Operating concept of circulating fluidized bed gasifier from the kinetic point of view

Circulating fluidized bed gasifiers used in integrated gasification and combined cycle (IGCC) possess unique design and operational features: high gas velocity, high solid loading, and a large amount of circulating chars. As a consequence, their performance is noticeably different from the conventional fluidized bed gasifier operating at lower gas velocity. At the same time, limited information has been published to show how to operate the circulating fluidized bed from the kinetic point of view. In order to study these aspects, kinetic models are developed using kinetic data from previous literature and two concepts of operating the circulating fluidized bed gasifier from the kinetic point of view are presented in this paper. The results of the kinetic analysis using Johnson's kinetic data of a constant specific gasification rate of char (0.001 1/s) show that all species approach their equilibrium values after travelling a distance of 2.5 m, which is set as the height of the gasifier. The resident time of the char in the bed is 0.86 s and the required circulation rate of char and lime are 11.8 and 4.5 kg/s, respectively. In addition to Johnson's kinetic data, Wen's kinetic model is also applied in the kinetic analysis. It shows a way to operate the gasifier in the fast fluidization regime with a superficial velocity between 3.6 and 4.9 m/s and a solid loading between 50 and 100 kg/m(3), these operating conditions are recommended by industrial experience. It points out the necessity to predict the performance of the gasifier from the kinetic point of view in order to design the optimal geometry and operating methods that will allow the gasifier to play a successful role in the clean coal technology system such as IGCC, high-performance power system (HIPPS), etc. (C) 2003 Elsevier Science B.V. All rights reserved.