EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION OF MIXING BEHAVIOUR OF BIOMASS WITH INERT SAND IN FLUIDIZED BED

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 8, Issue 9, September 2017, pp , Article ID: IJMET_08_09_027 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EXPERIMENTAL AND COMPUTATIONAL INVESTIGATION OF MIXING BEHAVIOUR OF BIOMASS WITH INERT SAND IN FLUIDIZED BED B.J.M. Rao Assistant Professor, Department of Mechanical Engineering, Vignan s University, Vadlamudi, Guntur Dist, Andhra Pradesh, India And Research Scholar, Department of Mechanical Engineering, JNTU Kakinada, Andhra Pradesh, India K.V.N.S. Rao Professor, Department of Mechanical Engineering, B. V. Raju. Institute of Technology, Hyderabad, Telangana, India G. Ranga Janardhana Professor, Department of Mechanical Engineering JNTUA, Anathapour, Andhra Pradesh, India ABSTRACT: Mixing characteristics of biomass and sand plays an important role in fluidized bed performance. Effective mixing leads to good thermal contact and leads to enhanced heat transfer. Thus uniform distribution of temperature can be achieved. Computational and experimental analysis of hydrodynamic mixing behavior of biomass and sand are carried out in this present research paper. Three different biomasses namely ground nut shell, rice husk and saw dusts are considered for the analysis. The experiment carried is based on segregation and mixing with inert sand. The effects of superficial velocities for different mixtures are studied and minimum fluidization velocity is evaluated. The computational analysis is carried out based on experimental findings to validate the methodology. The results based on experimental and computational studies are compared with existing theoretical results.it is found that with an increase in fluidizing velocity, biomass and sand particles segregation characteristics can be reduced and better mixing can be achieved. Keywords: Experiment, CFD, Hydrodynamic Mixing, Biomass, Fluidization Velocity Cite this Article: B.J.M. Rao, K.V.N.S. Rao and G. Ranga Janardhana, Experimental and Computational Investigation of Mixing Behaviour of Biomass with Inert Sand in Fluidized Bed, International Journal of Mechanical Engineering and Technology 8(9), 2017, pp editor@iaeme.com

2 B.J.M. Rao, K.V.N.S. Rao and G. Ranga Janardhana 1. INTRODUCTION Fluidized bed gasification offers clean energy conversion of biomass fuels. This operation involves suspension of fuel particles, which are less dense in inert particles, which are high dense in nature. The suspension of these solid particles in a cylindrical chamber is supported by flow of gas usually air. The biomass fuel particles are generally not spherical and inert particles such as sand and glass beads are uniform in shape and size. This may leads to nonuniform mixing and unsteady fluidization problems such as bridging and channeling. Channeling is the segregation of fuel particles along the cylindrical bed and decreases the heat and mass exchange rate [1][2]. These problems can be overcome by establishing proper operating conditions such as particle sizes, biomass weight fraction and type of inert bed materials. Many researchers have conducted experimental and numerical investigations to understand the effect of broad range parameters on fluidized bed behavior on mixing/segregation characteristics. Hoffmann et al. [3] carried experimental studies to understand the occurrence of segregation phenomenon in a gas-fluidized bed. The high density, inert particle is sand of continuous size distribution. The parameter varied is fluidization velocity. They found that composition is non-uniform along the bed height in axial direction for the velocities considerably higher than minimum fluidization velocity. They observed the segregation of mixture in to two-layer structure at lower velocities. Huilin et al. [4] theoretically and experimentally investigated the fluidization characteristics of binary mixture dissimilar in size in a gas bubbling fluidized bed. They showed that the particle size, mass fraction of small particles and gas velocity have significant effect on the segregation of binary mixture systems. Bosmaet al. [5] studied the effectiveness of sieve-like baffles on segregation of a binary mixture in a continuous gas-solid fluidized bed. They observed that baffles boost the segregation. Further, they found that the baffles enhance the flotsam phenomenon by decreasing the circulation in bed and enhance the jetsam layer with increase in the gas velocity. Palappanet al. [6] conducted experiment on binary mixture of fuel and sand inert particles in a fast-fluidized bed to study the impact of density on segregation phenomenon. The other parameters investigated are solids feed rate, feed composition and particle size. Norouzi et al. [7] carried numerical investigation on binary mixtures in the existence of fines to study the size segregation. They showed that presence of fines to the binary mixture resulted in the improvement of segregation due to decrease in inter-particle forces. The present paper studied the Parameters Effect on Hydrodynamics of a Gas-Solid Chamber experimentally and numerically. It also investigated computationally the influence of fluidization velocity on segregation/mixing characteristics of binary mixture composed of fuel and sand solid particles in a cylindrical fluidized bed. Simulations are carried with ANSYS FLUENT, CFD software. 2. EXPERIMENTAL PROCEDURE Fig. 1 shows the apparatus employed in experiment. The apparatus consist of cylindrical fluidized bed vessel made of transparent acrylic tube. The fluidized bed dimensions: internal diameter of 0.09m and height of 1.2m and thickness of 4mm. Distributor plate is located at the bottom of the fluidized bed chamber. It is designed on the method followed by Kunii et al. [8]. The plate is made of mild steel with straight orifices and the ratio of total open area to cross sectional area of Static pressure drop over the fluidized bed is estimated by the pressure taps situated at six different locations along the vessel surface in vertical direction. They are at equidistant of 135mm and are connected to water tube manometer. A pressure tap is provided below the distributor plate to determine the pressure drop in distributor section. A editor@iaeme.com

3 Experimental and Computational Investigation of Mixing Behaviour of Biomass with Inert Sand in Fluidized Bed centrifugal blower provided the air required for the fluidization. Superficial velocity of air is governed by throttle valve. The inert material for bed is chosen as sand with mean particle size diameter of 0.93mm. Three different fuels in grounded form are considered. They are rice husk, saw dust and ground nutshell. The fuel particles properties are presented in table 1. The sand is packed above the distributor plate up to a height of 30mm. In addition, above this the fuel particles are packed up to a total height of 60mm. the corresponding mass ratio of fuel to sand (MR) for the three fuel particles are rice husk, MR r = 1:13; saw dust, MR s = 1:5; and groundnut shell, MR g = 1:12. The air required for fluidization is sent through bottom of the bed. The flow velocity is maintained above the fluidization velocity for 900s and it is suddenly stopped. The bed materials are divided in to four equal sections in axial direction. Measurements of mass fractions of fuel and sand are done. Figure 1 Experimental setup for hydrodynamic studies Property Rice husk Saw dust Groundnut shell Mean particle size, mm Bulk density, kg/ m Particle density, kg/ m Table 1 Properties of fuels 3. CFD METHODOLOGY The geometry and mesh generated using ICEM CFD, advanced meshing software. The geometry is simplified to a cylindrical vessel without distributor plate and other components of experiment. Eulerian - Eulaerian multi-phase flow analysis is considered with one primary and two secondary fluids. The primary fluid is air as ideal gas. Kinetic theory of granular flow is applied to the secondary fluids, sand and fuel solid particles. The solid particles are with mass, density and velocity, where indicates the solid particles of classes A and B. Sand and fuel solid particles are modeled as elastic spheres of constant dia. Fuel particle's properties considered for computational analysis are shown in table 2. The turbulent model is employed for the gas phase computations. The laws of conservation of mass, momentum and granular temperature are employed for both gas and solid particles individually. The list of parameters employed for the simulations are shown in Table editor@iaeme.com

4 B.J.M. Rao, K.V.N.S. Rao and G. Ranga Janardhana Table 2 Fuel Particle Properties Property Rice husk Saw Dust Ground Nut shell Voidage at minimum fluidization Coefficient of elasticity Parameter Table 3 List of Parameters Model in (Fluent) Solid viscosity Gidaspow Solid bulk viscosity Lun et al. Frictional viscosity Scheaffer Solid pressure Lun et al. Radial distribution function Lun et al. Drag law (gas-solid) Gidaspow Drag law (solid-solid) Syamlal and O Brien symmetric The following equations are fluid flow governing equations for gas-solid two-phase flow. Continuity equation (gas phase): +. =0 (1) Continuity equation (solid phase with A, B solid classes): +. =0 (2) Momentum equation (Gas phase): +. = !,# (3) Momentum equation (solid phase with A, B solid classes): +. = $!,# &$ $ (4) Kinetic energy equation (solid phase with A, B solid classes) ' ) * +. ( +,= :. +.. / 3 + (5) In equation 5, the left-hand side denotes the time dependency of the granular energy; the second term is the convection of the granular energy. On the right-hand side, the first termed notes the creation of the granular energy, the second term is the diffusion of the granular temperature and the third term is the dissipation of granular energy due to inelastic particle particle collisions, and the last term is the dissipation due to fluid friction. Conservation equations of mass, momentum and granular temperature is solved for each solid classes using ANSYS Fluent, CFD software. 4. RESULTS AND DISCUSSION Experiments are conducted to analyze the mixing/segregation behavior at minimum and above fluidization velocity. Initial charging of fluidized bed can be done in different ways: initial static bed made of well mixed of sand and biomass particles, fully segregated layers made of each component or intermediate mixed condition of particles. All the experiments are conducted with initial charging of fluidized bed with separate layers of sand and biomass particles editor@iaeme.com

5 Experimental and Computational Investigation of Mixing Behaviour of Biomass with Inert Sand in Fluidized Bed Table 4 shows the mixing Characteristics of biomass fuel particles with inert sand Quarter % of Mass Fraction of Fuel Rice Husk Saw Dust Ground Nut Shell I II III IV Required % of mass fraction in each quarter Rice husk particles showed segregation behavior with inert sand under minimum fluidization velocity. Channeling effect is observed in the case of sawdust. This is indicated by maximum amount of sawdust collection in one selection and the calculated mass ratio is 49.71% where as expected average value in the section is 15.44%.Fairly well mixing behavior is observed in the case of groundnut shells. Effect of fluidization velocity The fluidization velocity is varied in order to study the impact on the mixing behavior of fuel particles with inert particles. The influence is measured in terms of bed expansion. Fig.2 shows the bed expansion for three different fuel and sand mixtures: rice husk, sawdust and groundnut shell. The bed expansion in terms of bed height is increased in initial stage and decreases thereafter with increase in velocity except for sawdust and sand mixture. The internal recirculation of particle is the cause for the above phenomenon. Figure 2 Effect of fluidization velocity on bed expansion for different fuel and sand mixtures Computational analysis is carried out to analyze mixing behavior of sand and biomass particles. Transient simulations are conducted with time step of 1ms for the total time of 10 seconds. All the simulations are conducted with ANSYS FLUENT. Initial static bed condition is same as that of experiment. The results are presented in the form of contours plotted with volume fraction of sand and biomass particles at an instant of 2 sec as shown in fig.3 to fig.5 Volume fraction contours for sand and rice husk shown in fig. 3 indicates the spread of solid particles layer. It is observed that rice husk at the top not totally mixed with sand particles editor@iaeme.com

6 B.J.M. Rao, K.V.N.S. Rao and G. Ranga Janardhana Figure 3 Contours of solid particles volume fraction for sand and rice husk at minimum fluidization velocity It is observed from the Figure 4 that no well mixed condition of solid particles. Sawdust particles layer expanded and rises well above the sand particles. Figure 4 Contours of solid particles volume fraction for sand and saw dust at minimum fluidization velocity Sand and groundnut shell are well mixed in all the regions of expanded bed except at the bottom of bed as observed from fig. 5. Figure 5 Contours of solid particles volume fraction for sand and groundnut shell at minimum fluidization velocity editor@iaeme.com

7 Experimental and Computational Investigation of Mixing Behaviour of Biomass with Inert Sand in Fluidized Bed 5. CONCLUSIONS: From the experimental and computational investigation, following conclusions are drawn Segregation behavior of rice husk particles with inert sand particles is observed Channeling effect is observed in the sawdust and sand mixture Fairly well mixing of groundnut shell with sand is observed REFERENCES [1] Beeckmans, J. M., Nilsson, J. and Large, J. F., 1985, Observations on the Mechanisms of Segregation in Flotsam-Rich, Fully Fluidized-Beds, Industrial & Engineering Chemistry Fundamentals, 24 (1) [2] Daleffe, R. V., Ferreira, M. C. and Freire, J. T., 2008, Effects of binary particle size distribution on the fluid dynamic behaviour of fluidized vibrated and vibro fluidized beds, Brazilian Journal of Chemical Engineering, 25 (1): [3] A.C. Hoffman, E.J. Romp, Segregation in a fluidized powder of a continuous size distribution, Powder Technology, vol. 66, 1991, [4] L Huilin, H Yurong, D Gidaspow, Y. Lidan, Q Yukun, Size segregation of binary mixture of solids in bubbling fluidized beds, Powder Technology, vol. 134, 2003, pp [5] J. C. Bosma, A. C. Hoffmann, On the capacity of continuous powder classification in a gas-fluidized bed with horizontal sieve-like baffles, Powder Technology, vol. 134, 2003, pp.1-15 [6] K. G. Palappan, P. S. T. Sai, Studies on segregation of binary mixture of solids in a continuous fast fluidized bed-part I. Effect of particle density, Chemical Engineering Journal, vol. 138, 2008, pp [7] H R. Norouzi, N Mostoufi, R Sotudeh Gharebagh, Effect of fines on segregation of binary mixtures in gas solid fluidized beds, Powder Technology, vol. 225, 2012, pp [8] Kunii, D., and Levenspiel, O Fluidization Engineering. New York: John Wiley. [9] Manish Kumar, Bireswar Paul and Dhananjay Singh Yadav, Effect of Moisture Content and Equivalence Ratio on the Gasification Process for Different Biomass Fuel. International Journal of Mechanical Engineering and Technology, 7(6), 2016, pp [10] M. Senthil Kumar and S. Vivekanandan, Mathematical Modelling To Predict the Cold Gas Efficiency of Rice Husk In Biomass Gasifier. International Journal of Mechanical Engineering and Technology, 7(6), 2016, pp [11] Muhammad Akram Akhund, Ali Raza Khoso, Ashfaque Ahmed Pathan, Uroosa Memon, Fida Hussain Siddiqui, Influence of Biomass Aggregate on Strength of Foam Concrete. International Journal of Civil Engineering and Technology, 8(8), 2017, pp editor@iaeme.com