Slagging and Fouling Potential The potential for slagging (i.e., fused slag deposits that form on furnace walls and other surfaces exposed to predominately radiant heat) is temperature and ash composition related. Slagging potential affects furnace sizing, arrangement of radiant and convective heating surfaces, and the number of sootblowers required [28]. Fouling deposits form primarily in lower temperature regions of the furnace and convective section and affect the design and maintenance of superheaters, reheaters, furnace waterwalls, air heaters, and the number of sootblowers required. The potential for fouling is linked to the alkaline content of the ash, specifically the active alkalis.

Ash fusibility has long been recognized as a tool for measuring the performance of coals related to slagging and deposit buildup. American Society for Testing Materials (ASTM) Standard D1857 specifies the experimental procedure to use to determine the ash fusion temperatures. The test is based on the gradual thermal deformation of a pyramid-shaped ash sample in either an oxidizing or reducing atmosphere. Four temperatures are obtained during the test: (1) initial deformation temperature, which is the temperature at which the tip of the pyramid begins to show evidence of deformation; (2) softening or fusion temperature, which is the temperature at which the ash sample has fused and the height equals the width; (3) hemispherical temperature, which is the temperature at which the sample has fused into a hemispherical shape for which the height is equal to half of the width at the base; and (4) fluid temperature, which is the temperature at which the sample has fused down into a nearly flat layer. The ash-softening temperature is related to the type and ease of deposit removal from heat-transfer surfaces. If ash particles arrive at heat-absorbing surfaces at temperatures below their softening temperature, they will not form a bonded structure, and ash removal is relatively easy. If the ash particles arrive at these surfaces after they have been subjected to temperatures above their softening temperature and have become plastic or liquid, the resulting deposit will be tightly bonded and more difficult to remove [8]. Also, the temperature difference between the initial deformation and fluid temperatures provides information on the type of deposit to expect on furnace tube surfaces [8]. A small temperature difference indicates a thin, running, tenacious, difficult-to-remove slag, whereas wider temperature differences indicate less-adhesive deposits.

Another measure of ash viscosity that is used to predict furnace slagging is the temperature of critical viscosity (Tcv) [29]. This is the temperature at which the viscosity properties of the molten slag change on cooling from those of a Newtonian fluid to those of a Bingham plastic and is believed to be the temperature at which solid phases start to crystallize from the melt.

Several slagging and fouling indices have been developed based on the ash composition. Many of these indices have been developed for eastern U.S.-type coals. These are characterized as high iron and low alkali and alkaline-earth content coals. The ash composition of such coals reflects a Fe2O3 level exceeding the combined CaO, MgO, Na2O, and K2O percentage. In addition, sulfur percentages are higher than western United States coals. The acidic ash constituents (reported as weight percent on an oxide basis) SiO2, Al2O3, and TiO2 are generally considered to produce high-melting-temperature ashes. Temperatures will be lowered by the relative amounts of basic oxides, Fe2O3, CaO, MgO, Na2O, and K2O, available in the ash. The base (B)/acid (A) ratio defined as:

B= Fe2O3 + CaO + MgO + Na2O + K2O A = SiO2 + Al2O3 + TiO2 ( - )

has been developed as an indictor to predict the relative performance of coal ash in the furnace. This index can be used for all ranks of coal. A base/acid ratio in the range of 0.4 to 0.7 reflects low-fusibility temperatures and a higher slagging potential [8].

The base/acid ratio has also been used to define a slagging index expressed as Rs = B/Ax S, where S is the weight percent sulfur in the dry coal.

The slagging index has been used with success to identify four types of slagging coals [30]:

Slagging Type

Slagging Index (Rs)




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