The understanding of the combustion performance
of blends is important in determining the economical and
environmental benefit of blending. The combustion performance of a
blend dictates the level of carbon in the ash, with high
carbon-in-ash equating to energy losses and increased ash disposal
costs. Emissions of NOx, SOx, CO2
and particulates are also influenced by blend composition.
This brief review is based on CoalTech's report
examining the performance of thermal coal blends for the Queensland
Government.
The combustion performance of a coal blend is
more complex than that of a single coal because it is not only
dependant on the combustion performance of each component coal but
also on possible interactions between coals. This interaction
between coals first occurs:
- in the
milling of the blend if there are
large differences in the hardness of each coal then there is
potential for large differences in the size distribution of the
component coals which will influence flame stability, burnout and
possible NOx formation.
- within
the burner flame the volatile release from the different blend
components will impact flame temperature and therefore can
influence flame stability, char reactivity and NOx
formation.
CoalTech's milling and combustion model shows how
size distribution of some component coals can impact on burnout
performance. For blends with similar milling behaviour the burnout
of blends is additive and can be determined from the burnout
performance of the component coals. When the milling performance of
component coals differs significantly then there will be
preferential grinding of the softer coal leading to a finer size
distribution for that coal. Usually the softer coal is of higher
rank, lower volatile content and poorer burnout performance. The
preferential grinding of the softer, lower volatile coals does
allow, depending on the coal, up to 30% of the lower volatile coal
to be used in blends that will have acceptable burnouts similar to
the higher volatile coal.
The model also indicates that when these higher
rank coals are used in proportions greater than 50% the burnout can
be similar or worse than the higher rank coal if it was burnt
unblended.
As the temperature at which the blend burns
increases, due to higher load, burner and boiler design ,the impact
of differential grinding decreases and burnout of the blend
approaches a linear relationship to the burnout of the component
coals.
The simple model developed does not allow for any
influence of different maceral types. There is some evidence that
fragmentation will increase the burnout of some lower volatile
coals with high vitrinite content. This fragmentation can be
increased when the heating rate within the flame is increased by
the addition of a higher volatile coal and there is sufficient
oxygen in the flame to oxidise the char surface.
Flame
stability of a blend is greatly enhanced when only
small amounts of a Surat Basin coal is included in a blend. This is
due to the high hydrogen content of this per- hydrous coal which
contributes to a high energy release within the flame.
Based on the findings in this report, lower
volatile Bowen Basin coals that are now excluded from the
international thermal coal market could be used in thermal coal
blends in proportions of up to 30% without unduly impacting on
combustion performance.
The early release of coal nitrogen within the
fuel rich region of a flame is the key to the minimising of
NOx
emissions. Second generation low-NOx
burners generate rapid heating and high temperatures at the centre
of the fuel-rich flame, which promotes the devolatilisation of the
coal and early release of fuel nitrogen leading to lower
NO
x emissions, better flame stability and carbon
burnout.
Coals with high volatile content and low nitrogen
content produce very low NOx emissions. The addition of
these coals to a thermal coal blend will significantly reduce
NOx emissions. It has been demonstrated in full-scale
burner tests that the medium and low volatile coals of the Bowen
Basin can be fired in second generation low-NOx burners
with acceptable NOxemissions.
The general application of correlations for the
prediction of
slagging and
fouling or electrostatic
precipitator behaviour for a wide range of coals,
based on ash analysis, needs to be treated with a degree of
caution. This is because it is the amount, size and type of
minerals in the coal and surface composition of the fly ash that
are important not the bulk composition. A wide range of slagging
and fouling indices are given in this report, their use will only
indicate potential problems.
The correlation used in the CoalTech report on
blending between ash properties and the performance of an ESP does
give a reasonable estimate for the outlet dust loading for most
single coals. But this correlation failed to predict the very good
ESP performance of Surat Basin coals which resulted in low
particulate emissions. When Surat Basin coals are used in blends
with a coal of poor ESP performance there is an improvement that is
far greater than if ESP performance was additive.