The simulations reveal gas composition, temperature, and pressure interdependencies along the height of the reactor. The product gas composition compares well with the experiment and the temperature profile demonstrate good consistency with the experiment. The developed model is used for a case study of Shubarkol coal gasification in the circulating fluidized bed reactor. Development of euler-lagrangian simulation of a circulating fluidized bed reactor for coal gasification. T1 - Development of euler-lagrangian simulation of a circulating fluidized bed reactor for coal gasification.
Fingerprint coal gasification. Coal gasification. Fluidized beds. Fluid dynamics. Chemical analysis. Reaction kinetics. Reaction rates. Dynamic models. Model predictions and experimental results showed good agreement in all cases. Taking into account departures from isothermality in the bed, jetting at the gas inlet, and particle size distribu- tions in the feed coal did not provide significant improvements in the ability of the model to correlate the data, except for elutriation rates, which are highly sensitive to the concentration of fines in the feed.
Sulfur gas formation is better predicted by equating sulfur and carbon conversion than by using pub- lished kinetic correlations. The effects of various operating parameters and phenomena on reactor performance were determined using the models. As expected, carbon conversion and make-gas production both increase with bed temperature, steam-to-carbon feed ratio, and solid- phase space time. Both also go up with pressure; but, above about 1. Research Triangle Park, NC.
5.2.3. Fluidized Bed Gasifiers
Introduction As a part of a continuing research program on the environmental aspects of fuel conversion, the U. Process control, data acquistion, and data logging systems, and an extensive analytical laboratory com- plete the facility. In experiments conducted to date, a devolatilized Western Kentucky bitumin- ous coal, a New Mexico subbituminous coal, a North Carolina peat, and a Texas lignite have been gasified with steam and oxygen.
The experimental results are summarized in several EPA reports. The primary function of the NCSU gasi- fication reactor is to provide a reproducible synthesis gas for studies of the potential environmental impact of coal gasification processes. The development of correla- tive and predictive models of the gasifier was felt to be an indispensable adjunct to planning and implementing the overall experimental program.
This report summarizes the principal features of both models, and reports the model correlations of the gasification data for both feedstocks studied. Model Description The three-stage well-mixed reactor model and the bubbling-bed model are described. Three-Stage Well-Mixed Reactor Model The first stage of the modeling studies was to formulate the simplest possible model which incorporates the principal gasification reactions and the gross physical characteristics of the reactor, and to determine the degree to which the gasifier performance could be correlated by this model.
A well-mixed fluidized-bed model was used for this purpose. The model assumes instantaneous devolatili- zation and oxidation, followed by gasifica- tion in a perfectly mixed bed. The model takes as input the average reactor bed temperature and pressure; the bed dimensions; feed rates of coal, steam, oxygen, and nitrogen; solids holdup in the bed; and ultimate analysis of the feed coal. It calculates carbon conversion and make-gas flow rate and composition.
Devolatilization is presumed to occur instantaneously where the coal enters the bed. The extent of devolatilization and composition of the volatile products are taken from rapid pyrolysis data obtained in a separate study at NCSU and additional data for a western subbituminous coal close in composition to the one used in this study. Reactions 1, 2, and 3 are the reactions with which the Institute of Gas Technol- ogy correlated gasification kinetics data.
These kinetic expressions, summarized in the body of the report, include as an adjustable parameter a char reactivity coefficient, f0, which has values ranging from 0. A model option allows the assumption of shift reaction equilibrium or the use of kinetic rate equations. The rate expression used contains an adjust- able parameter, fwg, which accounts for the varying catalytic activities of different chars.
The rate of Reaction 7 is estimated by assuming that the sulfur conversion equals the carbon conversion, and that all converted sulfur forms hydrogen sulfide by Reaction 7. The flow rate of COS is then determined by assuming equilibrium of Reaction 8. The other coefficients of the various rate and equilibrium equations are prescribed by the model.
Details of these equations and of the model computation- al procedure are given in the report. Bubbling-Bed Model The two-phase bubbling-bed model divides the reactor into two main sections: a bed region, and a freeboard region above the bed.
Coal Gasification Modeling in Fluidized Bed Reactors
Consideration of a jetting region at the gas inlet of the bed is a model option that can add a third section to the model. The NCSU gasifier uses a top coal feeding system, with the feed particles falling through the hot exiting gas stream before entering the bed. In the freeboard region, the particles heated by the gas stream are dried and devolatilized. In addition, the fines in the coal feed stream may be eluted by the exiting gas stream. The model of the freeboard section accounts for all of these phenomena.
The blowover solids are assumed to be devolatilized at the gas exit temperature. After the blowover calculation, the flow rate and size distribution of the remaining coal are determined. The devolatilization products for this stream are calculated, assuming equilibrium yields at the temperature at the top of the bed.
The devolatilization products are assumed to be evenly evolved along the length of the freeboard region. Heat transfer from the gas to the coal particles is calculated for each class of particle sizes using a single-sphere heat transfer correlation. It is assumed that there are no radial temperature gradients within particles. The latent heat of devolatilization is neglected. The heat of vaporization of moisture is included in the model, with an average value of It is assumed that the heat required to vaporize the moisture in the feed coal is taken from the gas phase.
Particles are assumed to have constant volumes in the freeboard region, so that the particle densities decrease as drying and devolatilization proceed. The model contains an option to consider a jetting region at the gas inlet of the bed. The jet penetration height and jet angle are calculated using earlier correla- tions. The gases in the fluidized bed are assumed to be in plug flow, while the bed solids are assumed to be well-mixed with respect to composition and size. Mass transfer between the jet and emulsion phases is determined using a mass transfer coefficient correlation. The region between the jet penetration height and the top of the bed is assumed to be a two-phase fluidized region, consisting of an incipiently fluidized emulsion, and a solids-free bubble phase.
Both gas phases are assumed to be in plug flow, while the solids are assumed to be perfectly mixed throughout the jet and fluidized-bed regions. The assumption of well-mixed solids throughout the bed is supported by experimental results.
Simulation of Olive Kernel Gasification in a Bubbling Fluidized Bed Pilot Scale Reactor
Mass transfer rates between the two phases are calculated using earlier correlations. For both the jet and fluidized regions, the temperature may be determined three ways: 1 a specified isothermal value; 2 an imposed temperature profile; and 3 a profile calculated assuming adiabatic reactor operation. For all, the solids and gases at any height are assumed to be at the same temperature.
Reactions are gasification reac- tions, with rates given by the previously discussed kinetic expressions. Carbon combustion Reactions 12 and 13 are described by an earlier rate equation. The combustion rate expression yields the rate of carbon consumption, but does not specify the gaseous products.
The combustion product split between CO and CO2 is therefore calculated as outlined in the context of the simple three-stage model. Gas phase combustion occurs accord- ing to Reactions The reactions are very fast, and are assumed to go to instantaneous completion. If oxygen is insufficient for complete combustion, the reactions probably compete for oxygen at different rates. However, the reactions are expected to have a small overall effect on the modet predictions.
So, for simpli- city, the three reactions are assumed to compete equally for available oxygen. In the emulsion phase, the reaction occurs predominantly on the char surface, which acts as a catalyst. A model option also allows for the assump- tion of shift reaction equilibrium.