Scale-up Considerations of the Ubfb Solar Receiver

Concentrated Solar Power plants (CSP) are a promising technology for electricity generation. Indirect particle receivers can operate at high receiver temperatures and foster the application of advanced power generation cycles. The present work specifically considers the scale-up of the CNRS concept, referred to as Upflow Bubbling Fluidized Bed (UBFB) system. Previous papers have provided detailed information concerning the on-sun testing of a single tube (36 mm I.D., 0.5 m long) experiments at the solar furnace of PROMES-CNRS in Font Romeu (FR) ^1-3 and of a 150 kW pilot module consisting of 16 parallel tubes⁴ (29.7mm I.D., each 1 m long). SiC, a Geldart A type powder, was used and fluidized at superficial gas velocities of ~0.03 to 0.25 m s^-1. To scale-up these vertical UBFBs, some phenomena require additional attention. These phenomena are 5-fold, and related to (i) the pressure balance over the receiver and the achievable solids' flux and air velocity relationship; (ii) the efficiency of the vertical transport; (iii) the possible particle choking; (iv) the gas-solid hydrodynamics gradually transforming from a freely bubbling into a slugging mode in long UBFB tubes; and (v) attrition and erosion. Pressure Balance, Achievable Solid Flux and Air Velocity Relationship

The dispenser bed is operated at a fluidization velocity close to the minimum bubbling (U_mb) superficial velocity of the powder (~1.2 to 1.5 U_mb). The major part of the conveying gas is injected at the bottom of the tube (about 10 cm above its inlet). In the loop, different factors should be considered. The solid fraction, , in the upward part of the circuit (the riser) is lower than in the downcomer parts, being 0.35-0.40 and 0.45-0.50 respectively.

To operate the loop in a stable flow mode, driving downflow pressures (including the pressurization of hopper) should exceed pressure drops of the upflow branch including acceleration and friction losses⁵. The transport equation reveals that for solids circulating flux to be positive, several conditions should be met on the pressure balance, the gas velocity as a function on the terminal settling velocity of the particles, the bed voidage and the tube length.

Efficiency of the UBFB Conveyor System

The efficiency of the UBFB conveyor system can be calculated by comparing the compression and uplift work. To pressurize this air flow to the required pressure of the operating ΔP, above the atmospheric pressure, work is needed in the compressor The efficiency of the UBFB will be determined for two powders as a function of solids circulating flux. A higher powder density reduces the conveying efficiency through the higher the bed. For the same reason, a similar reduction of the efficiency will occur for deeper beds (longer tubes).

Slugging

Extensive research at ambient conditions demonstrated that wall slugging was found at a bed level of ~ 50 cm above the air inlet ⁶, whereas axi-symmetric slugging occurred at a height exceeding ~ 150 cm above the air inlet ⁷. Slugs significantly reduce the bubble-induced particle mixing and associated excellent heat transfer. A long UBFB will suffer from slugging both towards the wall-to-bed heat transfer coefficient, as towards the considerable pressure fluctuations measured across the bed. At higher temperatures, and in-line with predictions by Kong et al. ⁷, the onset of slugging is deferred to higher bed levels. This finding is positive for the scaling up of the UBFB solar receivers that will involve multi-meter long tubes. Wall slugging is characterized by a slug frequency of 1 Hz. Axi-symmetric slugs move up the bed at a frequency of ~0.5 Hz. In this studies, a diagram of the bubble and/or slug frequency as a function of height at ambient temperature and 475°C temperature will be determined.

The Maximum Achievable Solid Flux

Common fluidized bed operations can be hampered in a specific superficial gas velocities (U) and solids circulation flux (G) range where choking occurs, being understood as the phenomenon where a small change in gas or solids flow rate prompts a significant change in the pressure drop and/or solids holdup: the stable upflow regime can no longer be maintained when -values exceed a certain limit for a low to moderate gas velocity. This choking can occur in dense upflow of particles when the superficial gas velocity and the driving pressure are no longer capable of entraining the particles. In the upflow bubbling fluidized bed concept, only -values up to 100 kg/m²s were tested, where the stability of the operation was confirmed. This is expected since the pressure balance indicates that the loop will operate in a stable manner provided the external pressure in the dispenser compensates the upflow bubbling fluidized bed pressure drop. At high -values, acceleration and friction losses, both proportional to (n=1 to 2) ⁸ will increase and will finally hamper the system stability. To assess the impact of () combinations, a non-choking criterion is established when considering that the particle slip velocity must remain positive. To operate the upflow bubbling fluidized bed at -values in excess of 100 kg/m²s, superficial air velocities should exceed 0.1 m/s, as commonly used in the single and multi-tube set-ups. In this work, a prediction of chocking limit at different superficial gas velocities and bed voidages will be determined.

Attrition of Particles

Since a fluidized bed is the key in operating the solar receiver at a high wall-to-bed heat transfer coefficient, attrition of particles was examined experimentally in order to select the type of Geldart A-powders less prone to attrition. This extensive research was fully reported by Zhang et al ⁹ and only essential features are summarized below. Although fast particle motion associates a high degree of mixing, it however causes inter-particle collision and bed-to-wall impacts, both leading to particle attrition. Attrition generates fines that can be lost in the dust collection system, whereas the particle size distribution of the bed will alter during the operation. Zhang et al. ⁹ clarified the influence of particle size and nature, bed height, fluidization velocity, action of jets and orifice diameter. An equation was developed and enables to predict attrition rates for different particles at different operating contribution. The total attrition rate combines the bubble-induced and jet-induced effects. It was moreover shown that particles with a high Abrasion Index (AI), as defined by CEMA ¹⁰, are less prone to attrition. The higher AI is however an indication of the expected wear of the equipment ¹¹.

Conclusions

The UBFB is a key part of the novel solar power concept. The research has dealt with specific considerations in view of its scale-up, including: (i) the effect of the diameter of the receiver tubes and the use of internal fins; (ii) the applicable circulation flux; (iii) the possible extension of the pipe length from 0.5 or 1 m to beyond 3 m; (iv) the limited effect of powder attrition and equipment wear.

Acknowledgements

This work was supported by the Beijing Advanced Innovation Center for Soft Matter Science and Engineering of the Beijing University of Chemical Technology and funding was obtained from the European Union's Horizon 2020 research and innovation program under grant agreement 727762, project Next-CSP.

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