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Equation-Oriented Rigorous Modelling, Simulation and Optimization for IGCC Operation
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
IGCC has arisen as a promising technology in hydrogen and electricity production due to its higher efficiency, lower emissions of CO2, capability for CO2 capture and flexibility with respect to feeds and products. However, IGCC is as yet not competitive with conventional technologies due to its high capital and operating costs and its low reliability. The aim of this research is to develop and apply Equation-Oriented rigorous modelling and flowsheet optimization in order to identify process designs / operating conditions that minimize operating and management costs. Challenges for process modelling, simulation and optimization of IGCC relate to: i) the large number of units present in the flowsheet; ii) the large number of chemical components which are described by different equations of state and iii) the complex flowsheet structure with multiple recycle streams. In this work, a general framework is proposed to incorporate rigorous thermodynamic property models, unit operation models, and flowsheet synthesis. The overall process is described by a set of equations that is solved simultaneously by the solver CONOPT in GAMS software. The objective is to minimize the total annualized cost or maximise the annual profit. Rigorous modelling of the IGCC process leads to a large-scale nonlinear problem which is difficult to solve. An initialization procedure is proposed to obtain initial values of flow rates and physical properties for streams and unit operations. A systematic algorithm is proposed to solve flowsheet models, accounting for recycles and complex unit operations. Two examples illustrate the proposed initialization procedure and solution algorithm. Two case studies apply the proposed method. The first case study is operational optimization of Acid Gas Removal, considering the chiller temperature, nitrogen purge flow rate and solvent recirculation flow rate. The second case study is operational optimization of a hydrogen-power plant. Decision variables include the choice of feedstock, gasifier oxidant flow rate, power production and other operation conditions. The optimization results of the proposed method are compared with those of rigorous simulation software and other optimization methods. The results show that the total annualized cost of acid gas removal can be reduced by 9%; the second case study shows that the profit can be increased by 2%, demonstrating the ability of the proposed method to balance accuracy, efficiency and optimality.