Verification of a solvent optimization approach for postcombustion CO2 capture using commercial alkanolamines
Introduction
Amine scrubbing is currently the leading technology for postcombustion CO2 capture (Rochelle, 2009). However, high energy consumption in solvent-based CO2 capture continues to be a major hurdle for commercialization. The U.S. Department of Energy (DOE)/National Energy Technology Laboratory (NETL) (DOE/NETL, 2013) estimates that the deployment of current postcombustion CO2 capture technology, aqueous monoethanolamine (MEA) solution based chemical absorption process, in a new pulverized coal power plant would decrease the plant efficiency by 30%. Clearly, an economical and energy efficient CO2 capture process is a prerequisite for global CO2 emission control from fossil energy sources to mitigate global warming. However, through optimization of solvent properties and the absorption/desorption process, it may be possible to reduce the energy penalty from CO2 capture (Oyenekan and Rochelle, 2006, Hoff et al., 2006).
Energy consumption for solvent regeneration, represented by reboiler heat duty, is an important parameter for design and operation of a cost-effective CO2 capture process. Some commonly used solvents are aqueous solutions of alkanolamines, such as MEA, diethanolamine (DEA), and methyl-diethanolamine (MDEA) (Kohl and Nielsen, 1997). MEA is the most widely used because it has a fast rate of reaction with CO2, reasonable degradation resistance, and low solvent cost. However, high desorption energy consumption, vaporization losses and equipment corrosion issue are disadvantages of MEA. Sterically hindered and tertiary amines, 2-amino-2-methyl-1-propanol (AMP) and MDEA are receiving increased attention recently due to their loading up to 1 mol of CO2/mol of amine and relatively low energy consumption for solvent regeneration, leading to significant savings in process costs (Sartori et al., 1994). MEA solvent requires high reboiler heat duty, from 3800 to 5400 kJ/kg CO2 (Sakwattanapong et al., 2005). The AMP with DEA activated solvent has a modest heat requirement of 3030 kJ/kg CO2 (Adeosun and Abu-Zahra, 2013). To compare among the CO2 capture processes, Oyenekan and Rochelle (2006) applied the concept of “equivalent work,” which is the equivalent loss of electricity in the power plant due to the steam extraction and the required power demand due to the CO2 compression.
Effects of various stripper configurations on energy consumption have been studied (Oyenekan and Rochelle, 2006, Jassim and Rochelle, 2006, Freguia and Rochelle, 2003) and both reboiler duty and the total equivalent work were reduced compared to a simple stripper by using configurations including multipressure stripping, vacuum stripping or stripping with vapor recompression. Because the reboiler heat duty relates inversely to lean-CO2 loading/rich-CO2 loading and alkanolamine concentration (Sakwattanapong et al., 2005), the reboiler energy saving can be also achieved by optimizing the lean solvent loading, the solvent concentration and the stripper operating pressure for MEA-based CO2 capture process but the optimal heat duty is still 3000 kJ/kg CO2 (Abu-Zahra et al., 2007).
Heat of reaction has been identified as a key property for optimization of the CO2 capture process (Hoff et al., 2006, Hopkinson et al., 2014). A phase equilibrium approach (Hopkinson et al., 2014) was developed to describe a conceptual solvent, which is characterized by the heat of reaction between CO2 and the solvent. Optimization of a conceptual solvent is achieved through quantification of the impact of heat of reaction on the sensible and stripping heat. The conceptual solvent was optimized for the least total equivalent work for a conventional absorption based post-combustion CO2 capture process. Results show that the least total equivalent work is about 0.1034 kWh/kg CO2 with a heat of reaction of 71 kJ/mol CO2 for tertiary or sterically hindered amines under typical solvent regeneration conditions of 2 atm operating pressure.
This paper verifies the phase equilibrium approach for optimization of conceptual solvents using commercial solvents, including a tertiary amine (MDEA aqueous solvent), and a sterically hindered amine (AMP aqueous solvent). In this study, the energy performance of the absorption/desorption process using MDEA or AMP was obtained through process simulations. The thermal performance of MEA aqueous solvent was also simulated using a similar optimization approach as a reference case. The findings obtained from this work provide new insights and guidance for identifying energy efficient solvents and develop a strategy for cost-effective postcombustion CO2 capture.
Section snippets
Overview
The verification of the phase equilibrium approach was conducted with process simulations (Hopkinson et al., 2014). Specifically, the thermal performance of a conventional absorption/desorption based CO2 capture process was simulated using ProTreat® software (Otimeas Treating Inc, 2009). ProTreat was developed for simulating processes for acid gas removal from a variety of high and low pressure gas streams by absorption into solutions including single/blended amines or physical solvents. The
Energy performance
In the simulation of the energy performance for the CO2 capture process depicted in Fig. 1, process conditions were established to be consistent with the assumptions made in the phase equilibrium approach (Hopkinson et al., 2014). First, the CO2 loading for the lean solution (the inflow to the absorber) was set as high as possible to allow its CO2 equilibrium pressure to approach the CO2 partial pressure in the flue gas at the top of absorber. Second, the solvent circulation flow rate is set as
Conclusions
This work verified a phase equilibrium approach for optimization of a conceptual solvent by using simulations of commercial solvents, aqueous MDEA and AMP, for a conventional absorption/desorption based postcombustion CO2 capture process. Simulation results of the energy performance for these solvents indicate the following conclusions:
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The simulated CO2 working capacities for the commercial solvents agree well with those obtained with the phase equilibrium approach for conceptual solvents with
Disclaimer
This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with URS Energy & Construction, Inc. Neither the United States Government nor any agency thereof, nor any of their employees, nor URS Energy & Construction, Inc., nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any
Conflict of interest
The authors declare no competing financial interest.
Acknowledgment
As part of the National Energy Technology Laboratory's Regional University Alliance (NETL-RUA), a collaborative initiative of the NETL, this technical effort was performed under the RES contract DE-FE0004000.
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