Employing Microalgae Feedstock for the Production Biodiesel

Name: Musfiq Islam
UW ID: 20300084
Course: BIOL 443
Professor: Dr. Owen Ward

Introduction
Global interests in biofuel production as a substitute for liquid transport fuel have grown substantially in recent years, mainly due to concerns over energy security and climate change (Yahya et al, 2012). The most common used biofuels are biodiesel and bio-ethanol, which can replace diesel and gasoline, respectively, with limited or no modifications of vehicle engines (Karthikeya, 2012). These first generation biofuels are typically extracted from food and oil crops including rapeseed oil, sugarcane, sugar beet and corn as well as vegetable oil and animal fats using traditional technologies (Brennan and Owende, 2009). However, the use of first generation biofuels have generated wide-scale criticisms, primarily due to raising competition with food production, increased water consumption, soil degradation, biodiversity loss, their low energy potential and their role in greenhouse gas emissions (Barbosa et al., 2011). A particular concern is that the demand for biofuels could place substantial additional pressure on the natural resources such as arable land. Currently, about 1% (14 million hectares) of the world’s available arable land is used for the production of biofuels, providing 1% of global transport fuels (Beal et al., 2012). However, with a fixed and possibly reducing amount to arable land, increasing the share of land devoted to biofuel production will have severe impact on world’s food supply as well as harmful environmental and social consequences.
In response to these criticisms, there have been concerted efforts on deriving second generation biofuels from lignocellulosic biomass such as wood and plant stocks (Takeshita, 2011). This would involve extracting wood, forest and switchgrass residues as the feedstock required for second generation biofuel production. However, using lignocellulosic biomass for biofuel production could possibly remove nutrients from surrounding environment and negatively impact forest biodiversity (Demirbas, 2011).
More importantly, cellulosic ethanol production only addresses the needs of the gasoline market; it does not however, address the need for higher energy density fuels that can replace diesel and jet fuel (Benemann, 2008). The demand for diesel in North America and Europe alone exceeds 120 billion gallons/year, and the current biodiesel production from oilseed crops does not remotely come close to meeting the worldwide diesel demand (IEA Bioenergy Conference, 2010). Therefore, alternative means of biodiesel production is necessary to meet the increasing demand for higher energy density liquid transport fuel (IEA Bioenergy Conference, 2010).
Employing microalgae for biodiesel production have been suggested since the 1950s and in particular during the oil crisis in the 1970s (Sheehan et al., 1998). However, only recently has there been a resurgence of microalgal biofuel research and commercial development prospects. The resurgence in the commitment to develop of algal biofuel by industries and governments has been triggered mainly by criticisms over first generation biofuels, record high crude oil prices, increasing energy demand, and environmental concerns as well as considerable developments in biotechnology (Wijffels and Barbosa, 2010).
This paper focuses on the effectiveness of employing microalgae as feedstock for biodiesel production. The advantages and limitations of using microalgae are discussed. Also, the production process and parameters affecting microalgal biodiesel production such as growth, harvesting and processing are explored. Lastly, the production economics and commercial viability of microalgae for biodiesel are examined.

Advantages of Employing Microalgae for Biofuel Production
Microalgae are unicellular or simple multicellular photosynthetic microorganisms that can grow rapidly and live in harsh conditions due to their simple structure (Li et al., 2008). Accordingly, microalgae have been described to have several benefits for biofuel production over other available feedstocks. Most importantly, they have much higher growth rates and productivity while requiring little to no land area and lower water utilization rates compared to conventional biodiesel feedstocks of agricultural origin (Pienkos and Darzins, 2009). Microalgae’s ability to thrive in areas that do not support agriculture can therefore, provide a way of resolving the potential conflict between the use of arable land for food or for biofuel production. Table 1 illustrates microalgae’s substantially higher oil and biomass yields compared to other biodiesel feedstock as detailed by Mata et al., 2010. Moreover, as photoautotrophs microalgae can reproduce almost daily using photosynthesis to convert sunlight into chemical energy (Beal et al., 2012). Their growth rates can be accelerated even further by the addition of specific nutrients and sufficient aeration (Renaud et al., 1999). Also, different microalgae species are adapted to live in a variety of environmental conditions. As a result, it is possible to find species best suited to local environments or specific growth characteristics, which is not possible to do with current biodiesel feedstocks such as soybean, rapeseed, sunflower and palm oil (Mata et al., 2010). More importantly, microalgae feedstock can be converted into third generation biofuels such biodiesel, methane and hydrogen ethanol (Amaro et al., 2011). Also, algae biodiesel is environmentally friendlier as they contain no sulfur, and reduce the emission of carbon monoxide and hydrocarbons (Radakovits, 2010).

Table 1. Comparison of microalgae with other biodiesel feedstocks (Mata et al., 2010)

Plant source content | Seed oil (% oil by wt in biomass) | Oil yield(L oil/ha year) | Land use(m2 year/kg biodiesel) | Biodiesel productivity(kg biodiesel/ha year) | Soybean | 18 | 636 | 18 | 562 | Camelina | 42 | 915 | 12 | 809 | Canola/Rapeseed | 41 | 974 | 12 | 862 | Sunflower | 40 | 1070 | 11 | 946 | Castor | 48 | 1307 | 9 | 1156 | Palm oil | 36 | 5366 | 2 | 4747 | Microalgae (low oil content) | 30 | 58,700 | 0.2 | 51,927 | Microalgae (med oil content) | 50 | 97,800 | 0.1 | 86,515 | Microalgae (high oil content) | 70 | 136,900 | 0.1 | 121,104 |

Current Limitations Microalgal Production Systems
The main problem found with microalgae for biodiesel production is the economics (Schulz, 2006). Biodiesel from microalgae is even more costly than biodiesel from other sources. In fact, microalgal biodiesel production is estimated to be 15 times higher than rapeseed biodiesel production (Kovacevic and Wesseler, 2010). This is mainly due to the substantial capital infrastructure investment required in additional to challenges in cost-effective downstream processing including collection and dewatering (Norsker et al., 2011). Although, there are currently many biodiesel projects and companies that are focusing on microalgal biodiesel in the US and Europe, they are mainly driven by government subsidies, and by not profitability (Brown, 2009).

Stages of biodiesel production from microalgae

There are several stages in the microalgal biodiesel production value chain as outlined in Figure 1. These critical stages include, selecting the optimal microalgae species depending on local conditions, designing and implementing the cultivation system for microalgal growth, harvesting, biomass processing and extraction of the oil which through transesterification will be converted to biodiesel (Chen et al., 2011).

Figure 1. Stages of biodiesel production from microalgae (Mata et al., 2010)
Selecting the Ideal Microalgae for Biofuel Production

The ideal microalgae should be robust with the ability to thrive in a wide range on environmental conditions with varying nutrients, temperature, light, and compete against other microalgae specie or bacteria. (Wijffels et al., 2010). Most importantly, for microalgae to be a viable option for commercial biodiesel production, it must have high energy conversion efficiency and lipid productivity (Chisti, 2008). The ideal microalgae should also display self-flocculation to ensure feasible downstream processing (Brennan and Owende, 2009). Figure 2 illustrate the ideal microalgal cell for production of biofuels. Currently, microalgae that exhibit all of the ideal traits do not exist (Aguirre et al., 2012). However, genetic engineering of microalgal species offers the possibility of strain improvement (Radakovits et al., 2010). For instance, microalgae lipid production is high in stressed conditions however, it is at the expense of higher growth rate. Therefore current developments in genomics and bioengineering, it may be possible to induce high lipid accumulation without having to apply stress factors, and thereby also maintain high growth rate (Li et al., 2008). Given the high research and industry interest in microalgal biofuel production, it is likely that the genome of more algal strains will be sequenced in the near term, allowing the bioengineering of the ideal microalgae.

Figure 2. The ideal microalgal cell for production of biodiesel (Wijffels et al., 2010)

Microalgal Cultivation Systems
There are two main groups of systems for cultivation of microalgae, open and closed systems (Lassing at al., 2008). The open ponds have their surface open towards the atmosphere, while the closed photobioreactors are closed vessels made of a transparent material allowing the light to reach the microorganisms inside (Wurts, 2010). 1 Open pond systems are typically shallow ponds in which algae are cultivated. Nutrients can be provided through runoff water from nearby land areas or by channelling the water from sewage/water treatment plants (Chen at.,2011). While open systems are less expensive and easy to build compared to closed systems, there are several technical and biological limitations that have given rise to the development of enclosed photobioreactors (Lassing at al., 2008). Open systems exhibit low productivity per area and volume, due to the low light over volume ratio (Morita et al., 2000). In addition, the open system can easily be contaminated by other microorganisms and harm the desired cultivation. There is also a high loss of water evaporation and diffusion of CO2 to the atmosphere in open systems (IEA Bioenergy Conference, 2010).
Microalgae production in closed photobioreactors is much more expensive than open ponds (Molina et al., 2000). However, the closed systems require much less light and land to cultivate the microalgae (Lassing at al., 2008). More importantly, the ability to control and optimize growth conditions in photobioreactors can potentially yield 58,000L of microalgal oil per acre per year, which over 200 times the yield from the best-performing plant/vegetable oils (Chisti, 2008). Therefore, as illustrated in figure 3, tubular photobioreactors designed to maximize sunlight capture are the favored system for cultivating algal biomass needed for biodiesel production (Molina et al., 2001).

Figure 3. Tubular photobioreactor system with fence like solar collectors (Chitsi, 2008)
Challenges in Harvesting and Downstream Processing of Microalgae Production
Harvesting, biomass processing and oil extraction constitute a large portion of microalgal biofuel production cost. In fact, the downstream purification processes constitute almost 50% of the total production cost (Chen et al., 2011). Currently, there is no unique process for biomass harvesting. The harvesting method selection depends on biomass characteristics (size, density and product value). Typical processes such as flocculation, filtration, flotation, and centrifugation are used for harvesting and purification (Mata et al., 2010). However, the effectiveness of recovering the algal biomass is often limited by the low cell densities and small size of the cells (Grima et al., 2003). Therefore, it is important to implement large scale microalgae technology in order to overcome the bottlenecks in downstream processing (Halim et al., 2011). According to Aguirre et al. 2010, the ideal harvesting methods must be have the lowest energy requirements, capital, operating cost and highest efficiency, while being compatible with all other processes. Table 2 provides a summary of advantages and disadvantages of currently used methods for harvesting, purification, extraction and cell wall disruption of microalgae for biofuel production.

Table 2. Advantages and disadvantages of different harvesting, purification, extraction and cell wall disruption methods (Brennan & Owende, 2010; Fu et al., 2010; and Halim et al., 2011) Method | Advantages | Disadvantages | Harvesting and dewatering | | | Flocculation | It can be operated in continuous fashion No shear stress on biomass | It is necessary to addflocculants | Flotation | It does not require chemicals addition | Little evidence of its technical and economic feasibility | Centrifugation | Allows treatment oflarge volumes | High cost of operation; Need for constant maintenance | Filtration | Effective | High cost of replacingmembranes and large-scale pumping | Sun Drying | Inexpensive | It is time consuming; requires large surfaces | Cell Wall disruption | | | Microwaves | Effective | High cost of operation | Cellulase Treatment | Low energy required; High selectivity; and Few side products | High cost of the cellulase | Heat pretreatment | Improve the recovery rate | Energy demanding | Lipid Extraction | | | Supercritical | Low toxicity | High cost of infrastructure andoperation | Carbon dioxide | Favorable masstransfer equilibriumSolvent-free extract | |

Conversion of Microalgal Lipids into Biodiesel
Upon extraction of the purified microalgal lipid, the process of transesterification is typically used to convert the algal oil into biodiesel. The biodiesel transesterification reaction in the presence of a catalyst proceeds as follows (Huan et at., 2010)):
Triglyceride + 3methanol ↔ Glycerine + 3methyl ester (Biodiesel)
Production Economics and Commercialization Prospects of Microalgal Biodiesel
As of 2009, there were over 50 algal biofuel companies worldwide, none of which were producing commercial-scale biodiesel at competitive prices (Pienkos and Darzin, 2009). This is primarily due to the substantial cost of downstream procession for oil collection which contributes to almost 50% of the total of microalgal biodiesel production cost (Chisti, 2008). The economics of microalgal biodiesel production could be improved by advances in photobioreactors and extractions processes for more efficient oil recovery (Milledge, 2009). The most important development in improving the economics of producing microalgal diesel would be to increase productivity of microalgae via genetic and metabolic engineering. (Lee et al.,2008). Bioengineering of microalgae could be used to: i) improve photosynthetic efficiency and increase biomass production; (ii) increase biomass growth rate; (iii) increase lipid productivity of microalgae; and (iv) improve tolerance to varying temperatures to eliminate the hefty expenses associated with cooling (Chisti, 2008). Recent economic analysis of microalgal production in tubular photobioreactors vs open ponds found that the cost of producing one kg dry biomass was €5.96 and €4.95, respectively (Norsker et al., 2011). It was also identified that the centrifuge, the culture circulation pump and the blower wheel proportionately contribute most to the total cost (Norsker et al., 2011). Accordingly, enhancing the efficiency of the most expensive process can reduce overall cost. In fact, Stephens et al. and Richardson et al. in 2010 have shown that small changes in the technological aspects of the downstream processing can improve economic viability, which increases the prospects for microalgae technology in the long term.
The current estimated costs of production of microalgal biodiesel lie between US $9 and $25 per gallon in ponds, and within the range US $15–$40 per gallon in photobioreactors (Amro et al., 2011). Broere in 2008, estimated that microalgal biodiesel could only compete with crude oil if the price of oil rose to $800/barrel (approx. $20/gallon). Therefore, it is evident that in order to make microalgae a commercially viable source of biodiesel, many economical constraints need to be overcome.
Currently, there many companies focused on the development photobioreactors or bioengineering of microalgae for the commercial production on biodiesel. Table 3 briefly describes only few of the many companies currently working with in this area
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Table 3. Selected companies involved in R&D and commercialization of large-scale microbal biomass production for biofuels (Websites visited in November 2012) Company | Description | Country | Website | Astaxa | Microalgal biotechnology company employing closed photobioreactors to produce industrial scale algal biomass | Garmany | www.algae-biotech.com/ | Subitec | Employs proprietary photobioreactors to produce microalgal biomass at industrial scale | Germany | www.en.subitec.com/ | Synthetic Genomics | In partnership with ExxonMobil, Synthetic Genomics employs genomics technologies to develop superior strains of algae for biofuel production | USA | www.syntheticgenomics.com | Diversified Energy Corporation | Develops and commercializes algal biomass production system | USA | www.diversified-energy.com/ | Evodos | Develops microalgal harvesting system | Netherlands | www.evodos.eu | Bodega Algae | Develops scalable algae photobioreactors | USA | www.bodegaalgae.com/ |

Conclusion
Global warming, increasing energy demands and depleting fossil fuels have prompted the development and use of biofuels. The most widely used biofuels, bioethanol, is generated from corn or sugarcane and biodiesel is produced from vegetable oil or animal fats. As both bioethanol and biodiesel are produced from limited agricultural resources, they compete for land and resources that could have otherwise be used for human consumption. Thus, the production of biodiesel from microalgae is an attractive alternative because it provides a renewable source of fuel and helps to mitigate environmental and social issues surrounding first generation biofuels.
However, several technical barriers need to be overcome before microalgae can be used as an economically viable biofuel feedstock. Currently challenges remain in scaling up the production of microalgae biomass. In addition, the substantially high cost of downstream processing of the lipid extraction and recovery is a major hurdle. Evidently, each stage in the value chain for microalgal biofuel production must be improved in order to increase the economic viability of commercial microalgae biodiesel. Currently, the lipid extraction process is the main bottleneck in the microalgal biodiesel process. Accordingly, new technologies and or bioengineered microalgae must be developed so that the lipids within the cell are released in an efficient and economical way.
With a concerted effort from the academia as well as the industry to effectively employ methods of systems biology, genetic engineering and biorefining to overcome current challenges, it is conceivable that microalgal biodiesel can be produced in a sustainable and economical way within the next 20-25 years.