INTRODUCTION Biomass can be converted to energy by biological or thermochemical methods. - direct combustion - indirect processes Microalgae - most promising renewable feedstock for biofuel production and biorefineries. Choice of biomass depends on: 1. Social 2. Environmental 3.Industrial factors
Microalgae (3 rd generation feedstock) i) does not compete for arable land and portable water. ii) reduce freshwater consumption. iii) low lignin content (high fermentable sugar). iv) their saccharification is much easier. v) being a more promising and sustainable biomass sources for bioethanol production (Harun et al., 2010; Ho et al., 2012).
C. vulgaris (CCAP 211/11B) C. vulgaris (P12) C. reinhardtii UTEX 90 C. reinhardtii Tetraselmis subcordiformis S. obliquus CNW-N C. vulgaris C. vulgaris FSP-E List of microalgaes
C. vulgaris FSP-E can achieve the highest carbohydrates productivity Information concerning the composition of carbohydrates produced by microalgae is also crucial for future applications of microalgae feedstock. Comparison of the carbohydrate production performance of different microalgae
Microalgae strainsBiomass productivity (g/L/d) Carbohydrate productivity (g/L/d) Reference C. vulgaris (CCAP 211/11B) 0.0370.021A.M.IIIMAN et al., 2000 C. vulgaris (P12)0.4850.199G. Dragone et al., 2011 C. reinhardtii UTEX 90 0.5070.304M.T. Nguyen et al., 2009 C. reinhardtii0.4840.257M.S. Kim et al., 2006 Tetraselmis subcordiformis N.D0.255Y. Zheng et al., 2011 S. obliquus CNW-N0.8210.383S.H. Ho et al.,2012 C. vulgaris0.2540.112Y.N. Liang et al., 2009 C. vulgaris FSP-E1.3630.687S.H. Ho et al., 2012 Comparison of biomass productivity and carbohydrate productivity of microalgae strains
Types of biofuels from microalgae 1)Biodiesel After extraction process, the resulting microalgal oil can be converted into biodiesel through a process called transesterification. Transesterification – reaction consist of transforming triglycerides into fatty acid alkaly esters in the presence of an alcohol, catalyst and glycerol as a by product (Vasudevan, 2008). Microalgal oil contains a high degree of polyunsaturated fatty acid compared to vegetable oils, which makes it susceptible to oxidation in storage and therefore reduces its acceptibility for use in biodiesel ( Dragone et al., 2010).
2) Bioethanol Current interest in producing bioethanol are focusing on microalgae as a feedstock for fermentation process. Microalgae provide carbohydrates and protein as a carbon sources for fermentation. Result showed a maximum bioethanol concentration of 3.83g/L obtained from 10g/L of lipid-extracted microalgae debris (Harun et al., 2010). Fermentation process requires less consumption of energy and simplified process compared to biodiesel production system. Carbon dioxide produced as by-product from fermentation process can be recycled as carbon sources to microalgae in cultivation process.
3) Biogas Produced naturally from break down of organic material. Microalgae can form biogas because of its high sugar content. Biogas can be burned directly in an engine or a cooker or upgraded for use in car engines or for generating electricity. Methane gas is the main ingredient of biogas which can be used as fuel. An anaerobe micro-organisms is used to breakdown the microalgae without oxygen.
Current Technologies 1)Cultivation System Open air System The classical open-air cultivation system comprise lakes and natural ponds, circular ponds, raceway ponds and inclined system (Dragone et al., 2010). These system are easier and less expensive to build, operate more durably and have a larger production capacity compare to closed system. They can utilize sunlight and nutrients can be provided through runoff water from nearby land area (Carlsson et al., 2007).
Generally ponds are susceptive to weather conditions, not allowing control of water temperature, evaporation and lighting, which make these system dependent on the prevailing regional climate conditions. Natural and artificial ponds i) only viable when a series of conditions are met. ii)the existence of favorable climatic conditions and sufficient nutrients in order to grow the microalgae. Inclined system i)the only open-air system which achieves high sustainable cell densities (up to 10 g/L). ii)very well suitable for algae such as Chlorella, which can tolerate repeated pumping ( Borowitzka, 2005).
iii) Turbulence is created by gravity cause the culture suspension flowing from the top to the bottom of a sloping surface. Raceway ponds i) Typically made of a closed loop, oval shaped recirculation channels, with mixing and circulation required to stabilize algae growth and productivity. ii) Algae broth and nutrients are introduced in front of the paddlewheel and circulated through the loop to the harvest extraction point. iii) The paddlewheel is in continuous operation to prevent sedimentation.
2) Photobioreactors (PBRs) reduced contamination risk, no carbon dioxide losses, reproducible cultivation conditions, controllable hydrodynamic and temperature, and flexible technical design (Pulz, 2001). Tubular photobioreactor a) can be horizontal, vertical, incline and conical shaped (Molina et al., 2001). b) microalgae can be circulated through the tubes by a pump, or preferably with airlift technology. c) relatively cheap, have a large illumination surface area and have fairly good biomass productivities. Flat photobioreactors a) a thin layer of a very dense culture is mixed or flown across a flat transparent panel, which allows radiation absorbance in the first few millimeter thickness.
b) Suitable for mass cultures of microalgae due to the low accumulation of dissolved oxygen and high photosynthesis efficiency (Brennan et al., 2010). Column photobioreactor a) Column PBRs are occasionally stirred tank reactor ( Sodczuk et al., 2006), but more often bubble column (Chini et al., 2006), or airlifts (Krichnavaruk et al., 2007). b) Offer the most efficient mixing, highest volumetric gas transfer rates and the best controllable growth conditions. c) Vertical bubble columns and airlift cylinders can attain substantially increased radial movement of fluid that is necessary for improved light–dark cycling.
Table 1 Advantages And Limitation Of Microalgae Culture System Source: Giuliano et al., 2010
UPSTREAM PROCESSING (Starch Liquefaction) Liquefaction – transforms starchy raw material into a fermentable mash
Saccharification Enzymatic Involving the use of cellulases, amylases and glucoamylases Lower equipment cost (conducted at mild conditions) Higher glucose yields without sugar-degradation products or toxic by-product which may affect fermentation Chemical Involving higher temperature, pressure, and addition of acid and alkali Short hydrolysis time required Resulting in production of inhibitors (furfural and 5- hydroxymethylfurfural) which repress biofuels production and also require costly downstream treatment of waste
SSF Simultaneous saccharification and fermentation Process of enzymatic hydrolysis occurs at the same time as fermentation in the same vessel Other available fermentation techniques: Separate hydrolysis and fermentation (SHF), but not cost effective Simultaneous saccharification and co- fermentation (SSCF), but not for this case.
Downstream process Distillation Distillation is a process of separating ethanol from mixture through vaporisation and condensation based on different volatility.
Dehydration Ethanol vapor under pressure passes through a bed of molecular sieve beads. The bead's pores are sized to allow absorption of water while excluding ethanol.
Solid state distillage can be digested for methane production to recover the remaining energy through an anaerobic digestion process. Solid state distillage can be used as Animal feed. Centrifuge is used to separate distillage from distillation of previous step into solid and liquid state. Centrifugation
Microalgae have the ability to remove toxic compounds from the wastewater. Microalgae play a major role in aerobic treatment of waste in the secondary treatment process. Microalgae cultivation system Liquid state distillage (Wastewater) that produced will be used to cultivate microalgae.
This stage is to aggregate microalgal cells from cultivation system of previous step to increase the size. Addition of flocculants neutralises or reduces the negative surface charge that carried by microalgae itself. Ferric chloride, aluminium sulphate and ferric sulphate. Self flocculation
Sedimentation Sedimentation of the flocculated algae cell to recover the microalgae biomass. This step is to prepare high concentrated algae cell for further process.
Environmental factors affecting microalgae carbohydrate production To enhance the economic feasibility of using algal carbohydrates for biofuels production, productivity needs to be improved. Carbohydrate content of microalgae could be enhanced by the use of cultivation strategies, for instance: i) Irradiance (A.Sukenik, 1991) ii) Nitrogen depletion (D’Sauza et al., 2000) iii) Temperature variation (De Oliveira et al., 1999) iv) pH shift (Khalil et al., 2010) v) Carbon dioxide supplement (Aroujo and Garcia, 2005)
Irradiance For autotrophic growth of microalgae – energy can be stored in form of carbohydrates or lipids in microalgal biomass. Configuration of microalgae cultivation should be designed to provide uniform and sufficient irradiance to the cells. Light intensity also found to affect carbohydrates accumulation in microalgae. Illumination can offer light energy that is further stored in the form of carbohydrates or lipids in the microalgal biomass Increase light intensity (30-400umol/m2s) could slightly increase the accumulation of carbohydrates(Carvalho et al., 2009). Accumulation of carbohydrates not only depends on light intensity, but also on other environmental parameter.
Nitrogen Depletion Nitrogen - an essential nutritional component for growth of microalgae (Turpin, 1991) Variety of nitrogen sources can be utilized by microalgae, while different nitrogen source may influence their biochemical composition. Microalgal strains could transform protein or peptides to lipids or carbohydrates as energy reserve component when it under nitrogen-depletion condition. There was a competition between carbohydrates synthetic and lipids because of the metabolic pathway associated with synthesis and degradation of energy-rich compound are closely linked (Ho et al., 2012; Rismani Yazdi et al., 2011; Y.Chisti 2007).
Starch biosynthesis of microalgae can directly proceed away from lipids synthesis. Degradation of starch provides metabolites for producing of acetyl-CoA, which is precursor of fatty acid synthesis (Rismani-Yazdi et al., 2011; Li et al., 2010). Decreasing starch degradation by genetic modification is necessary to block synthetic pathway of lipids (Radakovits et al., 2010).
Temperature Variation The effect of temperature on carbohydrates accumulation in microalgae is highly dependent on the microalgal strains used. No significant differences in biochemical composition under the stress of temperature variation in some microalgae species (Renaud et al., 2002). Temperature is potentially able to change the biochemical composition of microalgae.
pH Important environmental condition for metabolism of microalgae. Not only effecting the cell growth rate, but also biochemical composition of microalgae (Khalil et al., 2010). Adequate pH for carbohydrate accumulation differ based on the type of microalgal species used. For example total carbohydrate accumulation in both D.bardawil and C.ellipsoidae was reached pH 7.5 and 9.0 respectively (Khalil et al., 2010).
Carbon Dioxide supplementation It is considered to be positively related to the efficiency of photosynthesis, with the synthesis of carbohydrates as the end product. Under nitrogen starvation conditions and with an adequate supply of CO2 and light energy, the protein content in microalgae can be consumed as a nitrogen source, and the carbohydrate content may increase significantly during this process. Carbohydrate accumulation in microalgae is improved by increasing the percentage of carbon dioxide in the inlet gas (Xia and Gao, 2005; Giordano, 2001). According to Xia and Gao, increasing dissolved carbon dioxide concentration from 3 to 186umol/L in cultivation of C.pyrenoidosa and C.reinhardtii could elevate carbohydrate content from 9.30 to 21.0% and 3.19 to 7.40%(w/w) respectively.
Carbon dioxide also induces the synthesis of relevant proteins, which may influence the cell physiology. Increase in carbon dioxide concentration result in an increase in protein content, but decrease or no obvious change in carbohydrate content (Brown et al., 1997). Suitable addition of carbon dioxide is a key step to improve the autotrophic growth of microalgae cells, although it may not directly enhance carbohydrate accumulation in microalgae.
CONCLUSION Microalgae strains can accumulate over 50% carbohydrates intracellularly under appropriate cultivation condition. Microalgae is a prerequisite for developing more effective strategies to increase carbohydrates productivity. More economic and effective saccharification processes should be developed to enhance the efficiency of biofuels conversion through microalgae biomass. Large scale processes should be developed with appropriate photobioreactor design. Microalgae based biofuels producing system should be conducted to assess the commercial feasibility for biofuels production.
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