Plasticity refers to the melting and bonding
behavior of the coal and
- is an
indication of the initial softening, chemical reaction, gas
liberation and resolidification process within the coke oven.
- is an
important requirement in the coke blend and is required for end
product coke strength
fluidity of the plastic stage is a major factor in determining what
proportions of a coal is used in a proportions of a coal is used in
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