IJIRST –International Journal for Innovative Research in Science & Technology| Volume 4 | Issue 4 | September 2017 ISSN (online): 2349-6010
Modelling and Simulation of a Bubbling Fluidized Bed Gasifier Digambar H. Patil PG Student Department of Chemical Engineering T.K.I.E.T., Warananagar, India
J. K. Shinde Professor Department of Chemical Engineering T.K.I.E.T., Warananagar, India
Abstract Bubbling fluidized beds are commonly used for small to medium scale capacities. Gasification being order of magnitude slower than combustion process, handling it in fluidized bed gasifier pose many challenges as it may lead to loss of efficiency, unburnt carbon and so on. Hence study of fluidized bed gasifier is important from aspects like gas-solid hydrodynamics, reaction kinetics and inter-phase mass transfer that control the overall performance of the gasifier unit. In this paper, the objective of the study is to develop method for predicting the performance of the bubbling fluidized bed gasifier. This will critically evaluate the various correlations/methods available in literature which enables the research/ process designer to predict the performance of bubbling fluidized bed gasifier. The overall methodology will be to perform cold flow experiments with appropriate representative particles and generate the related information. Numerical model will be developed for bubbling fluidized bed gasifier and its validation will be performed based on experimental data. Keywords: Bubbling Fluidized Bed, Entrained Flow, Gasification, Hydrodynamics, Kinetics, Mass Transfer _______________________________________________________________________________________________________ I.
INTRODUCTION
The gasification is the process in which conversion of carbonaceous fuels like coals, pet coke, biomass, MSW (Municipal solid waste) etc. in to a synthesis gas (syngas/producer gas) that contains CO, H 2 and CH4 as major components and also contains CO2, H2O, NH3 etc. This is achieved by reacting the material at high temperatures (>700 °C), with a controlled amount of oxygen and/or steam. The resulting gas mixture is called syngas (from synthesis gas or synthetic gas) or producer gas and is itself a fuel. And this resulting gas from gasification process is used to generate the power or replacement of Furnace oil or to generate chemicals. There are several gasifiers are used for this gasification process including Fixed bed gasifier, Fluidized bed gasifier and Entrained flow gasifier. The fluidized bed gasifier consists of two different methods which are bubbling fluidized bed gasifier and circulating fluidized bed gasifier. Among these gasifiers, the bubbling fluidized bed gasifier is very effective and economical to use at smaller to medium scale capacity. So this study includes the evaluation of performance of bubbling fluidized bed gasifier. Bubbling fluidized bed gasifier, will be simulated using equilibrium approach based on minimization of Gibbs free energy. The other aspects like hydrodynamic and reaction kinetics will be also studied in this. The hydrodynamic study contains minimum fluidization velocity (Umf); superficial to minimum fluidization velocity ratio, bubble size formed at the distributor and bed level and such information will be sought from various correlations available in literature. Mass and heat balance will be performed using steady state solvers. Model predictions will compared with the experimental data from the literature or plant data. II. MODEL FORMULATION General Assumptions ď€ ď€ ď€ ď€
The system is in steady state and The hydrodynamic behaviour of the fluidized bed will be estimated based on correlations The fluidization state in the bed is maintained in the bubbling mode. The bubble size in the bed is variable with respect to the bed height. Conservation Equations Generalized Overall mass balance equation Accumulation of mass = mass in- Mass out đ?‘‘đ?‘€ đ?‘‘đ?‘Ą đ?‘‘ đ?‘‘đ?‘Ą
= đ?‘€đ?‘–đ?‘› − đ?‘€đ?‘œđ?‘˘đ?‘Ą
(1)
đ?œŒđ?‘‰ = đ?œŒđ?‘–đ?‘› đ??šđ?‘–đ?‘› − đ?œŒđ?‘œđ?‘˘đ?‘Ą đ??šđ?‘œđ?‘˘đ?‘Ą
(2)
Component Balance Accumulation of species ‘k’ = mass flow of species ‘k’ in- mass flow of species ‘k’ out + source term for species ‘k’ +/- species consumed or generated
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Modelling and Simulation of a Bubbling Fluidized Bed Gasifier (IJIRST/ Volume 4 / Issue 4 / 013)
�
đ?‘‘đ??śđ?‘˜đ?‘œđ?‘˘đ?‘Ą
= đ??š1 đ??śđ?‘˜đ?‘–đ?‘› − đ??šđ?‘œđ?‘˘đ?‘Ą đ??śđ?‘˜đ?‘œđ?‘˘đ?‘Ą + đ?‘&#x;đ?‘˜ đ?‘‰ Âą đ?‘„đ?‘˜ (3) đ?‘‘đ?‘Ą Energy Balance Accumulation of Enthalpy = Enthalpy in- Enthalpy out – heat transferred + heat generated due to reaction đ?‘‘ đ?‘‰đ?œŒđ??śđ?‘? đ?‘‡ = đ??šđ?‘–đ?‘› đ?œŒđ?‘–đ?‘› đ??śđ?‘?đ?‘–đ?‘› đ?‘‡đ?‘–đ?‘› − đ??šđ?‘œđ?‘˘đ?‘Ą đ?œŒđ?‘œđ?‘˘đ?‘Ą đ??śđ?‘?đ?‘œđ?‘˘đ?‘Ą đ?‘‡đ?‘œđ?‘˘đ?‘Ą + đ?‘„ (4) đ?‘‘đ?‘Ą
Bed Hydrodynamics The bubble size is one of the most critical parameters in modelling a fluidised bed. It affects bubble rise velocity, fractions of the bubble and emulsion phases, solids mixing, and interface mass transfer between the phases. Numerous experiments and analyses aiming at determining the bubble size in the fluidised bed have been carried out. Results have shown that bubbles form as soon as the gas velocity exceeds the minimum fluidisation velocity. Bubble coalescence and net flow lead to increasing bubble size with an increase in bed height. Correlations for Axial Distribution of Bubble Sizes đ??ˇđ??ľ0 = 0.347 (
đ??´(đ?‘˘0 −đ?‘˘đ?‘šđ?‘“ ) đ?‘›đ?‘‘
0.4
)
(5)
đ??ˇđ??ľđ?‘€ = 0.652[đ??´(đ?‘˘0 − đ?‘˘đ?‘šđ?‘“ )]
0.4
đ?‘‘đ?‘? = đ??ˇđ??ľđ?‘€ − (đ??ˇđ??ľđ?‘€ − đ??ˇđ??ľ0 )đ?‘’đ?‘Ľđ?‘? (
−0.3đ?‘§ đ??ˇđ?‘Ą
)
(6) (7)
Volume fractions occupied by bubbles đ??ľ=
đ??ť đ??ťđ?‘šđ?‘“
đ?œ€đ?‘? = 1 −
= 1.0 +
0.738 0.376 đ?œŒđ?‘ đ?‘‘đ?‘?1.006 0.937 0.126 đ?‘˘đ?‘šđ?‘“ đ?œŒđ?‘”
10.978(đ?‘˘0 −đ?‘˘đ?‘šđ?‘“ )
1 đ??ľ
(8) (9)
Bubble Rising Velocity đ?‘˘đ?‘? = đ?‘˘0 − đ?‘˘đ?‘šđ?‘“ + 0.711(đ?‘”đ?‘‘đ?‘? )0.5
(10)
đ?œ€đ?‘“ = đ?œ€đ?‘? + (1 − đ?œ€đ?‘? )đ?œ€đ?‘šđ?‘“
(11)
The Bed Void Fraction Hydrodynamic Data The cold flow model of bubbling fluidized bed gasifier contains fluidized bed with material like sand. Solids are in batch and air fed is in continuous mode. Air is introduced from the bottom of the fluidized bed and distributed through the bed by distributer plate in the form of bubbles. The velocity of gas in to the reactor must be sufficient to fluidize the particles within the bed and should not exceed the certain value so as to entrain them out of the bed. This velocity is called as minimum fluidization velocity. Cold flow experiments will be performed to obtain key hydrodynamic parameters and validation of the correlations available in literature. This will acts as input to the numerical model of the gasifier. The equation used for calculation of minimum fluidized bed is Umf = {(dp2.(Ď p-Ď f).g)/(150.Îźf)}*{(Ć?mf3.φ2)/(1-Ć?mf)} (12) The equation used for calculation of superficial velocity is Us = [{(1.25-1).Umf0.937.Ď f0.126}/{10.978.Ď p0.376.dp1.006}]1.355 + Umf (13) The equation for calculation of terminal velocity is Ut = [{4.(Ď p-Ď f).g.dp}/(3.CD.Ď f)]0.5 (14) Where dp is diameter of particle, Ď p & Ď f are the densities of the particle and flowing fluid, Ć? mf is a minimum porosity, φ is the sphericity, Îźf is the viscosity of the fluid and CD is the drag coefficient. Kinetic study The study of reaction kinetics will be taken by using the following reaction: 1) C + O2 → CO2 (-393.5 KJ/mol) 2) C + 0.5O2 → CO (-110.5 KJ/mol) 3) C + CO2 → 2CO (+171 KJ/mol) 4) C + H2O → CO + H2 (+118.9 KJ/mol) 5) C + 2H2 → CH4 (-74.9 KJ/mol) 6) H2 + 0.5O2 → H2O (-242 KJ/mol) 7) CO + H2O → CO2 + H2 (-41.09 KJ/mol) 8) CH4 + H2O → CO + 3H2 (+206 KJ/mol) The kinetic parameters and rate equations forms for all above reactions will be obtained from literature. The correlations used for this study is as follows K = K0 * e(E/RT) (15) This is known as Arrhenius law. Where K is the rate constant, K0 is the pre-exponential factor, E is the activation energy in J/mol, R is gas constant and T is the absolute temperature in K. And the rate equation is
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Modelling and Simulation of a Bubbling Fluidized Bed Gasifier (IJIRST/ Volume 4 / Issue 4 / 013)
-rA = K.CAn.CBm (16) Where (-rA) is rate of reaction, CA & CB are the concentrations of reactants A & B resp., n & m are the orders of the reaction with respect to A & B resp. Heat and Mass Transfer Heat and mass transfer between bubbles and the solid (emulsion phase) is quite important. The information’s and correlations available for this will be supplemented from literature. The correlation for heat transfer coefficient is as follows: Nup = 2 + 0.6.Rep0.5.Pr0.33 (17) Where Nup = Particle Nusselt no. = hgpdp / Kg, hgp is a heat transfer coefficient, Rep = particle Reynolds no. = (U-Up)dpρg / μ The correlation for mass transfer coefficient is as follows: Sh = 2.0 + 0.6.Re1/2.Sc1/3 (18) Where Sh = KL/D = Sherwood no. K = mass transfer coefficient. III. EXPERIMENTAL WORK Gasifier Unit The pilot gasifier unit is mainly consists of three sections as Plenum, Gasifier Bed and Gasifier freeboard. The plenum section is the bottom end part of the gasifier which contains the perforated plate called the distributor plate. The preheated air comes to plenum and passes through the distributor plate which helps for air distribution in the bed section. The bed section contains the bed material which is heated and fluidized by the preheated air. Also the feeding line is attached to this section above the bed. The upper section is called the freeboard section. Feeding Section The feeding unit is equipped with feeding hopper suitable for feeding high density coal as well as low density biomass. Feeding takes place by using the screw feeder. The feeding rate is controlled by using Variable Frequency Drive (VFD). The nitrogen line is also provided to feed line for smooth conveying of feed and to avoid back flow which prevents the feed flow. And one knife gate valve is also situated in the line to isolate the feeding system at the time of maintenance. Air Supply and Preheating section The air required for the gasification process is supplied by the roots blower situated near the gasifier unit. From the roots blower the air comes to the air header situated in line to gasifier. From the air header air is distributed to various parts of the gasifier like for coal conveying, purging required for view ports etc. Then there are two air heaters are present in the line of air which preheats the air up to 5500C. The preheated air finally introduced in to the gasifier from the bottom end of the gasifier. Syngas Sampling and Analysis The sampling line is kept at a constant temperature of ~300 0C in order to avoid tar condensation and sampling line fouling. Tar condensation and particle removal takes place in the gas cleaning section consisting of water washing, moisture trap, impinger bottles with iso-propanol, a fibre filter with silica gel filter. When the appropriate gasification conditions are achieved producer gas is sampled, using a membrane pump and air tight sampling bags. And the analysis is taken place in the Gas Chromatography system. The Gas Chromatograph is fitted with two columns HP-Plot Q with helium as carrier gas. The retention time of the analysis process was 20 min. The GC calibration composed of CO, H 2, CO2, N2, CH4 and O2. IV. SIMULATION WORK Simulation Software Engineering Equation Solver was selected for modelling the gasifier. Engineering Equation Solver (EES) is a commercial software package used for solution of systems of simultaneous non-linear equations. It provides many useful specialized functions and equations for the solution of thermodynamics and heat transfer problems, making it a useful and widely used program for mechanical engineers working in these fields. EES stores thermodynamic properties, which eliminates iterative problem solving by hand through the use of code that calls properties at the specified thermodynamic properties. Simulation Procedure 1) The simulation of the programme has been done by using various windows present in the software. 2) The information concerning a problem is presented in a series of windows.
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Modelling and Simulation of a Bubbling Fluidized Bed Gasifier (IJIRST/ Volume 4 / Issue 4 / 013)
3) 4) 5) 6)
Equations and comments are entered in the Equations window. After the equations are solved, the values of the variables are presented in the Solution and Arrays windows. The residuals of the equations and the calculation order may be viewed in the Residuals window. Additional windows are provided for the Parametric and Lookup Tables, a diagram and up to 10 plots. There is also a Debug window. V. RESULTS AND DISCUSSION
In order to validate the simulation results, experimental data from gasification of coal in a pilot-scale fluidized bed gasifier was used. Using EES simulator and fluidized bed gasifier the effect of different operating parameters (temperature, equivalence ratio) was studied and it was found that the most of trends were similar for both the case but the concentrations were different because of some simplified assumptions were considered for simulation model. Effect of Temperature on Product Gas Composition Figures 1 and 2 show the simulation results compared with experimental data for product gas composition versus different temperatures in the range of 800 0C – 900 0C. From these figures, we can say that as temperature increases the product gas composition also increases.
Fig. 1: Effect of Temperature on CO composition
Fig. 2: Effect of Temperature on H2 composition
Effect of ER on Product Gas Composition Simulation results and experimental data of gas composition versus different air equivalence ratios in the range of 0.2 to 0.4 are shown in figures 3 and 4. From these figures, we can say that as ER increases the product gas composition increases.
Fig. 3: Effect of ER on product gas composition
Fig. 4: Effect of ER on product gas composition
VI. CONCLUSION 1) The proposed combined transport and kinetic model is successfully validated with the experimental data reported in the literature. 2) The model predicted composition of producer gas matches very well with the experimental data. 3) The molar fractions of CO and CH4 increase and that of water vapour decreases with time in the pyrolysis zone of the gasifier due to a subsequent increase in the temperature of pyrolysis zone. 4) The molar fraction of water vapour in the gaseous phase is the highest in the pyrolysis zone and decreases as it travels through the oxidation and reduction zone of the gasifier.
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Modelling and Simulation of a Bubbling Fluidized Bed Gasifier (IJIRST/ Volume 4 / Issue 4 / 013)
5) The molar fractions of CO and CH4 in the gaseous phase increase in the pyrolysis zone as gas travels through it. However, the molar fractions of CO, H2 and CH4 decreases in the oxidation zone due to high temperature oxidation and hence the molar fraction of CO2 increases. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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