Reheat Steam Research Articles

Using molten-salt energy storage to decrease the minimum operation load of the coal-fired power plant

As the renewable energy fluctuating in the power grid, the traditional coal-fired power plant needs to operate on the extremely low load, so as to increase the share of renewable energy. This paper deals with thermodynamic simulation and exergy analysis of the coal-fired power plant integrated with the molten-salt energy storage system to explore the potential of reducing the minimum operation load. A steady-state simulation was performed to obtain the thermodynamic properties of process streams in a subcritical 600 MW unit. The results indicated that with simple main steam and re-heat steam energy storage plan, the storage efficiencies are 39.4-42.9% and 51.3-51.4%, the minimum operation load would decrease by 27.2 MW, 10.6 MW, respectively. Exergy analysis shows that the exergy loss mainly comes from the throttling process and temperature difference in the phase-changer heat transfer process. With optimized Molten-salt energy storage plans, the storage efficiencies increase to 72.6% and 78% via lead the exhaust drains or steam into higher pressure points; the minimum operation load decreases by 35.1 MW and 3 MW, respectively. Moreover, with the coupled plan including both optimized main steam and re-heat steam energy storage system, the minimum operation load would decrease by 80.7 MW, and storage efficiency is 75.1%.

Environmental impact and cost analysis of gas turbine cycles with steam injection and inter-stage turbine reheat

Inter-stage turbine reheat is an effective gas turbine retrofit which can easily be used with simple and steam injected gas turbines as well. Owing to the uncertainties relating to an efficiency comparison of steam injected and no-injection cycle designs, thermoeconomics and environmental impacts have been considered to evaluate reheat and steam injection which regards pollutant emissions and environmental costs together. Being the complemental of the authors previous studies on steam injected gas turbines, simple, reheat, steam injected (STIG) and reheat steam injected (RHSTIG) gas turbine cycles are compared. Economic feasibility of steam injection as well as inter-stage turbine reheat are discussed according to 2018 plant cost data. Optimal cycle parameters are determined using a new comprehensive cycle model which permits to work with realistic working fluids, simulates the combustion process regarding 14 exhaust species and determines NOx and CO emissions.

Dynamic characteristics of a solar‐aided coal‐fired power generation system under direct normal irradiance (DNI) feedforward control

AbstractA solar‐aided coal‐fired power generation (SAPG) system is a new type of power generation system in the field of solar thermal power generation. However, the instability of solar radiation will cause fluctuations in steam temperature, which is unfavorable for the operation of boilers and steam turbines. Based on a 600 MW subcritical coal‐fired power generation unit, TRNSYS software is applied to establish a transient simulation model of SAPG in this paper. A dynamic feedforward control system for steam temperature is set up. Some essential parameters, such as the fuel consumption rate, solar‐heated steam extraction rate, water spraying flow rate, and flue gas damper opening, are under the control of direct normal irradiance (DNI). Then, the dynamic characteristics of SAPG are analyzed using typical weather data. The simulation results show that under the fuel‐saving mode, when solar irradiation changes, the SAPG system achieves a coal reduction of 4.9% by using the steam temperature control system with DNI feedforward control, and the main steam temperature fluctuation is less than 5°C with the reheat steam temperature reduced within a reasonable range. The power generation fluctuation is less than 10 MW, and the solar‐to‐electricity efficiency reaches 22.5%. Some parameters on both the flue gas side and the water/steam side in the utility boiler are simulated to demonstrate the effectiveness of DNI control.

Enhancing peak shaving capability by optimizing reheat-steam temperature control of a double-reheat boiler

Double-reheat coal-fired power plants elicit much research attention due to their high efficiency and low emissions. However, steam temperatures fluctuate heavily, especially in transient processes, thereby compromising the load cycling capacity of these power plants and even threatening their stable operation. To overcome these challenges and optimize the operations of double-reheat boilers, a dynamic boiler model was established and validated with values measured from a real power plant. A revised reheat steam control logic was proposed that considers the heat storage variation in boiler metals and the deviation of steam temperatures during peak shaving transient processes. The performance of the original and revised reheat steam control logics in terms of temperature control and energy delivery characteristics was compared and discussed. Calculation results show that the revised control logic performs better than the original logic in large-range load cycling processes. The steam temperature fluctuations are alleviated, and the overshoots are reduced substantially. The revised reheat steam control logic ensures that the accessible maximum load variation rate is 4.0% BMCR min−1, which cannot be safely guaranteed by the original logic. These improvements are beneficial to increasing the threshold of the load variation rate and improve the flexibility of the power plant. The average thermal (ηI) and exergetic (ηII) efficiencies of the original and revised control logics in load cycling are compared. For loading up processes, ηI and ηII decrease by 1.04% and 0.28% at the most, respectively. For the loading down processes, the maximum increments in ηI and ηII are 1.44% and 0.77%, respectively.

Evaluation of Novel Configurations of Natural Gas Combined Cycle (NGCC) Power Plants for Load-Following Operation using Dynamic Modeling and Optimization

With increasing penetration of intermittent renewable energy sources into the electric grid, conventional thermal power plants are being forced to cycle their load and operate under low-load conditions much more frequently. A high-fidelity dynamic model of a natural gas combined cycle (NGCC) power plant with rigorous equipment level sub-models is developed in this work. A model of the gas turbine (GT) for estimating its performance under off-design conditions, a thermo-hydraulic model for the heat recovery steam generator (HRSG), and a model of the steam turbine (ST) with moisture detection and model adaptation capability are also developed. Five novel configurations are proposed for controlling the main steam and reheat steam temperatures while avoiding ‘spraying to saturation’ during fast load-following by considering final high-pressure superheater (HP SH2)/ reheater (RH) arrangement (in series or in parallel) and attemperation strategies (single-stage, two-stage, and damper-assisted attemperation). Lo...

Effect of Flue Gas Recirculation on Operating Performance of Coal-Fired Unit Boiler

Taking the flue gas recirculation system of 220MW coal-fired unit as the test object, effects of the operation mode of the flue gas recirculation system on the operating characteristics of the boiler under different working conditions was studied. The results show: To a certain extent, the operation of flue gas recirculation system will reduce boiler efficiency, which is more obvious under low load conditions. However, The operation of the flue gas recirculation system can effectively increase temperatures of reheat steam and superheated steam.Comparing the economics of the front and rear units of the flue gas recycling system, it can be find that net coal consumption of the unit under the load of 200MW and 140MW can be reduced by up to 1.51g/kW·h and 1.86g/kW·h respectively.

Study on economics of depth peak-shaving for 1000MW ultra supercritical unit

With the increase of the demand for deep peak regulation of power grid, large-capacity units have the advantages of strong peak regulation capacity. 1000MW units have been widely involved in deep peak regulation. In the process of deep peak regulation, the actual working condition of the unit deviates greatly from the designed working condition, and the unit efficiency decreases significantly, making the unit economy deteriorate seriously, especially in the 1000MW grade unit. In this paper, a 1000MW unit depth peak adjustment is experimentally studied and its economic indexes are calculated. Then, by analyzing the test data, the optimization Suggestions for the operation of the unit during the depth peak adjustment are put forward:(1)Improve the opening of the high pressure switch of the unit to reduce throttling loss and improve the unit efficiency; (2)Increase the reheat steam temperature to ensure the stable operation of the reheat steam temperature at 620 °C of the full load section; (3) Keep a circulating pump running at a low speed during deep peak adjustment to reduce power consumption.

Temporal Feature Selection for Multi-Step Ahead Reheater Temperature Prediction

Accurately predicting the reheater steam temperature over both short and medium time periods is crucial for the efficiency and safety of operations. With regard to the diverse temporal effects of influential factors, the accurate identification of delay orders allows effective temperature predictions for the reheater system. In this paper, a deep neural network (DNN) and a genetic algorithm (GA)-based optimal multi-step temporal feature selection model for reheater temperature is proposed. In the proposed model, DNN is used to establish a steam temperature predictor for future time steps, and GA is used to find the optimal delay orders, while fully considering the balance between modeling accuracy and computational complexity. The experimental results for two ultra-super-critical 1000 MW power plants show that the optimal delay orders calculated using this method achieve high forecasting accuracy and low computational overhead. Moreover, it is argued that the similarities of the two reheater experiments reflect the common physical properties of different reheaters, so the proposed algorithms could be generalized to guide temporal feature selection for other reheaters.

Axial Thrust Balancing in High-Temperature Cylinders of Steam Turbines during Transients in Combined-Cycle Units

The effect of magnitude and direction of the axial thrust during steam turbine transients on the starting technology is investigated. The behavior of the axial thrust during start-up of a steam turbine with a combined high/intermediate pressure cylinder operating in a single-bypass or two-bypass thermal cycle is presented. At different stages of start-up, varying steam pressure at the turbine inlet and in its flow path can result in a change in the temperature of thrust bearing pads and the magnitude and direction of the axial thrust acting on them. During start-ups with a two-bypass thermal scheme for steam admission to the turbine and its rotor acceleration, connection of the steam turbine generator to a grid and its initial loading are performed with steam supply through intermediate pressure-control valves. In this case, the high-pressure cylinder (HPC) is under “negative pressure” or “countercurrent” conditions passing steam from the exhaust via the bypasses of the check valves in the “cold” reheat steam lines and removing this steam to the condenser via the drains of high-pressure crossover pipelines and from the high-pressure cylinder casing. Connection of the turbine HPC for normal steam supply and change-over to the once-through scheme of steam admission to the turbine are carried out after connection of the turbine generator to the grid and initial loading of the turbine by partial opening of the HP control valve and full opening of the intermediate pressure (IP) control valves. At the same time, the valves of quick-acting pressure-reducing and cooling units (QAPRCU) BROU-1 and BROU-2 are closed to maintain the specified high and intermediate pressure of steam upstream from them. These process operations change the axial thrust acting on the thrust bearing pads with a corresponding change in the pad temperature. The effect of variable axial thrust on the steam turbine maneuverability is examined. Methods are proposed for balancing the axial thrust during start-ups of steam turbines in combined-cycle units having different thermal schemes.

Assessment of the Performance of a Nuclear–Hydrogen Power Generation System

The article deals with the assessment of the performance and competitiveness of a nuclear power plant (NPP) combined with a hydrogen power plant compared with a gas-turbine power plant (GTPP) and a pumped-storage hydroelectric power station (PSPS) to satisfy the peak electrical demand in a power system in terms of the peak electric power prime costs. The structure of installed capacities of various interconnected power systems is shown considering the increase in the capacity levels in the immediate future. The necessity of curtailing the load of the power-generating units during the periods of the nighttime off-peak demand is justified. For this purpose, the efficiency of curtailing the load of the power-generating units of various types is analyzed under variable electric loads. As an example, the power-generating units of an NPP equipped with a VVER-1200 reactor, a 300-MW condensation electric power plant, and a PGU-450 combined-cycle gas turbine are considered. The increase in the electric power prime cost serves as the efficiency criterion. To provide the NPP with the base load, variants of combining the NPP with a hydrogen power plant with steam–hydrogen superheating of the live steam upstream from the high-pressure cylinder of the primary turbine and the high-pressure cylinder of the auxiliary steam turbine accompanied by ejection of the heating reheat steam are presented. The forecasted prices of fuel gas and nuclear fuels, as well as of the nighttime electric power for 2020 and in the long term until 2035, are taken into account. It is shown that the employment of GTPPs results in an additional increase in the expenditures caused by curtailing the load of the NPP at nighttime and appears to be inefficient in some cases. The expenditures on the substituted power are accounted for in comparison with the pumped-storage power station in the hydrogen power plant variant. It is shown that the pumped-storage power stations can compete in the foreseeable future with the hydrogen power plants in terms of peak electric power prime costs at minimum specific capital investments of approximately 660 $/kW. In the long term, due to a considerable increase in the nighttime electricity cost, the pumped-storage power stations will not be able to compete with the hydrogen power plants.

Adjustment and research of 623 °C reheat steam temperature in the commissioning of 660 MW ultra-supercritical boiler

The 623 °C reheat steam temperature is adopted for 660 MW ultra-supercritical unit. During the operation, the materials of the high temperature reheater surface have less safety margin, leading to the result that the reheat steam temperature cannot reach the design value. During the commissioning process, the combustion adjustment plan was optimized. After a lot of adjustment of combustion deviation, the main steam temperature and reheat steam temperature reached to the design value. The temperature of the reheater surface was lower than the alarm value.

Performance Analysis of Reheat Steam Temperature Control System of Thermal Power Unit Based on Constrained Predictive Control

The reheat steam temperature control system of thermal power unit is a complex control object with time‐varying parameters and large delay. In order to achieve precise control of reheat steam temperature, the performance of the reheat temperature control system is analyzed according to the data that are obtained based on the constrained predictive control algorithm. Firstly, the process and mathematical model of reheat steam temperature control system are introduced. Then the principle of constrained predictive control algorithm is analyzed. Finally, the steady‐state values of control quantities of reheat steam temperature control system under different conditions are given by MATLAB simulation, and, by analyzing the steady‐state values and steady‐state time of the input and output of the system, the reference values and the regulating law of the control quantities and the specific constraint range of the control quantities of the system are given, which can provide reference data and theoretical basis for the field adjustment of the reheat steam temperature control system in power plant and improve the safety and effectiveness of the system.

Artificial Bee Colony Optimization of NOx Emission and Reheat Steam Temperature in a 1000 MW Boiler

This paper puts forward a new viewpoint on optimization of boiler combustion, namely, reducing NOx emission while maintaining higher reheat steam temperature rather than reducing NOx emission while improving boiler efficiency like traditional practices. Firstly, a set of multioutputs nonlinear partial least squares (MO‐NPLS) models are established as predictors to predict these two indicators. To guarantee better predictive performance, repeated double cross-validation (rdCV) strategy is proposed to identify the structure as well as parameters of the predictors. Afterward, some controllable process variables, taken as inputs of the predictors, are then optimized by minimizing NOx emission and maximizing reheat steam temperature via multiobjective artificial bee colony (MO‐ABC). Results show that our rdCV‐MO‐NPLS model with MO‐ABC optimization methods can reduce NOx emission synchronously and improve reheat steam temperature effectively compared with nondominated sorting genetic algorithm II (NSGA‐II) and combustion adjustment experimental data on a real 1000 MW boiler.

Parameters calculation of thermal power plant dynamic model using steam cycle data

Standard models are created to carry out power system dynamic studies. One of the major problems in this regard is the parameter identification of these models. These parameters are not constant and may change due to extensive use and the large number of repairs, modifications and overhaul of the units, and, consequently, they cannot provide a reliable basis for further use. Thus, actual parameters need to be determined. This paper determines the actual parameters of a real 150 MW tandem compound, single reheat steam turbine located in the Egyptian network since 1986 using the available unit steam cycle data. The time constants of Steam Chest, reheater and cross over are calculated. The power fractions of high pressure (HP), intermediate pressure (IP) and low pressure (LP) turbine stages are also estimated. The calculated parameters are compatible with IEEEG1 general steam turbine model. Finally, parameters validation is investigated by comparing the actual and simulated turbine power response. Also governor and turbine speed responses are provided. The simulation is carried out by DIgSILENT power factory software.

Comparative evaluation of different combined cycle configurations having simple gas turbine, steam turbine and ammonia water turbine

Gas/steam combined cycles have been increasingly used in power plants due to better energy utilization for getting better performance from them as compared to Brayton cycle based gas turbine plant or the Rankine cycle based steam turbine plant individually. However, there exists ample scope for further improvement in the performance of combined cycles through variations in their arrangements. The present study deals with the thermodynamic analysis of different combined cycle power plant configurations having the simple gas turbine with closed loop cooling of gas turbine blades and varying arrangements in the bottoming cycle. The variation in the bottoming cycle relies upon the effective utilization of energy available in the topping cycle. The bottoming cycle considered uses steam cycle or ammonia water cycle or its combination. The comparison of eight different combined cycle configurations considered in this study reveals that, the work output is maximum for simple gas turbine with bottoming cycle having reheat ammonia water turbine and steam turbine, with an output of 638 kJ/kg of air, first and second law efficiency of 54.95% and 57.87% respectively for ammonia mass fraction of 0.7. The exergy loss is found to be maximum for the combustion chamber followed by heat recovery vapor generator.

Design, modelling and techno-economic analysis of a solid oxide fuel cell-gas turbine system with CO2 capture fueled by gases from steel industry

The steel industry is one of the major sources of CO2 emissions that are released in the manufacture and process of steel as well as in related power production. Focused on reduction of CO2 emissions in the power production, this paper presents a novel solid oxide fuel cell-gas turbine combined heat and power system fed by coke oven gas. The solid oxide fuel cell-gas turbine system design consists of an adequate gas cleaning section for contaminants removal, solid oxide fuel cell as the main power producer and an anode offgas pressure swing adsorption based CO2 capture unit. This system is thermodynamically and techno-economically analyzed and compared with a reheat steam turbine. Furthermore, the reheat steam turbine is retrofitted with a CO2 capture unit. It is then compared to the solid oxide fuel cell-gas turbine system to analyse the difference in system efficiencies.The solid oxide fuel cell-gas turbine system yields an electrical efficiency of 64%, which is significantly higher than electrical efficiency achieved by both, a conventional reheat steam cycle (34.1%) and the retrofitted system (27.0%). Moreover, it depicts a combined heat and power efficiency of 73%. Results also reveal that the solid oxide fuel cell-gas turbine system can achieve a reduction of 50% in CO2 emissions for equal power production. Furthermore, techno-economic analysis lead to a payback period of 9 years, taking into account state-of-the-art taxes and variation in the cost of components over the lifetime, without taking into account the fuel cost.

Development of a Dynamic Model and Control System for Load-Following Studies of Supercritical Pulverized Coal Power Plants

Traditional energy production plants are increasingly forced to cycle their load and operate under low-load conditions in response to growth in intermittent renewable generation. A plant-wide dynamic model of a supercritical pulverized coal (SCPC) power plant has been developed in the Aspen Plus Dynamics® (APD) software environment and the impact of advanced control strategies on the transient responses of the key variables to load-following operation and disturbances can be studied. Models of various key unit operations, such as the steam turbine, are developed in Aspen Custom Modeler® (ACM) and integrated in the APD environment. A coordinated control system (CCS) is developed above the regulatory control layer. Three control configurations are evaluated for the control of the main steam; the reheat steam temperature is also controlled. For studying servo control performance of the CCS, the load is decreased from 100% to 40% at a ramp rate of 3% load per min. The impact of a disturbance due to a change in the coal feed composition is also studied. The CCS is found to yield satisfactory performance for both servo control and disturbance rejection.

Study on the influence of low nitrogen combustion on boiler operation

With the national emphasis on improving the quality of atmospheric environment and protecting the ecological environment, the standards for pollutant emission from power station boilers are becoming more and more strict. In response to national environmental requirements, low nitrogen combustion technology has been adopted in coal-fired power plants. However, the introduction of low nitrogen combustion technology has also brought a series of negative effects on boiler operation. It not only includes a dramatic rise in the high ash carbon content and heating water flow, slow load response rate and such influence on the unit economy, but also includes great reduction of reheat steam temperature under low load, rapid sharp rise of load steam temperature. The steam temperature decreases when load decreasing rate greatly reduces, the effects, such as big deviation between steam temperature and wall temperature, slagging furnace, high water wall, temperature corrosion and so on, have an impact on the stability of the security unit. This paper investigates 13 coal-fired power plants with capacity of 300 MW to 600 MW in Inner Mongolia autonomous region, to study the effects of low nitrogen combustion of boiler operation, pointing out the necessity of reasonable control of the export of nitrogen oxide content.

COMPARATIVE ENERGY AND EXERGY ANALYSIS OF A POWER PLANT WITH SUPER-CRITICAL AND SUB-CRITICAL

The aim of this study that how to effect live steam parameters reheat and feed water preheater numbers on efficiencies of energy and exergy at coal-fired power plants. Moreover, two desuperheaters and a regenerative turbine are added USCPP (Case 3) to approach best results. Soma Power Plant (Case 1) consists of one reheat stage, two HPRHs and four LPRHs with one DEA. It is operated sub-critic and coal is used for a fuel. Live steam conditions of Soma Power Plant set at 13,92 MPa and 540 °C, and the reheat steam is reheated to 540 °C. Supercritical Power Plant (Case 2) consists of the same main components of Case 1. However, steam parameters of Case 2 are increased to 262.5 Bar and 600 °C to determine impact of the steam parameters on power plant efficiencies. USCPP which consists of two reheat stages, four HPRHs, six LPRHs with one DEA is designed to generate live steam under nominal conditions of 30 Bar and 600 °C. Besides, reheat steam are heated to 620 °C. Simulations have been carried out Ebsilon Professional software and pressure drops at preheaters and reheats are also considered. Some assumptions are made in the analysis. The thermal and exergy efficiencies of USCPP increase by 9.241 and 8.06 percentage points compared with Soma power plant, respectively. The results of this study that live steam parameters which are increased from sub-critical values to super-critical values have enormous influence on energy and exergy efficiencies. Secondly, adding second reheat stage has positive impact to improve power plant efficiencies. Finally, augmenting feed water preheater number, adding two desuperheater and one regenerative turbine increase power plant efficiencies. However, optimum numbers of feed water preheaters are determined considering economic parameters.

Efficiency and Boiler Parameters Effects in Sub-critical Boiler with Different Types of Sub-bituminous Coal

It has been a practice for almost all countries to run coal-fired power plant as a base load supplying the electricity. Coal is selected as main fuel as the coal price remains lower as compared to dry natural gas, medium fuel oil and distillate. Boiler as the main device in power generation must be capable and reliable in ensuring the steam production is continuous without any disruption. During the commissioning stage, boiler operation will be designed and tuned as per coal specification considering the environmental factor. However, the source of available coal used during commissioning is limited and alternative coal must be found. Coal with different characteristic will affect the boiler performance and parameters. From this study, the results indicate that coal with different calorific value (CV) and properties gives different efficiency to the boiler. The results show that the sub-bituminous coal with CV 5013 kcal/kg performs similarly to designate coal with CV 4852 kcal/kg. Besides that, the superheated steam and reheater steam for coal CV 5013 kcal/kg perform at normal value which is close to 540 °C setting point. The desuperheater spray water flows operate between 18 and 25 t/h with minimal operation to achieve the target value, 540 °C.