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.     

Multiphase flow simulation of a simplified coal pulveriser

In coal-fired power plants, the first major component is pulveriser, whose performance dictates the total power station efficiency. Pulveriser is employed to grind the lumped coal and transport the fine coal powder to furnace chamber for an efficient combustion. In this study, we have simulated motion of air and coal particles inside a commercial-scale pulveriser. Multiphase flow simulation of a simplified pulveriser was carried out using a granular Eulerian–Eulerian approach. Due to inclined air-distributor vanes, the flow field within pulveriser was slightly asymmetric. Regions of exceptionally high velocities were predicted close to the outer walls of the pulveriser, indicating a strong probability of particles carryover within these regions. The 100 μm coal particles qualitatively followed the air path-lines. However, the velocity vectors for 500 μm particle deviated significantly from those of airflow. The results presented in the paper provide impetus for the development of a complex pulveriser model, which further could prove valuable to designers for optimisation of components within the mill.     

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 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.     

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.     

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.     

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.     

Hydrodynamics in a gravity settling vessel: CFD modelling with LDA validation

Vertical gravity settling vessels, usually referred to as primary separation vessels (PSV), are used in separating bitumen aggregates from slurry containing sand and fine clays. The hydrodynamics in the PSV influences the separation efficiency of recovered bitumen through the overall mean flow and turbulent interaction. In order to deepen our understanding of the hydrodynamic conditions in such vessels, this paper presents a combined study of the flow field using Laser Doppler Anemometry (LDA) to measure the velocity field, and computational fluid dynamics (CFD) simulations to validate the CFD model. The investigation shows that the flow geometry has a significant influence on the overall flow pattern in such vessels. It also demonstrates that the CFD simulation is a reliable tool in capturing the complex mean flow pattern observed in experiments. Use of different turbulent models such as the standard k‐ε model and Reynolds stress model has very little effect on the mean flow field.     

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.     

Ventilation design of industrial premises through CFD modelling

This work investigates a general ventilation system for reducing the health hazard in industrial premises. Several tests carried out in a laboratory mock‐up have been used to validate the reliability of a general‐purpose computational fluid dynamic model. The same model has been also used to compare the efficiency of different ventilation systems. This clearly indicated the different health hazard related to each configuration, thus allowing one to identify the configuration leading to the lower contamination in the breathing zone.     

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 of fast chemical reactions in turbulent liquid flows

In this work an in-house CFD code is used to simulate a single-phase acid–base neutralisation in a tubular reactor. The Reynolds averaged Navier–Stokes (RANS) equations including the k–ε-turbulence model is used to simulate the turbulent flow. Different models are tested to describe the chemical reaction, including the Eddy dissipation concept (EDC) and the presumed probability density function (PDF) models. The EDC-model was developed for gas phase reactions and the objective of this work was to modify the model to make it more suitable for liquid phase reactions. Two different PDF-models are tested, namely the battlement- and the beta-PDF. The simulation results are compared to experimental data and the results has shown that the standard EDC-model is not suitable for liquid phase reactions, a modified version of the model has shown good results. The most promising PDF-model is shown to be the beta-PDF-model.     

CFD modelling of a mixing chamber for the realisation of functionally graded scaffolds

Biological tissues are characterised by spatially distributed gradients, intricately linked with functions. It is widely accepted that ideal tissue engineered scaffolds should exhibit similar functional gradients to promote successful tissue regeneration. Focusing on bone, in previous work we proposed simple methods to obtain osteochondral functionally graded scaffolds (FGSs), starting from homogeneous suspensions of hydroxyapatite (HA) particles in gelatin solutions. With the main aim of developing an automated device to fabricate FGSs, this work is focused on designing a stirred tank to obtain homogeneous HA–gelatin suspensions. The HA particles transport within the gelatin solution was investigated through computational fluid dynamics (CFD) modelling. First, the steady-state flow field was solved for the continuous phase only. Then, it was used as a starting point for solving the multi-phase transient simulation. CFD results showed that the proposed tank geometry and setup allow for obtaining a homogeneous suspension of HA micro-particles within the gelatin solution.