Current trends in energy supply and use are unsustainable―economically, environmentally and socially. Without decisive action, energy related to greenhouse gas emissions will be more than double by 2050 and the growing demand for oil will increase concerns about security of supply. We can and must change the way we are today. The energy-carbon technologies will play a crucial role in the energy revolution needed to make this change happen [1] .

Biofuels marketed today are bioethanol and biodiesel. There is a wide variety of conventional conversion technology and advanced biofuels. The processes of conventional biofuels, despite already commercially available continue to improve in efficiency and economy. The advanced conversion routes are moving into the demonstration phase or are already there.

Biodiesel, a renewable source of biofuel derived for use in internal combustion engines with compression ignition, contributes to the reduction in global temperature of the planet and the health costs, provides employment and income, is fully miscible with mineral diesel oil increases Fuel lubricity improving engine performance, it is biodegradable and non-toxic [2] .

The raw material for biodiesel production can be divided into three categories: vegetable oils, animal fats and oils and waste fats. All vegetable oils and animal fats, waste or not, are triglycerides of fatty acids which can be processed into biodiesel. Form major sources used today, such as soybeans, have low productivity per area planted oil well that are used for food and feed [3] . That is why, as a promising source of lipids for biodiesel production chain, microalgae arise as a biomass having a high fat content (20% - 50%), this being obtained naturally from the carbon dioxide conversion atmospheric when using photosynthesis and helping with that, the removal of CO₂ from the atmosphere [4] .

The use of algae is of great importance also for the production of high value-added products (spirulina, astaxanthin, carotenoids, etc.). Despite the great interest that the use of algae for biofuel and byproducts has aroused, the problems are still viewed as important. The potential recognized: high oil yield (60%), rapid growth, land use and low cost opportunity resources do not compete with food and CO₂ capture and use. Therefore, investments have been significant, however, is even better understanding of the necessary basic principles for the commercial scale-up viable, fundamental R & D and applied even seem necessary. Within the main themes of research and development are the growing costs, harvesting and oil extraction, recycling of water and nutrients, as well as the added value of co-products.

Many authors admit that the cultivation of algae for biofuel industry cannot be profitable in itself. The use of co-products with high added value, such as “nutraceuticals”, fertilizer and energy production from waste biomass of algae is required.

No doubt the concept of biorefinery should be applied, since the oils represent only one of many products with commercial value. An integrated system requires not only that the algal biomass directly feeds the biorefinery, but also the direct use of waste and exhaust gases (e.g., synthesis gas, methane, heat, carbon dioxide, sewage, etc.) or the biorefinery power plant in the new algae production systems.

In the quest for viability of microalgae production process does need to address the concept of biorefinery where the benefits take advantages and the high value-added products can be exploited.

In the past, little attention was dedicated to the development of techniques for crops to maximize the value of the products obtained from microalgae (biorefining). The oils represent only one of many products with commercial value, and the choice of the body to grow requires the evaluation of all the materials potentially produced.

Depending on the type of algae chosen and the growing conditions, marine biomass is able to accumulate significant amounts of bio-based products (proteins. oils, carbohydrates, starch, vitamins); diatoms often accumulate oils, green algae: starch and oils, and the golden algae: oils and carbohydrates [5] .

They can also give compounds included in the field of fine chemicals such as PUFAs, pigments, sugars and antioxidants [6] . Its biochemical composition can be modulated by different growth conditions, are induced to produce high concentrations of great commercial importance components and the yield of oil can be significantly improved [7] .

In case of micro-algae systems, energy production has to be combined with for example waste water treatment and/or the co-production of value-added products (food, feed, materials, chemicals) to make the overall cultivation/processing concept economically feasible [8] .

After oil extraction, the remaining biomass can be processed for the production of bioethanol, used as animal feed and fertilizer, it is a source of nitrogen and phosphorus or simply burned to generate power [9] .

The technical potential of marine crops for greenhouse gas abatement has been recognized for many years, given their ability to use CO₂, and the possibility of achieving higher productivities than terrestrial crops. Biofuel production from these marine crops, including overall sustainability aspects, is now an Increasing topic of discussion. Microalgal biomass production can perform waste CO₂ biofixation (1 kg dry algal biomass using about 1.83 kg of CO₂) [7] ; therefore the CO₂ emitted in the burning biodiesel can be reabsorbed by plants and microalgae [9] .

The biomass production potentially could significantly be increased by the application of high-efficient fully integrated aquatic biomass cultivation-processing systems, i.e. “Marine Biorefineries” (Figure 1) [5] .

An integrated system requires not only that the algal biomass directly feeds the biorefinery, but also the direct use of waste and exhaust gases (e.g., synthesis gas, methane, heat, carbon dioxide, sewage, etc.) or the biorefinery power plant in our new algae production systems.

The general idea of the concept of Biorefinery Aquatic (applicable to microalgae) is the algal biomass processing for value-added products and by-products, heat and power. The use of aquatic biomass offers the possibility of a significant increase in the availability of domestic biomass to bio-refineries processing, it should be considered particularly for regions with limited biomass supply [10] .

Another study demonstrated economic viability refers to the microalgae production process for electricity generation, which is considered as a joint activity with ethanol in order to meet the expected increase in power demand. The production of this alternative energy source in conjunction with an ethanol plant maximizes both gains and brings benefits to the environment, with the microalgae capturing the carbon dioxide emitted by the ethanol plant. This process occurs through the burning of biomass of these microorganisms for each ton of biomass burned are generated 8.12 MWh. That would be a sustainable alternative to increase the supply of electric power in Brazil, for the burning of algal biomass does not release more CO₂ than was consumed in production, so it is a power “zero emission”, this also happens with the water from washing cane system [11] . This would certainly reduce the microalgae production costs for the generation of bioelectricity. On the other hand there is also the possibility of selling the O₂ released in photosynthesis process as additional revenue. It is estimated that for every ton of CO₂ absorbed release at approximately 500 kg of O₂.

According [12] for the use of biomass in the preparation of food and extraction of substances of interest it is necessary to separate the biomass from the culture medium, which involves solid phases: liquid as flocculation, centrifugation and filtration. Then the biomass can be dried by techniques such as sun drying, in a “spray drying” or lyophilization. The compounds of microalgae cells are extracted by using cell disruption methods homogenization, ultrasound, osmotic shock, solvents, enzymes, among others. From this point, substances of interest can be recovered by purification processes (ultrafiltration, chromatography or fractionation).

At present, it has thought much in water reuse alternative in industrial processes as well as reuse of wastewater and sewage and other effluents from other industries. In Figure 2 it is shown how the water can be reused to take advantage of the nutrients contained in it and reduce water consumption [36] .

Clearly for the production of biodiesel would be needed to optimize the cheaper cost system, with consequent cost savings in all areas (water use, nutrient, energy, infrastructure, etc.), an alternative that is being thought is the use of industrial or domestic wastewater (as a source of nutrients) and the own infrastructure (or additions) of an Effluent Treatment Plant (WWTP) with stabilization pond system, obtaining microalgae biomass [37] .

Microalgae as a way to minimize the costs of the final product (microalgae oil) can be obtained directly from a system where they are already naturally available due to providing the optimal environmental conditions for their development. Microalgae fed by effluent, in a stabilization pond system, may be used to obtain a cheaper oil when compared to oil obtained in culture systems [38] . This will lead to a saving of water and nutrients, as well as serve for the removal of effluent nutrients such as nitrogen and phosphorus. Aquaculture systems involving the production of microalgae and the waste water treatment (for example, amino acids, enzymes or food industry waste waters) seem to be very promising for the cultivation of microalgae combined with the biological purification [39] .

In studies on the potential of microalgae assessment of waste as an alternative to the biodiesel production chain it has concluded that the lipid content obtained (around 16%) from the microalgae of facultative pond resembles the levels obtained from microalgae biomass cultured in conventional processes, compared to literary data, having, therefore, potential for use in the production of biodiesel. Obtaining microalgae biomass directly from the facultative pond, shows up as an alternative to processes for the production of algal biomass traditionally used (fotobioreator and aerated tanks), as well as recover the microalgae biomass failing to launch it in the stream, and it can have an economic and environmental gain while the biodiesel production could minimize the environmental impacts of the high presence of this material in environment [39] .

The Spanish University of Malaga developed another interesting study in conjunction with the School of

Chemistry of UFRJ, which shows some results obtained with the cultivation of microalgae Scenedesmus dimor- phus using a source of swine wastewater. When the culture is fed with waste water introduced is more concentrated and more intense green color. When observed under a microscope, the colonies are significantly larger along the growing period. He has appeared in the residual medium, which growth continues for longer leading to a higher biomass concentration [10] .

The removal of nutrients in the crops Scenedesmus dimorphus using waste water was determined by comparing the concentrations of nitrates and nitrites, ammonia and phosphate in the culture medium at the beginning and end of the cultures. The results showed an almost complete absorption of these nutrients during the cultivation period. The absorption of phosphate was 89.1%, while the absorption of nitrogenous compounds and ammonia was above 99% (Table 2) [10] .

Given the possibility that the waste water can be obtained without additional costs, there is an opportunity to involve the growth of some species the treatment of swine waste water thus achieving: on the one hand, achieving a low cost biomass with significant lipid content, and on the other hand, treatment of these waters which are rich in nutrients such as phosphates and nitrates, which in the case of discharge to water bodies contribute to pollution and eutrophication of these ecosystems.

For future studies should seek a balance between nutrient removal in waste water and the production of lipids, aiming to maximize this last aspect [10] . The amounts of lipids and other components of Scenedesmus dimorphus biomass are shown in Table 3.

Currently, the level of CO₂ in the atmosphere is the cause of climate change that governments wish to control. Withdrawal mechanisms of CO₂ through reuse is an alternative offered by bio-fixation process for microalgae [40] .

Industrial activity and power generation in thermal cast into the atmosphere each year, about 20 billion tons of CO₂. Among the various alternatives to capture and use of CO₂ is the use of microalgae [41] .

To reduce the concentration of atmospheric CO₂ exists as a possibility to reduce emissions or absorption of the CO₂ produced in excess, called carbon sequestration. Some measures to reduce current levels of CO2 in the atmosphere have been proposed in the Kyoto Protocol in 1997, which states that developed countries would have to reduce by 5% the emission of gases that cause the “greenhouse effect” by 2012 [42] .

Among all photosynthetic and autotrophic organisms capable of removing CO₂ from gas streams, microalgae are the group that support up to 50% CO₂ and the presence of other compounds such as NO and SO₂ [41] [43] .

In this context, the microalgae are highlighted in reducing concentrations of greenhouse gases due to the photosynthetic capacity and its applicability in systems engineering in the solar energy leads to CO₂ fixation converting the thermal energy-rich biomass into energy with added value-added generating chemicals and food [44] .

It is estimated that carbon fixation by microalgae crop in top condition would surroundings of 11 - 36 ton. C ha/year, representing a yield of approximately three to thirteen times the values reported for the reforestation (3 - 4 ton C ha/year) [42] . Studies on the removal of CO₂ and NO generated during combustion of coal microalgae have reported removing maximum values of 22.97% CO₂, 27.13% NO and which can reduce costs of the nutrient culture medium and problems generated by issuing gases [43] .

A viable strategy for carbon sequestration based on photosynthetic microalgae as conceptually shown in Figure 3 is possible, where the CO₂ generated in the combustion of fossil fuels and nutrients are added to a photobioreactor in which the micro-algae use sunlight to convert photosynthetic CO₂ in high-value compounds. The advantages of using a system based on microalgae are [45] :

• High purity CO₂ gas is not necessary for the cultivation of algae. Combustion gases containing variable quantities of CO₂ can be fed directly to culture microalgae. This will simplify the separation of CO₂ from flue gas significantly.

• Some combustion products such as NOx or SOx can be effectively used as nutrients for microalgae. This could simplify flue gas cleaning the combustion system.

• The planned process is a renewable cycle with minimal negative impacts on environment.

A CO₂ recovery model from gas chimneys to redirect the microalgae cultivation tanks was developed by [46] , and the basic design based on typical thermal emissions, with potential 50e of 500 MW, and recovery efficiency of 86.60% per year. The estimated operating cost of the process, including the extraction of monoethanolamine (MEA), compression of 1500 psi, dehydration, transportation to the tanks (assuming distance of 100 Km, supplying approximately 100% of the CO₂ recovered the tanks, was estimated at $40 50 and $57.10 per ton of CO₂ redirected to the plants 500 MW and 50 respectively. The model also assessed the efficacy of direct use of the stack gas, however, this option had 40% more expensive costs than MEA process which has been used as a technology for CO₂ recovery with lower gas pressures. The model also concludes that the transport step equals approximately 8% of the operating costs of the MEA process and can reach 20% of the costs for direct pumping process from chimneys. These data demonstrate the advantages of the decentralized production and microalgae complex located adjacent to CO₂ emissions units, impacting directly on reducing process costs.

Another model is a process that recovers high purity CO₂ from flue gases using membranes. A stream of CO₂ and O₂ is emitted as a product, and O₂ can be recycled to enrich the gas stream entering the membrane concentrator. Preliminary economic studies are in favor, obtaining an operating cost of $21.00 per ton of CO₂ recovered [47] . This cost estimate is close to that obtained for a more recent model, which estimates an operating cost of $16.00 to $22.00 per ton of CO₂ recovered [48] .

However operating costs estimated for these models, available technology at the time, has been overcome by the use of new technologies cheaper, and some authors have consider that this process is available at low or no

cost [4] , however, it is necessary to analyze various parameters that significantly affect this estimate. Currently it can be assumed the operating cost of US $1.00 per tonne of CO₂ recovered, including the steps of collecting and forwarding to the CO₂ tanks, with a 99% efficiency in the recovery of CO₂. Implementation costs of these processes can fluctuate according to the sizing of equipment such as compressors and tubing, which are designed according to the characteristics of emissions from each chimney [49] .

Another study of microalgae Spirulina sp. and Scenedesmus obliquus with gas combustion of coal-fired Thermal Power Plant of President Medici (UTPM) operated by CGTEE (Thermal Generation Company of Electric Power), installed in southern Brazil. It demonstrated that the use of flue gas from the UTPM increased by 35.0% the production of biomass at the end of the growing Spirulina sp. a reduction of 24.2% of the CO₂ concentration of the combustion gas being biofixado 5.7% CO₂ for growth of microalgae. The final biomass of microalgae S. obliquus and Spirulina cultivated with flue gas had 6.2 and 4.8% lipids, and 40.6 and 46.8% protein, respectively. The results show that micro-algae can be grown in power plants for biofixar CO₂ from coal combustion gas and contribute to reducing the global warming [41] .

Concern for the environment has led the countries of the United Nations to sign an agreement stipulating control over human interventions in the climate. This agreement was born in December 1999 with the signing of the Kyoto Protocol. Thus, it was determined that signatory developed countries to reduce their emissions of green- house gases by 5.2% on average for the year 1990 between 2008 and 2012. This period was also known as the first commitment period. Not to compromise the economies of these countries, the protocol established that part of this reduction could be made through negotiation with nations through the flexibility mechanisms (en.wikipedia.org).

One of the flexibility mechanisms is the Clean Development Mechanism (CDM). The CDM carbon credits is called Certified Emission Reduction (CER)-or in English, Certified Emission Reductions (CER). One CER corresponds to one tonne of carbon dioxide equivalent. The carbon credits or Certified Emission Reduction (CER) certificates are issued to a person or company that has reduced its emissions of greenhouse gases (GHG).

Each ton of CO₂ that is absorbed (carbon sequestration) or the amount of greenhouse gases that are no longer produced by the company classified as pollution is converted into a carbon credit unit, which is traded in dollars on the world market. Reducing the emission of other gases, also generators of the greenhouse effect can also be converted into carbon credits, using the concept of Carbon Equivalent.

Buy carbon credits in the market is about to buy a permit to emit greenhouse gases. The price of this permission, traded on the market, must necessarily be lower than the fine that the issuer must pay to the government, to issue GHG. For the sender, so buy carbon credits in the market means, in practice, get a discount on the fine due. International agreements such as the Kyoto Protocol determine a maximum quota of greenhouse gases that developed countries can emit. The countries, in turn, create laws restricting GHG emissions. Thus, those countries or industries that fail to meet emissions reduction targets, become carbon credit buyers. On the other hand, those industries that have managed to reduce their emissions below certain dimensions, can sell at market prices, the surplus of “emission reduction” or “emission allowance” in the national or international market [50] . The carbon credit market is in full development, mainly because of so-called voluntary market. In it, even countries that do not need to lower their emissions or that have not signed the Kyoto Protocol are able to trade credits.

This paper, like the title suggest, has presented a brief overview about biodiesel from microalgae focused in biorefinery concept. It has been exposed some alternatives that can help to reduce the high cost of biodiesel production based in microalgae, turn it more feasible. It has become clear that when it manages to integrate the production of biodiesel with the other value-added products and integrating with other pollutants and industries with large wastewater emissions, could bring significant economic benefits.

We would like to acknowledge the valuable contributions of GranValle Biodiesel Factory, who gave the idea and the support to develop this review and to the financial agency (FAPERJ) for the support given.

Lauren Espinosa González,Gisel Chenard Díaz,Donato Alexandre Gomes Aranda,Yordanka Reyes Cruz,Mariana Monteiro Fortes, (2015) Biodiesel Production Based in Microalgae: A Biorefinery Approach. Natural Science,07,358-369. doi: 10.4236/ns.2015.77039