MODELING AND SIMULATION OF FLUIDIZED BED GASIFIER FOR INDIAN COAL

Abstract

The present work entitled “Modeling and Simulation of fluidized bed gasifier for Indian coal” is based on the modeling of pilot scale bubbling fluidized bed gasifier operating at central Institute of mining and fuel research (CIMFR) Dhanbad, India-a research wing of CSIR. For this purpose, a two dimensional model of the gasifier using quadrilateral cells has been developed using FLUENT 13 software. The model of the gasifier includes the mechanism of drying, volatilization, gasification and combustion process. Eulerian – Eulerian modeling approach is used to exchange mass, momentum and energy between gas and solid phase. Kinetic theory of granular flow is used to describe the phenomenon of solid phase. The gasifier model involves the kinetics of 8 homogeneous reactions as (carbon monoxide combustion reaction, water gas shift reaction, reverse water gas shift reaction, hydrogen combustion reaction, steam reforming reaction, methane combustion reaction and ethane combustion reaction) and 5 heterogeneous reactions as ( char oxidation reaction, char hydration reaction, char carbon dioxide combustion reaction, char hydrogenation reaction resulting in the formation of methane and ethane ). Eddy dissipation model of FLUENT describe the kinetics of homogeneous reactions and a user defined code written in C has been developed that describes the kinetics of heterogeneous reaction, involves the Arrhenius kinetics. Calibration and validation of the model has been done using the experimental data obtained from CIMFR. Dhanbad. The results shows that the computed exit gas composition of syngas are in good agreement with the experimental data, as the maximum error is found to be around 4.05% in the composition of CO, while the composition of hydrogen shows the error of about 2.68%. In an attempt to maximize hydrogen production, this model has been used to study the variation in hydrogen concentration in the syngas on the basis of different operating and design parameters. Simulation has been performed for the different grades of Indian coals data, and results are compared on the basis of hydrogen gas concentration. It has been found that the Indian coal having higher percentage of volatile matter content and carbon content, produces higher concentration of hydrogen as compared to others. The choice of gasifying medium for coal gasification affects the quality of syngas as well as its composition. Product gas has the lowest calorific value if air is used as gasifying agent because of the presence of large concentration of nitrogen in it. It has been observed that gasification with steam increases the calorific value of the product gas due to enhance in the rate of gasification reaction which produces more hydrogen. Thus, the present work incorporates the use of steam as a gasifying agent to investigate the increase in hydrogen gas concentration during gasification process. By the use of steam as a gasifying agent, the concentration of hydrogen in the product gas obtain is about 42.9% which is 52.38% higher as compared to base case model which uses air as gasifying agent. However, this technology has one drawback it decreases the temperature iv of the gasifier. To overcome this drawback, oxygen along with steam is supplied to the gasifier. Thus the study also includes the effect of variation of steam/oxygen ratio on exit hydrogen concentration as well as temperature of the gasifier. Present work also incorporates the parametric study of the variation in steam/coal ratio to see the effect on hydrogen gas concentration. The model has also been used to study the effect of temperature and freeboard height on gasifier performance. It has been observed that the temperature of the gasifier decreases along the height of gasifier as most of the exothermic reactions occur near the bed zone of the gasifier and results in an increase of temperature in this region. However, due to the lack of exothermic reactions, in the upper portion of the gasifier the temperature drops. When the height of the gasifier is varied from 3-5 m, the results show that the hydrogen concentration increases with the freeboard height of the gasifier this is primarily due to the fact that residence time of coal particles in reaction zone increases with freeboard height.