Flexibly using results of CFD and simplified heat transfer model for pulverized coal‐fired boilers

The efficient use of pulverized coal is crucial to the utility industries. The use of computational fluid dynamics (CFD)‐based numerical models has an important role in the design of new boiler furnaces or in retrofitting situations. The results of CFD simulations can be used to better understand the complex processes occurring within the boiler furnace. The use of these results to support boiler operation and training of operators requires that the CFD models can be easily accessed and the results are easily analysed. This paper discusses two ways to simulate the heat transfer process in boiler furnaces. The method directly applying CFD results is employed, in which the grid for solving the energy equation is the same as the flow grid in the CFD simulation while radiation heat transfer is solved in another relatively coarse grid. Comparison of the prediction results between CFD and Heat Transfer code (Simple model) is performed under boiler full load (100%) with one side wall fouling, as well as for different boiler loads (100, 98 and 95 per cent boiler full load, respectively). Finally, the flexible use of the results of CFD and the simple model for pulverized coal‐fired boilers is presented. To facilitate the use of the system, a user‐friendly interface was developed which enables the user to manipulate new calculations and to view results, namely performing ‘what–if’ analysis. Copyright © 2000 John Wiley & Sons, Ltd.     

Investigation of flame characteristics using various design parameters in a pulverized coal burner for oxy‐fuel retrofitting

In oxy‐coal combustion for carbon capture and storage, oxygen and recirculated CO₂ are used as oxidizers instead of air to produce CO₂‐rich flue gas. Owing to differences between the physical and chemical properties of CO₂ and N₂, the development of a burner and boiler system based on fundamental understanding of the flame type, heat transfer, and NOx emission is required. In this study, computational fluid dynamic analysis incorporating comprehensive coal conversion models was performed to investigate the combustion characteristics of a 30 MWth tangential vane swirl pulverized coal burner. Various burner design parameters were evaluated, including the influence of the burner geometry on the swirl strength, direct O₂ injection, and O₂ concentrations in the primary and secondary oxidizers. The flame characteristics were sensitive to the oxygen concentration in the primary oxidizer. The performance of direct O₂ injection around the primary oxidizer with low O₂ concentration was dependent on the mixing of the fuel and oxidizer. The predictions showed that swirl number adjustment and careful direct oxygen injection design are essential for retrofitting air‐firing pulverized coal burners as oxy‐firing burners.     

Analysis of Conduction and Radiation Heat Transfer in a Differentially Heated 2‐D Square Enclosure

Radiative heat transfer with and without conduction in a differentially heated 2‐D square enclosure is analyzed. The enclosure with diffuse gray boundaries contains radiating and/or conducting gray homogeneous medium. Radiatively, the medium is absorbing, emitting and scattering. On the south boundary, four types of discrete heated regions, viz., the full boundary, the left one‐third, left two third and middle one third, are considered. In the absence of conduction, distributions of heat flux along the south boundary are studied for the effect of extinction coefficient. In the presence of conduction, distributions of radiation, conduction and total heat fluxes along the south boundary are analyzed for the effects of extinction coefficient, scattering albedo, conduction–radiation parameter, and south boundary emissivity. Effects of these parameters on centerline temperature distribution are also studied. To assess the performance of three commonly used radiative transfer methods, in all cases, the radiative transfer equation is solved using the discrete ordinate method (DOM), the conventional discrete ordinate method (CDOM) and the finite volume method (FVM). In the combined mode problem, with volumetric radiative information known from one of the three methods, viz., DOM, CDOM, and FVM, the energy equation is solved using the finite difference method (FDM). In all cases, the results from FDM‐DOM, FDM‐CDOM, and FDM‐FVM are in good agreement. Computationally, all three sets of methods are equally efficient.     

CFD simulation of jet behaviour and voidage profile in a gas–solid fluidized bed

Based on an Eulerian–Eulerian approach, a computational fluid dynamic (CFD) model for gas–solid system has been developed to investigate the hydrodynamics in fluidized beds. With this model, jet penetration height, jet frequency, time‐averaged axial gas velocity profile, and time‐averaged voidage profile have been simulated in a two‐dimensional bed. The computational results indicate that the jet penetration height increases with increasing the jet gas velocity. The jet frequency decreases with increasing the jet gas velocity and decreasing particle diameter. The time‐averaged axial gas velocity profile becomes ‘lower’ and ‘wider’ and the time‐averaged voidage decreases with increasing distance from the jet nozzle. These conclusions appear in good agreement with the experimental and simulated data in the literature. Copyright © 2004 John Wiley & Sons, Ltd.     

Effects of coal characteristics to performance of a highly efficient thermal power generation system based on pressurized oxy‐fuel combustion

Because of its fuel flexibility and high efficiency, pressurized oxy‐fuel combustion has recently emerged as a promising approach for efficient carbon capture and storage. One of the important options to design the pressurized oxy‐combustion is to determine method of coal (or other solid fuels) feeding: dry feeding or wet (coal slurry) feeding as well as grade of coals. The main aim of this research is to investigate effects of coal characteristics including wet or dry feeding on the performance of thermal power plant based on the pressurized oxy‐combustion with CO₂ capture versus atmospheric oxy‐combustion. A commercial process simulation tool (gCCS: the general carbon capture and storage) was used to simulate and analyze an advanced ultra‐supercritical(A‐USC) coal power plant under pressurized and atmospheric oxy‐fuel conditions. The design concept is based on using pure oxygen as an oxidant in a pressurized system to maximize the heat recovery through process integration and to reduce the efficiency penalty because of compression and purification units. The results indicate that the pressurized case efficiency at 30 bars was greater than the atmospheric oxy‐fuel combustion (base line case) by 6.02% when using lignite coal firing. Similarly, efficiency improvements in the case of subbituminous and bituminous coals were around 3% and 2.61%, respectively. The purity of CO₂ increased from 53.4% to 94% after compression and purification. In addition, the study observed the effects of coal‐water slurry using bituminous coal under atmospheric conditions, determining that the net plant efficiency decreased by 3.7% when the water content in the slurry increased from 11.12% to 54%. Copyright © 2016 John Wiley & Sons, Ltd.     

Numerical study of radiative heat transfer effects on a complex configuration of rack storage fire

This work describes the application and the performance of a new radiation model in CFD calculations for the simulation of thermal radiation transfer effects on a fire scenario. A 3D Cartesian coordinates radiative heat transfer procedure based on coupling of the FTn finite volume method (FTnFVM) with the bounded high-order resolution CLAM scheme is developed. The narrow-band based weighted-sum-of-gray-gases (NB-WSGG) model is applied to take account of nongray effects by CO₂, H₂O and soot. To treat irregular boundaries, the present model used the blocked-off-region procedure. This radiation code is implemented in the Fire Dynamics Simulator (FDS), a Computational-Fluid-Dynamics-based fire model, where a the combustion is represented by means of the mixture fraction with a single step chemical reaction model and the Large Eddy Simulation (LES) is used to model the dissipative processes. Computational results with and without radiation effects are compared against available experimental data and quasi-steady state law correlations of in-rack storage fire, which consists a complex configuration of double tri-wall corrugated paper cartons placed onto a wood pallet. Sensibility analyses of spatial and angular grids demonstrate the improvements due to the FTnFVM and to the CLAM scheme in the configuration studied. Results show that the simulations of the flame height, the gas temperature and the gas velocity are strongly influenced by thermal radiation. Overall, simulations predicted closer profiles to the experimental results only when the nongray-sooting radiation model was incorporated and an over-prediction of the gas temperature and the flame height is found when radiation is neglected. A sensibility analysis has shown that the flame characteristics are strongly affected by the soot yield.