Fouling Of Heat Transfer Surfaces Research Articles

Heat Transfer Efficiency Prediction of Coal-Fired Power Plant Boiler Based on CEEMDAN-NAR Considering Ash Fouling

Ash fouling has been an important factor in reducing the heat transfer efficiency and safety of the coal-fired power plant boilers. Scientific and accurate prediction of ash fouling of heat transfer surfaces is the basis of formulating a reasonable soot blowing strategy to improve energy efficiency. This study presented a comprehensive approach of dynamic prediction of the ash fouling of heat transfer surfaces in coal-fired power plant boilers. At first, the cleanliness factor is used to reflect the fouling level of the heat transfer surfaces. Then, a dynamic model is proposed to predict ash deposits in the coal-fired boilers by combining complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) and nonlinear autoregressive neural networks (NARNN). To construct a reasonable network model, the minimum information criterion and trial-and-error method are used to determine the delay orders and hidden layers. Finally, the experimental object is established on the 300 MV economizer clearness factor dataset of the power station, and the root mean square error and mean absolute percentage error of the proposed method are the smallest. In addition, the experimental results show that this multiscale prediction model is more competitive than the Elman model.

Analysis of Fired Equipment within the Framework of Low-Cost Modelling Systems

Fired equipment suffers from local overloading and fouling of heat transfer surfaces, products are not of the required quality, and operating costs are increased due to the high pressure drop of process fluids. Such operational issues are affected by the non-uniform distribution of fluid flow and heat flux variability. Detailed numerical analyses are often applied to troubleshoot these problems. However, is this common practice effective? Is it not better to prevent problems from occurring by using quality equipment design? It is, according to the general consensus. Still, the experience of designing fired apparatuses reveals that the established standards do not reflect the real maldistribution sufficiently. In addition, as found from the given overview of modelling approaches, the radiant chamber and the convection section are usually analysed separately without significant continuity. A comprehensive framework is hence introduced. The proposed procedure clearly defines the interconnection of traditional thermal-hydraulic calculations and low-cost modelling systems for radiant and convection sections. A suitable combination of simplified methods allows for the reliable design of complex equipment and fast identification of problematic areas. The utilisation of selected low-cost models, i.e., the second phase of the systematic framework, is presented regarding the example of a steam boiler.

Анализ работы теплообменного аппарата при загрязнении теплопередающей поверхности

Анализ работы теплообменного аппарата при загрязнении теплопередающей поверхности

Ash fouling monitoring based on acoustic pyrometry in boiler furnaces

Unexpected fouling of heat transfer surfaces has always been one of the main operational concerns in coal-fired utility boilers. A cost-effective way to address this difficulty is the continuous monitoring of fouling tendencies. This article has adopted acoustic pyrometry to monitor the changes of the flue gas temperature near the heat surface and has proposed a new clean factor, &, based on this acoustic method to monitor the ash fouling. A laboratory study and field research using a boiler were conducted. The results indicated that the acoustic system could be adopted to monitor temperature changes near the water-cooling wall in the boiler furnace. The new clean factor, &, could be used to monitor the changes of the ash fouling on the heat surface. This study may help to develop a more intelligent use of soot blowers, save power station boiler furnace energy and reduce discharge.

Effects of Steam on the Release of Potassium, Chlorine, and Sulfur during Char Conversion, Investigated under Dual-Fluidized-Bed Gasification Conditions

The corrosion and fouling of heat-transfer surfaces and the agglomeration of bed materials in fluidized beds are some of the ash-related problems caused by the transformation and release to the gas phase of ash-forming elements from biomass during thermochemical conversion processes. The magnitudes of these problems are largely dependent upon the release of potassium (K), chlorine (Cl), and sulfur (S) from the biomass. We investigated the effects of steam on the release of K, Cl, and S during char conversion, under conditions relevant for dual-fluidized-bed gasification (DFBG). The study was carried out with wheat straw in a laboratory-scale bubbling fluidized-bed reactor in the temperature range of 800–900 °C. The release of K, Cl, and S from wheat straw during devolatilization, char gasification, and char combustion was quantified with a mass balance that linked the masses of these elements in the wheat straw to the mass of the solid residue obtained at the end of each experiment. To facilitate analyses of the experimental results, leaching and the Brunauer–Emmett–Teller surface area measurement of the wheat straw and some of the solid residues were carried out. The results show that, during devolatilization, the release of volatile salts, e.g., KCl, is significantly limited by intraparticle diffusion resistance, owing to a compact char matrix (i.e., negligible porosity). However, during char gasification, steam renders the char less compact by expanding and/or creating new pores in the char. As a result, intraparticle diffusion resistance decreases, thereby facilitating the evaporation of volatile salts of K and S from the char matrix. The conversion of the char is also conducive to the release of char-bound K and S, especially at 900 °C. At temperatures of >800 °C, the relative proportions of the elements released and char gasified indicate that the release of K can somewhat be decoupled from the release of S and Cl by maximizing the extent of char conversion in the gasification chamber. The results also show that, during char combustion, the proportions of the char that can be combusted and the extent of the release of the elements are influenced by the extent to which the char is gasified in the gasification chamber.

Transformation and Release of Potassium, Chlorine, and Sulfur from Wheat Straw under Conditions Relevant to Dual Fluidized Bed Gasification

The release and transformation of potassium (K), chlorine (Cl), and sulfur (S) from biomass during thermochemical conversion processes may lead to problems, such as the corrosion and fouling of heat transfer surfaces, agglomeration of bed material, and the poisoning of catalysts used in the downstream processes of gasifiers. To predict and to mitigate effectively these problems, information regarding the quantity and mechanism of the release of these elements under relevant operating conditions is required. In the present work, the release of K, Cl, and S from wheat straw under conditions relevant to dual fluidized bed gasification were quantified in a laboratory-scale bubbling fluidized bed reactor. During the pyrolysis step, the bed temperature ranged from 550 to 900 °C, while the residence time for the fuel in the reactor was fixed at 3 min. The char samples obtained from the pyrolysis step were partially combusted at the same temperature at which they were produced for an additional 3 min. The fractio...

Modeling of Sootblower Jets and the Impact on Deposit Removal in Industrial Boilers

Fouling of heat-transfer surfaces by fireside deposits can be of significant concern in industrial boilers burning poor-quality fuel. It is commonly controlled by sootblowers that blast deposits with high-pressure supersonic steam or air jets. However, sootblowing is expensive, which motivates efforts to fundamentally understand how sootblower jets behave and how they interact with heat-exchanger geometries and deposits, to guide efforts to improve and optimize sootblower use. Here, we report on the development of a computational fluid dynamics (CFD) model to predict the flow behavior of sootblower jets, work that began with the customization of a research code but has more recently led to an implementation using the commercial CFD software ANSYS Fluent, which makes the model more accessible to the wider engineering community. CFD model results are compared to experimental data that we obtained for jet flow within model tube bank geometries, which are representative of superheaters and generating banks in...

Online cleaning of tubular heat exchangers in water service systems using projectiles

In water service processes, tubular heat exchangers are commonly used to provide a heat transfer medium for cooling or heating of which both are subject to fouling of heat transfer surfaces. The deposit would dominantly build up in form of biofouling, crystallization, and/or particulate fouling depending on the operating and surface conditions. This, in turn, may result in more demand for excessive energy consumption due to higher pumping requirements, as well as for offsetting the impact of fouling. In this study, as part of a European project entitled “Clean-Ex,” a comprehensive set of experimental runs was carried out for crystallization fouling of CaSO4 with and without projectiles. Due to laboratory restriction, the fouling runs were performed at accelerated conditions to rigorously characterize the impact of projectile cleaning in terms of injection frequencies and various types of projectiles. The experimental results show that cleaning by using projectiles will only take place when no deposit is allowed to form on the surface or if formed, then it is just brittle; hence, the projectiles can remove it due to excessive shear forces. There is also a direct link between the rate of fouling and injection frequencies.

Development and Validation of a Numerical Model of Fouling within Spiral-Grooved Tubes

Fouling of heat transfer surfaces causes serious technical and economic problems in industry. A CFD based model is proposed for predicting fouling process in spiral-grooved tubes. This model considered both crystallization at heat transfer surface and deposition of crystal forming in the stream. The precision of this model was validated using experiment date. It was proved that the model developed in this work can predict fouling process in spiral-grooved tubes well.

Fouling of Milk on Heat Transfer Surface with and without the Addition of Bacillus stearothermophilus - A Laboratory Study

Fouling of heat transfer surfaces during milk processing is a major problem for dairy industry. In the current work, the nature and extent of milk fouling with and without the addition of Bacillus stearothermophilus in the feed solutions have been studied. A fouling detection system based on electric conductivity measurements has been used. The coresponding thermal resistance of the deposit layer has also been worked out. The fouling deposits have been analysed visually as well as with a scanning electron microscope (SEM). The results show significant variation in the fouling patterns with and without the large presence of bacteria in the flowing medium.

Particulate fouling in waste incinerators as influenced by the critical sticking velocity and layer porosity

Fouling of heat transfer surfaces introduces a major uncertainty into the design and operation of heat exchange equipment. Fouling layers as observed on the tube bundles of the economizer in a Dutch waste incinerator were thin and powdery. The fouling layer showed an asymptotic growth rate with a levelling off increase of the thickness. In this study, the influence of the critical sticking velocity on the growth rate of particulate fouling layers is described. The critical sticking velocity of an incident particle hitting a powdery layer is defined as the maximum impact speed at which the particle will stick to the layer. Since the critical sticking velocity is a key parameter in the deposition mechanism, a well-defined experimental set-up has been built to assign it. Experimental results showed that the critical sticking velocity increases with the porosity of the fouling layer. Literature shows that the porosity of a thin sintered powdery layer changes with the layer thickness. Based on the experimental results and the variation of porosity with thickness for a thin sintered powdery fouling layers, a correlation is developed which shows that the sticking velocity decreases exponentially as the fouling layer thickness increases. Therefore, fewer particles are likely to stick as the fouling layer builds up and consequently the deposition rate decreases. The change in the critical sticking velocity as the fouling layer builds up contributes to the explanation of the asymptotic growth of particulate fouling layers on the tube bundle of waste incinerators.

Nucleation, Growth, and Composition of Crystals Obtained from Solutions of Na2CO3 and Na2SO4

The formation of crystals from solutions containing sodium carbonate and sodium sulfate is a complex process that often results in fouling of heat-transfer surfaces. Complexity results from a dependence of the composition of crystals formed on the composition of the solution, and the solution composition changes with crystallization. In the present work, experiments involving evaporation at 115 °C were used to identify these species and the specific regimes in which they are formed. Crystals produced in these experiments were sodium sulfate, burkeite, a new species referred to as dicarbonate, and either sodium carbonate monohydrate or anhydrous sodium carbonate.

Numerical simulation of the fouling process

Fouling of heat transfer surfaces causes serious technical and economic problems in industry. The goal of this work is to simulate the aforementioned fouling process using the CFD code FLUENT. The obtained numerical results assist in designing and running heat exchangers. Based on models for the calculation of deposition and removal mass rates [S. Krause, Internat. Chem. Engrg. 33 (1993)], the crystallization fouling of calcium sulfate on flat heat transfer surfaces was simulated. The induction period, which occurs with almost all fouling processes, was therefore not considered. The simulation of real crystal growth requires a continuous variation of the geometric flow model and therefore considerable computational effort. For that reason fictitious crystal growth was simulated instead. This numerical simplification enabled an unsteady simulation to be obtained, of the fouling process and a realistic description of the temporal modification of both the flow and temperature field due to the continuous crystal growth. Based on experimental results of Hirsch [M. Bohnet et al., in: T.R. Bott et al. (Eds.), Understanding Heat Exchanger Fouling and its Mitigation, United Engineering Foundation and Begell House, New York, 1997, pp. 201–208], a model was developed which enables the calculation of the density of the fouling layer not only as a function of the local position within the fouling layer, but also as a function of the time-dependent total thickness of the fouling layer. In addition a model was developed, that enables a realistic distribution of the heat flux along the heat transfer surface during the simulation. Both models provide a more exact description of the complicated fouling process. Results of the numerical simulation are the prediction of the fouling resistance as a function of time and the calculation of the temperature distribution within the fouling layer. In view of the complexity of the fouling process during the incrustation of heat transfer surfaces and the fact that not all influences from the used models could be considered the agreement between calculated and experimentally obtained data is satisfactory.

Use of crude oil fouling threshold data in heat exchanger design

The existence of a `threshold' below which chemical reaction fouling of heat transfer surfaces by crude oil does not occur has been identified by Ebert and Panchal [Fouling Mitigation of Industrial Heat-Exchange Equipment, Begell House, 1997, 451–460] and clearly demonstrated by Knudsen et al. [Understanding Heat Exchanger Fouling and its Mitigation, Begell House, 1999, 265–272]. This phenomenon has important implications for the design and operation of heat exchangers in refinery pre-heat trains used for the processing of crudes. In this paper we show how a consideration of the fouling threshold condition can be incorporated into the design procedures for shell-and-tube heat exchangers. We then proceed to show how fouling can be mitigated through attention to heat exchanger design, particularly the choice of configuration. The cost of improperly designed units, based on the conventional use of `fouling factors', is demonstrated.

Further Evaporation and Final Treatment of Process-Water Concentrates

The study is part of a longer-term effort aimed at developing a separate treatment process for evaporation-concentrates of paper-mill process waters. This article deals with research on the two key processing steps; namely, the further evaporation and the final treatment of the concentrates. In laboratory experiments, various feed waters, including several different TMP filtrates, were evaporated to high dry-solids contents. The condensates recovered in most experiments contained relatively small amounts of organic matter. The extent of vapourisation of organic acids, relative to water, increased somewhat with increase in dry-solids content. Two TMP concentrates from large-scale evaporation plants were further concentrated in a pilot-scale forced-circulation evaporator. Viscosity was the factor limiting the extent of concentration. Using an evaporation temperature of about 80°C, the maximum dry-solids contents achieved with the two different concentrates were about 45 wt% and about 60 wt%, respectively. Fouling of heat-transfer surfaces was observed with both concentrates. A techno-economic evaluation of final-treatment options for alkali-rich concentrates is under way. According to intermediate results, molten-phase combustion processes would not require support fuel once the dry-solids content of TMP concentrate exceeds about 50 wt%. The aim of future work is to optimise the overall treatment process.

Fundamentals of corrosion fouling

This paper reviews the fouling of heat transfer surfaces caused by corrosion as a consequence of their exposure to flowing oxygenated water. The fundamental mechanisms of oxygen transport, deposit growth, and deposit removal are considered. A model for predicting the formation of a corrosion fouling deposit is derived and the utility of dimensionless variables deduced from the model is demonstrated. The interaction of corrosion fouling and other categories of fouling is discussed. Recommendations are made for future research in this area.

Further evaporation and final treatment of process-water concentrates

The study is part of a longer-term effort aimed at developing a separate treatment process for evaporation-concentrates of paper-mill process waters. This article deals with research on the two key processing steps; namely, the further evaporation and the final treatment of the concentrates. In laboratory experiments, various feed waters, including several different TMP filtrates, were evaporated to high dry-solids contents. The condensates recovered in most experiments contained relatively small amounts of organic matter. The extent of vapourisation of organic acids, relative to water, increased somewhat with increase in dry-solids content. Two TMP concentrates from large-scale evaporation plants were further concentrated in a pilot-scale forced-circulation evaporator. Viscosity was the factor limiting the extent of concentration. Using an evaporation temperature of about 80°C, the maximum dry-solids contents achieved with the two different concentrates were about 45 wt% and about 60 wt%, respectively. Fouling of heat-transfer surfaces was observed with both concentrates. A techno-economic evaluation of final-treatment options for alkali-rich concentrates is under way. According to intermediate results, molten-phase combustion processes would not require support fuel once the dry-solids content of TMP concentrate exceeds about 50 wt%. The aim of future work is to optimise the overall treatment process.

98/02249 Fouling of heat-transfer surfaces in oil-fired boilers for domestic heating

Domestic-heating boilers fired with distillate oil are subject to fouling of the convective section gas-side surfaces, and this fouling leads to a reduction in efficiency and to increased maintenance requirements. A rapid test to enable the composition of fouling deposits and the rates of fouling to be studied has been developed and used at two laboratory sites. It involves special fouling surfaces which can be removed for deposit analysis. Fouling rates are shown to be directly related to the sulphur content of the fuel. Also, operation with reduced excess air in these boilers can lessen fouling rates by reducing the conversion of fuel sulphur to sulphuric acid. Results are compared with those from tests on full-size boilers.

A Maintenance Strategy for Heat Transfer Equipment Subject to Fouling: A Probabilistic Approach

Fouling of heat transfer surfaces introduces a major uncertainty into the design and operation of heat transfer equipment. After a brief discussion on impact of fouling, we present and discuss a stochastic approach to the analysis of fouling models. In view of the performance indicator (U/Uc) of the heat exchangers, a maintenance strategy for planned maintenance schedules is presented. Various scenarios of reliability based maintenance strategy are introduced. The strategy is explained in terms of the scatter parameter (α) of the time-to-fouling distribution corresponding to a critical level of fouling, and the risk factor (p) representing the probability of tubes being fouled to a critical level after which a cleaning cycle is needed. In addition, the cost implications of the above mentioned strategy are explained and their impact on heat exchanger maintenance is highlighted.

Fundamentals of corrosion fouling

This paper reviews the fouling of heat transfer surfaces due to corrosion as a consequence of their exposure to flowing oxygenated water. The fundamental mechanisms of oxygen transport, deposit growth, and deposit removal are considered. A model for predicting the formation of a corrosion fouling deposit is derived, and the utility of dimensionless variables deduced from the model is demonstrated. The interaction of corrosion fouling and other categories of fouling is discussed. Recommendations are made for future research in this area.