Dennis Y.C. Leung

Hydrogen production over titania-based photocatalysts

Because of their relatively high efficiency, high photostability, abundance, low cost, and nontoxic qualities, titania-based photocatalysts are still the most extensively studied materials for the photocatalytic production of hydrogen from water. The effects of the chemical and physical properties of titania, including crystal phase, crystallinity, particle size, and surface area, on its photoactivity towards hydrogen generation have been identified by various investigations. The high overpotential for hydrogen generation, rapid recombination of photogenerated electrons and holes, rapid reverse reaction of molecular hydrogen and oxygen, and inability to absorb visible light are considered the most important factors that restrict the photoactivity of titania, and strategies to overcome these barriers have been developed. These issues and strategies are carefully reviewed and summarized in this Minireview. We aim to provide a critical, up-to-date overview of the development of titania-based photocatalysts for hydrogen production, as well as a comprehensive background source and guide for future research.

A review on the energy production, consumption, and prospect of renewable energy in China

China is the second largest energy consumer in the world. This paper reviews the production and consumption of traditional and renewable energy in China over the past three decades. It also presents an overview on the research and development of renewable energy, such as solar, biomass, geothermal, ocean and wind energy in China. Study indicated that the usage of renewable energy in China shows a promising prospect in the near future, of which biomass is found to be one of the most promising renewable energy resources that have great potential for development in China.

Low temperature catalytic oxidation of volatile organic compounds: a review

Volatile organic compounds (VOCs) are toxic and recognized as one of the major contributors to air pollution. The development of efficient processes to reduce their emissions is highly required. Complete catalytic oxidation is a promising way to convert VOCs, especially with low concentration, into harmless CO2 and water. This reaction is highly desirable to proceed at low temperature for the consideration of safety, energy savings, low cost and environmental friendliness. Great efforts have been devoted to develop efficient catalysts in order to reduce the temperature of catalytic oxidation of VOCs. The present review highlights recent important progress in the development of supported noble metal and metal oxide catalysts in this field. We examined several typical metals that are widely adopted as essential components for catalytic oxidation of VOCs and explored the effect of some important influencing factors such as the properties of metal and support, dispersion, particle size and morphology of metals. The specific mechanism that leads to superior catalytic activity towards low temperature VOC oxidation was discussed too.

Kinetic study of scrap tyre pyrolysis and combustion

Abstract This paper investigates the kinetic of pyrolysis and combustion of scrap tyre using thermogravimetric and derivative thermogravimetric analysis method. Three materials, namely tyre rubber powder, tyre fiber and wood powder were studied and compared with each other. The process parameters show that these three materials exhibit different thermal degradation patterns during pyrolysis and combustion process. Thermal degradation models were proposed to derive the kinetic parameters. It was found that the process and kinetic parameters vary with heating rates but are less dependent on the powder sizes. The simulations by the proposed models agreed well with experimental data.

Pyrolysis of tire powder: influence of operation variables on the composition and yields of gaseous product

The pyrolysis of tire powder was studied experimentally using a specially designed pyrolyzer with high heating rates. The composition and yield of the derived gases and distribution of the pyrolyzed product were determined at temperatures between 500 and 1000 °C under different gas phase residence times. It is found that the gas yield goes up while the char and tar yield decrease with increasing temperature. The gaseous product mainly consists of H2, CO, CO2, H2S and hydrocarbons such as CH4, C2H4, C2H6, C3H6, C3H8, C4H8 and C4H6 with a little other hydrocarbon gases. Its heating value is in the range of 20 to 37 MJ/Nm3. Maximum heating value is achieved at a temperature between 700 and 800 °C. The product distribution ratio of gas, tar and char is about 21:44:35 at 800 °C. The gas yield increases with increasing gas residence time when temperature of the residence zone is higher than 700 °C. The gas heating value shows the opposite trend when the temperature is higher than 800 °C. Calcined dolomite and limestone were used to explore their effect on pyrolyzed product distribution and composition of the gaseous product. It is found that both of them affect the product distribution, but the effect on tar cracking is not obvious when the temperature is lower than 900 °C. It is also found that H2S can be absorbed effectively by using either of them. About 57% sulfur is retained in the char and 6% in the gas phase. The results indicated that high-energy recovery could not be achieved if fuel gas is the only target product. In view of this, multi-use of the pyrolyzed product is highly recommended.

Effect of reduction treatment on structural properties of TiO2 supported Pt nanoparticles and their catalytic activity for formaldehyde oxidation

A series of highly active Pt/TiO2 catalysts were prepared by impregnation and deposition precipitation methods with different reduction processes. Their catalytic activities were evaluated by catalytic decomposition of formaldehyde (HCHO) at room temperature. The effects of reduction treatment on structural properties and catalytic activity were studied. Reduced Pt/TiO2 catalysts showed large differences in structural properties (such as particle size, oxidation state, surface content and electronic property of Pt nanoparticles, and surface oxygen) and catalytic activity for HCHO oxidation compared with the unreduced ones. Nearly 100% HCHO conversion was achieved on the former. Especially, sodium borohydride reduced Pt/TiO2 catalysts even with 0.1% Pt loading showed nearly complete oxidation of HCHO. Well-dispersed and negatively charged metallic Pt nanoparticles, and rich chemisorbed oxygen are probably responsible for their high catalytic activities.

Large-Eddy Simulation of Flow and Pollutant Transport in Street Canyons of Different Building-Height-to-Street-Width Ratios

This study employs a large-eddy simulation technique to investigate the flow, turbulence structure, and pollutant transport in street canyons of building-height-to-street-width (aspect) ratios of 0.5, 1.0, and 2.0 at a Reynolds number of 12 000 and a Schmidt number of 0.72. When the approaching wind is perpendicular to the street axis, a single primary recirculation is calculated for the street canyons of aspect ratios 0.5 and 1.0, and two vertically aligned, counterrotating primary recirculations are found for the street canyon of aspect ratio 2.0. Two to three secondary recirculations are also calculated at the corners of the street canyons. A ground-level passive pollutant line source is used to simulate vehicular emission. The turbulence intensities, pollutant concentration variance, and pollutant fluxes are analyzed to show that the pollutant removal by turbulent transport occurs at the leeward roof level for all aspect ratios. Whereas the ground-level pollutant concentration is greatest at the leeward corner of the street canyons of aspect ratios 0.5 and 1.0, the ground-level pollutant concentration in a street canyon of aspect ratio 2.0 occurs at the windward corner and is greater than the peak concentrations of the other two cases. Because of the smaller ground-level wind speed and the domination of turbulent pollutant transport between the vertically aligned recirculations, the ground-level air quality is poor in street canyons of large aspect ratios. The retention of pollutant in the street canyons is calculated to be 95%, 97%, and 99% for aspect ratios of 0.5, 1.0, and 2.0, respectively.

Physical Modeling of Flow Field inside Urban Street Canyons

Abstract The flow characteristics inside urban street canyons were studied in a laboratory water channel. The approaching flow direction was horizontal and perpendicular to the street axis. The street width was adjusted to form street canyons of aspect ratios 0.5, 1.0, and 2.0. The velocity field and turbulent intensity were measured with a laser Doppler anemometer at various locations within the street canyons, which were used to elucidate the flow pattern inside the street canyons. It was found that the previous numerical modeling results are in good agreement with the current experimental results at most locations. For the street canyon of aspect ratio 0.5, which belongs to the wake interference flow regime, the mean and fluctuating velocity components were more difficult to measure as compared with the other two cases because of its more complicated flow pattern. Some guidelines for numerical modeling were developed based on the measurement results. The data presented in this paper can also be used as...

Pollutant dispersion in urban street canopies

Abstract The effects of building configurations on pollutant dispersion around street canopies were studied numerically. The dispersion of pollutants emitted from ground sources was simulated by continuously discharging large number of particles into the computation domain. The mean wind velocities at each time-step were firstly computed by solving the time-dependent incompressible Navier–Stokes equations, while the fluctuated velocities were determined using a statistical procedure. The trajectories of the discharged particles were obtained from a Lagrangian particle model. Three categories of numerical simulation were conducted to study the effect of different canopy geometries on the pollutant dispersion. The computed wind field data were consistent with the wind field characteristics described in the previous wind tunnel studies. A counter-clockwise vortex was found resulting in high pollutant concentration at the windward side of the downstream building of the street canopy and low pollutant concentration at the leeward side of the upstream building. The increase in height of the urban roughness buildings would facilitate the pollutant dispersion in urban street canopy under certain building configurations. Two or more vortices stacked vertically in a street canopy were found when height of the upstream and downstream buildings of a street canopy was increased, preventing pollutants from escaping out of the canopy.

Theoretical study of heat transfer with moving phase-change interface in thawing of frozen food

A theoretical solution was obtained for a transient phase-change heat transfer problem in thawing of frozen food. In the physical model, a sphere originally at a uniform temperature below the phase-change temperature is suddenly immersed in a fluid at a temperature above the phase-change temperature. As the body temperature increases, the phase-change interface will be first formed on the surface. Subsequently, the interface will absorb the latent heat and move towards the centre until the whole body undergoes complete phase change. In the mathematical formulation, the nonhomogeneous problem arises from the moving phase-change interface. The solution in terms of the time-dependent temperature field was obtained by use of Greens function. A one-step Newton–Raphson method was specially designed to solve for the position of the moving interface to satisfy the interface condition. The theoretical results were compared with numerical results generated by a finite difference model and experimental measurements collected from a cold water thawing process. As a good agreement was found, the theoretical solution developed in this study was verified numerically and experimentally. Besides thawing of frozen food, there are many other practical applications of the theoretical solution, such as food freezing, soil freezing/thawing, metal casting and bath quenching heat treatment, among others.