Research on Heat Transfer Inside the Furnace of Large Scale CFB Boilers

Field tests in one unit of 135MWe and two units of 300MWe commercial Circulating Fluidized bed (CFB) boilers (AB Wirth (2); Andersson and Leckner (3); Basu and Nag (4)) and the results showed that the heating transfer coefficient from bed to heating surface was affected by the solids suspension density, particle size and bed temperature. Pagliuso, et al. (5) found that the profile of the heat transfer coefficient along the riser correlated well with that of the solid suspension density. They also found that the heat transfer coefficient increased with the increasing particle size, which became more obvious for smaller particles and higher suspension density. Breitholtz and Leckner (6) presented a correlation between heat transfer coefficient and suspension density based on measurements in six CFB boilers ranging from 12 to 300 MWth. In addition, with increasing bed temperature, Tb, thermal conductivity and radiation heat transfer of the gas increased accordingly, which lead to an increase of the heat transfer coefficient. This was confirmed by the test of Jestin, et al. (7) in a 125MWe CFB boiler and the experiment of Jeon, et al. (8). Many studies on gas-solid flow and heat transfer in CFB boilers were carried out in laboratory scale CFB combustors, which differ from industrial ones because of smaller height to diameter ratios. Zhang, et al. (9) and Noymer, et al. (10) found the height to diameter ratio of a riser has a significant impact on the gas-solid flow inside the furnace. Besides, considering the difference in temperature, particle size distribution and suspension density between laboratory scale CFB combustors and industrial ones, previous test results may not totally represent those found in industrial CFB boilers. At the same time, the heat transfer coefficient is also affected by boiler load, which was proven by Zhang et al. (11). Therefore, it is necessary to measure the distribution of the heat transfer coefficient inside an industrial CFB boiler for proper design and arrangement of heating surfaces. In recent years, large-scale CFB boilers have been developed, however, only a few test results on heat transfer in large-scale CFB boilers have been published. To compare and analyze heat transfer characteristics in different large-scale CFB boilers, field tests in one unit of 135MWe CFB boiler and two units of 300MWe CFB boilers with different structures were conducted. The influence of boiler load on the local heat transfer coefficient and thermal boundary layer distribution were studied in this work. EXPERIMENTAL The tests were carried out in a 135MWe CFB boiler and two 300MWe CFB boilers, which are all reheat, natural circulation boilers. The 135MWe CFB boiler has single furnace with height of 38m from distributor to the roof and with a cross section of 6.6m×13.1m. Two refractory-lined cyclones were used as the gas solid separators. Lean coal was burnt during the experimental test. Two 300MWe CFB boilers, referred to as A and B respectively, have different furnace structures. Boiler A has a pants leg furnace structure with height of 36m and a cross section of 14.8m×12.6m. Four cyclones are arranged on both sides of the furnace. A higher bed temperature (890°C) was adopted for coal with a low volatile content to improve combustion efficiency. The boiler was operated with a high solids circulation rate. Boiler B has a single furnace with height of 40m from the distributor to the roof, and a cross section of 8.4m×28.2m. There were two water cooled wing walls hung in the furnace. Three steam-cooled cyclones are used as gas solid separators. Local heat transfer coefficient (a), heat flux (b), the peripheral distribution of the heat transfer coefficient (c), the temperature profile near the wall (d) and the vertical distribution of the solid suspension density along the furnace height at different loads (e) were measured respectively. The local heat transfer coefficient, K, was measured by a water-cooled conductive heat flux meter designed by Tsinghua University, shown in Fig.1. According to the heat conduction law, the local heat transfer coefficient can be calculated from the axial temperature gradient inside the probe-defined as heat flux method (HFM). The heat flux, q, was calculated based on the temperature difference measured with thermocouples installed in the fin-tube (shown in Fig.2) with a two-dimensional finite element calculation method (FEM), which has been investigated by many researchers (Andersson and Leckner (12); Zhang, et al. (13); Wang, et al. (14); Zhang et al. (15)). The heat flux profile was obtained by arranging a number of measuring points peripherally at different heights along the furnace. The relationship between the heat transfer coefficient and the heat flux can be expressed as: