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 NOx 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.