Study Of The Moving-granular-bed Filtration System For High-temperature High-pressure Dust-removal

Advanced coal fired combined cycle power generation systems, such as integrated gasification combined cycle (IGCC) and pressurized fluidized bed combustion combined cycle (PFBC-CC), are promising in future for their high efficiency and very low emission. Hot gas cleanup is an important aspect of technological improvement for efficient IGCC and PFBC-CC systems. Gas cleanup facilities are necessary to protect the gas turbine and environment. However, it is not well developed worldwide at the present time. It is the aim of the study in this paper to lay a foundation for the engineering application of the technology of moving granular bed filter (MGBF). A unique counterflow moving granular bed filtration system (MGBFS) without screen, which would be used for high-temperature high-pressure gas cleanup, was developed and constructed for the first time in China. The system is an integration of MGBF and medium pneumatic regeneration system (MPRS). As a result of integration, the MGBF is more fit for continuous operation under high-temperature high-pressure conditions. The system was tested with air stream at the ambient temperature and under high-pressure. The results showed that the moving granular bed filtration and granular medium regeneration could be put into steady operation together. Based on the results of the air filtration experiments, the structures of the MGBF were improved and the key issue of MGBF for hot coal syngas systems was settled. Moreover, hot syngas filtration experiments were conducted on a pilot plant fluidized bed gasifier. These are the initiative experiments in China. The test results showed that the MGBFS could nearly meet the technical requirements for the hot syngas dust-removal of IGCC system. An efficiency of 99.65 ~ 99.81% were successfully obtained under the conditions of 400 ~ 500 ℃and 1.0 ~ 1.1 MPa pressure. The flowrate of hot syngas is 110 ~ 150m3/h at STP. The pressure, pressure drop and flowrate of the syngas and granules-conveying gas are steady in operation. The pressure drops of both the MGBF and MPRS are all very low. Their values are 4 ~ 6 kPa and 7 ~ 10 kPa respectively. The structure and operation optimization for MGBFS were conducted. Based on the fluidization and pneumatic conveyance theories, a mathematics model of the whole granular circulation system was established for the first time worldwide. According to the requirements of hot gas cleanup, the MGBF is divided into three sections and the flow pattern of each section was investigated. The calculation methods of pressure drop and granular flowrate were obtained for each section. The calculation methods for pressure drop and gas velocity of pneumatic conveyance and the design method of the feeder for granular circulation, which work under high-pressure condition, were provided in this paper. The design and operation control of MGBFS is determinative for its practical application. Now the problem has been solved. The analysis of flow pattern in MGBF showed that the pressure gradient on the granular layer seemed to be the main factor affecting the flow pattern and the pressure difference between the process gas and granules-conveying gas. The calculated results indicated that the temperature and pressure of the process gas would have great influence on the selection of filtration velocity. The moving granular bed filtration process was simulated with a direct numerical simulation method of discrete particles. It is presented for the first time to use the method of modifying adhesion force of bigger particles to simulate the mechanical action of fine particles. And the discrete element method (DEM), describing the interaction of particles, was amended. The parameters such as process gas velocity, granular diameter and dust particle diameter have great influence on the filtration performance of MGBFS. The numerical simulation of filtration process was carried out to study them. The results were qualitatively consistent with that of the experimental ones. The numerical simulation was also conducted for the case of different granularmixtures with varying granularity. The numerical results showed that the filtration efficiency would decrease with increasing fraction of coarser granules.