Ash fusion behavior

Table 3.14 Ash composition and ash fusion behavior under reducing atmosphere from coals of different rank [4,9).

Table 3.14 shows the ash fusion behavior under reducing conditions and the corresponding ash composition. The influence of the iron content on IDT and ST can be clearly seen in the case of the Kentucky no. 9 and Illinois no. 5 coals. 

To approach the physical background of coal ash fusion behavior, it is instructive to look at the ash composition and identify the bulk components. If the main three bulk components are identified, a common practice is to review literature [84] or even use thermodynamic software [85] to construct a ternary diagram that displays the isolines of the liquidus temperature. Because of the manifold interactions of the components, minima of the liquidus temperature can occur at distinct compositions (eutectics). 

Because the temperature of critical viscosity is, of course, related to the ash fusion behavior, it is suggested to be in the range of the ST up to the HT plus lllK [98,99]. Watt refined these estimations involving the main constituents of the slag on a wt% basis, where the sum of all indicated components yields... 

Alkali metals occur in salty coals typically as chlorides, humates, sulfates, and carbonates. The reducing atmosphere in the gasifier favors the continuous liberation of volatile minerals, resulting in steadily changing eutectics causing transitions of the ash fusion behavior, which are nearly unpredictable. 

These partly competing and overlapping devolatilization mechanisms demonstrate that salty coals can cause severe corrosion problems, which are mostly accompanied by abnormal and unstable ash fusion behavior, if gasified in dry-... 

If high cold gas efficiency is desired, the potential of all single-stage entrained-flow processes is limited by the ash fusion behavior, whereas gasifiers with nonslagging outlet conditions show the potential to achieve the global maximum. 

Several approaches are being taken in an attempt to relate petrographic composition with combustion behavior. Figure 17 shows how R. along with FS1 and ash fusion data may be used to predict the burning rate of coals in a specific combustion type (cross-feed stoker). In general, as R and FSI increase and ash fusion temperatures decrease, coals require more grate surface to insure optimum burn-out and minimum loss of combustibles in the ash. 

Ash fusion temperatures set of temperatures that characterize the behavior of ash as it is heated. These temperatures are determined by heating cones of ground, pressed ash in both oxidizing and reducing atmospheres. 

The most uniquely suited standardized analyses for ash deposition include ash chemistry and ash fusion temperature. The total ash content from proximate analysis and ash composition provide the fuel irrformation that goes into the majority of common empirical indices of ash behavior, along with ash fusion temperature. The ash chemistry analysis typically reports the ash elemental composition on an oxide basis. This does not mean that all of the species exist as oxides in the fuel (which they do not). It is a convenient method of checking the consistency of the data. The sum of the oxides shoirldbe about the same as the total ash content. The analysis is fundamentally an elemental analysis with no distinction of the chemical speciation of the inorganic species. 

The grade of a coal establishes its economic value for a specific end use. Grade of coal refers to the amount of mineral matter that is present in the coal and is a measure of coal quality. Sulfur content, ash fusion temperatures, that is, measurement of the behavior of ash at high tanperatures, and quantity of trace elements in coal are also used to grade coal. Although formal classification systems have not been developed around grade of coal, grade is important to the coal user. 

The ash fusion temperatures. They indicate the behavior of the ash residues from the coal at high temperatures and are mainly related to the chemical composition of the ash and the nature of the coal s mineral matter. They are nsed to indicate whether the ash will remain as a fine powder within the fnmace system after the coal is burned or whether some of it might melt to form a slag on the boiler s heat exchange surfaces. 

However, the main drawbacks of the usage of iron are mostly the insufficient contact between the catalyst and the carbon, the aggregation of the iron particles at the later stages of gasification, the upcoming sulfur poisoning, and the crystallization of char in the presence of iron [27,30,34], Additionally, iron influences significantly the fusion behavior of the ash (see Section 3.11.5.3). 

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