Slagging assessment

The following is a brief survey of the methods adopted for slagging assessment. Temperature indicators... 

Although in principle the lower the values of SR (over the range 0.5 - 0.8) see Table 16.6, the greater is the tendency for slagging to occur, the ratio is not considered to be a particularly reliable method for slagging assessment. Raask [1985] suggested that the iron oxide (Fe O ) content of ash is a better indicator of slagging propensity. 

The optimization of reactions involving a large number of variables, and in tire case of metal production diese might include die temperature, gas, slag and metal compositions, die state of motion of each phase, and die leiigdi of die refining period, could be analysed by die classical, so-called Newtonian mediod in which one variable is altered in a given series of tests while all odier variables are held constant, and die results are collected in order to assess die dependence of die productivity on diat variable. However, since each test would be expensive of time and labour on die industrial scale, an alternative may be adopted which reduces die number of plant trials required to separate die effects of die variables. 

The impact of alternative feeds on ash/slag quality/ marketability needs to be assessed. 

The extraction of calcium took place inside a water-filled, stirred reactor for 15 min, which resulted in a calcium hydroxide concentration of 1.1 gT1 for paper bottom ash and 0.46gT1 for steel slag. The formed hydroxide slurry was separated from the solids and carbonated by injecting it with C02 at a rate of 25 ml min-1. However, the feasibility of the process should be investigated with a cost and environmental assessment before any firm conclusions can be drawn, as was also the case for a process described by Gorset et al. [59]. 

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. 

Assessment of density models. An analysis of the uncertainties associated with the estimaticxi of densities with this model has not yet been completed. However on the basis of those values obtained so far the standard deviation of the factor ( pest" pexpt/ pexpt) is between 1 and 2% and less than that recorded using the method due to Bottinga et al (J. The experimental uncertainties associated with density measurements for slags are ca. + 2%. 

Prediction of ash deposit characteristics based solely on bench-scale fuel properties always requires substantial judgement and allows only a certain level of confidence. As discussed, the ash deposition process is so complex that detailed modelling of commercial systems based on fundamental data is presently unrealistic. However, current techniques can provide relative data which in most cases is sufficient to make accurate assessment of slagging and fouling potentials relative to other fuels. 

R. W. Borio, R. R. Narciso, Jr., The Use of Gravity Fractionation Techniques for Assessing Slagging and Fouling Potential... 

Because of its universal application to industrial processes there is a considerable background to attempts to characterise different types of coal in respect of their propensity to fouling and slagging. In general these are empirical assessments often involving the composition in terms of the major constituents of the coal. 

A number of indicators have been devised, based on the effects of temperature on the ash produced from the coal under carefully controlled conditions. The temperature indices are an attempt to assess, under standard conditions, the temperature at which the solid ash becomes soft and fluid thereby giving an indication of the temperature at which slagging is likely to occur. The major standards that are used for these tests are, in no particular order ... 

Despite these restrictions however, ash fusion tests are a useful way of assessing the slagging propensity associated with different coal samples. 

Since the characteristics of coal ash will depend on its elemental components and compounds, it is not unreasonable to attempt to assess fouling and slagging potential in terms of the chemical constituents of the coal in question. In general these indices are largely based on the common minerals found in coal. 

The elaborate equipment and the difficult experimental technique required coupled with the fact that ash used in the assessment may not be representative of that found during combustion (as discussed earlier), make the test unlikely to be universally acceptable as a method of assessing slagging potential. 

The experimental technique involved with phase equilibria is of necessity, elaborate. For this reason, as with viscosity measurement, the method is unlikely to be adopted as a method for the assessment of slagging potential. 

Temperature indices are useful but limited in application as discussed earlier in this section. Until improved methods of assessment become available predictions based on chemical composition are likely to yield the most useful data. Tables 16.4 and 16.5 probably provide the most reliable indication of slagging propensity along with the base to acid ratio... 

Because of the broad similarity to coal, tests to assess the potential for fouling (and/or slagging) have been based on those developed for coal, for instance the temperature indicators described in Section 16.3.2.1 Dabron and Rampling [1988] have given some ash fusion data for refuse derived fuel from three different UK incinerators and compared them with a coal as presented in Table 16.16. 

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