The heat content of the steam can be fully utilized or part of it is returned via the condensate recirculation to the power plant. The steam extraction design depends on the specific configuration and power plant unit, level of process integration, and steam requirement in the reboiler (also function of the solvent characteristics), which has been extensively discussed in the literature [44–46].
In order to compensate for the efficiency reduction in power generation introduced by the CO2 capture and conditioning processes, several studies have been conducted to increase the efficiency of the integrated process. Increasing process integration within the capture unit itself might lead to reduced specific reboiler duty, for example, by using lean vapor recompression, absorber intercooling, or solvent split flow to stripper [41, 47]. In addition, studies have shown the potential to reduce the specific reboiler duty by using a technique called exhaust gas recirculation (EGR), which consists of recirculating part of the CO2‐rich stream to increase the partial pressure of the exhaust [48] using supplementary firing to increase the partial pressure of CO2 [49] and/or even integrating part of the reboiler duty in the power plant [50]. These options can lead to lower capital and operational costs at the expense of higher integration between the power plant and the capture unit under operation.
1.3.2 Flexible Operation of Thermal Power Plants in Future Energy Systems
Thermal power plant operation is highly coupled to the operation of power systems and power markets. Flexible operation of thermal power plants has become an important issue in the past decades because the increased integration of renewables CCS must operate accordingly (e.g. [51]). In decarbonized power systems, thermal power plants must be operated in cycling mode in order to cope with variability in demand and generation [52, 53], following the main patterns:
Efficiency at part load: In power systems with high penetration of renewables, it is expected that thermal power plants will be operated during a significant number of hours at part load [52, 54, 55]. However, at part load, the efficiency of thermal power generation is generally reduced and specific emissions at minimum compliant load increase. Thermal power plant developers are striving to reduce minimum compliant load level (to minimize economic losses at times when marginal costs of operation are higher than electricity prices) and increase part load efficiency. Design and operation should take into consideration the part load performance of thermal power plants with CCS. An important aspect is to keep minimum specific reboiler duty and an economically suitable capture rate in the capture unit over the whole load range [43, 44, 56, 57].
More frequent changes in load: Faster ramping can be valuable for thermal power plants in order to be more competitive in day‐ahead power markets and balancing markets [54, 55] and the different time scales required for ramping the power plant load and the capture plant will be the key. Generally, thermal power plant load change is characterized by stabilization times in the order of 5–10 minutes, while the capture unit can take up to several hours [54] to stabilize under load changes because of the inertia of the chemical process [58–60]. Efforts are being made to develop operational and control strategies to improve the stabilization time and reduce the specific reboiler duty under transient conditions [57,61–63].
More frequent start‐up and shutdown events: The start‐up and shutdown increase CO2 emissions during start‐up and fuel utilization without any significant power output from the power plant. Efforts are being made in order to reduce the start‐up time to provide power on demand and/or reduce emissions during start‐up [64]. Because the start‐up of amine‐based post combustion CO2 capture is time and energy intensive, minimizing the start‐up time and emissions during the start‐up sequence might be relevant.
Several operational strategies are proposed to operate thermal power plants with CCS in flexible operation mode, being the main purpose to change the power output of the power plant by changing the operational conditions of the integrated process. The main goal from the power operator perspective is to maximize the profits, while from the power system operator perspective, the power plant would be providing variation management to the power system to accommodate the variability of renewables. In addition, flexible operation of post‐combustion capture might be required when integrated within industrial processes because of the inherent variability of the industrial process operations, such as in primary steelmaking [65]. The main strategies proposed for power plants with a carbon capture system can be summarized as follows:
Allowing the thermal power plant to follow load changes. The capture unit follows the power plant load change [58, 59].
Varying the CO2 capture rate, depending on CO2 costs and electricity prices [51]. In such case, the solvent regeneration is variable, using the large amount of loading capacity and large inventories of solvent as CO2 storage [66]. At times with high electricity prices, the steam is used for power production, while the regeneration takes place at low electricity prices.
Turning on‐and‐off the capture unit or flue gas bypass. The flue gases sent to the capture unit are bypassed to the stack of the power plant so that partial or no CO2 is being captured. Part of the flue gas is vented to the atmosphere. This allows part of the steam used for solvent regeneration to be used for power production in the steam turbine. This option might be viable in scenarios in which CO2 emission costs or prices are low.
Providing solvent storage to decouple plant operation from the capture unit. The capture rate is kept constant and the solvent is stored in tanks. The regeneration energy is shifted to times when electricity prices are low. Solvent storage can incur in significant capital expenditure required for solvent storage, which could be favorable in scenarios with high CO2 emission costs.
1.4 CO2 Capture in the Industrial Sector
The industrial sector was responsible for almost 25% of the CO2 emissions in 2014. CO2 is emitted on the fuel combustion, intrinsic reactions and indirectly on the use of electricity. IEA predicted a required reduction on the CO2 emissions of 3–6 Gt/yr to achieve the 2 degrees scenario (2DS) or B2DS. Although other measures such as increasing energy efficiency, developing new production process, using renewable energy or fuel switching, will reduce CO2 emissions, still there is a significant amount of CO2 from the process that can be only reduced through CO2 capture [20]. To achieve the B2DS, the contribution of CCS is estimated as 23%.
All the available CO2 capture technologies can be potentially installed in industrial facilities. However, while certain industries would have similar or even more favorable characteristics for the implementation of carbon capture utilisation and storage (CCUS) compared to power plants, the design of CO2 capture systems must be tailored for each facility. The heat and energy integration will be site specific and, together with the composition and CO2 emission stacks, will impact on the optimum capture rate and the CO2 avoidance cost.
An exhaustive description of the integration of certain CO2 capture technologies in the cement sector can be found, for example, in Refs. [67, 68]. A large scale chemical absorption system will be installed in the Norcem Brevik facility, after other technologies (solid sorbents and membranes) were tested at smaller scale [6]. Oxyfuel has been included in the Front End Engineering Design (FEED) studies within the European climate research alliance (ECRA) project and the LEILAC project will test the Calix technology (direct separation) [7]. Other technologies, such as chilled ammonia, membrane‐based capture combined with liquefaction, and calcium looping were studied, for example, in the CEMCAP project at modeling scale [69]. Moreover, partial capture configurations for several industries are being studied by the CO2STCAP project [70] and the CLEANKER project will scale up the calcium looping up to a TRL of 7.6
The peculiarity of the steelmaking sector is the heterogeneity of production processes that will be more
