Adhesion of Ash, Slag, and Carbon Deposits

Features of Ash and Slag Adhesion. The adhesion of ash and slag to the heating surfaces in boiler units is a serious detriment to good operation. Deposits on boiler tubes may interfere with circulation of the heat carrier, may bring about tube corrosion and rupture, may plug gas passages, and may reduce the boiler capacity. 

Deposits can be classified on the basis of location, chemical composition, or structure. 

Depending on the site of adhesion, ash and slag can be classed arbitrarily as tube deposits adhering to the heating surfaces, wall deposits on the furnace walls, flue deposits adhering to the walls of the gas passage, and hearth deposits settling on the furnace floor. The conditions for the formation and removal of wall and flue deposits were examined previously (see Section 40,41). In this section, therefore, we will be concerned primarily with tube deposits. 

In classifying deposits by structure, the adherent layer can be characterized as loose (ash and soot deposits), tacky, dense (sintered and cemented), or liquid. The properties of the adherent layer are determined by the particular stage at which the deposit formation process ends, this process consisting of the deposition of fly ash, adhesion of ash particles to the heating surface, sintering of the ash particles, and melting of the outer layer of deposits. We will be considering three types of deposits loose, tacky, and dense. 

Loose deposits consist of an adherent layer of solid particles. Tacky deposits are caused by the presence of tacky (liquid or oily) components. Dense deposits consist of a single dense mass for which the autohesive interaction between particles is much greater than the adhesive force. Dense deposits are formed, for example, in the combustion of Estonian shales at temperatures of 500-1000°C and also in the combustion of lean coal from the Moscow area. Also, dense deposits are formed by the combustion of certain residual fuel oils. Under the operating conditions encountered in practice, all types of deposits may be present at the same time, and it is sometimes difficult to observe the distinctions between these types. 

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