Installation of flow deflectors and wing baffles to reduce dead zone and enhance flashing light effect in an open raceway pond

doi:10.1016/j.biortech.2015.08.144

Bioresource Technology

Volume 198, December 2015, Pages 150-156

https://doi.org/10.1016/j.biortech.2015.08.144 Get rights and content

Highlights

•
A novel raceway pond with flow deflectors and wing baffles was developed.
•
Pressure loss and dead zone decreased with flow deflectors.
•
Period of average L/D cycle was significantly shortened with wing baffles.
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Biomass concentration in the novel raceway pond increased by 30.11%.

Abstract

To reduce the dead zone and enhance the flashing light effect, a novel open raceway pond with flow deflectors and wing baffles was developed. The hydrodynamics and light characteristics in the novel open raceway pond were investigated using computational fluid dynamics. Results showed that, compared with the control pond, pressure loss in the flow channel of the pond with optimized flow deflectors decreased by 14.58%, average fluid velocity increased by 26.89% and dead zone decreased by 60.42%. With wing baffles built into the raceway pond, significant swirling flow was produced. Moreover, the period of average L/D cycle was shortened. In outdoor cultivation of freshwater Chlorella sp., the biomass concentration of Chlorella sp. cultivated in the raceway pond with wing baffles was 30.11% higher than that of the control pond.

Introduction

Microalgae has caught more and more attention for their high potential to produce pigment, high-value chemicals and biofuels, to remove heavy metals, nitrogen and phosphorus nutrient from wastewater and flue gas (Borowitzka, 2013, He et al., 2015, Pruvost et al., 2011, Zhu et al., 2014). Currently, most widely used system for mass microalgal cultivation are open ponds, due to their low construction and operation cost. Nevertheless, open ponds have some serious drawbacks including the high risk of culture contamination, the lack of temperature control, the poor gas/liquid mass transfer and the low final biomass concentration (Carvalho et al., 2006, Posten, 2009).

Several studies have been performed to improve open raceway ponds performance, mainly focusing on ponds design and CO₂ utilization efficiency. Chiaramonti et al. (2013) investigated the actual energy consumption in a raceway pond and developed innovative solutions to improve the energy performance. Li et al. (2014) compared the effect of four blades configuration on power consumption in a 2.2 m² raceway pond. Ketheesan and Nirmalakhandan (2012) designed an airlift-driven raceway reactor for microalgal cultivation. Hadiyanto et al. (2013) evaluated the effect of velocity, ratio of channel length to width and culture depth on power consumption, dead zone volume and shear stress. Sompech et al. (2012) investigated the flow field of an open raceway pond with flow deflectors and island design, and found that the proportion of dead zone area decreased from 14.2% to 0%, compared to the control pond. In order to improve the CO₂ utilization efficiency in the raceway pond, devices such as sumps and mixing columns have been proposed as a means of increasing the gas/liquid contact time and the efficiency of CO₂ absorption (de Godos et al., 2014, Mendoza et al., 2013, Putt et al., 2011). However, when sumps are used, more power for mixing is required. In addition, Pawlowski et al. (2014) improved the CO₂ utilization efficiency of flue gas through controlling pH in a raceway reactor.

It has been reported that regular mixing, which make algae cells shuttle between light region near illumination surface and dark region, is beneficial to improve the efficiency of light utilization and photosynthesis (Janssen et al., 2000, Matthijs et al., 1996). Terry concluded that the photosynthetic efficiency can be improved at light/dark frequency higher than 1 Hz (Terry, 1986). However, the L/D frequency in closed photobioreactors and open pond system is in the medium frequency (0.01–1 Hz) range (Grobbelaar et al., 1996). Xue et al. (2011) found that the growth rate of Spirulina platensis increased with increasing L/D frequency in the range of 1 Hz or less with a thin-layer flat plate photobioreactor.

To increase the L/D frequency, many efforts have been made by introducing mixers or baffles into traditional photobioreactors (Degen et al., 2001, Perner-Nochta and Posten, 2007, Ugwu et al., 2002, Wang et al., 2014). Degen et al. (2001) set up baffles into a flat panel airlift photobioreactor and found that the biomass productivity of Chlorella vulgaris was 1.7 times higher than that in a randomly mixing bubble column with identical dimensions. Wang et al. (2014) developed a novel flat plate photobioreactor with horizontal baffles and cultivation test showed that the maximum biomass productivity in the novel optimized bioreactor was 1.88 times higher than that in a traditional bioreactor without baffles. Therefore, the introduction of static mixers or baffles is a very efficient way to increase biomass productivity for photobioreactors. However, for an open raceway pond with shallow bulk liquid volume, it is difficult to produce significant swirl flow with the introduction of ordinary static mixers or baffles. Therefore, special baffles or mixers should be developed to enhance flashing light effect in open raceway ponds.

In this study, in order to reduce the dead zone and enhance flashing light effect, a novel open raceway pond installed with flow deflectors and wing baffles was developed. The hydrodynamics and light characteristics were investigated using computational fluid dynamics (CFD), and the structural parameters of wing-baffle and flow defector were optimized. The effectiveness of the wing baffles for outdoor cultivation of Chlorella sp. was also experimentally investigated.

Section snippets

Description of the novel open raceway pond

The schematic diagram of the open raceway pond with flow deflectors and wing baffles for simulation is shown in Fig. 1. The raceway pond was 4.85 m long, 0.85 m wide and 0.2 m high. Water depth was kept at 0.1 m. The thickness of the central division wall is 0.05 m. In Fig. 1, l, w, s₁, s₂, s₃ and α represent the length of the wing baffles, the width of the wing baffles, the gap between the wing baffle and pond wall along width-direction, the gap between the two neighbouring wing baffles along

Comparison of different flow deflectors

Fig. 2a–f presents the simulated dead zone in the open raceway pond with the same wing baffles but different flow deflectors. The dimension of the wing baffles was l = 0.1 m, w = 0.1 m, s₁ = 0.05 m, s₂ = 0.1 m, s₃ = 0.5 m and α = π/6. The fluid inlet velocity was 0.3 m s⁻¹. The average fluid velocity, pressure loss and dead zone volume fraction in the raceway ponds with different flow deflectors were shown in Table 1.

As shown in Fig. 2a, dead zone widely existed along the semicircular wall of the pond and at the

Conclusions

In this study, a novel open raceway pond with flow deflectors and wing baffles was developed to reduce the dead zone and enhance flashing light effect. The results demonstrated that, with installing optimized flow deflectors in the raceway pond, the area of dead zone decreased by 60.42%. Compared to the control raceway pond, in the raceway pond with built-in wing baffles, significant swirl flow could be produced and the average L/D cycle period was shortened from 14.05 s to 4.42 s. The biomass

Acknowledgements

The authors greatly appreciate the support of National High-Tech 863 Program (2014AA022002).

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Installation of flow deflectors and wing baffles to reduce dead zone and enhance flashing light effect in an open raceway pond

Highlights

Abstract

Introduction

Section snippets

Description of the novel open raceway pond

Comparison of different flow deflectors

Conclusions

Acknowledgements

Appl. Energy

J. Biotechnol.

Bioresour. Technol.

Chem. Eng. J.

Bioresour. Technol.

Chem. Eng. J.

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Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

J. Biotechnol.

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Bioresour. Technol.

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Microalgae: Biotechnology and Microbiology