Investigation of gas-solid flow and temperature distribution uniformity of 350 MW supercritical CFB boiler with polygonal furnace
Graphical abstract
Introduction
During the scale-up process of high parameter circulating fluidized bed (CFB) boiler, the optimal designs of furnace structure and gas-solid separation system with multiple cyclones are the keys to the development of large-scale CFB boiler technology [[1], [2], [3]]. With the amplification of the CFB boiler capacity and the increase of the steam parameters, the size of the furnace increases, and the corresponding ratio of the water-cooled heat exchange surface to the furnace volume decreases. In order to ensure the full mixing of the gas-solid flow in the furnace and the penetrating ability of the secondary air, some new structures of the furnace usually need to be adopted, such as “pantleg furnace” and “annular furnace” and so on [[4], [5], [6]], the changes of the above furnace structures have a certain influence on the distribution of solid particles concentration and heat flow density along the height of the furnace [[7], [8], [9], [10]], and the flow control of the solid fuel becomes more complicated. For example, for the “pantleg furnace”, the material drift phenomenon is more prone to occur, which easily leads to the uneven distribution of the pressure in the furnace and the problem of turning over the bed. In order to adapt to the large-scale and high-parameter development of CFB boilers, the furnace structure of supercritical CFB boilers needs further optimized design [[11], [12], [13], [14], [15]].
The supercritical CFB boiler adopts once-through technology, which puts forward higher requirements for hydrodynamic safety and gas-solid flow uniformity in the furnace [[16], [17], [18]]. For the traditional rectangular furnace, four right corners of the furnace have the particles downflow aggregation phenomenon near the water-cooled wall, that is, there is obvious non-uniform gas-solid flow at furnace boundary [14,[19], [20], [21], [22], [23], [24]], which causes the problem of water-cooling wall explosion in serious cases. Based on the above problems, a innovative furnace structure of polygon furnace for the supercritical large-scale CFB boiler is put forward for the first time by the Institute of Engineering Thermophysics, the Chinese Academy of Sciences [25]. For the arrangement of three cyclone separators on the same side of CFB boiler, the traditional arrangement is shown in Fig. 1(a), but there is a large deviation of particle mass concentration among three cyclone separators in this arrangement, and the distribution trend is low in the middle and high at both ends. In order to improve the uniformity of gas-solid distribution among three cyclone separators, three main technical measures are adopted. First of all, the rotating directions of two separators on both sides are changed. Secondly, the inlet positions of two separators on both sides are closer to the middle of the furnace. Finally, two chamfered corners are added at the two right angles of the back wall of the furnace. The new arrangement is shown in Fig. 1(b). Through the improvement of the above arrangement, the aggregated particles in four right corners of furnace are dispersed to a larger extent, which reduces the local wear of four corners, and greatly decreases the mass concentration deviation among three cyclone separators, this type of furnace structure is especially suitable for the large-scale supercritical CFB boilers.
Based on the technical scheme of 350 MW supercritical CFB boiler with polygonal furnace, this paper firstly established a cold test device according to the scaling ratio of 1:20, and investigated the influence of three kinds of polygonal furnace structures (no chamfered-corner furnace type 1, two chamfered-cornerfurnace type 2, four chamfered-corner furnace type 3) on the distribution characteristics of the gas-solid flow in the furnace and the uniformity of the particle concentration distribution among three cyclones, and obtained the optimized furnace type of 350 MW supercritical CFB boiler with polygonal furnace, then the gas-solid flow characteristics in the furnace and three circulating loops of the above optimized furnace type were carried out by numerical calculation method, and the calculation results were compared with the test results. At the same time, the correlated characteristics of solid particle flow characteristics from small scale to large scale were established. Combined with cold experimental tests and numerical calculation results, the technical scheme of 350 MW supercritical CFB boiler with polygonal furnace was optimized and the engineering technology demonstration was carried out. Based on the actual boiler tests, the temperature distribution uniformity of furnace and three circulating loops for the first 350 MW supercritical CFB boiler with polygonal furnace was obtained; the research results will provide a very important reference for the development of supercritical large-scale CFB boiler technology in the future.
Section snippets
Cold experimental system
The cross-section size of the cold test bench of the polygonal furnace was reduced by a ratio of 1:20 according to the design scheme of the 350 MW supercritical CFB boiler. The cross-sectional dimension of the furnace was 1137 mm × 392 mm, and the height of the furnace was 8000 mm. In order to obtain more realistic gas-solid flow characteristics, the bed characteristics, fluidization velocity and circulation flow rate were as consistent as possible with the actual boiler. The design principles
Numerical simulations
Based on the method of numerical calculation, the 350 MW supercritical CFB boiler with polygon furnace (furnace type 2) was simulated by 1:1 structural scale, and analyzed the distribution characteristics of the gas-solid two-phase flow in the furnace and among three cyclones, which would provide a theoretical basis for the optimal design of a 350 MW supercritical CFB boiler with polygonal furnace.
Introduction of a commercial 350 MW supercritical CFB boiler
The 350 MW supercritical CFB boiler was supercritical parameters, the polygonal furnace (furnace type 2 with two chamfered corners) single air distribution plate, M-shaped arrangement, and a full steel frame supporting structure. The overall layout of the 350 MW supercritical polygonal furnace CFB boiler and the chamfered-corner structure of the furnace rear wall were shown in Fig. 13. The boiler was divided into three parts, the first part was arranged with the furnace; the second part was
Conclusion
Based on the cold tests, numerical calculation and actual boiler tests, the gas-solid flow characteristics of three polygon furnace structure (furnace type 1, furnace type 2, furnace type 3) were studied, and the temperature distribution uniformity of first 350 MW supercritical CFB boiler with polygon furnace were investigated. The main conclusions are as follows:
- 1)
After adopting the chamfered corners of the furnace, it not only greatly affects the circulating flow rates of three circulating
Nomenclature
- G
circulating flow rate, kg/(m2·s)
- d
diameter, μm
- u
fluidization velocity, m/s
- ρ
the bulk density of quartz sand, kg/m3
- ∆ h
the material stack height in the riser, m
- △t
thestacking time, s
- A
the cross-sectional area of the riser, m2
- ε
particle concentration
- U
voltage, V
- δ
particle concentration relative deviation, %
- σ
temperature relative deviation,%
- T
temperature, °C
Acknowledgments
This work is financially supported by the National Key Research & Development Program of China (Grant No. 2018YFB0605002) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA07030100).
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