Full Length ArticleA comparative study of novel activated AMP using 1,5-diamino-2-methylpentane vs MEA solution for CO2 capture from gas-fired power plant
Introduction
The need to reduce carbon emissions from large industrial sources has led to the increased study of carbon dioxide (CO2) capture, utilization and storage (CCU&S). It has been reported by the International Energy Agency that coal accounts for the most of the 2016 CO2 emissions in the energy sector [1]. According to the Environment and Climate Change Canada 45% of greenhouse gas (GHG) emissions was contributed by stationary combustion sources in 2016 [2]. This has led to an increase in the switch from coal to natural gas for power generation, and it is solely because natural gas is cleaner than coal. However, natural gas being a fossil fuel release CO2 into the atmosphere when combusted, hence it is imperative to capture CO2 from gas-fired power plants is imperative. Typical CO2 concentration in flue gases from natural gas fired boiler and natural gas combined cycle turbine can vary from 3 vol% to 8 vol% [3], [4], [5].
Several technologies for capturing CO2 include absorption (chemical or physical solvents), adsorption, membrane, cryogenic and biological processes. However, the most matured, most studied, can handle high gas flow rate and capable of 90% CO2 capture efficiency is the absorption technology using amine solvents [6], [7], [8], [9]. There are different classification of amine solvents namely primary amine (MEA, monoethanolamine), secondary amines (DEA, diethanolamine) and tertiary amines (MDEA, methyldiethanolamine). In addition, other specialty amines like sterically hindered amine (AMP, 2-amino-2-methyl-1-propanol) and reactive polyamines like piperazine (PZ) and diethylenetriamine (DETA).
There are various amine-based carbon capture projects at different stages, for example two CO2 capture plants integrated to coal-fired power plants are currently in operation in Canada and USA [10], [11], [12]. However, four projects are currently under construction which includes two in Canada, one in China and one in Australia [12].
The benchmark amine solvent and concentration is the primary amine, monoethanolamine (30 wt% MEA or 5 kmol/m3 MEA). This is because it possesses high mass transfer, low cost and high absorption rate, but it is plagued by high solvent regeneration energy, corrosion and high amine losses via degradation and vaporization [13], [14], [15], [16], [17], [18]. According to previous studies, the regeneration energy of MEA-based CO2 capture adds up to 70–80% of the plant operating cost [19], [20]. The regeneration energy is made up of the desorption heat (ΔHdes, GJ/ tonne CO2), sensible energy (Qsen, GJ/tonne CO2) and vaporization energy (Qvap, GJ/tonne CO2) (Eq. (1)).
Sensible energy is the heat is required to raise the temperature of the CO2 rich amine solution to the regeneration temperature. On the other hand, the vaporization energy is the amount of heat required to vaporize volatile components (mostly water) in the amine solution to strip the CO2 in the amine solution flowing downwards. Most studies have reported heat of vaporization considering only the amount of water, which is because the amount of water in the CO2 loaded amine solution is by far higher than specie [21], [22], [23], [24]. According to Chakma, a concentrated aqueous amine solution (contains smaller water concentration) will only require less latent heat of water vaporization [25]. Hence, 30 wt% MEA will possess higher heat of vaporization than 50 wt% TEA [25]. This was also confirmed by Nwaoha et al. in AMP-MDEA-DETA and AMP-PZ-MEA tri-amine solvent blends [26], [27].
The high energy penalty of single solvent MEA (5 kmol/m3 MEA) is the reason for blending amine solvents to utilize their specific potentials while minimizing their individual problems [28]. To achieve this, bicarbonate (HCO3−) forming amine solvents like tertiary amines (e.g. like methyldiethanolamine, MDEA) and/or sterically hindered amines (e.g. 2-amino-2-methyl-1-propanol, AMP) are blended with reactive monoamines and/or polyamines (e.g. MEA, PZ, DETA etc.) [20], [29], [30], [31], [32], [33], [34]. This is because amine desorption process is enhanced by the presence of HCO3− which in turn reduces the energy of regeneration. The higher kinetics and lower regeneration energy of sterically hindered amine (like AMP) compared to commonly studied tertiary amines is the reason why it is favored [31], [35]. Laboratory, pilot plant and process simulation studies have shown that AMP-based blended amine solution exhibit lower capital expenditure and operating cost, lower regeneration energy, increased absorption rates, cyclic loading, cyclic capacity and higher rich amine loading compared to MEA [36], [37], [38], [32], [31], [30], [39]. Several pilot plant studies have investigated AMP-PZ bi-amine solvent blends [30], [40]. However, there are underlying challenges accompanying this blend such as AMP precipitation and PZ crystallization due to limited solubility in water [36], [17], [41]. In addition, being a secondary amine, PZ has the tendency to produce more nitrosamines when compared to tertiary and primary amines respectively [42], [43]. Therefore, it is important to investigate new AMP-based blends that possess superior CO2 capture capability than MEA, without any operational problems like precipitation and crystallization of AMP-PZ blend. Reduction of nitrosamine formation due to secondary amine reaction with NOx needs to be taken into account.
The aim of this study is pilot plant investigation of CO2 capture from gas-fired power plant using novel bi-amine solvent blend containing AMP activated by 1,5-diamino-2-methylpentane (AMP-DA2MP) compared to the standard MEA solvent. Considering that AMP at high concentration can precipitate when it absorbs CO2 [17], [36], the AMP concentration in this study was kept at 2 kmol/m3 (19 wt%). In order not to exceed 40 wt% of amine so that there will be sufficient amount of water in the aqueous amine solution, the concentration of DA2MP was fixed at 1.5 kmol/m3 (20.3 wt% of water). This is the rational why the concentration of the amine blend was set at 2 kmol/m3 AMP-1.5 kmol/m3 DA2MP which is a total amine concentration of 3.5 kmol/m3 (39.3 wt%). The investigated parameters for comparative assessment are CO2 absorption efficiency (%), absorber overall volumetric mass transfer coefficient (KGav(ave), kmol/kPa h m3), desorber mass transfer coefficient (KLav, h−1), rich amine loading (αrich, mol CO2/mol amine), lean amine loading (αlean, mol CO2/mol amine), cyclic loading (CL, mol CO2/mol amine), cyclic capacity (CC, mol CO2/L-amine soln.), CO2 absorption rate (rabs, g-CO2/h), and regeneration energy (Qreg, GJ/tonne CO2). The effect of individual energies that contribute to the regeneration energy was also analyzed.
Section snippets
Chemicals
The skeletal structure, purity and supplier of the amine solvents are listed in Table 1. Acidification technique using Chittick apparatus was used to analyze the CO2 loading in the amine solution [44]. The 1N (1 kmol/m3) hydrochloric acid was bought from Fisher Chemical, United States.
Pilot plant experimental procedure
This study was performed using an absorber pilot plant (0.0508 m by 1 m; 316 stainless steel) as shown in Fig. 1. The laboratory structured packings (LDX Sulzer) supplied by Sulzer Chemtech Ltd., Switzerland
Amine loading
Fig. 4 displays the rich amine loading (αrich) and lean amine loading (αlean) of 5 kmol/m3 MEA compared to 2 kmol/m3 AMP-1.5 kmol/m3 DA2MP blend. Results showed that the rich amine loading of AMP-DA2MP blend (0.58 mol CO2/mol amine) is 43.21% higher than that of MEA (0.405 mol CO2/mol amine). This can be attributed to the higher equilibrium loading of AMP and DA2MP compared to the single solvent MEA. Additionally, the reactivity of DA2MP as a primary diamine enhanced the chemical interaction
Conclusion
Comparative analysis showed that 2 kmol/m3 AMP-1.5 kmol/m3 DA2MP blend offer reduced regeneration energy (13.8% lower) than the standard MEA. This shows that the steam consumption and associated reboiler for the amine blend is expected will be lower, hence minimized capital cost and operating cost. Apart from water concentration, higher desorber temperature profile increases the vaporization energy.
The absorber and desorber heights for the amine blend will be shorter compared to MEA system.
Acknowledgements
This publication was made possible by NPRP grant# 7 – 1154 – 2 – 433 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. The authors gratefully thank Clean Energy Technologies Research Institute (CETRi), Faculty of Engineering and Applied Science, University of Regina, S4S 0A2, Canada. We also acknowledge the research equipment support to the CETRi, from the followings organizations: Natural Sciences and
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