Once-through Boiler Research Articles

Comparative Analysis of the Dynamic Properties of Variants of Automatic Frequency and Power Control Systems for Steam Power Plants Equipped with Once-Through Boilers

Considerably complicated power-system problems that have to be solved by the Russian power-generating units and, consequently, the need to ensure the fully automated operation of the power-generating units under working loads necessitated improving the structure of the automatic frequency and power control systems (AFPCSs). Not all solutions adopted in the 1980s, in particular, the use of the classical variants of the automatic power control systems (APCS-1 and APCS-2), ensure the required quality of suppressing power-system disturbances. In this work, the authors performed comparative analysis of six AFPCS variants that correspond to both the classical APCS-1 and APCS-2 and their combinations. The research was carried out using a mathematical model under disturbances of characteristic types, viz., those caused by network frequencies, unit power settings, and boiler loads. The model was developed by approximating experimental step response curves of the 300-MW power-generating unit 3 equipped with a once-through boiler at Kashirskaya State District Power Station. The quality of control was analyzed by comparing the amplitude-frequency characteristics and the curves of the transient processes of the compared AFPCS variants and the integral quadratic criterion values and the maximum deviations of the controlled variables and control actions in the course of the above processes. The combined AFPCS and advanced APCS-2 proved to be the most preferable. The combined AFPCS variants offer advantages in terms of maintaining the steam pressure, while the advanced APCS-2 has an advantage as a means for controlling the unit power with the gain in the first case being somewhat greater than that in the second case. Both variants have been successfully used for a long time at numerous power-generating units of the country.

Development of a NO x emission model with seven optimized input parameters for a coal-fired boiler

Optimizing the operation of coal-fired power plants to reduce nitrogen oxide (NO x ) emissions requires accurate modeling of the NO x emission process. The careful selection of input parameters not only forms the basis of accurate modeling, but can also be used to reduce the complexity of the model. The present study employs the least squares support vector machine-supervised learning method to model NO x emissions based on historical real time data obtained from a 1000-MW once-through boiler. The initial input parameters are determined by expert knowledge and operational experience, while the final input parameters are obtained by sensitivity analysis, where the variation in model accuracy for a given set of data is analyzed as one or several input parameters are successively omitted from the calculations, while retaining all other parameters. Here, model accuracy is evaluated according to the mean relative error (MRE). This process reduces the parameters required for NO x emission modeling from an initial number of 33 to 7, while the corresponding MRE is reduced from 3.09% to 2.23%. Moreover, a correlation of 0.9566 between predicted and measured values was obtained by applying the model with just these seven input parameters to a validation dataset. As such, the proposed method for selecting input parameters serves as a reference for related studies.

Study of flue-gas temperature difference in supercritical once-through boiler

The 600 MW coal-fired once-through Boilers with opposed firing at a power plant are found to experience marked temperature variation and even overtemperature on the wall of the heating surface as a result of flue-gas temperature (FGT) variation in the boiler. In this study, operational adjustments were made to the pulverizing, combustion, and secondary air box systems in these boilers, in order to solve problems in internal combustion. The adjustments were found to reduce FGT difference and optimize the boiler’ combustion conditions. The results of this study can provide a reference for optimization of coal-fired boiler of the same type in similar conditions.

An experimental study on the hydrodynamic performance of the water-wall system of a 600 MW supercritical CFB boiler

The temperature distribution on the backside of the water-wall tubes in the first 600 MW CFB boiler operating at 60%, 80% and 100% of the boiler Maximum Continuous Rating (MCR) loads was experimentally determined and the thermal-hydrodynamic performance of water-wall system based on the mass and energy equations using the measured temperature was calculated. The results showed that the temperature of the water-wall tubes increased as the furnace height increased, showing a positive response characteristic in the water-wall system. However, the temperature of the water-wall tubes in the upper furnace was obviously declined at the places where the superheaters and cyclone separator inlet-channels were arranged. This suggests that the arrangement of the heat-transfer surfaces in the upper furnace was significantly influenced the temperature distribution of the water-wall tubes. Meanwhile, the calculated total pressure-drop and mass flux in the water-wall system increased as the boiler’s load raised, showing an energy-efficiency for the vertical once-through boiler. In addition, the metal temperature was calculated to check the working condition of the water-wall tubes and the results showed the highest metal temperature of 426.3 °C of the water-wall tubes was found at the tip of the fin at 100% MCR. This temperature was far below the permissible temperature of the metal, implying that a safe working condition of the metal at three loads was guaranteed. Nonetheless, analysis into the heat-transfer characteristics of the water in the water-wall reflects that the water temperature in the water-wall at 80% MCR load was adjacent to the boundary of the large specific-heat region, implying that heat-transfer deterioration was more likely to occur at such condition. Therefore, the metal temperature of the water-wall tubes operating at 80% MCR load requires the most concern and monitor to avoid overheating of the water-wall tubes.

Analyzing the temperature control of steam purging of 660mw ultra-supercritical once-through boiler with pressure-reducing method

This paper generally introduced the process of steam purging of the ultra-supercritical once-through boiler of Jiangxi Xinchang 2×660MW Power Plant with the pressure-reducing method. In this paper, the key-points of steam temperature control was importantly analyzed and summarized. The success experience can provide the reference for preventing steam overtemp of the similar ultra-supercritical once-through boilers with pressure-reducing method.

Dynamic simulation of the water-steam flow in a supercritical once-through boiler

Once-through steam generator systems with supercritical fluids are applied to achieve large capacity and high efficiency in power generation systems. A supercritical fluid is considered as a single-phase substance, but properties of the fluid sharply change near the pseudo point, where specific heat of the fluid reaches its maximum at the corresponding pressure. Therefore, properties calculation and correlations of the heat transfer coefficient and pressure drop have to be evaluated carefully to predict the dynamic behavior. In this study, a 1-dimensional discretized dynamic model was developed for the dynamic simulation of the once-through boiler with supercritical fluids. The mass, momentum and the energy balance were solved in each cell. Validation of the model was checked against the experimental data for the single tube, and the dynamic behavior was simulated for the case of the simple step disturbance. After checking the single tube, this model was applied to the water-steam flow system of the commercial 500 MWe once-through boiler. The steady state values were compared with the design data. To simulate the dynamic behavior of the once-through boiler system, the feed water flow rate and the heat duty of the evaporator were changed as the disturbance. Finally, the operation simulation of the water-steam flow of the once-through boiler was performed for the case of dynamic step-down from the supercritical pressure to the subcritical pressure.

Cycle chemistry monitoring system as means of improving the reliability of the equipment at the power plants

There are many problems in domestic energy at the present that require urgent solutions in the near future. One of these problems - the aging of the main and auxiliary equipment. Wear of equipment is the cause of decrease reliability and efficiency of power plants. Reliability of the equipment are associated with the introduction of cycle chemistry monitoring system. The most damageable equipment’s are boilers (52.2 %), turbines (12.6 %) and heating systems (12.3 %) according to the review of failure rate on the power plants. The most part of the damageability of the boiler is heated surfaces (73.2 %). According to the Russian technical requirements, the monitoring systems are responsible to reduce damageability the boiler heating surfaces and to increase the reliability of the equipment. All power units capacity of over 50 MW are equipped with cycle chemistry monitoring systems in order to maintain water chemistry within operating limits. The main idea of cycle chemistry monitoring systems is to improve water chemistry at power plants. According to the guidelines, cycle chemistry monitoring systems of a single unit depends on its type (drum or once-through boiler) and consists of: 20…50 parameters of on-line chemical analyzers; 20…30 «grab» sample analyses (daily) and about 15…20 on-line monitored operating parameters. The operator of modern power plant uses with many data at different points of steam/water cycle. Operators do not can estimate quality of the cycle chemistry due to the large volume of daily and every shift information and dispersion of data, lack of systematization. In this paper, an algorithm for calculating the quality index developed for improving control the water chemistry of the condensate, feed water and prevent scaling and corrosion in the steam/water cycle.

Enthalpy control for the feed water supply in a once-through boiler with disturbance estimation

In this article, a nonlinear modeling approach to describe the enthalpy dynamics between the inlet of the economizer and the outlet of the evaporator inside a once-through boiler is presented. Furthermore, a combined control and estimation approach based on an LQG controller consisting of an LQ regulator and an Extended Kalman Filter (EKF) is introduced. The EKF is thereby extended by a state in order to estimate the supplied heat, which constitutes a disturbance to the process. This can be due to fouling or varying heat values of the fuel during operation.

Experimental study on temperature distribution of membrane water wall in an ultra-supercritical pressure once-through boiler burning zhundong coal

Taking an ultra-supercritical pressure once-through boiler as an example, the temperature distribution of the lower membrane water wall is investigated experimentally, the conclusion reveals that increasing the proportion of Zhundong coal can effectively reduce the district heat load, which benefits the temperature uniformity in the lower membrane water wall. When the boiler being operated at middle load, the temperature deviation in lower membrane water wall increase simultaneously, one of the reasons is that the restriction orifice could not adjust the flow rate of working fluid as expected. By adjusting boiler performance, the temperature uniformity of lower membrane water wall can be improved to a certain degree.

Standardization of the water heat carrier quality at Russian thermal power plants

The necessity of developing a new industry-specific standard of the heat carrier quality for the operating, newly commissioned, and prospective power-generating units of the thermal power plants is substantiated. The need of extending the scope of the automatic chemical monitoring and the possibility of indirect measurements of some basic standardized and diagnostic indices of the water chemistry using the specific conductance are shown. Investigations proved the possibility of automatic chemical monitoring of the phosphating of the drum boilers and quantitative control of potentially acidic impurities in the feed water in oncethrough boilers. The normative STO NP INVEL document developed at OAO VTI in 2009 is proposed as the basis for alterations and amendments. A new index, the total organic carbon, is introduced into this document. The standardized value of this index in the drum boiler feed water and steam is 100 μg/dm3. According to the above normative document, the scope of the chemical monitoring should be extended by measurements of the specific conductance of the direct and H-cation samples of both the feed and the boiler water. The content of chlorides should also be standardized. For the first time, normative restrictions are suggested on amine-containing water chemistry of the power-generating units with the combined cycle gas turbines. Flowcharts are proposed for pretreatment of the make-up water on the basis of low-mineralized natural waters with high organic substance contents, which reduces the oxidizability by 70–80%.

Start-Up and dynamic processes simulation of supercritical once-through boiler

The rapid expansion of the electricity demand causes the gap between the peak load and valley load increasing in China. Since the coal-fired supercritical power plant needs to be started up frequently, the operations of power plant become more flexible. During the start-up process, the operation parameters are off-designed, the multi-transient phenomenon is inevitable and the intensive emission always appears during varying working condition, it is difficult to ensure the operator of the real power plant to start up the boiler following the design curve. In this work, a new dynamic model for simulating start-up dynamic process of a 600MW supercritical boiler is introduced in this paper. The six-equation solution of two-phase flow was applied to modeling the evaporator, and the process from ignition to the load at 30% Boiler Maximum Continuous Rating (BMCR) was simulated. The results demonstrate that the model is reasonable and dynamic behavior can be observed clearly. Also, these prove that the control strategy is advisable. Moreover, the fuel consumption of the start-up process can offer a reference to operator of the real power plant. The validated model is of high relevance for further optimizing the boiler start-up process.

A dynamic mathematical model of an ultra-supercritical coal fired once-through boiler-turbine unit

It is challenging and interesting to establish a precise dynamic model of an OTB (once-through boiler) power plant unit in order to meet large scale load demands from the power grid. This study proposes to establish such a dynamic mathematical model of an ultra-supercritical OTB unit under dry operating conditions. More precisely, the dynamic model structure was derived from mass and energy conservation laws as well as thermodynamic principles under some reasonable simplifications and assumptions. Then an IGA (immune genetic algorithm) was improved to identify the parameters, combined with running data. After this, to further enhance model performance, the dynamic mathematical model was extended to be the one with different sets of parameters and functions under different monotonous load ranges. Additionally, open- and closed-loop experiments were conducted in order to further validate the developed model. The experimental results show that the model outputs can approach the actual running data over a wide operating range with appropriate accuracy. More importantly, the dynamic model captures the essential dynamic characteristics of the unit. Therefore, the model can be feasible and applicable for simulation analysis and testing control algorithms.

Research of power fuel low-temperature vortex combustion in industrial boiler based on numerical modelling

The goal of the presented research is to perform numerical modelling of fuel low-temperature vortex combustion in once-through industrial steam boiler. Full size and scaled-down furnace model created with FIRE 3D software and was used for the research. All geometrical features were observed. The baseline information for the low-temperature vortex furnace process are velocity and temperature of low, upper and burner blast, air-fuel ratio, fuel consumption, coal dust size range. The obtained results are: temperature and velocity three dimensional fields, furnace gases and solid fuel ash particles concentration.

Improved boiler-turbine coordinated control of 1000 MW power units by introducing condensate throttling

Improving the operating flexibility of coal-fired units has become more and more important since an increasing number of fluctuant renewable energy connected to the power grid would make the power system easily unstable. However, it is difficult for the 1000MW power units to improve their load change capability because the large delay of their once-through boilers. In this paper, the condensate throttling and coordinated control are combined to accelerate the load response, and a 4-inputs 3-outputs control model is firstly set up. Then a dual controller is designed to realize the combination control of fuel flow and condensate water flow on turbine load; also, the dual controller can recover the condensate water flow and deaerator level to their former values. Furthermore, the feedforward control based on stable inversion is developed to advance the response of fuel regulation, and the main steam pressure control and separator outlet enthalpy control are decoupled for convenient control. Finally, simulation tests and analyses on a 1000MW unit are carried out to prove the effectiveness of our strategy.

Investigation of the thermal-hydraulic characteristics of supersonic steam injector

Steam injector (SI) is known as passive jet pump and heat exchanging device which operates without external power source or mechanical machineries. It utilizes direct contact condensation heat transfer between steam and water-jet as a driving mechanism of the operation and is capable of discharging subcooled water at higher pressure than the inlet fluids pressure. In addition, it has an excellent heat transfer capability, more than 1000 times that of shell and tube heat exchanger. In the present study, thermal-hydraulics characteristics of the water-jet-centered supersonic SI system were investigated from both experimental and analytical approaches. The SI body was manufactured with stainless steel equipped with overflow port. The water injection nozzle was designed with shaft-driving mechanism to freely adjust the axial location of the water nozzle and steam inlet cross sectional area. High pressure steam was supplied to the SI from once-through boiler, which is capable of supplying saturated steam at the maximum pressure of 0.63MPa for the current test facility. Water jet was injected at mass flow rate of 0.4–0.8kg/s. Pressure and temperature measurements were conducted at inlet and outlet of the steam injector system as well as at the overflow port to investigate the operation characteristics of the SI. Results showed the water-jet centered SI’s promising functionality as a passive coolant injection in view of its quick-start up, operable condition limits, discharge pressure and heat transfer capabilities at current inlet conditions. In addition, obtained experimental results were compared with analytical model to assess the predictive capability of discharge pressure value and reasonable agreement was obtained.

Modeling of ash deposition in a pulverized-coal boiler by direct simulation Monte Carlo method

In this paper, a mathematical model of the fly ash deposition in a 660MW supercritical once-through boiler was developed. The mechanism of particle-particle and particle-wall collisions has been considered. Particle collision is modeled by a direct simulation Monte Carlo (DSMC) method. Three submodels (impaction, thermophoresis, and sticking) were included in the mathematical model. In the ash deposition model, the critical velocity model was used to calculate the ash sticking rate during initial deposition, and the viscosity model was used for the outer layer deposition. An ash deposition probe was inserted into the peephole and ash deposits were collected. The morphology of these samples was analyzed using SEM. The deposit thickness and the distributions of adhesion particle size were obtained by mathematical model. The mean deposit thickness calculated by the DSMC method agreed with the value measured by the ash deposition probe with the relative error within 3.79%. The simulation results show that the deposition inner layer consists of fine particles less than 10μm in size and sticky fine particles were selectively captured during the initial deposition stage. The DSMC method may be a practical alternative for studies of ash deposit formation.

Results of examination of the TGMP-314 boiler superheaters of the power-generating units at Kashira state district power plant using a magnetic ferrite meter

The results of investigating the elemental composition of the scale and the metal of a tube circuit from the austenitic steel grade 12Cr18Ni12Ti are presented. The superheater is part of the high-pressure convection steam superheater of a TGMP-314 supercritical-pressure gas-and-oil-fired once-through boiler that had been operated for a long period of time. A structurally transformed layer, poor in iron, manganese, and chromium and rich in nickel was detected on the outer surface. The layer consisted predominantly of the FeNi3 phase with ferromagnetic properties. The heat-resistance test of the steel in contact with ash that simulated the combustion products of fuel oil showed that the higher the temperature and the longer the test time, the higher the content of the ferritic phase in the layer was. The established pattern of the structural transformations underlies a method for nondestructive control of the thermal nonuniformity and detection of “worst” tube circuits of superheaters from austenitic steel. The magnetic ferritometry complements the conventional selective thickness gauging that does not characterize the condition of the heating surfaces of hightemperature steel grades to the full extend. Data on damageability of high-pressure convection superheaters and low-pressure second-stage convection superheaters with rarefied tube banks of TGMP-314 boilers are presented. The damage is caused by overheating resulting from the nonuniform temperature field at the inlet and by the nonuniform flue gas velocity field in rarefied superheater banks. Sections of the tube circuits from the steel grade 12Cr18Ni12Ti, outlet superheater stages of the TGMP-314 boiler of the power-generating units at Kashira SDPP were examined using an MF-51NTs AKASKAN magnetic ferrite meter. Thermal nonuniformity was established and the “worst” superheater tube circuits were detected. It was shown that the zones with the “worst” and damaged tube circuits coincide. The results of examining the superheaters with the help of a magnetic ferrite meter, of dimensional control, and of metallographic investigation of the samples cut out of the tube circuits are in agreement with each other.

An experimental study on characteristics of post-CHF heat transfer in the high subcritical pressure region near to the critical pressure

Supercritical pressurized water cooled reactor (SCWR), which has a once-through water cooled reactor for supplying supercritical pressure steam at high temperature to a turbine system, is one of the next generation reactors for the purpose of improving economic efficiency and safety. In the SCWR system, water pressure passes through the critical pressure during startup, shutdown and in case of loss of coolant accident (LOCA). Similarly, in fossil-fired power plants, once-through boilers of sliding pressure type operate at supercritical pressure during nominal load, but they work near the critical pressure during partial load. In the high subcritical pressure region slightly below the critical pressure, critical heat flux (CHF) phenomenon readily occurs at relatively low heat flux and in a high subcooled region, and then it causes serious damages to fuel rods or boiler tubes due to the abrupt temperature rise in the rod or tube. In this study, experiments on post-CHF heat transfer in such the high subcritical pressure region near the critical pressure (reduced pressure range of 0.924 up to 0.992) in the range of mass flux 400-1000 kg/(m(2).s) with a circular tube of 4.4 mm ID in vertical upward flow. HCFC22 and HFC134a were used as the test fluids. Based on the obtained data, characteristics of the post-CHF heat transfer were examined and found to be classified into two distinctive types mainly according to flow rate conditions. The influences of pressure, flow rate and heat flux on the characteristics were also clarified.

The Regulation Trend and Combustion Technology on Industrial Once-Through Boiler

The Regulation Trend and Combustion Technology on Industrial Once-Through Boiler

A dynamic model used for controller design of a coal fired once-through boiler-turbine unit

Supercritical OTB (once-through boiler) units with high steam temperature and pressure have been widely used in modern power plants due to their high cycle efficiency and less emissions. To ensure the effective operation of such power generation systems, it is necessary to build a model for the design of the overall control system. There are already detailed models of once-through boilers; however, their complexity prevents them from being applied in the controller design. This study describes a lumped parameter dynamic model that has a relatively low complexity while faithfully capturing the essential overall plant dynamics. The model structure was derived by fundamental physical laws utilizing reasonable simplifications and data analysis to avoid the phase transition position problem. Parameter identification for the model structure was completed using operational data from a 1000 MW ultra-supercritical OTB. The model was determined to be reasonable by comparison tests between computed data and measured data for both steady and dynamic states. The simplified model is verified to have appropriate fidelity in control system design to achieve effective and economic operation of the unit.