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A 3D computational fluid dynamic (CFD) simulation is performed on a 35t/h grate boiler to investigate the compound combustion, consisting of grate firing of lump coal on the fire grate and suspensi...

This paper presented a coupled heat transfer model combining the combustion in the furnace and the ultra-supercritical (USC) heat transfer in the water wall tubes. The thermal analysis of the spiral water wall in a 1000 MW double reheat USC boiler was conducted by the coupled heat transfer simulations. The simulation results show that there are two peak heat flux regions on each wall of spiral water wall, where the primary combustion zone and burnt-out zone locate respectively. In the full load condition, the maximal heat flux of the primary combustion zone is close to 500 kW/m2, which is higher than that in the conventional single reheat USC boilers. The heat flux along the furnace width presents a parabolic shape that the values in the furnace center are much higher than that in the corner regions. The distribution of water wall temperature has a perfect accordance with the heat flux distribution of the parabolic shape curves, which can illustrate the distribution of water wall temperature is mainly determined by heat flux on the water wall. The maximal water wall temperature occurs at the middle width of furnace wall and approaches 530°C, which can be allowed by the metal material of water wall tube 12Cr1MoVG. In the primary combustion zone, the wall temperatures in half load are almost close to the values in 75% load condition, caused by the heat transfer deterioration of the subcritical pressure fluid under the high heat flux condition. The simulation results in this study are beneficial to the better design and operational optimization for the double reheat USC boilers.

The distribution of water wall temperature in the ultra-supercritical (USC) boilers was obtained by establishing a coupled heat transfer model. The reliability of this model has been validated thro...

The discrete element method (DEM) is used to simulate two types of mixing processes (radial mixing and axial mixing) of particles with and without cohesive interparticle forces in a two-dimensional (2D) fluidized bed. By using the Ashton index, the mixing degree is quantified and the differences between the mixing characteristics of particles with and without cohesive interparticle forces are compared. The cohesive interparticle forces, which impair bubble behaviors, can apparently weaken the mixing process and prolong the mixing time. This effect increases with increasing cohesive interparticle force. Radial mixing, which is mainly dependent on the convective mixing caused by bubbles, is relatively sensitive to cohesive interparticle force and shows a long mixing time, while axial mixing, which is mainly induced by the bubble wake, is less sensitive to cohesive interparticle force and presents a fast mixing process. Finally, to weaken the effect of cohesive interparticle force, an uneven air distribution is introduced, which greatly improves the mixing process by changing the bubble movement. This method may be helpful for the reduction of particle agglomeration in high-temperature fluidized beds.

Climate change is today’s common challenge faced by all humanity. The accumulated carbon dioxide emissions from the fossil fuels consumption have resulted in significant global warming. Among the commercial-ready technologies, the advanced ultra-supercritical (USC) technology is the most convenient solution for coal-fired power generation to address CO2 mitigation, especially in countries where electricity generation is primarily dependent upon coal. This paper reported the research and development of advanced ultra-supercritical technology in China in the past ten years and its technology roadmap. The USC R&D has suffered three stages: technology introduction, absorption, and innovation. Its characteristic is the transition to higher live steam parameters, larger capacity, and steam double–reheat. Except for the introduction of several milestones in China’s USC development, emphasis was placed on the first 1000 MW double-reheat USC unit in the Taizhou Power Plant and ongoing research for a future 700 °C USC program. This paper provided a status report on the progress achieved to date in China.

The combination of the advanced ultra-supercritical (AUSC) and oxy-fuel combustion technologies is considered the most feasible and promising choice for carbon capture and storage. The high efficiency of AUSC boilers compensates for the cost incurred in controlling CO 2 emissions. In this study, a new improved model was proposed for gas-radiative properties as a polynomial function of the continuous molar ratio of H 2 O to CO 2 and temperature, which was compiled as user-defined functions in C language. The modified radiative model was employed in the computational fluid dynamics analysis to simulate a tangentially fired 700 °C-USC boiler with a capacity of 300 MW under the oxy-fuel condition. The results show that the improved model has a good accuracy for gas-radiative properties compared to other classical models. In the oxy-fuel cases, the heat transfer obtained in the simulation is approximately 4–11% lower than that in the thermodynamic calculation because of the char gasification. The radiative capability of the flue gases in the oxy-fuel case is much greater than that in the air–fuel case with the increase in the molar fraction of oxygen. The heat transfer via the water wall is approximately 4–8% higher than that via air, when the molar fraction of oxygen increases from 26% to 29%. The increase in the heat absorption helps in better arrangement of the heat-exchanger surfaces in the furnace of the AUSC boiler.

The development of high parameter, large capacity, coal-fired utility boilers with improved performance, based on the operation and design experience of the current parameter level, is essential. In addition, the combination of theoretical algorithms and practical experimental data is of great significance for research into utility boilers. In this study, an experiment to observe the heat transfer characteristics of heating surfaces was conducted under advanced ultra-supercritical (A-USC) conditions on a coal-fired experiment platform with a coal feed capacity of 0.3 MWth. Four repeated experiments were performed to verify the reliability of the experimental data, the maximum relative error of all the experimental data was less than 7%, which indicates steady combustion performance and good repeatability. Significant heat transfer enhancement and deterioration was demonstrated and analyzed at the superheater near the pseudo-critical region at a pressure of 36 MPa. Outside of the pseudo-critical region, the heat transfer coefficients experienced continuous and smooth variation with changes in temperature. A distributed parameter model for temperature prediction was established based on the mass, momentum, and energy conservation law through the discretization of time and space. The results indicated that predicted temperatures were in good agreement with the experimental data. For the top and bottom furnace, the correlation by Mokry et al. was consistent with the experimental data. The correlations of Jackson et al., Swenson et al., and D.B et al. were used for accurate temperature predictions at the middle of the furnace, superheater, and reheater, respectively.

Due to the current qualitative method of the unburned carbon in fly ash for tangentially pulverized coal-fired boiler, the factor analysis was applied creatively to estimate quantitatively every influence on the unburned carbon in fly ash, considering the actual operations of 300MWe boiler. This study established the factor model to analysis the influences of unburned carbon in fly ash, and listed their weights in numerical form. The results of this study suggested the sequence of these influences is the boiler loading (coal-consumption), the angle of the tilting burner, the primary air pressure, the hot-primary air temperature, furnace air total volume and the hot-secondary air temperature, which is benefit to optimize the control of the boiler actual operation.