Subcritical Boiler Research Articles

Measurement of slagging thickness on the heating surfaces of a 330 MW subcritical boiler based on mid-infrared imaging technology

In this paper, a mid-infrared thermal imager and a Complementary Metal Oxide Semiconductor (CMOS) imaging device were employed to measure the slagging thickness on the heating surface in a 330 MW subcritical drum boiler. A way to establish the relationship between the slagging temperature and emissivity was proposed based on the Boom emissivity model and the Hadley thermal conductivity model, and then the slagging thickness was calculated by the Newtonian iterative method. In the superheater area, the average slagging thickness of the lower section was 8.6 mm, and it was 1.71 mm higher than that of the upper section. In the water-cooled wall area, the average slagging thickness of the right wall was 10.8 % larger than that of the left wall. To verify the accuracy of the calculated slagging thickness, the slagging image was measured by the direct visual probe, and the relative error was 7.69 %.

Influences of ultra-low load operation strategies on performance of a 300 MW subcritical bituminous coal boiler

To reveal the operational characteristics of subcritical boilers under ultra-low loads, numerical simulations were conducted based on a 300 MW subcritical bituminous coal boiler. The impacts of boiler load and low-load operation strategies on coal combustion behavior, heat transfer process, and NOx emission were thoroughly examined. Results indicate that acceptable aerodynamic and temperature fields can still be formed at 25 % ultra-low load, but the boiler performance is significantly reduced, with the maximum cross-sectional average combustion temperature being reduced by 225.98 K compared with the full-load condition. At 25 % load, the use of only two layers of burners results in a substantial increase in NOx emission from 290.32 mg/m3 to 445.56 mg/m3. The position of in-service burners affects the region where the intense coal combustion and heat release process occurs, and using the middle three layers of burners helps to maintain heat absorption by the suspended heating surfaces and to minimize NOx emissions. Furthermore, increasing primary air velocity at ultra-low loads can promote coal combustion and heat transfer processes in the lower furnace, but increase NOx emission by 14.23 % when primary air velocity is increased from 14.90 m/s to 23.00 m/s at 25 % load. These findings provide valuable insights for optimizing the ultra-low load operation of subcritical boilers engaged in deep peak-shaving processes under the carbon neutrality goal.

Effects of the addition of arch-supplied secondary air on the performance of the Foster Wheeler down-fired boiler: Air/particle flow, combustion and NOx emissions

This study proposes a low NOx combustion method for Foster Wheeler type down-fired boilers. The effectiveness of this technique was validated through laboratory-scale gas-particle two-phase experiments and industrial trials. The flow field characteristics within FW-type boiler furnaces equipped with arch-supplied secondary air injection nozzles were investigated. Furthermore, the application of this technique to a 660 MW subcritical FW-type boiler was examined, focusing on NOx emissions and combustion characteristics. Results demonstrate that optimizing the damper openings for F-layer and arch-supplied secondary air can significantly reduce NOx emissions by over 33 %. Additionally, increasing the mass flow rate of arch-supplied secondary air aids in reducing unburned carbon in fly ash and inducing a downward shift in the flame center within the furnace.

Leakage of water-cooled wall tube in subcritical boiler

Usually, heating surfaces used in thermal power plant boilers include economizers, water-cooled walls, superheaters, and reheaters, abbreviated as the “four tubes” of boilers. The leakage of the heating surface is a common form of unplanned shutdown in thermal power plants, accounting for more than 50% of abnormal shutdown accidents of units. Once the heating surface explosion occurs, the thermal power plant needs to shut down for treatment, resulting in huge losses in power generation and fuel consumption during startup and shutdown. This has a huge impact on the safety production and economic benefits of the enterprise. In this paper, the leaked water-cooled wall tube in the subcritical boiler was studied by means of microstructure analysis, hardness test, energy spectrum analysis, chemical composition test, macro-morphology inspection, and tensile test. The results revealed that the oxygen concentration difference between the area installed in ceiling pouring material and the area exposed to the air can result in significant differences in electrode potential between different areas of the front water wall tube passing through the ceiling. An oxygen concentration difference battery is formed. In this way, the water-cooled wall steel pipe continuously corroded and thinned. Finally, perforation leakage occurred under the pressure of the internal high-temperature medium.

Retrofit and application of pulverized coal burners with LNG and oxygen ignition in utility boiler under ultra-low load operation

An oxygen-rich and low NOx burner integrated with liquefied natural gas (LNG) was proposed to address unstable combustion and high NOx emissions from a 330 MW subcritical boiler under ultra-low load operation in China. To assess the effectiveness of the retrofit, Chemkin and Fluent softwares were utilized to construct a new NOx model and calculate NOx generation, based on the combustion of pulverized coal gas and LNG. Further, an eddy dissipation concept (EDC) model, which can reflect detailed chemical reactions, was applied to calculate gas-phase reactions in the furnace. The results showed that when performing the deep peak shaving after the retrofit, the combustion in the furnace was stable under 50% or more load, and NOx emission level at the furnace outlet was lower than 350 mg/m3 (6% O2 content, dry basis). Under 25% load, the oxygen-rich burner integrated with LNG was applied, and the pulverized coal flow entered the furnace in a state of high-intensity combustion, which effectively promoted the stability of combustion in the furnace. The reductive combustion state with reductive free radicals generated by LNG decomposition inhibited NOx formation. Consequently, NOx emissions from the furnace outlet decreased from 380 mg/m3 to 316 mg/m3.

BOILER SUPERHEATER PIPE FAILURE ANALYSIS THROUGH CORROSION RATE AND MECHANICAL PROPERTIES TESTING


 
 
 
 Superheater is a subcritical boiler component that functions to reheat saturated steam at constant working pressure so that it becomes saturated steam. This study aims to (1) analyze failures in superheaters, (2) test mechanical properties of superheater pipe materials, (3) calculate corrosion rates in superheater pipes with weight loss methods, (4) provide preventive maintenance solutions to minimize superheater pipe damage.
 The method used in this study is a qualitative approach with the type of experimental study. The method of failure analysis in this superheater pipe is by testing mechanical properties, calculating the corrosion rate of the superheater pipe and providing maintenance solutions.
 The results of this study are (1) the failure that occurs in this superheater pipe is the occurrence of corrosion and leakage, (2) after heat treatment with a temperature of 8500C on the superheater pipe material, the hardness value increases to 148 BHN, the impact value also increases to 43.26 joules and the phase that occurs in this material is ferrite and pearlite which shows the specimen is increasingly tenacious, (3) the corrosion rate that occurs in the superheater pipe is 0.0489 mmpy, (4) the workable maintenance solution is hardening to increase the durability and service life of the superheater pipe.
 
 Keywords: Superheater, Noise, Mechanical Properties, Heat treatment.
 
 
 

NOx emission prediction of coal-fired power units under uncertain classification of operating conditions

Coal-fired power units are widely involved in peak shaving in the future renewable energy grid, leading to frequent changes in operating conditions, which makes it more difficult to predict NOx emissions. Aiming to address this challenge, a NOx emission prediction method based on a deep learning algorithm and uncertainty classification is proposed. Firstly, in order to better capture the intrinsic information in the NOx time series data, the deep learning network is used to construct the original NOx sequence prediction model. Secondly, a prediction error classification model based on a support vector machine is proposed to consider the uncertain working conditions and prediction errors of thermal power units. Then, the original sequence prediction results are corrected at different levels to obtain the final prediction results. The simulation experiments were compared with the operation data of a 660 MW subcritical boiler in the thermal power unit. Compared with several traditional models, the mean absolute percentage error, root mean square error, mean absolute error, and determination coefficient of the proposed model are 2.60 %, 11.09 mg/m3, and 8.27 mg/m3 and 0.86, respectively, which has higher prediction accuracy.

Influence of collaborative disposal of sewage sludge in the fly ash partitioning and PM10 emission from a subcritical coal-fired boiler

Co-combustion of sewage sludge and coal in coal-fired power plants may be a reasonable alternative for intractable sewage sludge. The co-combustion experiments were conducted on a 600 MW subcritical coal-fired boiler to discuss the fly ash partitioning and PM10 emission characteristics. The sewage sludge was sent to the boiler through coal pulverizer with blending ratios of 5 and 10 wt% based on the coal mass. The results showed that the particle size distributions (PSDs) of PM10 present the typical two-peak distribution. The mass yields of PM1 dramatically decreased by 57.83 % and 61.67 % for the 5 % and 10 % blending ratios, respectively, compared with single coal combustion, corresponding to the increase in PM1-10 emissions. The reduced PM1 appears to be transferred toward PM1-10 during co-combustion. The Fe Al-silicate and the Fe-Ca-Si-Al species were formed by the co-combustion of sludge and coal, which were derived from the active Fe-rich fine particles in sludge and mainly mullite and Ca-Si-Al found in coal ash. These transformations can be intensified by the high temperature, intense turbulent flow fields and the consequent frequent contact and collision of minerals in the large capacity boiler to form molten aluminosilicates. Therefore, the condensable Ca and S vapor from coal combustion and fine Si and Al adhere to the surface of molten Fe Al-silicate to form PM1+, which is the main reason for PM1 reduction during co-combustion. In addition, Fe-rich and Ca-rich flocculants in sludge contribute to PM1-10 largely, while some of the fine Al-Si particles are partitioned from PM1 to PM1-10 as captured by the molten aluminosilicates, both contributing to the increased PM1-10 emission after co-combustion.

Study on The Improvement of Heat Recovery Steam Generator Efficiency – A Review

Boilers are widely used in industries to produce steam. In some sectors, the steam generated is utilized directly in the production line for heating. Certain industries use steam to produce electricity. Fire tube boilers are limited to generating steam for processing; meanwhile, water tube boilers are widely used in electricity generation besides steam generation for processing lines. Subcritical boilers, supercritical boilers, and Heat Recovery Steam Generator (HRSG) are types of boilers commonly used to produce high capacity steam. This review article focuses on the optimization of HRSG operational efficiency. Industry players are keen on the improvement of operational efficiency since these directly influence the operating cost. Steam pressure, steam output, heat transfer efficiency and temperature distributions are key areas comprehensively reviewed in this article. Generally, improvement studies on boilers are not feasible to conduct during operation. Therefore, the scaled-down model used in the experiment or the boilers CFD models are simulated to understand the characteristics of the boilers. This review article is expected to overview HRSG boiler efficiency improvements and factors influencing boiler operational parameters.

Application of Computational Fluid Dynamics and Process Modeling to Investigate Low-Load Operation of a Subcritical Utility-Scale Boiler

Abstract To enable the inclusion of intermittent renewable energy sources on existing power grids requires base-load coal-fired power plants to operate flexibly, including fast load changes and continuous low-load operation. Low-load operation of a 620 MWe subcritical boiler is analyzed with the aid of a cosimulation methodology that incorporates a detailed 3D computational fluid dynamics (CFD) model together with a one-dimensional process model. A discretized one-dimensional model of the water/steam circuit was developed for the furnace, the radiant superheaters, and the convective/back pass heat exchangers. This is coupled with a detailed CFD model incorporating the furnace and radiant superheaters using an Eulerian–Eulerian reference frame. The coupled model is used to investigate the combustion stability, water/steam side effects, and the radiant heat-exchanger operational limits for six different burner firing arrangements at a low boiler load of 32% of maximum continuous rating. The results show that certain firing arrangements can lead to a high risk of fire-side corrosion and overheating of heat-exchanger components. Based on the analyses of combustion stability, boiler efficiency, and the safe operation of heat-exchanger components, a mixed firing arrangement with a higher secondary air mass flowrate for nonfiring burners was selected as the best operational strategy at this low load for the boiler under investigation.

Analytical and numerical investigations on the high temperature upgrading solution of subcritical boilers

Currently there still exist a large base of subcritical coal-fired boilers remaining in operation worldwide. Improving their thermal efficiency is critical for these units to reduce the cost of electricity and meet the increasingly stringent efficiency regulations. This paper presented an economic high temperature upgrading retrofit solution for these subcritical units in which the final superheat and reheat steam temperatures are elevated from 540 °C to 600 °C level while the steam pressures remain at subcritical such that the retrofit can be made with minimum modifications to the boiler to reduce retrofit cost and technical risk. A systematic analysis was first conducted to deduce the best boiler heating surface modification option to achieve such significant increase of boiler steam temperatures. The analysis shows that replacing part of the furnace water wall below the furnace exit with wall superheater is the most viable and economic boiler modification design. A three-dimensional computational fluid dynamics (CFD) simulation was then conducted for a typical 320 MW subcritical boiler design to verify the feasibility of this design and determine the area of furnace wall that needs to be modified to achieve the target steam temperatures. The results show that with only 3.6 m height of furnace wall replaced with wall superheater both the superheat and reheat steam temperatures can reach the target 600 °C level even without any modification made to the reheaters. With this heating surface modification design, the efficiency of the existing subcritical power plants can be improved considerably with minimum boiler retrofit work. More importantly, this technology is going to help the plant owners meet the increasingly stringent efficiency and emission regulations mandated by the government and place them in a better position to continue operating these units.

A semi-analytical approach on critical thermal states in water wall tubes of a subcritical drum boiler of a thermal power plant

One of the most important issues in design of water tube boilers is the prediction of the critical heat flux and the corresponding critical quality of the steam and vice versa. This phenomenon directly affects growth rate of oxide/deposit layers, rising water wall tubes temperature, and consequently tubes burnout. In the present work, the critical situations in a furnace of a subcritical drum boiler were investigated. It was shown that by moving to higher altitudes of the investigated furnace, its water wall tubes were more sensitive to heat flux distribution, which consequently increased the possibility of dryness phenomenon and failure of the tubes. Besides, this study demonstrated that in the event of a disturbance in the fluid flow inside the tubes, all the length of water wall tube are at serious risk of burning and overheating. Microstructural evolutions, existence of thick deposits, and frequent failures at higher the altitudes confirmed the semi-analytical results.

Combined operation mode of sub-critical W-flame boiler and coal mill optimized numerical simulation

The flow, combustion, heat transfer and NOx emission characteristics of a 600 MW subcritical W-flame boiler were numerically simulated under different combined operation modes of coal mills, and compared with the measured results. The results show that the combustion, average residence time, burnout rate, NOx emission characteristics and temperature distribution near the side wall of pulverized coal particles in the furnace have different effects on the combined operation mode of pulverized coal. In the combustion efficiency of give attention to two or more things, screen superheater section of fly ash carbon content and flue gas temperature of entrance at the same time, compared with six coal mill run at the same time, 5 coal mill run, shut down near the side wall of the coal mill is beneficial to reduce NOx emission concentration, to achieve the emission reduction, at the same time under the wing wall and side wall area chamber of a stove or furnace slagging significantly reduce.

Performance evaluation of a supercritical circulating fluidized bed boiler for power generation under flexible operation

It is well known that the Łagisza power plant in Poland is the world’s first supercritical circulating fluidized bed (CFB) boiler, whose commercial operation started on June 2009. It has attracted a great deal of interest and operational data are publicly available, therefore it has been chosen as the object of the present study aimed at assessing load and fuel flexibility of supercritical CFB plants. First, the thermal cycle was modelled, by means of the commercial code Thermoflex®, at nominal and part load conditions for validation purposes. After having verified the validity of the applied modelling and simulation tool, the advantage of having supercritical steam combined with CFB boiler over subcritical steam and pulverized coal (PC) boiler, respectively, was quantified in terms of electric efficiency. As a next step, the designed fuel, i.e. locally mined hard coal, was replaced with biomass: 100% biomass firing was taken into account in the case of subcritical CFB boiler whereas the maximum share of biomass with coal was set at 50% with supercritical CFB boiler, consistently with the guidelines provided by the world leading manufacturers of CFB units. A broad range of biomass types was tested to conceive mixtures of fuel capable of preserving quite high performance, despite the energy consumption in pretreatment. However, the overall efficiency penalty, due to biomass co-firing, was found to potentially undermine the benefit of supercritical steam conditions compared to conventional subcritical power cycles. Indeed, the use of low-quality biomass in thermal power generation based on steam Rankine cycle may frustrate efforts to push the steam cycle boundaries.

Coupled heat transfer model for the combustion and steam characteristics of coal-fired boilers

This paper presents a coupled heat transfer model for coal-fired boilers that integrates boiler’s fire side computational fluid dynamics (CFD) solution with the boiler’s steam cycle, and thus, can predict the boiler steam temperatures subject to different boiler design and operation conditions. In this coupled model, the heat transfer submodels describing the gas-steam heat exchange for each part of boiler heating surfaces are integrated by exchanging the steam temperatures iteratively. This coupled heat transfer model is self-consistent since it ensures the fire side heat transfer solution to always coincide with the steam temperature changes across each boiler heating surface. This model is specifically designed to provide efficient computational solutions to engineering boiler problems in which the changes of boiler steam temperatures subject to different boiler design and operation conditions are of the major concerns. This model is applied to a 320 MW subcritical boiler to evaluate the impacts of coal switching and boiler operation adjustment on boiler’s heat transfer distributions and steam temperatures. The simulation results demonstrate that this model can effectively capture the complex coupled heat transfer behavior between the fire and steam sides of boiler and serve as an efficient predictive tool for engineering boiler problems.

Investigation of the mechanism for fireside corrosion in coal-fired boilers in South Africa

SYNOPSIS Furnace wall tubes from 600 MW subcritical boilers at three coal-fired power stations were sampled and the fireside deposits examined to determine the mechanism of fireside corrosion. This involved an in-depth investigation into the morphology and composition of the fireside deposits and the conditions of the furnace that enable this type of attack. SEM-EDS analysis revealed high concentrations of oxygen, iron, and sulphur, QEMSCAN and XRD analyses identified the presence of Fe3O4, Fe2O3, FeS, and FeS2. Differential thermal analysis showed thermal activities at temperatures of 500-600°C, 900-1100°C, and 1100-1250°C, which are associated, respectively, with FeS2 oxidation to FeS and Fe2O3, at 475-525°C, formation of aluminosilicates at 925-1100°C, and melting of FeS around 1190°C. The absence of sodium and potassium eliminates the contribution of molten alkali sulphates to the corrosion. The consistent coexistence of iron sulphide and iron oxide is indicative of the substoichiometric conditions in the furnace, while the detection of pyrite suggests that the coal is not completely combusted, which points to a poor combustion process. These observations were affirmed by gas analysis at one of the stations, where very high levels of carbon monoxide were measured at the furnace wall (> 14 000 ppm) and furnace exit (> 3500 ppm). The high CO concentrations are indicative of limited combustion caused by limited O2. These reducing conditions promote the formation of FeS-rich deposit, which is the corrosive species responsible for degradation. Keywords: fireside corrosion, sulphidation, coal-fired boiler, furnace wall tubes.

Data analytics for leak detection in a subcritical boiler

For decades, boiler leaks have been the leading cause of forced outages in the coal-fired unit. The leak occurrences are currently escalating since the existing plants must satisfy faster-ramping rates to support grid operation. Data analytics including Principal Component Analysis (PCA), Canonical Variate, and Fisher Discriminant Analysis (CV-FDA) were combined for detecting and characterizing the leak in a commercial 650 MW subcritical coal-fired power plant. The combined approach was shown to be highly effective in the fault investigation that would not have been easily achieved by an individual technique. The variability in both training and validation datasets was first evaluated using PCA. Then, the CV-FDA was employed to discriminate among faults, and to categorize the processed data into two main groups: no-leak (0) and leak (1), providing the timeframe and location of the leak occurrence. About 8,014 observations from 81 process variables were initially included in the calculation, while the variable count was reduced to 4 with less than 1% misclassification rate in total observations. Finally, the leak was isolated in the waterwall section. Thus, the outcome of this research may provide early detection and isolation of faulty operations in the coal-fired power plant that involves a considerable number of process variables.

Leak detection in a subcritical boiler

Thermal power plants experience cycling duty leading to the fatigue of the boiler and heat exchanger tubes. As a result, tube failures occur frequently in coal fired fleets leading to forced outages. Because the tube leaks have been the major source of unwanted shutdowns and the number of outages is increasing, present work focuses on the detection and isolation of the leak in a subcritical boiler based upon the process data from a commercial coal-fired power plant. The mass balance equation around the steam drum was analyzed using timeseries data collected from a 300 MW power plant. The ratio of the feed water mass flow rate to the steam mass flow rate was defined as a key parameter for detecting leaks. The difference in slope between the feedwater and steam mass flow rate during the normal and faulty operations was established as the upper control limit for real time monitoring. To reduce false alarm rates that arise when raw signal is directly compared against the threshold due to common process fluctuations, an optimal filter was derived for smoothing. It was found that the optimal filter reacted much more quickly to process changes than an exponential moving average filter, around 8 h earlier on average. Occurrence of relatively high false alarm rates even in the filtered responses was related to the cycling of the boiler from the base load condition. Variable threshold was established to keep false alarm rates to the minimum while maintaining the leak detection rate. Finally, the leak was located at the economizer and this could readily be isolated by investigating the magnitude of the mass flow rates ratio and the temperature at the economizer outlet.

Steam Temperature Of 600MW Subcritical Boiler Unit Research And Practice Of Upgrading To Ultra Supercritical Parameters

In order to save energy and improve the working efficiency of the unit, this paper analyzes the data improvement after the steam temperature of the subcritical boiler unit rises to the ultra supercritical parameters through the transformation of the unit and makes a summary of the change of the boiler efficiency. In order to effectively improve the economic benefits, environmental protection and energy saving effect.

CFD modeling of sodium transformation during high-alkali coal combustion in a large-scale circulating fluidized bed boiler

The ash-related problems of fouling and slagging occur easily during high-alkali coal combustion in boilers. In this study, numerical calculations were performed to investigate the sodium migration mechanism and ash deposition on heating surfaces in a large-scale circulating fluidized bed (CFB) boiler during the burning of high-alkali coal. The calculation principle was based on a comprehensive three-dimensional CFB combustion model and a sodium migration model from our previous study. The computing object of this study was a 300 MW subcritical CFB boiler. By performing a simulation, the distributions of NaCl(g) and Na2SO4(g) along the furnace were displayed. The rates of sodium deposition via condensation, inertial impaction, and thermophoresis on the water walls and suspended panels were analyzed. Additionally, the net deposition rates of fly ash on the water walls and suspended panels were determined. Then, the tendency of fouling and slagging in different areas of the CFB furnace could be predicted and judged. Calculation results for the gas–solid flow, furnace temperature, and gaseous species agreed with the measured data.