Power Station Boiler Research Articles

Elevated temperature solid particle erosion performance of Al<SUB align=right>2O<SUB align=right>3-3 wt% TiO<SUB align=right>2 composite coatings

The operating conditions in power station boilers are conducive to erosion, both in the furnace wall and in the super heater and reheater areas, and these effects cause tube wall thinning and premature failure. The present research work has been undertaken with an objective to explore the coating potential of Al2O3-3 wt% TiO2 deposited on boiler steel T11 with detonation gun spraying process for erosion resistance. Erosion performance at room temperature as well as elevated temperature by simulating the temperature to actual working conditions of the boilers was evaluated. The eroded samples were analysed with SEM/EDX and optical profilometer.

Burner Development for the Reduction of NOx Emissions from Coal Fired Electric Utilities

Pulverized coal combustion is widely used for electricity generation around the world. The oxides of nitrogen are among the important polluting species, emitted from coal combustion, having large scale adverse impacts on the environment and life. There can be different routes of the formation of NOx from coal combustion and their emission from electric utilities depends on several boiler operating parameters as well. Among the different options of reducing the NOx formation and emission from power station boilers, the introduction of low NOx burner is one of the less expensive alternatives, which can also be readily retrofitted in the existing plants. The present review has focused on the different design decisions in the burner technology for reducing the formation of NOx. Important patents in the field as well as several commercial designs of the burners have been analyzed. A discussion has also been incorporated on some of the current innovations for further reduction of NOx in order to achieve stringent regulations in the near future.

Study on Forming Quality Control of Bending Tube in Power Station Boiler

A FEM-numerical simulation system is developed to simulate the process of rotary draw bending of tube. The emphasis is to discuss some factors such as tube materials, bending moment of fan-shaped die, pushing force of sliding slot and tube relative bending radius which have an effect on tube bending quality. The quality parameters of ovality, thinning ratio and thickening ratio are the same as the measurement values in the practical production. Some improved methods are put forward to according to the simulation results. The experiment shows that the simulation system can forecast accurately the deformation in the process of tube bending, so the production manufacturing cost decreases greatly.

Identification of local heat flux to membrane water-walls in steam boilers

A heat flux entering steam generating tubes in power station boilers may be a critical factor in considering the safety of the tubes. The knowledge of the distribution and magnitude of this flux during the operation of the power boiler is very important. The design of a modern boiler furnace requires the computation of furnace wall metal temperatures for the proper selection of the tube thickness and material. These temperatures are functions of the heat fluxes and the internal heat transfer coefficients. In this study, a measuring device (flux-tube) and a numerical method for determining the heat flux in boiler furnaces, based on experimentally acquired interior tube temperatures, are presented. An inverse method was developed, which can estimate the following parameters: the absorbed heat flux, the heat transfer coefficient on the inner tube surface and the temperature of water-steam mixture from temperature measurements at several interior locations of the flux-tube. The least squares method was used to minimize the differences between the calculated and measured temperatures. The unknown parameters are found using the Levenberg–Marquardt method. The number of temperature sensors (thermocouples) is greater than three because the additional information can help in more accurate estimations of the unknown parameters. The temperature dependent thermal conductivity of the flux-tube material was assumed. The developed flux-tube can work for a long time in the destructive high temperature atmosphere of a coal-fired boiler.

Optimization of Gas Nozzles Geometry in Dual-Fuel Burners of Power Stations

Thermal power required by boilers in steam power stations is supplied through gas or heavy oil fueled burners. Incorrect functioning of these burners results, on one hand, in their inability to supply the utility’s needed steam and, on the other hand, in imposing damages to various parts of the boiler and burner’s structure itself. The present paper investigates problems related to the burners used in the boilers of Shazand 325 MW power station in Arak. Gas-fueled nozzles of these boilers had suffered burns, so that a large number of them would have to be either repaired or replaced in every six-monthly maintenance cycle. These difficulties are caused partly by design errors and partly by incorrect commissioning of these burners. In this paper, the burners’ faults are investigated from the following viewpoints: (1) Statistical assessment of failures. (2) Investigation of trend in the nozzles failure and microstructure changes during operations. (3) Calculation of nozzles temperature when using heavy oil and natural gas fuels. Investigations leading to solutions providing new designs for nozzles in this paper and later evaluation of temperature of the new nozzles in various conditions using natural gas and heavy oil fuels have been carried out.

The evaluation of waste tyre pulverised fuel for NO x reduction by reburning

The combustion of coal for power generation will continue to play a major role in the future, however, this must be achieved using cleaner technologies than we use at present. Scrap tyre arisings in the UK are 400,000 tonnes per year amounting to 30 million tyres and in the EU as a whole, more than 2.5 million tonnes of tyres per year are scrapped. The recent EC Waste Landfill Directive (1999) sets a deadline for the banning of whole and shredded tyres from landfill sites by 2006. Consequently, there is an urgent need to find a mass disposal route for tyres. We describe, in this paper, a novel use for tyre rubber pulverised fuel in a NO x reburning process which may have an application in power station boilers. This method of disposal could represent a way of combining waste disposal, energy recovery and pollution control within one process. A preliminary study of micronised tyre combustion was undertaken to identify the suitable size ranges for application in NO x reduction by reburning. Tests were performed in a down-fired, pulverised fuel combustor (PFC) operating at about 80 kW. Superior combustion characteristics, i.e. burnout were achieved with particle sizes less than 250 μm. A South African coal was used as the primary fuel in the reburn tests and the tyre was fed pneumatically via a separate feed system. Parameters studied, were, reburn zone stoichiometry and reburn fuel fraction. Additionally, the carbon content of the ash was carefully monitored for any effect on burnout at the fuel rich reburn stoichiometries. The NO x reductions achieved with tyres are compared with reburning with coal. NO x reductions up to 80% were achieved with tyres at half of the reburn fuel feed rate required to achieve the same reductions by coal. The results have been evaluated within the context of other studies available in the literature on NO x reburning by bituminous coal, brown coal, gas and biomass.

Erosion Wastage at High Temperature for Arc Spraying Coatings

A new novel cored wire for producing FeCrAl/WC intermetallic coating by arc spraying was developed. The erosion wastage at high temperature of the coating was researched with different impingement angles and temperatures. The results indicated that impingement angle and temperature greatly affected erosion wastage of the coating. At the same time, when impingement angle was 90°, different temperature had a little effect on the erosion wastage. And when temperature was 650°C, different impingement angle had a little effect on the erosion wastage too. As compared with water wall tube material (G20 steel) of power station boiler, the erosion resistance at high temperature of the coating was higher. The erosion resistance rate of the coating was 1.94~2.89 times of that of G20 steel when temperature was 650°C and particle was polygonal silica. The results indicated that erosive condition of high temperature flue gas and fly ash mixture in boiler tube of pulverized coal fired power station could be solved with the FeCrAl/WC coating.

Calculating the efficiency of a thermal power station boiler and systems on the basis of exergetic matrices

A method is considered in which the efficiencies of a boiler and thermal power station systems featuring a treelike distribution of energy flows and also having reversible or regenerative flows are calculated using exergetic matrices.

A maintenance optimisation framework in application to optimise power station boiler pressure parts maintenance

PurposeThe boiler pressure parts are a major asset of a power station, and the maintenance cost is often accountable for a huge portion of the annual budget. In the power generation industry, the outage costs due to loss of production, both planned or forced, are very significant and thus it is of interest to seek for a meaningful approach to the management of boiler pressure parts maintenance such that the enterprise performance is optimised. This paper aims to do this.Design/methodology/approachThis paper proposes a framework that introduces the division of the enterprise objectives into the three decision dimensions. The framework is applied to the case of power station boiler pressure parts maintenance to optimise maintenance outages decisions for Loy Yang, a power station in Victoria, Australia.FindingsThe study finds that the framework provides meaningful approach to optimising maintenance decisions and is generic for application in different cases.Originality/valueThe paper provides a new insight and integrated approach to optimising asset maintenance for an enterprise with the use of a case study.

Modelling and simulation of fluid flow and heat transfer in the convective zone of a power-generation boiler

The convective zone of a power-station boiler is a complex piece of engineering equipment, which comprises a multiplicity of inter-connected heat-exchanging elements. This paper presents a mathematical model of the convective zone, which allows for the calculation of the shell-side flow and the shell-side, tube-side and tube-wall, thermal fields, and of the shell-tube heat-exchange. The model is solved using conventional CFD techniques, in which the individual tubes are treated as sub-grid features. A geometrical model is used to describe the multiplicity of heat-exchanging structures, and the interconnections among them. The CFD model is first validated by comparison with simple heat exchanger geometries which can be solved with analytical methods, and then applied to an actual 350 MW(e) power-station boiler.

The effect of jet velocity ratio on aerodynamics of a rectangular slot-burner in the presence of cross-flow

In a typical coal-fired power station boiler the ignition and the combustion of the fuel is largely controlled by burner aerodynamics. An experimental and numerical study of the rectangular slot-burners widely used on power stations in Victoria, Australia has been conducted to improve understanding of jet development within the boiler. The 1:15 scale model burner consisted of a central (primary) rectangular fuel nozzle with two (secondary) rectangular air jets positioned above and below it. The burner jets entered the measurement vessel at an angle of 60° to the wall. A cross-flow jet was attached to the wall of the vessel to simulate the recirculation prevalent in power station boilers. Experiments were conducted using a primary to cross-flow jet velocity ratio ( φ) of 1.0 and secondary to primary jet velocity ratios ( ϕ) of 1.0 and 3.0. Laser Doppler Anemometry (LDA) was used to measure mean and turbulent velocity components in the near field and downstream regions of the jets. Cross-flow significantly influenced the near field flow development from the slot-burner by deviating both primary and secondary jets from their geometric axes towards the wall. The degree of deviation was greater for ϕ = 1.0 since the higher velocity secondary jets increased the overall momentum of the primary jet for ϕ = 3.0. A numerical investigation of the rectangular slot-burner was also performed. First, the numerical results were validated against the experimental results and then visualization of the developing flow field was used to reveal the finer details of the cross-flow/burner jet interaction. Agreement between numerical and experimental jet features was good, although the numerical results predicted a primary jet that was marginally too narrow. Also the predicted downstream behaviour for ϕ = 3.0 deviated more significantly from experimental observation. Using the SST turbulence model, the numerical results suggested that a twin vortex was generated behind the initial region of the primary jet and this would aid in mixing of gas and fuel between primary and secondary jets.

Optimization of baffle configurations to prevent aeroacoustic instabilities in heat exchangers: Preliminary experiments

Gas heat exchangers are prone to aeroacoustic instabilities, which often lead to severe noise levels, structural vibrations, and fatigue. Actually, this problem is solved by placing rigid baffles inside the container, which modify the acoustic modal fields and eventually inhibit the instability. For realistic industrial components using a restricted number of acoustical baffles, their optimal location is a challenging problem, as trial and error experimentation is often a costly and frustrating procedure. Recently, some strategies were proposed for the optimal location of a single baffle in a typical re‐heater from a power station boiler, based on simulated annealing as well as genetic algorithm approaches. In this paper and using the above‐mentioned optimization strategies, a more complex case of the problem—the optimal location of a given number (two or more) of baffles in a typical re‐heater, was addressed. Some preliminary experiments were performed and compared with the simulation results. From the discussion, improvements in the developed optimization functional were proposed. [This work has been endorsed by the Portuguese FCT and POCI 2010, with funding participation through the EC programme FEDER (Project No. POCI/EME/5728/2004).]

Heterogeneity of Ash Deposits Formed in a Utility Boiler during PF Combustion

This paper reports quantitative characterization of ash deposits from a 330 MWe power station boiler. The deposit samples were cut into a series of thin slices, and a systematic characterization wa...

Sulphur segregation and precipitation in HAZ of boiler steel thick section welds

Stress relief cracking can occur in weld heat affected zones (HAZ) after post-weld heat treatment (PWHT) and periods of service at elevated temperatures. Stress relief cracking is generally believed to occur by sulphur induced decohesion ahead of a growing sharp crack. The impurity segregation behaviour in a microalloyed steel, typical of that used in the construction of a power station boiler where intermittent cracks were observed along the weld fusion boundaries, has been assessed. In particular the type and amount of segregation in the coarse grained HAZ (CGHAZ) before and after PWHT has been determined. It was found that significant sulphur segregation occurred during the CGHAZ thermal cycle resulting in elemental sulphur on the prior austenite grain boundaries. Following PWHT some desegregation of sulphur, coupled with the formation of sulphides and carbides on the prior austenite grain boundaries, was observed; in addition, significant phosphorus segregation to the prior austenite grain boundaries and grain boundary precipitate/matrix interfaces was seen.

A Versatile Computer Simulation Model for Rotary Regenerative Heat Exchangers

A simulation technique for predicting the thermal performance of rotary regenerative heat exchangers, in particular those used for heating the intake air to power station boilers, has been developed and verified by means of site measurements. Various geometries of both rotating-hood and rotating-matrix types of air heaters can be accommodated in the simulation model, including packings of corrugated steel plates of various specified profiles and any given thicknesses and lengths in the flow direction. Rotational speed, leakage, blockage, and non-uniform inlet flow distribution may be taken into account as input variables. The heat transfer and pressure drop correlations for the various plates considered, which also form part of the input data, were determined experimentally using a single-blow transient test facility, constructed as part of this research program. The effect of erosion of the plates by fly ash particles carried in the outlet flue gas on heat transfer performance is also considered, and experimental results show that erosion has little effect on the thermal performance (up to the point that structural integrity is about to be compromised), but also that the pressure drop is reduced.

06/00520 Artificial neural network-based prediction of hydrogen content of coal in power station boilers: Yao, H. M. et al. Fuel, 2005, 84, (12–13), 1535–1542.

06/00520 Artificial neural network-based prediction of hydrogen content of coal in power station boilers: Yao, H. M. et al. Fuel, 2005, 84, (12–13), 1535–1542.

Reducing superheater corrosion in wood‐fired boilers

AbstractOne of the major drawbacks to the combustion of 100% biomass in power station boilers is the increase in the fouling and corrosion of superheaters. Experience shows that conventional superheater steels last no more than 20 000 h or four operating years before they must be replaced, if 100% wood‐based fuel is used and the steam temperature is higher than 480°C. Rapid corrosion of superheaters leads to increased maintenance costs while widespread deposit formation gives rise to a decrease in efficiency of the turbine and an increase in unplanned outages. This paper reports on measures taken to reduce superheater corrosion.Most biomass fuels have a high content of alkali metals and chlorine, but they contain very little sulphur compared to fossil fuels. Potassium chloride, KCl, is found in the gas phase, condenses on the superheater tubes and forms complex alkali salts with iron and other elements in the steels. These salts have low melting points and are very corrosive. The corrosion can be mitigated by use of an instrument for in‐situ measurement of alkali chlorides in the flue gases, in combination with the addition of ammonium sulphate. An ammonium sulphate solution, specially developed for the reduction of corrosion was sprayed into the flue gases and effectively converted KCl into potassium sulphate, K2SO4, much less corrosive than KCl.Deposit probe tests and long‐term corrosion probe tests have been performed in‐situ in two biomass‐fired fluidised bed boilers. One boiler, 105 MWtot, 540 °C steam, is the CHP plant in Nyköping, Sweden. The other producing 98 MWtot, 480 °C steam, is a bark‐fired boiler at a pulp and paper mill in Munksund, also in Sweden. Tests have been performed with a range of steel types, ferritic and austenitic, with ammonium sulphate additive and under normal conditions (no additive). Corrosion rates have been measured, deposit chemistry analysed and flue gas chemistry and KCl content measured. The structure and composition of the oxide with and without ammonium sulphate has been investigated. The results show that ammonium sulphate reduced the KCl levels in the flue gases, removed the chlorides from the deposits and the metal/oxide interface, greatly reduced the deposition rates and halved the corrosion rates for superheater materials. The alkali chloride measuring system and the use of ammonium sulphate for the reduction of corrosion have now been patented.

Artificial neural network-based prediction of hydrogen content of coal in power station boilers

Artificial neural networks (ANN) are powerful tools that can be used to model and investigate various highly complex and non-linear phenomena. This paper describes the development and training of a feed-forward back-propagation artificial neural network (BPNN), which is used to predict the hydrogen content in coal from proximate analysis. The ultimate objective is to enhance the performance of the combustion control system with the aid of regularly obtained knowledge of the elemental content of coal. In the present work, network modelling was performed using MATLAB with the Levenberg–Marquardt algorithm. Nine-hundred and three sets of data from a diverse range of coals have been used to develop the neural network architecture and topology. Trials were performed using one or two hidden layers with the number of neurons varied from 4 to 30. Validation data has been adopted to evaluate each trial and better model structure is determined to combat the over-fitting problem. As a result, it was found that a 4-12-1 or 4-8-4-1 network could give the most accurate prediction for this particular study. The regression analysis of the model tested gave a 0.937 correlation coefficient and the mean squared error of 0.0087. The average relative error is 5.46%. This has demonstrated that artificial neural networks have good potential for predicting elemental content of coal from frequently available proximate analysis data in power utilities.

Internal damage processes in low alloy chromium–molybdenum steels during high-temperature creep service

Results are presented of investigations into structure of low alloy Cr–Mo steels exhibiting internal damage after long-term creep service. It was demonstrated that intercrystalline cavitation cracks were the dominant factor in service damage of power station boiler components operating in creep regimes. Consecutive stages in development of internal damage involving intercrystalline cavitation cracking were discussed and illustrated with micrographs. The results seem to indicate that nucleation of creep cavities in materials under consideration is related to grain boundary slip. Evidence confirming the shear mechanism proposed by Sklenička and Saxl for cavity coalescence was obtained. Occurrence of intercrystalline crevice cracking was demonstrated. Micrographs were used to illustrate wedge crevice nucleation modes on triple junctions depending on the direction of slip according to Chang and Grant. A classification of internal damage in relation to life exhaustion was proposed for materials under consideration. A method used in industrial practice for evaluation and qualification of creep-damaged materials was presented.

An approach to power station boiler and turbine life management

The de-regulated electricity market which has existed in the UK for around 15 years has led to energy companies operating their power plant flexibly to maintain profitability in a very competitive commercial environment. To do this, the company maintains a very strong engineering capability in the area of structural integrity of boiler components and high temperature rotating plant. The key issues for the older 500 MW boiler plant, which is daily two-shifted and operates at temperatures well into the creep range for low-alloy steels, are fatigue and creep damage, the latter being influenced greatly by the operating hours seen by the older coal-fired units which have exceeded their design lives. Major components of the boiler which are subject to rigorous and thorough examination are headers, drums and high-energy steam pipework. Methodologies include on-line plant damage analysis for controlling degradation and optimising start-up procedures, plant modifications, innovative repair and retro-fitting with improved materials. Steam and gas turbine rotors operate at high speeds at elevated temperatures. Maintaining rotors in a good commercial and safe operational condition in compliance with any regulatory requirements can also bring increased efficiency and reduced costs, for example by incorporating proactive measures into planned overhauls. Procedures have been developed for both prevention of creep cavitation cracking and assessment of bore high strain fatigue life (for hollow rotors). These use material data, temperature and stress information along with the results of NDE inspections to determine any operational constraints (starts, temperatures, and running hours) which may be necessary. Vibration, which can be an indication of cracking in an operating rotor is monitored remotely and on-line.