Oxy-combustion Power Plants Research Articles

Energy-efficient CO2 liquefaction for oxy-combustion power plant with ASU-CPU integration enhanced by cascaded sub-ambient energy utilization and waste heat recovery

In the oxycombustion process, the cost of carbon capture is directly related to the energy requirements of the Air Separation Unit (ASU) and CO2 Processing Unit (CPU). In this paper, an integrated sub-ambient process is proposed to reduce the energy expense of these processes. Intermediate recuperative Rankine cycles (IRRC) driven by cryogenic energy from liquid N2 in the ASU are able to utilize waste heat from condensing CO2 and compression process in the CPU at high efficiency. Consequently, the high boiling point temperature difference between N2 and CO2 is converted from a problem of highly irreversible heat transfer to the benefit of efficient power recovery. Furthermore, a novel CPU process featuring post-separation direct pumping of CO2 is introduced to achieve 15.2% savings in power. Finally, the efficacy of these new processes is demonstrated and further improved by optimization of the ASU-CPU coupled system, based on an equation oriented modeling framework and large scale nonlinear programming (NLP) technology.

Dynamic exergy method and its application for CO2 compression and purification unit in oxy-combustion power plants

A systematic dynamic exergy method is proposed for identifying exergy performance dynamically to achieve efficient operation, and then examined through its applications for air compression process and CO2 compression and purification unit (CPU). In this method, the dynamic exergy calculation is based on complicated coupling procedures among steady-state simulation, dynamic simulation, and exergy calculation. Then, dynamic exergy property, energy consumptions for operating conditions, sensitivity of operating parameters, and the effect of control system can be acquired in the dynamic exergy evaluation. It is found that the proposed dynamic exergy method would be a powerful tool to uncover important information for operators and engineers. From the results of CPU, flue gas composition ramp up process runs more effectively than that of flow rate ramp down and composition ramp down processes. CPU is more sensitive to the variation of flue gas composition than that of flue gas flow rate since smaller sensitive factor is obtained in the latter case. Control system saves energy during flue gas composition ramp down process however consumes energy in flow rate ramp down and composition ramp up processes. Meanwhile, smaller control costs are required for flue gas flow rate ramp case than that of composition ramp case.

Oxy-combustion power plant integration in an oil refinery to reduce CO2 emissions

The refining sector comprises a lot of different processes, since a wide range of final products are obtained in refineries. In any case, CO2 emissions levels are high and three units can be considered as the main sources accounting the majority of carbon dioxide emissions to the atmosphere: the utility boilers system, the process heaters and the fluid catalytic cracking unit (FCC). In this study, the substitution of the utility boilers system for an oxy-combustion power plant is presented. The power plant is designed to cover the steam thermal and electricity demands of oil refinery. In addition, a heat integration methodology based on pinch analysis is applied to the whole concept for increasing efficiency. Final results show that more than 380,000tonnes of CO2 could be avoided. Afterwards, a complete economic approach is carried out. Although profitability is still low with current economic costs, it is expected that in the medium term, oxy-combustion could be economically feasible in refining sector. To this extent, sensitivity analysis shows affordable values for main economic parameters that can be reached in the next years. A 24€/MWh natural gas price reduces operational cost for oxy-combustion power plant below the conventional refinery value. An electricity price higher than 67€/MWh and a CO2 cost around 20€/tCO2 also seem to reduce operational cost below reference case ones. The study concludes with the analysis of some realistic future scenarios that prove the oxy-combustion technology both technically and economically feasible and payback periods between 9 and 11 years with IRR values around 7% can be affordable within the following years.

Oxy-combustion of coal, lignite and biomass: A techno-economic analysis for a large scale Carbon Capture and Storage (CCS) project in Romania

Power generation sector is facing important challenges to develop energy efficient solutions at the same time with reducing the greenhouse gas emissions (mainly CO2). Oxy-fuel combustion is a promising power generation technology for reducing both energy and cost penalties for CO2 capture. This paper presents a detailed techno-economic analysis for oxy-combustion power plant to generate about 350MW net power with a carbon capture rate higher than 90%. Both fossil fuels (coal and lignite) and renewable energy sources (sawdust) were used to fuel a super-critical power plant (live steam parameters: 582°C/29MPa). The assessment is based on numerical analysis, the models of various power plant sub-systems being built in ChemCAD and Thermflow software. As benchmark option used to quantify the CO2 capture energy and cost penalties, the same super-critical power plant without CCS was considered. The investigated coal, lignite and sawdust oxy-combustion cases show an energy penalty of 9–12 net efficiency percentage points, 37–50% increase of total capital investment, the O&M costs are increasing with 7–15% and the electricity cost with 54–95% (all compared to coal-fuelled non-CCS case). Sensitivity studies were also performed to evaluate the influence of various economic parameters on electricity and CO2 avoidance costs.

Techno-economic analysis of a 15 MW corn-rape oxy-combustion power plant

Oxy-combustion is a promising technology to enable the capture of CO2 from power plants based on diverse raw material. This paper describes and compares the results of technical and economic viability studies of a 15MW power plant based on a corn-rape blend oxy-combustion, a bituminous coal-combustion and a bituminous coal oxy-combustion located in the north of Spain. Technical simulations were implemented in Aspen Plus© software. The economic viability studies were based on the typical economic indicators such as net present value, rate of return, specific total plant investment, cost of electricity, cost of CO2 avoided and energy consumption for CO2 avoided. A sensitivity analysis was carried out in order to identify the most important factors affecting the investment risks. The use of a cryogenic air separation unit was associated with a reduction in oxy-combustion plant viability. While coal-combustion process reported an enough profitability, a supplementary public grant of 63€/MWh and 57€/MWh were required in coal oxy-combustion process and biomass oxy-combustion process respectively to obtain a positive profitability. A stability of raw material and electricity markets was found to be necessary to improve a risk reduction for the potential investment.

Plantwide control and operating strategy for air separation unit in oxy-combustion power plants

To ensure that air separation unit can bear high ramp rate and flexible operations when integrated into oxy-combustion power plant, this paper focuses on the investigations of plantwide control design and operating strategy for double-column air separation unit without or with liquid oxygen storage drum through dynamic simulation. It is found that three air separation unit types (normal, oversize, and downsize) with the proposed control structure configured with logic control loops (strategies 1, 2, and 3) can achieve 5%/min of high ramp rate and flexible operations such as peak and off-peak or/and energy storage. Compared between the latter two logical controls, strategy 2 would be better because its oxygen product qualities can be maintained closer to their targets. Three integrated patterns between air separation unit and oxy-combustion boiler are proposed to improve operating flexibility, among which oversize case would be more suitable for oxy-combustion power plant since its energy saving (20.33MW) is larger and operating strategy is more flexible than those in downsize (10.76MW) and normal cases. Process control for flexible operations would be an opportunity for engineering, and comprehensive dynamic model with specified control system provides possibility to integrate air separation unit with full-train oxy-combustion power plants.

Thermoeconomic cost analysis of CO2 compression and purification unit in oxy-combustion power plants

High CO2 purity products can be obtained from oxy-combustion power plants through CO2 compression and purification unit (CPU) based on phase separation method. To identify cost formation process and potential energy savings for CPU, detailed thermoeconomic cost analysis based on structure theory of thermoeconomics is applied to an optimized CPU (with double flash separators). It is found that the largest unit exergy cost occurs in the first separation process while the multi-stage CO2 compressor contributes to the minimum unit exergy cost. In two flash separation processes, unit exergy costs for the flash separator and multi-stream heat exchanger are identical but their unit thermoeconomic costs are different once monetary cost for each device is considered. For cost inefficiency occurring in CPU, it mainly derives from large exergy costs and thermoeconomic costs in the flash separation and mixing processes. When compared with an unoptimized CPU, thermoeconomic performance for the optimized CPU is enhanced and the maximum reduction of 5.18% for thermoeconomic cost is attained. To achieve cost effective operation, measures should be taken to improve operations of the flash separation and mixing processes.

Effect of operating pressure and fuel moisture on net plant efficiency of a staged, pressurized oxy-combustion power plant

The immediate need for a high efficiency and low cost carbon capture process has prompted the recent development of pressurized oxy-combustion. With an increase in combustion pressure, the dew point of the flue gas is increased, allowing for effective integration of the latent heat of flue gas moisture into the Rankine cycle. This increases the net plant efficiency and reduces costs. The Staged, Pressurized Oxy-Combustion (SPOC) process further enhances the efficiency and reduces costs by reducing the recycle of flue gas to near zero. Fuel staging and a novel boiler/burner design are used to control temperature and heat flux. Compared to a first generation oxy-combustion process, the SPOC process results in an increased net plant efficiency of about 6 percentage points. In this work, the effects of operating pressure and fuel moisture content on the net plant efficiency are discussed. It is shown that the effect of pressure is small for pressures higher than about 16bar, while the effect of fuel moisture is quite significant. Net plant efficiency gradually decreases with increasing fuel moisture until the heat saturation value, beyond which additional latent heat cannot be integrated into the steam cycle and the efficiency reduction is more dramatic. The effect of slurry feeding of coal on plant efficiency is discussed.

Mercury Retention by Fly Ashes from Oxy-fuel Processes

The objective of this study is to determine the mechanism of mercury retention in fly ashes, the main solid waste from coal combustion power plants, and to evaluate the interactions between the type of mercury and fly ashes. The work was based on the results of mercury speciation in the gas and the solid fly ash before and after mercury retention. The identification of the mercury species in the gas was performed using previously validated methods, but the speciation of the mercury retained in the fly ashes was carried out using a mercury temperature-programmed desorption technique (HgTPD) still under development. The fly ashes were sampled from conventional coal combustion in air and oxy-combustion power plants. The main mercury species identified in the raw fly ashes and after they were subjected to an oxy-combustion atmosphere were mercury bound to organic matter and HgS, the ratio of these species depending on the characteristics of the ashes. The results obtained indicate that fly ashes are the route of mercury oxidation in an oxy-combustion atmosphere, although they hardly retain any mercury unless the unburned carbon content is high. HgTPD analysis shows that the main mechanism for mercury retention in the fly ashes is via the carbon matter.

Optimization and control for CO2 compression and purification unit in oxy-combustion power plants

High CO2 purity products can be obtained from oxy-combustion power plants through a CO2 CPU (compression and purification unit) based on phase separation method. To ensure that CPU (with double flash separators) can be operated under optimal conditions, this paper focuses on single variable analysis, multi-variable optimization, dynamic simulation and control system design for CPU in oxy-combustion power plants. It is found that optimal operating conditions are 30 bar, 30.42 °C, −24.64 °C, and −55 °C for multi-stage CO2 compressor discharge pressure, flue gas temperature after compression, first flash separator temperature, and second flash separator temperature, respectively. The designed double temperature control structure based on a systematic top-down analysis and bottom-up design method is more suitable than the single temperature control structure under different operating scenarios (load change and flue gas composition ramp change). To bear operating disturbances, operating strategies like elevating temperature, manipulating valves and adjusting setpoints are proposed. Influence of SOx would be more obvious than that of NOx, whilst the kij mixing parameters in Peng–Robinson property method affects little on process optimization and control system design. Comprehensive dynamic model with specified control system provides possibility to integrate CPU with full-train oxy-combustion power plants.

A comparative thermodynamic, economic and risk analysis concerning implementation of oxy-combustion power plants integrated with cryogenic and hybrid air separation units

This paper presents a comparison of two types of oxy-combustion power plant that differ from each other in terms of the method of oxygen separation. For the purpose of the analysis, detailed thermodynamic models of oxy-fuel power plants with gross power of approximately 460MW were built. In the first variant (Case 1), the plant is integrated with a cryogenic air separation unit (ASU). In the second variant (Case 2), the plant is integrated with a hybrid membrane–cryogenic installation. The models were built and optimized using the GateCycle, Aspen Plus and Aspen Custom Modeller software packages and with the use of our own computational codes. The results of the thermodynamic evaluation of the systems, which primarily uses indicators such as the auxiliary power and efficiencies of the whole system and of the individual components that constitute the unit, are presented. Better plant performance is observed for Case 2, which has a net efficiency of electricity generation that is 1.1 percentage points greater than that of Case 1. For the selected structure of the system, an economic analysis of the solutions was made. This analysis accounts for different scenarios of the functioning of the Emission Trading Scheme and includes detailed estimates of the investment costs in both cases. As an indicator of profitability, the break-even price of electricity was used primarily. The results of the analysis for the assumptions made are presented in this paper. A system with a hybrid air separation unit has slightly better economic performance. The break-even price of electricity in this case is approximately 3.4€/MWh less than for the system with a cryogenic unit. The main risk factors concerning implementation of such technologies were identified. The analysis of risk connected with the selection of these technologies was performed with the use of two methods – sensitivity analysis and a Monte Carlo method. The values of the probability were calculated, and the main results are presented and discussed in the paper.

Influence of a CO2-enriched flue gas on mercury capture by activated carbons

The main environmental problem caused by the production of energy from coal combustion is the emission of CO2. One emerging technology designed for CO2 capture is oxy-combustion. Among other issues to be solved in oxy-combustion power plants is the presence of mercury as this may damage the CO2 compression unit. Hence the study of the behavior of mercury in oxy-combustion is of great interest both from an environmental and a technological point of view. The present study performed at laboratory scale evaluates the retention of mercury in a CO2-enriched flue gas using the same activated carbon before and after it has been impregnated with sulfur and proposes a mechanism to explain the interactions between mercury and activated carbons. The results show that carbonyl and quinone groups are responsible for mercury oxidation and retention in the carbons. Although the contact time between mercury and the activated carbon surface limits the amount of mercury that can be captured, high retention capacities can be achieved in an oxy-combustion atmosphere. The presence of water, in high concentrations in oxy-combustion, may compete for the same active sites (carbonyl groups) as mercury, thereby inhibiting mercury adsorption on the surface of the activated carbons. Moreover, the presence of sulfur in the impregnated material, which is the key to mercury capture in other atmospheres, does not modify mercury capture in oxy-combustion.

Dynamic modeling and control for pulverized-coal-fired oxy-combustion boiler island

Oxy-fuel combustion is one of competitive and promising carbon capture technologies for restraining CO2 emissions from power plants. Dynamic simulation of oxy-combustion system, which is essential for gaining its dynamic characteristics, can help to evaluate and improve process design, and to develop control system and operational strategies. This paper focuses on dynamic simulation and control design for a conceptual 600MWe oxy-combustion pulverized-coal-fired boiler. The steady-state model using a pseudo coal to substitute real coal is established, validated, and then transformed into the dynamic model which uses pressure-driven solution. The control system, which aims to automatically regulating flue gas O2 concentration within reasonable range (2–7mol.% in this study), is designed and applied to the dynamic boiler model. Three planned disturbances of load change, oxygen purity (from cryogenic air separation unit) ramp change and air in-leakage (from boiler) step change are conducted. Detailed dynamic behavior from water side and flue gas side is obtained and analyzed to help engineering operation in real plant. Mode switching process and alternative control strategy are simulated and discussed. Comprehensive dynamic model with specified control system provides possibility to study the dynamic characteristics of full-train oxy-combustion power plants.

Oxy-combustion coal based power plants: study of operating pressure, oxygen purity and downstream purification parameters

Oxy-combustion coal based power plants: study of operating pressure, oxygen purity and downstream purification parameters

A framework for efficient large scale equation-oriented flowsheet optimization

Despite the economic benefits of flowsheet optimization, many commercial tools suffer from long computational times, limited problem formulation flexibility and numerical instabilities. In this study, we address these challenges and present a framework for efficient large scale flowsheet optimization. This framework couples advanced process optimization formulations with state-of-the-art algorithms, and includes several notable features such as (1) an optimization-friendly formulation of cubic equation of state thermodynamic models; (2) a new model for distillation column optimization based on rigorous mass, equilibrium, summation and heat (MESH) equations with a variable number of trays that avoids integer variables; (3) improvements on the Duran–Grossmann formulation for simultaneous heat integration and flowsheet optimization; and (4) a systematic initialization procedure based on model refinements and a tailored multi-start algorithm to improve feasibility and identify high quality local solutions.Capabilities of the framework are demonstrated on a cryogenic air separation unit synthesis study, including two thermally coupled distillation columns and accompanying multistream heat exchangers. A superstructure is formulated that includes several common ASU configurations in literature. As part of the optimization problem the solver selects the best topology in addition to operating conditions (temperatures, flowrates, etc.) for coal oxycombustion applications. The optimization problem includes up to 16,000 variables and 500 degrees of freedom, and predicts specific energy requirement of 0.18 to 0.25kWh/kg of O2 depending on design assumptions. These results are compared to literature and plans to extend the framework to an entire coal oxycombustion power plant optimization study are discussed.

Optimal design and operation of membrane-based oxy-combustion power plants

This article focuses on optimal design and operation of AZEP (advanced zero emissions power) cycles, both zero and partial emissions. The first-law efficiency is maximized as a function of CO2 emissions with fixed ITM (ion transport membrane) size and consequently, variable power output. A two-step heuristic global optimization of the power cycle is performed. In the first step, the top cycle is optimized. In the next step, the bottoming cycle is optimized for fixed conditions of the top cycle. This procedure is repeated with different initial guesses for the optimization variables of the top cycle to obtain a near-global optimum. The optimization results in a significant increase in the efficiencies of AZEP100 and partial emissions cycles. This increase in efficiency is important with respect to viability of the partial emissions cycle compared to alternative power cycles. This viability is determined herein using a linear combination metric, which combines efficiency and CO2 emissions. Optimization and simulations have shown that reducing the maximum membrane temperature results in an increase in the efficiency till membrane temperature reaches 850 °C, after which the efficiency starts decreasing. However, reduced temperature results in dramatic drop in net power output of the power plant.

Process design and performance analysis of a Staged, Pressurized Oxy-Combustion (SPOC) power plant for carbon capture

Support for carbon capture, utilization and storage (CCUS) for coal power plants is limited, to some extent, by the high cost of electricity associated with available technologies. While a comparison of different capture methods favors oxy-combustion technology, the cost is still prohibitively high and the efficiency is low. The requirement that CO2 must ultimately be delivered at high pressure for storage or enhanced oil recovery has led to interest in pressurizing the combustion process, whereby the latent heat in the flue gas moisture can be largely recovered and integrated into the Rankine cycle, thus increasing the plant efficiency. In this work, the performance of a novel Staged, Pressurized Oxy-Combustion (SPOC) process is presented, which has the potential of increasing plant efficiency even further via reduction of auxiliary loads associated with flue gas recycle and flue gas cleanup. Heat flux and temperature are controlled with fuel staging and burner design. ASPEN Plus modeling of a conceptual power plant using the SPOC approach shows an improvement in efficiency of over 6 percentage points over first generation atmospheric oxy-combustion technology. The process design and results from ASPEN Plus modeling are described.

Coal Oxycombustion Power Plant Optimization Using First Principles and Surrogate Boiler Models

Numeric optimization has successfully been applied in the chemical industry to improve process performance, reduce emissions, create advanced control systems and intelligently explore process design alternatives. Regarding power plants, optimization methods provide a systematic approach to balance trade-offs when designing efficient and minimal cost carbon capture systems. Coal oxycombustion power plants are an ideal candidate for optimization, given the complex trade-offs regarding flue gas recycle strategies, heat integration and design of cryogenic separation systems.Many oxycombustion studies consider sensitivity analysis instead of multi-variable optimization, which neglect consequentially important interactions between subsystems. Furthermore, these studies rely on over- simplified boiler models available in commercial process simulators. In this paper we present a hybrid 1D/3D boiler model that balances accuracy and computational expense, making it well suited for optimization studies of an entire coal power plant. This model is then incorporated into an equation-based optimization framework for power plants. Trust region optimization methods are used to ensure convergence. Finally a proof-of-concept case study is presented, and future work is discussed.

Performance Simulation of Full-scale Wet Flue Gas Desulfurization for Oxy-coal Combustion

Abstract Oxy-coal combustion implies significant changes on the power plant design and operation compared to air-firing. In particular, the new operating conditions cause modifications on the process design and performance of the wet flue gas desulfurization (FGD) unit. It has been previously showed at pilot plant scale that high SO2 removal efficiency is achieved and no substantial problems have been observed while operating in oxy-combustion conditions. This study intends to meet the needs for a full-scale analysis of the process performances in order to investigate the wet FGD potential. To do so, a dynamic model already developed in a previous work for air-fired power plants has been adapted to deal with these new process conditions and used to perform an investigation of the wet FGD capability when operating under oxy-combustion conditions. The process design and operating parameters are determined to optimally achieve process performances (energetic, economics). There is no significant difficulty to achieve targeted performance. A conservative SO2 outlet of 200 mg/Nm 3 can be achieve easily both for low and high sulfur coal. Warm recycle increases the oxy-combustion power plant efficiency, reduces the flue gas to be treated but increases the SO2 content by 60%. To cope with this increase inlet SO2, the L/G ratio is increased by 80%; but the investment cost is reduced by 25% compared with cold recycle. Finally, a global design of the FGD+DCCPS equipment is needed in order to achieve optimal performance of the flue gas desulfurization at reasonable cost.

Exergy Analysis and Heat Integration of a Coal-Based Oxy-combustion Power Plant

Oxy-combustion is a competitive technology to enable the capture of CO2 from coal-based power plants. The main challenge to implement this technology is the large energy penalty and investment cost related to CO2 capture. A comprehensive exergy analysis has been performed for an entire coal-based oxy-combustion power plant. The exergy flows and irreversibilities are presented. The thermal efficiency penalty related to CO2 capture is about 9.4 percentage points (on the basis of the higher heating value) and is mainly caused by the air separation unit and the CO2 compression and purification unit. The theoretical minimum is 3.4 percentage points when the two units are assumed to be reversible, where the air separation unit contributes 1.4 percentage points and the CO2 compression and purification unit is responsible for 2.0 percentage points. The compression processes are causing the largest exergy losses related to CO2 capture. The case studies show that the integration of the compression heat with the steam cycle can increase the thermal efficiency by up to 0.72 percentage points.