Water Hyacinth Gasification, ANSYS Fluent CFD Simulation

Description

Water Hyacinth Gasification Process

In this project, a water hyacinth gasification reactor has been numerically modeled using the finite volume method. Figure 1 shows the geometry of the gasifier. The water hyacinth prepared for combustion enters the reactor from the fuel inlet section at a flow rate of 0.1 kg/s, and from the other two sides of fuel inlet, air as an oxidizer enters the computational area at a flow rate of 0.2 kg/s. In this way, after combustion and chemical reactions, syngas discharges from the top and biochar exits from the bottom of the gasifier. The simulation is done in two dimensions and considering symmetry.

The computational grid as well as the boundary conditions are also shown in Figure 2. The inlet mass flow boundary condition is used at the inlets. At the syngas outlet, an outlet pressure boundary condition is used, in which case the gas is discharged to the atmosphere. A wall was used at the biochar outlet with the assumption that biochar particles would be trapped.

The water hyacinth particles entering the gasifier from the fuel inlet area are heated. The heat transferred around these particles causes physical and chemical reactions. The kinetic model for the reactions for the oxidation and gasification of water hyacinth in the gasifier is used. However, the volatile, biochar, ash and moisture composition released from the water hyacinth decomposition are expressed by the following equation.

Which we will have in the above equation:

However, in this study, only two chemical reactions are considered. The first chemical reaction is specific to volatiles, which is expressed by the above equations, and the second reaction is related to the release of carbon dioxide, which is expressed by the following equation.

In this study, a water hyacinth sample from Dian Lake in Kunming, Yunnan province of China, which was powdered to a spherical particle size of 0.5 mm, was used. The proximate analysis and ultimate analysis of this water hyacinth powder are presented in the table below [1]. It should be noted that this powder has been converted into gasifier fuel at a standard pressure, and these properties are also related to the biomass fuel produced from this powder.

Thus, chemical reactions were simulated by the species transport model, and biochar particles were tracked using the DPM model. On the other hand, in the species transport model, eddy dissipation was used for the turbulent chemical interaction. Also, in the DPM model, issues including devolatilisation, surface combustion, and heat transfer are considered in the simulation. Therefore, when water hyacinth particles are injected into the computational domain, they will be affected by thermochemical degradation and surface combustion, as well as inert heat release. The results of this simulation are presented below.

Results

The temperature and velocity contours are shown in Figure 3. The maximum of both parameters occurred at the center of the gasifier. The maximum temperature reached more than 996 K, and due to the properties of the combustion products, which are considered as ideal compressible gases, the maximum velocity in those areas has also increased to about 26 m/s.

Contours for the volume fraction of water hyacinth (WH), as well as combustion products, are also presented in Figure 4. The reaction between H2O and other species is clearly evident in the pyrolysis zone.

Thus, DPM mass source and DPM concentration are shown in Figure 5. DPM mass source, which represents the total mass exchange of the continuous phase with the dispersed phase, indicates its maximum value near the air and fuel inlets (Where the biochar particle fallout occurred). The maximum amount of particle accumulation was also observed at the inlet and part of the downstream wall of the gasifier.

The flow lines as well as the volume fraction of biochar are shown in Figure 6. As can be seen, due to the density of spherical particles, their maximum amount accumulates in the lower part of the reactor, waiting to be discharged from the bottom of the system. However, the streamlines indicate a turbulent flow regime with the generation of vortices in different areas of the gasifier.

Finally, in Figure 7, the temperature and velocity of a multiphase flow consisting of interpenetrating gases are presented. In the pyrolysis zone, a temperature increases as well as flow drift is observed. This savior takes heat from the oxidation zone, and by reducing H₂O, produces syngas [2]. Also, in this area, an increase in velocity up to more than 25 m/s has been observed. This can also be seen from the contours in Figure 3.

Conclusion

In this project, a gasifier based on the production of syngas and biochar from water hyacinth has been simulated. The following achievements will be obtained by studying and training this project.

• Learning to simulate species transfer in the Ansys Fluent 2024R2.
• Using DPM to simulate water hyacinth powder injected into the gasifier.
• Investigating the behaviors of devolatilisation, surface combustion and heating particles by applying their laws in the software.
• How to apply the P1 radiation model to such closed systems.
• Training various techniques for convergence of residuals to achieve accurate results in such systems where fluid flow will be affected by complex physical and chemical phenomena.

[1] H. Yue, Y. Rao, J. Meng, X. Zhang, R. Chen, Gas emissions from combustion of water hyacinth and pistia stratiotes biomass particles under O2/CO2, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects  (2020) 1-15.

[2] U. Kumar, M.C. Paul, CFD modelling of biomass gasification with a volatile break-up approach, Chemical Engineering Science 195 (2019) 413-422.