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Heat-transfer enhancement for corn straw slurry from biogas plants by twisted hexagonal tubes
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 211816, PR China.
State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 211816, PR China.
State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 211816, PR China.
State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 211816, PR China.
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2020 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 262, article id 114554Article in journal (Refereed) Published
Abstract [en]

Heat-transfer geometries that enhance heat transfer performance for slurries increase the net raw biogas production in the bio-methane process. In this study, the precise temperature-dependent rheologies of corn straw slurry with 6 and 8% total solid were determined, collected, and modeled to conduct a numerical simulation via CFD, the first instance of such research. Subsequently, the reliability of the numerical results was verified with heat-transfer experiments. The heat-transfer performances of the circular, twisted square and twisted hexagonal tubes were estimated numerically, ultimately showing that the twisted hexagonal tube performed optimally with an enhancement factor of up to 2.0 in the turbulent region, compared to the circular tube. Based on the numerical results, the mechanism of heat-transfer enhancement was revealed, showing balanced radial mixing and the near-wall shear effect that leads to a strong and continuous shear rate under a considerable radial-flow intensity. An engineering equation was obtained for the performance evaluation, and the waste-heat recovery from corn straw slurry was analyzed, showing the twisted hexagonal tube can increase the net raw biogas production by up to 17.0% compared to the circular tube.

Place, publisher, year, edition, pages
Elsevier, 2020. Vol. 262, article id 114554
Keywords [en]
Corn straw slurry, Twisted hexagonal tube, CFD, Shear rate, Waste-heat recovery, Biogas production
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-78261DOI: 10.1016/j.apenergy.2020.114554ISI: 000517398200055Scopus ID: 2-s2.0-85078772361OAI: oai:DiVA.org:ltu-78261DiVA, id: diva2:1420851
Note

Validerad;2020;Nivå 2;2020-04-01 (alebob)

Available from: 2020-04-01 Created: 2020-04-01 Last updated: 2022-02-04Bibliographically approved
In thesis
1. Heat-transfer Enhancement for Slurries from Biogas Plants− Properties, processes, and thermal systems
Open this publication in new window or tab >>Heat-transfer Enhancement for Slurries from Biogas Plants− Properties, processes, and thermal systems
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

 Biomethane production from renewable residues with anaerobic digestion gains increasing attention as a crucial alternative to petroleum fuels. It has been vigorously developed, but the large amounts of subsidy from the government indicate that the process efficiency needs to be further improved. For biomethane production, on the one hand, a great amount of heat needs to be used for heating the feeding slurry, sanitation of slurry, and maintaining the temperature in the large-scale reactors. On the other hand, a large amount of thermophilic effluent slurries brings a huge amount of waste heat, which can be recovered. This makes it important to study how to increase production by improving the thermal efficiency of biogas plants with novel heat exchangers. 

The working fluids in the biogas plants are the non-Newtonian and high-viscous slurries, and the conventional heat exchangers in biogas plants always show much lower performance compared to those in other industries. Normally, the slurries in the biogas plant consist of different substrates, including straw, manure, food waste, municipal sludge, and their mixtures, and various factors such as the amount and type of solids, particle size, shear rate, and temperature impact the rheological properties of the slurries, which makes the complexity in the rheological properties and the difficulty in developing novel heat exchangers.

The development of heat exchangers calls for the rheological properties of slurries. However, to the best of our knowledge, only the rheology of manure slurry was systematically determined and modeled considering the effect of temperature. The lack of the rheological properties of slurries further hinders the design and development of novel geometries to enhance the heat transfer of the slurries. Correspondingly, the quantitative contribution and potential of the waste-heat recovery from the slurries to production using the enhanced geometry remain unclear. 

    In this thesis work, to design novel geometry with heat-transfer enhancement for different slurries and determine its potential in thermal cycles in the full-scale biogas plants, firstly, the temperature-dependent rheological properties of the slurries, including the corn straw, food waste, and mixed slurries, were tested and modeled. It was found that these slurries possess strong shear-thinning behavior, the temperature has a significant impact on their dynamic viscosity, and the power-law model combined with the Arrhenius equation can describe the rheology well.

    Subsequently, with the reliable models of the rheological properties as the key input,  Computational Fluid Dynamics simulations were conducted to screen different twisted geometries, determine the heat-transfer performance, and reveal the mechanism of the heat-transfer enhancement. Lab- and pilot-scale experiments were also conducted to validate the numerical results. The twisted hexagonal tubes show a positive enhancement factor up to 2.6 compared to normal heat exchangers in a wide range of operating conditions. The continuous and strong near-wall shear effect is the intrinsic reason for achieving a significant heat-transfer enhancement in the twisted hexagonal tubes. Moreover, the generalized engineering equations for predicting the effective shear rate and heat-transfer performance with measurable parameters were established and verified with both numerical and experimental results.

    Finally, the twisted-hexagonal-tube heat exchange was integrated with complete thermal cycles, including waste-heat recovery and external heating processes in the biogas plant, and the potential of increased production and profits were modeled and analyzed combined with the practical operating conditions in a full-scale biogas plant. It was found that for the waste-heat recovery using the twisted hexagonal tubes, the net raw biogas production can increase by up to 17.0 %, and for the external heating process, the increased profit equivalent to 39 % of total production can be achieved owing to energy conservation in external heating using the twisted-hexagonal-tube heat exchangers for a full-scale biogas plant. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022. p. 67
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
heat-transfer enhancement, slurry, rheological properties, computational fluid dynamics, twisted tubes, biogas production
National Category
Energy Systems Bioprocess Technology Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-89115 (URN)978-91-8048-026-0 (ISBN)978-91-8048-027-7 (ISBN)
Public defence
2022-03-31, Online (via Zoom) and L-E616A Green room, Lulea University of Technology, Lulea, 10:00 (English)
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Supervisors
Available from: 2022-02-07 Created: 2022-02-04 Last updated: 2025-02-17Bibliographically approved

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Chen, JingjingJi, Xiaoyan

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