Catalysts are important in hydrothermal liquefaction processes, and a range of catalysts has been proposed for the subcritical processing of biomass to tailor the reaction to a specific product and enhance the reaction rates of the proceeding reactions. These catalysts comprise homogeneous catalysts such as mineral acids, organic acids, and bases as well as heteroge­neous catalysts such as zirconium dioxide, anatase titania, and other materials (Moller et al., 2011).

11.1.2 Homogeneous

The addition of alkali salts has a positive influence on hydrothermal processes. It improves gasification, accelerates the water gas shift, and increases liquid yields (Watanabe et al., 2005; Yang and Montgomery, 1996; Mok et al., 1992). In addition, the catalysts raise the pH, thereby inhibiting dehydration of the biomass monomers. A high degree of oxygen removal in the form of dehydration instead of decarboxylization might result in unsaturated compounds that easily polymerize to char and tar. Indeed, alkali is also known to suppress char and tar formation (Toor et al., 2011).

Song et al. (Song et al., 2004) investigated the effect of the addition of 1.0 wt% of Na2CO3 on the liquefaction of corn stalk and concluded that the use of a catalyst increased the yield of

biocrude (from 33.4% to 47.2%); however, no elaboration on the action of the catalyst was made. Similarly, alkali in the form of K2CO3 was shown to have a positive effect on hydro­thermal treatment of wood biomass at 280°C for 15 min (Karagoz et al., 2006). In a similar study performed with the same equipment and utilizing wood biomass, the authors observed that potassium salts were more effective than sodium salts, and they ranked the salts in order of catalytic activity as follows: K2CO3 > KOH > Na2CO3 > NaOH. The catalysts improved liq­uid yields and decreased the amount of solid residue. Minowa et al. (Minowa et al., 1998) tested the catalytic action of Na2CO3 during hydrothermal conversion of cellulose. Above 300° C the catalyst decreased secondary tar formation from the oil product and catalyzed the gasification of the aqueous organics. The study shows how nicely cellulose is converted at different temperatures.

One important catalytic action of alkali during hydrothermal liquefaction is the accelera­tion of the so-called water gas shift, and thus it favors H2 and CO2 formation at the expense of CO. The produced hydrogen gas may act as a reducing agent, increasing the heat value and quality of the oil product. The mechanism proceeds via formation of a formate salt (Schmieder et al., 2000; Sinag et al., 2004) and is more thoroughly described next.

A formate salt (HCOO~K+) is formed when the alkali salt reacts with CO from the gasification:

K2CO3 + H2O! KHCO3 + KOH (11.1)

KOH + CO! HCOOK (11.2)

Hydrogen is obtained when formate reacts with water:

HCOOK + H2O! KHCO3 + H2 (11.3)

In the next step, CO2 is produced from KHCO3:

2KHCO3 ! H2O + K2CO3 + CO2 (11.4)

The overall reaction can be written as:

H2O + CO $ HCOOH $ H2 + CO2 (11.5)

There are also other positive effects of homogeneous catalysts, such as enhanced decarbox­ylation of fatty acids. For example, Watanabe et al. (Watanabe et al., 2006) improved the con­version of C17-acid (fatty acid) decomposition from 2% to 32% by addition of a KOH catalyst.

13.1.1.1 Cultivation System and Growth Medium

Microalgae cultivation is generally realized in two kinds of systems: open raceways (ORW) or photobioreactors (PBR). ORWs are shallow ponds (between 10 and 50 cm depth). They can be built in concrete (Lardon et al., 2009) or simply carved from the ground (Campbell et al., 2011) and can be recovered by a plastic liner made of high-density polyethylene (HDPE) (Collet et al., 2011) or polyvinylchloride (PVC). Ponds are generally open but can be sheltered under a greenhouse. This kind of system is commonly used in the industry to produce

microalgae used as foodstuffs (Shimamatsu, 2004; Del Campo et al., 2007). PBRs are closed systems that allow the intensification of the culture. There are numerous types and very dif­ferent designs of PBR. They can be tubular (TPBR) or made of flat panels (FPBR) (Jorquera et al., 2010) or more rustically made of simple polyethylene bags soaked in a thermostatic wa­ter bath (Batan et al., 2010).

The choice of growth medium can be made independently of the cultivation system. Depending on the chosen species, algae can be cultivated in fresh water, brackish water, or seawater. The use of wastewater has also been suggested by several authors (Clarens et al., 2010,2011; Sander and Murthy, 2010), offering the double advantage of an unreclaimed source of water and nutrients. However, it should be acknowledged that microalgae grown in wastewater could not be used afterward as feedstock for fish or cattle. Water consumption has

Climate change Human health Ecosystems Resources

FIGURE 13.4 Climate change and endpoint impacts of various fertilizers (percentage of the worst case by impact category).

been identified as one of the main environmental concerns of bioenergy production from microalgae. Consequently, some authors suggest growing algae in seawater in order to have an unlimited resource (Batan et al., 2010; Khoo et al., 2011). Brackish water from groundwater is also used in some systems (Clarens et al., 2011). It should be noted that fresh water is still required in these systems in order to stabilize the salinity.

Table 13.4 lists cultivation systems, growth media, and cultivated species mentioned in the selected studies.

Sedimentation through gravity is the most usual method of harvesting microalgal biomass from wastewater treatment plants. This is because of the large volumes handled and the low commercial value of the biomass formed (Munoz and Guieysse, 2006). The density and diam­eter of the cells influence the speed of sedimentation. The collection of microalgae by sedi­mentation can be carried out in sedimentation tanks (Uduman et al., 2010). However, this method can only be applied to large-cell microalgae (>70 gm) such as Spirulina. Flocculation is usually used to increase the efficiency of sedimentation (Chen et al., 2011).

Su-Chiung Fang

Biotechnology Center in Southern Taiwan, Academia Sinica
Agricultural Biotechnology Research Center, Academia Sinica
Tainan, Taiwan R. O.C.

3.1 INTRODUCTION

Compared to other biofuel feedstocks, microalgae are the preferred option for many reasons:

1. They grow extremely fast and hence produce high biomass yield quickly.

2. Microalgae-based fuels do not compete with the food supply and hence present no food security concerns.

3. Biofuels generated from microalgae are renewable and can be carbon-reducing [generation of 100 tons of algal biomass is equivalent to removing roughly 183 tons of carbon dioxide from the atmosphere (Chisti, 2008)].

4. Microalgal farming does not require arable land and can utilize industrial flue gas as a carbon source.

5. Selected oleaginous microalgae do not require fresh water and can grow in seawater, brackish water, or waste water.

6. Biodiesel fuels derived from microalgae can be integrated into the current transportation infrastructure.

During the past few years, there have been significant advances in uncovering molecular components required for production of fuel molecules in microalgae. The availability of ge­nomic sequences in the model green alga Chlamydomonas reinhardtii has accelerated forward genetic analysis and allowed for the use of reverse genetic approaches to uncover molecular mechanisms associated with fuel production (Merchant et al., 2007). Moreover, tran — scriptomics, proteomics, and metabolomics studies have provided new insights into gene regulation networks and coordinated cellular activities governing physiological flexibility and metabolic adaptation of microalgae. Understanding the basis of microalgal biology is important in laying the foundation for innovative strategies and for ultimate development of fuel surrogates. This review summarizes recent progress in elucidating molecular and cellular mechanisms of cellular physiology that are relevant to fuel production in microalgal systems, with an emphasis on developing metabolic engineering strategies to increase fuel production.

Gravity sedimentation is a process of solid-liquid separation that separates a feed sus­pension into a slurry of higher concentration and an effluent of substantially clear liquid. It is the most common concentration process for sludge treatment at wastewater treatment plants. To remove particles that have reasonable settling velocity from a suspension, gravity sedimentation under free or hindering settling is satisfactory. However, to remove fine par­ticles with a diameter of a few microns and for practicable operation, flocculation should be induced to form larger particles that possess a reasonable settling velocity. The thickened underflow of sludge is withdrawn from the bottom of the tank; the effluent or supernatant overflows a weir and is pumped back to the inlet of the treatment plant.

Gravity sedimentation is used for algae separation where the clarity of the overflow is of primary importance and algal feeds suspension is usually dilute (Mohn and Soeder, 1978; Mohn, 1980; Eisenberg et al., 1981; Venkataraman, 1980; Sukenik and She1ef, 1984) or where a thickening of the underflow and the algae feed slurry is usually more concentrated (Mohn, 1980).

A fast pyrolysis system consists basically of a fluidized bed reactor, a cyclone, a condenser, and a combustion chamber, generally constructed as shown in Figure 7.2.

The fluidized bed reactor is where pyrolysis actually occurs. The remaining constituents are responsible for phase separation. The reactor operates at around 450°C. Heating is done by an immersed electrical resistor covered with inert material (silicates, in general). The func­tion of this inert material is to increase the heat transfer between the air and the fluidizing material to be pyrolyzed through abrasive action, increasing the contact surface of the solids (DalmasNeto, 2012).

Once temperature is achieved, air feeding begins. Then heating stops and the material to be pyrolyzed is fed to the reactor. At this point, an initial temperature fall is observed, caused by air and material entrance in much lower temperatures. Reactor temperature can be

immediately reestablished by combustion of the pyrolysis’ incondensable gases or by con­trolled combustion of part of the material fed to the reactor.

The combustion of incondensable gases, such as CO, H2,andCH4 (Cortez et al., 2008), is the best option, generating enough heat for autothermal operation of the reactor, but this entails the acquisition of additional equipment. On the other side, controlled combustion of part of the material fed to the reactor is easier to be handled but means loss of product (about 10% of the material needs to be burned to maintain reactor temperature, according to Mesa-Perez, 2005).

Residence time is controlled based on material feeding rate, air flow, and reactor volume. Material characteristics such as density and size are taken into account to avoid dragging out the time. After pyrolysis, the gaseous mixture is sent to a cyclone by pneumatic conveying (by the fluidizing air itself). In the cyclone, gaseous and liquid components are separated by cen­trifugal force. The gaseous products enter the condenser. The condensable fractions are then separated by gravity: In the bottom an output is used for bio-oil gathering, while the acid ex­tract is collected at the middle of the condenser. Gases and very light particles enter a centri­fuge located at the top of the condenser, where some light particles condensate, increasing the yield of the liquid phases.

The condenser effluent gases are formed by four fractions. The first one is composed of inert atmospheric gases that adhered to biomass particles when the reactor was fed; the second one consists of inert gases fed with air in fluidization (nitrogen, CO2). The third fraction involves semioxidized pyrolysis gases such as CO and CH4; the fourth is composed of those gases that are combusted to provide energy to the system. Usually this gas phase is fed back to the system, especially due to the potential of the third fraction to provide energy to the system.

The combustion chamber is responsible for burning all combustible gases generated in the process. It acts as a restorative power cell besides being a security tool (preventing release of flammable gases into the atmosphere).

The following steps and reactions summarize pyrolysis processes (adapted from Gomes et al., 2008):

1. Drying: Humid material! solid material + H2O(g)

2. Pyrolysis: Dry material! coal + volatile products

3. Combustion reactions:

a. C(s) + O2!CO2(g) + energy

b. 2H2(g) + 02(g) ! 2H2O(g) + energy

4. Heat transfer

5. Mass transfer

The smooth operation of a fast pyrolysis system depends very little on the raw material conditions but strongly depends on its composition (organic matter amount). To be pyro — lyzed, the material might be dried and milled into particles smaller than 20 mm (Bridgwater et al., 1999). Low moisture content is desired to avoid wasted energy (or higher energy de­mand) and possible influence on calorific power of the final product. (High-moisture-content materials are frequently pyrolyzed but with the drawback mentioned previously.) Particle size might be big enough to avoid excessive biomass drag by fluidizing air (the flow of which is usually high), causing loss of nonpyrolyzed material, but also small enough to allow easy heat transfer and avoid secondary polymerization and carbonization reactions (this will cause coal yield increase, according to Sanchez, 2003).

Ganesh, 1990 found that both acid and alkaline catalysts tend to increase gas production. The same study noted that desmineralization caused an increase in the superficial area of coal.

Due to the high heating rate to which material is subjected in fast pyrolysis, the residence time might be very short, usually around 1 second (Gomez, 2002). In this condition, advanced stages of undesirable reactions (such as polymerization and/or decomposition) are avoided. Figure 7.3 presents the most probable mechanisms of formation of pyrolysis products.

Osmotic shock or osmotic stress is a sudden change in the solute concentration around a cell, causing a rapid change in the movement of water across its cell membrane (Fajardo et al., 2007). This shock causes a release in the cellular contents of microalgae. The method is more applicable for the strains cultivated in marine environments (eg. Nannochloropsis sp.). Os­motic shock is also induced to release cellular components for biochemical analysis (Mario, 2010). This method is also applied for Halorubrum sp. isolated from saltern ponds. The results showed increased lipid productivities and variations in lipid compositions (Lopalco et al., 2003).

Extraction of lipids is a key aspect involved in biomass-to-biodiesel production, the method directly influences the lipid productivity potential of the process. So far, several methods have been employed for extracting the cellular contents (lipids) of microalgae. Each method has its own advantages and disadvantages for practical applicability. Among the pro­cesses described, solvent extraction is suitable for extracting lipids from mass cultures but requires large volumes of solvent. The Soxhlet extraction method is applicable only when a single solvent is used and is not suitable for binary solvent applications. However, recovery and reusability of the solvent are possible with this method. The ultrasonic extraction method can perform well when coupled with the enzymatic treatment, but both methods lack cost effectiveness and feasibility for large-scale applications. Supercritical carbon dioxide extrac­tion (SC-CO2), pulse electric field procedure, osmotic shock, hydrothermal liquefaction, and wet lipid extraction require more optimization efforts for large-scale applications. A suitable method operatable with both binary and single solvents, applicable at large scales and yield­ing higher lipid productivities, is yet to be optimized for achieving enhanced microalgae lipid yields.

10.3.4.1 Chlorophylls

Chlorophylls are green, lipid-soluble pigments found in all algae, higher plants, and cyanobacteria that carry out photosynthesis (Rasmussen and Morrissey, 2007). Chlorophyll is converted into pheophytin, pyropheophytin, and pheophorbide in processed vegetable foods following ingestion by humans. These valuable bioactive compounds show antimutagenic effect so are thus likely play a significant role in cancer prevention— specifically via inhibition of myeloma cell multiplicity via pheophorbide (Simon, Alvin et al., 1999). Moreover, chlorophylls are used as a natural food-coloring agent and has anti­oxidant as well as antimutagenic properties.

The process of extracting chlorophyll from marine algae begins with dewatering and desalting the highly dilute culture (0.1-1% w/v, in the case of microalgae). Chlorophyll is then extracted from the dried biomass by organic solvent extraction or SFE. This process is followed by a fractionation step to separate the chlorophyll pigments and derivatives. Many studies have been carried out to optimize chlorophyll extraction and fractionation from algae (Liqun, Pengcheng et al., 2008; Hosikian, Lim et al., 2010).

Production

Man Kee Lam, Keat Teong Lee

School of Chemical Engineering, Universiti Sains Malaysia, Pulau Pinang, Malaysia

12.1 INTRODUCTION

Increasing energy demand coupled with serious environmental concerns over the last 10 years have made the search for renewable and sustainable energy a key challenge of this century (Singh and Gu, 2010). To date, many countries are still heavily dependent on crude petroleum as a source of transportation fuel, and the price of petroleum has always fluctuated in the global fuel market. In addition, 57.7% of the primary energy consumed has been used in the transportation sector, where the consumption rate of fossil diesel fuel was estimated to be 934 million tonnes per year (Kulkarni and Dalai, 2006; Lam et al., 2010). Nevertheless, fossil fuels are nonrenewable resources that are limited in supply and will one day be exhausted (Sharma and Singh, 2009). The concern regarding limited energy resources is caused by the rapid growth in human population and industrialization (Pimentel and Pimentel,

2006) . Due to our knowledge of the impending shortage of energy resources, the era of inex­pensive fossil fuel no longer exists. Instead, the world is facing a shortage in the fossil fuel supply, bitter conflicts, and an increasing number of undernourished people, especially in the undeveloped countries (Lam et al., 2010).

Lately, much attention has been devoted to the cultivation of algae for biofuel production. Algae are desirable for biofuel production compared to land-based plants because (1) algae are fast-growing microorganisms, with reproducibility 100 times faster than land-based plants and able to double their biomass in less than one day; (2) some algal strains can accu­mulate significant amounts of lipids within their cells (as high as 75% of their weight), and the lipids can be converted to biodiesel; (3) algal-based biofuels do not interfere with food secu­rity concerns; (4) fertile land is not required to cultivate algae and thus their cultivation does not compete with agricultural land, and (5) cultivation of algae could be coupled with waste­water treatment and biological CO2 mitigation that enhances the sustainability of algal-based biofuels (Chisti, 2007; Mata et al., 2010; Mutanda et al., 2011; Pittman et al., 2011). The residue of algal biomass after lipid extraction could be further converted to produce different types of biofuels such as bioethanol, biomethane, and biohydrogen (Harun et al., 2010). Examples of algal strains that have been widely evaluated for biofuel production include Botryococcus braunii, Neochloris oleoabundans, Nannochloropsis sp., Chlorella vulgaris, Dunaliella salina, and Haematococcus pluvialis (Chisti, 2007; Harun et al., 2010; Mata et al., 2010; Singh and Gu, 2010).

To date, large-scale algal cultivation still faces several technical challenges that hinder the commercialization potential of algal biomass as a renewable feedstock for biofuel production. The challenges can be divided into two categories: (1) upstream processes: algal selection and cultivation method, energy input for operating closed photobioreactors, nutrient sources, water reusability and footprint, and sensitivity of algae to surrounding environment; and (2) downstream processes: harvesting and drying techniques for algal cells, effective algal lipid extraction methods, algal biodiesel conversion technologies, and biodiesel quality and potential to diversify biofuel production from algal residue after the lipid-extraction process. Beyond the technical challenges, the economic feasibility of commercial algal biofuel produc­tion is still questionable because algal cultivation and associated biofuel production technol­ogies are still under development. As such, an in-depth understanding of these technical and economically related problems should be able to help identify possible solutions to enhance the commercial potential of algal biofuels (Bull and Collins, 2012). In this chapter, several im­portant technical problems surrounding the entire algal biofuels supply chain are discussed, especially from a thermodynamic (energy-balance) perspective. In addition, techno-economic assessments of algal biofuel production are included at the end of this chapter to benchmark the current status of algal biofuels compared to fossil fuels and other renewable fuels.

Because the aim of biofuel production is to provide a substitute for the use of fossil energy, it is important to check that the proposed system manages to create energy and does not use more energy than it produces. Publications use different metrics to evaluate the energy performances of the assessed systems. The net energy ratio (NER), defined as the ratio of produced energy/consumed energy, totals energy consumption as seen at the facility gate. This means that the consumption of 1 MJ of electricity will be accounted for as 1 MJ of invested energy. Other studies measure energy consumption in terms of cu­mulated energy ratio (CER); in that case the price of using 1 MJ of electricity will depend on how it has been produced and will measure the total quantity of primary energy used to create the MJ of electricity. Both approaches have their own interest; a NER will focus on the system technology, whereas a CER will also include the effect of the technological environment of the production system. None of the approaches considers the fraction of storable energy (which could have been directly used for transportation) mobilized by the process. Table 13.8 summarizes the environmental and energy assessment methods.