• Energy Research

Abstract Globally, low rank coals are responsible for about half of the world's total coal deposits. However, these low rank coals present a high moisture content, which significantly impacts their utilization processes, including lower power plant efficiency, increased transportation costs, higher CO 2 emission, and spontaneous combustion during storage. In order to decrease the energy consumption of low rank coal during the utilization processes, drying and dewatering technologies must be well designed. This review presents recent development in drying and dewatering technologies for low rank coals. Evaporative drying technologies, such as rotary-drying, fluidized-bed drying, hot oil immersion drying, hot oil-immersion drying, as well as non-evaporative drying technologies such as hydrothermal dewatering, mechanical/thermal dewatering, solvent extraction, are summarized in detail. Future research to upgrade low rank-coals, which are deposited in arid geological environments, is also suggested.

Many efforts have been dedicated to improve the solar steam generation by using a bi-layer structure. In this paper, a two-dimensional mathematical model describing the water evaporation in a bi-layer structure is firstly established and then the finite element method is used to simulate the effects of different influence factors on the evaporation rate. Results turn out that: besides the high solar energy absorptivity of the first-layer, an optimum porosity of the second-layer porous material should be applied and the optimum porosity is about 0.45 in this work. This optimum porosity is determined by the balance between the positive effect of the lowering effective thermal conductivity of the second layer and the negative effect of the reduced vapor diffusivity in the second layer when the porosity is decreased. The influence of the thermal conductivity of the second-layer porous material is negligible because the effective thermal conductivity of the second layer is determined by the porosity while a larger porosity means more water in the second layer. The ambient air velocity could greatly enhance the evaporation rate, and the evaporation rate will decrease linearly with the increase of the air relative humidity. This study is expected to supply some information for developing a more effective bi-layer solar steam generation system. arXiv admin note: substantial text overlap with arXiv:1806.06485

Abstract Solar steam generation has become one of the most promising techniques to realize water purification and desalination owing to the abundance and cleanness of solar energy. Here, we proposed a preheating chamber structure for high-efficiency solar steam generation with carbonized sawdust beds as the solar absorber. Effects of bed porosity on the water transportation, heat transfer and evaporation performance were studied. Experimental results turn out that with the increase of the bed porosity, the heat conductive loss will be increased while water transportation is promoted. When the positive effect of the increased water transportation and the negative effect of the increased heat losses on the evaporation are balanced, the evaporation efficiency reaches the maximum of about 91.5% at the porosity of 76% under one sun illumination in this work. In addition, the influence of water supply on the steam temperature under 3-sun irradiation was also investigated. It turns out that the evaporation efficiency and temperature can be coordinated by adjusting the water-supply area. When the ratio of water-supply area to the evaporation area reaches 58.8%, a proper balance between efficiency (33.8%) and steam temperature (115.8 °C) can be achieved. This work is expected to supply a method to obtain high temperature and also efficiency steam generation.

Abstract In recent years, solar steam generation has attracted many attentions due to its potential applications in desalination, etc. In the present work, a bi-layer solar steam generation system is prepared by daubing carbon particles on the sintered sawdust film, which possesses an advantage of adjustable porosities compared to widely used wood. Then, the influence of the porosity on the evaporation performance is explored. The experimental result indicates that: the porosity could significantly affect the water transportation in the film, and the water diffusivity increases almost linearly with the increase of the porosity. The evaporation efficiency increases with the increasing porosity, until the porosity reaches about 0.52 then decrease slowly. The positive effect of the increased water diffusivity and the negative effect of the increased thermal conductivity of the bottom film layer determine that the porosity of 0.52 is optimal for improving the evaporation efficiency. Under a solar light power of 1 kW·m−2, the optimal porosity gives an evaporation efficiency of 77.64%, which is comparable to the best performance of bi-layer systems reported in previous works. The conduction of heat through the bottom layer to the bulk water and the convection heat loss on the top surface contribute 83% to the total heat losses in the system, suggesting that the energy losses of these two modes should be further reduced in the future applications. Considering the accessible materials, easy preparation, low cost and high efficiency, we conclude that the 0.52-porosity system is suitable for being used as an efficient solar steam generation device.

Abstract Sawdust, a natural and renewable material from wood processing, has become more and more widely reused in biomass-based thermal insulating material preparation. However, there is still a lack of biomass-based thermal insulating material, which should possess a characteristic of easily preparation but high thermal insulation performance. Herein, we developed an eco-friendly thermal insulation material by firstly pre-pressing sawdust powder into packed bed and then carbonizing the sawdust bed into porous carbon bed. The influences of the bed structure, environmental temperatures and humidity on the thermal conductivity of the prepared beds were firstly investigated. Results turn out that the carbonized pine sawdust packed (CPP) bed could have a low thermal conductivity of about 0.062 W m - 1 K - 1 . A high intrinsic porosity of sawdust particle, high porosity of bed, and spherical shape of pores in the CPPs are preferred for reducing the thermal conductivity. Moreover, the prepared CPPs not only possess hydrophobicity but also show excellent reusability and structural stability. The thermal conductivity of CPPs can be fully recovered without obvious structure collapse, if dried again after placed under a relative humidity of 60% for more than 24 hours. Importantly, the CPPs has excellent flame-retardant and smoke-free property because of the carbonization treatment in the preparation, and it can self-extinguish within 6 seconds in the vertical burning test. This work provides a facile and sustainable strategy to manufacture thermal insulation material from recycling waste bioresources, which has potential for future energy saving in buildings.

Abstract To relieve the fresh water shortage, a skeleton double layer structure (SDLS) is developed in this work to give a high evaporation efficiency for solar steam generation. In the SDLS, the bottom layer is dug hollow to prevent heat dissipating from the bottom layer into the bulk water. The method to optimize structure of the SDLS is also given in this work. The increase of height of SDLS has a positive effect on reducing heat losses while negative effect on supplying water, thus a proper height should be selected. After obtaining the proper height, the optimal cross sectional area of the skeleton structure can be approximately calculated based on the mass conservation of water. Applying the optimal SDLS, both of our experiment and simulation methods show that the evaporation rate and the evaporation efficiency under a solar power illumination of 1 sun can be 1.5 kg m−2 h−1 and 90% respectively, which is much higher than most emerging structures. The simulation by finite element method further shows that the high evaporation efficiency of the SDLS arrives from the low energy losses. The good match between the simulation and experimental results suggests the reliability of our results. We concluded that the SDLS is a promising system for application in solar steam generation due to its high evaporation efficiency, reusability and also easy to prepare.

The thermal conductivity of mesoporous material has aroused the great interest of scholars due to its wide applications such as insulation, catalyst, etc. Mesoporous alumina substrate consists of uniformly distributed, unconnected cylindrical pores. Near-field radiative heat transfer cannot be ignored, when the diameters of the pores are less than the characteristic wavelength of thermal radiation. In this paper, near-field radiation across a cylindrical pore is simulated by employing the fluctuation dissipation theorem and Green function. Such factors as the diameter of the pore, and the temperature of the material are further analyzed. The research results show that the radiative heat transfer on a mesoscale is 2~4 orders higher than on a macroscale. The heat flux and equivalent thermal conductivity of radiation across a cylindrical pore decrease exponentially with pore diameter increasing, while increase with temperature increasing. The calculated equivalent thermal conductivity of radiation is further developed to modify the thermal conductivity of the mesoporous alumina. The combined thermal conductivity of the mesoporous alumina is obtained by using porosity weighted dilute medium and compared with the measurement. The combined thermal conductivity of mesoporous silica decreases gradually with pore diameter increasing, while increases smoothly with temperature increasing, which is in good agreement with the experimental data. The larger the porosity, the more significant the near-field effect is, which cannot be ignored.