Modelling and experimental investigation of devolatilizing wood in a fluidized bed combustor

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.     

Mass loss and apparent kinetics of a thermally thick wood particle devolatilizing in a bubbling fluidized bed combustor

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 (T_bed = 750, 850 and 950 °C), two inert bed material sizes (mean size d_p = 375 and 550 μm) and two fluidizing velocities (u = 5u_mf and u = 10u_mf). 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.     

A 1-D non-isothermal dynamic model for the thermal decomposition of a gibbsite particle

A 1-D mathematical model describing the thermal decomposition, or calcination, of a single gibbsite particle to alumina has been developed and validated against literature data. A dynamic, spatially distributed, mass and energy balance model enables the prediction of the evolution of chemical composition and temperature as a function of radial position inside a particle. In the thermal decomposition of gibbsite, water vapour is formed and the internal water vapour pressure plays a significant role in determining the rate of gibbsite dehydration. A thermal decomposition rate equation, developed by closely matching experimental data reported previously in the literature, assumes a reaction order of 1 with respect to gibbsite concentration, and an order of −1 with respect to water vapour pressure. Estimated values of the transformation kinetic parameters were k₀ = 2.5 × 10¹³ mol/(m³ s) for the pre-exponential factor, and E_a = 131 kJ/mol for the activation energy. Using these kinetic parameters, the gibbsite particle model is solved numerically to predict the evolution of the internal water vapour pressure, temperature and gibbsite concentration. The model prediction was shown to be very sensitive to the values of heat transfer coefficient, effective diffusivity, particle size and external pressure, but relatively less sensitive to the mass transfer coefficient and particle thermal conductivity. The predicted profile of the water vapour pressure inside the particle helps explain some phenomena observed in practice, including particle breakage and formation of a boehmite phase.     

Thermal properties of carbon fibers at very high temperature

Thermal properties such as specific heat C_p, thermal diffusivity a, and thermal conductivity λ of carbon fibers are important parameters in the behaviour of the carbon/carbon composites. In this study, the specific heat and the thermal diffusivity are measured at very high temperatures (up to 2500 K). The experimental thermal conductivity estimated by the indirect relation λ = aρC_p is presented as a function of the temperature. Validations are carried out on isotropic metallic (tungsten) and ceramic (Al₂O₃) fibers. Measurements were obtained on three carbon fibers (rayon-based, PAN-based and pitch-based). Thermal conductivity results allow us to classify fibers from the most insulated to most conductive. The main result is that insulated carbon fibers have an increasing thermal conductivity when the temperature and the heat treatment temperature rise. Relationships between thermal conductivity and the structural properties (L_c and d₀₀₂) of such carbon fibers are studied. We also describe the influence of heat treatment on the thermal conductivity of carbon fibers.     

Heat transfer in a pulsed bubbling fluidized bed

Surface-to-bed heat transfer and pressure measurements were carried out in a 0.17 m ID pulsed bubbling fluidized bed with glass bead and silica sand particles having mean diameters ranging from 37 μm to 700 μm to investigate the effects of flow pulsation on heat transfer and bed hydrodynamics. A solenoid valve was used to supply pulsed air to the bed at 1 to 10 Hz. The bed surface was found to oscillate with the frequency of pulsation, the oscillation's amplitude decreasing with frequency. The standard deviation of the bed pressure drop in the pulsed bed was found to be larger than that in the conventional bed due to the acceleration force imposed by pulsation. For both Geldart B and A particles, high frequency pulsation (7, 10 Hz) enhances the heat transfer compared to continuous flow, the enhancement diminishing with superficial gas velocity and particle size. For Geldart B particles, the effect of pulsation on heat transfer ceases around U_o/U_mf = 3.5, whereas 24% improvement in heat transfer coefficient was obtained for 60 μm glass bead particles (Group A) at superficial gas velocities as high as U_o/U_mf = 27. Furthermore, in the fixed bed (U_o/U_mf < 1) for Geldart B particles, 1 Hz pulsation was found to be very effective resulting in two- to three-fold increase in heat transfer coefficient compared to continuous flow at the same superficial gas velocity. The flow pulsation loses its effect on heat transfer with increasing static bed height, i.e., when H_bed/D > 0.85.     

Prediction of single particle settling velocities through liquid fluidized beds

Single particle settling velocities through water fluidized beds of mono-sized glass spheres (d_p = 0.645, 1.20, 1.94, 2.98 and 5 mm in diameter) were studied experimentally using a column, 40 mm in diameter. The settling spherical particles (D_p = 10 and 19.5 mm) had different densities (1237 to 8320 kg/m³), while the settling particles (D_p = 5 and 2.98 mm) were glass spheres. The pseudo-fluid model, which considers a liquid fluidized bed as a homogenous pseudo-fluid, predicts single particle settling velocities quite well if the ratio D_p/d_p is larger than about 10. With decreasing ratio D_p/d_p, the overall friction between the settling particle and the fluidized media increases. A method for predicting single particle settling velocities through a liquid fluidized bed is proposed and discussed. Following the approach of Van der Wielen et al. [L.A.M. Van der Wielen, M.H.H Van Dam, K.C.A.M. Van Luyben, On the relative motion of a particle in a swarm of different particles, Chem. Eng. Sci. 51 (2006) 995-1008], the overall friction is decomposed into a particle-fluid and a particle-particle component. The effective buoyancy force is calculated using the transition function proposed by Ruzicka [M.C. Ruzicka, On buoyancy in dispersion, Chem. Eng. Sci. 61 (2006) 2437-2446]. A simple model for predicting the collision force is proposed, as well as a correlation for the collision coefficient. The mean absolute deviation between the experimental and calculated slip velocities was 5.08%.     

Heat transfer enhancement by twisted tapes with alternate-axes and triangular, rectangular and trapezoidal wings

The effects of twisted tapes with alternate-axes and wings on heat transfer, flow friction and thermal performance characteristics in a round tube are investigated. The influence of wing shape including triangle, rectangle and trapezoid is also studied. The experiments are performed using twisted tape with three different wing-chord ratios (d/W) of 0.1, 0.2 and 0.3 and constant twist ratio (y/W) of 4.0. The alternate-axis is made by arranging each plane of twisted tape to 60 degree difference relative to the adjacent plane. The results show that both heat transfer rate and friction factor associated by all twisted tape are consistently higher than those without twisted tape. Under the similar operating conditions, Nusselt number, friction factor as well as thermal performance factor given by the tape with alternate-axes and trapezoidal wings are higher than those given by the others. It is also found that that Nusselt number, friction factor and thermal performance increase with increasing wing-chord ratio. The maximum thermal performance factor of 1.42 is found with the use of twisted tape with alternate-axes and trapezoidal wings at d/W = 0.3, where the heat transfer rate and friction factor, respectively increase to 2.84 and 8.02 times of those in the plain tube.     

Mass transfer from nanofluid drops in a pulsed liquid–liquid extraction column

Mass transfer in gas–liquid systems has been significantly enhanced by recent developments in nanotechnology. However, the influence of nanoparticles in liquid–liquid systems has received much less attention. In the present study, both experimental and theoretical works were performed to investigate the influence of nanoparticles on the mass transfer behaviour of drops inside a pulsed liquid–liquid extraction column (PLLEC). The chemical system of kerosene–acetic acid–water was used, and the drops were organic nanofluids containing hydrophobic SiO₂ nanoparticles at concentrations of 0.01, 0.05, and 0.1 vol%. The experimental results indicate that the addition of 0.1 vol% nanoparticles to the base fluid improves the mass transfer performance by up to 60%. The increase in mass transfer with increased nanoparticle content was more apparent for lower pulsation intensities (0.3–1.3 cm/s). At high pulsation intensities, the Sauter mean diameter (d₃₂) decreased to smaller sizes (1.1–2.2 mm), leading to decreased Brownian motion in the nanoparticles. Using an analogy for heat and mass transfer, an approach for determining the mass diffusion coefficient was suggested. A new predictive correlation was proposed to calculate the effective diffusivity and mass transfer coefficient in terms of the nanoparticle volume fraction, Reynolds number, and Schmidt number. Finally, model predictions were directly compared with the experimental results for different nanofluids. The absolute average relative error (%AARE) of the proposed correlation for the mass transfer coefficient and effective diffusivity were 5.3% and 5.4%, respectively.     

Three-dimensional numerical simulation of a horizontal rotating fluidized bed

Three-dimensional numerical simulations of a horizontal rotating fluidized bed (RFB) containing glass bead particles (d_s = 82 μm, ρ_s = 2450 kg/m³) and washed alumina (d_s = 89 μm, ρ_s = 1550 kg/m³) were performed. FLUENT 6.1 software was used to carry out our simulation. The numerical results were compared with the experimental data of Qian and Pfeffer et al. [G.H. Qian, I. Bagyi, I.W. Burdick, R. Pfeffer, H. Shaw, Gas-Solid Fluidization in a Centrifugal Field.” AIChE J. 47 (5) (2001) 1022-1034]. The rotating speed of the RFB was set at 325 rpm (34 rad/s), which is equivalent to a centrifugal acceleration of 7 g.The flow behavior of the solid particles was analyzed; the bed thickness and the calculated pressure drop were compared with the experimental results. Our calculated pressure drop agreed very well with the experimental results.     

Nitrogen dilution effect on flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The nitrogen dilution effect on flame stability was experimentally investigated in a lifted non-premixed turbulent hydrogen jet with coaxial air. Hydrogen gas was used as the fuel and coaxial air was injected to initiate flame liftoff. Hydrogen was injected into an axisymmetric inner nozzle (d_F = 3.65 mm) and coaxial air jetted from an axisymmetric outer nozzle (d_A = 14.1 mm). The fuel jet and coaxial air velocities were fixed at u_F = 200 m/s and u_A = 16 m/s, while the mole fraction of the nitrogen diluent gas varied from 0.0 to 0.2 with a 0.1 step. For the analysis of the flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF was performed. The stabilization point was in the region of the flame base with the most upstream region and was defined as the point where the turbulent flame propagation velocity was found to be balanced with the axial component of the local flow velocity. The turbulent flame propagation velocity increased as the nitrogen mixture fraction decreased. The nitrogen dilution makes the flame structure more premixed. That is, the stabilization mechanism shifts from edge flame propagation based mechanism toward premixed flame propagation based mechanism. We concluded that the turbulent flame propagation velocity was expressed as a function of the turbulent intensity and the axial strain rate, even though the mole fraction of the nitrogen diluent varied.     

Thermal conductivities from temperature profiles in the polymer electrolyte fuel cell

We report measured temperatures inside the single polymer fuel cell, and thermal conductivities and heat transfer coefficients calculated from these. Temperatures were measured next to the membrane on its two sides, and in the gas channels. Higher temperatures (5 °C or more at 1 A/cm²) were found at the membrane electrode surface than in the gas channels. The thermal conductivity of the membrane (λ^m) was small, as expected from the properties of water and polymer, while the heat transfer coefficient of the electrode surfaces (λ^s) was smaller, 1000±300 W/m² K for a layer thickness of 10 μm. The real coefficient is smaller, since the measured temperatures are systematically smaller than the real ones. The electrode surface heat transfer coefficient is not previously reported. The average value for the catalyst surface plus gas diffusion layer was 0.2 W/m K.     

Effect of annular fins on heat transfer of a horizontal immersed tube in bubbling fluidized beds

Experiments were conducted in a bubbling air-fluidized bed to investigate the effect of annular fins of constant thickness on heat transfer. Steady state time averaged local heat transfer coefficient measurements were made by the local thermal simulation technique in a cold bubbling fluidized bed (90 mm ID, 260 mm tall) with horizontally immersed tube initially with no fin and then with three fixed annular fins of constant thickness. Silica sand of mean particle diameter 307 μm and 200 μm were used as the bed materials. The superficial velocity of air was from minimum fluidization conditions, u_mf, to approximately 3 × u_mf. The results indicate that, although the heat transfer coefficient falls with the use of fins, the total heat transfer rises as a result of the greater surface area. Increasing the particle diameter reduces the heat transfer coefficient not only for unfinned horizontal tube but also for annular finned horizontal tube at the same conditions of fluidized bed. Based on the experimental data, correlations are proposed for predicting heat transfer coefficient from fluidized bed to horizontally immersed tubes with and without fins.     

PEM fuel cell voltage transient response to a thermal perturbation

This paper describes a transient model predicting PEMFC voltage response to a step change in the cooling water temperature. Its objectives are to put forward the main transport parameters and their corresponding time scales. The fuel cell is assumed isothermal with a time constant τ_t. The temperature variations result from the production of heat by the exothermic chemical reaction and by internal heat dissipation, and from heat transfer with the cooling circuit. The effects of temperature on fuel cell performances are taken into account through the variations in its thermodynamic voltage, in the kinetics of the half-reactions, and in the membrane ionic resistance. A dynamic and one-dimensional simulation of water transport in the membrane by electroosmotic drag and by diffusion is carried out: the relative humidity of gases varies with the cell temperature under the assumption that their specific humidity (i.e., the vapor content in the gas diffusion layers) remains unchanged.Two time constants characterize mass transfer in the membrane by water diffusion (τ_d) and by electroosmosis (τ_e). The Péclet number Pe which is equal to the ratio between τ_d and τ_e allows the comparison of the magnitude of these two transport mechanisms, both depending on current density and on the other operating conditions.The results of the model are compared to a set of experimental results obtained with a cell composed of a Nafion 115 membrane, and fed by hydrogen and pure oxygen. The average current density is 4000 A m⁻². In these conditions, the smallest time constant is the one characterizing the fuel cell thermal response τ_t (16 s). Therefore, the fuel cell voltage response to a temperature step occurs in two stages, the first one corresponding to the thermal regime. The second stage concerns water transport in the membrane; the best fit between numerical and experimental results yields to a Péclet number of about 16, which makes electroosmosis the most significant phenomenon.     

High permeability Ni-Cu-Zn ferrites through additive-free low-temperature sintering of nanocrystalline powders

Nanocrystalline Ni-Cu-Zn ferrite powders Ni_0.20Cu_0.20Zn_0.62Fe_1.98O_3.99 were prepared by thermal decomposition of an oxalate precursor. The particle size is 6 nm and 350 nm, respectively, for powders obtained through calcinations at 350 °C or 750 °C. The shrinkage behavior significantly changes with particle size; the temperature of maximum shrinkage rate is T_MSR = 700 °C for particles of 6 nm size and increases to T_MSR = 880 °C for particles 350 nm in size. Dense samples with a permeability of μ = 780 are obtained by sintering at 900 °C for 2 h. Mixtures of nanocrystalline and sub-micron powders allow tailoring of the shrinkage behavior. A maximum permeability of μ = 840 is obtained after sintering of a 1:1-mixture at 900 °C. This demonstrates the potential of nanocrystalline ferrites for co-firing without additives at 900 °C and integration of ferrite inductors into LTCC modules.     

Preliminary study on suspension plasma sprayed ZrO2 + 8 wt.% Y2O3 coatings

ZrO₂ + 8 wt.% Y₂O₃ powder of a mean diameter d_VS = 38 μm was milled to obtain fine particles having mean size of d_VS = 1 μm. The fine powder was used to formulate a suspension with water, ethanol and their mixtures. The zeta potential of obtained suspensions was measured and found out to be in the range from −22 to −2 mV depending on suspension formulation. The suspension was injected through a nozzle into plasma jet and sprayed onto stainless steel substrates. The plasma spray experimental parameters included two variables: (i) spray distance varying from 40 to 60 mm and (ii) torch linear speed varying from 300 to 500 mm/s. The microstructure of obtained coatings was characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The coatings had porosity in the range from 10% to 17% and the main crystal phase was tetragonal zirconium oxide. The scratch test enabled to find the critical load in the range of 9-11 N. Finally, thermal diffusivity of the samples at room temperature, determined by thermographic method, was in the range from 2.95 × 10⁻⁷ to 3.79 × 10⁻⁷ m²/s what corresponds to thermal conductivities of 0.69 W/(mK) and 0.97 W/(mK) respectively.     

Novel three-dimensional microfluidic device for process intensification

Microchannel heat exchangers and chemical reactors have extensive applications in process industries, especially in electronics cooling, air conditioning, and chemical industries. Microchannel devices provide enhanced heat and mass transfer characteristics by controlled flow conditions and high surface-to-volume ratios. The different designs of microchannels are based on planar structures from various fabrication technologies. In the present study a novel three-dimensional micro-structured device, the micro coiled flow inverter (MCFI) with 0.38–0.8 mm internal diameter, has been numerically investigated as a micro heat exchanger. In comparison micro helical coil (MHC) and straight tube of same heat transfer area (d_t = 0.5 mm, A = 3.9 × 10⁻⁴ m²) has been studied, too. The Reynolds and Prandtl numbers are varied from 25 to 1200 and 0.74 to150, respectively. The MCFI offers a four-fold heat transfer enhancement as compared to straight tube of same heat transfer area at N_Re = 1200 and N_Pr = 7. Furthermore, the heat transfer coefficient in MCFI augments by 38.5% as compared to MHC, with slight increase (5–16%) in friction factor. New design correlations are developed for Nusselt number and friction factor in MCFI. The MCFI device offers 1.15–1.7-folds higher thermal merit as compared to micro straight tube.     

Inter-particle heat transfer in a riser of gas-solid turbulent flows

Effects of the particle-particle heat transfer in a gas-solid turbulent flow in a riser were evaluated. An Eulerian/Lagrangian four-way interaction formulation including the particle collisions in conjunction with the k − τ and the k_θ − τ_θ model equations were used in the numerical simulation. Inter-particles and particle-wall interactions were accounted for with an inelastic collision model, where the restitution coefficient was evaluated for each collision. The special case when the flow initially contains two groups of hot and cold particles was treated in details. Particular attention was given to the nature of heat transfer to particles due to inter-particle interactions. The results showed that the effect of particle-particle heat transfer was more significant for smaller sizes, lower flow Reynolds numbers, and for higher loading ratios. Solid thermal properties, however, did not have a noticeable effect on the inter-particle heat transfer. The simulation results indicates that although the heat transferred to each group of hot and cold particles was significant, the mean values of gas and particle temperatures and suspension heat transfer was insensitive to the inter-particle heat transfer.     

A study of removal of Pb heavy metal ions from aqueous solution using lignite and a new cheap adsorbent (lignite washing plant tailings)

The present study determines the efficiency with which lignite and lignite washing plant tailings can adsorb Pb heavy metal ions. In the first experiment, the effect of size distribution on the absorbance capacity was investigated for the samples. Therefore, lignite sample was ground to five sizes (d₈₀ = 0.600, 0.355, 0.250, 0.106 and 0.063 mm) under nitrogen (N₂), and the tailings sample was classified into seven fractions, along with the original state (original state: d₆₀ = 0.063, −1 + 0.600, −0.600 + 0.355, −0.355 + 0.250, −0.250 + 0.106, −0.106 + 0.063 and −0.063 mm). The test results showed that the optimum size distributions for lignite and tailings were d₈₀ = 0.063 mm and the original state (d₆₀ = 0.063 mm), respectively. Simultaneously, the adsorption capacity results of the two optimum sizes were compared with each other, and the tailings sample (d₆₀ = 0.063 mm) gave the best results, with 9.30 mg/g Pb ions adsorbed value. Therefore, in the second study, a series of laboratory experiments using 2³ full factorial designs was conducted to determine the optimum pH, contact time and initial metal concentration using the original tailings sample. The experimental studies showed that pH 9, a 120 min contact time and 300 ppm initial metal concentration gave the best results, namely an adsorption of 29.92 mg Pb ions/g.     

Species-electrochemical transports in a free-breathing cathode of a PCB-based fuel cell

The species-electrochemical transport phenomena in a free-breathing cathode of a printed-circuit-board (PCB)-based fuel cell have been studied numerically. A perforated current collector is attached to the porous cathode that breathes the fresh air through an array of circular holes. Parametric studies include the breathing-hole diameter (d) and the cathode-layer thickness (δ). Gas flow in the porous cathode is governed by the Darcy equation with constant porosity and permeability. Electrochemical reaction on the surfaces of the porous matrices is depicted via the Butler-Volmer equation. The multi-species diffusive transports in the porous cathode are described using the Stefan-Maxwell equation. The charge transports in the porous matrices are dealt with the Ohm's law. The coupled equations are solved by a finite-element-based CFD technique. Results show that the mean flow of the gaseous mixture directs outward from the porous cathode to ambient. In addition, the species diffusion dominates the mass transports in the free-breathing cathode. Furthermore, the enhancement of species diffusion compromises the increase of Ohmic resistance shows the optimal breathing-hole diameter around d = 2.1 mm that provides the best electrochemical performance.     

Mass transfer in vertical liquid-solids flow of coarse particles

Wall-to-bed mass transfer in the hydraulic transport of spherical glass particles was studied. The experiments were performed by transporting spherical glass particles 1.20, 1.94 and 2.98 mm in diameter with water in a 25.4 mm I.D. tube. The mass transfer coefficients were determined by following rate of dissolution of a segment of the transport tube prepared from benzoic acid.In the runs in hydraulic transport, the Reynolds number of the tube varied between 1826 and 27597. The loading ratio (G_p/G_f) was between 0.026 and 0.474, and the fluid superficial velocity was between 0.267 · U_t and 4.904 · U_t, where U_t represents the single particle terminal velocity. For these ratios, the voidage ranged from 0.7123 to 0.9228.Also, wall-to-bed mass transfer in the single phase flow regime was studied. In the runs without particles, the Reynolds number of the tube varied between 122 and 39132. The data for the mass transfer factor (j_D) in single phase flow are correlated for turbulent flow regime, using the Chilton-Colburn's type equations, j_D = f(Re). Those investigations were conducted in aim to compare with results for wall-to-bed mass transfer in hydraulic transport.The data for wall-to-bed mass transfer (j_D) in hydraulic transport of spherical particles were correlated by treating the flowing fluid-particle suspension as a pseudofluid, by introducing a modified suspension-wall friction coefficient (f_w) and a modified Reynolds number (Re_m). The data for wall-to-bed mass transfer in the hydraulic transport of particles show that an analogy between mass and momentum transfer exists.