Comparison of energy production and renewable fuels method from algae with other ways of biodiesel production resources

Nowadays, the production of alternative fuels to fossil fuels is one of the most noticeable issues in dealing with environmental pollution. With the same approach, biodiesel is using to replace fossil diesel fuel. In general, this article examines four generations of biofuel production from different sources and economic comparisons of their production. In this review study of the previous reports, firstly, according to the approach of localizing alternative fuels, we discuss the introduction of microalgae's types, their reproduction process and environmental applications, production of biogas, bioethanol, and biodiesel. Afterward, in particular, the optimal method of producing biodiesel from microalgae is investigated, and eventually, a comparison has been made between biodiesel produced from microalgae and fossil diesel fuel and the ASTM standard of biodiesel. The results of numerous studies have demonstrated that due to the rapid growth of microalgae, high efficiency, and the low cost of their cultivation and production, it is possible to produce about 5000 to 15000 gallons of biodiesel from each hectare of microalgae cultivation; which makes the production of biodiesel from algae allocate a significant position compared to other sources of biodiesel production.


Introduction
Renewable energy sources provide a large part of humanity's future energy needs. One of these types of energy is alternative fuels, in other words, green fuels, and can meet the future human need for fuel for both transportation and electricity supply, and so on. Alternative fuels include four general categories of biodiesel, bioalcohols (including bioethanol and biomethanol), biogas, and liquid hydrogen. Biodiesel is a suitable fuel for diesel engines and diesel generators, etc., which arises from the reaction between the fats of plants or animals with alcohols in the presence of a catalyst [1].
Algae are the richest source of fat for biodiesel production, with a fat content of up to 60% by weight. Most importantly, the high speed of reproduction of these resources and the ease of work and the very low cost of producing fuel from these sources is one of the main reasons for turning to algae in different countries. This report will introduce microalgae and their types and describe the methods of cultivation, harvesting and production of fuel from them [1].
Microalgae are organisms with different structures in the form of one or more cells and in complex and variable forms such as rods, branches, flagella, etc., which reproduce in both sexual and asexual ways. Photosynthesis converts inorganic matter into organic matter. Microalgae live in wetlands, seas, freshwaters, mudflats, and even on rocks or as parasites on other plants. Microalgae need light, water and carbon dioxide as nutrients to grow. Microalgae are divided into three main families (brown, green and red microalgae), but there are sub-species that are divided into the following six groups depending on the breeding conditions, habitat and physical characteristics: Brown microalgae, golden microalgae, green microalgae, red microalgae, greenyellow microalgae and blue-green microalgae [2].

Applications of microalgae 2.1 Environmental applications of microalgae
Growing and multiplying in effluents, microalgae absorb heavy metals, absorb harmful bacteria with the help of phenolic acid, use carbon dioxide produced by industrial plants and machines for their growth and reproduction as feed and wastewater treatment. Among these, brown microalgae have the greatest effect on the absorption of heavy metals such as cadmium, nickel and zinc [3].

Biogas production
Due to anaerobic digestion of microalgae in digestive tanks, it is carried out by methane-producing bacteria. Due to the lack of lignin and the low amount of cellulose in the structure of microalgae, the production of biogas is done at a higher speed. The required temperature is 55-35 ° C [4].

Bioethanol production
With the help of carbohydrates and proteins in the cellular structure of microalgae and in dark conditions and fermentation tanks and with the help of anaerobic bacteria, bioethanol is produced and from each kilogram of carbohydrates and glucose in the structure of microalgae, 0.51 kg Ethanol is produced [4].

Biodiesel production
Due to rapid growth, low cost and high yields, about 5,000 to 15,000 gallons of biodiesel can be produced per hectare of microalgae cultivation per year. The following steps are generally required to produce biodiesel from microalgae. Reproduction, harvesting, dewatering, extraction and production of the final product [5]. One of the early stages of biodiesel production from microalgae is the stage of lubrication, in which lubrication of algae is done in 4 different ways (industrial and laboratory). There are several methods for extracting oil from microalgae, including:  pressure method  Solvent liquid method  Supercritical flow method  Ultrasonic method The flow chart of the biodiesel production process can be seen in Figure. 1.

Benefits of producing biodiesel from microalgae
 Rapid growth of microalgae (it takes 5 to 24 hours to double in size)  High biodiesel production  Lack of sulfur in biodiesel compared to petroleum diesel  It is non-toxic  Consumption of CO2 produced for growth and reproduction

Conclusion
Evaluation of biodiesel production from microalgae is of considerable importance and makes appropriate operational plans for the exploitation of this resource to be carried out and implemented. In this study, different types of microalgae were considered and also several methods for biodiesel production from microalgae were investigated. Studies show that the production of biodiesel from algae has a high efficiency compared to previous generations and other sources due to the high percentage of oil in their structure. Furthermore, based on the study of the advantages and disadvantages of using alternative fuels and the economic study of biofuel production from different sources, the low cost of biodiesel production from algae was determined. Therefore, these valuable resources can be used more in the production of biodiesel. The oil content of microalgae is higher than other plant-oil products and also the biodiesel produced from microalgae is in accordance with ASTM biodiesel standards. Finally, it should be noted that the production of third generation fuels does not reduce food production and also does not require arable land, fresh water and fertilizer. Highgrowth algae production for fuel production has not yet found its place on a commercial scale, however feasibility studies have been conducted to achieve high yields, and significant large-scale developments will soon take place. Detailed economic studies with regard to the current situation and resources of the country and the future and accurate comparisons of performance and costs and effects of algal and non-algal fuels in relation to fossil fuels are among the items that should be given special attention in the future.