Engineering Solutions for CO2 Conversion. Группа авторовЧитать онлайн книгу.
Figure 1.11 Schematic integration of a power plant with a post‐combustion CO2 capture system.
The energy required to run the chemical absorption–desorption process in the capture unit process is mainly due to the (i) mechanical work to drive the flue gas fan to compensate the pressure drop induced by direct contact cooler, absorber column, and water wash sections and ducting; (ii) mechanical work to drive the pumps for cooling water and solvent circulation pumps; (iii) steam for solvent reclaiming because of its degradation in order to keep the solvent fresh and contaminant free; and (iv) steam to feed the reboiler duty: regenerate the solvent, generate stripping vapors, heat up the solvent to saturation conditions, and evaporate the water.
The flue gas is sent to the capture unit and additional pressure drop is imposed, which is a function of the thermal power plant unit, its equipment for emissions control, and the boiler type. In gas plants, the heat recovery steam generator in the exhaust gas generally imposes the additional pressure drop downstream the gas turbine (in the order of 20–40 mbar [8]), and in boilers (coal/oil/gas fired), a fan is commonly used to keep it under slightly sub‐ambient pressure. The main flue gas line equipment inducing pressure drop would be as follows [42]:
The particle removal system (electrostatic precipitator [ESP]).
The flue gas desulfurization (FGD) unit (if existing).
The NOx scrubber (if required) 2–7 mbar [42].
The bypass stack and damper if installed to bypass the capture unit.
Flue gas recirculation ducting and/or bypass (if utilized).
Direct contact cooler, absorber column, and water washes 6–80 mbar [43].
Absorber duct and stack.
In general, a fan will be required to overcome those pressure drops, whose size will depend on the volumetric flow and pressure drop. In combined cycle thermal power plants, most of the pressure drop might be overcome by raising the back pressure of the gas turbine. However, in boilers, the pressure drop is generally overcome by one or more fans [43].
The extraction of steam from the power plant steam turbine could cover the requirements in the capture system. This strategy will reduce the power output at a lower degree than the amount of heat extracted because the exergy content of the steam is just a fraction of the heat [41]. When the extracted steam is superheated (typically at higher steam pressure), it is normally cooled down by high‐pressure water injection. 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