Photobioreactors have the ability to produce algae while performing beneficial tasks, such as scrubbing power plant flue gases or removing nutrients from wastewater (Carlsson et al. 2007). Photobioreactors are different types of tanks or closed sys­tems in which algae are cultivated (Richmond 2004).

Algae biomass can play an important role in solving the problem between the production of food and that of biofuels in the near future. Microalgae appear to be the only source of renewable biodiesel that is capable of meeting the global demand for transport fuels. Microalgae are photosynthetic microorganisms that convert sun­light, water, and carbon dioxide into algal biomass (Chisti 2007).

Most algal species are obligate phototrophs and thus require light for their growth. Phototropic microalgae are most commonly grown in open ponds and pho­tobioreactors (Patil et al. 2005). Open-pond cultures are economically more favor­able but raise the issues of land use cost, water availability, and appropriate climatic conditions. Photobioreactors offer a closed culture environment, which is protected from direct fallout and so is relatively safe from invading microorganisms. This technology is relatively expensive compared to the open ponds because of the infra­structure costs. An ideal biomass production system should use the freely available sunlight.

Tredici (1999) has reviewed mass production in photobioreactors. Many differ­ent designs of photobioreactor have been developed, but a tubular photobioreactor seems to be most satisfactory for producing algal biomass on the scale needed for biofuel production. Closed, controlled, indoor algal photobioreactors driven by ar­tificial light are already economical for special high-value products such as phar­maceuticals, which can be combined with the production of biodiesel to reduce the cost (Patil et al. 2008).

Photobioreactors have higher efficiency and biomass concentration (2 to 5 g/L), shorter harvest time (2 to 4 weeks), and higher surface-to-volume ratio (25 to 125/m) than open ponds (Lee 2001; Wang et al. 2008). Closed systems consist of numerous designs: tubular, flat-plated, rectangular, continued stirred reactors, etc. Photobiore­actors in general provide better control of cultivation conditions, yield higher pro­ductivity and reproducibility, reduce contamination risk, and allow greater selection of algal species used for cultivation. The bioreactor has a photolimited central dark zone and a better lit peripheral zone close to the surface (Chisti 2007). CO2-enriched air is sparged into the reactor creating a turbulent flow. Turbulent flow simultane­ously circulates cells between the light and dark zones and assists the mass transfer of carbon dioxide and oxygen gases. The frequency of light and dark zone cycling is
depended on the intensity of turbulence, cell concentration, optical properties of the culture, tube diameter, and the external irradiance level (Chisti 2007). Regulation of carbon dioxide and dissolved oxygen levels in the bioreactor is another key element to algal growth. The highest cost for closed systems is the energy cost associated with the mixing mechanism (Wijffels 2008).

Tubular photobioreactors consist of transparent tubes that are made of flexible plastic or glass. Tubes can be arranged vertically, horizontally, inclined, helically, or in a horizontal thin-panel design. Tubes are generally placed in parallel to each other or flat above the ground to maximize the illumination surface-to-volume ratio of the reactor. The diameter of tubes is usually small and limited (0.2-m diameter or less) to allow light penetration to the center of the tube where the light coefficient and linear growth rate of culture decrease with increasing unit diameter (Ogbonna and Tanaka 1997; Riesing 2006). The growth medium circulates from a reservoir to the reactor and back to the reservoir. A turbulent flow is maintained in the reactor to ensure distribution of nutrients, improve gas exchange, minimize cell sedimentation, and circulate biomass for equal illumination between the light and dark zones.

The most widely used photobioreactor is a tubular design, which has a number of clear transparent tubes, usually aligned with the sun’s rays. The tubes are generally less than 10 cm in diameter to maximize sunlight penetration. The medium broth is circulated through a pump to the tubes, where it is exposed to light for photosyn­thesis, and then back to a reservoir. A portion of the algae is usually harvested after it passes through the solar collection tubes, making continuous algal culture possi­ble. In some photobioreactors, the tubes are coiled spirals to form what is known as a helical-tubular photobioreactor. The microalgal broth is circulated from a reservoir to the solar collector and back to the reservoir (Chisti 2007).

Figure 4.1 depicts a tubular photobioreactor with parallel run horizontal tubes (Campbell 2008). A tubular photobioreactor consists of an array of straight trans­parent tubes that are usually made of plastic or glass. This tubular array, or the solar

collector, is where the sunlight is captured as seen in Figure 4.1. The solar collec­tor tubes are generally 0.1 m or less in diameter. Tube diameter is limited because light does not penetrate too deeply in the dense culture broth, which is necessary for ensuring a high biomass productivity of the photobioreactor. Figure 4.2 shows another type of tubular photobioreactor with parallel run horizontal tubes (Chisti 2007). Figure 4.3 shows a tubular (a) and a vertical (b) photobioreactor.

Flat-plated photobioreactors are usually made of transparent material. The large illumination surface area allows high photosynthetic efficiency, low accumulation of dissolved oxygen concentration, and immobilization of algae (Ugwu et al. 2008). The reactors are inexpensive and easy to construct and maintain. However, the large surface area presents scale-up problems, including difficulties in controlling culture temperature and carbon dioxide diffusion rate and the tendency for algae adhering to the walls.

An inclined triangular tubular photobioreactor was designed to install adjacent to a power plant utilizing flue gas as the feed gas. Flue gas entered the reactor from the bottom of the inclined tube. Gas bubbles traveled along the inner surface of the

Harvest

Degassing column

Fresh — medium

Cooling

water

Pump

(a) (b)

Figure 4.3 (a) Tubular and (b) vertical photobioreactors

tube generating eddies for mixing and preventing fouling. The upper surface of the inclined tube absorbed natural light. The mixing to the algal culture and the flow rate of flue gas influences the growth rate of algae. The system worked, and 15 to 30% of algae were harvested each day. The setup was able to remove 82% of the carbon dioxide on a sunny day and 50% of the carbon dioxide on a cloudy day. Nitrogen oxide was also lowered by 86% (Riesing 2006).

Rectangular tanks are another example of photobioreactors. Unlike the circular tank design, rectangular tanks do not require a stirring device when a sufficiently high gas velocity is used. Drain pipes and gas spargers are located at the bottom of the tank.

Continuous stirred tank reactors (CSTRs) consist of a wide, hollow, capped cylin­drical pipe that operates both indoors and outdoors with low contamination risk. A mechanical stirrer and a light source are inserted from the top of the reactor. Drain channels and gas injectors are positioned at the bottom (and midsection) of the reactor. The uniform turbulent flow established within the reactor promotes algal growth and prevents fouling.

Another photobioreactor uses helical coils made of plastic tubing placed across a columnlike structure. A group of helical coils make up one unit of photobiore­actor. Each helical coil runs independently with its own gas injector, pump, and gas removal system. The helical coils operate both indoors (fluorescent light) and outdoors (sunlight).

Similar to the helical coils, square tubular reactors consist of plastic tubing ar­ranged in a series of squares. One pump is used to provide algal flow through the series of squares and back. Compared with the helical coils, the square tubing is longer and holds more algal volume. The unit is also intended to be installed on the rooftop of a power plant and can operate both outdoor and indoor. However, light is a limitation where only one side of the square is exposed to the Sun at a time. To maximize light penetration, square tubular reactors cannot be packed as closely as the inclined triangular tubular reactor or the helical coils.

In 1858, Dr. Abraham Gesner, a Canadian physician and amateur geologist, devel­oped and patented the extraction of a lamp fuel from asphalt rock, which he named kerosene (Nova Scotia Museum 2008). At that time, kerosene, which was an ex­tremely high-value lighting fuel for lamps, was the primary product of petroleum refining. For a while, distillation of kerosene for lamps was the mainstay of the new petroleum industry. Gasoline was merely a byproduct of kerosene production from crude oil, and until the early 1900s there was no significant demand for it. The first petrochemical, aside from carbon black manufactured on an industrial scale, was isopropyl alcohol, made by Standard Oil of New Jersey in 1920.

When a mixture of two liquids of different boiling points is heated to its boiling point, the vapor contains a higher mole fraction of the liquid with the lower boiling point than the original liquid, i. e., the vapor is enriched in the more volatile com­ponent. If this vapor is now condensed, the resultant liquid has also been enriched
in the more volatile component. This is the principle of batch fractional distillation, and in a distillation column many, many such cycles are performed continuously, al­lowing almost complete separation of liquid components. A generalized distillation column is shown in Figure 7.4. The first step in the refining of crude oil, whether in a simple or a complex refinery, is the separation of the crude oil into fractions (fractionation or fractional distillation). These fractions are mixtures containing hy­drocarbon compounds whose boiling points lie within a specified range.

Crude oil is a complex mixture that is between 50 and 95% hydrocarbon by weight. The first step in refining crude oil involves separating the oil into different hydrocarbon fractions by distillation. An oil refinery cleans and separates the crude oil into various fuels and byproducts, including gasoline, diesel fuel, heating oil, and jet fuel. The main crude oil components are listed in Table 7.3. Since various components boil at different temperatures, refineries use a heating process called distillation to separate the components. For example, gasoline has a lower boiling point than kerosene, allowing the two to be separated by heating to different temper­atures. Another important job of the refineries is to remove contaminants from the oil, for example, sulfur from gasoline or diesel to reduce air pollution from the auto­mobile exhausts. After processing at the refinery, gasoline and other liquid products are usually shipped out through pipelines, which are the safest and cheapest way to move large quantities of petroleum across land (Demirbas 2009).

The refining of heavy oil requires extracting and thorough chemical, engineering, and computing processes. Before the actual refining begins, the stored heavy crude oil is cleaned of contaminants such as sand and water.

Industrial distillation is typically performed in large, vertical, steel cylindrical columns known as distillation towers or distillation columns with diameters ranging from about 65 cm to 11m and heights ranging from about 6 to 60 m or more. To improve the separation, the tower is normally provided inside with horizontal plates or trays, or the column is packed with a packing material. To provide the heat re­quired for the vaporization involved in distillation and also to compensate for heat loss, heat is most often added to the bottom of the column by a reboiler. Large — scale industrial fractionation towers use reflux to achieve more efficient separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation tower that is returned to the upper part of the tower. Inside the tower, the downflowing reflux liquid provides cooling and partial condensation of the upflowing vapors, thereby increasing the efficacy of the distillation tower. There are generally 25 to 45 plates or trays in a distillation tower. Each of the plates or trays is at a different temperature and pressure. The stage at the tower bottom has the highest pressure and temperature. Progressing upwards in the tower, the pressure and temperature decrease for each succeeding stage. Another way of improving the separation in a distillation column is to use a packing material instead of trays.

Three major refinery processes change crude oil into finished products: (1) sep­aration, (2) conversion, and (3) purification. The first step is to separate the crude oil into its naturally occurring components. This is known as separation and is ac­complished by applying heat through a process called distillation. Separation is per­formed in a series of distillation towers. The conversion processes have focused on reducing the length of some hydrocarbon chains. The primary purpose of con­version is to convert low valued heavy oil into high valued petrol. For example, catalytic reforming is a conversion process. The purpose of the reformer is to in­crease the octane number of the petrol blend components. Once crude oil has been through separation and conversion, the resulting products are ready for purification, which is principally sulfur removal. Common process units found in an oil refinery are presented in Table 7.4.

Petroleum refining is somewhat analogous to biorefining. Although biorefineries utilize different processing technologies, they separate and isolate components of biomass for the production of energy fuels, chemicals, and materials. Biorefiner­ies can be designed and built to produce desired outputs from the processing of a wide variety of biorenewable materials. These biorefineries will adopt and inte­grate a range of materials handling and preprocessing equipment, thermochemical and biochemical conversion technologies, and new extraction and purification sci­ences to produce a range of intermediate products, while using less energy and re­ducing effluents and emissions. The scale of the biorefining operations will range from medium-sized to very large (equivalent in size to existing chemical plants and pulp and paper mills). Adoption of biorefineries and related processes and product technologies depends on available research, development, and prevailing regulations during design and construction.

In some ways, biorefineries are analogous to oil refineries. Oil refineries take crude oil and fractionate it into many different useful parts. This is done using a sim­ple chemical distillation process. Biomass, like oil, consists of many different frac­tions that are separated and made into useful products in biorefineries. However, the processes involved in fractionating biomass are more complex than those used in oil refineries. Another important difference between biorefineries and oil refineries is their size. The term biorefinery was coined to describe future processing complexes that will use renewable agricultural residues, plant-based starch, and lignocellulosic materials as feedstocks to produce a wide range of chemicals, fuels, and bio-based materials. Biorefineries will most likely be limited in size, because biomass must be produced and transported economically from a limited catchment area. In contrast, oil is drilled and transported all over the world for processing.

Biomass can be processed into plastics, chemicals, fuels, heat, and power in a biorefinery. High-value components, for example essential oils, drugs, or fibers, can be recovered as a preprocessing step, with the remaining materials then pro­cessed downstream. Processing technologies are most advanced for chemicals and fuels. Biorefineries vary from small single-process plants to large multiprocess sites. Larger biorefineries will be able to integrate different technologies to obtain maxi­mum value from biomass feedstocks.

A biorefinery is an integrated plant producing multiple value-added products from a range of renewable feedstocks. This innovative approach responds to chang­ing markets for traditional forest products as well as new products such as energy, chemicals, and materials. The range of feedstocks, processes, and potential prod­ucts is large; each combination of feedstock, process, and product is characterized by its own unique combination of technical and economic opportunities, emerging technologies, and barriers.

Table 7.5 shows the classification of biorefineries based on their feedstocks. A forest biorefinery will use multiple feedstocks including harvesting residues, ex­tracts from effluents, and fractions of pulping liquors to produce fiber, energy, chem­icals, and materials. A lignocellulosic-based biorefining strategy may be supported

by biomass reserves, created initially with residues from wood product processing or agriculture. Biomass reserves should be used to support first-generation biorefin­ing installations for bioethanol production, development of which will lead to the creation of future high-value coproducts (Mabee et al. 2006).

Biorefineries can be classified based on their production technologies: first — generation biorefineries (FGBRs), second-generation biorefineries (SGBRs), third- generation biorefineries (TGBRs), and fourth-generation biorefineries.

The FGBRs refer to biofuels made from sugar, starch, vegetable oils, or animal fats using conventional technology. Table 7.6 shows the classification of biorefiner­ies based on their generation technologies. SGBRs and TGBRs are also called ad­vanced biorefineries. SGBRs made from nonfood crops, wheat straw, corn, wood, and energy crop using advanced technology.

Sugar and vegetables are used and converted into bioalcohols and biodiesel in FGBRs. The transition from FGBRs to SGBRs will mark a qualitative leap. Ligno — cellulosic residues such as sugar cane bagasse and rice straw feedstocks are used and converted into SGBs in SGBRs.

The first TGBR demonstration plant in the world was commissioned in Oulu, Finland, by Chempolis Oy. As far as is known, the world’s first TGBR producing paper fiber, biofuel, and biochemicals from nonwood and nonfood materials was launched in Finland. TGBRs start with a mix of biomass feedstocks (agricultural or forest biomass) and produce a multiplicity of various products, such as ethanol for fuels, chemicals, and plastics, by applying a mix of different (both small — and

large-scale) technologies such as extraction, separation, and thermochemical or bio­chemical conversion. However, large integrated TGBRs are not expected to become fully established until around 2020. Increasing quantities of agricultural residues will be needed to make paper in the future, as insufficient wood is available locally in the world’s growing paper markets, forest resources are declining, and growing environmental pressures are being put on the use of wood. Vegetable oil is used and converted into biogasoline in fourth-generation biorefineries.

Biorefineries can also be classified based on their conversion routes: biosyngas — based biorefineries, pyrolysis-based biorefineries, hydrothermal-upgrading-based biorefineries, fermentation-based biorefineries, and oil-plant-based biorefineries. Table 7.7 shows the classification of biorefineries based on their conversion routes. Biosyngas is a multifunctional intermediate for the production of materials, chemi­cals, transportation fuels, power, and heat from biomass.

Biofuels are oxygenated compounds. Oxygenated compounds such as ethanol, methanol, and biodiesel provide more efficient combustion and cleaner emissions. At a stoichiometric air:fuel ratio of 9:1 in comparison with gasoline’s 14.7:1, it is obvious that more ethanol is required to produce the chemically correct products of CO2 and water.

Ethanol has a higher octane number (108), broader flammability limits, higher flame speeds, and higher heats of vaporization than gasoline. These properties al­low for a higher compression ratio, shorter burn time, and leaner burn engine, which lead to theoretical efficiency advantages over gasoline in an internal combustion en­gine. The octane number of ethanol allows it to sustain significantly higher internal pressures than gasoline, before being subjected to predetonation.

Disadvantages of ethanol include its lower energy density than gasoline, its cor­rosiveness, low flame luminosity, lower vapor pressure, miscibility with water, and toxicity to ecosystems.

Methanol also allows one to take advantage of the higher octane number of methyl (114) alcohol and increase the engine compression ratio. This would increase the efficiency of converting the potential combustion energy to power. Finally, alco­hols burn more completely, thus increasing combustion efficiency. Some technical properties of fuels are presented in Table 5.7.

Biofuels such as bioethanol, biomethanol, biohydrogen, and biodiesel generally have lower emissions than fossil-based engine fuels. Many studies on the perfor­mances and emissions of compression ignition engines, fueled with pure biodiesel and blends with diesel oil, have been performed and are reported in the literature (Laforgia and Ardito 1994; Cardone et al. 1998).

Vegetable oils have become more attractive recently because of their environ­mental benefits and the fact that they is made from renewable resources. Dorado et al. (2003) describe experiments on the exhaust emissions of biodiesel from olive oil methyl ester as alternative diesel fuel in a diesel direct injection Perkins engine.

The methyl ester of vegetable oil was evaluated as a fuel in CIE by researchers (Dunn 2001), who concluded that the performance of the esters of vegetable oil did not differ greatly from that of diesel fuel. The brake power was nearly the same as with diesel fuel, while the specific fuel consumption was higher than that of diesel fuel. Based on crankcase oil analysis, engine wear rates were low but some oil di­lution did occur. Carbon deposits inside the engine were normal, with the exception of intake valve deposits.

The results showed that transesterification treatment decreased the injector cok­ing to a level significantly lower than that observed with diesel fuel (Shay 1993). Although most researchers agree that vegetable oil ester fuels are suitable for use in CIE, a few contrary results have also been obtained. The results of these stud­ies point out that most vegetable oil esters are suitable as diesel substitutes but that more long-term studies are necessary for commercial utilization to become practi­cal.

The use of biodiesel to reduce N2O is attractive for several reasons. First, biodiesel contains little nitrogen, as compared with diesel fuel, which is also used as a reburning fuel. The N2O reduction is strongly dependent on initial N2O concen­tration and only slightly dependent upon temperature, where increased temperature increased N2O reduction. This results in lower N2O production from fuel nitrogen species for biodiesel. In addition, biodiesel contains a virtually trace amount of sul­fur, so SO2 emissions are reduced in direct proportion to the diesel fuel replacement. Bioesel is a regenerable fuel; when a diesel fuel is replaced by a biodiesel, there is a net reduction in CO2 emissions. As an energy source used in a diesel engine it reduces the consumption of diesel fuels and thereby reduces the greenhouse effect. Additional effects are a reduction in the ash volume and the SOx and NOx emis­sions of a biodiesel-fueled CIEs. Neat BD and BD blends reduce particulate matter, hydrocarbons (HC), and carbon monoxide (CO) emissions and increase nitrogen ox­ide (NOx) emissions compared with diesel fuel used in an unmodified diesel engine (Grassi 1999).

The total net emission of carbon dioxide (CO2) from biodiesel is considerably less than that of diesel oil, and the amount of energy required for the production of biodiesel is less than that obtained with the final product of diesel oil. In addition, the emission of pollutants is somewhat less. CO2, one of the primary greenhouse gases, is a transboundary gas, which means that, after being emitted by a source, it is quickly dispersed in the atmosphere by natural processes. Table 5.8 shows the aver­age biodiesel emissions compared to conventional diesel, according to EPA (2002).

Table 5.9 shows the average changes in mass emissions from diesel engines using biodiesel mixtures relative to standard diesel fuel.

Results indicate that the transformities of biofuels are greater than those of fossil fuels, thus showing that more resources are required to obtain the environmental friendly product. This can be explained by the fact that natural processes are more efficient than industrial ones. On the other hand, the time involved in the forma­tion of fossil fuels is considerably different from that required for the production of biomass (Carraretto et al. 2004).

Different scenarios for the use of agricultural residues as fuel for heat or power generation have been analyzed. Reductions in net CO2 emissions are estimated at 77 to 104 g/MJ of diesel displaced by biodiesel. The predicted reductions in CO2 emissions are much greater than values reported in recent studies on biodiesel de­rived from other vegetable oils, due both to the large amount of potential fuel in the residual biomass and to the low-energy inputs in traditional coconut farming tech­niques. Unburned hydrocarbon emissions from biodiesel fuel combustion decrease compared to regular petroleum diesel.

Biodiesel, produced from different vegetable oils, seems very interesting for sev­eral reasons: it can replace diesel oil in boilers and internal combustion engines without major adjustments; only a small decrease in performance has been reported; it produces almost zero emissions of sulfates; it generates a small net contribution of CO2 when the whole life cycle is considered; the emission of pollutants is compara­ble with that of diesel oil. For these reasons, several campaigns have been planned in many countries to introduce and promote the use of biodiesel (Carraretto et al. 2004).

The brake power of biodiesel was nearly the same as with diesel fuel, while the specific fuel consumption was higher than that of diesel fuel. Based on crankcase oil analysis, engine wear rates were low but some oil dilution did occur. Carbon deposits inside the engine were normal, with the exception of intake valve deposits.

Several results related to the influence of micronutrition on oil yields have been reported (Nithedpattrapong et al. 1995; Tilman et al. 2006; Thamsiriroj 2007; Thamsiriroj and Murphy 2009). Suitable climatic and soil conditions have increased plant oil yields (Thamsiriroj 2007). Biomethane generated from grass requires four times less land than biodiesel from rape seed to produce the same gross energy (Thamsiriroj and Murphy 2009). Grass is a low-energy, carbon-negative input crop (Tilman et al. 2006; Thamsiriroj and Murphy 2009).

Biofuel consumption in the EU is growing rapidly, but major efforts will need to be undertaken if the EU’s objectives for 2010 and beyond are to be achieved. Cur­rent and future policy support therefore focuses on creating favorable economic or legal frameworks to accelerate the market penetration of biofuels. The EU member states will promote specific types of biofuels, depending on their main objectives and natural potentials. The main EU directives that have an impact on sustainable energy development are those promoting energy efficiency and use of renewable energy sources, those implementing greenhouse gas mitigation and atmospheric pollution reduction policies, and other policy documents and strategies targeting the energy sector. Promotion of use of renewable energy sources and energy effi­ciency improvements are among the priorities of the EU’s energy policy because the use of renewable energy sources and energy efficiency improvements has a pos­itive impact on energy security and climate change mitigation. In the EU, climate change has been the principal policy driver for promoting the use of energy from renewable resources. The spring European Council of 2007 set ambitious targets by 2020 of a 20% reduction in greenhouse gas emissions compared to 1990 lev­els and a 20% renewables share in the EU’s final energy consumption, including a 10% share of biofuels in each member state. The greenhouse gas emission savings from the use of biofuels will be at least 35% compared to fossil fuels (Demirbas 2009b).

On 23 January 2008, the Proposal for a Directive of the European Parliament and of the Council on the Promotion of the Use of Energy from Renewable Re­sources was issued. Article 15 of this proposal defined environmental sustainability criteria that both domestic and imported biofuels must satisfy. These criteria in­clude a minimum greenhouse-gas-reduction requirement, limits on the types of land where conversion into biofuel crop production is acceptable, and reinforcement of best agricultural practices. The first sustainability criterion defined in the European proposal is a 35% reduction in greenhouse gas emissions for biofuels relative to their fossil fuel counterpart. The second sustainability criterion relates to the preser­vation of diverse ecosystems. The greenhouse gas emission savings from the use of biofuels and other bioliquids shall be at least 35%. Biofuels and other bioliquids shall not be made from raw material obtained from highly biodiverse land and from land with high carbon stock. Agricultural raw materials cultivated in the European Community and used for the production of biofuels and other bioliquids shall be ob­tained in accordance with the requirements and standards under the provisions of the proposal, and in accordance with the minimum requirements for good agricultural and environmental conditions (Demirbas 2009b).

Specifically for biofuels and bioliquids the draft directive establishes environ­mental sustainability criteria to ensure that biofuels that are to count towards the target are sustainable. They must achieve a minimum level of greenhouse gas emis­sion reduction (35%) and respect a number of binding requirements related to envi­ronmental impact and biodiversity. The sustainability criteria aim to reduce green­house gas emissions and to prevent loss of valuable biodiversity and undesired land use changes. For the foreseeable future the EU will have to rely on first-generation biofuels to achieve the 10% target by 2020: vegetable oils for biodiesel and sugar and starch crops for bioethanol. These import requirements may be between 30 and 50%. The sustainability criteria under discussion in the EU constitute an important step forward, their shortcomings notwithstanding. They should be implemented in a nondiscriminatory way without conferring undue competitive advantage to domes­tic producers and allowing developing countries export opportunities for bioethanol and biomass. A speedy transition to second-generation biofuels is of particular im­portance. Sustainability criteria should progressively ensure that only advanced bio­fuels are available for end users (Demirbas 2009b).

The cost of producing microalgal biodiesel can be reduced substantially by using a biorefinery-based production strategy, improving the capabilities of microalgae through genetic engineering and advances in engineering of photobioreactors (Chisti 2007). Genetic and metabolic engineering are likely to have the greatest impact on improving the economics of microalgal diesel production (Dunahay et al. 1995).

Algae offer the greatest promise of energy security for three reasons. Algae yield more than ten times as much oil per unit area as palm oil, the most productive land- based biofuel crop. Algae’s yield depends upon exactly what species is used and especially upon the oil content, by weight, of that species. Algae’s greater yield comes from basic biological differences from land plants and could be improved with research over time.

Algal oil is unlikely to be suitable for a standard refinery, meaning that either new refineries will need to be constructed or existing ones would need to be retrofitted to handle the different types of oil. Better processes for the conversion of algae into oil and the conversion of algal oil into usable fuel would make algae a far more efficient fuel source. Algae are unlike corn or soybeans in that there are not separate sowing and harvesting seasons. Their great yield is due to algae’s ability to reproduce far more quickly than any land plant, and productivity should remain consistent through all seasons.

Over 50 countries worldwide possess the appropriate temperature range (293 to 303 K) for year-round cultivation of algae across Central America, South Amer­

ica, Africa, and Southeast Asia and Oceania. Algal productivity depends upon a set of climatic conditions prevailing across most of the Earth’s equatorial region, and a monopoly or oligopoly over algae is very difficult to envision. Both coastal and inland states have unique advantages for cultivation, as coastal states possess direct access to salt water for algal culture as well as bays and coasts where algae could be cultivated using no land at all. Inland states fitting this temperature range typically have desert climates ideal for high-yield culture.

Algae can be cultivated in almost any area as long as it is flat, hot, sunny, and has access to salt water. It requires no arable land and no fresh water, and so will not compete for agricultural resources that are often scarce in areas best suited for algal growth. A quick survey of states suitable for algal culture reveals that almost all lie within the developing world and include some of the poorest countries on Earth. Many of these states suffer from poverty at least in part because they contain large swaths of arid, unfarmable land. Algae thrive in land that is too dry and too hot for conventional crops and can be grown in these climates without displacing food plots. Algal culture represents a significant opportunity for these developing states for several reasons.

Algae could provide these states with a cash crop that does not compete with subsidized crops grown in Europe and the USA. Powerful domestic agricultural lobbies represent a significant obstacle to large-scale import of agricultural products, particularly from the developing world, but feel less threatened when the product in question is mainly produced elsewhere for reasons of climate. Algae can be grown in salt water, adding no additional pressure to rivers and lakes already overdrawn by freshwater irrigation and reducing the number of minerals and nutrients that need to be added to culture media for growth.

Gasification followed by FTS is currently the most promising method for upgrad­ing low-value coal and biomass to high-value liquid fuels and chemicals. The total biomass produced each year as waste material from agriculture and forest operations could be converted into roughly 40 billion gal/year of liquid fuels (roughly 25% of the current US gasoline usage).

Tijmensen et al. (2002) review the technical feasibility and economics of BIG — FT process and also point out the key R&D issues involved in the commercializa­tion of this process. Boerrigter and den Uil (2002) give a similar review identifying a potential BIG-FT process configuration. The FTS for the production of liquid hy­drocarbons from coal-based synthesis gas has been the subject of renewed interest for conversion of coal and natural gas to liquid fuels (Jin and Datye 2000).

Gasification is a complex thermochemical process that consists of a number of elementary chemical reactions, beginning with the partial oxidation of a biomass fuel with a gasifying agent, usually air, oxygen, or steam. The chemical reactions involved in gasification include many reactants and many possible reaction paths. The yield from the process is a product gas from thermal decomposition com­posed of CO, CO2, H2O, H2, CH4, other gaseous hydrocarbons, tars, char, inor­ganic constituents, and ash. The gas composition of product from biomass gasi­fication depends heavily on the gasification process, the gasifying agent, and the feedstock composition. A generalized reaction describing biomass gasification is as follows:

Biomass + O2 ! CO, CO2, H2O, H2, CH4 + other (CHs) + tar + char + ash (3.1)

The relative amount of CO, CO2, H2O, H2, and (CHs) depends on the stoichiometry of the gasification process. If air is used as the gasifying agent, then roughly half of the product gas is N2.

Most biomass gasification systems utilize air or oxygen in partial oxidation or combustion processes. These processes suffer from low thermal efficiencies and low Btu gas because of the energy required to evaporate the moisture typically inherent in the biomass and the oxidation of a portion of the feedstock to produce this energy.

Syngas (a mixture of carbon monoxide and hydrogen) produced by gasification of fossil fuels or biomass can be converted into a large number of organic com­pounds that are useful as chemical feedstocks, fuels, and solvents. Many conversion technologies were developed for coal gasification but process economics have re­sulted in a shift to natural-gas-derived syngas. These conversion technologies suc­cessively apply similarly to biomass-derived biosyngas. Franz Fischer and Hans Tropsch first studied the conversion of syngas into larger, useful organic compounds in 1923 (Spath and Mann 2000).

The reasons for using biofuels are manifold and include energy security, envi­ronmental concerns, foreign exchange savings, and socioeconomic issues related to the rural sector. Catalytic conversion will be a primary tool for industry to produce valuable fuels, chemicals, and materials from biomass platform chemicals. Catalytic conversion of biomass is best developed for synthesis gas, or syngas. Economic con­siderations dictate that the current production of liquid fuels from syngas translates into the use of natural gas as the hydrocarbon source. Biomass is the only renewable that can meet our demand for carbon-based liquid fuels and chemicals. Biofuels as well as green motor fuels produced from biomass by FTS are the most modern biomass-based transportation fuels. Green motor fuels are the renewable replace­ment for petroleum-based diesel. Biomass energy conversion facilities are important for obtaining bio-oil by pyrolysis. The main aim of FTS is the synthesis of long — chain hydrocarbons from a CO-H2 gas mixture. The products from FTS are mainly aliphatic straight-chain hydrocarbons (CxHy). Besides the CxHy, branched hydro­carbons, unsaturated hydrocarbons, and primary alcohols are also formed in minor quantities. The FTS process is a process capable of producing liquid hydrocarbon fuels from biosyngas. The large hydrocarbons can be hydrocracked to form mainly diesel of excellent quality. The process for producing liquid fuels from biomass, which integrates biomass gasification with FTS, converts a renewable feedstock into a clean fuel.

FTS is a process for producing mainly straight-chain hydrocarbons from a syn­gas rich in CO and H2. Catalysts are usually employed. Typical operating condi­tions for FTS are temperatures of 475 to 625 K and very high pressure depending on the desired products. The product range includes light hydrocarbons such as meth­ane (CH4) and ethane (C2H6), propane (C3H8), butane (C4H10), gasoline (C5-Ci2), diesel (C13-C22), and light waxes (C23-C33). The distribution of the products de­pends on the catalyst and the process conditions (temperature, pressure, and resi­dence time). The syngas must have very low tar and particulate matter content.

The literature dealing with the actual conversion of biosyngas to fuels using FTS is smaller. Jun et al. (2004) report experimental results of FTS carried out using a model biosyngas. In his review on biofuels, Demirbas (2007) considers FTS using biosyngas as an emerging alternative.

FTS was established in 1923 by German scientists Franz Fischer and Hans Tropsch. It is described by the following set of equations (Schulz 1999):

nCO C (n C m/2)H! CnHm C «H2O (3.2)

where n is the average length of the hydrocarbon chain and m is the number of hy­drogen atoms per carbon. All reactions are exothermic and the product is a mixture of different hydrocarbons where paraffin and olefins are the main constituents.

In FTS one mole of CO reacts with two moles of H2 in the presence of a cobalt (Co)-based catalyst to yield a hydrocarbon chain extension (-CH2-). The reaction of synthesis is exothermic (AH = —165kJ/mol):

CO C 2H2 ! — CH2- C H2O AH = — 165kJ/mol (3.3)

The — CH2- is a building block for longer hydrocarbons. A main characteristic re­garding the performance of FTS is the liquid selectivity of the process (Stelma- chowski and Nowicki 2003). When iron (Fe)-based catalysts are used in WGS re­action activity, the water produced in Reaction 3.2 can react with CO to form ad­ditional H2. The reaction of synthesis is exothermic (AH = —204kJ/mol). In this case a minimal H2/CO ratio of 0.7 is required:

2CO C H2 ! — CH2- C CO2 AH = -204 kJ/mol (3.4)

The kind and quantity of liquid product obtained in FTS is determined by the re­action temperature, pressure and residence time, type of reactor, and catalyst used. Fe catalysts have a higher tolerance for sulfur, are cheaper, and produce more olefin products and alcohols. However, the lifetime of Fe catalysts is short and in commer­cial installations generally limited to 8 weeks (Davis 2002). Bulk Fe catalysts are the catalysts of choice for converting low H2/CO ratio syngas produced by gasifi­cation of biomass or coal to fuels via FTS. These relatively low-cost catalysts have low methane selectivity and high WGS activity. However, development of a bulk Fe FTS catalyst that combines high FT activity, low methane selectivity, high attrition resistance, and long-term stability is still elusive and presents a widely recognized barrier to the commercial deployment of FTS for biomass conversion. The critical property determining the activity and deactivation of Fe catalysts for FTS appears not to be Fe in the metallic state but the carburized Fe surface.

The design of a biomass gasifier integrated with an FTS reactor must be aimed at achieving a high yield of liquid hydrocarbons. For the gasifier, it is important to avoid methane formation as much as possible and convert all carbon in the biomass to mainly carbon monoxide and carbon dioxide (Prins et al. 2004). Gas cleaning is an important process before FTS and is even more important for the integration of a biomass gasifier and a catalytic reactor. To avoid the poisoning of the FTS catalyst, tar, hydrogen sulfide, carbonyl sulfide, ammonia, hydrogen cyanide, alkali, and dust particles must be removed thoroughly (Stelmachowski and Nowicki 2003).

FTS has been widely investigated for more than 70 years, and Fe and Co are typ­ical catalysts. Co-based catalysts are preferred because their productivity is better than that of Fe catalysts thanks to their high activity, selectivity for linear hydrocar­bons, and low activity for the competing WGS reaction.

There has been increasing interest in the effect of water on Co FTS catalysts in recent years. Water is produced in large amounts with Co catalysts since one wa­ter molecule is produced for each C atom added to a growing hydrocarbon chain and due to the low WGS activity of Co. The presence of water during FTS may affect the synthesis rate reversibly as reported for titania-supported catalysts and the deactivation rate as reported for alumina-supported catalysts; water also has a sig­nificant effect on the selectivity for Co catalysts on different supports. The effect on the rate and on deactivation appears to depend on the catalyst system studied, while the main trends in the effect on selectivity appear to be more consistent for different supported Co systems. There are, however, also some differences in the selectivity effects observed. The present study deals mainly with the effect of water on the se­lectivity of alumina-supported Co catalysts, but some data on the activity change will also be reported. The results will be compared with those for other supported Co systems reported in the literature.

The activity and selectivity of supported Co FTS catalysts depends on both the number of Co surface atoms and on their density within support particles, as well as on transport limitations that restrict access to these sites. Catalyst preparation vari­ables available to modify these properties include Co precursor type and loading level, support composition and structure, pretreatment procedures, and the presence of promoters or additives. Secondary reactions can strongly influence product se­lectivity. For example, the presence of acid sites can lead to the useful formation of branched paraffins directly during the FTS step. However, product water not only oxidizes Co sites, making them inactive for additional turnovers, but it can inhibit secondary isomerization reactions on any acid sites intentionally placed in FTS re­actors.

Fe catalysts used commercially in FTS for the past five decades (Dry 2004) have several advantages: (1) lower cost relative to Co and ruthenium catalysts, (2) high WGS activity allowing utilization of syngas feeds of relatively low hydrogen con­tent such as those produced by gasification of coal and biomass, (3) relatively high activity for the production of liquid and waxy hydrocarbons readily refined to gaso­line and diesel fuels, and (4) high selectivity for olefinic C2-C6 hydrocarbons used as chemical feedstocks. The typical catalyst used in fixed-bed reactors is an unsup­ported Fe/Cu/K catalyst prepared by precipitation. While having the aforementioned advantages, this catalyst (1) deactivates irreversibly over a period of months to a few years by sintering, oxidation, formation of inactive surface carbons, and transforma­tion of active carbide phases to inactive carbide phases and (2) undergoes attrition at unacceptably high rates in the otherwise highly efficient, economical slurry bubble — column reactor.

It is well known that the addition of alkali to iron causes an increase in both the 1-alkene selectivity and the average carbon number of produced hydrocarbons. While the promoter effects on iron have been thoroughly studied, few and, at first glance, contradictory results are available for Co catalysts. In order to complete the experimental data, the carbon number distributions are analyzed for products obtained in a fixed-bed reactor under steady-state conditions. Precipitated Fe and Co catalysts with and without K2CO3 were used.

Activated carbon (AC) is a high-surface-area support with the very unique prop­erty that its textural and surface chemical properties can be changed by an easy treatment like oxidation, and these changes affect the properties of the resultant cat­alysts prepared with AC.

Fe catalysts have a higher tolerance for sulfur, are cheaper, and produce more olefin products and alcohols. However, the lifetime of Fe catalysts is short and in commercial installations generally limited to 8 weeks. Co catalysts have the advan­tage of a higher conversion rate and a longer life (over 5 years). Co catalysts are in general more reactive for hydrogenation and therefore produce fewer unsaturated hydrocarbons and alcohols compared to Fe catalysts.

Low-temperature Fischer-Tropsch (LTFT) reactors, either multitubular fixed-bed (MTFBR) or slurry reactor (SR) operating at approx. 25 bar and 495 to 525 K, use a precipitated FE catalyst. High-temperature Fischer-Tropsch (HTFT) fluidized bed reactors, either fixed (SAS) or circulating (Synthol) operating at approx. 25 bar and 575 to 595 K, use a fused Fe catalyst. In their experiments with Fe for CO2 hydro­genation, Riedel et al. (1999) found that alumina was the best support and potassium acted as a powerful promoter. Copper was added to the catalyst to enable its easy re­duction. They report that the hydrocarbon distribution from the H2/CO2 and H2/CO syngas is the same, but the reaction rate for CO2 syngas was about 43% lower than that of the CO-rich syngas.

The Al2O3/SiO2 ratio has a significant influence on Fe-based catalyst activity and selectivity in the process of FTS. Product selectivities also change significantly with different Al2O3/SiO2 ratios. The selectivity of low-molecular-weight hydro­carbons increases and the olefin-to-paraffin ratio in the products shows a monotonic decrease with an increasing Al2O3/SiO2 ratio. Table 3.6 shows the effects of the Al2O3/SiO2 ratio on hydrocarbon selectivity (Jothimurugesan et al. 2000). Recently, Jun et al. (2004) studied FTS over Al2O3- and SiO2-supported Fe-based catalysts from biomass-derived syngas. They found that Al2O3 as a structural promoter fa­cilitated the better dispersion of copper and potassium and gave much higher FTS activity. Table 3.7 shows properties of FT diesel and No. 2 diesel fuels.

Biosyngas consists mainly of H2, CO, CO2, and CH4. FTS has been carried out using a CO/CO2/H2/Ar (11/32/52/5 vol.%) mixture as a model for biosyngas on co­precipitated Fe/Cu/K, Fe/Cu/Si/K, and Fe/Cu/Al/K catalysts in a fixed-bed reactor. Some performances of catalysts that depended on the syngas composition have also been presented (Jun et al. 2004). The kinetic model predicting product distribution is taken from Wang et al. (2003) for an industrial Fe-Cu-K catalyst.

FTS for the production of transportation fuels and other chemicals from synthesis has attracted much attention due to the pressure from the oil supply. Interest in the use of Fe-based catalysts stems from its relatively low cost and excellent WGS reaction activity, which helps to make up the deficit of H2 in the syngas from coal gasification (Wu et al. 2004; Jothimurugesan et al. 2000; Jun et al. 2004). Riedel et al. (1999) have studied the hydrogenation of CO2 over both these catalysts. In the absence of any WGS reaction promoter like Mn, CO2merely behaves as a diluting gas as it is neither strongly adsorbed nor hydrogenated on Co catalysts. When Mn is added to Co catalysts, reverse WGS is possible. The FT chain growth on Co occurs due to strongly adsorbed CO on the surface. With a low partial pressure of CO, these inhibitions are removed and the regime moves from an FT to a methanation regime, yielding more CH4. It was observed that even when the r-WGS reaction was fast, the attainable CO concentration was not sufficient to attain an FT regime. It was hence concluded that CO2 hydrogenation is not possible even with a hybrid Co catalyst containing a shift catalyst like Mn.

Biodiesel is defined as the monoalkyl esters of vegetable oils or animal fats. The vegetable oils and fats as alternative engine fuels are all extremely viscous, with viscosities ranging from 10 to 17 times greater than petroleum diesel fuel. Biodiesel is produced by transesterifying the parent oil or fat to achieve a viscosity close to that of petrodiesel. The chemical conversion of the oil into its corresponding fatty ester (biodiesel) is called transesterification. The purpose of the transesterification process is to lower the viscosity of the oil. The transesterification reaction proceeds with or without a catalyst by using primary or secondary monohydric aliphatic alcohols having one to four carbon atoms as follows (Demirbas 2007):

Triglycerides + Monohydric alcohol! Glycerin + Monoalkyl esters (Biodiesel)

(6.1)

This is an equilibrium reaction (Figure 6.1) where triglycerides can be processed into biodiesel, usually in the presence of a catalyst, and alkali such as potassium hydroxide (Ma and Hanna 1999; Demirbas 2007).

Transesteriflcation and catalytic cracking has usually been adopted to convert fat in the cell of microalgae into gasoline and diesel. This kind of method was limited by low temperature, and the outcome function was highly influenced by the fat content. What was more, the fat content in the microalgae had to be very high; otherwise good economic performance would be hard to achieve (Xu et al. 2006).

Biodiesel is a biofuel commonly consisting of methyl esters that are derived from organic oils, plant or animal, through the process of tranesteriflcation. The biodiesel transesteriflcation reaction is very simple:

Catalyst

Triglyceride + 3 Methanol ————— ! Glycerine + 3 Methyl esters (Biodiesel)

(6.2)

An excess of methanol is used to force the reaction to favor the right side of the equation. The excess methanol is later recovered and reused.

The triglyceride is a complex molecule that plants and animals use for storing chemical energy; in more simple terms, it is fat. The process of making biodiesel occurs as follows. (1) Triglycerides, methanol, and catalyst are placed in a controlled reaction chamber to undergo transesteriflcation. (2) The initial product is placed in a separator to remove the glycerine byproduct. (3) The excess methanol is recovered from the methyl esters through evaporation. (4) The flnal biodiesel is rinsed with water, neutralized, and dried (Xu et al. 2006). Unlike petroleum fuels, the relative simplicity of biodiesel manufacture makes its production scalable. Many existing vendors are small-time producers. Biodiesel is a somewhat “mature” fuel and was used as a diesel alternative in the early 20th century (Demirbas 2007). This has allowed biodiesel to attain a level of “grassroots” popularity among environmental advocates and visionaries.

The energy density of biodiesel is comparable to petroleum diesel. The higher heating value of petroleum diesel is 42.7MJ/kg. Values for biodiesel vary depend­ing on the source of biomass. Typically, biodiesel derived from seed oils, such as rapeseed or soybean, produces 39.5 MJ/kg, while biomass derived from algae yields 41 MJ/kg (Demirbas 1998; Rakopoulos et al. 2006; Xu et al. 2006). Although the lower-energy biodiesels based on seed oils are the most common, they have enough energy density to make them a viable alternative to petroleum diesel.

Biodiesel can be made from virtually any source of renewable oil. Typical sources include restaurant waste oil, animal fats, and vegetable oils. The supply of waste oil is very limited; however, it is a popular source for small-scale, in­dependent producers. Large commercial producers often use vegetable oils, such as soybean, rapeseed, palm, and corn oils. Unfortunately, biodiesel derived from seed oil diverts from the food supply and the increasing competition for seed causes the oil, and resulting biodiesel, to become increasingly expensive (Camp­bell 2008).

The main advantages of biodiesel as diesel fuel its portability, ready availability, renewability, higher combustion efflciency, and lower sulfur and aromatic content (Demirbas 2007). Adopting biodiesel has a number of advantages. First, because the fuel is derived from biomass, it does not contribute to atmospheric CO2 emis­

sions. Second, biodiesel emissions are, except forNOx, lower than petroleum diesel. Combustion of biodiesel alone provides over a 90% reduction in total unburned hy­drocarbons, and a 75 to 90% reduction in polycyclic aromatic hydrocarbons (PAHs). Biodiesel further provides significantly greater reductions in particulates and car­bon monoxide than petroleum diesel fuel. Biodiesel provides a slight increase or decrease in nitrogen oxides depending on engine family and testing procedures. Many studies on the performances and emissions of compression ignition engines, fueled with pure biodiesel and blends with diesel oil, have been performed and are reported in the literature (Laforgia and Ardito 1994; Cardone et al. 1998). Third, the infrastructure needed for biodiesel already exists. Biodiesel can be used in ex­isting diesel engines blended with petroleum diesel, or it can be run unblended in engines with minor modifications (Crookes 2006; Rakopoulos et al. 2006; Bowman et al. 2006). Because biodiesel has twice the viscosity of petroleum diesel, its lu­brication properties can actually improve engine life (Bowman et al. 2006). Fourth, biodiesel has low toxicity and is biodegradable (Aresta et al. 2005; Demirbas 2007). The biodegradabilities of several biodiesels in the aquatic environment show that all biodiesel fuels are readily biodegradable. After 28 d all biodiesel fuels are 77 to 89% biodegraded; diesel fuel is only 18% biodegraded in the same period (Zhang 1996). The enzymes responsible for the dehydrogenation/oxidation reactions that occur in the process of degradation recognize oxygen atoms and attack them im­mediately (Zhang et al. 1998). Fifth, like petroleum diesel, biodiesel has a more complete combustion than gasoline, giving a cleaner burn (Bowman et al. 2006). The oxygen content of biodiesel improves the combustion process and decreases its oxidation potential. The structural oxygen content of a fuel improves its combus­tion efficiency due to an increase in the homogeneity of oxygen with the fuel during combustion. Because of this, the combustion efficiency of biodiesel is higher than that of petrodiesel. A visual inspection of injector types would indicate no difference between biodiesel fuels when tested on petrodiesel. The overall injector coking is considerably low.

The major disadvantages of biodiesel are its higher viscosity, lower energy con­tent, higher cloud point and pour point, higher nitrogen oxide (NOx) emissions, lower engine speed and power, injector coking, engine compatibility, high price, and greater engine wear. Biodiesel is not without problems. First, it does pro­duce increased NOx emissions, relative to petroleum diesel, owing to the higher compression ratios typically used in biodiesel engines (Crookes 2006; Pradeep and Sharma 2007). Second, biodiesel does reduce the power output of a diesel engine compared to petroleum diesel, although this is only around 2% overall (Schneider 2006). Third, the production of biodiesel results in glycerine byprod­ucts and wash wastewater. Fourth, the price of biodiesel is typically higher than that of petroleum diesel. Fifth, and most importantly, the biomass feedstocks for making biodiesel are diverted from other important uses, typically food produc­tion.

The algae that are used in biodiesel production are usually aquatic unicellular green algae. This type of algae is a photosynthetic eukaryote characterized by high growth rates and high population densities. Under good conditions, green algae can double their biomass in less than 24 h (Schneider 2006; Chisti 2007). Additionally, green algae can have huge lipid contents, frequently over 50% (Schneider 2006; Chisti 2007). This high-yield, high-density biomass is ideal for intensive agriculture and may be an excellent source for biodiesel production.

The annual productivity and oil content of algae is far greater than that of seed crops. Soybean can only produce about 450 L of oil per hectare. Canola can produce 1,200 L/ha, and palm can produce 6,000L. Now, compare that to algae, which can yield 90,000 L/ha (Haag 2007; Schneider 2006; Chisti 2007). It is possible that US demand for liquid fuel could be achieved by cultivating algae in one tenth the area currently devoted to soybean cultivation (Scott and Bryner 2006).

The process for producing microalgal oils consists of a microalgal biomass pro­duction step that requires light, carbon dioxide, water, and inorganic nutrients. The latter are mainly nitrates, phosphates, iron, and some trace elements. Approximately half of the dry weight of microalgal biomass is carbon, which is typically derived from carbon dioxide. Therefore, producing 100 tons of algal biomass fixes roughly 183 tons of carbon dioxide. This carbon dioxide must be fed continually during daylight hours. It is often available at little or no cost (Chisti 2008). The optimal temperature for growing many microalgae is between 293 and 303 K. A tempera­ture outside this range could kill or otherwise damage the cells.

There are three well-known methods to extract oil from algae: (1) expeller/press, (2) solvent extraction with hexane, and (3) supercritical fluid extraction. A simple process is to use a press to extract a large percentage (70 to 75%) of the oils from algae. Algal oil can be extracted using chemicals. The most popular chemical for solvent extraction is hexane, which is relatively inexpensive. Supercritical fluid ex­traction is far more efficient than traditional solvent separation methods. Supercrit­ical fluids are selective, thus providing the high purity and product concentrations (Paul and Wise 1971). This method alone can allow one to extract almost 100% of the oils. In supercritical fluid CO2 extraction, CO2 is liquefied under pressure and heated to the point where it has the properties of both a liquid and a gas. This liquefied fluid then acts as the solvent in extracting the oil.

The lipid and fatty acid contents of microalgae vary in accordance with culture conditions. Algal oil contains saturated and monounsaturated fatty acids. The fatty acids exist in algal oil in the following proportions: 36% oleic (18:1), 15% palmitic (16:0), 11% stearic (18:0), 8.4% iso-17:0, and 7.4% linoleic (18:2). The high propor­tion of saturated and monounsaturated fatty acids in this alga is considered optimal from a fuel quality standpoint, in that fuel polymerization during combustion would be substantially less than what would occur with polyunsaturated fatty-acid-derived fuel (Sheehan et al. 1998). Table 6.1 shows the oil contents of some microalgae. Oil levels of 20 to 50% are quite common (Chisti 2007; Carlsson et al. 2007; Demirbas 2009a).

After oil extraction from algae, the remaining biomass fraction can be used as a high protein feed for livestock (Schneider 2006; Haag 2007). This gives further value to the process and reduces waste.

Open ponds are the oldest and simplest systems for mass cultivation of microalgae. The pond is designed in a raceway configuration, in which a paddlewheel circu­lates and mixes the algal cells and nutrients. The raceways are typically made from poured concrete, or they are simply dug into the earth and lined with a plastic liner to prevent the ground from soaking up the liquid. Baffles in the channel guide the flow around the bends in order to minimize space. The system is often operated in a continuous mode, i. e., the fresh feed is added in front of the paddlewheel, and the algal broth is harvested behind the paddlewheel after it has circulated through the loop.

Open-pond systems are shallow ponds in which algae are cultivated. Nutrients can be provided through runoff water from nearby land areas or by channeling the
water from sewage/water treatment plants (Carlsson et al. 2007). The water is typi­cally kept in motion by paddlewheels or rotating structures, and some mixing can be accomplished by appropriately designed guides. Algal cultures can be defined (one or more selected strains), or are made up of an undefined mixture of strains. For an overview of systems used see Borowitzka (1999) and Chaumont (1993).

The only practicable methods of large-scale production of microalgae are race­way ponds (Terry and Raymond 1985; Molina Grima 1999) and tubular photobiore­actors (Molina Grima et al. 1999; Tredici 1999; Sanchez Miron et al. 1999).

Open architecture approaches (e. g., ponds or traditional racetracks), while pos­sibly the cheapest of all current techniques, suffer challenges with contamination, evaporation, temperature control, CO2 utilization, and maintainability.

Figure 4.4 shows open-pond “algae farm” systems. The ponds are “raceway” designs in which the algae, water, and nutrients circulate around a racetrack. Pad­dlewheels provide the flow. The algae are thus kept suspended in water. Algae are circulated back up to the surface on a regular frequency. The ponds are kept shallow because of the need to keep the algae exposed to sunlight and the limited depth to which sunlight can penetrate the pond water. The ponds are operated continuously; that is, water and nutrients are constantly fed to the pond while algae-containing water is removed at the other end. Some kind of harvesting system is required to recover the algae, which contain substantial amounts of natural oil.

Large-scale outdoor culture of microalgae and cyanobacteria in open ponds, race­ways, and lagoons is well established (Becker 1994). Open culture is used commer­cially in the USA, Japan, Australia, India, Thailand, China, Israel, and elsewhere to produce algae for food, feed, and extraction of metabolites. Open-culture systems allow relatively inexpensive production but are subject to contamination. Conse­quently, only a few algal species can be cultured in open outdoor systems. Species

that grow successfully in the open include rapid growers such as Chlorella and species that require a highly selective extremophilic environment that does not favor the growth of most potential contaminants. For example, species such as Spirulina and Dunaliella thrive in highly alkaline and saline selective environments, respec­tively. Algae produced in quantities in open systems include Spirulina, Chlorella, Dunaliella, Haematococcus, Anabaena, and Nostoc (Chisti 2006).

Algae farms are large ponds. The size of these ponds is measured in terms of sur­face area, since surface area is so critical to capturing sunlight. Their productivity is measured in terms of biomass produced per day per unit of available surface area. Even at levels of productivity that would stretch the limits of an aggressive research and development program, such systems require acres of land. At such large sizes, it is more appropriate to think of these operations on the scale of a farm. Such al­gae farms would be based on the use of open, shallow ponds in which some source of waste CO2 could be efficiently bubbled into the ponds and captured by the al­gae. Careful control of pH and other physical conditions for introducing CO2 into the ponds allows for more than 90% utilization of injected CO2. Raceway ponds, usually lined with plastic or cement, are about 15 to 35 cm deep to ensure ade­quate exposure to sunlight. They are typically mixed with paddlewheels, are usually lined with plastic or cement, and are between 0.2 to 0.5 ha in size. Paddlewheels provide motive force and keep the algae suspended in the water. The ponds are sup­plied with water and nutrients, and mature algae are continuously removed at one end.

Figure 4.5 shows a raceway pond (Chisti 2007). A raceway pond is made up of a closed-loop recirculation channel that is typically about 0.3 m deep. As shown in the figure, mixing and circulation are produced by a paddlewheel. Flow is guided

Figure 4.5 Arial view of a raceway pond

around bends by baffles placed in the flow channel, and raceway channels are built in concrete or compacted earth and may be lined with white plastic. During daylight, the culture is fed continuously in front of the paddlewheel where the flow begins. Broth is harvested behind the paddlewheel on completion of the circulation loop. The paddlewheel operates all the time to prevent sedimentation.

Photosynthesis is the most important biochemical process in which plants, al­gae, and some bacteria harness the energy of sunlight to produce food. Organisms that produce energy through photosynthesis are called photoautotrophs. Photosyn­thesis is a process in which green plants utilize the energy of sunlight to manufac­ture carbohydrates from carbon dioxide and water in the presence of chlorophyll (Viswanathan 2006).

Raceway ponds for mass culture of microalgae have been used since the 1950s. Extensive experience exists on the operation and engineering of raceways. The largest raceway-based biomass production facility occupies an area of 440,000 m2 (Spolaore et al. 2006). Productivity is affected by contamination with unwanted algae and microorganisms that feed on algae. Raceway ponds and other open cul­ture systems for producing microalgae are further discussed by Terry and Raymond (1985).

There are quite a number of sources of waste CO2. Every operation that involves the combustion of fuel for energy is a potential source. One program targeted coal and other fossil-fuel-flred power plants as the main sources of CO2. Typical coal — flred power plants emit flue gas from their stacks containing up to 13% CO2. This high concentration of CO2 enhances transfer and uptake of CO2 in ponds. Figure 4.6 shows the production of algae in open ponds.

For large-scale biofuel production, which would require systems of hundreds of hectares in scale, this would mean deploying tens of thousands of such repeating units, at great capital and operating cost. Open ponds, specifically mixed raceway ponds, are much cheaper to build and operate, can be scaled up to several hectares for individual ponds, and are the method of choice for commercial microalgae pro­duction. However, such open ponds also suffer from various limitations, including

Figure 4.6 Production of algae in open ponds

more rapid (than closed systems) biological invasions by other algae, algae grazers, fungi and amoeba, etc., and temperature limitations in cold or hot humid climates.

Microalgae can be cultivated in coastal areas. The raceway pond system of biomass culture must be approved to achieve high and sustained growth rates and oil yields that are essential to developing an algal-based biofuel industry.

A biorefinery system starts with the contract harvesting of whole crops (grain and straw), which are then stored and fractionated (including drying as necessary) into products and byproduct for sale. A biorefinery is a factory that processes crops, such as wheat, barley, and oilseed rape, to produce various refined specialized fractions, such as flour, gluten, starch, oil, straw chips, etc. The concept of a biorefinery, com­pared with, for example, a flour mill, is that the use and value of all the fractions into which the input can be separated is maximized (Audsley and Annetts 2003).

The analysis of a biorefinery system can be considered in three parts. The first is the effect on the farm of selling products to a biorefinery, on the assumption that the biorefinery contracts to harvest the crop using a wholecrop forage harvester. The second part is the impact of the type of biorefinery system on the profitability of the processing required to produce the various products. The third part is the transport of crop to the biorefinery, which is a function of the distribution of farms around the biorefinery location (Audsley and Annetts 2003).

As biomass hydrolysis and sugar fermentation technologies approach commer­cial viability, advancements in product recovery technologies will be required. For cases in which fermentation products are more volatile than water, recovery by dis­tillation is often the technology of choice. Distillation technologies that will allow the economic recovery of dilute volatile products from streams containing a variety of impurities have been developed and commercially demonstrated. A distillation system separates the bioethanol from water in the liquid mixture.

The first step is to recover the bioethanol in a distillation or beer column, where most of the water remains with the solids part. The product (37% bioethanol) is then concentrated in a rectifying column to a concentration just below the azeotrope (95%). The remaining bottom product is fed to the stripping column to remove additional water, with the bioethanol distillate from stripping being recombined with the feed to the rectifier. The recovery of bioethanol in the distillation columns in the plant is fixed at 99.6% to reduce bioethanol losses.

After the first effect, solids are separated using a centrifuge and dried in a rotary dryer. A portion (25%) of the centrifuge effluent is recycled to fermentation and the rest is sent to the second and third evaporator effects. Most of the evaporator condensate is returned to the process as fairly clean condensate (a small portion, 10%, is split off to wastewater treatment to prevent buildup of low-boiling-point compounds) and the concentrated syrup contains 15 to 20% by weight total solids.

One of the advantages of alkane production from biomass by aqueous-phase de- hydration/hydrogenation is that the alkanes spontaneously separate from the aque­ous feed solution, whereas ethanol produced during fermentation processes must be removed from water by an energy-intensive distillation step.

Biomass-derived oxygenates can be converted into hydrogen and alkanes (rang­ing from C1 to C15) via aqueous-phase processing (Audsley and Annetts 2003). These aqueous-phase processes could be used in an integrated biorefinery to pro­duce a range of fuels, as shown in Figure 7.5. The first step in the biorefining process is conversion of biomass into an aqueous sugar solution. Production of hydrogen for biorefining processes is accomplished by aqueous-phase reforming. The biorefinery can also produce light alkanes ranging from C1 to C6 by aqueous — phase dehydration/hydrogenation (Audsley and Annetts 2003). The light alkanes could be used as synthetic natural gas, liquefied petroleum gas, and a light naptha stream. Aqueous-phase processing can also produce larger alkanes ranging from C7 to C15 by combining the dehydration/hydrogenation reactions with an aldol conden­sation step prior to the aqueous-phase dehydration/hydrogenation step (Huber et al. 2005).

Biomass has been traditionally converted into liquid fuels by either (a) fermen­tation or (b) pyrolysis methods. Modern improvements to these classical processes are many in number but do not essentially change the type of product resulting from these two vastly different sets of reaction conditions. While ethanol production by fermentation has become more efficient, it is still limited to a 67% yield due to the loss of one third (1/3) of the available carbon as carbon dioxide gas. Pyrolytic reactions also lose carbon as gases and char but may achieve about 80% carbon con­version. While most thermochemical processes usually require nearly dry feedstock, the hydrothermal upgrading (HTU) process requires a 3:1 ratio of water to biomass. However, HTU produces only 50% biocrude, which still contains 10 to 15% oxygen. Obviously, there remains a need for a variety of fuels from many sources, especially conventional liquid fuels for transportation purposes. To resolve this fuel problem

and to use a renewable resource, a strategy was selected to prepare valuable liquid hydrocarbons from biomass by a new chemical process.

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 tertbutyl ethers, ethylene glycol, and propylene glycol. Glycerol has been pyrolyzed for the production of clean fuels such as H2 or a feedstock such as syngas for additional transportation fuel via FTS. The conver­sion of glycerol to H2 and CO takes place according to the following stoichiometric equation:

C3O3H8 ! 3CO C 4H2 (7.1)

The stoichiometry for conversion of glycerol into liquid alkanes, by the formation of synthesis gas coupled with FTS, is shown in Equation 7.2:

25C3O3H8 ! 7C8H18 C 19CO2 C 37H2O (7.2)

This overall reaction to produce liquid fuels from glycerol is slightly exothermic and the yield of liquid alkanes is 40% at 1.7 MPa pressure.

It is possible to produce light alkanes by aqueous-phase reforming of biomass — derived oxygenates such as sorbitol, which can be obtained from glucose by hy­drogenation (Huber et al. 2005; Metzger 2006). The production of alkanes from aqueous carbohydrate solutions would be advantageous because of the easy separa­
tion of the alkanes from water. Much hydrogen is needed to reduce biomass-derived oxygenates to alkanes as shown in Equation 7.3:

C6H14O6 C 6H2 ! C6H14 C 6H2O (7.3)

Production of ethanol (bioethanol) from biomass is one way to reduce both the con­sumption of crude oil and environmental pollution. Ethanol from lignocellulosic biomass has the potential to contribute substantially to bioethanol for transporta­tion. In the process evaluated, prehydrolysis with dilute sulfuric acid is employed to hydrolyze hemicellulose and make the cellulose more accessible to hydrolysis by enzymes. Residual biomass from hydrolysis and extraction of carbohydrates can be burned in a power plant to generate electricity and process steam. Figure 7.6 shows a flow diagram of pretreatment for fermentation of ethanol production from sugar crops and lignocellulosic feedstocks.

Carbohydrates (hemicelluloses and cellulose) in plant materials can be converted into sugars by hydrolysis. Fermentation is an anaerobic biological process in which sugars are converted into alcohol by the action of microorganisms, usually yeast. The resulting alcohol is bioethanol. The value of any particular type of biomass as feedstock for fermentation depends on the ease with which it can be converted into sugars. Bioethanol is a petrol additive/substitute. Bioethanol and the bioreflnery concept are closely linked. It is possible that wood, straw, and even household wastes may be economically converted into bioethanol. In 2004, 3.4 billion gal. of fuel

ethanol were produced from over 10% of the corn crop. Ethanol demand is expected to more than double in the next 10 years. For the supply to be available to meet this demand, new technologies must be moved from the laboratory to commercial reality. World ethanol production is about 60% from sugar-crop feedstock.

The corn-starch-to-fuel-ethanol industry has been developed over the past 30 years by bioethanol researchers. Most bioethanol researchers focus on the chal­lenge of producing bioethanol from lignocellulosic biomass instead of from corn starch. To this end, researchers have already developed effective technology to ther­mochemically pretreat biomass, to hydrolyze hemicellulose to break it down into its component sugars and open up the cellulose to treatment, to enzymatically hy­drolyze cellulose to break it down into sugars, and to ferment both 5-carbon sugars from hemicellulose and 6-carbon sugars from cellulose.

Cellulose is a remarkable pure organic polymer, consisting solely of units of an- hydroglucose held together in a giant straight-chain molecule. Cellulose must be hydrolyzed to glucose before fermentation into ethanol. Conversion efficiencies of cellulose to glucose may depend on the extent of chemical and mechanical pre­treatments to structurally and chemically alter the pulp and paper mill wastes. The method of pulping, the type of wood, and the use of recycled pulp and paper prod­ucts also could influence the accessibility of cellulose to cellulase enzymes.

Cellulose fraction of the structural components is insoluble in most solvents and has a low accessibility to acid and enzymatic hydrolysis. Hemicelluloses (arabino — glycuronoxylan and galactoglucomammans) are related to plant gums in composi­tion and occur in much shorter molecule chains than cellulose. The hemicelluloses, which are present in deciduous woods chiefly as pentosans and in coniferous woods almost entirely as hexosanes, undergo thermal decomposition very readily. Hemi — celluloses are derived mainly from chains of pentose sugars and act as the cement material holding together the cellulose micells and fiber. Hemicelluloses are largely soluble in alkali and as such are more easily hydrolyzed.

The term bio-oil is used mainly to refer to liquid fuels. When biomass is processed under high temperature in the absence of oxygen, products are produced in three phases: the vapor phase, the liquid phase, and the solid phase. The liquid phase is a complex mixture called bio-oil. The compositions of bio-oils vary significantly with the types of feedstock and processing conditions.

Thermochemical conversion is a process through which biomass in the absence of oxygen and at high temperature can be converted into various fuels including char, oil, and gas. The resulting bio-oils present an alternative to liquid biofuels with similarities to petroleum oil (Kishimoto et al. 1994). The process can be subdivided into pyrolysis and thermochemical liquefaction (Demirbas 2000).

Bio-oils are liquid or gaseous fuels made from biomass materials, such as agri­cultural crops, municipal wastes, and agricultural and forestry byproducts, via bio­chemical or thermochemical processes.

Bio-oil has a higher energy density than biomass and can be obtained by quick heating of dried biomass in a fluidized bed followed by cooling. The byproduct char and gases can be combusted to heat the reactor. For utilization of biomass in remote locations, it is more economical to convert the biomass into bio-oil and then transport the bio-oil. Bio-oil can be used in vehicle engines — either totally or partially in a blend.

Biomass is dried and then converted into an oily product known as bio-oil by very quick exposure to heated particles in a fluidized bed. The char and gases produced are combusted to supply heat to the reactor, while the product oils are cooled and condensed. The bio-oil is then shipped by truck from these locations to the hydrogen production facility. It is more economical to produce bio-oil at remote locations and then ship the oil, since the energy density of bio-oil is higher than biomass. For this analysis, it was assumed that the bio-oil would be produced at several smaller plants that are closer to the sources of biomass so that lower-cost feedstocks could be obtained.

The feasibility of producing liquid fuel or bio-oil via pyrolysis or thermochemical liquefaction of microalgae has been demonstrated for a range of microalgae (Dote et al. 1994; Sawayama et al. 1999; Peng et al. 2000,2001; Tsukahara and Sawayama 2005; Demirbas 2006).

Five moss samples (Polytrichum commune, Dicranum scoparium, Thuidium tamarascinum, Sphagnum palustre, Drepanocladus revolvens), one alga sample (Cladophora fracta), and one microalga sample (Chlorella protothecoides) were used in the earlier work (Demirbas 2006). The yields of bio-oil from the samples via pyrolysis are presented as a function of temperature (K) in Figure 5.3. 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 were 39.1, 34.3, 33.6, 37.0, 35.4, 48.2, and 55.3% of the sample for Polytrichum commune, Dicranum scoparium, Thuidium tamarascinum, Sphag-

num palustre, Drepanocladus revolvens, Cladophorafracta and Chlorella protothe — coides, respectively.

Most traditional biofuels, such as ethanol from corn, wheat, or sugar beets, and biodiesel from oil seeds, are produced from classic agricultural food crops that re­quire high-quality agricultural land for growth. An important parameter for biomass energy is the impact of land use. This impact category describes the environmen­tal impact resulting from land use for human activities. In particular, the land use category considers natural land as a resource and assumes that land occupation and management causes consumption of the resource. Natural land can be defined as land not damaged at the moment by human activities and the remaining natural land fraction under use. Land use is an impact indicator.

Land use impacts have been related to the area of land used, or physical land use. Specifically, they include impacts on biodiversity, biotic production potential (including soil fertility and use value of biodiversity), and ecological soil quality (including life support functions of soil other than biomass production potential). The use of land to produce biofuels in a well-established infrastructure has an impact on the environment.

The environmental impacts of irrigation are the changes in the quantity and qual­ity of soil and water as a result of irrigation and the ensuing effects on natural and social conditions at the tail-end and downstream of the irrigation scheme. Irrigated agriculture depends on supplies from surface or ground water. Irrigation agriculture will be an essential component of any strategy to increase the global food supply. Improving the environmental performance of irrigation agriculture is important for its long-term sustainability. Irrigation projects and irrigated agriculture practices can impact the environment in a variety of ways. The environmental impact of irrigation systems depends on the nature of the water source, the quality of the water, and how it is delivered to the irrigated land. The management of water application systems as well as the suitability of related agronomic practices has a dramatic influence on the environmental impact of irrigated agriculture.

The ecosystem diversity of fuel applications confers many environmental, eco­nomic, and consumer benefits. The use of crop residues for bioenergy production must be critically assessed because of its positive impact on soil carbon sequestra­tion, soil quality maintenance, and ecosystem functions (Lal 2005).

Energy crops include fast growing trees such as hybrid poplar, black locust, wil­low, and silver maple, in addition to annual crops such as corn and sweet sorghum and perennial grasses such as switchgrass. Bioenergy from biomass, both residues and energy crops, can be converted into modern energy carriers such as hydrogen, methanol, ethanol, and electricity.

The impact of energy cropping on habitat and ecosystem diversity depends not only on the previous land use and cultivation but also on the nature of the energy crop. The role of energy cropping can play in the context of sustainable develop­ment, such as via the use of bioethanol, and its potential impact on biodiversity is a subject that warrants more concerted research (Adsavakulchai et al. 2004).

Conflicts exist today in the use of land, water, energy, and other environmental resources required by both food and biofuel production. Bioenergy supplies can be divided into two broad categories: (a) organic municipal waste and residues from the food and materials sectors and (b) dedicated energy crop plantations. The term “food supply chain” refers to the strict correlation and the functional link existing between the agriculture sector and transformation industry.

Serious problems face the world food supply today. The rapidly growing world population and rising consumption of biofuels is increasing demand for both food and biofuels. This exacerbates both food and fuel shortages. Using food crops such as corn grain to produce ethanol raises major nutritional and ethical concerns. Grow­ing crops for fuel squanders land, water, and energy resources vital for the produc­tion of food for human consumption. Using corn for ethanol increases the price of US beef, chicken, pork, eggs, breads, cereals, and milk by 10 to 30% (Demirbas 2006; Pimentel et al. 2008).

Many problems associated with biofuels have been ignored by scientists and pol­icymakers. The environmental impacts of corn ethanol are enormous: (1) corn pro­duction causes more soil erosion than any other crop grown; (2) more than 1,700 gal. of water are required to produce 1 gal. of corn ethanol; (3) enormous quantities of carbon dioxide are produced during corn ethanol production by the large quantity of fossil energy used in production, during fermentation, and when the soil is tilled, leaving soil organic matter exposed and oxidized. In addition, the conversion of cropland for biofuel production contributes to the release of GHGs, all of which speeds global warming; (4) using corn for ethanol increases the price of other foods dependent on manufacturing or animal feeding with corn (Demirbas and Demirbas 2007; Pimentel et al. 2008).

The use of soybeans as a potential biofuel source puts cropland in competition with food production. On the other hand, extensive use of vegetable oils in biodiesel production may cause other significant problems such as starvation in developing countries (Demirbas and Demirbas 2007; Demirbas 2007).