Engineering Solutions for CO2 Conversion. Группа авторов

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СКАЧАТЬ on CO₂ emission reduction in power and industrial sectors. Carbon capture represents a contribution of 23% in the “Beyond 2 degrees scenario” (B2DS) modeled by the International Energy Agency (IEA)¹ and has other interesting characteristics that increase its value beyond its cost: (i) easiness to retrofit current power plants or industrial facilities,² (ii) simplicity to integrate that in the electricity grid and offer an interesting tool to cover the intermittency of renewables, (iii) ideal to cut down industrial process emissions that otherwise cannot suffer deep reductions, and (iv) current carbon budgets rely on negative emissions to compensate the use of fossil fuels [1]. Carbon capture combined with bioenergy (BECCS) can provide negative emissions at large scale in an immediate future.

      CO₂ capture (also called CO₂ sequestration or carbon capture) involves a group of technologies aiming to separate CO₂ from other compounds released during the production of energy or industrial products, obtaining a CO₂‐rich gas that can be stored or used for the obtention of valuable products. The main classification of CO₂ capture technologies relies on where in the process the CO₂ separation occurs. For the power sector, it can be divided into pre‐, oxy‐, and post‐combustion. For the industrial sector, the classification is similar, although their integration would be different. In addition, other new arrangements are emerging.

1.2 CO₂ Capture Technologies

      1.2.1 Status of CO₂ Capture Deployment

GCCSI reported in 2018 23 large‐scale CCS facilities in operation or under construction globally, summing up 37 MtCO₂ per year. This wide range of facilities shows the versatility of CO₂ capture processes.³

      In the power sector, the United States is leading the implementation deployment, although Europe has the highest CO₂ capture capacity. The Boundary Dam project (Canada) and Petra Nova (USA) are pioneers in reaching commercial scale. Moreover, based on the successful results of the Boundary Dam project, a CO₂ capture facility has been planned for the Shand power facility (Canada), incorporating not only learnings from the Boundary Dam but also enhanced thermal integration and tailored design. The results show a significant cost reduction [2]. Also in Canada, the Quest project completes the list of Canadian CCS projects in operation [3] and The National Energy Laboratory (NET) power project recently appeared in the United States as a potential significant reduction on CO₂ capture costs [4].

      In the industrial sector, cement, steel, refining, chemicals, heavy oil, hydrogen, waste‐to‐energy, fertilizers, and natural gas have been identified by the Carbon Sequestration Leadership Forum (CSLF; https://www.cslforum.org) as the main intensive emitter industries. As it is highlighted, the Norcem Brevik plant [5, 6], LEILAC [7] (cement production), and Al Redayah (steel production) are on the way to start running carbon capture systems in industrial facilities at pilot and large scales.

      1.2.2 Pre‐combustion

Pre‐combustion systems can be applied to natural gas combined cycles (NGCC) or integrated gasification combined cycle (IGCC) (Figure 1.1), where a syngas, comprising mainly CO and H₂, feeds a gas turbine (GT) combined cycle system to produce electricity. The potential advantages are higher conversion efficiencies of coal to electricity and cheaper removal of pollutants [8]. The syngas, based on the water shift reaction, can be converted into CO₂ and H₂O. This mixture is typically separated with physical solvents (as described in Section 1.2.4), membranes, or sorbents. However, hybrid technologies can also be used. Depending on the technology, further post‐treatment would be needed to avoid degradation and loss of efficiency.

The main theoretical advantage of pre‐combustion is the production of hydrogen, which will add value to the business model, and a lower energy penalty compared to using the traditional chemical absorption within a post‐combustion configuration. However, large projects demonstrated that this difference is only 1–2%, as reported by National Energy Technology Laboratory (NETL) [9].

      The most notable pre‐combustion project was the Kemper County IGCC plant in the United States, which stopped its operation in 2017.This demonstration facility would place this arrangement at high TRL, while other testing campaigns would reach up to a TRL of 6.

Figure 1.1 Diagram of pre‐combustion capture for power generation in IGCC.

      Source: Adapted from Jansen et al. [72].

      1.2.3 Oxyfuel

      In the oxyfuel process, the air is split into nitrogen and oxygen, generally using an air separation unit (ASU), for the combustion of fuel with nearly pure oxygen. The consequence is a higher flame temperature and a highly concentrated CO₂ stream (60–75%, wet and might contain impurities and incondensable components) that can be further purified to meet the final use specifications. The CO₂‐rich gas is typically recirculated to manage the unstable flame and its high temperature. Nowadays, the progress on oxyfuel combustion is focused on the reduction of air separation costs and the enhancement of process configuration to reduce capture costs. Further information can be found, for example, in Ref. [10]. Based on the current progress, the most advanced arrangements can be assessed as TRL 7.

An advanced oxyfuel process, called the Allam cycle (Figure 1.2), is being tested at large scale as part of the NET Power project in the United States [4]. This involves oxyfuel combustion and a high‐pressure supercritical CO₂ working fluid in a highly recuperated Brayton cycle, aiming to reduce CO₂ capture costs and prove stable operation. Based on that, there is a potential to progress to a TRL of 7 once the facility is fully operational.

      1.2.4 Post‐combustion

Post‐combustion refers to the group of technologies able to separate CO₂ from the flue gas emitted during the fuel combustion and/or other reactions in the industrial sector. This indicates that those systems are mainly installed as additional equipment downstream in new plants or during the retrofitting of the existing facilities. The latter represents the main advantage of post‐combustion technologies compared to pre‐ or oxy‐combustion, as a fundamental redesign or complex integration with the existing facilities would be minimal.

Figure 1.2 Process schematic of a simplified commercial scale natural gas Allam cycle.

      Source: Adapted from Allam et al. [4].

      1.2.4.1 Adsorption

      Adsorption refers to the uptake of CO₂ molecules onto the surface of another material. Based on the nature of interactions, adsorption can be classified into two types: (i) physical adsorption and (ii) chemical adsorption. In physical adsorption, the molecules are physisorbed because of physical forces (dipole–dipole, electrostatic, apolar, hydrophobic СКАЧАТЬ