A comparative study of novel activated AMP using 1,5-diamino-2-methylpentane vs MEA solution for CO₂ capture from gas-fired power plant

Abstract

Bench-scale pilot plant study of AMP and 1,5-diamino-2-methylpentane (DA2MP) blend for CO₂ capture from gas-fired power plant is investigated. The concentration of the amine blend is 2 kmol/m³ AMP-1.5 kmol/m³ DA2MP while that of single solvent MEA is 5 kmol/m³. Comparative analysis was based on CO₂ absorption efficiency (%), absorber mass transfer coefficient (K_Gav_(ave), kmol/kPa h m³), desorber mass transfer coefficient (K_Lav, h⁻¹), rich amine loading (α_rich, mol CO₂/mol amine), lean amine loading (α_lean, mol CO₂/mol amine), cyclic loading (C_L, mol CO₂/mol amine), cyclic capacity (C_C, mol CO₂/L-amine soln.), CO₂ absorption rate (r_abs, g-CO₂/h), and regeneration energy (Q_reg, GJ/tonne CO₂). The contribution of sensible energy (Q_sen, GJ/tonne CO₂), vaporization energy (Q_vap, GJ/tonne CO₂), and desorption heat (ΔH_des, GJ/tonne CO₂) towards Q_reg was also investigated. Results showed that the AMP-DA2MP blend possess higher K_Gav_(ave) (11.66%), K_Lav (7.67% higher), and CO₂ absorption efficiency (4.66% higher) than MEA. Also, the superior cyclic loading (51.5%) and cyclic capacity (6.7%), and lower regeneration energy (13.8% lower) was observed for the AMP-DA2MP blend. The desorption heat (ΔH_des) was the major contributor to the Q_reg of both amine systems however the ΔH_des of AMP-DA2MP was 23% lower than MEA. It was noticed that though the water concentration of the amine blend (60.7 wt%) is lower than MEA (70 wt%), the vaporization energy of the amine blend was 32.9% higher than MEA. Therefore, besides the amount of water concentration, higher desorber temperature profile, amine solvent vapor pressure and boiling point also increases the vaporization energy. The results is a revelation of possible reduction in capital cost and operating costs for the AMP-DA2MP blend compared to the standard MEA.

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 (ΔH_des, GJ/ tonne CO₂), sensible energy (Q_sen, GJ/tonne CO₂) and vaporization energy (Q_vap, GJ/tonne CO₂) (Eq. (1)).

$$Q_{\mathit{reg}}={\mathrm{\Delta }H}_{\mathit{des}}+Q_{\mathit{sen}}+Q_{\mathit{vap}}$$

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 (K_Gav_(ave), kmol/kPa h m³), desorber mass transfer coefficient (K_Lav, h⁻¹), rich amine loading (α_rich, mol CO₂/mol amine), lean amine loading (α_lean, mol CO₂/mol amine), cyclic loading (C_L, mol CO₂/mol amine), cyclic capacity (C_C, mol CO₂/L-amine soln.), CO₂ absorption rate (r_abs, g-CO₂/h), and regeneration energy (Q_reg, GJ/tonne CO₂). The effect of individual energies that contribute to the regeneration energy was also analyzed.