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The design of heat exchangers in the advanced supercritical power conversion system cannot be separated from the study of heat transfer issues. Half-side heating mode is often encountered for solar receiver and supercritical boiler. Here, the characteristics of supercritical CO2 (sCO2) convection heat transfer in vertical tubes with circumferentially half-side heating was numerically investigated through the SST k-ω turbulent model which matches well with the experimental data. Then, heat transfer between sCO2 upflow and downflow was compared. Similar to film boiling heat transfer at subcritical pressure, numerical results were processed according to the supercritical pseudo-phase transition hypothesis, with liquid-like phase in the tube core region and vapor-like film in the region near the heated tube wall. The structure of two layers was demarcated by pseudo-critical temperature Tpc. Therefore, sCO2 heat transfer was assessed according to double thermal resistances caused by vapor-like film near the wall and core liquid-like phase. The findings suggest that wall temperature for upflow is higher than that for downflow, which is attributed to larger thermal resistance in the fluid domain for upflow than that for downflow. The difference guarantees the excellent heat transfer performance for downflow than upflow. It is also further concluded that the formation of vapor-like film near the wall due to pseudo-phase transition plays a key role in dominating wall temperature and inducing heat transfer deterioration in half-side heating tubes. The present contribution is significant to the design of supercritical heat exchanger under half-side heating mode. [ABSTRACT FROM AUTHOR]

Roasting is an important step in the pretreatment of biomass upgrading. Roasting can improve the fuel quality of biomass, reduce the O/C and H/C ratios in the biomass, and provide the biomass with a fuel quality comparable to that of lignite. Therefore, studying the structure and component evolution laws during biomass roasting treatment is important for the rational and efficient utilization of biomass. When the roasting temperature is 200–300 °C, the cellulose and hemicellulose in the biomass undergo a depolymerization reaction, releasing many monocyclic aromatic hydrocarbons with high reactivity. The proportion of monocyclic aromatic hydrocarbons in biomass roasting products can be effectively regulated by controlling the reaction temperature, residence time, catalyst, baking atmosphere, and other factors in the biomass roasting process. This paper focuses on the dissociation law of organic components in the pretreatment process of biomass roasting. [ABSTRACT FROM AUTHOR]

The objective of this paper is to provide the optimal choice of single-reheating or double-reheating when considering residual flue gas heat in S-CO2 coal fired power system. The cascade utilization of flue gas energy includes three temperature levels, with high and low temperature ranges of flue gas heat extracted by S-CO2 cycle and air preheater, respectively. Two methods are proposed to absorb residual flue gas heat Qre in middle temperature range. Both methods shall decrease CO2 temperature entering the boiler T4 and increase secondary air temperature Tsec air, whose maximum value is deduced based on energy conservation in air preheater. The system is analyzed incorporating thermodynamics, boiler pressure drop and energy distribution. It is shown that at a given main vapor temperature T5, the main vapor pressure P5 can be adjusted to a value so that Qre is completely eliminated, which is called the main vapor pressure adjustment method. For this method, single-reheating is only available for higher main vapor temperatures. The power generation efficiency for single-reheating is obviously higher than double-reheating. If residual flue gas heat does exist, a flue gas heater FGC is integrated with S-CO2 cycle, which is called the FGC method. Both single-reheating and double-reheating share similar power generation efficiency, but single-reheating creates less residual flue gas heat. We conclude that single-reheating is preferable, and the pressure adjustment method achieves obviously higher power generation efficiency than the FGC method. [ABSTRACT FROM AUTHOR]

It is great significance to understand the mechanism of heat transfer deterioration of supercritical CO 2 for heat exchanger design and safe operation in the supercritical CO 2 Brayton cycle. Three-dimensional steady-state numerical simulation was performed to investigate the behavior of supercritical CO 2 heat transfer in heated vertical up-flow tube with inner diameter d i = 10 mm and heated length L h = 2000 mm. Based on the characteristics of inverted-annular film boiling at subcritical pressure, the heat transfer model of supercritical CO 2 flowing in the heated vertical tube was established in this paper. The mechanisms of heat transfer deterioration (HTD) and heat transfer recovery (HTR) for supercritical CO 2 were discussed. Numerical results demonstrate that HTD is affected by multiple factors, such as the thickness and property of vapor-like film near the wall, the turbulence intensity near the interface between liquid-like and vapor-like, and in the liquid-like core region as well as the distribution of radial velocity vector. Among the above factors, the change of turbulent kinetic energy caused by the buoyancy effect seems to be a more important contributor to HTD and HTR. Furthermore, the influences of heat flux and mass flux on the distribution of wall temperature were analyzed, respectively. The reasons for the difference in wall temperature at different heat fluxes and mass fluxes were explained by capturing detailed thermal physical properties and turbulence fields. The present investigation can provide valuable information for the design optimization and safe operation of a supercritical CO 2 heat exchanger.