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Cai H.,Shanghai Boiler Works Ltd. | Wu Y.,Xian Jiaotong University | Yang D.,Xian Jiaotong University
Zhongguo Dianji Gongcheng Xuebao/Proceedings of the Chinese Society of Electrical Engineering | Year: 2011

Under sub-critical, near-critical and supercritical pressure, this paper experimentally investigated the heat transfer characteristics of vertical upward low mass-flux optimized rifled tube, which is applied in supercritical W-shape flame boiler. The wall temperature distribution in rifled tube was obtained and the effects of pressure, inner wall heat flux and mass flux on heat transfer characteristics was analyzed. The results illustrate that low mass-flux optimized rifled tube has good heat transfer performance and can efficiently prevent the occurrence of departure from nucleate boiling (DNB). In sub-critical pressure region, with the increase of pressure and inner wall heat flux and the decrease of mass flux, dryout occurs ahead and the wall temperature in post-dryout region increases obviously. Compared with the heat transfer characteristics of rifled tube at sub-critical pressure, that in near-critical pressure region gets worse, the wall temperature significantly rises and the critical quality decreases with increasing pressure. At supercritical pressure, heat transfer enhancement occurs in pseudo-critical enthalpy region in rifled tube; while the pressure approaches critical pressure, the heat transfer is enhanced more; the wall temperature increases with increasing pressure and heat flux and with decreasing mass flux. © 2011 Chin. Soc. for Elec. Eng.

Dai B.-Q.,Monash University | Low F.,Monash University | De Girolamo A.,Monash University | Wu X.,Monash University | And 2 more authors.
Energy and Fuels | Year: 2013

In this paper, an intensive characterization of ash deposits collected from different positions of a pulverized-coal (PC) boiler has been conducted to diagnose the ash slagging and fouling issues within this boiler and to clarify the mass balance/flow of individual major elements and their role on ash slagging and fouling. A lab-scale drop-tube furnace has also been employed to elucidate the partitioning of the major metals during coal pyrolysis and char oxidation, to interpret the PC boiler results. The lignite tested is rich in Na and Ca, which are mostly present as organically bound cations and superfine mineral grains. In the air-fired boiler, the refractory minerals of silicates, aluminates, or aluminosilicates preferentially remained in fireside slag and bottom ash, forming low-temperature eutectics via the interaction with CaO and Fe2O3 on the receding char surface. The complex eutectic Ca-Al-Si consists of the liquidus matrix of the dense layer of fireside slag, in which Fe2+-bearing oxide was highly crystallized into a diamond-shape crystal on the water-tube surface. The ash fouling on Feston and superheater tubes was formed with a thinner Fe-rich layer that is followed by the deposition of Na2SO4 liquids. The abundance of Fe 2O3 and CaO in the char matrix is crucial, which triggered the formation of around 80% liquids in the fireside slag with a viscosity of approximating 100 poise at 1200 C. On the reheat tube surface, about 60% of the fully oxidized hematite was even reduced by the metallic iron into magnetite. Na2O and MgO in the char matrix preferentially escaped into flue gas as vaporized metallic vapor and fine oxide particles, respectively. The sulfation of Na-bearing vapor and CaO particle in flue gas was controlled by the partial pressure of Na2SO4 vapor and reaction rate, respectively. © 2013 American Chemical Society.

Chen K.,Xian Jiaotong University | Zhang J.,Shanghai Boiler Works Ltd. | Che D.,Xian Jiaotong University
Applied Thermal Engineering | Year: 2016

In order to provide references for designing boilers with separated over-fire air (SOFA), numerical simulation was applied for studying the effect of SOFA on temperature, heat flux and fuel combustion in a 3 MW pilot-scale facility. Simulation results agrees well with measureable data. Results show the temperature and the sensible heat flux of the flue-gas at the furnace outlet first decrease, and then increase with the excess air ratio in the combustion zone. As the SOFA ports rises, the temperature and the sensible heat flux of flue-gas decrease at the furnace outlet. These variations are caused by variations of the sensible heat flux of air injected into furnace. Moreover, as the SOFA ratio increases, both the total heat flux from furnace walls and the fuel combustion heat decrease. The maximum decrease of the heat flux from furnace walls occupies 10% of the total in the case without SOFA. The maximum decrease of the fuel combustion heat occupies 8% of the total, which is caused by the unburnt carbon in bottom ash. The SOFA position has little influence on the total heat flux and the fuel combustion heat. These results are valuable for designing boilers with SOFA. © 2016 Elsevier Ltd

Xiong J.,Huazhong University of Science and Technology | Xiong J.,Shanghai Boiler Works Ltd. | Zhao H.,Huazhong University of Science and Technology | Zheng C.,Huazhong University of Science and Technology
International Journal of Greenhouse Gas Control | Year: 2012

Oxy-combustion technology, through adding a cryogenic air separation unit process and a flue gas treatment unit process to a conventional combustion process, has been rapidly developed because it is considered as a feasible choice to capture CO 2 from power plants for sequestration. To reach a comprehensive understanding of the thermodynamic and economic characteristics of the oxy-combustion process, a detailed thermoeconomic cost analysis of a 600MW e oxy-combustion pulverized-coal-fired power plant was carried out. Based on the detailed results of exergy analysis and the latest data of investment cost, the results of exergy cost and thermoeconomic cost were obtained. It is found that, in comparison to the corresponding conventional supercritical plant with the same gross output (600MW e), the additional power consumption in the oxy-combustion system increases the unit exergy costs (or unit thermoeconomic costs) of products for about 10%. On the other hand, the additional monetary cost, including investment cost, interest, and operation and maintenance cost, in the oxy-combustion system increases the unit thermoeconomic costs of products for nearly another 10%. The relation between the unit thermoeconomic cost change and the unit exergy cost change in the oxy-combustion system comparing to the conventional system was established. Moreover, in this paper, a new decomposition method for exergy cost was proposed to explore the formation mechanism of the exergy cost, furthermore, the thermoeconomic cost. The unit exergy cost was decomposed into three parts: fuel, exergy destruction, and negentropy. Meanwhile, the reasonable unit exergy costs of exergy destructions happened in the system can be defined. It is found that the exergy destruction part is the most important factor that affects the unit exergy costs of products for these components in the same model (such as the boiler model). © 2012 Elsevier Ltd.

Prationo W.,Monash University | Zhang J.,Monash University | Abbas H.A.A.,Monash University | Wu X.,Monash University | And 3 more authors.
Industrial and Engineering Chemistry Research | Year: 2014

The influence of external clay additive and inherent minerals on the ignition of a Xinjiang lignite and its volatile flame propagation in air versus oxy-fuel combustion have been clarified in this work, through the use of a flat-flame burner reactor (FFBR) coupled with in-situ optical diagnosis tools. As has been confirmed, ignition of the lignite studied in this paper was initiated by homogeneous oxidation of a tarry volatile cloud. The removal of HCl-soluble metals shifted coal devolatilization toward higher temperatures in air and 21% O2 in CO2. The mixing of external clay with coal had little effect on the ignition time. However, it enhanced the decomposition of volatiles, leading to a larger volatile cloud shielding on the particle surface. The oxygen fraction in the bulk gas was found to be most influential. Increasing the oxygen fraction to 30% eliminated all of the discrepencies between raw lignite, acid-washed lignite, and a mixture of raw lignite and clay. © 2014 American Chemical Society.

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