Bioreactors
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Bioreactors

Sustainable Design and Industrial Applications in Mitigation of GHG Emissions

Lakhveer Singh, Abu Yousuf, Durga Madhab Mahapatra, Lakhveer Singh, Abu Yousuf, Durga Madhab Mahapatra

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eBook - ePub

Bioreactors

Sustainable Design and Industrial Applications in Mitigation of GHG Emissions

Lakhveer Singh, Abu Yousuf, Durga Madhab Mahapatra, Lakhveer Singh, Abu Yousuf, Durga Madhab Mahapatra

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About This Book

Bioreactors: Sustainable Design and Industrial Applications in Mitigation of GHG Emissions presents and compares the foundational concepts, state-of-the-art design and fabrication of bioreactors. Solidly based on theoretical fundamentals, the book examines various aspects of the commercially available bioreactors, such as construction and fabrication, design, modeling and simulation, development, operation, maintenance, management and target applications for biofuels production and bio-waste management. Emerging issues in commercial feasibility are explored, constraints and pathways for upscaling, and techno-economic assessment are also covered.

This book provides researchers and engineers in the biofuels and waste management sectors a clear, at-a-glance understanding of the actual potential of different advanced bioreactors for their requirements. It is a must-have reference for better-informed decisions when selecting the appropriate technology models for sustainable systems development and commercialization.

  • Focuses on sustainable bioreactor processes and applications in bioenergy and bio-waste management
  • Explores techno-economic and sustainability assessment aspects through a comparative approach, catering to diverse arrays and applications
  • Offers comprehensive coverage of the most recent technology, from fundamentals to applications

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Information

Publisher
Elsevier
Year
2020
ISBN
9780128215371
Chapter 1

Microalgae biofuel bioreactors for mitigation of industrial CO2 emissions

Corey A. Laamanen and John A. Scott, Bharti School of Engineering, Laurentian University, Sudbury, ON, Canada

Abstract

Microalgae are a third-generation biofuel feedstock and as they are photographic organisms, can capture and mitigate carbon dioxide (CO2) in industrial off-gas and use it as their carbon source. This technology has significant promise, but ideal microalgae strains must be identified, and bioreactors must be developed, that can optimize CO2 capture. Of the many proposed bioreactor designs available, variations of vertical columns (bubble and airlift) are regarded as the most suitable and have been studied extensively for their potential to utilize industrial off-gas.

Keywords

Microalgae; biofuel; bioreactor; carbon dioxide; mitigation

1.1 Introduction

The reliance on fossil fuels continues to be an unsustainable option due to the depletion of reserves and the production of greenhouse gases, in particular carbon dioxide (CO2), released by their combustion. This has led to a significant interest in renewable, sustainable energy sources, one of which is biofuels.
Biofuel sources have been extensively researched for the past several decades, and their production can be divided into three generations, namely food crops, energy crops, and microalgae [1]. The first generation is based on food crops such as corn being diverted into energy. In light of the food versus fuel debate, the second generation transitioned into the growth of dedicated energy crops such as Jatropha. While this marked a step change in areal energy production (from 172 L/ha for corn to 1892 L/ha) [2], arable land was still required to grow nonfood crops. To move away from arable land requirements, aquaculture represents the third generation of biofuels. This includes the cultivation of phototrophic microalgae, which use CO2 as their carbon source. Using microalgae to produce biodiesel represents another order of magnitude increase in areal energy production (58,700 L/ha for microalgae at 30% oil by weight) [2].
For the production of biodiesel, microalgae cells are selected that accumulate between 15% and 85% lipid content. These lipids (triglycerides) can be directly converted into fatty acid methyl esters (biodiesel) through a transesterification reaction. The triglycerides are reacted with methanol in the presence of a catalyst (typically sulfuric acid) to produce glycerol and biodiesel. Transesterification can be shown as:
image
(1.1)
Phototrophic microalgae, in addition to not requiring arable land, grow faster and are able to capture more CO2 per unit area than terrestrial plants [3]. The capture of anthropogenic CO2 can be further enhanced by passing through aqueous medium industrial off-gases that have an elevated CO2 content. That is, industrial CO2 can be mitigated by microalgae using it as their carbon source [4]. As it has been shown, microalgal biomass is approximately 50% carbon [3], which translates to:
image
(1.2)
That is, for every gram (dry weight) of microalgae, 1.83 g of CO2 is taken up. The concept of carbon capture using microalgae and subsequent biofuel (biodiesel) production is shown in Fig. 1.1.
image

Figure 1.1 Carbon dioxide (CO2) capture and biofuel production using microalgae.

1.2 Microalgae

Microalgae can be biologically classified as single-celled plants that accumulate different products as a response to their environmental conditions, which is central to biodiesel production. Through the control of the process parameters and the selection of microalgae strains, the outcome can be aimed toward the production of lipids, which are cellular energy storage compounds [5]. Microalgal cells can accumulate lipids to up to 85% of total cell dry weight [6].
To be able to best utilize different metabolic pathways a two-stage cultivation strategy is often utilized, the photosynthetic production of biomass and the accumulation of lipids. In the first stage, ideal growth conditions are provided to allow for the production of microalgal biomass. For any microalgal strain, these ideal conditions will be related to pH, temperature, and CO2 and nutrient levels. A second stage will then be used to apply stress by changing one or more of these conditions, thereby triggering the accumulation of lipids [5].
Another benefit of microalgae cultivation for the production of biofuels is that the biomass remaining after lipid extraction can also be of significant value. This biomass has been examined for human dietary benefits, a protein source for fish farms or livestock, cosmetics, fertilizers, pharmaceuticals, and nutraceuticals [711]. While biofuel is a relatively low-value bulk product, additional high value coproducts can provide the economic benefit to make the entire operation profitable.

1.3 Microalgae growth parameters

Microalgae growth depends on a number of different parameters for the operation including the availability of nutrients (in particular the macronutrients nitrogen and phosphorus), the concentration of dissolved CO2, pH, light intensity and photoperiod, and temperature. The typical ranges for operations are 15°C–30°C, a pH value between 4 and 11, and light intensities of 1000–10,000 lux [12]. These can be modified to promote biomass production, or to move away from ideal growth conditions in order to trigger other pathways that lead to the accumulation of lipids for biodiesel production [13].

1.3.1 Light

Microalgae use light as their source of energy, and both the intensity and duration (photoperiod) must be optimized. If the intensity is above the saturation limit (around 6500 lux) [14], then photoinhibition can occur, which reduces the efficiency of the system [15]. Wahidin et al. [14] found that for a Nannochloropsis sp., both the light intensity and photoperiod needed to be optimized, rather than maximized, to achieve the best growth rate.
Microalgae can only utilize wavelengths between 400 and 700 nm, which accounts for about 50% of sunlight. This limits their potential photosynthetic efficiency, which is further reduced through reflection and cellular respiration [15]. On a cellular basis, the maximum photoconversion efficiency that light is used for photosynthesis and biomass production is approximately 9% [16]. This represents, however, a significant increase compared to terrestrial plants, which have maximums in the 4.6%–6.0% range [16].
Light delivery, intensity, spectra, photoperiods, the frequency of light/dark cycles, and the amount of light exposed surface area have all been reported to have a significant impact on microalgal biomass formation. Light characteristics have also been shown t...

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