Fossil-fired Power Plants Research Articles

Potential Dynamics of CO2 Stream Composition and Mass Flow Rates in CCS Clusters

Temporal variations in CO2 stream composition and mass flow rates may occur in a CO2 transport network, as well as further downstream when CO2 streams of different compositions and temporally variable mass flow rates are fed in. To assess the potential impacts of such variations on CO2 transport, injection, and storage, their characteristics must be known. We investigated variation characteristics in a scenario of a regional CO2 emitter cluster of seven fossil-fired power plants and four industrial plants that feed captured CO2 streams into a pipeline network. Variations of CO2 stream composition and mass flow rates in the pipelines were simulated using a network analysis tool. In addition, the potential effects of changes in the energy mix on resulting mass flow rates and CO2 stream compositions were investigated for two energy mix scenarios that consider higher shares of renewable energy sources or a replacement of lignite by hard coal and natural gas. While resulting maximum mass flow rates in the trunk line were similar in all considered scenarios, minimum flow rates and pipeline capacity utilisation differed substantially between them. Variations in CO2 stream composition followed the power plants’ operational load patterns resulting e.g., in stronger composition variations in case of higher renewable energy production.

Mysterious failure in load-free superalloys under repeated thermal shocks

The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or “fingers” is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions.

Stress relief investigation using creep model considering back stress in welded rotor

Welded ferritic steel rotors are a critical dynamic component of fossil-fired power plants. It is essential to investigate residual stress distribution in welded rotors and investigate methods to reduce residual stress for improving structural integrity. In the present study, a creep model that considers back stress is introduced in the residual stress relief simulation of welded ferritic steel rotors in furnace-based uniform post weld heat treatment. Comparisons between the new and Norton–Bailey creep model were conducted numerically, and results indicate that the new creep model can inherit the as-welded residual stress distribution and simulate post weld heat treatment (PWHT) residual stress more accurately, which has been validated with experimental results. Furthermore, it is discovered that the dominant stress relief mechanism of ferritic steel is creep-strain-induced stress relaxation. The creep strain occurred primarily in a short time with a dislocation creep mechanism, and the holding time hardly affected the generation of creep strain. Increasing the PWHT temperature enables more stress relaxation at low temperatures. Additionally, stress relaxation almost no longer occurred when the PWHT temperature exceeded a critical value (approximately 520 °C in this study) that was lower than the recrystallisation temperature. Hence, it is important to control the welded residual stress.

Mysterious Failure in Load-Free Superalloys Under Repeated Thermal Shocks

Abstract The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or “fingers” is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions.

Dynamics for Sustainable Nuclear Buildup Based on LWR and FBR Technologies and Its Impact on CO2 Emission Reduction

In recent years, most of the growth in electricity demand is covered by renewable and nuclear energy sources. However, electricity generation in fossil-fired power plants is also increasing resulting in the increase of CO2 emissions. Nuclear energy has to be considered as one of the available tools to accomplish CO2 emission reduction in electricity sector. Light water reactors (LWR) are currently the dominant nuclear technology but their intensive application in long-term period is constrained by available uranium fuel resources. Fast breeder reactors’ (FBR) technology is not used on a larger scale. Plutonium resources are limited, but do have the potential of stronger buildup if light water reactors, as the source of plutonium, are used on a larger scale. The appropriate dynamics for LWR/FBR buildup till the end of the 21st century is developed under assumptions of different LWR life times, and different uranium fuel resources available. The possible CO2 emission reduction is calculated with World Energy Outlook 2015 development scenarios being set as reference ones. It is shown that nuclear fuel resources do not represent an obstacle for strong nuclear buildup leading to significant CO2 emission reduction. However, the reduction is mostly achieved in the second half of the century.

Validation of an alloy design strategy for stable Fe–Cr–Al–Nb-X ferritic alloys using electron microscopy and atom probe tomography

The Fe–Cr–Al–Nb ferritic alloys strengthened by A2B Laves phases, with superior oxidation and corrosion resistance, are being considered for high temperature operation within fossil-fired steam power plants to increase process efficiency and reduce CO2 emissions. In this study, new sets of alloys based on Fe–30Cr–3Al–1Nb (in weight percent) were designed with (i) a high microstructural stability of Laves phase precipitates in a BCC-Fe matrix and (ii) reduced precipitate free zones along the grain boundaries, targeting an operating temperature of 700 °C. Two alloys with titanium or tungsten additions were down-selected through thermodynamic calculations with a design strategy to maximize the amount of Laves phase at 700 °C and lower the BCC solvus temperature. To validate this design strategy, the candidate alloys were cast, heat treated and aged at 700 °C, and the resulting microstructure was characterized using scanning electron microscopy and atom probe tomography. The alloys with titanium addition (1 wt%) showed monotonic precipitate coarsening. On the other hand, the alloys with tungsten addition (6 wt%) showed a reduced coarsening rate of Laves phase precipitates in the matrix up to 1008 h aging, whereas an acceleration of the precipitate coarsening after that. Atom probe tomography of this alloy revealed the formation of a core-shell precipitate structure with a Nb/Si rich core and W-rich shell, while the alloy with Ti-addition did not show the core-shell structure. The detailed characterization results revealed that the core-shell structure was strongly correlated with the observed two-step coarsening mode in the tungsten containing alloy.

How Effective Was the UK Carbon Tax? A Machine Learning Approach to Policy Evaluation

Carbon taxes are commonly seen as a rational policy response to climate change, but little is known about their performance from an ex-post perspective. This paper analyzes the emissions and cost impacts of the UK CPS, a carbon tax levied on all fossil-fired power plants. To overcome the problem of a missing control group, we propose a novel approach for policy evaluation which leverages economic theory and machine learning techniques for counterfactual prediction. Our results indicate that in the period 2013-2016 the CPS lowered emissions by 6.2 percent at an average cost of € 18 per ton. We find substantial temporal heterogeneity in tax-induced impacts which stems from variation in relative fuel prices. An important implication for climate policy is that a higher carbon tax does not necessarily lead to higher emissions reductions or higher costs.

Component Testing and Numerical Calculation of a Bolted High Temperature Power Plant Pipe Flange Under Complex, Near-Service Loads

Abstract To ensure a reliable power supply with a minimum environmental impact in the future, further increases in efficiency and flexibility of fossil-fired power plants are major challenges to address in recent R&D activities. In order to counterbalance substantial fluctuations in the electricity grid due to the rising share of renewable resources, frequent start-ups and shut-downs of turbomachinery will be claimed by the market. Hence, along with the development of advanced materials for elevated steam parameters, contemplations with respect to altered boundary conditions of established materials, e.g., for bolted pipe flange connections, are required. In this paper, a selection of results from a recently finished research work on stress relaxation will be presented. Together with European power plant component manufacturers, a newly developed test rig comprising a scaled intermediate pressure (IP) steam turbine pipe flange now allows investigations under near-service conditions. Before, throughout, and after performance of the experiments, an extensive measurement setup enables the examination of effects that cannot be studied in standardized relaxation tests. Within this work, a loss in bolt pretension of up to almost 50% over a comparatively short period of experimental time was observed. By means of numerical calculations, creep deformation in the transition areas between pipe and flange plate sections was identified to be a major originator of these pretension drops. Particularly, good correlations between experiments and finite element method (FEM) simulations could be achieved through the implementation of enhancements concerning the fitting strategy of the Graham–Walles-type creep model which was used.

Current status of research on heat transfer in forced convection of fluids at supercritical pressures

Investigation of heat transfer at supercritical pressures began as early as the 1930s with the study of free-convection heat transfer to fluids at a near-critical point. In the 1950s, the concept of using supercritical “steam” to increase thermal efficiency of fossil-fired thermal power plants became an attractive option. Germany, USA, the former USSR, and some other countries extensively studied supercritical heat transfer during the 1950s till the 1980s. This research was primarily focused on bare circular tubes cooled with SuperCritical Water (SCW). However, some studies were performed with modeling fluids such as SC carbon dioxide and refrigerants instead of SCW. Currently, the use of supercritical “steam” in fossil-fired thermal power plants is the largest industrial application of fluids at supercritical pressures.Near the end of the 1950s and at the beginning of the 1960s, several studies were conducted to investigate a potential of using SCW as a coolant in nuclear reactors. However, these activities were abandoned for some time and regain momentum in 1990s. In support of development of SuperCritical Water-cooled nuclear-Reactor (SCWR) concepts, first experiments have been started in annular and various bundle-flow geometries. At the same time, more numerical and CFD studies have been performed in support of our limited knowledge on specifics of heat transfer at SC Pressures (SCPs) in various flow geometries.As the first step in this process, heat transfer to SCW in vertical bare tubes can be investigated as a conservative approach (in general, heat transfer in fuel bundles will be enhanced with various types of appendages, i.e., grids, end plates, spacers, bearing pads, fins, ribs, etc.). New experiments in 1990s–2000s were triggered by several reasons: 1) thermophysical properties of SCW have been updated from 1950s to 1970s, e.g., a peak in thermal conductivity in the critical/pseudocritical points was “officially” introduced in 1990s; 2) experimental techniques have been improved; 3) in SCWRs various bundle flow geometries will be used instead of bare-tube geometry; and 4) in SC steam generators of thermal power plants larger diameter tubes/pipes (20–40 mm) are used, however, in SCWRs hydraulic-equivalent diameters of proposed bundles will be within 5–12 mm.A comparison of selected SCW heat-transfer correlations has shown that their results may differ from one to another by more than 200%. Based on these comparisons, it became evident that there is a need for a reliable, accurate and wide-range SCW heat-transfer correlation to be used. Therefore, the objective of this paper is to summarise in concise form the work performed concerning with the heat transfer at SCPs, based mainly on examples of water, carbon dioxide, and R-12.

Analysis and calculation of enclosed coal yard ventilation by using roof vortex flow natural ventilation

As the unsatisfied environment of the fossil fired power plant become more controversial and disputable, the current tendency of enclosed coal yard which helps to decrease air pollution gains more application than those open ones. In order to improve the indoor environment and ventilation effect as well as to achieve good energy conservation at the same time, this paper investigates the ventilation effect of the roof vortex flow natural ventilator. By using multivariate nonlinear regression analysis and the theoretical calculation of natural ventilation, this paper proposes an instructive natural ventilation model and calculation method of enclosed coal yard based on the indoor pollutants concentration control, which is highly recommended as a good design application in similar enclosed coal yard buildings.

Transient Analysis and Execution-Level Power Tracking Control of the Concentrating Solar Thermal Power Plant

Concentrating solar power (CSP) is a promising technology for exploiting solar energy. A major advantage of CSP plants lies in their capability of integrating with thermal energy storage; hence, they can have a similar operability to that of fossil-fired power plants, i.e., their power output can be adjusted as required. For this reason, the power output of such CSP plants is generally scheduled to maximize the operating revenue by participating in electric markets, which can result in frequent changes in the power reference signal and introduces challenges to real-time power tracking. To address this issue, this paper systematically studies the execution-level power tracking control strategy of an CSP plant, primarily aiming at coordinating the control of the sluggish steam generator (including the economizer, the boiler, and the superheater) and the fast steam turbine. The governing equations of the key energy conversion processes in the CSP plant are first presented and used as the simulation platform. Then, the transient behavior of the CSP plant is analyzed to gain an insight into the system dynamic characteristics and control difficulties. Then, based on the step-response data, the transfer functions of the CSP plant are identified, which form the prediction model of the model predictive controller. Finally, two control strategies are studied through simulation experiments: (1) the heuristic PI control with two operation modes, which can be conveniently implemented but cannot coordinate the control of the power tracking speed and the main steam parameters, and (2) advanced model predictive control (MPC), which overcomes the shortcoming of PI (Proportional-Integral) control and can significantly improve the control performance.

How Effective Was the UK Carbon Tax? - A Machine Learning Approach to Policy Evaluation

Carbon taxes are commonly seen as a rational policy response to climate change, but little is known about their performance from an ex-post perspective. This paper analyzes the emissions and cost impacts of the UK CPS, a carbon tax levied on all fossil-fired power plants. To overcome the problem of a missing control group, we propose a novel approach for policy evaluation which leverages economic theory and machine learning techniques for counterfactual prediction. Our results indicate that in the period 2013-2016 the CPS lowered emissions by 6.2 percent at an average cost of € 18 per ton. We find substantial temporal heterogeneity in tax-induced impacts which stems from variation in relative fuel prices. An important implication for climate policy is that a higher carbon tax does not necessarily lead to higher emissions reductions or higher costs.

Design assessment of a 5 MW fossil-fired supercritical CO2 power cycle pilot loop

Abstract The concept of the supercritical CO2 (S-CO2) power cycle has been widely proved to be effective by several small scale test loops. However, more specific system layout for fossil-fired power generation and key components design assessment is still imperative to demonstrate the technology feasibility of commercial scale utility. The S-CO2 power cycle must be optimized to deal with fossil-fired system integration constraints. In the present study the technology adaption of S-CO2 power cycle for fossil-fired power plant has been evaluated in terms of both the whole thermodynamic cycle layout and the preliminary assessment of key components. The design considerations and selections of key parameters such as turbine inlet parameters, compressor inlet parameters and split flow ratios were analyzed by a self-developed code for the purpose of optimization design of a 5 MW fossil-based S-CO2 pilot test loop. The proposed recompression and reheat cycle with two split ratios tailored for fossil-fired power plants can achieve 33.49% net efficiency. As a first step to the final recompression cycle, the design of simple recuperated and reheat cycle after carefully considerations of the specific design assessment of core components such as boiler, turbines, compressor and compact heat exchangers has been accomplished and currently under construction.

Robust optimization of a post-combustion CO2 capture absorber column under process uncertainty

This article presents a study on the optimal design and operation of a pilot-scale absorber column in post-combustion CO2 capture in the presence of uncertainty. A mechanistic model describing the behaviour of a post-combustion CO2 absorber column is explicitly considered in the present robust optimization formulation. This formulation takes into account process uncertainty, which will impact the absorber column operation due to seasonal or unexpected changes in the operating policies of a fossil-fired power plant, e.g. changes in the flue gas stream. Different scenarios were assessed in order to evaluate the impact of uncertainty on the optimal process design and operation. The results show that environmental constraints determine the impact of process uncertainty in the operation of the absorber column. Similarly, the operation of the absorber unit may become infeasible even when process design specifications are estimated under limited information on process uncertainty.

How Effective Was the UK Carbon Tax?—A Machine Learning Approach to Policy Evaluation

Carbon taxes are commonly seen as a rational policy response to climate change, but little is known about their performance from an ex-post perspective. This paper analyzes the emissions and cost impacts of the UK CPS, a carbon tax levied on all fossil-fired power plants. To overcome the problem of a missing control group, we propose a novel approach for policy evaluation which leverages economic theory and machine learning techniques for counterfactual prediction. Our results indicate that in the period 2013-2016 the CPS lowered emissions by 6.2 percent at an average cost of € 18 per ton. We find substantial temporal heterogeneity in tax-induced impacts which stems from variation in relative fuel prices. An important implication for climate policy is that a higher carbon tax does not necessarily lead to higher emissions reductions or higher costs.

A comparative CFD analysis of NACA0012 and NACA4412 airfoils

Wind energy has been seen as one of the most suitable sources of renewable energy. Wind energy is low cost when compared the other sources. Therefore, wind energy can gain an edge over the fossil-fired power plants. Aerodynamic efficiency of the airfoil is very crucial for aerodynamic efficiency of the wind turbine. The primary purpose of our study was to analyze the NACA0012 and NACA4412 airfoil at various attack angles with constant Reynolds number and to examine the effects of the symmetrical and asymmetrical profiles of the airfoil. Analysis of aerodynamic performance of NACA0012 and NACA4412 airfoil were performed with using ANSYS Fluent program. Also, lift coefficients and drag coefficients were calculated at various attack angles. According to calculations, optimum attack angles were found for each profile. Finally, NACA0012 and NACA4412 airfoils were discussed and reported in terms of their airfoil performances.

Recent advances on first-principles modeling for the design of materials in CO2 capture technologies

Novel technologies in consideration of industrial sustainability (IS) are in urgent need to satisfy the increasing demands from the society. IS realizes the production of materials while maintaining environmental and resource sustainability. The chemical materials used in CO2 capture and storage (CCS) technologies play a significant role in the disposal of greenhouse gas emissions coming from large stationary fossil-fired power plants, which breaks the principle of IS and brings severe environmental problems. This study aims at providing a detailed review of first-principles modeling (density functional theory, DFT) of materials in CO2 capture technologies. DFT analysis provides insight into the atomic properties of the studied systems and builds an efficient guidance of the future design of the materials used in CO2 capture technologies. Major materials including oxygen carriers, metal organic frameworks, membranes, zeolites, ionic liquids and some other promising candidates are considered. The computational studies bring the outcomes of the adsorption behaviors, structural characteristics and accurate force fields of the studied materials in short turn-around times at low cost. This review can stimulate the design of novel materials with specific target of CO2 capture and promote the industrial sustainability of fossil fuel combustion technologies.

Assessment of the operability of a 20 MWth calcium looping demonstration plant by advanced process modelling

The Calcium Looping (CaL) process is a promising technology for post-combustion CO2 capture from fossil-fired power plants and carbon intense industry. Within a CaL system, a limestone based sorbent stream is forced to circulation between two interconnected circulating fluidized bed (CFB) reactors. The main part of the CO2, contained in the flue gas stream is absorbed by CaO within the carbonator, whereas it is released during regeneration in the oxy-fired calciner. The feasibility of this technology was proven by numerous experimental investigations in semi-industrial scale. The next step in the development of this technology is expected to be a demonstration plant in the scale of approximately 20 MWth. The focus of this paper is the determination of the heat and mass balances and the assessment of the operability of a newly designed 20 MWth CaL demonstration plant. The investigations are based on a steady-state process model, which has been validated by experimental data from 1 MWth pilot tests. The aspect of solid entrainment during part load operation are addressed. In the present design case, a stable operation of the demonstration plant at 40 % equivalent carbonator load is feasible without additional flue gas recirculation.

The Future of Fossil Fired Power Plants in Germany—A Lifetime Analysis

In many German energy projections, the assumption of power plant lifetimes plays a central role, since it is often used in projections for the existing fleet of power plants or as a criterion for decommissioning in model-based investigations. The result of these analyses is a power plant capacity retirement graph, which is then used to determine the replacement demand. Especially in the context of the German Energy transition (“Energiewende”) the amount and dynamics of replacement play an important role. Against this background, a large number of studies have been evaluated and fleet developments compared. Many studies refer to empirical values for the assumption of lifetimes without specifying them in greater detail. This approach was used to conduct an ex post lifetime analysis—accurate to each number of units—of German power plants that have been decommissioned since 1990. The analysis is conducted with the aid of a power plant database, which has been continuously updated for each individual unit since 1985. In addition to the power plants currently in operation, the database includes also includes power plant units that have been successively decommissioned over the past decades. The ex post analysis presents the first lifetime analysis for decommissioned German plants, which can serve as a basis for future power plant fleet projections. The analyses show that the lifetime of fossil-fired power plants has extended considerably. For example, whereas the real lifetimes of coal-fired power plants were in a range of 30 to 35 years in the 1990s, today they amount to 40 to 45 years on average.

Investigation on a creep-tested CB2 steel cross-weld sample welded with a matching flux-cored wire

Heavy cast components like steam turbine or valve casings made of GX13CrMoCoVNbNB9-2-1 (CB2 steel) are used in modern fossil-fired power plants for service temperatures up to 620 °C. For production welding of these huge components, high productivity and good out-of-position weldability are needed. To combine these requirements, flux-cored arc welding is a preferred welding process. A matching flux-cored wire consumable has been developed, which offers sufficient short-term mechanical properties at ambient temperature. To prove weld metal long-term properties at service temperature, uniaxial creep rupture tests at 625 °C have been started several years ago. The microstructure of a cross-weld sample, ruptured in the heat-affected zone, was investigated by light optical microscopy and scanning electron microscopy with energy dispersive X-ray analyses. Area fraction and mean radius of precipitated Laves phase and M23C6 carbides were compared to the results of MatCalc™ simulations.