ACARP Project Number: C21058
Published: September 14
Philip Bennett, Karen Steel, Matthew Dick,
Fengyuan Yu
Extended
Abstract
The development of new coking coal resources
around the world and the changing coal quality from existing
resources will require steel producers to examine new blend options
for the feed to their coke ovens to maintain coke quality. The
value of a coal depends on the contribution of that coal to the
quality of the coke produced from that blend, knowing this value
assists in the marketing of the coal. For coal producers to
investigate washing options, mine site blending or port blending
options to control coke strength for their customers they are
limited to costly testing in pilot scale ovens. The expense of this
type of testing limits the number of blends that are evaluated.
Alternatively, the producers must rely on potentially dubious
correlations between laboratory tests and coke quality.
Current work at ALS Coal and other researchers
has demonstrated that the contraction of the plastic layer is
related to the properties of the coal, the void space into which
the coal can expand (bulk density) and the pressure exerted on the
plastic layer (normally by the generation of internal coking
pressure). All these factors control the porous structure of the
coke and therefore influence the coke strength. The hypothesis
investigated in this project was; Can the contraction as measured
in the Sapozhnikov Test or Sole Heated Oven (SHO) be used to obtain
a better estimate of the impact of a non-hard coking coal on the
coke structure of a blend?
In this project the ASTM SHO test was adapted to
measure the contraction due to the pressure exerted on the plastic
layer during coking of a coal charge. The design of the modified
SHO aims to measure the contraction of the plastic layer under
controlled heating conditions and different loads. The method of
applying the load to the charge has the capability of being altered
during a test giving measurements of contraction at different
loads. The application of different loads during a modified SHO
test enabled data to be collected on how contraction changes with
applied force giving a better insight into how component coals may
interact in a coke charge. A penetrometer was used to determine the
height of the coke-coal interface during the test. Knowing the coke
height allows the change in plastic layer thickness under different
loads to be estimated.
This work has provided a number of key insights
into the factors influencing the behaviour of coals during
coke-making. It is known that porosity plays a key role in
influencing coke strength. This study has shown how both the
viscoelastic properties and the applied load on the plastic layer
influence the degree that bubble growth and coalescence occurs,
which in-turn influences the porosity. Controlling the viscoelastic
properties and/or the applied force could enable desired control of
pore structure development and hence coke strength. The SHO is a
useful tool for assessing the influence of applied force on bubble
coalescence and degree of contraction (shrinkage) of the charge,
both of which control pore structure development and, hence, coke
strength. It is important to note that applied force influences the
degree of pore coalescence taking place but will also influence the
degree of shrinkage of the pore network structure. Under constant
force, the degree of shrinkage is thought to be dominated by the
rate of volatile release. It follows that the SHO could be used to
screen blending scenarios and identify blends for which improved
strength is expected to be achieved under realistic applied forces.
Rheometry is a useful tool for assessing whether interactions are
occurring during blending, and the influence that this has on
bubble growth and coalescence behaviour.
This work also showed how the viscoelastic path
plays an important role in bubble growth/coalescence behaviour
whereby one coal studied (coal Y) tracked back through the bubble
growth region due to it retaining much of its elastic character.
The consequence of this is two periods where bubble growth
dominates. This behaviour is expected to have an influence on the
final coke strength achieved.