Investigation On Heat Flux And Hydrodynamics Of Water Wall Of A Supercritical Pressure Circulating Fluidized Bed Boiler

Supercritical pressure circulating fluidized bed (SCCFB) is more and more attractive because of its unique combination of higher power generation efficiency with lower cost emission control. The conceptual design of a 26.17MPa, 604℃and once reheat 800MWe SCCFB fired high sulphur coal was suggested, of which the furnace is single with pants leg, and the membrane water wall of once through vertical smooth tube with throttle while without intermediate mixing. The tubes with thickness of 8mm for erosion protection are safe at various load conditions because of the relative higher mass flow rate of 1428kg/(m~2-s) under B-MCR. In the combustion system, there are 6 insulated cyclones, 6 water cooled mechanical distribution valves and 6 external heat exchangers (EHE). An internal start-up system with recalculating pumps was also suggested. It operates with sliding pressure at 40~91%T-MCR, while with fixing pressure at higher than 91%T-MCR or lower than 40%T-MCR. With the limestone particles added, the desulphurization efficiency is higher than 90%. The efficiency of the boiler is 93.12% at B-MCR. And emission of NOX and CO can be kept lower than 200mg/Nm3 and 100mg/Nm~3, respectively. In order to investigate the hydrodynamics in water wall, a semi-empirical local heat transfer (HT) model was developed according to the experimental results from the present measurements of conduct heat probe, where the effect of the membrane structure is considered. The model prediction agrees well with the experimental results from literatures. Then an empirical 3-dimensions solid suspension density (SD) model was established, which is necessary for HT model. The parameters of the SD model are defined from field measurements. However, the circulating material rate in SD model is predicted with an improved 1-demision material balance model. The SD model prediction consists well with the other experimental results. The heat transfer coefficient of bed to wall was measured in the furnace of real CFB boilers with cross section of 3m×6m,5m×10m,7m×14m based on the finite element method to check HT model. The HT model results of which solid suspension density is predicted with SD model agree well with these measured results, with less than 5% error. Besides, a heat transfer model of EHE is also suggested by improving the previous heat transfer model of the immersed tube of bubbling fluidized bed with 7% error compared with the data from a real boiler EHE. A water hydrodynamic (HD) model of water wall panel was proposed in supercritical and subcritical region. With HD model, the hydrodynamics and heat transfer of water wall were predicted in three cases of without (1) or with (2) the consideration of header effect and with throttles (3) in six loads. The model results show that for case 3 the temperature of water at the outlet of the tube near the furnace corner is highest as well as that of the inner or outer tube wall and the fin middle. The highest temperature difference occurs at 60%T-MCR. While the biggest mass flow rate difference occurs at B-MCR with the lowest mass flow rate in the tube near middle of the water wall. The highest temperature of the tube and fin at any load is less than the permitted working temperature over 100 oC for the designed tube material of SA-213T11. The heat flux of SCCFB is more uniform besides much less than that of supercritical pulverized coal boilers. The heat flux decreases with the increase of the distance to the distributor. The horizontal heat flux distribution is related to the horizontal position of tube and boiler load. The heat flux difference at the top of the tube is highest, which is about 20% at B-MCR. And it is found that if the solid suspension density is over 20kg/m3, the impact of it is no longer sensitive on the bed to the wall heat transfer coefficient. The general model combining the suggested models with the common heat transfer models of back pass was built to predict the SPCFB thermal performance of various loads including the material balance. The results show that the main steam and reheat steam temperature can reach the rated value for sliding pressure operating ranging from 40%T-MCR to B-MCR. Finally, the further investigations of SPCFB were suggested, such as the design comparison, material balance, dynamic model of SPCFB, etc.