The increased demand for alternative fuels has been observed in the past several years especially since the beginning of 21st century. Fossil fuel resources have been depleting since the beginning of rapid industrialization, it is said that the amount of fossil fuel consumed in the last 50 years is much more than that ever consumed in the human history. Another important factor in this discussion is the volatility of the prices of fossil fuels, which continuously encourage the investors and researchers to look for alternative fuel sources. Several substitutes have come into existence in the recent years and many more are on their way to get established as a sustainable fuel alternative. This also prompts the policy makers to consider different approaches towards energy storage point of view.
Biodiesel has rapidly gained importance as one of the most important substitutes for the depleting fossil fuels. There are many advantages associated to the use of biodiesel as fuel. An important feature is the similarity of combustion properties of biodiesel to those of petroleum diesel. Biodiesel is also non-degradable, non-toxic and renewable as well. The emissions from the combustion of biodiesel contains less harmful by products such as Carbon monoxide, unburnt Hydrocarbons, Sulphur oxides and particulate matter. However, the emissions of NOx, poor combustion and oxidation stability needs to be solved and researched thoroughly.
Biodiesel can be derived from multiple sources, but most popularly from edible oil seed crops such as sunflower, palm, rapeseed, soybean, coconut, etc. which are also considered as first generation biodiesel feedstocks. However, the use of such feedstocks for biodiesel production has faced problems as they disturb the overall worldwide balance of food reserves and safety which is an important concern for the world food authorities and nourishment societies.
The non-edible seed crops of jatropha, karanja, jojoba, mahua and waste cooking oil, grease, animal fats, etc. have gained importance in the last few years as second generation feedstocks for biodiesel production. It is important to mention that reuse of waste cooking oil has been prominent in strong economic countries such as China and America. But an important issue related with the second generation technologies is the recovery after the initial usage. Also these second generation feedstocks are not sufficient to entirely substitute the present transportation needs.
In the last few years focus is on microalgae as the third generation feedstock. The important reasons for using microalgae as the feedstock for generation are
· High photosynthetic efficiency
· Higher biomass production
· No competition for land
· Can even grow in brackish saline water
However there are several concerns associated with this technology as well such as, increase in the lipid content of microalgae and cultivating microalgae on large scale for establishing it as a stable source of alternative fuels.
In the present context, a brief description about the growth medium, conditions, growth evaluation and harvest of the algae, handling of the products and byproducts is explained. The growth medium, conditions and evaluation are explained with reference to three important species of microalgae Chlorella, Spirulina and pond water algae, for characterizing their growth potential and usage as feedstock for production of biofuel.
The following study and the respective procedure is excerpted from Centre for Enrergy Studies Indian Institute of Technology, Delhi and it takes into perspective especially countries like India where access to clean water is very limited and current usage of fossil fuels is very high as compared to renewables. This report will also highlight the prospects of using microalgae as a next generation fuel important for energy storage purposes.
1. Microalgae, growth medium and conditions:
The following study and the respective procedure is excerpted from Centre for Enrergy Studies Indian Institute of Technology, Delhi. The study uses Spirulina platensis and Chlorella pyrenoidosa which were procured from the National Collection of Industrial Microorganisms (NCIM), National Chemical Laboratory (NCL), Pune, (Maharashtra) India. The algae sample from a local pond was also collected from India gate, New Delhi, India. All the three samples were maintained in conical flasks containing 50 mL sterile BG11 media. Proper shaking conditions were also maintained at 120 rpm and 24 °C for half an hour and then placed in the direct sunlight. Regular sub culturing was performed after every 15–18 days. In a growth chamber, the cultures were incubated under illumination (2000 lx) at 25 ± 1 °C with light: dark cycles of 12:12 h for 15 to 18 days with bubbling air at normal pressure required to maintain stirring of the cultures. With the concern for large scale cultivation of algae, growth was also attempted in tap water without BG 11 medium.
2. Growth evaluation:
Growth of all the three species was monitored both in the presence and absence of BG 11 media for the total period of 27 days. Growth of the species was evaluated based on two parameters:
(i) Chlorophyll a content
(ii) Dry cell weight.
i) Chlorophyll content: To calculate the amount of chlorophyll in the microalgae biomass a known volume was suspended and centrifuged for 10 minutes at the speed of 800 rpm. Biomass collected after centrifugation was again suspended in a known volume of methanol.
This mixture is then submerged in water at a temperature of 60oC for a time of 30 minutes, hence collecting the chlorophyll content in the end. In the end of the process, the concentration of the chlorophyll was determined by using UV visible specter, the technology is more commonly known as UV Spectrophotometer. The absorbance values were then substituted in Eq. (1) used for chlorophyll estimation:
Chl a = 16.29 (A665.2 – A750) – 8.54 (A652 – A750)
In the previously mentioned equation A750, A665.2, A 652 are referred as the absorbance of algae biomass-chlorophyll suspension in methanol at 750, 665.2 and 652 nm, respectively.
ii) Dry cell weight: To calculate the growth with respect to the weight of the dry cell, a known quantity of microalgae biomass is taken and centrifuged for 10 minutes at revolution speed of 800 rpm. The collected biomass is the rinsed properly through water in order to remove all the kind of contaminations especially salts. The biomass which is collected at the end of the process is then dried for a t least 12 hours at a temperature of 60oC. The collected biomass is then weighed.
3. Algal production and harvest:
Since we have previously mentioned as well that microalgae could be cultivated in sea water or freshwater, and also since this article and the possibilities reviewed are keeping in view of scarcity of fresh water in India and other developing countries of this region, we are only reviewing the production possibilities of the species which are compatible with seawater i.e. Chlorella. Also it is important to mention that the production capability of microalgae by using sea water is encouraging than the conventional resources.
It is also vital to mention that since microalgae absorbs CO2, therefore an economic and environmental feasible approach would involve using of a higher concentration source of CO2 than already available in the atmosphere. In this was the cultivation of algae can be made economical, reduction of CO2 in the atmosphere is made possible and the storage of energy factor is also contributed to. Carbon capture from flue gas emissions, particularly from power plants that burn fossil fuels could be a promising option to provide high concentration of CO2 (up to 20%) and lengthen carbon emissions life at the same time.
This study involves the usage of the available stream of CO2 which is already cleaned from NOx and Sox emissions. The composition of Chlorella vulgaris (dry mass basis) was assumed to be protein (50%), carbohydrates (15%) and lipids (25%) others (10%).
After the completion of this process the biomass grown must be separated from the medium in which it was grown. Although there are several technical and conceptual technologies which are used in harvesting of algal biomass, however in this study the harvesting stage has been divided in three consecutive steps: settling tanks coupled with the cultivation ponds, a dissolved air flotation (DAF) unit and centrifugation unit. An organic coagulant (Chitosan) was considered to be used in the DAF unit. Dewatering, homogenization and pumping of liquids were also included in the harvesting section.
4. Oil extraction and biodiesel production:
Oil in the algae biomass was assumed to be extracted using hexane solvent in a stripper column with 91% extraction efficiency. For the production of biodiesel, transesterification of lipids with methanol was assumed to be performed in the presence of alkaline (KOH/NaOH) catalysts. Free fatty acids (FFA) are produced by the hydrolysis of oils and fats. The level of FFA depends on time, temperature and moisture content because the oils and fats are exposed to various environments such as storage, processing, heating or frying. The free fatty acid (FFA) content of oil was considered to be 7% and the materials and energy data required for processing oil with 7% FFA to biodiesel using a two-step process (acid esterification and alkali transesterification) modeled. The energy requirement for the purification of crude glycerol obtained in the biodiesel process is included in the biodiesel section.
5. Algae protein production:
The protein content of de-oiled algae biomass was assumed to be separated using ethanol (95%) and methanol (5%) extraction method mentioned in different research and review articles. The protein extraction method is similar to that of oil extraction method where the de-oiled algae are mixed with solvent (ethanol and methanol) for a desired temperature range (50–75oC) and for a time of (1 to 4 h). In order to recover the maximum amount of the protein and also to recover the solvent, it is done by evaporation, the protein is recovered multiple times. Therefore, an important assumption to be made here is that the amount of energy for oil extraction would be the same for energy required for protein extraction process. The protein output from the extraction process is considered to contain a similar quality of soybean protein used in animal feed.
2.4. Succinic acid production:
The carbohydrate remaining in the biomass after oil and protein extraction was assumed to be converted to succinic acid through fermentation using Escherichia coli. The processes involved in succinic acid production from carbohydrate are saccharification, fermentation, ultra-filtration, crystallization, centrifugation, drying.
Initially the algae carbohydrate is converted to glucose through saccharification using cellulase and amylase, and after saccharification the glucose rich slurry is fed into the fermentation tank along with other nutrients and seed culture. After fermentation at (36 _C) the broth is filtered to separate the solid and liquid fraction. Later the liquid fraction containing succinic acid is crystallized and centrifuged to separate the succinic acid, and dried to get the final product. The solid fraction containing biomass and water are sent to evaporator to remove water and the remaining cake is sent to boiler as fuel to generate part of energy required in the process.
As this succinic acid production technology is under research and there are no process data on succinic acid production from algae carbohydrate, the life cycle data of bio succinic production from starch using similar technology mentioned above in 25 was used in this paper to assess the environmental impact of succinic acid combined with other products.
2.5. Co-products handling
There are various options for using the remaining algal biomass such as combustion, biogas and fertilizers. In this paper, the algal biomass remaining after oil and protein extraction or oil, protein and carbohydrate extraction are assumed to be used in anaerobic digester to produce biogas that fuels the combined heat and power plant installed within system to generate partial process heat and electricity. The glycerol produced during biodiesel production, protein and succinic acid are assumed to be sold to the market replacing conventional products.