Full metadata record
| DC Field | Value | Language |
|---|---|---|
| dc.contributor | Department of Mechanical Engineering | en_US |
| dc.contributor.advisor | Cheung, C. S (ME) ; Leung, C. W. (ME) | - |
| dc.creator | Zhou, Quan | - |
| dc.identifier.uri | https://theses.lib.polyu.edu.hk/handle/200/10477 | - |
| dc.language | English | en_US |
| dc.publisher | Hong Kong Polytechnic University | - |
| dc.rights | All rights reserved | en_US |
| dc.title | Fundamental investigation on laminar premixed combustion and flame dynamics of bio-syngas | en_US |
| dcterms.abstract | Biomass, as one of the important renewable energy resources for maintaining sustainable energy development and reducing air pollutions as well as greenhouse gas emissions, has attracted global attention in recent years. The synthesis gas obtained from the gasification of biomass, known as bio-syngas, has complex fuel composition due to the different feedstocks and processing techniques and thereby it is increasingly difficult to control the combustion process of bio-syngas in practical combustion systems. Many studies have been conducted on the laminar burning characteristics of single-component fuels such as H₂ and CH4, or binary fuels such as H₂-CO and H₂-CH4 mixtures, but the related investigation of bio-syngas, which is composing of H₂/CO/CH4/N2/CO2, is rarely conducted. Therefore, a comprehensive understanding of the laminar premixed combustion characteristics of bio-syngas is the most important issue at present. In this study, the laminar premixed combustion and flame dynamics of bio-syngas has been systematically studied using a constant volume combustion bomb at an initial temperature of 303 K, equivalence ratios of 0.6-1.5 and initial pressures of 0.1-0.5 MPa for a wide range of H₂/CO/CH4 fuel compositions and N2/CO2 dilution ratios (0%-45%). The research includes investigating the laminar flame speeds, analyzing the instability of the flame front propagation, and studying the explosion characteristics of the bio-syngas/air mixtures. Moreover, a one-dimensional freely propagating flame was simulated using the PREMIX code in the CHEMKIN package with the Li mechanism for analyzing the experimental results. The main conclusions and innovative achievements are as follows: For the laminar flame speeds of various bio-syngas/air mixtures, the experimental data and predicted results show good agreement with each other under various fuel compositions and dilution ratios at atmospheric pressure, while there exists discrepancy in the laminar flame speeds at elevated pressures between the experimental data and predicted results. The laminar flame speed decreases with the increase of initial pressure under the tested equivalence ratios which is mainly due to the increasing unburned mixture density and decreasing key radicals (H, OH, and O) concentrations. With the increase of H₂ fraction in the fuel, the laminar flame speed increases significantly, but the CH₄ enrichment flame has the lowest laminar flame speed. With the increase of CO fraction in the fuel, the laminar flame speed does not change much. The thermal and chemical kinetic analyses indicate that the CO addition has more effect on the adiabatic flame temperature but only plays a small role in the chemical effect compared to that of the H₂ addition. Moreover, the laminar flame speed decreases with the increase of N₂/CO₂ dilution ratio in the fuel mixture. CO₂ dilution has stronger dilution effect, thermal effect, and chemical effect than those of N₂ dilution, and thereby it can substantially decrease the laminar flame speeds of H₂/CO/CH₄/air mixtures. For the intrinsic instability of the flame front propagation at atmospheric pressure for various bio-syngas/air mixtures, the results of flame morphology and Markstein length show that, the flame front can remain fairly smooth at atmospheric pressure except for some large wrinkles caused by the ignition disturbance. The Markstein length is decreased with the increase of H₂ fraction in the fuel mixture indicating the decrease of stability of the flame front, while the Markstein length is slightly increased with CH₄ or CO addition suggesting the enhancement of the flame front stability. With N₂/CO₂ dilution, the Markstein length decreases at all dilution ratios, suggesting that the addition of N₂/CO₂ promotes the flame instability at atmospheric pressure. | en_US |
| dcterms.abstract | For the intrinsic instability of the flame front propagation at elevated pressures for various bio-syngas/air mixtures, the results clearly show that with the increase of initial pressure, irregular wrinkles appeared at the flame surface and the higher the initial pressure, the more advance the onset of cellular instability of the flame is observed. The cellular instability of the flame front is significantly promoted with the increase of H₂ fraction in the fuel mixture and the earlier onset of cellular flame structure is also observed, while the cellular instability is suppressed and the moment for onset of cellular flame structure is postponed with the increase of CO fraction in the fuel mixture. With increasing CH₄ concentration in the fuel mixture, the cellular instability is found to be significantly inhibited and the moment for onset of cellular flame structure is also significantly delayed. On the other hand, the suppression effect of cellular instability with N₂/CO₂ dilution is observed and CO₂ dilution has stronger effect in suppressing the cellular instability at elevated pressures. For the explosion characteristics of various bio-syngas/air mixtures, the results show that the concentration of H₂ or CH₄ in the fuel mixture has a great influence on the explosion characteristics of bio-syngas/air mixtures, while the concentration of CO in the fuel mixture plays a mild role in affecting the explosion behaviors. Moreover, it is observed that the addition of N₂/CO₂ to the fuel mixture can significantly reduce the potential of explosion hazards of bio-syngas/air mixtures, and CO₂ dilution has stronger suppression effect on the explosion characteristics due to its larger negative influence on the thermal effect and the chemical effect in comparison to that of N₂ dilution. The corresponding correlations of Pmax/P0=f(Zdilution), tc =f(Zdilution), and (dP/dt)max =f(Zdilution) are developed for predicting the explosion behaviors of N₂/CO₂ diluted bio-syngas/air mixtures at various dilution ratios. In conclusion, since the diverse biomass feedstock and processing techniques lead to considerable variations in the fuel composition of bio-syngas, its fundamental combustion characteristics should be well understood because the components of a bio-syngas and their proportions have significant influences on the laminar flame speed, flame instabilities and explosion characteristics of the bio-syngas. It can be concluded that H₂-enriched condition has much higher laminar flame speed than other conditions, but the intrinsic instabilities of flame and explosion hazard are significantly prompted. For the CH₄-enriched condition, the behaviors are quite the contrary to that of the H₂-enriched condition. CO addition is found to play a mild role in affecting the laminar burning and explosion characteristics of bio-syngas. On the other hand, it can be concluded that N₂/CO₂ diluted bio-syngas has much lower laminar flame speed and suppressed intrinsic flame instabilities as well as explosion hazard than that of non-diluted bio-syngas. Therefore, the different combustion behaviors of diverse bio-syngas fuels need to be fully considered for optimizing the combustion of bio-syngas in practical combustors. | en_US |
| dcterms.extent | xix, 222 pages : color illustrations | en_US |
| dcterms.isPartOf | PolyU Electronic Theses | en_US |
| dcterms.issued | 2020 | en_US |
| dcterms.educationalLevel | Ph.D. | en_US |
| dcterms.educationalLevel | All Doctorate | en_US |
| dcterms.LCSH | Biomass energy | en_US |
| dcterms.LCSH | Synthesis gas -- Combustion | en_US |
| dcterms.LCSH | Hong Kong Polytechnic University -- Dissertations | en_US |
| dcterms.accessRights | open access | en_US |
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|---|---|---|---|---|
| 991022385359503411.pdf | For All Users | 4.81 MB | Adobe PDF | View/Open |
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