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[Display omitted] • A novel mechanism of standpipe vibration of CFB boiler was proposed, which was also proven in engineering. • The influence of several factors on standpipe vibration of CFB boiler was analyzed by calculation. • Engineering solutions to the standpipe vibration of CFB boiler were proposed, especially for large-capacity CFB boiler. Standpipe vibration of circulating fluidized bed (CFB) boiler will threaten its safe and stable operation. In order to solve the standpipe vibration, the mechanism of standpipe vibration was discovered based on the type B choking theory in the CFB system and the Ledinegg instability criteria for the gas-liquid flow. And the influence of several factors on the gas-solid flow, especially the reverse flow in the standpipe, was qualitatively analyzed according to the pressure and mass balance in the whole loop. The standpipe vibration is caused by the mismatch between the driving pressure and the resistance pressure drop in the whole loop. To suppress the standpipe vibration, it is necessary to optimize the flow and pressure parameters under stable working conditions, which can be achieved by four main methods of reducing the resistance of the returning valve and cyclone, increasing the circulating rate and the solid height in the standpipe. In engineering application, these four methods can be realized by appropriately increasing the aerated air velocity and the sectional area ratio between the riser and the standpipe, reducing the length of the passage and the height of the weir, increasing the effective material fraction and the inlet area of the cyclone. This paper theoretically analyzes the mechanism of standpipe vibration and proposes corresponding engineering solutions to guide the operation and design of CFB boilers. [ABSTRACT FROM AUTHOR]

• Detailed field test data of 330 MW CFB boiler was introduced. • Mechanism of bed pressure deviation of 330 MW CFB boiler was analysed. • The impact of gas velocity deviation on bottom bed was validated with experiment. • The bed pressure deviation could be weakened with uniform air distribution. The bed inventory overturn in pant-leg CFB boilers and bed pressure wave in single air distributor CFB boilers were the two well-known non-uniformity problems in large scale CFB boilers. However, a special gas solid flow non-uniformity problem, bed pressure deviation, occurred in a single air distributor 330 MW CFB boiler. The bed pressure in one side was 2 kPa constantly higher than that in the other side and lasted for a long time, resulting to the local over temperature and endangering the boiler performance. Mechanism analysis showed that the primary air temperature deviation caused the serious primary air rate deviation, further forming a stable solid concentration deviation in the bottom bed and bed pressure deviation, which was totally different from above two non-uniformity problems. The similar phenomenon was replicated in a laboratory CFB test rig, validating the gas velocity deviation had a serious effect on the bed pressure deviation. To solve the problem, solutions, like increase the primary air ratio, were put forward. To solve the primary air non-uniform distribution, further study on optimizing air chamber and air distributor is urgent, so that other large scale CFB boilers can avoid such a problem. [ABSTRACT FROM AUTHOR]

Graphical abstract The effects of the secondary air on the gas–solid flow are studied systematically by experiment. With the increasing of the secondary air ratio (SAR), the particles preferred to accumulate at the bottom of the riser, as a result, the particle concentration in the dense phase zone increased. Correspondingly, the particle concentration in the dilute phase zone decreased gradually and the circulating rate will also decrease. Highlights • The effects of the secondary air on the gas–solid flow are studied by experiment. • The solids carrying capability of secondary air is weaker than the primary air. • The solids concentration in the dense bed mainly depends on the primary air rate. • One transitional state was found in the transition zone. • The circulating rate with the secondary air can be predicted by a modified model. Abstract The design and operation of the secondary air (SA) have a great influence on the axial distribution of particles, the structure of the redox atmosphere, and the gas–solid mixture diffusion in the circulating fluidized bed (CFB) boiler. In this paper, the effects of the SA on the gas–solid flow in the CFB boiler, such as the axial particle distribution and the circulating rate were studied systematically through experimental analysis. All experiments were carried out under the ambient temperature and atmospheric pressure, and quartz sand was selected as the bed material in the 4.5 m high CFB test rig made of Plexiglas. The results showed that the SA whose capability to carry particle is affected by the particle concentration at the SA injection port, has a weaker effect on the entrainment of particles than the primary air (PA). The particle concentration in the dense phase zone mainly depends on the PA rate. The increasing of the secondary air ratio (SAR) may lead to the accumulation of particles in the dense phase zone. In addition, one transitional state was found in the transition zone. The variation of particle concentration in the transition zone with the increasing of the SAR is determined by the magnitude of the particle concentration at the SA injection port. The circulating rate decreased with the increasing of SAR or the SA injection height. The effect of the SA injection on the circulating rate in a circulating fluidized bed can be predicted by a modified model with the dimensionless parameters. [ABSTRACT FROM AUTHOR]

Graphical abstract The fluctuations of pressure drop across the standpipes. Highlights • Severe pressure fluctuations in parallel external loops were experimentally studied. • Two different mechanisms contributing to the pressure fluctuation were found. • Aeration air distribution would cause periodical fluctuation in external loops. • Parallel external loops would be more unstable at high solid circulation rate. Abstract Severe pressure fluctuations in parallel external loops were found in the circulating fluidized bed (CFB) boilers recently, so a CFB test rig with two external loops was established to investigate the unstable performance of parallel external loops. A long-length horizontal section was used in the loopseal to intensify the impact of gas flow inside horizontal section on solid circulation rate, and CO 2 tracer method was used to detect the variation of gas flow rate in the horizontal section. The results showed that the solid circulation rate would significantly influence the performance of parallel external loops. The pressure drop in the parallel external loops at low solid circulation rate was relatively stable if aeration air distributed evenly between the two parallel external loops, and the high-frequency pressure fluctuation in the riser was ascribed to the bubble behavior at turbulent fluidization. However, the fluctuation of gas flow rate inside the horizontal section aggravated at high solid circulation rate, leading to the unstable solid recycling rate, as well as the low-frequency and high-amplitude fluctuation of pressure drop in the parallel external loops. For this reason, a single blower is not recommended to use to supply the aeration rate among parallel external loops. [ABSTRACT FROM AUTHOR]

Highlights • The CPFD method was used to simulate the gas-solid flow in a cyclone and standpipe. • Gas leakage reduced the swirling intensity in the cyclone. • Gas leakage made the downward vortex turn upward in advance. • Gas leakage increased the pressure in the standpipe and the cyclone cone. • The particle circulations in the cyclone might be enhanced by gas leakage. Abstract This paper, the second part of a two-paper series, discusses the computational particle fluid dynamics (CPFD) simulation results to clarify the effects of gas leakage on the separation performance of a cyclone. Compared with the experimental results in Part 1 of this two-paper series, the CPFD simulation results showed good agreements on the pressure drop, the overall separation efficiency, and the grade efficiency. Gas leakage would reduce the swirling intensity in the cyclone, make the downward vortex turn upward in advance, and increase the pressure in the standpipe and the cyclone cone. The particle circulations in the cyclone might be enhanced by gas leakage. [ABSTRACT FROM AUTHOR]

Highlights • Experiments were carried out in a cold model of a cyclone. • Gas leakage fraction less than 8% did not affect the pressure drop over a cyclone. • Gas leakage fraction above 4% decreased the separation efficiency of a cyclone. • Gas leakage worsened the separation efficiency on fine particles of diameter less than 200 μm. Abstract This paper, first of a two-paper series, analyzes the experimental results to clarify the effects of gas leakage on the separation performance of a cyclone. Experiments were carried out in a cold model of a cyclone. Varied parameters were inlet gas velocity, inlet solid concentration, and gas leakage velocity. In the experimental range, the pressure drops over a cyclone showed small variation with the increase in gas leakage velocity. When the gas leakage fraction exceeded 4%, experiments showed that the separation performance deteriorated significantly. Gas leakage re-entrained separated fine particles in the standpipe into the cyclone, indicated by a significant decrease of the separation efficiency for fine particles. [ABSTRACT FROM AUTHOR]

It is difficult to control pollutant emissions during fire banking and start-up in CFB boilers, which poses threats to power plant operation, safety and environmental protection. A 350 MW supercritical CFB boiler was taken as the research object. Using the CPFD method and Barracuda software, a full-loop combustion and pollutant emission model was constructed to simulate periodic fire banking and start-up, and the gas solid flow characteristics and pollutant emission characteristics were analysed. The results showed that the bed material at the bottom of the furnace exhibits obvious stratification after being stationary, and this stratification has an impact on the generation, distribution and emission of NO x and SO 2 in the furnace. Eighty-five percent of the bottom bed material is lower layer, and the NO x and SO 2 concentrations are negatively correlated with the volume fraction of particles. The average NO x and SO 2 concentrations in the lower layer are 120.05 mg/m3 and 1155.50 mg/m3, respectively. During the hot start-up process, the NO x concentration decreases with increasing bed height. The residence time of the desulfurization reaction is prolonged, the desulfurization efficiency is improved, and the SO 2 concentration is reduced. These results provide a theoretical basis and a certain reference value for the periodic fire banking and start-up of supercritical CFB boilers. [Display omitted] [ABSTRACT FROM AUTHOR]

The effects of the inlet duct length on the performance of the dense medium cyclones were investigated in this study. The pressure drops and separation efficiencies of the dense medium cyclones with different inlet duct lengths were measured in a cold test rig. A numerical simulation using the four-way coupling method was also conducted to estimate the flow field inside the dense medium cyclone. The experimental results show that a longer inlet duct length results in a lower static pressure drop. The overall separation efficiency first increases and then keeps almost unchanged as the inlet duct becomes longer. The simulated results show that the energy loss decreases as the inlet duct length increases. The changing trends of the static and total pressure drops with the inlet duct length are similar. According to the simulated flow fields, lengthening the inlet gas reduces tangential velocity near the vortex finder and therefore reduces the gas swirling loss. The inlet duct length affects the separation performance mainly by affecting the swirling particle roof. A longer inlet duct leads to fewer particles moving in the swirling particle roof in the dense medium cyclone, which accounts for a better separation efficiency. It was also found that the conventional one-way coupling method fails to qualitatively analyze the effects of inlet duct length on the separation performance of the dense medium cyclone. • Cyclones with different inlet duct lengths were tested in a cold test rig. • Flows in dense medium cyclones were simulated using a four-way coupling method. • The inlet duct length only affects the gas tangential velocity near the roof. • The change of separation performance was mainly due to the swirling particle roof. • The simulated results of the one-way and four-way coupling methods were compared. [ABSTRACT FROM AUTHOR]

• Solid particle flow characteristics in the cone valve were studied. • Valve structure was regarded as an in-series connected double-orifice system. • The solid particle flow rate model of the cone valve was proposed and verified. • Recommended values of model parameters were provided. • Model prediction accuracy was acceptable for industrial application. The solid particle flow characteristics inside the cone valve were experimentally and numerically studied. The experiments were conducted at different valve sizes, valve opening positions, pressure drops, and fluidization velocities. Numerical simulation using the computational particle fluid dynamic method was also carried out over broader conditions. It is found that the solid particle flow inside the cone valve demonstrated the three-stage phenomena as the solid particle flew passing through two in-series connected orifice systems. The explanation of the three-stage phenomena of the solid particle flow rate to the valve opening was given according to the double-orifice flow model. Based on the physical nature of the solid particle flow characteristics, a semi-empirical model of the solid particle flow rate inside the cone valve was proposed. The model parameters, such as the saturated solid flow rate, the saturated valve opening, were determined using the experimental data and the existing knowledge of orifice systems. The model was then verified using the experimental data and presented acceptable prediction accuracy. [ABSTRACT FROM AUTHOR]

• Fe 2 O 3 composition is prone to be removed due to attrition. • The fly ash make-up is mainly from attrition of coal ash. • Chemical composition deviation is enhanced due to attrition. The Samcheok CFBC (circulating fluidized bed combustion) power generation plant, the world largest, consists of 4 units of 550 MWe USC (ultra-supercritical) CFB boilers, started operation since the end of 2016. The years' operation proves that there lacks circulating material and low solid hold-up in the upper furnace when burning Indonesia coal with low ash content of 4.5%. To solve this problem, the present study studied the mass balance of the Samcheok CFB boiler by investigating the ash formation and attrition characteristics of the feed coal in the lab. The ash formation PSD (particle size distribution) proves that the burnout-produced fly ash and fragmentation-produced fly ash are not the main sources of the production of fly ash. The heterogeneity of specific chemical compositions of particles in terms of particle sizes is proved to be one of the key factors for determining the attrition rate. For large particles, SiO 2 and Al 2 O 3 compositions dominate and results in low attrition rate, while for small particles, Fe 2 O 3 and CaO compositions dominate and results in high attrition rate. The fine particles produced due to attrition have very high Fe 2 O 3 contents, indicating that the heterogeneity of chemical composition is enhanced due to attrition. These findings show good agreement with the phenomenon of the high content of Fe 2 O 3 in fly ash in the CFB boiler. [ABSTRACT FROM AUTHOR]