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Abstract
A brief overview is presented of ash fouling of heat transfer surfaces of boiler and the thermophysical properties of deposits. Loose and bound ash deposit formation on transverse-flow boiler tubes is treated. Composition of the bound deposits differs from the composition of fly ash passing through the heat transfer surface. Ash deposited on boiler tubes is examined as a porous material with the effective thermal conductivity. At higher temperatures, heat exchange in a porous body also becomes affected by radiant heat transfer inside the material. The radiation properties of ash deposits are also discussed. The radiation flux originating from the ash particle layer and deposit layer is spectral in the region of short wavelengths but for a smooth slag layer the selectivity is negligible.Keywords:ash fouling;ash particle layer;boiler tubes;deposits;radiant heat transfer;thermophysical properties
Ash deposition always brings boilers some trouble due to fouling or slagging. In this paper, a completely controlled system was developed to study the growth of ash deposit. A novel sampling probe was designed to online measure the heat flux through ash deposit. Additionally, the thickness of ash deposit can be obtained by an online figure collecting system. The results of this research showed that as the thickness of ash deposit increased, the heat flux decreased. It was also found that at the initial stage of ash deposition when the thickness of ash deposit is approximately 1 mm, the heat flux through ash deposit had a sharp reduction. An effective method was attempted to situ measure the effective thermal conductivity of the ash deposit in the simulated combustion flue gas. It was found that temperature of the ash deposit layer had no obvious effect on its value. It was concluded that the structure of ash deposit had no obvious change in a short deposition time of 30 min with varied surface temperatures of the probe head between 400 °C and 600 °C.
Thermal conductivity coefficient for deposits formed on boiler tube surfaces in the process of coal combustion were measured. The samples were taken from Krasnoyarsk heat and electric power cogeneration plants and Berezov steam electric power plant. Three types of deposits differing in the thermal conductivity coefficient values were revealed. Those were: the anhydride (enriched with CaSO4), primary (ferrous) and secondary (slag) ones. The formulae for anhydride deposit thermal conductivity coefficient estimation (according to known porosity value) were obtained.
As shown by preliminary pilot-scale experiments on oil shale combustion in circulating fluidized bed, deposition of very fine particles in the convective part of boiler takes place. Assuming a quite complete binding of sulphur in CFB combustor and taking into account low-temperature level of this combustion process, there will not be adequate conditions for formation of bounded deposits on convective boiler surfaces and the fouling process will be controlled mainly by thermophoretic, diffusion and adhesion forces. That might invoke a forced increase in deposits with high thermal resistance. The prognosis of that process is presented in the given paper.
The paper deals with the behaviour of fuel mineral matter in combustion process. The results are primarily based on research and experience with Estonian oil shale, as a high carbonate, alkali metal and chlorine content fuel. The ECD (extent of carbonates decomposition) in PF (pulverized firing), AFBC (atmospheric fluidized bed combustion) and PFBC (pressurized fluidized bed combustion) conditions was investigated experimentally. The relationship between the heating value of the fuel and CO2 concentration in the flue gas as a function of the ECD was established. A description of fouling and high-temperature corrosion of heat transfer surfaces is presented.
This book offers a presentation of the fundamentals of energy, mass, and momentum transport, with emphasis on convection and diffusion in chemically reacting flow systems of current technological importance. The author has designed the book to be easily accessible to all engineers and applied scientists. This book gives guidelines to formulate and solve many important problems involving rates of energy, mass or momentum transport in fluids that may be chemically reacting. Numerous new sample problems, graphs and illustrations are included to facilitate the understanding of transport phenomena in chemically reacting systems, and the correlation of experimental or computer-generated data for the purpose of problem-solving and design. The contents are: Introduction to transport processes in chemically reactive systems; Governing conservation principles; Constitutive laws - the diffusion flux laws and their coefficients; Energy transport mechanisms, rates and coefficients; Mass transport mechanisms, rates and coefficients; Similitude analysis with application to chemically reactive systems-overview of the role of experiment and theory; Problem-solving techniques, aids, philosophy: forced convective heat and mass transfer to a tube in cross-flow; and Index.
The effective thermal conductivity of coal ash deposits strongly influences heat transfer in pulverized coal-fired boilers. In this study thermal conductivity measurements were performed over a wide range of temperatures for fly ash, slagging deposits, and fouling deposits. The effects of ash particle size, thermal history, and physical structure of the deposit are discussed. Thermal history and deposit structure were observed to have the greatest influence on the local thermal conductivity, which increased by an order of magnitude with particle melting. Conductivities for solid-porous deposits were twice those of the same sample in particulate form.
Boiler operation problems associated with coal mineral impurities impede the efficient production of electrical energy form coal. This book was written to provide information on this problem. It includes a lengthy bibliography with full citations and a detailed subject index.
A laser-Doppler velocimeter (LDV) study of velocity profiles in the laminar boundary layer adjacent to a heated flat plate revealed that the seed particles used for the LDV measurements were driven away from the plate surface by thermophoretic forces, causing a particle-free region within the boundary layer of approximately one half the boundary-layer thickness. Measurements of the thickness of this region were compared with particle trajectories calculated according to several theories for the thermophoretic force. It was found that the theory of Brock, with an improved value for the thermal slip coefficient, gave the best agreement with experiment for low Knudsen numbers, λ/ R = O (10 ⁻¹ ), where λ is the mean free path and R the particle radius.
Data obtained by other experimenters over a wider range of Knudsen numbers are compared, and a fitting formula for the thermophoretic force useful over the entire range 0 [les ] λ/ R [les ] ∞ is proposed which agrees within 20% or less with the majority of the available data.
This paper describes a technique developed to make in situ, time-resolved measurements of the effective thermal conductivity of ash deposits formed under conditions that closely replicate those found in the convective pass of a commercial boiler. Since ash deposit thermal conductivity is thought to be strongly dependent on deposit microstructure, the technique is designed to minimize the disturbance of the natural deposit microstructure. Traditional techniques for measuring deposit thermal conductivity generally do not preserve the sample microstructure. Experiments are described that demonstrate the technique, quantify the experimental uncertainty, and determine the thermal conductivity of highly porous, unsintered deposits. The average measured thermal conductivity of loose, unsintered deposits is 0.14 ± 0.03 W m-1 K-1, approximately midway between rational theoretical limits for deposit thermal conductivity.
A one-dimensional heat transfer method was used to determine the thermal conductivity for a range of coal ash and synthetic ash samples at elevated temperatures. The effect of parameters such as temperature, porosity, and sintering time were investigated. The thermal conductivity of the samples was generally observed to increase with increasing temperature. During heating of the samples, softening of minerals and sintering reactions resulted in changes in the physical structure of the ash, which then altered the observed thermal conductivity. The thermal conductivity of sintered ash samples was found to be higher than that of unsintered samples. The sintering temperature and sintering time were found to increase the observed thermal conductivity irreversibly. A decrease in sample porosity was also observed to increase the thermal conductivity. Chemical composition was found to have little effect on the thermal conductivity, apart from influencing the extent of sintering.Predictions of the thermal conductivity of ash samples based on Rayleigh's model are also presented. The thermal conductivity of slag and particulate structures was modelled by considering spherical pores distributed in a continuous slag phase. A particulate layer structure was modelled by considering solid particles dispersed in a continuous gas phase. The Brailsford and Major model of random distribution for mixed phases gives results within 20% of the measured values for a partially sintered sample.
A range of the published models, both unit cell and pseudohomogeneous chosen to illustrate the different approaches to modelling effective thermal conductivity, are reviewed. Their predictions are compared with experimental values obtained for packed beds of alumina and sand within the temperature range from 400 to 950°C. None predicts the high degree of temperature dependence observed. Their inadequacy is attributed to the assumption that the particles are opaque to radiation whereas both alumina and silica are partially transmissive. Deficiencies in the prediction of the conductive component by some models is attributed to their inability to characterize the effects of particle shape and size distribution. The Godbee and Ziegler (J. Appl. Phys. 37, 55 (1966)) and Kunii and Smith (A.I.Ch.E. Jl 6,71 (1960)) models gave reasonable predictions for the conductive component of the silica beds at the lower temperatures for which radiant transfer is negligible.
The review summarizes heat transfer mechanisms for highly porous chars. It provides a brief discussion on global models for thermal conductivity that consider mostly total porosity and a more detailed discussion of models considering details of the microstructure and phonon heat transport within microcrystals. The review includes recent methods for experimental determination of thermal conductivity of single small particles. A description of how thermal conductivity of highly porous chars changes under oxidation, in the range of zero to full burnout, was provided. This review emphasizes the necessity of a detailed description of all aspects that may contribute to the thermal conductivity of highly porous chars, such as macro- and microstructures, microcrystal orientation, microcrystal dimensions and connectivity.
A numerical model has been developed to describe the main deposition processes of ash particles with a predetermined chemical composition. Thermophoresis and inertial impaction are considered, the latter being the predominant process for particles > 10 μm. A single drop-tube furnace and a horizontal p.f.-fired furnace have been modelled in this study. Predicted results show that the denser (iron-rich) particles are more inclined to deposit than less dense (silica-rich) particles. Boiler aerodynamics are also shown to be a controlling factor in the slagging process. Ash layer formation is shown to enhance the deposition process further owing to the increased surface temperature of the deposit. Inclusion of a variable deposit temperature results in good agreement with experimentally measured values. The effect of gas temperature is shown to influence the deposition process owing to the effects on particle viscosity. Predicted results show good agreement with both laminar drop tube and p.f.-fired furnaces for fouling coals, but it has not yet been possible to model non-fouling coals accurately, since ash erosion and deposit breakage have not been considered.
The aerodynamic capture efficiency of small but nondiffusing particles suspended in a high-speed stream flowing past a target is known to be influenced by parameters governing small particle inertia, departures from the Stokes drag law, and carrier fluid compressibility. By defining an effective Stokes number in terms of the actual (prevailing) particle stopping distance, local fluid viscosity, and inviscid fluid velocity gradient at the target nose, it is shown that these effects are well correlated in terms of a 'standard' (cylindrical collector, Stokes drag, incompressible flow, sq rt Re much greater than 1) capture efficiency curve. Thus, a correlation follows that simplifies aerosol capture calculations in the parameter range already included in previous numerical solutions, allows rational engineering predictions of deposition in situations not previously specifically calculated, and should facilitate the presentation of performance data for gas cleaning equipment and aerosol instruments.
The study of thermophysical properties of boiler furnace deposits
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