Yudong Ding

Synthesizing MgO with a high specific surface for carbon dioxide adsorption

In this work, highly porous MgO was synthesized for CO2 adsorption by a simple and economic thermal decomposition of basic magnesium carbonate and magnesium oxalate. Characterizations of the synthesized samples were accomplished using scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and nitrogen adsorption–desorption isotherms. The results showed that the synthesized MgO possessed a high BET surface area in a range of 161–252 m2 g−1 and a highly porous structure. Thermo gravimetric analysis revealed that the synthesized MgO not only showed good selectivity to CO2 but also yielded a CO2 adsorption capacity of as high as 7.59 wt%. Besides, in situ FTIR spectroscopy and CO2 TPD curves demonstrated that the adsorption mechanism of synthesized MgO was mainly attributable to chemisorption and it could be regenerated at relatively low temperature. This work provides a new way to synthesize MgO with a highly porous structure for CO2 adsorption.

Experimental and theoretical study on dissolution of a single mixed gas bubble in a microalgae suspension

Aiming at technology for biofixation of carbon dioxide by microalgae in photobioreactors, a basic phenomenon, that is, the dissolution and consumption of a mixed gas bubble consisting of CO2 and air in a microalgae suspension, was investigated by visualization experiments using the promoted bubble grafting method. Furthermore, a theoretical model based on non-equilibrium theory at the gas–liquid interface was also proposed to predict the CO2 dissolution and fixation characteristics of bubbles in a microalgae suspension. The effects of the initial CO2 volume fraction, initial bubble size and microalgae concentration were discussed respectively. It was found that the bubble radius gradually decreased with time and trended towards a constant thereafter. The dimensionless Biot number in the promoted dissolution model was determined as 0.65 for the microalgae suspension. The bubble with a larger initial CO2 volume fraction experienced faster shrinkage and had a higher dissolution rate and CO2 fixation efficiency, while slight photosynthesis inhibition emerged at the beginning of dissolution when the initial CO2 volume fraction in the bubble was larger than 15%. A smaller initial bubble size resulted in a lower dissolution rate but greater CO2 fixation efficiency by photosynthesis. Higher microalgae concentration facilitated bubble dissolution and CO2 fixation especially when OD680 nm of the microalgae suspension was less than 1.0. These findings can be a guide to the design of a photobioreactor and aerator.

Optimizing the gas distributor based on CO2 bubble dynamic behaviors to improve microalgal biomass production in an air-lift photo-bioreactor

Dynamic behavior of bubbles would significantly affect CO2 mass transfer and may cause microalgae cells uneven distribution due to the bubble carrying effect. To improve microalgae growth, the gas distributor and aeration conditions was optimized according to the bubble rising behavior. The CO2 bubble rising trajectory is similar to a Zigzag. The amplitude and wavelength of the Zigzag, which reflected the influenced zone of microalgae suspension in horizontal direction and disturbance intensity on culture, respectively, was controlled by the structure of gas distributor and aeration conditions. An optimized round gas distributor that full of holes with an inner diameter of 0.5mm and spacing of 1.5mm was designed. When cultivated with the optimized gas distributor aerating 5% CO2 gas at 0.250vvm, the maximum biomass concentration of Chlorella pyrenoidosa achieved 2.88gL-1, increased by 83.44% compared to that of 1.57gL-1cultivated with the commercial micro-bubbles aerator.

Influence of the precursor on the porous structure and CO2 adsorption characteristics of MgO

MgO is a promising candidate for CO2 capture. In general, MgO is prepared from different precursors, which have an important effect on its CO2 adsorption performance. Meanwhile, the effect of precursor on the porous structure and CO2 adsorption of MgO remains largely unknown. In this work, porous MgO was prepared from different precursors to investigate the effect of precursor source. Experimental results showed that the morphology of MgO was greatly influenced by that of the precursor and was similar to that of its precursor. Due to the emission of by-products during decomposition, MgO prepared from a precursor with a high molecular weight per single Mg atom exhibits a highly porous structure, large pore volume and pore size. The highly porous structure is favorable for the CO2 adsorption of MgO. The pores formed on the decomposition of precursor play an important role in CO2 adsorption performance, which is affected by the molecular weight per single Mg atom. Narrow pores of MgO from precursors with low molecular weights hinder CO2 diffusion and lead to a low CO2 capacity at fixed adsorption time. The results indicate that the adsorption performance of MgO could be enhanced through the regulation of precursor source and its morphology. This work provides a useful guidance for the synthesis of porous materials with high performance in CO2 capture.

Dynamic behaviour of the CO2 bubble in a bubble column bioreactor for microalgal cultivation

Carbon dioxide (CO2) gas is a major carbon source for microalgal cultivation. It is usually sparged into photobioreactors in the form of bubbles. The behaviour of the bubbles significantly affects mass transfer, distribution and microalgae growth. In this study, the dynamic behaviour of the CO2 bubbles was compared between a microalgal suspension and pure water. These investigations were carried out via visual methods. The movement and distribution of microalgae at the gas–liquid interface were observed. The effects of gas flow velocity, CO2 concentration and capillary orifice size were analysed. The results indicated that much of the microalgal cells adsorbed onto the surface of the CO2 bubbles in the microalgal suspension, when compared with that in pure water. This resulted in an easier detachment of the bubbles in the microalgal suspension. The growth status of the bubbles were divided into two states according to changes in the Eötvös number and the behaviour of the CO2 bubbles as influenced by gas flow velocity: steady and unsteady state. The critical gas velocity between the two states was achieved. The CO2 bubble rising trajectory can be divided into three main phases: the vertical acceleration phase, the transition phase, and the oscillatory rising phase. During the oscillatory rising phase, the amplitude of the bubble rising trajectory was approximately two times greater than the bubble diameter. In addition, the wavelength of the bubble rising trajectory was approximately 16–18 times the bubble diameter in the microalgal suspension. A smaller capillary orifice size and larger CO2 concentration led to a decrease in the bubble detachment diameter, an increase in velocity and an enlargement in the zone of bubble influence in the horizontal direction. These are advantageous for CO2 transportation. These findings are beneficial for optimizing the design and operation of microalgal photobioreactors.

Photoautotrophic Microalgal Cultivation and Conversion

As the largest photoautotrophic microorganisms communities, microalgae are important materials for biological carbon dioxide fixation through photosynthesis and biodiesel production due to their fast growth rate, significant carbon dioxide capture capacity, high lipid content, etc. They are also regarded as the promising feedstocks for the third-generation biofuels. For the cultivation of microalgae, photobioreactors (PBRs) are necessary apparatuses that provide appropriate growth conditions for the proliferation of microalgal cells. Nevertheless, the design of an efficient PBR is associated with many thermodynamic challenges, such as light, CO2, and inorganic nutrients transfer in microalgal culture, all of which have a significant impact on the performance of the PBR. Up to now, various attempts have been devoted to optimize the light distribution, CO2, and nutrients transfer within microalgal cultures that employ light-guiding materials, hollow fiber membranes, anion exchange membranes, etc. In this chapter, we provided an overview and generated a comprehensive comparison on PBR performance enhancement methods from the perspectives of light and mass transfer, as well as potential approaches for the concentrating and conversion of photoautotrophic microalgal cells to biofuels.

Numerical study of water droplets impacting on cylindrical heat transfer pipes

ABSTRACT Poor performance in the condensers in power plants and chemical plants is due to the fact that condensed water is deposited on the heat transfer pipes. The dynamics of condensed water droplets forming on the surface of heat transfer pipes have a significant effect on the heat transfer efficiency of heat exchangers. In the present study, a numerical approach using a coupled level-set and volume of fluid (CLSVOF) method was adopted to investigate the impact of water droplets on cylindrical pipes. The numerical model was verified by an experiment, and both sets of results showed qualitative and quantitative agreements. The effects of the surface wettability, impact velocity and relative size of the droplet to the pipe on the droplet impact dynamics are systematically investigated. Moreover, the regularities of the contact area between the liquid and the pipe during the impacting process as well as the volume of residual liquid remaining on the pipe post-impact are also analyzed; these two parameters are the key factors which affect the heat transfer efficiency of heat transfer pipes, and they cannot be acquired very accurately using experiments.

Pore Network Modeling of Oxygen Diffusion in Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells

In the present study, a three dimensional pore network, consisting of spherical pores and cylindrical throats, is developed to simulate the oxygen diffusion and liquid water permeation in gas diffusion layer (GDL) in low-temperature fuel cell. Oxygen transport in the throats is described by Fick’s law and liquid water permeation in the network is simulated using percolation invasion algorithm. The effects of heterogeneity of GDL, connectivity of pores, and liquid water saturation on oxygen effective diffusivity are investigated respectively. The simulation results show that the GDL structure has a significant influence on the oxygen and water transport in the GDL. The oxygen effective diffusivity increases with increasing pore connectivity and decreasing heterogeneity. The shielding effect of large throats by smaller ones enhances with increasing heterogeneity of the network. Furthermore, the oxygen transportation is blocked in the presence of liquid water permeation. Thus the oxygen effective diffusivity drops significantly with increasing water saturation.Copyright