IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

In the present paper, the stability of the new oxyfuel combustion regime titled as Intermediate temperature Diluted Oxygen Combustion (INDOCS +) has been demonstrated theoretically. The operating conditions for the INDOCS are different from those of conventional oxyfuel combustion: the concentration by mass of oxygen in the oxidant stream is between 16 to 20% by mass (as opposed to 24% by mass in conventional oxyfuel combustion) and the temperature of the oxidant at entry to the furnace is in the range of 600 to 850 K (as opposed to an ambient temperature of about 300 K in conventional oxyfuel combustion). The method relies on the well-established fact that the stability of the flame increases with preheating of the oxidant (as demonstrated in High Temperature Air Combustion (HITAC) and FLAMELESS combustion regimes). The attendant high peak temperature of the flame can be reduced, to minimize NOx formation, by diluting the oxygen concentration. The overall benefits of this method are stable combustion at diluted oxygen concentrations, lower NOx (if N2 is present due to incomplete separation of oxygen from air), higher concentration of carbondioxide in the flue gases for a more efficient carbon sequestration. Flame structure calculations using the flamelet model of combustion with 35-step methane-air simplified reaction mechanism are used to demonstrate the stability of the flame. Simulations have also been carried out for the geometry of the 300 kW IFRF burner test facility; these show that the decreasing concentration of H and OH radicals in the quarl zone with increasing dilution by CO2 is counteracted by increasing the temperature of the oxidant. One of the principal advantages of this method is that, unlike in the case of HiTAC, the preheat requirements are mild which can be achieved by mixing part of the flue gas with the oxygen separated from air. It can be therefore readily implemented as a retrofitting measure in existing coal-fired plants.

Department Of Chemical Engineering

ASME International Mechanical Engineering Congress and Exposition, Proceedings

A new stable operating regime for oxyfuel combustion

Oxy-fuel combustion is one of the emerging technologies to capture and store CO2 emissions generated from thermal power plants. Research programs are in full swing to find out the possibility of retrofitting existing coal fired power plants with oxy-fuel mode combustion. Most of the Indian thermal power plants are pulverized coal based and use sub-bituminous Indian coals. These coals differ from the foreign coals in respect of maceral composition, ash content and combustion kinetics. It is imperative to understand the burning characteristics of Indian coals in oxy-fuel environment to evolve the suitable design for the retrofit of Indian boilers with oxy-fuel mode for the efficient and economic carbon capture without losing thermal efficiency. The present study involves the assessment of the kinetics of pyrolysis and char oxidation of three different Indian coals with varying ash contents viz. 33 to 44% in normal air and in oxy-fuel environment with three different CO 2/O2 concentrations (60:40, 70:30 and 80:20) by Thermo Gravimetric method (TGA). The results are compared in respect of weight loss pattern, peaking temperatures and weighted mean activation energy. The results have shown that the combustion kinetics of the selected high ash Indian coals in normal air combustion is more comparable with 70:30 and 80:20 than 60:40 CO2/O2 concentrations. The methodology adopted in the present study is found useful for comparing the combustion kinetics of various types of coals in normal air and oxy-fuel environment. Copyright © 2008 by ASME.

Burning profile of high ash Indian coals in oxy-fuel environment

The spreading of burning liquid sodium has been investigated using a depth-averaged shallow water equation for isothermal and non-isothermal (burning) conditions. In the latter case, the spreading is one-way coupled with the flame through a separate energy equation for the pool, with appropriate source terms for radiative and conductive heat transfer from the flame, and a sink term (for the continuity equation) to account for loss due to burning. Pool fires on soil and concrete surface have been considered with appropriate friction and heat transfer terms in the momentum and energy equations, respectively. Using this model, numerical simulations have been carried out for a wide range of leak rates, and for a range of burning rates of liquid sodium. Results obtained from the non-isothermal model show that the non-isothermal effects of liquid sodium spreading can safely be neglected for the case or spreading of burning liquid sodium on a typical ground surface such as concrete or soil. Based on these conclusions, dimensionless correlations are proposed for the prediction of spreading parameters such as, equilibrium pool radius, pool formation time, and for mass inventory under pool fire conditions for liquid sodium. These parameters which are obtained from the spreading code can be specified, as input parameters for the existing sodium fire safety codes. © 2009 Elsevier B.V. All rights reserved.

GENERAL

Nuclear Engineering and Design

Dynamics of liquid sodium pool spreading under sodium fire conditions

Experiments have been conducted using liquefied petroleum gas (LPG) as fuel and O2/CO2 mixture as oxidizer in a horizontally-fired swirl stabilized co-axial, 15 kW burner. Visual observations of the flame as well as temperature profiles across the flame have been carried out for different oxidant mixture compositions and two different swirl ratios. Comparative measurements have also been made with air as the oxidizer. Results from these experiments show that stable combustion is possible and that, from the point of view of temperature profiles, oxy-LPG flames are equivalent to conventional air flames when the oxygen concentration is 34% by volume in the oxidizer. Increase in swirl velocity results in stable and symmetric flames. The oxyfuel flame is typically less bright and more bluish than its normal air counterpart. These results will be useful in retrofitting existing combustion systems for operation under oxyfuel combustion mode. © 2009 Combustion Institute. All rights reserved.

7th Asia-Pacific Conference on Combustion, ASPACC 2009

Experimental study of oxy-fuel combustion in a swirl-stabilized burner

In the present paper, a new design of oxy-coal gasification combined cycle power plant is proposed based on the Intermediate temperature Diluted Oxygen Carbon sequestration (INDOCS) concept. Unlike in conventional exhaust gas recycling, the synthesis gas from coal gasification (syngas) is burnt in the presence of a preheated oxidant consisting of a reduced concentration of oxygen and CO2-enriched recycled flue gas. The oxidant is constituted by mixing oxygen from an air separator unit with exhaust gases from the gas turbine from which steam is partially removed in a flue gas conditioner. The reduced oxygen concentration will lead to lower conversion of N2 to NOx while recycling of the CO2-enriched, preheated exhaust gas leads to a higher mass flow rate and lower volumetric flow rate of the gaseous mixture. Thus, a more compact gas turbine can be used to generate the same power. The resulting flue gas is also significantly richer in CO2 at the gas turbine exit enabling a more efficient sequestration of CO2. © 2009 Combustion Institute. All rights reserved.

CO2-enriched flue gas recycling for more effective CO2 sequestration

A numerical study of oxy-fuel combustion has been carried out in the pressure range of 0.1-3 MPa with methane as the fuel and carbondioxide-diluted oxygen with trace amount of nitrogen (termed here as c_air) as the oxidant. The flame structure and NO generation rate have been calculated using the flamelet model with the detailed GRI 3.0 mechanism for two oxygen concentrations of 23.3% and 20% by weight in the oxidant at a strain rate of 40 s-1 (corresponding to a scalar dissipation rate of 1 s-1). It is observed that, for the reference case of 23.3 wt.% of oxygen, as the pressure increases, the peak temperature of the flame increases rapidly up to a pressure of 0.5 MPa, and more gradually at higher pressures. The concentrations of important intermediate radicals such as CH3, H and OH decrease considerably with increasing pressure while NO concentration follows the same trend as the temperature. Reducing the oxygen concentration to 20% by weight leads to an order of magnitude reduction in NO concentration. Also, for pressures greater than 0.3 MPa, the NO concentration decreases with increasing pressure in spite of the increasing peak flame temperatures. This can be attributed to the increasing domination of recombination reactions leading to less availability of the intermediate radicals H and OH which are necessary for the formation of NO by the thermal route. It is concluded that a stable, low NOx oxy-fuel flame can be obtained at high pressures at slightly increased dilution of oxygen. © 2008 Elsevier Ltd. All rights reserved.

Energy Conversion and Management

Flame structure and NO generation in oxy-fuel combustion at high pressures

Power plant ducting generally designed with simple shapes has to undergo many changes of shape to accommodate interfacing equipment associated with plant operation leading to higher pressure drop, higher power consumption and flow maldistribution zones having higher or lower velocities. To redress this situation, baffles, guide vanes and other internals are used to streamline the flow through ducts, especially in bends. A basic disadvantage in coal fired plants of using baffles is that they get punctured/eroded due to impact of high velocity ash particles in flue gas ducting, and the effectiveness of baffles is lost in short duration. To overcome the above disadvantages, a new method is developed to change the shape of the duct in such a way that a more streamlined flow is maintained across any cross section. The velocity profile, obtained using computational fluid dynamics (CFD) calculations, across the cross-section is examined at several locations along the duct. Wherever high velocity compared to average velocity is found, the cross-section is increased and where the velocity is low. the cross-section is reduced. A new grid is created through the revised cross-section and a fresh CFD analysis is made to identify zones of flow maldistribution. The flow simulation is done in an iterative manner, alternately calculating the flow domain and modifying the local cross-section based on the local velocity distribution. The method has been found to be more robust and led, after a few iterations, to a shape of the duct which resulted in a significant reduction in the pressure drop without using any baffles or inserts. Copyright © 2008 by ASME.

Shape optimization of power plant ducting using CFD

Oxy-fuel combustion, in which a conventional hydrocarbon fuel is burned in presence of oxygen diluted with carbon dioxide (called henceforth as oxy-fuel), is an emerging technology that accommodates CO2 sequestration while offering the prospect of low emissions. Dilution by CO2 (from flue gas recirculation) prevents high peak temperatures thereby reducing material damage, high NOX formation etc. Studies [1] reveal that CO 2-diluted flames are unstable as compared to N2-diluted flames and that a higher molar fraction of oxygen, 30% by volume, is necessary to ensure stable combustion. Nevertheless, as the heat transfer properties of CO2/O2-70:30 mixtures are different to that of normal air, the flow profiles and heat flux distributions might vary within the furnace. While considering the retrofit of existing normal air operated coal fired furnaces with oxy-fuel mode, it is imperative to investigate these flow variations in detail. In the present study, Computational Fluid Dynamics (CFD) based simulations have been done for a typical 210 MW Indian pulverized coal fired furnace under normal air combustion and under oxy-fuel combustion (CO2/O2-70:30 volume %) mode and the results in terms of CO2 concentration, temperature distribution, velocity of flue gas, particle trajectories and devolatilisation characteristics have been compared. Copyright © 2008 by ASME.

Optimization of a coal-fired furnace for oxy fuel combustion

The flow field is one of the main components of a fuel cell, which distributes the reactants to the active area of the cell and evacuates the products formed. Interdigitated flow field (IFF) is one among the different types of flow field designs that forces the reactants or products to flow through the electrode, thereby increasing the cell performance by decreasing concentration polarization loss, however, at the cost of higher-pressure drop. Prior understanding of the reactant and water vapour distribution in a flow field helps in obtaining the best flow field design. In the present paper, a model for the flow distribution and the pressure drop in an IFF has been developed using the analogy between fluid flow and electrical network in which the pressure is made analogous to the voltage and the flow rate to the current. The model, which ultimately reduces to the solution of a set of simultaneous algebraic equations, is capable of predicting the flow split among a set of inlet and outlet channels of an interdigitated flow field as well as the overall pressure drop for laminar, turbulent and two-phase flow conditions for arbitrary number of parallel channels. The results from the hydrodynamic network model have been validated against CFD simulations. This model can therefore be used for the optimization of interdigitated flow field design. © 2009.

International Journal of Hydrogen Energy

A hydrodynamic network model for interdigitated flow fields

Polymer electrolyte membrane direct methanol fuel cells (PEM-DMFCs) have several advantages over hydrogen-fuelled PEM fuel cells; but sluggish methanol electrochemical oxidation and methanol crossover from the anode to the cathode through the PEM are two major problems with these cells. In the present work, a comprehensive one-dimensional, single phase, isothermal mathematical model is developed for a liquid-feed PEM-DMFC, taking into account all the necessary mass transport and electrochemical phenomena. Diffusion and convective effects are considered for methanol transport on the anode side and in the PEM, whereas only diffusional transport of species is considered on the cathode side. A multi-step reaction mechanism is used to describe the electrochemical oxidation of methanol at the anode. Stefan-Maxwell equations are used to describe multi-component diffusion on the cathode side and Tafel type of kinetics is used to describe the simultaneous methanol oxidation and oxygen reduction reactions at the cathode. The model fully accounts for the mixed potential effect caused by methanol crossover at the cathode. It shows excellent agreement with literature data of the limiting current density for different low methanol feed concentrations at different operating temperatures. At high methanol feed concentrations, oxygen depletion on the cathode side, due to excessive methanol crossover, results in mass-transport limitations. The model can be used to optimize the geometric and physical parameters with a view to extracting the highest current density while still keeping a tolerably low methanol crossover. © 2008 Elsevier B.V. All rights reserved.

Journal of Power Sources

Comprehensive one-dimensional, semi-analytical, mathematical model for liquid-feed polymer electrolyte membrane direct methanol fuel cells

The spreading of an accidental spill of Liquefied Natural Gas (LNG) on sea water has been studied for many years and several theoretical models have been proposed and successfully used. Many modeling techniques have been used by researchers for the spreading of LNG. However, most of these neglect the heat transfer aspects related to the spreading, and the effect of temperature dependent properties such as density, thermal conductivity and specific heat of LNG is not included in the analysis. In the present study, this situation is redressed by including the depth-averaged energy equation in a one-dimensional model of the spreading of LNG on sea water. The thermophysical and transport properties of the fluid are made temperature-dependent and heat transfer to the pool from the water below and the flame above are included. The resulting set of coupled one-dimensional mass, radial momentum and energy balance equations are solved numerically using an explicit, second order-accurate finite difference method-based discretization of the governing equations. Results obtained in the present study show that the incorporation of the variable properties gives significantly improved predictions over conventional models. The predicted results are compared with the experimental results of Raj et al [1], and with a conventional, constant-properties model of Fay [2] for the test case #12. Excellent agreement is found between the current model predictions and the experimental data while the conventional model overpredicts the pool diameter for longer times. It is demonstrated that the present approach is inherently capable of distinguishing between the spreading of different LNG mixtures, and can therefore be readily extended to the analysis of the accidental spill of any other hazardous substance. Copyright © 2008 by ASME.

Energy considerations in the spreading of LNG on sea water

Combustion of liquid sodium is of interest in the safety assessment of liquid metal cooled fast breeder reactor systems. In the present study, a detailed thermodynamic analysis of sodium-air system has been carried out for equivalence ratios in the range of 0.1-1.9 and for flame temperatures ranging from 1100 to 1950 K. In addition to this, decomposition calculations presented for product aerosols such as sodium oxide (Na2O), sodium dioxide (Na2O2) and sodium hydroxide (NaOH) in normal oxygen and oxygen-deficient conditions (which are some form of phase diagrams of these aerosols) are used rigorously to find out the predominant aerosol that should be present in and outside the burn pan for a pool fire of liquid sodium. The conditions of occurrence of various sodium oxides under two different fire conditions namely pool and jet fires have been worked out. It is established that heterogeneous reactions involving sodium oxide are responsible for the formation of sodium dioxide and sodium hydroxide. It is necessary to take account of the rates of these heterogeneous reactions as well as the equivalence ratio-dependent decomposition calculations to correctly estimate the aerosol product mix in practical situations. © 2008 Elsevier B.V. All rights reserved.

Equilibrium considerations in aerosol formation during sodium combustion

The effect of oscillations on the heat transfer in a vertical tube has been studied experimentally. A vertical tube was mounted on a plate and the whole plate was subjected to oscillations in the vertical plane using a mechanical oscillator to provide low frequency oscillations. A section of the tube in the middle is subjected to a constant heat flux. The effect of the oscillations on the heat transfer coefficient has been examined. It was found that the heat transfer coefficient increased with oscillations in the laminar regime. In turbulent flow regime (Re > 2,100) it is found that the effect of oscillations did not show any change. A correlation has been developed for enhancement of the local Nusselt number in terms of the effective acceleration and Reynolds number. Using this, an expression has been proposed to calculate the mean Nusselt number as a function of the tube length. © 2007 Springer-Verlag.

Heat and Mass Transfer/Waerme- und Stoffuebertragung

Convective heat transfer in single-phase flow in a vertical tube subjected to axial low frequency oscillations

Heat transfer and other equipment mounted on off-shore platforms may be subjected to low frequency oscillations. The effect of these oscillations, typically in the frequency range of 0.1-1 Hz, on the flow rate and pressure drop in a vertical tube has been studied experimentally in the present work. A 1.75 m-long vertical tube of inner diameter 0.016 m was mounted on a plate and the whole plate was subjected to oscillations in the vertical plane using a mechanical simulator capable of providing low frequency oscillations in the range of 8-30 cycles/min at an amplitude of 0.125 m. The effect of the oscillations on the flow rate and the pressure drop has been measured systematically in the Reynolds number range 500-6500. The induced flow rate fluctuations were found to be dependent on the Reynolds number with stronger fluctuations at lower Reynolds numbers. The effective friction factor, based on the mean pressure drop and the mean flow rate, was also found to be higher than expected. Correlations have been developed to quantify this Reynolds number dependence. © 2007 Elsevier B.V. All rights reserved.

Flow and pressure drop fluctuations in a vertical tube subject to low frequency oscillations

Serpentine flow-fields have long been used in polymer electrolyte membrane (PEM) fuel cells for effective transportation of reactants from the flow-field to the reaction sites. It has been observed in the literature that localized flooding near the U-bend region of serpentine flow-fields occurs at high current densities. This has been attributed to the boundary layer separation and recirculation of flow in the U-bend. In the present study, it is established, using computational fluid dynamics (CFD) simulations, that this is due to lower channel-to-channel cross-flow in the electrodes between consecutive serpentine channels rather than to the flow-field in the gas distribution channels. © 2008 Elsevier B.V. All rights reserved.

Effect of channel-to-channel cross-flow on local flooding in serpentine flow-fields

The performance of a circulating air classifier has been studied systematically by conducting experiments over a range of process and design variables. It is found that while the overall induced flow rate is proportional to the speed of rotation of the wheel, the circulation pattern inside the classifier depends on the configuration of stationary guide vanes. This is found to have a significant effect on the range of operability of the classifier. Results show that the cut size, sharpness of separation and the bottom and top size selectivity increments are influenced strongly by the stationary guide vane configuration. Using experimental data from a dense material (fly ash) and a light material (rice husk), a model based on the reduced efficiency curve approach, originally proposed for hydrocyclones, has been developed to predict the performance of the classifier. © 2007.

Chemical Engineering and Processing: Process Intensification

A reduced efficiency approach-based process model for a circulating air classifier

Proper water management is critical for near-ambient temperature operation of a fuel cell. A simplified water balance model has been developed to predict when incipient flooding/dehydration may take place. The present model is based on multicomponent diffusion in the electrodes and molar balance in the flow channels. The overall model is in the form of a closed-form expression for the critical or threshold or balance current density at which the water production rate and the water vapor evacuation rate are exactly balanced. The model incorporates the influence of the operating conditions, properties of electrodes, and flow and geometric parameters in the gas channels on the balance current density. Predictions from the model of the state - incipient flooding or dehydration - of operation of the fuel cell agree well with the available experimental data. Using the model, a parametric study has been conducted over a range of parameters. Copyright © 2007 by ASME.

Journal of Fuel Cell Science and Technology

Simplified model to predict incipient flooding/dehydration in proton exchange membrane fuel cells

The existing literature on CFD-based coal combustion modelling is applicable mainly for coals of low ash content and the calculations are done on an ash-free basis. In Indian coals, the ash content may be significantly higher, up to 40% or more. Studies reported in the literature show that the mineral matter in the coal may have a number of effects on the combustion characteristics. In the present study, a sensitivity analysis is performed, using the CFD code CFX of AEA Technology, on the likely effect of ash content on the char reactivity, oxygen diffusion rate for char combustion and on the radiative heat transfer parameters. The results show that the effect of enhanced char reactivity is negligible whereas reduced oxygen diffusion rates due to a thicker ash layer may result in a significant reduction in char oxidation rates with a resultant decrease in the peak temperature in the furnace. The global parameters such as the peak temperature and the flue gas temperature remain relatively insensitive to the presence of high ash content. These results are consistent with the experimental observations of Kurose et al. [R. Kurose, M. Ikeda, H. Makino, Combustion characteristics of high ash coal in pulverized coal combustion, J. Fuel 80 (2001) 1447-1455]. © 2006 Elsevier Inc. All rights reserved.

Fulltext

Applied Mathematical Modelling

Assessment of the effect of high ash content in pulverized coal combustion

AIChE Journal

An eulerian/lagrangian study of solid suspension in stirred tanks

Measurements of two-phase flow pressure drop have been made during a phase-change heat transfer process with refrigerant (R-134a) as a working fluid for a wide range of pressures right up to the critical pressure. The experiments were conducted in a uniformly heated vertical tube of 12.7 mm internal diameter and 3 m length over a heat flux range of 35-80 kW/m2, mass flux range of 1200-2000 kg/m2 s, exit quality range of 0.19-0.81 and for reduced pressures ranging from 0.24 to 1 with a fixed inlet subcooling of 3 °C. The measurements were compared with the predictions from the homogeneous flow model, a separated flow model using correlations drawn from the literature for void fraction and frictional pressure drop, and finally, using a flow pattern-based predictive method accounting specifically for bubbly, slug and annular flow regimes. It was found that the best results were obtained with the flow pattern-based approach with a mean deviation of ±20% over the entire pressure range. © 2006 Elsevier Ltd. All rights reserved.