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Abstract Segregation exists extensively in the circulating fluidized bed (CFB) system, with solid particles of different sizes and densities. This paper studied the segregation of binary particles in a CFB system. Experimental results showed that segregation in the riser became weaker with increasing fluidizing gas velocity, u g , and solid circulating rate, G s. In the case of high u g (6.0 m/s) and high G s , segregation almost disappeared and the binary particles in the whole system became complete mixing. The average jetsam mass fraction along the riser height showed different relations with the initial jetsam mass fraction in the literature. In order to explain the discrepancy in the literature, this paper indicated the exit geometry influenced the segregation in the whole CFB loop, i.e. not only in the riser, but also in the external loop. For the abrupt exit, the internal refluxing of solids always existed in the sharp bend for the inertial separation of particles, and more coarse particles rebounded at the riser exit than the fine particles. Thus more coarse particles stayed in the riser than in the standpipe. Graphical abstract Unlabelled Image Highlights • Influencing factors on segregation of binary particles were studied. • Three exit structures were studied in a cold CFB test rig. • Separation effect of the T-shape exit was discovered. • The controversy in literature was explained by the impact of the exit. [ABSTRACT FROM AUTHOR]

The effect of the pressure drop across the furnace on heat transfer inside large scale CFB boilers was investigated with a 1-D CFB combustion model developed at Tsinghua University. The results from the 1-D model agree well with the field test data carried out in a 135 MW e CFB boiler. It was found that, at a certain load and with fixed size distribution of the coal particles fed to the boiler, a decrease in furnace pressure drop yields an increase in furnace temperature and reduced heat transfer coefficient. According to the heat balance, when the pressure drop decreases by 1 kPa, the heat transfer coefficient is expected to decrease approximately by 2%. The increase in temperature with decreasing pressure drop is less steep, when the boiler load increases or smaller sized coal particles are fed to the boiler. In both cases, more fine particles are carried to the upper parts of the furnace which results in a more uniform solid volume fraction distribution along the furnace height. The influence of the pressure drop on heat transfer by changing the solid suspension density weakens. Operation of the bed at relatively low pressure drop by reducing the size of coal particles will ensure adequate heat transfer. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics of outlet steam temperature and the metal temperature of the medium temperature platen superheater at different boiler loads was obtained in a 300MWCFB boiler, the result shows that the imbalance rate of tube mass flux is determined by the structure of header and tube. Generally Z type arrangement is one of the reasonable types arrangement. [Copyright &y& Elsevier]

Abstract: The heat transfer of the platen heating surface of circulating fluidized bed (CFB) boilers is a complex process. Because of technical protection and difficult measurement, at present there is no public calculation method which can be applied to engineering. Three 300MW CFB boilers in actual operation in different field sites were conducted, the parameters of temperature and pressure at the side of steam and flue gas were measured respectively. Based on the tests at different loads, the effects of solids suspension concentration and average temperature parameters etc. on the heat transfer were investigated, and then the heat transfer coefficient calculation model of platen heating surface at the flue gas side was established, which takes the effects of structure and dimension of platen heating surface, furnace temperature, steam temperature, heat transfer coefficient at the steam side, emissivity of tube wall and fluidization velocity etc. into consideration. The field test data obtained from another 300MW CFB boiler were used to verify the model. After comparing the calculated results and the values from the actual field test, it is found that the error is lower than 5%, which is within the precision permission range of engineering application. It is indicated that the calculation results using this model agree well with the experimental results on the site, confirming the reliability of this heat transfer model, which means that it can be used to calculate the heat transfer of platen heating surface of 300MW CFB boiler and can provide reference for design, commissioning, operation of 300MW CFB boilers. [Copyright &y& Elsevier]

Abstract: In order to investigate the heat transfer characteristics of fluidized bed heat exchanger (FBHE), a series of experiments was carried out in a commercial 300MW circulating fluidized bed (CFB) boiler with FBHE. The parameters of steam, solids and air in FBHE were measured at different boiler loads, based on which the absorbed heat and heat transfer coefficient were calculated. Further study indicates that when the calculated results are applied to the design of large-scale CFB boilers, the bed side heat transfer coefficient in FBHE can be simplified as the function of solids temperature and flow. Therefore, the empirical model of heat transfer coefficient at bed side is put forward. The deviation between calculated results and measured values is acceptable in engineering application. This model provides strong support for the FBHE design in 600MW supercritical CFB boilers. [Copyright &y& Elsevier]

Coal type adaptability for CFB boilers is of great importance because it influences whether the unit can operate safely, economically and environmentally when the coal type is changed. Research is carried out on a 300 MW CFB boiler located in Longyan power plant, Fujian province by burning three types of coal, i.e. Fujian anthracite, Shenhua bituminous coal and Indonesia lignite. The results show that all the three coals are capable to meet the basic requirement of bed temperature. But the fly ash carbon content and exhaust gas temperature depend on the coal type. Compared with Fujian anthracite, Shenhua bituminous coal and Indonesia lignite tend to possess lower fly ash carbon content and lower exhaust gas temperature under high boiler load, which helps to increase the boiler efficiency. The auxiliary power consumption rates under all the three types of coal are all lower than 4.3% with the successful utilization of lower energy consumption technology based on state specification design theory. Besides, when the fuel is changed from Fujian anthracite to Shenhua bituminous coal and Indonesia lignite, SO 2 emission and dust emission decreases, while the NO x emission increases. [ABSTRACT FROM AUTHOR]

The uneven gas–solid flow in parallel cyclones, which is called as the mal-distribution in publications, has been widely confirmed. Although there is non-uniform gas–solid flow in the riser, the parallel cyclones would influence the distribution of gas–solid flow due to the pressure drop constraint. Based on the pressure drop constraint and an empirical correlation for cyclone pressure drop, this paper analytically investigates the mal-distribution of gas–solid flow in double identical parallel cyclones, and measures to limit the mal-distribution are suggested. Due to the effects of wall friction and solid acceleration, the change of pressure drop with the increase of solid loading shows a nonlinear characteristic. The inflection point (at where the cyclone has the minimum pressure drop) changes depending on the wall friction and solid acceleration. If the upstream solid loading ( G s / U g / ρ g ) is far away from the solid loading at the inflection point, the uneven solid distribution in the pair of cyclones could be limited although there exists uneven gas–solid flow in the riser. If the upstream solid loading is almost equivalent to the solid loading at the inflection point, the wall friction and solid acceleration both significantly influence the mal-distribution, and the solid distribution in the cyclones would be much more non-uniform compared with the dilute phase and the dense phase. [ABSTRACT FROM AUTHOR]

The synthesis of titanium diboride by carbothermal reduction of TiO 2 , B 2 O 3 and C powder mixtures milled for 48, 100 and 200 h was investigated. The resultant powders were examined after isothermal annealing by X-ray diffraction (XRD) and X-ray photoelectronic spectroscopy (XPS) to determine the synthesis mechanism. During milling, a high-pressure phase, TiO 2 (II), transformed from anatase-TiO 2 was observed and crystallization of amorphous-B 2 O 3 was found to occur. After milling, a submicron lamelliform particle with a cladding structure was obtained. The synthetic reaction was observed to be finished as low as 1200 °C for the 200 h-milled powder. In this research, two reaction paths have been found that TiB 2 was formed through intermediates TiBO 3 and TiO. The predominance of TiBO 3 increased and the reactive temperature of the synthesis of TiB 2 decreased with prolonged milling time. [ABSTRACT FROM AUTHOR]

Based on different flow regimes classified by the saturation carrying capacity in the riser, a comprehensive hydrodynamic model for circulating fluidized beds was proposed and validated with experimental data. The model can describe the different axial solids distributions or regimes in previous literatures, especially the boundaries among them. The calculation of model shows good agreement with experiments not just for gas solids behavior in riser, but also for that in standpipe. The influences of the riser height on the axial solids distribution in riser were discussed. With the increase of the distance between the upper end of dense phase and the bottom of riser, the axial solids distribution in low riser appears as the exponential profile, the quasi exponential profile and the quasi S-shaped profile continuously. For the tall riser, the quasi exponential profile is replaced by the S-shaped profile due to the dilute phase in the upper. In addition, the saturation carrying capacity prevails in the tall riser, and with greater inventory solids circulation rate would exceed the saturation carrying capacity to form the quasi S-shape profile. [ABSTRACT FROM AUTHOR]

The effect of particle properties on gas solid flow in the whole circulating fluidized bed (CFB) system was investigated. Two Geldart-B particles with different sizes were used and the differences in pressure balance and material balance of the whole CFB system with a loop seal were described. The results show that coarse particles have a larger transport velocity U tr to fall into the fast bed regime, which means accumulation in the riser is easier for the coarse particles at the same fluidization velocity U r . The coarse particles also have a smaller solid saturation carrying rate G s , max and a lower height of the acceleration region in the fast fluidization regime. Considering the pressure balance of the system, a higher pressure head should be supplied by the standpipe at the aeration tap of the loop seal under the same G s when coarse particles are used. The pressure drop through the cyclone is lower for coarse particles under the same solid concentration at the entrance of it. In the loop seal, coarse particles need more aeration gas to reach the same G s because of its higher minimum fluidization velocity. Coarse particles show a higher pressure drop through the weir part and a lower pressure drop through the horizontal part of the loop seal. In the standpipe, coarse particles have a lower drag coefficient and need a higher gas solid slip velocity to build up the same pressure gradient, so fine particles suffer less bypassing gas into the cyclone and have higher material seal capability. However, because of the same density of the two particles, the maximum pressure gradient built up in the standpipe is similar. [ABSTRACT FROM AUTHOR]

In a circulating fluidized bed (CFB) boiler, loop seal is one of the most commonly used non-mechanical valves. When the solids are conveyed horizontally from the supply chamber to the recycle chamber through the horizontal section, the length of the horizontal section will cause much flow resistance, which not only influences the fan burden but also the pressure and mass balance between the furnace and the external loop. In this paper, a new structure loop seal with a slope passage connecting the supply and recycle chambers (named N-type) is described. The flow characteristic of quartz sand (Geldart-B particles) in the new loop seal was studied by comparison with the traditional loop seal (named U-type). The experimental data show that the passage design connecting the two chambers of a loop seal has an obvious effect on the gas solid flow not only in the loop seal, but also in other components in the whole loop. For the N-type loop seal, the minimum aeration rate required for the onset of solid flow is equal to the minimum fluidization velocity of the bed materials, much less than with the U-type loop seal. The solid recycling capability of the N-type loop seal is better than of U-type one at low aeration rate, but this status will be reversed at high aeration rate. A small amount of vertical aeration into the bottom of the standpipe can further improve the solid recycling capability of the N-type loop seal, by reducing the inter-particle friction and build up a higher pressure drop gradient in the standpipe. At the same time, less air bypassing upward into the cyclone through the standpipe is found for the N-type loop seal at high level of aeration rate, while, at the same condition the U-type suffers severe gas bypassing with a fluidized state in the standpipe. At last, the pressure drop of the new type loop seal is much less than the pressure drop of the traditional one with the same solid flow rate, which provides an advantage for reducing fan power and sustaining more solids in the boiler furnace. [ABSTRACT FROM AUTHOR]

Abstract: The Computational Particle Fluid Dynamics (CPFD) numerical method was used to study the gas solid flow characteristics in a circulating fluidized bed (CFB) with a loop seal in this work. The influences of operating parameters, including loop seal aeration rate (Q l), fluidized air velocity in the riser (U r) and total bed inventory (M p) on the solid circulation characteristics were mainly investigated in this part. The solid circulating rate (G s), total pressure drop in the riser (ΔP r), particle packed height (H s), pressure gradient (ΔP s/H s) and gas flow rate (G g) in the standpipe, as well as the particle concentration and pressure distributions in the loop were carefully analyzed under different operating conditions. The simulation results were compared with the experimental results also reported in this work and good agreements were obtained. The results showed that the increase of Q l resulted in a decrease of H s and increases of G s, ΔP r and ΔP s/H s. By increasing U r, the increase of G s was limited and the maximum value was approaching. An increase of M p also had a positive effect on G s while an elevated H s was obtained as a result of mass and pressure balance. No gas bypass flow phenomenon occurred in the operating cases of this work from the simulation results. The pressure drop in the riser could be properly predicted while the gas–solid interactions in the standpipe and loop seal were slightly underestimated by the CPFD method, where a relatively lower ΔP s/H s was obtained for CFB modeling. [Copyright &y& Elsevier]

Abstract: The Computational Particle Fluid Dynamics (CPFD) method was applied in the simulation of the gas solid flow in a circulating fluidized bed (CFB) with a loop seal in this work, and the modeling parameters were mainly investigated in this part. The influences of some crucial modeling parameters in CPFD simulation, such as mesh size, particle close pack volume fraction, interphase drag model and particle size distribution (PSD) on the flow behaviors in the whole loop of CFB were carefully analyzed with the help of experimental data conducted on the same CFB test rig. A medium mesh size was proved to be appropriate and used for the CFB modeling in this work. The particle close pack volume fraction largely affected the particle packed condition in the dense loop seal and standpipe. A value of 0.58 was proved to be suitable to describe the particle circulation and pressure distribution characteristics. Different gas–solid drag models showed certain influence on the particle circulation behaviors, and the combination of Wen-Yu and standard Ergun model was determined as the optimum choice. The consideration of the real PSD in CPFD scheme would better predict the overall flow characteristics for CFB system than the mono-size simplifications. The determination of modeling parameters in this work provides the base for simulating the gas–solid flow in the whole loop of CFB reactors by CPFD method, especially for the dense flow behavior in the standpipe and loop seal. [Copyright &y& Elsevier]