Biomass, Biofuels, Biochemicals

Biofuels from Algae

Ashok Pandey,Duu Jong Lee,Jo-Shu Chang,Yusuf Chisti,Carlos Ricardo Soccol

• Publisher: Elsevier
• Year: 2018
• ISBN:
  9780444642141
• Edition: 2
• Topic: Tecnología e ingeniería
• Subtopic: Ingeniería química y bioquímica

Biomass, Biofuels and Biochemicals: Biofuels from Algae, Second Edition provides information on strategies for commercial microalgae based biofuel production, including their cultivation, pre-treatment and conversion methods. The book discusses methods for producing microalgal biomass in large scale by outdoor culturing and outlines new technologies for their use. In addition, it explains how modern genetic engineering enables the generation of recombinant strains that generate higher quantities of feedstock. The complete utilization of microalgal biomass, which can also be obtained from valorizing nutrients from wastewater and industrial exhaust gases, can be efficiently converted to energy rich biofuels and high value pharmaceuticals in a well-defined biorefinery.

• Includes the current technologies for the cultivation and conversion of energy rich microalgal biomass into biofuels
• Provides information on all the conversion methods – biochemical and thermochemical conversions
• Covers other high value products from microalgae and less conventional applications, such as fine chemical production and aviation fuel generation
• Discusses the economics of microalgal biofuel production and how to accomplish cost competitive results

Chapter 1

Introduction to algal fuels

Yusuf Chisti School of Engineering, Massey University, Palmerston North, New Zealand

Abstract

An overview is provided of the potential fuels from microalgae. Production of biomass and its thermochemical processing (pyrolysis, gasification, hydrothermal liquefaction) to potential fuels are outlined. Algal crude oil and production of biodiesel and other liquid hydrocarbon fuels are discussed. Fuel alcohols derived from algae directly, or through fermentation of sugars obtained via hydrolysis of algal starch, are examined. Biogas and biohydrogen technologies are charted. Hurdles to commercialization of algal fuels are delineated.

Keywords

Microalgae; Biofuels; Biodiesel; Bioethanol; Starch; Biohydrogen; Biogas; Pyrolysis; Biomass gasification; Hydrothermal liquefaction

1 Introduction

Algae biomass and biochemicals are potential renewable feedstocks for future production of fuels and chemicals [1]. This chapter provides an overview of the possible biofuels from microalgae and cyanobacteria. Algae are primitive plants. Microalgae are microscopic in dimensions whereas macroalgae (Fig. 1), or seaweeds, are much larger. Cyanobacteria are bacteria that carry out the exact same kind of photosynthesis as green algae, but are more primitive. Algae and cyanobacteria use atmospheric carbon dioxide, water, and sunlight to photosynthesize their biomass.

Fig. 1 Some marine macroalgae: (A) Red seaweed photographed by Eric Moody; (B) giant kelp Macrocystis pyrifera (photo by Shane Anderson); (C) bull kelp (Durvillaea antarctica) on rocky shore, South Island, New Zealand; (D) seabed covered with macroalgae, Royal National Park, New South Wales, Australia, photographed by Toby Hudson. Public domain photos from Wikipedia.

Some components of algal biomass are potential fuels, or precursors of potential fuels [2–6]. Cyanobacteria are also potential sources of biofuels [7–11]. Cyanobacterial fuels span biohydrogen [12–19]; ethanol [20–24], butanol [25,26] and other alcohols [27–30]; isobutyraldehyde [31]; isoprene [32,33] and other hydrocarbons [27,34–37]; acetone [38]; and fatty acids [39,40].

Microalgae found in seawater, brackish water and freshwater are of particular interest as potential sources of biofuels. Although macroalgae have been suggested as potential fuel sources, they are not suitable for this purpose, for reasons explained later in this chapter (see Section 3). In principle, microalgal biomass can substitute the higher plant biomass in various processes that have been developed for obtaining energy from lignocellulosic biomass. For example, processes exist for gasifying crop biomass to produce syngas [41,42], a mixture of carbon monoxide, hydrogen and carbon dioxide (see Section 5.1.1). Compared with lignocellulosic plant biomass, microalgal biomass is expensive, and this is a major hurdle to its use for bioenergy. Production of algal biomass and biofuels can be greatly enhanced through genetic and metabolic engineering. These aspects have been reviewed by Majidian et al. [11] and others [43–46]. Potential renewable biofuels from microalgae are further discussed by others [4–6].

2 Sunlight as the Source of Algal Fuels

Sunlight is the source of all energy stored in algal biomass. Photosynthesis converts energy in sunlight to biochemical energy in a two-stage process (Fig. 2). The first stage involves light-dependent reactions, or light reactions, in which photons are captured and used to generate biochemical energy in the form of adenosine triphosphate (ATP) and the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) (Fig. 2). Photon capture occurs in the light-harvesting complex, or the antenna complex of the algal cell. This complex is comprised of proteins and chlorophyll molecules embedded in the thylakoid membrane of the chloroplast. The second stage of photosynthesis uses light-independent reactions, or dark reactions, which use the NADPH generated by the light reactions. In the dark reactions, the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) captures carbon dioxide from the atmosphere and fixes it to produce a 3-carbon sugar (glyceraldehyde-3-phosphate; G3P) in the Calvin-Benson cycle (Fig. 2). The 3-carbon sugar G3P is the ultimate source of glucose, sucrose, starch, and cellulose. This type of photosynthesis, leading to production of a 3-carbon sugar as one of its first products, is known as C3 photosynthesis. Green microalgae and a great majority of higher plants thrive on C3 photosynthesis. All other chemicals and fuel precursors found in algal biomass are produced within the cell from the products of photosynthesis (Fig. 2).

Fig. 2 Photosynthesis in the chloroplast. Light (photons) is captured by the antenna complex comprised of photosystems I and II. The energy in the photons is used to remove four electrons from two water (H₂O) molecules, producing four H⁺ ions and a molecule of oxygen (O₂). This process produces ATP and NADPH, the energy sources that drive the dark reactions of the Calvin-Benson cycle. In the dark reactions of photosynthesis, the enzyme RuBisCO fixes carbon dioxide (CO₂) to the sugar glyceraldehyde-3-phosphate (G3P), the source of the other key metabolites such as starch. Some of the G3P leaves the chloroplast and is used for the biosynthesis of many other metabolites in the cytosol. Based on Tahir [113]. NADP⁺, nicotinamide adenine dinucleotide phosphate; ADP, adenosine diphosphate; P_i, inorganic phosphate.

Certain microalgae and cyanobacteria are capable of heterotrophic growth [47–49]: growth on dissolved organic carbon compounds in the dark. This type of growth may be useful for converting organic waste, in wastewater for example, into biomass for feed, or starch for ethanol or other applications. Heterotrophic growth tends to be much more productive than photosynthetic growth and typically allows a high concentration of the biomass to be achieved. For example, a heterotrophic culture may easily achieve a dry biomass concentration in excess of 20 g L^{− 1}, whereas a photosynthetic culture will achieve barely 20% of this amount. This high productivity notwithstanding, heterotrophic growth requires assimilable organic carbon that ultimately must be created via photosynthesis [2,49]. As a consequence, heterotrophic growth is less efficient in converting solar energy to algal biomass when compared with photoautotrophic growth. Heterotrophic growth is incapable of supplying large quantities of biofuels [50].

3 Potential Advantages of Microalgae Over Higher Plants

Almost all the biofuels that can be produced using photoautotrophic algae can also be produced using terrestrial crops. The interest in algae is driven by their high productivity relative to land plants, the possibility of their being grown using abundantly available seawater, and a production process that does not require arable land [1,2]. In principle, algal biofuels could be produced with a reduced competition for certain resources that are in demand for production of human food and animal feeds.

Biomass productivity of microalgae is high relative to land plants. For example, the median value of the maximum specific growth rate of microalgal species is nearly 1 day^{− 1} whereas for higher plants it is around 0.1 day^{− 1}, or less [1]. Biomass productivity of microalgae beats higher plants for several reasons [1]. Each algal cell is photosynthetically active, but only a fraction of the plant biomass photosynthesizes. Each algal cell can absorb nutrients directly from its surroundings, but much of the biomas...