| Summary: | 碩士 === 國立成功大學 === 航空太空工程學系碩博士班 === 98 === As the turn of the new century, energy crisis and global warming have become important and urgent issues threatening the human beings and modern civilization. The supply of energy resources falls short of rapidly growth of demand. For all the efforts to partly relieve the coming energy crisis, the use of blended fuels has been proposed and becoming one of the important methods. For the gaseous blended fuels, gasified fuels from biomass and low-grade coals, with H2,CO, and CH4 as the primary compositions, attract intensive research attentions recently. In the present study, the blended fuels have also been the subjects for researches, and From the previous results reported by our group, it has been shown that the maximum laminar burning velocity for the CH4/CO blended fuels varies with the ratio of the fuel components at the same equivalence ratio, and the maximum burning velocity for the blended fuels far exceeds the burning velocity for each individual component. Also, extensively reported in the literature that addition of small amount of hydrogen in fuel will generally increase its burning velocity apparently. Therefore, in this study, the effects of H2 addition in stoichiometric CH4/CO blended fuels on the resultant combustion characteristics and flame structures for are investigated by experiments and numerical simulations.
Numerical prediction results of the laminar burning velocity for CO/CH4/H2 blended fuels in stoichiometric condition reveal that not only all the laminar burning velocity is increased with H2 addition, but also the CH4/CO fuel ratio of the maximum laminar burning velocity is changed from 80%CO and 20% CH4 to 100%CO when H2 addition reaches 50% of the total fuel amount. This finding is validated by both numerical simulations using Chemkin collection 3.5 with GRI-Mech 3.0 and experiments using the premixed H_2/CH_4/CO/Air opposed-jet flames. When 10%H2 and 20%H2 are used in the H2/CH4/CO flames, the CH4/CO fuel ratio of the maximum burning velocity varies from 90%CO and 10% CH4 to 94%CO and 6% CH4 respectively
Based on the analysis of flame structures, it’s shown that the variation in burning velocity with H2 addition isprimary due to the transition of dominant chemical kinetics paths. Although the CH4/CO fuel ratio of maximum laminar burning velocity is varied with H2 addition, the reaction rate of CO fast “wet” oxidation（R99）is still dominant as the major contributor in the sensitivity study, and CO fast oxidation（R99）provides primary and most heat-release rate. However, it becomes different that the reaction rate of H2 oxidation（R84）exceeds CH4 oxidation（R98）in the position where major oxidation happens in terms of reaction rate, and heat-release rate as the concentration of H2 addition is increased. Finally, The results of sensitivity analysis also shows that the variation of the importance in reaction steps is not affected by H2 addition, and the most important reaction step is changed from the chain-branching reaction（R38）to the reaction of CO fast oxidation（R99）when CO in CH4/CO is increased to 80%.
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