Keywords

2.1 Introduction

Fuel is the backbone of today’s life. In 1947, usage of fossil fuels started on commercial scale, yet they have been basic need of contemporary civilization since the beginning of modernization. Although our growth has been significantly reliant on fossil fuels obtained from natural resources under the Earth’s surface in “solid (coal), liquid (petroleum oil) and gaseous states (natural gas)”. However, continuing to consume these finite naturally occurring fossil fuels has a negative effect on the environment and is therefore not very manageable. Concerns about overuse of petroleum products and high “non-renewable energy” consumption lead to increase in the cost of raw petroleum. This causes changes in the environment, extreme weather patterns and distortion of ecosystems have prompted. Policymakers, researchers, governments and scientists are looking for an alternatives to conventional energy sources such as solar, wind and biofuels (Alam et al. 2012). Growing energy and transportation sectors have been connected to total greenhouse gas (GHG) emissions of up to 60 and 70 by percentage, respectively, while agrarian countries such as India and China account for only approximately 9% of global greenhouse gas emissions (Mata et al. 2010).

The continuous and gradual increase in the utilization of fossil fuels in transportation and to fulfil other energy demands has led to enhancement in the emission of greenhouse gases that is badly effecting our environment and ultimately biosphere (Vologni et al. 2013; Chandrasekhar et al. 2015). Additionally, the increased emission of greenhouse gases is the leading cause of global warming. Excess soluble bicarbonates specifically CO2 in the aquatic ecosystems can alter the pH and leads to destruction of both the sea and land habitats which ultimately result in destruction of sea food chain and also the food supply to human. Furthermore, fossil fuels are a finite source for energy production and are not a continuing process as they are not renewable source of energy and take hundreds of years to form. So in order to get a continuous and long lasting source of energy and also to reduce the emission of greenhouse gases, we have to find out alternative sources of energy. For this purpose we are in need of an eco-friendly and renewable source of energy. These problems lead scientist towards the production of biofuels. In the beginning food crops like corn and soya-bean had been used to produce biofuels (also called as first generation biofuels). But its production was limited because their use for fuel production may lead to food shortage, especially in those areas where they are used as stable food. Then cultivation of special crops like Jatropha (second generation biofuels) for producing biodiesel on the land previously used for food crops cultivation was also an issue. Because it also leads to food shortage and ultimately increase in food price (Mata et al. 2010).

Direct combustion of biomass is currently 95% to 97% source of energy. The driving force behind the production of biofuels was complete combustion of natural biomass along with the combustion of semi-natural biomass (industrial waste, municipal solid waste, agro-industrial waste and also the solid fossil fuels like coal, peat, lignite, etc.) in many countries worldwide (Vassilev et al. 2013). Ecofriendly fuels are actually biofuels, as the name represents are biodegradable and non-toxic fuels that are obtained from natural sources like plants, animals and microbes (Vogel et al. 2019; Xu et al. 2006). Figure 2.1 indicates the different generations of biofuels. In comparison to geochemically produced fuels, these modern biological sources have acquired great attention due to presence of their sustainable feedstock and also with the minimum emission of greenhouse gases. The type of biofuels produced from these sources depends upon the production and accumulation of intracellular component of these biological sources such as lipid and carbohydrate content. Important debates for the global production of these biofuels comprise criteria for production of biofuels, energy balance between fossil and biofuels and emission of greenhouse gases by biofuels (Ullah et al. 2015). For example, first generation biofuels have issue of food shortage, indirect increase in CO2 and use of land so the production of biofuels from plants was not further extended (Ullah et al. 2015; Noraini et al. 2014; Trivedi et al. 2015). That is why we are in need of sustainable feedstock for the production of biofuels that reduce emission of greenhouse gases, ecofriendly and must be non-agricultural non-food feedstock, including microbes, algae as well as plants that can grow on low quality land (Farrell and Gopal 2008).

Fig. 2.1
The diagram describes the four types of biofuels: conventional sources, first-generation biofuels, second-generation biofuels, and third-generation biofuels.

Different types of biofuels

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Due to simple cultivation and other production parameters, microbial feedstocks have gained attention for the production of biofuels, because of certain restriction for the plant feedstock like water usage, land, long growth period and food vs. fuel issues. Additionally we can manipulate microbial sources on gene level with the help of genetic engineering, to produce recombinant strains having more lipid content. Different algal strains are the best known feedstock as microbial source for production of biofuels including biodiesel and bioethanol. Algae can be easily cultivated in natural (open/covered ponds) or artificial systems on large scale or even in simple continuous stirred tank reactors, (CSTR). Biofuels from algae are also called as third generation biofuels. Requirement of water for algal biomass production can be fulfilled by wastewater sludge, already containing sufficient amount of some nutrients and carbon (Mathiyazhagan and Ganapathi 2011). Microalgae and macroalgae both have potential to produce biofuels. Algae seem to be best feedstock for biofuels due to a number of reasons which are listed below (Anto et al. 2020).

  1. 1.

    Algae is not involve in food fuel and is not used as food in most of world.

  2. 2.

    Small period required for full growth.

  3. 3.

    Low nutrient requirement.

  4. 4.

    Low quality land can be used for its cultivation.

  5. 5.

    Artificial photobioreactors (as discussed in the next part).

2.2 Classification of Algae

Research has been begun on the use of several algal strains to convert them into biofuels through numerous techniques starting from the esterification to anaerobic digestion. Algae are mainly classified into two major groups: microalgae and macroalgae. The macro- and micro algae of bioenergy production must meet the following criteria (Carlsson et al. 2007):

(1) Both types must be very high yielding; (2) they must be easy to harvest; (3) they must be capable of resisting water currents from high sea level; (4) their production must be equal to or lower than those from other available sources (Kraan 2013).

2.2.1 Microalgae

Generally, microalgae are photosynthetic microorganisms found in both freshwater and marine habitats. Microalgae have been classified according to various features such as pigmentation, photosynthetic storage product, the arrangements of photosynthetic membranes and other morphological characteristics. Currently, there are four types of microalgae, namely “Diatoms (Bacillariophyceae), Green algae (Chlorophyceae), Blue green algae (Cyanophyceae) and Golden algae (Chrysophyceae)” (Khan et al. 2009). The dominant types of Microalgae for commercial scale production comprise Chaetoceros, Arthrospira (Spirulina), Dunaliella and Chlorella (Lee 1997). Several strains of Chlorella, which is a microalgae, can switch from phototrophic mode to heterotrophic mode of nutrition (Xiong et al. 2008; Xu et al. 2006). Just like heterotrophic organisms, the algae are also dependent upon carbon sources like glucose for obtaining energy and carbon metabolism. Few algal species have the ability to grow in the presence of mix nutrients. Bio-molecules like proteins, carbohydrates, nucleic acid and lipids are ordinary components of microalgae.

Recent research companies have shown that biomass of microalgae seems to be the one of favourable sources which can be used as renewable biodiesel and can meet global demand. The oil content of microalgal biomass can be as high as 80% by dry weight, depending on the species (Rodolfi et al. 2009). It is common for oil to range from 20% to 50% (Chisti 2007). It is possible to define oil production yield as the volume of oil produced per unit volume of microalgal culture each day as a function of microalgae growth rate and biomass. Various algal strains manufacture high levels of lipids about 50% to 60% of dry weight which acts as storage material. This stored lipid is chemically similar to oil-seed lipids which are obtained from other crops, making an encouraging source for production of biodiesel (Griffiths and Harrison 2009).

For more efficient lipid extraction from microalgae, many approaches have been used like expellers or oil pressing, ultrasonic procedures, extraction of super critical fluid and solvent extraction are the most popular methods. These methods of extraction should have the following characteristics: they must be quick, non-destructive and effective for the lipids removed and they should be easily scaled up (Medina et al. 1998). The modified Bligh and Dyer method for lipid extraction is frequently utilized (Mutanda et al. 2011). Different methods like simultaneous extraction, direct esterification and transesterification could be used to extract microalgal fatty acids from diverse biomass types and making it flexible process for production of biofuels. This process involved several steps that require a combination of solvent extraction, ultrasonication, heating at high-pressure, filtration, separation on the basis of solvent density, oil and liquids recovery via evaporation to dryness. Mondal et al. 2017 described that along with the production of biofuels some species of microalgae can also produce valuable chemicals that could be used in medicines as well as in other applications as described in Table 2.1.

Table 2.1 Different chemicals produced by microalgae and their uses
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2.2.2 Macroalgae

Macroalgae account for the most integral part of the marine ecosystem, because they protect marine resources by stopping pollution and eutrophication (Notoya 2010). Macroalgae are inferior plants because they do not have roots, stems or leaves. Instead, they are made up of thallus (leaf-like) and sometime stems and foot. Some species are filled with gas in enclosed structures to aid buoyancy. They can grow very fast, up to tens of metres in size (Lüning and Pang 2003). Macroalgae are different in several aspects, such as morphology, life span and physiology. According to their pigmentation, they are divided into Phaeophyta (brown), Rhodophyta (red) and Chlorophyta (green) algae (Chan et al. 2006). The growth of macroalgae on rocky substrates results in a stable multi-layered perennial vegetation that is capable of capturing nearly all of the available photons in the natural environment. Almost 200 species of macroalgae are utilized all over the world, in which about ten are more commonly cultivated like Laminaria japonica, Phaeophyta, Rhodophyta, Undaria pinnatifida, Gracilaria, Eucheuma, Porphyra, Kappaphycus Monostroma, Kappaphycus and Chlorophyta Enteromorpha (Lüning and Pang 2003).

Some macroalgae species collect a large number of carbohydrates that can be used as a substrate in microbial conversion activities, such as the manufacture of biofuels with a high product cost (Kraan 2013). Table 2.2 indicates different algal strains that can produce different fuels. Maceiras et al. (2011) recently discovered that a transesterification process could be used to produce biodiesel from triglycerides by using a variety of macroalgae, including Codium tomentosum, Ascophyllum nodosum, Fucus spiralis, Enteromorpha intestinalis, Sargassum muticum, Pelvetia canaliculata, Saccorhiza polyschides and Ulva rigida. It has been observed that macroalgae have a higher water content (80–85%) than terrestrial plants, making them better suitable for the thermochemical conversion of microbial conversion than the direct combustion process. Macroalgae like Gracilaria spp., Sargassum spp., Laminaria spp., Prymnesium parvum and Gelidium amansii are best targets for production of bioethanol (Adams et al. 2009; Wi et al. 2009).

Table 2.2 Different algal strains from the production of biofuels
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2.3 Cultivation of Algal Biomass

The cultivation of algae raw materials is comparatively an easy process and may evolve with limited or no supervision. Algal biomass can be cultivated by using wastewater which has harmful effect if consumed by human being and at the same time absorbing CO2 from the atmosphere (Bharathiraja et al. 2015). Algae absorb sunlight for the process of photosynthesis, which is biochemical process and used for growth of plant, then they produce chemical energy from sunlight (Brennan and Owende 2010). The CO2 formed is converted into distinct configuration of chemical energy through photosynthesis, like lipids, carbohydrates and protein. That is why the growth and reproduction of algae require only the supply of basic nutrients like sunlight, CO2 and water required for regular photosynthesis of plants, and the conversion of solar energy by fixing CO2 (Hallenbeck et al. 2016). Algae biomass cultivation basically has the following two types, as illustrated in Fig. 2.2.

Fig. 2.2
The flowchart describes the cultivation of algal biomass through natural and artificial processes. Natural include open ponds and covered ponds. Artificial includes photo-bio reactors.

Different types of cultivation systems

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2.3.1 Natural System for Cultivation of Algae

The natural system for cultivation of algae includes natural habitat of algae like lakes, ponds, lagon, etc. These are classified on the basis of structure and depth of water bodies such as circular pond, shallow ponds, open ponds, covered ponds, etc. These specialized ponds for algal production are constructed in such a way that they are situated above level of ground (Suganya et al. 2016).

2.3.1.1 Open Ponds

These are the best suited places for the cultivation of algae from economical point of view, as they are simply natural ponds containing fresh or salted water according to the selected strain of algae to produce biofuels. These ponds can be scaled up easily up to many hectors. But the main limiting factor for scaling up of these ponds are grazers, fungal growth or contamination of selected strains with unwanted algal strains. Although about 98% of the algal biomass production is obtained from open pond system. Algae can grow really fast so it can produce about 15–20 tons of biomass after drying from each hector per annum. All high yielding strains of algae contain about 50–60% of lipid content by weight that is further processed to obtain biofuels (Singh et al. 2015). There are a number of microalgae strains that could be cultivated by using open raceway ponds. One of these species is “Chlorella pyrenoidosa” in the pond containing wastewater and can generate about 1.7 g of biomass per litter of water (Bell and Strang 2020). Reports show that the dried biomass productivity of Dunaliella salina in open pond was 0.097 g/l per day and that of Nannochloropsis sp. is 0.207 g/l per day (Ghorbani et al. 2018).

2.3.1.2 Ponds

The problems associated to open ponds can be sorted out by covered ponds such as production of unwanted strains, fungal growth and protection from grazers. A huge water loss by evaporation in open pond is solved in covered ponds also. But one of the problems with these closed systems is the rise in temperature since the ponds are covered. This issue can be sorted out providing continuous agitation to the system that may increase cultivation cost (Carvalho et al. 2006). Many manipulations in the set-up of both open covered system are being made for significant increase in the yield of biomass of selected strains of algae. In a study (Marchin et al. 2015), a cascade system for the production of algae was made on the roof top, while the force of gravity was used on inclined surface to keep the system in motion simply to provide thorough agitation. The culture of algae was collected at the end of inclined surface in a tank and then the water is pumped back to the roof. The variation in the volume of system due to evaporation in very hot sunny days and in rainy day was managed in the tank that acts as buffering agents. Several different systems were also used by researchers to grow algae in their natural environment (Devi and Mohan 2012).

2.3.2 Artificial System for Algae Cultivation

Artificial cultivation of algae has been achieved by the use of photobioreactors (Lee and Lee 2016). These closed cultivation systems of algae have controlled environmental condition that improves overall yield (Kandiyoti et al. 2016). Water, CO2 and other nutritional requirements for growth of algae are added in the photobioreactors, while light, pH, temperature and density of the system are maintained at optimum level required for the growth of a particular algal strain (Trivedi et al. 2015). All the limitations that can affect the overall yield of the algal biomass in the natural system could be sorted out by using photobioreactors but limiting factor in the systems is capital cost (Brennan and Owende 2010). The main reason behind the invention of this artificial technology was to overcome the challenges as in the natural systems such as contamination and pollution, fungal attacks that hinder the production of good quality yield of algal biomass at large scale (Kandiyoti et al. 2016).

Photobioreactors are actually manmade cultivation system that enhances growth of specific algal species in the presence of ideal growth parameters including pH, light and temperature (Ullah et al. 2015). They are several forms of photobioreactors on the basis of their shape (Anto et al. 2020). Some types are listed below:

  1. 1.

    Tubular photobioreactors

  2. 2.

    Plate photobioreactors

  3. 3.

    Helical photobioreactors

  4. 4.

    Horizontal photobioreactors

  5. 5.

    Foil type photobioreactors

The algal culture is continuously pumped and recirculated in the photobioreactor for proper growth (Carvalho et al. 2006). The photobioreactors are made up of glass or acrylic (as represented in Fig. 2.3) but are translucent so do not allow natural sunlight to reach the algal biomass. So in order to minimize the need of natural sunlight, light emitting diodes are fitted inside the reactors and a yield of 100 g/per hour can be achieved. Increase or decrease in light intensity affects the rate of photosynthesis (Ullah et al. 2015). These manmade reactors have many advantages over natural system, a few are listed below (Anto et al. 2020):

  1. 1.

    As the cultivation systems are fully closed it eliminates the chances of contamination by unwanted algal strains or fungal attacks.

  2. 2.

    Water loss by evaporation is minimum due to no direct contact with sun that reduces the water need as compared to open ponds.

  3. 3.

    More efficient dissipation of heat and nutrients that leads to uniform and maximum growth.

  4. 4.

    Controlled parameters like CO2, nutrient, pH, etc. maintain the uniform environment.

  5. 5.

    Can also produce biomass at night by photosynthesis due to availability of light by artificial diode system which reduces the cultivation period.

Fig. 2.3
The diagram represents a setup of a photobioreactor labeled degassing column, fresh medium, water out, colling water in, air, exhaust, pump, and harvest.

Schematic representation of photobioreactor

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2.4 Harvesting

The next step right after the production of algae is to harvest its biomass present in aqueous environment. Usually cultivation of algal biomass in the aqueous environment faces many hurdles like its down-streaming and to make it water free. The cost of harvesting of “algal biomass is about 20–30%” of the total operating cost of the algal biomass production. This cost could be increased or decreased by different factors such as type of harvesting method used, type/species of algae and its density (Karemore and Sen 2016). That is why the increment in algal biomass and reduction in the overall volume of culture medium will automatically lead to decrease in the reduction of expenditure on the down-streaming. Some of the mostly used techniques for harvesting algal cell biomass are electrophoresis, centrifugation, flocculation, filtration and flotation, etc. (Karemore et al. 2016). Comparison of different types of harvesting techniques is shown in Table 2.3.

Table 2.3 Comparison of different methods of harvesting algal biomass
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Nichols and Scott (2012) opened a gateway for the growth and easy down-streaming of algal biomass by using “polycationic flocculants”. The process involves use of optimum amount of chitosan or chitin in the aqueous environment for the production and harvesting of algal biomass. It is used to

  1. 1.

    increase algal biomass

  2. 2.

    more aggregated algae

  3. 3.

    enhance over-all lipid concentration in the algal cell to produce biodiesel

  4. 4.

    downstream the biomass after removing extra water

That lipid containing algal biomass can be further processed to produce chemicals, biofuels like biodiesel, biogas, bioethanol or biohydrogen, for animal and fish feed and in some cases for human dietary purposes. Some anionic polymers could also be used like pectin, xanthan gum and alginate for harvesting (Karemore et al. 2016).

Another method to increase concentration of biomass after harvesting is the use of organic flocculant in their low concentration for the production of freely moving single cell algal biomass, which has relatively less diameter of about 5 μm, for example, Nannochloropsis. This process is economically very suitable to produce algae for biofuel production at large scale (Bazarnova et al. 2018). One of the other invented methods to obtain higher concentration of algae is filtration that is performed without addition of any supporting material, and algae have to bear no thermal or any other shock, that is why biomass remains protected. Additionally this technique is used to do harvesting in continuous manner and harvested biomass can be directly used for the production of biofuels or other products. To carry out the algal biomass will have to pass through a series of concentrators that work on the principle of passive filtration. The filtrating device can also be processed by using gravitational force (Kabakian 2014). Centrifugation can also be used to harvest algae in a “mechanical based system” and the biomass is collected with higher concentration. In recent experimentation the combination of all these methods is performed to obtain more concentrated product (Chowdhury et al. 2019).

2.5 Production of Different Types of Biofuels from Algae

There are different kinds of biofuels that can be obtained from algae by using different processing as shown in Fig. 2.4.

Fig. 2.4
The diagram describes six types of applications of algae as fuel, namely bio-hydrogen, direct combustion, bio-diesel, bio-gas, bio-ethanol, and bio-fuels blends emulsifies.

Different types of biofuels that can be produced from processing of algae

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2.5.1 Production of Biodiesel

Biodiesel is simply methyl or ethyl ester of fatty acid that is produced by the process of trans-esterification (in the presence of alcohols) of triglycerides. So we can say that biodiesel is alkyl fatty acids (having chain length of C14 to C25), esterified in the presence of alcohols mostly CH3OH or C2H5OH. Biodiesel is most comparable with the diesel from fossil fuels in characters like flash point, “cetane number”, viscosity and heating value (Demirbas 2009). Due to these characters biodiesel seems to be the most demanding fuel in future because of limited resources fossil fuels. The other most demanding feature of biodiesel is its eco-friendly nature as it reduces emission of CO2 by 77% as compared to conventional fuels (Atadashi et al. 2010).

It is the type of fuel that can be degraded by microbes (biodegradable) and non-toxic that is derived from renewable natural resources (Hossain et al. 2008). It also enhances engine performance as well as reduces the emission of sulfur and other particulate matter during combustion (Miao and Wu 2006; Scragg et al. 2002). For the production of biodiesel, triglycerides are transesterified by acids or any catalyst like metals to produce biodiesel and glycerol as by-product (Chisti 2007; Johnson and Wen 2009). By keeping in view all the above mentioned points biodiesels are acceptable in order to fulfil demand of fuel, but its current production by terrestrial biomass is not enough to meet energy demands. In 2007 Chisti noted that about three times of the area currently available for cropping in the USA is needed to fulfil the only half of the demand of fuel for transportation per annum. However algal biodiesel can fulfil this demand with the low quality cultivating land and less efforts (Chisti 2007). But up till now there is no report on the large scale production of biodiesel from algae (Lardon et al. 2009). Different algal species produce biodiesel in different quantities as described in Table. 2.4.

Table 2.4 Production of biodiesel from different strains of algae and per Kg lipid content
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The conversion of algal biomass into diesel is a chemical conversion process that needs the reaction of lipids from algal biomass with alcohols (Methanol/ethanol) by adding a suitable catalyst (Suganya et al. 2016; Johnson and Wen 2009; Kandiyoti et al. 2016; Lee and Lee 2016). This process is very compulsory in order to lower the viscosity of algal oils that is greater than the petroleum diesel. That viscosity lowering process automatically increases fluidity of the biodiesel. While glycerol is produced as by-product and used to form different pharmaceutical and cosmetic products. Transesterification can be done by two methods (Johnson and Wen 2009) as illustrated in Fig. 2.5.

  1. 1.

    Conventional method

  2. 2.

    Direct method

Fig. 2.5
The flowchart represents the production of biodiesel through biomass. The process is done by two methods conventional method and the direct method.

Production of biodiesel through direct and conventional methods of transesterification

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2.5.1.1 Conventional or Traditional Method of Trans-esterification

This is a two step process that includes extraction of lipids from algal biomass before the conversion process (Lee and Lee 2016). For this method, there is an essential step of pretreatment that is performed to extract the lipid from algal biomass and removal gums by the action of non-reacting solvents such as hexane (Hossain et al. 2018). These additional methods lead to high yield of extra refined biodiesel that could be directly used for high speed engines (Salam et al. 2016). The addition of these steps increases the over all production cost (Martinez-Guerra and Gude 2018). These processes not only increase the production cost by increasing energy demand but also increase the time of production (Jazzar et al. 2015).

2.5.1.2 Direct Transesterification

This is also called as one step process and involves the extraction of lipid content of algal biomass (Lee and Lee 2016; Ehimen et al. 2010). The wet and unwashed biomass of algae is directly placed in reaction chamber to perform direct transesterification (Jazzar et al. 2015). Excessive methanol is added in the reaction chamber that eliminates the need of processes like pretreatment, removal of gums and extraction of lipids. That is why it gives more yield than the traditional method (Lawayzy et al. 2014).

It was highlighted that yield of biodiesel from the transesterification by direct method of dry biomass of algae is greater than that of wet biomass, pointing about the need of dryness of biomass. In comparison to this use of conventional method for the biodiesel produces equal yield either the biomass is wet or dry (Johnson and Wen 2009). If solar drying techniques are developed, biodiesel synthesis from any biomass became more practical. But the over all yield and the amount of lipid extracted from solar drying of biomass are not currently known (Lardon et al. 2009). To reduce the production cost utilized in drying, pretreatment to reduce the energy consumption for production of algal biodiesel at large scale new and well-established technology is needed (Levine et al. 2009; Sialve et al. 2009). To enhance the over all biodiesel production by increasing lipid content of algal biomass needs to grow algae in nutrient rich medium. It was also described that in order to increase the lipid content of algae genetic engineering is a promising technique. But the previous experimentation in this regard was not very successful (Sheehan et al. 1998a, b). Thus a lot of research work is need in order to manipulate the genes of algal strains for greater yield of biodiesel (Hu et al. 2008).

2.5.2 Biohydrogen Production

Biohydrogen is the gaseous energy source that is considered as the future of energy field stock because of its high energy density and environment friendly nature (Oncel 2013). The traditional method of the production of biohydrogen is by the use of steam that consumes a lot of energy (to produce 1 kJ of biohydrogen 3–3.5 kJ of energy is required). The metabolic processes of microalgae can produce biohydrogen directly (Melis and Happe 2001). Similarly dark fermentative bacteria like (Clostridium and Enterobacter) can also produce biohydrogen by using macroalgae as substrate. Production of biological hydrogen is relatively simple process that requires less energy supply in mild conditions rather than the high energy consuming traditional method but the biological production of hydrogen is not currently on large scale due to lack of suitable instrumentation. One other main limitation is incomplete knowledge about the metabolic production of biohydrogen from bacteria that hinders the way for genetic engineering to do the process on large scale. Actually, hydrogen is the waste product of metabolic process of algae for the balancing of redox reaction in the absence or presence of oxygen (Dębowski et al. 2013).

But the metabolic machinery for production of biohydrogen in algae and cyano-bacteria is different. According to current studies, production of biohydrogen from algae was done only on ice lab scale or pilot-scale, so there are many challenges to upgrade the process for hydrogen production. The metabolic hydrogen enzymes are very sensitive to the presence of oxygen so the process has to be performed in strictly controlled environment without oxygen. But two phase reaction can solve this problem by separating the production of biohydrogen into two phase:

  1. 1.

    Biomass production (aerobic phase)

  2. 2.

    Hydrogen production (anaerobic phase)

To produce biohydrogen on large scale, the aerobic condition can be provided in open ponds to reduce the cost. However, the transfer of biomass after harvesting to the anaerobic environment is still time and cost consuming process. Similarly it is very difficult to prevent inhibition of hydrogen enzymes that is extremely sensitive to O2 (Márquez-Reyes et al. 2015) even by solving the problem of reduction process, there is still challenge to store hydrogen gas due to its low energy density (Hallenbeck et al. 2016).

2.5.3 Production of Biogas

Biogas can be produced by conversion of organic waste that is obtained from algal biomass. This process involves the breakdown of algal biomass into methane and carbon dioxide that can be used for domestic purpose or as a fuel for automobile engines (Lee and Lee 2016). By this process there is a huge recycling of carbon, in the form of methane (Adeniyi et al. 2018). This process can also covert biomass having high content of moisture into methane (Trivedi et al. 2015). Production of biogas (methane has the following four steps as illustrated in Fig. 2.6).

  1. 1.

    Hydrolysis of biomass:

    The first step of biogas involves the breakdown of algal cell wall by bacterial or enzymatic action in the digestion tank (Ehimen et al. 2013), which leads to conversion of algal substrate into soluble sugars and amino acids (Trivedi et al. 2015).

  2. 2.

    Fermentation of digested matter:

    This step involves the aerobic digestion of digested organic substrate (Ehimen et al. 2013). Acidogenic bacteria perform their action by converting sugars and amino acids into H2 (hydrogen), CO2 (carbon dioxide) and NH3 (ammonia) (Trivedi et al. 2015).

  3. 3.

    Acetogensis by oxidation:

    This step involves the oxidation of products of fermentation into acetate that has to be utilized further for methane production. In order to oxidize the substrate partial pressure is applied by using hydrogen (Ehimen et al. 2013).

  4. 4.

    Production of methane (biogas):

    The final and last step of this process involves the production of CH4 (methane) also called as methanogenesis (Trivedi et al. 2015). This is achieved by the help of methanogens (such as Methanosarcina barkeri). After the production of biogas, there would be enough nutrients as left over that can be utilized for further production of algal biomass (Ehimen et al. 2013).

Fig. 2.6
The flowchart describes the production of biogas through algal biomass harvesting by centrifugation, flotation, filtration, and sedimentation.

Production of biogas from algal biomass

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2.5.4 Bioethanol

Bioethanol/ethyl alcohol is the type of ethanol that is obtained by processing of any natural biological source. It could be utilized as replacement of petrol (Nahak et al. 2013). Demand for bioethanol as a transportation fuel is increased with time. Many countries like Brazil, China and India have started the production of bioethanol and using it as fuel source (Lee and Lee 2016). Over conventional fuels which are obtained from fossil fuels, bioethanol is preferred because of its source and impact over environment. It has lower sulfur content than petrol that is why it emits less greenhouse gases upon combustion. The scope for using bioethanol is high because it is renewable source. Bioethanol is produced after the break down of sugars and starch which are obtained from first or second generation feedstock like lignocellulosic biomass, wheat bran corn, etc. (John et al. 2011). But continuous consumption of second generation feedstock would lead to depletion of food in the world which is already a big issue of recent times. If they continue to use second generation feedstock as a fuel source there would be debate about the water use, use of arable land and food versus fuel.

This pitfall of using second generation feedstock is overcome by algae. Algae are third generation renewable source and are the best substrate to produce Bioethanol. Algae can grow on wastewater also either it is from municipal or industries. Production of bioethanol by using algae would help in bioremediation because algae use CO2 and other nutrients from wastewater for the process of “photosynthesis” and resulting in treated water (John et al. 2011). Mainly it is obtained through the process of fermentation of cellulose which is the main component of cell wall or by the starch that is a storage material (Ullah et al. 2015). Different species of algae can store starch and other sugars which is stored by Chlorella vulgaris is 37% of the dry weight of its biomass. “Blue green algae” like.

Chlorococum sp. and Spirogyra species can store high level of polysaccharide. Commonly used algal species (illustrated in Table 2.4) for the production of bioethanol are Prymnesium parvum, Chlorella, Scenedesmus, Sargassum Gracilaria, Spirulina, gracilis porphyridium, Dunaliella and Chlamydomonas (Chaudhary et al. 2014) (Table 2.5).

Table 2.5 Bioethanol production from different strains of algae
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2.5.5 Direct Combustion

It is an irreversible chemical change in which algal biomass is converted into gases that are hot and produce energy (Lee and Lee 2016). An interaction among a biomass of algae and O2 at 100 °C in the steam turbines, furnace or boiler etc., which produces hot mist (steam) that has the power to operate a turbine which drives a “generator” for the production of electricity (Suganya et al. 2016). For the reduction of air pollution direct combustion can be accomplished if the moisture level is lower than 50% (Sheehan et al. 1998a, b). Prior to combustion of biomass pretreatment processes like drying, grinding/milling is necessary (Trivedi et al. 2015). Alkali residues and high ash composition reduces the efficiency of overall conversion process. A new method called fluidized bed method was suggested by Milledge et al. (2013, 2014) which was the best method for the reduction of effect of alkali and high ash content. But for the application of this method algal biomass should be converted into smaller particles, therefore grinded algal biomass required another form of pretreatment (Lee and Lee 2016). Brennan and Owende (Kandiyoti et al. 2017) recommended that the heat which is produced during conversion process is utilized immediately, then the cost of this extra pretreatment process can be lessened (Adeniyi et al. 2018).

2.6 Algal Biofuels Blends and Emulsifiers

Due to the unfavourable effects and depletion of fossil fuels different researches have been done for the improvement of biofuels (Acharya et al. 2017). Due to physiochemical properties biofuels have to encounter several challenges (Rahman et al. 2015; Echim et al. 2012). For producing fuels with desired physiochemical properties blends of biofuels are produced. For this purpose different types of fuels in different proportion are mixed which as a result improve the properties of fuel which helps in the improvement of engine performance (Neto et al. 2013). Diesel produced by using algae can be used as additive or blended with bioethanol or petroleum diesel (Sheehan et al. 1998a, b; Acharya et al. 2017; Barabás et al. 2010), which can be used without any modification to existing engines (Nair et al. 2013). Blending of algal biodiesel and petroleum diesel was begun by Nagane and Choudhari (Patel et al. 2014). They blend about 20% of algal biodiesel which results in good performance of engine. To upgrade the efficiency level of 4-cylinder diesel engine, oxygenating activity of algal biodiesel was combined with addition of butanol. Makarevičienė et al. (2014) found that a blend of 10% algal biodiesel, 60% petroleum and 30% butanol was environmentally benign, with lower NOx and CO emissions (Makarevičienė et al. 2014).

This is due to the fact that direct use of some of these fuels might cause the problems of engine such as incomplete combustion, engine fouling, pollution of lubricating fuel and low fuel atomization (Mofijur et al. 2016). However the blends of these fuels have improved properties as compared to individual fuels (Nair et al. 2013). Because of the oxygenating effect of blends successful depletion in emission of exhaust gases occurs. But when these blended fuels are used in engines there instability causes a big problem, and it might result in failure of engine. Proper biofuel solubility, stability and durability are required, while the temperature, specific gravity and viscosity should be in mind (Khalife et al. 2017).

2.6.1 Blending

There are several methods of blending algal biofuels with petroleum fuels:

  • Splash mixing method: It is the least precise method of all the blending methods of biofuels that is why it is not used commonly (Mofijur et al. 2016). In this method fuel is pumped into another tank which contains petroleum fuel. The temperature of fuel which is present in tank should be colder than 8 °C and the temperature of pumped fuel should be 18–20 °C.

  • Mixing by in-line method: for this process an empty vessel called collecting vessel is present which is used for final product collection. In this vessel two different fuels which are used for blending are pumped in exact same ratio through a pipe or hose (Mofijur et al. 2016). For blending of large volumes of fuel this method is perfect but it is preferable to not keep the bended fuel in this tank for long period of time. In this method the blend consistence is much better than splash method but the final product still requires proper adjustment.

  • Mixing by injection method: Being the most correct and preferable method of blending. It applies control by valves. In this method variable valves are used for the injection of different fuels in a required ratio. This method is used before delivery into final tank at the production point (Mofijur et al. 2016).

2.6.2 Additives

Additives can also be used for the modification of fuel properties like octane number, energy content, lubricity and stability. Few selected additives are tested for better performance of engines (Roy et al. 2016).

2.6.2.1 Types of Additives

Different types of additives are used to increase the performance of algal biofuels as shown in Fig. 2.7.

  • Oxygenated additives: These additives are used to control compromised combustion which is usually caused by inadequate supply of oxygen (Mata et al. 2010). Oxygenated compounds are added in a specific ratio to fulfil the oxygen need which helps in avoiding incomplete combustion.

  • Additives based upon metals:

    This type of additives helps to minimize the temperature for oxidation and upgrade the soot oxidation. During soot oxidation they directly react with atoms of carbon and upon reaction with water produce hydroxyl radicals.

  • Water: In the ignition chamber, upon entry of water the temperature of local combustion is lowered and as a result reduce the emission of NO.

  • Antioxidants: Antioxidants terminate the chain reaction and prevent the oxidation of other molecules. In this way they increase the shelf life of fuels.

  • Polymeric based additives:

    This type of additives is used for the improvement of engine parameters and engine performance.

Fig. 2.7
The diagram represents five types of additives, namely polymeric additives, oxygenated additives, water, antioxidants, and additives based on metals.

Different types of additives and their function

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When two different, multifunctional and organic additives are combined it is estimated that upon injection they will exert positive physiochemical effect on engine. Ethanol can also be used as good additive which is used for better emission and combustion performance in engine. Bhale et al. (2009) recommended ethanol blended biodiesels which are renewable and can be used as alternative to fossil fuels. These blended also boost the cold flow behaviour and does not affect the performance of engine in cold climate. Sadeghinezhad et al. (2014) examined the factors that influence ethanol solubility in diesel oil and establish a connection between the blend temperature and the water content. When the ambient temperature is warm, dry ethanol can mix with diesel fuel, but when the temperature is lower than 10 °C, both fuels started to isolate. The inclusion of an emulsifier, which holds these blended diesels together by floating microscopic drops of ethanol in diesel, could address this problem. The careful mixing of biofuels like bioethanol or biodiesel with petroleum diesel is a key technique for lowering diesel engine emissions. Although these tidy blends are insufficient for the removal of all pollutants that is why water fuel emulsion approach was developed (Hoseini et al. 2017). For the improvement of thermodynamic stability of fuels micro emulsions are preferable (Karthikeyan and Prathima 2016). These emulsions are evenly diffused in mixtures of liquids comprising polar and non-polar parts that use organic molecules known as emulsifiers or surfactants to lower the interfacial tension among the water and oil phases (Xu et al. 2016). This is the process of creating thermodynamically stable mixes that are transparent, isotropic and nonpolarized (Nair et al. 2013). “Emulsions” are also referring as fuels those are hybrid of diesel since they can combine biodiesel with low-molecular-weight alcohols to lower consistency by using an amphipathic compound referred as a “surfactant” (Dunn 2010).

Several commonly used surfactants are used as cosurfactants with butanol and water such as cetyl trimethyl ammonium bromide (CTAB) and sorbitan monooleate (Span80). These are used for the improvement of emission from the engines that use algal biofuels blends (Xu et al. 2016). Hagos et al. (2017) tried to make a difference between blending and emulsification techniques which are based on boiling point of mixtures. For obtaining neat fuel mixture, blending of those fuels that have similar boiling point should be done. While for emulsion, fuels that have different boiling pints are used. These fuels of different boiling points show phenomenon of small eruption in fuel during mixing of fuel with any oxidizer. Mixture of diesel or biodiesel with water is used to produce emulsions of fuels is an alternative procedure of using microalgae. This emulsified water extends emulsion stability and provides help for interaction between mixtures. This process is described by (Al-lwayzy and Yusaf 2013). Ultrasound treatment is applied to microalgae aggregates which improve the disintegration of these aggregates in dispersed cells and result in fine droplets with wide spray angle in the emulsion. Lei et al. (2012) developed a new emulsifier which is called CLZ. This is based on the firmness of ethanol in blends of biodiesel. It was noticed after testing or using CLZ emulsifiers there was remarkable improvement in engine performance. For the possible reduction of NO and CO2 emulsion should be used instead of tidy biodiesel blends (Haik et al. 2011).

2.7 Economics of Algae

Petroleum’s shifting price continues to establish the economic benchmark that all biofuels must reach in order to be competitive. Lipid production of the microalgae has pronounced potential for renewable fuel production. Cost information of fuel produced by algae is limited to biodiesel but various processes use several forms of microalgae for fuel production. The estimate of cost will be on assumption until unless production plant produces minimum amount of biofuel which is one million gallons per year. Even the best assumptions about the cost of production are most probably may be in precise. For large scale production, economic cost and cost of resources such as water, air and land are unknown. Heterotrophic and autotrophic both processes are used for the production of biodiesel. In heterotrophic process the cost of carbon sources (carbohydrate) or feedstock is reported about 60–75% of total cost of biodiesel production (Brennan and Owende 2010; Borowitzka 1997). The most recent cost of biodiesel which is produced by using Solazyme is about $67 per gallons. That is why it is considered that Solazyme has attained the major milestone in this developing market of biodiesel (Rapier 2010).

Comparison of PBR and PRP was done by Chisti (Chisti 2007). Both techniques are used for the production of 100 s of tons of biomass and used extensively for commercial productions (Spolaore et al. 2006; Tredici 2010; Pulz 2001). First step of obtaining the target product from algal biomass is recovery of algal biomass from the cultural broth. Several techniques such as centrifugation, filtration, etc. are used for the separation of algal biomass (Grima et al. 2003). The techniques which are used for the separation of algal biomass affect the overall cost of process. The recovery costs of PBR technique are somewhat less as compared to the costs of PRP process because in PBR process higher biomass concentration is obtained.

The cost for the recovery of algal biomass from PBR process is somewhat lower as compared to those of PRP process. Because in PBR process there is higher concentration almost 30 times greater of biomass as compared to PRP process. Producing 1KG of microalgal biomass through PBR process would cost almost $2.95 and that same amount of biomass production through PRP process would cost almost $3.80 (Grima et al. 2003; Terry and Raymond 1985). If the amount of production is increased and annual biomass level is almost 10,000tons, then the cost of biomass per KG would decrease which is most probably $0.47 for PBR process and $0.60 for PRP process. For 30% oil extraction efficiency of algal biomass by weight is assumed and the cost for the 1 L of oil which is extracted from biomass would cost almost $1.40 and $5.30/gal via PBR process and $1.81 and $6.85/gal from PRP process (Grima et al. 2003; Chisti 2007).

Algal derived biofuels have standard efficiency about 60%, their conversion would cost about $8.03 via PBR process and $10.38 through PRP process. Improvements, such as growth technique improvements, process efficiency and stability and efficiency of GMOs, also continue to bring the cost of biodiesel made from algal biomass closer to the biodiesel made up of other feedstock such as soybean, canola, palm and petroleum. Recently no such commercial algal plants are established which work consistently for the production of biofuel (Menetrez 2012).

These all theoretical examples about cost which are given above are best putative estimates. Algae-derived biodiesel would be competitive with other biodiesel feedstocks and petroleum-derived diesel at a price of around $4/gal. This hypothetical situation would also have the added benefits of lowering wastewater nutrient loads, lowering plant water demands and CO2 sequestration. In addition to this importance of nonlipid part of algal biomass cannot be ignored. Ethanol can be produced from high carbohydrate levels and animal feed can be provided from high protein content (Menetrez 2012).

2.8 Genetic Engineering

The illegibility of recombinant algae production to maximize both of the previously mentioned cultivation methods is dependent on its quality of modified algal biomass to modify the standardized attributes of algal biomass in order to generate maximum yields of both the by-products and main products (Snow and Smith 2012). Cultivation process involves purposeful changes in genes of algae in order to make a more efficient substrate for different uses (Tabatabaei et al. 2011). Most of the algal species used a specialized structure called as “antennae” for capturing radiations from sun to perform photosynthesis and is responsible to produce maximum yield. Modification of the DNA which is responsible for these antennae would allow for the expression of genes that are small in sequence, allowing the greater light absorption by the cells of algae. To transfer DNA into algae cells, a variety of transformation methods have been used, including electroporation, particle bombardment, artificial transposons, virus infection, in the presence of glass beads and DNA agitation of cell suspension is done, silicon carbide whiskers, agro-bacterium infection and most recently, agro bacterium-mediated transformation.

Highest transformation rate is obtained in particle bombardment and electroporation methods (Rismani-Yazdi et al. 2011). For reengineering of algal DNA genetic engineering gradually has become the most methodical and time saving method. The reason for DNA restructuring most commonly is that processes like hybridization, sequencing, enhanced evolution and metagenomics can generate or enhance the likelihood of survival in harsh environments (Dana et al. 2012). For achieving the essential metabolism of specific area genomic DNA of algae is modified which also improves the performance in harsh environment. Nontransgenic methods require proper assessment for obtaining new characteristics and it also improves performance and produces best algal strains which can survive under broad scale of abiotic and biotic circumstances (Flynn et al. 2010). But after doing these changes it improvse one characteristic but at the same time it makes other traits harmful, for example, after metabolic changes fuel and synthesis of energy is enhanced but simultaneously it results in the production of such algal strains which are harmful for food and non-food applications (Hall and Benemann 2011). The reason for having such side effects of metabolic changes and genetic improvements is that these changes are made by focusing only on by-products which are produced during process like medical products or cosmetics. By-products are focused because they are used as a compensation for the production cost (Rismani-Yazdi et al. 2011).

This technique of algal engineering was suggested by Snow and Smith (2012) for both organic and inorganic growth since about all of the algal strains can be separated, restructured and hybridized to rapid cultivation even in harsh environments. Snow and Smith (2012) concluded that genetically engineered algae technology can be used to supplement both organic and inorganic cultivation. However, only with the aid of the public and business investors this will become a realistic possibility. As a result of this support, the high price of a working commercialized PBR could be significantly reduced (Davison 2005). Tabatabaei et al. (2011) identified low rates of growth and an impoverished gene as the two most significant problems of recombinant algal production, both of them could have an impact on the global marketing of algae in the future. Organizations like “Sapphire Energy and Monsanto” are trying to develop few new synthetic genes that could enhance the growth as well as other beneficial characteristics.

Few genes and pathways might affect biofuel production by “Dunaliella tertiolecta” that is flagellate from marine ecosystem, these are described by Rismani-Yazdi et al. 2011. He described that DNA modification can increase the growth rate and ability to use nitrogen. But there should be a suicidal gene which controls the survival of dangerous algal strains in the environment after an accidental escape (Tabatabaei et al. 2011; Perls 2017). Control of these dangerous algal strain is necessary as they have high risk of polluting environment (Quinn and Davis 2015).

2.9 Effect on Environment

The interaction between the algal fuel industry and the environment is critical to realizing the vision of a continuous and limited industry of fuel based upon algae. By keeping all challenges in consideration all remedies of these problems must be environment friendly and must not confronted with the same issues as the traditional fuel industry. Few techniques like foot printing are used to find out nature of biofuels whether they are green and environment friendly or not (Dassey et al. 2014; Farooq et al. 2015). Therefore emerging techniques should be according to claims and action and focus on clean manufacturing of renewable fuels. Farming of seaweed is the traditional way of harvesting microalgae for many applications. But it has negative effect on environment (Dunningham and Atack 2014; Titlyanov and Titlyanova 2010).

If the farms are not built in accordance with the rules, they could pose a serious issue to aquatic assets and ecosystem balance. As a result of this, the farm should be built in a location where the seaweed planting would not restrain with the natural aquatic environment nor will it pose a threat to the biological clock of the local fish. Aquatic ecosystems like sea possess a synergistic relationship with farming systems for fishes. According to some estimates, the population of fish may act as danger for farming of a few aquatic weed species, however, fluctuations in temperature of water may cause fish populations to migrate in the direction of the seaweed farm, allowing it to be established. In other words, to restrict the fish population in a limited area, it is an even more complex problem issue than the growth of microalgae in aquatic system. Contrastingly cultivation of wild microalgae in beach cast area and eutrophic zones might have positive effect in the cleaning of coastal zones.

Enormous amount of seaweed is accumulated which is used for production of biofuels and aid in balancing the environment and ecology as well as help in biofuels market. Marine algae plays important role in ocean environment. It manages the function and balance of marine habitats and helps in stability of marine environment. If there is no control and cultivation is not used properly then harvesting of macroalgae in an open sea farm might be invasive. Because it results in depletion in biodiversity, changes in environmental structure and can cause economic loss as well (Wang et al. 2013). When water balance is changed, attack on marine environment by wild macroalgae has been reported. This would allow for contaminated seawater to clean up and huge accumulations of algal biomass to accumulate and process it to fuel instead of cultivating macroalgae for fuel algae. Although this method is in favour of production of biofuel from algae but periodic changes and environment of sea may not be stable for providing a consistent amount of algal biomass for industrial scale. Another issue inside the macroalgae farmlands is the impact of human activity on the environment. As a result of the growing demand for macroalgae-derived compounds, the number of manufacturing facilities in developing countries is also increasing. Farmlands with a low level of expertise and untrained workers can be terribly damaging to water ecology. In order to ensure long-term production, the overall process should be modified and comprehensive to account for any potential instrumentation or other problems that may arise, which is difficult given the volume of continuous growth. Regulations in the USA and the European Union, as well as international and local legislation, secure the rights of individuals, the concerns about biofuels, such as algae (Benson et al. 2014; McGraw 2009).

Targets are set by European Commission about greenhouse gas emission, CO2 reduction, development of continuous technology and usage of algal biofuel, etc. it is estimated that till the year 2020 reduction of 20% was the main goal (Ribeiro et al. 2015). But to secure energy is the main problem of developing countries (Saifullah et al. 2014). In order to talk about energy and management problems, the most important issues to consider are access to electricity, utilization of fuels for utilization of man, and presuming enough power for today fast growing world and in the succeeding era (Rimmer et al. 2015). Political fluctuations, reliance on the few limited petroleum providing organization, high prices of fuel and the occupation of limited countries on trading of energy reservoirs all contribute to issues of energy insecurity. There should be balance between energy which is produced from fossil fuels and any other energy sources which provide reliable, safe, cheap energy for consumption at large scale (Kose and Oncel 2017).

2.10 Conclusion

From above discussion it is concluded that biofuels are the basic need of today’s world because they are environment friendly. Fuels obtained from fossil fuels are harmful for environment; result in emission of greenhouse gases and cause global warming. So we need to move towards safe fuels that do not cause any harm to the environment. Algae are best option for producing biofuels because algae not only could be processed into all types of fuels like bioethanol, biohydrogen, etc. algal fuels can be used for making blend and emulsifiers which improve the engine performance but also have easier and simple procedure for cultivation. From future perspective it is important to do more research on genetic engineering of algae so that we produce specifically that species of algae which gives maximum fuel production because fuel production from algae is eco-friendly, cheap and time saving. It might be possible that in future algal biofuels or biofuels may totally replace fossil fuels.