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Show lJhilst exam.in1ng .data it has been observed tbat there ls, ' in many cases, evidence to suggest an op:1~~m heat : l~ x l evel at which to sootblow. This observation has been substantiated by lndepend~nt investigations conducted by the Central Electricity Generat1ng Board <Ref 1 ). Further ~ata an~lysis is being carried out with the view to relat1ng this --behaviour to the physical properties of tbe ash (Figure 15>. A possible explanat10n of the phenomenon could then be : The initial ash depos1t1on on 'clean' tubes will be in an uns1ntered, particulate phase. As the aGh thickness progressivly increases so the surface temperature of the ash will rise, and when this temperature is in the region of lOOOdeg C the surface ash will start to s1nter, this s1ntering process tend1ng to bind the p~rticulate surface of the ash layer into a solid block. ruther format10ns of ash are subject to similarly high temperatures and so progressivly develope as a sintered mass. Sootblowing before surface , s1ntering requires each individual ash particle to be phys1cally dislodged from the wall, as very little particulate interconnection will be present 1n unsintered ash. However after s1nter1ng bas commenced and the surface has high particulate agglomeration the ash will tend to fall in blocks, thus ~king sootblow1ng tar more effective. CASE STUDY 3 A major problem in foss1l fuelled boilers is tube corrOSlon and failure. This study illustrates and 1dentlfies practical boller conditions where tube corrosion mech&n1sms will be p~esent, ie. high heat fluxes and tube surface temeratures causing both fires1de ~nd wa~ers1de corrosion. Figure 12 shows the heat flux measurement in an area of boiler wall instrumented with four Fluxtubes, No . 1, 2, 5, 6. Approx1matly 10 mlns into the study time per10d a full Eequential sootblow of the ent1r~ back wall was conducted. The reg10n arou~d Flux~ube 1 1S seen ~o clear and the heat flux in this area rises in a typical manner but to an abnormally higb level. The areas aro~nd Fluxtubes 2, 5 and 6 show no evidence of slag removal and an investigation revealed that the area around Fluxtube 1 was the only clear. area in the top section of the evaporator. Vhen the burner w~lls are in a slagged condition the flame temperature 1s high due to reradiat10n from the w~lls. A generalreduct10n of slas would result 1n a corresponding d~op in flame temperature. However in this instance, as onlya s~ll area of wall has cleaned, the higly absorptive surface is subjected ~o high lncident anc absorptive heat flux levels, and will continue to be as long as there is s uf!1c1ent heat absorptiO~ potential in the back w!ll to s1gnificantly reduce the flame temper~: 'J re . Figure 16 ShO~' 2 :hiG heat flux with t~ e inner thermocouple te~? (which is slightly h1 8 ~ er than the ~~ cisturbed tube suri~ c e te~peratwre ). It ce ~ ce seen th~t the heat flux ri6es to about 57 0 K·.' I YL2 wi th a corrEEpor.~:':ig s~rface tempera ture cf about 695 deg C, cc~=i:io~s which will al~c 2 : certa1nly cause s i gnificant levels of f ireside : ube wastage ei~ h er by oxidation 1n ~ x vgen rich conditions or ~ y reduct~on in oxygen l een cond1tions . The I r.} ,--' I I |