Ash Properties Melting Properties
For all gasifiers the ash-softening and ash-melting or fusion temperatures are important variables. For fluid-bed gasifiers these properties govern the upper operating temperature at which agglomeration of the ash is initiated. For entrained-flow gasifiers it is essential to ensure that the ash flows continuously and that the slag tap does not freeze up. The method for determining these temperatures is specified in ASTM D1857, “Fusibility of Coal and Coke Ash,” or similar specifications, such as ISO 540. In these methods the temperatures measured relate to the behavior of an ash sample under specified conditions and are reported as IDT (initial deformation temperature), ST (softening temperature), HT (hemispherical temperature), and FT (fluid temperature). For gasifier applications the ash-melting characteristics should be determined under reducing conditions, as these data may differ considerably (generally, but not universally lower) from data for oxidizing conditions.
An additional property required for slagging gasifiers is the slag viscosity - temperature relationship. It is generally accepted that for reliable, continuous slagtapping a viscosity of less than 25 Pa. s (250 Poise) is required. The temperature required to achieve this viscosity (T250) is therefore sometimes used in the literature (Stultz and Kitto 1992). Some slags are characterized by a typical exponential relationship between viscosity and temperature over a long temperature range. For others this relationship is foreshortened at a critical temperature (Tcv) at which the viscosity increases very rapidly with decreasing temperature. For a slagging gasifier to operate at a reasonable temperature, it is necessary for the slag to have a Tcv<1400°C.
The relationship between ash-melting characteristics and composition is a complicated one and is dependant largely on the quaternary Si02-Al203-Ca0-Fe0 (Patterson, Hurst, and Quintanar 2002). In general, slags that are high in silica and/ or alumina will have high ash-melting points, but this is reduced by the presence of both iron and calcium—hence the use of limestone as a flux. However, the Si02/Al203 ratio is also important, and where the calcium content is already high (as in some German browncoals), there can be some advantage to lowering the ash melting point by adding Si02. Properties of some typical ashes are given in Table 4-6.
In dry ash moving-bed gasifiers and in fluid-bed gasifiers, coals with a high ash melting point are preferred, whereas in slagging gasifiers, coals with a low ash melting point are preferred.
The caking properties of a coal and the melting characteristics of its ash are the reason that there are forbidden temperature ranges that have to be taken into account, both in design and during operation. In entrained-flow gasifiers only the ash properties are important.
The ash that is produced in gasifiers always has a lower density than the minerals from which they originate, due to loss of water, decomposition of carbonates, and other factors, and the presence of some carbon. The bulk density of the ash in particular may be low due to the formation of hollow ash particles (cenospheres). This means that special attention has to be given to the transport of such ashes.
Slag is very different from ash as it has been molten and is in fact a fusion-cast material similar to glass. Ideally, slag becomes available as an inert, fine, gritty material with sharp edges due to the sudden temperature drop upon contact with a water bath. Because lumps of solid slag will form during process upsets, a slag breaker is sometimes installed between the water bath and the slag depressurizing system.
