Batch and repeated-batch oil production by microalgae

G. Benvenuti

Research output: Thesisinternal PhD, WU

Abstract

Microalgal triglycerides (TAGs) are promising feedstocks for the commodity markets (i.e. food, chemical and biofuel). Nevertheless, microalgal TAGs are not yet economically feasible due to the high production costs. To reduce these costs, TAG productivity needs to be maximized.

The aim of this thesis was to increase microalgal TAG productivity by investigating the effects of biological and engineering parameters (i.e. production strain and operational strategy). 

We first screened seven marine species on their TAG productivity under nitrogen (N) starvation. Nannochloropsis sp. was identified as the most suitable species as it retained its photosynthetic activity while accumulating large amounts of TAGs ensuring the highest TAG productivity. Therefore, Nannochloropsis sp. was used in all following studies.

Next, we aimed at optimizing TAG productivity by investigating the effect of initial-biomass-specific (IBS) light availability (i.e. ratio of light impinging on reactor ground area divided by initial biomass concentration per ground area) in batch outdoor cultivations carried out in horizontal and vertically stacked tubular reactors at different initial biomass concentrations at the start of the TAG accumulation phase, over different seasons. Based on the observed trends of TAG productivity for the Dutch climate, optimal initial biomass concentrations were suggested to achieve high areal TAG productivities for each reactor configuration and season.

Subsequently, repeated-batch processes were investigated to further increase TAG productivity compared to batch processes. For this, repeated-batch cultivations were tested and compared to batch cultivations both at lab-scale under day/night cycles and in two identical, simultaneously operated, outdoor vertically stacked tubular reactors over different seasons. Although at lab-scale, batch and repeated-batch cultivations led to similar TAG productivities, outdoor repeated-batch processes were always outcompeted by the batch. It was concluded that repeated-batch processes require further optimization.

For this, the physiological responses of Nannochloropsis sp. to N-starvation and N-replenishment were determined under continuous light in lab-scale batch and repeated-batch cultivations and condensed into a mechanistic model describing both cultivation strategies. Scenarios for improved TAG yields on light were simulated and, based on the optimized yields, a comparison of the two processes was performed. It was concluded that under continuous light, an optimized batch process will always result in higher TAG productivities than an optimized repeated-batch process.

Finally, a techno-economic analysis for a two-step-continuous TAG production process (i.e. growth reactors are operated in continuous mode such that multiple batch-operated stress reactors are inoculated and sequentially harvested) is performed for a hypothetical 100 ha-scale plant in southern Spain using vertically stacked tubular reactors. Photosynthetic efficiencies based on outdoor pilot data were used as model input. By optimizing both photosynthetic efficiency and process technology, the production cost could be decreased from 7.4 to 3.0 €·kg-1 of TAG-enriched biomass. We believe to be on the right track to achieve an economically feasible TAG production platform provided that photosynthetic efficiency is further improved, the whole biomass is valorized and cheaper reactors are designed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Wijffels, Rene, Promotor
  • Bosma, Rouke, Co-promotor
  • Barbosa, Maria, Co-promotor
Award date4 Mar 2016
Place of PublicationWageningen
Publisher
Print ISBNs9789462576582
Publication statusPublished - 2016

Keywords

  • algae
  • chlorella
  • arthrospira
  • dunaliella
  • nostoc
  • triacylglycerols
  • light
  • lipogenesis
  • photosynthesis
  • oil products
  • productivity
  • biomass conversion
  • economic analysis

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  • Cite this

    Benvenuti, G. (2016). Batch and repeated-batch oil production by microalgae. Wageningen: Wageningen University.