Adhesion of Ash and Slag

The conditions for the formation and removal of wall and flue incrustation were considered earlier (see 32, 33). In this section we shall therefore devote principal attention to pipe incrustation. 

A free-running or loose deposit is an adhering layer of solid particles. A tacky deposit is caused by the presence of tacky (liquid, oily) constituents. Dense deposits are formed, for example, in the combustion of Estonian shales at 500-1000°C and also of the poor coal under Moscow. Dense deposits are also formed in the combustion of certain kinds of fuel oil. In practical conditions, all forms of deposit may be present at the same time, and it is sometimes difficult to observe the boundary between them, 

Free-Running Deposits. The composition (as regards particle size) and quantity of free-running deposits precipitated from a gas flow depends on the size distribution and concentration of the particles in the flow, the operating time of the plant, the velocity and direction of the flow, and also the diameter and mutual arrangement of the pipes. 

The particle-size distribution of deposits usually differs from that of the dust in the gas flow, since small particles tend to stick to the pipes most, so that the proportion of these particles in the deposits is greater than in the flow. This is confirnied by particle-size analysis [454] (carried out by the method of air separation) of the ash from Moscow coal, both in the flow and in the deposits on pipes 38 mm in diameter  

The number of particles falling out of the flow depends both on their concentration in the flow and on the time of operation of the plant. The longer the plant has been working, the more deposits are formed however, this increase is not without limit, and for any particular conditions a state of saturation is reached after a certain time, i.e., the quantity of deposits formed reaches a maximum. It has been found [454] that this saturation usually sets in after 7 h operation, the deposition taking place, not uniformly, but chiefly in the first 1-3 h. Thus, half the maximum amount of deposits are formed in 2.5 h if their concentration in the flow is 7 g/vc and in 1.5 h if the concentration equals 21 g/m. Hence, the 

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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. 

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. 

The results in Figure 7 show that the strength of adhesive bond between the ferritic steel sample and boiler deposit increased exponentially with temperature in the range of 775 to 900 K. Similar results were obtained by Moza et al. (34) who used a droplet technique to measure the adhesive bond of coal ash slag on a ferritic steel target in the temperature range of 700 to 950 K. 

The corrosion product, a mixture of oxide, sulphide at the metal interface and sulphate outside, has a weak adhesive bond to the metal surface and cannot support large deposit masses. It is therefore unusual to find excessive amounts of sintered ash deposits and fused slag in the exact localities where severe high temperature corrosion occurs. Conversely, a strongly adhering matrix of sintered ash deposit in the absence of sulphate, sulphide or chloride phases is not markedly corrosive. 

This leads to the conclusion that the Si02 content will not react with other ash constituents but will only alter the portion of slag-relevant constituents for coals in which Si02 appears largely as quartz. The microspheres generated by fusion primarily determine the adhesion process in the liquid state and have a chemical composition different from the average ash analysis. 

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