| Recently, MILD (Moderate & Intensive Low Oxygen Dilution) combustion has been popular among industries and academia for its energy saving and emission reduction characteristics, and it is lauded as one of the most promising combustion technologies in the 21 century. Although this technology has been investigated for more than 20 years, so far the related researches are mainly on gaseous fuels and using highly preheated air. Besides, air is used as oxidizer in most of the studies, while less investigations are conducted under Oxy-fuel condition. Considering the energy and environment situations, carrying out the researches of MILD combustion under low preheating conditions for different fuels is relatively important for generalizing this technology in applications, especially under Oxy-fuel condition.First, this paper studies the effects of geometric parameters of combustion apparatus of the natural gas (NG) MILD combustion, and explores the NG combustion characteristics using low temperature air, both numerically and experimentally. The results show, not only burners'geometric parameters, but also furnace chambers' have important impacts on the realization of MILD combustion and its behaviors. Expanding the divergence between furnace sidewalls and roof would facilitate the achievement of MILD regime, and improve the temperature distribution uniformity as well as reduce NO emission. Stable MILD combustion can also reached in industrial furnaces using low temperature air, and increasing the switching temperature can enhance the stability of the transition process of MILD combustion. Simultaneously, when combustion status is transferred from conventional combustion to MILD combustion, the reacting region will gradually moving from upstream to downstream inside the furnace, and hence, the overall temperature level can be promoted.Next, this paper studies the Oxy-MILD combustion characteristics of coal under low preheating temperature condition and analyzes the effects of CO2 and H2O on coal Oxy-MILD combustion using both numerical and experimental methods. The results reveal under MILD operation, when enhancing the oxygen concentration to 30% in O2/CO2, the combustion temperature can reach the similar level to that in air. Under Oxy-MILD combustion, the addition of H2O can accelerate coal ignition and inhibit fuel-NOx conversion. CO2 is beneficial to reaching MILD combustion as compared to H2O. Coal MILD combustion can also be achieved under low preheating condition, however, the flame appearance just shows "semi-flameless" due to the heterogeneous reactions in the char surface. As compared to conventional Oxy-fuel combustion or Air-MILD combustion. Oxy-MILD combustion obtains the largest potential of temperature uniformity and NOx reduction. With comparison to conventional combustion, the coal burnout becomes worse under MILD combustion, mainly because of the insufficient resistance time of coal particle in low oxygen atmosphere.Last, this paper studies the jet-in-coflow (JHC) diffusion flames of CH4/H2, and reveals the physical and chemical effects of CO2 on the early ignition stage of MILD combustion using numerical simulations as well as reaction kinetics methods. The results suggest, with respect to N2, CO2 can lower the peak combustion temperature not only from the physical effects of CO2, but also from its chemical effects. In the early ignition stage, the chemical effects of CO2 can suppress the formation of OH radical, while the physical effects of CO2 can accelerate the distribution uniformity of OH radical. In Oxy-fuel atmosphere, the chemical effects of CO2 are the main reason for the delayed ignition under MILD combustion, meanwhile, this can make fuel ignite more homogeneously. Under MILD mode, the physical effects of CO2 have no significant impacts on CH4 oxidation process, while the chemical effects of CO2 can suppress CH4 oxidation through routes ? as well as ?, and facilitate the dissociation of CO2 to CO. |
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