Optimal Control of Load Changes for Molten Carbonate Fuel Cell Systems: A Challenge in PDE Constrained Optimization

Dynamic Operation and Simulation of Post-Combustion CO2 Capture

Compact steam bottoming cycles: Model validation with plant data and evaluation of control strategies for fast load changes

This paper presents a framework for the system integration and optimization of a molten carbonate fuel cell (MCFC) working under stationary conditions using process integration. Here, the analysis is focused on two systems in terms of the efficiency and process requirements: (i) an MCFC system alone and (ii) an MCFC system integrated with the steam turbine cycle, now onwards referred to as fuel cell combined cycle system for electric power generation. In the first system, a steady state direct internal reforming MCFC system is being simulated using desulphurized natural gas. A heat exchanger network is developed using process integration so that a minimum amount of external thermal energy is provided to the fuel cell system for electric power generation. In the second analysis, a steam turbine system is added to the first (fuel cell) one to form a fuel cell combined cycle system. The procedure for developing a network of heat exchangers and proper integration of the steam turbine system with an optimized minimum temperature difference is discussed. The results of the study elucidate the advantages of properly designed fuel cell combined cycle system to reach power demand with 17% higher efficiency as compared with the system without a combined cycle.

As a part of various research projects, a methanation pilot plant with a slurry bubble column reactor (SBCR) was commissioned and operated. The plant has a nominal load of a 100 kW methane output (lower calorific value) with a reactor diameter of 260 mm and a reactor length of 2500 mm. First experimental data on the steady-state and dynamic operation of catalytic CO2 methanation are presented. Steady-state results from laboratory-scale studies (<1 kW methane output) published previously were confirmed qualitatively at the pilot plant eliminating wall effects unavoidable in small-scale reactors. As predicted, high H2/CO2 ratios increase CO2 conversion, but excess H2 apparently promotes decomposition of the liquid phase (dibenzyltoluene) used in the bubble column reactor. Additionally, due to the increased reactor dimensions compared to laboratory equipment, it was now possible to observe a thermal response of the SBCR under conditions of rapid gas load changes characteristic of envisaged power-to-gas applications with volatile renewable electricity. As the predicted robustness of the SBCR-concept toward a dynamic operation with fast load changes was demonstrated successfully, it offers an attractive alternative to the established fixed-bed methanation technologies with their inherent limitations on dynamic operability.

최근 대체에너지에 대한 관심이 높아짐에 따라 수소에너지를 기반으로 하는 차세대 발전 장치인 연료전지 관련 기술 개발이 활발하게 이루어지고 있다. 특히, 고온 연료전지의 대표적인 형태인 용융 탄산염 연료전지(MCFC: Molten Carbonate Fuel Cell, 이하 MCFC)는 전력사업용으로의 높은 가능성을 인정받아 화석연료를 대체할 발전방식으로 평가 받고 있다. 본 연구에서는 Aspen Custom Modeler(<TEX>$ACM^{TM}$</TEX>)에서 평형반응식을 이용하여 스택 모델을 구성한 후, Aspen <TEX>$Plus^{TM}$</TEX>에서 BOP(Balance of Plant) 시스템과 스택을 연결하여 전체 MCFC 발전 시스템의 정상상태를 모사하였다. 모델의 유효성을 입증하기 위해서 전류밀도, 연료이용률, S/C ratio, 재순환 흐름 비와 같은 주요 조업변수에 따른 셀 전압, 전력, 효율 등 시스템의 성능을 분석하였다. 그리고 Aspen <TEX>$Dynamics^{TM}$</TEX>에서 PID제어 방식을 적용하여 제어루프를 구성하였고 부하변화, 설정점 변화, 재순환 흐름비 변화에 따른 각각의 사례연구를 통하여 전체 시스템의 성능변화를 예측하였다. 그 결과 연료이용률과 전류밀도의 변화에 따른 전체 시스템의 최대 발전 효율 및 출력전압을 위한 운전조건을 제안하였다. Recentincreasing awareness of the environmental damage caused by the <TEX>$CO_2$</TEX> emission of fossil fuelsstimulated the interest in alternative and renewable sources of energy. Fuel cell is a representative example of hydrogen energy utilization. In this study, Molten Carbonate Fuel Cell system is simulated by using <TEX>$Aspen^{TM}$</TEX>. Stack model is consisted of equilibrium reaction equations using <TEX>$ACM^{TM}$</TEX>(Aspen Custom Modeler). Balance of process of fuel cell system is developed in Aspen <TEX>$Plus^{TM}$</TEX> and simulated at steady-state. Analysis of performance of the system is carried out by using sensitivity analysis tool with main operating parameters such as current density, S/C ratio, and fuel utilization and recycle ratio.In Aspen <TEX>$Dynamics^{TM}$</TEX>, dynamics of MCFC system is simulated with PID control loops. From the simulation, we proposed operation range which generated maximum power and efficiency in MCFC power plant.

Molten carbonate fuel cells (MCFC) are a promising alternative power source for distributed or residential power plants. Therefore, thermodynamic models are built in an Aspen customer modeler for the externally reformed (ER) MCFC and internally reformed (IR) MCFC. These models are integrated in Aspen Plus™. In this article the performance of internal and external reforming molten carbonate fuel cell systems are investigated. To this end the gas temperature at the anode inlet is varied to be able to exam the effect of operating temperature on the operating conditions for different modes of MCFC systems in a range between 600 and 700°C. It is found that the operating temperature has more effect on the cell voltage of IR-MCFC system compared to ER-MCFC system. Simulations show that the IR-MCFC system is more efficient than the ER-MCFC system. The cycle efficiency is rather independent of the operating temperature for as well ER-MCFC as IR-MCFC systems.

Early 1996 a three year (1996 - 1998) joint European project was launched under the name {open_quote}Advanced DIR-MCFC Development{close_quote}, aiming at the development of Direct Internal Reforming (DIR) Molten Carbonate Fuel Cell (MCFC) systems for cogeneration applications for the European market. In this project participate: Brandstofcel Nederland BV (BCN), British Gas pic (BG), Gaz de France (GDF), Netherlands Energy Research foundation (ECN), Stork, Royal Schelde and Sydkraft AB. The European Fuel Cell User Group (EFCUG) supports the project as an advisory board. Whereas the US and Japanese programmes are aimed at large-scale demonstrations of the MCFC technology, this project focusses on the development of concepts and technology, required for MCFC systems that will be competative on the cogeneration market. The project partners provide the essential expertise: from end-user, system engineering, stack development up to fundamental material research.

In this paper a coal-fired power plant with CO2 capture by integrating MCFC (molten carbonate fuel cell) system is studied. Aspen Plus software is used to establish the overall system model. The thermal and economic performances of the new hybrid system are investigated and compared with the benchmark system of coal-fired power plant without CO2 capture and the benchmark system with CO2 capture by using the MEA (Mono ethanol amine) method. The effects of the key parameters on the thermal and economic performances of the new hybrid system are studied. The research results show that compared with the benchmark system without CO2 capture, the efficiency of the new hybrid system is increased and the maximum CO2 capture rate can be 96.21%. While ensuring the high CO2 capture rate, the total efficiency of the new system with CO2 capture can be improved by4.05 percent points. Compared with the conventional CO2 capture method with the MEA, the CO2 capture method based on the electrochemical method of MCFC proposed in this paper has the obvious thermal performance advantage, however, its economic performance should be further improved. Achievements in this paper will provide the valuable reference for CO2 capture of coal-fired power plant with low energy consumption.

The load-following capabilities of power plants became increasingly important in recent years as a means of ensuring a reliable operation of future power systems. In this work, we propose a generic approach, based on reachability analysis, to rigorously verify the safety of critical components that often pose limitations on the flexibility of conventional power plants to perform fast load changes. The proposed reachability algorithm makes it possible to compute the bounds of all possible trajectories for a range of operating conditions while simultaneously meeting the practical requirements of a real power plant. As an example, we consider the verification of the water level inside a drum unit. In contrast to previous work, our results are based on measurement data of a realistic configuration of a boiler system located within a 450 MW combined cycle plant in Germany. We use an abstract model which considers the modelling errors to ensure that all dynamic behaviors of the process are replicated by the abstraction. Through the implementation of our abstract model, we formally guarantee that the water level inside the drum always remains within safe limits for load changes equivalent to 40 MW which, as a result, exploits the power plant's adaptability and load-following capabilities.

A feasibility assessment of a retrofit Molten Carbonate Fuel Cell coal-fired plant for flue gas CO2 segregation

Prefeasibility techno-economic assessment of a hybrid power plant with photovoltaic, fuel cell and Compressed Air Energy Storage (CAES)

ADVANCED FUEL CELL SYSTEM CONTROL THROUGH STATE SPACE MODELING

IntroductionThe molten carbonate fuel cell (MCFC) is one of the most advanced high-temperature and clean power generating devices. To date >8 billion kWh of electricity has been produced commercially using this new technology. The high operating temperature of MCFC (550-650 °C) dramatically improves the reaction kinetics and eliminates the need for a noble metal catalyst. The electrochemical reactions taking place during cell operation involve the CO2 transfer from cathode to anode in the form of carbonate ions (Figure 1). Therefore, MCFC stack technology can be used for power generation and simultaneously, as an effective CO2 separator. MCFC systems can be combined with conventional combustion-powered generators (coal and/or natural gas based power plants) to efficiently separate CO2 in the exhaust gas and produce additional electricity.The state of the art MCFC cathode is porous lithiated nickel oxide. The cathode stability and performance are affected by several factors such as gas composition, temperature, electrode structure and electrolyte chemistry. To ensure long-term performance and material stability against NiO dissolution, polarization loss, and carbon capture efficiency, major considerations need to be taken into account in material selection, operating conditions, and electrode structure design.FCE has tested many single cells (250 cm2) and technology stacks (30 kW) under carbon capture operating conditions (4-5% CO2 in the cathode inlet as opposed to >15% in baseline MCFC systems) to understand parameters affecting performance, life, and to investigate design solutions for further enhancement. Figure 2 highlights the effect of CO2 concentration on the cathode lithiation. These measurements were conducted using an out-of-cell test (OCT) where cathode samples were exposed to different oxidizing environments. It appears that lean-CO2 gas atmospheres (<5%CO2) lead to increased lithium content in the NiO cathode. This may impact the electrolyte distribution and the cathode microstructure. Therefore, the electrolyte inventory (fill level), the lithium content and the basicity need to be considered to achieving optimal operating conditions.This paper will review the cathode material stability, microstructure, and durability under long-term carbon-capture operations. The effect of parameters such as electrolyte fill, gas composition, and electrolyte chemistry, as well as approaches to enhance the CO2 capture efficiency and life, will be discussed. Figure 1

The concept of hybrids combining fuel cell and gas turbine systems is without question neoteric, and probably is less than eight years old. However, this concept is in a sense a logical development derived from the many early systems that embodied the key features of rotating machinery to compress air. It was the introduction of high temperature fuel cells such as the solid oxide fuel cell (SOFC) that allowed the concept of hybrid gas turbine fuel cell systems to take root. The SOFC with an operating temperature circa 1000° C matched well with small industrial gas turbines that had firing temperatures on the same order. The recognition that the SOFC could be substituted for the gas turbine combustor was the first step into the realm of fuel cell topping systems. Fuel cells in general were recognized as having higher efficiencies at elevated pressures. Thus the hybrid topping system where the gas turbine pressurized the fuel cell and the fuel cell supplied the hot gases for expansion over the turbine promised to provide a high level of synergy between the two systems. Bottoming systems using the exhaust of a gas turbine as the working fluid of a fuel cell such as the molten carbonate fuel cell (MCFC) have been identified and are potential future power generation hybrid systems. The MCFC is especially well suited to the bottoming role because of the need to have carbon dioxide present in the inlet air stream. The carbon dioxide in the gas turbine exhaust allows the high temperature blower, normally used to recirculate and inject exhaust products into the inlet air, to be eliminated. Hybrid systems have the potential of achieving fossil fuel to electricity conversion efficiencies on the order of 70% and higher. The costs of hybrid systems in dollars per kilowatt are generally higher than say an advanced gas turbine that is available today but not by much. The net energy output over the life of a hybrid topping system is similar to that of a recuperated gas turbine but possibly lower than a high-efficiency simple-cycle machine, depending on the efficiency of the hybrid. Methodologies to aid in the selection of the hybrid system for future development have to be developed and used consistently. Life cycle analyses (LFA) provide a framework for such selection processes. In particular the concept of net energy output provides a mechanism to assign relative worth to competing concepts.

Optimization of boiler control to improve the load-following capability of power-plant units