I am simulating the coal gasification process in a fluidized bed using Ansys Fluent 2020.
The heterogeneous and homogenous reactions were included. The simulation is running fine and the species are produced. But there is mass addition to the coal phase as the reactions are taking place which shouldn't happen. This is leading to divergence.
Initially, the simulations were run for a few time-steps without reactions for better stabilization. The moment reactions are included, initial few time-steps it ran fine but the mass addition to the coal phase is leading to the divergence.
Could anyone comment on how to deal with this?
A one-dimensional and three dimensional steady state mathematical model and a numerical algorithm can be developed to simulate the coal gasification process in fluidized bed. The model incorporates two phases, the solid and the gas. The gaseous phase participates in the emulsion (with the solid phase) and forms the bubble. Coal pyrolysis, homogeneous reactions and heterogeneous reactions were considered. The model can predict temperature, converted fraction, and particle size distribution for the solid phase. For the gaseous phase, in both emulsion and bubble, it can predict profiles of temperature, gas composition, velocities, and other fluid-dynamic parameters. The reaction rates of homogeneous reaction and heterogeneous reaction were determined by Arrhenius–Eddy dissipation reaction rate and Arrhenius-diffusion reaction rate, respectively. Flow patterns, gas velocities, particle velocities, composition profiles of gas product and distributions of reaction rates were obtained. Some calculated values were compared with experimental data. The validated model is thereupon can be used to compare Bubbling Fluidized Bed (BFB) and Circulating Fluidized Bed (CFB) gasifiers to highlight the effect of a change in fluidization regime from bubbling to fast fluidization on hydrodynamics, temperature and gas composition. Feed temperature as well as Air/Coal (A/C) and Steam/Coal (S/C) ratios are kept constant, whereas the velocity of the feed (Air-Steam) is increased so as to get into the fast fluidization regime. It concluded that the flue gas from the CFB is richer in desired gases, i.e., CO and H2 than that from BFB. H2 remains approximately the same, CO2 and CH4 decreased to a negligible amount and CO approximately doubles when the regime is changed. In addition, tar content in the gas also decreases
https://www.researchgate.net/publication/223174196_Modeling_and_simulation_of_coal_gasification_process_in_fluidized_bed
https://www.researchgate.net/publication/223350496_Three-dimensional_simulation_of_fluidized_bed_coal_gasification
http://www.scielo.br/scielo.php?pid=S0104-66322019000301289&script=sci_arttext
https://link.springer.com/article/10.1007/s40789-019-00288-x
https://opensiuc.lib.siu.edu/dissertations/1534
Perhaps you are partially oxidizing the coal creating a char phase which is heavier than the original coal because it has been oxidized but not yet vaporized
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