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3-E (Energy, Exergy, and Environment) analysis of Pressurized Pulverized Combined Cycle (PPCC) SubCritical (SubC) and SuperCritical (SupC) power plants of net capacity 400 MW e using High Ash (HA) Indian coal under Indian ambient conditions is performed to assess the potential of PPCC power plant for electricity generation in Indian energy sector. The study also includes the comparative performance of the plant using an imported Low Ash (LA) coal and a parametric study is performed to understand the effect of various parameters affecting the energy and exergy efficiencies of the PPCC plant. It is found that the plant energy efficiency of PPCC SubC and SupC plants using HA coal are 42.44% and 43.46%, respectively. The exergy efficiency of the same plant using HA coal are 38.94% and 39.87%, respectively. The energy balance shows that the maximum energy loss is observed in cooling water (24%) followed by loss in stack (22%). The exergy balance shows that the maximum exergy destruction in combustor (32%) followed by stack (7%). The environmental analysis reveals that the CO 2 , NO x , and SO x emissions are 426, 3.54, and 3.20 g/kWh, respectively.

In India, more than two-thirds of the total electricity is generated from coal-based power plants. The country will be heavily dependent on coal for electricity generation at least for a few more decades. A low carbon scenario for 2030 is projected and the results show that coal-based power plant will contribute to 58 % of electricity generated. This paper reports the sustainability assessment of several Advanced Coal Technologies (ACT), with and without a CO2 capture system, for application in Indian coal-based power plants. In order to assess sustainability quantitatively, an index called Sustainability Index is estimated using a sequential combination of the Analytic Hierarchy Process and a multi-criteria decision-making method. The results show that ACTs like supercritical and ultra supercritical power plants with a carbon capture system have higher sustainability indices. These ACTs will help in reducing the CO2 emissions and combat climate change. This study helps academicians, industrialists, policymakers, and other stakeholders to develop a technology roadmap for sustainable development in India.

Methanol crossover from anode to cathode is a process which adversely affects the performance of a DMFC (direct methanol fuel cell). Increasing the electrolyte membrane thickness and addition of a MPL (microporous layer) using PTFE (polytetrafluoroethylene) loading are two techniques used to reduce methanol crossover by increasing the mass transfer resistance. This paper reports experiments carried out to study the effect of membrane thickness and PTFE loading in anode MPL on methanol crossover in a 25 cm2 DMFC. The rate of methanol crossover is indirectly measured by measuring the CO2 concentration at the cathode exit. The influence of PTFE content (0–20%) and membrane thickness (183 μm and 254 μm) on limiting current density, peak power density and cell efficiency are reported. The experimental results show that the methanol crossover current density is reduced by 24% using thicker membrane compared to MEA (membrane electrode assembly) with thin membrane. This leads to enhanced peak power density of 22 mW/cm2 with cell efficiency of 10%. About 20% of methanol crossover current density is reduced by 10% PTFE loading in anode MPL, which helps in improving peak power density from 13 to 24 mW/cm2 with cell efficiency of 8% compared to membrane as mass transfer resistance layer.

More than two third share of electricity come from coal fired power plants in India. Coal fired power plants are the largest source of anthropogenic CO2 emissions per unit of electricity generation among all fossil fuel based power plants. There has been climate change and global warming globally due to increasing anthropogenic emission of greenhouse gas (GHG) into the atmosphere. This paper examines life cycle GHG emission such as CH4, CO2 and N2O of a National Thermal Power Corporation (NTPC) Limited power plant using life cycle approach. The various stages involved in the assessment of life cycle GHG emissions in the present study include coal mining, transportation of coal to the power plant and coal combustion for electricity generation. The results show that direct CO2 emission from coal combustion is about 890 g CO2-e/kWh, whereas life cycle GHG emissions amount to 929.1 g CO2-e/kWh. Indirect GHG emissions add up to 4.2% of total emissions. Coal mine methane leakage into atmosphere in India is low since more than 90% of the coal mining is surface mining.

This study deals with the performance analysis of 500 MWe pulverized coal-fired power plants based on subcritical (SubC), supercritical (SupC), and ultrasupercritical (USC) steam parameters with postcombustion CO2 capture using high ash (HA) Indian coal. The performance of power plants is evaluated in terms of plant energy efficiency and CO2 avoided. Furthermore, the effect of an imported low ash (LA) coal on power plant performance parameters is also determined compared with the HA Indian coal. SupC and USC power plants result in an increase of about 0.8% and 6.4% points, respectively, in the plant energy efficiency compared with the SubC power plant. Consequently, a reduction of about 3% and 16.5% in CO2 emission is observed for the same plants using both HA and LA coals that, in turn, will aid in significant gain in carbon credits for a developing country such as India. However, incorporation of postcombustion CO2 capture unit in the proposed power plant configurations result in a significant drop of about 8% - 13% points in the plant energy efficiency. � 2013 Taylor and Francis Group, LLC.

SUMMARY A thermodynamic analysis of a 500-MWe subcritical power plant using high-ash Indian coal (base plant) is carried out to determine the effects of carbon dioxide (CO2) capture on plant energy and exergy efficiencies. An imported (South African) low-ash coal is also considered to compare the performance of the integrated plant (base plant with CO2 capture plant). Chemical absorption technique using monoethanolamine as an absorbent is adopted in the CO2 capture plant. The flow sheet computer program “Aspen Plus” is used for the parametric study of the CO2 capture plant to determine the minimum energy requirement for absorbent regeneration at optimum absorber–stripper configuration. Energy and exergy analysis for the integrated plant is carried out using the power plant simulation software “Cycle-Tempo”. The study also involves determining the effects of various steam extraction techniques from the turbine cycle (intermediate-pressure–low-pressure crossover pipe) for monoethanolamine regeneration. It is found that the minimum reboiler heat duty is 373 MWth (equivalent to 3.77 MJ of heat energy per kg of CO2 captured), resulting in a drop of plant energy efficiency by approximately 8.3% to 11.2% points. The study reveals that the maximum energy and exergy losses occur in the reboiler and the combustor, respectively, accounting for 29% and 33% of the fuel energy and exergy. Among the various options for preprocessing steam that is extracted from turbine cycle for reboiler use, “addition of new auxiliary turbine” is found to be the best option. Copyright © 2011 John Wiley & Sons, Ltd.

Fossil fuels, coal, and natural gas, contribute a major share of electricity generated in India as well as the world and will continue to do so well into the future. With the justified concern of their rapid depletion and the environmental impacts associated with their large-scale use, there is a quest for advanced coal-fired power generation technologies which are energy efficient and environmental friendly. This study analyses the thermodynamic performance of an existing 62.5 MWe conventional Rankine cycle power plant using pulverized coal firing (PF) (reference plant with atmospheric pulverized coal combustion) repowered with a new and potentially advantageous technology, namely, pressurized pulverized coal firing in a combined cycle (PPCC) power plant. The performance of power plants is determined based on energy and exergy analysis. The energy and exergy efficiencies of the PF plant are estimated to be 28.3 and 25.9 per cent, respectively, whereas the PPCC power plant results in a significant increase of 6.3 and 5.7 per cent points, respectively, over the PF plant. Moreover, PPCC power plant results in an increase of about 2.6 times in the gross plant output (162 MWe) compared with the PF power plant. The gas turbine alone contributes to an additional 100 MWe when the steam turbine output is maintained constant at 62.5 MWe as in reference PF power plant.

Devolatilization of a single wet wood particle in a hot atmospheric fluidized bed is studied through a mathematical model and experiments. A typical tropical woody biomass, “Casuarina Equisetifolia”, is used in the study. A three-dimensional (3D) transient model with detailed consideration of (i) fuel anisotropy, (ii) fuel moisture, (iii) shrinkage during drying and devolatilization, (iv) heat generation, and (v) variable properties with a suitable reaction scheme is developed to determine the devolatilization time (τd) and char yield (Yc) of cubic/cuboidal wood particles at various initial fuel moisture contents and bed temperatures. Experiments are conducted for 10, 15, 20, and 25 mm cube-shaped wood particles at bed temperatures of 1023, 1123, and 1223 K. The model predictions agree well with the measured data (present experiments and those reported in the literature) within ±10% for the devolatilization time and ±11% for the char yield, supporting the validity of the overall structure of the model. A ...

This work presents the results of experiments conducted to determine the mass loss characteristics of a cylindrical wood particle undergoing devolatilization under oxidation conditions in a bubbling fluidized bed combustor. Cylindrical wood particles having five different sizes ranging from 10 to 30 mm and aspect ratio (l/d = 1) have been used for the study. Experiments were conducted in a lab scale bubbling fluidized bed combustor having silica sand as the inert bed material and air as the fluidizing medium. Total devolatilization time and mass of wood/char at different stages of devolatilization have been measured. Studies have been carried out at three different bed temperatures (Tbed = 750, 850 and 950 �C), two inert bed material sizes (mean size dp = 375 and 550 ?m) and two fluidizing velocities (u = 5umf and u = 10umf). Devolatilization time is most influenced by the initial wood size and bed temperature. Most of the mass is lost during the first half of the devolatilization process. There was no clear influence of the fluidization velocity and bed particle size on the various parameters studied. The apparent kinetics estimated from the measured mass history show that the activation energy varied narrowly between 15 and 27 kJ/mol and the pre-exponential factor from 0.11 and 0.45 s-1 for the wood sizes considered. � 2009 Elsevier Ltd. All rights reserved.

A three dimensional, steady state, non-isothermal, single phase model was developed and simulations were carried out in order to find the effect of cathode channel dimensions (width, depth and height) on the performance of an air-breathing fuel cell. The model was solved using commercial CFD package Fluent (version 6.3). Separate user defined functions were written to solve the electrochemical equations and the water transport through the membrane along with the other governing equations. Analyses were carried out for three different channel widths (2, 4 and 6�mm), for three different channel depths (2, 6 and 10�mm) and for three different cell heights (15, 30 and 45�mm). Cell characteristics like current distribution, species distribution, oxygen mass transfer coefficient, cell temperature, cathode channel velocities and net water transport coefficients are reported. The results show that the cell performance improves with increase in cathode channel width, channel depth and with decrease in cell height. Maximum power density obtained was 240�mW/cm2 for a channel width of 4�mm and channel depth of 6�mm. When the channel depth was 2�mm the performance was limited mainly due to the resistance offered by the channel for the buoyancy induced flow. For channel depths higher than 2�mm, the diffusion resistance of the porous GDL also contributed significantly to limit the performance to low current densities. At low current densities the fuel cell is prone to flooding whereas at high current densities ohmic overpotential due to dehydration of the membrane significantly contributes to the overall voltage loss. � 2009 Elsevier Masson SAS. All rights reserved.

Effect of cathode design on the performance of an air-breathing fuel cell is studied using a three dimensional, non-isothermal, steady state and single phase model developed using commercial CFD software FLUENT (version 6.3). Performances of ducted (channel) and ribbed (planar) cathode designs are compared and the cell characteristics such as current density, species, temperature distributions, velocity and net water transport coefficient are presented. Peak power density obtained for the cell with ducted cathode is 205 m W/cm2, whereas with ribbed cathode it is 232 m W/cm2. The limiting current density of the cell with ribbed cathode (690 m A/cm2) is much higher than that of the cell with ducted cathode (430 m A/cm2). The performance curves as well as the cell characteristics show that the ribbed cathode design is a better configuration compared to ducted design. Cell orientation has a significant effect on the cell performance. Best performance is obtained when the cell is oriented vertically for both ribbed and ducted cathode designs. � 2009 Professor T. Nejat Veziroglu.

This work proposes a transient heat transfer model to predict the thermal behaviour of wood in a heated bed of sand fluidized with nitrogen. The 2-D model in cylindrical coordinates considers wood anisotropy, variable fuel properties, fuel particle shrinkage, and heat generation due to drying and devolatilization. The influence of initial fuel moisture content, thermal diffusivity, particle geometry, shrinkage, external heat transfer coefficient, chemical reaction kinetics and heats of reaction on temperature rise is presented. The cylindrical wood particles chosen for the study have length (l) = 20�mm, diameter (d) = 4�mm and l = 50�mm and d = 10�mm, both having an aspect ratio (l/d) of 5. The bed temperature is 1123�K. The model prediction is validated using measurements obtained from literature. The temperature rise in the wood particle is found to be sensitive to changes in the moisture content and thermal diffusivity and heat of reaction (in larger particles) while it is less sensitive to the external heat transfer coefficient and chemical kinetics. Also shrinkage is found to have a compensating effect and it does not have any significant influence on the temperature rise. Beyond an aspect ratio of three, the wood particle behaves as a 1-D cylinder. � 2008 Elsevier B.V. All rights reserved.

A mathematical model capable of estimating the temperature and stress distribution in a devolatilizing cylindrical wood particle is proposed. The model assumes wood as orthotropic and comprises a thermal submodel and a stress submodel. It takes into account the contributions to stress by the temperature rise, shrinkage during drying and devolatilization, and mechanosorption. The thermal submodel estimates the temperature distribution and shrinkage as a function of space and time to be used as inputs to the stress submodel. Calculations have been carried out for cylindrical wood particles of aspect ratio l/d = 1 and with dimensions of 10, 20, and 30 mm. The model predictions compared with the results of an available drying model are found to be satisfactory. During devolatilization of the wood particle, the model predicts high compressive stresses at the center while those at the surface are tensile in nature and moderate in magnitude. A modified von Mises criterion predicts failure at the axial center of the cylinder, with the size of the crack formed by the failure increasing from 0.4 to 1.4 mm in diameter as the size of the wood particle increases from 10 to 30 mm. The predicted crack radius and the time of crack initiation are compared with the measurements carried out on wood particles undergoing devolatilization in a fluidized bed combustor. The predicted crack radius and time of crack initiation are found to be sensitive to the magnitude of the mechanical properties. � 2008 American Chemical Society.

This paper presents a two-dimensional, steady state, single phase, non-isothermal and complete model for a vertical, fully planar, Air Breathing Proton Exchange Membrane Fuel Cell (AB-PEMFC) with hydrogen fuel supplied by forced convection at the anode. It is applied to a cell with an active area of 6 cm 2 operating at ambient atmospheric conditions of 23 °C and 20% relative humidity. The transport characteristics in terms of the velocity, and heat and mass transfer coefficients in the various components of the fuel cell are reported for several values of the operating current density. The effect of cathode height (1–5 cm) and operating atmospheric conditions (10–40 °C and 20–80% relative humidity), on the cell performance is also reported. Further, the applicability of the model to a non-planar AB-PEMFC is examined by comparison with available experimental data. The average mass transfer coefficients for oxygen and water vapor at the cathode GDL surface are found to be of the order of 10 −3 m s −1 . The operating current density is seen to substantially affect the variation of the local current density distribution and the cathode surface temperature along the height of the fuel cell as also the temperature variation across the MEA thickness. The maximum power density and the corresponding current density, herein defined as the optimum current density, are found to increase with decreasing height of the fuel cell, decreasing ambient temperature and increasing ambient relative humidity. However, the local cell temperature at high current densities is found to increase beyond the safe operating limits for short fuel cells. Comparison of the model predictions with available experimental data points to its applicability in the ohmic polarization zone of a non-planar cell. The cell performance at high current densities deteriorates due to mass transport limitation and electrolyte dehydration.

The shrinkage characteristics of Casuarina wood in terms of the effect of fuel particle shape and size on the longitudinal, transverse, and volumetric shrinkage during devolatilization in a laboratory scale bubbling fluidized bed combustor are presented. Shrinkage of single fuel particles was measured in the longitudinal and transverse directions (with respect to the wood fibre) for various fuel particle shapes-like disc ( l / d = 0.2 – 0.6 7 ), cylinder ( l / d ∼ 1 ) and rod ( l / d = 2 – 1 0 ). The fuel particle dimensions ranged from 5 to 100 mm. The effect of the bed temperature on the shrinkage was studied by varying the bed temperature in the range of 650–850 °C. Fuel particle shape and size were found to influence the shrinkage in the two mutually perpendicular directions. The variation in the fuel particle heating rates for various shapes and sizes was found to be the cause of the variation in the shrinkage values. For all the shapes and sizes considered, the longitudinal shrinkage was found to be in the range of 6.5–24%, the transverse shrinkage from 14% to 29%, and the volumetric shrinkage from 35% to 58%. The average volumetric shrinkage was estimated to be 47% with a standard deviation of ±3.8%. Shrinkage increased negligibly with the increase in bed temperature. Increase in fuel particle density led to a decrease in volumetric shrinkage, however, this effect was not conclusive because of the effect of other factors-like chemical composition and wood type. Correlations for estimating the shrinkage coefficients in the two principal directions are presented.

Experimental surface-average heat transfer coefficients of a vertical tube located at various positions along the axis of a Circulating Fluidized Bed riser (CFB) are presented. Experiments were carried out on the tube in a 100 mm ? 100 mm cross-section and 5.5 m tall CFB cold unit using silica sand of mean particle size 363 ?m as the bed material. The copper tube was 9.6 mm OD and 0.6 m high placed in the core of the riser with hot water flowing though it. The tube was located axially at distances of 0.97 m, 1.62 m, 3.0 m and 4.0 m from the distributor plate. The fluidizing air velocity and solid circulation flux were varied in the range of 4.5-7.3 m�s-1 and 21-72 kg�m-2�s-1 respectively. The measured average heat transfer coefficient varied in the range of 58-101 W�m-2�K-1 and showed a decreasing trend from the riser bottom to the riser exit. At a given location, increasing the fluidizing velocity resulted in reduced heat transfer coefficient while increasing the solid circulation flux enhanced it. At a given average suspension density, the local dynamics of gas and solids adjacent to the tube surface affected the heat transfer. Gas convective contribution was found to be substantial at high gas velocities especially at higher tube locations. Using the experimental data for the axial tube, two empirical heat transfer correlations were developed in terms of the important design and operating parameters including a non-dimensional height parameter. Comparisons were made with literature available under similar but not identical operating parameters. ? 2002 �ditions scientifiques et m�dicales Elsevier SAS. All rights reserved.

Thermodynamic optimization of power plants based on supercritical (SupC) and ultrasupercritical (USC) steam parameters is reported in this article. The objective is to compute the maximum attainable power plant efficiency in Indian climatic conditions using high ash (HA) indigenous coal. A unit size of 800 MWe presently under development in India is considered for energy and exergy analysis of power plants. Commercially established steam turbine parameters are used for the optimization of SupC power plant, whereas advanced steam turbine parameters currently under research and development are used for the optimization of USC power plant. The plant energy efficiency of the optimized SupC and USC power plant based on air-coal combustion (ACC) show considerable increases of 2.8 and 5.2% points, respectively compared with the current SupC ACC power plant (reference plant) being commissioned in India. The increases in plant exergy efficiency for the same power plants are 2.6 and 4.8% points and the corresponding CO2 reductions are about 6 and 11%, respectively. The maximum possible plant energy efficiency in Indian climatic conditions using HA Indian coal is about 42.7% (USC power plant). The effect of low ash coal on plant energy and exergy efficiencies compared with HA coal is also presented. Further, the effect of oxy-coal combustion (OCC) on the plant energy and exergy efficiencies compared with the ACC is studied for the double reheat SupC and USC power plants to account for the impact of CO2 capture. A significant reduction of 8.8 and 6.6% points in plant energy efficiency is observed for SupC and USC OCC power plants, respectively compared with the reference SupC ACC power plant. � 2011 American Society of Mechanical Engineers.

Considerable amount of investigation on the subject of devolatilization of wood is found in the open literature. However, a systematic study of the effect of initial particle size and shape, and bed temperature on devolatilization time and char yield of wood in a hot fluidized bed is still missing. This paper attempts to fill this gap through a systematic experimental investigation to determine the devolatilization time and char yield of a typical woody biomass, "Casuarina equisetifolia" particles of different initial sizes and shapes at various fluidized bed temperatures. Experiments are conducted using 10, 15, 20, and 25 mm Casuarina wood particles of three shapes, namely, cube, cylinder, and sphere at bed temperatures of 1023, 1123, and 1223 K. It is found that the initial wood particle size has the strongest influence on devolatilization time followed by the shape of initial wood particle and the bed temperature. Correlation for devolatilization time (?d) as a function of initial wood particle size (deq), sphericity (?), and bed temperature (Tb), is developed using 573 experimental data points exhibiting a correlation coefficient of 0.96 and predictions falling well within a deviation band of �20%. The predictions of the present correlation are compared with the predictions of the existing correlations in literature for conditions also out of the present study and the deviation is found to be �30%. Char yield, defined as the ratio of the residual mass at the end of devolatilization process to the initial mass of the wood particle is found to be in the range of 9-14% for all sizes, shapes, and bed temperatures. Char yield does not depict any definite trend with the variation of initial particle size, shape and bed temperature. � 2011 Elsevier B.V. All rights reserved.

The efficiency of coal-fired power plant depends on various operating parameters such as main steam/reheat steam pressures and temperatures, turbine extraction pressures, and excess air ratio for a given fuel. However, simultaneous optimization of all these operating parameters to achieve the maximum plant efficiency is a challenging task. This study deals with the coupled ANN and GA based (neuro-genetic) optimization of a high ash coal-fired supercritical power plant in Indian climatic condition to determine the maximum possible plant efficiency. The power plant simulation data obtained from a flow-sheet program, " Cycle-Tempo" is used to train the artificial neural network (ANN) to predict the energy input through fuel (coal). The optimum set of various operating parameters that result in the minimum energy input to the power plant is then determined by coupling the trained ANN model as a fitness function with the genetic algorithm (GA). A unit size of 800. MWe currently under development in India is considered to carry out the thermodynamic analysis based on energy and exergy. Apart from optimizing the design parameters, the developed model can also be used for on-line optimization when quick response is required. Furthermore, the effect of various coals on the thermodynamic performance of the optimized power plant is also determined. � 2011 Elsevier Ltd.

The objective of the study is to identify the 'best' possible power plant configuration based on 3-E (namely energy, exergy, and environmental) analysis of coal-based thermal power plants involving conventional (subcritical (SubC)) and advanced steam parameters (supercritical (SupC) and ultrasupercritical (USC)) in Indian climatic conditions using high ash (HA) coal. The analysis is made for unit configurations of three power plants, specifically, an operating SubC steam power plant, a SupC steam power plant, and the AD700 (advanced 700�C) power plant involving USC steam conditions. In particular, the effect of HA Indian coal and low ash (LA) reference coal on the performance of these power plants is studied. The environmental impact of the power plants is estimated in terms of specific emissions of CO2, SOx, NOx, and particulates. From the study, it is concluded that the maximum possible plant energy efficiency under the Indian climatic conditions using HA Indian coal is about 42.3% with USC steam conditions. The results disclose that the major energy loss is associated with the heat rejection in the cooling water, whereas the maximum exergy destruction takes place in the combustor. Further, the sliding pressure control technique of load following results in higher plant energy and exergy efficiencies compared to throttle control in part-load operation. Copyright � 2009 John Wiley & Sons, Ltd.

Solar aided feedwater heating (SAFWH) appears to be a prospective option for using solar thermal energy in existing or new coal-fired thermal power plants. This article deals with the 4-E (namely energy, exergy, environment, and economic) analysis of solar thermal aided coal-fired power plants to establish their techno-economic viability. An operating coal-fired subcritical (SubC) and the first supercritical (SupC) power plant being commissioned in India are considered as reference power plants for SAFWH. The 4-E analysis is reported assuming operation of coal-fired power plants with SAFWH for 8h/day in either fuel conservation or power boosting mode. An instantaneous reduction of about 14-19% in coal consumption is observed by substituting turbine bleed streams to all the feedwater heaters including deaerator with SAFWH in "fuel conservation mode". The substitution of turbine bleed stream to high pressure feedwater heater alone with SAFWH results in about 5-6% instantaneous improvement in coal consumption and additional power generation for the fuel conservation and power boosting modes, respectively compared with the same values in reference power plants. The annual savings in fuel cost alone correspond to Indian Rupee (INR) 73.5-74.5 millions. The performance of solar thermal aided coal-fired power plants is also measured in terms of energy and exergy performance index and it is observed that the utilization of solar energy for feedwater heating is more efficient based on exergy rather than energy. The environmental analysis shows that about 62,000 and 65,000t of CO2 are reduced annually from 500MWe SubC and 660MWe SupC coal-fired power plants, respectively using the best possible SAFWH option. However, the cost/tonne of CO2 avoided is about 7775-8885 and 8395-9790 INR (~200 USD) for solar thermal aided coal-fired SubC and SupC power plants, respectively far higher than the most mitigation measures under consideration today. Furthermore, SAFWH is found to be a not very cost effective measure based on the cost of saved fuel (coal). � 2010 Elsevier Ltd.

A two-dimensional (2D) mathematical model in cylindrical coordinates has been formulated and used to predict the progress of conversion in a cylindrical wood particle undergoing pyrolysis in a fluidized bed. The transient model takes into account the heat consumed and shrinkage during drying and pyrolysis reactions in addition to anisotropy and changing thermal properties during the conversion. Predicted centre temperature has been compared with measurements found in the literature for validating the model. Calculations have been carried out for a 10 mm long and 10 mm diameter wooden cylinder in a bed of sand at 1107 K and fluidized by nitrogen. Developing profiles of temperature, solid-phase density, moisture content and degree of conversion at various stages have been presented. It is found that the conversion progresses faster and the gradients are steeper across the grains than along them. A comparison with a 1D model shows that thermal wave propagation predicted by the 2D model is faster than that of 1D. � 2008 Elsevier B.V. All rights reserved.

In the present work, heat transfer measurements are reported in a 100mm square, 5.5 m tall, cold CFB. The test section is a 19 mm OD electrically heated heat transfer tube, 4.64 m tall (covering more than 80% of the CFB height), sandwiched between two equally tall dummy tubes of 19mm OD, thus simulating a water wall geometry, forming one wall of the CFB. Narrow cut sand particles of mean diameters 156, 256, and 362 micrometers, and a wide cut sample of mean diameter 265 micrometer were used as the bed material. The superficial gas velocity ranged from 4.2 to 8.2 m/s, and the solids recycle flux varied from 17 to 110 kg/m2s. Local heat transfer coefficient at the simulated water wall varies, as expected from a low value at the top of the riser to a high value at the bottom, with an interesting increasing and decreasing trend in between. The average heat transfer coefficients were compared with those available in open literature. Correlations for average heat transfer coefficient are presented, both in terms of an average suspension density and also in terms of important nondimensional numbers, namely, Froude number, relative solids flux and velocity ratio. Comparisons are also made with predictions of relevant heat transfer models. Based on the present fifty-five experimental data points, the following correlation was presented with a correlation coefficient of 0.862 and maximum error is ± 15 %.

Thermal behavior and stresses developed in a wood particle are predicted using a mathematical model formulated for drying in fluidized bed. Drying experiments have been carried out on 10 mm long and 30 mm diameter cylindrical disks at a bed temperature of 403 K (130°C). Drying time, mass reduction, and shrinkage during drying have been measured and used to validate the thermal model. The stress model is validated with an analytical solution available in literature. The drying time is experimentally found to be 630 s. The wood particle shrinks by 4.6% in the radial direction and by 1.0% in the longitudinal direction. The model indicates splitting failure at the circumference, which was also noticed during experiments.

Casuarina equisetifolia, a hard wood, and a popular energy crop in many tropical countries, was investigated experimentally for its char fragmentation in a laboratory scale atmospheric bubbling fluidized bed combustor. The effect of fuel shape and size on wood char fragmentation was studied. Wood particles of spherical, cylindrical (aspect ratio of 1), and cubical shapes of different sizes ranging from 10 to 25�mm were used in the experiments. Fragmentation of wood char was quantified in terms of various parameters, such as Number of Fragments (NF), Percentage of Fragmentation Events, Frequency of Fragmentation, Timing interval of Fragmentation, Size distribution of char and Fragmentation Index (FI). Also, qualitative observations on the evolution of char in terms of deformation, cracks and surface texture are discussed. It was observed that Casuarina equisetifolia wood of sizes greater than 15�mm, of all shapes undergoes primary fragmentation during the devolatilization phase. Furthermore, chars fragment at the early stages (1st or 2nd quarter) of the char combustion phase, underscoring the significance of the phenomenon in fluidized bed combustion. For all the shapes of wood considered, there appears to be a cut-off size of the initial wood, below which its char certainly undergoes fragmentation. It was observed that the average char particle size at any instance during its combustion falls in a narrow range of 3.7-6.9�mm, 3-6.6�mm and 3-9.5�mm for spherical, cylindrical and cubical wood particles, respectively. Wood of initially cylindrical shape undergoes extensive fragmentation when compared with spherical and cubical shapes. � 2008 Elsevier B.V. All rights reserved.

A two-dimensional model is developed for the determination of devolatilization time and char yield of cylindrical wood particles in a bubbling fluidized bed combustor. By using the concept of shape factor, the model is extended to particles of cuboid shape. The model prediction of the devolatilization time agrees with the measured data (present and those reported in the literature) for cylindrical and cuboidal shaped particles within �20% while the char yield is predicted within �17%. Influence of some important parameters namely, thermal diffusivity, external heat transfer coefficient and shrinkage, on the devolatilization time and char yield are studied. Thermal diffusivity shows noticeable influence on devolatilization time. The external heat transfer coefficient shows little influence beyond a value of 300 W/(m2 K). However particle shrinkage shows negligible effect on the devolatilization time but has a significant influence on the char yield. � 2008 Elsevier Ltd. All rights reserved.

In some large capacity Circulating Fluidized Bed (CFB) boilers with water walls, additional heat transfer surfaces are necessary to control the furnace temperature while restricting the boiler height to about 40 m due to commercial considerations. These surfaces could be in the form of (1) External Heat Exchanger (EHE) in the solids recycle loop, (2) wing wall and omega panels at the very top of the furnace, or (3) a division or a curtain wall running through the core region of the upper dilute zone of the boiler. It is necessary to know the heat transfer characteristics for these core surfaces from design and operating considerations. In this context, experimental heat transfer studies have been conducted on a 0.6-m high single vertical tube, 0.5- and 1.0-m high U-tubes, and a 0.5-m high finned U tube, located in the top 10% height of a 100 mm?100 mm cross-section and 5.5-m tall CFB riser using silica sand of three mean sizes in the range of 143 to 363 ?m as the bed material. The fluidizing air velocity and solid circulation flux were varied in the range of 4-8 m/s and 13-80 kg/m2/s, respectively. The resulting cross-sectional average suspension density and measured total (gas convective plus particle convective) surface-average convective heat transfer coefficients for U-tubes were in the range of 9 to 25 kg/m3 and 30 to 105 W/m2/K, respectively. For single vertical tube at axis, they were in the range of 10 to 20 kg/m3 and 60 to 128 W/m2/K, and at off-axis positions, they were in the range of 10 to 20 kg/m3 and 70 to 140 W/m2/K, respectively. The results confirmed the increasing trend of the heat transfer coefficient from the axis towards the riser wall, thus reflecting the core-annulus flow structure of the CFB riser. It was observed that the length of the tube influences the heat transfer coefficient and fins augmented the heat transfer rate. Gas convective contribution was found to be substantial especially at high gas velocities and for large particles. Experimental data were compared with published heat transfer results under somewhat similar conditions. Using the experimental data points for the U-tubes, two empirical heat transfer correlations were developed in terms of the important design and operating parameters. Copyright ? 2002 Elsevier Science B.V.

Horizontal pressure gradient fluctuations caused by a bubble passage in the neighbourhood of the probe are simulated using Davidson's pressure field around a circular bubble. The salient features of these fluctuations are studied with regard to their sensitivity to the obliquity and randomness of the bubble traversing path. The utility aspects of horizontal pressure differential measurements over vertical pressure differentials are highlighted and a qualitative comparison between the simulated and those experimentally obtained from a two-dimensional bed, showing strong similarities are presented.Horizontal pressure gradient fluctuations caused by a bubble passage in the neighbourhood of the probe are simulated using Davidson's pressure field around a circular bubble. The salient features of these fluctuations are studied with regard to their sensitivity to the obliquity and randomness of the bubble traversing path. The utility aspects of horizontal pressure differential measurements over vertical pressure differentials are highlighted and a qualitative comparison between the simulated and those experimentally obtained from a two-dimensional bed, showing strong similarities are presented.

Whether or not the maximum in the differential pressure fluctuation due to bubble passage occurs when the bubble boundary touches the probe is examined, based on the Davidson's theoretical model for the pressure field around a bubble. The predictions based on the various inflection points in a differential pressure fluctuation record are examined carefully with regard to the characterization of apparent bubble pierced length. Experimental evidence is presented to substantiate these. The use of point pressure fluctuations in retrieving the true bubble pierced length from the apparent bubble pierced length is illustrated. The effect of practical limiting factors in probe design on the estimation of bubble parameters is studied in order to obtain a sound interpretation of the resultant probe response.

In this paper, an emulsion layer model is presented which predicts the thickness of a downward-moving emulsion layer along the wall of a circulating fluidized bed, the mean solids velocity, and the solids flux in the layer. Also presented is a heat transfer model which, in combination with the emulsion layer model, predicts the low-temperature data very well. An alternate slab model proposed for the radiative component in a high-temperature circulating fluidized bed agrees well with experimental data. The heat transfer predictions of the overall model for such operating parameters as solid circulation flux, suspension temperature, length of the heat transfer surface, superficial gas velocity, and mean particle size are in good agreement with the published data for long surfaces.

A three dimensional non-isothermal model is developed for anode side of Direct Methanol Fuel Cell (DMFC) to study the heat and mass transfer characteristics on cell performance. Electrochemical reaction is coupled with cell current density by Tafel kinetic expression. Commercial software “Fluent 6.3”is used for computation. Methanol and temperature distribution in the anode side are predicted. Double channel flow field is used to investigate the methanol distribution and its effect on cell performance. Methanol and water crossovers in the cell are the major controlling parameters which control the cell performance. The model is also used to predict the methanol crossover effect on Fuel Utilisation Efficiency (FUE) and cell performance. The cell efficiency increases from 7 to 13% with decreasing methanol concentration of 1 to 0.25 M. Net water transfer coefficient is high at low current density and decreases with increasing current density. Experiments were conducted with varying cell voltage and the model results are compared with experimental data.