Plasticity refers to the melting and bonding behavior of the coal and

During the heating of coal an unstable intermediate phase, called metaplast, is formed after the moisture is driven from the coal. The metaplast is responsible for the plastic behaviour of coal. On further heating a cracking process takes place in which tar is vaporized and non-aromatic groups are split off. This cracking process is accompanied by recondensation and formation of semicoke.

When a coal/blend is coked in slot-type ovens, two principal layers of plastic coal are formed parallel to the oven walls. They are linked near the sole and the top of the charge by two secondary plastic layers forming an envelope of plastic coal. As carbonization proceeds, the plastic layers move progressively inward eventually meeting at the oven center. It is within these plastic layers that the processes which result in particulate coal being converted into porous, fused semi-coke take place. The semi-coke undergoes further devolatisation and contracts which results in fissures in the final coke.

The only small-scale methods which have stood the test of time and have been accepted as standard plasticity tests are the crucible swelling number, Gray-King coke type, dilatation characteristics, Gleseler plasticity and, in some countries, the Rogas index and the Sapozhnikov test.  All of these are essentially empirical in nature and many are subjective, at least to some degree.  The Gieseler test is the only one which attempts to measure the actual extent of the plasticity of fluidity attained.  The Gieseler test is used to characterise coals with regard to thermoplasticity and is sometimes an important tool used for coal blending for commercial coke manufacture.  Several authors have published methods to predict coke strength using the plastic temperature range and/or maximum fluidity as determined by the Gieseler test.  The maximum fluidity determined by the Gieseler is very sensitive to weathering (oxidation) of the coal.

Proton Magnetic Resonance Thermal Analysis (PMRTA) measures hydrogen protons and therefore acts as a probe to the structure of, and of motions, molecular units within the molecular lattice as the coal is heated to over 600 °C. A PMRTA determined parameter Fmax is a measure of plasticity, the maximum extent to which a material has fused corresponds to Fmax=100% and a solid material has a Fmax=0%.  Fmax has been shown to be better parameter than any Gieseler results in predicting changes in coke strength with weathering.

Steel and coworkers[ 1] combined high-temperature rheometry and 1H NMR to assess the microstructural changes taking place during carbonization.  They showed a relationship exists between the logarithm of the material’s complex viscosity and the fraction of hydrogen present in rigid structures for the resolidification region in which the material is liquid-like with small amounts of dispersed solid.

Recent research conducted by ALS Coal has shown that the plastic layer behaves like a viscoelastic solid and the research results were found to agree with simple theories of how the viscoelastic properties may change with blend composition if each blend component is treated as an individual viscoelastic solid. These findings have begun to identify of the mechanisms that lead to coke strength, notably when coke strength is non-additive with blend composition.

1  Karen M. Steel, Miguel C. Diaz, John W. Patrick, Colin E. Snape, "UNDERSTANDING THE MICROSTRUCTURE OF COAL DURING CARBONIZATION USING RHEOMETRY AND 1H NMR", 2005 ICCS&T Okinawa - October 2005