CO2 Biofixation by Microalgae E-Book

Sihem Tebbani

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 188

Veröffentlichungsjahr: 2014

Beschreibung

Due to the consequences of globa l warming and significant greenhouse gas emissions, several ideas have been studied to reduce these emissions or to suggest solut ions for pollutant remov al. The most promising ideas are reduced consumption, waste recovery and waste treatment by biological systems. In this latter category, studies have demonstrated that the use of microalgae is a very promising solution for the biofixation of carbon dioxide. In fact, these micro-organisms are able to offset high levels of CO2 thanks to photosynthesis. Microalgae are also used in various fields (food industry, fertilizers, biofuel, etc.). To obtain a n optimal C O2 sequestration us ing micr oal gae, their cul tivatio n has to be c arried ou t in a f avorable e nvironment, corresponding to optimal operating conditions (temperature, nutrients, pH, light, etc.). Therefore, microalgae are grown in an enclosure, i.e. photobioreactors, which notably operate in continuous mode. This type of closed reactor notably enables us to reduce culture contamination, to improve CO2 transfer and to better control the cultivation system. This last point involves the regulation of concentrations (biomass, substrate or by-product) in addition to conventional regulations (pH, temperature). To do this, we have to establish a model of the system and to identify its parameters; to put in place estimators in order to rebuild variables that are not measured online (software sensor); and finally to implement a control law, in order to maintain the system in optimal conditions despite modeling errors and environmental disturbances that can have an influence on the system (pH variations, temperature, light, biofilm appearance, etc.).

Leseprobe

Contents

Introduction

1 Microalgae

1.1. Definition

1.2. Characteristics

1.3. Uses of microalgae

1.4. Microalgae cultivation systems

1.5. Factors affecting algae cultivation

1.6. Conclusion

2 CO2 Biofixation

2.1. Selection of microalgae species

2.2. Optimization of the photobioreactor design

2.3. Conclusion

3 Bioprocess Modeling

3.1. Operating modes

3.2. Growth rate modeling

3.3. Mass balance models

3.4. Model parameter identification

3.5. Example: Chlorella vulgaris culture

3.6. Conclusion

4 Estimation of Biomass Concentration

4.1. Generalities on estimation

4.2. State of the art

4.3. Kalman filter

4.4. Asymptotic observer

4.5. Interval observer

4.6. Experimental validation on Chlorella vulgaris culture

4.7. Conclusion

5 Bioprocess Control

5.1. Determination of optimal operating conditions

5.2. Generalities on control

5.3. State of the art

5.4. Generic Model Control

5.5. Input/output linearizing control

5.6. Nonlinear model predictive control

5.7. Application to Chlorella vulgaris cultures

5.8. Conclusion

Conclusion

Bibliography

Index

First published 2014in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.

Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:

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© ISTE Ltd 2014The rights of Sihem Tebbani, Filipa Lopes, Rayen Filali, Didier Dumur and Dominique Pareau to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.

Library of Congress Control Number: 2014939765

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISSN 2051-2481 (Print)ISSN 2051-249X (Online)ISBN 978-1-84821-598-6

Introduction

Due to the consequences of global warming and large emissions of greenhouse gases, solutions for the sustainable development and the safeguarding of land resources are currently the focus of reflection and international actions. One of the major lines of action relates to the stabilization or reduction of greenhouse gases concentrations in the atmosphere. CO2 is the most important greenhouse gas, due to the increase of the world’s production, deforestation and intensive use of fossil energy [PAC 07]. Furthermore, the proposal for a strategy for the reduction of CO2 concentration in the atmosphere is a subject of research in full expansion. Several approaches are proposed, such as the reduction of emissions of this gas, its storage (geological, in the oceans or by mineralization), its adsorption or its absorption by chemical, geological or biological means [PIR 11].

This book focuses more particularly on CO2 fixation by biological means through the use of microalgae, in order to minimize the environmental impact of this sequestration. Carbon dioxide (CO2) biofixation by microalgae is a very promising CO2 mitigation strategy since these microorganisms are the most efficient to sequester CO2, in comparison with terrestrial plants. In the presence of light, microalgae are capable of assimilating CO2 to grow while producing oxygen and secondary metabolites, via photosynthesis. The use of microalgae for environmental purposes is thus a very promising solution due to their potential, the various advantages related to their growth rate and their high tolerance regarding high concentrations of CO2. In addition, they have a large number of uses on laboratory and industrial scales in different areas: they can be used to produce high value-added molecules in pharmaceuticals and cosmetics, animal feed and human food, and they represent a feedstock for the production of renewable energy such as hydrogen, methane and biodiesel. Thus, the major aspect of CO2 biofixation by microalgae combines this sequestration with the production of value-added molecules.

The strategy of optimal CO2 fixation by microalgae relies on a specific approach, which is based initially on a selection phase of the algal species, that present a high CO2 fixation ability; and in a second phase, on the optimization of the operating conditions of the culture process and on maintaining the bioprocess at these optimal conditions.

At the present time, the industrial exploitation of microalgae cultivation for the sequestration of CO2 is scarce, essentially due to the difficulties of instrumentation, measurement and modeling of this type of process. Also, an important and compulsory step lies in the culture of microalgae on a small scale (laboratory scale), in order to develop powerful and robust tools, capable of controlling the culture of microalgae to effectively sequester CO2 on the one hand, and on the other hand that can be transposed and applied to large-scale cultivation systems.

In this context, the purpose of this book is not only to make a non-exhaustive assessment of recent research concerning microalgae culture for CO2 sequestration, but also to propose estimation and advanced control strategies, illustrated by experimental assays on Chlorella vulgaris cultures in an instrumented photobioreactor.

The design of an effective bioprocess for CO2 biofixation through microalgae culture observes the following implementation approach:

– Modeling: a crucial step that conditions the effectiveness of subsequent stages is the development of a relevant model of microalgae growth in the reactor. This step is all the more delicate since the system to model is very complex and could be time-varying. Also, the present challenge is to develop a simple model accurate enough to faithfully reproduce the behavior of the system in order to control it. Generally, two modeling types are proposed: macroscopic modeling based on a mass balance [BAS 90] and metabolic modeling [BAR 13]. The macroscopic approach is preferred in the context of the development of the control law because it leads to a simpler model with fewer parameters so that their identification is less complex than for the metabolic model [HEI 13]. The macroscopic model for microalgae growth is strongly nonlinear and involves not only biological elements conventionally used in bacterial culture bioprocessing, but also light whose intensity/quality/duration conditions the consumption of CO2. The identification phase of the parameters of this model is a delicate phase due to the nonlinearity of the model. In addition, this model could be time-varying since the system involves living organisms. At the end of this step, a nonlinear model capable of reproducing effectively the macroscopic behavior of the bioprocess is available.

– Estimation: the measurement of microalgae concentration in a reactor, important for the bioprocess control, is generally only available offline by sample analysis. There is indeed a lack of physical sensors, either at a reasonable price or accurate enough to undertake online measurements of this parameter. We must therefore develop observers that combine the previously identified model of the bioprocess with simple physical measurements available online (for example pH, light intensity, CO2 and O2 partial pressure, concentrations of biological variables, etc.), in order to estimate the system’s variables that are not accessible in real-time. The quality and accuracy of the estimation depends mainly on the quality of the model under consideration and conditions the effectiveness of the control law.

– Control: this last step aims at the establishment of robust control strategies regarding the uncertainties of the model’s parameters and external disturbances, in order to maintain the bioprocess at optimal operating conditions. To maximize the biofixation of CO2 emissions, it is necessary to consider strategies of advanced control, which are the sole guarantee of a good bioprocess performance. Instrumented bioprocessing on industrial scales usually uses simple control laws, namely for microalgal culture [BER 11, ZHA 14], hence it causes a limitation in the performance of CO2 biosequestration. Since the macroscopic model of the bioprocess is nonlinear, the latest research in the literature is moving toward the use of control strategies dedicated to nonlinear systems (linear control strategies have been tested and their limitations demonstrated based on the strong nonlinearity and uncertain nature of the process model).

This book proposes an implementation procedure of the optimal control strategy of CO2 biofixation by microalgae culture in a photobioreactor. The microalgae C. vulgaris is here more specifically studied, in order to assess the performance of the proposed growth strategies. The book is organized as follows:

– Chapter 1 presents microalgae and their areas of exploitation and use. It then describes the different types of cultivation systems of microalgae and lists their most influential growth factors;

– Chapter 2 is more particularly concerned with the biofixation of CO2 by microalgae, with emphasis on the most influential parameters of bioprocess optimization;

– Chapter 3 presents the modeling of microalgae cultivation in photobioreactors in the general case from the state of the art, and then more specifically in the case of the C. vulgaris culture;

– Chapter 4 considers the estimation of cell concentration from available online measurements, and three types of estimation strategies proposed: Extended Kalman Filter, asymptotic observer and interval observer. These estimators are applied to experimental data derived from cultures of C. vulgaris, and their performances are then compared;

– Chapter 5 deals with the optimal control of microalgae culture. The optimal operating conditions are first determined. Keeping the bioprocess around these optimal conditions is then studied. Three types of control laws are finally studied and implemented: the Generic Model control law, the input/output linearizing control law and the nonlinear model predictive control law;

– finally, a conclusion allows us to put forward the results obtained and to draw a balance of the proposed procedure. In particular, the transposition of strategies developed in microalgae cultures on industrial scale is discussed.

This multidisciplinary research is located at the junction of two major areas: Chemical Engineering and Automatic Control. The theoretical developments presented in the wake of this book are therefore related to skills and knowledge of these two areas to achieve in the end, a robust and reliable solution that optimizes the consumption mechanism of CO2. Also, basic concepts related to the two areas have been recalled for the reader’s convenience.

1

Microalgae

1.1. Definition

Algae are photosynthetic organisms that develop in varied habitats, predominantly in aquatic environments, capable of converting light energy and carbon sources, such as carbon dioxide (CO2), into “biomass”. Depending on their size, they can be classified into two broad categories: “macroalgae” and “microalgae”. Macroalgae are multicellular algae of around one centimeter in size which usually grow in ponds of natural fresh water or salt water. Microalgae have a size measured in micrometers and are considered to be single cell algae which grow in suspension, mainly in aqueous solutions [WEN 09].

These microorganisms are considered to be the first producers of oxygen (O2). Their existence in the oceans dates back to more than three billion years ago. They are responsible for transforming the composition of the atmosphere (CO2 fixation and O2 emission) and have allowed the emergence of plant and animal life on Earth. Also referred to as Phytoplankton, microalgae represent a food source from the earliest stages of larval life right up to human beings, owing to their specific biochemical composition.

Their adaptation and survival capacities are such that they are able to colonize all types of environments. They are found in thermal waters as well as in ice, in acidic or even hyper saline waters, in caves, in symbiotic relationships with any other type of living organisms, and as parasites, even on humans. They are also able to develop on hard surfaces, such as walls or tree trunks, and even on immersed structures. Certain species can withstand very low or paradoxically extreme temperatures. This faculty of adaptation is the result of their morphological properties as well as their capacity to synthesize different varieties of secondary metabolites.

1.2. Characteristics

Through photosynthesis, these microorganisms synthesize O2 and primary organic metabolites such as carbohydrates, lipids and proteins. From a cell structure perspective, a microalga has a nucleus, a plasma membrane and contains organelles, essential to its operation, such as chloroplasts, amyloplasts, elaioplasts and mitochondria. It contains three main types of pigments: chlorophylls, carotenoids and phycobiliproteins.

Microalgae take a variety of forms (Figure 1.1): spherical (Porphyridium), crescent-shaped (Closterium), spiral-shaped (Arthrospira), droplet-shaped (Chlamydomonas) and even star-shaped (Staurastrum).

From a nutritional point of view, microalgae are predominantly photoautotrophic1 but they can also be heterotrophic or mixotrophic [CHE 11b]. An autotrophic metabolism uses inorganic carbon such as CO2 or bicarbonate as a carbon source while a heterotrophic metabolism is characterized by a consumption of organic carbon as a carbon source for their development; mixotrophs use both types of carbon sources.

Figure 1.1.Morphological diversity of microalgae [SUM 09]. a) Gephyrocapsa; b) Haematococcus lacustris; c) Spirulina platensis; d) Chlorella vulgaris; e) Dunaliella tertiolecta; f) Chaetoceros calcitrans; g) Chaetoceros calcitrans; h) Dinophysis acuminate; i) Alexandrium; j) Bacillariophycea; k) Raphidophceae; l) Botryococcus. The length of the line in each figure is equal to 10 μm [SUM 09]

1.3. Uses of microalgae

Microalgae offer interesting perspectives for applications in diverse areas such as the pharmaceutical industry, agriculture, environment and renewable energy. The main uses are detailed below.

1.3.1. Nutrition

Microalgae represent an excellent source of nutrients. They are used for animal feed, as a human food source and in aquaculture. They are used in the manufacture of natural colorants in the food industry. Polysaccharides (hydrosoluble polymers) from microalgae are used in the food industry as gelling agents or thickeners. Glycerol (the molecule involved in the osmoregulatory systems of microalgae) is exploited in the food industry as a sweetener.

1.3.2. Pharmaceuticals

Microalgae are an interesting source of bioactive molecules and toxins that have notably been used in the development of new medicines for the treatment of cancerous diseases [PUL 04]. Polysaccharides extracted from microalgae allow the synthesis of antioxidant, antiviral, antitumor and anticoagulants agents. Microalgae are capable of synthesizing vitamins and natural antioxidants.

1.3.3. Cosmetics

Several species of microalgae are used industrially in the cosmetics industry [PUL 04, SPO 06], mainly the two species Arthrospira and Chlorella. Algae extracts with antioxidant properties are used in the manufacture of hair care products, anti-wrinkle products and sun creams. Pigments derived from microalgae are also used for cosmetics.

1.3.4. Energy

Algal biomass offers benefits in the production of energy in the form of electricity and/or heat by direct combustion, or in the form of biomethane or biofuels. However, these benefits are only competitive in cases with strong biomass productivity, using simple mechanical harvesting techniques and which present lower production costs than those involved in processes using other types of biomass [CAR 07].

1.3.4.1. Biomethane production

Several research projects have confirmed the technical and commercial feasibility of biomethane production from marine biomass, showing great potential [CHY 02]. However, technical obstacles such as the accessibility of the nutrients and high production costs limit the use of microalgae for this application. A way to reduce costs would be, for example, to link the production of methane with the production of secondary high value-added metabolites. Species such as Gracilaria sp. and Macrocystis are excellent methane-producing organisms.

1.3.4.2. Biofuel production

Considering the current global context (increase in the price of oil, depletion of fossil resources, production of greenhouse gases, etc.), it is interesting to consider microalgae as a source of production of different types of biofuel: bio-oil and biodiesel [PAN 11].

Bio-oil from microalgae represents an interesting alternative to liquid biofuels. It is produced by the thermochemical conversion of biomass at high temperatures in the absence of O2. Two different processes are used: pyrolysis and thermochemical liquefaction. Several studies have been carried out based on the implication of microalgae in bio-oil synthesis [DOT 94, SAW 99, DEM 06]. Areas for improvement for this type of process include the reduction of production costs, the optimization of the culture system, and the improvement of separation and harvesting steps.

The most promising approach involves the production of second and third generation biofuels (ethanol production from lignocellulosic materials and biodiesel production from microalgae). The third generation biofuel addresses the major drawbacks observed in first and second generation biofuels (competing with food production, excessive water consumption and deterioration of soil). Due to certain valuable properties (important biomass productivity, high photosynthetic activity, large lipid storage potential up to 20–50% dry weight), microalgae are 500 to 1,000 times more effective than terrestrial species for biodiesel synthesis.

1.3.4.3. Biohydrogen production

Biohydrogen is an effective source of renewable energy and is currently the subject of extensive research and applications. The process of biohydrogen synthesis can take two forms: direct photolysis and indirect photolysis. Direct photolysis is based on the transfer of electrons from water molecules to protons, coupled with a reduction of ferredoxin (protein intervening at the level of the algae photosystem in the transport of electrons and protons) inducing hydrogen synthesis with hydrogenase enzymes [BEN 00]. The indirect method is based on the conversion of starch stored by algae to hydrogen under anaerobic conditions and sulfur limitation [CAR 07]. Several species of microalgae have shown interesting properties in relation to indirect processes, i.e. a large capacity for biohydrogen synthesis under sulfur deprivation conditions. Accordingly, the production of hydrogen from microalgae is a promising niche but requires a better understanding of microalgae metabolism and engineering [BEE 09].

1.3.5. Environmental field

The main environmental applications of microalgae are in wastewater treatment and consumption of CO2 as a method for reducing greenhouse gas emissions.

1.3.5.1. Wastewater treatment

Their capacity to assimilate numerous nutrients necessary for their growth means that microalgae offer an interesting solution for the elimination of these elements; they are also able to fix heavy metals. They thus constitute the main biological element of certain municipal and industrial water treatment systems (mainly tertiary treatment). Due to the assimilation of nitrogen and phosphorus, they contribute in reducing the phenomenon of eutrophication (i.e. degradation) of certain aquatic environments.

In order to reduce the economic costs of these water treatment processes, generated microalgae biomass can be used to produce molecules with high added value, (such as biodiesel, methane, hydrogen etc.). These processes are typically coupled with the elimination of CO2 in industrial gas emissions, leading to integrated processes.

1.3.5.2. Agriculture

Algal biomass constitutes a valuable asset as manure, fertilizer and soil stabilizer in agriculture, and also as a crop accelerator and protector by limiting the proliferation of epiphytes and parasites. Microalgae allow particle adhesion and storage of water in the soil as well as nitrogen fixation. The synthesis of bioactive molecules means that they are likely to influence the growth of terrestrial plants. Microalgae are used, for example, in the production of rice, ensuring nitrogen fixation in tropical and subtropical agriculture. They are also used for surface strengthening in arid regions in order to combat erosion.

1.3.5.3. Life-support systems

During space missions, the development of a life-support system for the crew is essential. These systems must fulfill four basic needs: regeneration of a breathable atmosphere (O2 supply), water recycling, waste treatment and provision of food. In order to respond to these constraints, closed-loop regenerative life support systems use biological systems, such as algae and terrestrial plants. The Micro-Ecological Life Support System Alternative (MELISSA) project has been developed by the European Space Agency (ESA) for thesepurposes [GOD 02]. The microalga Spirulina is produced and then dried to be consumed or incorporated into food.

1.3.5.4. CO2 sequestration

The current alarming situation concerning climate change has triggered worldwide awareness. The growing concentration of greenhouse gases (known as “GHG”) in the atmosphere has an increasingly important effect on climate change [MAT 95].

Natural absorption no longer compensates for the high production rate of these types of gas; CO2 having the highest effect, representing more than 68% of total emissions [MAE 95, KON 07, ROM 07]. A dramatic increase in the release of CO2 in the atmosphere has been observed due to anthropogenic sources. Indeed, released CO2 was about 7.4 billion tons in 1997, and is estimated to be about 26 billion tons by 2100. There is need to remind the reader of the catastrophic results of climate warming on desertification, increase in the frequency of extreme weather events, disruption of ecosystems and melting of non-polar glaciers resulting in rising sea levels [MOR 97].

In order to reduce the levels of greenhouse gases in the atmosphere, intensive research has focused on the development of new CO2 reduction techniques (Figure 1.2) [IPC 05]. There are three main types of processes: geological sequestration, chemical processes and bioprocesses [PIR 11].

Geological sequestration relies mainly on the storage of liquid or gaseous CO2 in geological formations, in the soil [HER 01] or in deep ocean storages [ISR 09]. However, these technologies have many disadvantages such as the possibility of leakage, the contamination of drinking water aquifers, the increase in the acidity of water, the disruption of the marine ecosystem and significant financial costs.

Figure 1.2.Schematic view of the capture and storage of carbon dioxide [IPC 05]

Chemical processes include absorption by alkaline solutions [DIA 04], the use of multi-walled carbon nanotubes [SU 09], and adsorption–neutralization on amine enriched carbon [PLA 07]. These methods are expensive and energyintensive [WAN 08].

The use of biological systems is a very promising alternative solution, relatively efficient, economically feasible and sustainable. These methods are based essentially on photosynthesis, with the transformation of CO2 into biomass [KON 07, DEM 07]. Two CO2 sequestration biological pathways exist: one using terrestrial plants and the second photosynthetic microorganisms. In the first method, forests are used to convert CO2 into cellulosic structures for plants (namely trees) and into humus for soils [ZAM 10]. However, due to limited conversion efficiency, low growth rate, economic and technical disadvantages (possibility of release of stored carbon following a forest fire or damage to trees), numerous studies have focused on the second pathway, which is the implementation of photosynthetic organisms such as microalgae. CO2 sequestration by microalgae is the main subject of this book and will be discussed in Chapter 2.

1.4. Microalgae cultivation systems

Given the diversity of industrial applications and nutritional and environmental requirements of microalgae culture, the establishment of an efficient cultivation system is a specific and crucial step which depends on the application in question. There are two main categories of cultivation systems: open systems (natural and artificial ponds characterized by a low surface-to-volume ratio) and closed systems (photobioreactors with a high surface-to-volume ratio).

1.4.1. Open systems