of CO2 is mainly carried out by chemical absorption with amines or by physical absorption with solvents. The resulting pure H2 can be used as clean fuel —its combustion generates water vapour— or as raw material in the chemical industry and refineries.
Figure 2. General diagram on the different carbon capture technologies (Source: PTECO2)
Carbon capture technologies have reached a great importance, proved by the existence of 23 large-scale plants in operation or under construction all around the world. They have been designed to capture around 40 million annual tons of CO2. Other 28 pilot and demonstration plants have been built, that capture more than 3,000 annual tons of CO2 in total. (See Figure 3)
Figura 3. Distribución mundial de plantas de captura de CO2 (Fuente: Global CCS Institute)
The project Boundary Dam in Canada was the first carbon capture plant that was commissioned (in 2014) in a 120MW coal thermal plant. It captures over 1Mt a year using amine chemical absorption (see Figure 4). The captured CO2 is transported 70km away and is used to improve the extraction of oil, it is permanently stored inside the deposit.
The largest carbon captured plan linked to a thermal plant (Petro Nova) was commissioned in 2017 in Texas, EEUU. It can capture 90% of the CO2 generated, up to 1.4 million tons a year, equivalent to 240MW. CO2 is then used in oil extraction. Its construction is expected to be recouped in less than 10 years.
China is a country that has hugely committed to these technologies. The project Yanchang CCUS wants to avoid over 400 thousand CO2 tons a year in two coal gasification plants. In Europe there are two industrial carbon capture plants located in Norway: Slepiner and Snohvit linked to natural gas processing plants. They have been in operation for almost 30 years and have captured more than 20 million tons of CO2 and stored it in the Northern Sea. In the United Kingdom there are other two plants under construction: White Rose, that will generate energy by burning biomass in oxy-fuel combustion and Peterhead, that will implement carbon capture technology in a combined cycle power plant.
Figure 4. Boundary Dam carbon capture plant, Canada. (Source: Saskpower)
Up to now, in Spain there has been limited developments in this field. At the National Coal Institute (INCAR-CSIC), there is a research group that has been developing highly efficient carbon capture technologies for 15 years. This group was a pioneer in the use of CaO as CO2 sorbent at high temperatures —called Calcium Looping—. It has also developed effective processes that can be applied to cement factories, steelworks or energy plants. The research cover from laboratory tests to pilot plants and include the modelling of gas-solid reactions, reactors and processes. Thanks to the participation in several European projects financed in the FP6 and FP7 programmes (publication | Cordis Project | Ascent Project), H2020 (Cemcap Project | Cleanker Project) and RFCS (Flexical Project), the INCAR-CSIC carbon capture group is playing an important role in the scaling of these technologies, such as in the case of the 1.7MW pilot plant in La Pereda thermal plant in Asturias where Calcium Looping technology was validated to generate energy without carbon emissions in a pre-industrial scale (see Figure 5) or the demonstration plant under construction in a cement factory in Piacenza, Italy.
Figure 5. 1.7MW pilot plant at La Pereda thermal plant, Asturias, based on Calcium Looping technology.
- 2014. Climate change 2014: Synthesis report. Contribution of Working Group I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. International Panel on Climate Change, Geneva, Switzerland.
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- Global Carbon Atlas
- Global CCS Institute