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Overview and Assessment of AlgalBiofuels Production Technologies

Ganti S. Murthy*Biological and Ecological Engineering, Oregon State University

*Corresponding author: E-mail: [email protected]

1 INTRODUCTION

World economy is critically dependent on fossil fuels. Fossil fuels, in addition to being lim-ited non-renewable resources, are concentrated in unstable regions of theworld constituting anational security threat. Global climate change has been shown to be a direct consequence ofanthropogenic carbon dioxide in atmosphere over the last two centuries. As the world looksfor for alternatives to fossil fuels, first-generation biofuels such as corn ethanol and soybeanbiodiesel were an initial effort to produce transportation fuels domestically and mitigategreen house gas emissions. However, due to production capacity limitations, first-generationbiofuels cannot meet the transportation fuel needs. Additionally, some of the issues such asfood versus fuel debate, intensive use of agricultural inputs such as fertilizers and pesticides,low net energy balance, and uncertain environmental impacts have led to investigation of sec-ond-generation biofuels.

Second-generation biofuels such as ethanol from grass straws, corn stover, switch grass,and other herbaceous crops do not directly compete with food sources and have highernet energy than corn ethanol. However, even the second-generation biofuels do notcompletely eliminate the need for fertilizers, pesticides, arable land, and fresh water in theirproduction. Recalcitrance of cellulosic feedstocks poses additional technological challenges inconversion of these feedstocks to ethanol. Due to the challenges in production, harvesting,and processing technologies, ethanol from cellulosic feedstocks is yet (as in 2010) to be pro-duced in significant quantities. Algae biofuels have the potential to be a sustainable alterna-tive to produce transportation fuels without concerns about food versus fuel or use ofvaluable agricultural lands (Table 1). This has led to research interest in third-generationbiofuels such as biodiesel and ethanol from algae.

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TABLE 1 Comparison of Different Generations of Biofuel Alternatives

First Gen.

(Corn Ethanol)

Second Gen.

(Cellulosic Ethanol)

Third Gen.

(Algal Biofuels)

Enhance energy security

Minimum impact on currenttransportation infrastructure

Food versus fuel debate �Limited capacity to meet the fuel needs � �Intensive use of agricultural inputs �Net energy balance studies � (1.16-1.67) (3-5) (9-10?)

Impact on environment � � �Need for arable lands � �Need for fresh water � �

416 18. OVERVIEW AND ASSESSMENT OF ALGAL BIOFUELS PRODUCTION TECHNOLOGIES

This increased attention to algal biofuels can be describedmore as a reawakening than startof a novel paradigm as in last 50 years, algae have been investigated as sources of food, feed,and fuels. Perhapsmost famous of these efforts is the USDOE’s Aquatic Species Program thatstarted in late 1970s and continued till 1995. The closeout report from this program (Sheehanet al., 1998) contains an extensive summary of the efforts to convert algae biomass intobiofuels. Similar efforts were also underway in Japan during the same timeframe.

Current production of micro- and macroalgae around the world is around 10,000 tons/year. Macroalgae have been traditionally used mostly for food applications. While microandmacroalgae have been investigated for other applications such as production of pigmentsand neutraceuticals, most of the algal biofuels research has been focused on microalgae. Thischapter mostly focuses on production technologies, resource use, energetic, and life-cycleassessment of utilizing microalgae biofuels production.

Microalgae are microscopic plants that can grow in diverse environmental conditions.Some strains of algae accumulate lipids up to 60% of their body weight, while other strainsaccumulate starch. Compared to oil yield of 455 L/ha (48 gal/acre) from soybeans and5685 L/ha (600 gal/acre) from oil palms grown in tropical regions, algae have been shownto yield upto 7760 L/ha (819 gal/acre). Algae have a potential to produce up to 40,000 L/ha(4222 gal/acre) of biodiesel (Weyer et al., 2010). Due to their higher productivities (�8 timescompared to soybeans) and higher lipid content, their potential for biodiesel production hasbeen extensively investigated. Aquatic species program of the U.S. Department of Energyidentified, among others, cost of production as the principal obstacle in the adoption of algaetechnology on a large scale (Sheehan et al., 1998).

In order to achieve a sustainable biofuels production using algae, three critical aspects ofalgae system need to be mastered: production, recovery, and processing. Efficient productionof algae biomass by leveraging waste water treatment, use of flue gases, and waste heat willreduce the production cost of algae as a feedstock for fuels and chemicals. Second aspect of an

4172 AUTOTROPHIC PRODUCTION TECHNOLOGIES

integrated system is the separation/recovery of algae from the medium. The third aspect isthe production of fuels and chemicals from algae. A brief overview of the challenges andopportunities in each of the three aspects is discussed below.

2 AUTOTROPHIC PRODUCTION TECHNOLOGIES

Algal growth in autotrophic production systems occurs through photosynthesis convertinglight energy into chemical energy. Open ponds and closed photobioreactor (PBR) systems arethe two main types of autotrophic cultivation systems that have been investigated. Both typesof systems have different advantages and disadvantages. Therefore, hybrid systems withfeatures of both open pond and closed PBR systems have also been proposed.

2.1 Open Ponds

Open pond cultivation systems generally consist of shallow raceways (0.15-0.45 m depth)constructed as concrete, clay, or plastic-lined ponds. Capital costs for construction of openraceway ponds are lower compared to closed PBRs (Benemann and Oswald, 1996). Paddlewheels are used to provide the necessary mixing to maintain algae culture in suspension.While the mixing energy requirement of paddle wheels is relatively low, efficiency of gastransfer is also lower. Sometimes, aerators may be used to supplement CO2 for maximizingthe algae growth. Pond temperature is not generally controlled and the light intensity isdependent on incoming solar insolation. Hence, efficiency of the open ponds is dependenton the local diurnal variations in temperature and solar insolation.While evaporative coolingof open ponds partially regulates the temperatures of open ponds, it also leads to significantloss of water (DOE, 2010).

It is difficult to maintain algae monocultures in open ponds due to contamination withnative algae and algae grazers. Many strategies such as operating at higher salinity, pH, ortemperatures have been proposed tomaintain an exotic microenvironment in the open pondsthat minimizes contamination with other strains. Successful commercial cultivation ofSpirilunamonocultures in open ponds has beenmade possible bymaintaining the open pondsat high pH (9.0-11.0). Similarly, b-carotene production from Dulaliella sp. in open ponds ispossible due to high salinity levels in the open ponds. Although other systems such as circularponds have been utilized for production of high-value products, most common open pondconfiguration for algal biofuels production is the open raceway ponds (Figure 1).

2.2 Closed PBRs

PBRs are closed bioreactors that permit exchange of light and energy without materialexchange from surroundings. Over the years, PBRs in tubular, flat plate, column, spiral, plas-tic bag, vertical bag configurations have been proposed.

Growing algae in PBRs confers many advantages such as reduced land area requirement,greater productivity at higher culture densities, reduced contamination, lower evaporativewater losses, and better control compared to open pond systems. However, PBR scalability isone of the challenges that still has to be addressed comprehensively. Temperature regulation

FIGURE 1 Lab-scale open pond.


of PBRs is a second challenge due to absence of evaporative cooling. Light penetration, fouling/biofilm formation on the PBRwalls, gradients of pH, dissolved oxygen andCO2within the PBR,and hydrodynamic stresses are some of the additional challenges that have to be addressed foralgae cultivation in PBR (Brennan and Owende, 2010; DOE, 2010; Pulz, 2001).

2.3 Hybrid and Novel Systems

Since cost of production, land area requirement, evaporative loss of water, and contami-nation are some of the critical aspects of economically viable algal biomass production,combinations of open ponds and low-cost PBRs have been proposed. In these proposedsystems, first stage usually consists of closed PBRs to culture the initial inoculumwith robustgrowth characteristics and minimum contamination. This inoculum is transferred during asecond stage to an open pond for maximizing the biomass growth and lipid accumulation.Additional system configurations such as open ponds coveredwith transparent plastic sheetsto minimize contamination and evaporative losses, low-cost PBRs utilizing plastic bagssupported on vertical stands to maximize space and light utilization efficiency have also beenproposed. Presently, most of the PBR systems are experimental- or laboratory-scale deviceswith very few examples of pilot-scale systems in continuous operation.

As any type of land-based algae cultivation system is restricted by land availability, opencoastal/offshore systems for production of macro- and microalgae have been proposed.Large-scale systems for commercial production ofmacroalgae in offshore/near shore systemsexist in China (WSA, 2009) and South Korea. One of the novel systems for microalgae produc-tion is offshore membrane enclosures for growing algae (OMEGA) (WSA, 2009).

3 HETEROTROPHIC AND MIXOTROPHIC PRODUCTION

Heterotrophic growth of algae, using organic carbon substrates such as sugars, has beensuccessfully demonstrated for production of algae biomass and other metabolites. Sincethe algae do not depend on photosynthesis during heterotrophic growth phase, algae can

4194 HARVESTING AND PROCESSING OF ALGAL BIOMASS

be cultured using standard industrial fermenters to high culture densities. With greater con-trol of culture conditions, it is possible to obtain desired product profiles such as higher lipidcontent during heterotrophic growth. Higher cell densities also result in lower capital andoperating costs. However, long-term sustainability of such systems needs to be evaluatedcarefully as production of organic carbon substrate such as sugars can be energy intensiveand has similar limitations as first- and second-generation biofuels.

Mixotrophic growth of the algae for production of algal biofuels and high-value productshas been proposed. Some of the proposed systems supplement the initial inoculum prepara-tion with organic carbon substrates to achieve higher cell densities before introduction toopen ponds for final increase in biomass and contamination reduction by outcompetingthe other algae. The main advantage of such systems is increasing productivities by utilizingboth autotrophic and heterotrophic growth phases. However, as with heterotrophic systems,long-term sustainability of utilizing organic carbon substrates for production of algal biomassneeds to be evaluated.


4.1 Harvesting/Dewatering Technologies

Algae after cultivation in open raceway ponds or closed PBRs exist as dilute solution ofalgae (0.1-10 g/L). Recovering algae biomass from such dilute solutions poses manychallenges, especially for open pond cultivation systems. Therefore, development of efficientprocesses to recover algae is critical for economic viability of algal biodiesel.

Over the years, many of the technologies such as bio/chemical flocculation, sedimentation,dissolved air floatation; various types of centrifuges and filtration systems, hydrocyclones,vacuum filters have been proposed and utilized (Mohn, 1980; Molina Grima et al., 2003).Electroflocculation and ultrasonic-assisted algae concentration techniques have also beenproposed recently (DOE, 2010). A summary of the performance of harvesting/dewateringtechnologies is presented in Table 2.

Most of the proposed strategies to recover algae from the growth media such ascentrifuges, screens, and bio/chemical flocculation are expensive or unreliable in a continu-ous large-scale operation. Although some of these processes are used in commercial produc-tion of Spiriluna, the economics of operations are different economics as it is sold as humanfood. Therefore, to produce biodiesel from algae, it is critical to use simple, reliable, and low-cost algae recovery processes. Suitability of a particular harvesting technology is criticallydependent on the strain of the algae. Therefore, pilot-scale tests must be conducted beforeany decision regarding the optimum harvesting technology can be made.

4.2 Processing Technologies

Over the years, many processing technologies (Figure 2) have been investigated toconvert algal biomass into useful products. A brief overview of the technologies is providedbelow.

TABLE 2 Harvesting and Dewatering Technologies (Data from Mohn, 1980; Molina Grima et al., 2003)

Harvesting

Suspended

Solids (%)

Concentration

Factor

Energy Use

(kWh/m3) Continuous?

Sedimentation tank 1.5 15 0.1 D

Lamella separator 1.6 16 0.1 D

Self-cleaning disk stack 12 120 1 C

Nozzle discharge 2-15 20-150 0.9 D

Decanter bowl 22 11 8 C

Hydrocyclones 0.4 4 0.3 C

Netzsch chamber filter 22-27 245 0.88 D

Netzsch belt filter 18 180 0.5 C

Suction filter 16 160 - D

Cylindrical sieve rotators 7.5 75 0.3 C

Filter basket 5 50 0.2 D

Nonprecoat vacuum filter 18 180 5.9 C

potato starch precoat vacuum filter 37 2-18.5 - C

Vacuum Suction filter 8 80 0.1 D

Vacuum belt filter 9.5 95 0.45 C

Filter thickener 5-7 50-70 1.6 D

Nonprecoat vacuum filter 18 180 5.9 C

Solar drying 85 8.5 0.01 D

Thermal drying 90 9 0.627 Kwh/kg H2O C


4.2.1 Anaerobic Digestion

One of themost direct approaches for utilization of algae biomass ismethane production inanaerobic digester. This approach has the advantage of utilizing wet algae without the needfor additional drying. Many researchers had tried anaerobic digestion of both microandmacroalgae biomass since 1950s. In addition to production of a clean combustible gas, sludgefrom the anaerobic digestion can be used as a nitrogen-rich organic fertilizer. Anaerobicdigestion can be integrated in waste water treatment systems. While anaerobic digestionhas many advantages, the process cannot be used to produce liquid transportationfuels and takes comparatively longer time for algae processing. Additionally buildingindustrial-scale anaerobic digestion facilities that can handle millions of tons of biomassannually can pose significant challenges.

FIGURE 2 Processing technologies for algal biomass.


4.2.2 Thermochemical Conversion

Various types of thermochemical processes including direct combustion, fast pyrolysis,gasification, and pyrolysis have been suggested for utilizing algae biomass.Main advantageof the thermochemical conversion processes is that algae or any other biomass with varyingcomposition can be used. However, thermochemical processes also result in a wide range ofproducts and require additional processing to produce usable fuels. Different temperaturesranges and oxygen levels differentiate various thermochemical conversion processes(Figure 3). Direct combustion is combustion of biomass in excess of oxygen and mainlyleads to heat energy which can be utilized for steam and/or electricity production. Gasifi-cation involves combustion of the biomass in presence of limited amount of oxygen andproduces syngas and char as the main products (Balat et al., 2009a). Pyrolysis, on theother hand, involves heating of dry biomass in complete absence of oxygen and producessyngas, bio-oil, and bio char as the main products (Balat et al., 2009b). Direct hydrothermalliquefaction is very similar to pyrolysis except that it occurs at much higher feedstock mois-ture contents. Product composition can be controlled in these processes by controlling thebiomass composition, catalysts, operating temperatures, and pressures. Syngas producedfrom these technologies can be further upgraded to liquid fuels through Fischer-Tropcshprocess.

Biomass feedstock

Combustion

Hot gases

Steam, process heat,electricity

IC engine Fuel gases/methane Liquid fuels (methanol,gasoline)

Fuel oil and distillates

Low CV gas Medium CV gas Char Hydrocarbons

Gasification Pyrolysis Hydrothermalliquefaction

FIGURE 3 Overview of thermochemical conversion processes.


One of the simplest approaches for utilization of algae biomass is combustion for combinedheat and power production. However, energy costs for drying the biomass may be prohibi-tive. However, with the advent of new types of fluidized bed burners, even wet biomass with50% moisture can be directly combusted. Cofiring of coal-algae mixture in coal-fired powerplants has also been suggested and could result in lower green house gas emissions.

Gasification of algae has been extensively studied. A novel system of low-temperature gas-ification that converts all the nitrogen in algae into recoverable ammonia has been proposed(Minowa and Sawayama, 1999). Hirano et al. (1998) studied the gasification of Spiriluna andobtained theoretical yields of methane (64% biomass) with an energy balance of 1.1 (ratio ofmethanol produced to total energy required).

Pyrolysis has been used to produce bio-oil from algae and other biomass. Bio-oil can berefined in existing petroleum refineries after some additional processing such ashydrotreating and hydrocracking (DOE, 2010). Three types of pyrolysis have been reportedin literature and are defined based on the temperatures, residence time, and heating rates(Balat et al., 2009b; McKendry, 2002). In general, liquid product yields are favored by fasterheating rates, shorter residence times, andmoderate temperatures. Since efficiency of the pyrol-ysis is dependent on the feedstock particle size, microalgae have an advantage over lignocellu-losic feedstocks. However, since pyrolysis can be performed only on low moisture (<15%)feedstocks, the need to dry the algae before pyrolysis would offset any particle size advantage.

Direct hydrothermal liquefaction process can be used to produce liquid fuels fromwet algal biomass (Aresta et al., 2005; Minowa et al., 1995). This process is conducted at mod-erate temperatures (300-350�C) and pressures (5-20 MPa) in presence of a catalyst to yield bio-oil (Brennan and Owende, 2010). Use of microwave heating has been suggested for efficient

4235 CHALLENGES IN LARGE-SCALE CULTIVATION OF ALGAE

heating of wet biomass. Subcritical water reacts with the biomass and results in formation ofenergy dense biocrudewith lower heating values of 30-35 Mk/Kg (Patil et al., 2008). Biocrudeyields up to 37% of the biomass have been reported (Minowa et al., 1995) with properties sim-ilar to heavy oil (Amin, 2009). Thermal efficiency of this process has been claimed to be up to75%. Since wet biomass can be utilized, hydrothermal liquefaction process has much morefavorable energetic balance compared to gasification and pyrolysis.

4.2.3 Solvent Extraction

For liquid transportation fuels production, algae cells must be disrupted and oil presentextracted. Most common technology for lipid recovery from dried algae is solvent extractionusing one of the many polar solvents such as hexane, chloroform, petroleum ether, butanoland methanol. Hexane extraction is the most common solvent due to nontoxicity of hexanecompared to other solvents. Solvent extraction has been successfully used for extraction ofb-carotene and asthaxanthan from algae. In addition to the traditional solvent extraction ofdead algae, ‘milking’ of live algae cells using decane and dodecane as solvents has also beenproposed (DOE, 2010). Using this approach, the live algae cells can be stripped of their lipidsand returned to production system for continued accumulation of triglycerides.

Thermal pretreatment of the algae cells has been recently demonstrated to enhance solventrecovery (Kita et al., 2010). Among the different methods for cell disruption such asautoclaving, bead-beating, sonication and microwave heating prior to solvent extractionusing chloroform andmethanol, microwave heating method was identified as the most effec-tive and simple method of cell disruption (Lee et al., 2010).

While conversion of oil in algae to biodiesel can be accomplished using standard commer-cially available technology, extraction of oil is one of the key challenges. Remaining solids richin protein and carbohydrates can be used as animal feed.

4.2.4 Fermentation

Significant fraction of algal biomass is carbohydrates. Carbohydrate fraction of thealgae biomass can be hydrolyzed and fermented using yeast to produce ethanol.The carbohydrates in algae can exist as starch, cellulose, and other structural. Hydrolysisof starch is an established technology andwidely used in production of corn ethanol; similarextension to algae starch hydrolysis should not pose significant difficulties. Hydrogen-bonding pattern differences in cellulose (O’Sullivan, 1997) from algae (1a) and higher plants(1b) could result in their different response to cellulases and thus different hydrolysis yields.With a modest pretreatment process, the hydrolysis efficiencies can be increased. Algaecells contain other structural carbohydrates such as pectin and chitin. There is a need foradditional research to identify enzymes and pretreatments for effective hydrolysis of algaecarbohydrates.

5 CHALLENGES IN LARGE-SCALE CULTIVATION OF ALGAE

Many of the open ponds and closed PBR systems have been demonstrated at laboratoryand pilot scale. A larger commercial scale facility would face additional challenges in culturemixing, gas exchange, contamination control, and process management. These challenges


had been identified in the Aquatic species program (Sheehan et al., 1998). Some of thesechallenges are only beginning to be addressed in a comprehensive way. In a recent report,the US DOE has identified that stability of large-scale cultures, system productivity, accessto nutrient sources, water management and recycling, control of systems, and coproducts willplay a crucial role in the success of any algal biofuels project (DOE, 2010).

6 RESOURCE CONSTRAINTS FOR MASS PRODUCTIONOF MICROALGAE

One of the biggest challenges for large-scale production of algal biofuels is in the sustain-able use of resources. Although, algae can be produced theoretically at higher productivitiesthan terrestrial plants, it must be noted that any large-scale production must address thechallenges in land, water, and nutrient availability.

6.1 Nutrients

Algae are photosynthetic organisms and when produced under autotrophic conditionsrequire light, nitrogen, phosphorous, CO2, and trace metals for their growth. Although sun-light is virtually free, access to solar insolation is not, as distribution of sunlight varies withlatitude and time of the year.

Nitrogen is one of the essential macronutrients for algae growth. Nitrogen can be suppliedas chemical fertilizers or from other waste streams such asmunicipal waste water. Productionof urea and other nitrogen fertilizers is an energy-intensive process that uses large amounts ofnatural gas. About 42 MJeq of natural gas is required and 1346 Kg CO2 are released for everykg of urea produced (GREET, 2010). Therefore, integration of waste water treatment andalgae production systems has been proposed to simultaneously treat the waste water andsupply nitrogen for growth of algae.

Advances in modern agriculture are dependent upon application of chemical fertilizers.Theoretically, sufficient nitrogenous fertilizers can be produced if there is access to largeamount of low-cost energy source. Of much greater concern is the use of phosphorousfertilizers asmost of the phosphorous used inmodern agriculture ismined from few locationsaround the world (Cordell et al., 2009). Once the phosphorous mines are depleted, entiremodern agriculture will be in jeopardy. This has led to some authors to term this imminentcrisis as peak phosphorous (Figure 4). Currently, over 90% of the demand for phosphate fer-tilizer is in food production and with large-scale increase in biofuels production, concernscould be raised about food versus fuel in the context of phosphorous use. In view of long-termsustainability, it is important to integrate waste water treatment systems for utilization ofnitrogen and phosphorous.

Algae growth has been demonstrated to be faster when supplemented with CO2. Utili-zation of CO2 from power plants, cement plants, ethanol plants, petroleum refineries, andother large emitters of CO2 was a driver for earlier research in microalgae. Many authorshave reported use of flue gases, concentrated CO2 streams for enhancing the algae growth(Benemann, 1997; Brown, 1996; Maeda et al., 1995). Significant barriers for CO2 use fromlarge stationary sources of CO2 emissions such as power plants are optimizing pumping

35Peak phosphorus curve

ActualModelled

30

25

20

Pho

spho

rus

prod

uctio

n (M

T P

/yr)

15

10

5

19000

1920 1940 1960 1980 2000Year

2020 2040 2060 2080 2100

FIGURE 4 Issue of peak phosphorous (figure from Cordell et al., 2009).

4256 RESOURCE CONSTRAINTS FOR MASS PRODUCTION OF MICROALGAE

costs, transportation logistics, and efficiencies. Maximum utilization efficiency is limited to20-30% due to mass transfer limitations in open ponds and diurnal light availability varia-tion even in an optimistic scenario. Additionally, availability of land and water sourcesaround the CO2 sources is a challenge that needs site-specific solutions.

6.2 Climate

Solar insolation, mean temperature, temperature variations, amount and distribution ofrainfall, relative humidity, and cloud cover are some of the climatic variables that affectthe algae productivity. Range and mean values of temperature are one of the importantvariables and van Harmelen and Oonk (2006) in a preliminary analysis indicated the areaswith mean annual temperatures of �15�C to be suitable for algae production. However, asidentified by Lundquist et al. (2010), it is important to consider the variations in temperaturesalong with the mean temperature while assessing the suitability of a location for algae pro-duction. It must be recognized that site-specific analyses are needed to estimate actual pro-ductivity and the aforementioned guidelines are only indicators of productivity. For example,a typical power plant operates at 40% efficiency thus producing significant quantities of wasteheat which could be used for heating algae ponds in locations with lower mean annualtemperatures.

6.3 Land

On a global scale in year 2000, about 12% (12.2-17.1 million sq. Km) of the earth’s ice-freeland surface was used for agriculture, while 28% (23.6-30.0 million sq. Km) was used aspastures (Ramankutty et al., 2008). Producing algae biofuels on large scale requires large


tracts of land. Land slope, permeability of the soils, alternative uses of the land and economicvalue are important factors in determination of the land suitability for algae biomass produc-tion. Since the cost of land preparation can be substantial, only lands with moderate slopes(<5%) are suitable for large-scale algae production.

While nonarable marginally lands can be used for construction of algae productionfacilities, in reality many of these lands are located in areas where other challenges such asnutrient and water availability exist. For example, while large tracts of lands are theoreticallyavailable in southwestern United States for algae production, water availability and highevapotranspiration rates are two of the important challenges. Additional constraints in termsof nutrient availability and CO2 availability could limit the areas for mass production of algaeto few locations around the world. A global map of favorable climate (<15�C annual averagetemperature), lands (<5% slopes, <500 m altitude, <250 persons/Km2 indicating low landcost, >50 persons/Km2 indicating adequate infrastructure), and availability of nutrient-richwaste streams prepared by van Harmelen and Oonk (2006) indicates that suitable microalgaeproduction regions are limited and demonstrates the importance of considering the resourceconstraints.

6.4 Water

Water is used in production and processing of algae. Primary sources of fresh water aresurface waters (rivers, lakes, streams, and lakes) and ground water. Water use is generallycategorized as consumptive and nonconsumptive use. Consumptivewater use refers towaterthat is not returned to the original source such as the water lost in due to evaporation in algaeponds. Evapotranspiration losses represent the most important consumptive water use inagriculture and any potential algae production system in open ponds. Closed PBR minimizeevapotranspiration losses and thus can be beneficial in areas with high evapotranspirationand lower water availability. Depending on the climatic factors such as temperature, relativehumidity, and wind speed, evapotranspiration losses can be significant up to 2.7 m/year(Lundquist et al., 2010).

Increasingly, overuse of groundwater and surfacewaters for use in agriculture, industries,and other uses has resulted in lowering of water table in many areas (IWMI, 2006; Pate et al.,2007;WEC, 2010). Due to uneven availability and allocation of fresh water sources around theworld, many areas will face increasing water stress (Figure 5) and projected global wateravailability scenario is a cause of concern (IWMI, 2006; WEC, 2010). One of the advantagesof algae over terrestrial crops is that many strains of algae can be grown in water with differ-ent salinity levels such as waste water, brackish or sea water. These sources of water do notimpact the use or allocation of freshwater, and therefore algae production using these sourcesof water could aid in sustainable use of water resources.

7 ENERGY ANALYSIS

Multiple pathways for production, harvesting, and processing algae biomass have beenproposed (Figure 2). Suitability of a pathway is dependent on various factors including loca-tion, availability of resources, and end use for the algae biomass. A technoeconomic analysis

Little or no water scarcity

Physical water scarcity

Approaching physical water scarcity

Economic water scarcity

Not estimated

FIGURE 5 Overviewofworldwater stress. (Figure fromReport: ComprehensiveAssessment ofWaterManagementin Agriculture, InternationalWaterManagement Institute, 2006 (IWMI, 2006).) Physical water scarcity: Red,>75% riverflows already in use; Light Red,>60% rivers flows already used; Orange: Economic Water scarcity, <25% used due toeconomic reasons; Blue: Water resources available,<25% is with drawn for human purposes. (For interpretation of thereferences to color in this figure legend, the reader is referred to the Web version of this chapter.)

4277 ENERGY ANALYSIS

for each of these pathways must be performed before the viability of the pathways can becommented upon. For example, most common pathway of algae production in open ponds,harvesting using a combination of flocculation and centrifugation, thermal drying followedby solvent extraction to produce algae oil and cake may not be feasible in terms of energyreturns as the thermal energy input exceeds the recoverable energy from algae oil andcoproducts (Sander andMurthy, 2010). However, if the algae can be dried using solar energyor waste heat, the energetics may be more favorable. Therefore, it is important to conductenergetic and sustainability assessment for each of the algae biomass utilization pathwaysbefore large implementation.

7.1 Theoretical Production Estimates

Determining theoretical maximum productivity limits based on thermodynamics andother laws of nature is important for realistic assessment of the algal biofuels potential.One of the methods for estimating theoretical maximum algal biomass and lipid productivitywas discussed byWeyer et al. (2010). The model proposed byWeyer et al. (2010) consists of anumber of factors such as full-spectrum solar energy (FSSE, MJ/m2-year), photosyntheticallyactive radiation (PAR), photon energy (PE,MJ/mol), efficiencies of photon transmission (ZPT)and utilization (ZPU), quantum requirement for conversion of PE into chemical energy(QR, mol photons/mol carbohydrate), energy in carbohydrates (CE, KJ/mol), biomass


accumulation efficiency (ZBiomass), biomass energy content estimate (BE, KJ/g), cell oilcontent (CO, %), and cell oil density (CD, Kg/m3).

Basic photosynthesis equation used to estimate the quantum requirement for conversion ofPE into chemical energy (QR, mol photons/mol carbohydrate) is:

CO2 þH2Oþ 8 Photons ! CH2OþO2:

Energy content of the biomass is estimated based on the proximate analysis of the dry
algae as:
BEðKJ g�1Þ ¼ 15:7ðKJ g�1Þ Proteinþ 16:7ðKJ g�1Þ Carbohydrateþ37:6ðKJ g�1Þ Lipids:

Using these assumptions, the maximum daily productivity can be estimated as:

Maxmimum daily growthðg m2 day�1Þ ¼ FSSE�PAR��PT��PU�CE��Biomass

PE�QR�BE�365 :

Similarly, annual lipid production can be estimated from the maximum daily growth as:

Anual oil productionðLHa year�1Þ ¼Maxmimumdaily growthðgm2 day�1ÞCO�365� 10;000

CD:

An estimate of the theoretical and best case algal lipid production scenarios along with
estimates for open raceway ponds and PBRs is provided in Tables 3 and 4. These estimates
TABLE 3 Production Model (Based on Production Model from Weyer et al., 2010).

Description Units Th. Max Best

Open Ponds PBR

Min Max Min Max

Full-spectrum solar energy MJ/sq m-year 11,616.0000 5623.0000 4700 4700 4700 5621

PAR - 0.4580 0.4580 0.458 0.458 0.458 0.458

Photon energy MJ/mol 0.2253 0.2253 0.2253 0.2253 0.2253 0.2253

ZPhoton transmission - 1.0000 0.9500 0.8 0.9 0.95 0.95

ZPhoton utilization - 1.0000 0.5000 0.5 0.5 0.7 0.7

Quantum requirement - 8.0000 8.0000 8 8 8 8

Carbohydrate energy content KJ/mol 482.5000 482.5000 482.5 482.5 482.5 482.5

ZBiomass accumulation - 1.0000 0.5000 0.5 0.7 0.6 0.8

Biomass energy content KJ/g 21.9000 21.9000 21.9 21.9 21.9 21.9

Cell oil content - 0.5000 0.5000 0.1 0.15 0.15 0.25

Oil density Kg/m3 918.0000 918.0000 918 918 918 918

TABLE 4 Production Estimates

Production metric Units Th. Max Best

Open Ponds PBR

Min Max Min Max

Maximum daily growth g/sq m-day 178.2 20.5 14.4 22.7 28.8 45.9

Annual oil production L/ha year 354,202 40,722 5733 13,543 17,155 45,592

Annual oil production gal/ac year 37,383 4298 605 1429 1811 4812

4297 ENERGY ANALYSIS

provide a preliminary guide to productivities, and more accurate results would be obtainedusing site- and strain-specific parameters in the model.

7.2 Energy Use in Processing Algae Biomass

Similar to the importance of algae production estimates, energy used in processing the algaebiomass is critical in assessing the energy use in the production of biofuels. As there are nolarge-scale algal biofuel production facilities in operation, there is presently little informationavailable on the production processes. Nevertheless, an initial estimate for the energy used inprocessed can be useful in identifying the energy intensive unit operations. A framework forreporting the algal biofuels production has been recently suggested (Beal et al., 2010). Some ofthe estimated energy use for algae harvesting and dewatering technologies based on lab-scaledata are presented in Table 2. Similarly, energy contained in some of the products from dif-ferent processing technologies with estimated process efficiencies is presented in Table 5.Using the production estimates, energy used in harvesting and processing steps, an initialenergetic assessment of an algal biofuel production pathway can be conducted.

A systematic energy assessment of a biofuels pathway has to incorporate the energy usedin all subprocesses, energy embodied in the fuels and the coproducts. In evaluating the ener-getic assessment of biofuels, solar energy is not considered as an energy input. Thus, the totalenergy input only includes the nonrenewable energy sources used in the production. These

TABLE 5 Products from Different Processing Technologies

Processing Fuel

Max yield (Kg or

L/Kg Biomass) Efficiency

HHV (MJ/L

or MJ/kg)

Direct combustion Biomass 1.0 of biomass 80 18.15

Solvent extraction Biodiesel 1.0 of lipid content 80 35.7

Anaerobic digestion Biogas (62%CH4)

475.8 L/Kg of biomass 95 2.375 � 10-2

Fermentation Ethanol 0.51 of carbohydrate 85 23.4

Thermochemical conversion(fast pyrolysis)

Bio-oil 0.553 of biomass 90 33.64


values are used to calculate total, net energy input, net energy balance, and net energy ratio asfollows:

Total energy input ¼X

Sub process energy inputs;

Net energy input ¼ Total energy input� Coproduct allocation;

Net energy balance ¼ Net energy input� Energy in functional unit;

Net energy ratio ¼ Net energy input

Energy in functional unit

Net energy return on energy invested ðEROEIÞ ¼ Energy in functional unit

Net energy input:

Use of net energy as the sole metric has been debated and is of limited use in complete assess-
ment of biofuels advantages (Dale, 2008).While a positive net energy is essential, it is importantto note that biofuelswere not produced just for energetic advantages. Therefore, a complete life-cycle assessment is needed for a comprehensive assessment of any biofuels technology.
8 LIFE-CYCLE ASSESSMENT

One of the important aspects of long-term algal biofuels production is the sustainability ofthe resource use. Although sustainability is a more abstract concept and is difficult to quan-tify, a widely accepted definition of sustainability was given by Brundlandt (1987) as “Sus-tainable development is a social development which fulfils the needs of presentgenerations without endangering the possibilities of fulfillment of the needs of futuregenerations.” The first basic condition for a sustainable process is that resource consumptionrate should be slower than the resource regeneration rate. For example, a sustainable processwould release less carbon dioxide than it would consume during the production, use, andrecycle processes. The second condition relates to the emissions during the life cycle of a prod-uct: the emissions must not harm the environment or at the minimum be lower than theassimilative capacity of ecosystem (Dewulf et al., 2000).

Life-Cycle Assessment (LCA) is a tool to assess impact of products, processes, and serviceson the environment. All elements of the process are considered startingwith the acquisition ofraw materials, production, manufacturing, and disposal. LCA studies were first developedby the U.S. Environmental Protection Agency (EPA) to estimate the resource use in produc-tion of different materials. Researchers at Argonne National Laboratory have developed atransportation sector-specific version called Well to Wheels LCA. GREET, MS Excel-basedsoftware developed by Argonne National Laboratory researchers, has been used forconducting LCA analysis for transportation fuels (Wang, 2005).

LCA consists of four iterative steps (Figure 6):

1. Goal definition and scoping: Defining the functional unit, scope of the LCA, and the systemboundary identification.

2. Inventory analysis: Compilation of the mass and energy flows for all the inputs, outputsentering/leaving the system boundary.

Goal definitionand scoping

Inventoryanalysis

Impactassessment

Inte

rpre

tatio

n

FIGURE 6 Steps in life-cycle assessment (from

SAIC, 2006)


3. Impact assessment: Assess the environmental, human and societal effects of the materialsuse and emissions due to mass and energy flows into/out of the system boundary.

4. Interpretation: Interpret the results of the impact assessment in the context of functionalunit, assumptions, quality of data source, and uncertainty in the data.

Since impact assessment and interpretation involve many more qualitative assumptions,many LCA are performed only until Step 2. Such LCA are called Life-Cycle Inventories (LCI).

While research in algal biofuels area is rapidly expanding, there are relatively few studieson the sustainability and life-cycle assessment of the algae pathways (Aresta et al., 2005; Batanet al., 2010; Campbell et al., 2011; Clarens et al., 2010; Lardon et al, 2009; Luo et al., 2010; Sanderand Murthy, 2010; Stephenson et al., 2010).

Lardon et al. (2009) performed a comparative LCA study of two different cultureconditions (nominal nitrogen or nitrogen starvation) and two different extraction options(wet or dry extraction technology) for a virtual facility. They concluded that while microalgaecould have lower impacts than corn ethanol, it is imperative to lower energy and fertilizerconsumption. They suggest using low nitrogen input production and wet extraction technol-ogy to accomplish these goals.

Clarens et al. (2010) compared the environmental impacts of producing microalgae,corn, switch grass, and canola. Contrary to Lardon et al. (2009), they concluded thatmicroalgae production has higher energy use, green house gas emissions, and waterconsumption regardless of cultivation location. Algae production had lower impacts in theeutrophication and land area requirement categories. However, as indicated by the authors,most of these results can be attributed to energy use in fertilizer production and CO2 delivery.Additionally, processing of algae biomass into fuels and coproducts was not considered inthis LCA.

Sander and Murthy (2010) performed a complete well to pump LCA incorporating anobjective system boundary definition. Alternative technology pathways including carryingcomposition of algae, filter press/centrifuges, solar/thermal drying were considered. Theiranalysis (Table 6.) indicated that algae biodiesel could result in lower GHG emissions andpositive net energy. These results were critically dependent on the large coproducts creditsand the harvesting technology. Only carbohydrate fraction was considered for coproductscredits, and the potential uses of algal protein for animal feed or organic fertilizers were

TABLE 6 Energy and CO2 Emissions for Algal Biodiesel Production (Data from Sander and Murthy, 2010)

Functional Unit: 1000 MJ of Algal Biodiesel Energy (MJ) CO2 Emissions (Kg)

Filter press (centrifuge) primary dewatering

Growth 15.43 (15.43) 0.00 (0.00)

Harvest 2915.27 (5743.32) 241.87 (398.48)

Separation 165.03 (165.03) 6.33 (6.33)

Transportation 8.79 (8.79) 0.65 (0.65)

Biodiesel conv. 36.02 (36.02) 3.18 (3.18)

MeOH prod. and transport 72.24 (72.24) 0.06 (0.06)

Biodiesel transport and dist. 9.66 (9.66) 0.66 (0.66)

Natural gas prod. 69.52 (69.52) 0.00 (0.00)

Coproduct allocation �9971.77 (�9971.77) �273.60 (�273.60)

Total �6679.81 (�3777.63) �20.90 (135.71)


not included in the LCA. They identified water loss due to evapotranspiration as a significantsink of water and demonstrated that thermal dewatering technologies were not suitable forsustainable algae biofuels production.

Batan et al. (2010) performed a “well to pump” LCA for a low-cost PBR system usingGREET 1.8c model as the basis. The CO2 was not purchased in their modeled production sys-tem although fertilizers were used as input for providing nitrogen and phosphorous. Theyreported an NER of 0.93 MJ consumed/MJ produced and 75 g avoided CO2/MJ of energyproduced.

Stephenson et al. (2010) compared the performance of raceways and air-lift tubularbioreactors for production of microalgal biodiesel in the UK. They considered a two-stageproduction system with a first nitrogen-sufficient stage to accumulate biomass followed bya nitrogen-starvation phase to accumulate lipids. They concluded that open pond cultivationhas lower energy consumption compared to air-lift tubular PBRs. The results were sensitiveto oil productivity, energy use in circulation, recycling of culture media, and concentration ofCO2 in flue gas.

Campbell et al. (2011) performed LCA for an open pond system in Australia under threedifferent CO2 supplementation and two production scenarios. They concluded that algaehave reduced green house gas emissions when the algae production facilities are co-locatednext to power plants or ammonia plants. They identified the algae productivity as one of thekey variables that influence the LCA results.

Luo et al. (2010) conducted LCA for direct ethanol production from blue-green microalgaethrough intracellular photosynthesis-fermentation. In their analysis, direct ethanol secretionfrom live algal cells was assumed to reach concentrations between 0.5% and 5.0% in theculture media consisting of sea/brackish water in a flexible-film PBR. The culturebroth was distilled to produce ethanol. They reported NER as 0.55-0.2 MJ/MJ ethanol and


29.8-12.3 CO2Eq/MJ ethanol. The CO2 values represented 67-87% reduction in carbonfootprint compared to gasoline on an energy-equivalent basis.

Aresta et al. (2005) developed COMPUBIO software to conduct LCA for conversion ofmacroalgae into biofuels. The modeled system consisted of CO2-supplemented macroalgaeproduction in sea shore facilities using nutrients from effluent waters, harvesting of thebiomass, and user-defined conversion technology. In the simulated best case scenario,macroalgae had higher net energy (11,000 MJ/ton dry algae) compared to microalgae gasifi-cation (9500 MJ/ton dry algae).

The difference in the results from many of these LCAs and even contrasting conclusions isnot surprising. Such contrasting results were also obtained in the LCAs for corn ethanol andhave been subject of intense debates (Wang, 2005).

Some of the factors that can lead to differences in the LCAs are:

1. System boundaries: Unit processes, inputs included in the system definition.2. Data quality, accuracy: Age of data, sources of data, geographical context3. Allocation of coproducts: Mass, energy, and displacement methods4. Nitrous oxide emissions: Varying fertilizer use for different crops5. Indirect land use change (ILUC): Variation in ILUC among first-, second-, and third-

generation fuels.6. Reported units, for example, GHG emissions/MJ of fuel or GHG emissions/Ha of land.

While LCA methodology can be used to assess the impact on environment and thus infersustainability of a technology, some limitations are inherent in the LCA. Some of thelimitations and of LCA are

1. Economic or risk assessment is not performed.2. Energy quality is not considered.3. Natural resource (e.g., water) use in LCA.4. Local versus Global impacts is not characterized well.5. Potential direct/ILUC is difficult to study.6. Policy driven change is not incorporated.7. Cannot predict the influence of game-changing technologies.

In recent years, many methods have been proposed to improve the LCA methodology toovercome some of its limitations. Some of the proposed improvements are:

1. Systematic boundary definition (Raynolds et al., 2000),2. Enhancing LCA inventories using thermodynamics (Hau et al., 2007),3. Dynamic LCA (Pehnt, 2005),4. Accounting for water use (Koehler, 2008),5. Incorporating alternative technology pathways.

8.1 Water Use in LCA

Water is one of themost important resources used in algal production. Current LCA frame-work does not incorporate the effect of water use. While this may not be very relevant forfossil fuel LCAs, it is very important to consider the impacts of water use in any biofuels


production system. This is mainly due to the intensive water use in both production andprocessing of first, second, or third generation of biofuels. Due to large-scale production ofbiofuels, understanding the interdependencies between energy and water will be crucialfor sustainable use of water resources (Pate et al., 2007).

One of the approaches for accounting the water use in different life-cycle assessment wasproposed by Mulder et al. (2010). Mulder et al. (2010) developed a generalized indicator forwater use intensity of different energy production technologies by defining energy return onwater invested (EROWI) similar to energy return on energy invested (EROEI). A metric, NetEROWI, was proposed as a function of EROWI and EROEI to estimate water use in differentfuel production systems as follows:

o ¼ EROEI

EROEI� 1;

Net EROWI ¼ Gross EROWI

o:

Based on the water consumption data, Mulder et al. (2010) estimated the EROEI and EROWI
for different fuels (Table 7). Similar analysis or algal biofuels indicates that the EROWI foralgae using fresh water is similar to other biofuels crops and hence would lead to significantwater scarcity issues if produced on large scale. However, by utilizing waste water/sea waterfor algae production, net EROWI can be improved significantly (Table 8).
TABLE 7 EROEI and EROWI for Different Fuels

Water usage (L/MJ) EROWI (MJ/L) EROEI (MJ/MJ) Net EROWI

Nuclear electric 1.162(0.145) 0.861(1.517) 10 0.775 (1.137)

Coal electric 0.560(0.488) 1.786 (2.049) - -

Conv. diesel 0.0035 285.3 5.01 228.4

Biodiesel

Rapeseed 100-175 0.010-0.0057 2.33 0.0057-0.0033

Algae (ponds) 20.142a 0.004965a 3.33a 0.03475a

Ethanol

Sugarcane 38-156 0.026-0.0065 8.3 0.023-0.0057

Corn 73-346 0.014-0.0029 1.38 0.0039-0.00081

Lignocellulosic Crops

Ethanol 11-171 0.091-0.0058 4.55 0.0071-0.0045

Hydrogen 15-129 0.067-0.0078 4.67 0.053-0.0062

Electricity 13-195 0.077-0.0051 5.0 0.062-0.0041

a Calculated based on data from Sander and Murthy (2010). 1000 MJ �27.89 L (7.36 gal) biodiesel.

(Data from Mulder et al., 2010).

TABLE 8 EROEI and EROWI for Algal Fuels: Alternative Scenarios (Murthy, 2010)

Scenario

Energy

Output (MJ)

Energy

Input (MJ)

Net

Energy

EROEI

(NER)

Water Usagec

(L/MJ) EROWIc Net EROWIc

Base casea 10,971.77 3291.96 7679.81 3.33 20.142 (0.403) 0.05 (2.48) 0.035 (1.74)

Improvedharvestingb

10,971.77 1105.51 9866.26 9.925 20.142 (0.403) 0.05 (2.48) 0.045 (1.74)

Withoutcoproductcredits

1000 3291.96 �2291.96 0.308 20.142 (0.403) 0.05 (2.48) �0.114 (�5.69)

WasteWater/SeaWater

10,971.77 1105.51 9866.26 9.925 0.1 10 9.25

a Base case as in Sander and Murthy (2010). 1000 MJ �27.89 L (7.36 gal) biodiesel.b Improved harvesting assumes a 75% reduction in energy to harvest and drying algae.c Numbers in parentheses indicate photobioreactor case, assuming a 50 times lower water consumption than an equivalent open pond.

4359 FUTURE PERSPECTIVES: CHALLENGES AND OPPORTUNITIES

9 FUTURE PERSPECTIVES: CHALLENGES AND OPPORTUNITIES

Algal biofuels due to their higher productivity compared to traditional biofuels hold greatpromise for sustainable biofuels production. Due to their versatility, algae can be grown inmany locations around the world and can use diverse water sources that are not suitablefor crop production or other industrial uses. While many researchers have demonstratedthe technical feasibility of producing algal biofuels, some of the challenges for large-scale pro-duction of algal biofuels remain.

Challenges in production technologies still need research and the engineering challenge ofmanaging large-scale ponds has to be addressed. Control systems for automated nutrientsupply, harvesting, and contamination control of large ponds need to be developed. Devel-opment of low-cost PBRs that have higher productivity than open ponds and yet do not sufferfrom some of their disadvantages will reduce the algal biomass production cost.

There is a critical need to develop wet processing technologies for converting wet algalbiomass into biofuels. Although the importance of coproducts utilization has beendemonstrated, research into utilization of algae carbohydrate and protein fractions needsto be performed.

Resource availability and constraints for algae production need to be assessed in a compre-hensive way. Presently, there are a large number of disparate sources and there is a need todevelop unified analysis of resource availability using GIS-based tools. Comprehensive LCAusing standard boundary definitions, incorporating water use can be the first steps inassessing various technologies for algal biomass processing.

In view of the challenges that the world faces in terms of limited fossil fuels, limitedresources such as land, fresh water access, and global climate change, the opportunities thatalgal biofuels provide for a long-term sustainable solution are much greater than the techno-logical challenges.


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