Ash deposition in boilers

Ash fusion characteristics are important in ash deposition in boilers. Ash deposition occurring on the furnace walls is termed slagging, whereas accumulation on the superheater and other tubes is termed fouling. A variety of empirical indexes have been developed (60,61) to relate fouling and slagging to the ash chemical composition through parameters such as acidic and basic oxides content, sodium, calcium and magnesium, and sulfur. 

Fuel Characteristics Fuel choice has a major impact on boiler design and sizing. Because of the heat transfer resistance offered by ash deposits in the furnace chamber in a coal-fired boiler, the mean absorbed heat flux is lower than in gas- or oil-fired boilers, so a greater surface area must be provided. Figure 27-42 shows a size comparison between a coal-fired and an oil-fired boiler for the same duty. 

The ash deposits resulting from the combustion of solid and oil fuels often contain appreciable quantities of other corrodants in addition to vanadium pentoxide. One of the more important of these is sodium sulphate, and the effects of this constituent in producing sulphur attack have been mentioned. The contents of sodium sulphate and vanadium pentoxide present in fuel oil ash can vary markedly and the relative merits of different materials depend to a great extent upon the proportions of these constituents. Exposure of heat-resisting alloys of varying nickel, chromium and iron contents to ash deposition in the super-heater zones of oil-fired boilers indicated a behaviour pattern depending on the composition of the alloy and of the ash... 

Ash deposition in biomass combustion systems has been the focus of numerous research efforts.559,659 The basic mechanism for deposit formation in biomass combustion systems starts with the vaporization of alkali metals, usually chlorides, in the combustor. Fly ash particles, which are predominantly silica, impact and stick to boiler tube surfaces. As the flue cools the alkali metal vapors and aerosols quench on the tube surfaces. When the ash chemistry approaches equilibrium on the surface and the deposit becomes molten, the likelihood increases that additional fly ash particles will stick, and deposits grow rapidly. Ash deposits can also accelerate the corrosion or erosion of the heat transfer surfaces. This greatly increases the maintenance requirements of the power plant often causing unscheduled plant interruptions and shutdown. 

Often coal ash deposit effects are inter-related. For example, slagging will restrict waterwall heat absorption changing the temperature distribution in the boiler which in turn influences the nature and quantity of ash deposition in downstream convective sections. Ash deposits accumulated on convection tubes can reduce the cross-sectional flow area increasing fan requirements and also creating higher local gas velocities which accelerate fly ash erosion. In-situ deposit reactions can produce liquid phase components which are instrumental in tube corrosion. 

Ash deposits on boiler tubes can be keyed to the surface of metal oxide by mechanical and chemical bonds. Mechanical bonding is enhanced by extending surface at the interface as shown in Figure 6a. Boiler tubes are not polished and thus have an extended surface that is further increased by oxidation and chemical reactions between the oxide layer and ash deposits. It is therefore evident that a comparatively rough surface of boiler tubes constitute an anchorage for keying ash deposits to the heat exchange elements. 

The galvanic reactions will take place at a much faster rate in the low viscosity phase of sulphates in boiler deposit than in highly viscous silicate glass. However, rapid reactions at the tube surface/deposit interface may not be necessary or appropriate for development of a strong bond between the ash deposit and boiler... 

King et al. (22) have suggested that in order to obtain good adherence of enamel coatings on metals, the enamel material at the interface must become saturated with the metal oxide, e.g. FeO of ferritic steels. Coal ash deposit on boiler tubes contains between 5 and 25 per cent iron oxide and thus the layer at the tube/ deposit interface becomes saturated with FeO. The chromium and nickel contents of ash deposit are low and thus the same chemical compatibility stage is not reached at the austenitic steel/deposit interface. 

The coal ash deposits on boiler tubes have frequently a separate zone structure with a sulphate rich layer up to 2 mm thick under the matrix of sintered ash (3 ). The outer layer is porous and it constitutes a pathway for the enrichment of alkali-metals in the deposit layer next to tube suface. The diffusable species may be sulphate, chloride, oxide or hydroxide, but the thermodynamic data W and the results of deposition measurements in coal fired boilers (Figure 3) suggest that sodium and potassium sulphates are the principal vapour species which diffuse through a porous matrix of silicate ash deposit. 

Corrosion of boiler tubes appears to be initiated in some cases with the formation of a white layer of general composition (Na,K)2Al(S0 2-Conditions for initiation of the deposit are favored by coals having high alkaU and sulfur contents. The white layer bonds to the tubes and permits growth of ash deposits that insulate the layer and permit further corrosion. 

The need to remove alkali material from the biogas stream depends on the end use for the gas. Problems associated with alkali vapor formation and deposition are critical in systems where the hot biogas is to be used without significant cool-down. Moderate gas cooling followed by removal of bulk particulates provides adequate cleaning for simple boiler systems that can tolerate some ash deposition. Other applications such as gas turbines require cleaner fuel gases. Since turbines operate at high rotational speeds, deposition can... 

Hot corrosion may also occur in breeder reactors when the fission products deposit on the stailness steel cladding as complex salts based on Csl and Cs20. This mode of corrosion occurs in boilers and turbines burning high-ash coal or residual fuel oil well as well as heat exchangers, alkali-carbonate-based fuel cells and carbonate storage systems. 

Ash fouling, the accumulation of deposits on boiler tube surfaces in utility boilers, is a severe operating problem in many power plants fired with low-rank coals. Ash fouling is a complex phenomenon the extent of which is related to the boiler design, the method of operating the boiler, and the coal properties. In extreme cases it is necessary to schedule frequent shutdowns for removing the deposits or to derate the boiler. A recent survey of six power plants estimated that total costs of curtailments due to ash-related problems were 20.6 million over a six-month period (20). 

Benson, S., Steadman, E. N., Zygarlicke, C. J. Erickson, T. A. (1996), Ash Formation, Deposition, Corrosion, and Erosion in Conventional Boilers. Applications of Advanced Technology to Ash-Related Problems in Boilers, Plenum Press, New York, p. 1. 

The flame imprinted characteristics of pulverized coal ash relevant to boiler slagging, corrosion and erosion have been discussed previously (1,2). Silicate minerals constitute between 60 and 90 per cent of ash in most coals and boiler deposits are largely made up from the silicious impurity constituents. This work sets out first to examine the mode of occurrence of the silicate mineral species in coal followed by a characterization assessment of the flame vitrified and sodium enriched silicate ash particles. The ash sintering studies are limited to investigations of the role of sodium in initiating and sustaining the bond forming reactions to the formation of boiler deposits. 

Decomposition of Sulphate on Silicate Ash Sintering. So di urn sulphates in the initial material deposited on boiler tubes will be decomposed by the pyrochemically acidic silicates in ash when the deposit temperature exceeds 1085 K. The transfer of sodium from the sulphate to silicate phase reduces the viscosity of the glassy material of silicate ash thus increasing the rate of sintering. 

The formation of ash deposits within coal-fired boilers may cause serious operating problems and reduction in thermal efficiency. 

To be able to relate the results of this study to the phenomena of boiler deposits it is necessary to look at the mineralogy of deposits. Table X is a comparison of the crystalline phases observed in deposits from six utility boilers with the primary phase predicted from the quaternary system. Three deposits are of western type and three are of eastern type. The normalized compositions of the deposits have been included in the Table and plotted on the appropriate plane of the equilibrium system. In the case of the western type deposits the quaternary systan correctly predicts the primary phase. This was also the case for the eastern type deposits except for Ironbridge where iron spinel was the primary phase whereas anorthite was predicted. This anomaly may be due to the presence of significant amount of ferric iron and the effect of minor components. Nevertheless the system CaO-FeO-Al2O -SiO2 appears to govern the crystallization of coal ash deposits to a significant extent. 

Clearly there is a need for improved techniques for predicting the behavior of mineral matter. This paper will provide a statement of the ash deposition problem in pulverized coal fired boilers it will present an assessment of the older, traditional methods for predicting mineral matter behavior and it will address some of the newer techniques that have been suggested as better ways of characterizing coal ash behavior. Additionally some areas of uncertainty will be identified which require the development of better predictive techniques. 

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