CFD researched on rice husk gasification in a pilot fixed bed up-draft system

Finding alternative energy sources for fossil fuels was a global matter of concern, especially in developing countries. Rice husk, an abundant biomass in Viet Nam, was used to partially replace fossil fuels by gasification process. The study was conducted on the pilot plant fixed bed up-draft gasifier with two kind of gasification agents, pure air and air-steam mixture. Mathematical modeling and computer simulations were also used to describe and optimize the gasification processes. Mathematical modeling was based on Computational Fluid Dynamics method and simulation was carried by using Ansys Fluent software. Changes in outlet composition of syngas components (CO, CO2, CH4, H2O, H2) and temperature of process, in relation with ratio of steam in gasification agents, were presented. Obtained results indicated concentration of CH4, H2 in outlet was increased significantly when using air-steam gasification agents than pure air. The discrepancies among the gasification agents were determined to improve the actual process.


INTRODUCTION
With the continuous development of economy and technology, people's living standards were constantly being enhanced and thereby energy demand surged in Vietnam.The As an agricultural country with a high proportion of the economy, Vietnam has huge biomass energy sources, specially rice husk, the by-products of rice production.So if it takes advantage of the energy from the abundant byproducts of rice, it can meet 27% of demand for primary energy consumption [2].Gasification was a potential technology can replace fossil energy sources.Therefore, the study of gasification, sophisticated technology, was one of the urgent issues.Modeling methods, was carried out in recent years, can be divided into 4 groups: thermodynamic equilibrium, kinetic, Computational fluid dynamics (CFD), Artificial neural network [3].Computational Fluid Dynamics (CFD) can be employed to investigate this process in detail by linking experimental data and numerical simulation and helping to reduce the complexity of experimental work.Gasification was a multiphase model that was mixed with chemical reactions.To solve this model, there were two approaches: the discrete element method (DEM) and Eulerian approaches.For DEM-based simulation, the framework for the application of the natural and physical models was provided.But it was computationally expensive, especially when the chemical reactions were supplemented [4].
In this research, the model was simulated on Ansys Fluent combined UDFs (User -Defined Functions) and C code with Eulerian approaches to model the gasification process.

Geometry
dimensions, temperature of combustion zones, height of combustion zone in the model were obtained from the pilot updraft gasification system of rice husk.The purpose of this study was to improve the gasification of rice husk and towards optimizing the operational processes.

MODEL DESCRIPTION
Figure 1.The pilot updraft gasification system.
The pilot updraft gasification system was showed in Figure 1, the gas obtained on the top of gasifier.AutoCAD software was used to create geometry for this system, ICEM CFD was used for the meshing process.Pilot equipment whose height was 740mm in cylindrical section, 260 mm in cone section.

Trang 98
Rice husk was a complex mixture of organic substances consisting mainly of components: Carbon, Hydrogen and Oxygen.Proximate and ultimate analyzes of rice husk were given in Table I and Table II [14].The identification of the chemical formula of biomass was quite complicated, some approximation method was employed to determine relatively its chemical formula.One approach was based on utilization of elemental composition from ultimate analysis of dry biomass and could be displayed as in Eq (1-3) which was based on a single atom of carbon [5] Typical chemical formula of biomass was Based on data from Table II, the amount of oxygen was calculated by subtracting the amount of carbon and hydrogen, the formula of the husks was obtained.The molecular mass of biomass was estimated as:

Governing Equations
The mass, energy and species equations of the gas phase and solid phase were described as follow: 3) Species Equation GasPhase

5) Porous media
Porous Media Model was used for describe flow through packed beds.Porous media were modeled by the addition of a momentum source term to the standard fluid flow equations.The source term was composed of two parts: a viscous loss term and an inertial loss term [6].
The permeability and inertial loss coefficient in each component direction could be identified as:

1) Drying
The moisture in the biomass was evaporated as the high-temperature: 2) Pyrolysis The pyrolysis reaction was expressed as: , 2 Pyrolysis reaction was based on practical assumptions that have been supported by the experimental results of biomass pyrolysis.Several hypotheses for present pyrolysis zone model have been employed which was based on the fact that the connection between H and O was far higher than that of C and O [5 Kinetic constants for the gasification reactions were listed in Table 4.

2) Char Combustion
In Combustion Zone, rice husk char oxidized with the supplied air Similarly, to the gasification reactions an overall reaction was introduced [7]   Kinetic constants for the combustion reactions were also listed in Table 3.   6) Figure 4-7 described the changes of the molar fraction over time from 3s to 120s.
Overall, the composition of gas with using air-steam agent were higher than using air agent, except CO.Comparing the effect of different steam/air ratio, it could be seen that increase of the ratio result in higher molar fraction of gas (C 2 H 2 , CH 4 , H 2 ) caused by the transition of hydrogen from steam to syngas through gasification reactions and water-gas shift reaction.
In Figure 4, the amount of CO in process using air agent was increased dramatically at 70s because the drying process was reached equilibrium.A decrease in moisture content drove a water gas shift reaction toward the side with more CO.This model using non-continuous approach resulted in decrease of combustible gas over time.6)  6) Figure 8 showed that the temperature surrounding air-supplying door was high dramatically It could be explained that combustion reactions occurred strongly and combustion zone gradually expanded over time because this was a batch-system.The amount of biomass reduced while gasification agents were fed constantly.
In Figure 8-12, while the steam content of the gasification agents was increasing, the temperature of the process decreased.
Combustion process decreased and gasification one increased gradually which demonstrated gasification process was significantly affected by steam-air gasification agents.
Figure 12 showed that when steam-air gasification agent consisted of 80% steam, the temperature of areas in the process had the temperature ranging from 600K to 700K leading to gasification process operated more effectively.
Figure 13 shows validation of present model with experimental data [15] which was in well agreement.The average composition of CO2, CO, CH4, H2, N2 were compared with Raharjo 's calculation and experimental data from the rice husk gasification system using air agent.Table VII provides

CONCLUSION
The CFD model of gasification process with Euler-Euler approach combining with UDFs code was applied in this research.Entire process model was able to be simulated by 2D CFD model, it was important means in understanding mechanism of process and the composition of syngas, outlet temperature, velocities and reaction rates for the gas and solid phase in function of time and space.With non-continuous approach, the main disadvantage of this model was long computational time.The result from this research demonstrated the promising way to predict the effect of various gasification agents on composition of outlet-gas.This outcome can be used to maximize efficiency for operating 3.9%, from 38 million tons of oil equivalent (MToe) in 2008 to 109 MToe by 2030.Vietnam was expected to become a country subjected to significant dependence on energy and an economy importing energy after 2020 [1].Besides, Vietnam was located in the tropical monsoon area so the plants grow faster.

Figure 2 .Figure 3 .
Figure 2. Fied-bed updraft gasifier Gasification model was divided into 4 zones: drying, pyrolysis, gasification and combustion.Figure 2 showed various zone from updraft gasifier system.Rice husk was entered in accordance with the composition of the proximate analysis: Combustibles matter, Moisture and Ash.Gasification Scheme was showed in Figure3.

Figure 7 .
Figure 7. Changes of the molar fraction (%) of H 2 versus time (s) in various cases (Table6)

Figure 9 .Figure 10 .Figure 11 .Figure 12 .
Figure 9. Contour of solid temperature at (a)30s, (b) 60s, (c)90s, (d)120s in case 2 (Table6) syngas composition results using air without steam as gasifying agent according to the experimental data, calculations and CFD model.The H2/CO of CFD model obtained from Table VII was 0.362, it was quite similar to the experimental data (0.348).

Figure 14
Figure 14 showed steam/air ratio was proportional with composition of H2 in both CFD model and experimental date [15].It's also displayed experimental points was located quite close to the modelling points, so the simulation was quite suitable with experiment.

Table 1 .
Proximate analysis of rice husk

Table 2 .
Ultimate analysis of rice husk

Table 3 .
Stoichiometry of pyrolysis reaction

Table 5 .
Kinetic data of gas-phase reaction

Table 6 .
Operation conditions for running simulation Figure 4. Changes of the molar fraction (%) of CO versus time (s) in various cases (Table 6) Figure 6.Changes of the molar fraction (%) of CH 4 versus time (s) in various cases (Table6)