Sensitivity Analysis and Control of Gasifier Using Recycled Carbon Dioxide Gas in Integrated Gasification Combined Cycle

Abstract

Integrated Gasification Combined Cycle (IGCC) is rapidly emerging as one of the most promising technologies in power generation and is able to meet the most stringent emissions requirements. The IGCC systems are extremely clean and much more efficient than traditional coal fired power systems. The gasifier is the heart and most complicated unit operation with high temperatures and pressure variations in the Integrated Gasification Combined Cycle (IGCC). The dynamics inside a gasifier is very rapid and it is extremely complicated to control the high temperatures inside the gasifier and the synthesis gas (H2 + CO) composition coming out of the gasifier. This paper describes a novel technique that has been developed for varying and controlling the synthesis gas compositions coming out of the gasifier and temperature inside the gasifier by carbon dioxide gas recycle to the gasifier. In order to vary the synthesis gas composition, the gasifier temperature and other key parameters, the carbon dioxide recycle gas to the gasifier is manipulated instead of varying the coal, water and oxygen mass flow rates. A sensitivity analysis on the gasifier has been completed by varying the mass flow rate of carbon dioxide recycle gas and the oxygen gas without varying the coal slurry (coal +water) mass flow rate to analyze the synthesis gas composition, temperature, char conversion and other important parameters. Using the sensitivity analysis it has been showing that enhanced controllability of the gasifier in the IGCC is possible to meet the varying demands by recycling CO2 without the need of varying the coal, water and oxygen flow rates. The method is based on the idea that different output compositions can be achieved by making the carbon in coal slurry react with as an oxidizer instead of O2 with the help of Boudouard reaction (CO2 + C → 2CO). The two-stage oxygen blown entrained flow gasifier has been modeled using Computational Fluid Dynamics (CFD) and the coal gasification process was modeled using the Discrete Phase Method implemented in the Finite Volume FLUENT® CFD software. The physical and chemical processing of the coal slurry gasification is implemented by using FLUENT® User Defined Function’s (UDF’S). These UDF’s define the mechanism through which thecoal partiticles undergo moisture release, vaporization devolatilization, char oxidation and gasification processes. One important property of the gasifier in the Integrated Gasification Combined Cycle (IGCC) is that it can simultaneously produce synthesis gas for chemical processes as well as electricity, how much is generated of each can be balanced according to market demand.