Concerned by potential climate change-related damages (including changes to coastlines and the spread of tropical diseases, among others), the US faces the necessity of finding solutions for the 17.7%-reduction of GHG emissions (Lokey, 2007). Because of the fact that the electrical sector accounts for 40% of all GHG emissions, investments in cost-competitive renewable energy sources, such as wind, geothermal and hydroelectricity, have been recommended. Given the ample solar resources that exist in the US, it has a plethora of untapped sources for renewable-energy generation (Flavin et al., 2006). The Biomass Program of the US Department of Energy (launched in 2000) recommended 5% use of biofuels by 2010, 15% by 2017, and 30% by 2050. However, it is predicted that the ethanol market penetration for transportation should attain ~50% of gasoline consumption by 2030 (Szulczyk et al., 2010). Currently, maize and other cereals (such as sorghum) are the primary feedstocks for US ethanol production. At 40 Ml of ethanol per day, maize is still considered a low-efficiency biofuel crop because of its high required input, excessive topsoil erosion (10 times faster than sustainable) and other negative side effects (Donner & Kucharik, 2008; Laurance, 2007; Sanderson, 2006; Scharlemann & Laurance, 2008). By comparison, biodiesel from soybean requires lower inputs. However, neither of these biofuels can displace fossil fuel without impacting food supplies. Even if all US corn and soybean production were dedicated to biofuels, only 12% of the gasoline and 6% of the diesel demand, respectively, would be met (Hill et al., 2006). However, agricultural, municipal, and forest wastes could together sustainably provide 1 Gt of dry matter annually and should complement the other biofuel crops (Vogt et al., 2008). It was proposed that 3.1-21.3 Mha of land should be converted to biomass production (Schmer et al., 2008). Algal biodiesel is also being included in an integrated renewable-energy park (Singh & Gu, 2010; Subhadra, 2010).

Bioethanol from Brazil results in over 90% GHG savings (Hill et al., 2006). In addition to the PROALCOOL program, the Brazilian government created the PRO-OLEO program in 1980 and expected a 30% mixture of vegetable oils or derivatives in diesel and full substitution in the long term. Unfortunately, after the price drop of crude oil on the international market in 1986, this program was abandoned and was only reintroduced in 2002. Because of its great biodiversity and diversified climate and soil conditions, Brazil has a variety of plant-oil feedstocks, including mainly soybean, sunflower, coconut, castor bean, cottonseed, oil palm, physic nut and babassu (Nass et al., 2007). Brazil celebrated the inauguration of the Embrapa Agroenergia research center in 2010 to promote the integration of the oil from these feedstocks into the network of biodiesel sources. The National Program of Production and Use of Biodiesel (PNPB) was launched in 2004 with the objective of establishing the economic viability of biodiesel production together with social and regional development. The current diesel consumption in Brazil is approximately 40 Gl/yr and the potential market for biodiesel currently of 800 Ml and that should achieve 2 Gl by 2013. In addition, B5 has been mandatory since 2010. Auction prices have varied between US$ 0.3 and 0.8/l according to the area of production (Barros et al., 2006). Between 1975 and 1999, US$ 5 bn were invested in bioenergy resulting in the creation of 700,000 new jobs and US$ 43 bn saving in gasoline imports (Moreira & Goldemberg, 1999). The rate of job creation related to biodiesel production has been estimated to be 1.16 jobs/Ml of annual production (Johnston & Holloway, 2007). However, the recent trend of business centralization is expected to reduce this rate (Hall et al., 2009). Petrobras is now processing (with a capacity of 425,000 t) a mixture of plant oil and crude oil under the name of "H-Bio". With a tropical climate in the major part of its extention, the country has a potential 90 Mha that could be used for oleaginous crop production and that extends over Mato Grosso (southwest), Goias, Tocantins, Minas Gerais (center), Bahia Piaui, and Maranhao (northeast).

The EU accounts for 454 million people (i. e., 7% of the world’s population and 50% more people than live in the US) (Solomon & Banerjee, 2006). The EU is dedicated to a long-term conversion to a hydrogen economy. Renewable energy sources and eventually advanced nuclear power, are envisioned as the principal hydrogen sources on the horizon for use in 2020-2050 (Adamson, 2004). However, even for the distant future, the EU foresees hydrogen production from fossil fuels with carbon sequestration still playing a major role (together with renewable energy and nuclear power). Because of their renewability, biodiesel and bioethanol in the EU have been calculated to result in 15-70% GHG savings when compared to fossil fuels. Frondel and Peters (2007) found that the energy and GHG balances of rapeseed biodiesel are clearly positive.

Bioethanol from sugar beets or wheat and biodiesel from rapeseed are currently the most important options available to the EU for reaching its target biofuel production. Because of increased land use for biofuel production, biofuel crops are now competing with food crops (Odling-Smee, 2007) and they are expected to have substantial effects on the economy. The European consumption of fossil diesel fuel is estimated to be approximately 210 Gl and that of biodiesel to be 9.6 Gl (Malga & Freire, 2011). The EU produces over ~2 Mha (i. e., ~1 Gl) of rapeseed (0.5 kl/ha) and sunflower (0.6 kl/ha) (Fischer et al., 2010), which shows that it depends heavily on importation of biofuels to approach the recommended target of B5.75. Given the higher energy potential of synfuel from biomass and the constraints on the availability of arable land, second-generation biofuels should soon enter the race for biofuel production (Fischer et al., 2010; Havlik et al., 2010).

The price for biodiesel that meets the EU quality standard (EN 14214) is approximately € 730/t. By subtracting the biodiesel export value from the EU market price, one obtains the profit obtained by selling biodiesel from abroad on that market. The export value includes production and exportation costs. Production costs are made up of the plant oil or animal fat production plus the biodiesel processing minus the value of by-products (glycerol for example). Exportation costs include scaling, insurance, taxes and administrative costs (see the calculations in Johnston & Holloway, 2007). The price of US$ 0.88/l for biodiesel was 45% higher than the price of fossil diesel fuel during the same period (2006). Although this price is a convenient baseline, the biodiesel price on the EU market can change quickly depending on factors such as current domestic production, fossil diesel-fuel prices, agricultural yields, and legislation. The same rules will apply to emerging markets in China. Based on volume and profitability estimated in this manner, the top five countries that have the best combination of high volumes and low production costs are Malaysia, Indonesia, Argentina, the US, and Brazil. Collectively, these countries account for over 80% of the total biodiesel production. Plant oils currently used in biodiesel production account for only approximately 2% of global vegetable — oil production, with the remainder going primarily to food supplies.

Despite the fact that India has not attained the high level of ethanol production seen in Brazil, it is the largest producer of sugar in the world. Indian ethanol is blended at 5% with gasoline in nine Indian states and an additional 500 Ml would be needed for full directive implementation. The total demand for ethanol is approximately 4.6 Gl (Subramanian et al., 2005). The country burns 3 times more fossil diesel fuel than gasoline (i. e., roughly 44 Mt), mainly for transportation purposes.

Because India imports 70% of its fuel (~111 Mt), any source of renewable energy is welcome. Therefore, India has established a market for 10% biodiesel blends (Kumar & Sharma, 2008). Because India is a net importer of edible oils, it emphasizes non-edible oils from plants such as physic nut, karanja, neem, mahua and simarouba. Physic nut and karanja are the two leaders on the Indian plant list for biodiesel production.

Of its 306 Mha of land, 173 Mha are already under cultivation. The remainder is classified as either eroded farmland or non-arable wasteland. Nearly 40% (80-100 Mha) of the land area is degraded because of improper land use and population pressures over a number of years. These wasted areas are considered candidates for restoration with physic nuts (Kumar & Sharma, 2008). Nearly 80,000 of India’s 600,000 villages currently have no access to fuel or electricity, in part because there is not enough fuel to warrant a complete distribution network. Physic nuts could bring oil directly into the villages and allow them to develop their local economies (Fairless, 2007). This also applies to developing areas of Brazil and Africa.

In addition to the biodiesel initiative, regular motorcycles with 100 cm3 internal combustion engines have been converted to run on hydrogen. The efficiency of these motorcycles has been proven to be greater than 50 km/ charge. This development has had great significance because 70% of privately owned vehicles in India are motorcycles and scooters. Efforts are also underway to adapt light cars and buses to hydrogen, a move that will likely be helped by the growing number of electric and compressed natural gas (CNG) vehicles in and around New Delhi (Solomon & Banerjee, 2006).

In China, the area of arable land per capita is lower than the world’s average. As a result, most edible oils are imported and the demand for edible oils in 2010 is projected to be 13.5 Mt. Because of its large population, China desperately needs sustainable energy sources. Because little arable land is available, China is exploring possibilities for the production of second — and third-generation biofuels (Meng et al., 2008). China is a large developing country that has vast degraded lands and that needs large quantities of renewable energy to meet its rapidly growing economy and accompanying demands for sustainable development. The energy output of biomass grown on degraded soil is nearly equal to that of ethanol from conventional corn grown on fertile soil. Biofuel from biomass is far more economic than conventional biofuels such as corn ethanol or soybean biodiesel. Potential energy production from biomass could reach 6,350,971 terajoules per year (TJ/yr) and an increased value of biomass in China’s energy portfolio is considered unavoidable (Zhou et al., 2008).

Taking advantage of seawater availability, biodiesel from microalgae could also be conveniently grown along the 18,000 km Chinese coastline (Song et al., 2008). Marine microalgae production requires unused desert land, seawater, CO2 and sunshine. Given the abundant areas of mudflats and saline lands in China, there is great potential to develop biodiesel production from marine microalgae.

Sales of electric bicycles and scooters in China have grown dramatically in the last 10 years and now total over 1 million per year. The growth of this demand has been facilitated by bans on gasoline-fueled bicycles and scooters in Beijing and Shanghai (among other large cities) because of increasing concerns about pollution (Solomon & Banerjee, 2006). For this reason, China has become one of the largest potential markets for hydrogen fuel cells in the transportation sector.

Frequent droughts in many Asian countries have made it difficult for them to replicate Brazil’s success with sugarcane, which needs an abundant water supply. Thailand and Indonesia are tapping the potential with palm oil.

Because of its need to retain its position as the high-tech superpower for new technologies, Japan has become one of the most important players in the international development of a hydrogen-based economy. Following Japanese estimations, the hydrogen production potential from renewable energy in Japan is 210 GNm3/ yr (Nm3 is the gas volume in m3 at 0 °C and one atmosphere), which is 4 times more than what it will actually need in 2030. However, hydrogen based on renewable sources is only expected to contribute approximately 15% of the hydrogen consumed by 2030. It is estimated that on-board reforming of methanol or gasoline for fuel cell propelling would be the most practical technology in the near term, but the long-term goal is to adopt pure hydrogen (Solomon & Banerjee, 2006).

Nanotechnology can be simply defined as the discipline of building machines/devices on the scale of molecules, a few Nanometers (10"9m) wide, way smaller than a cell. Table 1 below show some practical applications of nanotechnologies and confirms the vastness of their domain [7]:

In the practically important area of polymers, nanotechnologies originate nano-structured polymers, where applications can be found in support structures; manufacturing processes; diagnostics and therapy; pharmaceuticals; medical and dental prosthesis; and thin films for surface treatment. The main chemicals involved in nano-structured polymers are: poly­oxides; poly-acrylates; poly-vinylics; poly-saccharides; and poly-ethylenes. The main materials incorporated into polymer nano-matrices are silicon, chromium and carbonTurning algae into biofuels

As shown in Fig. 1, biofuels derived from algae offer a great potential in view of the possible high yields and smaller area requirements. In addition, algae can play a role in carbon mitigation, as one way of growing algae is to feed them carbon-dioxide (CO2), besides water and sunlight. Algae can be fed other substrates as well, because to grow, cost-effectively, on carbon dioxide there would be a need of concentrated sources of the gas, such as found in combustion off-gases from fossil fueled power plants.

Oil can form up to 50% of the algae mass, in contrast with the best oil-bearing plants — oil palm trees — where less than 20% of the biomass is made out of foil. Algae carbohydrates can also be made into ethanol or gasified into bio-gas, or methane or hydrogen [9].

But, algae development into biofuels must overcome a number of challenges before algae can become significant sources of commercial biofuels. Since algae also need water to grow,
expansion of algae production may create a dilemma of water versus fuel, similar to food versus fuel dilemma discussed previously. Another challenge is the low natural carbon dioxide concentration in the atmosphere, hence the consideration of additional sources of carbon for algal growth in a commercial biofuels system. One response to these challenges may include the use of nanotechnology to turn algae into biofuels.

As way of examples, in 2009, the company QuantumSphere received a grant from the California Energy Commission to develop a nano-catalyzed algae biogasification. Also in California, the Salton Sea receives large amounts of agricultural runoff, which sometimes create large algae blooms. These algae and similar biomass have been turned experimentally into methane, hydrogen and other gases [10].

One nanotechnology relevant to algae development is the use of nanoparticles as no-harm harvesters of biofuel oils from algae, as illustrated in Fig. 3 [11]. The nano particles are shown on the left hand side of the photograph before the oil pregnant algae are added. The right hand side shows the contacting between the algae and the nano particles, which results in extracting the oil without harming the algae. Maintaining the algae alive can dramatically reduce production costs and the generation cycle.

One possible downside of the nano-harvesters is the risk that they may be released into the environment, although the spherical nano-particles are made of calcium compounds and sand [12]. The pores of the spheres are lined with chemicals, which extract algal oil without breaking the cell membrane. Nevertheless, prior to commercial market penetration of nano­harvesting, there would be a need to carry out due diligence to ensure the safety of these processes.

The ethanol industry is dynamic and has been evolving over the years in order to overcome various challenges associated with both fuel and coproduct processing and use (Rosentrater,

2007) . A modern dry grind ethanol plant is considerably different from the inefficient, input­intensive Gasohol plants of the 1970s. New developments and technological innovations, to name but a few, include more effective enzymes, higher starch conversions, better fermentations, cold cook technologies, improved drying systems, decreased energy consumption throughout the plant, increased water efficiency and recycling, and decreased emissions. Energy and mass balances are becoming more efficient over time. Many of these improvements can be attributed to the design and operation of the equipment used in modern ethanol plants. A large part is also due to computer-based instrumentation and control systems.

Many formal and informal studies have been devoted to adjusting existing processes in order to improve and optimize the quality of the coproducts which are produced. Ethanol companies have recognized the need to produce more consistent, higher quality DDGS which will better serve the needs of livestock producers. The sale of DDGS and the other coproducts has been one key to the industry’s success so far, and will continue to be important to the long-term sustainability of the industry. Although the majority of DDGS is currently consumed by beef and dairy cattle, use in monogastric diets, especially swine and poultry, continues to increase. And use in non-traditional species, such as fish, horses, and pets has been increasing as well.

Additionally, there has been considerable interest in developing improved mechanisms for delivering and feeding DDGS to livestock vis-a-vis pelleting/densification (Figure 10). This is a processing operation that could result in significantly better storage and handling characteristics of the DDGS, and it would drastically lower the cost of rail transportation and logistics (due to increased bulk density and better flowability) (Figure 11). Pelleting could also broaden the use of DDGS domestically (e. g., improved ability to use DDGS for rangeland beef cattle feeding and dairy cattle feeding) as well as globally (e. g., increased bulk density would result in considerable freight savings in bulk vessels and containers).

There are also many new developments underway in terms of evolving coproducts. These will ultimately result in more value streams from the corn kernel (i. e., upstream fractionation) as well as the resulting distillers grains (i. e., downstream fractionation) (Figure 12). Effective fractionation can result in the separation of high-, mid-, and low-value components. Many plants have begun adding capabilities to concentrate nutrient streams such as oil, protein, and fiber into specific fractions, which can then be used for targeted markets and specific uses. These new processes are resulting in new types of distillers grains (Figure 13).

Fig. 13. Examples of traditional, unmodified DDGS and some fractionated products (e. g., high-protein and low-fat DDGS) which are becoming commercially available in the marketplace.

For example, if the lipids are removed from the DDGS (Figure 14), they can readily be converted into biodiesel, although they cannot be used for food grade corn oil, because they are too degraded structurally. Another example is concentrated proteins, which can be used for high-value animal feeds (such as aquaculture or pet foods), or other feed applications which require high protein levels. Additionally, DDGS proteins can be used in human foods (Figure 15). Furthermore, other components, such as amino acids, organic acids, or even nutraceutical compounds (such as phytosterols and tycopherols) can be harvested and used in high-value applications.

Mid-value components, such as fiber, can be used as biofillers for plastic composites (Figure 16), as feedstocks for the production of bioenergy (e. g., heat and electricity at the ethanol plant via thermochemical conversion) (Figure 17), or, after pretreatment to break down the lignocellulosic structures, as substrates for the further production of ethanol or other biofuels.

In terms of potential uses for the low-value components, hopefully mechanisms will be developed to alter their structures and render them useful, so that they will not have to be landfilled. Fertilizers are necessary in order to sustainably maintain the flow of corn grain into the ethanol plant, so land application may be an appropriate venue for the low value components.

As these process modifications are developed, validated, and commercially implemented, improvements in the generated coproducts will be realized and unique materials will be produced. Of course, these new products will require extensive investigation in order to determine how to optimally use them and to quantify their value propositions in the marketplace.

Fig. 15. As a partial substitute for flour, high-value DDGS protein can be used to improve the nutrition of various baked foods such as (A) bread, (B) flat bread, and (C) snack foods, by increasing protein levels and decreasing starch content.

5. Conclusion

The fuel ethanol industry has been rapidly expanding in recent years in response to government mandates, but also due to increased demand for alternative fuels. This has become especially true as the price of gasoline has escalated and fluctuated so drastically, and the consumer has begun to perceive fuel prices as problematic. Corn-based ethanol is not the entire solution to our transportation fuel needs. But it is clearly a key component to the overall goal of energy independence. Corn ethanol will continue to play a leading role in the emerging bioeconomy, as it has proven the effectiveness of industrial-scale biotechnology and bioprocessing for the production of fuel. And it has set the stage for advanced biorefineries and manufacturing techniques that will produce the next several generations of advanced biofuels. As the biofuel industry continues to evolve, coproduct materials (which ultimately may take a variety of forms, from a variety of biomass substrates) will remain a cornerstone to resource and economic sustainability. A promising mechanism to achieve sustainability will entail integrated systems (Figure 18), where material and energy streams cycle and recycle (i. e., upstream outputs become downstream inputs) between various components of a biorefinery, animal feeding operation, energy (i. e., heat, electricity, steam, etc.) production system, feedstock production system, and other systems. By integrating these various components, a diversified portfolio will not only produce fuel, but also fertilizer, feed, food, industrial products, energy, and most importantly, will be self-sustaining.

3.1.2.1 What is compost?

Similarly like in the evaluation of digestate when the daily practice has simplified the problem very much because the main functions of mineral and organic fertilisers are not distinguished from each other, the simplification of the problem of composting and application of composts has also led to an absurd situation. In many countries the compost is understood to be a more or less decomposed organic material, mostly from biodegradable waste, which contains a certain small amount of mineral nutrients and water. The main requirement, mostly defined by a standard, is prescribed nutrient content, minimum amount of dry matter, absence of hazardous elements and the fact that the particles of original organic material are so decomposed that the origin of such material cannot be identified. Such ‘pseudo’ composts are often offered to farmers at a very low cost because the costs of their production are usually paid by producers of biodegradable waste who want to dispose of difficult waste.

The producers of such composts often wonder why farmers do not intend to buy these composts in spite of the relatively low cost. It is so because the yield effect of fertilisation with these composts is minimal, due to a low content of nutrients it is necessary to apply tens of tons per 1 ha (10 000 m2), which increases transportation and handling costs. In comparison with so called "green manure", i. e. ploughing down green fresh matter of clover, lucerne, stubble catch crops and crops designed for green manure, e. g. mustard, some rape varieties, etc., the fertilisation with these false composts does not have any advantage. The highly efficient decomposing activity of soil microorganisms, supported by equalising the C : N ratio to the value 15 — 25 : 1, works in the soil similarly like the composting process in a compost pile where the disposal of biodegradable material is preferred at the cost of a benefit to farmers.

What should the real compost be like? It is evident from the definition: the compost is a decomposed, partly humified organomineral material in which a part of its organic component is stabilised by the mineral colloid fraction. It is characterised by high ion — exchange capacity, high buffering capacity and is resistant to fast mineralisation. The reader of this text has surely noticed that the nutrients have not been mentioned here at all. Of course, they are present in the compost, their amount may be higher or a lower, but it is not important. It is crucial that the compost will maintain nutrients in the soil by its ion — exchange reactions and that it will protect them against elution from topsoil and subsoil layers to bottom soil or even to groundwater, no matter whether these plant nutrients originate from the compost itself or from mineral fertilisers or from a natural source — the soil-forming substrate in the soil-forming process. In the production of such "genuine" compost it is necessary to ensure that organic matter of the original composted mixture will be transformed not only by decomposing mineralisation, exothermic oxidation processes but also partly by an endothermic humification process that is not a decomposing one, but on the contrary, it is a synthetic process producing high-molecular, polycondensed and polymeric compounds, humic acids, fulvic acids and humins, i. e. the components of soil humus. It is to note that we should not confound the terms "humus" and "primary soil organic matter"; these are completely different mixtures of compounds, of quite different properties! Humus is characterised by high ion-exchange capacity and very slow mineralisation (the half-time of mineralisation of humic acids in soil conditions is 3 000 — 6 000 years!) while primary organic matter, though completely decomposed but not humified, has just opposite properties. Sometimes it may have a high sorption capacity but not an ion — exchange capacity.

The high ion-exchange capacity of humified organic matter is a cause of other two very important phenomena: huge surface forces of humus colloids in soil lead to a reaction with similarly active mineral colloids, which are all mineral soil particles of silicate nature that are smaller than 0.001 mm in size. These particles are called "physical clay" in pedology. The smaller the particles, the larger their specific surface, which implies their higher surface activity. Clay-humus aggregates are formed, which are adsorption complexes, elementary units of well-aerated, mechanically stable and elastic soil microaggregates that may further aggregate to macroaggregates and to form the structured well-aerated soil that has a sufficient amount of capillary, semi-capillary and non-capillary pores and so it handles precipitation water very well: in drought capillary pores draw water upward from the bottom soil while in a rainy period non-capillary pores conduct water in an opposite direction. The basic requirement for soil productivity is met in this way. It is often much more important than the concentration of nutrients in the soil solution (and hence in the soil).

The other important phenomenon related to ion-exchange properties of compost or soil is buffering capacity, the capacity of resisting to a change in pH. Soils generally undergo acidification, not only through acid rains as orthodox ecologists often frighten us but also mainly by electrolytic dissociation of physiologically acid fertilisers and intensive uptake of nutrients from the soil solution by plants. By the uptake of nutrient cations plants balance electroneutrality by the H+ ion, which is produced by water dissociation, so that the total electric charge does not change. If it were not so, each plant would be electrically charged like an electrical capacitor. The humus or clay or clay-humus ion exchanger in compost or in soil, similarly like any other ion exchanger, behaves in the same way as the plant during nutrient uptake: when any ion is in excess in the environment, e. g. H+ in an acidifying soil, the plant binds this H+ and exchanges it for another cation that was bound by it before. The

H+ ion is blocked in this way and the pH of soil does not change. High buffering capacity is a very favourable soil property and is typical of soils with a high content of mineral or organic colloid fraction, i. e. of heavy-textured soils and of organic soils with a high degree of humification of soil organic matter.

As described above, it is quite obvious what soils should be fertilised with real genuine composts preferentially: these are mainly light sandy and sandy-loam soils in which mineralisation processes are so fast due to high aeration that the organic matter of potentially applied organic fertilisers factually "burns". Mineral nutrients are released from an organic fertiliser but very soon there is a lack of necessary organic matter in such a soil. Energy for the soil microedaphon is not sufficient, ion-exchange capacity is low because decomposed organic matter fails to undergo humification. Such a soil does not hold water while rainfall quickly leaches nutrients from the soil. Only the application of genuine composts can markedly improve the productivity of these soils. Their clay-stabilised organic matter resists the attack of oxygen excess and remains decomposable, so it is able to maintain the required microbial activity of soil.

Pressure Swing Adsorption (PSA) is the second most employed techniques for biogas upgrading. Several companies develop and commercialize this technology: Carbotech (www. carbotech. de), Acrona (www. acrona-systems. com), Cirmac (www. cirmac. com), Gasrec (www. gasrec. co. uk), Xebec Inc (www. xebecinc. com), Guild Associates (www. moleculargate. com), etc. Small scale plants (flowrate of 10 m3/hour of biogas) are in operation, but this technology is also available for much higher flowrates (10000 m3/hour of biogas).

In PSA processes, biogas is compressed to a pressure between 4-10 bar and is fed to a vessel (column) where is putted in contact with a material (adsorbent) that will selectively retain CO2. The adsorbent is a porous solid, normally with high surface area. Most of the adsorbents employed in the commercial processes are carbon molecular sieves (CMS) but also activated carbons, zeolites and other materials (titanosilicates) are employed. The purified CH4 is recovered at the top of the column with a very small pressure drop. After certain time, the adsorbent is saturated with CO2, and the column needs to be regenerated by reducing the pressure (normally to vacuum for biogas upgrading). The adsorption of H2S is normally irreversible in the adsorbents and thus a process to eliminate this gas should be placed before the PSA. Alternatively, depending on the choice of the adsorbent, the humidity contained in the biogas stream can be removed together with CO2 in the same unit. Multi-column arrays are employed to emulate a continuous process. For small applications subjected to discontinuities, a single column with storage tanks may be used. One of the most important properties of the PSA process is that is can be adapted to biogas upgrading in any part of the world since it does not depend on the availability of cold or hot sources. A detailed explanation of this process follows.

Extensive bioethanol and biodiesel implantation has been followed by a panoply of economic, sociopolitical and environmental issues (Guerrero-Compean, 2008). It is worth noting the strong dependency of these biofuels industries on crops used for human nourishment and the feeding of livestock (UNCTAD, 2010). Although a large number of patents have been proposed to solve many technical problems, the sudden peak in demand for biofuels has uncovered serious technical limitations of the currently used production systems. As a consequence, a growing controversy about the real sustainability and environmental friendliness of the actual biofuels industry has been generated (Fortman et al., 2008; Abdullah et al., 2009; Demirba§, 2009; Yee et al., 2009; Jaruwat et al., 2010).

In addition, the consequences of biofuel production for farming practices or food markets, as well as real greenhouse gases (GHG) emission reduction along the biofuel life cycle, represent an important issue that, frequently, is not clearly treated. Parameters such as the kind of biofuel under study, feedstock, and energy inputs needed to maintain the process of transformation need to be taken into account. Also, the possibility of cogeneration of electricity or the exchange of energy between the biofuel synthesis and the feedstock transformation processes must be added to the model. Thus, wide variations in the net energy gain and consumption of resources can occur owing to the different assumptions made to calculate the overall benefits and drawbacks. Timilsina and collaborators draw a general picture of this issue over the OECD estimations. According to these authors, the most efficient biofuel production scheme is represented by sugarcane-based bioethanol in Brazil, with a 90% GHG reduction with respect to the gasoline equivalent. This high efficiency relies mainly on the high yield of this crop and the usage of sugarcane as an energy source for production plants and the cogeneration of electricity. Second-generation biofuels based on cellulosic feedstocks present a 70-90% GHG reduction relative to gasoline or diesel. Combined with electricity cogeneration, this kind of biofuel could have an even greater effect on GHG reduction, but they are still under development. Ethanol from sugar beet GHG reduction ranges from 40 to 60%, while wheat-based ethanol presents a 30-50% GHG reduction. The corn-based production of bioethanol is the least GHG-reducing biofuel and presents a low efficiency at GHG reductions varying from 0 (even negative in some cases) to 50% compared to gasoline (OECD, 2008; Timilsina & Shrestha, 2010).

Ethanol provides the first model for biofuel commercialization. However, in order to make the cellulosic ethanol process economically viable, both government subsidies and scientific

R&D are still required. And it is generally accepted that ethanol alone is not going to provide a long-term solution to meet society’s energy needs (Hill et al., 2006). It suffers from a somewhat low energy density, inability to be transported through pipelines and fairly high cost for extraction from fermentation broths. This is opening the door to developing many other molecules as replacements for ethanol and thus, discovering new fuel molecules to be produced via microbial biotechnology.

There are two basic procedures for transforming solid biomass into liquid or gaseous biofuels. The first is to transform it by microbiological fermentation (Gavrilescu & Chisti,

2005) (i. e., to convert the polysaccharides into alcohols such as bioethanol or biobutanol) or to convert raw plant biomass by anaerobic fermentation into biomethane (Demirbas & Balat,

2006) . One of the main drawbacks of the anaerobic system is that it is slow (because of the small amount of energy that is available to the organisms). Therefore, the amount of methane that can be produced is limited and this technology is only sustainable under selected scenarios (Asam et al., 2011). However, the introduction of even a small installation for transforming agricultural and human wastes into methane can have an enormous effect on the living standards of small communities (Arthur et al., 2011; Parker, 2002).

The second procedure aims to thermochemically convert the total biomass into a synthesis gas of high calorific value (also called syngas, i. e., H2 + CO) with subsequent production of various liquid and gaseous fuels (Tijmensen et al., 2002). The production of syngas is a potential area for large-scale CO2 conversion and utilization. The reforming of CO2 to CH4 has been extensively studied and reported on in the literature (Song & Pan, 2004). The catalytic reduction of CO2 to form methanol (or even CH4) using renewable energy sources could become a viable alternative to scarce or expensive fossil resources.

Biodiesel from plant oils and bioalcohol from sugar use only a portion of the total biomass. Next-generation processes are being developed to convert biomass to syngas (Baker & Keisler, 2011; Fagernas et al., 2010) that can then be converted into fuels or chemicals by a synthetic process (the so-called Fischer-Tropsch, or FT, process).

CH0.8 + 0.7O2 ^ CO + 0.4H2O (1)

CH0.8 + H2O ^ CO + 1.4H2 (2)

CO + H2O ^ CO2 + H2 (3)

(2n + 1)H2 + nCO ^ CnH2n+2 + nH2O (4)

Considering that coal inputs supply a 0.8 to 1 ratio of H/C, the whole FT process can be briefly written as follows. The partial oxidation of coal by oxygen gives equation (1). The interaction of water with carbon monoxide through "steam reforming" produces equation (2). Subsequently, the H/CO ratio is improved by "shifting" (transferring) the oxygen from the molecular water to CO, producing an additional hydrogen and carbon dioxide following equation (3). After removing the sulfur and carbon dioxide contaminants, the syngas is reacted over a catalyst in the FT reactor to produce high-quality clean fuels following the formula (4) (Greyvenstein et al., 2008).

Biomass is more reactive than coal and (depending on the technology) is usually gasified at temperatures of between 550 °C and 1,500 °C and at pressures varying between 4 and 30 bars (Damartzis & Zabaniotou 2011; Leibold et al., 2008; Steinberg, 2006). Typically, biomass is burned in an electrically heated furnace consisting of several multiple-tube units that can be heated separately up to 1,350 °C (Theis et al., 2006). Alternatively, the conversion of fossil fuel or biomass can be performed in hydrogen plasma. The temperature induced by an electric arc in hydrogen plasma is very high (~1,500 °C); therefore, this technology produces hydrogen and CO gas with a conversion rate of near 100% (Steinberg, 2006). FT synthesis generates intermediate products for synthetic fuels (Liu et al., 2007). The thermal efficiency of producing electricity and hydrogen through hydrogen plasma and carbon fuel cells varies from 87% to 92%, depending on the type of fuel and the biomass feedstock. This is more than twice as efficient as a conventional steam plant that burns coal and generates power with a ~38% efficiency. In addition, coupling hydrogen plasma and carbon fuel-cell technologies allows for a 75% reduction in CO2 emission per unit of electricity (Steinberg, 2006).

Because FT produces predominantly linear hydrocarbon chains, this process is currently attracting considerable interest. FT has already been applied on a commercial scale by Sasol, Petro S. A. and Shell, mainly to produce transportation fuels and chemicals (the feedstock being coal or natural gas). This fuel option has several notable advantages. First, the FT process can produce hydrocarbons of different lengths (typically <C15, Liu et al., 2007) from any carbonaceous feedstocks; these hydrocarbons can then be refined to easily transportable liquid fuels. Secondly, because of their functional similarities to conventional refinery products, the synthetic fuels (synfuels) produced by the FT process (i) can be handled by existing transportation systems; (ii) can be stored in refueling infrastructure for petroleum products; (ii) are largely compatible with current vehicles; and (iv) can be blended with current petroleum fuels (Tijmensen et al., 2002). Thirdly, synfuels are of high quality (this is especially true for FT diesel), have a very high cetane number and are free of sulfur, nitrogen, aromatics, and other contaminants typically found in petroleum products. The principal drawbacks of the FT process are that the capital cost of FT-conversion plants is relatively high and that the energy efficiency for the production of FT liquids by conventional techniques is lower than the energy efficiency for the production of alternative fuels (Takeshita & Yamaji, 2008).

3.1 Use of rapeseed SVO in diesel engines

Rapeseed oil can be used as fuel in diesel engines. Other vegetable oils can also be used as SVO to fuel diesel engines because they have similar properties. In Table 2 the properties of different oils are shown. The differences in the oil properties are small. However, to replace diesel fuel, some modifications are required to adjust the physical properties of the oil to be pumped to the engine and pulverized in diesel common injectors.

The modifications are aimed to heat the rapeseed oil to reduce its viscosity and density. During start-up, the vehicle runs with diesel to avoid the engine working at low temperatures with straight vegetable oil. Once the engine has warmed, it will be able to heat and use SVO. Note that the engine shouldn’t be stopped for a long time when using SVO, otherwise it will be complicated to cold start the engine with SVO.

The components that need to be installed in the fuel supply system:

— an additional deposit for the start-up diesel

— a water-oil heat exchanger

— a temperature sensor

— two solenoid valves to select the fuel to be used

— filters for oil and diesel fuels

The use of vegetable oil as fuel started long ago. Rudolf Diesel used peanut oil to run a diesel engine at the World Exhibition in Paris in 1900 (Baquero et al., 2010). He also suggested that vegetable oils could be the future fuel for diesel engines, but diesel fuel from oil substituted vegetable oil due to its abundance and price.

The use of SVO in diesel engines carries also some difficulties, namely:

— difficulties in operating the motor itself because of the different ignition temperatures of the two fuels. These difficulties can be solved just by preheating the vegetable oil.

— problems of engine durability due to deposit formation in the combustion chamber and mix of the vegetable oil with the engine lubricating oil. The first problem is solved by increasing the vegetable oil temperature, so it decreases its viscosity and density, which allows a correct injection and burning of the vegetable oil. The second problem is solved by reducing the life of the engine lubricant, (Agarwal et al., 2008; Vaitilingom et al., 2008). Despite these difficulties, it is noteworthy that both fuels have very similar energy content: 34.42 MJ/l for rapeseed SVO and 35.81 MJ/l for diesel fuel. This makes the engine performance and consumption very similar for both fuels. If we compare the performance of both fuels in the same engine, experimental results show that the performance of a vehicle running on diesel is optimal at low loads, whereas working with vegetable oil is optimal at high loads.

Microalgae can be cultivated in open-culture systems such as lakes or (raceway) ponds, and in closed-culture systems called photobioreactors (PBRs). Open-culture systems are normally cheaper to build and operate, more durable and have a higher production capacity than PBRs. However, open systems are more energy expensive in terms of nutrient distribution owing to mass transfer problems, and have their depth limited to 15 cm, to ensure that the microalgae receive enough light to grow. Moreover, ponds are more sensitive to weather changes, and temperature, evaporation and light intensity controls are not feasible. Furthermore, these open systems require more land area than PBRs, and are more susceptible to contamination, both from bacteria and from microalgae present in the surroundings of culture installations (Manzanera, 2011).

In contrast, PBRs are more flexible and are intensive land-usage systems that can be configured according to the specific physical-chemical requirements of the algae of choice, allowing the cultivation of species unsuited to open ponds. Nutrient homogenization, light distribution, pH, temperature, CO2 and O2 control can be achieved in photobioreactors. Thus, closed systems provide more stable and appropriate growing conditions, allowing higher cell densities and minimizing contamination. Nevertheless, PBRs have several technical problems that make them non-competitive in applications that can be achieved in raceway ponds. Such problems are overheating, bio-fouling, shearing stress, oxygen accumulation, scaling-up difficulties and the high costs of building, operation and maintenance (Chen et al., 2011).

Within these problems, it is worth highlighting capital building investment and high operation costs. PBRs biomass production costs may be one order of magnitude higher than in open systems. If the biomass added value is high, PBRs can be competitive. Otherwise, open ponds will be the preferred option. However, the evaluation of performance of open and closed systems is complex and depends on several factors, such as algal species or productivity computation method. Three parameters are commonly used to evaluate productivity in microalgae cultivation installations. Firstly, volumetric productivity (VP), that is, productivity per unit of reactor volume (g/l • d). The second parameter is area productivity (AP), defined as productivity per unit of ground area occupied by the reactor (g/ m2 • d). The third one is illuminated surface productivity (ISP), namely the productivity per unit of reactor illuminated surface area (g/m2 • d). Nevertheless, the election of closed or open systems relies on more aspects apart from productivity, as will be discussed below (Richmond, 2010).