CFD based investigations into optimization of coal pulveriser performance: Effect of classifier vane settings

In coal-fired power plant, pulveriser is the first major component, whose performance dictates the total power plant efficiency. Uniform flow rate and desired size fraction at outlet pipes along with higher classifier efficiency are three important measures which decide the pulverizer performance. Optimization of pulverizer at its best operating conditions has been considered as a potential area that needs to be addressed for improving unit performance, emissions, operations, and maintenance. The best operating conditions are optimum air velocity and classifier vane settings. In this investigation, numerical simulations of a typical pf coal based pulveriser have been carried out for different classifier vane settings to evaluate uniform flow rate and desired size fraction at outlet pipes along with high optimum classifier efficiency. The optimum opening for the vanes has been determined based on the above measures, which not only reduces unburnt CO, SO_x and NO_x emissions at boiler end but also minimise energy consumption of mill (in terms of reductions in regrinding cost). Computational Fluid Dynamics (CFD) simulations of the coal classifier physical model indicate good agreement with the plant data, in terms of internal flow patterns, particle collection efficiency and desired cut size. From the simulation studies, optimum opening for the vanes is found to be 65% for selected utility which leads to closest uniformity with 60% classifying efficiency wherein 70% particles pass through 75 μm sieve.     

CFD modelling of a liquid-solid fluidized bed

A multifluid Eulerian computational fluid dynamics (CFD) model with granular flow extension is used to simulate a liquid-solid fluidized bed. The numerical simulations are evaluated qualitatively by experimental data from the literature and quantitatively by comparison with new experimental data. The effects of mesh size, time step and convergence criteria are investigated. Varying the coefficient of restitution did not alter the results significantly. The Gidaspow drag relationship predicted a higher voidage than the Wen and Yu drag law. Two different liquid distributors (uniform and non-uniform) were simulated and compared, but a better representation of the geometry of the distributor plate did not greatly influence the results. Qualitatively, the simulations show trends similar to experimental trends reported by various authors. The predictions are also compared with new experimental results for 1.13 mm glass spheres at a wide variety of superficial liquid velocities (0.0085-0.110 m/s) and two different temperatures (12 and ) significantly affecting the liquid viscosity. The CFD model predictions are within 5% of the steady-state experimental data and show the correct trend with variation in viscosity.     

Prediction of ignition behavior in a tangentially fired pulverized coal boiler using CFD

Prediction of pulverized coal ignition behavior in a 40 MW tangentially fired commercial boiler is studied. Pulverized coal combustion simulation is performed considering radiation properties of particles. Coal devolatilization and char combustion are modeled and the first order spherical harmonic approximation is used to model the radiative transfer equation. To confirm the accuracy of the simulation method, the results are confirmed by available operating data, design data, and the ignition image in the boiler whose inside is observed by the developed high temperature resistant CCD video camera system. The work indicates that the simulation method can be applied to commercial boilers and predict the ignition behavior with considering not only coal properties but also boiler operating conditions.     

CFD study of a sudden-expanding coal combustor using Euler–Euler and Euler–Lagrange models

In this study, the Euler–Euler (E–E) and Euler–Lagrange (E–L) models designed for the same chemical mechanism of heterogeneous reactions were used to predict the performance of a typical sudden-expanding coal combustor. The results showed that the current E–E model underestimated the coal burnout rate because the particle temperature fluctuation on char combustion is not adequately considered. A comparison of the E–E and E–L simulations showed the underestimation of heterogeneous chemical reaction rates by the E–E model.     

On the CFD modelling of Taylor flow in microchannels

With the increasing interest in multiphase flow in microchannels and advancement in interface capturing techniques, there have recently been a number of attempts to apply computational fluid dynamics (CFD) to model Taylor flow in microchannels. The liquid film around the Taylor bubble is very thin at low Capillary number (Ca) and requires careful modelling to capture it. In this work, a methodology has been developed to model Taylor flow in microchannel using the ANSYS Fluent software package and a criterion for having a sufficiently fine mesh to capture the film is suggested. The results are shown to be in good agreement with existing correlations and previous valid modelling studies. The role played by the wall contact angle in Taylor bubble simulations is clarified.     

Investigation of liquid maldistribution in trickle-bed reactors using porous media concept in CFD

A three-dimensional CFD model for simulating two-phase flow in trickle-bed reactors (TBRs) is presented. Based on porous media concept, a two-phase Eulerian model (rather than computationally demanding traditional three-phase Eulerian model) describing the flow domain as porous region is presented to understand and forecast the liquid maldistribution in TBRs under cold-flow conditions. The drag forces between phases have been accounted by employing the relative permeability concept [Sàez, A. E., Carbonell, R. G., 1985. Hydrodynamic parameters for gas-liquid cocurrent flow in packed beds. A.I.Ch.E. Journal 31, 52-62].The model predictions are validated against experimental data reported in literature, notably using the liquid distribution studies of Marcendelli [1999. Hydrodynamique, Transfert de Chaleur Particule-Fluide et Distribution des phases dans les Reacteurs a lit Fixe a Ecoulement a Co-courant Descendant de Gaz et de Liquide. Doctoral Thesis. INPL, Nancy, France]. Various distributor configurations reported therein have been recreated in the CFD model and sensitivity studies have been performed. Good agreement is obtained between the reported experimental results and this proposed first-principle based CFD model.Finally, the concept of distribution uniformity is discussed and applied to the CFD model predictions. The CFD model is subjected to a systematic sensitivity study in order to explore better liquid distribution alternatives.     

CFD modelling of liquid fluidized beds in slugging mode

Computational Fluid Dynamics (CFD) modelling has been used to simulate a liquid fluidized bed of lead shot in slugging mode. Simulations have been performed using a commercial code, CFX4.4. The kinetic model for granular flow, which is already available in CFX, has been used during this study. 2D time-dependent simulations have been carried out at different water velocities. Simulated aspects of fluidization such as voidage profiles, slug formation, pressure drop and pressure fluctuations have been analysed. The fluid-bed pressure drop was found to be greater than the theoretical one at all velocities, in agreement with experimental observations reported for fully slugging fluidized beds. Power spectral density analysis of the pressure signal was used to investigate the development of the flow pattern and the structure of the fluid-bed with increasing fluidizing velocity. A comparison between experimental and simulated results is also reported.     

CFD modelling of the hydrodynamics and kinetic reactions in a fluidised-bed MTO reactor

This study presents a computational investigation of the hydrodynamics and kinetic reactions in a fluidised-bed MTO reactor. By integrating a kinetic model of methanol conversion with a two-fluid flow model, a gas–solid flow and reaction model was established. CFD analyses were performed, and the influences of various operating parameters were evaluated. The results indicate that the velocity, volume fraction and species concentration were considerably non-uniform in the axial and radial directions of the MTO reactor. Methanol conversion rate and product yields were more sensitive to the reaction temperature and pressure than to the initial methanol content in the feedstock. A gas velocity of 2.5–3.0 m/s and a catalyst circulation rate of 100–120 kg/(m² s) were found to be ideal for the current reactor. Coke deposition significantly affected the methanol conversion rate, product distribution and species selectivity. The ethylene-to-propylene ratio could be adjusted by varying the amount of coke on the catalyst.     

CFD modelling of oxy-coal combustion in an entrained flow reactor

Oxy-fuel combustion is seen as one of the major options for CO₂ capture for both new and existing coal fired power stations. Coal is burned with a mixture of oxygen and recycled flue gas to obtain a rich CO₂ stream ready for sequestration. Computational fluid dynamics (CFD) tests for coal combustion under different O₂/CO₂ (21-35% vol O₂) atmospheres in an entrained flow reactor (EFR) were carried out using three coals of different volatile matter content. The temperature profiles, burning rates, burnout and concentration of major species, such as O₂, CO₂, CO, were predicted and compared with an air reference case. A decrease in gas temperature and burning rate was observed for 21% O₂/79% CO₂ environment in comparison to the air reference case due to the difference in gas properties between N₂ and CO₂. Experimental coal burnouts obtained in the EFR, were used to test the accuracy of the CFD model. The numerical results showed a decrease in coal burnout when N₂ was replaced by CO₂ for the same oxygen concentration (21%), but an improvement in the O₂/CO₂ atmosphere for an oxygen concentration higher than 30%. The numerical results for oxy-coal combustion were in good agreement with the experimental results.     

CFD modelling and scale-up of Confined Impinging Jet Reactors

Confined Impinging Jet Reactors (CIJRs) are appealing devices for precipitation of nanoparticles because of their high mixing efficiency. In fact, since precipitation processes are generally very fast, mixing plays a crucial role and it is of great importance to operate under very fast mixing conditions. In this work mixing and reaction in CIJRs are studied by means of Computational Fluid Dynamics (CFD). Mixing at the molecular level is modelled with a presumed Probability Density Function (PDF) approach: the Direct Quadrature Method of Moments coupled with the Interaction by Exchange with the Mean (DQMOM-IEM) model. The influence of operating conditions and reactor geometry on mixing is also evaluated and a scale-up criterion for CIJRs is developed, showing that scaling up by means of CFD is a practicable path, worth of further investigation.     

Comparative study of droplet drying models for CFD modelling

CFD simulation is used to study wall deposition and agglomeration phenomena commonly encountered in industrial spray dryers. This paper initially provides a comparison of two drying kinetics models: Characteristic Drying Curve (CDC) and Reaction Engineering Approach (REA). Comparisons are made with experimental data with application to carbohydrate droplet drying obtained from past workers. These models were then extrapolated to actual drying conditions to assess their performance. The REA model predicts the progressive reduction in drying rate better than the CDC model for the carbohydrate droplets. A modified CDC model incorporating a convex falling rate produced better agreement than the conventional linear falling rate model. Further analysis showed that the REA model can be extended to simulate the particle surface moisture which may affect the agglomeration process. The proposed concept was compared with reported simulation results from a diffusion model which showed reasonable fit with data.     

Investigation of free radicals produced during coal liquefaction using ESR

Coal liquefaction experiments are described which were carried out to dissolve a subbituminous coal in low and high hydrogen content solvents, in which liquid samples were periodically withdrawn and examined by electron spin resonance spectroscopy. The objectives of this study were to gain information on the free radical processes involved in coal liquefaction, to determine the relative effectiveness of two solvents (H-donor and non-donor) in influencing the radical processes involved in coal liquefaction and to investigate changes in radical concentration on storage.     

Three-dimensional modelling of in-furnace coal/coke combustion in a blast furnace

A three-dimensional mathematical model of the combustion of pulverized coal and coke is developed. The model is applied to the region of lance-blowpipe-tuyere-raceway-coke bed to simulate in-furnace phenomena of pulverized coal injection in an ironmaking blast furnace. The model integrates not only pulverized coal combustion model in the blowpipe-tuyere-raceway-coke bed but also coke combustion model in the coke bed. The model is validated against the measurements under different conditions. The comprehensive in-furnace phenomena are investigated in the raceway and coke bed, in terms of flow, temperature, gas composition, and coal burning characteristics. The underlying mechanisms for the in-furnace phenomena are also analysed. The simulation results indicate that it is important to include recirculation region in the raceway and the coke bed reactions for better understanding in-furnace phenomena. The model provides a cost-effective tool for understanding and optimizing the in-furnace flow-thermo-chemical characteristics of the PCI operation in full-scale blast furnaces.     

CFD modelling of inorganic membrane modules for gas mixture separation

This work is aimed at investigating the capability of a computational fluid dynamics (CFD) approach to reliably predict the fluid dynamic and the separation performances of inorganic membranes modules for gas mixture separation.The simulations are based on the numerical solution of the Navier-Stokes equations on the three dimensional domain representing quite closely the selected module geometry. The membrane is modelled as a selective layer, which allows the permeation of different components as a function of the transport mechanism and the driving force.The computational strategy is strictly evaluated by comparing the results with available experimental data. The simulation predictions show fairly good agreement with the measured permeation data and allow to recognise the critical local fluid dynamic features of the separation module.     

CFD modeling of MPS coal mill with moisture evaporation

Coal pulverizers play an important role in the functioning and performance of a PC-fired boiler. The main functions of a pulverizer are crushing, drying and separating the fine coal particles toward combustion in the furnace. It is a common experience that mill outlet pipes have unequal coal flow in each pipe and contain some coarse particles. Unequal coal flow translates into unequal air-to-fuel ratio in the burner, deviating from the design value and thus increasing unburned carbon in fly ash, NO_x and CO. Coarser particles at the mill outlet originate from poor separation and decrease the unit efficiency. In addition, coarser particles reduce burner stability at low load. Air flow distribution at the mill throat, as well as inside the mill, significantly influences the mill performance in terms of separation, drying, coal/air flow uniformity at the mill outlet, wear patterns and mill safety. In the present work, a three-dimensional computational fluid dynamics (CFD) model of the MPS Roll Wheel pulverizer at Alliant Energy's Edgewater Unit 5 has been developed. The Eulerian-Lagrangian simulation approach in conjunction with the coal drying model in Fluent, a commercial CFD software package, has been used to conduct the simulation. Coal drying not only changes the primary air temperature but it also increases the primary air flow rate due to mass transfer from coal. Results of the simulation showed that a non-uniform airflow distribution near the throat contributes significantly to non-uniform air-coal flow at the outlet. It was shown that uniform velocity at the throat improves the air and coal flow distribution at the outlet pipes. A newly developed coal mill model provides a valuable tool that can be used to improve the pulverizer design and optimize unit operation. For example, reject coal rate, which is controlled by the air flow near the mill throat, can be reduced. The model can also be used to further aid in identifying and reducing high temperature or coal-rich areas where mill fires are most likely to start.     

CFD modelling of flow and heat transfer in the Taylor flow regime

Transport phenomena in the Taylor flow regime for gas–liquid flows in microchannels have received significant attention in recent years. Whilst the hydrodynamics and mass transfer rate in the Taylor flow regime have been studied extensively using experimental and numerical techniques, studies of heat transfer in Taylor flow have been neglected. In this work, the flow and heat transfer in this regime is studied using the volume of fluid (VOF) and level-set techniques to capture the gas–liquid interface, as implemented in the ANSYS Fluent and TransAT codes, respectively. The results obtained from the two different codes are found to match very closely. Fully-developed flow and heat transfer are studied using the VOF method for a Reynolds number (Re) of 280, Capillary number (Ca) of 0.006 and homogeneous void fraction (β) of 0.51 for constant wall heat flux (H) and constant wall temperature (T) boundary conditions. The Nusselt numbers obtained for both cases are 2.5 times higher than those for liquid-only flow. The effects of the mixture velocity and the homogeneous void fraction on flow and heat transfer are also studied.     

Investigation of flow and temperature patterns in direct contact condensation using PIV,PLIF and CFD

In this study, experiments have been performed for the steam injected centrally at the bottom of a vertical rectangular water vessel. Instantaneous velocity and temperature field near the plume as well as in the downstream have been measured in a vertical plane through the central axis. For this purpose, Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) have been employed. The velocity profile of the region above the condensation region was found to be self-similar with a small downward velocity near the wall due to recirculation. The instantaneous (20-ns integration time) thermal images obtained from PLIF had a spatial resolution of 100 μm with a field of view of 100×100 mm. The time averaged velocity and temperature profiles are computed from an ensemble of 100 velocity/temperature images. The present work is also concerned with CFD simulation by employing k-ε and large eddy simulation (LES) turbulence models. All the measurements and simulations were carried out by varying nozzle upstream pressure in the range of 0.3–0.35 MPa (corresponding nozzle velocities were in the range of 286–304 m/s) with the nozzle diameter of 1 mm.