A COMPUTATIONAL STUDY OF DEVOLATILIZATION AND KINETICS IN ENTRAINED FLOW COAL GASIFICATION

Author

AALIA RAKHSHI, Southern Illinois University CarbondaleFollow

Date of Award

5-1-2018

Degree Name

Doctor of Philosophy

Department

Engineering Science

First Advisor

WILTOWSKI, TOMASZ

Abstract

Computational fluid dynamics (CFD) modeling is an integral part of the design cycle of modern entrained flow gasifiers. Gasification modeling considers an interacting multi-phase media in a turbulent environment. Several gasification sub-models have been developed and validated over the last thirty five years related to turbulence, gaseous combustion, devolatilization and char reactions. The CFD model of gasification must be valid for a range of operating conditions, reactor designs and feedstock compositions. Although tremendous strides have been made in every aspect of gasification modeling, shortcomings do exist in many of the sub-models. Two problems of practical significance are studied in this dissertation. The first is related to devolatilization and the second deals with the accuracy of the gaseous and char combustion models. A focus and consideration of the two problems will improve the predictive capability of the gasification model. In the first, a new framework for volatile breakdown is developed for entrained flow gasification modeling. The framework is based on an optimized solution of an under-determined system of equations formulated using a two-step Moore-Penrose generalized matrix approach. The approach permits the determination of the volatile composition using just the Proximate-Ultimate analysis data of coal. The method can be utilized for all coal types irrespective of origin.The accuracy and consistency of the framework is demonstrated by direct comparison with available devolatilization breakdown data. The overall performance of the framework is also appraised by incorporating it in a CFD simulation of an actual entrained flow gasifier, the 2550 TPD ConocoPhillips EGas technology based two stage oxygen blown gasifier. The reactor exit syngas composition from the simulation is favorably compared with available experimental data. In the second problem, a kinetics assessment of the quasi-global homogeneous and heterogeneous reaction mechanisms is carried out for entrained flow coal gasification modeling. Accurate closure of the chemical source term in gasification modeling necessitates a detailed study of turbulence-chemistry interaction. Towards this end, time-scale analysis of the homogeneous reactions is discussed using eigenvalue analysis of the reaction rate Jacobian matrix. A singular value decomposition of the stoichiometric reaction matrix is performed to assess the behavior of the homogeneous reactions in a reduced species vector space. The significant factors affecting the heterogeneous char reactions is assessed and the relative importance of bulk diffusion and inherent char kinetics is analyzed in a gasifier. The overall study is carried out using numerical and experimental results of an actual pilot scale gasifier, the 200 TPD (tons per day) Mitsubishi Heavy Industries (MHI) pilot scale air blown gasifier.