Gasification

Caking and Swelling Properties

Another important property of a coal is the swelling index. The swelling index is determined by heating a defined sample of coal for a specified time and temperature, and comparing the size and shape taken by the sample with a defined scale. There are a number of different scales defined in, for example, ASTM D 720-91, BS 1016 (Gray - King method), or ISO 335 (Roga method). The swelling index is an indicator for the caking properties of a coal and its expansion on heating. Softening/caking does not occur at a certain temperature but over a temperature range. It is an important variable for moving-bed and fluid-bed gasifiers. For the gasifiers of entrained-flow systems, the coal softening point has no relevance. However, the softening point may limit the amount of preheating of the pulverized coal feedstock used in dry coal feed gasifiers.

Hardness

Physical properties are not very relevant for the operation of a gasifier as such. The hardness of the coal is, for example, mainly important for the milling and grinding upstream of the gasifier. The hardness of a coal is usually dependant on the nature and quantity of its ash content, although some coals, such anthracites, are also hard. A high ash content or a very high hardness of the ash in the coal can make a feedstock unattractive for gasification because of the high cost of milling and grinding. Ashes with high silica and/or alumina contents have a high hardness. The hardness is generally characterized by the Hardgrove grindability index (ASTM D 409).

Density

The density is primarily of importance for the transport of the coal. In this connection, it is important to discriminate between the particle density and the bulk density of the coal. The bulk density is always lower, as is shown in Table 4-5.