Date of Award
Master of Science (MS)
Evaluation of ultra-superheated-steam (USS) gasification efficiency of coals to produce hydrogen enriched syngas is the research motive of this joint project between the University of North Dakota and Ohio University. A new USS bubbling fuidized bed gasifier was built in Ohio and several tasks were assigned to assess the gasifier's performance and the feasibility of producing tar-free and hydrogen-rich producer gas.
This thesis presents a thermo-equilibrium model of USS gasification. The model calculates the syngas composition and heating value for the base case fuel Clarion 4A coal using input data from experiments completed at Ohio. The RGIBBS reactor module in ASPEN PLUS, which performs calculation using the Gibbs free energy minimization concept is used to simulate the gasification process. The model compositions were then compared with the experimental syngas composition. The simulation was performed with four other coals and the output for all coals is compared on both wet and dry basis. A sensitivity analysis estimated the effect of temperature and steam flow rate variation on syngas composition and heating value on a wet and dry basis.
The model estimates the syngas composition of mainly 39% H2, 19% CO, 13% CO2, and 28% H2O and a heating value of 4300 Btu/lb (254 Btu/scf). The composition comparison among all coals provided a favorable syngas composition trend for the low-moisture Clarion 4A and Pittsburgh #8 coals, but gave almost the same H2 composition when gas compositions were compared on a dry basis. This implies that drying of high moisture coal before gasification would imporve gas composition. Temperature variation for all coals gives the same trend for gas composition and heating value. The data on temperature variation suggests that gasification at 1320degF gives the maximum H2 composition in the syngas and a gasification temperature of 1410degF produces the highest heating value syngas for this gasifier. The steam flow reate was varied and the effect of H2O/C on syngas composition and heating value was evaluated and compared with experimental data. H2 concentration decreased with an increasing H2O/C range of 0.85 to 3.5 compared on a dry basis.
Comparisons of model results to experimental data indicate a higher CO and lower H2 composition for the experimental data as compared to the model. This indicates the water-gas shift reaction may not be in equilibrium. Since this is a fast reaction it indicates there may be transport/diffusino limitations in the experimental gasifier.
This work tested two hypotheses. The first hypothesis, that a zero-order thermo-equilibrium model accurately predicts the performance of experimental set-up USS gasifier at Ohio with syngas composition and heating value calculation was not valid. A three-dimensional model that includes both kinetics and transport phenomena is required. The model sensitivity analysis determined the maximum gasification temperature and H2O to carbon ratio for maximum hydrogen concentration in syngas. This proves the hypothesis that the model provides useful information for the improvement of the experiment.
Karim, Taskin Binte, "Modeling Of USS Gasification To Evaluate New Gasifier Design In Aspen Plus" (2007). Theses and Dissertations. 756.