Study On Combustion Characteristics Of Dimethyl Ether Flame And Engineering Application

In China, since 90% industrial boilers are powered by coal, the pollution problems of the coal-fueled industrial boilers are particularly significant, and it has became one of the major sources of air pollutants nowadays. Given the abundant coal reserve of China, developing clean coal chemical industry to convert the coal to clean alternative fuels is not only an effective way for clean utilization of coal, but also can reduce the import quantum of oil and gases and guarantee the national energy security. Dimethyl ether(DME, CH3OCH3) is a new kind of clean alternative fuel, which can be efficiently produced from coal in large amounts. It is officially identified as the only ideal clean alternative fuel for the replacement of traditional fuels such as LPG and diesel oil, etc. At present, DME is mainly applied to the compression ignition engines, and the combustion and operational performances of the DME-fueled engines were systematically studied across the world. With regard to the industrial boiler and kiln furnace, the combustion characteristics and induatrial application of DME in them are not studied as thoroughly.This paper introduces DME into the field of industrial boiler to solve its pollution problem and reduce the overcapacity of DME in China. Flames in the industrial boilers can be divided into two types: premixed flame and non-premixed flame. In this paper, the fundamental combustion characteristics of the DME/air jet diffusion flame are systematically investigated, including the flame dimensions, stability, NOx and CO emission, and thermal radiation behaviors. Additionally, a medium-scaled gas combustion test platform is also desingned and constructed to study the combustion characteritics of the DME premixed flame in the industrial boiler, including the combustion, flow, and reaction zone size behaviors of the DME premixed flame inside the water-cooled cylindrical furnace, NOx and CO emission behaviors, as well as the convective heat transfer characteristics of the DME combustion products when longitudinally passing through the high-temperature flue gas tube and transversely flowing across the convective tube bank. Most of the researche results are presented as valuable empirical correlations and calculation methods, which may be helpful for engineering design and operation of the DME-fueled industrial boiler and kiln furnace. The main contents and originalities of this paper include:① Experiments are conducted to study the dimensional and stability behaviors of the DME/air jet diffusion flame. Correlations for the length, width, and volume, as well as the blow-out limit, of the DME jet diffusion flame are developed in this paper. Additionally, the above-mentioned flame behaviors of DME are also compared with those of natural gas and LPG, which may be valuable for the design of DME-fueled industrial boiler and kiln furnace, or for an appropriate retrofit of the existing units fueled by natural gas or LPG for applying DME to them. Moreover, the analytical similarity solutions for dimensions of the jet diffusion flame are also developed presently, based on which new forms of correlations for the jet diffusion flame length, width, and volume with higher prediction accuracy and rationality are proposed in this paper.② The distribution and emission index behaviors of NOx and CO of the DME/air jet diffusion flame are tested by experiments. The effects of fuel jet velocity and air co-flow velocity on NOx and CO emisson indices are analyzed. Additionally, by employing the CFD-CRN method, reaction path and sensitivity analyses, and a detailed chemical kinetic mechanism of DME, the conversion relationship among the nitrogenous species, NOx reaction rate behavior, and NOx formation mechanism and pathway inside different zones of the DME jet diffusion flame are also studied. The formation and emission mechanism of pollutants inside the DME jet diffusion flame are clarified, which may be helpful for working out an appropriate measure of pollutant emission reduction for the DME-fueled combustion systems.③ Experiments are conducted to test and analyze thermal radiation behavior of the DME/air jet diffusion flame. Finally, correlations between the flame radiation fraction of the DME jet diffusion flame and fuel nozzle diameter df, fuel jet Reynolds number Ref, and air co-flow velocity uco are developed presently. The model and method for predicting radiative heat flux outside DME/air jet diffusion flame are also suggested. They may be valuable for design of the radiation heating surface in the DME-fired non-premixed jet combustion systems.④ For the first time at home and abroad, the temperature and species distribution behaviors of the DME/air premixed flames inside the water-cooled cylindrical furnace under various excess air ratios α and thermal loads Q are tested by experiments on the gas combustion test platform. The fundamental characteristics such as combustion, flow, and heat and mass transfer behaviors, etc., of the DME/air premixed flames inside the furnace are analyzed, and the effects of α and Q on flame characteristics are also studied as well. Moreover, the flame structure and reaction zone size behaviors of the DME/air premixed flame inside the water-cooled cylindrical furnace are also studied by numerical simulations. Valuable correlations and methods for estimating the mean reaction rate, reaction zone volume and length of the DME/air premixture are developed in this paper.⑤ Experiments are carried out, for the first time, to test NOx and CO distribution and emission index behaviors of the DME/air premixed flame inside the water-cooled cylindrical furnace under different α and Q. The effects of α and Q on NOx and CO emission indices are analyzed. Additionally, the CFD-CRN method, as well as reaction path and sensitivity analysis methods, is also employed to study the NOx formation mechanism and pathway in different zones of the DME premixed flame inside the furnace. Finally, the optimal operational parameters and optimization method of furnace dimentions for pollutant emission reduction of the DME-fueled industrial boilers are developed in this paper.⑥ Finally, experiments are conducted on the convective-heat-transfer sections of the gas combustion test platform to study the convective heat transfer characteristics of the DME combustion products when longitudinally passing through the high-temperature flue gas tube and transversely flowing across the convective tube bank. The corresponding Nu correlations are developed, which may be valuable for thermal design calculations of the convection heating surfaces(including high-temperature flue gas tubes and convective tube bank) of the DME-fueled industrial boiler or kiln furnace.