The combustion products of hydrogen when it is burned completely with air consist of water, oxygen, and nitrogen. However, it has been suggested that hydrogen is too valuable to burn. Laboratory tests conducted on internal combustion engines burning hydrogen demonstrate good performance (Berry et al. 1996). In comparison with an engine burning gasoline, the emission of nitrogen oxides is far less for the engine — fueled hydrogen. The product of hydrogen combustion with air is water vapor and negligible pollution when the peak temperature is limited. Some oxides of nitrogen (NOx/ are formed at very high combustion temperatures (<2,300K); fortunately, the autoignition temperature of hydrogen is only 858 K.

Hydrogen has good properties as a fuel for internal combustion engines in au­tomobiles. Some of the characteristic properties of a hydrogen-air mixture that can definitely influence engine design and performance are low ignition energy, low density, wide range of ignition limits, high diffusion speed, and high flame speed (Plass Jr. et al. 1990).

The main disadvantage of using hydrogen as a fuel for automobiles is the huge on-board storage tanks that are required because of hydrogen’s extremely low den­sity. Hydrogen may be stored on board a vehicle as compressed gas in ultra-high- pressure vessels, as a liquid in cryogenic containers, or as a gas bound with certain metals in metal hydrides.

Hydrogen is one of the most promising alternative energy technologies. Hydro­gen can be generated in a number of ways, such as electrochemical processes, ther­mochemical processes, photochemical processes, photocatalytic processes, or pho­toelectrochemical processes (Momirlan and Veziroglu 1999, 2002). Biohydrogen production by anaerobic fermentation of renewable organic waste sources has been found to be a promising method for the recovery of bioenergy (Han and Shin 2004).

In this method, anaerobic bacteria use organic substances as the sole source of elec­trons and energy, converting them into hydrogen.

The use of hydrogen as a fuel for transportation and stationary applications is receiving much favorable attention as a technical and policy issue (Cherry 2004). Hydrogen gas is being explored for use in combustion engines and fuel-cell electric vehicles. It is a gas at normal temperatures and pressures, which presents greater transportation and storage hurdles than exist for liquid fuels. Several hydrogen tech­nologies are under development; the most promising of these is the fuel cell. Fuel cells use hydrogen, oxygen, a catalyst, and an electrolytic solution to produce energy in the form of heat and electricity.

Most current research on oil extraction is focused on microalgae to produce biodiesel from algal oil. The biodiesel from algal oil in itself is not significantly different from biodiesel produced from vegetable oils.

Dilution, microemulsification, pyrolysis, and transesterification are the four tech­niques applied to solve the problems encountered with high fuel viscosity. Of the four techniques, transesterification of oil into its corresponding fatty ester (biodiesel) is the most promising solution to the high viscosity problem. This is accomplished by mixing methanol with sodium hydroxide to make sodium methox — ide. This liquid is then mixed into vegetable oil. The entire mixture then settles and glycerin is left on the bottom while methyl esters, or biodiesel, is left on top. Biodiesel can be washed with soap and glycerin using a centrifuge and then filtered. Kinematic viscosities of the fatty acid methyl esters vary from 3.23 to 5.61 mm2/s (Knothe 2005). Methanol is preferred for transesterification because it is less expen­sive than ethanol (Graboski and McCormick 1998).

For production of biodiesel, a macroalga (Cladophora fracta) sample and a mi­croalga (Chlorella protothecoides) sample were used in one study (Demirbas 2009b). Proximate analysis data and higher heating values of algae samples are given in Ta­ble 6.1. As seen in Table 6.2, the higher heating value of Chlorella protothecoides (25.1 MJ/kg) is also higher than that of Cladophora fracta (21.1MJ/kg). Moisture content was determined by drying a 3- to 5-g sample at 378 K to constant weight (Demirbas 1999), ashing was carried out at 1,025 K for 2h (Demirbas 2001), and protein content was determined by the block digestion method and ether-extractable intramuscular fat content by solvent extraction (Boccard et al. 1981). Table 6.3 shows the average chemical composition of algae samples. The oil proportion from the lipid fractions of Chlorella protothecoides is considerable higher than that of Cladophora

fracta (Demirbas 2009b). Figure 6.2 shows the production of biodiesel from algae.

Oils were obtained by extracting algae with hexane in a Soxhlet extractor for 18 h. Transesteriflcation of algal oils was performed in a 100-mL cylinder using su­percritical methanol according to earlier methods (Kusdiana and Saka 2001; Demir — bas 2002). The fatty acids of the algal oils were fractionated into saturated, mo-

Figure 6.2 Production of biodiesel from algae

nounsaturated, polyunsaturated, and free forms by a preparative chromatographic thin layer on a glass plate coating with a 0.25-pm polyethanol succinate.

The fatty acid compositions of algal oils are given in Table 6.4. Fatty acids come in two varieties: saturated and unsaturated. Saturated fats come from animal prod­ucts such as meat and dairy. Most vegetable oils are unsaturated. The properties of the various individual fatty esters that comprise biodiesel determine the overall fuel properties of the biodiesel fuel. As seen in Table 6.4, the average polyunsat­urated fatty acids of Chlorella protothecoides (62.8%) are also higher than those of Cladophora fracta (50.9%). Algae generally produce a lot of polyunsaturates, which may present a stability problem since higher levels of polyunsaturated fatty acids tend to decrease the stability of biodiesel. However, polyunsaturates also have much lower melting points than monounsaturates or saturates; thus algal biodiesel should have much better cold weather properties than many other bio-oils (Demir — bas 2009b).

Xu et al. (2006) used Chlorella protothecoides (a microalga) for the production of biodiesel. Cells were harvested by centrifugation, washed with distilled water, and then freeze dried. The main chemical components of heterotrophic C. protothe — coides were measured as in a previous study (Miao et al. 2004). Microalgal oil was prepared by pulverization of heterotrophic cell powder in a mortar and extraction with n — hexane.

Biodiesel was obtained from heterotrophic microalgal oil by acidic transester — iflcation. Figure 6.3 shows the process flow schematic for biodiesel production (Xu et al. 2006). The optimum process combination was 100% catalyst quantity (based on oil weight) with 56:1 molar ratio of methanol to oil at a temperature of 303 K, which reduced product-specific gravity from an initial value of 0.912 to a fi­nal value of 0.864 in about 4 h of reaction time (Xu et al. 2006).

The technique of metabolic control through heterotrophic growth of C. protothe­coides was applied, and the heterotrophic C. protothecoides contained a crude lipid content of 55.2%. To increase the biomass and reduce the cost of algae, corn powder hydrolysate instead of glucose was used as an organic carbon source in heterotrophic culture medium in fermenters. The result showed that cell density significantly in­creased under heterotrophic conditions, and the highest cell concentration reached 15.5 g/L. A large amount of microalgal oil was efficiently extracted from the het­erotrophic cells using n-hexane and then transmuted into biodiesel by acidic trans­esterification (Xu et al. 2006).

Other system designs for algae production are possible. The Japanese, French, and German governments have invested significant R&D dollars on novel closed biore­actor designs for algae production. The main advantage of such closed systems is that they are not as subject to contamination with whatever organism happens to be carried in the wind.

When designing a photobioreactor, design parameters such as reactor dimen­sion, flowrate, light requirements, culture condition, algae species, reproducibility, and economic value need to be taken into consideration. Depending on the reac­tor dimensions, site location, and local climate, these parameters can determine the type of cultivation system needed (open versus closed). Reactor design should have good mixing properties, efficiency, and reproducibility and be easy to maintain and sterilize. An efficient photobioreactor not only improves productivity but also is used to cultivate multiple strains of algae. The performance of a photobioreactor is measured by volumetric productivity, areal productivity, and productivity per unit of illuminated surface (Riesing 2006). Volumetric productivity is a function of biomass concentration per unit volume of bioreactor per unit of time. Areal productivity is defined as biomass concentration per unit of occupied land per unit of time. Produc­tivity per unit of illuminated surface is measured as biomass concentration per area per unit of time.

Closed bioreactors support up to fivefold higher productivity with respect to re­actor volume and consequently have a smaller “footprint” on a yield basis. Besides saving water, energy, and chemicals, closed bioreactors have many other advantages that are increasingly making them the reactor of choice for biofuel production, as their costs are lower (Schenk et al. 2008). Closed bioreactors permit essentially single-species culture of microalgae for prolonged periods. Most closed bioreactors are designed as tubular reactors, plate reactors, or bubble column reactors (Weiss — man et al. 1988; Pulz 2001). Other less common designs like semihollow spheres have been reported to run successfully (Sato et al. 2006).

Enclosed photobioreactors have been employed to overcome the contamination and evaporation problems encountered in open ponds (Molina Grima et al. 1999). These systems are made of transparent materials and are generally placed outdoors for illumination by natural light. The cultivation vessels have a large surface-area — to-volume ratio.

The main problems in the large-scale cultivation of microalgae outdoors in open ponds are low productivity and contamination. To overcome these problems,

a closed system consisting of polyethylenes sleeves was developed. In a study con­ducted outdoors. The closed system was found to be superior to open ponds with respect to growth and production in a number of microalgae. In both closed and open systems, growth and production under continuous operation were higher than in batch cultivation (Cohen et al. 1991).

Sananurak et al. (2009) have designed, built, and operated a closed, recirculat­ing, continuous culture system to produce microalgae and rotifers in seawater (25% salinity) for larval fish culture. The system opens up a new perspective in terms of automated production of rotifers without labor cost. Rotifers can be easily harvested daily by a conical harvest net, and there is no routine maintenance work. This new, automated system has three components: a microalgae culture, a rotifer culture and storage with harvest, and a water treatment and reuse component.

The preferred alternative is closed photobioreactors, where the algae fluid re­mains in a closed environment to enable accelerated growth and better control over environmental conditions. These glass or plastic enclosures, often operated under modest pressure, can be mounted in a variety of horizontal or vertical configura­tions and can take many different shapes and sizes. Rigid frameworks or structures are usually used to support the photobioreactor enclosures.

Open systems using a monoculture are also vulnerable to viral infection. The en­ergy that a high-oil strain invests in the production of oil is energy that is not invested in the production of proteins or carbohydrates, usually resulting in the species being less hardy or having a slower growth rate. Algal species with lower oil content, not having to divert their energies away from growth, have an easier time in the harsher conditions of an open system.

Closed systems (not exposed to open air) do not have the problem of contami­nation by other organisms blown in by the air. The problem for a closed system is finding a cheap source of sterile CO2. Several experimenters have found the CO2 from a smokestack works well for growing algae.

In hybrid systems, both open ponds as well as closed bioreactor system are used in combination to get better results. Open ponds are a very proficient and lucrative method of cultivating algae, but they become contaminated with superfluous species very quickly. A combination of both systems is probably the most logical choice for cost-effective cultivation of high yielding strains for biofuels. Open ponds are in­oculated with a desired strain that had invariably been cultivated in a bioreactor, whether it is as simple as a plastic bag or a high-tech fiber-optic bioreactor. Impor­tantly, the size of the inoculums needs to be large enough for the desired species to establish in the open system before an unwanted species. Therefore, to minimize contamination issues, cleaning or flushing the ponds should be part of the aquacul­ture routine, and as such, open ponds can be considered as batch cultures (Schenk etal. 2008).

Abundant light, which is necessary for photosynthesis, is the third requirement. This is often accomplished by situating the facility in a geographic location with abundant, uninterrupted sunshine such as the American Southwest (Brown and Zeiler 1993). This is a favored approach when cultivating in open ponds. When working with bioreactors, sunlight quantity and quality can be further enhanced through the use of solar collectors, solar concentrators, and fiber optics in a system called photobioreactors (Scott and Bryner 2006; Chisti 2007). These technologies allow optimal sunlight to reach algal cells either by allowing them to float in arrays of thin, horizontal tubes or by directing light, through a fiber-optic matrix, through the bioreactor chamber itself.

The pH level generally increases as the microalgae consume CO2. Addition of carbon dioxide along the reactor would sustain microalgal growth by preventing car­bon limitation and an excess rise in pH. However, tubular photobioreactors do not work well in large-scale production because the surface-to-volume ratio is lower, causing poor light absorption. Length of tubes is another concern of tubular photo­bioreactors. As the tube length increases, the time for microalgae exposure to light increases, hence increasing the absorption of available CO2 and increasing photo­synthesis rate. However, the dissolved oxygen level also increases, which can easily lead to oxygen poisoning, and photoinhibition can result from the excess light ex­posure (Ogbonna and Tanaka 1997).

Biorefinery refers to the conversion of biomass feedstock into value-added chemi­cals and fuels with minimal waste and emissions. Some current aspects of biore­finery research and development since the early 1990s are presented, revealing that integrated processes, biomass upgrading technology, and biorefinery technol­ogy have become objects of research and development. Many of the currently used biorenewable-based industry products are the results of direct physical, catalytic, or chemical treatment and processing of biomass feedstocks.

The primary objective refining of pyrolysis products is to obtain valuable fuels like gasoline, diesel and jet fuel, and chemicals from biomass. Figure 7.7 shows the products from biomass by pyrolysis-based refining.

In vacuum pyrolysis, biomass is heated in a vacuum in order to decrease the boiling point and avoid adverse chemical reactions. In flash vacuum thermolysis

(FVT), the residence time of the substrate at the working temperature is limited as much as possible, again in order to minimize secondary reactions.

Vacuum pyrolysis is the thermal degradation of a feedstock in the absence of oxygen and under low pressure to produce a bio-oil and char as main products, to­gether with water and noncondensable gases. Both the bio-oil and char have a high energy content and may be used as fuels. An incredible number of chemical com­pounds are also found in the bio-oil, and these compounds can be extracted and sold as high-value chemicals.

Vacuum pyrolysis is a relatively new variant of pyrolysis with many recycling applications. During vacuum pyrolysis of biomass, the biorenewable feedstock is thermally decomposed under reduced pressure. The quick removal of the vapors reduces the residence times of the macromolecules and hence minimizes secondary decomposition reactions such as cracking, repolymerization, and recondensation, which occur during atmospheric pyrolysis. Temperatures between 675 K and 775 K and pressures of about 0.15 atm are typically used. Table 7.8 compares pyrolysis and gasification processes.

Bio-oil contains the thermally cracked products of the original cellulose, hemi- celluloses, and lignin fractions present in biomass. It also contains a high percentage of water, often as high as 30%. The total oil is often homogeneous after quenching but can easily be separated into two fractions, a water-soluble fraction and a heav­ier pyrolytic lignin fraction. The addition of more water allows the pyrolytic lignin fraction to be isolated, and the majority of it consists of the same phenolic polymer as lignin but with smaller-molecular-weight fragments. Bio-oils are composed of a range of cyclopentanone, methoxyphenol, acetic acid, methanol, acetone, furfural, phenol, formic acid, levoglucosan, guaioco, l and their alkylated phenol derivatives.

Products, wt%

Pyrolytic lignin is a better feedstock for liquid fuel production than the water-soluble fraction because of its lower oxygen content, and therefore the study focused on evaluating it as a potential feedstock for the production of highly aromatic gasoline (Demirbas 2000).

The pyrolysis of biomass is a thermal treatment that results in the production of charcoal, liquid, and gaseous products. Among the liquid products, methanol is one of the most valuable. The liquid fraction of the pyrolysis products consists of two phases: an aqueous phase containing a wide variety of organooxygen compounds of low molecular weight and a nonaqueous phase containing insoluble organics of high molecular weight. This phase is called tar and is the product of greatest interest. The ratios of acetic acid, methanol, and acetone of the aqueous phase are higher than those of the nonaqueous phase.

The bio-oil formed at 725 K contain high concentrations of compounds such as acetic acid, 1-hydroxy-2-butanone, 1-hydroxy-2-propanone, methanol, 2,6-dimeth — oxyphenol, 4-methyl-2,6-dimetoxyphenol, 2-cyclopenten-1-one, etc. A significant characteristic of bio-oils is the high percentage of alkylated compounds, especially methyl derivatives (Demirbas 2007).

The concept of using algae as a fuel was first proposed by Meier (1955) for the pro­duction of methane gas from the carbohydrate fraction of cells. This idea was further developed by Oswald and Golueke (1960), who introduced a conceptual technoeco­nomic engineering analysis of digesting microalgal biomass grown in large raceway ponds to produce methane gas. In the 1970s, as the cost of conventional fuels began rising rapidly, the possibility of using algae as a fuel source received renewed atten­tion. A more detailed design and engineering analysis of this concept was carried out by Benemann et al. (1978), who concluded that such systems could produce biogas competitively with projected fossil fuel prices.

Anaerobic digestion of biowastes occurs in the absence of air, and the resulting gas, known as biogas, is a mixture consisting mainly of methane and carbon dioxide. Biogas is a valuable fuel that is produced in digesters filled with feedstock like dung or sewage. The digestion is allowed to continue for a period of 10 d to a few weeks.

Algal biomass can be used for biogas production. In Poland, there are numer­ous active biogas installations, from large-scale ones to small ones fed with straw and green plant fuel that serve a few farms; so far, however, algae have not been used as a fuel. Some macroalgal species like Macrocystis pylifera and genera such as Sargassum, Laminaria, Ascophyllum, Ulva, Cladophora, Chaetomorpha, and Gracilaria have been explored as potential methane sources (Filipkowska et al. 2008). But in spite of the large seaweed biomass in various regions of the world, anaerobic digestion for biogas generation appears to be unsatisfactory and therefore uneconomical (Gunaseelan 1997; Caliceti et al. 2002).

Anaerobic digestion of algal waste produces carbon dioxide, methane, and am­monia. Leftover nitrogen and phosphorus compounds can be reused as fertilizer for the algal process. Using methane as an energy source can further enhance energy recovery from the process.

Researchers have highlighted some key issues to be addressed in microalgal pro­duction:

— Sodium (in salt) can inhibit the anaerobic digestion process when using marine algae, although researchers say that suitable bacteria (anaerobic digesters) can adapt.

— Digestion of algae can be enhanced and the methane yield increased by physical or chemical pretreatment to break down cell walls and make the organic matter in the cells more accessible.

— The nitrogen content of certain algae can be high, resulting in greater levels of ammonia, which can also inhibit the digestion process. One strategy to overcome this problem uses a “codigestion” process, whereby other organic waste, which is higher in carbon and lower in nitrogen, is added to the algal waste.

3.9.1 Glycerol-based Fuel Oxygenates for Biodiesel and Diesel Fuel Blends

Glycerol (1,2,3-propanetriol or glycerine) is a trihydric alcohol. It is a colorless, odorless, sweet-tasting, syrupy liquid. It melts at 291 K, boils with decomposition at 563 K, and is miscible with water and ethanol (Perry and Green 1997). The chem­ical formula for glycerol is OH-CH2-CH(OH)-CH2-OH. Glycerol is present in the form of its esters (triglycerides) in vegetable oils and animal fats.

Glycerol is a byproduct obtained during the production of biodiesel. As biodiesel production is increasing exponentially, the crude glycerol generated from the trans­esterification of vegetables oils has also been generated in large quantities (Pachauri and He 2006). With the increasing production of biodiesel a glut of glycerol has been created, causing market prices to plummet. This situation warrants finding al­ternative uses for glycerol. Glycerol is directly produced with high purity levels (at least 98%) by biodiesel plants (Ma and Hanna 1999; Bournay et al. 2005). Re­search efforts to find new applications of glycerol as a low-cost feedstock for func­tional derivatives have led to the introduction of a number of selective processes for converting glycerol into commercially valued products (Pagliaro et al. 2007). The principal byproduct of biodiesel production is crude glycerol, which is about 10 wt% of vegetable oil. For every 9 kg of biodiesel produced, about 1 kg of a crude glycerol byproduct is formed (Dasari et al. 2005).

Oxygenated compounds such as methyl tertiary butyl ether (MTBE) are used as valuable additives as a result of their antidetonant and octane-improving properties. In this respect, glycerol tertiary butyl ether is an excellent additive with a large potential for diesel and biodiesel reformulation.

Glycerol can be converted into higher-value products. The products are 1,3- propanediol, 1,2-propanediol, dihydroxyacetones, hydrogen, polyglycerols, suc­cinic acid, and polyesters. The main glycerol-based oxygenates are 1,3-propanediol, 1,2-propanediol, propanol, glycerol tert-butyl ethers, ethylene glycol, and propylene glycol.

Algae biomass cultivation confers four important potential benefits that other sources don’t have. First, algae biomass can be produced at extremely high volumes, and this biomass can yield a much higher percentage of oil than other sources. Second, al­gal oil has limited market competition. Third, algae can be cultivated on marginal land, fresh water, or sea water. Fourth, innovations to algae production allow it to be­come more productive while consuming resources that would otherwise be consid­ered waste (Campbell 2008).

Biodiesel derived from oil crops is a potential renewable and carbon-neutral al­ternative to petroleum fuels. Unfortunately, biodiesel from oil crops, waste cooking oil, and animal fat cannot realistically satisfy even a small fraction of the existing demand for transport fuels. Microalgae appear to be the only source of renewable biodiesel that is capable of meeting the global demand for transport fuels.

Biodiesel production from biorenewable sources has a number of problems. First, most biorenewable sources, such as waste oil, animal fat, and vegetable oil, have a limited supply (Ma and Hanna 1999). Second, many of these sources have com­petitive uses, such as food or cosmetic production. Extensive use of renewable oils may cause other significant problems such as starvation in poor and developing countries (Demirbas 2007). Third, the resources that were used to create the biomass have competition with other uses, and this includes arable land. Third, because of the limited supply and competition, many sources of biomass have become increas­ingly expensive (Haag 2007).

Like plants, microalgae use sunlight to produce oils, but they do so more ef­ficiently than crop plants. Oil productivity of many microalgae greatly exceeds the oil productivity of the best producing oil crops. Approaches to making mi­croalgal biodiesel economically competitive with petrodiesel have been discussed (Chisti 2007). Biodiesel derived from green algae biomass has the potential for high- volume, cost-effective production (Campbell 2008).

Laboratory studies exploring methods to maximize both density and oil content have demonstrated that there is yet much unrealized potential. Xu et al. (2006) cul­tivated the algae Chlorella protothecoids in a light-deprived, heterotrophic environ­ment with inexpensive hydrolyzed corn starch as the sole food source. The algae were not only able to adapt to this environment, but they reached a high population density of 15.5 g/L.

Biodiesel from microalgae seems to be the only renewable biofuel that has the potential to completely displace petroleum-derived transport fuels without adversely affecting the food supply and other crop products. Most productive oil crops, such as oil palm, do not come close to microalgae in being able to sustainably provide the necessary amounts of biodiesel. Similarly, bioethanol from sugar cane is no match for microalgal biodiesel (Chisti 2008).

Microalgae contain lipids and fatty acids as membrane components, storage products, metabolites, and sources of energy. Algae present an exciting possibil­ity as a feedstock for biodiesel, especially when you realize that oil was originally formed from algae.

In order to have an optimal yield, these algae need to have CO2 in large quanti­ties in the basins or bioreactors where they grow. Thus, the basins and bioreactors need to be coupled with traditional electricity-producing thermal power centers that produce CO2 at an average rate of 13% of the total flue gas emissions. The CO2 is put into the basins and assimilated by the algae. It is thus a technology that recycles CO2 while also treating used water.

Algae can grow practically anywhere where there is enough sunshine. Some al­gae can grow in saline water. All algae contain proteins, carbohydrates, lipids, and nucleic acids in varying proportions. While the percentages vary with the type of algae, there are algae types that are comprised of up to 40% of their overall mass by fatty acids (Becker 1994). The most significant distinguishing characteristic of algal oil is its yield and, hence, its biodiesel yield. According to some estimates, the yield (per acre) of oil from algae is over 200 times the yield from the best­performing plant/vegetable oils (Sheehan et al. 1998). Microalgae are the fastest growing photosynthesizing organisms. They can complete an entire growing cycle every few days. Approximately 46 tons of oil/ha/year can be produced from diatom algae. Different algae species produce different amounts of oil. Some algae produce up to 50% oil by weight. The production of algae to harvest oil for biodiesel has not been undertaken on a commercial scale, but working feasibility studies have been conducted to arrive at the above number.

Microalgae are very efficient solar energy converters, and they can produce a great variety of metabolites (Chaumont 2005). The culture of algae can yield 30 to 50% oil (Chisti 2007; Dimitrov 2008). Oil supply is based on the theoretical claims

that 47,000 to 308,000L/ha/year of oil could be produced using algae. The calcu­lated cost per barrel would be only $ 20 (Demirbas 2009a). Currently, a barrel of oil in the US market sells for over $ 100. Despite all the claims and research dating from the early 1970s to date, none of the projected algae and oil yields have been achieved (Patil et al. 2005). Algae, like all plants, require large quantities of nitrogen fertilizer and water, plus significant fossil energy inputs for the functioning system (Goldman and Ryther 1977). Harvesting the algae from tanks and separating the oil from the algae are difficult and energy-intensive processes (Pimentel et al. 2004; Pimentel 2008).

Fatty acids come in two varieties: saturated and unsaturated. Saturated fats come from animal products such as meat and dairy. Most vegetable oils are unsaturated. The properties of the various individual fatty esters that comprise biodiesel de­termine the overall fuel properties of the biodiesel fuel. Algae generally produce a lot of polyunsaturates, which may present a stability problem since higher levels of polyunsaturated fatty acids tend to decrease the stability of biodiesel. However, polyunsaturates also have much lower melting points than monounsaturates or sat­urates; thus algal biodiesel should have much better cold-weather properties than many other bio-oils (Demirbas 2009b). Algae are theoretically a very promising source of biodiesel. The lipid and fatty acid contents of microalgae vary in accor­dance with culture conditions. In some cases, lipid content can be enhanced by the imposition of nitrogen starvation or other stress factors. Which is the best species of algae for biodiesel? There is no one strain or species of algae that can be said to be the best in terms of oil yield for biodiesel. However, diatoms and secondly green algae have shown the most promise. Scenedesmus dimorphus is a unicellular alga in the class Chlorophyceae (green algae). While this is one of the preferred species for oil yield for biodiesel, one of the problems with Scenedesmus is that it is heavy and forms thick sediments if not kept in constant agitation. The strain known as Dunaliella tertiolecta has an oil yield of about 37% (organic basis). D. tertiolecta is a fast growing strain, which means it has a high CO2 sequestration rate as well (Demirbas 2009a, b; Ozkurt 2009). Table 6.5 shows the yield of various plant oils.

Certain algae strains also produce polyunsaturated fatty acids (omega-3s) in the form of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) generally found in fish oils. Phototrophic microalgae are used to provide polyunsaturated fatty

acids (omega-3 and omega-6) for aquaculture operations. These additional products greatly enhance the overall marketability and economics of producing algae (Volk — man et al. 1989; Yaguchi et al. 1997; Vazhappilly and Chen 1998).

A selection of algae strains with the potential to be used for the production of oils for biofuel is presented in Table 6.6. A major current problem for the com­mercial viability of biodiesel production from microalgae is the low selling price of biodiesel (less than US$ 1.38/kg). Microalgal oils can potentially completely replace petroleum as a source of hydrocarbon feedstock for the petrochemical industry.

Table 6.6 Oil content in selected microalgal species Species Oil content (% dw) Reference

Producing microalgal biomass is generally more expensive than growing crops. Pho­tosynthetic growth requires light, carbon dioxide, water, and inorganic salts. Tem­perature must remain generally within a range of 293 to 303 K. To minimize ex­pense, biodiesel production must rely on freely available sunlight, despite daily and seasonal variations in light levels (Chisti 2007).

Algae cultivation has four basic, and equally important, requirements: carbon, water, light, and space. By maximizing the quality and quantity of these require­ments, it is possible to maximize the quantity of oil-rich biomass and the return on investment. In order to maximize algal growth, CO2 needs to be provided at very high levels, much higher than can be attained under natural conditions. Rather than becoming an expense, this need for CO2 fertilization creates a unique opportunity to offset costs by consuming air pollution. The flue gases from industrial processes, and in particular from power plants, are rich in CO2 that would normally be released directly into the atmosphere and thereby contribute to global warming. By divert­ing the CO2 fraction of the flue gas through an algae cultivation facility, the CO2 can be diverted back into the energy stream and the rate of algal production can be greatly increased (Pulz 2007). Although most of the CO2 will ultimately be de­posited in the atmosphere, we can realize a greater energy return for each molecule of carbon.

Water, containing the essential salts and minerals for growth, is the second re­quirement. Fresh water is a valuable resource as are the salts and minerals needed; however, algae cultivation can be coupled to another type of environmental remedi­ation that will enhance productivity while mitigating pollution. High nutrient waste­water from domestic or industrial sources, which may already contain nitrogen and phosphate salts, can be added to the algal growth medium directly (Schneider 2006).

This allows for inexpensive improvement in algae production along with simulta­neous treatment of wastewater. Alternatively, salt water can be used, either from a saline aquifer or sea water. This means that competition for water will be low.

The main advantages of second-generation microalgal systems are that they (1) have a higher photon conversion efficiency, (2) can be harvested batchwise nearly year round providing a reliable and continuous supply of oil, (3) can utilize salt and wastewater streams, thereby greatly reducing freshwater use, (4) can cou­ple CO2-neutral fuel production with CO2 sequestration, and (5) produce nontoxic and highly biodegradable biofuels. Current limitations exist mainly in the harvest­ing process and in the supply of CO2 for high efficiency production (Schenk et al. 2008).

Prior economic-engineering feasibility analyses have concluded that even the simplest open-pond systems, including harvesting and algal biomass processing equipment, would cost at least $ 100,000 per hectare, and possibly significantly more. To this would need to be added operating costs. Algae production requires a site with favorable climate, available water (which can be saline, brackish, or wastewater), a ready and essentially free source of CO2, nearly flat land, and a clay soil or liner, as plastic liners would be too expensive.

Nutrients such as phosphorus must be supplied in significant excess from phos­phates complex with metal ions; therefore, not all the added phosphorus is bioavail­able. Sea water supplemented with commercial nitrate and phosphate fertilizers and a few other micronutrients is commonly used for growing marine microalgae (Molina Grima et al. 1999). Genetic and metabolic engineering is likely to have the greatest impact on improving the economics of production of microalgal diesel (Roessler et al. 1994; Dunahay et al. 1996).

Growth media are generally inexpensive. Microalgal biomass contains approx. 50% carbon by dry weight (Sanchez Miron et al. 2003). All of this carbon is typi­cally derived from CO2. Producing 100 tons of algal biomass fixes roughly 183 tons of carbon dioxide. Feeding controlled in response to signals from pH sensors mini­mizes loss of CO2 and pH variations.

Algae can grow practically anywhere where there is enough sunlight. Some al­gae can grow in saline water. All algae contain proteins, carbohydrates, lipids, and nucleic acids in varying proportions. While the percentages vary with the type of algae, there are algae types whose overall mass is comprised of up to 40% fatty acids (Becker 1994). The most significant distinguishing characteristic of algal oil is its yield and, hence, its biodiesel yield. According to some estimates, the yield (per acre) of oil from algae is over 200 times the yield from the best-performing plant/vegetable oils (Sheehan et al. 1998). Microalgae are the fastest-growing pho — tosynthesizing organisms. They can complete an entire growing cycle every few days. Approximately 46 tons of oil/ha/year can be produced from diatom algae. Different algae species produce different amounts of oil. Some algae produce up to 50% oil by weight. The production of algae to harvest oil for biodiesel has not been undertaken on a commercial scale, but working feasibility studies have been conducted to arrive at the above number. Specially bred mustard varieties can pro­duce reasonably high oil yields and have the added benefit that the meal left over after the oil has been pressed out can act as an effective and biodegradable pesticide (Demirbas 2009c).

Microalgae are very efficient solar energy converters and can produce a great variety of metabolites (Chaumont 1993). A culture of algae can yield 30 to 50% oil. Oil supply is based on the theoretical claims that 47,000 to 308,000L/ha/year of oil could be produced using algae. The calculated cost per barrel would be only $ 20. Currently, a barrel of oil in the US market is selling for over $ 100 per barrel. Despite all the claims and research dating from the early 1970s, none of the projected algae and oil yields has been achieved (Dimitrov 2008; Demirbas 2009c). Algae, like all plants, require large quantities of nitrogen fertilizer and water, plus significant fossil energy inputs for a functioning system (Goldman and Ryther 1977).

Harvesting algae from tanks and separating the oil from the algae is a difficult and energy-intensive process (Pimentel et al. 2004). One difficulty in culturing algae is that the algae shade one another and thus there are different levels of light saturation in the cultures, even under Florida conditions. This influences the rate of growth of the algae. In addition, wild strains of algae invade and dominate algae culture strains, and oil production by the algae is reduced (Biopact 2008). Another major problem with the culture of algae in ponds or tanks is the harvesting of the algae. This problem was observed at the University of Florida where algae were being cultured in managed ponds for the production of nutrients for hogs. After 2 years without success, the algal-nutrient culture was abandoned (Pimentel 2008).

In recent years, there has been increasing interest in greenhouse gas mitigation technologies. As a consequence, there has been renewed interest in microalgae mass culture and fuel production from the perspective of CO2 utilization. This is not a new concept, as Oswald and Golueke (1960) had previously emphasized the potential for microalgae systems to reduce and avoid CO2 emissions and thus reduce the potential for global warming. Indeed, microalgae have a rather unique attribute: they can utilize concentrated CO2 for growth, rather than the atmospheric levels of CO2 used by higher plants. Flue gas can be utilized in algal ponds.

Microalgae wastewater treatment uses less energy, and thus fossil fuels, than con­ventional treatment processes, resulting in a reduction in greenhouse gas emissions. Wastewater treatment processes could provide a near-term pathway to developing large-scale microalgae production processes and could find applications in the real world.

Open ponds can be categorized into natural waters (lakes, lagoons, ponds) and artificial ponds or containers. The most commonly used systems include shallow big ponds, tanks, circular ponds, and raceway ponds. The major capital costs for an open-pond system are tabulated in Table 4.1 (Weissman and Goebel 1987; Shee­han et al. 1998). Polymers can be used in very small amounts, without contributing a major cost to the overall process. The base case (30g/m2/d) capital costs were estimated at almost $ 72,000/ha, without working capital, or almost twice as high as the prior effort (Benemann et al. 1982). This was due to higher costs for many com­ponents, such as earthworks, which were several-fold higher. Among other things, higher costs were assumed for rough and fine (laser) grading, which depends on the type of site available. Also, the 1987 study estimated about $5,000/ha to provide

a 3- to 5-cm crushed-rock layer to reduce the suspension of silt from the pond bot­tom. There is, however, little evidence that such erosion prevention is needed, except perhaps for some areas around the paddlewheel and perhaps the turns. Further, the

Weissman and Goebel (1987) study selected slip form poured concrete walls and di­viders (baffles) as the design of choice. A power generation system can be specified to produce electricity from the methane generated from the algal residues (at about 10% of total costs).

Table 4.2 shows the operating costs for an open-pond system (Sheehan et al. 1998). The operating costs were discussed in terms of mixing, carbon utilization, nutrient, flocculants, salt disposal, maintenance, labor, and the accumulation of pho­tosynthetically produced oxygen (Benemann 2008).

Table 4.3 shows the comparative economics of open ponds and closed photo­bioreactors.

8.1 Introduction

During the last 200 years, developed countries have shifted their energy consump­tion toward fossil fuels. Renewable energies have been the primary energy source in the history of the human race. Wood was used for cooking and water and space heating. The first renewable energy technologies were primarily simple mechanical applications and did not reach high energetic efficiencies. Industrialization changed the primary energy use from renewable resources to sources with a much higher energetic value such as coal and oil. The promise of unlimited fossil fuels was much more attractive, and rapid technical progress made the industrial use of oil and coal economical.

Developing renewable sources of energy has become necessary due to the lim­ited supply of fossil fuels. Global environmental concerns and decreasing resources of crude oil have prompted demand for alternative fuels. Global climate change is also the major environmental issue of current times. Global warming, the Kyoto Protocol, the emission of greenhouse gases, and the depletion of fossil fuels are the topics of environmental pleadings worldwide. Due to rapidly increasing energy re­quirements along with technological development around the world, research and development activities have perforce focused on new and renewable energy.

Competition of renewable liquid fuels into petroleum liquid fuels will be impor­tant in the near future. This can be achieved by research and development (R&D), technological development, and industrial mobilization by implementing a proper energy tax system that takes into account the environmental and social costs of con­ventional energies, by making calculations based on the entire energy system and not only the cost of one technology but of the entire energy chain, by calculating the burdens for the national economy of every country from importing fossil energies, etc. This requires a comprehensive view on energy.

The calculation of energy prices should be designed to maintain into the fu­ture the equilibrium between demand and supply, taking into account the costs of planned investments. They should also take into account the rest of the economy and

A. Demirbas, M. Fatih Demirbas, Algae Energy DOI 10.1007/978-1-84996-050-2, © Springer 2010

the environment. Two very important characteristics of energy prices are equity and affordability. Energy prices must reflect the cost imposed by the specific consumer category on the economy. Since energy prices based on apparent long-run marginal costs may not be sufficient to finance the development of the energy sector, prices should be adjusted so that the energy sector can be financed without subsidies to enhance its autonomy. In competitive markets this form of adjustment may not be possible. Energy pricing policy should not be employed as an anti-inflationary in­strument. It should be applied in such a way that it does not create cross-subsidies between classes of consumers.

Hydrogen is an important fuel with wide applications in fuel cells, liquefaction of coal, and upgrading of heavy oils (e. g., bitumen). Hydrogen can be produced bio­logically by a variety of means, including the steam reformation of bio-oils, dark and photo fermentation of organic materials, and photolysis of water catalyzed by special microalgal species.

The chemical compositions of algae are given in Table 5.10 (Demirbas 2007). Algae are mainly composed of proteins, lipids, and water-soluble carbohydrates.

Two moss samples (Polytrichum commune, Thuidium tamarascinum), one alga sample (Cladophora fracta), and one microalga sample (Chlorella protothecoides) were subjected to pyrolysis and steam gasification for producing hydrogen-rich gas (Demirbas, unpublished work).

The temperature of the reaction vessel was measured with an iron-constantan thermocouple and controlled at ±3 K. The pyrolysis experiments were performed at temperatures of 575, 625, 675, 725, 775, 825, and 925 K. The steam gasification experiments were carried out at temperatures of 825, 875, 925, 975, 1,025, 1,075, 1,125, 1,175, and 1,225K (Demirbas, unpublished work).

Table 5.11 shows the proximate analysis data and higher heating values (HHVs) of samples. The HHV (MJ/kg) of the moss and alga samples as a function of fixed carbon (FC) wt% can be calculated from

HHV = 0.322 (FC) + 10.7123 (5.3)

The HHVs can be calculated using Equation 5.3 and represent high correlation ob­tained by means of regression analysis. The correlation coefficient r is 0.999.

The yields of bio-oil from the samples via pyrolysis are presented as a func­tion of temperature (K) in Figure 5.4. The yield of bio-oil from pyrolysis of the samples increased with temperature, as expected. The yields were increased up to 750 K in order to reach the plateau values at 775 K. The maximum yields for Poly­trichum commune, Thuidium tamarascinum, Cladophorafracta, and Chlorella pro — tothecoides were 31.6, 37.3, 45.0, and 50.8% of the sample at 925 K, respectively. The bio-oil yields of pyrolysis from algae were higher than those of mosses. Bio-oil comparable to fossil oil was obtained from microalgae (Miao and Wu 2004). In the pyrolysis process, the yield of charcoal decreases with increasing pyrolysis temper­ature. The yield of the liquid product is highly excessive at temperatures between 625 and 725 K.

The HHVs for bio-oils from mosses 21.5 to 24.8MJ/kg and the HHVs for bio­oils from algae and microalgae 32.5 and 39.7MJ/kg, respectively, were obtained by pyrolysis at temperatures ranging from 775 to 825 K. In general, algae bio-oils are of a higher quality than bio-oils from mosses.

Figure 5.5 shows the effect of temperature on yields of gaseous products from the samples by pyrolysis. As can be seen in Figure 5.4, the yields of gaseous products from the samples of Polytrichum commune, Thuidium tamarascinum, Cladophora fracta, and Chlorella protothecoides increased from 5.3 to 40.6%, 6.5 to 42.2%, 8.2 to 39.2%, and 9.5 to 40.6% by volume, respectively, while the final pyrolysis temperature was increased from 575 to 875 K.

Figure 5.6 shows the plots for yields of hydrogen in gaseous products from the samples by pyrolysis. The percent of hydrogen in gaseous products from the sam­ples of Polytrichum commune, Thuidium tamarascinum, Cladophora fracta, and Chlorella protothecoides increased from 21.3 to 38.7%, 23.0 to 41.3%, 25.8 to 44.4%, and 27.6 to 48.7% by volume, respectively, while the final pyrolysis tem­perature was increased from 650 to 875 K.

Figure 5.7 shows the plots for yields of hydrogen in gaseous products from the samples by steam gasification. The percent of hydrogen in gaseous products from the samples of Polytrichum commune, Thuidium tamarascinum, Cladophora fracta, and Chlorella protothecoides increased from 21.8 to 50.0%, 23.5 to 52.0%, 26.3 to 54.7%, and 28.1 to 57.6% by volume, respectively, while the final gasification temperature was increased from 825 to 1,225 K.

Figure 5.8 shows the plots for yields of hydrogen in gaseous products from mi­croalgae and wood samples by pyrolysis. The percent of hydrogen in gaseous prod­ucts from the samples of beech wood and spruce wood increased from 31.5 to 40.5% and 33.3 to 42.3% by volume, respectively, while the final pyrolysis temperature was increased from 650 to 875 K (Demirbas and Arin 2004). Microalgae gaseous prod­ucts are higher quality than gaseous products from wood (Figure 5.8). In general, algal gaseous products are of higher quality than gaseous products from mosses.

Table 5.12 shows the yields of bio-oil by pyrolysis from moss and algae samples (Demirbas 2006). As can be seen from Table 5.12, the bio-oil yield for Chlorella protothecoides (a microalgae sample) rose from 12.8 to 55.3% as the temperature rose from 575 to 775 K, and then gradually decreased to 51.8% was obtained at 875 K with a heating rate of 10K/s. The bio-oil yield for Polytrichum commune (a moss sample) rose from 10.3 to 39.1% as the temperature rose from 575 to 775 K,

and then gradually decreased to 36.7% was obtained at 875 K with a heating rate of 10K/s (Demirbas 2006). For algae, maximum bio-oil yields of between 48.2 and 46.8%, and for microalgae 55.3 and 53.7% were obtained at temperatures ranging from 775 to 825 K, whereas for wood, cotton stalk, tobacco stalk, and sunflower bagasse, maximum oil yields between 39.7 and 49.4% were obtained at tempera­tures ranging from 775 to 825 K (Putun 2002; Gercel 2002).

Table 5.13 shows the yields of gaseous product by pyrolysis from moss and algae samples (Demirbas 2006). As shown in Tables 5.12 and 5.13, the yields of gaseous products for Chlorella protothecoides increased from 9.5 to 39.5% as the tempera-

ture rose from 575 to 875 K. The char yields of pyrolysis from mosses were higher than those of algae.

With the interaction of water and char from decomposition of biomass occur in­termediate products, which leads to more hydrogen-rich gas yield by steam reform­ing. The pyrolysis is carried out at moderate temperatures and steam gasification at the highest temperatures. In order to clarify the steam gasification mechanism in detail, more kinetic study is necessary. These results suggest that the fundamen­tal information obtained in the gasification of each component could possibly be used to predict the composition of product gas generated in air-steam gasification of biomass.