Super-hydrophobic coatings have great application potential in the fields of surface self-cleaning, fluid drag reduction, anti-fog and anti-icing, and microfluidic control. The controls of morphologies for the super-hydrophobic coating inside a circular tube have not been investigated. In this paper, the polydopamine (PDA) was coated on the inner wall of stainless-steel cylinder by electrochemical deposition under different values of shear stress, and n-dodecyl mercaptan (NDM) was used to modify the PDA surface. This PDA/NDM coating shows super-hydrophobic behavior. Scanning electron microscopy (SEM), contact angle tester (CA), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction tester (XRD) were used to analyze the characterization of the coating. The results show that PDA deposition can be divided into two stages under the effect of shear stress. The first stage is the agglomeration of PDA particles on the stainless steel substrate. The second stage is PDA in-situ growth based on PDA particles aggregate, and the growth process is controlled by shear stress. When the shear stress is 1.85 mPa, the coating surface shows coral shaped balls with about 15—24 μm. When the shear stress is 7.41 mPa, the coating surface has a flaky structure with a particle size of about 1—4 μm. The wetting angles of the prepared PDA/NDM coating are greater than 150°, belongs to super-hydrophobic properties. And the coating has good chemical stability and heat resistance wear resistance and corrosion resistance. The work has certain guiding for the regulation and control of the surface nano/microstructure during the preparation of the inner surface coating of the pipe.

The low mass transfer rate in porous materials hinders the use of adsorbed natural gas as vehicle fuel. Fundamentally, the mass transfer rate depends on the structures of the adsorbents and the operating conditions. Therefore, in this study, the effects of adsorbent (activated carbons) structure and operating conditions on the mass transfer rate of methane (main component of natural gas) were investigated quantitatively, providing a theoretical basis for the synthesis of efficient adsorbent materials. By performing Monte Carlo and molecular dynamics simulations and utilizing a non-equilibrium thermodynamics linearization transfer model, the mass transfer behavior of methane in porous carbon materials was quantitatively evaluated, specifically focusing on the material structure, operating conditions, and feasibility of using natural gas as vehicle fuel. The proposed linear non-equilibrium thermodynamics mass transfer model is applicable to interfacial gas species and provides a valuable tool for gas separation.

Agitation power consumption (P) in the anaerobic digestion of biogas plants is a major consumer of electric energy. To reduce P by adjusting the rheological properties, in this work, the rheological properties of the corn-straw slurry were studied systematically considering the effects of TS, temperature and particle-size, and P was calculated based on the rheological behavior of the corn-straw slurry. The investigation shows that the corn-straw slurry is a non-Newtonian fluid and exhibit shear-thinning behavior, and the rheological properties can be well described with the power law model. The size-reduction is more effective compared to the option of temperature-increase to improve the agitation power efficiency, and the value of P can be reduced by up to 48.11 %. Since the size-reduction can also increase the methane yield, the reduction of the particle-size is a promising option to save P, especially at relatively high TSs and for the thermophilic AD process.

With the increasing demand for environmental protection and renewable energy, bioenergy technology has been attracting considerable attention. Anaerobic digestion (AD) is the process to convert the low-grade biomass into bioenergy, in which both heat-recovery and -recycling should be treated carefully in order to improve the process efficiency. In this work, the heat-recovery and its utilization processes were reviewed, and different types of heat exchangers as well as their advantages in biogas engineering were surveyed. It shows that the recovery and utilization of the waste heat from biogas plants with an internal system, such as slurry effluent unit, the combined heat and power unit, the sanitation unit, and the internal recycle unit, are important for improving the AD efficiency of biogas production. For example, the recovery and recycling of waste heat from the effluent can result in a 2-3 °C temperature increase for the inlet manure slurry. For thermophilic AD, the heat recovery from effluent can save about 50% of the total heat requirement. The external heating process is more suitable for large- and medium-scale biogas plants, and the heat transfer coefficient of external heating (850-1000 W/m2 K-1) is almost two-times higher than that of the internal heating (300-400 W/m2 K-1). To utilize the waste heat in biogas plants, heat exchangers have been designed for biogas slurry. However, further improvement on the heat exchangers with anti-blockage, anti-fouling, high efficiency, and low investment is still needed. Moreover, the heat exchanger suitable for a low-temperature-difference system is specially needed in China, but the development is still in its infancy. Therefore, to tailor to the Chinese national conditions, special external heating processes should be designed and reoriented to the diversity of biomass, the climatic environmental conditions, and the renewable Chinese policies

Waste-heat recovery from discharged slurries can improve the net raw biogas production in the bio-methane process in order to meet the demand for a next-generation of anaerobic digestion. In this study, a numerical model of a scraped-surface heat exchanger was proposed with the consideration of the complete and precise rheological behaviour of the slurry of animal manure for the first time for achieving highly efficient waste-heat recovery. The rheological model results were verified with new experimental data measured in this work. Subsequently, the convective heat-transfer coefficient of the scraped-surface heat exchanger was calculated numerically with the proposed numerical model, and the performance was determined. Then, the contributions of waste-heat recovery from the slurry to the biogas production using a general shell-and-tube heat exchanger and the scraped-surface heat exchanger were calculated quantitatively and compared. For the case of scraped-surface heat exchanger, the increase of net raw biogas production can be up to 8.53%, which indicates that there is a great potential to increase the net raw biogas production in the bio-methane process using a scraped-surface heat exchanger with low-cost equipment and a compactible structure.

Heat-transfer enhancement in manure slurry is crucial for increasing the efficiency and production of biogas during anaerobic digestion in biogas plants. In this study, a novel heat exchanger with an optimal twisted geometry was developed based on the numerical screening of the twisted tubes with equilateral polygons, and experiments were conducted to validate the numerical results. It was observed that the SST k-ω model is more efficient than other turbulence models in representing the heat transfer performance of the twisted tubes, and the numerical model with a thermostatic wall can be used to reliably screen the twisted geometries. The twisted hexagonal tube has the optimal geometry, with enhancement capability of up to 1.4 times compared to that of the circular tube. The formation of high continuity regions with relatively strong heat-transfer rates along the heat-exchange wall is the main reason for the high performance during heat transfer. The external heating process was integrated in a full-scale biogas plant, and the model and algorithm were developed and validated with additional experiments to describe the overall performance of both conventional and screened optimal geometries under different conditions. It was observed that a profit equivalent to 39% of total production for a large-scale biogas plant can be achieved owing to energy conservation in external heating with the twisted hexagonal tubes.

Waste-heat recovery from discharged slurries can improve the net raw biogas production in bio-methane process in order to meet the demand of a new generation of anaerobic digestion. In order to achieve a high efficient waste-heat recovery, in this work, a mathematical model of waste-heat recovery process with surface scraped heat exchanger (SSHE) was proposed with the consideration of the shear rate and temperature-dependent rheological behaviour. The convective heat transfer performance of SSHE was calculated numerically where slurry was considered. The contribution of waste heat recovery from the slurry to biogas production by SSHE and general shell-and-tube heat exchanger (STHE) were firstly calculated quantitatively, and the increase of net raw biogas production could be over 13.5% by SSHE with need of heat exchange area less than a quarter of STHE's, which showed a great potential to increase the net raw biogas production in bio-methane process with low equipment investments and more compactible structure.

Biogas is one of the most crucial renewable energy and achieving high-efficient heat exchangers is the key to improve its production. In this study, the effect of pulsating flow on heat transfer in a twisted hexagonal tube with manure slurry was investigated for the first time by using computational fluid dynamics CFD. The performances of pulsating flows were simulated under different conditions, including the inlet velocity, frequency, and amplitude of pulsating flow in the twisted hexagonal tube with different torques. Pressure drops at different frequencies were further investigated. Moreover, the mechanism of heat-transfer enhancement was revealed with the evolution of the heat-transfer coefficient over time. It was found the pulsating flow achieves an 18.9% enhancement at low torque.

Heat transfer geometries with enhanced performance for the slurries with high viscosity can improve the net raw biogas production in bio-methane process. In this study, the rheological properties of different slurries were tested, correlated and implemented to computational fluid dynamics (CFD). CFD was then used to screen a new geometry based on the twisted tube combined with mechanism study, and experimental testing was conducted for verification. It shows that the twisted hexagonal tube (THT) has the highest performance. The mechanism for enhancing the heat transfer with THT was mainly due to the effective shear rate. Furthermore, the waste-heat recovery with the THT heat exchanger in biogas production was estimated quantitatively and compared with the normal heat exchanger and scraped-surface heat exchanger (SSHE). Compared to the normal heat exchanger, for THT, the increase of net raw biogas production δ_NRBP can be up to 17%, while it is only up to 8.53% for SSHE. Besides, the external heating up processes with THT and normal heat exchanger were studied to estimate the heating time for different temperature fluctuations and power requirements of boiler. It is found that the process with THT can save 25-38% heating time for the anaerobic reactor compared to the normal heat exchanger. Therefore, designed THT heat exchanger is promising, and the developed methods can also be beneficial for studying other heat transfer processes.

Waste-heat recovery from discharged slurries is important to improve the biogas production efficiency but still remains challenge duo to the special properties of slurries in anaerobic digestion process. In this work, numerical study was carried out to investigate the flow field, and heat transfer performance of slurry from biogas plant in the twisted hexagonal and other twisted tubes was simulated with computer fluid dynamic (CFD) for the first time. The numerical method was validated with experimental data from the literature. The heat transfer performance and flow resistance of twisted hexagon tube were calculated and compared with other types of twisted tubes. The enhancement factor of the twisted hexagonal tube reached to 2 and kept optimum at turbulence flow region compared to the twisted tubes with square and elliptical cross section. Meanwhile, the mechanism of heat transfer enhancement with different twisted tubes was further studied, and the optimal field synergy and minimum local circulation flow near the wall are the main reasons for the high performance and low flow resistance of the twisted hexagonal tube.

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.