Original Research PaperExperimental study on gas-solid flow characteristics in an internally circulating fluidized bed cold test apparatus
Graphical abstract
Introduction
As a new type of clean coal combustion technology, the circulating fluidized bed (CFB) combustion technology which was developed in the 1960s can burn low grade coal and at the same time release less pollutants [1]. It is also characterized by high combustion efficiency, wide load adjustment range and therefore has been widely used worldwide [2]. With the development of the CFB combustion technology, researchers around the world not only systematically investigated how to control externally circulating systems and enhance heat transfer intensity [3], [4], but also studied the flow and heat transfer characteristics of the internally circulating systems [5], [6], [7]. Some engineering application solutions have been proposed. Wu and Wang [8] investigated the influences of fluidization air velocity, particle diameter and bed temperature on convection heat transfer coefficient between immersed tubes and bed materials in the in-bed heat exchanger of a 2.5 MW pilot internally circulating fluidized bed reactor with heat transfer probes. Fang et al. [9] designed a double fluidized bed solid circulating system, which could be used for biomass gasification to produce middle heating value town gas. Plastic balls and fluidized bed ash were used as bed materials. The effects of bed material, operation velocity, and bed structure on solid circulating rate between two beds were studied in a small-scale test facility by using photographic method, and its reasonable operation and design parameters were put forward. Huang et al. [10] investigated the particle flow characteristics in a clapboard-type internal circulating fluidized bed reactor using two kinds of bed material, i.e. rice husk and quartz sand. The effects of fluidization velocity, structure dimension and amount of side air on the solids circulating rate between high velocity zone and low velocity zone were studied. The recommended parameters for the design of the clapboard-type internal circulating bed gasifier were given based on the experimental data. The studies above show that, with the different purpose of engineering application, researchers focus on different factors which can influence gas-solid flow characteristics in internally circulating fluidized bed reactors. The engineering applications for the studies carried out by Fang and Huang were biomass gasifiers, therefore the test fluidization air velocities were relatively low (the fluidization number was only 1.2–2.2 and 1.8–4.5 respectively). The studies need to accurately forecast the inner loop gas-solid flow and heat transfer characteristics in different furnace structures.
High gas-solid concentration not only enhances the intensity of heat transfer, but also facilitates the abrasion of heating surfaces in the existing CFB furnaces [11]. This can be solved by installing internal heat exchanger with high heat transfer intensity in the furnace, and at the same time reducing the gas-solid concentration in the upper space of the furnace [12]. This work puts forward a new kind of internally circulating fluidized bed furnace [13] in order to develop a thermal oil furnace which strictly requires security of the heating surfaces and controllable heat transfer intensity. In the past research on internally circulating system, most of the particles flow down along the furnace wall from upper space of furnace and form the internally circulating system. Therefore, the gas-solid flow characteristics in the upper space of furnace have been at the state of fast fluidization. To avoid abrasion of heating surfaces, the solids holdup in the upper space of the furnace should be reduced as much as possible. Consequently, particles should be blown to the internal heat exchanger instead of being entrained upwards to the upper space of the furnace. In this new kind of internally circulating fluidized bed furnace, the gas-solid flow characteristics is at dilute phase flow state in the upper space of furnace and is at fast fluidization state in the middle part of the furnace. This new kind of internally circulating fluidized bed furnace makes full use of buried tube heat transfer in the internally circulating system and is characterized by high efficiency and safety performances. In this new kind of furnace, the furnace is separated into separate chambers: the main chamber and the heat exchange chamber. The two chambers are placed in the furnace parallelly and are separated by partition wall. They share the upper space in the furnace. The fluidization air velocity in the main chamber is higher than that in the heat exchange chamber. In this way, the coarse and fine particles will be separated. Therefore, combustion and heat transfer will be decoupled [14]. The heat exchange chamber is in fact a bubbling fluidized bed with fine particles where the gas-solid flow is not severe. In this kind of internally circulating fluidized bed furnace, the amount of the particles carried by the fluidization air from the main chamber to the heat exchange chamber directly influences the heat transfer intensity in the heat exchange chamber. Enough and continuous particles which circulate internally are the key to control the heat transfer in the furnace. In order to design and operate this kind of internally circulating fluidized bed furnace successfully, a good knowledge of the particle internal circulation rate (G) is required.
This work investigated the gas-solid flow characteristics in a self-designed and built internally circulating fluidized bed cold test apparatus.
Section snippets
Experimental apparatus
As shown in Fig. 1, the main body of the internally circulating fluidized bed cold test apparatus consisted of a centrifugal blower, two uniform-pressure wind boxes, a furnace, a partition wall, a cyclone and a loop seal. The furnace with a height of 3200 mm was separated into two chambers: a main chamber with a cross section of 390 mm × 265 mm and a heat exchange chamber with a cross section of 390 mm × 610 mm. The two chambers were placed in the furnace parallelly and were separated by partition wall.
Effect of fluidization air velocity in the main chamber (ug) on the particle internal circulation rate (G)
Fig. 6, Fig. 7 show the effect of fluidization air velocity in the main chamber (ug) on the particle internal circulation rate (G) at different partition wall height (Hw) at Hi = 300 mm and Hi = 400 mm. From Fig. 6, Fig. 7, it can be seen that regardless of the height changes in partition wall and initial static bed, with the increase of fluidization air velocity in the main chamber, the particle internal circulation rate firstly increased, reaching the maximum at ug = 6umf and then decreased. With the
Conclusions
In order to develop a CFB thermal oil furnace which strictly requires security of the heating surfaces and controllable heat transfer intensity, this work proposed a technology solution for the first time through enhancing internally circulating system and reducing the externally circulating system. In this new mode of gas-solid flow, fast fluidization state exists in the middle part of the furnace and dilute phase flow state exists in the upper space of the furnace. The gas-solid flow
Acknowledgements
This work was financially supported by the National Key Research & Development Program of China [grant number 2016YFB0600201].
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2021, Advanced Powder TechnologyCitation Excerpt :Compared with the circulating fluidized bed, the ICFB has the advantages of simple and compact bed structure, low bed height and low internal heat loss, making it widely used in biomass gasification technology, coal gasification technology, and out-of-stock, waste pyrolysis, and so on. Song et al. [9] studied the fluidization of particles in the upper space of a new type of internally circulating fluidized bed furnace. Yang et al. [10] studied the fluidization of bifunctional particles in ICFB.
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2021, EnergyCitation Excerpt :Until now, plenty of experiments regarding the thermophysical properties of biomass gasification in the CFB have been conducted. Song et al. [4] built a cold test-rig of CFB unit to investigate the effect of the operating conditions on the particle internal circulation rate and the solids holdup. They demonstrated that increasing partition wall height affects the particle internal circulating rate, and increasing initial bed height benefits the solid flux.
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2021, Powder TechnologyCitation Excerpt :Huang et al. [7] also carried out experimental studies on the characteristics of the particle internally circulating flow in a cold test apparatus with similar structure. Song et al. [8] proposed a thermal oil furnace with two fluidized beds, and studied the influences of the fluidization air velocity, the partition wall height and the initial bed height on the internal particle circulation rate. The correlation of the structure size of the furnace, the operating parameters and the internal particle circulation rate was obtained.
Axial flow structure of solids holdup in an 18-m high-density CFB riser based on pressure measurements
2021, ParticuologyCitation Excerpt :It is thus important to set up a pilot or industrial-scale CFB system for the study of flow structures in an HDCFB, especially in the fully developed zone. Hydrodynamic parameters can be obtained by making various measurements (Song et al., 2017), such as measurements with capacitance probes (Guo & Werther, 2004) and optical fiber probes (Liu, Grace, & Bi, 2003b) and electrical capacitance volume tomography (Guo, Ye, Yang, & Liu, 2019; Wang et al., 2018; Weber, Bobek, Breault, Mei, & Shadle, 2018), high-speed image capture (Xu et al., 2018), and particle image velocimetry (Gopalan & Shaffer, 2013; Xu et al., 2017). Among those measurements, the use of pressure transmitters is the most common method adopted for fluidized beds owing to the ease of measuring under various conditions (Ommen et al., 2011; Tahmasebpour, Zarghami, Sotudeh-Gharebagh, & Mostoufi, 2013).