Microalgae-Based Biofuels and Bioproducts

From Feedstock Cultivation to End-Products

Raul Muñoz, Cristina Gonzalez-Fernandez

• Publisher: Woodhead Publishing
• Year: 2017
• ISBN:
  9780081010273
• Topic: Technology & Engineering
• Subtopic: Renewable Power Resources
• Index: Technology & Engineering

Microalgae-Based Biofuels and Bioproducts: From Feedstock Cultivation to End Products compiles contributions from authors from different areas and backgrounds who explore the cultivation and utilization of microalgae biomass for sustainable fuels and chemicals.

With a strong focus in emerging industrial and large scale applications, the book summarizes the new achievements in recent years in this field by critically evaluating developments in the field of algal biotechnology, whilst taking into account sustainability issues and techno-economic parameters. It includes information on microalgae cultivation, harvesting, and conversion processes for the production of liquid and gaseous biofuels, such as biogas, bioethanol, biodiesel and biohydrogen. Microalgae biorefinery and biotechnology applications, including for pharmaceuticals, its use as food and feed, and value added bioproducts are also covered.

This book's comprehensive scope makes it an ideal reference for both early stage and consolidated researchers, engineers and graduate students in the algal field, especially in energy, chemical and environmental engineering, biotechnology, biology and agriculture.

• Presents the most current information on the uses and untapped potential of microalgae in the production of bio-based fuels and chemicals
• Critically reviews the state-of-the-art feedstock cultivation of biofuels and bioproducts mass production from microalgae, including intermediate stages, such as harvesting and extraction of specific compounds
• Includes topics in economics and sustainability of large-scale microalgae cultivation and conversion technologies

1

Photobioreactors for the production of microalgae

F.G. Acién⁎; E. Molina⁎; A. Reis^†; G. Torzillo^‡; G.C. Zittelli^‡; C. Sepúlveda^§; J. Masojídek⁎⁎^{,†† ⁎} University of Almería, Almeria, Spain
^† National Institute of Industrial Engineering and Technology, Lisbon, Portugal
^‡ CNR-Institute for Ecosystem Study, Sesto Fiorentino, Italy
^§ University of Antofagasta, Antofagasta, Chile
^⁎⁎ Czech Academy of Sciences, Třeboň, Czech Republic
^†† University of South Bohemia, České Budějovice, Czech Republic

Abstract

Microalgae have been proposed as valuable microorganisms for several applications, from production of pharmaceuticals to wastewater treatment. Whatever the final application, the core of the process is the photobioreactor in which microalgae are produced. To design adequate photobioreactors, it is mandatory to understand the major phenomena limiting the performance of microalgae cells such as light availability, nutrients supply including CO₂, environmental conditions including temperature and solar radiation, and mixing. To fulfill the requirements of microalgae cells, different technologies has been proposed such as raceway and thin-layer open reactors, in addition to tubular and flat-plate closed reactors. These technologies are still being upgraded and improved to maximize the biomass production capacity and to reduce the production cost. Additionally, the control and modeling of these reactors is a hot topic for the industrial development of microalgae-based processes. This chapter summarizes the current state of the art on photobioreactor design and operation, discussing the major challenges to be solved to achieve a massive expansion of microalgae-based technologies.

Keywords

Photobioreactors; Irradiance; Raceway; Flat-plate; Thin-layer

1.1 Introduction

Microalgae have a large biotechnological potential for producing valuable substances for feed, food, nutraceutical, and pharmaceutical industries (Spolaore et al., 2006). Furthermore, other applications can be attributed to the photosynthetic process performed by these microorganisms such as CO₂ mitigation, wastewater treatment, and biofuels production (Acien et al., 2012; Chisti, 2007). Whatever the process, it must be designed considering the specific characteristics of these microorganisms. Thus microalgae (according to applied phycology) are photosynthetic microorganism able to perform oxygenic photosynthesis. Both cyanobacteria with a prokaryotic cell structure and microalgae with a eukaryotic cell structure are usually included in this category. These microorganisms are photoautotrophs, although they may also grow under mixotrophic or heterotrophic conditions. For the production of microalgae under phototrophic conditions, it is necessary to use photobioreactors that must be adequately designed, built, and operated to satisfy the requirements of the selected microalgae. Multiple designs and configurations of photobioreactors have been proposed, but no optimal design still exists. For whatever application, the photobioreactor to be used must be adequately selected according to the requirements of process. Thus the establishment of the requirements of the biological system to be used is required to adequately design the optimal photobioreactor, which constitutes the starting point when designing a microalga-based process.

Major requirements to be satisfied in phototrophic microalgae-based processes are the supply of light and nutrients (carbon, nitrogen, phosphorous, etc.), the maintenance of adequate culture conditions (pH, temperature, etc.), and mixing to avoid gradients of these parameters that reduces the yield of biological system (Acién Fernández et al., 2013; Posten, 2009; Tredici and Zittelli, 1997). To satisfy these requirements at laboratory or small-scale conditions is relatively easy although costly, but to carry it out at large-scale is more difficult especially when it must be performed at sensible costs (Acién et al., 2012; Norsker et al., 2011).

Two major categories of photobioreactors are considered: open and closed. As open cultivation systems (having direct contact with the environment), artificial ponds, tanks, raceways (shallow racetracks mixed by paddle wheels), and thin-layer (i.e., inclined-surface systems) platforms are often used. As closed cultivation systems (having no direct contact between the culture and the atmosphere), bubble columns, tubular loops, and flat-panels are typically used. At present, open systems are feasible for the production of thousands of tons of biomass significantly cheaper than that from closed systems (Benemann, 2013). Open systems have certain advantages: easy cleaning, direct exposure to sun, self-cooling by evaporation, and lower oxygen accumulation by releasing it into the atmosphere. On the downside, open systems are strongly dependent on the weather/climate, they have increased risk of microbial contamination and high CO₂ losses and present higher area requirements compared to closed systems. Yet the cost of construction is about one order of magnitude lower than that of closed systems (Chisti, 2012, 2013). Due to the limited control of cultivation conditions and contamination, the use of open cultivation units is restricted to a relatively small number of microalgae species. Hence, these units are suitable for “robust” microalgae strains (e.g., Chlorella, Scenedesmus, and Nannochloropsis) that grow rapidly or under very selective conditions (e.g., Arthrospira, Dunaliella). Alternatively, due to the fact that closed photobioreactors support a controlled environment, potentially free of contaminants, much wider selection of strains can be produced. Thus sensible strains such as Haematococcus pluvialis, Isochrysis galbana, and Porphyridium cruentum among others, are produced in closed photobioreactors. In this chapter the major requirements of microalgae cultures and cultivation systems are reviewed to fulfill production at large scale.

1.2 Requirements of photosynthetic microorganisms

1.2.1 Light availability

The most important factor in the growth and productivity of photosynthetic microorganisms is light availability. Light is the energy input for photosynthetic microorganisms; thus it must be maximized. However, excess of light can damage the photosynthetic apparatus, particularly when coupled with suboptimal temperature or high oxygen level (Tredici and Zittelli, 1998). Therefore light supply to the cultivation system must be optimized by adequate design of its geometry and orientation (Acién Fernández et al., 2001; Tredici et al., 2015). The growth of microalgae is determined by the photosynthesis rate, which is a direct function of the irradiance to which the cells are exposed inside the culture. The irradiance is defined as the amount of radiation reaching a point from all directions in space, at every wavelength. However, photosynthetic microorganisms can only exploit the photosynthetically active radiation (PAR) in the range from 400 to 700 nm. From whatever light source (lamps, LEDs, sun), only PAR is used by microalgae to perform photosynthesis. However, the irradiance inside microalgae cultures is not homogeneous. Due to mutual shading the irradiance inside microalgae cultures gets attenuated as a function of light intensity, culture depth, and biomass concentration. Thus cells in the outer part of the culture can be exposed to high irradiances, whereas in the inner part of the culture, cells can be in complete dark. To solve this problem the concept of average irradiance was proposed (Fernández-Sevilla et al., 1998). According to this concept the average irradiance at which the cells are exposed to inside a culture is calculated as the volumetric integral of the corresponding irradiance in all the points inside the culture. This local irradiance can be c...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• Acknowledgments
• 1: Photobioreactors for the production of microalgae
• 2: Heterotrophic and mixotrophic microalgae cultivation
• 3: Microalgae cultivation in wastewater
• 4: Applications of genome-scale metabolic models of microalgae and cyanobacteria in biotechnology
• 5: Harvesting of microalgae: Overview of process options and their strengths and drawbacks
• 6: Cell disruption technologies
• 7: Biogas production from microalgae
• 8: Breakthroughs in bioalcohol production from microalgae: Solving the hurdles
• 9: Biohydrogen production from microalgae
• 10: Biodiesel from microalgae
• 11: Pyrolysis of microalgae for fuel production
• 12: Biogas upgrading using algal-bacterial processes
• 13: Synthetic biology of cyanobacteria for production of biofuels and high-value products
• 14: Biorefinery of algae: Technical and economic considerations
• 15: Microalgal proteins for feed, food and health
• 16: Microalgal fatty acids—From harvesting until extraction
• 17: Cyanobacterial toxins as a high value-added product
• 18: Trends in red biotechnology: Microalgae for pharmaceutical applications
• 19: Extraction of value-added compounds from microalgae
• 20: Economics of microalgae production
• 21: Environmental impacts of full-scale algae cultivation
• Index