OCR Text |
Show surface temperature was maintained constant at 600°C appropriately varying the cooling eir flow as heat flux decreased linearly wi th the build-up rate as shown in Fig.7. Wi th the South-African coal a loose layer of very fine particles deposi ted thermophoretically on the trailing edge around the circumference of the tube. This yellow-white deposit is visible at the beginning of the run on both sides of the tube, but only on the leading edge after half an hour is it transformed into a denser brown hollow layer which easily breaks down and spalls. The Colombian coal shows similar behaviour but the volumetric deposition rate, see fig.S, is lower due to the lower ash content in the coal. Apparently, this deposit adheres more strongly to the tube surface. The outer layer surface, after a three hours run, was brown and glassy in appearance and grew in small stalacti tes that frequently broke off a few hours after the combustor was turned off. We recognize that a shortcoming of our pilot scale measurements is the inadequate duration of testing. Transformation of loose fine particle deposi t into a dense layer usually takes weeks. Nevertheless, in the full-scalle boiler soot-blowing is done a few times a day and newly fresh deposited layers may well reform after each blow. On the basis of visual and video recording observations, we divided the ash behaviour of the two coals as follows: South African coal: Particles arrive at the tubes in dry form, producing deposi ts wi th a low adhesive strength. As soon as deposit builds up the outer layer becomes more cohesi ve, sti 11 retaining a high porosi ty between agglomerates. It sti 11 adheres loosely to the tube and easily breaks off during the run. Colombian coal: Deposi ts are laid in an unsintered form. They then start to grow and sinter and a sort of hollow |