INDONESIAN SCHOLARS JOURNAL –2014-ISJ-003-00049

Microalgae Oil for Biodiesel Production as Renewable Energy Resource – A Review

1Nyayu Aisyah, 1Ade Clara Pretty Sundari &1Khusnul Khotimah,

2Erwana Dewi2K.A Ridwan

Chemical Engineering Department, State Polytechnic of Sriwijaya, Palembang, South Sumatera, IndonesiaEmail: 1aisyahnyayu6@gmail.com, 1adeclaraalexander@gmail.com, 1khusnul486@gmail.com

Abstract. Nowadays biodiesel, as an alternative fuel, is attracting increasing attention. Microalgae appear as a solution for this crucial problem. As renewable energy resource, microalgae capable of meeting the global demand for transport fuels. Like plants, microalgae use sunlight, carbon dioxide and several nutrients to produce oils but they do so more efficient than crop plants. Oil productivity of microalgae greatly exceeds the oil productivity of the best producing oil crops. Due to their high biomass productivity, rapid lipid accumulation, and ability to survive in saline water, microalgae have been identified as promising feedstocks for industrial-scale production of carbon-neutral biodiesel. This paper reviews the reasons for choosing microalgae as renewable energy resources and the process of biodiesel production, including growth rate and productivity.

Keywords: Microalgae, renewable resource, biodiesel, cultivation, harvesting, extraction, transesterefication

1. INTRODUCTION

The idea of using microalgae as a source of biodiesel is not new, but it is now being taken seriously because of the rising price of petroleum and more significantly, the emerging concern about global warming that is associated with burning of fossil fuels1]. The utilization of microalgae for biofuels production offers the following advantages over higher plants: (1) microalgae synthesize and accumulate large quantities of neutral lipids (20–50% dry weight of biomass) and grow at high rates; (2) microalgae are capable of all year round production, therefore, oil yield per area of microalgae cultures could greatly exceed the yield of best oilseed crops; (3) microalgae need less water than terrestrial crops therefore reducing the load on freshwater sources; (4) microalgae cultivation does not require herbicides or pesticides application; (5) microalgae sequester CO2 from flue gases emitted from fossil fuel-fired power plants and other sources, thereby reducing emissions of a major greenhouse gas (1 kg of dry algal biomass utilize about 1.83 kg of CO2); (6) microalgae could be applied as bioremediation agent of wastewater by removal of NH4

+ , NO3-, PO4

3- from a variety of wastewater sources (e.g. agricultural run-off, concentrated animal feed operations, and industrial and municipal wastewaters); (7) combined with their ability to grow under harsher conditions and their reduced needs for nutrients, microalgae can be cultivated in saline/brackish water/coastal seawater on non-arable land, and do not compete for resources with conventional agriculture; (8) depending on the microalgae species, other compounds may also be extracted, with valuable applications in different industrial sectors, including a large range of fine chemicals and bulk products, such as polyunsaturated fatty acids, natural dyes, polysaccharides, pigments, antioxidants, high-value bioactive compounds, and proteins 2].

One of the characters that form the basis in selecting microalgae as biodiesel feedstock is because microalgae can growth in extreme environments3]. There are several types of microalgae that have been known to have high lipid content, such as Botryococcus braunii, Chlorella sp., Schizochitrium sp., Nannochloropsis sp. However, Indonesian microalgae has not been explored and studied intensively as feedstock biodiesel makers.

Table 1. Oil content of some microalgae4]

No MicroalgaeOil content

(% dry weight)

1 Botryococcus braunii 25-752 Chlorella sp. 28-323 Crypthecodinium cohnii 204 Cylindrotheca sp. 16-375 Dunaliella primolecta 236 Isochrysis sp. 25-337 Monallanthus salina >208 Nannochloropsis sp. 31-689 Neochloris oleoabundans 45-4710 Nitzschia sp. 45-4711 Phaeodactylum tricornutum 20-3012 Tetraselmis sueica 15023

Microalgae are present in all existing earth ecosystems, not just aquatic but also terrestrial, representing a big variety of species living in a wide range of environmental conditions. It is estimated that more than 50,000 species exist, but only a limited number, of around 30,000, have been studied and analyzed5].

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During the past decades extensive collections of microalgae have been created by researchers in different countries.

2. RESULT AND DISCUSSION

What is biodiesel?Biodiesel (monoalkyl esters) is one of alternative biofuel,

which is obtained by the transesterification of triglyceride oil with monohydric alcohols. Biodiesel is biodegradable and non toxic (less COx and NOx emissions) alternative fuel from renewable energy sources, such vegetable oils or animal fats. Since vegetable oils may also be used for human consumption, it can make the price of food-grade oils increasing. So we need the raw material or feedstock which is not compete with vegetable oils.

Studies have shown that some species of algae can produce 60% or more of their dry weight in the form of oil. This oil can then be turned into biodiesel which could be sold for use in automobiles. Regional production of microalgae and processing into biofuel will provide economic benefits to rural communities.

Why microalgae can be used as feedstock for biodiesel?Microalgae have also been identified as attractive sources

of biodiesel because different species can produce a variety of fuel products. Various microalgae species have the ability to produce large quantities of lipid while sequestering CO2, particularly neutral lipids in the form of triacylglycerol (TAG), which can be converted to fatty acid methyl esters (FAMEs), the main components of biodiesel6], through trans-esterification, or refined into other fuel constituents7]. Total lipids and other biomass constituents can be converted into crude oil alternatives through thermo-chemical processes such as hydrothermal liquefaction (Barreiro et al. 2013). Microalgae carbohydrates can be fermented into ethanol, and some species can produce bio-hydrogen8]. In addition to their diversity of products, microalgae are attractive as fuel sources because many species grow relatively fast.

Microalgae are prokaryotic or eukaryotic photosynthetic microorganisms that can grow rapidly and live in harsh conditions due to their unicellular or simple multicellular structure9]. They reproduce themselves using photosynthesis to convert sun energy into chemical energy, completing an entire growth cycle every few days. Moreover they can grow almost anywhere, requiring sunlight and some simple nutrients, although the growth rates can be accelerated by the addition of specific nutrients and sufficient aeration10].

Different microalgae species can be adapted to live in a variety of environmental conditions. Thus, it is possible to find species best suited to local environments or specific growth characteristics, which is not possible to do with other current

biodiesel feedstocks (e.g. soybean, rapeseed, sunflower and palm oil). They have much higher growth rates and productivity when compared to conventional forestry, agricultural crops, and other aquatic plants, requiring much less land area than other biodiesel feedstocks of agricultural origin, up to 49 or 132 times less when compared to rapeseed or soybean crops, for a 30% (w/w) of oil content in algae biomass.

How microalgae produce biodiesel?Microalgae need a light source, carbon dioxide, water

and inorganic salts to grow. The temperature should be between 15 and 30oC for optimal growth. For making biofuel there are several steps that we must do (Cultivation, harvesting, extraction and transesterification).

The first step is cultivation. There are two types of microalgae cultivation, open system and enclosed system. Each system has its own advantages and disadvantages. In this paper we concern on enclosed system. Photobioreactor are the example of enclosed system. Photobioreactor can overcome the problems of contamination and evaporation encountered in open ponds6]. The biomass productivity of photobioreactors can be 13 times more than the traditional raceway pond on average10]. Photobioreactors are often turbular to allow for a greater amount of light penetration. A tubular photobioreactor consists of an array of straight transparent tubes that are usually made of plastic or glass. This tubular array, or the solar collector, is where the sunlight is captured (Fig. 1). The solar collector tubes are generally 0.1 m or less in diameter. Tube diameter is limited because light does not penetrate too deeply in the dense culture broth that is necessary for ensuring a high \biomass productivity of the photobioreactor. Microalgae broth is circulated from a reservoir (i.e. the degassing column in Fig. 1) to the solar collector and back to the reservoir. Continuous culture operation is used, as explained above.

Fig 1. Continuous culture operation of the degassing column in solar collector

The second step is harvesting. Microalgae harvesting consists of biomass recovery from the culture medium that may contribute to 20–30% of the total biomass production cost. In order to remove large quantities of water and process large algal biomass volumes, a suitable harvesting method may involve one or more steps and be achieved in several physical, chemical, or biological ways. Most common harvesting methods include sedimentation, centrifugation,

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filtration, ultra-filtration, sometimes with an additional flocculation step or with a combination of flocculation–flotation.

Turning wet algal biomass into combustible fuel has proven challenging. After harvesting the algae, the biomass is typically processed in a series of steps, which can differ based on the species and desired product; this is an active area of research. Often, the algae is dehydrated and then a solvent such as hexane is used to extract energy-rich compounds like triglycerides from the dried material. Then, the extracted compounds can be processed into fuel using standard industrial procedures. For example, the extracted triglycerides are reacted with methanol to create biodiesel via transesterification. The unique composition of fatty acids of each species influences the quality of the resulting biodiesel and thus must be taken into account when selecting algal species for feedstock.

Coagulation is the process of chemically changing colloids so that they are able to form bigger particles by coming close to one another11]. It involves the formation of chemical flocs that absorb, entrap, or otherwise bring together suspended matter that are colloidal12]. The aggregation process of these particles to form flocs is described as colloidal destabilization. Chemical flocculants like alum and ferric chloride are used to harvest microalgae, but it is often too expensive for large operations (Hung et al., 2010).

Other options, such as autoflocculation, rely on increased pH either by CO2 consumption through photosynthesis or the addition of alkali resulting in excess calcium and phosphate ions. In the presence of excess calcium ions, the calcium phosphate precipitate is positively charged and therefore adsorbed on the negatively charged algal cells agglomerating them and promoting algal flocculation13]. Calcium and phosphate are limiting reagents in autoflocculation and generally become coagulant additions to induce floc formation. To attain flocs within the pH range of 8.5-9.0, the culture should contain 0.1-0.2 mM orthophosphate and between 1.0-2.5 mM calcium14]. A combination of methods can also be used, e.g. a pre-concentration with a mechanical dewatering step such as coagulation/flocculation, a post- concentration by means of a screw centrifuge or a thermal drying. After separation from the culture medium algal biomass (5–15% dry weight) must be quickly processed lest it should get spoiled in only a few hours in a hot climate.

The final step is transesterification. This is a multiple step reaction, including three reversible steps in series, where triglycerides are converted to diglycerides, then diglycerides are converted to monoglycerides, and monoglycerides are then converted to esters (biodiesel) and glycerol (by-product). The overall transesterification reaction is described in Fig. 2 where the radicals R1, R2, R3 represent long chain hydrocarbons, known as fatty acids.

For the transesterification reaction oil or fat and a short chain alcohol (usually methanol) are used as reagents in the presence of a catalyst (usually NaOH). Although the alcohol : oil theoretical molar ratio is 3:1, the molar ratio of 6:1 is generally used to complete the reaction accurately. The relationship between the feedstock mass input and biodiesel mass output is about 1:1, which means that theoretically, 1 kg of oil results in about 1 kg of biodiesel.

A homogeneous or heterogeneous, acid or basic catalyst can be used to enhance the transesterification reaction rate, although for some processes using supercritical fluids (methanol or ethanol) it may not be necessary to use a catalyst.

Fig.2. Transesterification of triglycerides

Most common industrial processes use homogeneous alkali catalysts (e.g. NaOH or KOH) in a stirred reactor operating in batch mode.

Recently some improvements were proposed for this process, in particular to be able to operate in continuous mode with reduced reaction time, such as reactors with improved mixing, microwave assisted reaction, cavitation reactors and ultrasonic reactors.

Another form of biofuels that can be produced from microalgae

Butanol can be made from algae or diatoms using only a solar powered biorefinery. This fuel has an energy density 10% less than gasoline, and greater than that of either ethanol or methanol. In most gasoline engines, butanol can be used in place of gasoline with no modifications. In several tests, butanol consumption is similar to that of gasoline, and when blended with gasoline, provides better performance and corrosion resistance than that of ethanol.

Biogasoline is gasoline produced from biomass. Like traditionally produced gasoline, it contains between 6 (hexane) and 12 (dodecane) carbon atoms per molecule and can be used in internal-combustion engines.

Methane, the main constituent of natural gas can be produced from algae in various methods, namely Gasification, Pyrolysis and Anaerobic Digestion. In Gasification and Pyrolysis methods methane is extracted under high 22temperature and pressure. Anaerobic Digestion is a straight forward method involved in decomposition of algae into

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simple components then transforming it into fatty acids using microbes like acidific bacteria followed by removing any solid particles and finally adding methanogenic bacteria to release a gas mixture containing methane. A number of studies have successfully shown that biomass from microalgae can be converted into biogas via anaerobic digestion. Therefore, in order to improve the overall energy balance of microalgae cultivation operations, it has been proposed to recover the energy contained in waste biomass via anaerobic digestion to methane for generating electricity.

3. CONCLUSIONAlgae Fuel or algae biofuel is an alternative to fossil fuel that uses microalgae as sources of natural deposite. The crisis of energy has ignited interest in developing and processing microalgae as feedstock for making biodiesel and other biofuels. This is due to many factors, including their high lipid content, the ability to grow on non arable land and/or salt water, their fast rate of growth, and the fact that they do not compete with food crops. Microalgae are one of nature’s smallest gifts that we believe hold the key to unlock an endless word of possibilities in the fuel oil resources today.

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