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1. INTRODUCTIONWith increasing demand for energy, depleting primary energy sources (i.e. coal and oil) and deteriorating environment, it has become essential not only to use the existing energy sources efficiently but also to develop alternate or non-conventional sources of energy. Of the various renewable energy sources available, biomass appears to offer a promising solution to tackle the ever increasing energy demand (Basu, 2006). Biomass is an organic matter produced by plants, both terrestrial (those grown on land) and aquatic (those grown in water) and their derivatives. It includes forest crops and residues and animal manures. Biomass is the term used in the context of energy for a range of products which have been derived from photosynthesis. Thus everything which has been derived from the process of photosynthesis is a potential source of energy. Biomass constitutes a significant, clean and renewable energy source and has very desirable option. Photosynthesis or photo-biological process is a continuous activity creating organic carbon that burns with less air pollution than fossil fuels. Photosynthesis helps to remove carbon dioxide from the atmosphere and generates oxygen, the life sustaining gas. Thus it helps to remove environmental pollution. Since plants use carbon dioxide for their growth, greater sources on biomass production may help to restore clean environment. Biomass energy is thus environmentally a very acceptable resource.CFD modelling is a powerful tool for development of new ideas and technologies and for fundamental understanding of fluidsolid interactions. CFD has enabled the correct theoretical prediction of various macroscopic phenomena encountered in fluidized beds by quantifying the physical and chemical processes in the biomass thermo- chemical reactors. Accurate simulations can help to optimize the system design and operation thereby helping to understand the dynamic process inside the reactors. Thus with the CFD simulations flow behaviour within the gasifier can be understood properly. Technologies to convert biomass in to energy fall in two categories as mention below. i. Bio chemical conversion (anaerobic digestion, fermentation) process ii. Thermo chemical conversion (combustion and gasification) process. 1.1 Thermo-chemical Conversion Gasification and direct combustion are two examples of thermo-chemical conversion process. Direct combustion is probably the most common conversion process whereby solid biomass is burnt in a confined container, stove or boiler. Gasification is a process of turning solid biomass into combustible gas. The solid biomass is partially burnt in presence of air or oxygen to produce gases of low or medium calorific value.

1.2 Advantages of Biomass Gasification Advantages of biomass energy utilization include ensuring the sustainability of energy supply in the long term as well as reducing the impact on the environment. As biomass energy uses agricultural waste as fuel, it is considered CO2 neutral and emissions of sulfur dioxides and nitrogen oxides are very low, making it a good option as clean fuel for the environment. Indeed, among the technologies available for using biomass for producing electricity, gasification is relatively new. Gasification is primarily a thermo-chemical conversion of organic materials at elevated temperatures with partial oxidation. In gasification, the biomass or any other organic matter is converted to combustible gases (i.e. mixture of CO, CH4 and H2), with char, water, and condensable as minor products. 1.3 Advantage of FBG The concern for climatic variations has triggered the interest in biomass gasification making fluidized bed gasifiers as one the popular options, occupying nearly 20% of the market. (i) Fluidized Bed gasifier can handle all types of dry small sized biomass wastes. (ii) It can be operated batch wise and in continuous manner. FBG handling biomass produces syn-gas of high colorific value and solid wastes with less ash content. Time taken for biomass conversion is less and density of char is less. Wastes from agro industry, timber industry, sugar industry etc. can also be used for power generation. In rural areas, biomass samples are readily available for which power problem can easily be solved with proper gasification technology. 1.4 Application of Biomass Gasification1. To recover energy and reduce emission to atmosphere2. Heat and steam generation3. Electrical power generation1.5 Objectives Objective of the present work has been framed in the following manner. a) Experimental analysis on production of H2 from different biomass samples using FBG c) To study the performance of gasifier by carrying out energy analysis of different biomass samples d) To carry out CFD analysis for fluidized bed Gasifier for different biomass samples in the following manner :i).by simulating the hydrodynamic behaviors of fluidized bed gasifier at isothermal condition. ii).by investigating the thermo-flow behavior inside the gasifier with Particles. 2. LITERATURE 2.1 Fluidized Bed GasifierAir or oxygen is injected upward at the bottom. Gasifier allows an intensive mixing and a good heat transfer to take place. Granular inert solids (usually silica sand) along with the feedstock are fluidized by the gasifying agent. Reactions take place simultaneously in the bed as it has no separated Reduction zone. Four distinct stages in the gasifier are as follows.1. Drying, 2. Pyrolysis, 3. Reductionand 4. Combustion Schematic diagram of the gasifier with different zones are shown below.

Fig. 1.1: Fluidized Bed Gasifier Fig. 1.2: Basic Process Chemistry2.2 Chemistry of ReactionPyrolysis generally produces the following three products Gases like H2, CO, CH4, H2O, and CO2 Char, a solid residue containing carbon Tar, a black, viscous and corrosive liquid.Combustion ZoneC + O2 = CO2 H2 + 0.5O2 = H2O Reaction Zone C + CO2 = 2CO C + H2O = CO + H2CO + H2O = CO2 + H2 C + 2H2 = CH4

2.3 Previous work Table 2.1- work on Fluidized bed Gasifier.AuthorWork on FBG

Keijo et al.(1995)Studied co-combustion and gasification of various biomass samples using steam gasification. Wood based fuel and waste agricultural wastes, waste paper etc. were used for heat and power generation.

Schiffer et al.(1995)gasified different biomass samples including pulp and paper sludge to municipal sludge. They used high temperature Winkler (HTW) process where solid feed stocks are gasified in a fluidized bed at elevated pressure using oxygen plus steam or air as gasification agents. They observed that biomass and waste materials often incorporate a higher amount of volatile matter, different proportions and compositions of inorganic matter having a significant variety of physical properties in comparison with coal. Therefore, gasification or co-gasification of peat, wood, sewage sludge has consequences with regard to feed stock preparation, gasification behavior, corrosion, emissions and residues. Thus, they recommended that HTW process is favourable for the conversion of Biomass.

Chern et al.(1998)Used an empirical stoichiometric equation for wood chip gasification in a commercial-scale moving bed downdraft gasifier. The equation is based on an analysis of overall and elemental material balance for experimental data obtained with the gasifier. A thermodynamic analysis of the gasifier has also been performed. Resultant empirical efficiencies of the gasifier have been evaluated for four different operating models at three different output temperatures. The resultant empirical stoichiometry was found to be in agreement with the experimental observations.

Natarajan et al.(1998)Determined agglomeration tendencies of some common agricultural residues in fluidized bed combustion and gasification system. It is observed that the combustion zone temperature is in the order of 900 10000C as in moving bed gasifiers and 800-9000C in fluidized bed gasifiers. The ashes of biomass feed stocks were observed to have ash fusion temperatures in the range of 8000C to 15000C.

Warnecke et al.(2000)Carried out a comparative study on gasification process between fluidized and fixed bed gasifier using different feed samples. Other aspects such as technology involved in the process, energy consumption for the process, environmental problem caused by the process and overall economy of the process were also analyzed by him. It was concluded that there is no significant advantage with fixed bed gasifier or fluidized bed gasifier.

Rao et al.(2002)Worked on thermo chemical characterization of various biomass samples using down draft gasifier and fixed bed and fluidized bed gasifiers. They observed that producer gas obtained is contaminated with tars, chars and ash particles to different degree depending upon the reactor type and feed stock utilized. The moisture content varies over a wide range from oven dry to about 90% on wet basis and ash content varies from 0.5 to 22%. Highest heating value of 12-18 MJ.N/m3 was observed with producer gas.

Murakami et al. (2006)Discussed on some process fundamentals for biomass gasification in dual fluidized bed. The dual uidized bed gasication technology is prospective because it produces high calorie product gas, free of N2 even when air is used to generate the heat required for gasication via in situ combustion. The necessary process fundamentals for development of a bubbling uidized bed (BFB) biomass gasier coupled with pneumatic transported riser (PTR) char combustor were also studied by them.

Ramirez et al.(2007)Suggested on the basic design of a pilot scale Fluidized Bed Gasifier for handling Rice Husk. According to them the gasifier was divided in seven parts or sub-systems Intending to produce an energetic gas. Experimental tests conducted with such a gasifier showed that the developed procedure is adequate with a maximum deviation of 50% for the operational performance variables.

Kumar et al.(2009)Modified steam and air fluidized bench-scale FBG. The effects of furnace temperature, steam to biomass ratio and equivalence ratio on gas composition, carbon conversion efficiency and energy conversion efficiency of the product gas were studied by them.

Table 2.2- CFD on Fluidized bed Gasifier. Authors

Models usedParameter studied

Fletcher et al. (2000)

Model is based on the CFX package. Biomass particulate is modelled via a Lagrangian approach.

Turbulent fluid flow, heat transfer, species transport, devolatilization, particle combustion, and gas phase chemical reactions are described

Dimitrios S. (2001)

3-D, multi-fluid Eulerian approach for bubbling fluidized bedStudied drying and devolatilization of biomass, heterogeneous reactions of char.

Liang Yu (2007)

kinetic theory of granular flow to simulate coal gasification in a bubbling fluidized bed gasifierStudied different cases for coal feed rate, air supply, steam supply and bed temperatures, instantaneous drying and devolatilization in the feed zone

Gerun et al. (2008)

2D axisymmetric CFD model for the oxidation zone in a two-stage downdraft gasifier

Verified temperature profile, stream function, gas path line tar concentration and compared with experimental data.

Papadikis (2008)

Euler-Euler approach to model the behavior of the sand and Euler-Lagrange for investigation of momentum transport to one biomass particle

Study the complex hydrodynamics of fluidized bed.

Yiqun Wang and LifengYan (2008)

Three-dimensional CFD model of a fluidized bed for sewage sludge gasifier

Model described complex physical and chemical phenomena in the gasifier, including turbulent flow, heat and mass transfer, and chemical reactions

S. Gerber et al. (2010)

Eulerian multiphase approach for modelling the gasification of wood in fluidized bed.

Wood pyrolysis, char gasification and homogeneous gas phase reactions are modelled. product gas and tar concentrations datas compared with experimental data

Tingwen Li et al. (2010)

Details of high resolution simulations of coal injection in a gasifier using CFD techniques

Studied effects of grid resolution and numerical discretization scheme on the predicted behavior of coal injection and gasification kinetics.

3. CFD MODELLING3.1 PROBLEM DESCRIPTIONThe bed dynamics, thermal-flow and gasification process in a fluidized bed gasifier are studied. A two-dimensional three-phase flow model is simulated using Air as continuous phase and binary mixtures as dispersed phase. An Eulerian Granular Multiphase model has been used and simulations are carried out using the commercial CFD package ANSYS Fluent 15.0.0.Inert material sand considered as the bed material, Biomass (Sugarcane-bagasse) as the feed sample. Air is used as fluidizing medium. Both homogeneous (gas-gas) reaction and heterogeneous (gas-solid) reactions are simulated in this study.3.2 GEOMETRY & MESH

Fig. 3(a): Geometry of fluidized bed (b) 2-D MeshTable 3.1 MeshingMinimum mesh size0.0005 m

Maximum mesh size0.005 m

Number of nodes17773

Number of elements17327

3.3 EULERIAN MULTIPHASE MODEL GOVERNING EQUATIONSContinuity Equation:

Momentum Equation (For Gas Phases)

Momentum Equation (For Solid Phases)

Conservation of Energy

Species transport equations

TABLE 3.2- FLOW MODELS USED IN FLUENTParameter

Model

Solid viscosity

Gidaspow

Solid bulk viscosity

Lun et al.

Frictional viscosity

Scheaffer

Solid pressure

Lun et al.

Drag law (gas-solid)

Gidaspow

solid-solid interaction

Syamlal and OBrien symmetric

3.4 METHODOLOGYBoundary Conditions: Inlet- velocity Outlet- pressure Wall- no slip Pressure Velocity Coupling: Phase coupled Simple Algorithm Spatial Discretization: Gradient: Least Squares Cell BasedMomentum: Second Order Upwind Volume fraction: QUICK schemeTurbulent Kinetic Energy: Second Order Upwind Turbulent Dissipation Rate: Second Order Upwind Species Equations Second Order Upwind

4. HYDRODYNAMIC STUDY4.1 ASSUMPTIONS FOR HYDRODYNAMIC STUDYIsothermal non-reactive, unsteady state gas-solid cold model FB gasifier. Operating conditions: temperature 300K and pressure of 1 atm. solid initially in static condition inside the fluidized bed column. Solid particle velocity is set at zero.The single pressure field shared for all three phases, in proportion to their volume fractionsTABLE 4.1 - PROPERTIES OF MATERIALPropertySand

Sugarcane-bagasseCoconut-coir

Gas

Mean particle size, (m)385

530, 856

1025

Apparent density, (kg.m-3)

2650

120.17581.2

Porosity

0.41

0.62

0.96

TABLE 4.2- PARAMETERS FOR SIMULATIONParameter

Value

Static bed height, m

0.1

Superficial gas velocities , m/s

0.2,0.5, 0.7

restitution coefficient, e

0.9

Time Steps(sec)

0.001

5. RESULTS & DISCUSSION5.1 Contours of Solid Volume Fraction

Fig.5.1- contour plot of volume fraction against time for sugarcane bagasse at air velocity of 0.2m/s for initial static bed height of 0.1m.

Fig.5.2- contour plot of volume fraction against time for coconut-coir at air velocity of 0.2m/s for initial static bed height of 0.1m.

Fig.5.3- contour plot of volume fraction against time for sand (sugarcane-bagasse) at air velocity of 0.2m/s for initial static bed height of 0.1m.

Fig.5.4- contour plot of volume fraction against time for sand (coconut-coir) at air velocity of 0.2m/s for initial static bed height of 0.1m. The contour plots of the sugarcane bagasse , coconut-coir and sand with an inlet velocity of 0.2m/s have been shown in fig.5.1,fig. 5.2,fig.5.3 and fig.5.4 respectively. It is observed from fig.5.1 and fig.5.2 that bubbles are formed only within the static bed height without any noticeable bed expansion. The reason may be attributed to the fact that bubbling occurs at the surface only. In other words, solids in the bottom section of the bed are in pneumatic transport while fluidization in the upper section is in freely bubbling state.



Fig.5.7- contour plot of volume fraction against time for sand at air velocity of 0.5m/s for initial static bed height of 0.1m.

Fig.5.8- contour plot of volume fraction against time for cc sand at air velocity of 0.5m/s for initial static bed height of 0.1m.

Fig.5.5 and fig.5.6 shows the variation in the bed profile with time for sugarcane bagasse and coconut-coir at air velocity of 0.5m/s. the contour plot has been plotted with time step of 10secs. While simulating the fluidized bed, it is observed that the bed profile changes with time. But after some time significant change is observing the bed profile. This indicates that the fluidized bed has come to a quasi-steady state. The contour plot in Fig.5.5 and fig.5.6 shows higher solid volume fractions along the walls compared to the core region. This may be due to the segregative tendencies of the particles towards the walls or gulf streaming. Thus the solid particles slide down along the wall of the reactor without too much resistance from the upward gas flow.



Fig.5.11- contour plot of volume fraction against time for sand at air velocity of 0.7m/s for initial static bed height of 0.1m.

Fig.5.12- contour plot of volume fraction against time for cc sand at air velocity of 0.7m/s for initial static bed height of 0.1m.

Fig.5.13-Air volume fraction with Air Velocity for sugarcane-bagasse.

Fig.5.14-Air volume fraction with Air Velocity for coconut-coir.Fig.5.9, fig.5.10, fig. 5.11, fig.5.12, fig. 5.13 and fig.5.14 show the contours of volume fractions of sugarcane bagasse. ,coconut-coir Sand and air obtained at air velocity of 0.7m/s for initial static bed height 0.1m in 2-D fluidized bed after the quasi steady state is achieved. The contour scale given to the left of each contours gives the value of volume fractions corresponding to any particular colour. The contours for sugarcane bagasse , coconut-coir and sand illustrates that bed is in fluidized condition. The contour for air illustrates that volume fraction of the gas is less in fluidized section than the solid particles. 5.2. Phase VelocityThe velocity vectors show magnitude of velocity with direction and thud helpful to determine the flow pattern in fluidized bed. The velocity vector of sugarcane bagasse, coconut-coir , sand and air in the column obtained after the quasi steady state at air velocity of 0.7 m/s with initial static bed height of 0.1m are shown in fig.5.15, fig.5.16 ,fig. 5.17 and fig.5.18.

Fig.5.15- Velocity vector of sugarcane bagasse and sand at air velocity 0.7 m/s.

Fig.5.16- Velocity vector of sugarcane bagasse and sand at air velocity 0.7 m/s.

Fig.5.17-Velocity contour and vector of air (sugarcane-bagasse) at air velocity 0.7 m/s.

Fig.5.18-Velocity contour and vector of air (coconut-coir) at air velocity 0.7 m/s. From velocity of solid phase (fig.5.15 and fig. 5.16), it is observed that there is vigorous movement of solid particles throughout the bed implying that the velocity at the bottom is less. In the central region of the bed, direction of velocity near the wall is observed to be downwards while that in the region away from wall is upwards. In the upper part of fluidizing section there is circulatory motion/ downward motion of the solid particle near the wall and upward motion in the central region of the bed. The velocity vector of gas phase in the column(fig.5.17 and fig.5.18) indicate that there is an upward flow throughout the column which implies that velocity of air is very less within the bed compared to that in remaining part of the column. This is due to very small volume fraction of air within the bed compared to solids in the region. In the upper section of the column, air velocity is high thus it carries air bubbles but in the lower section of the column solid particles obstruct the movement of bubbles thereby reduces air velocity. 5.3. Bed pressure drop The axial pressure drop in a fluidized bed varies from higher value at the bottom of the bed to zero value at the top of the column. The bed pressure drop can be determined from the difference of pressure at the inlet and outlet. Fig.5.19 and fig.5.20 shows the contours of static gauge pressure. It is evident from the figure that the pressure is higher in the inlet and gradually decreases and became zero at the outlet.

Fig.5.19: contour of bed pressure drop against air velocity for the fluidized bed for sugarcane-baggase.Fig.5.20: contour of bed pressure drop against air velocity for the fluidized bed for coconut-coir.5.4. Effects of inlet velocities The volume fraction distribution for the particles using the Gidaspow model with three inlet velocities, i.e., 0.2 m/s, 0.5 m/s and 0.7 m/s, are shown in fig.5.21 and fig. 5..22 for particles with a diameter of 530 m. If the gas velocity does not exceed V the particles fall back down to the particle bed. This is referred to as a bubbling bed and is shown in fig.5.21 and fig. 5.22 Exceeding V means the suspended particles can be carried with the gas phase and continue up the riser. This fast fluidization state has been shown in fig.5.21 and fig. 5.22. The contour plot of fig.5.21 and fig. 5.22 Shows bubbles increasing in size and distorting with increase height. This is due to the coalescence of the bubbles with smaller bubbles rising from the base of the reactor. As the velocity increases, the bubbles sizes increase and the solid-gas mixture appears more dilute particularly towards the top of the bed. The solids descend to the base of the reactor as the solids and gas compromise.

Fig.5.21: Particle volume fraction and velocity vector for Sugarcane-bagasse.

Fig.5.22: Particle volume fraction and velocity vector for coconut coir.

The fast fluidizing states in fig.5.21 and fig. 5.22 Show very dilute distributions in comparison to the bubbling models. The particle volume fraction and particle velocity are shown in fig.c.at gas velocity i.e., 0.7m/s which is only slightly lower than the terminal velocity. Increasing the gas velocity allows for a faster flow of gas to push the collection particles higher up the bed. fig.5.21 and fig. 5..22. Shows the particle volume fraction and particle velocity at gas velocity 0.7 m/s. So terminal velocity in the present study is found to be approximately 1.9m/s.5.6. Particle distributionsFig.5.13 illustrates radial variation of solid concentration at different bed heights at air velocity 0.7 m/s which shows higher particle volume fraction along the walls compared to the core region. The result confirms that the solid volume fraction is not symmetrical. According to the axial solid volume concentration profile (fig.5.13) the riser is axially divided into the lower zone and upper zone. The lower region of FB riser is denser than the upper-dilute region even through the solids mainly accumulate in both sides the wall for 2D model. The computed time averaged volume fraction of sugarcane-bagasse and sand particles for a bed height of 0.15 m and a gas velocity of 0.5 m/s are compared (Fig.5.14) the volume fraction of particles is observed to be lower in the central region than the region near the wall. From the simulation result as shown in the figures, the hydrodynamic model is able to describe quantitatively the accumulation of solids near the wall. Solid concentrations appear flat in the central region and increase towards the wall. This is due to the segregative tendencies of the particles towards the walls.

Fig.5.23- Sugarcane-bagasse particle concentration against the radial position for different bed height at air inlet velocity of 0.7 m/s

Fig.5.24- comparison of distributions of sugarcane-bagasse and sand at air velocity 0.5 m/s. 6. THERMAL FLOW BEHAVIOR WITH SOLIDS (NO REACTIONS) 6.1 Thermal flow Behaviour with Solids (No Reactions) This case analysis the thermal flow behaviour with particles as well as the fluidization in the geometry. Sand and sugarcane-bagasse particles are patched up to a static bed height 0.1m. The air enters at a velocity of 0.7 m/s at 673 K and 1273 K flow through the bed. All the other boundary conditions, simulation model parameter and solution techniques used in this study are the same as taken for previous hydrodynamic study.6.2 Result and DiscussionFig.6.1 fig.6.2, fig.6.3and fig.6.4 shows the particle velocity field versus air velocity field. It can be clearly seen that all air streams move upward whereas particles circulate within the fluidized bed in the bottom part of the domain. At 0.7 m/s inlet air velocity, no particles are seen in the upper part of the domain. A sequence of volume fraction distributions of sugarcane- bagasse and Coconut - coir are shown in Fig 6.5 , fig 6.6, fig.6.7 and fig.6.8 at different seconds. Bubbles are formed above the inlet due to the fast supply of air at a rate of 0.7 m/s. Then bubbles continue to rise towards the top of the bed along the wall. The bubbles also appears too elongated and circle back round towards the walls. This indicates the solid particles in the bed move in a circular motion there by influencing and distorting the bubble back towards the wall. This is more clearly evident in Fig.6.1 fig.6.2, fig.6.3and fig.6.4 which displays the particle velocity vectors. Since no reactions are simulated in this case, the temperature inside the domain is considered to be uniform (Fig.6.9, fig. 6.10, fig6.11 and fig.6.12).

Fig.6.1 Velocity vector plot for Sand, Sugarcane-bagasse and air coloured by static pressure (Pascal) at Temperature 673 K.

Fig.6.2 Velocity vector plot for Sand, coconut - coir and air coloured by static pressure (Pascal) at Temperature 673 K.

Fig.6.3 Velocity vector plot for Sand, Sugarcane-bagasse and air coloured by static pressure (Pascal) at Temperature 1273 K.

Fig.6.4 Velocity vector plot for Sand, Coconut coir and air coloured by static pressure (Pascal) at Temperature 1273 K.

Fig.6.5 Distribution of volume fraction of sugarcane bagasse with time at air velocity 0.7 m/s and Temperature 673K.

Fig.6.6 Distribution of volume fraction of coconut-coir with time at air velocity 0.7 m/s and Temperature 673K.

Fig.6.7 Distribution of volume fraction of sugarcane bagasse with time at air velocity 0.7 m/s and Temperature 1273K.Fig.6.8 Distribution of volume fraction of coconut-coir with time at air velocity 0.7 m/s and Temperature 1273K.

Fig.6.9-Temperature profile at different time intervals inside the fluidized bed at temperature- 673 K for sugarcane-bagasse..

Fig.6.10-Temperature profile at different time intervals inside the fluidized bed at temperature- 673 K for coconut-coir.

Fig.6.11 -Temperature profile at different time intervals inside the fluidized bed at temperature- 1273 K for sugarcane-bagasse.

Fig.6.12 -Temperature profile at different time intervals inside the fluidized bed at temperature- 1273 K for coconut-coir.7. CONCLUSIONCFD modelling of fluidized bed biomass gasifier is carried out by using Eulerian granular multiphase model. Increasing superficial gas velocity makes the flow development faster.The bed expansion behavior is found to vary with gas velocity. Able to describe quantitatively the accumulation of solid at the wall. Results give information concerning the thermal-flow behaviour and gasification process8. FURTHER WORK PLAN Future work needs to be carried out to observe the effect of bed height on minimum fluidization velocity for various other particle systems of mean size in the range as mentioned in the Fluidized bed Biomass Gasifier. High temperature studies will also be carried out using a High temperature fluidized bed Gasifier different particles as coconut coir, Shaw dust, rice straw and wood chips (Air temperature up to 1273 K). 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