Dynamic modelling and analysis of post-combustion CO2 chemical absorption process for coal-fired power plants

Post-combustion capture by chemical absorption using MEA solvent remains the only commercial technology for large scale CO₂ capture for coal-fired power plants. This paper presents a study of the dynamic responses of a post-combustion CO₂ capture plant by modelling and simulation. Such a plant consists mainly of the absorber (where CO₂ is chemically absorbed) and the regenerator (where the chemical solvent is regenerated). Model development and validation are described followed by dynamic analysis of the absorber and regenerator columns linked together with recycle. The gPROMS (Process Systems Enterprise Ltd.) advanced process modelling environment has been used to implement the proposed work. The study gives insights into the operation of the absorber-regenerator combination with possible disturbances arising from integrated operation with a power generation plant. It is shown that the performance of the absorber is more sensitive to the molar L/G ratio than the actual flow rates of the liquid solvent and flue gas. In addition, the importance of appropriate water balance in the absorber column is shown. A step change of the reboiler duty indicates a slow response. A case involving the combination of two fundamental CO₂ capture technologies (the partial oxyfuel mode in the furnace and the post-combustion solvent scrubbing) is studied. The flue gas composition was altered to mimic that observed with the combination. There was an initial sharp decrease in CO₂ absorption level which may not be observed in steady-state simulations.     

Control strategies for flexible operation of power plant with CO2 capture plant

About 20% power output penalties will be incurred for implementing CO₂ capture from power plant. This loss can be partially compensated by flexible operation of capture plant. However, daily large variations of liquid and gas flows may cause operation problems to packed columns. Control schemes were proposed to improve the flexibility of power output without causing substantial hydraulic disturbances in capture plant is presented. Simulations were implemented using ASPEN Plus. In varying lean solvent flow strategy, the flow rate of recycling solvent was manipulated to control the CO₂ capture rate. The liquid flow of the absorber and gas flow of the stripper will vary substantially. In an alternative strategy, the lean solvent loading will be varied. Variation of gas throughput in the stripper is avoided by recycling part of CO₂ vapor to stripper. This strategy provided more stable hydraulics condition in both columns and is recommended for flexible operation. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Optimization of CO2 capture process with aqueous amines using response surface methodology

Amine is one of candidate solvents that can be used for CO₂ recovery from the flue gas by conventional chemical absorption/desorption process. In this work, we analyzed the impact of different amine absorbents and their concentrations, the absorber and stripper column heights and the operating conditions on the cost of CO₂ recovery plant for post-combustion CO₂ removal. For each amine solvent, the optimum number of stages for the absorber and stripper columns, and the optimum absorbent concentration, i.e., the ones that give the minimum cost for CO₂ removed, is determined by response surface optimization. Our results suggest that CO₂ recovery with 48 wt% DGA requires the lowest CO₂ removal cost of $43.06/ton of CO₂ with the following design and operating conditions: a 20-stage absorber column and a 7-stage stripper column, 26 m³/h of solvent circulation rate, 1903 kW of reboiler duty, and 99°C as the regenerator-inlet temperature.     

Preliminary analysis of process flow sheet modifications for energy efficient CO2 capture from flue gases using chemical absorption

The energy penalty associated with solvent based capture of CO₂ from power station flue gases can be reduced by incorporating process flow sheet modifications into the standard process. A review of modifications suggested in the open and patent literature identified several options, primarily intended for use in the gas processing industry. It was not immediately clear whether these options would have the same benefits when applied to CO₂ capture from near atmospheric pressure combustion flue gases. Process flow sheet modifications, including split flow, rich split, vapour recompression, and inter-stage cooling, were therefore modelled using a commercial rate-based simulation package. The models were completed for a Queensland (Australia) based pilot plant running on 30% MEA as the solvent. The preliminary modelling results showed considerable benefits in reducing the energy penalty of capturing CO₂ from combustion flue gases. Further work will focus on optimising and validating the most relevant process flow sheet modifications in a pilot plant.     

Dynamic modeling of CO2 absorption from coal-fired power plants into an aqueous monoethanolamine solution

Among carbon capture and storage (CCS), the post-combustion capture of carbon dioxide (CO₂) by means of chemical absorption is actually the most developed process. Steady state process simulation turned out as a powerful tool for the design of such CO₂ scrubbers. Besides steady state modeling, transient process simulations deliver valuable information on the dynamic behavior of the system. Dynamic interactions of the power plant with the CO₂ separation plant can be described by such models. Within this work a dynamic process simulation model of the absorption unit of a CO₂ separation plant was developed. For describing the chemical absorption of CO₂ into an aqueous monoethanolamine solution a rate based approach was used. All models were developed within the Aspen Custom Modeler^® simulation environment. Thermo physical properties as well as transport properties were taken from the electrolyte non-random-two-liquid model provided by the Aspen Properties^® database. Within this work two simulation cases are presented. In a first simulation the inlet temperature of the flue gas and the lean solvent into the absorber column was changed. The results were validated by using experimental data from the CO₂SEPPL test rig located at the Dürnrohr power station. In a second simulation the flue gas flow to the separation plant was increased. Due to the unavailability of experimental data a validation of the results from the second simulation could not be achieved.     

Dynamic analysis of the absorption/desorption loop of a carbon capture plant using an object-oriented approach

Post-combustion capture of CO₂ is regarded as a possible technology in order to reduce CO₂ emission to the atmosphere. This paper provides a dynamic analysis of the absorption/desorption loop of a carbon capture plant with the help of a simulation model, built using the object-oriented Modelica Library Thermal Separation. The solvent used is an amino-acid salt.The dynamic behaviour is investigated for a reduction in regeneration heat flow rate but constant flue gas flow rate. Hereby four different control strategies are compared, one keeping the lean solvent loading constant, one keeping the solvent flow rate constant, one where flue gas bypasses the capture plant and a last one where an additional solvent tank is introduced. The simulation shows i.e. that for a constant lean solvent loading the response of the absorbed CO₂ flow rate is much faster than for a constant solvent flow rate.Also the effect on the dynamic behaviour is investigated, comparing the whole cycle model to a stand-alone desorber model and to a stand-alone absorber model respectively. It was found that the dynamic responses on a short time scale are very similiar, but different on a long time scale.     

Reduction of amine mist emissions from a pilot-scale CO2 capture process using charged colloidal gas aphrons

In the CO₂ capture process from coal-derived flue gas where amine solvents are used, the flue gas can entrain small liquid droplets into the gas stream leading to emission of the amine solvent. The entrained drops, or mist, will lead to high solvent losses and cause decreased CO₂ capture performance. In order to reduce the emissions of the fine amine droplets from CO₂ absorber, a novel method using charged colloidal gas aphron (CGA) generated by an anionic surfactant was developed. The CGA absorption process for MEA emission reduction was optimized by investigating the surfactant concentration, stirring speed of the CGA generator, and capture temperature. The results show a significant reduction of MEA emissions of over 50% in the flue gas stream exiting the absorber column of a pilot scale CO₂ capture unit.     

Absorber intercooling in CO2 absorption by piperazine‐promoted potassium carbonate

Intercooling was evaluated as a process option in CO₂ absorption by piperazine (PZ) promoted potassium carbonate. The system performance with 4.5 m K⁺/4.5 m PZ was simulated by a model in Aspen Plus® RateSep?. The absorber was evaluated for use with a double matrix stripper by optimizing the position of the semilean feed and intercooling stages to maximize CO₂ removal. Additionally, a simple absorber system was modeled to observe the effect of intercooling on systems with variable CO₂ lean loading. Intercooling increases CO₂ removal by as much as 10% with the double matrix configuration. With a simple absorber, the effectiveness of intercooling depends on solvent rate. Near a critical liquid/gas ratio (L/G) there is a large improvement with intercooling. This is related to the position of the temperature bulge. An approximation is proposed to estimate the critical L/G where intercooling may maximize removal. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

FAU zeolite membranes for dewatering of amine-based post-combustion CO2 capture solutions

In amine-based CO₂ capture processes, aqueous amine solvent is circulated between absorber (CO₂ absorption) and stripping (solvent regeneration) columns. To reduce solvent regeneration energy demand, a selective membrane can dewater and enrich the CO₂ concentration in solution prior to the stripper, lowering steam requirements for solution heating. In this work, a facile synthesis strategy was developed to prepare faujasite (FAU) zeolite membranes built upon polydopamine (PDA) modified α-Al₂O₃ substrates. PDA facilitated the attachment of zeolite phases onto the substrate surface to form a 3 μm membrane layer. Membrane permeation flux of 4.45 kg m⁻² h⁻¹ and 95% rejection rate calculated by either CO₂ loading or total alkalinity was achieved in dewatering of CO₂ loaded 30 wt% monoethanolamine (MEA) solution. The effects of temperature on membrane dewatering performance and stability were investigated. This study highlights the potential for process integration of membrane technology in amine-based post-combustion CO₂ capture operations.     

The use of carbonic anhydrase to accelerate carbon dioxide capture processes

The chemical absorption of CO₂ into a monoethanolamine solvent is currently the most widely accepted commercial approach to carbon dioxide capture. However, the subsequent desorption of CO₂ from the solvents is extremely energy intensive. Alternative solvents are more energy efficient, but their slow reaction kinetics in the CO₂ absorption step limits application. The use of a carbonic anhydrase (CA) enzyme as a reaction promoter can potentially overcome this obstacle. Native, engineered and artificial CA enzymes have been investigated for this application. Immobilization of the enzyme within the gas absorber or in a membrane format can increase enzyme stability and avoid thermal denaturation in the stripper. However, immobilization is only effective if the mass transfer of carbon dioxide through the liquid phase to reach the immobilization substrate does not become rate controlling. Further research should also consider the process economics of large‐scale enzyme production and the long‐term performance of the enzyme under real flue gas conditions. © 2014 Society of Chemical Industry     

Modeling and Simulation for Post-Combustion Carbon Dioxide Capture from Power Plant Flue Gas with Economic Analysis

In this study three configurations, viz. conventional MEA, interstage absorber, and interstage absorber with two stripper configuration have been techno-economically evaluated for the optimized result by carrying out simulations using ASPEN PLUS. An economic model defined for carbon capture using 30 wt% MEA to reduce CO₂ in flue gas to 0.5 mol% of 550 MWe coal fired power plant resulted in increase in COE of power plant by 20.6, 17.4, and 15.6 percents for the three configurations, respectively. The CO₂-avoided cost for the three configurations are 65.94, 64.05, and 63.09 ($ /tonne of CO₂ avoided), respectively.     

Investigation into a gas–solid–solid three-phase fluidized-bed carbonator to capture CO2 from combustion flue gas

A two-dimensional (2D) transient model was developed to simulate the local hydrodynamics of a gas (flue gas)–solid (CaO)–solid (CaCO₃) three-phase fluidized-bed carbonator using the computational fluid dynamic method, where the chemical reaction model was adopted to determine the molar fraction of CO₂ at the exit of carbonator and the partial pressure of CO₂ in the carbonator. This investigation was intended to improve an understanding of the chemical reaction effects of CaO with CO₂ on the CO₂ capture efficiency of combustion flue gases. For this purpose, we had utilized Fluent 6.2 to predict the CO₂ capture efficiency for different operation conditions. The adopted model concerning the reaction rate of CaO with CO₂ is joined into the CFD software. Model simulation results, such as the local time-averaged CO₂ molar fraction and conversion of CaO, were validated by experimental measurements under varied operating conditions, e.g., the fraction of active CaO, chemical reaction temperature, particle size, and cycle number at different locations in a gas–solid–solid three-phase fluidized bed carbonator. Furthermore, the local transient hydrodynamic characteristics, such as gas molar fraction and partial pressure were predicted reasonably by the chemical reaction model adopted for the dynamic behaviors of the gas–solid–solid three-phase fluidized bed carbonator. On the basis of this analysis, capture CO₂ strategies to reduce CO₂ molar fraction in exit of carbonator reactor can be developed in the future. It is concluded that a fluidized bed of CaO can be a suitable reactor to achieve very effective CO₂ capture from combustion flue gases.     

Simulations of Membrane Gas Separation: Chemical Solvent Absorption Hybrid Plants for Pre- and Post-Combustion Carbon Capture

Solvent absorption and membrane gas separation are two carbon capture technologies that show great potential for reducing emissions from stationary sources such as power plants. Here, plants combining chemical solvent absorption and membrane gas separation are considered for post-combustion capture as well as pre-combustion capture. In all ASPEN HYSYS simulations the membrane stage initially concentrates CO₂ into either the permeate or the retentate stream, which is then passed to a monoethanolamine (MEA) based solvent absorption process. In particular, post-combustion capture scenarios examined a membrane that is selective for CO₂ against N₂, while for the pre-combustion scenario a H₂-selective membrane was studied. It was found the energy demand of the combined hybrid plant was always more than that of a stand alone MEA solvent process. This was mainly due to the need to generate a pressure driving force upstream of the membrane in the post-combustion scenario or to recompress downstream gas streams in the pre-combustion scenarios. For both scenarios concentrating the CO₂ in the feed to the solvent system reduced the absorber column height and diameter, which could represent a CAPEX saving for the hybrid plant, dependent upon the membrane price. The use of a hydrogen selective membrane downstream of an oxygen fired gasifier was identified as the most prospective scenario, as it led to significant reductions in absorber size, for a relatively small membrane area and energy penalty.     

CO2 absorption intensification using three-dimensional printed dynamic polarity packing in a bench-scale integrated CO2 capture system

Postcombustion carbon capture using a chemical absorbent is a promising technology to reduce CO₂ emission. However, the overall construction and operating costs remain a major challenge. In order to intensify the absorption process and to reduce these costs, a novel dynamic polarity structured packing (DP packing) with alternate patterns of surface polarity has been developed to enhance local macro-scale turbulence within the advanced viscous solvent to reduce the mass transfer diffusion resistance. Three DP structured packings that incorporate multiple polymeric materials were fabricated using three-dimensional printing technique and evaluated through parametric testing using a bench-scale integrated CO₂ capture unit with 76.2 mm ID absorber. At optimized operating conditions, the DP packing showed a relative 22.7% increase in absorption and 20.0% decrease in energy penalty.     

燃煤电厂烟气二氧化碳胺法捕集工艺改进研究进展

有机胺化学吸收法是当前最具大规模工业化应用前景的燃煤电厂烟气二氧化碳捕集技术,但目前仍存在能耗较高的缺点,而通过工艺改进将是降低碳捕集系统能耗的有效方法之一。本文从吸收端和解吸端分别对十余类工艺改进方法的原理和研究进展进行了阐述,其中吸收端涉及吸收塔内部冷却、富液循环、贫液分配流和旋转塔4类工艺;解吸端则包括解吸塔塔级再热、富液分流、闪蒸再生、闪蒸压缩、多压力解吸、多效解吸塔和直接蒸汽解吸共7类工艺。分析表明,富液分流、闪蒸压缩、多效解吸塔和直接蒸汽解吸工艺展示了较好的改进效果。并进一步介绍了多种工艺联合改进的碳捕集系统综合优化方法,特别对综合优化过程中需要重点关注的各工艺间相互作用以及吸收剂与优化工艺匹配性进行了讨论说明,由此指出联合采用新型胺吸收剂和复合工艺改进是未来开发先进胺法碳捕集系统的重要研究方向。     

A model of acid gas absorption/stripping using methyldiethanolamine with added acid

A nonequilibrium stage model was developed for the absorption and stripping of H₂S and CO₂ using aqueous methyldiethanolamine (MDEA). Heat and mass transfer are calculated for each stage assuming the liquid is well mixed and the gas moves in plug flow. The vapour-liquid equilibrium is represented by an empirical expression that was fit to experimental data. The mass transfer enhancement factor for CO₂ is based on the surface renewal theory with approximations made to the reaction term by the method of DeCoursey. Calculation of H₂S absorption assumes an instantaneous reaction rate at the gas/liquid interface and accounts for enhancement by equilibrium chemical reactions. Results were generated at Claus tail gas conditions using available equilibrium and rate data for 50 wt% MDEA. The amount of H₂S in the absorber outlet gas, or H₂S leak, was used to measure system performance. The base case resulted in a H₂S leak of 98 ppm with 20 absorber stages, 25 stripper stages, and a steam rate of 1.7 lb/gal solvent. Adding 0.05 equivalents of acid per mole of MDEA to the aqueous solution reduced the H₂S leak to 6 ppm and the steam rate to 1.2 lb/gal. Reducing the base case stripper pressure of 2.0 atm to 1.0 atm reduced the H₂S leak to 22 ppm. Analysis of McCabe-Thiele plots generated by the model showed that system performance improved after adding acid or reducing the stripper pressure because the H₂S equilibrium in the stripper was linearized.     

Numerical Study of the CO2 Absorber Performance Subjected to the Varying Amine Solvent and Flue Gas Loads

The paper is devoted to the amine-based post-combustion carbon dioxide capture technology. The aim of the paper was to analyze the effect of varying flow conditions on the CO₂ capture efficiency of the absorber column. As a research tool, a numerical model of the chemical absorption with aqueous monoethanolamine solution in a packed bed was employed. A complex physio-chemical process including two-phase flow hydrodynamics, heat transfer, and absorption chemistry was simulated by Ansys Fluent commercial software. The parametric study was focused on CO₂ capture efficiency in terms of varying loads of amine solvent (liquid) and flue gas. The corresponding changes of liquid holdup, species concentration, temperature and reaction rate distributions are discussed in detail allowing to better understand the absorption column operation. The simulation results have shown clearly the mutual interactions of partial processes and the sensitivity of the system to varying column loads. They have been found to be useful in defining the optimal ranges of operational parameters.     

Determination of cost-effective operating condition for CO2 capturing using 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) ionic liquid (IL) is considered for CO₂ capturing in a typical absorption/stripper process. The use of ionic liquids is considered to be cost-effective because it requires less energy for solvent recovery compared to other conventional processes. A mathematical model was developed for the process based on Peng-Robinson (PR) equation of state (EoS). The model was validated with experimental data for CO₂ solubility in [BMIM][BF4]. The model is utilized to study the sorbent effect and energy demand for selected operating pressure at specific CO₂ capturing rates. The energy demand is expressed by the vapor-liquid equilibrium temperature necessary to remove the captured CO₂ from the spent solvent in the regeneration step. It is found that low recovery temperature can be achieved at specific pressure combination for the absorber/stripper units. In fact, the temperature requirement is less than that required by the typical monoethanolamine (MEA) solvent. The effect of the CO₂ loading in the sorbent stream on the process performance is also examined.     

Dynamic modeling and simulation of absorption of carbon dioxide into seawater

In order to elucidate the dynamic performance of the CO₂ ocean disposal process, effects of operating parameters, such as gas flow rate, salinity and temperature, on the absorption of CO₂ into seawater were examined. The rate-based model consisting of the rates of chemical reaction and gas-liquid mass transfer was developed for simulating dynamic process of CO₂ ocean disposal. In modeling, non-ideal mixing characteristics in the gas and liquid phases are described using a tanks-in-series model with backflow. Experiments were performed to verify dynamic CO₂ absorption prediction capability of the proposed model in a cylindrical bubble column. The operation was batch and continuous with respect to liquid phase and gas phase, respectively. Experimental results indicate that the CO₂ gas injection rate increased the absorption rate but the increase in salinity concentration caused inhibition of the absorption of CO₂. The proposed model could describe the present experimental results for the dynamic changes and the steady-state values of dissolved CO₂ concentration and hydrogen ion concentration. The proposed model might effectively handle the prediction of the absorption of CO₂ into seawater in the CO₂ ocean disposal.     

Post-combustion CO2 capture with chemical absorption: A state-of-the-art review

Global concentration of CO₂ in the atmosphere is increasing rapidly. CO₂ emissions have an impact on global climate change. Effective CO₂ emission abatement strategies such as Carbon Capture and Storage (CCS) are required to combat this trend. There are three major approaches for CCS: post-combustion capture, pre-combustion capture and oxyfuel process. Post-combustion capture offers some advantages as existing combustion technologies can still be used without radical changes on them. This makes post-combustion capture easier to implement as a retrofit option (to existing power plants) compared to the other two approaches. Therefore, post-combustion capture is probably the first technology that will be deployed. This paper aims to provide a state-of-the-art assessment of the research work carried out so far in post-combustion capture with chemical absorption. The technology will be introduced first, followed by required preparation of flue gas from power plants to use this technology. The important research programmes worldwide and the experimental studies based on pilot plants will be reviewed. This is followed by an overview of various studies based on modelling and simulation. Then the focus is turned to review development of different solvents and process intensification. Based on these, we try to predict challenges and potential new developments from different aspects such as new solvents, pilot plants, process heat integration (to improve efficiency), modelling and simulation, process intensification and government policy impact.