ACARP Project Number: C24056
Published: February 17
Philip Bennett and Karen
Steel
Extended
Abstract
The objective of this project was to explore the
relationships between the expansion and contraction behaviour of
coals and blends of coals during carbonisation and then relate this
behaviour to the properties of the coke produced. Two type of coke
are recognised in this project:
- Cokes
where the coke strength is strongly controlled by the bond between
fused coal particles due to either a deficient in fusible
components or low fluidity of the fusible components. These are
identified as coke with adhesion controlled breakage;
- Cokes
where the coke strength is strongly controlled by the pore size and
pore wall thickness of the fused material. These are identified as
cokes with porosity controlled breakage.
Samples of three widely-traded Australian coking
coals were tested. Coal A represents a coal that would have strong
adhesion control breakage producing poor coke quality if coked at
the pilot scale or small scale (7kg). Coal B is a hard coking coal
that produces some oven wall force when coked in a movable wall
coke oven. Coal C is a typical Australian soft coking coal.
Testing was conducted in ALS's modified Sole
Heated Oven (SHO) on a range of blends made from combinations of
these coals at different applied loads and different coal bulk
densities.
This project has clearly demonstrated that the
plastic layer can be assumed to be a viscoelastic solid and the
associated mechanical properties of that plastic layer can be
determined in the ALS SHO. The Strength Modulus, defined as
stress/deformation (MPa/m), was determined from the SHO
results.
These 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
foam. These findings have begun to identify of the mechanisms that
lead to coke strength, notably when coke strength is non-additive
with blend composition.
At a constant load an increase in bulk density
will result in a greater expansion of the coke in the SHO. Due to
this expansion there is no change in the porous structure of the
coke thus there is little variation in coke strength. That is, the
applied force is controlling the coke strength.
The SHO testing of Coal A with both Coals B and C
demonstrates how the addition of binder, Coals B or C, assists in
improving the coke strength of an adhesion controlled breakage
coal. The porosity varies linearly with blend composition while the
I600 does not. In a blend series, the point where the I600 starts
to decrease rapidly with decreasing percent of binding coal
indicates the maximum carrying capacity of the binder coal. Coal B
has a higher carrying capacity than coal C for SHO tests at a load
of 5kPa.
For blends of Coal A with Coal B or Coal C there
are strong linear relationships between the blend composition with
Strength Modulus and coke porosity. For these blends the plastic
layer of the individual coals are behaving as individual
foams.
The rheometry and SHO results for expansion,
Strength Modulus, coke porosity and I600 on the blends of Coal B
with Coal C showed non-linear behaviour with blend composition. The
strong linear link between the SHO determined Strength Modulus and
an estimate of the theoretical coke strength for these blends
demonstrates that the mechanical properties of the plastic layer
are controlling the coke strength for these blends.
A possible mechanism for this non-linear
behaviour is that Coal C physically flows into the inter-particle
voids of Coal B that are normally not filled when Coal B is coked
alone. This mechanism was tested with a simple model that assumes
the porous structure of Coals C and B are determined just by the
applied load and Coal C filled the inter-particle voids of Coal B.
The predicted blend porosity of this simple model matched the
measured porosity of the blends at different applied loads and bulk
densities supporting this hypothesis.
This project demonstrates the reduction in
expansion, as measured in the SHO, of Coal B (a high OWP coal) by
the addition of Coal C (a low OWP coal). This reduction in
expansion is not linear with blend composition. Several authors
have linked expansion measured in the ASTM SHO test with the OWP.
The mechanism for the reduction in expansion is explained by the
mechanism controlling porosity discussed above.
Future SHO work should further examine the
non-linear behaviour of blends similar to blends of Coals B and C,
but at more blend compositions and at least three SHO loads
starting at below 3kPa and up to 10kPa. This expanded dataset
together with the modelling of the SHO temperature profile should
allow the testing of theories developed in polymer science for
blending of viscoelastic solids.
