Boiler Surface Research Articles

Models for evaluating the thermal engineering reliability of the heating surfaces of boilers under steady operating conditions

Existing indicators characterizing the reliability of boiler unit heating surfaces are systematized. Classifications of deterministic and stochastic models for evaluating the thermal engineering reliability of tube heating surfaces are developed. A few actual cases were considered to show that the deterministic approach to determination of the reliability of heating surfaces lacks consistency. A model for stochastic assessment of the reliability of boiler unit heating surfaces is proposed in general outline.

Development of an ash particle deposition model considering build-up and removal mechanisms

Slagging and fouling on heat exchanger surfaces in power boilers fired with fossil fuels and fuel mixtures has a significant influence on boiler efficiency and availability. Mathematical modelling is since long considered a suitable method to assist boiler operators to determine optimized operating conditions for an existing furnace. The ultimate goal in ash deposition prediction is hereby the determination of the total amount of material deposited and hence the determination of the total reduction in efficiency. Depending on the fuels fired the total deposited mass is a combination of ash particle deposition and ash particle erosion due to non-sticky particles.The novel ash particle deposition model presented in this work considers deposition of sticky ash particles, cleansing of deposit by non-sticky sand particles and sticking of sand due to contact with sticky ash.The steady-state modelling results for the total amount of ash deposited on the deposition probe of an entrained flow reactor presented in this work agree well with the experimental data. Only at very high fractions of sand added as non-sticky material, a significant influence of the sand on the overall mass deposited was found.Since the model considers sticking of non-sticking sand due to contact with sticky ash, the fraction of sand deposited on the probe was especially studied. Using a correction factor to consider the influence of operating time on the steady-state simulations led to good agreement between simulations and experimental data.

Mathematical modelling of transient processes in convective heated surfaces of boilers

The paper presents a one-dimensional mathematical model for simulating the transient processes which occur in convective heated surfaces of boilers, namely superheaters and economizers. The proposed model is based on solving equation describing the energy conservation and is simplified because the mass and momentum balance equations are omitted. The suggested method considers the superheater or economizer model as one with distributed parameters. The temperature of the separating wall is determined from the equation of transient heat conduction. In order to obtain a grater accuracy of the results, the wall was divided into two control volumes making it possible to compute the temperatures on its both surfaces. All thermo-physical properties of the operating media and the material of the separating walls are computed in real time. The space-time heat transfer coefficients are also computed on-line considering the actual tube pitches and assuming cross-flow of the combustion gases. In the proposed model the boundary conditions can be time-dependent. In order to experimentally verify the proposed method for modelling the processes occurring in convective surfaces of power boilers, a series of measurements of the platen superheater and economizer in an OP-210 boiler (with steam capacity of 210×103 kg/h) were carried out. Comparing the results of measurements of the steam and feed water temperature with computation results, satisfactory convergence is found.

Results of steam-water-oxygen treatment of boiler units for the 300-MW power-generating sets at the Novocherkassk State Area Power Station

A steam-water-oxygen treatment, passivation, and preservation (SWOT, P, and Pr) of the internal heating surfaces of TPP-110, P-210, and P-210A boilers for the 300-MW power-generating sets have been successfully introduced at the Novocherkassk State Area Power Station (SAPS). The condition of the treated heating surfaces of the boilers while in service or on stand-by was investigated to determine the required periodicity of the SWOT, P, and Pr under the service loads and existing water-chemical regime. Specific results of the investigation are presented.

Some aspects of using piezoelectric probes for ultrasonic inspection of thin welded joints

The choice of parameters of dedicated piezoelectric probes is considered with allowance for a surface wave arising at probe angles close to the second critical angle. Recommendations are given for the choice of optimal probe angles and probe x-values. Examples are given to show the application of the dedicated probes, the choice of their type, and the parameters (frequency, probe angle, x-value) for the inspection of butt-welded joints of tubes on the heating surfaces of boilers at thermoelectric power stations.

Cleaning of boiler heating surfaces

Basic methods and facilities for the external cleaning of the heating surfaces of boilers designed for the combustion of low-grade solid fuels are discussed. Water and steam blastings, which are the basic means of cleaning furnace shields, and semi-radiative and convective heating surfaces have the greatest range of application.

ASSESSMENT OF REMAINING LIFE OF SUPERHEATER AUSTENITIC STEEL TUBES IN OIL SHALE PF BOILERS

Superheater surfaces in oil shale-fired steam boilers are subject to intensive corrosion, which causes thinning of tube wall and increased stresses. It leads to reduction of creep life of superheater tubes and results in the necessity of superheater repair every 3-4 years with replacement of up to 30-50% of austenitic tubes. The ability to predict accurately the remaining life of superheater tubes allows to reduce the amount and cost of repair. The method of assessment of remaining life for superheater austenitic steel tubes operating in conditions of intensive high-temperature corrosion is presented in this paper. The method is based on measurements of tube wall thickness and kinetic diagram of corrosion resistance of a particular steel.

Boilers with fixed (stationary) layer of catalyst

In the presented paper, the special features of boilers with fixed catalyst layer and the types of catalyst fixing are described. The model of blackbody and radiant heat transfer from it to the heat-absorbing surface of boiler is also presented. The meaning of Catalytic Heating Element CHP is introduced, and the corresponding CHP constructions were developed for it. They are intended for the different arrangements and applications. Many efforts were made on selecting the efficient catalyst. The specific power characteristics of some catalysts are compared. It is recommended to take into account the obtained results in designing the heat generators with fixed catalyst layer.

03/02477 Corrosion of low pressure heated water boiler and its prevention: Wang, F.-U. Heilongjiang Dianli, 2002, 24, (4), 305–307. (In Chinese)

Analyses the acid corrosion of low temperature heated surface of coal-fuelled boiler, and the fume side corrosion of boiler water pipes.

Technical note: Anticorrosive behaviour of octadecylamine for protection of boiler surfaces

The anticorrosive behaviour of octadecylamine (ODA) films has been investigated via weight loss and electrochemical methods. Tests were conducted on specimens of carbon steel, brass, and austenitic stainless steel in the presence and absence of amine films. Inhibitor efficiency values in excess of 94·5% were obtained in this series of experiments. These results have been applied for the protection of boiler surfaces when out of service. For surfaces that were coated with ODA before lay up began, the optimum concentration of ODA for protection while out of surface was found to be 0·2 ppm.

Ash deposition prediction in biomass fired fluidised bed boilers ? combination of CFD and advanced fuel analysis

Operational problems caused by ash such as slagging, fouling and corrosion of boiler surfaces continue to be the most usual single reason for unscheduled shut downs of these boilers. The amount and distribution of various ash forming elements in the fuel and the local operational conditions in a boiler strongly affect the type of fly ash formed during the firing process, which again will affect the deposition behaviour. To avoid these operational uncertainties, it will be very beneficial to be able to predict ash deposition behaviour based on advanced fuel analysis combined with computational fluid dynamic (CFD) calculations. This paper presents for the first time a concept combining CFD calculations and advanced fuel analysis to predict the ash deposition behaviour on heat exchanger surfaces and boiler walls in the freeboard of a 105 MW biomass fired bubbling fluidised bed boiler. Extensive experimental investigations of the untreated fuel deliver the composition of the ash forming elements in the fuel. These fuel specific data are used as input for advanced thermodynamic equilibrium analysis leading to a detailed description of the temperature dependent melting behaviour of the ash. Based on this melting behaviour, a fuel-specific stickiness criterion is determined. This stickiness criterion, experiences from field studies and the operational set up of the boiler serve as boundary conditions for the CFD calculations. In these calculations physical and chemical processes occurring in the freeboard region of a bubbling fluidised bed combustor - starting from the bed up to the second super heater - are predicted in the form of gas phase and ash particle trajectory simulations. The exact positions of ash particle impacts on the boiler surfaces are recorded and the particle temperatures at these locations are the linking parameter to the fuel-specific stickiness criterion. The predicted locations of high ash deposition probability on boiler walls and super heater surfaces are compared with observations made in the boiler and very good qualitative agreement is found.

Saving energy lost from steam boiler vessels

This study is concerned with two important and inter-related issues: the issue of energy conservation and the issue of environment protection. Fortunately, the success in management and solution of the first issue will produce positive impact on the second one. That means that a reduction in fuel consumption, in the form of energy dissipated, tends to reduce the environmental pollution caused by objectionable products of combustion (e.g. oxides of carbon, sulpher and nitrogen as well as ash and particulate). These statements have been applied to a steam boiler produced by the NASR company. For an insulated steam boiler burning heavy fuel (e.g.C 13H 28) producing energy of 13.25 MWh and operating continuously for one year (i.e. 8000 h), the results showed that the calculated optimum insulation thickness is about 0.03 m which is required to minimize the total costs of insulation material and fuel consumption in the form of heat lost from the boiler surface. The corresponding emission gases to the environment are 60 Ton/y CO 2 for complete combustion, and 35 Ton/y CO 2 and 15.2 Ton/y CO for incomplete combustion. The results showed also that when the insulation conductivity increased by 50%—as a result of diffusion of atmospheric water vapor into the macrostructure of the insulation material—the air pollution has increased by 25 Ton/y CO 2 for complete combustion and 14.5 Ton/y CO 2 and 6.5 Ton/y CO for the case of incomplete combustion. Accordingly it is intensively recommended that the maintenance routine program of insulated hot surfaces must be carefully implemented to save much energy and reduce the environment pollution simultaneously. Finally, it has been shown that the heat lost, and consequently the air pollution, are relatively large while the cost is minimal. To this end, there must be a compromise between the total costs of the insulation material and the fuel consumed on one hand, and the economic damage and public health deterioration caused by the pollutant gases on the other.

High-Temperature Corrosion of Martensite Alloy Under Oil Shale Fly Ash

Superheater surfaces in oil shale steam boilers, constructed from pearlitic or austenitic alloys, are subject to intensive corrosion, primarily from the presence of chlorine in external deposits. The applicability of martensitic alloys X10CrMoVNb91 and X20CrMoV121 for superheaters has been examined and empirical equations to predict alloy corrosion resistance have been developed for the operational temperature range. Alloy 10CrMoVNb91 is found highly suitable for superheaters in boilers firing fossil fuels that contain alkaline metals and chlorine. An abnormal dependence of corrosion resistance of martensitic alloys on temperature is revealed, namely that corrosion in presence of oil shale fly ash is more severe at 580°C than at 620°C.

00/01418 Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study

00/01418 Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study

Influence of deposit formation on corrosion at a straw-fired boiler

Straw-fired boilers generally experience severe problems with deposit formation and are expected to suffer from severe superheater corrosion at high steam temperatures due to the large alkali and chlorine content in straw. In this study, deposits collected (1) on air-cooled probes and (2) directly at the existing heat transfer surfaces of a straw-fired boiler have been examined. Deposits collected on air-cooled probes were found to consist of an inner layer of KCl and an outer layer of sintered fly ash. Ash deposits formed on the heat transfer surfaces all had a characteristic layered structure, with a dense layer of K 2SO 4 present adjacent to the metal surface. It is argued that the K 2SO 4 layer present adjacent to the metal surface may lead to reduced corrosion rates at this boiler. A discussion of the deposit structure, the K 2SO 4 layer formation mechanism, and the influence of the inner layer composition on the corrosion of the superheaters is provided.

Too hot to handle: An unusual exposure of HDI in specialty painters

Background Hexamethylene Diisocyanate (HDI) is a color stable aliphatic isocyanate that is used in specialty paints as a hardener. Due to the lower vapor pressure of its commercial biuret form, it is considered a relatively “safe” isocyanate from an exposure standpoint. This case series reports on an unusual toxic exposure to HDI. Between November 1993 and May 1994, seven specialty painters and one boiler maker who were working at three different power plants were examined at the Institute of Occupational and Environmental Health at West Virginia University. At their respective work sites, HDI was applied to the hot surfaces of boilers that were not shut down, and allowed sufficient time to cool. Consequently, these workers were exposed to volatile HDI and its thermal decomposition products. Methods All of these workers underwent a complete physical examination, spirometry, and methacholine challenge testing. Results All 8 workers complained of dyspnea, while 4 of the 8 also complained of rash. On examination 3 workers were methacholine challenge positive and 2 had persistent rash. At follow-up 4 years later, 5 workers still had to use inhalation medication and one had progressive asthma and dermatitis. All 8 workers, by the time of the follow-up, had gone through economic and occupational changes. Conclusions This case series reports on an unusual exposure to HDI. It is unusual in that: 1) There were two simultaneous sentinel cases with two different Material Safety Data Sheets (MSDS) for the same product, 2) Exposure was to volatile HDI and its decomposition products and 3) Hazardous conditions of exposure occurred at three different sites. Am. J. Ind. Med. 37:431–437, 2000. © 2000 Wiley-Liss, Inc.

Deposition of potassium salts on heat transfer surfaces in straw-fired boilers: a pilot-scale study

The fate of inorganic salts in straw-fired combustion systems was studied in Sandia's Multifuel Combustor. Special attention was drawn to the deposition of potassium, chlorine, and sulphur and the sulphation of potassium chloride to potassium sulphate. The experiments included wheat straw-firing under different combustion conditions and with elevated sulphur levels. Investigations of deposit formation on a simulated superheater tube placed in cross flow with the gas revealed that the deposit was ellipse-shaped and had an inner layer of white condensed salts. The inner layer had a dendritic structure and consisted of potassium chloride and potassium sulphate. Part of the water-soluble potassium in the straw was transformed into potassium silicates and a mixed mineral of potassium, calcium, and silicon. A novel probe was designed to investigate condensation phenomena. SEM-analyses revealed that the structure of the potassium salts consisted of individual angular particles of primarily KCl (1–2 μm) and a sponge-like matrix of sub-micron particles consisting primarily of K 2SO 4, which may represent vapour condensate agglomerates. Mechanisms of deposit formation and the fate of chlorine, potassium, and sulphur in the system are discussed.

Remediation of ash problems in pulverised coal-fired boilers

This paper reports the control methods for mitigating fouling and slagging in brown coal-fired power utilities. Two control methods were investigated, i.e. wet pretreatment of coal and the use of mineral additives. By the first technique, samples of brown coal were treated with aluminium solutions to adjust the levels of Na, Al and Cl. The effect of cleaning was tested under closely controlled conditions in the range of 1000–1400°C. At a gas temperature of 1250°C, the deposition behaviour of treated and raw coals were compared using a deposition probe maintained at temperatures of 500, 600, 700 and 800°C. The second technique involved experiments with mixtures of candidate mineral additives and sodium compounds at temperatures ranging from 1000 to 1400°C.Experiments showed that both the methods were effective in reducing ash-related problems. Between 1000 and 1400°C, the quantity of condensable salts generated was found to depend only on the amount of sodium in coal. With the wet method, aluminium reduces the sodium level and thus fouling by ion exchange. The stickiness of sodium silicates from this coal was also shown to be reduced by aluminium. Deposition model calculations based on sticking probability also indicated that Al-treated coals result in less ash deposition on boiler surfaces. Experiments with mineral additives showed that sodium could be captured by clay minerals, particularly kaolin. For Victorian brown coal-based power station boilers, 10–20μm kaolin making up 2–3wt.% of the feed was estimated to effectively reduce the ash problems.

Possibilities for gas turbine and waste incinerator integration

The aggressive nature of the flue gases in municipal waste incinerators does not allow the temperature of steam in the boiler to rise above 400°C. An increase in steam temperature can be achieved by external superheating in a heat recovery steam generator positioned behind a gas turbine, so that steam of a higher energy content becomes available for electricity production. The paper addresses two basic schemes. In one case, steam generated at a waste-to-energy plant is superheated in a combined-cycle plant that operates in parallel. In the other case, the exhaust from a gas turbine plant is sent through a superheater section to the waste incinerator's boiler providing preheated combustion air. Performance of these configurations together with two modified schemes was analyzed in terms of efficiency, natural gas consumption and boiler surface area. An exergy analysis of the cases was carried out. The results showed that the integrated options can effect a substantial increase in efficiency. The hot windbox configuration was found the most effective solution, offering a smaller boiler surface area along with a moderate rate of natural gas consumption.

Rapid water quenching and formation of thermal fatigue cracks on the surface of a tube

In the process of combustion of low-grade solid fuels with complex composition of the mineral part, the heat exchange surfaces of boilers are covered with rapidly growing slag and ash deposits. Water cleaning is one of the most efficient methods of removal of deposits of this sort from the heated surfaces of boilers. As a result of rapid cooling of the outer surface of the metal, the protective oxide coating is destroyed and this accelerates the high-temperature corrosion of the metal and the formation thermal fatigue cracks. The criterion of appearance and the intensity of growth of thermal fatigue cracks strongly depend on the temperature field in the outer layer of the metal. We present the solution of the nonstationary heat conduction equation under the conditions of rapid cooling. The coefficient of convective heat transfer from the heated metal surface to the water jet is determined experimentally. We also study the conditions necessary for the initiation of thermal fatigue cracks and the intensity of their growth under laboratory and industrial conditions. A special formula is deduced for the evaluation of the intensity of crack growth depending on the number of cleaning cycles.