The effects of impregnation of coal by alkali salts upon carbonization properties

The co-carbonization of coking and caking coals with potassium and sodium salts destroys the coking and caking capacities of the coals. Further, the resultant char is of high surface area and exhibits a high chemical reactivity to oxidizing gases because of the catalytic influence of potassium retained within the char. This article attempts to explain the above phenomena, i.e. the loss of coking mechanisms, the development of high surface areas and the retention of the potassium. Initially, current theories are outlined of coking mechanisms which establish the anisotropic, carbonaceous structural units within resultant cokes. These structural units are best observed as isochromatic areas in colours of blue, yellow or purple, using a polarized light microscope and a half-wave retarder plate in conjunction with polished surfaces of coke. When a coking coal is carbonized, it first softens and melts to form an isotropic, pitch-like fluid. On further heating, anisotropic units of irregular shape develop from within this fluid phase. In coal systems, these units grow to about 0.5–5.0 μm at which stage they join or fuse together but do not coalesce. Their identity is maintained, and they establish what are termed finemozaics. At the same time, the macro-properties of coke, e.g. porosity, are established. The formation of these anisotropic mozaics occurs via the growth of lamellar nematic liquid crystals containing stacked lamellar molecules. The liquid crystals possess the crystalline order which is transferred to the solid coke substance. It is the plasticity of the liquid crystals which allows the growing anisotropic units to fuse together, and the introduction of disclinations which impart desirable properties to the coke substance. The addition of potassium salts to coking coals is thought to reduce the fluidity of the coals primarily by increasing the number of cross-links which normally exists between the aromatic and hydroaromatic constituent molecules (building blocks) in the coal. Such an increase results, in turn, in an increase in molecular weight of the coal, decrease in its fluidity upon heat treatment, and the consequent decrease in mobility of planar regimes preventing their alignment to form the liquid crystals and then the anisotropic mozaics. It is suggested that the presence of potassium results in a higher oxygen content being present in the coal upon heating, either by reducing the rate of oxygen evolution from the coal as CO, CO2 and/or water or by acting as an intermediate to extract additional oxygen from the steam added as a reactant to the system (that is, steam gasification). Thus, an increased oxygen content results in more cross-linking in the structure probably via ether linkages between aromatic and/or hydroaromatic regimes. This increase in cross-linkage creates the isotropic carbon of the char, the spaces between the cross-linked constituent molecules being microporosity responsible for the high surface area of the char. The potassium could be retained within the microporosity by being bonded to the oxygen attached to the carbon.