Simplification of detailed rate-based model of post-combustion CO₂ capture for full chain CCS integration studies

As post-combustion CO₂ capture (PCC) technology nears commercialisation, it has become necessary for the full carbon capture and storage (CCS) chain to be studied for better understanding of its dynamic characteristics. Model-based approach is one option for economically and safely reaching this objective. However, there is need to ensure that such models are reasonably simple to avoid the requirement for high computational time when carrying out such study. In this paper, a simplification approach for a detailed rate-based model of post-combustion CO₂ capture with solvents (rate-based mass transfer and reactions assumed to be at equilibrium) is presented. The simplified model can be used in model-based control and/or full chain CCS simulation studies. With this approach, we demonstrated significant reduction in CPU time (up to 60%) with reasonable model accuracy retained in comparison with the detailed model

Carbon capture from natural gas combined cycle power plants: Solvent performance comparison at an industrial scale

Natural gas is an important source of energy. This article addresses the problem of integrating an existing natural gas combined cycle (NGCC) power plant with a carbon capture process using various solvents. The power plant and capture process have mutual interactions in terms of the flue gas flow rate and composition vs. the extracted steam required for solvent regeneration. Therefore, evaluating solvent performance at a single (nominal) operating point is not indicative and solvent performance should be considered subject to the overall process operability and over a wide range of operating conditions. In the present research, a novel optimization framework was developed in which design and operation of the capture process are optimized simultaneously and their interactions with the upstream power plant are fully captured. The developed framework was applied for solvent comparison which demonstrated that GCCmax, a newly developed solvent, features superior performances compared to the monoethanolamine baseline solvent

Dynamic Modelling of Carbon Dioxide Absorber for Different Solvents

In the recent era, the green house alleviation technologies have been fully emphasized and implemented due to the rapid climate change. Fossil fuelled power plants contribute to the globally carbon dioxide (CO2) emission. Post combustion CO2 capture (PCC) and natural gas processing plant CO2 capture processes have attained a high consideration and interest which led to significant progress in CO2 capture. Chemical absorption is the well-established and commercialized technology compare to other technologies in CO2 capture. Choice of solvent is very important to optimize the performance of absorber. The aim of this project is to simulate the rate based CO2 absorber model using two different types of solvent to check the CO2 removal efficiency considering the same operational conditions of the absorption column. This project emphasis only on the solvent parameters since the manipulate variable of this project is solvent, while the inlet gas stream is considered constant parameters. For this project, the CO2 absorption process modelling was done using air as the gas inlet stream. The developed models for each solvent were implemented in Matlab. The developed models are comprised of mass balance, energy balance, rate transfer, interface and hydraulic model. The scope of this project involved the evaluation of three different solvents which are monoethanolamine (MEA) and sodium hydroxide (NaOH). The results of developed models are validated with the literature. The analysis of simulation results highlights that the CO2 absorption process more likely happens at the lower segment of the absorber. As the gas flow from bottom to top of the absorber, the gas phase CO2 concentration and the gas temperature decrease. The solvent concentration declines from upper to lower segment of the absorber while the solvent temperature rises from upper to lower segment of the absorber. Both MEA and NaOH solvents results are agreed to the pilot scale experimental results. The developed model can be used to evaluate the efficacy and capability of ANY novel solvent in a simulated environment before it is being tested on actual experimental set-up. Therefore the best solvent can be determined. The objective of the project is accomplishe

Dynamic Operation and Simulation of Post-Combustion CO2 Capture

AbstractThermal power need to operate, on a daily basis, with frequent and fast load changes to balance the large variations of intermittent energy sources, such as wind and solar energy. To make the integration of carbon capture to power plants economically and technically feasible, the carbon capture process has to be able to follow these fast and large load changes without decreasing the overall performance of the carbon capture plant. Therefore, dynamic models for simulation, optimization and control system design are essential.In this work, we compare the transient behavior of the model against dynamic pilot data for CO2 absorption and desorption for step-changes in the flue gas flow rate. In addition we investigate the dynamic behavior of a full-scale post-combustion capture plant using monoethanolamine (MEA) and piperazine (PZ). This analysis demonstrates the good agreement between the developed model (dCAPCO2) and the pilot measurements at both, transient and steady-state conditions. It outlines how the time needed to reach a new steady-state varies with respect to amine type and concentration. The simulation study reveals that it is essential to control the lean solvent flow to avoid sudden changes in the CO2 removal rate and to avoid increased heat demand of solvent regeneration. In addition, it shows how storage tanks (liquid hold-up of the system) can be designed to accommodate significant upstream changes in the power plant management. This flexibility is especially needed for operation in future mixed green energy market

Modeling and Control of Post-Combustion CO2 Capture Process Integrated with a 550MWe Supercritical Coal-fired Power Plant

This work focuses on the development of both steady-state and dynamic models for an monoethanolamine (MEA)-based CO2 capture process for a commercial-scale supercritical pulverized coal (PC) power plant, using Aspen PlusRTM and Aspen Plus DynamicsRTM. The dynamic model also facilitates the design of controllers for both traditional proportional-integral-derivative (PID) and advanced controllers, such as linear model predictive control (LMPC), nonlinear model predictive control (NMPC) and H? robust control.;A steady-state MEA-based CO2 capture process is developed in Aspen PlusRTM. The key process units, CO2 absorber and stripper columns, are simulated using the rate-based method. The steady-state simulation results are validated using experimental data from a CO2 capture pilot plant. The process parameters are optimized with the goal of minimizing the energy penalty. Subsequently, the optimized rate-based, steady-state model with appropriate modifications, such as the inclusion of the size and metal mass of the equipment, is exported into Aspen Plus DynamicsRTM to study transient characteristics and to design the control system. Since Aspen Plus DynamicsRTM does not support the rate-based model, modifications to the Murphree efficiencies in the columns and a rigorous pressure drop calculation method are implemented in the dynamic model to ensure consistency between the design and off-design results from the steady-state and dynamic models. The results from the steady-state model indicate that between three and six parallel trains of CO2 capture processes are required to capture 90% CO2 from a 550MWe supercritical PC plant depending on the maximum column diameter used and the approach to flooding at the design condition. However, in this work, only two parallel trains of CO2 capture process are modeled and integrated with a 550MWe post-combustion, supercritical PC plant in the dynamic simulation due to the high calculation expense of simulating more than two trains.;In the control studies, the performance of PID-based, LMPC-based, and NMPC-based approaches are evaluated for maintaining the overall CO2 capture rate and the CO2 stripper reboiler temperature at the desired level in the face of typical input and output disturbances in flue gas flow rate and composition as well as change in the power plant load and variable CO2 capture rate. Scenarios considered include cases using different efficiencies to mimic different conditions between parallel trains in real industrial processes. MPC-based approaches are found to provide superior performance compared to a PID-based one. Especially for parallel trains of CO2 capture processes, the advantage of MPC is observed as the overall extent of CO2 capture for the process is maintained by adjusting the extent of capture for each train based on the absorber efficiencies. The NMPC-based approach is preferred since the optimization problem that must be solved for model predictive control of CO2 capture process is highly nonlinear due to tight performance specifications, environmental and safety constraints, and inherent nonlinearity in the chemical process. In addition, model uncertainties are unavoidable in real industrial processes and can affect the plant performance. Therefore, a robust controller is designed for the CO2 capture process based on ?-synthesis with a DK-iteration algorithm. Effects of uncertainties due to measurement noise and model mismatches are evaluated for both the NMPC and robust controller. The simulation results show that the tradeoff between the fast tracking performance of the NMPC and the superior robust performance of the robust controller must be considered while designing the control system for the CO2 capture units. Different flooding control strategies for the situation when the flue gas flow rate increases are also covered in this work

Simulation of fixed bed adsorption processes with simplified models. Application for CO2 adsorption in biogas purification

Biogas production has grown remarkably in recent years and this growth is expected to continue in the coming years. This growth leads to an increase in the field of biogas purification and upgrading research. Among the variety of existing purification and upgrading techniques, one of the most used today is adsorption, specifically Pressure Swing Adsorption (PSA) as it is convenient when a cost-efficiency balance is made. To describe the dynamic behavior of an adsorption column, it is necessary to know the effluent concentration-time profile, also called the breakthrough curve. A good prediction of the breakthrough curve is essential to ensure the normal and safe operation of the process. Among the existing mathematical models to make this prediction, the Bohart-Adams model is one of the oldest and simplest in terms of mathematical application. This model considers that the operation is carried out at a constant velocity, but when the feed concentration is high, as in the case of biogas purification where we can have CO2 concentrations of up to 50%, the velocity cannot be considered constant. In this work, the different forms of the Bohart-Adams equation were studied and it was observed that this model, Tomas's and Yoon Nelson's are the same with minor modifications. The main advantage of the use of a mathematically simple model such as Bohart-Adams for the design and scale-up in an adsorption process is to shorten the times in determining the design variables and construction parameters of the equipment and this advantage is accompanied by a reduction in costs in the design and calculation stage of the process. Seeking to transfer the mathematical simplicity of this equation to cases in which high concentrations are used, tests were performed using the Bohart-Adams logistic form, studying the changes in the fit of the experimental data when a correction for variable velocity is applied in the stoichiometric time calculation. These tests were carried out with three types of adsorbents along with three different concentrations, the three adsorbents were: Activated Carbon, Pellets CuBTC and Bulk CuBTC and the concentrations used were 20%, 33% and 50% CO2. It was found that using the sigmoid or logistic form of Bohart-Adams, good adjustments were achieved even with initial concentrations of 50%. In the end, it is shown how the parameters obtained from this model are useful to make a scale-up in a simple way.A produção de biogás cresceu notavelmente nos últimos anos e espera-se que esse crescimento continue nos próximos anos. Este crescimento leva a um aumento no campo da purificação do biogás e pesquisa de atualização. Entre a variedade de técnicas de purificação e atualização existentes, uma das mais usadas hoje é a adsorção, especificamente a Adsorção por Variação de Pressão (PSA), pois é conveniente quando é feito um equilíbrio de custo-benefício. Para descrever o comportamento dinâmico de uma coluna de adsorção, é necessário conhecer o perfil concentração-tempo do efluente, também chamado de curva de ruptura. Uma boa previsão da curva de ruptura é essencial para garantir o funcionamento normal e seguro do processo. Dentre os modelos matemáticos existentes para fazer essa previsão, o modelo Bohart-Adams é um dos mais antigos e simples em termos de aplicação matemática. Este modelo considera que a operação é realizada a uma velocidade constante, mas quando a concentração de alimentação é alta, como no caso da purificação do biogás onde podemos ter concentrações de CO2 de até 50%, a velocidade não pode ser considerada constante. Neste trabalho, as diferentes formas da equação de Bohart-Adams foram estudadas e observou-se que este modelo, o de Tomas e o de Yoon Nelson são iguais, com pequenas modificações. A principal vantagem do uso de um modelo matematicamente simples como Bohart-Adams para o projeto e aumento de escala em um processo de adsorção é encurtar os tempos na determinação das variáveis de projeto e parâmetros de construção do equipamento e esta vantagem é acompanhada por um redução de custos na fase de desenho e cálculo do processo. Em busca de transferir a simplicidade matemática desta equação para os casos em que são utilizadas altas concentrações, foram realizados testes utilizando a forma logística de Bohart-Adams, estudando as alterações no ajuste dos dados experimentais quando uma correção para velocidade variável é aplicada no estequiométrico cálculo do tempo. Estes testes foram realizados com três tipos de adsorventes juntamente com três concentrações diferentes, os três adsorventes foram: Carvão Ativado, Pellets CuBTC e Bulk CuBTC e as concentrações utilizadas foram 20%, 33% e 50% CO2. Verificou-se que usando a forma sigmóide ou logística de Bohart-Adams, bons ajustes foram alcançados mesmo com concentrações iniciais de 50%. Ao final, mostra-se como os parâmetros obtidos neste modelo são úteis para fazer um scale-up de forma simples

Numerical simulation of CO2 adsorption behaviour of polyaspartamide adsorbent for post-combustion CO2 capture

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering. 10 February, 2017.Climate change due to the ever-increasing emission of anthropogenic greenhouse gases arising from the use of fossil fuels for power generation and most industrial processes is now a global challenge. It is therefore imperative to develop strategies or modern technologies that could mitigate the effect of global warming due to the emission of CO2. Carbon capture and storage (CCS) is a viable option that could ensure the sustainable use of cheap fossil fuels for energy generation with less CO2 emission. Amongst existing CCS technologies, absorption technology using monoethanolamine (MEA) is very mature and widely embraced globally. However, the absorption technology has a lot of challenges such as, low CO2 loading, high energy requirement for solvent regeneration, corrosive nature etc. On this note, the adsorption technology using solid sorbents is being considered for CO2 capture due to its competitive advantages such as flexibility, low energy requirement for sorbent regeneration, non-corrosive nature etc. On the other hand, adsorbents have a very vital role to play in adsorption technology and there is need to understand the behaviour of adsorbents for CO2 capture under different operating conditions in order to adapt them for wider applications. On this note, the study contained in this dissertation investigated the adsorption behaviour of a novel polymer-based adsorbent (polyaspartamide) during post-combustion CO2 capture using experimental study and mathematical modelling approach. Polyaspartamide is an amine-rich polymer widely used in drug delivery. In addition, its rich amine content increases its affinity for CO2. Its porosity, thermal stability and large surface area make it a promising material for CO2 capture. In view of this, polyaspartamide was used as the adsorbent for post-combustion CO2 capture in this study. This dissertation investigated the kinetic behaviour, the diffusion mechanism and rate limiting steps (mass transfer limitation) controlling the CO2 adsorption behaviour of this adsorbent. Furthermore, effect of impurities such as moisture and other operating variables such as temperature, pressure, inlet gas flow rate etc. on the CO2 adsorption behaviour of polyaspartamide was also investigated. Existing mathematical models were used to understand the kinetics and diffusion limitation of this adsorbent during CO2 capture. Popularly used gas-solid adsorption models namely; Bohart- Adams and Thomas model were applied in describing the breakthrough curves in order to ascertain the equilibrium concentration and breakthrough time for CO2 to be adsorbed onto polyaspartamide. Lagergren’s pseudo 1st and 2nd order models as well as the Avrami kinetic models were used to describe the kinetic behaviour of polyaspartamide during post-combustion CO2 capture. Parameter estimations needed for the design and optimization of a CO2 adsorption system using polyaspartamide were obtained and presented in this study. The Boyd’s film diffusion model comprising of the interparticle and intra-particle diffusion models were used to investigate the effect of mass transfer limitations during the adsorption of CO2 onto polyaspartamide. Data obtained from continuous CO2 adsorption experiments were used to validate the models in this study. The experiments were conducted using a laboratory-sized packed-bed adsorption column at isothermal conditions. The packed bed was attached to an ABB CO2 analyser (model: ABB-AO2020) where concentrations of CO2 at various operating conditions were obtained. The results obtained in this study show that temperature, pressure and gas flow rate had an effect on the adsorption behaviour of polyaspartamide (PAA) during CO2 capture. Polyaspartamide exhibited a CO2 capture efficiency of 97.62 % at the lowest temperature of 303 K and pressure of 2 bar. The amount of CO2 adsorbed on polyaspartamide increased as the operating pressure increased and a decrease in the adsorption temperature resulted in increased amount of CO2 adsorbed by polyaspartamide. The amounts of CO2 adsorbed on polyaspartamide were 5.9, 4.8 and 4.1 mol CO2/kg adsorbent for adsorption temperatures of 303, 318 and 333 K, respectively. The maximum amount of CO2 adsorbed by polyaspartamide at different flow rates of 1.0, 1.5 and 2.5 ml/s of the feed gas were 7.84, 6.5 and 5.9 mmol CO2/g of adsorbent. This shows that higher flow rates resulted in decreased amount of CO2 adsorbed by polyaspartamide because of low residence time which eventually resulted in poor mass transfer between the adsorbent and adsorbate. Under dry conditions, the adsorption capacity of polyaspartamide was 365.4 mg CO2/g adsorbent and 354.1 mgCO2/g adsorbent under wet conditions. Therefore, the presence of moisture had a negligible effect on the adsorption behaviour of polyaspartamide. This is very common with most amine-rich polymer-based adsorbents. This could be attributed to the fact that CO2 reacts with moisture to form carbonic acid, thereby enhancing the CO2 adsorption capacity of the material. In conclusion, this study confirmed that the adsorption of CO2 onto polyaspartamide is favoured at low temperatures and high operating pressures. The adsorption of CO2 onto polyaspartamide was governed by film diffusion according to the outcome of the Boyd’s film diffusion model. It was also confirmed that intra-particle diffusion was the rate-limiting step controlling the adsorption of CO2 onto polyaspartamide. According to the results from the kinetic study, it can be inferred that lower temperatures had an incremental effect on the kinetic behaviour of polyaspartamide, external mass transfer governed the CO2 adsorption process and the adsorption of CO2 onto polyaspartamide was confirmed to be a physicochemical process (both physisorption and chemisorption).MT201