Author: Wei, Zhilong
Title: Combustion, thermal and emission characteristics of laminar premixed biogas-hydrogen bunsen flame
Advisors: Leung, C. W. (ME)
Cheung, C. S. (ME)
Degree: Ph.D.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Department: Department of Mechanical Engineering
Pages: xviii, 228 pages : color illustrations
Language: English
Abstract: The objective of this study is to improve the understanding on the fuel characteristics of biogas-hydrogen mixture and then promote the real applications of biogas fuel. Both experimental measurements and numerical simulations were carried out so as to investigate the combustion, thermal and emission characteristics of biogas-hydrogen flames. In the study, biogas is composed of CH₄ and CO₂ and represented by BGXX, with XX representing the volumetric ratio of CH4 in the biogas, while hydrogen is adopted as the addition to improve fuel features of biogas. Heat fluxes, flame temperature and pollutant emissions of laminar premixed biogas-hydrogen impinging flames were measured experimentally. In addition, a one-dimensional computational model was developed using the Chemkin software to calculate the heat release rate and obtain the relative significances of exothermic reactions of biogas-hydrogen flames while a two-dimensional computational model was developed using the STAR-CD software to calculate the pollutant emissions of biogas-hydrogen impinging flames. In this study, effects of H₂, CO₂, nozzle-to-plate distance (H), equivalence ratio (φ) and Reynolds number (Re) on the heat release, heat transfer and pollutant emission features of laminar premixed biogas-hydrogen flames were investigated quantitatively. For the heat release features of biogas-hydrogen flames, the complete combustion results in the maximum global heat release rate (HRR) at φ=1.0, while the exothermic recombination of intermediates gives rise to the enhanced HRR in the post flame region at φ=1.2. The primary exothermic reactions of biogas-hydrogen mixture are dominated by φ, and O+CH₃=H+CH₂O, H+CH3(+M)=CH₃(+M), OH+H2=H+H₂O, OH+CO=H+CO₂ and O+CH₃=>H+H₂+CO can play critical roles in HRR consistently with the varied φ. With the H₂ enrichment, H+HO₂=2OH and H+O₂=OH+H are accelerated effectively, leading to the enhanced HRR of the biogas-hydrogen flame, and the contributions of OH+H₂=H+H₂O and OH+H+M=H₂O+M on HRR are enhanced most evidently. As the CO₂ proportion in the biogas is increased, the global HRR of the biogas-hydrogen flame is reduced by the dilution/thermal effect and further aggravated by the chemical effect, and the contributions of H+CH₃(+M)=CH₄(+M) and OH+H₂=H+H₂O on HRR are enhanced. Furthermore, the O+CH₃ reactions are found to have the stable contributions on HRR of the biogas-hydrogen flame, and the O×CH₃ product is demonstrated to be a reliable indicator to reflect HRR variations in the premixed flame with the unclear equivalence ratio.
With respect to heat transfer characteristics of biogas-hydrogen flames, the total heat transfer rate of biogas-hydrogen impinging flame can be enhanced with the increased H₂. This improvement can be attributed to the enhanced flame temperature leading to the larger temperature difference and the stronger fuel diffusivity resulting in the larger heating area with high temperatures. With the increased CO₂ proportion in the biogas, its dilution and chemical effects lead to the worst heating performance of BG50-H₂ flames. Whereas, a suitable proportion of CO₂ in the fuel can reduce the decline of flame temperature in the wall jet region thanks to the larger specific heat capacity and the increased flame height, which leads to the better heating performance of the BG75-H₂ flame than that of the CH₄-H₂ flame. Thus the BG75-H₂ fuel could be a substitute of CH₄-H₂ fuel for the flame impingement heating. In addition, an empirical correlation, as a function of Pr, u/SL and φ, is obtained to determine the optimal heating distance of laminar premixed Bunsen flame quantitatively. For the pollutant emissions of biogas-hydrogen flames, the EICO is increased initially and then dropped as H is increased. The contributions of N₂O and thermal routes on the EINOx are enhanced with H, while that of prompt NO is decreased, and the contribution of NNH routes keeps relatively stable with H. The EINO₂ and the NO₂/NOx ratio achieve their peak values as the H is slightly less than the flame height. With the increased φ, EICO is enhanced evidently due to the incomplete fuel oxidization while contributions of different routes on the EINOx are changed, and the improved prompt NO leads to the higher EINOx, as well as its different profile with H, at the fuel-rich condition. The EINO₂ is increased with φ while the NO₂/NOx ratio is codetermined by concentrations of H and O₂ in the air mixing region and contribution variations of different routes on NO formation. With the increased Re, the EICO is dropped at small H but enhanced at large H while the EINOx is increased due to the improved prompt NO production. In addition, with the increased Re, the EINO₂ and the NO₂/NOx ratio are both decreased at small H but increased at large H. With the hydrogen addition, the EICO is enhanced steadily due to the H₂ oxidation competing for the OH radical. In addition, with the increased H₂, the EINOx is enhanced due to the enhanced flame temperature and the increased active radicals, and the contribution of prompt NO is decreased while that of other routes are increased steadily. With the increased H₂, EINO₂ is enhanced steadily thanks to the improved HO₂ production in the air mixing region, while the NO₂/NOx ratio is increased at small H but decreased at large H. As the CO₂ content is decreased in the biogas, the enhanced EICO at small H is caused by the better premixed combustion, while the declined EICO at large H is resulted from the larger post-flame region, intensive air entrainment and longer residence time. With the decreased CO₂ content, the EINOx is enhanced owing to increased active radicals and higher flame temperature, and the contributions of thermal NO and N₂O route are increased, while that of prompt NO and NNH route are dropped gradually. With the decreased CO₂ content, the EINO2 is increased steadily while the NO₂/NOx ratio is decreased gradually.
Rights: All rights reserved
Access: open access

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