The influence of coal-blending methods, such as out-furnace and in-furnace methods, on unburned carbon and NO emissions of blending coal combustion in a drop-tube furnace (DTF) has been analyzed using experimental and numerical approaches for binary coals (sub-bituminous and bituminous coals) used by Korean power plants. In the out-furnace method, as the blending ratio of sub-bituminous coal based on bituminous coal (SBR) increases up to 50%, the unburned carbon fraction gradually decreases. The worst condition is found at a SBR of 75%, which is caused by the occurrence of highly oxygen-deficient conditions in the furnace. Furthermore, the emission index of NO (EINO) is proportional to the SBR. In addition, the in-furnace blending method is applied using a modified DTF to improve the efficiency of unburned carbon fractions and the involvement of NO emissions. The results show that unburned carbon fractions as well as the EINO decrease over time because of improvements in oxygen deficiency and NO reduction mechanisms. The numerical modeling strongly supports these phenomena and provides insights into the mechanisms that affect the blending methods during the combustion of coal blends.

Nowadays, it is widely recognized that biomass combustion processes can contribute to the mitigation of greenhouse gas emissions, and thus, these processes have become a viable option as an alternative energy source for the power industry. Among various biomasses, the herbaceous biomass is regarded as an abundant and relatively inexpensive fuel. However, it contains high ashes (especially high levels of alkali metals), leading to operation troubles such as slagging and fouling inside a heat exchanger or deterioration in efficiency. Accordingly, we herein propose an economical method to remove the inherent ashes in the biomass using 16.6 M acetic acid pretreatment at 60 °C for 10 min. Seven different biomasses were investigated to validate the effects of method. The Kenaf shows the total mineral rejection of 93.48%. In particular, the potassium and sodium elements in the Kenaf, which are major factors influencing fouling and slagging in a boiler, were removed up to 99.46 and 100%, respectively. Furthermore, the proposed wet treatment was more effective for biomass with higher surface areas.

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The influence of coal blending methods, such as out-furnace (external or premixed) blending and in-furnace (initially nonmixed) blending, with different excess oxygen (highest, medium, and lowest stoichiometric conditions) on unburned carbon and NO x emissions of blend combustion in an entrained flow reactor has been analyzed, using experimental and numerical approaches for binary coals used by Korean power plants. The results confirm that, under the medium condition, contrasting processes, such as reactive and unreactive effects, occur with SBRs in the out-furnace blending method. The in-furnace blending method results in an improvement in the efficiency of unburned carbon fractions and a further reduction in the NO x emission. Under the highest condition, the unburned carbon fraction in both the out-furnace and the in-furnace blending methods corresponds with the tendency under the medium condition with contrasting processes of lower magnitude, whereas the NO x emission in the highest condition increases slightly. Under the lowest conditions, the unburned carbon fraction in the out-furnace blending method gradually decreases as SBR decreases, without a competition effect. The reduction of NO x emission under the lowest conditions is more effective than those under other conditions for the two blending methods because of homogeneous and heterogeneous NO x reduction mechanisms. These results show that the phenomenon that occurs with coal blending methods under different excess oxygen conditions has been demonstrated and the in-furnace blending method below medium conditions would be an effective method to improve combustibility and NO x emission due to penalty of NO x under the highest condition. In general, the numerical results are in agreement with the measured values and give insight into the phenomena that affect the blending methods under different excess oxygen conditions.

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